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†

Abstract

Total rate constants of decay (k(t)) as a function of temperature from -45 to +65 degrees 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 S1. This reactive intermediate returns to starting material faster than it rearranges, and therefore an activated internal conversion is a major pathway for deactivation of S1.

Steric effects in the phototransposition reactions of dialkylbenzenes

“…T-23 is particularly obvious because of the very low A and E a values obtained. Similar low values of A (~10 8 s -1 ) and E a (<10 kJ/mol) have been reported previously for activated intersystem crossing (23,25). Therefore, the k r value obtained from eq.…”

“…The first conclusion to be reached, one that reinforces our previous reports (22,23), is the general one that although the k r process results in significant quantum yields, the actual quantum yields of reaction are considerably smaller. Therefore, internal return of the prefulvene biradical 1 (Scheme 1) is one of the dominant modes of deactivation of S 1 for substituted benzenes.…”

“…Recently, we have shown that the measurement of excited singlet state lifetimes (τ) or quantum yields of fluorescence (Φ f ) as a function of temperature provide a simple method for obtaining rate constants of reaction and other modes of decay of the excited singlet states of substituted benzene derivatives (22,23). This method has also been used very effectively by the laboratories of Lewis for arylalkenes (24) and Zachariasse for arylamines (25).…”

“…103 (13), 102 (10), 91 (12), 90 (18), 89 (60), 88 (27), 87 (22), 86 (11), 77 (18), 76 (23), 74 (17), 71 (72), 65 (22), 64 (25), 63 (61), 62 (38), 61 (14), 60 (26), 59 (16), 57 (43), 52 (15), 51 (45), 50 (21). HR-MS calcd.…”

A detailed kinetic study of the direct photooxidation of 2,4,6-trichlorophenol

“…As the temperature increased from 5.0 to 35.0 C, the rate of TCP degradation increased to 2.14-fold. Following some literature precedents [36][37][38][39][40][41], this dependence was interpreted by assuming that both k 1 and k 2 have Arrhenius type temperature dependencies with pre-exponential factors A 1 and A 2 and activation energies of E a1 and E a2 . The temperature dependence of the quantum yield is simply interpreted in this line of thought as follows:…”