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 (ESA) 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 ps 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.