The influence of chain length and electron acceptor residues in 3-substituted 7- N, N-diethylaminocoumarin dyes

Abstract

Several 7- N, N-diethylaminocoumarin dyes with vinylogously extended electron acceptor groups in the 3-position have been prepared by condensing the 3-aldehyde with various active methylene compounds. Structures vary according to both the nature of the electron withdrawing moiety and the length of the conjugation pathway between the 7-amino group and the electron acceptor residue. Dyes with colours ranging from red to cyan have been prepared, and their light absorption properties are discussed in terms of the electron acceptor and the extent of conjugation. Steric factors play a major role in influencing the colour properties, and this was exemplified in one extreme case where lengthening the conjugation chain by one vinyl unit caused a hypsochromic shift due to increased steric inter-actions. In general, the vinylogous donor-acceptor coumarin dyes were not as fluorescent as the more conventional 7-dialkylaminocoumarins which have a shorter conjugation path between the amino group and the 3-acceptor group. Two cationic dyes have been prepared by condensation of the coumarin aldehyde with enamines, and these show typical cyanine dye characteristics, with intense long wavelength absorption.