Effects of chromophore interaction in photophysics and photochemistry of cyanine dyes

Spectral and fluorescent properties of ketocyanine dyes (polyenic bis-ω,ω′-aminoketones) and cation-anionic polymethine dyes of various structures were studied. The symmetric ketocyanines were shown to have a long-wavelength absorption band bathochromically shifted in comparison with that of the asymmetric ketocyanines with the same total length of the polyenic chain. The nonlinear ketocyanines exhibit the additional short-wavelength band in their absorption spectra, which can be more intense than the longwavelength band. The absorption spectra of ion pairs of cation-anionic dyes with overlapping cation and anion bands contain a new intense short-wavelength band inactive in fluorescence excitation. These spectral peculiarities are explained on the basis of chromophore interaction model. It has also been shown that the T1 levels of ketocyanine chromophores do not essentially interact with each other in a ketocyanine molecule in nonpolar solvents; in polar solvents this interaction becomes appreciable due to lowering the potential barrier for conjugation.


INTRODUCTION
Dyes with single chromophores have wide applications in various fields of science and technology. Along with such simple dyes, a number of bichromophoric dyes were synthesized (e.g., biscyanine dyes [1]), which possess interesting photochemical properties different from those of simple dyes. In the three recent decades, two new classes of bichromophoric cyanine dyes were obtained, ketocyanines [2,3,4] and cationanionic dyes [5]. Ketocyanine dyes (polyenic bis-ω, ωaminoketones) consist of two ω-aminopolyenic units bound to the central carbonyl group; cation-anionic dyes contain in their structure cationic and anionic polymethine dyes. This work is devoted to investigation of the spectral and photochemical properties of these dyes and their features owing to interaction of chromophores in the dye molecules.

EXPERIMENTAL
Ketocyanine dyes studied in this work were synthesized as described earlier [3,4,6]. The structures of the ketocyanines are presented in Table 1. The cationanionic dyes obtained as described earlier [5] and contained the following dye anions: Acetonitrile, chloroform, toluene, dioxane, ethanol, n-propanol, and isopropanol were used as solvents. Chloroform was passed through an Al 2 O 3 column to purify from traces of acids. Ethyl acetate was stored over molecular sieves; toluene was refluxed over metal sodium and stored over molecular sieves. The absorption spectra were recorded on a Specord UV-VIS spectrophotometer using cells with 1 cm path length. The fluorescence spectra were recorded using an Aminco-Bowman spectrofluorimeter equipped with an R136 photomultiplier (the spectra were not corrected for spectral sensitivity of the spectrofluorimeter). Concentrations of the dyes were in the range of 10 −6 -10 −5 M.
All measurements were performed at 20±2 • C.

RESULTS AND DISCUSSION
Ketocyanine dyes. Table 1 presents the absorption and fluorescence maxima for the dyes studied. It can be seen that the symmetric dyes (with two aminopolyenic fragments of the equal length) have the absorption bands shifted bathochromically compared to the asymmetric dyes with the same total length of the polyenic chain (n+m) ( Table 1, cf. dyes 2 and 3; 5 and 7; 11 and 12; even dye 6 with asymmetric structure and longer total polyenic chain than that of dye 7 has the absorption spectrum located in the short-wavelength region with respect to the symmetric dye 7). This can be explained on the basis of interaction of two aminopolyenic chromophores in the dye molecule. The π -electronic structure of a ketocyanine dye can be represented as intermediate between two limiting resonance structures (structures 1 and 2; see Scheme 1). Structure 1 is characterized by alternating polyenic bonds and absence of charge separation; structure 2 has completely equalized polyenic bonds (complete π -electron conjugation along the whole polyenic chain) and separate charges (zwitterionic structure). The latter structure of a polyenic chain is characteristic for a cationic polymethine having the most bathochromic absorption band, so the shift of the real structure of a ketocyanine (structure 3) toward structure 2 (e.g., due to solvation in polar and, in particular, protic solvents) will lead to a bathochromic shift of the ketocyanine absorption band. This can explain strong positive solvatochromism and thermochromism of ketocyanines observed experimentally [10].
The spectral properties of polymethine dyes are known to be described well by the "free electron" model [11]. In this approach, a dye chromophore is represented by a potential box, whose width corresponds to the chromophore length. Thus, ketocyanine can be represented by two potential boxes corresponding to two aminopolyenic units (two chromophores) separated by Table 1. Maxima of absorption (λ max abs ) and fluorescence (λ max fl ) spectra and extinction coefficients (ε) of ketocyanines (for dyes 1-19 in ethanol; λ max fl in n-propanol).
Interaction of the chromophores (their excited singlet states) in symmetric and asymmetric ketocyanines. a potential barrier (see Scheme 1). This barrier is due to the presence of two single bonds at the carbonyl group of the dye; it is highest for structure 1 and vanishes for structure 2 with complete (barrierless) π -electron conjugation. So the real structure of a ketocyanine has a moderate potential barrier.
Interaction of the excited singlet levels of the chromophores in a ketocyanine can occur by two ways: dipole-dipole interaction and conjugation through the potential barrier. The first one corresponds to exciton coupling in Davydov's model [12], the second corresponds to electron tunneling through the barrier. The dipole-dipole interaction results in symmetric splitting of the chromophore singlet levels [11]; the conjugation leads to further splitting and lowering of these levels [13] (see Scheme 2). Therewith, the maximum energy of the interaction (and the maximum splitting of the singlet electronic levels) occurs for the dyes with the sim-ilar chromophores (for symmetric ketocyanines). For the linear ketocyanines 1-17, the radiative transition S 0 ←→ S 2 is symmetry forbidden and the transition S 0 ←→ S 1 is allowed, so the chromophore interaction in linear ketocyanines manifests itself as bathochromic shift of the long-wavelength absorption band (S 0 → S 1 ) of the dyes. Thus, the bathochromic shift of the absorption band of symmetric ketocyanines compared to that of asymmetric ones can be explained by stronger chromophore interaction in symmetric ketocyanines.
The S 0 ←→ S 2 transition, forbidden in linear ketocyanines, is allowed in nonlinear ones, so the energy gap ∆ν between the absorption bands S 0 → S 1 and S 0 → S 2 can serve as a measure of the chromophore interaction in the nonlinear ketocyanines. Indeed, for the nonlinear symmetric ketocyanine 18 the value of ∆ν is greater than that for the nonlinear asymmetric dye 19 (see Figure 1; ∆ν 1 = 4400 cm −1 , ∆ν 2 = 3500 cm −1 ). Generally, the intensities of the long-wavelength and the short-wavelength bands in the dipole-dipole approximation are related by the following expression [12,14]: where θ is the angle between the chromophores; r = (λ 1 ε 1 )/(λ 2 ε 2 ); λ 1 ,ε 1 and λ 2 ,ε 2 are the wavelength and extinction coefficients in the maxima of the shortwavelength and long-wavelength band, respectively. The estimation of the θ value from expression (1) for dyes 18 and 19 gives θ ∼ 120 • C, which corresponds to the simple geometrical evaluation.
The absorption spectra of ketocyanines 20-22 are unusual: they consist of the long-wavelength band with relatively low intensity and the intense shortwavelength band (see Figure 2 and Table 1). This can be explained on the basis of the model of chromophore interaction. According to this model, ketocyanines with acute angles between the chromophores should have the short-wavelength absorption band more intense than the long-wavelength band. Indeed, the estimation of the angle θ from expression (1) gives θ = 75 • , 81 • and 83 • for dyes 20, 21, and 22, respectively.
It is of interest to note that even for the polymethine salts whereas other similar polymethine salts (even the polymethine salts-derivatives of the nonlinear ketocyanines 18 and 19) have only one absorption band that is located in the long-wavelength region. So we have to conclude that there is no complete conjugation in the whole polymethine chain characteristic for conventional cationic polymethines, and the central pyran unit in the structure of salts 23-25 creates some potential barrier for the conjugation. In this case we can use the model of chromophore interaction for evaluation of the angle θ between the chromophores; it was found from expression (1) that for dye 23 θ = 71 • .
Apart from the singlet levels, the triplet levels of ketocyanine chromophores can interact with each other only by conjugation, with no dipole-dipole coupling, because the dipole transition moment for the S 0 ←→ T 1 transitions is close to zero. This interaction could cause splitting and lowering of the triplet lewels similar to that for the singlet levels (see Scheme 3), and we might expect lower T 1 levels for symmetric ketocyanines compared to corresponding asymmetric ones.
Interaction of the chromophore triplet states in ketocyanines. Table 2. Triplet energy levels (E T ) for ketocyanine dyes in toluene and n-propanol [15].  Table 2 presents the triplet energy levels for some ketocyanines [15]. It can be seen that ketocyanine symmetry is not a decisive factor in determining these levels, and the dyes with the same longer chromophore (the same n) have essentially the same E T in toluene independent of their symmetry (cf. dyes 2, 4, and 7; 26 and 27 in Table 2). Hence the triplet level of the whole ketocyanine molecule in toluene is determined only by the length of the longer chromophore (longer polyenic fragment of the molecule), and there is no essential influence of the shorter chromophore in the molecule. So we can conclude that the T 1 triplet levels of ketocyanine chromophores do not essentially interact with each other in ketocyanine molecule (in toluene), which is probably due to their low triplet energy insufficient to overcome the potential barrier for chromophore interaction (conjugation). In the polar and protic solvent, n-propanol, this barrier is lowered due to solvation, and interaction of the triplet levels becomes appreciable, which is reflected in the slightly lower E T value for the symmetric dye 7 compared to those for the asymmetric dyes 2 and 4 ( Table 2).
Cation-anionic dyes. The photophysical and photochemical properties of cation-anionic polymethine dyes, in which both a cation and an anion are dyes, have some interesting features making them perspective for technical application, e.g. in the systems of solar energy conversion, laser technology, information stor-age etc. In polar solvents cation-anionic dyes behave as equimolar mixtures of corresponding simple cationic and anionic dyes, whereas in weakly polar and nonpolar solvents, where the dyes form ion pairs, their properties can change drastically as a result of the interaction between the cation and anion chromophores [16]. We might expect that in ion pairs the cation and the anion will have parallel orientation (the angle θ between the cation and anion chromophores would be equal to zero). The maximum interaction effect should be expected for the S 1 electronic levels of the chromophores having the same energy, i.e., for a cation and an anion with coinciding or close absorption bands. Indeed, in going from polar solvents, ethanol and acetonitrile, to weakly polar chloroform and dioxane and nonpolar toluene, no essential changes are observed in absorption spectra of the dyes with completely different position of cation and anion absorption bands (dyes 28-32, 34, 36 in Table 3; only slight bathochromic shift of the bands is observed), whereas a new short-wavelength band appears in the absorption spectra of the dyes with overlapping cation and anion absorption bands accompanied by simultaneous diminishing of the initial, longwavelength band (dyes 33, 35, 37; see Table 3 and Figure 4). This new band is inactive in fluorescence excitation and appears as a result of interaction of the S 1 levels of the cation and anion chromophores in the cation-anionic ion pair (Scheme 4). In this case, the   (1), chloroform (2), and dioxane (3).
long-wavelength transition S 0 ←→ S 1 is forbidden, whereas the short-wavelength transition S 0 ←→ S 2 is allowed (see expression (1) for θ = 0), so we should observe only the short-wavelength band S 0 ←→ S 2 in the absorption spectrum of the ion pair. After absorbing light, this cation-anionic pair undergoes transition to the S 2 state followed by fast nonradiative deactivation to the S 1 state. Because the radiative S 1 ←→ S 0 transition is forbidden, the ion pair is nonfluorescent.