A detailed study of the visible spectra in solution and the infrared spectra in the solid state have been made for the following vinylogous series of dyes: [2-bis(3-ethylbenzothiazolyl)] cyanine iodides and [2-bis 3-ethylbenzoselenazolyl)] cyanine iodides. Each dye, to be acceptable for study, had to be chromatographically pure, give a correct microchemical elemental analysis, and be free of electron-spin resonance (free radical) signals. The characteristic red shift of the principal absorption maxima was observed for these dyes in the visible as the number of methine linkages increased. Assignments of vibrational modes to separate absorption regions have been made for these vinylogous series of dyes. Each vinylog has a characteristic pattern of resonant-conjugated stretching modes in the region 1600–1400 cm<sup>−1</sup>. These modes exhibit a low-frequency shift as the resonant-conjugated chain length increases. There are bands absorbing in the regions 1594–1572 cm<sup>−1</sup> and 1470–1453 cm<sup>−1</sup> which are present in all the vinylogs and which have been assigned to the aromatic stretching vibrations of the fused phenyl rings present in all these dyes. There is a band appearing in the region 1000–900 cm<sup>−1</sup> which changes systematically with an increase in the number of hydrogens on the bridge and with substitution on the bridge. This band has been assigned to the out-of-plane bending vibrations of the hydrogens in a <i>trans</i> configuration on the bridge. No evidence of a <i>cis</i> isomer was observed in the spectra. There is a band near 760 cm<sup>−1</sup> which is split into a doublet and has been assigned to the aromatic CH out-of-plane bending vibrations of the four adjacent hydrogens on the fused phenyl rings. This splitting has been attributed to a crystal field effect which gives rise to in-phase and out-of-phase vibrations of the same groups in two different molecules.
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