Role of Skeletal and O-H Vibrational Motions in the Ultrafast Excited-State Relaxation Dynamics of Alizarin.

The role of two skeletal (C═C and C═O stretch) and O-H vibrational motions in the internal conversion dynamics associated with the coupled S(ππ*, A') -S(nπ*, A″) potential energy surfaces of alizarin are investigated theoretically. Quantum wavepacket dynamics simulations reveal a nonadiabatic population transfer from the "bright" S(ππ*, A') to "dark" S(nπ*, A″) state on a time scale of 10 fs. A detailed analysis of computed structural parameters, energetics, and time-dependent observables suggest that these vibrations promote the nonadiabatic dynamics before initiating the proton transfer process. We also discuss how the simultaneous evolution of multidimensional dynamics involving several vibrational degrees of freedom would increase the complexity, while analyzing the spectral and kinetic data of time-resolved spectroscopy measurements.