Intersystem Crossing Rates in Photoexcited Rose Bengal: Solvation versus Isolation

Area of Science:

• Photochemistry
• Physical Chemistry
• Spectroscopy

Background:

• Rose Bengal (RB) is a photosensitizer with applications in photodynamic therapy.
• Understanding its photophysical properties, particularly intersystem crossing (ISC), is crucial for optimizing its use.
• Comparing RB in solution versus isolated ions provides insights into environmental effects on photophysics.

Purpose of the Study:

• To compare the intersystem crossing (ISC) rate of Rose Bengal (RB) in aqueous solution with gas-phase RB anions.
• To investigate the excited-state dynamics and lifetimes of isolated RB-derived anions and dianions.
• To elucidate the role of solvation and ion structure on RB photophysics.

Main Methods:

• Time-resolved photoelectron spectroscopy (TRPES) for mass-selected gas-phase anions.
• Transient absorption spectroscopy (TAS) for RB in aqueous solution (pH 12).
• Femtosecond temporal resolution (ca. 50 fs) for both techniques.

Main Results:

• The singly deprotonated RB monoanion ([RB-H]⁻) showed an S₁ lifetime of ~80 ps.
• The solution ensemble (primarily solvated dianions) exhibited a similar S₁ lifetime of ~70 ps.
• The isolated doubly deprotonated RB dianion ([RB-2H]²⁻) displayed a significantly longer S₁ lifetime.
• Both solution and gas-phase species showed oscillatory features attributed to librational motion.

Conclusions:

• Solvation significantly influences the intersystem crossing rate and excited-state lifetime of Rose Bengal.
• Isolated RB dianions exhibit distinct photophysical behavior compared to their solvated counterparts.
• Librational oscillations play a role in the excited-state relaxation dynamics of RB species.
• Findings aid in interpreting solution-phase measurements and understanding RB photophysics.