Time-Resolved Fluorescence Monitoring of Aromatic Radicals in Photoinitiated Processes

Abstract

Photolytic initiation of free radical reactions is important to many areas of technology; time-resolved monitoring of submicromolar concentrations of radicals produced during the course of these reactions is needed to provide information about the rate of initiation and its competition with radical recombination. In this work, time-resolved laser-induced fluorescence is evaluated for monitoring of diphenylketyl radicals produced by photoreduction of the triplet state of benzophenone. Fluorescence from the doublet-doublet transition of the radical is excited with a continuous wave laser and provides a sensitive method to detect these intermediates at nanomolar concentrations and to study their kinetics in solution on time scales from a few microseconds to hundreds of milliseconds. The ketyl radical fluorescence measurements of radical initiation reactions allowed the H atom abstraction rate constant by triplet benzophenone from both 2-propanol and benzhydrol to be determined, where k(H) = (2.1 ± 0.1) × 10(6) M(-)(1) s(-)(1) for 2-propanol and k(H) = (4.4 ± 0.1) × 10(6) M(-)(1) s(-)(1) for benzhydrol. The diphenylketyl radical recombination rate constant was also determined by time-resolved fluorescence monitoring of the decay of the radical population and found to be k(r) = (1.9 ± 0.2) × 10(8) M(-)(1) s(-)(1). Formation kinetics could be measured on a microsecond time scale from radical populations as low as 45 nM; decay kinetics could be followed on a millisecond time scale from 20 nM radical concentrations.

Reaction Kinetics at Dispersed-Colloid/Solution Interfaces: Benzophenone Triplet-State Quenching by Methylated Silica Particles

“…Subtraction of the probe-only and pump-only transients was necessary to resolve the ketyl radical fluorescence from triplet-benzophenone phosphorescence and background fluorescence. 33 Photoexcited ketyl radical fluorescence was collected at the lowest colloid concentration, 2.7 mg/mL, to avoid excessive laser scattering.…”

“…The excited-state decay kinetics are compared to homogeneous free-solution quenching of the triplet state by methanol and hexamethyldisiloxane, and the rate constant for H-atom abstraction from methyl groups bound to the colloid surface by triplet benzophenone is determined. To check the mechanism of quenching and yield of free radicals, the diphenylketyl radical was detected using time-resolved fluorescence, 33 and rates of ketyl-radical recombination were measured.…”

Monitoring the Formation and Decay of Transient Photosensitized Intermediates Using Pump-Probe UV Resonance Raman Spectroscopy. II: Kinetic Modeling and Multidimensional Least-Squares Analysis

“…Depletion of triethylamine radical (TEAx ) proceeds similarly: Of the rate constants of the radical decay processes described in Eq. 5, only the benzopinacol formation rate constant (k ra 5 2.0 (60.6) 3 10 8 M 2 1 s 2 1 ) is found in the literature, 30,31 m ost recently from benzophenone ketyl radical uorescence decay experiments in acetonitrile containing dilute benzhydrol as the photoreducing agent. 31 Therefore, application of Eqs.…”

“…5, only the benzopinacol formation rate constant (k ra 5 2.0 (60.6) 3 10 8 M 2 1 s 2 1 ) is found in the literature, 30,31 m ost recently from benzophenone ketyl radical uorescence decay experiments in acetonitrile containing dilute benzhydrol as the photoreducing agent. 31 Therefore, application of Eqs. 12 and 13 to the data presents a challenge as two separate recombination rate constants (k rb , k rc ) as well as the initial benzophenone triplet concentration ([ 3 BP*] t5 0 ) are not accurately known and must be determined from the data.…”

Monitoring the Formation and Decay of Transient Photosensitized Intermediates Using Pump-Probe UV Resonance Raman Spectroscopy. I: Self-Modeling Curve Resolution

“…Flash photolysis measurements reveal broad electronic absorption bands of excited-state or radical intermediates. [21][22][23] Measuring excited doublet-state uorescence from aromatic radicals 24 allows the detection of low concentrations of transient aromatic radical intermediates. Measurem ents based on electronic transitions do not reveal structural inform ation about the intermediates and the broad spectral features make the resolution of com ponent behavior dif cult.…”