Highly Stable Supramolecular Donor-Acceptor Complexes Involving a Bis(18-Crown-6)azobenzene as Weak Donor: Structure-Property Relationships.

The physicochemical properties of highly stable supramolecular donor-acceptor (D-A) complexes of a bis(18-crown-6)azobenzene (weak π-donor) with a series of bis(ammonioalkyl) derivatives of viologen-like molecules (π-acceptors) in acetonitrile were studied using cyclic voltammetry, UV-vis absorption spectroscopy, H NMR spectroscopy, and density functional theory (DFT) calculations. The crystalline structures of the bis(crown)azobenzene and its complex with a bis(ammoniopropyl) derivative of 2,7-diazapyrene were determined by X-ray diffraction analysis. In solution, all of the supramolecular D-A complexes studied have a pseudocyclic structure owing to ditopic coordination of the ammonium groups of the acceptor to the crown ether moieties of the donor. These complexes show somewhat lower stability as compared with the previously studied complexes of the related derivative of stilbene (strong π-donor), which is explained by the relatively weak intermolecular charge-transfer (CT) interactions. Time-dependent DFT calculations predict that the low-energy CT transition in the D-A complex of the bis(crown)azobenzene with a bis(ammoniopropyl) derivative of 4,4'-bipyridine lies between the local ππ* and π* transitions of the azobenzene. The absorption band associated with the CT transition is indiscernible in the spectrum since it is overlapped with broad and more intense ππ* and π* bands. It was found that the → photoisomerization quantum yield of the bis(crown)azobenzene decreases by almost an order of magnitude upon the complexation with the 4,4'-bipyridine derivative. This effect was tentatively attributed to the intermolecular electron transfer that occurs in the ππ* excited state of the azobenzene and competes with the ππ* → π* internal conversion.