Phenylthiophene-dipicolinic acid-based emitters with strong solution blue and solid state green emission

The synthesis and characterization of highly efficient blue emitting phenyl-thiophene compounds derivatized with dipicolinic acid bis(diethylamide) and dipicolinic acid diethyl ester 1, 2, and 3 are reported. Quantum yields were determined to be between 97% and 76% in methanol and 99% and 49% in toluene solution. The solvent dependences of the solution luminescence behavior are discussed in terms of solvent refractive index and dielectric constant as well as hydrogen-bonding capability. Luminescence is also observed in the solid state for the three compounds. A bathochromic shift in the emission wavelength is accompanied by solid-state quantum yields between 29% and 4%. HOMO and LUMO energies, in the range -5.55 to -5.71 eV and -2.20 to -2.48 eV, respectively, were determined experimentally from cyclic voltammetry and absorption spectroscopy experiments. The electrochemical characterization revealed that cyclic voltammetry can be utilized to deposit thin films of 3. An X-ray quality single crystal of 1 was isolated. This compound crystallizes in the monoclinic space group P2(1)/c with cell parameters a = 15.224(3) A, b = 8.9383(18) A, c = 17.234(3) A, beta =114.36(3)degrees, and V = 2136.4(7) A(3). Appreciable solid state interactions in the form of pi-pi or S-S short contacts are not present, only weaker C-H center dot center dot center dot pi. A large torsion angle of 30.28 degrees between the phenyl and thiophene moieties and a small torsion angle of -3.37 degrees between the pyridine and the thiophene moieties are seen.