Facile one-pot synthesis of triply fused, fluorescent chlorin–porphyrin heterodimers with near-IR absorption: impact of π-conjugation on optical properties

Abstract

Herein, we report a facile one-pot synthesis of a remarkably stable dication diradical of a triply fused dizinc(II)chlorin–porphyrin heterodimer along with its neutral form and free base heterodimer under mild conditions. The transformation occurs via a radical-induced oxidative rearrangement in which a dipyrromethene bridge completely fuses between two porphyrin macrocycles with the formation of two new C–C bonds and one C–N bond, leading to the formation of two six-membered rings and one seven-membered ring. The two macrocycles exhibit extensive π-conjugation through the bridge, which results in antiferromagnetic coupling between the two π–cation radicals of the macrocycles. Reduction of the dication diradical complex using NaBH₄ produces the neutral product, which, upon demetallation under mild acidic conditions (5% HCl), enables us to isolate a stable, green free-base heterodimer in a quantitative yield. Due to their structural rigidity and extended π-conjugation, the neutral triply fused dimers of zinc and the freebase macrocycle exhibit unique spectral features that include near-IR absorption bands, a narrow HOMO–LUMO band gap and intense fluorescence with increased quantum yields (ϕ_f). Apart from this, these dimers exhibit strong nonlinear optical properties involving two-photon absorption TPA (σ₂) due to their rigid π-conjugated structures, while the Zn complex exhibits much higher (σ₂) values than their Ni and Cu analogs. In contrast to the neutral heterodimer, a large enhancement in the TPA coefficient was observed for their singlet dication diradical complex with the same structural framework.