Extending the porphyrin core: synthesis and photophysical characterization of porphyrins with π-conjugated β-substituents

Abstract

New free-base and Zn-porphyrins with either vinyl groups or p-phenylene vinylene groups substituted on the β carbon have been synthesized and studied from a photophysical viewpoint in toluene. A bathochromic shift of all absorption bands together with a pronounced decrease in the molar absorption coefficient at the wavelength of maximum absorption (ε_max) and a broadening of the Soret band was observed with increasing conjugation of the substituent. A similar bathochromic shift trend was also observed in the corresponding emission spectra. The results are discussed in terms of delocalization of the frontier molecular orbitals of the porphyrin ring onto the substituent π-orbitals, and semi-empirical calculations confirming this interpretation are presented. High reactivity with oxygen was displayed by the triplet excited states of both free-base and Zn-porphyrins, and sensitized singlet oxygen (¹Δ_g), detected via its luminescence, was formed with high yields. Moreover, the transient absorption spectra of the lowest triplet displayed interesting features in the near-infrared region: a clear enhancement in the molar absorption coefficient in the range 600–900 nm was observed with increasing conjugation of the substituent.