| Abstract: | 10-Methylphenothiazine and p-(methylthio)anisole were compared to polymers which contained these donor molecules on the side chains of N-acyl-substituted polyethylenimines. Charge-transfer absorption spectra were compared for these donors with the acceptors: dichlorodicyanobenzoquinone, tetracyanoquinodimethane, tetracyanoethylene, and 2,4,5,7-tetranitrofluorenone. Benesi-Hildebrand plots show that the formation of the polymer complexes have 3 to 50 times higher equilibrium constants than those of the corresponding model complexes. This can be explained by complexing parallel to the polymer backbone. The polymer has the proper geometry for complexing (6.4 Å, repeat distance in the polymer backbone), and an acceptor molecule can therefore be inserted between two adjacent donor molecules for increased stability. Shifts of the absorption maxima to longer wavelength for some of the polymer complexes can be rationalized by the probability that in the polymer, an acceptor is sandwiched between two donors and thus forms 2:1 complexes; the extra resonance energy may shift the absorption maximum to longer wavelength. A second possible explanation is based on solvation of the complex which reduces the energy of the excited state. Polymers absorb mainly in the complex form. Model compounds absorb mainly by contact charge transfer, which is nonsolvated and thus occurs at higher energy or shorter wavelength. Extinction coefficients are higher for the model complexes than for the polymer complexes. Contact charge transfer, which can contribute in greater proportion to the model than to the polymer complexes, explains this. The amount of contact charge transfer can be calculated simply from the probability of a donor being in the solvent shell of an acceptor. Complex decomposition rates were determined based on measuring changes in the intensities of the charge-transfer absorption spectra. Dichlorodicyanoquinone complexes were unstable, while the other complexes were stable. |
