Highly Fluorescent Open-Shell NIR Dyes: The Time-Dependence of Back Electron Transfer in Triarylamine-Perchlorotriphenylmethyl Radicals

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Abstract

Triarylamine-perchlorotriphenylmethyl radicals (TARA-PCTM) may be viewed as open-shell mixed valence donor−acceptor compounds that exhibit strong charge-transfer (CT) bands in the visible to NIR spectral region. While open-shell molecules generally do not fluoresce at RT, we observed a surprisingly strong fluorescence from the highly polar excited CT state of the TARA-PCTM radicals in the visible and NIR spectral region. The fluorescence quantum yield is enhanced by a factor of up to 150 compared to the unsubstituted perchlorotriphenylmethyl radical. The enhancement depends on the donor strength of the TARA moiety which was tuned by small substituents (OMe, Me, Cl, CN, and NO2) attached to the phenyl groups, thus forming a series of donor−acceptor species that mainly differ by the free energy difference of the excited CT state and the ground state. The reorganization parameters of the CT process were extracted by Bixon−Jortner fits to either the absorption or the fluorescence bands. The dynamics of the nonradiative back-electron transfer were investigated by time-resolved fluorescence and transient absorption spectroscopy in the ps to ns time regime. We observed a strong deviation of the back-electron transfer rate from the expected inverted Marcus behavior which might be due to anharmonic effects.

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