title: An integrated experimental and theoretical approach to the spectroscopy of organic-dye-sensitized TiO₂ heterointerfaces: disentangling the effects of aggregation, solvation, and surface protonation.
abstract: We report a joint experimental and computational study into the spectroscopic properties of a prototypical D5 organic dye, both in solution and adsorbed on a TiO2 surface, with the aim of modeling and quantifying the UV/Vis spectral shifts that occur in the different explored environments. Going from the dye in solution to dye-sensitized TiO2, various factors may shift the position of the UV/Vis absorption maximum, both towards longer and shorter wavelengths. Here we have focused on the effect of dye aggregation on TiO2, surface protonation, and solvent effects. The D5 dye forms stable aggregates on the TiO2 surface that cause spectral blueshifts. We used different sensitization conditions to vary the dye loading and thus the extent of dye aggregation. For each sensitization condition, we explored protonated and native TiO2 films. Computational modeling of different dimeric aggregates with increasing intermolecular interactions and simulation of the associated optical responses also confirm the observed spectral blueshifts. Our results show that both the presence of surface protons and solvent stabilize the excited state of the adsorbed dye molecules, which causes a marked redshift in the absorption maximum and thus moves in the opposite direction to the shift due to the increase in the surface coverage.
authors.affiliation: Computational Laboratory for Hybrid and Organic Photovoltaics, Istituto CNR di Scienze e Tecnologie Molecolari via Elce di Sotto 8, 06123, Perugia (Italy); Dipartimento di Chimica, Università degli studi di Perugia via Elce di Sotto 8, 06123, Perugia (Italy).
authors.forename: Gabriele
authors.initials: G
authors.lastname: Marotta
authors.forename: Maria Grazia
authors.initials: MG
authors.lastname: Lobello
authors.forename: Chiara
authors.initials: C
authors.lastname: Anselmi
authors.forename: Gabriella
authors.initials: G
authors.lastname: Barozzino Consiglio
authors.forename: Massimo
authors.initials: M
authors.lastname: Calamante
authors.forename: Alessandro
authors.initials: A
authors.lastname: Mordini
authors.forename: Mariachiara
authors.initials: M
authors.lastname: Pastore
authors.forename: Filippo
authors.initials: F
authors.lastname: De Angelis
keywords: aggregation; density functional calculations; dye-sensitized solar cells; dyes/pigments; titanium oxide