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Solvent effects on the structure of the triplet excited state of xanthone: a time‐resolved resonance Raman study
Author(s) -
Ravi Kumar Venkatraman,
Umapathy Siva
Publication year - 2016
Publication title -
journal of raman spectroscopy
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.748
H-Index - 110
eISSN - 1097-4555
pISSN - 0377-0486
DOI - 10.1002/jrs.4954
Subject(s) - chemistry , raman spectroscopy , excited state , resonance raman spectroscopy , resonance (particle physics) , triplet state , absorption band , photochemistry , solvent effects , solvent , acetonitrile , absorption (acoustics) , analytical chemistry (journal) , molecule , atomic physics , materials science , organic chemistry , optics , physics , composite material
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 (TR 3 ) 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 (a 2 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 1 3 ππ* (1 3 A 1 ) and 1 3 nπ* (1 3 A 2 ) excited states. This was further confirmed by depolarization ratio measurements of some of the representative TR 3 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 1 3 ππ* 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.