Two-photon absorption in butadiyne-linked porphyrin dimers: torsional and substituent effects

Dyes exhibiting efficient two-photon absorption (2PA) are in demand for a wide range of applications, and meso–meso butadiyne-linked porphyrin dimers (bis-porphyrins) are promising chromophores in this area. As part of an investigation of the structure–property relationships controlling the performance of these dyes, we present the synthesis of eight π-extended dimers, with substituents providing diverse push and pull electronic effects, and high solubility. We show that the peak 2PA cross sections can be increased from 3000 GM to 20 000 GM by attaching terminal electron-withdrawing or -donating groups, and by converting the free-base dimers to zinc complexes. The two-photon excited fluorescence spectra of porphyrin dimers in viscous media, under conditions such that excited states do not planarize prior to emission, reveal that dimers in planar conformations dominate the two-photon absorption.