The photophysical properties of two pyrene-bodipy molecular dyads, composed of a phenyl-pyrene (Py-Ph) linked to the meso position of a bodipy (BD) molecule with either H-atoms (BD1) or ethyl groups (BD2) at the 2, 6 positions, are investigated by stationary, nanosecond and femtosecond spectroscopy. The properties of these dyads (Py-Ph-BD1 and Py-Ph-BD2) are compared to those of their constituent chromophores in two solvents namely 1,2 dichloroethane (DCE) and acetonitrile (ACN). Stationary spectroscopy reveals a weak coupling among the subunits in both dyads. Excitation of the Py subunit eads to emission that is totally governed by the BD subunits in both dyads pointing to excitation energy transfer (EET) from the Py to BD chromophore. Femtosecond fluorescence and transient absorption spectroscopy reveal that EET takes place within 0.3-0.5 ps and is mostly independent of the solvent and the type of the BD subunit. The EET lifetime is in reasonable agreement with that predicted by Förster theory. After EET has taken place, Py-Ph-BD1 in DCE and Py-Ph-BD2 in both solvents decay mainly radiatively to the ground state with 3.5 - 5.0 ns lifetimes which are similar to those of the individual BD chromophores. However, the excited state of Py-Ph-BD1 in ACN is quenched having a lifetime of 1 ns. This points to the opening of an additional non-radiative channel of the excited state of Py-Ph-BD1 in this solvent, most probably charge separation (CS). Target analysis of the TA spectra has shown that the CS follows an inverted kinetics and is substantially slower than the recombination of the charge-separated state. Occurrence of CS with Py-Ph-BD1 in ACN is also supported by energetic considerations. The above results indicate that only a small change in the structure of the BD units incorporated in the dyads, significantly affects the excited state dynamics leading either to a dyad with long lifetime and high fluorescence quantum yield or to a dyad with an intramolecular CS ability.

The excited state and electron injection dynamics of three new organic sensitizers, comprising a triphenylamine moiety connected by an ethenylene (C−C double-bond) or ethynylene (C−C triple-bond) Ï€-spacer to an electron-withdrawing benzothiazole bearing a cyanoacrylic acid anchoring group, have been studied using a combination of steady-state and femtosecond-resolved spectroscopies. The measurements were carried out for the three dyes in predominantly neutral and completely deprotonated forms in liquid solutions and bound on nanocrystalline TiO₂ and Al₂O₃ thin films. In addition, quantum-chemical calculations were performed to predict absorption spectra of the sensitizers and their corresponding cation radicals. Time-resolved fluorescence (TRF) measurements on TiO₂ indicate that electron injection takes place on a <0.2 ps time scale. Transient electronic absorption (TA) measurements provide evidence for the formation of radical cations not only in dye-sensitized TiO₂ films but also in Al₂O₃ ones. The cation lifetime in Al₂O₃ is significantly shorter compared to TiO2, indicating a faster recombination of injected electrons with the dye cations. In addition, the ground-state bleach band in dye-sensitized TiO₂ films experiences a gradual red-shift, which is indicative of a transient Stark effect. Finally, femtosecond transient absorption measurements in the IR region point to an ultrafast generation of injected electrons for all dyes. A faster recombination of the injected electrons with the dye cations is observed for the sensitizer decorated with auxiliary electron-donating methoxy groups on the triphenylamine moiety.