Femto- to Millisecond Photophysical Characterization of Indole-Based Squaraines Adsorbed on TiO2 Nanoparticle Thin Films

Abstract

In this paper, we address femto- to millisecond transient absorption studies of TiO2 nanoparticle (NP) thin films sensitized with four squaraine (SQ) molecules, with and without a deaggregating agent, chenodeoxycholic acid (CDCA). On the femto- to picosecond time scale, we determined the presence of three transient species by using singular value decomposition (SVD) analysis, i.e., S1 of the SQ monomers, S1 of the SQ H-aggregates, and the SQ radical cation formed after the electron injection. Both monomers and H-aggregates are proven to inject electrons to the TiO2 conduction band, being 5 times faster in the monomers (e.g., keimon = 5.1 × 1011 s–1 and keiH-agg = 1.1 × 1011 s–1 for SQ 41). Besides, the undesired singlet–singlet annihilation is an active process in these samples, constituting the drain of a high percentage of the absorbed photons. The coadsorption of CDCA on the TiO2 NP avoids the formation of H-aggregates, and therefore, only two transient species are present in these samples: S1 of the m...