Theoretical Studies on Excited-State Properties of Au(III) Emitters with Thermally Activated Delayed Fluorescence

Thermally activated delayed fluorescence (TADF) phenomena have been found in many organometallic complexes, but Au(III) complexes with TADF are rarely reported, possibly due to the existence of efficient nonradiative channels for luminescence states. Recent experiments identified two cyclometalated Au(III) aryl molecules with TADF; however, the underlying photophysical and luminescence mechanisms are elusive. Here, we have employed M06 and TD-M06 methods combined with polarizable continuum model and quantum mechanics/molecular mechanics approaches to comprehensively study the excited-state structures and properties of these two Au(III) complexes in toluene solution and crystal phases, respectively. We have found that both S-1 and T-1 states are of ligand-to-ligand charge transfer character. The significant twisting between C boolean AND N boolean AND C and aryl groups leads to good separation and negligible overlap of the highest occupied molecular orbital and lowest unoccupied molecular orbital. This results in a pretty small S-1-T-1 energy gap, which, in conjunction with strong spin-orbit coupling, facilitates the reverse intersystem crossing (rISC) process. In terms of the results of electronic structure calculations, we have calculated the related radiative and nonradiative rates. The forward intersystem crossing (ISC) and rISC processes are estimated to occur on the timescale of 10(10) s(-1), which is significantly faster than the fluorescence and phosphorescence emission rates (10(6) and 10(3) s-(1)). The faster rISC process relative to the phosphorescence one enables the TADF process. The low-frequency vibrational modes are found to have important contribution to the Huang-Rhys factors and to enhance the ISC and rISC rates. Moreover, environmental effects are found to be important and cannot be completely ignored in realistic simulations. Finally, the substituted -F and -OEt groups have a small influence on geometric structures but visible effects on electronic structures and related radiative and nonradiative rates, which implies that the TADF performance of the Au(III) complexes could be further enhanced through chemical tailoring or tuning these substituting groups.