Optical and electrochemical properties of peripherally substituted chromium and manganese phthalocyanines

Abstract

New chromium phthalocyanine complex peripherally substituted with 8-quinolinoxy group was synthesized from modified precursor 4-(8-quinolinoxy) phthalonitrile in presence of CrCl3·6H2O in DMAE, using zinc dust as the reducing agent. The product μ-oxodimer of Cr(III)QPc [Cr(III)QPc-O-Cr(III)QPc] exhibited green florescence, in contrary to the typical red emissive behaviour of phthalocyanines. The detailed study on the emission spectrum of Cr(III)QPc-O-Cr(III)QPc revealed dual peaks with abroad band situated at 510 nm and a narrow peak situated at 686 nm, where the former broad band was found to be responsible for the intense green emission. The emission at 686 nm was the expected ligand based transition of Pc core from excited state singlet to ground state, S1 → S0, while the abnormal peak at 510 nm was found to be due to the combined effect of radiative relaxation of second excited singlet state to ground state (S2 → S0) and the LMCT of Pc2- to Cr(III) metal center. The emission spectra of Cr(III)QPc-O-Cr(III)QPc also exhibited a reciprocal relationship in the intensity of the dual peaks when the excitation wavelength was varied from 350 to 440 nm. The presence of an isoemission point at 648 nm indicated that two emission bands were brought about by two different excitonic transitions. Substituted chromium phthalocyanines are very less reported to date with remarkable fluorescent features, and in this work, the modified cyclotetramerization method applied by using zinc dust have paved new ways for their preparation. The preparation of unstable manganese phthalocyanine, MnQPc, was also rarely reported owing to its aerobic oxidation to the more stable and often synthesized analogue, MnQPc(CH3COO). Here, the synthesis of MnQPc was achieved under normal cyclization conditions and the effect of acetic acid and chlorinated solvents on the oxidative stability of MnQPc were also studied.