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.