Extending conjugation in porphyrin dimer carbocations

Abstract

The addition of alkynyl porphyrins to carbonyl compounds was used to prepare a series of porphyrin dimer tertiary alcohols. Treatment of these alcohols with acid gave conjugated carbocations with three to nine carbon atoms bridging between the porphyrins. All these carbocations show strong absorption in the near-IR region between 1000–1800 nm. The absorption spectra exhibit a length-dependence similar to that of polymethine cyanines, with a bathochromic shift of approximately 200 nm (900 cm⁻¹) per alkyne. The longest of these chromophores has an absorption maximum at 1623 nm (in CHCl₃ with 2% trifluoroacetic acid). The symmetry of the π-system has a strong effect on the position, intensity and width of the absorption bands. The stability of the cations in solution decreases with increasing bridge length.