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W. WEST, S. PEARCE, AND F. GRUM

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tensions below 30 dynes/cm. Experimental data on yo for liquids of high surface tension and strong hydrogen bonding are needed; the data in Table I1 for

liquids with surface tensions above 30 dynes/cm indate that such a determination is experimentally feasible.

Stereoisomerism in Cyanine Dyes-meso-Substituted Thiacarbocyanines

by W. West, Sandra Pearce, and F. Grum Research h b 0 r d W i e 8 , Eaatman Kodak Co., Rochestsr, New York 14660 (Received September IS, 1966)

The visible absorption spectra of alcoholic solutions of thia- and selenacarbocyanines containing methyl or ethyl substituents in the meso position of the polymethine chain are found to be resolved at low temperatures into two component bands, interpreted to indicate the coexistence in equilibrium of two stereoisomers, one in an extended all-trans configuration of the chromophoric chain and the other in a bent mono-cis configuration. At temperatures from about -90 to -120°, the two isomers are photochemically interconvertible. Nonequilibrium mixtures are stable at temperatures of about - 100' and below, but they spontaneously revert to equilibrium as the temperature is increased, slowly at about -80' and with increasing rapidity as the temperature is increased. No photoconversion was observed in a rigid glass at -196'. Dimerization and H aggregation were marked in alcoholic solutions of those dyes at low temperatures, expecially in the more concentrated solutions. The all-trans configuration is assigned to the isomer absorbing at the longer wavelength; preferential dimerization of this isomer was observed. The observed isomerization of 9-alkylthia- and selenacarbocyanines explains the anomalously short wavelength of the absorption maximum of the two former classes of dye with respect to the absorption of the corresponding 9-H dyes.

In the course of various spectroscopic studies of alcoholic solutions of cyanine dyes, it was observed that thiacarbocyanines containing alkyl substituents in the mew position of the methine bridge (Figure 1) exhibited visible absorption spectra that were resolved into two component bands at low temperature.' The components are not attributable to the normal vibrational structure that is generally accentuated in the spectra of these dyes at low temperature, as is illustrated in Figure 2 for the dye, 3,3'-dimethyl-9phenylthiacarbocyanine iodide. Photochemical experiments showed that the two bands of the mesoalkylthiacarbocyanines must be ascribed to independent molecular species. The dye samples were analytically pure and, at room temperature, chromaThe Journal of Physical Chemietry

tographically and electrochromatographically homogeneous on all substrates and toward all developing media examined; the independent species seem therefore likely to be stereoisbmers that rapidly interchange at room temperature but are individually thermally stable at low temperature. In the structure of cyanine dye cations (Figure l), two heterocyclic nuclei axe linked by a conjugated chain containing an odd number of methine carbon atoms. It is known from X-ray analysis that the cations of 3,3'-diethylthiacarbocyanine bromide, dye I1 (Table I), are present in the crystalline state in the (1) The first observation of this splitting waa made by Dr. W. Cooper in experiments on the luminescence of cyanine dyes carried out in

collaboration with W. West.

STEREOISOMERISM IN CYANINE DYES

Et

Et (a)

-

a l l trans

(b)

mono

- cis

Figure 1. trans and cis forms of thiacarbocyanines.

Mmp)

Figure 2. Absorption spectrum of 9-phenyl-3,3’-dimethylthiacarbocyanine iodide in an alcoholic solvent a t various temperatures, showing increased resolution of the dominant vibrational progression a t low temperatures. The feeble shoulder indicated by the arrow may represent a trace of stereoisomer. (1) 23’; (2) -50’; (3) -101’; (4) -196’.

extended all-trans configuration of Figure la.2 General considerations of resonance stabilization suggest that, in general, an all-trans configuration of cyanine dyes will be that of minimum energy, unless steric impingemerit of componentatomsor groups in this configuration might be relieved by the adoption of an alternative configuration. Among cyanine dyes, the possibility Of the existence of geometrical isomers formed by rotation about a

1317

bond in the conjugated chain has long been di~cussed.~-~ Experimental realization of this possibility dates from the observation that the heptamethine dye, 3,3’diethylthiatricarbocyanine chloride, could be chromatographically resolved at room temperature into three components considered to be stereoisomers. These components, however, placed individually into solution at room temperature gave rise to the same equilibrium mixture, whose spectrum showed no definite structure attributable to the separate isomers.‘ A definite photochemical production of a stereoisomer of the tricarbocyanine, 1,5-N,N’-dipyrrolidylpentamethine perchlorate, and of similar dyes by irradiation within the main visible absorption band of an alcoholic solution at low temperatures, e.g., - 1M0, was observed by Scheibe and co-workers, along with thermal reconversion to the trans f ~ r m . ~Among ,~ some of the dyes of longer chain lengths, the equilibrium coexistence of spectrally distinct isomers before irradiation was also o b ~ e r v e d . ~Flash photolysis of solutions of cyanine dyeslo*llhas also shown the production of transient species formed presumably by trans-cis photoconversions and subsequent spontaneous reconyersion. cis-trans isomerization has long been studied among other classes of conjugated compounds than cyanine dyes, including stilbenes,12polyenes and carotinoid corn pound^,'^ indigo and thioindigo dyes, l 4 azo compounds,16-17 anils, l8 conjugated nitriles,lgand other compounds.

(2) P.J. Wheatley, J. Chem. Soc., 3245, 4096 (1959). (3) G. Scheibe, Angew. Chem., 5 2 , 631 (1939). (4) L.G. S. Brooker, F. L. White, R. H. Sprague, S. G. Dent, Jr., and G. Van Zandt, Chem. Rev., 41, 325 (1947). (5) L. G. S. Brooker, F. L. White, D. W. Heseltine, G. H. Keyes, S. G. Dent, Jr., and E. Van Lare, J . Phot. Sei., 2, 173 (1953). (6) H.Kuhn, Helv. Chim. Acta, 34, 1308 (1951). (7) L. Zechmeister and J. H. Pinckard, Ezperkntia, 9 (l), 16 (1953). (8) F. Baumgartner, I. Gthther, and G. Scheibe, 2. Elektrochem., 60, 570 (1956). (9) G. Scheibe, J. Heiss, and K. Feldman, Ber. Bunsenges. Physik. Chem., 70, 52 (1965). (10) F. Dorr, J. Kotschy, and H. Kausen, ibid., 69, 11 (1965). (11) R. J. McCartin, J . Chem. Phys., 42, 2480 (1965). (12) See, for example, G. S. Hammond, J. Saltiel, A. A. Lamola, N. J. Turro, J. 8. Bradshaw, D. 0. Cowan, R. C. Counsell, V. Vogt, and c. Dalton, J. Am. c h m . SOC., 86, 3197 (1964). (13) See review of cis-trans isomerization, G. M. Wyman, Chem. Rev., 5 5 , 625 (1957); L. Zechmeister, “Cis-trans Isomeric Carotinoids, Vitamins A and Arylpolyenes,” Academic Press, New York, N. Y., 1962. (14) W. R.Brode, E. G. Pearson, and G. M. Wyman, J . Am. Chem. Soc., 7 6 , 1034 (1954). (15) W. R. Brode, J. H.Godd, and G. M. Wyman, ibid., 74, 4641 (1952).

Volume. 71, Number 6 April 1967

1318

W. WEST, S. PEARCE, AND F. GRUM

Table I: Dyes Examined

R Name

Dye

I I1 I11 IV V VI VI1 VI11 IX X XI

XI1

R

3,3’-Dimethylthiacarbocyanine-p-toluenesulfonate 3,3’-Diethylthiacarbocyanine-p-toluenesulfonate 3,3 ’,9-Trimethylthiacarbocyanine-p-toluenesulfonate 9-Ethyl-3,3’-dimethylthiacarbocyanine bromide 3,3’-Diethyl-9-methylthiacarbocyanine bromide 3,3‘,9-Triethylthiacarbocyanine bromide 3,3’-Dimethyl-9-phenylthiacarbocyanineiodide 3,3’-Dimethyloxacarbocyanineiodide 3,3’-Diethyloxacarbocyanine-p-toluenesulfonate 9-Ethyl-3,3’-dimethyloxacarbocyanineiodide 3,3’-Diethyl-9-methyloxacarbocyanineiodide 3,3’-Diethyl-9-methylselenacarbocyaninebromide

In the present paper it will be shown that appropriately chain-substituted trimethinethia- and selenacarbocyanines exist at room temperature and at lower temperatures as an equilibrium mixture of two stereoisomers, from which the separate forms have been obtained fairly pure by irradiation at low temperatures, although they have not been isolated at room temperature. In addition to the all-trans structure for thiacarbocyanine, illustrated in Figure la, two other configurations can be formally derived from Figure l a by rotating one or both heterocyclic nuclei 180’ about the methine bridge. If both nuclei are so displaced in the $alkyl dye, gross crowding of the N-alkyl groups on the meso-alkyl group will prevent coplanarity of the atoms in the chromophoric chain; hence, this structure of relatively high energy will not contribute appreciably to any isomeric equilibrium a t room temperature or below. The structure of the 9-alkyl dyes in which one of the heterocyclic nuclei is rotated 180’ with respect to the other is also more crowded than that of Figure l a and since, experimentally, it is absent in the crystal of the chain-unsubstituted dye12it seems unlikely to participate to any great extent in isomeric equilibrium of the 9-alkyl dye at room temperature. Even in the structure of Figure la, the close approach of the van der Waals “surface” of the two S atoms to that of the meso-alkyl group makes it doubtful that the atoms in the chromophoric chain are completely coplanar, especially when the alkyl group is ethyl.20 The crowding of the two S atoms on the 9-alkyl group in structure l a is not much less than that of the S‘ atom on the 8-H atom in the mono-cis The Journal of Physicel Chemistry

Y

R

X

S S S S S S S

Methyl Ethyl Methyl Methyl Ethyl Ethyl Methyl Methyl Ethyl Methyl Ethyl Ethyl

H H Methyl Ethyl Methyl Ethyl Phenyl H H Ethyl Methyl Methyl

0 0 0 0 Se

structure of Figure lb, derived from structure l a by rotation of the right-hand benzothiazole nucleus about the 9-10 C-C bond. Structure l b for the 9alkyl dyes is therefore expected to be of comparable stability to that of l a and both might exist in dynamic equilibrium. I n the chain-unsubstituted dye, the crowding in the cis configuration l b is about the same as for the meso-substituted dye and is distinctly greater than in the trans configuration la; hence, the latter structure may be expected to be so much more stable than the cis structure that it constitutes by far the dominant component in the equilibrium mixture a t room temperature or below. The three other cis forms of the 9-alkylthiacarbocyanines that exist formally with the same disposition of C-C bonds as in Figure lb, derived from l b by rotating one or the other or both of the heterocyclic nuclei 180’ with respect to the trimethine chain, are probably less stable than structure l b because of increased crowding and greater deviation from coplanarity of the atoms in the chromophoric chain. The latter structure therefore appears to be the cis ~

~

~

~~

~~~~~

~

(16) G. Zimmerman, L-y. Chow, and U. Paik, J. Am. C h m . SOC., 80, 3528 (1958).

(17) 8. Malkin and E. Fischer, J . Phy8. Chem., 66, 2482 (1962), and other papers. (18) D. G. Anderson and G. Wettermark, J. Am. C h a . SOC., 87, 1433 (1965). (19) E. Lippert and W. Ltkder, J . Phy8. Chem., 66, 2430 (1962). (20) The me80-alkyl groups considered in this paper are limited to methyl and ethyl. Dyes containing larger alkyl groups in the me80

position, such as tbutyl, cannot possibly be coplanar with respect to the atoms of the chromophoric chain and, according to models, may be less crowded in a nonplanar cia configuration than in a nonplanar trans.

STEREOISOMERISM IN CYANINE DYES

structure of minimum energy and that most likely to coexist in equilibrium in amounts comparable to that of the trans structure la.

Experimental Procedure The dyes discussed are listed in Table I. The absorption spectra in an alcoholic solvent were measured by a Cary-14 automatic spectrophotometer at temperatures from 23 to - 196'. For measurements below room temperature, the solutions were placed in conventional absorption cells of square cross section and 1 cm in optical path length and mounted in an aluminum holder immersed in a helium atmosphere cooled by a surrounding volume of liquid nitrogen in a glass or quartz dewar flask provided with plane windows for the passage of the photometric beam. Temperatures were adjusted by a heating coil placed near the sample in the helium atmosphere and were measured by a copper-constantan thermocouple. I n Figures 3 and 4,the spectral curves are shown as optical density for a given sample of dye measured initially at 23' and then at a lower temperature, without other irradiation than that of the spectrophotometric beam, and then measured after various exposures at low temperatures to intense monochromatic light from an external source. The solvent was a mixture of ethanol 90%, methanol 5y0, and 2-propanol 5%, chosen to avoid crystallization at low temperatures; at temperatures below about - 140' the solution was in the state of a rigid glass. The increased concentration of the dye in the sample caused by contraction of the solvent at low temperatures was sufficiently accurately estimated from published values of the density of ethanol at temperatures down to -110°21 and from extrapolated values at lower temperatures. The optical densities plotted in Figures 3 and 4 for temperatures below 23' are adjusted to the initial concentration to eliminate the effect of contraction, and integrated optical densities, JDdv, are compared on the same basis. Phototropic changes in the dye were brought about at low temperatures by irradiation by monochromatic light of wavelengths near those of the maxima MI and AI,, respectively, illustrated in Figure 3. A typical irradiation within band Mz was accomplished by exposing for 15 min to the light of a 500-w tungsten projection lamp transmitted by an interference filter for wavelength 540 mp, 14-mp half-band width, combined with a Kodak Wratten Filter No. 12 to absorb blue and ultraviolet light. The corresponding filter for irradiation within band M1 was a similar interference filter for wavelength 560 mp, combined with a Kodak Wratten Filter No. 21.

1319

I

"

"

'

'

I

"

"

Figure 3. Phototropic changes showing stereoisomers, M1, Mz, of 9-ethyl-3,3'-dimethylthiacarbocyanine bromide, initial concentration, 6.2 X lo4 M : (1) at 23'; (2)at -120'; (3)after irradiation within MZband a t -120'; (4)after additional irradiation within M1 band a t -120'; (5)after a n increase in temperature to 23'. I.,,

,

,

,

,

,

,

I

Almrl

Figure 4. Phototropic changes showing stereoisomers, MI, Mz, of 3,3',9-triethylthiacarbocyaninebromide. Initid concentration 1.01 X M: (1) at 23'; (2)at -125'; (3)after irradiation within MZband at -125'; (4)after raking temperature to -89'; ( 5 ) after irradiation within MI band a t -89'; (6)after an increase in temperature to 23'.

Illustrative Experimental Results. The essential spectroscopic observations showing that O-alkylthiacarbocyanines consist of two isomers are illustrated in Figure 3 for 9-ethyl-3,3'dimethylthiacarbocya,nine bromide, dye IV. The appearance of the visible absorption band at 23', curve 1, is, at first sight, similar to that frequently observed among cyanine dyes, showing a main maximum on the long-wavelength side with a shoulder, in this case feeble, at shorter wavelengths attributable, according to general experience with the spectra of these dyes, to vibrational structure. At - 120' absorption curve 2 differs from that at room temperature in two spectral regions; the main maximum is resolved into two bands, MI and M2, a t wavelengths 559 and 543 mp, respectively, and absorption is increased in the region from 500 to 520 mp, with the (21) C.

P. Smythe and W. N. Stoops, J. Am. Chem. SOC.,51, 3318

(1929).

Volume 71, Number 6 April 1887

1320

appearance of a slight maximum, D1, a t 510 mp. I n the dark, the spectrum a t - 120' remained constant with time. I n the subsequent discussion, we shall refer to the isomer whose absorption maximum in the monomeric state is the band MI as the MI isomer and correspondingly with respect to band M2, although according to experimental conditions the absorption within the individual bands as revealed by the spectrophotometer may contain overlapping contributions from both isomers. On irradiation within the R/Iz band a t -120°, the intensity of this band was decreased and that of 311 increased (curve 3). The integrated density for the irradiated sample (curve 3) was 11% less than for the sample at -120' before irradiation, suggesting some loss of dye by bleaching during the irradiation; this was confirmed in other experiments in which the sample was remeasured a t 23' immediately after irradiation a t - 120'. Since, however, as is shown by comparison of curves 2 and 3, the absolute intensity at band M1was increased by irradiation, the photochemically induced intensity changes in the bands ,111 and iclz cannot be ascribed merely to selective bleaching of the A12 isomer and a photoconversion of the 1 4 2 to the AT1 form must have taken place. Curve 4 shows photoconversion of the MI to the 1 1 2 form, effected by irradiation within the M 1band of a solution at -120' containing a high relative concentration of M1 to Mz isomers. The integrated density after the irradiation (curve 4)was about 8% lower than before (curve 3). This loss in absorption is probably not entirely the result of bleaching, since the molar extinction coefficient of the J12 isomer, as will be discussed later, is probably lower than that of the allisomer. Finally, curve 5 shows the effect of bringing to room temperature the solution relatively rich in the M2 isomer a t -120°, indicated by curve 4. The initial state of spectrally unresolved isomers was regenerated with an over-all loss in optical density denoting some 10% bleaching of the dye by the two irradiations. This loss of dye by irradiation a t low temperature was not eliminated by removal of oxygen from the solution. The general conclusion drawn from these experiments, along with the previously mentioned facts of analytical purity and the chromatographic homogeneity of the dye at room temperature, is that the dye in alcoholic solution consists of two isomers in equilibrium in comparable proportions, with monomeric absorption maxima AI1 and M2, respectively, resolvable a t low temperatures but masked as separate spectral entities by overlap a t room temperature. No separation has been chromatographically effected at room temperature, nor does monochromatic irradiation a t the wavelengths The Journal of Physded Chemiatry

W. WEST,S. PEARCE, AND F. GRUM

540 or 560 mp, respectively, cause any change in the shape of the bands at room temperature observable by static spectroscopy. At sufficiently low temperatures, about -80 to -130°, the isomers have been observed to be mutually photo-interconvertible with, in addition, some permanent destruction of dye and a nonequilibrium state thus produced is stable; as the temperature is raised, the equilibrium state is spontaneously regenerated, slowly at about -SOo, but very rapidly at room temperature. No photoconversion was observed in this dye in a rigid glass a t - 196'. We shdl now consider the changes in the spectral regions of wavelength 500-520 mp caused by cooling and by irradiation at 540 and 560 mp. The key fact in accounting for the maximum D1a t 510 mp, which appears on cooling from 23 to -120' (Figure 3, curve 2) and which appears more clearly after irradiation within the band MZ(Figure 3, curve 3), is that this band is intensified with increasing concentration of the dye. The effect of concentration on the spectrum of dye I V at low temperatures in alcoholic solution is illustrated by comparing Figure 3 and Figure 5 for which the con-

Mmp)

Figure 5. Absorption spectrum of 9-ethyl-3,3'-dimethylthiacarbocyaninebromide in an alcoholic solvent (ethanol 90%, methanol 5%, isopropanol 5%) a t various temperatures, showing partially resolved isomeric bands MI and Mz and dimerization a t low temperature. Dye concentration, 9.8 X 10-6 M : (1) 23'; (2) -100'; (3) 196O. P indicates the permitted transition of the dimer, N the "forbidden" transition, and H, the H aggregate.

-

centrations a t room temperature were 6.2 X 10" and 9.8 X M , respectively. As the temperature was lowered, the band a t 510 mp in the spectrum of the more concentrated solution ultimately overshadowed the maxima M I and Mz. The peak a t 510 mp is therefore to be attributed to an associated form of the dye, most probably to a dimer, examples of which, in alcoholic solution of cyanine dyes a t low temperature,

STEREOISOMERISM IN CYANINE DYES

have already been reported22*2aand found, as will be reported in a subsequent communication from these laboratories, among many cyanine dyes. These concentration-dependent bands observed in alcoholic solution at low temperature have characteristics similar to those of the concentration-dependent bands, usually attributed to the formation of dimers, found in aqueous solutions of cyanine dyes at room temperature. They consist of a strong branch, P, hypsochromic to the monomeric maximum, and a very feeble branch, N, bathochromic to this maximum, at wavelength separations from the monomeric maximum similar to those observed for the dimeric bands in aqueous The increased absorption at low temperatures shown in curves 2 and 3 at shorter wavelengths than the dimeric maximum can be attributed to higher H aggregate^.^^ As a result of photolytic conversion of the M2 to the M1isomer at - 130' (Figure 3, curve 3), the dimeric peak of dye IV a t 510 mp was found to be strong in a solution containing preponderantly the isomer M1; hence, the peak at 510 mp must be attributed to dimerization of the M1 isomer. No definite dimeric band attributable to the Mz isomer of this or of any of the dyes examined has been observed; it appears, therefore, that the isomer absorbing at the longer wavelength, M1, dimerizes preferentially in alcoholic solution at low temperature. The separation between the monomeric maximum, MI, and the dimeric maximum, D1, of dye IV is 1700 cm-l, larger than the dimer-monomer separation of 1526 cm-l observed in the spectra of aqueous solutions of dye 11, 3,3'-diethylthiacarbocyanine-p-toluenesulfonate. Since, however, a correlation has been observed between the tendency of cyanines to dimerize and the magnitude of the dimer-monomer separation, 24 the observed value of the separation for the dimerizing isomer, &I1, of dye IV is not unreasonable and indicates a greater dimerizing capacity of this isomer than of the chain-unsubstituted dye. For comparison, the dimer-monomer separation of 1840 cm-l for the chain-unsubstituted dye of next longer chain length than dye I1 may be cited; for the next longer one, again the separation is 2235 cm-l. Both dyes of longer chain length than dye I1 exhibit greater dimeric association constants than does dye II.24 The dimeric band at 510 mp is superposed on absorption of the monomeric isomers. The small peak, VI, at 520 mp (Figure 3, curve 3), discernible after partial elimination of the overlapping absorption band of the 3% isomer by photolytic conversion to the MI isomer at -120°, can be identified as a band in the vibrational progression of the monomeric spectrum of

1321

the MI isomer, of which the 0-0 band is the M1band at 560 mp. As the proportion of M2 isomer in the isomeric mixture is decreased by appropriate irradiation, the VI band becomes more distinct and its peak moves to about 525 mp. The interval between the V1 and M1 bands is 1200 cm-l, consistent with the interval in the progression of dominant vibrational bands that determine the structure of the monomeric spectrum of cyanine dyes. Similarly, after appropriate irradiation at low temperature has yielded an isomeric mixture relatively rich in the M2 isomer, the vibrational band, V2, in the spectrum of the M2 isomer can be recognized at a position 1200 cm-l hypsochromic to the main maximum, M2, of the isomer, curve 4. The effects of irradiation within the bands M1 and M2,respectively, and of altering the temperature on the absorption in the spectral region 500 to 530 mF in which the contributions of D1, VI, and V2 overlap are consistent with these interpretations. Further illustrations of the phototropic and temperature changes in the spectrum of 9-alkylthiacarbocyanine dyes are given in Figure 4 for 3,3',9-triethylthiacarbocyanine bromide, dye VI. In this series the MI isomer was formed with only a small residue of the Ma isomer by irradiation within band Ma a t - 125' (curve 3) and the dimeric band, D1, and the vibrational band, VI, of the M1 isomer are prominent in the spectrum. An increase in temperature of the irradiated solution to -89' caused the dimeric peak to disappear as a result of thermal dissociation to monomer and the vibrational band, V1, of the isomer Ml now clearly appears (curve 4) with little overlap of dimeric absorption. After subsequent irradiation within the M1 band at -89' (curve 5), the main constituent of the mixture was the && isomer and the vibrational band, VZ, of that isomer, overlaid by some contribution from the short-wavelength side of V1, can be seen in this curve. The integrated density for the solution of dye VI, after it had been cooled to -125' and before irradiation and after correction for the contractional increase in the dye concentration, was 12% greater than for the initial solution at 23'. On the other hand, we have found that the integrated molar extinction coeEcients of the monomeric band of a number of cyanine dyes whose spectra were not resolved into isomeric com(22) G . Levinson, W. T. Simpson, and W. Curtis, J . Am. Chena. Soc., 79, 4314 (1957). (23) Dimerisation of dye I V at low temperature in an alcoholic solvent has recently been reported by G . Scheibe, Angew. Chem., 78, 304 (1966). (24) W. West and S. Pearce, J . Phys. Chem., 69, 1894 (1965).

Volume 71,Number 6 April 1967

1322

ponents is independent of temperature. Since a lowering of temperature from 23 to -125' inverts the relative optical densities at wavelengths 563 and 548 mp (the position of the maxima of the absorption spectra of the M1 and M2 isomers, respectively (Figure 4, curves 1 and 2)), it is clear that the ratio of the concentration of the MI to that of the M2 isomer increases as the temperature is lowered; the observed increase in the integrated optical density at the lower temperature then shows that the molar extinction coefficient of the Ml isomer is greater than that of the M2 isomer. The observed enrichment of the solution in the MI isomer in equilibrium as the temperature is lowered shows, according to the Clapeyron-Clausius equation, that the conversion of the M2 to the M1 isomer of dye VI is exothermal; AMl is the more stable isomer of this dye. The conclusion that isomer M1 of dye VI has the higher molar extinction coefficient is supported by observations on the increase in integrated density accompanying the thermal conversion of an isomer mixture photolytically enriched in A42 at -89' to the equilibrium mixture at 23' (Figure 4, curves 5 and 6). As the temperature is increased, the concentration of the M2 isomer decreases relative to that of the MI isomer, but the integrated density at 23' is 15% higher than that at -89', correction having been made for the concomitant concentration change. Again, the M1isomer of dye VI must have the higher molar extinction coefficient. Because of uncertainty regarding the concentration of the ;LI1 and 112 isomers in the monomeric state introduced by dimerization and further association at low temperatures, our present data do not permit an accurate evaluation of the molar extinction coefficients of the pure isomers. Tentative limiting values, however, can be calculated from measurements of partially dimerized solutions of dye VI containing, as a result of isomeric photoconversion, only a few per cent of the Mz isomer. An average value of 21 X lo4 was found for the apparent molar extinction coefficient at the maximum of the M1 band (at -120°), the relevant total dye concentration having been determined, after correction for contraction, from the known molar extinction coefficient of the solution at 23' and its absorption measured at this temperature immediately after the measurement at low temperature. Since some of the dye is dimerized, this value is therefore a minimum for the monomeric molar extinction coefficient of the M1isomer of dye VI at its absorption maximum. From an analysis of absorption curves for dye V I containing preponderantly the M2 maximum, such as curve 5, Figure 4, a maximum value of the 17 X lo4for The Journal of Physicat Chemistry

W. WEST,S. PEARCE, AND F. GRUM

the molar extinction coefficient of the M2 isomer at its absorption maximum is obtained. Although these values are tentative, they indicate qualitatively a considerably higher value for the monomeric molar extinction coefficient of the M1 than for the Mz isomer of dye VI, in agreement with the conclusions based on comparison of the integrated optical densities. I n addition to the isomeric changes, some permanent loss of dye VI was caused by the irradiations at low temperature. Comparison between the initial and final curves, 1 and 6, Figure 4, shows that, over-all, a loss of 30% of dye was caused by the two irradiations at low temperature. General Survey of Spectral Indications of Isomerism among Thia- and Oxacarbocyanine Dyes. The most definite indications of isomerism among thiacarbocyanine dyes is the resolution of the visible absorption spectrum of the meso-alkyl dyes, 111, IV, V, and VI, into M1and Mz components similar to those illustrated in Figures 3 and 4 for the two latter dyes; the M1and M2 isomers of all of these dyes are mutually photointerconvertible at low temperatures, as illustrated in these figures. Of all the oxa-, thia-, and selenacarbocyanines examined (Tables I and II), the meso-alkyl derivatives of the latter two classes alone show wellresolved isomeric bands at low temperature, although a few others at -196' show feeble shoulders hypsochromic to the main maximum that may indicate the existence of a small amount of an isomer in equilibrium with a dominant isomer probably analogous to the M1 isomer of the 9-alkylthiacarbocyanines. At room temperature, the shapes of the absorption band of all of the carbocyanines examined are similar on casual examination, but the presence of appreciable amounts of different isomers of the 9-alkylthiacarbocyanines in equilibrium is signaled by an abnormally great width of the main monomeric absorption band. In Table I1 are summarized data on the wavelengths of maximum absorption of the dyes examined at 23" and at -196' in solution in the alcoholic mixture and on the widths of the main absorption band. At 23', overlap on the main band of the vibrational component on its short-wavelength side renders the conventional half-width unsuitable for comparison of band widths. To circumvent this difficulty, we have expressed the width of the band at 23' as A V ~ .the ~ ~ wavenumber , interval between the points on either side of the main maximum at which the optical density is two-thirds of that at the absorption maximum. A t -196', as is illustrated in Figure 2, the vibrational structure is usually sufficiently well resolved to make the conventional half-width, a meaningful description of the width of the main band. When the isomeric

STEREOISOMERISM IN CYANINE DYES

1323

Table 11: Data on the Visible Absorption Spectra of Oxac and Thiacarbocyanines in an Alcoholic Solvent OX8C8rbOCY8nhSubstituents

Dye

VI11

IX X XI

3

3'

Me Et Me Et

Me Et Me Et

3

IV V VI

VI1

9

H H Et Me

XQl8X

8t -196',

mlr

mr

485 49 1 492 493

559 560 540 543 545 550 562

488 494

AW.87 8t 23",

cm -1

AW.6

at -I9So, om -1

493, (486)'

860 840 830 840

680 720 860 810

563, (545)" 564 537, 555b 541, 560b 542,357b 546, 562* 565, (548)'

840 770 1130 1180 975 990 610

805 750 1167 Uncertainc 1145 1050 440

...

Thiocarbocy8nine~-

7

I I1 I11

xma.

a t 23O,

Me Et Me Me Et Et Me

3'

Me

Et

9

H H

Me Me

Me

Et Et

Me Et

Me

'p

Et

The wavelengths in brackets apply to very feeble shoulders that are distinct only a t -196". * The underlined wavelength is that Overlap of the of the more intense band as resolved at low temperatures, uncorrected for overlap of the MI and MI components. dimeric on the monomeric band makes the half-width of the latter uncertain.

components, MI and M2, are resolved at -196' aa overlapping bands but with distinct maxima, the halfwidth in Table I1 is given as the wavenumber separation of points on the composite MI, Mz band at which the optical density is one-half of that at the more intense maximum; reference will also be made later in the text to the half-widths of the separate isomeric bands. As is shown in Table 11,the main absorption bands of the 9-alkylthiacarbocyanines are distinctly wider than those of the chain-unsubstituted dyes and wider than that of the 9-phenylthiacarbocyanine1 which exhibits the narrowest band of the series listed (Figure 2). The 9-alkyloxacarbocyanines1 X and XI, are not resolved into distinct isomeric maxima at -196' and their band widths at 23' are about the same as those of the corresponding chain-unsubstituted oxacarbocyanines VI11 and IX, the somewhat greater width of the 9-alkyloxa dyes at -196' than of the corresponding 9-H dyes being attributable to overlap with a feeble dimeric band at the low temperature. The spectrum of the dye 3,3'-dimethyl-9-phenylthiacarbocyanine-p-toluenesulfonate,dye VII, Figure 2, is typical of the first vibronic spectrum of thiacarbocyanines that show only a single maximum in their main absorption band at low temperatures, although at - 196' a very feeble shoulder, indicated in the figure by an arrow, is discernible in the spectrum of this dye at 548 mp, separated from the main maximum by about the same interval as the M2 band is from the MI band, in the meso-alkylated dyes. As has been pointed out by Bruylants, van Dormael, and NyslZ6

the phenyl group in dye VI1 appears to be oriented perpendicularly to the direction of the chromophoric chain; coplanarity of the atoms in the methine bridge with those of the heterocyclic nuclei is thereby secured. Since the van der Waals thickness of the benzene ring, 3.4 A, is less than the corresponding diameter of the methyl group, about 4 A, the crowding in the alltrans configuration of the 9-phenyl dye appears to be sufficiently lower than in the cis configuration, lb, to preclude the existence of more than a small amount of the cis isomer under equilibrium conditions, whereas the energies of the all-trans and mono-cis isomers of the 9-alkyl dyes are sufficiently close to each other for both to coexist in comparable amounts in equilibrium. Of the dyes examined, the 9-alkylthia- and selenacarbocyanines alone show resolution of their main molecular absorption bands into well-defined components MI and Mzat temperatures above -196'. The spectra of several other dyes, however, including besides the 9-phenylthiacarbocyanine dye just mentioned 3,3'-dimethylthiacarbocyanine-p-toluenesulfonate and 9-ethyl-3,3'-dimethyloxacarbocyanine iodide, show at -196' a very feeble shoulder hypsochromic to the main molecular maximum by an interval similar to that between the MPand MI isomers of the mesoalkylthiacarbocyanines (Table 11). Although this shoulder may represent a vibrational transition outside the dominant progression characteristic of cyanine dyes, it can also be attributed, perhaps more prob(25) P. Bruylants, A. van Dormael, and J. M. Nys, Bull. Classe Sci., Acad. Rog. Belg., [ 5 ] 34,703 (1948).

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ably, to a trace of stereoisomer in the equilibrium state of the dye analogous to the R/lz isomer of the mesoalkylthiacarbocyanines. This latter possibility is strengthened by a comparison of the half-widths of the main absorption bands of thiacarbocyanines such as dyes I and 11, whose spectra are unresolved into discrete isomeric components at low temperature, or, a t most show the merest trace of such resolution, with the half-widths of the pure M1 isomer of dyes IV and V. The half-width of these isomers at -130' can be determined from measurements on solutions from which the other isomer has been largely removed by photolytic conversion to the M1form. Its value is about 480 cm-l, significantly less than the values of 700-800 cm-1 observed a t low temperatures for the main band of dyes I and 11. It therefore seems probable that, as surmised by Kuhn16 even chain-unsubstituted carbocyanines contain small amounts of stereoisomers in equilibrium with the alltrans isomer a t room temperature and at lower temperatures. Since the observed bands in the spectra of these dyes fall off in intensity more gradually on the short-wavelength side of the maximum than on the long-wavelength side, the bands associated with the stereoisomers present in small proportion, which in these cases can scarcely be other than cis isomers, must be at shorter wavelengths than that of the trans isomers. The 9-alkyloxacarbocyanines, whose spectra are unresolved into M 1and Mzbands at low temperature, except for a very slight shoulder in the spectrum of 3,3'diethyl-9-methyloxacarbocyanine iodide discernible at -196O, must exist, according to the principle of maximum resonance stabilization, mostly in the all-trans form. Because of the small van der Waals radius of the 0 atoms in the benzoxazole nuclei of these dyes, the crowding of these atoms on the 9,-alkyl group in the trans configuration is so diminished in comparison with that in the corresponding thiacarbocyanines that the energy of the trans isomer is too low with respect to that of the cis isomer to allow much of the latter to participate in the equilibrium mixture. This circumstance, of course, does not exclude the possibility of generating nonequilibrium mixtures containing larger amounts of cis isomers among these and all the other dyes considered in this study by photochemical or catalytic thermal reactions. The half-widths of the main monomeric bands of the oxacarbocyanines at -196' are greater than the half-width of the MI isomer of the 9-alkylthiacarbocyanines at that temperature, suggesting the possible presence of small amounts of stereoisomers even of the oxa dyes in equilibrium with the all-trans form. It has long been realized that the absorption maxima The Journal of Physical Chemistry

of 9-alkylthia- and selenacarbocyanines in alcoholic solution a t room temperature occur at wavelengths anomalously shorter than those of the maxima of corresponding dyes containing the hydrogen atom on the 9-position. Whereas for the oxa dyes, the presence of an alkyl group on the meso-C atom causes the expected small bathochromic shift, the presence of these groups in the chain of the thia and selena dyes causes a hypsochromic shift of 10-20 mFz6 (Table 11). The anomaly can now be largely explained. When, as in the oxacarbocyanines, the 9-H and 9-alkyl dyes are nearly all in uncrowded trans configurations, the alkyl group exerts its normal small bathochromic effect. A 9-alkylthia- or selenacarbocyanine is, on the other hand, a mixture of isomers whose spectra a t room temperature combine to give an unresolved band t,he center of gravity of whose absorption is at a wavelength only slightly longer than the unperceived maximum of the A42 isomer. The fact that the observed maximum of the unresolved band is nearer that of the Mz isomer than to that of the Rfl does not necessarily indicate a preponderant intensity of the R l z band, since, because of the skewness of the bands toward the shorter wavelengths, the MI band can contribute much more absorption a t the position of the Mz maximum than vice versa. A preliminary analysis of the observed, partly resolved overlapping and I f 2 bands of the 9-alkylthiacarbocyanines in isomeric equilibrium at -120' into the separate spectra of the M1 and MZ isomers, corrected for overlap, indicates a greater intensity of the true band of the IT2isomer than of the MI for dye 111, 3,3',9-trimethylthiacarbocyanine bromide, approximately equal intensities of the two bands for dyes IV and V, 3,3'-dimethyl-9ethylthiacarbocyanine bromide and 3,3'-diethyl-9methylthiacarbocyanine bromide, and a greater intensity of the band of the TUl isomer than of the other for dye VI, 3,3',9-triethylthiacarbocyanine bromide. Nature of the Isomers. It has been implicit in our discussion of the Ml and Mz isomers of the 9-alkylthiacarbocyanine dyes that they are the all-trans and monocis isomers of the structures illustrated in Figures l a and lb. Several criteria have been advanced to discriminate between the fully extended all-trans configuration and cis configurations of polyenes and polymethine dyes.6 ,28 (1) The all-trans isomer shows the highest intensity of absorption in the electronic transition of longest wavelength since, in essence, vn

(26) L. G. 9. Brooker and F. White, J . Am. Chem. SOC.,57, 2485

(1936). (27) R. 5. Mulliken, J . Chem. Phvs., 7 , 364, 570 (1939). (28) L. Zechmeister, A. L. Le Rosen, W. A. Schroeder, A. PolgAr, and L. Pauling, J. Am. Chem. SOC., 65, 1940 (1943).

STEREOISOMERISM IN CYANINE DYES

the chromophoric chain of that isomer is longest and the transition moment greatest. (2) Among carotenes, the absorption maximum of the all-trans isomer is at a greater wavelength than that of the cis isomers.28 (3) The deficiency of absorption of cis isomers in the first electronic spectrum is replaced by greater absorption of this isomer than of the trans in the second electronic spectrum; a more or less prominent “cis band” appears in the second electronic t r a n s i t i ~ n . ~These ~ criteria are likely to become blurred if, in addition to the bending of the conjugated chain, differences in planarity exist between the isomers. The data on the intensities of the All and M 2 isomers of dye VI already discussed show that the ratio of the extinction coefficient at the maximum of the MI isomer to that of the Mzis greater than 1.24, pointing to the MI isomer as the all-trans. The longer wavelength of the absorption band of this isomer is also consistent with this assignment. On the other hand, no decided change in the intensity of the ultraviolet bands of dyes IV and VI (the only dyes for which we yet have measured the ultraviolet spectra) has been observed to accompany interconversion of the isomers. Among the carotenes, cis peaks are not always much more intense than the corresponding band for the trans isomerm and the possibility of some degree of nonplanarity in the isomeric structures of 9-alkylthiacarbocyanines may perhaps make our inability to associate a cis peak with one or the other of the isomers not an insuperable objection to the proposed assignments. We suggest that the preferential dimerization of the MI isomer of dye VI in alcoholic solvent at low temperatures may also be a criterion pointing to the all-

1325

trans structure of this isomer. The relatively low intensity of the visible absorption of cis isomers indicates a correspondingly low polarizability of the chromophoric chain and hence a relatively low tendency of these isomers to dimerize by dispersion force interaction, as compared with the all-trans isomer. Experimentally, we have found that thiacarbocyanine dye molecules structurally constrained to adopt the trans configuration dimerize more readily than similar molecules forced to adopt a cis configuration. For example, as is shown in Figure 6, 3,3’-diethyl-8,10ethylenethiacarbocyanine iodide (formula I) in which the linking of the ethylene group between the 8- and 10-C atoms forces the chromophoric chain into the alltrans configurationlsl dimerizes more readily than 3,3’diethyl-9,lO-trimethylenethiacarbocyaninebromide, in which, by a similar device, the chromophore is forced into the mono-cis configuration (formula 11).

I

II

htmpi

Figure 6. Absorption spectra in water containing 2% by volume of methanol of thiacarbocyanines constrained to adopt an all-trans and mono-cis configuration, respectively, a t 23”. (A) 3,3’-diethyl-8, 10-ethylenethiacarbocyanineiodide (trans): (1) 1.06 x 10-6 M ; (2) 1.06 X 10-4 M , (B) 3,3’-diethyl-9,10-trirnethylenethiacsrbocyanine iodide (cis): (1) 1.00 x lo* M ; (2) 1.00 x 10-4 M .

It therefore seems highly probably that the alltrans structure can be assigned to the MI isomer of dye VI and the cis structure Ia to the M2 isomer. It seems likely that the MI and Mz bands of the other three 9-alkylthiacarbocyanines are also attributable to the all-trans and mono-cis isomers, respectively. Bleaching by irradiation at low temperatures and dimerization introduce difficulties into the determination of the relative intensities of the bands of the pure isomers, but our data for dyes I V and V indicate qualitatively that the MI isomer of these dyes, like the (29) L.ZechmeisterandA.Pol&, J.Am. Chem. SOC., 65,1522(1943). (30) L. Zechmeister and L. Wallcave, ibid., 75, 5341 (1953). (31) C. E. K. Mees, “Theory of the Photographic Process,” rev ed, The Macmilian Co., New York, N. Y., 1954, p 409.

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same isomer of dye VI, possesses the more intense absorption band and dimerizes more readily than the Mz isomer. Our present data on dye I11 are incomplete. Other arguments for the assignment of a cis structure to the Mz isomer of the meso-alkylthiacarbocyanines absorbing a t the shorter wavelength can be drawn from McCartin’sll observation of a photochemically induced isomer of 3,3’-diethylthiacarbocyaninep-toluene sulfonate (dye 11) whose absorption maximum is hypsochromic to the normal monomeric maximum by about the same frequency interval of the Mz from the MI maximum of the meso-alkyl dyes. The probability that the main constituent of the isomeric equilibrium of dye I1 possesses the trans configuration is so overwhelmingly high that the isomer absorbing at shorter wavelengths can hardly be other than cis. For the same reason, if the very feeble shoulder, displaced hypsochromically from the main maximum by about the same interval as the MZ from the MI maxima of the 9-alkyl dyes, that appears at -196’ in the spectrum of 3,3‘-dimethylthiacarbocyanineiodide and 3,3‘diethyl-9-methyloxacarbocyanine iodide originates in a trace of stereoisomer, the configuration of this isomer is by far most likely to be cis. Nevertheless, a consideration of all the existing data on isomerism of cyanine dyes does not suggest a clear and unique relation between the wavelengths of the absorption maxima of trans and cis isomers in the first vibronic spectrum. The photochemically formed

isomers of the cyanine dyes studied by Scheibe and co-workers,! N,N’-piperidinopolymethine cyanines and similar dyes, exhibit absorption maxima a t longer wavelengths than those of the unphotolyzed dyes, as do also the products of flash photolysis of these and other cyanines observed by Dorr and co-workers, lo in contradistinction to the corresponding product from 3,3’-diethylthiacarbocyanineobserved by McCartin.l‘ The spectra of these photoproducts8-10 conform to some of the criteria of cis structure-relatively low molecular coefficient in the first vibronic spectrum and accentuated absorption in the region of the ultraviolet cis band. Among the carotenes, the hypsochromic shift with respect to the trans isomer caused by the introduction of a single cis linkage is smallB -leas than that observed between the MI and Mz isomers in the present study of meso-substituted thiacarbocyanines-and relatively small differences in structural detail, e.g., in crowding and coplanarity of the atoms in the chromophoric chain, may possibly cause the cis-trans spectral shift to veer in one direction or the other. The study of these and other details regarding isomerization of cyanine dyes constitutes a field of investigation for further development. ,@

Achwledgments. We are indebted to Dr. L. G. S. Brooker for the supply of dyes. We wish to acknowledge helpful discussions with Dr. Brooker and with Dr. J. E. ,Jones of these laboratories.