Solvatochromic and solid-state emissive azlactone-based AIEE-active organic dye: Synthesis, photophysical properties and color-conversion LED application

Abstract Here, we report one-pot synthesis of a new azlactone-based dye bearing isoxazole moiety as a donor group (i.e., Dye-Iso), which displays fluorescence emission both in solution and solid-state, and a systematic study of its photophysical properties. The analyses of steady-state photoluminescence (PL) spectroscopy measurements reveal that Dye-Iso exhibits positive solvatochromic behavior accompanied by the decreasing trend in PL quantum yield (QY), as the polarity of solvent increases. In addition, time-resolved fluorescence (TRF) spectroscopy shows that the PL lifetime is shortening and overall effective nonradiative decay rate becomes accelerated with increasing polarity of the medium, which explain well the decrease in PL QY. These findings strongly suggest that the dipole-dipole interactions between the dye molecule having intramolecular charge transfer (ICT) characteristic and solvent are responsible for the redshift in PL emission with the increase in solvent polarity. Furthermore, the electronically ground- and excited-state dipole moments were determined by using linear correlation methods having different solvent polarity functions. The results depict that the Dye-Iso has a larger excited-state dipole moment as against that in the ground-state, which reflects the ICT mechanism. The spectroscopic studies show that the Dye-Iso exhibits aggregation-induced emission enhancement (AIEE) under aggregate formation and is AIEE-active compound. The PL QYs of the Dye-Iso in aggregation- and solid-state were also found to be higher in comparison with those in solutions, which has been elucidated by the increase in the radiative decay rate due to reduced restriction of intramolecular motions (RIM). Finally, we have demonstrated that the AIEE-active Dye-Iso can be used as a color-conversion material for solid-state lighting applications by employing that on a commercial near-ultraviolet LED.