Apparent vibrational side bands in rr-conjugated systems: the case of distyrylbenzene

The photoluminescence sPLd spectra of dilute solution and single crystals of distyrylbenzene show unique temperature-dependent vibronic structures. The characteristic single frequency progression at high temperatures is modulated by a low-frequency progression series at low temperatures. None of the series side-band modes corresponds to any of the distyrylbenzene Raman frequencies. We explain these PL properties using a time-dependent model with temperature-dependent damping, in which the many-mode system is effectively transformed to two "apparent" modes and then to a single apparent mode as damping increases.