Observation of Long-Lived Charge-Separated States in Anthraquinone-Phenothiazine Electron Donor-Acceptor Dyads: Transient Optical and Electron Paramagnetic Resonance Spectroscopic Studies

We prepared a series of phenothiazine (PTZ)-anthraquinone(AQ) electron donor-acceptor dyads to study the relationshipbetween molecular structures and the possibility of charge transfer(CT) and intersystem crossing (ISC). As compared to the previouslyreported PTZ-AQ dyad with a direct connection of twounits via a C-N single bond, the PTZ and AQ units are connectedvia a p-phenylene or p-biphenylenelinker. Conformation restriction is imposed by attaching ortho-methyl groups on the phenylene linker. UV-vis absorptionspectra indicate electronic coupling between the PTZ and AQ unitsin the dyads without conformation restriction. Different from thepreviously reported PTZ-AQ, thermally activated delayedfluorescence (TADF) is observed for the dyads containing one phenylenelinker (PTZ-Ph-AQ and PTZ-PhMe-AQ). Theprompt fluorescence lifetime in cyclohexane is exceptionally long(& tau;(PF) = 62.0 ns, population ratio: 99.2%) and 245.0ns (93.5%) for PTZ-Ph-AQ and PTZ-PhMe-AQ, respectively (normally & tau;(PF) <20 ns); the delayedfluorescence lifetimes for these two dyads were determined as & tau;(DF) = 2.4 & mu;s (6.5%) and 7.6 & mu;s (0.8%), respectively.For the dyad containing a biphenylene linker (PTZ-Ph ( 2 ) Me-AQ), no TADF was observed. Charge-separated(CS) states were observed for PTZ-Ph-AQ and PTZ-PhMe-AQ, and the lifetimes were determined as 7.0 and 1.3 & mu;s, respectively,indicating the triplet spin multiplicity of the CS state. The (CS)-C-3 state lifetimes are shortened to 100 ns and 440 ns forthe two dyads, respectively, in the polar solvent acetonitrile. Fordyads with a longer linker, i.e., PTZ-Ph ( 2 ) Me-AQ, the CS state lifetime is not sensitiveto solvent polarity (& tau;(CS) = 1.8 and 1.3 & mu;s incyclohexane and acetonitrile, respectively). In reference dyads, wherethe PTZ unit is oxidized to sulfoxide, no CT absorption band and TADFwere observed, which is attributed to the increased CS state energy(>3 eV) becoming higher than that of the AQ triplet ((3)AQ*)state (ca. 2.7 eV). These experimental evidence show that the presenceof (CS)-C-1, (CS)-C-3, and (LE)-L-3 (LE: locallyexcited) states sharing similar energy is essential for the occurrenceof TADF. Population of the long-lived (CS)-C-3 state (with alifetime of a few & mu;s) does not produce by itself TADF, becausethe ISC process of (CS)-C-1 & RARR;(CS)-C-3 is nonsufficient.Femtosecond transient absorption spectra show that charge separation(CS) occurs readily (<5 ps) for most dyads, even in nonpolar solvents.Nanosecond pulsed laser-excited time-resolved electron paramagneticresonance (TREPR) spectra show that either a spin correlated radicalpair (SCRP) is formed, with the electron exchange energy 2J = +2.14 mT, or radical pairs with stronger interaction,|2J| > 6.57 mT. These studies are useful for in-depthunderstanding of the CS and ISC in compact electron donor-acceptordyads and for design of efficient TADF emitters.