SIZE DEPENDENCE OF EXCITED-STATE DYNAMICS FOR J-AGGREGATES AT AGBR INTERFACES

The dependence of fluorescence lifetime and relative quantum yield on temperature and aggregate size has been investigated for the J-aggregate of pseudoisocyanine (PIC) on an AgBr surface, varying the average physical size of the aggregate in a statistical sense by diluting it with a close structural analogue. The dominant feature controlling the excited-state dynamics is found to be energy transfer to a defect state which is nonradiative at room temperature. The rate of this transfer process increases with aggregate size. At large aggregate sizes, a weak superradiant enhancement of the J-aggregate radiative rate is also observed, with a temperature dependence which suggests strong coupling of the J-aggregate exciton to a low-frequency phonon. Since both the energy transfer to the defect state and the radiative decay compete with the desired process of electron transfer from the aggregate excited state to the AgBr conduction band, the sensitizing efficiency of the J-aggregate is expected to decrease with increasing aggregate size. Measurement of this size-dependent sensitizing efficiency shows a smaller loss than expected, indicating that the electron-transfer rate from the aggregate excited state to the AgBr conduction band increases with increasing aggregate size.