Synthesis, Optical Properties and DNA‐Binding Behavior of a Quinoxaline Ring‐Fused π‐Elongated Chlorin – Efforts Towards Preparation of Long Wavelength Absorbing Porphyrinoids

Abstract

A porphyrinoid containing cis‐diol pyrroline subunit and fused nitro‐quinoxaline π‐elongated unit at its peripheral β,β’‐positions was prepared. The UV‐visible spectrum exhibits typical chlorin‐like sharp Qy‐band, but significantly more red‐shifted (687 nm) and more intense than for a normal diol chlorin. The precursor porphyrin exhibited weak Q‐bands slightly above 650 nm. The same porphyrin exhibited a low pH dependent ∼40 nm red‐shift in its UV‐visible spectrum. Analysis of the frontier molecular orbital energies using electrochemical techniques and theoretical analysis of the lowest energy geometry of the porphyrin confirmed the roles of non‐planar conformation and electronic charge redistribution in inducing this spectral feature. In addition, biophysical and computational analysis of DNA‐binding behavior of these porphyrinoids indicated that groove binding induced by Van der Waals contacts was the preferred mode of interaction.