Ellinger Y., Defranceschi M. (eds.) Strategies and applications in quantum chemistry (Kluwer, 200

the positive charge, instead of charge separation as in the classic TICT donor-acceptor systems, e.g. DMABN [6]). As previously argued [24], the CT nature of the perp forms

decay of cyanine-like systems has been firmly established [26,58,59], henceforth we will consider only the properties of the and potential surfaces involved in the

respectively. The results of the CS INDO CI calculations for the 2-3 and 3-4 one-bond and 2-3, 4-5 two-bond isomerizations are represented in Fig. 2. The concerted two-bond

sufficient proof to rule out definitely simultaneous isomerizations at two C-C bonds. On the other hand, the potential curves for both 2-3 and 3-4 single isomerizations are in keeping with the observed photophysical and photochemical behaviours. The main

for the isomerizations around 2-3 and 3-4 links, respectively) and through which decay to the ground state trans and cis isomers may rapidly occur (the barriers for thermal back isomerization of 2-3 cis and 3-4 cis isomers being 25.4 and 24.2 kcal respectively). From the comparison between the calculated barriers the formation of the 3-4 cis isomer appears to be decidedly favoured in agreement with both the CS

INDO CI calculations on PC [24] and the analysis of the spectrum of the phototropic form of BMPC in methanol solution [25].

In conclusion almost all the experimental observations concerning photophysics and photochemistry of BMPC in alcoholic solutions appear to be fairly accounted for by

calculations performed in the “free space” approximation. Of course, this requires supplementary investigations since, in principle, one should expect both the photoinduced and the thermal isomerization dynamics to be influenced by solvent polarity, owing to the charge localization phenomenon in the and perp forms (Fig. 3), as well as by solvent viscosity, as has been observed with many other polymethine cyanines [16].

As for the theoretical treatment, we could only try to include the electrostatic solutesolvent interactions and, in fact, we corrected the electronic potential energies for the

[eq. (2)]. The resulting potential curves are to be seen as effective potentials at equilibrium, i.e. reflecting orientational equilibrium distributions of the solvent dipoles around the charged atoms of the solute molecule. In principle, the use of potentials thus corrected involves the assumption that solvent equilibration is more rapid than internal rotation of the solute molecule. Fig. 4 points out the effects produced on the potential

cation charge begins to localize at highly twisted conformations, solvation manifests

affected by the solvent polarity, while the barriers to trans-cis isomerization in S0, located at undergo a significant, yet not very large, decrease when the solvent polarity increases. Anyway, some 50% of the limit polarity effect is obtained on passing

atomic charges obtained by rather extended CI wavefunctions (Fig. 5). Not surprisingly, appreciable changes were found only for the excited state and were confined to a smaller

Further heating causes a much slower lifetime shortening. Such a behaviour is parallel to that of the solvent viscosity, which undergoes a steep change between 100 and 130 K due to matrix melting [61]. This indicates that solvent friction considerably affects the photoisomerization dynamics of BMPC and poses the very question whether the photoisomerization of BMPC is a barrierless process, so that its kinetics would be affected by temperature only through the variation of the solvent viscosity. The results reported in Fig. 7 clearly show that the answer to this question is "no". The fluorescence quantum yields of BMPC in several linear alcohols at 298 K increase with solvent viscosity much more slowly than when measurements are carried out in ethanol at different temperatures. It is apparent that, in ethanol, a temperature lowering causes an increase of the quantum yield not only indirectly, by increasing the solvent viscosity to values comparable with those of the longer-chain alcohols, but also in a direct way: the photoisomerization of BMPC in alcohols features a significant intramolecular barrier.

Such a qualitative conclusion is supported by the observation that the room- temperature fluorescence spectrum of BMPC in alcohols (Fig. 8) is a good mirror image

In order to go further into the experimental check we constructed Arrhenius plots of the fluorescence quantum yield of BMPC in a few solvents (methanol, ethanol, propanol, hexanol and methylene chloride), all of which showed good linearity. The activation energies and ratios, calculated from the slopes and intercepts of those plots, are collected in Table 1. The smooth increase of both parameters in the alcohol series is mainly associated with the increase of solvent viscosity. On the other hand, decrease of the solvent dielectric constant from 32.7 (methanol) to 8.9 (dichloromethane) causes a small but significant increase of the activation energy; also, this increase is probably somewhat compensated by the decrease of the viscous-flow

#2 The oscillator strength of the longest wavelength absorption band of BMPC (1.1, [25]) is very similar to those of two previously studied carbocyanines (DOC and DTC) [45] so that we can expect that, for

394 F. MOMICCHIOLI ET AL.

activation energy of the latter solvent with respect to the former one#3. However, in view of its slight size this change is not contrary to the theoretically predicted "insensitiveness" of the conversion barrier to solvent polarity.

With the aim of getting a quantitative evaluation of the "intramolecular" activation energy for the photoisomerization of BMPC in alcohols, a parameter which can be directly compared with calculated barriers, isoviscosity plots were drawn at 2, 6 and 10 cP using data obtained in methanol, ethanol, propanol and hexanol (Fig. 9). In all cases, the hexanol points lie slightly above the lines drawn through the points of the three shorter alcohols. This is probably a manifestation of the saturation of viscosity effects emphasized by Fig. 7: as the size of the alcohol molecule increases, the microscopic friction felt by the isomerizing solute is less and less adequately described by the solvent shear viscosity. Therefore, in spite of the fact that, due to the very low barrier and associated frequency, an almost diffusive reaction dynamics is expected [16], shear viscosity only provides a rough description of the frictional interaction between the twisting solute and the solvent molecule. This is confirmed by the finding that the slopes of the isoviscosity plots, determined omitting the hexanol points, decrease with viscosity. The "intramolecular" activation energies obtained at 2, 6 and 10 cP were equal

approximations in our analysis, related to the problematic use of shear viscosity as a measure of solvent friction, we can only provide an estimate of the "intramolecular"

#3 Comparison of these results with those found for DOC and DTC , whose activation energies in dichloromethane were equal or even smaller than in methanol [55], indicates that the effect of solvent polarity on the photoisomerization barrier, although still small, is more pronounced for the open-chain cyanine BMPC than for the carbocyanines.

activation energy for the photoisomerization of BMPC in short-chain linear alcohols: This finding strongly supports the theoretical prediction that photoisomerization of BMPC proceeds efficiently by twisting about the 3-4 bond in the

surface. In fact, the calculated barriers to conversion in highly polar solvents ranged between 1.10 kcal and 1.75 kcal (Fig. 5) according to whether the atomic charges derived from single configuration or extended-

CI wavefunctions were used to evaluate (the calculated barrier in the isolated molecule being

Finally, the dependence of the back isomerization kinetics of BMPC in the ground state on solvent polarity was investigated by measuring spectrophotometrically the rate constant of this process in methanol, dichloromethane, chlorobenzene and toluene. These solvents have similar room-temperature viscosities (from 0.45 to 0.8 cP) and viscous-flow activation energies (from 1.60 to

[66]). Because of this, and of the probably small relative factional contribution to the overall activation energy connected with the high curvature of the potential function at the barrier top [16], solvent polarity effects can be evaluated by direct inspection of the measured activation energies. These were calculated as usual from Arrhenius plots and are shown in Table 2. As the solvent dielectric constant decreases, the measured activation energy increases on going from methanol to dichloromethane, decreases slightly in chlorobenzene and, finally, drops to a substantially lower value in toluene. The preexponential factor, too, shows a strong decrease in toluene with respect to the