PHOTORELAXATION PROCESSES IN COVALENTLY-LINKED INDOCARBOCYANINE AND THIACARBOCYANINE DYES

The photophysical and photochemical properties of monomeric thiacarbocyanine (B-1) and two bis-derivatives which are Linked by three or six methylene groups in one N,N'-chain (B-2 and B-3, respectively) as well as of indocarbocyanine (A-1) and bis-derivatives which are linked by two and, respectively, four methylene groups in one (A-2 and A-3) and two chains (A-4 and A-5) were studied in ethanol. The quantum yield of fluorescence at 297 K is Phi(f) = 0.02-0.07 and increases substantially with decreasing temperature, the smallest change occurring for A-4, where intersystem crossing and internal conversion at the trans side are largest. Photobleaching contains two parts one from trans --> cis photoisomerization and another from the T-T transition in the range of ground state absorption. Population and decay of the triplet state were observed in the presence and absence of sensitizers, the lifetime at room temperature being in the millisecond range. For the cyclic bis-cyanines A-4 and A-5, where trans --> cis photoisomerization is hindered, the quantum yield of intersystem crossing approaches Phi(isc) approximate to 0.1 and 0.03, respectively, and Phi(isc) is even smaller for the other dyes. The quantum yield of trans --> cis photoisomerization is substantial for the monomers; for example, Phi(t-->c) = 0.25 for B-1 at 297 K and drops with decreasing temperature. The smaller values for the single-chain bis-cyanines, e.g., Phi(t-->c) = 0.05 for A-2, are due to the reduced flexibility. The activation parameters for the thermal back-reaction are analyzed. The characteristic features of structure influencing the mechanism, i.e., the photochemical deactivation pathways of the excited trans isomers, are discussed.