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

Kumar, Venkatraman Ravi and Umapathy, Siva (2016) Solvent effects on the structure of the triplet excited state of xanthone: a time-resolved resonance Raman study. In: JOURNAL OF RAMAN SPECTROSCOPY, 47 (10). pp. 1220-1230.

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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 614cm(-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)* (1(3)A(1)) and 1(3)n* (1(3)A(2)) 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 614cm(-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)* 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 (c) 2016 John Wiley & Sons, Ltd.

Item Type:	Journal Article
Publication:	JOURNAL OF RAMAN SPECTROSCOPY
Additional Information:	Copy right for this article belongs to the WILEY-BLACKWELL, 111 RIVER ST, HOBOKEN 07030-5774, NJ USA
Department/Centre:	Division of Chemical Sciences > Inorganic & Physical Chemistry
Date Deposited:	03 Dec 2016 10:14
Last Modified:	03 Dec 2016 10:14
URI:	http://eprints.iisc.ac.in/id/eprint/55417

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