Lowest electronic excited states of poly (para-cyclobutadienylenecyclopentadienylcobalt)butadiynylene
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abstract
The copolymer poly(para-cyclobutadienylenecyclopentadienylcobalt)butadiynylene was studied with electronic absorption, Raman, and resonance Raman spectroscopies to determine the nature of the lowest visible absorbance in the optical spectrum. The electronic absorption suggests the presence of two major transitions within 5000 cm-1 of one another in the lowest energy absorption band. Vibrational assignments were made by comparing the 457.9 nm excited resonance Raman frequencies and depolarization ratios to ab initio calculations, and enhancement values were calculated. Depolarization ratios show that the Raman enhancement of several modes results from two or more electronic transitions. These data show that the C-C triple bonds experience significant excited-state distortion in at least one of these transitions. However, we also find that several other vibration modes exhibit significant enhancement upon electronic excitation. These changes are consistent with a charge-transfer assignment for one of the lowest energy singlet excited states between a nonbonding butadiene-centered orbital and an antibonding conjugated orbital. Because of metal involvement in the nonbonding orbital, this transition has significant MLCT character. A second transition is suggested that may be localized on the metal complex portion of the molecules. 2000 American Chemical Society.
