Continuous Tuning\nof Intersystem Crossing Times in\nRose Bengal Water/Methanol Solutions

We use femtosecond transient broadband absorption spectroscopy\n(TAS) to characterize Rose Bengal in water/methanol solutions and\nreveal a continuous tunability of intersystem crossing (ISC) times\nby changing the mole fraction of the solvents. We find that the transients\nof excited state absorptions (ESAs) in Rose Bengal at ∼430\nnm can be attributed to transitions from the singlet state S<sub>1</sub>, with decay times of 74 ps via ISC in pure water and up to 405 ps\nin pure methanol. TA measurements at near-infrared wavelengths, on\nthe other hand, reveal the rise of an ESA at ∼1080 nm from\nthe triplet state T<sub>1</sub> with time constants of 68 and 491\nps in pure water and methanol, respectively, strongly supporting the\nassociated UV–vis TAS data. Solvent mixtures show a quasi-linear\nrise of the ISC times with increasing mole fractions of methanol and\nindicate that Rose Bengal in varying solvent mixtures can be used\nas a model system to study their influence on excited state photophysics.