UV-Vis, fluorescence spectroscopy and time resolved spectroscopic technique on picosecond and femto- second scales have been utilized to investigate the effect of steric hindrance and multiple hydroxyl groups of solvents on the barrierless isomerization in excited state of 3, 3′- di(3-sulfopropyl)thiacyanine triethylaminium salt. The re-sults show that the factors mentioned above may be respon-sible for a different 搒hort-time behavior?of the bleaching signal on the ground state and nonlinear viscosity depend-ence of the excited-state lifetime in isopropanol and glycol. These phenomena are distinguished from the situation re-ported in the long-chained monohydroxyl alcohol solvents. However, the dye molecule still exhibits the analyzing- wavelength dependence of the observed fluorescence lifetime, that is, the fluorescence lifetime increases with the increasing of analyzing-wavelength. The conclusions are in agreement with those drawn from femtosecond upconversion fluores-cence experiments and further exemplify the barrierless isomerization model in the excited state.
As a structural analogue of adenine, 2-aminopurine (2Ap) is often used as a fluorescent probe to study the intramolecular charge transfer reaction in DNA. We have designed and synthesized a series of model DNA helix with the variation in the distance between the 2Ap probe and the GGG sequence, and have investigated, by means of picosecond time-resolved fluorescence spectroscopy, the effect of the length of the bridge (consisting of a number of inosines, I) separating the electron donor (???GGG???) and the acceptor (2Ap) on the charge transfer dynamics. The fluorescence dynamics of 2Ap exhibited three exponential decay components, the one with a time constant of a few hundred picoseconds is assigned to the intramolecular charge transfer from GGG to 2Ap. Within 2.4 nm of the donor-acceptor separation, the rate of charge transfer decreased exponentially upon increasing the separation, from which the decay factor β is determined to be 1.3 nrrf-1. Beyond 2.4 nm, however, the rate started toincrease, this abnormal behavior of charge transfer is interpreted in terms of the match of elec- tronic energies between the l-bridge and the donor/acceptor couple.
