10,11 -dihydro-5h-dibenzo[b,f]azepine derivatives as photoinitiatiors in photopolymerisation for use in photocurable compositions

Abstract

The present invention relates to photoinitiators of formula (I) having improved performances and to their use in photopolymerization compositions. The invention also relates to a process for the photopolymerization of compositions comprising said photoinitiators as well as their use in articles of manufacture, including printed, coated, and fabricated assemblies.

Description

[0001] NOVEL PHOTOINITIATORS ABSTRACT The present invention relates to novel photoinitiators having improved performances and to their use in photopolymerization compositions. The invention also relates to a process for the photopolymerization of compositions comprising said photoinitiators as well as their use in articles of manufacture, including printed, coated, and fabricated assemblies. PRIOR ART The dynamic growth of UV curing is dependent on continued innovation to support this technology in the overcoming of always new challenges and this is reflected by rapid development of new materials needed for the formulations. In particular, one key component is the photoinitiator (PI), its role is that to convert light to chemical energy in the form of reactive intermediates. These intermediates are radicals able to initiate radical polymerization of the double bonds present in the formulation to be cured. The development in the field of photoinitiators is stimulated by various factors. Firstly, there is a continuous improvement of photoinitiators for existing applications such as coatings, inks, adhesives or electronics because there is no single photoinitiator that is able to meet all the requirements of each application, like for instance line speed, surface curing or solubility. Secondly, the increasing number of photoinitiators banned as toxic or reprotoxic. Thirdly, the introduction of new lamps, such as LED lamps, stimulated the development of photoinitiators tuned more in the UVA than the UVC region of the spectra. So, the research of new photoinitiators is always required in order to mimic standard photoinitiators or to overcome the problems just described. In particular many efforts were made in order to design photoinitiator able to work at LED wavelengths (mainly 365-405 nm). Some new classes were developed or improved (e.g. ketoquinolones (Polym. Chem., 2022, 13, 2659), acylgermanium photoinitiators (EP3150641, EP2649981), ketocoumarins (WO2017 / 216699), coumarin glyoxylate (WO2021 / 070152), but none of them was able to achieve the required performance. There is therefore a constant need of new technical solutions able to provide good performances during polymerization. AIMS OF THE INVENTION It is a first aim of the invention to provide for novel photoinitiators (PIs), their use as photoinitiators and photocurable compositions comprising them. It is a further aim of the invention to provide for photocurable compositions comprising the novel PIs of the invention. Is a further aim of the invention to provide for processes for photocuring ethylenically unsaturated compounds using the novel PIs of the invention, as well as articles of manufacture made by said process. SUMMARY OF THE INVENTION Now, surprising, we discovered that the presence of an iminodibenzyl core in a photoinitiator, is able to increase the reactivity of any photoinitiator group. These results are more outstanding when comparing the iminodibenzyl core with the carbazole core, from which differ only for a carbon atom. The use of the iminodibenzyl core led to an increase of reactivity of any class of photoinitiator tested showing the exceptional versatility of this core. Indeed, it was surprisingly found that photoinitiators bearing an iminodibenzyl core better react to UVA, UVB and UVC wavelength ranges and more preferably to LED sources emitting in the range from 350 to 420 nm, compared to photoinitiators disclosed in the prior art. DESCRIPTION OF THE INVENTION According to one of its aspects, the present invention relates to a compound of formula (I) wherein R1 and R2 are, each independently H or a photoinitiating group; and R3 is selected from H, -C1-C6-alkyl; -C(=O)-C1-C6-alkyl; -CH2-phenyl; and -phenyl; provided that R1 and R2 are not both H; and provided that when R1 is benzoyl and R3 is ethyl, then R2 is not H. The expression “photoinitiating group” herein designate a chemical moiety which, upon an appropriate light stimulus, generates reactive species which activate polymerization reactions. An appropriate light stimulus is for instance a visible or UV light stimulus. Reactive species are for instance radicals, cations and anions. According to the invention, R1 and R2 are, each independently, H or a photoinitiating group, preferably both a photoinitiating group, which activates photopolymerization of at least ethylenically unsaturated compounds. According to a preferred embodiment, the present invention relates to a compound of formula (Ia) Preferably, photoinitiating groups are selected from: - benzophenones - acetophenones - hydroxyacetophenones - diketones - alpha-hydroxyketones - alpha-aminoketones - alpha-OSi(alk)3 - alpha-OSi(alk)2Si(alk)3 - benzoin alkyl ethers - benzyl ketals - ketoxime esters - oxime esters - ketosulfones - glyoxylates - acyloximino esters - anthraquinones - phosphine oxides - acylphosphines - acylphosphine oxides - acylphosphine sulfides - benzoylformate esters - alpha-amino acids - polysilanes and - peroxy derivatives More preferably, R1 and R2 are each independently selected from H and one group (A) to (D):

[0002] wherein R4, R5, R6, R8, R9 and R10, each independently, are selected from H, C1- C6-alkyl, -CH2-phenyl and phenyl; n is 0 to 3; the wavy lines show the bonds linking to the benzene rings in formula (I) or (Ia); provided that when R1 is a group (A) wherein n is zero and R3 is ethyl, then R2 is not H; R7 is selected from OH, C1-C4alkoxy, dimethylamino, diethylamino, morpholino and a group of formula (E) to (G) wherein R11 is H or C1-C6-alkyl; m is 0 to 3; R12, R13, R14, R15, R16, R17, R18 and R19, each independently, are selected from phenyl, C1-C6alkyl; the wavy lines show the bonds linking to the carbon atom of group (B). Preferably, R3 is selected from H or -C1-C6-alkyl, more preferably methyl or ethyl. In the present description the expressions "alkyl” or “alkyl group" mean, where not differently indicated, a linear or branched, saturated alkyl chain containing the given number of carbon atoms and includes all possible isomer for each number of carbon atoms in the alkyl group, i.e. for three carbon atoms: n-propyl and isopropyl; for four carbon atoms: n-butyl, isobutyl and tertiary-butyl; for five carbon atoms: n-pentyl, 1,1-dimethyl-propyl, 2,2- dimethylpropyl and 2-methyl-butyl, and the like. Preferably, R11, R12, R13, R14, R15, R16, R17, R18 and R19 are C1-C4alkyl, more preferably methyl. Particularly preferred compounds of formula (I) and (Ia) are disclosed in the examples. According to one of its aspects, the present invention relates to the use of a compound of formula (I’) wherein R1 and R2 are, each independently H or a photoinitiating group; and R3 is selected from H, -C1-C6-alkyl; -C(=O)-C1-C6-alkyl; -CH2-phenyl; and -phenyl; as a photoinitiator. According to an embodiment, the present invention relates to the use of a compound of formula (Ia’) wherein R1 and R2 are, each independently H or a photoinitiating group; and R3 is selected from H, -C1-C6-alkyl; -C(=O)-C1-C6-alkyl; -CH2-phenyl; and -phenyl; as a photoinitiator. According to a preferred embodiment, in formula (I’) and (Ia’), R1, R2 are selected from groups (A) to (D) as above defined and R3 is selected from H or -C1-C6-alkyl, more preferably methyl or ethyl. The compounds of formula (I), (Ia), (I’) and (Ia’) can be prepared according to any suitable process. For example, it can be prepared by subjecting a compound of formula (II) wherein R3 is as above defined, to a Friedel-Crafts reaction, optionally in the presence of an appropriate solvent and a suitable catalyst, such as for instance AlCl3, to provide the compound of formula (I), (Ia), (I’) and (Ia’) wherein R1 and / or R2 are a group of formula (A) to (C). Compounds of formula (I), (Ia), (I’) and (Ia’) wherein R1 and / or R2 are a group of formula (D) the acyl derivative can be obtained by reaction of the corresponding acyl derivative with a suitable hydroxy-amine followed by esterification of the hydroxy group. Compound of formula (II) is known in the art or can be prepared according to known methods. When R3 is H, the nitrogen atom shall be protected with conventional protecting groups before the subjecting compound (II) to the Friedel-Crafts reaction. The skilled in the art is certainly able to prepare the compounds of the invention according to known analogy processes. Details of the process of the invention are anyway reported in the Experimental Section of the present description. Preferred compounds of formula (I), (Ia), (I’) and (Ia’) are the following:

[0003] According to another of its aspects, the present invention relates to a photopolymerizable composition comprising: a) from 50 to 99.7%, preferably from 70 to 98.9% by weight, based on the total content of the composition, of at least one ethylenically unsaturated compound; b) from 0.1 to 35%, preferably from 0.1 to 20%, and more preferably from 0.2 to 15% by weight, based on the total content of the composition, of at least one compound of formula (I), (Ia), (I’) or (Ia’) as above defined; and c) from 0 to 20% by weight, preferably from 0 to 15%, and more preferably from 0.2 to 15% by weight, based on the total content of the composition, of an accelerator and / or of a coinitiator. According to the present invention, the terms “photocuring” and “photopolymerizing” and related terms, are synonyms. The expression “based on the total content of the composition” means that the % weight amounts of any of the components is calculated with respect to the sum of the weight of all the components of the composition, including any possible further additional components (in addition to a), b) and c) above), but possible water and / or solvents which may be present in the composition are not considered for the calculation of said % weight amounts. According to another of its aspects, the present invention relates to a process for photocuring photopolymerizable compositions coatings, adhesives and inks, which process comprises: i. providing a photopolymerizable composition as above defined; ii. coating or printing said photopolymerizable composition onto a substrate, and iii. photocuring said coated or printed composition with a light source on said substrate. According to another of its aspects, the present invention relates to a process for three-dimensional printing which comprising photocuring with a light source a mixture comprising the composition as above defined. According to another of its aspects, the present invention relates to an article of manufacture obtained by the process of the invention. According to a preferred embodiment, the photopolymerizable composition used the processes of the invention comprises at least components (a), (b), (c), more preferably at least (a), (b) and (c). The photoinitiators of the invention can be used in photocurable compositions comprising ethylenically unsaturated compounds (a). Said unsaturated compounds (a) can contain one or more olefinic double bonds. They can be low-molecular weight (monomeric) or high-molecular weight (oligomeric) compounds. Examples of suitable low molecular weight monomers (monomeric compounds) having one double bond are alkyl or hydroxyalkyl acrylates or methacrylates, such as methyl-, ethyl-, butyl-, 2-ethylhexyl-,2-hydroxyethyl- or isobornyl-acrylate; and methyl or ethyl methacrylate. Further examples are resins modified with silicon or fluorine, e.g. silicone acrylates. Further examples of these monomers are acrylonitrile, acrylamide, methacrylamide, N-substituted (meth)acrylamides, styrene, alkylstyrenes and halogeno styrenes, vinyl esters such as vinyl acetate, vinyl ethers such as iso-butyl vinyl ether, N-vinylpyrrolidone, vinyl chloride or vinylidene chloride. Examples of monomers having more than one double bond are the ethylene glycol diacrylate, propylene glycol diacrylate, neopentyl glycol diacrylate, hexamethylene glycol diacrylate, bisphenol A diacrylate, 4,4'-bis-(2- acryloyloxyethoxy)-diphenylpropane, trimethylolpropane triacrylate, pentaerythritol triacrylate or tetraacrylate, vinyl acrylate, divinyl benzene, divinyl succinate, diallyl phthalate, triallyl phosphate, triallyl isocyanurate or tris-(2-acryloylethyl) isocyanurate. Examples of high-molecular weight (oligomeric) polyunsaturated compounds are acrylated epoxy resins, acrylated or vinyl-ether- or epoxy-group- containing polyesters, acrylated polyurethanes or acrylated polyethers. Further examples of unsaturated oligomers are unsaturated polyester resins which are usually prepared from maleic acid, phthalic acid and one or more diols and which have molecular weights of from about 500 Da to 3,000 Da. Such unsaturated oligomers can also be referred to as prepolymers. Examples of compounds (a) which are particularly suitable for the implementation of the present invention, are esters of ethylenically unsaturated carboxylic acids and polyols or polyepoxides, and polymers containing ethylenically unsaturated groups in the chain or in side groups, e.g. unsaturated polyesters, polyamides and polyurethanes and copolymers thereof, alkyl resins, polybutadiene and butadiene copolymers, polyisoprene and isoprene copolymers, polymers and copolymers having (meth)acrylic groups in side chains, as well as mixtures thereof. Illustrative examples of unsaturated carboxylic acids or anhydrides, useful for the preparation of the above esters, are acrylic acid, methacrylic acid, maleic anhydride, crotonic acid, itaconic acid, cinnamic acid and unsaturated fatty acids such as linolenic acid and oleic acid. Acrylic and methacrylic acid are preferred. Examples of polyols, which can also be esterified, are aromatic and aliphatic and cycloaliphatic polyols, preferably aliphatic and cycloaliphatic polyols. Aromatic polyols are, for example, hydroquinone, 4,4'-dihydroxydiphenyl, 2,2-di(4-hydroxyphenyl) propane, as well as novolaks and resoles. Polyepoxides, which can be esterified, include those based on the said polyols, especially the reaction products between aromatic polyols and epichlorohydrin. Also suitable as polyols are polymers and copolymers that contain hydroxyl groups in the polymer chain or in side groups, for example polyvinyl alcohol and copolymers thereof or polymethacrylic acid hydroxyalkyl esters or copolymers thereof. Further suitable polyols are oligoesters carrying hydroxyl terminal groups. Examples of aliphatic and cycloaliphatic polyols include alkylenediols containing preferably from 2 to 12 carbon atoms, such as ethylene glycol, 1,2- or 1,3-propanediol, 1,2-, 1,3- or 1,4-butanediol, pentanediol, hexanediol, octanediol, dodecanediol, diethylene glycol, triethylene glycol, polyethylene glycols having molecular weights of preferably from 200 Da to 1,500 Da, 1,3-cyclopentanediol, 1,2-, 1,3- or 1,4-cyclohexanediol, 1,4- dihydroxymethyl cyclohexane, glycerol, tris(E-hydroxy-ethyl)amine, trimethylolethane, trimethylolpropane, pentaerythritol, dipentaerythritol and sorbitol. Further suitable ethylenically unsaturated compounds (a) are unsaturated polyamides obtained from unsaturated carboxylic acids and aromatic, aliphatic and cycloaliphatic polyamines having preferably from 2 to 6, preferably from 2 to 4, amino groups. Examples of such polyamines are: ethylenediamine, 1,2- or 1,3-propylenediamine, 1,2-, 1,3- or 1,4- butylenediamine, 1,5-pentylenediamine, 1,6-hexylenediamine, octylenediamine, dodecylene diamine, 1,4-diaminocyclohexane, isophoronediamine, phenylene diamine, bisphenylenediamine, di-(E- aminoethyl) ether, diethylene triamine, triethylenetetramine and di(E- aminoethoxy)- and di(E-aminopropoxy)ethane. Other suitable polyamines are polymers and copolymers which may contain additional amino groups in the side chain and oligoamides containing amino end groups. Specific examples of such unsaturated polyamides are methylenebisacrylamide, 1,6-hexamethylene bisacrylamide, diethylenetriamine trismethacrylamide, bis(methacrylamidopropoxy) ethane and N-[(E-hydroxyethoxy)ethyl]-acrylamide. Unsaturated polyurethanes are also suitable for the implementation of the present invention as components (a), for example those derived from saturated or unsaturated diisocyanates and unsaturated or saturated diols. Polybutadiene and polyisoprene and copolymers thereof may also be used. Suitable monomers include, for example, olefins, such as ethylene, propene, butene and hexene, (meth)acrylates, acrylonitrile, styrene and vinyl chloride. Polymers having unsaturated (meth)acrylate groups in the side chain can also be used as component (a). They may typically be reaction products of epoxy resins based on novolac with (meth)acrylic acid; homo- or copolymers of vinyl alcohol or hydroxyalkyl derivatives thereof that have been esterified with (meth)acrylic acid; and homo- and co-polymers of (meth)acrylates that have been esterified with hydroxyalkyl (meth)acrylates. According to a preferred embodiment, the photocurable composition further comprises a coinitiators (c), also referred to as accelerators. Suitable examples of accelerators / coinitiators (c) are alcohols, thiols, thioethers, amines or ethers that have an available hydrogen, bonded to a carbon adjacent to the heteroatom, disulfides and phosphines, e.g. as described in EP 438 123 and GB 2 180 358. Suitable examples of amine accelerators / co-initiators include, but are not limited to, aliphatic, cycloaliphatic, aromatic, aryl-aliphatic, heterocyclic, oligomeric or polymeric amines. They can be primary, secondary or tertiary amines, for example butyl amine, dibutyl amine, tributyl amine, cyclohexyl amine, benzyldimethyl amine, di-cyclohexyl amine, N-phenyl glycine, triethyl amine, phenyl-diethanol amine, triethanolamine, piperidine, piperazine, morpholine, pyridine, quinoline, esters of dimethylamino benzoic acid, Michler’s ketone (4,4'-bis-dimethyl aminobenzophenone) and derivatives thereof. As the amine accelerators / co-initiators, an amine-modified acrylate compound can be used; examples of such amine-modified acrylate include acrylates modified by reaction with a primary or secondary amine that are described in US 3,844,916, EP 280222, US 5,482,649 or US 5,734,002. Multifunctional amine and polymeric amine derivatives are also suitable as co-initiators some examples are those presently marketed as Omnipol®ASA from IGM Resins B.V., Genopol®AB-2 from Rahn A.G., Speedcure®7040 from Lambson Limited or those described in US2013 / 0012611. The photocurable compositions of the present invention can also comprise one or more of the following components: (d) photosensitizers and / or (e) further photoinitiators and / or (f) conventional additives, in addition to compounds (a), (b) and, when present, (c). The photocurable compositions of the present invention can also be formulated in compositions further comprising water and / or solvents, such as organic solvents. Photosensitizers (d) can be present in an amount comprised between 0.01 and 15% by weight, based on the total content of the composition, preferably between 0.01 and 10% by weight. Examples of photosensitizers are those commonly used in the art, aromatic carbonyl compounds, e.g. benzophenones, thioxanthones, anthraquinones, coumarins and 3-acylcoumarin derivatives, terphenyls, styryl ketones, and 3- (aroylmethylene)-thiazolines, camphorquinones and also eosin, rhodamine and erythrosine dyes. Examples of thioxanthones are thioxanthone, 2-isopropyl thioxanthone, 2- chloro thioxanthone, 2-dodecyl thioxanthone, 2,4-diethyl thioxanthone, 2,4- dimethyl thioxanthone, 1-methoxycarbonyl thioxanthone, 2-ethoxycarbonyl thioxanthone, 3-(2-methoxyethoxycarbonyl) thioxanthone, 4- butoxycarbonyl thioxanthone, 3-butoxycarbonyl-7-methyl thioxanthone, 1- cyano-3-chloro thioxanthone, 1-ethoxycarbonyl-3-chloro thioxanthone, 1- ethoxycarbonyl-3-ethoxy thioxanthone, 1-ethoxycarbonyl-3-amino thioxanthone, 1-ethoxycarbonyl-3-phenylsulfuryl thioxanthone, 3,4-di[2-(2- methoxyethoxy)ethoxycarbonyl] thioxanthone, 1-ethoxycarbonyl-3-(1- methyl-1-morpholinoethyl) thioxanthone, 2-methyl-6-dimethoxymethyl thioxanthone, 2-methyl-6-(1,1-dimethoxybenzyl) thioxanthone, 2- morpholinomethyl thioxanthone, 2-methyl-6-morpholinomethyl thioxanthone, N-allylthioxanthone-3,4-dicarboximide, N-octylthioxanthone- 3,4-dicarboximide, N-(1,1,3,3-tetramethylbutyl)-thioxanthone-3,4- dicarboximide, 1-phenoxy thioxanthone, 6-ethoxycarbonyl-1-2- methoxythioxanthone, 6-ethoxycarbonyl-2-methylthioxanthone, thioxanthone-2-polyethylene glycol ester, 2-hydroxy-3-(3,4-dimethyl-9-oxo- 9H-thioxanthon-2-yloxy)-N,N,N-trimethyl-1-propanaminium chloride, or those described in the patent application PCT / EP2011 / 069514, such as n- dodecyl-7-methyl-thioxanthone-3-carboxylate and N,N-disobutyl-7-methyl- thioxanthone-3-carbamide. Also suitable are polymeric thioxanthone derivatives (e.g. those presently marketed as Omnipol®TX from IGM Resins B.V., Genopol®TX-1 from Rahn A.G., Speedcure®7010 from Lambson Limited). Example of benzophenones are benzophenone, 4-phenyl benzophenone, 4- methoxy benzophenone, 4,4'-dimethoxybenzophenone, 4,4'-dimethyl benzophenone, 4,4'-dichloro benzophenone, 4,4'-dimethylamino benzophenone, 4,4'-diethylamino benzophenone, 4-methyl benzophenone, 2,4,6-trimethyl benzophenone, 4-(4-methylthiophenyl) benzophenone, 3,3'- dimethyl-4-methoxy benzophenone, methyl 2-benzoyl benzoate, 4-(2- hydroxyethylthio) benzophenone, 4-(4-tolylthio) benzophenone, 4-benzoyl- N,N,N-trimethylbenzene methanaminium chloride, 2-hydroxy-3-(4- benzoylphenoxy)-N,N,N-trimethyl-1-propanaminium chloride monohydrate, 4-(13-acryloyl-1,4,7,10,13-pentaoxatridecyl) benzophenone, 4-benzoyl- N,N-dimethyl-N-[2-(1-oxo-2-propenyl)oxylethyl-benzene methanaminium chloride, or those described in US9938231 (e.g. Omnirad®991 from IGM Resins B.V.). Also suitable are polymeric benzophenone derivatives (e.g. those presently marketed as Omnipol®BP, Omnipol®2702 and Omnipol®682 all from IGM Resins B.V., Genopol®BP-2 from Rahn A.G. and Speedcure®7005 from Lambson Limited). Examples of 3-acylcoumarin derivatives are 3-benzoyl coumarin, 3-benzoyl- 7-methoxy coumarin, 3-benzoyl-5,7-di(propoxy) coumarin, 3-benzoyl-6,8- dichloro coumarin, 3-benzoyl-6-chloro coumarin, 3,3'-carbonyl-bis[5,7- di(propoxy) coumarin], 3,3'-carbonyl-bis(7-methoxy coumarin), 3,3'- carbonyl-bis(7-diethylamino coumarin), 3-isobutyroyl coumarin, 3-benzoyl- 5,7-dimethoxy coumarin, 3-benzoyl-5,7-diethoxy coumarin, 3-benzoyl-5,7- dibutoxy coumarin, 3-benzoyl-5,7-di(methoxyethoxy) coumarin, 3-benzoyl- 5,7-di(allyloxy) coumarin, 3-benzoyl-7-dimethylamino coumarin, 3-benzoyl- 7-diethylamino coumarin, 3-isobutyroyl-1,7-dimethylamino coumarin, 5,7- dimethoxy-3-(1-benzoyl) coumarin, 5,7-dimethoxy-3(1-benzoyl)-coumarin, 3-benzoylbenzo [f]coumarin, 7-diethylamino-3-thienoyl coumarin, 3-(4- cyanobenzoyl)-5,7-dimethoxy coumarin, or those described in EP2909243 and WO2017216699. Examples of 3-(aroylmethylene) thiazolines are 3-methy-1,2- benzoylmethylene-E-benzo thiazoline, 3-methyl-2-benzoylmethylene-benzo thiazoline, 3-ethyl-2-propionylmethylene^E^benzo thiazoline. Examples of other aromatic carbonyl compounds are acetophenone, 3- methoxyacetophenone, 4-phenylacetophenone, benzil, such as that described in WO 2013 / 164394, 2-acetylnaphthalene, 2-naphthaldehyde, 9,10-anthraquinone, 9-fluorenone, dibenzosuberone, xanthone, 2,5-bis(4- diethylaminobenzylidene) cyclopentanone, D-(para-dimethylamino benzylidene), ketones, such as 2-(4-dimethylamino-benzylidene)-indan-1- one or 3-(4-dimethylaminophenyl)-1-indan-5-yl-propenone, 3- phenylthiophthalimide, N-methyl-3,5-di(ethylthio) phthalimide. Particularly preferred are thioxanthones, coumarins and 3-acylcoumarins. It was observed that the above components (d) increase the activity of photoinitiators (b) without shortening the shelf life of the compositions. Moreover, such compositions have the special advantage that an appropriate choice of the photosensitizer (d) allows the spectral sensitivity of photoinitioator (b) to be shifted to any desired wavelength region. The skilled in the art is able to select the suitable photosensitizer (d) to make the photoinitiator(s) (b) work at any desired wavelength region. The further possible photoinitiators (e) can be present in an amount comprised between 0.5 and 15 % by weight, of the total content of the composition, preferably between 1 and 10% by weight of the composition. Examples of other suitable photoinitiators (e) are camphorquinone, benzophenone, benzophenone derivatives, acetophenone, acetophenone derivatives, dialkoxyacetophenones, D-hydroxyketones, D-aminoketones, 4- aroyl-1,3-dioxolanes, benzoin alkyl ethers and benzil ketals, e.g. benzil dimethyl ketal, ketosulfones, e.g 1-[4-[(4-benzoyl-phenyl)-thio]-phenyl]-2- methyl-2-[(4-methyl-phenyl)-sulfonyl]-propan-1-one (Esacure®1001, from IGM Resins B.V.), 3-ketocoumarins, for example as described in EP2909243 and WO2017216699, phenylglyoxylates and derivatives thereof, dimeric phenyl glyoxylates, peresters, e.g. benzophenonetetracarboxylic acid peresters, for example as described in EP 126 541, acylphosphine photoinitiators (which can be selected from mono-acylphosphine oxides, bis- acylphosphine oxides, tris-acylphosphine oxides and multifunctional mono- or bisacylphosphine oxides), halomethyltriazines, hexaaryl bisimidazole / coinitiator systems, e.g. ortho-chlorohexaphenylbisimidazole in combination with 2-mercaptobenzothiazole, ferrocenium compounds or titanocenes, for example dicyclopentadienyl-bis(2,6-difluoro-3-pyrrolo- phenyl)titanium, O-acyloxime ester photoinitiators. Examples of D-hydroxyketones and D^aminoketones are 1-hydroxy cyclohexylphenyl ketone, 2-hydroxy-2-methyl-1-phenyl-propane-1-one, 1- [4-(2-hydroxyethoxy)phenyl]-2-hydroxy-2-methyl-1-propane-1-one, 2- hydroxy-1-{4-[4-(2-hydroxy-2-methyl-propionyl)-benzyl]phenyl}-2- methyl-propane-1-one), 2-hydroxy-1-{4-[4-(2-hydroxy-2-methyl- propionyl)-phenoxy]-phenyl}-2-methyl-propan-1-one, 2-methyl-1-(4- methylthiophenyl)-2-morpholinopropane-1-one), 2-benzyl-2- dimethylamino-1-(4-morpholinophenyl)-butane-1-one, and (2- (dimethylamino)-2-[(4-methylphenyl)methyl]-1-[4-(4-morpholinyl) phenyl]- 1-butanone). Examples of O-acyloxime ester photoinitiators are 1,2-octanedione,1-[4- (phenylthio)phenyl]-2-(O-benzoyloxime), ethanone 1-[9-ethyl-6-(2- methylbenzoyl)-9H-carbazole-3-yl] 1-(O-acetyloxime) or those described in GB 2339571. Examples of acylphosphine photoinitiators include, but are not limited to, bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide, bis(2,6- dimethoxybenzoyl)-2,4,4-trimethylpentyl phosphine oxide, bis(2,4,6- trimethylbenzoyl)-(2,4-dipentyloxyphenyl), 2,4,6-trimethylbenzoyl-diphenyl phosphine oxide, ethyl (2,4,6-trimethylbenzoyl) phenylphosphinate, and phenyl(2,4,6-trimethylbenzoyl)phosphinic acid glycerol ethoxylated triester (Omnipol TP from IGM Resins B.V.). Examples of the halomethyltriazines based photoinitiators are 2-[2-(4- methoxy-phenyl)-vinyl]-4,6-bis-trichloromethyl [1,3,5]triazine, 2-(4- methoxy-phenyl)-4,6-bis-trichloromethyl [1,3,5]triazine, 2-(3,4- dimethoxyphenyl)-4,6-bis-trichloromethyl [1,3,5]triazine, 2-methyl-4,6-bis- trichloromethyl [1,3,5] triazine. Cationic photoinitiators can be also used as the further photoinitiators (e), when the photocurable compositions according to the invention are used in hybrid systems (which in this connection mean mixtures of free- radically and cationically curing systems). Examples of suitable cationic photoinitiators are aromatic sulfonium, phosphonium or iodonium salts, as described e.g. in US4,950,581, or cyclopentadienylarene-iron(II) complex salts, e.g. isopropylbenzene)(K5-cyclopentadienyl) iron(II) hexafluorophosphate or photolatent acids based on oximes, as described, for example, in GB 2 348 644, US4,450,598, US4,136,055, WO 00 / 10972 and WO 00 / 26219. The photocuring composition according to the invention may also comprise conventional additives, from 0 to 10% based on the total content of the composition. Additives (f) can be, for example, thermal initiators, binders, stabilizers, and mixture thereof. The choice of additives is governed by the field of use in question and the properties desired for that field. The additives (f) described above are known in the art and are accordingly used in the amounts conventionally used in the art. For instance, especially in the case of pigmented compositions, the composition may also comprise, as additional additive (f), a thermal initiator, a compound that forms free radicals when heated, e.g. an azo compounds, such as 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), a triazene, diazosulfide, pentazadiene or a peroxy compound, for example a hydroperoxide or peroxycarbonate, e.g. tert-butyl hydroperoxide, as described e.g. in EP 245 639. Binders may also be added to the photocurable composition of the invention. The addition of binders is particularly advantageous when the photocurable compounds are liquid or viscous substances. The amount of binder may be, for example, from 5 to 60% by weight, preferably from 10 to 50% by weight, based on the total content of the composition, excluding possible water and solvents. The choice of binder is made in accordance with the field of use and the properties required therefor, such as developability in aqueous and organic solvent systems, adhesion to substrates and sensitivity to oxygen. Suitable binders are, for example, polymers having a weight average molecular weight (Mw) of approximately from 5,000 Da to 2,000,000 Da, preferably from 10,000 Da to 1,000,000 Da. Illustrative examples are: homo- and copolymers of acrylates and methacrylates, e.g. copolymers of methyl methacrylate / ethyl acrylate / methacrylic acid, poly(methacrylic acid alkyl esters), poly(acrylic acid alkyl esters); cellulose esters and ethers, such as cellulose acetate, cellulose acetate butyrate, methylcellulose, ethylcellulose, polyvinylbutyral, polyvinylformal, cyclised rubber, polyethers such as polyethylene oxide, polypropylene oxide, polytetrahydrofuran, polystyrene, polycarbonates, polyurethanes, chlorinated polyolefins, e.g. polyvinyl chloride, co-polymers of vinyl chloride / vinylidene chloride, co-polymers of vinylidene chloride with acrylonitrile, methyl methacrylate and vinyl acetate, polyvinyl acetate, co-poly (ethylene / vinyl acetate), polymers such as polycaprolactam and poly(hexamethylene adipamide), polyesters such as poly(ethylene glycol terephthalate) and poly(hexamethylene glycol succinate). Suitable stabilizers are, for example, thermal inhibitors, such as hydroquinone, hydroquinone derivatives, p-methoxyphenol, E-benzol or sterically hindered phenols, e.g. 2,6-di(tert-butyl)-p-cresol, which prevent premature polymerization. In order to increase dark storage stability it is possible to use, for example, copper compounds, such as copper naphthenate, stearate or octoate, phosphorus compounds, for example triphenylphosphine, tributylphosphine, triethyl phosphite, triphenyl phosphite or tribenzyl phosphite, quaternary ammonium compounds, e.g. tetramethylammonium chloride or trimethylbenzylammonium chloride, or hydroxylamine derivatives, e.g. N,N-diethylhydroxylamine. For the purpose of excluding atmospheric oxygen during polymerization it is possible to add paraffin or similar wax-like substances which, being insoluble in the polymer, migrate to the surface at the beginning of the polymerization and form a transparent surface layer which prevents air from entering. It is also possible to add a light stabilizer, such as UV absorbers, e.g. hydroxyphenylbenzotriazole, hydroxyphenylbenzophenone, oxalic acid amide or hydroxyphenyl-s-triazine type. Such components can be used on their own or in the form of mixtures, with or without the use of sterically hindered amines (HALS). The photocurable compositions according to the invention may also comprise, as further additives (f), photoreducible dyes, e.g. a xanthene, benzoxanthene, benzothioxanthene, thiazine, pyronin, porphyrin or acridine dye, and / or radiation cleavable trihalomethyl compounds. These compounds are described, for example, in EP445624. Further customary additives (f) are, depending upon the intended use, optical brighteners, fillers, pigments, both white and colored pigments, colorants, antistatics, wetting agents or flow improvers. Additives conventionally used in the art, e.g. antistatics, flow improvers and adhesion enhancers, can also be used. In addition to the above components, other components may be present in the composition of the invention. It is also possible for chain-transfer reagents conventionally used in the art to be added to the compositions according to the invention. Examples are mercaptans, amines and benzothiazole. The composition of the invention may also comprise colorants and / or colored pigments. Depending upon the intended use, both inorganic and organic pigments may be used. Such additives are well known to the person skilled in the art; some examples are carbon black, iron oxides, such as iron oxide yellow, iron oxide red, chromium yellow, chromium green, nickel titanium yellow, ultramarine blue, cobalt blue, bismuth vanadate, cadmium yellow and cadmium red. Examples of organic pigments are mono- or bis-azo pigments, and also metal complexes thereof, phthalocyanine pigments, polycyclic pigments, e.g. perylene, anthraquinone, thioindigo, quinacridone or triphenylmethane pigments, and also diketo-pyrrolo-pyrrole, isoindolinone, e.g. tetrachloroisoindolinone, isoindoline, dioxazine, benzimidazolone and quinophthalone pigments. The pigments may be used in the formulations on their own or in admixture. Depending upon the intended use, the pigments can be added to the formulations in amounts conventionally used in the art, for example in an amount from 0.1 to 30% by weight or from 10 to 25% by weight, based on the total weight of the composition. The composition may also comprise, for example, organic colorants of an extremely wide variety of classes. Examples are azo dyes, methine dyes, anthraquinone dyes and metal complex dyes. Usual concentrations are, for example, from 0.1 to 20% wt, especially from 1 to 5% wt, based on the total weight of the composition. The photocurable compositions of the invention may comprise water. The photocurable compositions of the invention are suitable for various purposes, for example as a printing ink, such as screen printing inks, flexographic printing inks, offset printing inks and inkjet printing inks, as clearcoats, as colored coats, for example for wood or metal, as powder coatings, as coating materials inter alia for paper, wood, metal or plastics, as daylight-curable paints for marking structures and roads, for photographic reproduction processes, for holographic recording materials, for image- recording processes or in the production of printing plates that can be developed using organic solvents or using aqueous-alkaline media, for the production of masks for screen printing, as dental filling compounds, as adhesives, as pressure-sensitive adhesives, as laminating resins, as photoresists, e.g. galvanoresists, as etch resists or permanent resists, both liquid and dry films, as photostructurable dielectrics, and as solder masks for electronic circuits, as resists in the production of color filters for any type of display screen or in the creation of structures during the manufacture of plasma displays and electroluminescent displays, in the production of optical switches, optical gratings (interference gratings), in the manufacture of three-dimensional articles by bulk curing (UV curing in transparent moulds) or according to the stereolithography process, as described, for example, in US4,575,330, in the manufacture of composite materials (e.g. styrene polyesters which may include glass fibers and / or other fibers and other adjuvants) and other methods of printing in three dimensions well-known to one skilled in the art, in the coating or sealing of electronic components or as coatings for optical fibers. The photocurable compositions of the invention are also suitable for the production of optical lenses, e.g. contact lenses or Fresnel lenses, in the manufacture of medical apparatus, aids or implants, in dry film paints. The photocurable compositions of the invention are also suitable for the preparation of gels having thermotropic properties. Such gels are described for example in DE 19700064 and EP 678534. An article comprising a compound of formula (I), (Ia), (I’) or (Ia’), or comprising a photocurable composition of the invention, represents a further subject-matter of the invention. The compounds and compositions according to the invention may also be used as free-radical photoinitiators or photoinitiating systems for radiation- curable powder coatings. The photocurable compositions according to the invention are suitable, for example, as coating materials for all kinds of substrate, for example wood, textiles, paper, ceramics, glass, plastics, such as polyesters, polyethylene terephthalate, polyolefins and cellulose acetate, especially in the form of films, and also metals, such as Al, Cu, Ni, Fe, Zn, Mg or Co and GaAs, Si or SiO2, to which a protective layer is to be applied or an image is to be applied e.g. by imagewise exposure. A large number of the most varied kinds of light source may be used in the process according to the invention, the light source emitting at wavelengths from approximately 200 nm to approximately 800 nm. Both point sources and planiform radiators (lamp carpets) are suitable. Examples are: carbon arc lamps, xenon arc lamps, medium pressure, high pressure and low pressure mercury arc radiators, doped, where appropriate, with metal halides (metal halide lamps), microwave-excited metal vapour lamps, excimer lamps, superactinic fluorescent tubes, fluorescent lamps, argon incandescent lamps, flash lamps, photographic floodlight lamps, light-emitting diodes (LED), electron beams, X-rays and lasers. According to one embodiment, said light source comprises UV light in at least one of the UVA, UVB and UVC ranges. According to a preferred embodiment, said light source is a LED source, particularly preferred are LED light source emitting at wavelengths comprised between 365 nm and 420 nm, more preferably at 365 nm, 385 nm and 395 nm. According to the invention the distance between the lamp and the substrate to be exposed may vary according to the intended use and the type and strength of the lamp, e.g. from 0.1 cm to 150 cm, preferably from 1 cm to 50 cm. Said photopolymerizable composition may also be applied over a substrate already comprising a coated or printed layer. Said photopolymerizable composition may, after photopolymerization with said light source, be overprinted or overcoated with one or more compositions suitable for printing or coating. The article obtained by applying said photopolymerizable composition to said substrate by said means of coating or printing, and photopolymerizing by said light source, with or without further elaboration of the article by further coating or printing, represents a further subject-matter of this invention. As said, we surprisingly found that compounds of formula (I), (Ia), (I’) and (Ia’) have a very high reactivity under UVA, UVB and UVC wavelengths compared to that of the prior art. The new compounds showed their great improvement in surface curing under LED and Hg lamps especially in pigmented systems. The invention is illustrated in detail below by the following examples, which are illustrative and not limiting. EXPERIMENTAL SECTION In case of inconsistencies between the chemical name and the chemical structure herein indicated, the chemical structure prevails.1H NMR spectra were recorded with a Bruker Ascend 300 MHz NMR Spectrometer. EXAMPLE 1 7.37 g (184.250 mmoles) of sodium hydride (60 % dispersion in mineral oil) were added in portions to a stirred solution of 30.00 g (153.641 mmoles) of Iminodibenzyl in 150 mL of dimethylformamide. The reaction was stirred at room temperature for 30 min, then 28.76 g (184.394 mmoles) of Iodoethane were cautiously added dropwise. After the addition was finished, the reaction mixture was stirred at room temperature for 24 hours. The reaction was followed by TLC. Then additional 4.91 g (122.750 mmoles) of sodium hydride (60 % dispersion in mineral oil) were added in portions. After stirring for 30 minutes at room temperature, additional 19.17 g (122.908 mmoles) of Iodoethane were added dropwise. After the addition was finished, the reaction mixture was stirred at room temperature for further 24 hours. Then the reaction was poured into 300 mL of cold water. The mixture was extracted with toluene and the organic phase washed in sequence with 100 mL of water (x2) and brine. The organic layer was dried over anhydrous sodium sulfate filtered and the solvent removed by distillation under vacuum obtaining the crude. The crude product was purified by flash column chromatography on silica gel (toluene / eptane 75:25) followed by crystallization from 160 mL of methanol obtaining 30.25 g of the title compound as white solid (yield 88%).1H-NMR (DMSO-d6, į ppm): 1.06 (t, 3H), 3.08 (s, 4H), 3.73 (q, 2H), 6.91 (m, 2H), 7.05-7.18 (m, 6H). EXAMPLE 2 To an ice-cooled mixture of 4.05 g (18.135 mmoles) of EXAMPLE 1 and 2.66 g (19.949 mmoles) of anhydrous aluminum chloride in 40 mL of DCM, a solution of 2.677 g (19.044 mmoles) of benzoyl chloride in 10 mL of dichloromethane was added dropwise. Then the reaction was stirred at room temperature for further 1.5 hours and poured into 100 mL of ice / water. The mixture was extracted with ethyl acetate and the organic layer was separated, washed again with water and brine, dried over anhydrous sodium sulfate, filtered and the solvent removed by distillation under vacuum obtaining the crude. The crude product was purified by flash column chromatography on silica gel (toluene / eptane 85:15) obtaining 2.83 g of the title compound as yellow oil (yield 48%).1H-NMR (DMSO-d6, į ppm): 1.09 (t, 3H), 3.11 (m, 4H), 3.84 (q, 2H), 7.00 (m, 1H), 7.05-7.18 (m, 4H), 7.46-7.56 (m, 4H), 7.59-7.70 (m, 3H). EXAMPLE 3 To a mixture of 5.00 g (22.389 mmoles) of EXAMPLE 1 and 9.44 g (67.155 mmoles) of benzoyl chloride, 8.96 g (67.197 mmoles) of anhydrous aluminum chloride were cautiously added in portions. Make sure that the temperature does not rise above 30-35°C during the addition. After the addition was finished, the reaction was allowed to react at 35°C for further 2.5-3 hours and then cooled in an ice-bath. Then 50 mL of ethyl acetate were added dropwise under stirring. After the addition was finished, 100 mL of cold water were added dropwise under stirring. Make sure that the temperature does not rise above 40°C during the addition. The organic layer was separated, washed in sequence with 50 mL (x2) of NaOH 1M and then with brine, dried over anhydrous sodium sulfate, filtered and the solvent removed by distillation under vacuum obtaining 11.00 g of crude. The crude material was subjected to fractional distillation to remove benzoic acid (4.00 g) and to recover compound of EXAMPLE 1 (1.60 g). The final residue (5.40 g) was crystallized from 25 mL of toluene, then diluted with n-heptane and the additional precipitate collected by filtration to obtain 2.73 g of the title compound as beige solid (yield 60% considering the recovery of compound of EXAMPLE 1).1H-NMR (DMSO-d6, į ppm): 1.15 (t, 3H), 3.18 (m, 4H), 3.94 (q, 2H), 7.33 (d, 2H), 7.52-7.75 (m, 14H). EXAMPLE 4 To an ice-cooled mixture of 2.00 g (8.956 mmoles) of EXAMPLE 1 and 1.345 g (9.851 mmoles) of ethyl chlorooxoacetate in 25 mL of dichloromethane, 2.627 g (19.702 mmoles) of anhydrous aluminum chloride were cautiously added in portions. Then the reaction was stirred at room temperature for further 1.5 hours and poured into 100 mL of ice / water. The mixture was extracted with ethyl acetate and the organic layer was separated, washed again with water and brine, dried over anhydrous sodium sulfate, filtered and the solvent removed by distillation under vacuum obtaining the crude. The crude product was purified by flash column chromatography on silica gel (toluene / eptane 85:15) obtaining 0.69 g of the title compound as yellow oil (yield 24%).1H-NMR (DMSO-d6, į ppm): 1.07 (t, 3H), 1.32 (t, 3H), 3.10 (m, 4H), 3.88 (q, 2H), 4.40 (q, 2H), 7.08 (m, 1H), 7.18-7.29 (m, 4H), 7.61 (d, 1H), 7.68 (dd, 1H). EXAMPLE 5 To an ice-cooled mixture of 5.00 g (22.389 mmoles) of EXAMPLE 1 and 3.63 g (23.481 mmoles) of o-toluoyl chloride in 65 mL of dichloromethane, 3.28 g (24.599 mmoles) of anhydrous aluminum chloride were cautiously added in portions. Then the reaction was stirred at room temperature for further 1.5 hours and poured into 150 mL of ice / water. The mixture was extracted with ethyl acetate and the organic layer was separated, washed in sequence with 50 mL (x2) of NaOH 1M, water and brine, dried over anhydrous sodium sulfate, filtered and the solvent removed by distillation under vacuum obtaining the crude. The crude product was purified by flash column chromatography on silica gel (toluene 100%) obtaining 2.17 g of the title compound as yellow oil (yield 28%).1H-NMR (DMSO-d6, į ppm): 1.08 (t, 3H), 2.19 (s, 3H), 3.08 (m, 4H), 3.85 (q, 2H), 7.03 (m, 1H), 7.16-7.35 (m, 7H), 7.37-7.49 (m, 3H). EXAMPLE 6 To an ice-cooled mixture of 2.00 g (5.857 mmoles) of EXAMPLE 5 and 0.552 g (7.032 mmoles) of acetyl chloride in 30 mL of dichloromethane, 1.56 g (11.699 mmoles) of anhydrous aluminum chloride were cautiously added in portions. The reaction mixture was stirred at room temperature for further 1.5 hours. Then additional 2.207 g (28.115 mmoles) of acetyl chloride and 0.78 g (5.850 mmoles) of anhydrous aluminum chloride were cautiously added. After stirring at room temperature for 1.5 hours, the reaction was stirred under reflux for further 1.5 hours and then poured into 100 mL of ice / water. The mixture was extracted with ethyl acetate and the organic layer was separated, washed with water, brine, dried over anhydrous sodium sulfate, filtered and the solvent removed by distillation under vacuum obtaining the crude. The crude product was purified by flash column chromatography on silica gel (toluene / EtOAc 97.5:2.5) obtaining 2.20 g of the title compound as yellow oil (yield 98%).1H-NMR (DMSO-d6, į ppm): 1.08 (t, 3H), 2.19 (s, 3H), 3.14 (m, 4H), 3.88 (q, 2H), 7.24-7.37 (m, 5H), 7.39-7.51 (m, 2H), 7.54 (d, 1H), 7.77 (m, 2H). EXAMPLE 7 0.58 g (8.347 mmoles) of hydroxylamine hydrochloride and 0.86 g (10.484 mmoles) of sodium acetate were mixed in 25 mL of ethanol / water (80:20 w / w) and stirred at room temperature for 30 minutes. Then 2.00 g (5.215 mmoles) of EXAMPLE 6 were added and the reaction mixture heated to reflux. After stirring under reflux for 1 hour, the reaction was diluted with water and extracted with ethyl acetate. The organic layer was separated, washed with water, dried over anhydrous sodium sulfate, filtered and the solvent removed by distillation under vacuum obtaining 1.77 g of the title compound as yellow oil (yield 85%).1H-NMR (DMSO-d6, į ppm): 1.08 (t, 3H), 2.13 (s, 3H), 2.19 (s, 3H), 3.10 (m, 4H), 3.83 (q, 2H), 7.15-7.35 (m, 5H), 7.38-7.52 (m, 5H), 11.05 (s, 1H). EXAMPLE 8 0.82 g (8.032 mmoles) of acetic anhydride were added to a solution of 1.60 g (4.015 mmoles) of EXAMPLE 7 in 10 mL of dichloromethane. After stirring at room temperature for 20 hours, the reaction was diluted with ethyl acetate and washed with water. The organic layer was separated, dried over anhydrous sodium sulfate, filtered and the solvent removed by distillation under vacuum obtaining the crude. The crude product was purified by flash column chromatography on silica gel (toluene / EtOAc 97.5:2.5) obtaining 1.66 g of the title compound as yellow oil (yield 100%).1H-NMR (DMSO-d6, į ppm): 1.09 (t, 3H), 2.19 (s, 3H), 2.21 (s, 3H), 2.33 (s, 3H), 3.14 (m, 4H), 3.88 (q, 2H), 7.22-7.37 (m, 5H), 7.39-7.49 (m, 2H), 7.52 (d, 1H), 7.60 (m, 2H). EXAMPLE 9 To an ice-cooled mixture of 1.41 g (4.306 mmoles) of EXAMPLE 2 and 0.55 g (5.162 mmoles) of isobutyryl chloride in 30 mL of dichloromethane, 1.15 g (8.625 mmoles) of anhydrous aluminum chloride were cautiously added in portions. The reaction mixture was stirred at room temperature for further 1.5 hours. Then additional 2.20 g (20.648 mmoles) of isobutyryl chloride and 0.58 g (4.350 mmoles) of anhydrous aluminum chloride were cautiously added. The reaction was stirred under reflux for further 2 hours, then cooled too room temperature and poured into 100 mL of ice / water. The mixture was extracted with ethyl acetate and the organic layer was separated, washed in sequence with 50 mL (x2) of NaOH 1M, water, brine, dried over anhydrous sodium sulfate, filtered and the solvent removed by distillation under vacuum obtaining the crude. The crude product was purified by flash column chromatography on silica gel (toluene / EtOAc 97.5:2.5) obtaining 1.24 g of the title compound as yellow oil (yield 72%).1H-NMR (DMSO-d6, į ppm): 1.12 (m, 9H), 3.17 (m, 4H), 3.61 (m, 1H), 3.91 (q, 2H), 7.30 (m, 2H), 7.51-7.59 (m, 4H), 7.61-7.74 (m, 3H), 7.80 (m, 2H). EXAMPLE 10 To a stirred solution of 1.10 g (2.767 mmoles) of EXAMPLE 9 in 20 mL of dichloromethane, a solution of 0.486 g (3.041 mmoles) of bromine in 5 mL of dichloromethane was slowly added dropwise in 30 min. After stirring at room temperature for 1.5 hours under nitrogen atmosphere, an additional solution of 0.243 g (1.521 mmoles mmoles) of bromine in 1 mL of dichloromethane was slowly added dropwise and the reaction stirred at room temperature for further 1 hour under nitrogen atmosphere. Then the reaction was washed in sequence with water, sodium hydrogen carbonate saturated solution, water, dried over anhydrous sodium sulfate, filtered and the solvent removed by distillation under vacuum obtaining the crude. The crude product was dissolved in 15 mL of dimethylformamide, then 0.567 g (3.182 mmoles) of sodium p-toluenesulfinate were added in one portion and the reaction mixture stirred at room temperature for 2 hours. Then additional 0.567 g (3.182 mmoles) of sodium p-toluenesulfinate were added and the mixture stirred at room temperature for further 1 h. Then the reaction was poured in 60 mL of water and extracted with toluene. The organic layer was separated, washed with water, dried over anhydrous sodium sulfate, filtered and the solvent removed by distillation under vacuum obtaining the crude. The crude product was purified by flash column chromatography on silica gel (toluene / EtOAc 97.5:2.5) obtaining 1.09 g of the title compound as yellow oil (yield 71%).1H-NMR (DMSO-d6, į ppm): 1.12 (t, 3H), 1.62 (s, 6H), 2.41 (s, 3H), 3.14 (m, 4H), 3.91 (q, 2H), 7.24-7.35 (m, 2H), 7.42 (m, 2H), 7.51-7.74 (m, 10H), 7.80 (dd, 1H). EXAMPLES 11 and 12 To a stirred solution of 3.29 g (8.276 mmoles) of EXAMPLE 9 in 40 mL of dichloromethane, a solution of 1.45 g (9.073 mmoles) of bromine in 10 mL of dichloromethane was slowly added dropwise over the course of 1 hour. After stirring at room temperature for 1.5 hours under nitrogen atmosphere, an additional solution of 0.725 g (4.537 mmoles) of bromine in 5 mL of dichloromethane was slowly added dropwise and the reaction stirred at room temperature for further 1 hour under nitrogen atmosphere. Then the reaction was washed in sequence with water, sodium hydrogen carbonate saturated solution, water, dried over anhydrous sodium sulfate, filtered and the solvent removed by distillation under vacuum obtaining 3.60 g of crude bromo- intermediate. 3.60 g (7.557 mmoles) of crude product were dissolved in a mixture of 40 mL of methanol / toluene (30:10 v / v), then 1.47 mL (7.911 mmoles) of sodium methoxide 30% solution in methanol were added dropwise. After stirring at room temperature for 10 minutes, the reaction mixture was diluted with toluene and concentrated under reduce pressure to remove methanol. Then the reaction was filtered to remove sodium bromide and the solvent removed by distillation under vacuum obtaining the crude. The crude product was dissolved in 40 mL of acetonitrile, then 8.56 g (98.255 mmoles) of morpholine and 5.63 g (52.919 mmoles) of lithium perchlorate were added in sequence and the reaction stirred at 60°C for 5 hours under nitrogen atmosphere. Then the reaction was poured into 200 mL of water and extracted with toluene. The organic layer was separated and washed with 100 mL of water. Then the organic layer was extracted with 50 mL(x2) of 6M hydrochloric acid. The two phases were separated and worked in two different ways: a) The pH of the aqueous layer was adjusted to 8-9 with 50% sodium hydroxide solution and extracted with toluene. The organic layer was separated and washed with brine, dried over anhydrous sodium sulfate, filtered and the solvent removed by distillation under vacuum obtaining the crude. The crude product was purified by flash column chromatography on silica gel (toluene / EtOAc 97.5:2.5) obtaining 0.72 g of EXAMPLE 11 as yellow oil (yield 18%). b) The organic layer was washed with water, dried over anhydrous sodium sulfate, filtered and the solvent removed by distillation under vacuum obtaining the crude. The crude product was purified by flash column chromatography on silica gel (toluene / EtOAc 95:5) obtaining 2.30 g of EXAMPLE 12 as yellow oil (yield 58%).1H-NMR EXAMPLE 11 (DMSO-d6, į ppm): 1.13 (t, 3H), 1.23 (s, 6H), 2.46 (m, 4H), 3.17 (m, 4H), 3.58 (m, 4H), 3.91 (q, 2H), 7.24-7.34 (m, 2H), 7.52-7.61 (m, 4H), 7.62-7.75 (m, 3H), 8.30 (d, 1H), 8.41 (dd, 1H).1H-NMR EXAMPLE 12 (DMSO-d6, į ppm): 1.13 (t, 3H), 1.40 (s, 6H), 3.16 (m, 4H), 3.91 (q, 2H), 5.60 (s, 1H), 7.23-7.33 (m, 2H), 7.49-7.60 (m, 4H), 7.61- 7.74 (m, 3H), 7.98 (d, 1H), 8.08 (dd, 1H). Test Examples Representative compounds of the invention were evaluated with the aim of verifying their reactivity respect to commercial photoinitiators, such as Omnirad EMK, Omnirad ITX and (9-ethyl-9H-carbazole-3,6- diyl)bis(phenylmethanone) (herein referred to as “carbazole”, disclosed in Polym. Chem., 2020, 11, 3551–3556), having the following formulae: OMNIRAD ITX (9-ethyl-9H-carbazole-3,6-diyl)bis(phenylmethanone) Test Example 1 Tack-free in clear formulation The photopolymerizable compositions for the test were prepared by dissolving the PIs at the concentration of 3% by weight and the coinitiator, Omnirad EDB, at 3% by weight in a solution of Photomer 3016 (Bisphenol A epoxy diacrylate) 50%, Photomer 4335 (PETIA) 15%, Photomer 4600 (DPHA) 15%, Photomer 4172 (PPTTA) 20%. The PIs were tested with - a UV Hg Lamp at 120 W / cm power, - a UV LED lamp 365 nm 16 W / cm2power, and - a UV LED lamp 395 nm 16 W / cm2power. The photopolymerizable composition is spread with a thickness of 12 g / m2on a BYK coated cardboard. The results are expressed in meters per minutes as the maximum speed at which the tack-free is reached in Table 1. The higher the value obtained the better the reactivity of the PIs. Results Table 1: Applicative evaluation results Compound UV Hg 120 W / cm Tack-free m / min Omnirad EMK 25 EXAMPLE 3 43 EXAMPLE 10 47 carbazole 40 Compound UV LED365 nm 16 W / cm2UV LED395 nm 16 W / cm2Tack-free m / min Tack-free m / min Omnirad EMK 27 17 EXAMPLE 3 36 17 carbazole 32 <10 All molecules tested under UV Hg lamp 120 W / cm lamp showed a higher reactivity compared to Omnirad EMK, and almost comparable when LED lamps were used. Instead, when we compared the new photoinitiators with the carbazole derivatives, we observed an unexpected increased of reactivity under LED lamps. Test example 1.2 Trough-cure in Black Offset Ink and Cyan Offset Ink The test formulations were prepared dissolving the photoinitiator at a concentration of 3% by weight (wt) in an industrial black offset ink or an industrial cyan offset ink. The coinitiator (Omnirad EDB) was added at the compositions in the same amount (3% by weight). The test formulations were homogenized with a mechanical stirrer for 1 hour at 50°C and applied onto a white cardboard at 1.5 microns of thickness using IGT repro-tester equipment. The formulations were cured using: - a UV Hg Lamp at 120 W / cm, - a UV LED lamp 365 nm, and - a UV LED lamp 395 nm at different power. Omnirad EMK was used as reference. The through cure test is a measurement of the complete ink cure obtained at a defined speed and checked by “thumb twist pressure test”. Higher speed corresponds to higher reactivity. The results are shown in Figure 1. Looking at the results in Cyan Offset ink, it is clear that all PIs under LED lamps at 16 W / cm2 reach the maximum speed limit of the belt (100 m / min). So, to have a better understanding of the real performance of the new PIs in comparison to the reference the power of the LED lamps was decreased showing the incredible advantage given from the PIs of the invention; in fact Example 3 and Example 10 continue to achieve 100 m / min at the 50% of the LED power. As it can be seen, all the tested PIs showed a reactivity higher than the reference in any condition. The best performances were obtained for EXAMPLE 10 and EXAMPLE 3 that showed a reactivity twice the reference in cyan offset ink. Test Example 2 Tack-free in clear formulation The photopolymerizable compositions for the test were prepared by dissolving the PIs at the concentration of 5% by weight and the coinitiator, Omnirad EDB, at 5% by weight in a solution of Photomer 3016 (Bisphenol A epoxy diacrylate) 50%, Photomer 4335 (PETIA) 15%, Photomer 4600 (DPHA) 15%, Photomer 4172 (PPTTA) 20%. The PIs were tested with: - a UV LED lamp 365 nm 8 W / cm2power, and - a UV LED lamp 395 nm 8 W / cm2power. The photopolymerizable composition is spread with a thickness of 12 g / m2on a BYK coated cardboard. The results are expressed in meters per minutes as the maximum speed at which the tack-free is reached in Table 1. The higher the value obtained the better the reactivity of the PIs. Omnirad ITX was used as reference. Compound UV LED365 nm 8 W / cm2UV LED395 nm 8 W / cm2Tack-free m / min Tack-free m / min Omnirad ITX 75 65 EXAMPLE 11 100 75 The new PI showed a reactivity higher than the reference with all lamps. Through-cure black offset ink The test formulations were prepared dissolving the photoinitiator (pre solubilized in a 1:1 solution of dichloromethane) at a concentration of 3% by weight (wt) in an industrial black offset ink. The coinitiator (Omnirad EDB) was added at the compositions in the same amount (3% by weight). The test formulations were homogenized with a mechanical stirrer for 1 hour at 50°C and applied onto a white cardboard at 1.8 microns of thickness using IGT repro-tester equipment. The formulations were cured using: - a UV Hg Lamp at 120 W / cm, - a UV LED lamp 365 nm 8 W / cm2power, and - a UV LED lamp 395 nm 8 W / cm2power. Omnirad EMK and carbazole were used as reference. Results Table 3: Applicative evaluation result using UV Hg lamp 120 W / cm power Through cure m / min. (Thumb twist test) 120 W / cm Omnirad EMK 18 EXAMPLE 3 40 carbazole <10 Table 4: Applicative evaluation result using UV LED 395 nm lamp Through cure m / min. (Thumb twist test) 8 W / cm2Omnirad EMK 30 EXAMPLE 3 40 carbazole <10 Table 5: Applicative evaluation result using UV LED 365 nm lamp Omnirad EMK 65 EXAMPLE 3 96 carbazole 15 The PIs tested showed a reactivity higher than both reference with all lamps. These results are completely unexpected due also to the results obtained with the carbazole PI, the improvement in reactivity in this case increased more than 600% under LED 365 nm showing all the benefits that the iminodibenzyl ring is able to give to the new PIs.

Claims

Claims 1. A compound of formula (I)wherein R1 and R2 are, each independently H or a photoinitiating group; and R3 is selected from H, -C1-C6-alkyl; -C(=O)-C1-C6-alkyl; -CH2- phenyl; and -phenyl; provided that R1 and R2 are not both H; and provided that when R1 is benzoyl and R3 is ethyl, then R2 is not H.

2. The compound according to claim 1, of formula (Ia)3. The compound according to claim 1, characterized in that: R1 and R2 are each independently selected from H and one group (A) to (D):wherein R4, R5, R6, R8, R9 and R10, each independently, are selected from H, C1-C6-alkyl, -CH2-phenyl and phenyl; n is 0 to 3; the wavy lines show the bonds linking to the benzene rings in formula (I); provided that when R1 is a group (A) wherein n is zero and R3 is ethyl, then R2 is not H; R7 is selected from OH, C1-C4alkoxy, dimethylamino, diethylamino, morpholino and a group of formula (E) to (G)wherein R11 is H or C1-C6-alkyl; m is 0 to 3; R12, R13, R14, R15, R16, R17, R18 and R19, each independently, are selected from phenyl, C1-C6alkyl; the wavy lines show the bonds linking to the carbon atom of group (B).

4. The compound according to claim 2, characterized in that: R1 and R2 are each independently selected from H and one group (A) to (D):wherein R4, R5, R6, R8, R9 and R10, each independently, are selected from H, C1-C6-alkyl, -CH2-phenyl and phenyl; n is 0 to 3; the wavy lines show the bonds linking to the benzene rings in formula (Ia); provided that when R1 is a group (A) wherein n is zero and R3 is ethyl, then R2 is not H; R7 is selected from OH, C1-C4alkoxy, dimethylamino, diethylamino, morpholino and a group of formula (E) to (G)wherein R11 is H or C1-C6-alkyl; m is 0 to 3; R12, R13, R14, R15, R16, R17, R18 and R19, each independently, are selected from phenyl, C1-C6alkyl; the wavy lines show the bonds linking to the carbon atom of group (B).

5. The compound according to any one of claims 1 to 4 selected from6. Use of a compound according to any one of claims 1 to 5, as a photoinitiator.

7. A photopolymerizable composition comprising: a) from 50 to 99.7%, preferably from 70 to 98.9% by weight, based on the total content of the composition, of at least one ethylenically unsaturated compound; b) from 0.1 to 35%, preferably from 0.1 to 20%, and more preferably from 0.2 to 15% by weight, based on the total content of the composition, of at least one compound of formula (I) or (Ia), as above defined; andc) from 0 to 20% by weight, preferably from 0 to 15%, and more preferably from 0.2 to 15% by weight, based on the total content of the composition, of an accelerator and / or of a coinitiator.

8. The photocurable composition according to claim 7, further comprising one or more of the following components: d) from 0.01 to 15% by weight based on the total content of the composition, of one or more photosensitizer; and / or e) from 0.5 to 15% by weight based on the total content of the composition, one or more further photoinitiators.

9. The photocurable composition according to claim 7 or 8, characterized in that said accelerator and / or coinitiator is an amine, preferably a tertiary amine.

10. A process for photocuring photopolymerizable compositions, coatings, adhesives and inks, which process comprises: (i) providing a photopolymerizable composition according to any one of claims 7 to 9; (ii) coating or printing said photopolymerizable composition onto a substrate; and (iii) photopolymerizing said coated or printed composition on said substrate with a light source.

11. A process for three-dimensional printing which comprises providing a photopolymerizable composition according to any one of claims 7 to 9 and photocuring said composition with a light source.

12. The process according to claims 10 or 11, characterized in that said light source comprises UV light in at least one of the UVA, UVB and UVC ranges.

13. The process according to any one of claims 10 to 12, characterized in that said light source is a LED source emitting in the range from 350 to 420 nm.

14. An article of manufacture obtained according to the process of any one of claims 10 to 13 or by three-dimensional printing of a composition of any one of claims 7 to 9.

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