Polymer-soluble thermostable phosphate-ester copper complexes for near-infrared absorbing dyes with weak absorbance in the visible region

Abstract Near-infrared absorbing dyes must be thermally stable above their processing temperatures. Here, polymer-soluble thermostable phosphate-ester copper complexes that meet these requirements are reported for use as near-infrared absorbing dyes with a wide absorption range from 700 to 1200 nm, and a weak absorbance in the visible region. C5–C12 alkyl phosphate-ester copper complexes were synthesized and their thermal stabilities were evaluated by thermogravimetric analysis. The previously reported acetoxy-butyl-phosphate copper complex, which contains an ester group, was also synthesized for comparison. The mass of the alkyl-phosphate-ester copper complexes remained constant up to 170 °C, whereas that of the acetoxy-butyl-phosphate copper complex started to decrease below 100 °C. Some copper complexes showed decomposition temperatures above 250 °C. The thermal-decomposition behavior of the n-pentyl-phosphate copper complex, cyclohexyl-phosphate copper complex and 2-ethylhexyl-phosphate copper complex was investigated by thermal-decomposition gas analysis measured by purge-and-trap gas chromatography/mass spectrometry. Cleavage of the O–C bond was the main decomposition reaction for both complexes. The difference in the thermal stabilities of the cyclohexyl-phosphate copper complex and other alkyl-phosphate-ester copper complexes is presumed to result from their different tendencies towards cleavage of the O–C bond. The absorption spectra of the 2-ethylhexyl-phosphate copper complex and the isodecyl-phosphate copper complex showed a broad absorbance from 700 to 1200 nm and a weak absorbance in the visible region. Therefore, these compounds are suitable for window materials that require heat-shielding properties and transparency. The spectra also indicated that the structures of these copper complexes are similar regardless of the substituent on the ligand.