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

Mixed substitution at the beta-position of porphyrins influences their photophysical and electrochemical redox properties. Two new series of asymmetrically mixed beta-octasubstituted porphyrins viz. MTPP (Ph)(2)Br5X (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)(2)Br-5 showed a nonplanar saddle shape conformation of the macrocyclic core. Furthermore, the fully optimized geometries confirmed the saddle shape conformation of H2TPP(Ph)(2)Br5X (X = NO2 or Br). Electronic spectra revealed a significant bathochromic shift by appending both electron donor and acceptor substituents at the beta-position of the meso-tetraphenylporphyrin skeleton, which reflects the following order H2TPP < H2TPP(NO2) < H2TPP(NO2)(Ph)(2) < H2TPP(Ph)(2)Br-6 < H2TPP(NO2)(Ph)(2)Br-5. H2TPP(Ph)(2)Br5X (X = NO2 or Br) exhibited a significant bathochromic shift (Delta lambda(max) = 53-61 nm) in the Soret band and (Delta lambda max = 90-95 nm) in the longest wavelength Qx(0,0) band as compared to H2TPP. Nonplanar conformations and electron withdrawing beta-substituents induce higher protonation and deprotonation constants for H2TPP(NO2)(Ph)(2)Br-5 and H2TPP(Ph)(2)Br-6 as compared to precursor porphyrins viz. H2TPP, H2TPP(NO2) and H2TPP(NO2)(Ph)(2). The electronic spectral properties and redox potentials of MTPP(Ph)(2)Br5X (X = NO2 or Br and M = 2H, Co-II, Ni-II, Cu and Zn-II) are affected by beta-substituents at the periphery of the porphyrin core. Redox tunability was achieved by appending push-pull substituents at the beta-position of the MTPP (M = 2H, Co-II, Ni-II, Cu-II, and Zn-II) skeleton of the macrocycle. CuTPP(Ph)(2)Br-6 and CuTPP(NO2)(Ph)(2)Br-5 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 beta-substituents and nonplanarity of the porphyrin core.