First Synthesis of meso-Chlorinated Tetrabenzoporphyrins

meso-Chlorinated bicyclo[2.2.2]octadiene-fused porphyrins 2 (Cl(n)TBCODPs; n = 1-4) have been synthesized by chlorination of the free base TBCODP-H-2 (1-H-2) using N-chlorosuccinimide (NCS). The chlorinated derivatives were easily separated by column chromatography on silica gel to give 2-H-2 in good yields. This method for the chlorination of porphyrin using NCS is mild, safe, and economic compared with the established procedure for the meso-chlorination of porphyrins. We also discovered the unexpected meso-chlorination of the porphyrin with dichlorodicyanobenzoquinone (DDQ) by single-electron transfer. Cl(n)TBCODPs 2 were converted into tetrabenzoporphyrins (Cl(n)TBPs) 3 in 100% yields by retro-Diels-Alder reaction. The derived compounds were characterized by NMR spectroscopy, UV/Vis spectrophotometry, cyclic voltammetry (CV), X-ray crystallography, and organic field-effect transistor (OFET) characteristics. The introduction of chlorine at the meso positions of the TBPs drastically changed their electronic and structural properties. The maxima of protonated Cl4TBP-H-2 (3d-H-2i lambda(max) = 494, 649, and 707 nm) were redshifted by around 50-65 nm relative to those of protonated TBP-H-2 (lambda(max) = 431, 605, and 660 nm). The two Q-band maxima of the Cl4TBP dication are similar to those of a tetraacenaphthoporphyrin dication (lambda(max) = 525, 646, and 702 nm), which has one of the most redshifted Q-bands. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)