Fluorescences and underlying mechanisms of DAST solid-films and solutions

The outstanding organic nonlinear optical material of 4-N,N-dimethylamino-4′-N′-methyl-stilbazolium tosylate (DAST) also exhibits fluorescence. While a strong fluorescence would add value to DAST in opening door to new applications such as sensing, it's underlying mechanism remains unclear and is expected to vary with the experimental conditions. To this end, DAST in the solution phase, with solutions in various polar solvents, and in the solid phase, with thin films annealed at different temperatures, were prepared and investigated. Results show that only one fluorescence peak at around 600–610 nm was present in the DAST solutions, and the increase of the solvent polarity leads to a red-shift of the fluorescence peak and a decrease of the fluorescence intensity. Rather differently, two fluorescence emission peaks were measured from the DAST solid thin films, and particularly, the peak at the higher frequency in 570–589 nm is excited by the charge transfer from the donor (dimethylamino group) to acceptor (pyridyl groups) of DAST cations, while the other peak at the lower frequency in 710–723 nm is assigned to the charge transfer from the anions to cations in DAST. After having been annealed at a high temperature of 150 °C, the fluorescence intensity of the peak at 587 nm of DAST solid film decreases evidently, due to the molecular reabsorption. This work provides insight to the fluorescence properties of DAST and points to avenues to alter or improve the emission characteristics and efficiency.