Holmium chloride hexahydrate (HoCl 3 • 6 H 2 O), ethanol, chloroform, and n-hexane were purchased from Kanto Chemical Co., Inc. (Tokyo, Japan). Sodium ethoxide was purchased from FUJIFILM Wako Pure Chemical Co. (Osaka, Japan). Dibenzoylmethane was purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). Deionized water with a resistance of 18.2 MΩ•cm was obtained by passing water through a Millipore Simplicity UV system.