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

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 NO 2 ) 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.

Computational and spectroscopic studies of organic mixed-valence compounds: where is the charge?

“…Due to their neutral character, it is expected that solvent effects may be less pronounced for radicals 11-17 than for the cations 1-10. The first experimental studies on such unsymmetrical compounds have been carried out recently, 57,101 with particular emphasis on 11, 12 and 13. As the donor is clearly on the triarylamine side, these systems are best represented as localized class II cases, possibly with only one minimum along the ET coordinate.…”

Organische gemischtvalente Verbindungen: ein Spielplatz für Elektronen und Löcher

“…[92] Die diabatische Dipolmomentdifferenz Dm 12 kann aus Gleichung (10) [271] Maxima der IV-CT-Absorptionsbanden,ñ max (abs), und Emissionsbanden, n max (em), sowie durch Bixon-Jortner-Anpassung der Absorptions-(abs) und Emissionsbanden (em) in Cyclohexan bestimmte ET-Parameter. [ [270] extrahierte Übergangsmomente m ab , durch EAOM bestimmte adiabatische Dipolmomentdifferenzen Dm ab , durch Gleichung (10) Die ET-Dynamik von Verbindung 207 wurde in verschieden polaren Lçsungsmitteln durch zeitaufgelçste transiente Spektroskopie im Femtosekundenbereich untersucht. [273] Diese Studien ergaben eine dreifach hçhere BETGeschwindigkeitskonstante in Benzonitril als in Hexan.…”

Luminescence, Stability, and Proton Response of an Open‐Shell (3,5‐Dichloro‐4‐pyridyl)bis(2,4,6‐trichlorophenyl)methyl Radical

“…[2] While almost all known luminescent molecules are of closed-shell nature in the ground state, the characteristic luminescent properties of organic radicals with open-shell character have attracted interest recently. [3,4] Open-shell luminescent materials exhibit luminescence at long wavelengths without extended pconjugated structures. Monoradicals (or triplet biradicals) display a spin-allowed fluorescent transition from the doublet (or triplet) lowest excited state to the doublet (or triplet) ground state.…”