Absorption of the mononuclear Fe(III) coordination compounds

Abstract

This paper presents the study of wide-gap semiconductors in the form of two new Fe(III) mononuclear coordination compounds ([Fe(H2L)(H2O)2]∙X2∙2.5H2O, where X=NO3-, ClO4-) based on the 2,6-diacetylpyridine bis(picolinoylhydrazone) (H2L).

Optical absorption spectra were measured using UV-Vis Spectrophotometer (PerkinElmer Lambda 25) at 300 K in the wavelength range 200-500 nm measured. The UV-vis spectral titration of H2L with Fe3+ were carried out by gradually increasing the amount of Fe3+ (10-100 μL) in the ethanol media of ligand. The spectra of three concentrations of Fe3+ (Fig.1) represent a series of curves characterized by the position of the absorption maxima at the same wavelength and differing only in their intensities. Upon successive addition of Fe3+ the absorbance bands gradually decreases in intensity, accompanied by the appearance of shoulder peaks in the region of 250-400 nm.

To estimate the band gap Eg, Tauc coordinates were used [1]: (α∙hν)1/n = hν - Eg, where α is the absorption coefficient, Eg is the band gap, n is a constant depending on the type of electronic transition in the material.

An analysis of the optical absorption spectra under the assumption of direct allowed transitions (n=1/2) is presented in the inset to figure 2a. Approximation of the intrinsic absorption edge constructed in coordinates (αhν)2 from (hν) gives the value of the direct band gap (Egdir).Approximation of the intrinsic absorption edge plotted in coordinates (αhν)1/2 from (hν), which corresponds to allowed indirect transitions, shows the value of the indirect band gap (Egindir).