Synthesis, spectral characterization and fluorescence study of functional [5]Helicene derivatives: Experimental and theoretical investigation

Abstract

Two D–π–A structured [5]Helicene derivatives, labeled HL1 and HL2, were synthesized in a high overall yield through a short synthetic photochemical route using mild conditions. Structural analyses, photophysical and electrochemical properties and theoretical studies were investigated in a comparative manner. These compounds show deep green color in both solution and solid state and they are highly soluble in common organic solvents and even in non-polar solvents such as toluene and cyclohexane. The shapes of experimental and calculated absorption spectra of these helical scaffolds in a dichloromethane solution were found to be approximately the same, indicating that the alkoxy groups could efficiently prevent intermolecular contact and aggregation. These Helicene derivatives were found to emit a green light in the fluorescence process inducing a Stokes shift of 0.39–0.41 eV (3200–3303 cm−1). They exhibit an irreversible one-electron oxidation and one-electron reduction and their HOMO and LUMO energy levels were electrochemically estimated to be, respectively, -5.88 and -3.20 eV for HL1 and -5.76 and -3.26 eV for HL2. Complementary density functional theory (DFT) calculations have also been performed in order to rationalize the different behaviors in the target [5]Helicenes and to gain insight into their geometric, electronic and spectroscopic properties. The corresponding calculated results were found to be in good agreement with the experimental data.