Spectroscopic properties and photophysics of the synthesized compound 5-nitro-benzo[b]thiophene-2-carboxylic acid in non-polar/polar media and in the presence of TiO2 nanoparticles

The present work investigates by electrochemical and steady-state and time-resolved spectroscopic methods a synthesized compound 5-nitro-benzo[b]thiophene-2-carboxylic acid (5NBTC), both in normal solvents and in the presence of TiO2 nanoparticles to reveal the nature of the photophysical processes involved. From the present experimental observations it is inferred that both in the ground state and the excited electronic state S1, there exists a strong binding between –COOH functionality of 5NBTC and TiO2 nanoparticles. However, the rupture of this binding in the presence of excess TiO2, as apparent from the steady-state and time-resolved spectroscopic measurements, is responsible for the increase in radiative transitions. Formations of aggregations of TiO2 nanoparticles at higher concentrations appear the cause of such rupturing. The redox potential measurements by cyclic voltammetry and theoretical computations by time-dependent density functional theory (TD-DFT) with B3LYP/6-311 G(d, p) basis function implemented in the Gaussian package confirm the electron accepting nature of 5NBTC and hence no electron transfer is possible between the organic compound and TiO2 nanoparticles. It is most likely that the interaction model between 5NBTC and TiO2 nanoparticles should be that the –COOH group of 5NBTC molecule coordinates either directly or through a hydrogen bond to the TiO2 surface.