Theoretical insight into electronic structures and spectroscopic properties of [Pt2(pop)4]4-, [Pt2(pcp)4]4-, and related derivatives (pop = P2O5H2(2-) and pcp = P2O4CH4(2-)).

The structures of [Pt2(pop)4]4-, [Pt2(pcp)4]4-, and related species [Pt2(pop)4X2]4- and [Pt2(pop)4]2- in the ground states (pop = P2O5H2(2-), pcp = P2O4CH4(2-), and X = I, Br, and Cl) were optimized using the second-order Møller-Plesset perturbation (MP2) method. It is shown that the Pt-Pt distances decrease in going from [Pt2(pop)4]4- to [Pt2(pop)4X2]4- to [Pt2(pop)4]2-. This is supported by the analyses of their electronic structures. The calculated aqueous absorption spectra at the time-dependent density functional theory (TD-DFT) level agree with experimental observations. The unrestricted MP2 method was employed to optimize the structures of [Pt2(pop)4]4- and [Pt2(pcp)4]4- in the lowest-energy triplet excited states. The Pt-Pt contraction trend is well reproduced in these calculations. For [Pt2(pop)4]4-, the Pt-Pt distance decreases from 2.905 A in the ground state to 2.747 A in the excited state, which is comparable to experimental values of 2.91-2.92 A and 2.64-2.71 A, respectively. On the basis of the excited-state structures of such complexes, TD-DFT predicts the solution emissions at 480 and 496 nm, which is closer to the experimental values of 512 and 510 nm emissions, respectively.