Title: Revealing the spin–vibronic coupling mechanism of thermally activated delayed fluorescence

Authors: Marc K. Etherington, Jamie Gibson, Heather F. Higginbotham, Thomas J. Penfold and Andrew P. Monkman

Published: 30 Nov 2016

Journal: Nature Communications http://www.nature.com/articles/ncomms13680

Advances in knowledge about light emission have brought breathtaking innovations in lighting and display technologies. From cathode ray tubes (CRT) to liquid crystal display (LCD), and now to organic light emitting diodes (OLEDs), our displays nowadays have undergone many transitions. These are all brought about by better understanding and discoveries of light-emitting mechanisms. Among these, fluorescence and phosphorescence have attracted much focus in the academic world.

In both fluorescence and phosphorescence, the molecule is excited by light energy absorbed.  The molecule then falls back (or relaxes) to its original state by giving out light energy. In the terminology of the quantum world, fluorescence is a transition where both states are of the same spin. On the other hand, phosphorescence occurs when light is emitted from an excited state of different spin compared to the original state. For instance, emission of light from triplet state to singlet ground state is phosphorescence. The process begins when light excites the molecule to a higher energy singlet state. It then undergoes intersystem crossing (ISC) to an intermediate triplet state, before it relaxes back to its original singlet ground state (Figure 1). However, this emission from the triplet states often faces many obstacles such as instability issues or favorable non-emissive competitive pathways that limit the amount of light emitted by the process.

Figure 1a and b show a schematic diagram for fluorescence and phosphorescence process, respectively.

Thermally activated delayed fluorescence (TADF) is of great interests to scientists due to its potential ability to fully utilize the absorbed energy of the incoming light. This article presents a model to illustrate this process from molecular perspective. A specially designed molecule, with donor-acceptor units, have been chosen for illustration and potential ways to trigger TADF process are suggested.

The authors built a donor-acceptor-donor molecule, DPTZ-DPTO2, as shown in Figure 2a. This molecule possesses intramolecular charge-transfer (CT) states between the donor and acceptor units. It was originally thought that the singlet (¹CT) and triplet (³CT) states give rise to the TADF process. Yet, results from the studies show that another excited state, the local exciton triplet state (³LE) plays a crucial role in ISC as well as reverse ISC (rISC) between the CT states. Through changing the polarity of solvents and doping hosts, as well as conducting time-dependence and temperature dependence measurements, the authors realized the role of ³LE state in the TADF process. The process is summarized in Figure 2b.

Figure 2a shows the structure of DPTZ-DPTO2 molecule and Figure 2b shows the proposed model for the TADF mechanisms.

In the presence of non-polar solvents or doped with a rigid host, ³LE is well below ¹CT (Type I). On the other hand, in a less rigid host or polar solvents, ³LE state is well above ¹CT (Type III). In both cases, the coupling between the ³LE and the CT states is not effective. Type II is the ideal case of the most efficient TADF system. The molecular structure as well as the polarity and rigidity are controlled in such that ³LE state couples effectively with the CT states. This favors highly efficient rISC process and in turn TADF.

This model has demonstrated the importance of material design, as well as provided a better understanding of photophysical systems. The advances in such knowledge will aid in developing new technologies such as lighting and displays where the manipulation of excited states can lead to exciting new discoveries.