Mechanism of the Photodecomposition of Stable Triarylmethyl Radicals

Luminescent radicals, the vast majority of which are derivatives of tris(trichlorophenyl)methyl (TTM), are of significant recent interest because of the unique photophysical properties of the doublet excited state. Though they show high chemical stability, most trityl radicals show very poor photostability, which hinders their application as magnetic, optical and quantum-related materials. In this work, we use density functional theory to study the mechanism of photodegradation of TTM. We isolate the photodecomposition products and characterize them via mass spectrometry, NMR, EPR, UV-Vis absorption spectroscopy, cyclic voltammetry (CV), and X-ray crystallography. We show that the reaction proceeds by a 5-electron electrocyclization followed by an unusual 1,8-sigmatropic chloride shift, affording two fluorenyl radicals, which slowly oxidize and hydrolyze to form semiquinone products. We carefully examine the reported photostability of >80 substituted triarylmethyl radicals and demonstrate that other common triarylmethyl radicals, including benchmark luminescent derivatives with the highest photostability, the carbazole-appended TTMs, photodecompose through the same cyclization mechanism, and thus the DFT-calculated activation energy of cyclization can be used to guide the design of photostability in new luminescent triarylmethyl radicals.