Annihilation of singlet excitons in <i>J</i> aggregates of pseudoisocyanine (PIC) studied by pico- and subpicosecond spectroscopy

Abstract

The excited-state dynamics of J aggregates of PIC have been studied by means of picosecond and subpicosecond absorption spectroscopy as well as integrated fluorescence yield measurements. The results of these measurements show that both the lifetime and the fluorescence yield are strongly dependent on excitation pulse intensity. At relatively high light intensity (1014 photons cm−2 pulse−1) the S1 lifetime is essentially pulse limited (&amp;lt;1 ps) and the fluorescence yield is very low. Upon decreasing the light intensity a gradual increase of the excited-state lifetime and fluorescence yield is observed. At very low excitation intensity (1010 photons cm−2 pulse−1) a single exponential lifetime of 400 ps is observed. At intermediate intensities the excited-state decay is strongly nonexponential. The observed intensity dependence of the excited-state dynamics is attributed to efficient exciton–exciton annihilation between the highly mobile singlet excitons. By applying an expression for bimolecular exciton annihilation, derived and used for photosynthetic antenna systems [Paillotin et al., Biophys. J. 25, 513 (1979)], and treating the energy migration as a hopping motion, information about aggregate size and exciton hopping rate was obtained.