Optically Detected ESR (OD ESR) of ion-radical Pairs in coloured solutions. Observation of transienttrans-azobenzene radical cation

Trans-azobenzene dissolved in different liquid hydrocarbons absorbs fluorescence arising from all acceptors previously used in Fluorescence Detected Magnetic Resonance (FDMR) and Optically Detected ESR (OD ESR) spectroscopy making optical detection impossible. In this report a new acceptor,rubrene, having sufficient quantum yield of fluorescence in the red band 550–620 nm, has been proven successful. OD ESR spectra of the radical-ion pairtrans-azobenzene⁺/rubrene^? were detected in liquid squalane (2,6,10,15,19,23-hexamethyl-tetracosane) solution in the temperature range 294–243 K. The experimental isotropic hyperfine splittings of the radical cation oftrans-azobenzene (a _N=1.4 mT) have been compared with those from MNDO/INDO calculations and with those of earlier work using freon matrix studies.