Dec., 1962
2403
hVEBTIG.4TION O F SPECTRAL SESSITIZATION
of the excited singlet state of the sensitizer (a process in which spin is not conserved and which therefore is expected to be slow), and the triplet state of the supersensitizer reacts with the silver halide. This mechanism is based on experiments involving vinylogous series of supersensitizers with the same sensitizer, d i i c h seem to indicate that supersensitization ocrurred only when the triplet level of the supersensitizer was low enough t o b~ excited by the excited singlet level of the scnsitizcr. S c w physicwhemical techniques promise to be iiscful in the study of spectral sensitization. For cxumple, the production of free radicals in pre-
cipitates of silver bromide dyed with sensitizing dyes on illumination a t - 196" with light absorbed by the dye has been shown by electron paramagnetic resonance experiment^.^^ Transient fading of sensitizing dyes adsorbed to emulsion grains has been recently observed by A. Buettner, of the Eastman Kodak Research Laboratories. These experiments are not sufficiently developed to merit further discussion a t present, but it is a t least clear that short-lived species are produced from sensitizing dyes under certain conditions of photographic exposure. (43) W. C. Needler, R. L. Griffitli, and W. West, .Yature, 191, 90% (1961).
INVESTIGATIOS OF SPECTRAL SESSITIZATIOS. I. SOME PROPERTIES OF SESSITIZISG ASD DESESSITIZING DYES BYJAMES E. LUYALLE,As.&LEIFER,P. 11. DOUGHEHTY, A X D 11. KORAL Defense Products Division, Pairchild Cumera and Instrument Corporation, Syosset, L. I . , S.Y Received June BO, 1962
The e.s.r. signal from crystalline cyanine and merocyanine dyes has been shown to arise from traces of impurities. I t is not due to the crystallinity of the dye or to the extended structure. It also has been found that certain sensitizing and desensitizing dyes will initiate the photopolymerization of acrylonitrile when illuminated with light of wave length beyond 5000 A. The data a t present is ambiguous as to the mechanism; Le., electron transfer or energy transfer from dye to monomer.
Introduction This paper ii the hrst in a series on the investigation of spectral sensitization. The electron spin resonance (em-.) signals arising from some crystalline sensitizing dyes and the ability of some of these dyes to initiate polymerization when exposed to light will constitute the subject matter of this paper. During the past two years, several papers have appeared in which e.s.r. signals from crystalline sensitizing dyes and other dyes have been disc ~ s s e d . 2 ~The ~ authors of the first two papers2a.b attributed the resonance to the crystal structure of the dye. They concluded that the resonance signal v a s characteristic of long unsaturated chains and that it also was related to the nature of the heterocyclic nuclei attached to the unsaturated chains. The third paperzcdid not attempt to explain the obqcrved em-. signal for some crystalline dyes. The fourth paper3discussed some phenothiazine dyes and suggested the cxplanation given by Chernyakovakii, ct al.*b
During the past year, some thirty-odd sensitizing and desensitizing dyes have been synthesized and purified for the investigation of spectral sensitization. I n view of the preceding papers, i t was decided to investigate the source of the em-. signal from crystalline dye samples. (1) This work was supported In part b y Contracts AF 33(616)-8167 a n d A F 33(616)-8249 with ASD a n d monitored by Wnglit Patterson Air Force Base. ( 2 ) fa) Y i i Sh. lfoshkovskii, Dokl. A k a d . .&'auk SSSR,130, 1277 f l W 0 ) ; (b) F. P. Chernyakmakii, 5'. P. Kalmanson, a n d L. A. Blyiiin~nfeld,Optzka 2 Spektroskopya, 9, 786 (1960): (c) W.C. Needler, R L Gnffith, a n d W. West, A-atature, 191, 902 (1901). (3) C. Layercrantz and hf. Yhland, A c t a Chem. Scand., 15, 1204 (1961).
Experimental A simple set of experiments was devised. Asample of
each dye that exhibited a radical signal (whose radical signal had been measured) wa9 dissolved in chloroform and the e.s.r signal of the solution measured. Then the dissolved dye was recrystallized and the e.8.r. signal of the crystallized dye obtained. The following assumptions were made in setting up this experiment: (1) if the signal were due to a stable radical, it would persist in the chloroform solution and in the recrystallized dye; (2) if the signal were due to the crystal structure of the dye, it would disappear when the dye was dissolved in chloroform and reappear when the dye was recrystallized; (3) if the signal were due to an unstable radical, stabilized while adsorbed to the surface of the dye crystal, it would disappear in the chloroform solution arid not reappear in the recrystallized dye. Another experiment on the ability of sensitizing dyes t,o initiate polymerization also was performed. At present, it appears that sensitizing dyes act by either exchanging energy with the substrate or passing electrons t o the snb~ t r a t e . Recently, ~ Needler, Griffith, and Westza have reported an e.8.r. signal that appears when some sensitizing dyes are adsorbed on silver bromide and illuminated with light, which they attribute to free radicals derived from tllc dye molecules. T h k signal was observed a t liquid nitrogen temperatures but not at room temperature.*CJ If unstable free radicals were formed upon illumination, the Sensitizing dyes might serve as polymerization initiators. Hence, II simple experiment was set up. A sensitizing dye waa dissolved in deoxygenated acrylonitrile and exposed to light from a 100-watt tungsten ribbon filament lamp for 2 hr. a t = 13" and a t 55". Controls were held a t the same temperatures without illumination. The criterion for polymerization initiation waa the appearance of a visible precipitate of polymer. Thus, any dye which caused only a minor amount of polymerization would not be classified as a polymerization initiator by these experiments. Obviously, if the dye forms a stable frre radical, polymerization would not take place, but the build-up of a free radical should be detected. It may be argued that an adsorbed dye is in a different state (4) B.
(5)
H. Carroll, P h o t . Sci. Eng., 6, 65 (1961).
W.J. West, J . chim. phys.,
672 (1958).
2404
J. u. LUVALLE,h.LEIFER,P. H. DOUGHElZTY, .\ND 11. I < o ~ ~ A L
TABLEI CRYSTALLINE CYASINEASD MEROCYANINE DYESTHAT WEREEXAMINED FOR DARKE.s.R. SIGKAL AKD INITIATE POLYMERIZATION Number
I I1 I11 IV
v
1.1 VI1 VI11
IX X
SI
s11 SI11 XIV
xi-
SYI
siTII ZVIII
XIS
xx
SXI XXII XXIII
XXIV XXT’ SXVI SXVII SXVIII ZSIX
sxs
XXXI XSXII SXXIII XXXIY
1-01. 66
FOR
ABILITYTO
Dye
4-Bis- [ 1-ethylquinolyll-pentarnethinecyanine iodide 4-Bis- [ 1-ethylquinolyll-heptamethinecyanine iodide 2-Bis- [3-ethylthiazolinyl]-hept amethine cyanine iodide 2-Bis- [ l-ethyl-3,3-dimethyl indolinyl 1-heptamethine cyanine iodide 2-Bis- [triphenylphosphonium cycloprntadienyll-pentamethinechloride 2-Bis- [3-ethyl-4,5-diphenylthiazolyl]-pentamethine cyanine iodide 2-Bis- [3-ethylbenzothiazolyl]-hrptarnethine cyanine iodide 2-Bis- [3-ethylbenzoselrnazolyl]-pentamethinecyanine iodide 2-Bis- [3-ethyl-p-naphthiazolyl]-pentamethine cyanine iodide [2-(3-Ethylbcnzothiazolrl)1- [4-(1-ethylquinolyl) l-pentamethine cyanine iodidc [2-(3-Ethylbenzothiazolyl)]- [4-(1-ethy1quinolyl)l-heptamethinecranine iodide [2-( 1-Ethylquinolyl) 1- [4-( 1-ethylquinolyl) l-pentamethine cyanine iodide [2-(3-Ethylbenzothiazolyl)]- [4-( 1-ethylquinolyl) 1-trimethine cyanine iodide [2-( 3-Ethylbenzothiazolyl)]- [2-( 3-ethylbenzoselenazolyl) l-pentamethine cyaniiic, ioditlts [ 2-( 3-Ethylbenzothiazoly1)l-[5-rhodanine 1-hexamethine merocyanine 12-(3-Ethylbenzothiazolyl)]-5-(3-phenyl rhodanine)-hexamethine merocyanine [%-(3-Ethylbenzothiazolyl)]- [triphenylphosphonium-2-cyclopentadien~~l]-hexaniettiinr iotlidc [2-(3-Ethylbenzoxazolyl)]- [triphenylphosphoniumS-c~clopentadien~l]-hexamethinc iodides [4-(1-Ethylquinolyll ]- [t~riphcn~lphosphonium-2-cyclopentadieiiyl]-hexainethine iodidr 2-Bis- [I-ethylquinolj-I]-trimethine cyanine iodide 2-Bis- [:3-ethylbenzothiazolyl] -pentamethine cyanine iodide 2-Bis- [3-ethylbenzothiazolyIj-heptainethinecyanine iodide cyanine iodide 2-Bis- [.?-ethylbenzothiazolyl]-9-methyltrimethine [2-(3-Ethylbenzothiazolyl)1- [2-(1-ethylyuinoly1)l-pentamethine cyanine iodide 2-Bis- [ 1-eth ylquinolyl l-pentamethine cyanine iodide [2-(3-Ethrlbenzothiazo1y1) 1- [5-rhodanine]-tetramethine merocyanine 2-Bis- [3-ethylbenzothiazolyl]-9- ~1,3-dimethyl-5-barbiturenolide]-pent~methine CJ aiiinc [ 1,3-diethj I-5-thiobarbiturenolidel-pentamethinecyaniiic 2-Bis [-3-ethylbenzothiazolyl]-94-131s- [ 1-ethylquinolyl1-11-bromopentamethine cyanine iodide 2-Bis- [ 1-ethylquinolyll-11-bromopentamethine cyanine iodide 2-Bis- [3-ethylbenzothiazolyl]-lO-bromopentamethine cyanine iodide 2- [3-Ethvlbenzothiazolyl][p-dimethylaminotoly1]-9-( 1,3-diethvl-5-thiobarbiturj l)-tetruinc~thiiiecyaiiint. 2-Bis- [3-ethylbenzothiazolyl]-9-dicyanomethylene pentamethine cyanine 2-His- [ l-ethyl-3,3-dimethylindolyl]-trimethinecyanine iodide
than the dye dissolved in acrylonitrile and hence a negative result would not show that free radicals were not formed in the adsorbed Jtate; on the other hand, a positive result would indicate either radical formation or energy transfer.
fine structure. Attempts to recrystallize dye S X I resulted in decomposition. Dyes XXIT’, XXV, and XXYI exhibited e.s.r. signals in the dark. I n all cases, the fraction of Experimental Results dye exhibiting the resonance signal was quite small. The experiments described above were per- The resonance signal disappeared when each of formed on some thirty-odd sensitizing and de- these dyes was dissolved in chloroform. When sensitizing dyes. In Table I, the names of some each of these dyes was recrystallized. the resonance 34 cyanine and merocyanine dyes are listed. Xote signal did not appear in the recrystallized dye. that nine symmetrical dyes were investigated, all Some additional experiments were performed on being penta- or heptamethines. Ten unsym- dye XXT’I. Oxygen was bubbled through the metrical dyes also mere investigated, with one rrystalline dye. If the resonance signal were exception all being pentamethines or higher. due to aerial oxidation of the surface of the crystal, According t o Jloshkovskii, virtually all of these the signal should increase with time. Iigure 2 dyes should have given an e.s.r. signal in the shows the spectra obtained from this dye at I , crystalline state; however, not one exhibited an 2 . 5 , and 19 hr. It is obvious that oxygen had no e.s.r. signal. effect upon the radical. Il’eedler, Griffith, and Westzc reported e.8.r. Some of dye XXT-I was recrystallized; the signals in the crystalline state for dyes XX, XXI, recrystallized dye gave no e.s.r. signal. A sample XXII, and XXIII. When these dyes were freshly of the recrystallized dye then was illuminated for prepared, none of them exbibited an e.s.r. signal. 135 min. with the imaged light from a 300-watt ,4fter three months, dyes XXI, XXII, and also zirconium arc lamp. S o radical signal was dye IT exhibited a small e.s.r. signal. The signals detected a t the end of this experiment. X sample from these dyes are shown in l?ig. 1 . The width of the recrystallized dye was illuminated at 5.5”. of all signal3 ip about 25 gauss. The fraction of for 135 min. S o radiral signal waq detected at the dve exhibiting the resonanre signal was quite small cnd of this experiment. Thus, neither. oxygen, in :ill canici ‘I’hc. .;ignnl from dyc TI indic:~tc..; light, hc:it, iior heat niid light ( ~ n i i i c c thcl l r:idic~:il
Dee., 1‘3G2;
k
signal to appear or in the case of oxygen to increase. However, long periods of time do cause the radical signal to appear. Of a total of 33 cyanines and merocyanines tested, three exhibited resonance signals. I n all cases, the resonance signal disappeared on solution or upon recrystallization. The signal appears to reside on the surface of the dye crystal. A sample of dye XXC’I was shaken in tetrahydrofuran, in which the dye is only slightly soluble. Some traces of dyc dissolved. The e.s.r. signal disappeared. This experiment strongly indicates that the signal arises from the surfaw of the dye. All of the data appear to confirm the conclusions that the signal is due to an impurity rather than the long-conjugated chains, the heterocyclic nuclei, or the crystal structure-unless the slow appearance of the signal with time is caused by a change in crystal structure of the surface molecules. These dyes were stored in brown bottles in a refrigerator. Hence, light has little chance to reach the dyes. The signal map arise from peroxide formation; however, this has not been confirmed. It has been suggested that the signal may arise from cosmic. ray homhardmcnt. If w ,thc damage apparently migrntw to tlic siirf:iw
The low percentage of dyes which exhibited a resonance signal may hare been due to the strict purification procedure. All dyes were prepared as pure as possible by the usual synthetic techniques. They then were chromatographed, and the hot solution was filtered and concentrated to obtain the dye. Finally, the dye was fractionally recrystallized. If the infrared spectrum of the dye changed appreciably during this final procedure, the procedure was repeated until the infrared spectrum of the crystalline material was unchanged. Only two of the 34 cyanines and merocyanines tested, dyes XXT‘II and XXVIII, initiated polymerization lvith visible light. These dyes required both heat and light to initiate polymerization. Both of these dyes are trinuclear dyes and both are such that conjugation extends throughout all three nuclei. Table I1 gives the structure of these two dyes. Dyes XXIX, XXX, and XXXI, in which the third nucleus was a bromine atom, did not initiate polymerization. Dye XXXII, in which a henzothiazole group of dye XXVIII was replaced hy a p-dimethylaminotolyl group, did not initiate polymerization. I)ye XXXIII, in which thc 1)arhitrirtnolitlcgroup of‘ d y s SXl7II
J. E. LUVALLE,A. LEIFEI~, P. H. DOUGHERTY, ASD M. KORAL
24OG
CS
/
and XXT'III was replaced by (2'
\
, did not initiate
L)ESENSITIZING
TABLE I11 DYESTHAT INITIATED
Dye XXXV
Pyronin Y
Dye XXXVI
Acridine red
Dye XXXVII
Rhodamine E
Vol. 66
POLYMERIZATION
cs
polymerization. Thus, the presence of a bisethylbenzothiazole pentamethine cyanine is not sufficient for the initiation of polymerization with visible light. The presence of the barbiturenolide nucleus is not sufficient for the dye to act as a photoinitiator. Apparently, the ability of a cyanine to act aa a photoinitiator is a complex of properties, The structure of the end groups, the structure of the third nucleus, the number of atoms in the polymethine chain, and the location of the third nucleus (along the chain all are involved. A group of tri- and tetranuclear dyes is being synthesized to evaluate these factors. TABLE I1
STRCTTCREOF C:YANISE DYESWHICH INITIATED Massnx P O LYYERIZATIOS
Dye XXVII
2-Bia- [3-etliylbenzothiazolyll-9-(1,3-dimethyl-~-barbitureno1ide)-pentamethine cyanine
I
Et
Et
-oe
o=c I CHB-N,
I ,N-CHJ C
I/
0 Dye S X V I I I
2-I~i~:-[:~-ottiylbonzothiazol~-l]-~-(l,3-diethyl-d-thiubarbitureno1ide)-pentamethine cyanine
I
I El.
Et
o=c'
C
*C-Oe
I C'2Hj-N,
/
,N-CzH;
C I1
S
Several desensitizing dyes also were tested for their ability to initiate polymerization. All of the dyes in Table I11 initiated polymerization. These dyes all belong to the xanthene class. This group of dyes initiates polymerization with light longer than 5000 A. and is much faster-acting than the trinuclear dyes which also utilize light beyond 5000 A. The structures of the xanthene dyes do not indicate why these dyes act as polymerization initiators. Sone of the dyes that failed to initiate polymerization exhibited a radical signal a t the end of the illumination period. However, many of these dyes undergo a color change, not merely a dye fading, when illuminated for a period with a ribbon filament lamp. Several of the xuritliPiie dyes were dissolved in chloroform and illuminated in the e.s.r. cavity a t room temperature. No radical signal was observed either at g = 2 or g = 4. Thus, a t present, there is no positive evidence that the cyanine dyes
or xanthene dyes that initiate polymerization utilize electron transfer or energy transfer. The failure to observe e.s.r. signals from the illuminated cyanine dyes indicates that a stable free radical is not formed at room temperature in solution. The failure to observe polymerization initiation by the large majority of the illuminated cyanine and merocyanine dyes indicates that reactive free radicals are not formed by these dyes in solution. The observation that many of these dyes undergo somewhat drastic color changes upon illumination in solution indicates that a phot'ochemical reaction takes place. As the dye is present in considerably smaller amounts than the acrylonitrile, each dye molecule probably is present in a cage of acrylonitrile molecules; hence, dissociation and recombination is not very probable.
Conclusions The data in this paper clearly show that e.s.r. signals arising from crystalline sensitizing dyes in the dark are due to impurities and do not arise from the crystalline structure of the dye. The data concerning photopolymerization initiation do not lead to a nice clean conclusion. The mechanism whereby some of the dyes initiate polymerization is not clear; considerable work must be done on this phase of the problem. I n subsequent papers, the results of investigations of these dyes in the adsorbed state and in solid solutions will be discussed.
