Ultrafast Proton-Transfer and Coherent Wavepacket Motion of Electronically Excited 1,8-Dihydroxyanthraquinone in Liquid Benzyl Alcohol Solution

Ultrafast pump-probe experiments with a time-resolution of 30 fs have been carried out to explore the non-radiative relaxation dynamics of electronically excited 1,8-dihydroxyanthraquinone (DHAQ) in polar liquid solution. The results are discussed in terms of a Lippincott-Schroeder double-minimum potential along the proton-transfer reaction coordinate for the ground (S₀) and first excited singlet states (S₁) of DHAQ. The 400-nm pump-visible probe data reveal a strongly Stokes-shifted stimulated emission due to the proton-transferred 1,10-quinone configuration in the S₁-state. A dominant fraction of this stimulated emission appears instantaneously implying that cross-well excitation directly from the ground-state 9,10-quinone form into the excited-state 1,10-quinone configuration takes place. A smaller fraction of the stimulated emission appears delayed with a time constant of approximately 300 fs. This component may be due to proton-transfer in the S₁-state following optical excitation. Further, a transient absorption is observed on the red edge of the linear absorption spectrum of ground-state DHAQ due to higher-lying electronic states presumably from the 1,10-configuration. Periodic modulations of the transient absorption due to wavepacket motion in the 9,10-quinone form are partially consistent with previous fluorescence excitation spectra of the molecule taken under jet-cooled conditions.