Design, Synthesis and Properties of 2/6-Aryl Substituted Azulene Derivatives

Six 2/6-aryl substituted azulene derivatives 1 similar to 6 were designed and synthesised. Compounds 1 similar to 3 and 4 similar to 6 are 2and 6-substituted derivatives, respectively, where the arly substituents were pentafluorobenzene, benzene and a-thiophene. The UV-Vis spectra, fluorescence spectra, electrochemical properties and proton-responsive properties of 1 similar to 6 were studied. To investigate the molecular sturcture, absorption spectra and energy levels of compounds 1 similar to 6, density functional theory (DFT) calculations were carried out. In comparison with the UV-Vis spectra of azulene, the absorption of S-0 -> S-2 transition of 1 similar to 6 showed red-shift (Delta lambda= 6 similar to 68 nm). Owing to the strong electron-donating ability of a-thiophene group, remarkable bathochromic shifts of 3 and 6 (Delta lambda= 68 and 48 nm, respectively) were obseved. The fluorescence spectra revealed that 4 (phi(F)= 0.082) has the highest fluorescence quantum yield of 1 similar to 6, while 1-H+ (phi(F)= 0.359) has the highest fluorescence quantum yield of the protonated compounds 1-H+similar to 6-H+, benefiting from the electron-withdrawing pentafluorophenyl group of 1 and 1-H+. Moreover, the electrochemical analysis and DFT calculations demonstated that the introduction of electron-withdrawing pentafluorophenyl unit in the 2/6-positon of azulene can significantly lower the energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). In comparison with the HOMO/LUMO energy levels of azulene, those of 1 and 4 shift downward with Delta E-HOMO/Delta E-LUMO of -0.23/-0.18 and -0.20/-0.15 eV, respectively. The investigations of physical/chemical properties of 2/6-aryl substituted azulene derivatives will provide v aluable insights for developing azulene-based organic functional molecules.