Tuning the strength of intramolecular charge-transfer of triene-based nonlinear optical dyes for electro-optics and optofluidic lasers

Organic conjugated dyes have received extensive attention due to their broad applications in nonlinear optics and light-emitting and fluorescent sensors. Herein, we report the two-step synthesis of donor-acceptor dyes with triene-conjugated bridges. Via Vilsmeier formylation and Knoevenagel condensation, the dyes 1-4 with respectively varied electron-withdrawing moieties were successfully synthesized with high yields in short duration times. Photophysical properties showed gradually enhanced intramolecular charge-transfer (ICT) strength from dye 1 to dye 4 and diverse absorption and fluorescence properties. This structure-property tuning ensured their different applications in optofluidic lasers and nonlinear optics. Moreover, an optofluidic laser obtained using dye 1 in toluene as the gain material was demonstrated. Narrow linewidth (1.6 nm) of the optofluidic laser (peak wavelength: 610 nm) with a threshold of 94.2 μJ mm^-2 was obtained. In electro-optic (EO) activities, four dyes in guest-host EO films showed the step-up EO coefficients from dye 1 (3 pm V^-1) to dye 4 (76 pm V^-1). This trend was in accordance with the results of ICT energy gap and DFT calculations, and it showed the effective tuning of structure-property relationship for electro-optics. The facile synthesis of NLO dyes, fine-tuning of their intramolecular charge-transfer, as well as the experimental demonstration in nonlinear optics and optofluidic lasers indicated their significant applications in organic photonic devices.