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 Angstrom, but the molecule retains its D-2h symmetry. The calculated electronic structures clearly show that the acetylene group contributes to the pi-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 (IF) 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 PH2. LDA-Delta SCF results for the first three valence IPs indicate changes of 0.1 to 0.2 cV 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. (C) 1998 American Institute of Physics.