摘要
Hemicyanines are currently widely used as photosensitizers in many biological applications. Understanding their excited state dynamics is essential in improving their performance. The photophysical properties of three hemicyanines—diphenyl aminostyryl pyridinium (DPPy), dianisole aminostyryl pyridinium (DAPy), and carbazolyl aminostyryl pyridinium (CBPy), including monomer and aggregate forms—were studied using time-resolved fluorescence techniques. Aggregation is found in less polar solvents and has superior fluorescence yields. The decay of the fluorescence intensity of these aggregates display a time component τR,A of 30–300 ps and is assigned to the non-radiative process—dissociation. The composition of the aggregates is determined from the fluorescence anisotropic decay: The time constant of the diffusive rotational relaxation is proportional to the volume of the rotational species. Accordingly, the major composition of aggregates is dimer. From density functional theory calculations, the structure of the dimer is proposed to be a head-to-tail anti-parallel π–π stacking configuration. The time-resolved fluorescence of the S1 state of monomer yields a third time component τR1,m ranging from 3 to 10 ps. This rapid decay component is tentatively assigned to a conical intersection. A fourth time component τR2,m (tens of ps) is assigned to the process induced by a twisting internal rotation. These two rapid pathways result in extremely low emission yields in monomer, whereas in dimer, they are hindered, resulting in high fluorescence quantum yields and longer lifetimes.
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•Dimer aggregates of hemicyanines in non-polar solvent yield high emission yield than monomer.•DFT calculations show a head-to-tail antiparallel H dimer structure.•Relaxation of the S1 monomer undergoes two rapid non-radiative decay processes.