Reorganization Energies for Interfacial Electron Transfer across Phenylene Ethynylene Rigid-Rod Bridges

A family of three ruthenium bipyridyl rigid-rod compounds of the general form [Ru(bpy)₂(LL)](PF₆)₂ were anchored to mesoporous thin films of tin-doped indium oxide (ITO) nanocrystals. Here, LL is a 4-substituted 2,2-bipyridine (bpy) ligand with varying numbers of conjugated phenylenethynylene bridge units between the bipyridine ring and anchoring group consisting of a bis-carboxylated isophthalic group. The visible absorption spectra and the formal potentials, E^o(Ru^III/II), of the surface anchored rigid-rods were insensitive to the presence of the phenylene ethynylene bridge units in 0.1 M tetrabutyl ammonium perchlorate acetonitrile solutions (TBAClO₄/CH₃CN). The conductive nature of the ITO enabled potentiostatic control of the Fermi level and hence a means to tune the Gibbs free energy change, -ΔG^°, for electron transfer from the ITO to the rigid-rods. Pseudo-rate constants for this electron transfer reaction increased as the number of bridge units decreased at a fixed -ΔG^°. With the assumption that the reorganization energy, λ, and the electronic coupling matrix element, H_ab, were independent of the applied potential, rate constants measured as a function of -ΔG^° and analyzed through Marcus-Gerischer theory provided estimates of H_ab and λ. In rough accordance with the dielectric continuum theory, λ was found to increase from 0.61 to 0.80 eV as the number of bridge units was increased. In contrast, H_ab decreased markedly with distance from 0.54 to 0.11 cm^-1, consistent with non-adiabatic electron transfer. Comparative analysis with previously published studies of bridges with an sp³-hybridized carbon indicated that the phenylene ethynylene bridge does not enhance electronic coupling between the oxide and the rigid-rod acceptor. The implications of these findings for practical applications in solar energy conversion are specifically discussed.

Charge-transfer excited state in pyrene-1-carboxylic acids adsorbed on titanium dioxide nanoparticles

The electronic structure of excited photosensitizer adsorbed at the surface of a solid is the key factor in the electron transfer processes that underlie the efficiency of dye-sensitized solar cells and photocatalysts. In this work, Stark effect (electroabsorption) spectroscopy has been used to measure the polarizability and dipole moment changes in electronic transitions of pyrene-1-carboxylic (PCA), -acetic (PAA) and -butyric (PBA) acids in ethanol, both free and adsorbed on colloidal TiO₂, in glassy ethanol at low temperature. The lack of appreciable increase of dipole moment in the excited state of free and adsorbed PAA and PBA points that two or more single bonds completely prevent the expansion of π-electrons from the aromatic ring towards the carboxylic group, thus excluding the possibility of direct electron injection into TiO₂. In free PCA, the pyrene's forbidden S₀→S₁ transition has increased intensity, exhibits a long progression in 1400cm^-1 A_g mode and is associated with |∆μ| of 2 D. Adsorption of PCA on TiO₂ causes a broadening and red shift of the S₀→S₁ absorption band and an increase in dipole moment change on electronic excitation to |∆μ|=6.5 D. This value increased further to about 15 D when the content of acetic acid in the colloid was changed from 0.2% to 2%, and this effect is ascribed to the surface electric field. The large increase of |∆μ| points that the electric field effect can not only change the energetics of electron transfer from the excited sensitizer into the solid, but can also shift the molecular electronic density, thus directly influencing the electronic coupling factor relevant for electron transfer at the molecule-solid interface.

Unravelling the effect of anchoring groups on the ground and excited state properties of pyrene using computational and spectroscopic methods

Anchoring groups play an important role in dye sensitized solar cells (DSCs). In order to acquire a suitable anchoring group for DSCs, a deeper understanding of the effect of anchoring groups on the ground and excited state properties of the dye is significant. In this context, various anchoring group connected pyrene derivatives are successfully synthesized and well characterized by using (1)H, (13)C-NMR, FT-IR and EI-MS spectrometry. The anchoring groups employed are carboxylic acid, malonic acid, acrylic acid, malononitrile, cyanoacrylic acid, rhodanine and rhodanine-3-acetic acid. The optimized geometries, HOMO-LUMO energy gap, light harvesting efficiency (LHE) and electronic absorption spectra of these dyes are studied by using density functional theory (DFT) calculations. The results show that pyrene connected with anchoring groups with weak electron pulling strength (PC, PAC and PMC) has a larger HOMO-LUMO energy gap, whereas that connected with anchoring groups with strong electron pulling strength (PCC, PMN, PR and PRA) has a reduced HOMO-LUMO energy gap. These molecules with a reduced energy gap are primarily preferred for DSC applications. Moreover, P, PC, PAC and PMC molecules undergo π→π* transition, whereas PCC, PMN, PR and PRA molecules show significant charge transfer along with π→π* transition. UV-visible absorption spectral studies on these dyes reveal that connecting various anchoring groups with different electron pulling abilities enables the pyrene chromophore to absorb in the longer wavelength region. Notably, an efficient bathochromic shift is observed for PCC, PMN, PR and PRA molecules in both electronic absorption and fluorescence spectral measurements, which suggests that the excitation is delocalized throughout the entire π-system of the molecules. Both theoretical and spectral studies reveal that dyes with an ICT character (PCC, PMN, PR and PRA) are suitable for dye sensitized solar cell applications.

Tuning the photophysical properties of pyrene-based systems: a theoretical study

Recently new molecular systems based on the pyrene moiety were developed for photovoltaic applications. Here we present the results of a quantum chemical study focused on the effects induced by some different substituents on the electronic properties of pyrene, to obtain general hints for the molecular design of new pyrene-based systems. In particular, a series of electron-donating (hydroxy, amino, acetylamino) and electron-withdrawing (cyano, carbamoyl, formyl, ethynyl, ethenyl) groups were considered. Furthermore, in addition to the single pyrene molecule, two pyrene units linked by ethenylene, ethynylene, 2,5-thienylene, and ethynylene-p-phenylene containing chains of different lengths were taken into account. For all of the model structures presented, the ground state geometries have been optimized using the density functional approach, while the vertical transition energies were calculated using the time-dependent density functional theory. We will show that the tuning of the lowest electronic excitation energy (i.e., the HOMO-LUMO energy gap) as well as the localization of the spatial distributions of the frontier molecular orbitals (i.e., the nature of the electron-hole pair, generated by photon absorption) can be obtained through the analysis of the pyrene frontier molecular orbitals. This approach allows to evaluate the most suitable position of the substituents on the pyrene moiety giving rise to enhanced electronic effects also in function of their electronic nature. In this way, pyrene-structures with tailored electronic properties could be modeled. Our screening shows that promising candidates for photovoltaic applications could be molecular structures formed by two pyrene units joined/linked by a short conjugated bridge containing double or triple bonds (henceforth pyrene-linked dimers). As far as the single pyrene units are considered, the most significant reduction of the transition energy of the lowest optical electronic excitation is obtained with disubstituted pyrenes with push-pull character.