Vibrational Damping Reveals Vibronic Coupling in Thermally Activated Delayed Fluorescence Materials.

Matthias Hempe,Mark A Fox,Martin R Bryce,Andrew P Monkman,Nadzeya A Kukhta,Andrew Danos,Andrei S Batsanov

doi:10.1021/acs.chemmater.0c03783

Abstract

We investigate a series of D–A molecules consisting of spiro[acridan-9,9′-fluorene] as the donor and 2-phenylenepyrimidine as the acceptor. In two of the materials, a spiro center effectively electronically isolates the D unit from (consequently) optically innocent yet structurally influential adamantyl side groups. In a third material, adamantyl groups attached directly to the acceptor strongly influence the electronic properties. Steady-state and time-resolved photophysical studies in solution, Zeonex polymer matrix, and neat films reveal that the substituents impact the efficiency of vibronic coupling between singlet and triplet states relevant to reverse intersystem crossing (rISC) and thermally activated delayed fluorescence (TADF), without significantly changing the singlet–triplet gap in the materials. The adamantyl groups serve to raise the segmental mass and inertia, thereby damping intramolecular motions (both vibrational and rotational). This substitution pattern reveals the role of large-amplitude (primarily D–A dihedral angle rocking) motions on reverse intersystem crossing (rISC), as well as smaller contributions from low-amplitude or dampened vibrations in solid state. We demonstrate that rISC still occurs when the high-amplitude motions are suppressed in Zeonex and discuss various vibronic coupling scenarios that point to an underappreciated role of intersegmental motions that persist in rigid solids. Our results underline the complexity of vibronic couplings in the mediation of rISC and provide a synthetic tool to enable future investigations of vibronic coupling through selective mechanical dampening with no impact on electronic systems.

Highlights

We investigate a series of D−A molecules consisting of spiro[acridan-9,9′-fluorene] as the donor and 2-phenylenepyrimidine as the acceptor
A very recent study correctly notes that the model neglects “the underlying physics encoded in the empirical preexponential factor of the Arrhenius equation,” which are vital in promoting reverse intersystem crossing (rISC), especially for larger ΔEST materials.[14]
The synthesis of the materials 1a−c used in this study (Figure 1) is based on the modular substitutions of the respective acridine donor and phenyl−pyrimidine acceptor segments. 5,5′-Dimethyl-substituted acridine−pyrimidine-based thermally activated delayed fluorescence (TADF) molecules have been shown to emit at a wavelength of 448 nm in toluene solution,[19] while the intrinsic fluorescence wavelength of the fluorene structure is onset at 320 nm (3.8 eV),[32] with a lowest excited triplet state energy of 3.1 eV

Summary

Introduction

We investigate a series of D−A molecules consisting of spiro[acridan-9,9′-fluorene] as the donor and 2-phenylenepyrimidine as the acceptor. Since spin−orbit coupling (SOC) of singlet and triplet states of the same character is formally forbidden, recent studies of the TADF mechanism instead indicate that a coupling of excited triplet charge-transfer states (3CT) with localized excited triplet states (3LE) allows for rapid rISC toward excited singlet charge-transfer states (1CT).[9,10] This kind of coupling requires a small energy difference between the states involved, as well as the existence of coupling vibrational modes (primarily D−A dihedral angle rocking), which further lower the energetic activation barrier of the rISC.[11−13] In this context, successful applications of Arrhenius models to the temperature dependence of experimental rISC rates demonstrate the high relevance of ΔEST to rISC and TADF. It was observed that despite very similar energy gaps in a series of D−A−D molecules, severely reducing the dihedral twisting motion between the donor and acceptor segments by substituent effects, or changing the excitonic character (CT versus LE), can lead to large variations of the rISC rate of the respective materials.[15,16] This former result indicates a strong correlation between the TADF efficiency and the intramolecular (intersegmental) freedom of movement, while the latter demonstrates that other molecular factors encompassed by the Arrhenius preexponential factor can, in some cases, be just as important as ΔEST

Results

Discussion

Conclusion