ULTRAFAST FLUORESCENCE QUENCHING BY ELECTRON-TRANSFER AND FLUORESCENCE FROM THE 2ND EXCITED-STATE OF A CHARGE-TRANSFER COMPLEX AS STUDIED BY FEMTOSECOND UP-CONVERSION SPECTROSCOPY

Photoinduced electron transfer dynamics between fluorescer (acceptor, A) and quencher (donor, D) was investigated by measuring the fluoresce nce quenching using femtosecond up-conversion spectroscopy. The measur ements were made in a quencher concentration range of 0.15 M-l M and a lso in a neat quencher solvent. Fluorescence decay at times longer tha n 5 ps can be explained by combining the diffusion equation with the M arcus equation of electron transfer. At higher quencher concentrations (>0.3 M), an additional component with a time constant of similar to 250 fs appears. At these concentrations, the fluorescers (9-cyanoanthr acene, CA and 9,10-dicyanoanthracene) and the quenchers (N,N-dimethyla niline, DMA) were found to form ''weak'' CT complexes. Fluorescence fr om the S-1 state of the CA-DMA complex was detected by steady state sp ectroscopy. The excitation spectrum observed at the maximum intensity of this fluorescence indicates the existence of an excited S-2 State o f the CT complex ni-ar the energy of D . A (the locally excited state of the pair). Excitation of CA at 400 nm leads to simultaneous excita tion of the CT complex to the S-2 state. It was concluded that the fas t component is the fluorescence from the S-2 State of the complex. Thi s was confirmed by the concurrent rise of the S-1 fluorescence of the CA-DMA complex. The fast decay of similar to 250 fs is caused by the c ompetition between the radiative transition S-2-->S-0 and the nonradia tive internal conversion S-2-->S-1. The fast S-2-->S-1 nonradiative tr ansition can be regarded as a charge separation process. (C) 1998 Amer ican Institute of Physics.