Hydrogen-Bonded Self-assembly of Supramolecular Donor-Acceptor Complexes of (E)-Bis(18-crown-6)azobenzene with Bis(ammoniopropyl) Derivatives of Bipyridine and Dipyridylethylene in Acetonitrile

The supramolecular complex formation between donor (E)-bis(18-crown-6)azobenzene ((E)-1) and bis(ammoniopropyl) derivatives of bipyridine and dipyridylethylene acceptors (2, (E)-3) was studied in MeCN using steady-state absorption and fluorescence spectroscopy, fluorescence kinetics, and quantum-chemical (DFT, TDDFT, QDPT) calculations. The complexes are quite stable, but they undergo decomposition via substitution by alkali and alkaline-earth metal cations. The formation of 1:1 and 1:2 complexes between (E)-1 and alkali and alkaline-earth metal cations (Mn+) was confirmed. Spectral-kinetic parameters and stability constants were calculated. The determined stability constants are generally lower than those for bis(crown)stilbene complexes due to moderate electron-donor ability of azobenzene. The plot of fluorescence quantum yields of 1 center dot(Mn+)(2)vs charge density of the metal cation shows two separate linear relations for unipositive and dipositive ions. The results of quantum-chemical calculations supported the conclusions derived from steady-state experiments. The most stable conformations of pseudocyclic complex (E)-1 center dot 2 and two metal complexes were found. The latter involve 4 or 6 molecules of acetonitrile per each complex: (E)-1 center dot(Ca2+)(2)center dot(MeCN)(4), (E)-1 center dot(K+)(2)center dot(MeCN)(6). Based upon quantum-chemical calculations of model species, the character of absorption bands of crowns and their complexes with actual energy conversion pathway are discussed.