Thermally activated delayed fluorescence processes for Cu(i) complexes in solid-state: a computational study using quantitative prediction

Abstract

The photophysical properties of four representative Cu(I) complex crystals have been investigated using the combination of an optimally tuned one- and two-dimensional range-separated hybrid functional with the polarizable continuum model, and the thermal vibration correlation function (TVCF) approach. The calculated excited singlet–triplet energy gap, radiative rates and lifetimes match the experimentally available data perfectly. At 300 K, the reverse intersystem crossing (RISC) proceeds at a rate of k^dir._RISC ≈ 10^6–8 s⁻¹, which is 4–5 orders of magnitude larger than the mean phosphorescence rate, k_P ≈ 10^2–3 s⁻¹. At the same time, the ISC rate k^dir._ISC ≈ 10⁹ s⁻¹ is again 2 orders of magnitude larger than the fluorescence rate k_F ≈ 10⁷ s⁻¹. In the case of k^dir._RISC ≫ k_F and k^dir._RISC ≫ k_P, thermally activated delayed fluorescence should occur. Vibronic spin–orbit coupling can remarkably enhance the ISC rates by the vital “promoting” modes, which can provide crucial pathways to decay. This can be helpful for designing novel excellent TADF Cu(I) complex materials.