Quenching of diphenylmethyl radical fluorescence by cyanoaromatics and phenols

Abstract

We have used double-laser nanosecond laser flash photolysis to study the kinetics of bimolecular quenching of the lowest doublet excited state of the transient diphenylmethyl radical ( 2 Ph 2 CH Á ) in acetonitrile at 25°C. We chose a series of cyanoaromatic compounds as acceptor-type quenchers and a series of 4-substituted phenols as donor-type quenchers. The observed bimolecular quenching rate constants (k Q ) are in the range 2 9 10 6 -2 9 10 10 M -1 s -1 for cyanoaromatics and 1 9 10 5 -3 9 10 9 M -1 s -1 for phenols. The Hammett plots of the k Q values for these two series have positive and negative slopes, respectively, and establish the enhanced behavior of the photo-excited diphenylmethyl radical as both a donor and an acceptor. For cyanoaromatics, the trend in the dependence of 2 Ph 2 CH Á * quenching rate constants on the free energy change of electron transfer (DG el ) coincides well with that of RehmWeller data on acceptor-type and donor-type quenchers of singlet excited states. An approximate fit of the k Q data for cyanoaromatics to the Agmon-Levine model of electron transfer gave a value of 4.5 kcal mol -1 for the free energy of activation at DG el = 0. For phenols, the trend in the dependence of k Q on the free energy change of electron transfer deviates significantly from that of cyanoaromatics data. This deviation was traced to the use of irreversible oxidation potentials of phenols to calculate DG el . No isotope effect is observed from substitution of phenolic hydrogen by deuterium, i.e., k Q,H /k Q,D & 1. Phenolate ions quench 2 Ph 2 CH Á * with large rate constants, 1-2 9 10 10 M -1 s -1 , i.e., well in the limit of diffusion control.

Visible Light‐Induced Sulfonylation/Arylation of Styrenes in a Double Radical Three‐Component Photoredox Reaction

“…The desired product 1 was obtained in appreciable yields (61 % and 63 %) when employing either phenanthrene (Phen, 50 mol %) or fac ‐Ir(ppy) 3 (2 mol %) as strongly reducing photoredox catalysts ( E 1/2 ([Ir IV (ppy) 3 ] + / 3 [Ir III (ppy) 3 ]*) = −1.73 V, E 1/2 (Phen . + / 1 [Phen]*) = −2.07 V, E 1/2 (4‐CP/4‐CP .− ) = −1.66 V, all values in MeCN vs. SCE). In further optimization studies (Supporting Information, Tables S2 and S3), the yield of 1 could be increased to 94 % with concomitant reduction of the catalyst loading to 1 mol %.…”

“…Upon excitation by visible light, fac ‐Ir(ppy) 3 is converted to a long‐lived (1.9 μs emission lifetime in degassed MeCN at 25 °C) 3 MLCT‐state being a strong reductant ( E 1/2 ([Ir IV (ppy) 3 ] + / 3 [Ir III (ppy) 3 ]*) −1.73 V vs. SCE) . Oxidative quenching by a heteroaromatic nitrile such as 4‐CP ( E 1/2 (4‐CP/4‐CP .− )=−1.66 V) should be thermodynamically feasible. Indeed, the luminescence of the excited catalyst is quenched efficiently by this nitrile but not by TolSO 2 Na (Supporting Information, Figure S5).…”

Density Functional Theory and ab Initio Computational Evidence for Nitrosamine Photoperoxides: Hammett Substituent Effects in the Photogeneration of the Nitrooxide Intermediate

“…For example, Hammett studies have been correlated in photolysis of arylchlorodiazirine and their lifetimes , and fluorescence emission wavelengths of arylamidazopyridines . Hammett studies have also been used to understand fluorescence quenching of diphenylmethyl radical , thermoluminescence , organotelluride‐catalyzed photooxidation of phosphites , fluorescence and photoisomerizaion of p ‐nitrophenyl‐substituted ethenylindoles , photophysical properties of perylene diimides , photocatalyzed cadmium sulfide‐mediated isomerization of cis ‐4‐substituted stilbenes , photophysical properties of monomeric Ru II complexes , photodecarbonylation in crystalline 1,3‐diarylacetones , photosensitized oxidation of 4‐substituted thioanisoles and the photochemistry of substituted phthalimides .…”

Visible Light-Induced Sulfonylation/Arylation of Styrenes in a Double Radical Three-Component Photoredox Reaction

“…First, the difunctionalization of styrene (2.0 equiv) using p-TolSO 2 Na (2.0equiv) and 4-CP (1.0 equiv) in aqueous MeCN with some common photoredox catalysts was investigated (SupportingI nformation,T ableS1). The desired product 1 was obtained in appreciable yields (61 %a nd 63 %) when employing either phenanthrene (Phen, 50 mol %) or fac-Ir(ppy) 3 (2 mol %) as strongly reducing photoredox catalysts (E 1/2 ([Ir IV (ppy) 3 ] + / 3 [Ir III (ppy) 3 ]*) = À1.73 V, [8a] E 1/2 (PhenC + / 1 [Phen]*) = À2.07 V, [21] E 1/2 (4-CP/4-CPC À À ) = À1.66 V, [22] all values in MeCN vs. SCE). In further optimization studies (Supporting Information, Ta bles S2 and S3), the yield of 1 could be increased to 94 %w ith concomitant reduction of the catalystl oading to 1mol %.…”

“…Upon excitation by visible light, fac-Ir(ppy) 3 is converted to al ong-lived( 1.9 mse mission lifetimei n degassed MeCN at 25 8C) [26] 3 MLCT-state being as trong reductant (E 1/2 ([Ir IV (ppy) 3 ] + / 3 [Ir III (ppy) 3 ]*) À1.73 Vv s. SCE). [8a] Oxidative quenching by ah eteroaromatic nitrile such as 4-CP (E 1/2 (4-CP/ 4-CPC À À ) = À1.66 V) [22] should be thermodynamically feasible. Indeed, the luminescence of the excited catalyst is quenched efficiently by this nitrile but not by To lSO 2 Na (Supporting Information, Figure S5).…”