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We have carried out ultrafast pump−probe measurement of four TPM dyes, malachite green (MG), brilliant green (BG), crystal violet (CV), and ethyl violet (EV), with a time resolution of 30 fs. The pump−probe signal showed that solvent dependence arose first in the femtosecond time regime, e.g., the decay of n-butanol solution was clearly slower than the methanol solution just 50 fs after the initial photoexcitation. The signal decays in a multiexponential manner and the slower components showed stronger linear dependence on the solvent viscosity than did the faster components. We have also carried out temperature-dependent measurement of ethanol solution and calculated the activation energies from the Arrhenius plots of each components. The activation energies and effective volumes were larger for slower decays. The activation energy of the viscosity of ethanol was larger than that of the decay components of TPM dyes. These observations are explained with a combined effect of microviscosity and intramolecular relaxation. The lifetime of the transient absorption appearing at the red edge of the ground state absorption was longer than any of the reported lifetimes of the excited state absorption around 400 nm. Therefore, the red-edge absorption is assigned to the unrelaxed ground state molecule with the twisted phenyl group.

We investigated the pH-dependent fluorescence spectra, quantum yields, and lifetimes of firefly luciferin in aqueous solutions by varying the pH and excitation wavelength. Green fluorescence peaking at 540 nm and red fluorescence peaking at about 620 nm were detectable in the pH range between 1 and 10. The lifetime of the green fluorescence decreased significantly from about 5 to 0.5 ns with decreasing pH from 10 to 1, while the red fluorescence lifetime was almost constant, about 0.4 ns, in the measurable pH range between 1 and 4. The pH dependence of the greenfluorescence lifetime had analogous pH-dependence with that of the fluorescence quantum yield. This reflects the contribution of the emissionefficiency variation of green fluorescence due to the pH-sensitivity of a non-radiative decay that competes with a pH-insensitive radiative decay. #

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