Yasutake Takahashi

Ultrafast charge separation and exciplex formation induced by strong interaction between electron donor and acceptor at short distances

We have measured the decay of fluorescence from the electron acceptors (A) dissolved in donor (D) solvent by femtosecond up-conversion spectroscopy with a time resolution of 75 fs. The measurements have been made for several acceptors (9-cyanoanthracene, CA; 9,10-dicyanoanthracene, DCA; and 1,2,9,10-tetracyanoanthracene, TCA) in the donor solvent (N,N-dimethylaniline, DMA, and aniline, ANL). The decay times obtained are between 150 fs (CA–ANL) and 280 fs (TCA–DMA). Observation of the decay of acceptor fluorescence and the concurrent rise of fluorescence from the exciplex state indicates that the excited acceptor (A*) state directly relaxes to the exciplex state. Charge separation rates (kCS) determined from the decay of the acceptor fluorescence lie between 3.6×1012–6.7×1012/s for six combinations of donors and acceptors. Very weak energy gap (ΔE) dependence of kCS was observed in the region of ΔE of 0.36–1.47 eV. This relation between kCS and ΔE is quite different from the charge recombination rate (kCR)...

A comparative study on electron-transfer fluorescence quenching by aliphatic and aromatic amines

Abstract The electron-transfer (ET) fluorescence quenching of aromatic compounds by aliphatic amines (DABCO, ABCO, and triethylamine) was studied in acetonitrile. The fluorescence quenching rate constant kq suddenly decreases when the free energy change (ΔGfet ) of full ET in fluorescence quenching increases from −0.6 to −0.4xa0eV, indicating that the downhill region shifts by ca. −0.5xa0eV compared with the Rehm–Weller plot, where both fluorescer and quencher are aromatic. The fluorescence quenching distance rq increases with decreasing ΔGfet and the rate kbet of back ET within geminate radical pairs shows a bell-shaped dependence on the free energy change (ΔGbet) of back ET. Both free energy dependence of rq and kbet are the same as those of aromatic electron donor and acceptor system.