Tin(IV) Porphyrins Containing β‐Substituted Bromines: Synthesis, Conformations, Electrochemistry and Photophysical Evaluation

Abstract

Tin(IV) porphyrins containing bromine atoms at 2,3,12,13 β‐pyrrole positions are synthesised using four different bisaxial ligands; namely, chloro 2, hydroxo 3, phenolato 4 and 4‐nitrophenolato 5. The compounds are characterised using 1H NMR spectroscopy, ESI‐MS and elemental analysis. Despite steric loading at the β‐pyrrole positions, the X‐ray diffraction crystal structure analysis of 4 reveals a planar macrocyclic conformation. However, the structure of 5 shows a nonplanar conformation, with a predominant out‐of‐plane wave distortion mode. Normal‐coordinate structural decomposition (NSD) analysis, as well as analysis of the Sn–N distances, shows that the relief of steric strain in 4 is through in‐plane core elongation, while in 5, both in‐plane and out‐of‐plane distortions are evident. The effect of the highly charged central metal ion, SnIV, together with β‐bromine atoms and different axial ligands on the photophysical and electrochemical redox properties of the porphyrin π‐ring system, is studied with UV/Vis, fluorescence and transient‐absorption spectroscopy. Photophysical studies disclose redshifted absorption bands and reduced fluorescence quantum yield, as a result of the presence of bromine atoms. Cyclic voltammetric measurements indicate that the porphyrin π system becomes much easier to reduce. Preliminary experiments on the singlet‐oxygen‐generation efficiency reveal the role of the axial ligands in tuning such properties.