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 (PM) 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 PM and PBA points that two or more single bonds completely prevent the expansion of pi-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 S-0 -> S-1 transition has increased intensity, exhibits a long progression in 1400 cm(-1) A(g) mode and is associated with vertical bar Delta mu vertical bar of 2 D. Adsorption of PCA on TiO2 causes a broadening and red shift of the S-0 -> S-1 absorption band and an increase in dipole moment change on electronic excitation to vertical bar Delta mu vertical bar = 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 vertical bar Delta mu vertical bar 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. (C) 2018 Elsevier B.V. All rights reserved.