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 pyrene (Py) subunit leads 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 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 ability to undergo CS.
Copper-free Huisgen cycloaddition reaction was applied to post grafted dyes on mesoporous electrodes. It enhances the stability towards desorption and offers the possibility of dye functionalization directly performed on the electrodes.
This study addresses a practical aspect of hybrid dye-sensitized photoelectrochemical cells by exploring a simple method to prepare multicomponent systems. Building on a previously reported methodology based on a copper-free click chemistry dipolar cycloaddition of azide with activated alkyne, a naphthalene diimide (NDI) derivative substituted with two propiolic esters was clicked on a NiO photocathode already coated with a diketopyrrolopyrrole (DPP) dye bearing two azido groups. A detailed photophysical study by transient absorption spectroscopy demonstrates that optical excitation of DPP dye leads to an effective electron transfer chain from the NiO valence band to the NDI passing via the DPP dye, resulting to a long-lived charge-separated state (hole in NiO/NDI radical anion) of 170 s. p-type dye-sensitized solar cells were also fabricated with the above molecular components and confirm the occurrence of the electron transfer as the performances of the solar cells were improved in terms of Voc and Jsc compared to the DPP dye lacking the NDI unit. The above clicked system was also compared to a covalently linked DPP-NDI dyad, whose performances are 30% superior to the clicked system probably due to longer mean distance between the NiO surface and the NDI with the dyad. This finding paves the way for the design of multicomponent hybrid dyesensitized photoelectrochemical cells by chemistry on the electrode.
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