Highly pure synthesis, spectral assignments, and two-photon properties of cruciform porphyrin pentamers fused with benzene units.

Tetrameric porphyrin formation of 2-hydroxymethylpyrrole fused with porphyrins through a bicyclo[2.2.2]octadiene unit gave bicyclo[2.2.2]octadiene-fused porphyrin pentamers. Thermal conversion of the pentamers gave fully pi-conjugated cruciform porphyrin pentamers fused with benzene units in quantitative yields. UV/Vis spectra of fully pi-conjugated porphyrin pentamers showed one very strong Q absorption and were quite different from those of usual porphyrins. From TD-DFT calculations, the HOMO level is 0.49 eV higher than the HOMO-1 level. The LUMO and LUMO+1 levels are very close and are lower by more than 0.27 eV than those of other unoccupied MOs. The strong Q absorption was interpreted as two mutually orthogonal single-electron transitions (683 nm: 86 %, HOMO-->LUMO; 680 nm: 86 %, HOMO-->LUMO+1). The two-photon absorption (TPA) cross section value (sigma((2))) of the benzene-fused porphyrin pentamer was estimated to be 3900 GM at 1500 nm, which is strongly correlated with a cruciform molecular structure with multidirectional pi-conjugation pathways.