Photophysical Studies of Dipolar Organic Dyes That Feature a 1,3-Cyclohexadiene Conjugated Linkage: The Implication of a Twisted Intramolecular Charge-Transfer State on the Efficiency of Dye-Sensitized Solar Cells

A detailed study of the synthesis and photophysical properties of a new series of dipolar organic photosensitizers that feature a 1,3-cyclohexadiene moiety integrated into the pi-conjugated structural backbone has been carried out. Dye-sensitized solar cells (DSSCs) based on these structurally simple dyes have shown appreciable photo-to-electrical energy conversion efficiency, with the highest one up to 4.03%. Solvent-dependent fluorescence studies along with the observation of dual emission on dye 4b and single emission on dyes 4a and 32 suggest that dye 4b possesses a highly polar emissive excited state located at a lower-energy position than at the normal emissive excited state. A detailed photophysical investigation in conjunction with computational studies confirmed the twisted intramolecular charge-transfer (TICT) state to be the lowest emissive excited state for dye 4b in polar solvents. The relaxation from higher-charge-injection excited states to the lowest TICT state renders the back-electron transfer process a forbidden one and significantly retards the charge recombination to boost the photocurrent. The electrochemical impedance under illumination and transient photovoltage decay studies showed smaller charge resistance and longer electron lifetime in 4b-based DSSC compared to the DSSCs with reference dyes 4a and 32, which further illustrates the positive influence of the TICT state on the performance of DSSCs.