Spectral Study of 1,4-bis(3-(2-pyridyl)pyrazol-1- ylmethyl)benzene (PPB): UV-VIS Absorption Spectra Investigation in Single and Binary Solvents and Spectrophotometric Determination of the Dissociation Constant (pKb)

Abstract

The effect of solvents of different polarity and hydrogen bonding ability on electronic
absorption spectra of PPB was investigated. UV absorption spectra (200-400 nm) were
recorded in five single solvents and one binary solvent (DCM-EtOH). PPB has two
absorption maxima (250 and 280 nm) due to pyridyl pyrazole and benzene systems. The
spectral shifts and absorption maxima in various solvents were correlated with the Kamlet
and Taft parameters (α, β and π*) using linear solvation energy relationships. The
multiparametric analysis indicates that hydrogen bond donor ability of the solvent (for band
I) and non-specific dipolar interactions of the solvents (for band II) play an important role
in absorption maxima of PPB in pure solvents. Index of preferential solvation was calculated
as a function of solvent composition. Preferential solvation by ethanol (band I) and by
dichloromethane (band II) was detected in DCM-EtOH solvent mixture. The pKb of PPB in
two solvent mixtures; EtOH – H2O & THF – H2O, were determined using UV-Vis
spectrophotometry. Two graphical methods were used to estimate the base dissociation
constant (pKb) using absorbance measurements. The resulting average of pKb values in the
two solvent mixtures using the two methods were 10.77 and 11.14 with a standard deviation
of 0.03 & 0.33, respectively