Ultrafast excited state dynamics of a stilbene-viologen charge transfer complex and its interaction with alkanediammonium salts

The spectral and thermodynamic properties of charge transfer complexes DA and DAD between (E)-bis(18-crown-6) stilbene (D) and 1,1'-bis(2-ammonioethyl)-4,4'-bipyridlnium tetraperchlorate (A) in MeCN were studied. The complex DA is highly stable, while the complex D center dot A center dot D is weakly stable. D center dot A does not fluoresce due to fast intramolecular processes of direct and reverse electron transfer. The efficiency of DA fluorescence ignition upon adding alkanediammonium salts depends on the length of a carbon chain. The spectral and kinetic characteristics of D center dot A and D center dot A center dot D CT states were obtained by femtosecond transient absorption spectroscopy. It was established that the characteristic time of back electron transfer in the CT state of D center dot A center dot D (770 fs) is significantly higher than that in the CT state of D center dot A (400 fs). In a number of charge transfer complexes, formed by derivatives of 4,4'-bipyridinium and di-(4-pyridinium)-ethylene with ammonioethyl and ammoniopropyl N-substituents, the rate of back electron transfer depends weakly on the acceptor nature, but is determined by the length of the carbon chain of terminal groups.