Theoretical characterization of a tridentate photochromic Pt(II) complex using density functional theory methods

Density functional theory methods have been used to characterize a tridentate photochromic Pt(II) complex [Pt(AAA)Cl], its acetonitrile complex [Pt(AAA)Cl·CH₃CN], and the transition state in the complexation reaction. B3LYP/6-31G* (effective core potential for Pt) optimized geometries of Pt(AAA)Cl and Pt(AAA)Cl·CH₃CN are found to be in reasonably good agreement with most of the applicable parameters for the available experimental crystal structures of Pt(AAA)Cl and a Pt(AAA)Cl-triphenylphoshine complex, with the exception of one of the dihedral angles, the deviation of which is determined to be due to a steric cis versus trans effect. Vibrational frequencies are calculated for Pt(AAA)Cl and cis-Pt(AAA)Cl·CH₃CN, and the predicted shift in the benzaldehyde carbonyl frequency is found to be in line with that observed experimentally. Singlet vertical excitation energies are calculated for Pt(AAA)Cl and cis-Pt(AAA)-Cl·CH₃CN using time-dependent density-functional theory and are found to be in good agreement with the experimental transition energies, although for cis-Pt(AAA)CI-CH3CN, the calculations suggest a reassignment of the experimental S₁ and S ₂ transitions. Single point energies are calculated at the B3LYP/6-311 +G(2d,2p) level (effective core potential for Pt) and the calculations predict the complexation reaction (dark reaction) to be exothermic and, after a correction to the entropy, to be exoergic at 298 K and to proceed with a reasonable activation energy. Based on singlet and triplet vertical excitation energies, it is speculated that the photoreaction occurs via an intersystem crossing from S₁ to T₁ for cis-Pt(AAA)Cl·CH₃CN followed by an adiabatic reaction along the T₁ surface and then nonradiative intersystem crossing to the S₀ state of Pt(AAA)Cl.