Synthesis, optical, electrochemical and photovoltaic properties of a D-π-A fluorescent dye with triazine ring as electron-withdrawing anchoring group for dye-sensitized solar cells

The D-π-A fluorescent dye OUJ-1 with 1,3,5-triazine ring as electron-withdrawing anchoring group and (diphenylamino)carbazole containing a thiophene ring as D-π moiety have been newly developed and their optical and electrochemical properties, adsorption states on TiO nanoparticles, and photovoltaic performance in dye-sensitized solar cell (DSSC) were investigated. The absorption maximum (λabsmax) for the intramolecular charge-transfer (ICT) absorption band of the D-π-A dye OUJ-1 occurs at a longer wavelength than those of D-π-A dye NI-6 with a pyridyl group and D-π-A dye OUK-1 with a pyrzayl group. Moreover, the dye OUJ-1 exhibits significant fluorescence solvatochromic properties, that is, a bathochromic shift of the fluorescence band and a decrease in the fluorescence quantum yield (Φ) due to a change from the ICT excited state to the twisted intramolecular charge transfer (TICT) excited state with increasing solvent polarity were observed. The photovoltaic performance of a DSSC based on OUJ-1 is lower than those of NI-6 and OUK-1 due to the low dye loading of OUJ-1 on TiO electrode. It was found that the dye OUJ-1 is adsorbed onto the TiO surface through the formation of trizainium ions at the Brønsted acid sites (surface-bound hydroxyl groups, Ti-OH) on the TiO surface, although the dye NI-6 was predominantly adsorbed on the TiO through coordinate bonding between the pyridyl group of the dye and the Lewis acid sites (exposed Ti cations) on the TiO surface, and the dye OUK-1 was adsorbed on the TiO surface through both the formations of hydrogen bonding of pyrazyl groups and pyrazinium ions at Brønsted acid sites on the TiO surface. This work revealed that the binding mode of D-π-A dye sensitizers with azine rings on the TiO surface can be changed by control of the basicity and electron density of the azine rings.