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-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 DṡA^* (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.