Spectroscopy and Dynamics of YD2-o-C8 in Solution and Interacting with Alumina Nanoparticles Electrode Maria Rosaria di Nunzio, † Boiko Cohen, † Shyam Pandey, ‡ Shuzi Hayse, ‡ Giovanni Piani, † and Abderrazzak Douhal* ,† † Departamento de Química Física, Secció n de Químicas, Facultad del Medio Ambiente and INAMOL, Universidad de Castilla-La Mancha, Avda. Carlos III, S.N. 45071 Toledo, Spain ‡ Kyushu Institute of Technology, 2-4 Hibikino, Wakamatsu, Kitakyushu, Japan * S Supporting Information ABSTRACT: 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). 1. INTRODUCTION Nature has elected chlorophyll as the focal point of photosyn- thesis. Apart from its brilliant green color, chlorophyll is an efficient molecule, which directly takes part in all the steps of the solar energy conversion: light harvesting, energy, and electron transfer. 1 Photosynthesis is a powerful enterprise since it is estimated that the total energy conversion by plants (100 TW) 2 is well above the total amount of energy used by humans (∼13 TW in 2005). 3 The success of photosynthesis encourages many efforts through the scientific community to emulate it in the laboratories. Biomimetic systems can propose artificial power sources leading to potential application of artificial photosynthesis to solar energy conversion. Porphyrins’ introduction as sensitizers dates back to 1993, when Kay and Grä tzel reported on the first porphyrin- sensitized solar cell. 4 The idea was to mimic the light harvesting processes occurring in nature and based on chlorophyll: for that they used a copper chlorophyllin sensitizer, obtaining a cell efficiency η = 2.6%. Thereafter, porphyrin-based dyes have been widely used and studied: the porphyrin core has been carefully engineered in order to build more and more efficient donor-π- acceptor systems for dye-sensitized solar cells (DSSCs). 5 In particular, the efforts have been focused to optimize (a) the electronic nature of the donor and acceptor moieties, and (b) the conjugation of the π-linker. Asymmetric porphyrins have been built, resulting in a splitting of π and π* energy level and in a decrease of the HOMO-LUMO separation. 6 The absorption band undergoes broadening and red shifting, and the Q-band intensity is increased with respect to that of the Soret band. Moreover, the introduction of ancillary ligands has been investigated to broaden even more the absorption band. 7 Furthermore, the electron transfer dynamics can be modified as a result of a strong correlation between the tilt angle and the electron transfer rate. 8,9 Very recently, the fastest (<0.2 ps) electron transfer from a zinc porphyrin to a semiconductor was observed for ZnO nanorods modified by a 5 nm-layer of TiO 2 (titania). 10 Nevertheless, in general, efficiencies below 8% had to be expected 11-14 before the introduction of the so-coded YD2 dye, 15 for which 11% power-conversion efficiency (PCE) was measured, when used on titania substrate, in conjunction with iodide/triiodide redox electrolyte. In YD2, the electron donor moiety consists of a diarylamine group attached on the zinc porphyrin core. On the opposite side of the core, an ethynyl benzoic substituent acts as electron acceptor and it is Received: April 8, 2014 Revised: May 2, 2014 Published: May 6, 2014 Article pubs.acs.org/JPCC © 2014 American Chemical Society 11365 dx.doi.org/10.1021/jp503449q | J. Phys. Chem. C 2014, 118, 11365-11376