Electronic structures and optical properties of neutral substituted fluorene-based cyclometalated platinum(II)–acetylide complexes: A DFT exploration

We report a combinational DFT and TD-DFT study of the electronic and optical properties of several tridentate cyclometalated mononuclear [Pt(C^N^N)(Ctriple bond; length of mdashCR)] (1–3), [Pt(C^N^N)(Ctriple bond; length of mdashCRCtriple bond; length of mdashCH)] (4), and dinuclear [Pt(C^N^N)(Ctriple bond; length of mdashCRCtriple bond; length of mdashC)Pt(C^N^N)] (5 (C2 symmetry) and 5′ (Cs symmetry)) platinum(II) complexes with σ-acetylide ligand bearing fluorene substituents, where HC^N^N = 6-aryl-2,2′-bipyridine, R = fluorene-2,7-diyl 1, 4, 5 and 5′, R = 9,9-dimethylfluorene-2,7-diyl 2, R = 9,9-diethylfluorene-2,7-diyl 3. The structural and electronic properties of the ground- and lowest triplet state and the EA and IP values of the complexes are discussed. It is found that all of the lowest-lying absorptions are categorized as the LLCT combined with the MLCT transitions. The oscillator strengths of the lowest energy absorptions get a remarkable enhancement for the dinuclear complexes 5 and 5′compared to 1–4 due to the increase of electronic delocalization on the more planar molecular geometry. In general, the phosphorescent emissions of these complexes in CH2Cl2 are the reverse process of their lowest energy absorption transitions, except that of 4 is assigned as 3[π∗−π]/3MLCT transition because of the strengthened electronic localization effect and the interaction with the solvent in the lowest triplet state. In addition, these complexes hold promise as a new kind of nonlinear optical material owing to their large static first hyperpolarizabilities (β0). The β0 value has increased in the dinuclear complexes in contrast to those of the mononuclear ones owing to their larger transition moment and smaller transition energy.