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

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 1 H 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, Sn IV , 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.