Three-component synthesis of cryofluorescent 2,4-disubstituted 3H-1,5-benzodiazepines -: Conformational control of emission properties

2,4-Disubstituted benzodiazepines are readily synthesized in good yields from acyl chlorides, terminal alkynes, and benzene-1,2-diamines by a consecutive one-pot, three-component Sonogashira coupling/Michael addition/cyclocondensation sequence. These diazepines display intense solid-state fluorescence, but only weak emission in solution at room temperature. The absorption and emission maxima can be controlled by the substitution pattern. Upon cooling, however, the phenomenon of cryofluorescence can be observed. X-ray structure analysis and temperature-dependent NMR indicate that freezing out of conformational interconversions is the source of the thermochromicity observed upon UV excitation. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008).