Lateral Intermolecular Electronic Interactions of Diketopyrrolopyrrole D-π-A Solar Dye Sensitizers Adsorbed on Mesoporous Alumina

The development of push-pull organic dyes utilizing the donor-(pi-conjugated bridge)-acceptor (D-pi-A) motif has induced a paradigm shift in the design of efficient nanocrystalline dye-sensitized solar cells (DSSCs), offering control over the aesthetic properties and performance. Because of the large transient dipole moment characterizing this type of dye upon photoexcitation, these molecules are intrinsically subjected to significant lateral electronic interactions once adsorbed on a solid surface, which can greatly affect the efficiency of the electron injection in DSSCs. Here, we investigated the detailed intermolecular interactions upon photoexcitation of D-pi-A diketopyrrolopyrrole (DPP)-based dye molecules, denoted (E)-3-(5-(4-(4-(5-(4-(bis(2',4'-dibutoxy-[1,1'-biphenyl]-4-yl)amino)phenyl)thiophen-2-yl)-2,5-bis-(2-ethylhexyl)-3,6-dioxo-2,3,5,6-tetrahydropyrrolo [3,4-c] pyrrol-1-yl) phenyl)furan-2-yl)-2-cyanoacrylic acid (DPP_A) and commercially available under the name "Dyenamo Blue", using ultrafast transient absorption spectroscopy. A comparison of the excited-state properties of the molecule with those of a similar dye lacking the triarylamine donor moiety (DPP_R) revealed the critical role played by the donor in determining the mechanisms of intermolecular interactions. The results showed that the intermolecular interactions between DPP_A molecules in solution caused partial delocalization of excitons in a process involving the participation of the triarylamine moiety. Lateral charge transfer (CT) between DPP_A molecules adsorbed on the redox-inactive surface of Al2O3 was observed upon photoexcitation and yielded CT excitons between neighboring dyes. Furthermore, on the basis of the experimental evidence obtained from the pump fluence-dependent transient absorption and excitation spectra, the DPP_R molecules adsorbed on the Al2O3 film exhibited intermolecular pi-pi interactions, inducing the formation of excimer-like excited states. These results suggest that lateral intermolecular electronic interactions between dye sensitizer molecules adsorbed on the surface of a semiconductor can have a strong influence on the dynamics of electron injection in DSSCs and should thus be considered in the molecular design of new efficient dyes.