Observation of Long-Lived Charge-Separated States in Anthraquinone-Phenothiazine Electron Donor-Acceptor Dyads: Transient Optical and Electron Paramagnetic Resonance Spectroscopic Studies
Year: 2023
Authors: Yu Z., Sukhanov A., Xiao X., Iagatti A., Doria S., Butera V., Zhao J., Voronkova V.K., Di Donato M., Mazzone G.
Autors Affiliation: Dalian Univ Technol, Frontiers Sci Ctr Smart Mat, Sch Chem Engn, State Key Lab Fine Chem, Dalian 116024, Peoples R China; Russian Acad Sci, Zavoisky Phys Tech Inst, FRC Kazan Sci Ctr, Kazan 420029, Russia; LENS European Lab Nonlinear Spect, I-50019 Florence, Italy; INO CN R, I-50125 Florence, Firenze, Italy; ICCOM CNR, I-50019 Florence, Italy; Brno Univ Technol, CEITEC Cent European Inst Technol, Brno 61200, Czech Republic; Xinjiang Univ, Coll Chem, State Key Lab Chem & Utilizat Carbon Based Energy, Urumqi 830017, Peoples R China; Univ Calabria, Dipartimento Chim & Tecnol Chim, I-87036 Arcavacata Di Rende, Italy.
Abstract: We prepared a series of phenothiazine (PTZ)-anthraquinone (AQ) electron donor-acceptor dyads to study the relationship between molecular structures and the possibility of charge transfer (CT) and intersystem crossing (ISC). As compared to the previously reported PTZ-AQ dyad with a direct connection of two units via a C-N single bond, the PTZ and AQ units are connected via a p-phenylene or p-biphenylene linker. Conformation restriction is imposed by attaching ortho-methyl groups on the phenylene linker. UV-vis absorption spectra indicate electronic coupling between the PTZ and AQ units in the dyads without conformation restriction. Different from the previously reported PTZ-AQ, thermally activated delayed fluorescence (TADF) is observed for the dyads containing one phenylene linker (PTZ-Ph-AQ and PTZ-PhMe-AQ). The prompt fluorescence lifetime in cyclohexane is exceptionally long (τPF = 62.0 ns, population ratio: 99.2%) and 245.0 ns (93.5%) for PTZ-Ph-AQ and PTZ-PhMe-AQ, respectively (normally τPF <20 ns); the delayed fluorescence lifetimes for these two dyads were determined as τDF = 2.4 μs (6.5%) and 7.6 μs (0.8%), respectively. For the dyad containing a biphenylene linker (PTZ-Ph2Me-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 μs, respectively, indicating the triplet spin multiplicity of the CS state. The 3CS state lifetimes are shortened to 100 ns and 440 ns for the two dyads, respectively, in the polar solvent acetonitrile. For dyads with a longer linker, i.e., PTZ-Ph2Me-AQ, the CS state lifetime is not sensitive to solvent polarity (τCS = 1.8 and 1.3 μs in cyclohexane and acetonitrile, respectively). In reference dyads, where the PTZ unit is oxidized to sulfoxide, no CT absorption band and TADF were observed, which is attributed to the increased CS state energy (>3 eV) becoming higher than that of the AQ triplet (3AQ*) state (ca. 2.7 eV). These experimental evidence show that the presence of 1CS, 3CS, and 3LE (LE: locally excited) states sharing similar energy is essential for the occurrence of TADF. Population of the long-lived 3CS state (with a lifetime of a few μs) does not produce by itself TADF, because the ISC process of 1CS→3CS 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 paramagnetic resonance (TREPR) spectra show that either a spin correlated radical pair (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-depth understanding of the CS and ISC in compact electron donor-acceptor dyads and for design of efficient TADF emitters. © 2023 American Chemical Society
Journal/Review: JOURNAL OF PHYSICAL CHEMISTRY B
Volume: 127 Pages from: 5905 to: 5923
More Information: J.Z. would like to thank the NSFC (U2001222), the Research and Innovation Team Project of Dalian University of Technology (DUT2022TB10), the Fundamental Research Funds for the Central Universities (DUT22LAB610), and the State Key Laboratory of Fine Chemicals for financial support. A.A.S. and V.K.V. acknowledge financial support from the government assignment for FRC Kazan Scientific Centre of RAS. M.D.D. would like to thank the European Union´s Horizon 2020 research and innovation program under grant agreement no. 871124 Laserlab, Europe, for the support.KeyWords: Acetonitrile; Aromatic compounds; Cyclohexane; Electron spin resonance spectroscopy; Fluorescence; Insecticides; Ketones; Paramagnetic resonance; Spectroscopic analysis; Ultraviolet spectroscopy; Charge transferDOI: 10.1021/acs.jpcb.3c02723ImpactFactor: 2.800Citations: 2data from “WEB OF SCIENCE” (of Thomson Reuters) are update at: 2024-11-24References taken from IsiWeb of Knowledge: (subscribers only)Connecting to view paper tab on IsiWeb: Click hereConnecting to view citations from IsiWeb: Click here
