In the series [(4,4'-(X),-bpy)Re1(CO)3(py-PTZ)]+ (X = W H 3 , CHI, H, C(0)NEtz, C02Et) and [(bpz)Re1(C0),(py-PTZ)]+ (bpz is 2,2'-bipyrazine), dr(Re)r*(4,4'-(X),-bpy) excitation is followed by rapid, intramolecular electron transfer to give the redox-separated states, [(4,4'-(X)2-bpy')Re1(CO)3(py-PTZ'+)] ' . They return to the ground states by intramolecular, back electron transfer, [ (4,4'-(X)2-bpy')Re'(CO)3(py-PTZ'+)]+ [ (4,4'-(X),-bpy)Re(CO),(py-PTZ)]+. The back electron transfer reactions are highly exergonic, AGO --1.18 to -1.93 eV in 1,2-dichloroethane, and occur in the inverted region.As determined by transient absorbance measurements following laser flash photolysis, k b varies from 6.7 X lo6 s-l for X = OCH3 to 9.1 X lo7 s-l for the bpz complex. The energies of the related MLCT excited states, [(4,4'-(X),-bpy'-)ReI1-(C0),(4-Etpy)]+* (4-Etpy is 4-ethylpyridine) fall in the range Eo = 1.77-2.31 eV in the same medium. In this series the variations in the rate constant for nonradiative decay with driving force, k,, = 2.6 X 105-2.8 X IO's-I, are in accord with the energy gap law. Nonradiative decay has been analyzed quantitatively based on the results of a Franck-Condon analysis of emission spectral profiles. For back electron transfer in the inverted region, kb decreases logarithmically with -AGO as predicted by the energy gap law. The slope of the correlation between In ( k b X 1s) and -AGO is that between In (knr X Is) and Eo. An analysis based on the energy gap law provides an explanation for the decrease. The implications of our findings for the design of long-lived, redox-separated states are presented.(1) (a) Marcus, R. A.

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The major goal of this project was to use FT-IR spectroscopy to monitor the effects of chain length and temperature on small, helix-forming peptides of the general form, Ac-W(EAAAR) n A-NH 2 , where n ) 1, 3, 5, and 7, in aqueous solutions. FT-IR spectra were collected in D 2 O as a function of temperature in the range of -4 to 95°C. The spectral range of interest, 1500-1725 cm -1 , contains the amide-I′ band of the fully-exchanged H f D peptide bond. Even in these simple peptides, the amide-I′ region of the IR spectra is complex and congested, composed of features derived from the conformation of the peptide backbone and from the contributions of amino acid side chains. Unambiguous resolution of peak positions and intensities is thus extremely difficult, particularly when assessing subtle differences between two data sets. Two-dimensional correlation analysis (Noda, I. J. Am. Chem. Soc. 1989, 111, 8116. Noda, I. Applied Spectrosc. 1990 was used to guide and verify the results of the peak-fitting procedure and thereby facilitate physical interpretation of the temperature-dependent spectra. The results of the two-dimensional analysis and fitting procedure show that the spectral bands, particularly those of the amide-I′ band, exhibit significant frequency shifts and bandwidth and intensity changes as a function of temperature and chain length. For the amide-I′ modes arising from the helical and random conformations of the peptide bond, the normalization of peak intensities to units of molar absorptivity is discussed in terms of different models. Two different molar absorptivity calculations are presented, the first using the length-dependence of R-helical frequencies as predicted by perturbation theory, and the second assuming a more rigorous two-state transition. The results from each are discussed in terms of the effects of chain length on R-helix stabilization and in terms of a mechanism of helix unfolding.

Dynamic infrared linear dichroism (DIRLD) measurements of films of a thermo-plastic polyester/polyurethane random copolymer (Estane 5703, B. F. Goodrich) are reported. Step-scan Fourier transform infrared (FT-IR) is used for dynamic measurements, and a photoelastic modulator (PEM) is used to create broadband polarization modulation as the carrier frequency for the strain modulation. A novel modulation/demodulation strategy has been employed that simplifies the triple-modulation experiment into a double-modulation experiment; the theory is thoroughly discussed. Both static and dynamic dichroic absorption difference spectra have been measured on the prestretched polymer film. The results are of high signal-to-noise ratio (SNR) and clearly indicate the static and dynamic orientation of the transition dipole moments due to the tensile deformation. The dynamic orientation responses are primarily in phase with the perturbation. The orientation magnitudes of the infrared absorption bands are quantified and compared, and the orientations of the hard and soft domains are differentiated. To assist in the interpretation of the dynamically measured data, we also describe a static linear dichroic measurement using a wire-grid polarizer and FT-IR in the rapid-scan mode for a sample incrementally drawn until the point of breaking. The orientation functions of selected bands have been calculated, and the static results agree with the dynamic data, indicating the dependency of the dynamic orientation response on the preorientation state.