Microviscosity inside a nanocavity: A femtosecond fluorescence up-conversion study of malachite green

Femtosecond fluorescence up-conversion measurements of malachite green (MG) have been carried out to confirm the relaxation pathway and subsequently to probe the microviscosity of water trapped in a nanoconfined environment using an AOT (sodium dioctylsulfosuccinate, aerosol-OT) reverse micelle as a model system. The experimental results reveal a strong dependence of S₁ state relaxation dynamics of MG on solvent viscosity while a very weak dependence has been observed for the S₂ state relaxation. The time-dependent density functional theory (TD-DFT) calculations have been used to construct potential energy surfaces of MG by pursuing an intramolecular rotation along the torsional coordinate of the phenyl rings. On synchronization with the experimental observations, the computational results comprehend the existence of a conical intersection along the S₁ and S₀ potential energy surfaces, which leads to mixed vibrational levels of S ₁ and S₀ characteristics. The results suggest that the conical intersection is along the torsional coordinate of N,N-dimethyl substituted phenyl ring. Correlating the observed dynamics of MG in a confined system with the relaxation time of MG in different glycerol-water mixtures, we assert the determination of the microviscosity of water inside the AOT reverse micelle. The data confer that the microviscosity of water in an AOT water pool of w₀ = 2 (9 cP) is almost 9 times higher than the bulk water. As we increase the w₀ from 2 to 40, the microviscosity decreases monotonically to 5.68 cP, and the decrease is observed to be exponential in nature.