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

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

doi:10.1021/acs.jpclett.0c00498

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

J Phys Chem Lett. 2020 May 7;11(9):3305-3312. doi: 10.1021/acs.jpclett.0c00498. Epub 2020 Apr 15.

Authors

Yun Long¹, Masashi Mamada^{2

3

4}, Chunyong Li¹, Paloma Lays Dos Santos¹, Marco Colella¹, Andrew Danos¹, Chihaya Adachi^{2

3

4

5}, Andrew Monkman¹

Affiliations

¹ Department of Physics, Durham University, South Road, Durham DH1 3LE, United Kingdom.
² Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan.
³ JST, ERATO, Adachi Molecular Exciton Engineering Project c/o Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, Nishi, Fukuoka 819-0395, Japan.
⁴ Academia-Industry Molecular Systems for Devices Research and Education Center (AIMS), Kyushu University, Nishi, Fukuoka 819-0395, Japan.
⁵ International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, Nishi, Fukuoka 819-0395, Japan.

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. (Cao, Y.; Eng, J.; Penfold, T. J. Excited State Intramolecular Proton Transfer Dynamics for Triplet Harvesting in Organic Molecules. J. Phys. Chem. A 2019, 123, 2640-2649).