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Transition from Charge‐Transfer to Largely Locally Excited Exciplexes, from Structureless to Vibrationally Structured Emissions
Author(s) -
Young Ralph H.,
Feinberg Adam M.,
Dinnocenzo Joseph P.,
Farid Samir
Publication year - 2014
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1111/php.12380
Subject(s) - excited state , charge (physics) , atomic physics , materials science , chemical physics , photochemistry , chemistry , physics , quantum mechanics
Exciplexes of 9,10‐dicyanoanthracene ( DCA ) with alkylbenzene donors in cyclohexane show structureless emission spectra, typical of exciplexes with predominantly charge‐transfer ( CT ) character, when the donor has a relatively low oxidation potential ( E ox ), e.g . hexamethylbenzene ( HMB ). With increasing E ox and stronger mixing with a locally excited ( LE ) state, vibrational structure begins to appear with 1,2,3,5‐tetramethylbenzene and becomes prominent with p ‐xylene ( p ‐Xy). A simple theoretical model reproduces the spectra and the radiative rate constants, and it reveals several surprises: Even in this nonpolar solvent, the fractional CT character of a highly mixed exciplex varies widely in response to fluctuations in the microscopic environment. Environments that favor the LE (or CT ) state contribute more to the blue (or red) side of the overall spectrum. It is known that sparsely substituted benzene radical cations, e.g ., p ‐Xy •+ , are stabilized more in acetonitrile than the heavily substituted HMB •+ . Remarkably, ion pairing with DCA •– in cyclohexane leads to even larger differences in the stabilization of these radical cations. The spectra of the low‐ E ox donors are almost identical except for displacements that approximately equal the differences in E ox , even though the exciplexes have varying degrees of CT character. These similarities result from compensation among several nonobvious, but quantified factors.