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,
1
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
n
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
n
π* bands. It was found
that the
E
→
Z
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
1
ππ* excited
state of the azobenzene and competes with the
1
ππ*
→
1
n
π* internal conversion.