Basic photophysical analysis of a thermally activated delayed fluorescence copper(i) complex in the solid state: theoretical estimations from a polarizable continuum model (PCM)-tuned range-separated density functional approach

Abstract

A quantitative understanding of photophysical processes is fundamental for designing novel thermally activated delayed fluorescence (TADF) materials. Taking a Cu(pop)(pz₂Bph₂) crystal as a typical TADF molecular model, we computed the conversion and decay rates of the first excited singlet state (S₁) and triplet state (T₁) at different temperatures by employing the thermal vibration correlation function (TVCF) approach. For the consideration of the solid-state environment, a methodology, which is based on the combination of a nonempirical, optimally tuned range-separated hybrid functional with the polarizable continuum model, was applied. Our calculated results are in excellent agreement with the experimentally available data. It is found that the reverse intersystem crossing (RISC) from T₁ to S₁ proceeds at a rate of k_RISC = 6.34 × 10⁵ s⁻¹ and can compete with the radiative decay rate (k^T_r = 3.29 × 10³ s⁻¹) and nonradiative intersystem crossing rate (k⁰_ISC = 1.48 × 10² s⁻¹) of T₁ at 300 K. This implies that the S₁ state can be repopulated from the T₁ state, TADF should be observed and the TADF decay time was found to be τ (300 K) = 9.68 μs by fitting calculations. In addition, the calculations indicate that the free rotation of the phenylene ring in the pop ligand can provide an important channel to energy conversion between T₁ and S₁. But, at a low temperature of T < 100 K, the situation will experience a larger change. The RISC rate becomes very small, k_RISC ≪ k^T_r or k_ISC, and it cannot induce an occurrence of delayed fluorescence. As a consequence, Cu(pop)(pz₂Bph₂) is a highly attractive candidate for applications of TADF.