Photophysics of TADF guest-host systems: introducing the idea of hosting potential

Abstract

The thermally activated delayed fluorescence (TADF) donor-acceptor (D-A) molecule, DMAC-TRZ, is used as a TADF emitter ‘probe’ to distinguish the environmental effects of a range of solid state host materials in guest-host systems. Using the guest’s photophysical behaviour in solution as a benchmark, a comprehensive study using a variety of typical TADF OLED hosts with different characteristics provides a clearer understanding of guest host interactions and what effects emitter performance in solid state. We investigate which are the key host characteristics that directly affect change transfer (CT) state energy and singlet triplet energy gaps. Using time-resolved photoluminescence (PL) measurements, we use the CT state energy distribution obtained from the FWHM of the emission band and correlate this with other photophysical properties such as the apparent dynamic redshift of CT emission on-set to estimate the disorder induced heterogeneity of D A dihedral angles and singlet triplet gaps1,2. Further, the delayed emission stabilization energy value, and time dependent CT band FWHM are shown to be related to a combination of host’s rigidity, emitter molecule packing and the energy difference between guest and host lowest energy triplet states. Concentra-tion dependence studies, show that emitter dimerization/aggregation can improve as well as reduce emission efficiency, depending on the characteristics of the host. Two similar host materials, mCPCN and mCBPCN, with optimum host characteristics show completely different behaviour and their hosting potential is extensively explored. We demonstrate that that type I and type III TADF emitters behave differently in the same host and that the materials with intrinsic small ΔEST have the smallest disorder induced CT energy and rISC rate dispersion. We also present an optimized method to define the actual triplet energy of a guest host system, a crucial parameter in understanding the overall mechanism of the TADF efficiency of the system.