Ten novel small-molecule fluorophores containing two electron-accepting imidazo[1,2-a]pyridine (ImPy) units are presented. Each ImPy core is functionalized at its C6 position with groups featuring either electron accepting (A) or donating (D) properties, thus providing emitters with general structure X-ImPy-Y-ImPy-X (X=either A or D; Y=phenyl or pyridine). The molecules bear either a phenyl (series 4) or a pyridine (series 5) π bridge that connects the two ImPys via meta (phenyl) or 2,6- (pyridine) positions, yielding an overall V-shaped architecture. The final compounds are synthetized straightforwardly by condensation between substituted 2-aminopyridines and α-halocarbonyl derivatives. All the compounds display intense photoluminescence with quantum yield (PLQY) in the range of 0.17-0.51. Remarkably, substituent effect enables tuning the emission from near-UV to (deep-)blue region while keeping Commission Internationale de l'Éclairage (CIE) y coordinate ≤0.07. The emitting excited state is characterized by a few nanoseconds lifetime and high radiative rate constant, and its nature is modulated from pure π-π* to intramolecular charge transfer (ICT) by the electronic properties of the peripheral X substituent. This is further corroborated by the nature of the frontier orbitals and vertical electronic excitations computed at (time-dependent) density functional level of theory (TD-)DFT. Finally, this study enlarges the palette of bright deep-blue emitters based on the interesting ImPy scaffolds in view of their potential application as photo-functional materials in optoelectronics.
Keywords: N-heterocycles; chromophores; density functional theory; fluorescence; fluorescent probes.
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