- Email: [email protected]
Photochemistry and photopolymerization activity of novel 4-alkylamino benzophenone initiators-synthesis, characterization, spectroscopic and photopolymerization activity
Fur. Polym. J. Vol. 26, No. 12, pp. 1345-1353, 1990
0014-3057/90 $3.00 + 0.00 Copyright © 1990 Pergamon Press plc
Printed in Great Britain. All rights reserved
PHOTOCHEMISTRY A N D PHOTOPOLYMERIZATION ACTIVITY OF NOVEL 4-ALKYLAMINO BENZOPHENONE INITIATORS--SYNTHESIS, CHARACTERIZATION, SPECTROSCOPIC A N D PHOTOPOLYMERIZATION ACTIVITY NORMAN S. ALLEN, EDWARD LAM and EDWARD M. HOWELLS Department of Chemistry, Faculty of Science and Engineering, Manchester Polytechnic, Chester Street, Manchester MI 5GD, England PETER N. GREEN and ARTHUR GREEN International Biosynthetics Ltd, Halebank Industrial Estate, Lower Road, Widnes, Cheshire WA8, 8NS, England FERNANDO CATALINA and CARMEN PE1NADO Instituto Plasticos y Caucho, CSIC, 3, Juan de la Cierva, 28006 Madrid, Espafia (Received 9 March 1990)
Abstract--Fifteen novel 4-substituted alkylaminobenzophenones have been synthesized and characterized using NMR, mass spectrometry and microanalysis. Their spectroscopic properties have been determined using u.v. absorption and phosphorescence analysis and the data related to their behaviour on photocuring using both Fourier Transform Infra-Red (FTIR) analysis and hardness testing in a triacrylate monomer. Absorption and phosphorescence spectra are similar to those of benzophenone although the quantum yields are lower with longer emission lifetimes indicative of the presence of some degree of charge-transfer in the lowest excited triplet state. Using FTIR analysis for photocuring, the compounds are in many cases more effective than benzophenone itself. Using hardness testing on the other hand, all the compounds proved to be more effective; the alicyclic amine structures are the least effective followed by the aliphatic and then the aromatic amines. For structural analogues, photocuring was more effective for those containing a propoxy link in the substituent than for those containing an ethoxy link. This difference is related to the ability of the molecule in the former case to form an intra-molecular exciplex with the aromatic ketone group. The alicyclic compounds are less effective in hydrogen atom or electron abstraction reactions. Differences observed in the methods of curing are related to the quenching effects of oxygen
INTRODUCTION
The photo-initiated polymerization and photocrosslinking of polymers are well established fields of industrial and academic research and development [1-3]. M a n y efforts have been directed towards an understanding of the mechanistic action of photoinitiators under various conditions in order either to improve formulations or to develop better quality and more efficient initiator types. One important area of industrial development is based on the synthesis of new photo-initiators for specialized applications. Problems associated with many conventional photo-initiators are poor solubility and migration which can result in the loss of adhesion and consequent loss of gloss of the coating.
The extractability of components by food in contact with u.v. cured coatings excludes their use for food packaging materials, and one of the methods to overcome this problem is to develop photoinitiators (e.g. benzophenone derivatives) with builtin functional groups capable of reacting with the monomer. Another is to avoid the use of liquid amine co-synergists by using solid amines or more recently through the use of alkylamino groups built into the initiator chromophore [4]. Mechanistic studies to date [4] on benzophenone indicate a primary photochemical process of hydrogen atom abstraction from the solvent, m o n o m e r or tertiary amino co-initiator by the photo-excited triplet state of the ketone group to produce a ketyl radical, solvent or alkylamino radical thus:
O
O
I
SH = Solvent
H 1345
R
NORMAN S. ALLEN et al,
1346
Either of the latter two species is then believed to initiate polymerization and this view has been confirmed through the use of Fourier Transform i.r. (FTIR) analysis for the alkylamino radicals [5]. In this case the tertiary amine is co-reacted into the monomer. Thus, the purpose of developing benzophenone derivatives with built-in alkylamino groups is not only to develop a more efficient initiator type with a built-in synergist but also to produce an initiator which will co-react with the m o n o m e r and thus not migrate out of the coating. Whilst simple alkylamino substituted benzophenone initiators have been examined in the past, such as Michler's ketone [4], a more detailed synthetic study of various structural types and their inter-relationship with photopolymerization activity has not been undertaken. In this work 15 novel 4-substituted alkylaminobenzophenones have been synthesized, including acyclic and alicyclic types, and characterized; their structures have been related to photopolymerization activity and spectroscopic properties. EXPERIMENTAL PROCEDURES
Materials Glycerol propoxylate triacrylate monomer was supplied by Harcross Chemicals Ltd, Eccles, Manchester, U.K. Acetone and ethanol were obtained from Vickers Laboratories Ltd, U.K. aluminium oxide, methanol, magnesium sulphate, potassium carbonate and sodium from Fisons Ltd., U.K., conc. H2SO4 from M&B Chemicals Ltd, U.K. 4-hydroxybenzophenone from Lancaster Synthesis Ltd, U.K., AR cyclohexane, ethyl acetate, chloroform and 2-propanol from BDH Chemicals Ltd, U.K. and l-bromo-2-chloroethane, 1-bromo-3-chloropropane, 2-chloroethanol, 3-chloropropan-l-ol, 1-(2-chloroethyl)piperidine monohydrochloride, l-(3-chloropropyl)piperidine monohydrochloride, 4-(2-chloroethyl)morpholine hydrochloride, 2-(dimethylamino ethyl chloride) monohydrochloride, 3-(diethylamino ethyl chloride) monohydrochloride, 2-(di-isopropylamino ethyl chloride) monohydrochloride, 4-N-dimethylamino benzoic acid, N-ethyl-2methyl-allylamine, 1-(2-hydroxyethyl) piperazine, morpholine, piperazine, 1-methyl piperazine and sodium ethoxide were all obtained from the Aldrich Chemical Company Ltd, U.K.
Synthesis (I) 4-(2-Chloroethoxy) benzophenone [6]. 4-Hydroxybenzophenone (19.8 g) was added to a solution of sodium ethoxide (3 g Na and 200 cm 3 ethanol); after boiling for 5 rain, l-bromo-2-chloroethane (14.3 g) was added followed by refluxing for 24 hr, cooling and filtration of the precipitate. The filtrate was then evaporated under reduced pressure followed by separation in a water/chlorofom mixture. The bottom layer was then collected and washed several times with 1M NaOH dried and then evaporated. After 1 hr of standing, the crystalline product was removed and recrystallized from methanol to give 19.5 g of product with a m.p. of 79°C (Yield 75%). (II) 4-(3-Chloropropoxy) benzophenone [6]. As in (I), the sodium salt of 4-hydroxybenzophenone was reacted with 1-bromo-3-chloropropane (15.7 g) to give the desired product after 24 hr of reflux with stirring. The crude product after purification gave 19.2 g (Yield 70%) with a m.p. of 60°C. (ill) 4-(2-Hydroxyethoxy) benzophenone [7]. 4-Hydroxybenzophenone (19.8 g) was added to a solution of NaOH (8 g in 200 cm 3 of distilled water) and after a 5 min boil 2-chloroethanol (8 g) was added. The mixture was then
refluxed for 24 hr, cooled, filtered and washed with distilled water. The product after recrystallization from methanol gave 18.1 g (Yield 75%) with a m.p. of 87-88°C. (IV) 4-(3-Hydroxypropoxy) benzophenone. This derivative was prepared as for (III). The product was recrystallized from methanol to give 18.6 g (Yield 73%) with a m.p. of 68-70°C.
(V) [4-[2-(4-Morpholinyl) ethoxy] phenyl} phenyl methanone [8]. 4-Hydroxybenzophenone (19.8 g) was added to a solution of sodium ethoxide (6 g Na in 200 cm 3 of ethanol) followed by boiling for 5 rain. The 4-(2-chloroethyl) morpholine hydrochloride (18.6 g) was then added, followed by refluxing for 24 hr. The NaCI was filtered off and the filtrate evaporated under reduced pressure. The crude product was then dissolved in chloroform followed by washing with NaOH solution, then with distilled water and finally dried o v e r M g S O 4. The product was purified on an alumina column using chloroform as eluant. After evaporation and recrystallization from methanol, the product was 18.6g (yield 60%) with a melting point of 93-94°C. Calculated C, H and N are 73.29, 6.8 and 4.5% respectively. Values found were 73.32, 6.85 and 4.46% respectively.
(VI) [4-[3-(4-Morpholinyl) propoxy] phenyl] phenyl methanone. 4-(3-Chloropropoxy) benzophenone (27.5g) and K2CO 3 (14.0 g) were mixed with 100 cm 3 of morpholine followed by refluxing for 12 hr, cooling and removing the precipitate by filtration. Unreacted morpholine was removed under reduced pressure. The oily product was then distilled under vacuum to give 21.2 g (Yield 65%) with a b.p. of 260-270°C at 3 mm Hg. Calculated C, H and N were 73.83, 7.12 and 4.31% respectively. Values found were 72.62, 7.12 and 4.17% respectively.
(VII) [4-[2-(4-Piperidinyl) ethoxy] phenyl] phenyl methanone [9]. 4-Hydroxybenzophenone (19.8 g) was added to a solution of sodium ethoxide (6 g Na in 200 cm3 ethanol) followed by boiling for 5 rain and then adding 1-(2-chloroethyl) piperidine monohydrochloride (18.4g) and then refluxing for 24 hr, cooling and filtering. Following the purification procedure for (V), the product was recrystallized from ethanol to give 15.5 g (Yield 50%) with a m.p. of 64-65°C. Calculated C, H and N are 77.63, 7.49 and 4.53% respectively. Values found were 77.71, 7.59 and 4.47% respectively.
(VIII) [4-[3-(4-Piperidinyl) propoxy) phenyl] phenyl methanone. This compound was prepared as for (VII) but with l-(3-chloropropyl) piperidine monohydrochloride (19.8 g). After filtration, the filtrate was evaporated under reduced pressure followed by dissolving the crude product in chloroform and then washing in dilute NaOH, distilled water and drying over MgSO4. After evaporation, the crude product was distilled under vacuum to give 17.8 g (Yield 55%) with a b.p of 240-250°C at 4 mm Hg. Calculated C, H and N are 77.98, 7.79 and 4.33%. Values found were 77.81, 7.6 and 4.34% respectively.
(IX) [4-[2-[(Dirnethylamino) etho xy] phenyl] phenyl metha none [10]. 4-Hydroxybenzophenone (19.8 g) was added to a solution of sodium ethoxide (6 g Na in 200 cm 3 ethanol) followed by boiling for 5 rain and then the addition of 2-dimethylaminoethyl chloride monohydrochloride (14.4 g) and then refluxing with stirring for 24 hr. The whole was then cooled and the precipitate filtered off. Following the purification procedure as for (VIII), the product gave 12.9 g (Yield 48%) with a boiling point of 188-191°C at 1 mm Hg. Calculated C, H and N are 75.81, 7.11 and 5.2% respectively. Values found were 75.76, 7.08 and 4.90% respectively.
(X) [4-[3-(Dimethylamino) propoxy] phenyl] phenyl methanone. As for (IX), the sodium salt of 4-hydroxybenzophenone was reacted with 3-dimethylaminopropyl chloride monohydrochloride for 24 hr under reflux. After the same purification procedure, the product obtained gave 13.9g (Yield 49%) with a boiling point of 201-208°C at 1 mm Hg. Calculated C, H and N are 76.29, 7.47 and
Photochemistry and polymerization of novel 4-alkylamino benzophenone initiators 4.49% respectively. Values found were 76.3, 7.38 and 4.68% respectively.
1347
7.90% respectively. Values found were 71.08, 7.33 and 7.75% respectively.
(XI) [4-[2-(Diethylamino) ethoxy] phenyl] phenyl methanone [11]. As for (IX), 2-diethylaminoethyl chlor-
(XIX) [4'[2'[Ethyl(2-methyl-2-propen-l-yl) amino] ethoxy]phenyl]phenylmethanone. 4-Hydroxybenzophenone
ide monohydrochloride (17.2g) was reacted with the 4-hydroxybenzophenone in sodium ethoxide. The product gave 13.4 g (Yield 45%) with a boiling point of 202-203°C at 1 mm Hg. Calculated C, H and N are 76.73, 7.80 and 4.71% respectively. Values found were 76.58, 7.56 and 4.04% respectively.
(19.8g) and K2CO 3 (14g) were mixed with N-ethyl-2methylallylamine (26 cm 3) followed by refluxing for 24 hr with stirring. The whole was then cooled and filtered and evaporated under reduced pressure. The product gave 16.9 g (Yield 52%) with a boiling point of 197-200°C. Calculated C, H and N are 77.98, 7.79 and 4.33% respectively. Values found were 77.81, 7.80 and 4.44% respectively.
(XII) [4-[2-(Diisopropylamino) ethoxy] phenyl] phenyl methanone. As for (IX), 2-diisopropylaminoethyl chloride monohydrochloride (20.15 g) was used. The product gave 15.3g (Yield 47%) with a boiling point of 211-212°C. Calculated C, H and N are 77.50, 8.36 and 4.30% respectively. Values obtained were 77.50, 8.13 and 3.98% respectively.
(XIII) [4-[2-(4-Methylpiperazin- l-y l)ethoxy] phenyl] phenyl methanone. 4-(2-Chloroethoxy) benzophenone (26 g), K2CO 3 (14 g) and l-methylpiperazine (100 cm ~) were refluxed for 24 hr and purified as described for (V). The product gave 16 g (Yield 49%) with a m.p. of 93-94°C at 0.9 mm Hg. Calculated C, H and N are 74.04, 7.45 and 8.36% respectively. Values found were 74.24, 7.54 and 8.37% respectively.
(~7V) [4-[3-(4-Methylpiperazin- l-y 1)propoxy] phenyl] phenyl methanone. As for compound (XIII), 4-(3-chloropropoxy) benzophenone (27.4g) was used in the reaction mixture. After distillation under vacuum, the product gave 15.2 g (Yield 45%) with a boiling point of 208-210°C at 1 mm Hg. Calculated C, H and N are 74.52, 7.74 and 8.28% respectively. Values found were 74.56, 7.74 and 7.97% respectively.
(XV) [l,4-Piperazinediyl bis(l,2-ethoxy- l,4-phenylene)] bis phenyl methanone. 4-(2-Chloroethoxy) benzophenone (26 g), and K2CO 3 (14 g) were mixed with acetone 200 cm 3 followed by boiling for 5 min and then adding piperazine (4.3 g). The whole was then refluxed for 24 hr followed by hot filtration and then cooling in ice. The crude product was purified on an alumina column using chloroform as eluant. The product gave 20.3 g (Yield 38%) with a melting point of 159-160°C. Calculated C, H and N are 74.01, 5.95 and 3.60% respectively. Values obtained were 74.07, 5.97 and 3.57% respectively.
(XVI) [4-[2-(4-Dimethylaminobenzoate) ethoxy] phenyl] phenyl methanone. 4-(2-Hydroxyethoxy) benzophenone (24.2 g), 4-N-dimethylaminobenzoic acid (82.6 g) and conc. H2SO 4 (5cm 3) were mixed with 100cm 3 of toluene. The mixture was then refluxed for 24 hr using a Dean and Stark column. The reaction mixture was then evaporated and taken up in chloroform followed by purification on an alumina column using chloroform as eluant. The product after recrystallization from methanol gave 26.5g (yield 68%) with a m.p. of 92-93°C. Calculated C, H and N are 74.42, 6.25 and 3.47% respectively. Values found were 74.19, 6.34 and 3.65% respectively.
(::VII) [4-[3-(4-Dimethylaminobenzoate) ethoxy] phenyl] phenyl methanone. As for (XVI), 4-(3-hydroxypropoxy) benzophenone (25.6 g) was used in the reaction mixture. The recrystallized product gave 26.2 g (Yield 65%) with a m.p. of 118-120°C. Calculated C, H and N are 76.38, 6.41 and 5.24% respectively. Values found were 75.46, 6.44 and 5.17% respectively.
(XVIII) [4-[2-(1-(2-Hydroxyethyl) piperazin-l-yl) ethoxy] phenyl] phenyl methanone. 4-Hydroxybenzophenone (19.8 g) and K2CO 3 and (14g) were mixed with acetone (200 cm 3) followed by boiling for 5 min and then the addition of l-(2-hydroxyethyl) piperazine (14 g). The whole was then refluxed for 24 hr followed by pouring into iced water. After filtration, the product was purified on an alumina column using chloroform as eluant and then recrystallized from ethyl acetate to give 18.4 g (Yield 52%) with a m.p. of I02°C. Calculated C, H and N are 71.16, 7.39 and
Instrumentation Characterization. ~H-NMR Spectra were recorded on a Perkin-Elmer R12B instrument at a frequency of 60 MHz and a field strength of 1.409 Tesle using TMS for reference. Mass spectra were recorded on a VG 7070 machine using the electron ionization mode. The micro-analyses were carried out by C.H.N. Analysis Ltd, Leicester. Photopolymerization. Films of Photomer 4094 containing 0.1 and 1% w/w of the photo-initiators were irradiated using a 100 watt high pressure Hg lamp for various periods. Cure rate was assessed using a Pendulum Hardness Meter (Sheenan Instruments Ltd, London, U.K.) as well as by measuring the reduction in the absorption band at 812 cmrelative to that at 1740cm -I of coatings between two salt fiats using a Mattson Alpha Centauri Fourier Transform i.r. instrument. Spectroscopic measurement. Absorption spectra were recorded using a Perkin-Elmer Lambda 7 absorption spectrometer. Phosphorescence spectra and emission quantum yields and lifetimes were obtained at 77 K using a Perkin-Elmer LS-5 luminescence spectrometer. Benzophenone in ethanol was used as the standard and assumed to have a quantum yield of 0.74 [12]. RESULTS AND DISCUSSION
Photopolymerization T h e chemical structures a n d n a m e s of the 15 p h o t o i n i t i a t o r s are listed in Table 1 for reference. They will be referred to t h r o u g h o u t by their numbers. One industrial m e t h o d of measuring the rate o f cure o f a coating is to determine its hardness. This m e a s u r e m e n t is m a d e using a p e n d u l u m device which consists of stainless steel bearings resting o n the surface of the cured coating. T h e p e n d u l u m is set in m o t i o n a n d the n u m b e r of swings or oscillations is directly related to the h a r d n e s s of the cured resin. The greater the n u m b e r o f oscillations the h a r d e r is the coating. A poorly cured coating will be softer a n d so impose more drag on the bearings. The results for a 1% w/w c o n c e n t r a t i o n o f p h o t o - i n i t i a t o r in the P h o t o m e r 4094 are c o m p a r e d in Table 2 after 11 rain o f irradiation. All 15 initiators are m u c h more effective t h a n b e n z o p h e n o n e which is referred to as comp o u n d 0; the m o s t effective is c o m p o u n d 13 which c o n t a i n s two b e n z o p h e n o n e units in the same molecule. These data are illustrated more effectively in Fig. 1 using a b a r chart. F r o m this i n f o r m a t i o n , an interesting structural correlation is observed. C o m p o u n d s 5 to 8 a n d 15 c o n t a i n only aliphatic alkylamino groups in the 4-substituent a n d are m o r e effective t h a n the c o m p o u n d s c o n t a i n i n g alicyclic a m i n e substituents viz. c o m p o u n d s 1 to 4, 9 a n d 10 a n d 14. In the former case this effect m a y be related to the lower ionization potential of the amine substituent. This view is confirmed by the fact t h a t the
NORMAN S. ALLEN et al.
1348
Table 1. Nomenclature and structures of the photo-initiators Compound number
Structure
O-iQ
IUPAC name
O(CH2)aN ~
[4-[2-(4-Morpholinyl) ethoxy] phenyl] phenyl methanone
O(CH:)3 N ~
[4-[3-(4-Morpholinyl) propoxy] phenyl] phenyl methanone
O
O
[4-[2-(4-Pipcridinyl) ethoxy] phenyl] phenyl methanone O
[4-[3-(4-Piperidinyl) propoxy] phenyl] phenyl methanone O
~
i-~L-O(CH2)2 N(CH3)2
[4-[2-(Dimethylarnino) ethoxy] phenyl] phenyl methanone
O
~-~
i~'~
O(CH2)3N(CHa)2
[4-[3-(Dimethylamino) propoxy] phenyl] phenyl methanone
0(CH2)2N(C2Hs)2
[4-[2-(Diethylamino) propoxy] phenyl] phenyl methanone
0
~"-i
"--~ O
~
i ~~'-'-O (CH2)2N(CH(CH3)2)2
[4-[2-(Diisopropylamino) ethyl] phenyl] phenyl methanone
O
O(CH2) 2Nk....../N~CH3
[4-[2-(4-Methylpiperazin-l-yl) ethoxy] phenyl] phenyl methanone
O 10
O(CH2)3 N
~
CHa
[4-[3-(4-Methylpiperazin-l-yl) propoxy] phenyl] phenyl methanone
0
li 0
i
[4-[2-(4-Dimethylaminobenzoate) ethoxy] phenyl] phenyl methanone Continued
Photochemistry and polymerization of novel 4-alkylamino benzophenone initiators
1349
Table l--continued Compound number
Structure
IUPAC name
0
CH3
[4-[3-(4-Dimethylaminobenzoate) propoxy] phenyl] phenyl methanone
FI 0 13 N
N
[I,4-Piperazinediyl bis(I,2-ethoxy-l,4-phenylene)] bis phenyl methanone
14 O(CH~h
N
N (CHa), OH
x___/
[4-[2-(l-(2-hydroxyethyl) piperazin-l-yl) ethoxyl] phenyl] phenyl methanone
0
<
~
C-~--x~
II
0
O(CH2)a NCH2CH2C= C H z
I
C2Hs
I
[4'[2'[Ethyl(2-methyl-2-propen-l-yl) amino] ethyl] phenyl] methanone
CH3
compounds (11 and 12) containing aromatic (acyclic) amine substituents are also more effective than the alicyclic compounds. Compound 13 is anomalous in that it contains two benzophenone chromophores. Another interesting feature of these results is that, for the pairs of analogous compounds 1 and 2, 3 and 4, 9 and 10 and 11 and 12, the second of the pairs containing a propoxy group are more effective photoinitiators than their corresponding ethoxy analogue. Thus, the compounds containing a propoxy link must be important for hydrogen atom abstraction. Future publications using kinetic nanosecond laser flash photolysis will confirm that this mechanism occurs via an intramolecular process. The only exception here is for compounds 5 and 6 which contain a dimethylamino substituent. Here the presence of a methyl group alpha to the nitrogen from which a hydrogen atom may be abstracted is present in both cases and may compensate.
On a more scientific basis, the rate of curing may be monitored using i.r. spectroscopy. Here, the coating is irradiated between two salt flats in order to exclude the quenching effect of oxygen. In the above method, all the coatings were exposed to the CURE RATES OF PHOTOINITIATORS 200
I00
Table 2. Influence of the photo-initiator: (benzophenone, compounds 1-15) 1% w/w on the hardness of u.v. triacrylate resin (Photomer 4094) Compound Benzophenone Compound 1 Compound 2 Compound 3 Compound 4 Compound 5 Compound 6 Compound 7 Compound 8 Compound 9 Compound 10 Compound I 1 Compound 12 Compound 13 Compound 14 Compound 15
No. of oscillations after 11 rain of radiation 40 73 86 67 87 167 149 139 142 73 79 155 164 173 88 155
0
0
1 2
3 4
5
6 7 t
9 tO II 12 13 14 15
COMPOUND Fig. 1. Cure rate of Photomer 4094 measured by the hardness method in oscillations and initiated by c o m p o u n d s 0-15.
1350
NORMAN S. ALLEN et al.
Table 3. Photopolymerization conversion of an oil-soluble triaerylate resin (Photomer 4094) by benzophenone initiator: (benzophenone, compounds 1-15) 0.1% w/w
~g CONVERSION VERSUS EXPOSURE TIME 0.1~ w/w
% Conversion at various exposure times (see)
Compound Benzophenone Corn )ound 1 Com )ound 2 Corn )ound 3 Corn )ound 4 Com )ound 5 Corn )ound 6 Corn )ound 7 Corn )ound 8 Corn )ound 9 Com )ound 10 Corn )ound I 1 Corn ~ound 12 Com )ound 13 Corn )ound 14 Corn )ound 15
1
5
10
20
30
40
50
60
18 34 32 17 18 16 46 31 15 19 24 12 33 30 6 49
27 46 51 25 26 26 64 47 25 25 44 31 57 36 26 63
52 65 69 48 51 52 74 64 36 46 65 47 68 65 47 70
65 72 76 61 57 63 77 71 52 49 73 58 75 68 59 74
70 75 77 65 63 67 79 72 57 55 78 63 78 72 63 75
72 77 78 65 67 70 81 75 64 59 79 70 78 72 64 76
73 77 78 66 67 72 81 76 64 62 80 72 78 73 67 77
73 80 78 66 68 74 82 76 67 68 80 73 78 75 71 77
80-
60
Z m
'" ).
BENZOPHENONE
40
d,.
COMPOUND I
Z
COMPOUND 2
z
o
COMPOUND 3
a t m o s p h e r e a n d t h e g r e a t e r efficiency o f t h e c o m pounds containing the 4-alkylamino substituents c o m p a r e d w i t h t h a t o f b e n z o p h e n o n e is d u e in t h e m a i n to t h e i r ability to s c a v e n g e o x y g e n w h i c h m a y o t h e r w i s e q u e n c h t h e p h o t o - e x c i t e d triplet s t a t e o f t h e k e t o n e [1, 2]. I n t h e F T I R m e t h o d u s e d here, t h e r a t e o f c u r e is r e l a t e d to t h e d e c r e a s e in t h e v i n y l (acrylic) a b s o r p t i o n at 8 1 2 c m -1 relative to t h a t o f t h e e s t e r b a n d at 1740 c m - ~ w h i c h s h o u l d n o t c h a n g e a n d so c o m p e n s a t e s for c o n t r a c t i o n in t h e film d u r i n g cure. T h e d a t a for t h e c o n v e r s i o n ; u s i n g 0 . 1 % w / w p h o t o i n i t i a t o r in P h o t o m e r 4094 a r e s h o w n in T a b l e 3; t h o s e f o r a I % w / w o f p h o t o i n i t i a t o r a r e s h o w n in T a b l e 4. It is s e e n t h a t at t h e h i g h e r c o n c e n t r a t i o n t h e c u r e d u r i n g t h e first 20 sec o f i r r a d i a t i o n is m u c h f a s t e r b u t t h e r e a f t e r t h e r a t e b e g i n s to p l a t e a u . T h i s effect is d e m o n s t r a t e d m o r e clearly b y t h e p l o t s o f F i g s 2 - 5 for t h e 0 . 1 % w / w c o n c e n t r a t i o n a n d F i g s 6 - 9 f o r t h e 1.0% w / w c o n c e n t r a t i o n o f p h o t o - i n itiator. T h u s , at 0 . 1 % w / w c o n c e n t r a t i o n , c o m p o u n d s 3, 4, 5, 8, 9, 11 a n d 14 a r e less effective than benzophenone; at 1.0% w/w concentration, o n l y c o m p o u n d 13 p r o v e d to be less effective. T h u s , under these curing conditions, the exclusion of oxyg e n is s e e n to be i m p o r t a n t . H o w e v e r , t h e o n e c o n s i s t e n t t r e n d in t h e s e r e s u l t s is s e e n at t h e 0 . 1 %
COMPOUND 4
20
01 0
I '0
2'0
5'0
4'0
5'0
60
EXPOSURE TIME (Sees)
Fig. 2. Conversion vs exposure time of Photomer 4094 using F T I R spectroscopy, initiated by c o m p o u n d s l ~ t at 0.1% w/w, relative to that of benzophenone.
~g CONVERSION VERSUS EXPOSURE TIME O. 1 ~g w / w
100"
80"
60"
Table 4. Photopolymerization conversion of an oil-soluble triacrylate resin (Photomer 4094) by benzophenone initiator: (benzophenone, compounds 1-15) 1% w/w % Conversion at various exposure times (see)
Compound Benzophenone Compound 1 Compound 2 Compound 2 Compound 3 Compound 4 Compound 5 Compound 6 Compound 7 Compound 8 Compound 9 Compound 10 Compound 11 Compound 12 Compound 13 Compound 14 Compound 15
1
3
10
20
30
40
50
60
29 45 45 45 46 46 49 50 48 52 41 50 52 55 12 35 52
55 62 61 61 56 59 69 67 66 70 61 65 62 68 27 53 70
61 69 68 68 66 62 76 72 72 78 69 75 68 71 37 67 78
67 75 72 72 71 69 80 76 78 83 75 81 74 76 55 71 82
71 76 75 75 74 72 81 78 79 85 77 81 76 78 59 74 83
72 79 76 76 75 73 82 80 81 85 79 83 76 80 59 76 84
75 79 77 77 76 74 82 81 81 85 80 83 78 80 65 78 84
76 80 78 78 79 75 83 82 82 86 80 83 80 81 68 80 85
40"
20"
0
0
IO
2'0
~
COMPOUND 8
" ~
BENZOPHENONE
3b
4'0
5'0
60
EXPOSURE TIME (SEC5)
Fig. 3. Conversion vs exposure time of Photomer 4094 using F T I R spectroscopy, initiated by c o m p o u n d s 5-8 at 0.1% w/w, relative to that of benzophenone.
Photochemistry and polymerization of novel 4-alkylamino benzophenone initiators
1351
;~ CONVERSIONVERSUSEXPOSURET l l l E 1.0 :g w / w
:g CONVER310N VERSUS EXPOSURE TIME O.l :g w / w 80
80
70 60
60
Z 0 e~
:::,. z o
Z C~
COMPOUND 12
40 ,I,
C(NIPOUND I I
:
COMPOUND I 0
~,'
dE
50
Z C~ ¢,J
g~
O01'1POUND I (X~IPOUND 2 COMPOUND 5
COMPOUND 9
40 ¸
20
t
BENZOPHENONE
(~OrlPOUND4
50 ¸
0
0
I0
~ 20
30
40
50
60 20
lb
EXPOSURE TIME (SECS) Fig. 4. Conversion vs exposure time of Photomer 4094 using FTIR spectroscopy, initiated by compounds 9-12 at 0.1% w/w, relative to that of benzophenone.
2b
3b
4b
':-b
60
EXPOSURETIME (5ECS)
Fig. 6. Conversion vs exposure time of Photomer 4094 using FTIR spectroscopy, initiated by compounds 1-4 at 1% w/w, relative to that of benzophenone.
% GONVERSION VERSUS EXPOSURE TIME O.1%w/w
:g CONVERSIONVERSUSEXPOSURETI ME 1,0 :g wlw
80
90 ¸
8O
60
Z f __q r'Y
40-
>. z
O ¢_)
20
~
COMPOUND t 5
JL
COMPOUND 14
~
COMPOUND 13
"
O011POUND 7
•
COMP(XJND B
- - { 3 - - BENZOPHENONE ~
/ 01 0
20
1'0
2'0
5'0
4'0
5'0
60
EXPOSURE TIME (5E05) Fig. 5. Conversion vs exposuretime of Photomer 4094 using
FTIR spectroscopy, initiated by compounds 13-15 at 0. I% w/w, relative to that of benzophenone.
0
lb
2b
3b
4b
sb
60
EXPOSURETIHE (SECS)
Fig. 7. Conversion vs exposure time of Photomer 4094 using FTIR spectroscopy, initiated by compounds 5-8 at 1% w/w, relative to that of benzophenone.
1352
NORMANS. ALLENet al. % CONVERSION VERSION EXPOSURE TIME 1.0 % w l w
% CONVERSION VERSUS EXPOSURE TIME 1.0 % w/w
100
9O
8o
~
o
80
7O
g
Z o
50
~
~-
."IIl 2
0
>-
COIIpOUHD 9
d
o
~
BENZOPHENONE
z
o u
-:::::::0 ~
60
IR
dO
CCII'POUN I D 13 COMPOUND 14
~COMPOUND 11
COMPOUND I15 20
0
10
20
30
40
50
,o
60
EXPOSURE TIME (SECS)
2b
~
~
sb
60
EXPOSURE TIME (SECS)
Fig. 8. Conversion vs exposure time of Photomer 4094 using FTIR spectroscopy, initiated by compounds 9-12 at 1% w/w, relative to that of benzophenone.
Fig. 9. C o n v e r s i o n vs e x p o s u r e t i m e o f P h o t o m e r 4 0 9 4 u s i n g FTIR spectroscopy, initiated by compounds 13-15 at 1% w / w , relative to t h a t o f b e n z o p h e n o n e .
w/w concentration where in the analogous pairs 1 and 2, 3 and 4, 5 and 6, 9 and 10 and 11 and 12 the second compound containing the propoxy link in the 4-substituent is the more effective initiator. At 1.0% w/w concentration, the relative differences in cure rates are small after 20 sec of irradiation.
4.7. The absorption maxima occur in the range 277-310 nm and are little affected by solvent polarity although there is a small red shift for the benzophenone derivatives indicating that this band is associated with the main n - ~ * transition of the benzene rings. None of the compounds studied here exhibited any fluorescence but all exhibited phosphorescence with an emission spectrum the same as that of benzophenone itself. The emission maxima are shown in Table 6 together with their corresponding emission quantum yields and triplet lifetimes at 77 K in liquid N2. Whilst the emission maxima are seen to be the same, it is noted that the phosphorescence quantum
Spectroscopic properties The absorption properties of all 15 compounds are compared in Table 5 in cyclohexane, chloroform, ethyl acetate and propan-2-ol. The blank spaces indicate difficulties in solubility. In general, the extinction coefficients are little affected by solvent polarity and all appear to range between 4.1 and
Table 5. u.v. Absorption properties of oil-soluble substituted benzophenones (10 -4 M) Cyclohexane
Chloroform
Ethyl acetate
Propan-2-ol
Compound
nm Max
log ~M=~
nm Max
log ~M,x
nm Max
log EM,x
nm Max
log ~u,x
I 2 3 4 5 6 7 8 9 l0 I1 12 13 14 15
277.2 281.2 282.0 281.2 280.4 282.4 282.4 283.0 281.6 283.2 290.4 293.8
4.13 4.28 4.24 4.23 4.20 4.13 4.21 4.20 4.21 4.18 4.40 4.45
282.8 283.2 283.2 283.2 282.8 283.6 283.8 282.0 283.2 285.2 291.2 303.8 282.0 285.0 282.6
4.22 4.24 4.31 4.23 4.18 4.19 4.27 4.23 4.19 4.24 4.48 4.39 4.63 4.30 4.31
4.26 4.20 4.28 4.25 4.19 4.19 4.30 4.3 I 4.26 4.23 4.49 4.39
4.25 4.22
4.24 4.25 4.21 4.18 4.19 4.19 4.23 4.22 4.17 4.18 4.47 4.44 4.52 4.27 4.25
286.0 286.8 285.6 285.6 284.0 286.6 290.8 291.6 285.2 291.4 302.8 300.0
282.4 282.0
286.4 288.4 288.8 289.6 286.0 289.2 289.2 290.0 287.6 288.4 310.4 299.2 287.2 288.8 290.4
289.2 290.4
4.35 4.44
Photochemistry and polymerization of novel 4-alkylamino benzophenone initiators Table 6. Phosphorescence properties of benzophenone photoinitiators (10 5M) Phosphorescence (Propan-2-ol) E m
Photo-initiator Benzophenone Compounds 1 Compounds 2 Compounds 3 Compounds 4 Compounds 5 Compounds 6 Compounds 7 Compounds 8 Compounds 9 Compounds 10 Compounds 11 Compounds 12 Compounds 13 Compounds 14 Compounds 15
m a x
(nm) 446 446 446 446 446 446 446 446 446 446 446 446 446 446 446 446
417 417 417 417 417 417 417 417 417 417 417 417 417 417 417 417
~p 0.74 0.37 0.36 0.43 0.43 0.38 0.37 0.39 0.37 0.55 0.55 0.17 0.16 0.32 0.32 0.29
478 478 478 478 478 478 478 478 478 478 478 478 478 478 478 478
msec 6.4 11.2 I 1.9 8.3 8.4 10.8 12.8 12.6 10.2 7.8 8.1 18.4 18.3 8.0 8.5 8.2
yields of all 15 derivatives are lower than that of benzophenone. This effect is associated with the fact that, whilst the lowest excited triplet state is essentially ~ - ~ * in nature, the 4-alkylamino substituent introduces some degree of charge-transfer character into the molecule and will thus partly mix with the lowest excited triplet state and reduce the quantum yield of phosphorescence. This effect will be expected to enhance the emission lifetime as confirmed by the data in Table 6. The results in Fig. 10 show a plot of the phosphorescence quantum yields vs the emission lifetimes. Whilst it is not expected to be linear QUANTUM YIELD VERSUS LIFETIME 20
because of structural differences in the molecules, there is an inverse trend in that the lifetime decreases with increase in the phosphorescence quantum yield. This is related to the degree of charge-transfer character in the lowest triplet state. A direct correlation between the phosphorescence quantum yield and the photo-initiation efficiency of the compounds was not observed. This suggests in some cases that greater curing efficiency in the presence of oxygen is associated with a lower excited triplet state with a shorter triplet lifetime and hence greater degree of chargetransfer content which would be less effected by oxygen. CONCLUSIONS This paper presents information regarding the synthesis and characterization of 15 novel 4-aikylamino substituted benzophenone compounds. All the compounds operate as effective photo-initiators, the effect being dependent upon the conditions of cure and method of measurement. In the presence of oxygen, all the compounds are more effective than benzophenone itself because of the oxygen scavenging ability of the 4-alkylamino substituent. The ionization potential and degree of chargetransfer induced by the substituent appear to have an important influence on the photo-initiation efficiency. The rate of photocuring decreases in the order acyclic amino groups > aliphatic amino groups > alicyclic amino groups. C o m p o u n d s containing substituents with a propoxy link are more effective than those containing an ethoxy link. W o r k on nanosecond laser and conventional microsecond flash photolysis confirms the involvement of the lowest excited triplet state in inter and intra molecular hydrogen atom abstraction reactions with the 4 alkylamino substituent. Acknowledgement--The authors thank International Biosynthetics for co-operation in providing facilities and partial financial support of one of them (EL).
LIFETIME
15
REFERENCES
la. ..d
•
"° °
I0
\
•
,
O.O
1353
0.1
.
,
0.2
.
,
0.3
.
,
0.4
.
,
0.5
.
,
0.6
•
,
0.7
.
0.8
QUANTUM YIELD
Fig. 10. Phosphorescence quantum yields at 77 K for compounds 0-15 vs emission lifetime (in msec).
1. S. P. Pappas (Ed.) UV Curing: Science and Technology, Vol. I. Technology Marketing, Conn., U.S.A. (1978). 2. S. P. Pappas (Ed.), UV Curing: Science and Technology, Vol. II. Technology Marketing, Conn., U.S.A. (1985). 3. S. Tazuke. In Developments in Polymer Photochemist r y - 3 (edited by N. S. Allen), Chap. 2, p. 53. Elsevier Applied Science, London (1982). 4. H. Hageman. In Photopolymerisation and Photoimaging Science and Technology (edited by N. S. Allen), Chap. 1, p. 1. Elsevier Science, London (1989). 5. N. S. Allen, F. Catalina, J. Luc-Gardette, P. N. Green, W. A. Green, K. O. Fatinikun and W. Chen. Eur. Polym. J. 24, 435 (1988). 6. Wm. S. Merrill Co. U.S. Pat. 2,914,516-4 (1959). 7. Eastman Kodak Co. U.S. Pat. 2,831,768 (1958). 8. Wm. S. Merrill Co. U.S. Pat. 2,914,529 (1959). 9. O. N. Tolkachev, A. A. Cherkasova and N. A. Preobrazhenskii. Zh. obshch. Khim. 29, 1627 (1959). 10. P. L. Coe and C. E. Scriven. J. chem. Soc., Perkin Trans. 1 3, 475 (1986). 11. Wm. S. Merrill Co. U.S. Pat. 2,914,561 (1959). 12. J. G. Calvert and J. N. Pitts. Photochemistry. Wiley Interscience, New York (1966).
0014-3057/90 $3.00 + 0.00 Copyright © 1990 Pergamon Press plc
Printed in Great Britain. All rights reserved
PHOTOCHEMISTRY A N D PHOTOPOLYMERIZATION ACTIVITY OF NOVEL 4-ALKYLAMINO BENZOPHENONE INITIATORS--SYNTHESIS, CHARACTERIZATION, SPECTROSCOPIC A N D PHOTOPOLYMERIZATION ACTIVITY NORMAN S. ALLEN, EDWARD LAM and EDWARD M. HOWELLS Department of Chemistry, Faculty of Science and Engineering, Manchester Polytechnic, Chester Street, Manchester MI 5GD, England PETER N. GREEN and ARTHUR GREEN International Biosynthetics Ltd, Halebank Industrial Estate, Lower Road, Widnes, Cheshire WA8, 8NS, England FERNANDO CATALINA and CARMEN PE1NADO Instituto Plasticos y Caucho, CSIC, 3, Juan de la Cierva, 28006 Madrid, Espafia (Received 9 March 1990)
Abstract--Fifteen novel 4-substituted alkylaminobenzophenones have been synthesized and characterized using NMR, mass spectrometry and microanalysis. Their spectroscopic properties have been determined using u.v. absorption and phosphorescence analysis and the data related to their behaviour on photocuring using both Fourier Transform Infra-Red (FTIR) analysis and hardness testing in a triacrylate monomer. Absorption and phosphorescence spectra are similar to those of benzophenone although the quantum yields are lower with longer emission lifetimes indicative of the presence of some degree of charge-transfer in the lowest excited triplet state. Using FTIR analysis for photocuring, the compounds are in many cases more effective than benzophenone itself. Using hardness testing on the other hand, all the compounds proved to be more effective; the alicyclic amine structures are the least effective followed by the aliphatic and then the aromatic amines. For structural analogues, photocuring was more effective for those containing a propoxy link in the substituent than for those containing an ethoxy link. This difference is related to the ability of the molecule in the former case to form an intra-molecular exciplex with the aromatic ketone group. The alicyclic compounds are less effective in hydrogen atom or electron abstraction reactions. Differences observed in the methods of curing are related to the quenching effects of oxygen
INTRODUCTION
The photo-initiated polymerization and photocrosslinking of polymers are well established fields of industrial and academic research and development [1-3]. M a n y efforts have been directed towards an understanding of the mechanistic action of photoinitiators under various conditions in order either to improve formulations or to develop better quality and more efficient initiator types. One important area of industrial development is based on the synthesis of new photo-initiators for specialized applications. Problems associated with many conventional photo-initiators are poor solubility and migration which can result in the loss of adhesion and consequent loss of gloss of the coating.
The extractability of components by food in contact with u.v. cured coatings excludes their use for food packaging materials, and one of the methods to overcome this problem is to develop photoinitiators (e.g. benzophenone derivatives) with builtin functional groups capable of reacting with the monomer. Another is to avoid the use of liquid amine co-synergists by using solid amines or more recently through the use of alkylamino groups built into the initiator chromophore [4]. Mechanistic studies to date [4] on benzophenone indicate a primary photochemical process of hydrogen atom abstraction from the solvent, m o n o m e r or tertiary amino co-initiator by the photo-excited triplet state of the ketone group to produce a ketyl radical, solvent or alkylamino radical thus:
O
O
I
SH = Solvent
H 1345
R
NORMAN S. ALLEN et al,
1346
Either of the latter two species is then believed to initiate polymerization and this view has been confirmed through the use of Fourier Transform i.r. (FTIR) analysis for the alkylamino radicals [5]. In this case the tertiary amine is co-reacted into the monomer. Thus, the purpose of developing benzophenone derivatives with built-in alkylamino groups is not only to develop a more efficient initiator type with a built-in synergist but also to produce an initiator which will co-react with the m o n o m e r and thus not migrate out of the coating. Whilst simple alkylamino substituted benzophenone initiators have been examined in the past, such as Michler's ketone [4], a more detailed synthetic study of various structural types and their inter-relationship with photopolymerization activity has not been undertaken. In this work 15 novel 4-substituted alkylaminobenzophenones have been synthesized, including acyclic and alicyclic types, and characterized; their structures have been related to photopolymerization activity and spectroscopic properties. EXPERIMENTAL PROCEDURES
Materials Glycerol propoxylate triacrylate monomer was supplied by Harcross Chemicals Ltd, Eccles, Manchester, U.K. Acetone and ethanol were obtained from Vickers Laboratories Ltd, U.K. aluminium oxide, methanol, magnesium sulphate, potassium carbonate and sodium from Fisons Ltd., U.K., conc. H2SO4 from M&B Chemicals Ltd, U.K. 4-hydroxybenzophenone from Lancaster Synthesis Ltd, U.K., AR cyclohexane, ethyl acetate, chloroform and 2-propanol from BDH Chemicals Ltd, U.K. and l-bromo-2-chloroethane, 1-bromo-3-chloropropane, 2-chloroethanol, 3-chloropropan-l-ol, 1-(2-chloroethyl)piperidine monohydrochloride, l-(3-chloropropyl)piperidine monohydrochloride, 4-(2-chloroethyl)morpholine hydrochloride, 2-(dimethylamino ethyl chloride) monohydrochloride, 3-(diethylamino ethyl chloride) monohydrochloride, 2-(di-isopropylamino ethyl chloride) monohydrochloride, 4-N-dimethylamino benzoic acid, N-ethyl-2methyl-allylamine, 1-(2-hydroxyethyl) piperazine, morpholine, piperazine, 1-methyl piperazine and sodium ethoxide were all obtained from the Aldrich Chemical Company Ltd, U.K.
Synthesis (I) 4-(2-Chloroethoxy) benzophenone [6]. 4-Hydroxybenzophenone (19.8 g) was added to a solution of sodium ethoxide (3 g Na and 200 cm 3 ethanol); after boiling for 5 rain, l-bromo-2-chloroethane (14.3 g) was added followed by refluxing for 24 hr, cooling and filtration of the precipitate. The filtrate was then evaporated under reduced pressure followed by separation in a water/chlorofom mixture. The bottom layer was then collected and washed several times with 1M NaOH dried and then evaporated. After 1 hr of standing, the crystalline product was removed and recrystallized from methanol to give 19.5 g of product with a m.p. of 79°C (Yield 75%). (II) 4-(3-Chloropropoxy) benzophenone [6]. As in (I), the sodium salt of 4-hydroxybenzophenone was reacted with 1-bromo-3-chloropropane (15.7 g) to give the desired product after 24 hr of reflux with stirring. The crude product after purification gave 19.2 g (Yield 70%) with a m.p. of 60°C. (ill) 4-(2-Hydroxyethoxy) benzophenone [7]. 4-Hydroxybenzophenone (19.8 g) was added to a solution of NaOH (8 g in 200 cm 3 of distilled water) and after a 5 min boil 2-chloroethanol (8 g) was added. The mixture was then
refluxed for 24 hr, cooled, filtered and washed with distilled water. The product after recrystallization from methanol gave 18.1 g (Yield 75%) with a m.p. of 87-88°C. (IV) 4-(3-Hydroxypropoxy) benzophenone. This derivative was prepared as for (III). The product was recrystallized from methanol to give 18.6 g (Yield 73%) with a m.p. of 68-70°C.
(V) [4-[2-(4-Morpholinyl) ethoxy] phenyl} phenyl methanone [8]. 4-Hydroxybenzophenone (19.8 g) was added to a solution of sodium ethoxide (6 g Na in 200 cm 3 of ethanol) followed by boiling for 5 rain. The 4-(2-chloroethyl) morpholine hydrochloride (18.6 g) was then added, followed by refluxing for 24 hr. The NaCI was filtered off and the filtrate evaporated under reduced pressure. The crude product was then dissolved in chloroform followed by washing with NaOH solution, then with distilled water and finally dried o v e r M g S O 4. The product was purified on an alumina column using chloroform as eluant. After evaporation and recrystallization from methanol, the product was 18.6g (yield 60%) with a melting point of 93-94°C. Calculated C, H and N are 73.29, 6.8 and 4.5% respectively. Values found were 73.32, 6.85 and 4.46% respectively.
(VI) [4-[3-(4-Morpholinyl) propoxy] phenyl] phenyl methanone. 4-(3-Chloropropoxy) benzophenone (27.5g) and K2CO 3 (14.0 g) were mixed with 100 cm 3 of morpholine followed by refluxing for 12 hr, cooling and removing the precipitate by filtration. Unreacted morpholine was removed under reduced pressure. The oily product was then distilled under vacuum to give 21.2 g (Yield 65%) with a b.p. of 260-270°C at 3 mm Hg. Calculated C, H and N were 73.83, 7.12 and 4.31% respectively. Values found were 72.62, 7.12 and 4.17% respectively.
(VII) [4-[2-(4-Piperidinyl) ethoxy] phenyl] phenyl methanone [9]. 4-Hydroxybenzophenone (19.8 g) was added to a solution of sodium ethoxide (6 g Na in 200 cm3 ethanol) followed by boiling for 5 rain and then adding 1-(2-chloroethyl) piperidine monohydrochloride (18.4g) and then refluxing for 24 hr, cooling and filtering. Following the purification procedure for (V), the product was recrystallized from ethanol to give 15.5 g (Yield 50%) with a m.p. of 64-65°C. Calculated C, H and N are 77.63, 7.49 and 4.53% respectively. Values found were 77.71, 7.59 and 4.47% respectively.
(VIII) [4-[3-(4-Piperidinyl) propoxy) phenyl] phenyl methanone. This compound was prepared as for (VII) but with l-(3-chloropropyl) piperidine monohydrochloride (19.8 g). After filtration, the filtrate was evaporated under reduced pressure followed by dissolving the crude product in chloroform and then washing in dilute NaOH, distilled water and drying over MgSO4. After evaporation, the crude product was distilled under vacuum to give 17.8 g (Yield 55%) with a b.p of 240-250°C at 4 mm Hg. Calculated C, H and N are 77.98, 7.79 and 4.33%. Values found were 77.81, 7.6 and 4.34% respectively.
(IX) [4-[2-[(Dirnethylamino) etho xy] phenyl] phenyl metha none [10]. 4-Hydroxybenzophenone (19.8 g) was added to a solution of sodium ethoxide (6 g Na in 200 cm 3 ethanol) followed by boiling for 5 rain and then the addition of 2-dimethylaminoethyl chloride monohydrochloride (14.4 g) and then refluxing with stirring for 24 hr. The whole was then cooled and the precipitate filtered off. Following the purification procedure as for (VIII), the product gave 12.9 g (Yield 48%) with a boiling point of 188-191°C at 1 mm Hg. Calculated C, H and N are 75.81, 7.11 and 5.2% respectively. Values found were 75.76, 7.08 and 4.90% respectively.
(X) [4-[3-(Dimethylamino) propoxy] phenyl] phenyl methanone. As for (IX), the sodium salt of 4-hydroxybenzophenone was reacted with 3-dimethylaminopropyl chloride monohydrochloride for 24 hr under reflux. After the same purification procedure, the product obtained gave 13.9g (Yield 49%) with a boiling point of 201-208°C at 1 mm Hg. Calculated C, H and N are 76.29, 7.47 and
Photochemistry and polymerization of novel 4-alkylamino benzophenone initiators 4.49% respectively. Values found were 76.3, 7.38 and 4.68% respectively.
1347
7.90% respectively. Values found were 71.08, 7.33 and 7.75% respectively.
(XI) [4-[2-(Diethylamino) ethoxy] phenyl] phenyl methanone [11]. As for (IX), 2-diethylaminoethyl chlor-
(XIX) [4'[2'[Ethyl(2-methyl-2-propen-l-yl) amino] ethoxy]phenyl]phenylmethanone. 4-Hydroxybenzophenone
ide monohydrochloride (17.2g) was reacted with the 4-hydroxybenzophenone in sodium ethoxide. The product gave 13.4 g (Yield 45%) with a boiling point of 202-203°C at 1 mm Hg. Calculated C, H and N are 76.73, 7.80 and 4.71% respectively. Values found were 76.58, 7.56 and 4.04% respectively.
(19.8g) and K2CO 3 (14g) were mixed with N-ethyl-2methylallylamine (26 cm 3) followed by refluxing for 24 hr with stirring. The whole was then cooled and filtered and evaporated under reduced pressure. The product gave 16.9 g (Yield 52%) with a boiling point of 197-200°C. Calculated C, H and N are 77.98, 7.79 and 4.33% respectively. Values found were 77.81, 7.80 and 4.44% respectively.
(XII) [4-[2-(Diisopropylamino) ethoxy] phenyl] phenyl methanone. As for (IX), 2-diisopropylaminoethyl chloride monohydrochloride (20.15 g) was used. The product gave 15.3g (Yield 47%) with a boiling point of 211-212°C. Calculated C, H and N are 77.50, 8.36 and 4.30% respectively. Values obtained were 77.50, 8.13 and 3.98% respectively.
(XIII) [4-[2-(4-Methylpiperazin- l-y l)ethoxy] phenyl] phenyl methanone. 4-(2-Chloroethoxy) benzophenone (26 g), K2CO 3 (14 g) and l-methylpiperazine (100 cm ~) were refluxed for 24 hr and purified as described for (V). The product gave 16 g (Yield 49%) with a m.p. of 93-94°C at 0.9 mm Hg. Calculated C, H and N are 74.04, 7.45 and 8.36% respectively. Values found were 74.24, 7.54 and 8.37% respectively.
(~7V) [4-[3-(4-Methylpiperazin- l-y 1)propoxy] phenyl] phenyl methanone. As for compound (XIII), 4-(3-chloropropoxy) benzophenone (27.4g) was used in the reaction mixture. After distillation under vacuum, the product gave 15.2 g (Yield 45%) with a boiling point of 208-210°C at 1 mm Hg. Calculated C, H and N are 74.52, 7.74 and 8.28% respectively. Values found were 74.56, 7.74 and 7.97% respectively.
(XV) [l,4-Piperazinediyl bis(l,2-ethoxy- l,4-phenylene)] bis phenyl methanone. 4-(2-Chloroethoxy) benzophenone (26 g), and K2CO 3 (14 g) were mixed with acetone 200 cm 3 followed by boiling for 5 min and then adding piperazine (4.3 g). The whole was then refluxed for 24 hr followed by hot filtration and then cooling in ice. The crude product was purified on an alumina column using chloroform as eluant. The product gave 20.3 g (Yield 38%) with a melting point of 159-160°C. Calculated C, H and N are 74.01, 5.95 and 3.60% respectively. Values obtained were 74.07, 5.97 and 3.57% respectively.
(XVI) [4-[2-(4-Dimethylaminobenzoate) ethoxy] phenyl] phenyl methanone. 4-(2-Hydroxyethoxy) benzophenone (24.2 g), 4-N-dimethylaminobenzoic acid (82.6 g) and conc. H2SO 4 (5cm 3) were mixed with 100cm 3 of toluene. The mixture was then refluxed for 24 hr using a Dean and Stark column. The reaction mixture was then evaporated and taken up in chloroform followed by purification on an alumina column using chloroform as eluant. The product after recrystallization from methanol gave 26.5g (yield 68%) with a m.p. of 92-93°C. Calculated C, H and N are 74.42, 6.25 and 3.47% respectively. Values found were 74.19, 6.34 and 3.65% respectively.
(::VII) [4-[3-(4-Dimethylaminobenzoate) ethoxy] phenyl] phenyl methanone. As for (XVI), 4-(3-hydroxypropoxy) benzophenone (25.6 g) was used in the reaction mixture. The recrystallized product gave 26.2 g (Yield 65%) with a m.p. of 118-120°C. Calculated C, H and N are 76.38, 6.41 and 5.24% respectively. Values found were 75.46, 6.44 and 5.17% respectively.
(XVIII) [4-[2-(1-(2-Hydroxyethyl) piperazin-l-yl) ethoxy] phenyl] phenyl methanone. 4-Hydroxybenzophenone (19.8 g) and K2CO 3 and (14g) were mixed with acetone (200 cm 3) followed by boiling for 5 min and then the addition of l-(2-hydroxyethyl) piperazine (14 g). The whole was then refluxed for 24 hr followed by pouring into iced water. After filtration, the product was purified on an alumina column using chloroform as eluant and then recrystallized from ethyl acetate to give 18.4 g (Yield 52%) with a m.p. of I02°C. Calculated C, H and N are 71.16, 7.39 and
Instrumentation Characterization. ~H-NMR Spectra were recorded on a Perkin-Elmer R12B instrument at a frequency of 60 MHz and a field strength of 1.409 Tesle using TMS for reference. Mass spectra were recorded on a VG 7070 machine using the electron ionization mode. The micro-analyses were carried out by C.H.N. Analysis Ltd, Leicester. Photopolymerization. Films of Photomer 4094 containing 0.1 and 1% w/w of the photo-initiators were irradiated using a 100 watt high pressure Hg lamp for various periods. Cure rate was assessed using a Pendulum Hardness Meter (Sheenan Instruments Ltd, London, U.K.) as well as by measuring the reduction in the absorption band at 812 cmrelative to that at 1740cm -I of coatings between two salt fiats using a Mattson Alpha Centauri Fourier Transform i.r. instrument. Spectroscopic measurement. Absorption spectra were recorded using a Perkin-Elmer Lambda 7 absorption spectrometer. Phosphorescence spectra and emission quantum yields and lifetimes were obtained at 77 K using a Perkin-Elmer LS-5 luminescence spectrometer. Benzophenone in ethanol was used as the standard and assumed to have a quantum yield of 0.74 [12]. RESULTS AND DISCUSSION
Photopolymerization T h e chemical structures a n d n a m e s of the 15 p h o t o i n i t i a t o r s are listed in Table 1 for reference. They will be referred to t h r o u g h o u t by their numbers. One industrial m e t h o d of measuring the rate o f cure o f a coating is to determine its hardness. This m e a s u r e m e n t is m a d e using a p e n d u l u m device which consists of stainless steel bearings resting o n the surface of the cured coating. T h e p e n d u l u m is set in m o t i o n a n d the n u m b e r of swings or oscillations is directly related to the h a r d n e s s of the cured resin. The greater the n u m b e r o f oscillations the h a r d e r is the coating. A poorly cured coating will be softer a n d so impose more drag on the bearings. The results for a 1% w/w c o n c e n t r a t i o n o f p h o t o - i n i t i a t o r in the P h o t o m e r 4094 are c o m p a r e d in Table 2 after 11 rain o f irradiation. All 15 initiators are m u c h more effective t h a n b e n z o p h e n o n e which is referred to as comp o u n d 0; the m o s t effective is c o m p o u n d 13 which c o n t a i n s two b e n z o p h e n o n e units in the same molecule. These data are illustrated more effectively in Fig. 1 using a b a r chart. F r o m this i n f o r m a t i o n , an interesting structural correlation is observed. C o m p o u n d s 5 to 8 a n d 15 c o n t a i n only aliphatic alkylamino groups in the 4-substituent a n d are m o r e effective t h a n the c o m p o u n d s c o n t a i n i n g alicyclic a m i n e substituents viz. c o m p o u n d s 1 to 4, 9 a n d 10 a n d 14. In the former case this effect m a y be related to the lower ionization potential of the amine substituent. This view is confirmed by the fact t h a t the
NORMAN S. ALLEN et al.
1348
Table 1. Nomenclature and structures of the photo-initiators Compound number
Structure
O-iQ
IUPAC name
O(CH2)aN ~
[4-[2-(4-Morpholinyl) ethoxy] phenyl] phenyl methanone
O(CH:)3 N ~
[4-[3-(4-Morpholinyl) propoxy] phenyl] phenyl methanone
O
O
[4-[2-(4-Pipcridinyl) ethoxy] phenyl] phenyl methanone O
[4-[3-(4-Piperidinyl) propoxy] phenyl] phenyl methanone O
~
i-~L-O(CH2)2 N(CH3)2
[4-[2-(Dimethylarnino) ethoxy] phenyl] phenyl methanone
O
~-~
i~'~
O(CH2)3N(CHa)2
[4-[3-(Dimethylamino) propoxy] phenyl] phenyl methanone
0(CH2)2N(C2Hs)2
[4-[2-(Diethylamino) propoxy] phenyl] phenyl methanone
0
~"-i
"--~ O
~
i ~~'-'-O (CH2)2N(CH(CH3)2)2
[4-[2-(Diisopropylamino) ethyl] phenyl] phenyl methanone
O
O(CH2) 2Nk....../N~CH3
[4-[2-(4-Methylpiperazin-l-yl) ethoxy] phenyl] phenyl methanone
O 10
O(CH2)3 N
~
CHa
[4-[3-(4-Methylpiperazin-l-yl) propoxy] phenyl] phenyl methanone
0
li 0
i
[4-[2-(4-Dimethylaminobenzoate) ethoxy] phenyl] phenyl methanone Continued
Photochemistry and polymerization of novel 4-alkylamino benzophenone initiators
1349
Table l--continued Compound number
Structure
IUPAC name
0
CH3
[4-[3-(4-Dimethylaminobenzoate) propoxy] phenyl] phenyl methanone
FI 0 13 N
N
[I,4-Piperazinediyl bis(I,2-ethoxy-l,4-phenylene)] bis phenyl methanone
14 O(CH~h
N
N (CHa), OH
x___/
[4-[2-(l-(2-hydroxyethyl) piperazin-l-yl) ethoxyl] phenyl] phenyl methanone
0
<
~
C-~--x~
II
0
O(CH2)a NCH2CH2C= C H z
I
C2Hs
I
[4'[2'[Ethyl(2-methyl-2-propen-l-yl) amino] ethyl] phenyl] methanone
CH3
compounds (11 and 12) containing aromatic (acyclic) amine substituents are also more effective than the alicyclic compounds. Compound 13 is anomalous in that it contains two benzophenone chromophores. Another interesting feature of these results is that, for the pairs of analogous compounds 1 and 2, 3 and 4, 9 and 10 and 11 and 12, the second of the pairs containing a propoxy group are more effective photoinitiators than their corresponding ethoxy analogue. Thus, the compounds containing a propoxy link must be important for hydrogen atom abstraction. Future publications using kinetic nanosecond laser flash photolysis will confirm that this mechanism occurs via an intramolecular process. The only exception here is for compounds 5 and 6 which contain a dimethylamino substituent. Here the presence of a methyl group alpha to the nitrogen from which a hydrogen atom may be abstracted is present in both cases and may compensate.
On a more scientific basis, the rate of curing may be monitored using i.r. spectroscopy. Here, the coating is irradiated between two salt flats in order to exclude the quenching effect of oxygen. In the above method, all the coatings were exposed to the CURE RATES OF PHOTOINITIATORS 200
I00
Table 2. Influence of the photo-initiator: (benzophenone, compounds 1-15) 1% w/w on the hardness of u.v. triacrylate resin (Photomer 4094) Compound Benzophenone Compound 1 Compound 2 Compound 3 Compound 4 Compound 5 Compound 6 Compound 7 Compound 8 Compound 9 Compound 10 Compound I 1 Compound 12 Compound 13 Compound 14 Compound 15
No. of oscillations after 11 rain of radiation 40 73 86 67 87 167 149 139 142 73 79 155 164 173 88 155
0
0
1 2
3 4
5
6 7 t
9 tO II 12 13 14 15
COMPOUND Fig. 1. Cure rate of Photomer 4094 measured by the hardness method in oscillations and initiated by c o m p o u n d s 0-15.
1350
NORMAN S. ALLEN et al.
Table 3. Photopolymerization conversion of an oil-soluble triaerylate resin (Photomer 4094) by benzophenone initiator: (benzophenone, compounds 1-15) 0.1% w/w
~g CONVERSION VERSUS EXPOSURE TIME 0.1~ w/w
% Conversion at various exposure times (see)
Compound Benzophenone Corn )ound 1 Com )ound 2 Corn )ound 3 Corn )ound 4 Com )ound 5 Corn )ound 6 Corn )ound 7 Corn )ound 8 Corn )ound 9 Com )ound 10 Corn )ound I 1 Corn ~ound 12 Com )ound 13 Corn )ound 14 Corn )ound 15
1
5
10
20
30
40
50
60
18 34 32 17 18 16 46 31 15 19 24 12 33 30 6 49
27 46 51 25 26 26 64 47 25 25 44 31 57 36 26 63
52 65 69 48 51 52 74 64 36 46 65 47 68 65 47 70
65 72 76 61 57 63 77 71 52 49 73 58 75 68 59 74
70 75 77 65 63 67 79 72 57 55 78 63 78 72 63 75
72 77 78 65 67 70 81 75 64 59 79 70 78 72 64 76
73 77 78 66 67 72 81 76 64 62 80 72 78 73 67 77
73 80 78 66 68 74 82 76 67 68 80 73 78 75 71 77
80-
60
Z m
'" ).
BENZOPHENONE
40
d,.
COMPOUND I
Z
COMPOUND 2
z
o
COMPOUND 3
a t m o s p h e r e a n d t h e g r e a t e r efficiency o f t h e c o m pounds containing the 4-alkylamino substituents c o m p a r e d w i t h t h a t o f b e n z o p h e n o n e is d u e in t h e m a i n to t h e i r ability to s c a v e n g e o x y g e n w h i c h m a y o t h e r w i s e q u e n c h t h e p h o t o - e x c i t e d triplet s t a t e o f t h e k e t o n e [1, 2]. I n t h e F T I R m e t h o d u s e d here, t h e r a t e o f c u r e is r e l a t e d to t h e d e c r e a s e in t h e v i n y l (acrylic) a b s o r p t i o n at 8 1 2 c m -1 relative to t h a t o f t h e e s t e r b a n d at 1740 c m - ~ w h i c h s h o u l d n o t c h a n g e a n d so c o m p e n s a t e s for c o n t r a c t i o n in t h e film d u r i n g cure. T h e d a t a for t h e c o n v e r s i o n ; u s i n g 0 . 1 % w / w p h o t o i n i t i a t o r in P h o t o m e r 4094 a r e s h o w n in T a b l e 3; t h o s e f o r a I % w / w o f p h o t o i n i t i a t o r a r e s h o w n in T a b l e 4. It is s e e n t h a t at t h e h i g h e r c o n c e n t r a t i o n t h e c u r e d u r i n g t h e first 20 sec o f i r r a d i a t i o n is m u c h f a s t e r b u t t h e r e a f t e r t h e r a t e b e g i n s to p l a t e a u . T h i s effect is d e m o n s t r a t e d m o r e clearly b y t h e p l o t s o f F i g s 2 - 5 for t h e 0 . 1 % w / w c o n c e n t r a t i o n a n d F i g s 6 - 9 f o r t h e 1.0% w / w c o n c e n t r a t i o n o f p h o t o - i n itiator. T h u s , at 0 . 1 % w / w c o n c e n t r a t i o n , c o m p o u n d s 3, 4, 5, 8, 9, 11 a n d 14 a r e less effective than benzophenone; at 1.0% w/w concentration, o n l y c o m p o u n d 13 p r o v e d to be less effective. T h u s , under these curing conditions, the exclusion of oxyg e n is s e e n to be i m p o r t a n t . H o w e v e r , t h e o n e c o n s i s t e n t t r e n d in t h e s e r e s u l t s is s e e n at t h e 0 . 1 %
COMPOUND 4
20
01 0
I '0
2'0
5'0
4'0
5'0
60
EXPOSURE TIME (Sees)
Fig. 2. Conversion vs exposure time of Photomer 4094 using F T I R spectroscopy, initiated by c o m p o u n d s l ~ t at 0.1% w/w, relative to that of benzophenone.
~g CONVERSION VERSUS EXPOSURE TIME O. 1 ~g w / w
100"
80"
60"
Table 4. Photopolymerization conversion of an oil-soluble triacrylate resin (Photomer 4094) by benzophenone initiator: (benzophenone, compounds 1-15) 1% w/w % Conversion at various exposure times (see)
Compound Benzophenone Compound 1 Compound 2 Compound 2 Compound 3 Compound 4 Compound 5 Compound 6 Compound 7 Compound 8 Compound 9 Compound 10 Compound 11 Compound 12 Compound 13 Compound 14 Compound 15
1
3
10
20
30
40
50
60
29 45 45 45 46 46 49 50 48 52 41 50 52 55 12 35 52
55 62 61 61 56 59 69 67 66 70 61 65 62 68 27 53 70
61 69 68 68 66 62 76 72 72 78 69 75 68 71 37 67 78
67 75 72 72 71 69 80 76 78 83 75 81 74 76 55 71 82
71 76 75 75 74 72 81 78 79 85 77 81 76 78 59 74 83
72 79 76 76 75 73 82 80 81 85 79 83 76 80 59 76 84
75 79 77 77 76 74 82 81 81 85 80 83 78 80 65 78 84
76 80 78 78 79 75 83 82 82 86 80 83 80 81 68 80 85
40"
20"
0
0
IO
2'0
~
COMPOUND 8
" ~
BENZOPHENONE
3b
4'0
5'0
60
EXPOSURE TIME (SEC5)
Fig. 3. Conversion vs exposure time of Photomer 4094 using F T I R spectroscopy, initiated by c o m p o u n d s 5-8 at 0.1% w/w, relative to that of benzophenone.
Photochemistry and polymerization of novel 4-alkylamino benzophenone initiators
1351
;~ CONVERSIONVERSUSEXPOSURET l l l E 1.0 :g w / w
:g CONVER310N VERSUS EXPOSURE TIME O.l :g w / w 80
80
70 60
60
Z 0 e~
:::,. z o
Z C~
COMPOUND 12
40 ,I,
C(NIPOUND I I
:
COMPOUND I 0
~,'
dE
50
Z C~ ¢,J
g~
O01'1POUND I (X~IPOUND 2 COMPOUND 5
COMPOUND 9
40 ¸
20
t
BENZOPHENONE
(~OrlPOUND4
50 ¸
0
0
I0
~ 20
30
40
50
60 20
lb
EXPOSURE TIME (SECS) Fig. 4. Conversion vs exposure time of Photomer 4094 using FTIR spectroscopy, initiated by compounds 9-12 at 0.1% w/w, relative to that of benzophenone.
2b
3b
4b
':-b
60
EXPOSURETIME (5ECS)
Fig. 6. Conversion vs exposure time of Photomer 4094 using FTIR spectroscopy, initiated by compounds 1-4 at 1% w/w, relative to that of benzophenone.
% GONVERSION VERSUS EXPOSURE TIME O.1%w/w
:g CONVERSIONVERSUSEXPOSURETI ME 1,0 :g wlw
80
90 ¸
8O
60
Z f __q r'Y
40-
>. z
O ¢_)
20
~
COMPOUND t 5
JL
COMPOUND 14
~
COMPOUND 13
"
O011POUND 7
•
COMP(XJND B
- - { 3 - - BENZOPHENONE ~
/ 01 0
20
1'0
2'0
5'0
4'0
5'0
60
EXPOSURE TIME (5E05) Fig. 5. Conversion vs exposuretime of Photomer 4094 using
FTIR spectroscopy, initiated by compounds 13-15 at 0. I% w/w, relative to that of benzophenone.
0
lb
2b
3b
4b
sb
60
EXPOSURETIHE (SECS)
Fig. 7. Conversion vs exposure time of Photomer 4094 using FTIR spectroscopy, initiated by compounds 5-8 at 1% w/w, relative to that of benzophenone.
1352
NORMANS. ALLENet al. % CONVERSION VERSION EXPOSURE TIME 1.0 % w l w
% CONVERSION VERSUS EXPOSURE TIME 1.0 % w/w
100
9O
8o
~
o
80
7O
g
Z o
50
~
~-
."IIl 2
0
>-
COIIpOUHD 9
d
o
~
BENZOPHENONE
z
o u
-:::::::0 ~
60
IR
dO
CCII'POUN I D 13 COMPOUND 14
~COMPOUND 11
COMPOUND I15 20
0
10
20
30
40
50
,o
60
EXPOSURE TIME (SECS)
2b
~
~
sb
60
EXPOSURE TIME (SECS)
Fig. 8. Conversion vs exposure time of Photomer 4094 using FTIR spectroscopy, initiated by compounds 9-12 at 1% w/w, relative to that of benzophenone.
Fig. 9. C o n v e r s i o n vs e x p o s u r e t i m e o f P h o t o m e r 4 0 9 4 u s i n g FTIR spectroscopy, initiated by compounds 13-15 at 1% w / w , relative to t h a t o f b e n z o p h e n o n e .
w/w concentration where in the analogous pairs 1 and 2, 3 and 4, 5 and 6, 9 and 10 and 11 and 12 the second compound containing the propoxy link in the 4-substituent is the more effective initiator. At 1.0% w/w concentration, the relative differences in cure rates are small after 20 sec of irradiation.
4.7. The absorption maxima occur in the range 277-310 nm and are little affected by solvent polarity although there is a small red shift for the benzophenone derivatives indicating that this band is associated with the main n - ~ * transition of the benzene rings. None of the compounds studied here exhibited any fluorescence but all exhibited phosphorescence with an emission spectrum the same as that of benzophenone itself. The emission maxima are shown in Table 6 together with their corresponding emission quantum yields and triplet lifetimes at 77 K in liquid N2. Whilst the emission maxima are seen to be the same, it is noted that the phosphorescence quantum
Spectroscopic properties The absorption properties of all 15 compounds are compared in Table 5 in cyclohexane, chloroform, ethyl acetate and propan-2-ol. The blank spaces indicate difficulties in solubility. In general, the extinction coefficients are little affected by solvent polarity and all appear to range between 4.1 and
Table 5. u.v. Absorption properties of oil-soluble substituted benzophenones (10 -4 M) Cyclohexane
Chloroform
Ethyl acetate
Propan-2-ol
Compound
nm Max
log ~M=~
nm Max
log ~M,x
nm Max
log EM,x
nm Max
log ~u,x
I 2 3 4 5 6 7 8 9 l0 I1 12 13 14 15
277.2 281.2 282.0 281.2 280.4 282.4 282.4 283.0 281.6 283.2 290.4 293.8
4.13 4.28 4.24 4.23 4.20 4.13 4.21 4.20 4.21 4.18 4.40 4.45
282.8 283.2 283.2 283.2 282.8 283.6 283.8 282.0 283.2 285.2 291.2 303.8 282.0 285.0 282.6
4.22 4.24 4.31 4.23 4.18 4.19 4.27 4.23 4.19 4.24 4.48 4.39 4.63 4.30 4.31
4.26 4.20 4.28 4.25 4.19 4.19 4.30 4.3 I 4.26 4.23 4.49 4.39
4.25 4.22
4.24 4.25 4.21 4.18 4.19 4.19 4.23 4.22 4.17 4.18 4.47 4.44 4.52 4.27 4.25
286.0 286.8 285.6 285.6 284.0 286.6 290.8 291.6 285.2 291.4 302.8 300.0
282.4 282.0
286.4 288.4 288.8 289.6 286.0 289.2 289.2 290.0 287.6 288.4 310.4 299.2 287.2 288.8 290.4
289.2 290.4
4.35 4.44
Photochemistry and polymerization of novel 4-alkylamino benzophenone initiators Table 6. Phosphorescence properties of benzophenone photoinitiators (10 5M) Phosphorescence (Propan-2-ol) E m
Photo-initiator Benzophenone Compounds 1 Compounds 2 Compounds 3 Compounds 4 Compounds 5 Compounds 6 Compounds 7 Compounds 8 Compounds 9 Compounds 10 Compounds 11 Compounds 12 Compounds 13 Compounds 14 Compounds 15
m a x
(nm) 446 446 446 446 446 446 446 446 446 446 446 446 446 446 446 446
417 417 417 417 417 417 417 417 417 417 417 417 417 417 417 417
~p 0.74 0.37 0.36 0.43 0.43 0.38 0.37 0.39 0.37 0.55 0.55 0.17 0.16 0.32 0.32 0.29
478 478 478 478 478 478 478 478 478 478 478 478 478 478 478 478
msec 6.4 11.2 I 1.9 8.3 8.4 10.8 12.8 12.6 10.2 7.8 8.1 18.4 18.3 8.0 8.5 8.2
yields of all 15 derivatives are lower than that of benzophenone. This effect is associated with the fact that, whilst the lowest excited triplet state is essentially ~ - ~ * in nature, the 4-alkylamino substituent introduces some degree of charge-transfer character into the molecule and will thus partly mix with the lowest excited triplet state and reduce the quantum yield of phosphorescence. This effect will be expected to enhance the emission lifetime as confirmed by the data in Table 6. The results in Fig. 10 show a plot of the phosphorescence quantum yields vs the emission lifetimes. Whilst it is not expected to be linear QUANTUM YIELD VERSUS LIFETIME 20
because of structural differences in the molecules, there is an inverse trend in that the lifetime decreases with increase in the phosphorescence quantum yield. This is related to the degree of charge-transfer character in the lowest triplet state. A direct correlation between the phosphorescence quantum yield and the photo-initiation efficiency of the compounds was not observed. This suggests in some cases that greater curing efficiency in the presence of oxygen is associated with a lower excited triplet state with a shorter triplet lifetime and hence greater degree of chargetransfer content which would be less effected by oxygen. CONCLUSIONS This paper presents information regarding the synthesis and characterization of 15 novel 4-aikylamino substituted benzophenone compounds. All the compounds operate as effective photo-initiators, the effect being dependent upon the conditions of cure and method of measurement. In the presence of oxygen, all the compounds are more effective than benzophenone itself because of the oxygen scavenging ability of the 4-alkylamino substituent. The ionization potential and degree of chargetransfer induced by the substituent appear to have an important influence on the photo-initiation efficiency. The rate of photocuring decreases in the order acyclic amino groups > aliphatic amino groups > alicyclic amino groups. C o m p o u n d s containing substituents with a propoxy link are more effective than those containing an ethoxy link. W o r k on nanosecond laser and conventional microsecond flash photolysis confirms the involvement of the lowest excited triplet state in inter and intra molecular hydrogen atom abstraction reactions with the 4 alkylamino substituent. Acknowledgement--The authors thank International Biosynthetics for co-operation in providing facilities and partial financial support of one of them (EL).
LIFETIME
15
REFERENCES
la. ..d
•
"° °
I0
\
•
,
O.O
1353
0.1
.
,
0.2
.
,
0.3
.
,
0.4
.
,
0.5
.
,
0.6
•
,
0.7
.
0.8
QUANTUM YIELD
Fig. 10. Phosphorescence quantum yields at 77 K for compounds 0-15 vs emission lifetime (in msec).
1. S. P. Pappas (Ed.) UV Curing: Science and Technology, Vol. I. Technology Marketing, Conn., U.S.A. (1978). 2. S. P. Pappas (Ed.), UV Curing: Science and Technology, Vol. II. Technology Marketing, Conn., U.S.A. (1985). 3. S. Tazuke. In Developments in Polymer Photochemist r y - 3 (edited by N. S. Allen), Chap. 2, p. 53. Elsevier Applied Science, London (1982). 4. H. Hageman. In Photopolymerisation and Photoimaging Science and Technology (edited by N. S. Allen), Chap. 1, p. 1. Elsevier Science, London (1989). 5. N. S. Allen, F. Catalina, J. Luc-Gardette, P. N. Green, W. A. Green, K. O. Fatinikun and W. Chen. Eur. Polym. J. 24, 435 (1988). 6. Wm. S. Merrill Co. U.S. Pat. 2,914,516-4 (1959). 7. Eastman Kodak Co. U.S. Pat. 2,831,768 (1958). 8. Wm. S. Merrill Co. U.S. Pat. 2,914,529 (1959). 9. O. N. Tolkachev, A. A. Cherkasova and N. A. Preobrazhenskii. Zh. obshch. Khim. 29, 1627 (1959). 10. P. L. Coe and C. E. Scriven. J. chem. Soc., Perkin Trans. 1 3, 475 (1986). 11. Wm. S. Merrill Co. U.S. Pat. 2,914,561 (1959). 12. J. G. Calvert and J. N. Pitts. Photochemistry. Wiley Interscience, New York (1966).