Solvent effects on the structure of the triplet excited state of xanthone: a time-resolved resonance Raman study

Abstract

Solvent effects, especially intermolecular hydrogen bonding, play a central role in the photophysics and photochemistry of aromatic ketones. To gain insight into the solute–solvent interactions and their implications for structure and reactivity, we studied xanthone (XT) in two different solvents of similar dipolarity: acetonitrile (ACN; aprotic) and methanol (MeOH; protic), using time‐resolved resonance Raman (TR3) spectroscopy in conjunction with time‐dependent density functional theory calculations. Raman excitation profiles of XT in ACN followed the triplet‐triplet absorption band with a shoulder at the blue end, but for MeOH, they followed the triplet‐triplet absorption band quite closely; therefore, we propose that the resonance enhancement of Raman peaks are from two states in ACN and from a single state in the MeOH solvent. Furthermore, a resonance Raman peak at 614 cm−1 (a2 symmetry) that appeared in ACN but not in the MeOH solvent has been identified as a vibronic active mode that could be involved in coupling the two lowest 13ππ* (13A1) and 13nπ* (13A2) excited states. This was further confirmed by depolarization ratio measurements of some of the representative TR3 peaks in ACN, which showed a depolarized intensity for the 614 cm−1 peak while the other peaks were polarized. Interestingly, we also observed blue shifting of some of the vibrational frequencies of XT in the 13ππ* state compared with the ground state with increasing solvent polarity. This anomalous blue shift casts doubt on the general use of the resonance canonical structure to explain the structure of the excited states. In summary, we propose that the different hydrogen bonding mechanisms exhibited by the two lowest triplet states of XT separate them further in energy and that this can contribute to its low reactivity towards H atom abstraction in protic solvents. Copyright © 2016 John Wiley & Sons, Ltd.

Solvent Effects on Ultrafast Photochemical Pathways

“…Comparison of the early time kinetics obtained in CD 3 OD (τ ISC = 2.5 ps) and CH 3 OH (τ ISC = 1.7 ps) solvents reveals the role of fluctuating hydrogen bonding in populating the T 2 ( 3 ππ*) state. The S 1 ( 1 nπ*) and T 2 ( 3 ππ*) states exhibit hydrogen bond weakening and strengthening, ,− respectively, which is to say the S 1 state is destabilized in energy and the T 2 state is stabilized by intermolecular H-bonds to methanol. The consequent breaking of the near degeneracy of the S 1 and T 2 states of Bzp induced by accepting a H-bond is proposed to slow the ISC rate, which becomes more favorable again when the transient H-bond breaks.…”

Intermolecular Hydrogen Bonding Controlled Intersystem Crossing Rates of Benzophenone

“…Detailed descriptions of the various experimental methods adopted in this work are published elsewhere, ,− and the Supporting Information provides a brief summary of these and the computational methods used. To verify that REEE applies to Bzp in methanol, we carried out resonance Raman spectroscopy experiments because they show selective enhancement of the vibrational modes coupled to the chromophore upon resonant electronic excitation. ,, Figure S1 of the Supporting Information shows resonance Raman spectra of Bzp in methanol at several different excitation wavelengths across the π* ← n carbonyl band. The spectra reveal two carbonyl peaks corresponding to distinct solvation states, namely, the DC (for excitation to longer wavelengths in band I) and the HBC (to shorter excitation wavelengths).…”

“…The S 1 /T 1 (nπ*) and the T 2 (ππ*) states of Bzp and similar aromatic ketones ,,, undergo hydrogen bond weakening and strengthening, respectively, compared to the ground state (S 0 ). Hydrogen bonding destabilizes the S 1 state, and photoexcitation of Bzp with a HBC to the S 1 state will induce reorganization along the HBC solvation coordinate.…”

Visible-Light Photocatalytic E to Z Isomerization of Activated Olefins and Its Application for the Syntheses of Coumarins

“…It is likely that these solvents stabilize the radical cation intermediate involved, while acetonitrile helps to elongate the lifetime of triplet excited states [25][26][27][28][29] or stabilize biradical intermediates in an energy transfer mechanism as suggested for our work. Acetone, a typical solvent which can be used as a sensitizer, was not effective enough by itself to achieve good transformation in this reaction.…”