Density functional theory study of electronic absorption spectra and intermolecular interactions of porphyrin-borate complexes

Electronic absorption spectra and intermolecular interactions of complexes formed between meso-tetrakis (1-methylpyridinium-4-yl) porphine (TMPyP) and borates (tetraphenylborate (TPB) anion or fluorinated derivatives of TPB (TFPB, TFMPB)), namely TMPyP-TPB, TMPyP-TFPB and TMPyP-TFMPB, are investigated by density functional theory (DFT) and time-dependent DFT (TDDFT). It is revealed that the frontier molecular orbitals and electronic transitions for TMPyP have varied when TMPyP is incorporated with borates. TDDFT calculated Q (0, 0) and B (0, 0) bands of TMPyP exhibit a large bathochromic shift when TMPyP turns from its monomeric to complexed forms, and the obtained absorption maxima are in good agreement with the experimental data. The extension of Cα-Cm bonds for TMPyP in complexes leads to the weakened asymmetric Cα-Cm stretching vibration, and then moves Qx (0, 1) transition to longer wavelengths. The flattening of TMPyP in complexes is suggested to be the main structural origin of the red-shift event. The calculated interaction energies between TMPyP and borates are in the range of 9.13-18.82 kcal/mol with intensity sequence of TMPyP-TPB > TMPyP-TFPB >> TMPyP-TFMPB. The C-H⋯π static attraction is proposed to be mainly responsible for the stability of the complexes.