Structure of charge-transfer complexes formed by biscrown stilbene and dipyridylethylene derivatives as probed by surface-enhanced Raman scattering spectroscopy

Charge-transfer (CT) complexes between bis(18-crown-6) stilbene (1) and bisammonium derivative of dipyridylethylene (2) were studied by surface-enhanced Raman scattering spectroscopy. It was revealed that a molar excess of 2 favored the 1:1 CT complex ([1.2]) formation, whereas an excess of 1 stabilized the sandwichlike [1.2.1] CT complex. The considerably less stable 1:1 CT complex was also detected for I and 1-ammoniopropyl-4-methylpyridine. On the basis of the spectral analysis, the following was concluded: in the ground state these CT complexes can be considered as usual molecular complexes, the shift of electronic density from the donor to the acceptor occurred upon interaction of the molecular complex with a photon, and a large shift of the electronic density from donor to acceptor was a property of the excited state. Using model compounds, it was shown that interactions of the ammonium groups with the crown ether moieties played a critical role in the CT complex formation by stabilizing the molecular complex. Concerning the [1.2.1] complex, it was concluded that each ammonium group was coordinated with a single but not with both adjacent crown ether cycles. The mere interaction between the pi-electronic systems of stilbene and dipyridylethylene was not sufficient to form a tight complex required for CT. The ethylene group was not essential for CT, and the donor-acceptor pair was produced solely by the trisubstituted benzene and pyridine rings. Both [1.2] and [1.2.1] contained two equivalent donor-acceptor pairs, but a pair alone was supposed to participate in CT.