Enhanced photochromism of chromen-based colorants near silver nanorods in sol–gel matrix

Abstract

In this article, Ag rods with four different aspect ratios were synthesized at 4, 16, 24, and 32 °C (denoted as Ag4, Ag16, Ag24, and Ag32, respectively) in a sol–gel matrix and coated with a photochromic dye, 8-N,N-dimethylamino-2,2-dimethyl-2H-pyrano[3,2-c]chromen-5-one (NN-DDPC-CF, CF denotes the ring-closed form), forming AgX-NN-DDPC-CF (X = 4, 16, 24, and 32). They were prepared, structurally characterized and proven to enhance the reversible photochromism of NN-DDPC. Upon irradiation with UV light at various irradiation wavelengths, the NN-DDPC adsorbed onto the Ag rods in the sol–gel underwent a ring opening reaction to produce the opened forms and had a bright yellow reflective appearance. The kinetic results clearly account for the fact that the reaction rate of NN-DDPC-coated Ag rods with different aspect ratios is about 6–11 times faster than NN-DDPC alone due to the excitation of localized surface plasmon resonance (LSPR). A sol–gel matrix was also used as a soft environment for NN-DDPC so as to decrease the barrier of the closed form to the opened form and hence, increase the photochromic reaction efficiency. The noteworthy photochromic reaction efficiencies of NN-DDPC-covered Ag rods in the sol–gel under irradiation at different irradiation wavelengths were also evaluated. The enhancement factor (E) and the volume percentages of the enhanced areas (FE) in the entire shell of the forward photochromic reaction of Ag4- to Ag32-NN-DDPC, depending on the irradiation wavelengths, i.e. 365, 385, 400, and 415 nm, can be deduced to be 12–42 (13–24%), 6–55 (11–16%), 7–38 (12–22%), and 5–39 (12–22%), respectively. It should be noted that for Ag4- to Ag32-NN-DDPC, the E factors in the forward process show more promising values than the reported results. E values of <30 have been reported in past relevant studies. Moreover, the recovery efficiencies and FE values of Ag4- to Ag32-NN-DDPC have been evaluated. It should be noted that without the addition of Ag nanorods, the recovery process of NN-DDPC is negligible. On the other hand, reversibility experiments for Ag32-NN-DDPC under alternating UV and visible irradiation were carried out for fifteen repeated cycles and the around 90% recovery of the absorbance for Ag32-NN-DDPC in each cycle demonstrated the high degree of reproducibility. Accordingly, it is clear that a future extension of this type of photochromic dye with the assistance of the LSPR of inert-metal nanorods should be versatile and prospective.