Synthesis of p-cumylphenoxy-1,8-naphthalimide derivatives as brighteners

Abstract

A series of 4-alkoxy or 4-aryloxy-N-aryl-1,8-naphthalimides were synthesized and evaluated for their optical properties as well as thermal and photo stabilities. The synthesis began with the imidation of the commercially available 4-bromo-1,8-naphthalic anhydride with various aniline derivatives. Substitution of the bromine with phenol or aliphatic alcohol provided the target molecules in good yields. All compounds had absorption wavelengths around 364-368 nm and emission wavelengths of 425-436 nm, which were desirable properties for optical brightening agents (OBA). In comparison with the commercially available OBA - Tinopal - their photostability were closely similar but their thermal stabilities are more superior and could significantly be improved by substitution at the 4-position with a cumylphenoxy group. In this research, it was found that their quantum efficiencies were exclusively dependent on the substituents at the 4-position. Compounds with alkoxy and phenoxy groups exhibited high quantum efficiencies whereas those compounds with substituted aryloxy groups showed lower quantum efficiencies. It is postulated that the electronic effect of the substituents on the 4-aryloxy groups influences the photo-induced electron transfer (PET) process, which causes variations in fluorescence signal.