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)t(AAA)Cl center dot CH3CN], and the transition state in the complexation reaction. B3LYP/6-31 center dot G* (effective core potential for Pt) optimized geometries of Pt(AAA)Cl and Pt(AAA)Cl center dot 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 center dot 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 center dot 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)Cl center dot CH3CN, the calculations suggest a reassignment of the experimental S-1 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 2.98 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 S-1 to T-1 for cis-Pt(AAA)Cl center dot CH3CN followed by an adiabatic reaction along the T, surface and then nonradiative intersystem crossing to the S-0 state of Pt(AAA)Cl.