Thermally activated delayed fluorescence: excited state engineering towards efficient organic light emitting diodes

Abstract

Thermally activated delayed fluorescence (TADF) is a rare phenomenon occurring in systems where a triplet state sits very close in energy to the lowest excited singlet. Once the triplet state is populated, either upon intersystem crossing (ISC) following photoexcitation or upon injection of charges in a device, it may transfer its population to the nearby singlet state via a reverse ISC (RISC), provided that the energy gap between singlet-triplet is very low. The TADF requirement of singlet and triplet states lying close in energy is easily met in dyes with low-energy charge transfer (CT) states, provided the conjugation between the electron-donor (D) and electron acceptor (A) is weak. Dyes with the D and A units arranged almost orthogonally were immediately recognized as target systems. However, strictly orthogonal (non-conjugated) systems also have vanishingly small spin–orbit coupling between relevant states, hindering RISC, as well as negligible transition dipole moments from the excited singlet to the ground state, strongly suppressing emission intensity. An enormous effort towards the design of novel and more efficient TADF dyes includes multipolar dyes, where several D and A groups are linked together in different geometries, macromolecular and dendritic systems. Conformational flexibility, modulating the D-A conjugation, and hence affecting both the singlet–triplet energy gap and spin–orbit coupling is crucial to get efficient TADF. To make the picture even more intricate, the matrix properties, including dielectric properties, mobility, viscosity etc., may affect in different ways states with different nature, with dramatic effects on the relative energies of excited states. Mastering TADF then requires reliable models able to accurately account for the different interactions while maintaining the global picture in control.

This thesis provides an guide and fundamental understanding towards this direction. Having charge-transfer as well local-excited states for TADF dyes, one of the most delicate issue is the choice of the functional to treat all the states properly. The dielectric properties of local environment also largely affect the behaviour of TADF dyes. Typically used continuum models leads to wildly different results because of the adiabatic approximation used for solvent electronic degrees of freedom. An antiadiabatic approached is proposed and validated for different dyes. Later an essential-state model is built without adjustable parameters, which can satisfactorily reproduce the absorption and fluorescence spectra (in terms of band positions and bandshapes) and their evolution with the solvent polarity, as well as absorption, fluorescence and phosphorescence spectra in a solid matrix for a representative TADF dye. The model offers a safe guidance on environmental effects on the photophysics of the TADF dye. This is a very important step towards the so-called smart-matrix approach, where the dye and the matrix are optimized together for best device performances.

This work provides an original and fundamental approach towards reliable models of the TADF process, accounting for the delicate role of environmental effects. It provides the scientific and technological community a useful tool as needed to guide the design and synthesis of concurrently optimized TADF dyes and matrices.