Ground and Excited State Electronic Structures of d⁸-Squared Planar Platinum(II) and Gold(III) Complexes Bearing Cyclometallated 2,6-Diphenylpyridine and Pyrene-Derived N-Heterocyclic Carbene

Abstract

Electronic structures of four pyrene-derived N-heterocyclic carbene complexes of platinum(II) and gold(III) bearing cyclometallated 2,6-diphenylpyridine have been investigated theoretically. The structures of all complexes in gas phase have been first optimized. Different functionals and basis sets have been tested to find the best theoretical method. The results show that B3PW91 (LANL2DZ/6-31G*) combination give the best description of the molecules as compared with experimentally determined structural features. Using the calculation data from that best functional and mixed basis set combination, frontier molecular orbitals of the complexes have been examined. To gain understanding on the nature of electronic transitions in the UV-vis absorption spectra of the compounds, TD-DFT calculations have been carried out. A details analysis of the vertical excitation reveals that the nature of all the vertical electronic transitions in the complexes. Notably, the lowest energy transitions are π_cb + d_Pt → $\pi _{{{\text{cb}}}}^{{\text{*}}}$ (for Pt¹ and Pt²) and π_CNC → d_Au + $\pi _{{{\text{CNC}}}}^{{\text{*}}}$ (for Au¹ and Au²) charge transfer in nature. A sharp contrast in lowest energy triplet excited state for d⁸‑squared planar Au¹ and Pt¹ has been demonstrated. While no contribution from gold(III) d orbital to the SOMO and SOMO – 1 orbital of Au¹, there is significant contribution of platinum(II) d_yz orbital in the SOMO – 1 orbital of Pt¹, suggesting possible interaction of this singly occupied orbital with radical of π-symmetry substrates.