Donor Effect on the Photoinduced Interfacial Charge Transfer Dynamics of D-π-A Diketopyrrolopyrrole Dye Sensitizers Adsorbed on Titanium Dioxide

In this report, we investigate the effects of a strong electron-donating group on the electron injection and recombination processes of diketopyrrolopyrrole (DPP)-sensitized TiO2 films by comparing the dynamics of triphenylamine donor-installed DPP_A and donor-absent DPP_R dyes. We show that the electron-donating group enhances the intramolecular charge transfer (ICT) characteristics of DPP_A, as demonstrated by a significant spectral redshift in the steady-state measurement of DPP_A compared to that of DPP_R. By comparing the ultrafast transient behaviors of the dye-coated films through femtosecond transient absorption (fs-TA) measurements, we provide insight into the high efficiency of DPP_A: most of the electrons from DPP_A are injected into the conduction band of TiO2 within the time resolution of TA (<150 fs), while DPP_R displays a biphasic electron injection process of ca. 340 ps (including slow decay). Additionally, due to the existence of lateral intermolecular interactions on TiO2, a phenomenon that takes place on the time scale of several picoseconds and was only recently investigated in our prior work, the charge injection process becomes inefficient. More precisely, lateral interactions act as a barrier to efficient charge injection when the electron donor is absent by opening another faster pathway before the delayed electron injection. Through further flash photolysis measurements, we demonstrate that this intermolecular interaction affects the recombination of electrons with dye cations, causing delayed recombination decay components.