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Temperature Effects, Arrhenius Activation Parameters and Rate Constants for the Photochemical Reactivity of Cyano, Boronato, Trifluoromethyl and Methoxy Substituted Toluenes in the Excited Singlet State †
Author(s) -
Gonzalez Carlos,
Pincock James
Publication year - 2006
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 - 10.1562/2005-04-27-ra-502
Subject(s) - trifluoromethyl , photochemistry , arrhenius equation , reaction rate constant , chemistry , excited state , reactivity (psychology) , singlet state , singlet oxygen , kinetics , activation energy , organic chemistry , oxygen , alkyl , atomic physics , physics , medicine , alternative medicine , quantum mechanics , pathology
Total rate constants of decay ( k t ) as a function of temperature from –45 to +65°c for the compounds 1 and 2 in AN and TFE and 3 and 4 in AN have been determined by fluorescence lifetime measurements. The data have been fit to an equation that assumes that the rate constants of fluorescence ( k f ) and intersystem crossing ( k isc ) are temperature independent, that k ic = 0 and that the rate constant of reaction ( k r ) is activated according to the Arrhenius expression. For compounds 1–3, values of k f and k isc were found to be independent of solvent for any given compound, but k r was consistently greater in TFE than AN. For the anisoles 4, the temperature effect was very small, indicating that k r did not compete with k f + k isc and suggesting that an activated intersystem crossing was the dominant temperature‐dependent process. The k r , A and E a values obtained for compounds 1–3 were rationalized in terms of their known photochemistry, phototransposition reactions in AN and photoadditions in TFE. The critical reactive intermediate in all cases is a bicycle[3.1.0]hexenyl biradical/zwitterion that is formed in an activated process from S 1 . This reactive intermediate returns to starting material faster than it rearranges, and therefore an activated internal conversion is a major pathway for deactivation of S 1 .