The UV-vis diffuse reflectance spectra of the crystalline powder were measured using either a Lambda 650 spectrometer (PerkinElmer) or a V-650 spectrometer (JASCO) at 20 °C. The powder was mounted on a glass plate and covered with a quartz glass plate. Measurements were performed before and after photoirradiation (365 nm, 10 mW cm -2 ) on the quartz glass side using a UV-LED (8332A, CCS Inc.). Measurements were performed in the range of 250-800 nm at 1 nm resolution with a scan speed of 200 nm min -1 .