Unsymmetrical nonplanar ‘push–pull’ β-octasubstituted porphyrins: facile synthesis, structural, photophysical and electrochemical redox properties

Abstract

Mixed substitution at the β-position of porphyrins influences their photophysical and electrochemical redox properties. Two new series of asymmetrically mixed β-octasubstituted porphyrins viz. MTPP(Ph)₂Br₅X (X = NO₂ or Br and M = 2H, Co(II), Ni(II), Cu(II), and Zn(II)) have been synthesized and characterized by various spectroscopic techniques. The single crystal X-ray structure of H₂TPP(NO₂)(Ph)₂Br₅ showed a nonplanar saddle shape conformation of the macrocyclic core. Furthermore, the fully optimized geometries confirmed the saddle shape conformation of H₂TPP(Ph)₂Br₅X (X = NO₂ or Br). Electronic spectra revealed a significant bathochromic shift by appending both electron donor and acceptor substituents at the β-position of the meso-tetraphenylporphyrin skeleton, which reflects the following order H₂TPP < H₂TPP(NO₂) < H₂TPP(NO₂)(Ph)₂ < H₂TPP(Ph)₂Br₆ < H₂TPP(NO₂)(Ph)₂Br₅. H₂TPP(Ph)₂Br₅X (X = NO₂ or Br) exhibited a significant bathochromic shift (Δλ_max = 53–61 nm) in the Soret band and (Δλ_max = 90–95 nm) in the longest wavelength Q_x(0,0) band as compared to H₂TPP. Nonplanar conformations and electron withdrawing β-substituents induce higher protonation and deprotonation constants for H₂TPP(NO₂)(Ph)₂Br₅ and H₂TPP(Ph)₂Br₆ as compared to precursor porphyrins viz. H₂TPP, H₂TPP(NO₂) and H₂TPP(NO₂)(Ph)₂. The electronic spectral properties and redox potentials of MTPP(Ph)₂Br₅X (X = NO₂ or Br and M = 2H, Co, Ni, Cu and Zn) are affected by β-substituents at the periphery of the porphyrin core. Redox tunability was achieved by appending push–pull substituents at the β-position of the MTPP (M = 2H, Co^II, Ni^II, Cu^II, and Zn^II) skeleton of the macrocycle. CuTPP(Ph)₂Br₆ and CuTPP(NO₂)(Ph)₂Br₅ exhibited a dramatically reduced HOMO–LUMO gap with a difference of 0.55 V and 0.62 V, respectively as compared to CuTPP due to the push–pull effect of β-substituents and nonplanarity of the porphyrin core.