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

Abstract

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–1 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 ∼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 S1 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 S2 state of the CT complex near 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 S2 state. It was concluded that the fast component is the fluorescence from the S2 state of the complex. This was confirmed by the concurrent rise of the S1 fluorescence of the CA-DMA complex. The fast decay of ∼250 fs is caused by the competition between the radiative transition S2→S0 and the nonradiative internal conversion S2→S1. The fast S2→S1 nonradiative transition can be regarded as a charge separation process.