Theoretical Study and Design of Phosphorescent Cyclometalated (CC*)PtII(acac) Complexes: The Substituent Effect Controls the Radiative and Nonradiative Decay Processes

Density functional theory (DFT) and time-dependent DFT calculations were performed to evaluate the influence of substituent effect of (1) R = 4-Me, (2) R = 4-OMe, and (3) R = 2,3-OC6H4 on the phenyl ring of ((CC)-C-boolean AND*)Pt-II(acac) ((CC)-C-boolean AND* = phenylitnidazole, acac = acetylacetone), respectively, on absorption and phosphorescent spectra properties, as well as the radiative and nonradiative processes. We found that emissions of complexes, 2 and 1 originate from the Kasha-like T-1 state, whereas that of complex 1 originates from non-Kasha. T-2 state. Compared with the emission of complex 1, the emission peaks of 2 and 3 are red shifted, which is attributed to p-pi and, pi-pi conjugation effects resulting from the electron-donating groups -OCH3, and OC6H4 with ligand (CC)-C-boolean AND*, respectively. The radiative rate constants (kappa(r)) of 2 and 3 are larger than that of 1, naively, kappa(r)(1) < kappa(r)(2) < kappa(r)(3), indicating:that K-r can be efficiently increased by enlarging pi-conjugation at the main ligand of ((CC)-C-boolean AND*)Pt-II(acac), which can cause the increase :of spin orbit coupling (SOC) matrix elements. At the same time, the activation energy barriers for the rate-limiting step can be largely raised accompanied by enlarging the ability of electron-donation of the substituent group at the main ligand of ((CC)-C-boolean AND*)Pt-II(acac), which can cause the decrease of the nonradiative rate constant (kappa(nr)) namely, kappa(nr)(1) > kappa(nr)(2) > kappa(nr)(3). According to phi(P) = kappa(r/)(kappa(r) + kappa(nr)) the quantum yields should have the sequence phi(P)(1) < phi(p)(2) < phi(p)(3), which is in accordance with the experiment. In addition; to guide experimental Synthesis of highly efficient ((CC)-C-boolean AND*)Pt-II(acac), a new complex 4 through extending the pi-conjugation in the (CC)-C-boolean AND* ligand "of ((CC)-C-boolean AND*)Pt-II(acac) was theoretically designed, which has a larger quantum yield than 1-3.