The Resonance Raman Spectra of Some Cyanine Dyes?

J.-P. YaogS and R. H. Callenderg Physics Department, City College of The City University of New York, New York, New York 10031, USA

The resonance Raman spectra of a series of cyanine dyes, with various lengths of the methine chain, have been obtained. This series consists of the I,l'-diethyL2,2'-quino group basic structure. Raman spectra of some dyes with closely related structures were also measured. The Raman bands are assigned to normal modes arising from particular molecular moieties. For comparison purposes, the IR spectra of these dyes are also presented.

INTRODUCTION

The spectroscopic properties of cyanine dye:. are of interest for several reasons. They have been used as photographic sensitizers,' and a wealth of knowledge concerning biological systems, most notably mem- branes, has been obtained from studies using cyanine dyes as fluorescence probes.* Of importance to quantum chemistry, these dyes form a distinct class of T electron molecules. Cyanines have an odd number of atoms that share the n electron system and a symmetrical structure, and these properties permit a high degree of n electron delocalization. The methine bonds have approximately equal bond lengths; there are no essentially single and double bonds. In contrast, the linear polyenes, which have an even number of n elecron atoms, are character- ized by substantially less T electron delocalization and contain essentially single and double bonds. It should be possible to treat these systems within the context of a single theoretical framework and some success in this regard has been achieved in analysing their absorption properties (see, e.g., Refs 3 and 4). Much less is known about the vibrational properties of polymethines, partly because there are few available Raman spectroscopic studies. The polymethines are characterized generally by high fluorescence quantum yields and this, of course, makes Raman spectroscopy difficult.

We present here the Raman and infrared spectra from a vinylogous series of l,l'-diethy1-2,2'-quinocyanine iodide salts, hereafter referred to as quinocyanine( n > where n = 0, 1, 2 ,3 (see Fig. la). In addition, data were obtained for 1,1',3,3,3',3'-hexamethylindodicarbo- cyanine [ indocyanine( 2)], where the quinoline moiety of quinocyanine(2) is disrupted (Fig. lb). The resonance Raman and IR spectra of quinocyanine(0) has been reported previou~ly.~ Our goal here is to assign the spectral features to one or another particular molecular group, leading eventually to a detailed spectroscopic assignment and characterization of these dyes.

t This work was supported by The National Science Foundation,

$' On leave from Chemistry Department, Zhongshan (Sun Yat Sen)

g Author to whom correspondence should be addressed.

grant number PCM-8202840.

University, Guangzhou, People's Republic of China.

CCC-0377-0486/85,

@ Wiley Heyden Ltd, 1985

MATERIALS AND METHODS

1,1 '-Diethyl-2,2'-quinocyanine iodide [ quinocya- nine( O)], l,l'-diethyl-2,2'-quinodicarbocyanine iodide [quinocyanine(2)] and 1,1'3,3,3',3'-hexamethylindo- dicarbocyanine iodide [indocyanine(2)] were pur- chased from Eastman Kodak Co. (Rochester, NY). 1,l- Diethyl-2,2'-quinocarbocyanine iodide [quinocya- nine( l ) ] and l,l'-diethyl-2,2'-quinotricarbocya- nine iodide [quinocyanine( 3)] were purchased from Accurate Chemical and Scientific Corporation (Hicksville, NY).

All cyanine dyes were dissolved in spectral-grade methanol (Fisher Scientific Co., Fairlawn, NJ). Raman spectra were obtained with a spectrometer system con- sisting of a double spectrometer (Model 1401, Spex Industries, Metuchen, NJ) controlled by Spex Model CD2A Compudrive controller, a cooled RCA 31034 photomultiplier and photon counting electronics, inter- faced to LSI-11 minicomputer, which was also used for data storage and analysis. A Spectra-Physics (Mountain View, CA) Model 165 argon ion laser and a Coherent (Palo Alto, CA) Model 52 krypton laser were used to produce monochromatic radiation. All spectra were measured at room temperature. Most samples were stable during measurement. Quinocyanine(3), however, can be bleached with time by the laser. Bleached samples

n=O, 1,2.3

Figure 1. Chemical structure of (a) quinocyanine(n) and (b) indocyanine(2).

(0016-03 19 $01.50

JOURNAL OF RAMAN SPECTROSCOPY, VOL. 16, NO. 5,1985 319

J.-P. YANG AND R. H. CALLENDER

I I I I I I I , I

I I I

FREOUENCY ( c rn- ' 1 I700 I500 I300 I too

Figure 2. Resonance Raman spectra of (a) quinocyanine(3), (b) quinocyanine(2), (c) quinocyanine( 1) and (d) quinocyanine(0). The exciting laser irradiation was at 488 nm for (a) and (c) and 514.5 nm for (b) and (d). Resolution was 8 cm-' and band assignments are accurate to within =3 cm-'. The 1041 cm-' band is a remnant of unsubtracted methanol spectra.

gave no Raman signal under the experimental condi- tions. Raman peaks arising from the solvent were sub- tracted.

Potassium bromide pellets of the dyes were used to obtain the infrared absorption spectra. Typically, 1 mg of sample was added to about 250 mg of spectrographic- grade KBr powder (International Crystal Laboratories, Elizabeth, NJ). Infrared spectra were obtained with an IBM IR/85 spectrometer (IBM Instruments, Danbury, CT). All spectra were obtained at room temperature. Contributions from the KBr powder and water vapor were subtracted.

RESULTS AND CONCLUSIONS

The Raman spectra of quinocyanine( n ) in methanol, for n=0, 1, 2, 3, are shown in Fig. 2. Raman spectra of quinocyanine(2) in chloroform, aniline, pyridine, acetone, ethanol, nitrobenzene and formamide are iden- tical with that in methanol (Fig. 2b). Hence, the Raman bands can be assigned to the dyes and the effect of interaction of the solvent is minimal. The spectra in Fig. 2 are remarkably similar to each other, particularly for n = 0 , 1 and 2. There are, however, two spectral features which appear to depend on the methine chain length. The band at 1519 cm-' for quinocyanine(0) appears to shift to 1454 cm-' for the n = 1 pigment and to 1404 cm-' for n = 2. In addition, the most intense resonance Raman bands near 1350 cm-' appear as a triplet for n = 0 , a doublet for n = 2 and as a singlet for n = 1 and 3. Apart from those features, there is a close similarity amongst the four spectra in both the frequencies and intensities of the observed Raman bands. This suggests assignment

I 1 L

FREQUENCY (ern-' )

Figure 3. Resonance Raman spectra of indocyanine(2). The exciting laser irradiation was at 482.5 nm; resolution was 8 cm-'.

I700 1500 1300 1100

of those bands not sensitive to the length of the methine chain to the quinoline end groups.

The intense bands at ca 1350 cm-' are often thought to be due to the quinoline moieties because the Raman spectrum of quinoline contains a very intense band at 1372 cm-', assigned to aromatic ring breathing motion.6 Since there are two quinoline rings associated with each quinocyanine( n), there should be two such modes. However, a virtual degeneracy of the two modes would be expected based on symmetry considerations. The triplet observed in the Raman spectrum of quino- cyanine(0) near 1350 cm-' is therefore surprising. It has been speculated that various isomers of quino- cyanine(0) are present in solution in distinct configur- a t i o n ~ . ~ Three isomers are possible with respect to the locations of the ethyl groups relative to the central methine hydrogen (cis/ cis, cis/ trans, trans/ trans). However, NMR investigations have suggested that this is not the ~ a s e . ~ - ~

Our own data suggest that the 1350 cm-' bands arise from two distinct sources: methine chain motions and motions of the quinoline moiety. Contrasting Fig. 2b with Fig. 3, we observe that the doublet structure found in the spectrum of quinocyanine(2) near 1350 cm-' is replaced by a narrow singlet peak in indocyanine(2). The disruption of the quinoline structure results in the disappearance of one of the doublet bands in quino- cyanine(2), suggesting one of the two ca 1350cm-' bands is indeed associated with the quinoline moiety. On the other hand, calculations5 (and our own unpub- lished work) show that stretching motions of the poly- methine chain carbons can also lie in this frequency region. Moreover, our preliminary results for simpler cyanines, where the 7~ electron system consists of only the central methine chain, show a dominating Raman band in this spectral region. It is certainly reasonable that the ca 1350 cm-' bands in all the spectra arise from either the quinoline or the methine chain.

The band at ca 1629 cm-' has been assigned to C=N stretching motion," and our results are consistent with this assignment. The band is essentially unchanged as the methine chain length is varied and shifted slightly down field to 1601 cm-' cm-' for indocyanine(2). It has been suggested that the bands as 1226, 1250 and

320 JOURNAL OF RAMAN SPECTROSCOPY, VOL. 16, NO. 5,1985

THE RESONANCE RAMAN SPECTRA OF SOME CYANINE DYES

FREQUENCY ( c m - ' )

Figure 4. FTIR spectra of (a) quinocyanine(3). (b) quinocyanine(2). (c) quinocyanine(1) and (d) quinocyanine(0).

1283cm-' in the quinocyanine(1) data arise from motions of the ethyl group attached to the aromatic m ~ i e t y . ~ Results (data not shown) from a compound where these ethyl groups are replaced by butyl groups confirm this assignment. The Raman spectrum of this compound shows a triplet at 1206,1233 and 1269 cm-', which replace the 1200-1300 cm-' triplet observed in quinocyanine( 1); in all other respects the spectra from these compounds are identical to within the signal-to- noise ratio.

We can assign the 1519 cm-' band found in the quino- cyanine(0) (Fig. 2d) data to (probably stretching) motions of the methine carbons. The apparent shift of this band to a lower frequency (1454 cm-' for n = 1 and 1404 cm-' for n = 2), correlated with the methine chain length of the cyanine, is interesting. A similar correlation is found in the spectra of molecules which contain a polyene T electron structure.""2 In the polyene struc- tures for fixed end-groups of varying polyene chain length, the strongest observed Raman band, associated with C=C stretching motions, shifts to lower frequency as a function of the chain length or, equivalently, A,,,. Extension of the correlation to quinocyanine(3) results

Table 1. Assigoment of the quinocyanine(0) Raman bands

Band Band (ern-'] Molecular moiety (cm-'1 Molecular moiety

1630 C=N 1369 Polymethine chain 161 1 Quinoline (two bands) 1574 Quinoline 1354 1519 Polymethine chain 1283 Ethyl groups 1476 Quinoline (?) 1241 Ethyl groups 1389 Quinoline (one band) 1226 Ethyl groups

1174 Quinoline 1129 Quinoline

in the prediction of a band at ca 1330 cm-'. A shoulder on the dominant 1350cm-' band is observed at this position (Fig. 2a), but it is unclear whether or not the correlation can be extended to quinocyanine(3). Note that the 1404 cm-' band of quinocyanine(2) appears to be essentially unaffected by disruption of the quinoline moiety, as suggested by the indocyanine(2) data, despite a substantial change in A,,,. Therefore, if the correlation suggested here were to be verified by further measure- ment, it would be one of band position to chain length.

The IR absorption spectra of quinocyanine( n ) , for n =0, 1 ,2 and 3, are shown in Fig. 4. The IR spectra are relatively much more complicated and richer than the Raman spectra, and we are unable to provide a detailed analysis of the results. It is clear, however, that much of the observed IR spectra arises from atomic motions in the central methine chain. The number of observed bands increases markedly as n increases.

The assignments of the observed Raman bands of quinocyanine(0) are summarized in Table 1. Most of the bands belong to the quinoline moieties. In a future publication, we shall consider the factors which give rise to the relatively small resonance enhanced Raman cross- sections of polymethine normal modes and, hence, the relatively small number of observed bands.

Acknowledgement

We appreciate discussions with Professor A. Waggoner and generous gift of similar cyanine dyes. We thank Professor K. Nakanishi of Columbia University for the use of the FTIR spectrometer in his laboratory.

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Received 2 October 1984

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