Continuous Tuning of Intersystem Crossing Times in Rose Bengal Water/Methanol Solutions

We use femtosecond transient broadband absorption spectroscopy (TAS) to characterize Rose Bengal in water/methanol solutions and reveal a continuous tunability of intersystem crossing (ISC) times by changing the mole fraction of the solvents. We find that the transients of excited state absorptions (ESAs) in Rose Bengal at ∼430 nm can be attributed to transitions from the singlet state S₁, with decay times of 74 ps via ISC in pure water and up to 405 ps in pure methanol. TA measurements at near-infrared wavelengths, on the other hand, reveal the rise of an ESA at ∼1080 nm from the triplet state T₁ with time constants of 68 and 491 ps in pure water and methanol, respectively, strongly supporting the associated UV-vis TAS data. Solvent mixtures show a quasi-linear rise of the ISC times with increasing mole fractions of methanol and indicate that Rose Bengal in varying solvent mixtures can be used as a model system to study their influence on excited state photophysics.