Azabora[5]helicene Charge-Transfer Dyes Show Efficient and Spectrally Variable Circularly Polarized Luminescence

Three helicenes based on a borylated arylisoquinoline skeleton have been prepared in their enantiopure forms and characterized with respect to their photophysical properties, including the use of chiroptical spectroscopies. The dyes show varying charge-transfer characteristics and efficient emission (quantum yields between 0.13 and 0.30, in toluene), which is governed by the electron-donor substitution (p-MeO-phenyl, p-Me2N-phenyl) at the helicene. Marked differences in the emission wavelength and Stokes shift are observed, with the dimethylamino-substituted derivative emitting most red-shifted (maximum at ca. 590 nm) and displaying the highest Stokes shift (ca. 6000 cm(-1)) in toluene. The helicenes show electronic circular dichroism (ECD) and significant circularly polarized luminescence (CPL) with dissymmetry factors of up to 3.5x10(-3). The sign of the ECD band corresponding to the first transition and of the CPL spectrum depend sensibly on the electron-donor substitution.