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

Abstract

Femtochemistry / Wavepacket Dynamics / Proton TransferUltrafast 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 S 1 -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 jetcooled conditions.

Direct Observation of Nuclear Motion during Ultrafast Intramolecular Proton Transfer

“…Due to the insensitivity of the ESIPT to details of the reaction center we conclude that the proposed mechanism should be quite general for compounds exhibiting an ultrafast ESIPT within a H-chelate ring. And indeed, the experimental results on DHAQ [30], BBXHQ [31], and 10-HBQ [32] are in agreement with the model.…”

“…11.10). A coherent excitation of very similar vibrational modes was found in all ESIPT molecules we investigated and also in DHAQ [30], BBXHQ [31], and 10-HBQ [32]. In HBT and HBO this mode also modulates the 361 Figure 11.11 Coherently excited vibrational modes of HAN.…”

“…11.6 and 11.7 as insets. This observation was also made for a number of other ESIPT compounds [30][31][32].…”

“…In the following the emphasis is put on HBT due to the extensive analyses available for this molecule. Meanwhile, however, similar dynamics have also been observed in 1,8-dihydroxyanthraquinone (DHAQ) [30], 2,5-bis(2¢-benzaoxazolyl)-hydroquinone (BBXHQ) [31], and 10-hydroxybenzo[h]quinoline (10-HBQ) [32]. The general features of ESIPT will be elaborated by comparison between HBT and the other compounds.…”

Infrared-induced coherent vibration of a hydrogen-bonded system: Effects of mechanical and electrical anharmonic couplings

“…Recently, several groups have studied excited-state intramolecular proton transfer ͑ESIPT͒ using UV and visible ultrashort pulses and observed excited-state coherent vibrations in the time domain. [42][43][44][45][46][47][48] In these studies, nuclear wavepacket motion due to lowfrequency skeletal vibrations remains even after proton transfer, and their role in the proton transfer process was discussed. Proton transfer reactions are closely related to hydrogen-bond interactions because these two phenomena involve similar nuclear coordinates, along which the hydrogen atom changes its position between the donor and the acceptor atoms.…”