Nonadiabatic coupling reduces the activation energy in thermally activated delayed fluorescence

Abstract

The temperature dependent rate of a thermally activated process is given by the Arrhenius equation. The exponential decrease in the rate with activation energy, which this imposes, strongly promotes processes with small activation barriers. This criterion is one of the most challenging during the design of thermally activated delayed fluorescence (TADF) emitters used in organic light emitting diodes. The small activation energy is usually achieved with donor-acceptor charge transfer complexes. However, this sacrifices the radiative rate and is therefore incommensurate with the high luminescence quantum yields required for applications. Herein we demonstrate that the spin-vibronic mechanism, operative for efficient TADF, overcomes this limitation. Nonadiabatic coupling between the lowest two triplet states give rise to a strong enhancement of the rate of reserve intersystem crossing via a second order mechanism and promotes population transfer between the T to T states. Consequently the rISC mechanism is actually operative between initial and final state exhibiting an energy gap that is smaller than between the T and S states. This contributes to the small activation energies for molecules exhibiting a large optical gap, identifies limitations of the present design procedures and provides a basis from which to construct TADF molecules with simultaneous high radiative and rISC rates.

The theory of thermally activated delayed fluorescence for organic light emitting diodes

“…However recently, Gibson et al 42 proposed that the nonadiabatic coupling associated with the spin-vibronic mechanism could also play a significant role in this. Indeed, the nonadiabatic coupling active in the spin-vibronic mechanism, forms an equilibrium between the two lowest triplet states, T 1 (ππ * ) and T 2 ( 3 CT), which leads to significant population transfer, even without thermal activation.…”

“…SSSE have been shown to be especially important in 3 rd generation OLEDs exploiting TADF 17,37,38,[53][54][55][56][57][58] . The origin of this derives from recent work demonstrating that TADF is not simply a cyclic process between the singlet and triplet CT manifolds, but involves coupling to other states 12,38,40,42,47,59,60 , such as local triplet ππ * ( 3 LE) 12 or nπ * states 40 . The effectiveness of the mixing between the states and therefore of the overall TADF process crucially depends energy gap between them.…”

The influence of molecular geometry on the efficiency of thermally activated delayed fluorescence

“…Recent literature highlights the importance of spin-vibronic coupling, 28,29 and mixing of charge transfer and localized electronic states to promote RISC. 15,26,30,31 The motivation for the present study comes from literature reports of a series of phenothiazine (D) and dibenzothiophene-S,S-dioxide (A) TADF molecules where the donor is substituted with side bulky groups, aiming to force near D-A orthogonal geometry. 32 Remarkably, while in the unsubstituted D-A and D-A-D molecules strong TADF is observed, even in solid thin films, for the substituted emitters the TADF emission is suppressed, and is almost entirely replaced by room temperature phosphorescence (RTP) in some molecules.…”

“…32 Remarkably, while in the unsubstituted D-A and D-A-D molecules strong TADF is observed, even in solid thin films, for the substituted emitters the TADF emission is suppressed, and is almost entirely replaced by room temperature phosphorescence (RTP) in some molecules. 30,31 Here, we expand the study of D-A and D-A-D molecules, with additional analogues utilizing bulky substituents in different positions of both the D and A units. The effect of the substituents on the energy of the singlet and triplet excited states, and on molecular geometry is studied in detail with a variety of spectroscopic techniques and supporting density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations.…”

Computational Design of Thermally Activated Delayed Fluorescence Materials: The Challenges Ahead

“…On the one hand, the reverse IC in the triplet manifold of excited states which is a thermally-activated process, is expected to assist RISC by promoting the formation of higher-lying triplet excited states, from which the conversion to the singlet manifold might occur more efficiently because it is associated with a larger exergonic character. This interconversion channel is expected to compete with the direct conversion from T 1 to S 1 and features smaller activation energies 66 . On the other hand, non-radiative recombination to the ground state should take place mainly from S 1 and contribute as the main monomolecular pathway to molecular excitation loss.…”