H*- and J-Aggregates of an Indotricarbocyanine Dye: Steady-State Spectral Properties and Excited-State Dynamics

H*-aggregates are a rarely observed type of molecular aggregates, with many of their properties yet to be investigated. The indotricarbocyanine dye under study is the first known compound able to produce both H*- and J-aggregates. Steady-state spectral properties, morphology, photoelectrochemical response, and excited-state dynamics of the H*-aggregates are investigated for the first time. Under 400 nm pumping, the deactivation of electronic excitations in the H*-aggregates follows two paths involving the top and bottom of the exciton band. This is consistent with the presence of a fast 3 ps and a slow 33 ps component in the decay trace of the ground-state bleaching. Under 800 nm pumping, the H*-aggregates are promoted to the bottom of the exciton band. Consequently, only the slow component is present in the decay trace of the ground-state bleaching. For the J-aggregates, the ground-state bleaching follows biexponential decay with the time constants of 1 and 20 ps under both 400 and 800 nm pumping. The relaxation involves only the bottom of the exciton band. The faster deactivation of electronic excitations in the J-aggregates is probably due to greater exciton delocalization: 9 molecules in the J-aggregates versus 4 molecules in the H*-aggregates. The obtained results can be used to create photonic devices containing the H*-aggregates.