Role of charge transfer and spin-orbit coupling in fluorescence quenching : a case study with oxonine and substituted benzenes

Abstract

Fluorescence quenching of oxonine in methanol was investigated by means of a computerized dye laser flash spectrometer for the ∆G°et, dependence of the quenching rate constant (kq) and the efficiencies of induced dye triplet formation (ηT), reduced dye radical formation (ηR), and induced internal conversion. A total of 34 substituted benzenes including 20 monohalogenated benzenes, toluenes, and anisoles were used as quenchers spanning a range of -0.85 ≤ ∆G°et ≤ 1.4 eV for a possible photoelectron-transfer reaction with the dye singlet as electron acceptor. Induced internal conversion was the dominating channel in all cases. Free-radical formation was less than 2.0% throughout. Induced triplet formation was only detectable with heavy-atom-substituted quenchers and with ∆G°et ≥ -0.13 eV such that the free energy of the radical pair was not below the triplet energy of oxonine. In spite of a wide variation of kq the efficiency ηT was almost constant for a set of quenchers with the same halogen substituent.