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 character 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 4 ×
103. The sign of the ECD band corresponding to the first transition
and the CPL spectrum depend sensibly on the electron-donor
substitution.