Electronic structures and spectroscopic properties of mono- and binuclear d8 complexes:: A theoretical exploration on promising phosphorescent materials

The structures of trans-[M-2(CN)(4)(PH2CH2PH2)(2)] (M = Pt (1), Pd (2), and Ni (3)), trans-[Pt2X4(PH2CH2PH2)(2)] (X = Cl (4) and Br (5)), and trans-[M(CN)(2)(PH3)(2)] (M = Pt (6), Pd (7), and Ni (8)) in the ground state were optimized using the MP2 method. Frequency calculations reveal that the weak metal-metal interaction is essentially attractive for 1, 2, 4, and 5 but not for 3. The TD-DFT calculations associated with the polarized continuum model (PCM) were performed to predict absorption spectra in CH2Cl2 solution. Experimental spectra are well reproduced by our results. With respect to analogous mononuclear d(8) complexes (6-8), a large red shift of the absorption wavelength was calculated for the binuclear d(8) complexes (1-3). Relative to 1 with unsaturated CN- donors, introduction of saturated halogen donors into 4 and 5 changes their electronic structures, especially the HOMO and LUMO. The TD-DFT and subsequent unrestricted MP2 calculations predict that 1 produces the lowest-energy d -> p emission while 2-5 favor the d -> d emissions, agreeing with experimental observations.