Red Light‐Emitting Thermally‐Activated Delayed Fluorescence of Naphthalimide‐Phenoxazine Electron Donor‐Acceptor Dyad: Time‐Resolved Optical and Magnetic Spectroscopic Studies

Abstract

We prepared an orthogonal compact electron‐donor (phenoxazine, PXZ)‐acceptor (naphthalimide, NI) dyad (NI‐PXZ), to study the photophysics of the thermally‐activated delayed fluorescence (TADF), which has a luminescence lifetime of 16.4 ns (99.2 %)/17.0 μs (0.80 %). A weak charge transfer (CT) absorption band was observed for the dyad, indicating non‐negligible electronic coupling between the donor and acceptor at the ground state. Femtosecond transient absorption spectroscopy shows a fast charge separation (CS) (ca. 2.02∼2.72 ps), the majority of the singlet CS state is short‐lived, especially in polar solvents (τCR = 10.3 ps in acetonitrile, vs. 1.83 ns in toluene, 7.81 ns in n‐hexane). Nanosecond transient absorption spectroscopy detects a long‐lived transient species in n‐hexane, which is with a mixed triplet local excited state (3LE) and charge separated state (3CS), the lifetime is 15.4 μs. In polar solvents, such as tetrahydrofuran and acetonitrile, a neat 3CS state was observed, whose lifetimes are 226 ns and 142 ns, respectively. Time‐resolved electron paramagnetic resonance (TREPR) spectra indicate the existence of strongly spin exchanged 3LE/3CT states, with the effective zero field splitting (ZFS) |D| and |E| parameters of 1484 MHz and 109 MHz, respectively, much smaller than that of the native 3NI state (2475 and 135 MHz). It is rare but solid experimental evidence that a closely‐lying 3LE state is crucial for occurrence of TADF and this 3LE state is an essential intermediate state to facilitate reverse intersystem crossing in TADF systems.

Detection of the Dark States in Thermally Activated Delayed Fluorescence (TADF) Process of Electron Donor‐Acceptor Dyads: Insights from Optical Transient Absorption Spectroscopy

“…Recently we used the ns-TA spectroscopy to directly observe these states. [43,44,46] We prepared a compact electron donor-acceptor dyad 1 (Figure 2), [46] with 9,10-anthraquinone (AQ) as the electron acceptor (E Red = À 1.40 V, vs. Fc/Fc + ), and phenoxazine (PXZ) as the electron donor (E Ox = + 0.23 V, vs. Fc/Fc + ). AQ chromophore shows efficient, fast (~0.4 ps) ISC.…”

“…[26,[39][40][41] The topic has been summarized in a recent review article and a few research papers. [42][43][44] Another issue in study of the photochemistry of the electron donor-acceptor type of TADF molecules is the ISC mechanism.…”

A long-lived charge-separated state of spiro compact electron donor–acceptor dyads based on rhodamine and naphthalenediimide chromophores

“…To simulate such a spectral shape, we used a routine based on some functions of the EasySpin package for MatLab (note that exchange interaction in EasySpin is introduced within a spin Hamiltonian , whereas ref. 51 and 52 use ). 74 …”

“…18 Given that the electronic coupling between the donor and acceptor was strong, a charge transfer (CT) absorption band (S 0 / 1 CT transition) should be observed. [50][51][52] It is interesting that even for Rho-NDI, in which the donor and acceptor are separated by three s bonds, and the electronic coupling at the ground state is weak. The Rho moiety is well known for the reversible spirolactam 4 amide transformation in the presence of an acid or base.…”

Donor-Acceptor Dyads and Triads Employing Core-Substituted Naphthalene Diimides: A Synthetic and Spectro (Electrochemical) Study

“…Although some NDI-phenothiazine dyads have been reported [1], little research has been described employing the analogous donor species phenoxazine. Recent studies of the related species naphthalimide-phenothiazine [24,25] or naphthalimide-phenoxazine [26] show promise for creating donor-acceptor dyads with these and related components. Herein we report the synthesis of a series of NDI-based electron donor-acceptor systems composed of an NDI acceptor furnished with phenothiazine or phenoxazine donor appendages in combination with other core-substituents.…”