Spectroscopic studies on the intermolecular charge transfer interaction of Fe(II)- and Fe(III)-phthalocyanines with 2,3,5,6-tetrachloro-1,4-benzoquinone and its application in colorimetric sensing of amino acids and amines

The interactions of FeIIPc and FeIIIPc with π-acceptor 2,3,5,6-tetrachloro-1,4-benzoquinone (p-chloranil, p-CHL) have been investigated spectroscopically (UV/vis and FT-IR) and spectrofluorimetrically at three different temperatures. The stoichiometry of the complexes was found to be 1:1. The results of electronic spectral studies indicated that the formation constant for FeIIPc-p-CHL system is found to be higher than that for FeIIIPc-p-CHL system. This observation is well supported by the results of fluorescence quenching studies and the association constants calculated for FeIIPc-p-CHL system is 4.2 × 103 mol L−1 and that for FeIIIPc-p-CHL system is 2.2 × 103 mol L−1. The data are discussed in terms of physico-chemical parameters viz. molar extinction coefficient, oscillator strength, dipole moment, ionization potential, dissociation energy and thermodynamic parameters. The results indicated that the formation of π–π CT complex is spontaneous and endothermic. Preliminary studies indicated that the CT complex can effectively be used as a colorimetric agent for sensing amino acids and amines.

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► The results indicated that FeIIPc-p-CHL complex was relatively stronger than FeIIIPc-p-CHL complex.
► Spectrofluorimetric and thermodynamic studies corroborate the observations.
► The FeIIIPc-p-CHL CT complex colorimetrically senses amino acids and amines.