Synthesis and spectroscopic properties of axial phenoxide and para amino phenoxide incorporated indium (III) porphyrins

Abstract

Tetrakis-4-methoxyphenyl indium(III) porphyrins containing phenolic and para amino phenolic moieties in the apical sites have been successfully prepared and characterized spectroscopically. The phases and structure evolutions of prepared porphyrins were deduced from powder X-ray diffraction (PXRD). The axial coordination of phenolic moieties induces slight shift of the 1H NMR signals of indium porphyrin macrocycles. The measurements of UV–Visible absorptions of porphyrins in CH2Cl2 and CH3COOC2H5 reflects that axially ligated metal chelates at neutral pH display hyperchromicity of absorption bands compared to unligated metal porphyrin. The most striking feature was the structural stability of indium porphyrins under basic (pH = 10) as well as acidic media (pH = 4). On the other hand, the metal free ligand 5, 10, 15, 20-Tetrakis-(4-methoxyphenyl) porphyrin, H2TMP (I) undergoes J type aggregation in acidic media (pH = 4). The robustness of indium porphyrins especially in acidic media are in strong contrary to other sitting atop (SAT) or out of plane (OOP) complexes which are usually demetallized under acidic pH. All the prepared macro rings exhibit negative solvatochromism reflected by the blue shift of absorption bands in CH3COOC2H5 (more polar) than in CH2Cl2 (less polar). The photoluminescence emissions in indium(III) species are significantly quenched relative to metal free ligand (I) due to internal heavy atom effect and distortion of ligand plane. The quenching of fluorescence bands are more pronounced in axially ligated metal porphyrins. The lesser excited state lifetimes of indium(III) porphyrins relative to free base analog (I) may be ascribed to spin orbit coupling induced by indium(III) ion in the respective metal chelates.