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

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.

Charge Transfer and Intersystem Crossing in Compact Naphthalenediimide–Phenothiazine Triads: Synthesis and Study of the Photophysical Property with Transient Optical and Electron Paramagnetic Resonance Spectroscopic Methods

“…We assign this new band to the CT band with contribution from the presence of the PTZ moiety as an electron donor. ,, The support for this conclusion comes from NDI-Ph-PTZ-O (Figure b). For this triad, the PTZ unit is oxidized to the sulfoxide structure, the electron-donating ability is reduced, ,, and as a result, the CT absorption band beyond 450 nm virtually disappears. However, the shoulder absorption band centered at 420 nm is persistent.…”

Triplet Photosensitizers and Their Applications in Triplet–Triplet Annihilation Upconversion

“…Electron donor–acceptor-type TADF molecules are known for their small Δ E ST . − This is due to the orthogonal orientation of the electron donor–acceptor planes, which spatially separate the HOMO and LUMO; thus, the J is small and ensures the small S 1 /T 1 states energy gap (2 J ). Due to their high triplet quantum yield (Φ T , 10–40%) and long triplet-state lifetimes (on the microsecond time scale), they also have great application potentials in TTA-UC.…”

The effect of thionation of the carbonyl group on the photophysics of compact spiro rhodamine-naphthalimide electron donor–acceptor dyads: intersystem crossing, charge separation, and electron spin dynamics

“…The absorption of the NI radical anion (NI À ) without thionation of the carbonyl group was observed at 420 nm, 491 nm and 830 nm, which is different from that of the NIS À of NIS and Rho-NIS. [82][83][84]…”