Electrogenerated chemiluminescence of a series of donor - Acceptor molecules and X-ray crystallographic evidence for the reaction mechanisms

Three series of donor-acceptor pi-conjugated (D-pi-A) molecules 1-3 have been synthesized with a 2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]quinolinyl (Julolidine group), N,N-dimethylamino, or N,N-diphenylamino group as the donor moiety, a phenylvinyl or thienylvinyl unit as the bridge, and a bromide or aldehyde group as the acceptor moiety. The photophysical, electrochemical, and electrogenerated chemiluminescence (ECL) characters of these compounds have been studied in a 1:1 PhH/MeCN solution. Three different categories of ECL mechanisms for each of the three families of compounds are discussed, respectively. Compounds 1a-c produce typical and simple monomer ECL emission resulting from the annihilation of their radical cations and radical anions. The ECL emission of compounds 2a-c can be ascribed as an excimer emission. Compounds 3a-c exhibit an aggregate ECL emission. X-ray crystal structures of compounds 1b, 2a, and 3a provide further proof for the above-mentioned reaction mechanisms. All these compounds show stable ECL emission via the singlet excited state without the addition of any co-reactant or additional compound.