TRANSIENT RESONANCE RAMAN AND RAMAN SPECTROELECTROCHEMICAL STUDIES OFCUI COMPLEXES WITH POLYPYRIDYL LIGANDS

Studies by laser flash photolysis, transient Raman spectroscopy, and R aman and UV-vis spectroelectrochemistry are described in which the tec hniques have been used in parallel to compare the lowest energy charge -transfer excited states of Cu (1) complexes ([Cu(L)2]+ and [ (PPh3)2C u(L)]+ [L = 2,2'-biquinoline (BIQ) or 6,7-dihydro-5,8-dimethyldibenzo[ b,j] [1,10]-phenanthroline (DMCH)) with the species produced by electr ochemical reduction in the same group of complexes. Transient resonanc e Raman spectra for the metal-to-ligand charge-transfer (MLCT) states of [Cu(DMCH)2]+ (1), [Cu(BIQ)2]+ (2), [Cu(DMCH)(PPh3)2]+ (3), and [Cu( BIQ)(PPh3)2]+ (4) are compared with the resonance Raman spectra of the same group of complexes following one-electron electrochemical reduct ion of the DMCH and BIQ ligands. The UV-vis and resonance Raman eviden ce suggests that the electrochemical reduction of the [Cu(I)L2]+ speci es proceeds according to the sequence [LCu(I)L]+ -->e- [LCu0L] -->e- [ L.-Cu(I)L.-]-. Several features assignable to modes of the electrochem ically generated DMCH.-and BIQ'- radical anions exhibit a close corres pondence in both frequency and relative intensity with counterparts in the spectra of the MLCT states of 1 and 2. A notable exception is a b and near 1590 cm-1 in the spectra of the electrochemically reduced spe cies which occurs some 15 cm-1 lower in the corresponding spectra of t he excited-state species. It is suggested that the shift may reflect t he change in oxidation state of the metal center from Cu(I) to Cu(II) which occurs as a result of charge-transfer excitation.