Photophysics of trioxatriangulenium ion. Electrophilic reactivity in the ground state and excited singlet state

Abstract

Trioxatriangulenium (TOTA⁺, 4,8,12-trioxa-4,8,12,12c-tetrahydro-dibenzo[cd,mn]-pyrenylium) is a closed shell carbenium ion, which is stable in non-nucleophilic polar solvents at ambient temperatures. In alcohols, small quantities of the leuco ether are formed in a reversible reaction. The physical and chemical properties of the excited singlet state of the trioxatriangulenium (TOTA⁺) carbenium ion are investigated by experimental and computational means. The degeneracy of the lowest excited states is counteracted by Jahn–Teller-type distortion, which leads to vibronic broadening of the long wavelength absorption band. A strong fluorescence is observed at 520 nm (τ_fl = 14.6 ns, ϕ_fl = 0.12 in deaerated acetonitrile). The fluorescence is quenched by 10 aromatic electron donors predominantly via a dynamic charge transfer mechanism, but ground state complexation is shown to contribute in varying degrees. Quenching is also observed in the presence of halide ions. Quenching rate constants are derived from lifetime measurements while charge transfer (CT) complex formation constants follow from the steady-state Stern–Volmer plots. CT-complex formation with three discogenic triphenylenes is studied separately. Phosphorescence spectra, triplet lifetimes, and triplet–triplet absorption spectra are provided. In the discussion, TOTA⁺ is compared to the unsubstituted xanthenium ion and its 9-phenyl derivative with respect to the excited state properties.