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

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., k ei mon = 5.1 × 1011 s–1 and k ei H‑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 monomer and the SQ radical cation with k ei mon = 6.7 × 1011 s–1 for SQ 41. On the microsecond scale, we only observed the transient feature of the radical cation of the SQ that permits one to study its recombination dynamics. Similar lifetimes (94–150 μs) of the four SQ radial cations are obtained when only monomers are present in the sample. In the absence of CDCA, the presence of H-aggregates contributes to shorten the lifetime of the radical cation (e.g., from 110 to 45 μs in the case of SQ 41). This fact can be explained by considering a stronger electronic coupling of H-aggregates/TiO2 surface with respect to the monomers. These results explore the photodynamics of this family of SQs adsorbed on TiO2 NP in a very large time window and will enable a better understanding of the influence of aggregates in the kinetics of these SQs used as sensitizers in DSSCs.