Effect of electron withdrawing anchoring groups on the optoelectronic properties of pyrene sensitizers and their interaction with TiO2: A combined experimental and theoretical approach

A series of pyrene sensitizers (PC, PN, PMN, PR) with various electron withdrawing anchoring groups have been synthesized to understand their consequence on the optoelectronic properties. All the sensitizers have been characterized by NMR, mass spectroscopic and IR techniques. Absorption measurements revealed that charge transfer transition was enhanced by introducing electron withdrawing groups. Effect of solvents on the absorption and emission properties of the sensitizers was probed through multi-parameter Catalلn solvent scales. Thermal stability of the sensitizers was found to be influenced by the presence of COOH group. DFT/TDDFT calculations were performed to gain insight into the structure and electronic properties of the pyrene sensitizers. To investigate the binding affinity of pyrene sensitizers with TiO2, absorption measurement was performed and the results suggest that the sensitizer having COOH group along with a CN group (PC) show higher binding affinity over other sensitizers containing COOH group (PR) and NO2 group (PN). Further, laser flash photolysis measurement was carried out to study the electron transfer process between the sensitizers and TiO2. Electron injection into the conduction band of TiO2 was confirmed by the detection of cation radical of the sensitizer. We envisage that the results from this work will pave the way to design new efficient sensitizers with predetermined electron withdrawing anchoring groups and their photophysical properties for photovoltaic applications.