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The synthesis and characterization of a THF pendant, dimeric Mo cyclopentadienyl complex is reported and the structure characterized by NMR, FTIR, and UV-vis spectroscopies. The dimer was found to undergo metal-metal bond homolysis under either photochemical or electrochemical conditions to yield an odd-electron complex. Cyclic voltammetry under varying scan rate conditions from 25 to 10 000 mV/s demonstrates a dynamic equilibrium process that we assign to the formation of either an intramolecular 19-electron (18 + delta) complex or a bare 17-electron complex. Photolysis of the dimer in room temperature solution could be monitored as a loss of the dpi-dsigma transition at 503 nm by transient absorption spectroscopy. The recovery of the transient absorption signal at 410 nm follows first-order kinetics at a rate of 5 x 10(3) s(-1).

A detailed study of the visible spectra in solution and the infrared spectra in the solid state has been made for the following vinylogous series of cyanine dyes: [2-bis(3-ethylbenzoxazolyl)] cyanine iodides, [2-bis-(1-ethyl-3,3-dimethylindolinyl)] cyanine iodides, and [2-bis-(1-ethylquinolyl)] 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 range as the number of methine linkages increased. A careful comparison of the visible spectral data of the [2-bis(3-ethylbenzoxazolyl)] cyanine iodides with those of the corresponding [2-bis(3-ethylbenzothiazolyl)] and [2-bis(3-ethylbenzoselenazolyl)] cyanine iodides, [Leifer et al., Appl. Spectry. 20, (1966)] indicates that the electronegativity of the atom S, Se, or O in the heterocyclic rings probably affects the wavelength of the principal absorption maximum. As the electronegativity of the Group VI.A atom increases, the principal absorption maximum shifts slightly toward the blue. 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−1. Some of these modes exhibit a low frequency shift as the resonant-conjugated chain increases. Comparison of the benzoxazolyl modes with those of the corresponding benzothiazolyl and benzoselenazolyl modes indicates that they are probably a function of the electronegativity of S, Se, or O in the heterocyclic rings. As the electronegativity of the VI.A atom increases, the resonant-conjugated stretching modes shift to higher frequencies. There are also characteristic bands in the 1600–1400 cm−1 region which are present in all the vinylogs of each series of these dyes. These bands have been assigned to the stretching modes of the fused phenyl rings present in these dyes. It appears that the fused phenyl stretching modes are a function of the groupings C(CH3)2, O, S, Se present in the indolinyl, benzoxazolyl, benzothiazolyl, and benzoselenazolyl heterocyclic rings, respectively. The sulfur and selenium atoms affect these modes the least while the oxygen atom affects these modes the most. Assignments have been made for the aromatic CH out-of-plane bending modes in the region 800–700 cm−1 for these vinylogous series of cyanine dyes. In the spectra of the 2-bis-indolinyl and 2-bis-quinolyl cyanine iodides, there is a band appearing in the region 1000–900 cm−1 which changes systematically with an increase in the number of hydrogens on the bridge. This band has been assigned to the out-of-plane bending vibrations of the hydrogens in a trans configuration on the bridge. No evidence of a cis isomer was observed in the spectra.

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−1. These modes exhibit a low-frequency shift as the resonant-conjugated chain length increases. There are bands absorbing in the regions 1594–1572 cm−1 and 1470–1453 cm−1 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−1 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 trans configuration on the bridge. No evidence of a cis isomer was observed in the spectra. There is a band near 760 cm−1 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.