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 TiO₂ 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 ¹ICT 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 TiO₂ electrode. It was found that the dye OUJ-1 is adsorbed onto the TiO₂ surface through the formation of trizainium ions at the Brønsted acid sites (surface-bound hydroxyl groups, Ti–OH) on the TiO₂ surface, although the dye NI-6 was predominantly adsorbed on the TiO₂ through coordinate bonding between the pyridyl group of the dye and the Lewis acid sites (exposed Tiⁿ⁺ cations) on the TiO₂ surface, and the dye OUK-1 was adsorbed on the TiO₂ surface through both the formations of hydrogen bonding of pyrazyl groups and pyrazinium ions at Brønsted acid sites on the TiO₂ surface. This work revealed that the binding mode of D–π–A dye sensitizers with azine rings on the TiO₂ surface can be changed by control of the basicity and electron density of the azine rings.