Cyanines during the 1990s: A Review

Multiple relaxation pathways in push-pull polyenes

Subpicosecond absorption and gain spectroscopy are used to investigate the excited-state behavior of push-pull polyenes made of a diethylthiobarbituric acid electron-acceptor group and a dibutylaniline electron-donor group linked by a pi-conjugated chain. Four polyenes of increasing length, ranging from n = 2 to 5 double bonds, are compared. The relaxation path and relaxation kinetics are studied in dioxane and in cyclohexane, a polar and a nonpolar solvent, respectively. In dioxane, the results provide evidence for the formation of an emissive transient state on an ultrashort time scale (2-3 ps) attributed to a charge transfer (CT) state. The regular shift of the gain peak of this transient state with increase in the chain length (ca. 100 nm per added double bond) indicates that its structure is similar to that of a cyanine, i.e. with a fully conjugated polyenic chain. Its lifetime ranges from a few tens to a few hundreds of picoseconds depending on the chain length. When the number of double bonds increases from n = 2 to 3, the lifetime increases, then decreases continuously for longer chains. In cyclohexane, where the transient CT state is not formed, the decay of the initial excited state follows the same trend when the chain length increases but the lifetimes are shorter than that of the CT state in dioxane. In both solvents, the characterization of long-lived photoproducts by synchronizing two low repetition-rate subpicosecond laser systems demonstrates a change in the relaxation route as the chain length increases. Isomerization occurs for n = 2, whereas intersystem crossing to the triplet state occurs for n = 4. The change in the relaxation channel is observed for n = 3 in both solvents with however a solvent-dependent behavior. In dioxane, relaxation to the triplet state is already observed for n = 3, while an intermediate regime with a relaxation directly to the ground state is observed in cyclohexane. The photophysics of the studied push-pull polyenes is tentatively compared to that of polymethine cyanines and substituted carotenoids.

Dynamics of photoinduced changes in birefringence and dichroism in a nematic phase containing an azobenzene as dopant

Changes in birefringence and dichroism induced by light were monitored after pulse excitation of a nematic phase consisting of a mixture of a low molecular weight liquid crystal (ZLI-1132) and a dopant 4-(N,N-dimethylamino)-azobenzene. After excitation in the UV, large changes in the light transmitted by the sample, placed between polarizers at various angles or in the polarized absorption spectra, could be detected and were monitored as a function of time and temperature in samples with 1-10% w/w of dopant. Birefringence experiments, performed with different monitoring wavelengths and with dopants that degrade all the absorbed energy as heat, demonstrate that all the changes observed are due to the photoisomerization process of the dopant. The recovery of the optical delay in the sample and of the polarized absorption follows the same monoexponential kinetics, indicating that E-Z isomerization of the dopant governs the order of the nematic phase. When samples are irradiated near the clearing point, a photoinduced nematic-isotropic phase change is observed.

Nonradiative deactivation of the electronic excitation energy in cyanine dyes: influence of binding to DNA

The processes of nonradiative deactivation of electronic excitation energy in cyanine dyes determine their quantum yield. Because of that, the study of the influence of cyanines binding to DNA on these processes can provide information on the causes leading to the cyanines fluorescence intensity enhancement in the presence of DNA. In the presented paper, the activation energies of nonradiative degradation of electronic excitation, quantum yields and rate constants of nonradiative transitions of several cyanines in free state and in the presence of DNA were established and compared. The mechanisms of nonradiative deactivation of dye excitation energy were discussed.