Schiff base compounds, charge transfer

Early experiments with bacteriorhodopsin (228) interpreted the Raman spectrum in terms of an unprotonated Schiff base, forming a charge-transfer complex with a protein functional group (210,212). This interpretation of the Raman data, essentially based on a comparison with the frequencies of model Schiff bases in solution, was criticized by Honig and Ebrey (48), who pointed out that consistency could also be obtained with a protonated Schiff base model. The latter hypothesis was subsequently confirmed by deuteration experiments similar to those described for rhodopsin (229,230), and by Raman spectra in denatured systems (231). In variance with the clear-cut similarity observed between the resonance-Raman spectra of rhodopsin and isorhodopsin, and those of the 11-cis and 9-cis model compounds, respectively,... 

Pal et al. [89] have reported on substituted ferrocenyl compounds, where one of the cyclopentadienyl rings is linked to an aromatic Schiff base, that were synthesized and analyzed for their second-order nonlinearity ((3). Their results indicate that the metal to ligand charge transfer (MLCT) transition dominates their second-order response. These compounds form charge transfer (CT) complexes with acceptors such as Ij, p-chloranil (CA), 2,3-dichloro-5,6-dicyano-l,4-bcnzoquinonc (DDQ), tetracyanoethylene (TCNE), and 7,7,8,8-tetracyanoquinodimethane (TCNQ). The CT complexes exhibit much higher second-order response. Bisferrocenyl complexes where two ferrocene moieties are linked through the same aromatic Schiff base... 

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