Next, 1.36 g (5.0 mmol) of N-methylquinolinium iodide (Compound III) (FW 271.10), 7.0 mL (50 mmol) of triethylamine (FW 101.19, d=0.726), and 100 mL of methylene chloride were added to 3.24 g of mixture (crude product) containing Compound 112 (3-(4-carboxybutyl)-2-methylbenzothiazolium bromide) and 2-methylbenzothiazolium bromide. As a result, a transparent solution was obtained. This solution was stirred at 25° C. for 16 hours. Thereafter, the solvent was evaporated under reduced pressure. Acetone (200 mL) then was added to the residue and the precipitate obtained thereby was filtered, which then was washed with acetone. The residue thus obtained was dried under reduced pressure, and the red residue obtained after drying was washed with distilled water (50 mL). This further was filtered, which was washed with distilled water and then dried under reduced pressure. Thus a target substance (Compound 107) was obtained as red powder (654 mg, 1.39 mmol, 28%). This red powder was confirmed to be the target substance (Compound 107) through a comparison in 1HNMR spectrum with the reference value. Peak values from 1HNMR and 13CNMR (DMSO-d6) and the measured values of HRMS (ESI) are indicated below. Furthermore, FIG. 6 shows the 1HNMR spectrum (DMSO-d6) of Compound 107.
