Synthesis, spectral, and electrochemical studies of electronically tunable β-substituted porphyrins with mixed substituent pattern.

Two new families of porphyrins with mixed substituent pattern, viz. 2-nitro-12,13-disubstituted-meso-tetraphenylporphyrins (H2TPP(NO2)X2, X = Ph, phenylethynyl (PE), 2-thienyl (Th), Br, and CN) and 2-nitro-7,8,12,13,17,18-hexasubstituted-meso-tetraphenylporphyrins (H2TPP(NO2)X6, X = Br, Ph, PE, and Th), and their metal (Co(II), Ni(II), Cu(II), and Zn(II)) complexes have been synthesized and characterized. H2TPP(NO2)X6 exhibited remarkable red shift in the Soret band (45-70 nm) and longest wavelength band, Qx(0,0) (65-90 nm), as compared to H2TPP(NO2). Single-crystal structures of MTPP(NO2)X2 (M = Zn(II) and Ni(II); X = Br, Ph, and PE) showed quasi-planar conformation, whereas H2TPP(NO2)Th2 and NiTPP(NO2)Br6 exhibited moderate and highly nonplanar saddle-shape conformations, respectively. Further, DFT fully optimized geometries of H2TPP(NO2)X2 and H2TPP(NO2)X6 revealed moderate and severe saddle-shape nonplanar conformations, respectively. The imino proton resonances of H2TPP(NO2)X6 are significantly downfield shifted (Δδ = 1.10-1.80 ppm) relative to H2TPP(NO2). Mixed substituted highly nonplanar porphyrins exhibited higher protonation and deprotonation constants relative to H2TPPX8 (X = Cl and Br). The redox tunability was achieved by introducing electron donor (Ph and Th) and acceptor (PE, Br, and CN) groups on the MTPP(NO2) backbone. The unusual variation in the spectral and electrochemical redox properties of mixed substituted porphyrins are interpreted in terms of both an inductive and a resonance interaction of substituents on the porphyrin π-system as well as nonplanarity of the macrocycle.