Studies on some new meso-aryl substituted octabromo-porphyrins and their Zn(II) derivatives

Abstract

A series of porphyrins with various tolyl (H 2TTHP) and naphthyl substituents (H 2TNHP) at the meso-positions, their octabromoderivatives (H 2TTBP and H 2TNBP) with Br substituents at β-pyrrole positions and also their Zn(II) derivatives have been synthesised and characterised by 1H NMR, electronic, fluorescence and electrochemical studies. These have the meso-carbons bonded to tolyl moieties at ortho-, meta- and para-positions of the tolyl groups and at the α- or β-position of the naphthyl group. For the octabromoporphyrins, pronounced deshielding of NH protons and a moderate shift of meso-aryl protons to a lower δ value are observed compared to their nonbrominated species. The electronic spectra of ZnTTHP and ZnTNHP have almost the same B and Q bands while the B band of their free-base analogues have H 2TNHP absorbing at a higher wavelength than H 2TTHP. All the octabromoderivatives exhibit a pronounced red shift for both B and Q bands (compared to their nonbrominated forms) and show meso-substituent dependent change in both free-base and metallated forms. The above observations are interpreted in terms of moderate conjugative interaction of the aryl substituent with the π framework and also in terms of energy level reordering which alters the HOMO–LUMO gap. Consistent with the absorption spectral data the emission bands of all the bromoporphyrins were also seen to be red shifted considerably. Significant decrement in quantum yield ( φf) was observed for the bromocompounds. While the φf of nonbrominated porphyrins is seen to be higher than their Zn(II) derivatives the reverse order is observed for the bromoderivatives. The ability of the Zn 2+ ion to make the bromoporphyrins resistant to distortion by bridging the central cavity can be attributed as the cause for this interesting observation. Cyclic voltammetric studies exhibit characteristic quasi-reversible/irreversible oxidation–reduction features for all the free-bases and Zn(II) derivatives. The bromoderivatives on the other hand manifest marginally harder oxidation and very easy reduction features. The data are interpreted in terms of electron withdrawing ability of Br atoms and to reordering of HOMO and LUMO levels due to distortion in the porphyrin ring.