Poly(propylene amine) dendrimers decorated with dimethoxybenzene units.: Photophysical and electrochemical properties

We have investigated the absorption spectra, the fluorescence spectra and decay and the electrochemical redox processes of five dendrimers Gn (n = 1 to 5) that contain 2(n+1) -2 (i.e., 62 for G5) tertiary amino groups in the interior and 2(n+1) (i.e., 64 for G5) 1, 2- dimethoxybenzene (DMB) units in the periphery. In dichloromethane solution the absorption spectrum of the dendrimers is quite similar to that of 3,4-dimethoxy-N-propylbenzene-1-sulfonamide, a model compound of the peripheral DMB units. The fluorescence band of the DMB unit (lambda(max) = 319 nm, tau = 0.95 ns, Phi = 0.08 for the model compound), however, is much weaker in the dendrimers, which also display a broad emission tail above 450 nm (tau from 2 to 7 ns, depending on dendrimer generation), assigned to exciplex formation between the peripheral dimethoxybenzene groups and the inner tertiary amino groups. Upon addition of trifluoroacetic acid, which causes protonation of the inner amino groups of the dendrimers, the intensity of exciplex emission decreases and the intensity of the 319 nm band of the DMB units increases, reaches a maximum, and then decreases with increasing protonation of the dendrimer interior, presumably because of excimer formation between peripheral DMB units. Electrochemical investigations have shown that in acetonitrile the dendrimers exhibit a reversible, multielectron oxidation wave at about +1.7 V vs SCE, assigned to the peripheral DMB units and broad anodic peaks in the region +0.8/+1.5 V, assigned to oxidation of the inner tertiary amino groups.