STRUCTURE OF MEROCYANINE DYES WITH A HYDROCARBON CHAIN STUDIED BY VIBRATIONAL SPECTROSCOPY .1. DEPENDENCE OF THE SOLID-STATE STRUCTURE UPONTHE CHAIN-LENGTH

IR and non-resonance Raman spectra have been measured for thylbenzothi azonlin-2-ylidene)ethylidene]rhodanine (EtMD) and zothiazolin-2-yliden e)ethylidene]rhodanine(C(n)MD, 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 ch ains) in the solid state to derive new information on their structures . Spectral analysis in the C=O stretching region of the IR spectra ind icates that the stable isomers of C(2)MD, C(3)MD, C(12)MD, C(14)MD, C( 16)MD and C(18)MD have intramolecular hydrogen bonding between the ket o and carboxylic groups while those of C(4)MD, C(5)MD, C(6)MD, C(7)MD, C(8)MD and C(10)MD form a so-called 'ring dimer' via the carboxylic g roups. These results suggest that the structure of the MDs changes wit h the length of the hydrocarbon chain bonded to the MD chromophore. Se veral intense bands due to the stretching modes for the central conjug ated systems of the MDs suggest that intramolecular hydrogen bonding i ncreases the delocalization of n electrons in the conjugated systems.