Long-Lived Charge-Separated State in Naphthalimide-Phenothiazine Compact Electron Donor-Acceptor Dyads: Effect of Molecular Conformation Restriction and Solvent Polarity

Year: 2023

Authors: Xiao X., Yan Y., Sukhanov A. A., Doria S., Iagatti A., Bussotti L., Zhao J., Di Donato M., Voronkova V. K.

Autors Affiliation: Dalian Univ Technol, Frontier Sci Ctr Smart, Sch Chem Engn, State Key Lab Fine Chem, Dalian 116024, Peoples R China; RAS, FRC Kazan Sci Ctr, Zavoisky Phys Tech Inst, Kazan 420029, Russia; LENS European Lab Nonlinear Spect, I-50019 Florence, Italy; CNR, ICCOM, I-50019 Sesto Fiorentino, SI, Italy; CNR, INO, I-50125 Florence, FI, Italy; Xinjiang Univ, Coll Chem, State Key Lab Chem & Utilizat Carbon Based Energy, Urumqi 830017, Peoples R China.

Abstract: To study the charge separation (CS) and long-lived CS state, we prepared a series of dyads based on naphthalimide (NI, electron acceptor) and phenothiazine (PTZ, electron donor), with an intervening phenyl linker attached on the N-position of both moieties. The purpose is to exploit the electron spin control effect to prolong the CS-state lifetime by formation of the 3CS state, instead of the ordinary 1CS state, the spin-correlated radical pair (SCRP), or the free ion pairs. The electronic coupling magnitude is tuned by conformational restriction exerted by the methyl groups on the phenyl linker. Differently from the previously reported NI-PTZ analogues containing long and flexible linkers, we observed a significant CS emission band centered at ca. 600 nm and thermally activated delayed fluorescence (TADF) with a lifetime of 13.8 ns (population ratio: 42%)/321.6 μs (56%). Nanosecond transient absorption spectroscopy indicates that in cyclohexane (CHX), only the 3NI* state was observed (lifetime τ = 274.7 μs), in acetonitrile (ACN), only the CS state was observed (τ = 1.4 μs), whereas in a solvent with intermediate polarity, such as toluene (TOL), both the 3NI* (shorter-lived) and the CS states were observed. Observation of the long-lived CS state in ACN, yet lack of TADF, confirms the spin-vibronic coupling theoretical model of TADF. Femtosecond transient absorption spectroscopy indicates that charge separation occurs in both nonpolar and polar solvents, with time constants ranging from less than 1 ps in ACN to ca. 60 ps in CHX. Time-resolved electron paramagnetic resonance (TREPR) spectra indicate the existence of the 3NI* and CS states for the dyads upon photoexcitation. The electron spin-spin dipole interaction magnitude of the radical anion and cation of the CS state is intermediate between that of a typical SCRP and a 3CS state, suggesting that the long CS-state lifetime is partially due to the electron spin control effect.

Journal/Review: JOURNAL OF PHYSICAL CHEMISTRY B

Volume: 127 (31)      Pages from: 6982  to: 6998

More Information: J.Z. thanks 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 Center of RAS. M.D.D. thanks the European Union´s Horizon 2020 research and innovation program under Grant Agreement No. 871124, Laserlab, Europe, for the support.
KeyWords: Electron spin resonance spectroscopy; Electrons; Fluorescence; Insecticides; Magnetic moments; Organic solvents; Paramagnetic resonance; Spin dynamics; Transient absorption spectroscopy
DOI: 10.1021/acs.jpcb.3c02595

Citations: 5
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