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: State Key Laboratory of Fine Chemicals, Frontier Science Center for Smart Materials, School of Chemical Engineering, Dalian University of Technology, E-208 West Campus, 2 Ling Gong Road, Dalian, 116024, China
Zavoisky Physical-Technical Institute FRC Kazan Scientific Center of RAS, Sibirsky Tract 10/7, Kazan, 420029, Russian Federation
LENS (European Laboratory for Non-Linear Spectroscopy), via N. Carrara 1, Firenze, Sesto Fiorentino (FI), 50019, Italy
ICCOM-CNR, via Madonna del Piano 10, Sesto Fiorentino (FI), 50019, Italy
INO-CNR, Largo Enrico Fermi 6, (FI), Firenze, 50125, Italy
State Key Laboratory of Chemistry and Utilization of Carbon Based Energy Resources, College of Chemistry, Xinjiang University, Urumqi, 830017, 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

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: 3
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