Spectroscopy and Dynamics of YD2-o-C8 in Solution and Interacting with Alumina Nanoparticles Electrode

In this paper, we report on the absorption, emission, and photodynamics of a push–pull zinc porphyrin (YD2-o-C8) in solution and adsorbed on Al 2 O 3 (alumina) nanoparticles (NPs). The shift in the absorption and emission spectra for the dye adsorbed on alumina NPs with respect to the solution suggests the presence of an anchoring effect of the dye to the NPs’ surface. Indeed, different molecular populations (monomers and aggregates) coexist for the solid film, as it is confirmed by both steady-state and time-resolved measurements. The emission decays, while monoexponential in acetonitrile (ACN) solution (τ = 1.5 ns) become multiexponential (140 and 550 ps) for the dye interacting with the alumina NPs. These lifetime values increase (210 and 930 ps) upon addition of CDCA (a disaggregating agent). The deactivation (vibrational cooling) of the hot S 1 to cold S 1 state, which occurs in 4–6 ps in ACN, shows a biexponential behavior in the solid state (2 ps, 20 ps). These two ps components, whose values upon excitation at 460 nm do not change with respect to those observed without CDCA, become longer in the presence of the coadsorbent and exciting at 640 nm (4 ps, 50–60 ps), where the aggregates mostly absorb. The charge separation (2 ps in ACN, 15 ps in toluene) becomes faster in the solid state in absence of CDCA (∼500 and 110 fs exciting at 460 and 640 nm, respectively), while, by adding the coadsorbent, the process occurs on a time scale that is below our time-resolution (<50 fs). The obtained results are of potential interest to understand the dynamics of the porphyrin used in the up-to-now world-record-efficient dye-sensitized solar cell (DSSC).