Photophysics of Cyanine Dyes:  Subnanosecond Relaxation Dynamics in Monomers, Dimers, and H- and J-Aggregates in Solution

The photophysics of three cyanine dyes (i) 1,1‘-diethyl-2,2‘-cyanine iodide (pseudoisocyanine, PIC), (ii) 3,3‘-didodecyldithia-2,2‘-carbocyanine bromide (dye 1), and (iii) 3,3‘-diethyldithia-2,2‘-carbocyanine iodide (dye 2) have been examined by picosecond-laser photolysis in aqueous and methanolic-aqueous media. At moderately high concentration, solutions of PIC in 5 M NaCl/water contain monomers, H-aggregates, and J-aggregates; dye 1 water/methanol solutions consist mostly of monomers and H-aggregates (dimers and higher n-mers); aqueous dye 2 solutions contain only monomers and dimers. Photolysis of H- and/or J-aggregates in PIC and dye 1 cyanine solutions leads to photobleaching of the respective aggregate absorption bands and subsequently decays by biphasic kinetics. Two mechanisms are discussed for the deactivation of excited aggregates. In the first, nonradiative decay of the excited singlet states results in considerable heating of the aggregates together with their surrounding solvent shells causing the probe laser light in a pump−probe experiment to be strongly attenuated after excitation (see, e.g., J. Phys. Chem. 1995, 99, 11952). This heating, which subsequently leads to partial dissolution of the aggregates (deaggregation), later reformed slowly on cooling, can also arise from another mechanism. That is, relaxation of singlet excited states of H-aggregates can also occur, in part, by exciton−exciton annihilation as occurs in J-aggregates at high laser pump intensities. In this case, the longer lived excited singlet states of H-aggregates, relative to those of J-aggregates, are likely due to a less efficient (slower) exciton−exciton annihilation process in H-aggregates which would infer a weaker exciton coupling in comparison to the strong coupling known to prevail in J-aggregates. For dye monomers, singlet excited state lifetimes increase (i) with an increase in the length of the aliphatic residues attached to the dye molecule and (ii) with dimerization of the dye molecules. Dimerization restricts torsional dynamics along the dyes polymethine chain and diminishes the nonradiative deactivation channel. An important conclusion from this work is that the thermal energy stored in the aggregates in a short time after picosecond-laser excitation, and the subsequent heating of the surrounding solvent shells, leading to dissolution to smaller aggregates and monomers, provides another pathway to S1−S1 annihilation in the deactivation of excited dye agg