Ultrafast fluorescence quenching by electron transfer and fluorescence from the second 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 fluorescence quenching using femtosecond up-conversion spectroscopy. The measurements were made in a quencher concentration range of 0.15 M-l M and also in a neat quencher solvent. Fluorescence decay at times longer than 5 ps can be explained by combining the diffusion equation with the Marcus 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-cyanoanthracene, CA and 9,10-dicyanoanthracene) and the quenchers (N,N-dimethylaniline, DMA) were found to form "weak" CT complexes. Fluorescence from the S-1 state of the CA-DMA complex was detected by steady state spectroscopy. The excitation spectrum observed at the maximum intensity of this fluorescence indicates the existence of an excited S-2 State of 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 excitation of the CT complex to the S-2 state. It was concluded that the fast component is the fluorescence from the S-2 State of the complex. This 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 competition between the radiative transition S-2-->S-0 and the nonradiative internal conversion S-2-->S-1. The fast S-2-->S-1 nonradiative transition can be regarded as a charge separation process. (C) 1998 American Institute of Physics.