Spectral and luminescent properties of an indotricarbocyanine dye are studied in solutions and after deposition on quartz or silicon substrates. It is found that the dye molecules self-assemble in aqueous EtOH solutions to form H * -aggregates. The absorption band of the H * -aggregates shows a hypsochromic shift of 192 nm (5291 cm -1 ) relative to the absorption maximum of dye monomers (706 nm) and has a full width at half maximum of 21 nm (797 cm -1 ). The morphology of the H * -aggregates of the indotricarbocyanine dye is studied for the fi rst time. It is found that the aggregates are rod-like species ~10 nm high, 100 nm wide, and several micrometers long. H-aggregates with a fl uorescence maximum at 560 nm and Stokes shift of 325 cm -1 in addition to non-fl uorescent H * -aggregates form in aqueous EtOH solutions and are nanoparticles with a height of 1-3 nm and lateral dimensions of ~100 nm.

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.

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