We report on photophysical studies of lumichrome (Lc) in water at different pHs, and interacting with the human serum albumin (HSA) protein and β-cyclodextrin (β-CD) in neutral aqueous solutions. We used steady-state and picosecond time-resolved emission spectroscopy to investigate the structural changes of Lc at the ground and excited states, as well as the rotational dynamics of the complexes with HSA and β-CD. In neutral water, the predominant neutral alloxazine-type structure of Lc coexists with a small population of the anionic form. In the presence of HSA, we observed an increase in the absorption band intensity at 450 nm. This increase is due to a preferential complexation (1:1 stoichiometry, K=8600 M(-1)) of the Lc anion structures within the protein. This change is not observed when β-CD is added, in which the Lc neutral form is exclusively complexed, giving a 1:1 stoichiometry. The fluorescence lifetimes of Lc in neutral water solutions are 4.2 and 2.3 ns, assigned to anionic and neutral alloxazinic forms, respectively. Using β-CD, the lifetime of the 1:1 complexes is 0.74 ns, while in the case of HSA complexes we observed two lifetimes (0.83 and 0.14 ns), which we explained in terms of different interactions of the anions with the protein. The rotational relaxation time of free Lc in neutral water is 75 ps. For Lc:β-CD complexes this time is 0.44 ns, in full agreement with the expected value from the hydrodynamic theory. For HSA solutions, we obtained a distribution of values between ∼1 and 4.5 ns, suggesting a site heterogeneity of complexation and a different strength of binding for the involved Lc anionic forms. Our results give information about the different photorelaxation behavior of Lc within chemical and biological cavities, and might help in a better design of nanosystems for drug carriers and delivery.

We report on femto-to nanosecond emission studies of the interaction of an organic dye (TPC1) for solar cells, of (electron-donor)À(π-spacer)À(electron-acceptor) structure, with different semiconductor particles and aluminum-doped MCM-41 silicate mesoporous material, in a dichloromethane (DCM) suspension. We used ZnO, ZrO 2 , and Al 2 O 3 nanoparticles employed in dye-sensitized solar cells as active electron collection materials or insulating layers. Steady-state absorption and emission spectra reflect strong complex formation between TPC1 and the used materials. The femto-to nanosecond emission transients of the interfacial systems show a nonexponential behavior with an averaged half lifetime of 4, 11, and 150 ps for ZnO, ZrO 2 , and Al 2 O 3 , respectively. For the latter, we observed the effect of the dye's concentration indicating the action of a fluorescence self-quenching mechanism. For ZnO and ZrO 2 samples, the lifetime of the complexes is determined by an electron injection rate to the conduction band and trap states of these semiconductor samples. The electron injection does not occur efficiently from the high vibrational levels of TPC1 at the S 1 state, and the subpicosecond dynamics is dominated by solvation with a time similar to that of TPC1/DCM (1.4 ps). It is in contrast with the previously observed strong emission quenching of the hot S 1 state when interacting with titania. We observed a remarkable very efficient deactivation of excited TPC1 (with half-lifetime of 1.5 ps) when interacting with Al-doped MCM-41, probably due to an electron transfer from the dye to the aluminum-doped silica framework having an acid character, and a different Al orbital configuration than that of Al 2 O 3 . We believe that the results presented here will enable a better understanding of the interaction of organic dyes with the surface of nanomaterials used in photovoltaics and help in exploring alternative charge-collective materials for the solar cell improvements.

In this paper, we address femto-to millisecond transient absorption studies of TiO 2 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., S 1 of the SQ monomers, S 1 of the SQ Haggregates, and the SQ radical cation formed after the electron injection. Both monomers and H-aggregates are proven to inject electrons to the TiO 2 conduction band, being 5 times faster in the monomers (e.g., k ei mon = 5.1 × 10 11 s −1 and k ei H-agg = 1.1 × 10 11 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 TiO 2 NP avoids the formation of H-aggregates, and therefore, only two transient species are present in these samples: S 1 of the monomer and the SQ radical cation with k ei mon = 6.7 × 10 11 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/TiO 2 surface with respect to the monomers. These results explore the photodynamics of this family of SQs adsorbed on TiO 2 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.

We present femto-to-millisecond studies of the photodynamics of seven types of indole-based squaraine molecules (SQs) in solvents of different H-bonding ability and viscosity. These SQs can be classified into two families: SQs with two carboxylic groups in the side indole groups (symmetrical SQs) and with only one carboxylic group (asymmetrical SQs). Steady-state absorption and fluorescence techniques show narrow absorption and emission bands, with a small Stokes shift (about 300 cm(-1)). The femtosecond transient absorption spectra give a very short (∼100 fs) dynamics (assigned to IVR) and the associated spectra show two excited species assigned to two stereoisomers. A trans-cis photoisomerization occurs in a very fast time through a conical intersection. Pico-to-nanosecond emission experiments also reveal the presence of two fluorescing trans stereoisomers whose lifetimes show similar sensitivities to the nature of solvent. For example, lifetimes of 1.72, 0.46 and 0.29 ns were determined for the trans photoisomer of the SQ 41 in triacetin, dichloromethane and acetonitrile, respectively, reflecting the short decay of the S(1) state in highly polar and low viscous solvents. Flash photolysis experiments gave the transient absorption signals of the cis photoisomer that is formed after the twisting process at S(1). The cis-to-trans photoisomerization at the ground state happens in the μs time scale (1-4 μs), and it depends on the H-bonding ability and viscosity of the solvent. Thus, combining fs-ns and ns-μs experiments suggests that in the conical intersection region, only a small fraction of the twisted trans isomers are converted to the cis ones in the excited states. These results bring detailed and global insight into the large time window photodynamics of this family of SQs in solution.