Effect of electron withdrawing substituent and extended π- conjugation on photophysical properties of Ruthenium polyterpyridine D-P-A complexes and interfacial studies with semiconducting TiO 2 nanoparticle: Experimental and computational evidences

Abstract

Abstract The necessity of improving the excited state properties for efficient interfacial charge transfer to semiconducting TiO 2 material drag us to design complex 2 [(L2)Ru(L4)]2PF 6 and complex 3 [(L2)Ru(L1)Ru(L3)]4PF 6 based on D-P-A functionalisation (D = Donor, P = Photosensitizer and A = Acceptor). The study involves a comparative account of complexes 1–3, where our previously reported complex 1 considered as a basic motif. We have improved the excited state properties of complex 1 by substituting with an additional electron withdrawing group and extended π-conjugation in mononuclear complex 2 and binuclear complex 3 respectively. Compared to the average excited state lifetime (τ avg ) of 5.54 ns for complex 1, both complexes 2 and 3 show improved excited state lifetime of 10.24 ns and 22.06 ns respectively. Further, the anchoring of complexes on high bandgap TiO 2 semiconductor surface to perform interfacial charge transfer have been shown via Time-Correlated Single Photon Counting (TCSPC) studies, which displayed quenched decay pattern and decreased excited state lifetime in the complex 2/3-TiO 2 system compared to their bare complex solution. Therefore, the obtained average excited state lifetime in complex 2 and 3 is sufficient to perform interfacial charge transfer study in the semiconductiong TiO 2 nanoparticle. This interfacial charge transfer further strengthened by the femtosecond transient absorption spectroscopic studies in colloidal TiO 2 nanoparticle which give the evidence for the presence of cation radical and injected conduction band electron (e − CB) through their characteristic absorption. The Cyclic voltammetry experiment exhibit first reduction potential ( E red 1 ) for complex 2 and 3 at −1.22 V and −1.17 V respectively compared to −1.27 V for complex 1. The higher value of E red 1 in complex 2 and 3 indicate the higher electron affinity of the ligand for reduction due to an additional electron withdrawing group and extended π- conjugation respectively. Also, the TDDFT studies using B3LYP exchange correlations functional and LANL2DZ basis set along with salvation effect in Gaussian 09 programs give an overview of transitions involved in 1 MLCT and are in support with the experimental UV–vis absorption spectra of complexes. The trend of LUMO energy level for complexes obtained from electrochemical studies is also in agreement with the LUMO energy level distribution obtained from DFT analysis.