Structure of merocyanine dyes with a hydrocarbon chain studied by vibrational spectroscopy. Part 1.—Dependence of the solid-state structure upon the chain length

Abstract

IR and non-resonance Raman spectra have been measured for 3-ethyl-5-[2-(3-ethylbenzothiazonlin-2-ylidene)ethylidene]rhodanine (EtMD) and 3-carboxymethyl-5-[2-(3-alkylbenzothiazolin-2-ylidene)ethylidene]rhodanine (C_nMD, n= 2, 3, 4, 5, 6, 7, 8, 10, 12, 14, 16 and 18; where n represents the number of carbon atoms in the alkyl chains) in the solid state to derive new information on their structures. Spectral analysis in the CO stretching region of the IR spectra indicates that the stable isomers of C₂MD, C₃MD, C₁₂MD, C₁₄MD, C₁₆MD and C₁₈MD have intramolecular hydrogen bonding between the keto and carboxylic groups while those of C₄MD, C₅MD, C₆MD, C₇MD, C₈MD and C₁₀MD form a so-called ‘ring dimer’via the carboxylic groups. These results suggest that the structure of the MDs changes with the length of the hydrocarbon chain bonded to the MD chromophore. Several intense bands due to the stretching modes for the central conjugated systems of the MDs suggest that intramolecular hydrogen bonding increases the delocalization of π electrons in the conjugated systems.