Excited State Dynamics of Thermally Activated Delayed Fluorescence from an Excited State Intramolecular Proton Transfer System.

Yun Long,Marco Colella,Masashi Mamada,Chunyong Li,Andrew Monkman,Chihaya Adachi,Andrew Danos,Paloma Lays Dos Santos

doi:10.1021/acs.jpclett.0c00498

Abstract

We describe the photophysical processes that give rise to thermally activated delayed fluorescence in the excited state intramolecular proton transfer (ESIPT) molecule, triquinolonobenzene (TQB). Using transient absorption and time-resolved photoluminescence spectroscopy, we fully characterize prompt and delayed emission, phosphorescence, and oxygen quenching to reveal the reverse intersystem crossing mechanism (rISC). After photoexcitation and rapid ESIPT to the TQB-TB tautomer, emission from S1 is found to compete with thermally activated ISC to an upper triplet state, T2, very close in energy to S1 and limiting photoluminescence quantum yield. T2 slowly decays to the lowest triplet state, T1, via internal conversion. In the presence of oxygen, T2 is quenched to the ground state of the double proton transferred TQB-TC tautomer. Our measurements demonstrate that rISC in TQB occurs from T2 to S1 driven by thermally activated reverse internal conversion from T1 to T2 and support recent calculations by Cao et al. (CaoY.; EngJ.; PenfoldT. J.Excited State Intramolecular Proton Transfer Dynamics for Triplet Harvesting in Organic Molecules. J. Phys. Chem. A2019, 123, 2640−264930848598).

Highlights

Recent research has revealed that a low ΔEST and molecular design efficient reverse intersystem crossing mechanism (rISC) can be strategies,[12−14] with achieved the most by several successful materials exhibiting either a donor−acceptor (D−A) or a donor−acceptor−donor (D−A−D) structure and possessing strong intramolecular charge transfer character.[15−19] The mechanism underpinning high-efficiency Thermally activated delayed fluorescence (TADF) in these systems was found to be a second-order spin-vibronic coupling between the 1CT and 3CT charge transfer states, which is mediated by a locally excited 3LE triplet state.[20,21]
These measurements define an experimental ΔEST of 0.18 eV for TQB-TB, in good agreement with the value estimated from previously reported activation energies determined from Arrhenius plots.[38]
This realization implies the involvement of a second triplet state higher in energy than the T1 state revealed by zeonex film phosphorescence, which we identify as T2

Summary

Introduction

The absorption and emission spectra and fluorescence lifetime (Figure S2) revealed that alkyl substitution of the TQB structure has little influence on the excited state dynamics. Photoluminescence emission is thought to arise from the C N···H−O−C keto form (TQB-TB structure), due to the ESIPT process being orders of magnitude faster than fluorescence.[35] An unstructured emission peak is observed in steady state measurements centered at 537 nm (2.31 eV peak, 2.71 eV onset), which agrees with previous calculations for this tautomer (2.31 eV).[38] The TQB-TB tautomer has been identified as the most stable keto form, with the lowest excited singlet state energy.

Results

Conclusion