Density functional theory studies of meso-alkynyl porphyrins

Geometry optimizations and electronic structure calculations are reported for porphyrins with meso-acetylene substituents using density functional theory (DFT). The addition of the acetylene group alters the nearest C–C bond length in the porphyrin ring by 0.01 to 0.02 Å, but the molecule retains its $D_{\text{2h}}$ symmetry. The calculated electronic structures clearly show that the acetylene group contributes to the π-electron conjugation along the porphyrin ring for the HOMO and LUMO, and reduces significantly the HOMO-LUMO gap, being consistent with experimental results [H. L. Anderson, Tetrahedron Lett. 33, 101 (1992)]. The core ionization potential (IP) of the central nitrogen calculated using the so-called local density approximation effective core (LDAEC) method, is increased by 0.24 eV, indicating a more positive electrostatic potential than in ${\mathrm{PH}}_2$ . LDA-ΔSCF results for the first three valence IPs indicate changes of 0.1 to 0.2 eV relative to those of the free-base porphin, with the gap between the first and second IPs being over twice as large in the acetylated molecule. The protective trimethylsilyl (TMS) group further reduces the HOMO-LUMO gap, but by a small amount.