Reverse intersystem crossing from high-level triplet excited electronic states of fluorescent organic molecules

Abstract

Reverse intersystem crossing (RISC) from an excited electronic triplet state, T_m, to an excited electronic singlet state, S_n, of fluorescent organic molecules used as light emitters for organic light-emitting devices (OLEDs) offers the prospect of harvesting all electrically generated singlet and triplet excitons, enabling OLEDs with high external quantum efficiency. The latest generation of high-efficiency light-emitting organic compounds for OLEDs is designed to utilize thermally-activated delayed fluorescence (TADF) which arises as a result of RISC from the lowest-energy triplet excited state, T₁, to the fluorescent singlet excited state, S₁. RISC from upper-level (“hot”) triplet excited states (“hRISC”) in fluorescent organic molecules offers an important route to overcoming lingering technical limitations associated with the relatively long lifetime of the T₁ state in most TADF emitters but has been much less commonly reported. In this work we report the synthesis and characterization of two exemplars, DPOTAB and DPOTABCN, of a new class of fluorescent organic molecules designed specifically for hRISC. The energy difference between the T₄ and S₁ states for these two molecules is calculated to be very small, whereas the calculated energy gap between their T₄ and T₃ states and between their S₁ and T₃ states is large. These features coupled with their high photoluminescence (PL) quantum efficiency in solution satisfy key criteria required for prospective emitters in fluorescence-based OLEDs utilizing hRISC. Strong evidence for hRISC from T₄ to S₁ in DPOTAB is provided by steady-state and time-resolved PL emission measured at temperatures from 77 K to room temperature, including emission arising from energy transfer from ketone triplet sensitizers. This is one of the first examples of hRISC from a triplet excited state higher than the T₃ level in a fluorescent organic molecule.