DFT and TD-DFT Investigation of a Charge Transfer Surface Resonance Raman Model of N3 Dye Bound to a Small TiO₂ Nanoparticle

• Main Authors: Ronald L. Birke, John R. Lombardi
• Format: Article
• Language: English
• Published: MDPI AG 2021-06-01
• Series: Nanomaterials
• Subjects: Raman; surface enhance Raman scattering; charge transfer; surface geometry; UV-VIS; DSSC
• Online Access: https://www.mdpi.com/2079-4991/11/6/1491

Raman spectroscopy is an important method for studying the configuration of Ru bipyridyl dyes on TiO₂. We studied the [Ru(II)(4,4′-COOH-2,2′-bpy)2(NCS)2)] dye (N3) adsorbed on a (TiO₂)5 nanoparticle using Density Functional Theory, DFT, to optimize the geometry of the complex and to simulate normal Raman scattering, NRS, for the isolated N3 and the N3–(TiO₂)5 complex. Two configurations of N3 are found on the surface both anchored with a carboxylate bridging bidentate linkage but one with the two NCS ligands directed away from the surface and one with one NSC tilted away and the other NCS interacting with the surface. Both configurations also had another –COOH group hydrogen bonded to a Ti-O dangling bond. These configurations can be distinguished from each other by Raman bands at 2104 and 2165 cm⁻¹. The former configuration has more intense Normal Raman Scattering, NRS, on TiO₂ surfaces and was studied with Time-Dependent Density Functional Theory, TD-DFT, frequency-dependent Raman simulations. Pre-resonance Raman spectra were simulated for a Metal to Ligand Charge Transfer, MLCT, excited state and for a long-distance CT transition from N3 directly to (TiO₂)5. Enhancement factors for the MLCT and long-distance CT processes are around 1 × 103 and 2 × 102, respectively. A Herzberg–Teller intensity borrowing mechanism is implicated in the latter and provides a possible mechanism for the photo-injection of electrons to titania surfaces.