title: Vertically π-expanded imidazo[1,2-a]pyridine: the missing link of the puzzle.
abstract: The dehydrogenative coupling of imidazo[1,2-a]pyridine derivative has been achieved for the first time. In cases in which the most-electron-rich position of the electron-excessive heterocycle was blocked by a naphthalen-1-yl substituent, neither oxidative aromatic coupling nor reaction under Scholl conditions enabled the fusion of the rings. The only method that converted the substrate into the corresponding imidazo[5,1,2-de]naphtho[1,8-ab]quinolizine was coupling in the presence of potassium in anhydrous toluene. Moreover, we discovered new, excellent conditions for this anion-radical coupling reaction, which employed dry O2 from the start in the reaction mixture. This method afforded vertically fused imidazo[1,2-a]pyridine in 63% yield. Interestingly, whereas the fluorescence quantum yield (Φ(fl)) of compound 3, despite the freedom of rotation, was close to 50%, the Φ(fl) value of flat naphthalene-imidazo[1,2-a]pyridine was only 5%. Detailed analysis of this compound by using DFT calculations and a low-temperature Shpol'skii matrix revealed phosphorescence emission, thus indicating that efficient intersystem-crossing from the lowest-excited S1 level to the triplet manifold was the competing process with fluorescence.
authors.affiliation: Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw (Poland), Fax: (+48) 22-6326681.
authors.forename: Dikhi
authors.initials: D
authors.lastname: Firmansyah
authors.forename: Marzena
authors.initials: M
authors.lastname: Banasiewicz
authors.forename: Irena
authors.initials: I
authors.lastname: Deperasińska
authors.forename: Artur
authors.initials: A
authors.lastname: Makarewicz
authors.forename: Boleslaw
authors.initials: B
authors.lastname: Kozankiewicz
authors.forename: Daniel T
authors.initials: DT
authors.lastname: Gryko
keywords: dehydrogenation; density functional calculations; fluorescence; heterocycles; optical properties