Synthetic Control of Photophysical Process and Circularly Polarized Luminescence of [5]Carbohelicene Derivatives Substituted by Maleimide Units

A series
of [5]­carbohelicene derivatives substituted by electron-withdrawing
maleimide and electron-donating methoxy, such as maleimide-substituted
[5]­carbohelicene (HeliIm) and methoxy-substituted HeliIm (MeO-HeliIm),
were newly designed and synthesized to examine the electrochemical
properties, excited-state dynamic and circularly polarized luminescence
(CPL). First, electrochemical measurements and DFT calculations of
[5]­carbohelicene derivatives were performed by comparing with the
structural isomers: picene derivatives. Introduction of an electron-withdrawing
maleimide group onto a [5]­carbohelicene core contributes to the stabilized
LUMO state in HeliIm as compared to that of [5]­carbohelicene (Heli),
whereas the energy level of HOMO state in MeO-HeliIm increases by
introducing electron-donating methoxy (MeO) groups onto a HeliIm skeleton.
The HOMO–LUMO gap of MeO-HeliIm is smaller than those of HeliIm
and Heli, which is similar to the steady-state spectroscopic measurements.
The absolute fluorescence quantum yield (ΦFL) of
HeliIm (0.37) largely increased as compared to [5]­carbohelicene, Heli
(0.04), whereas ΦFL of MeO-HeliIm (0.22) was slightly
smaller than that of HeliIm. Theses photophysical processes including
intersystem crossing are successfully explained by the kinetic discussions.
Since [5]­carbohelicene derivatives show the chirality, measurements
of circular dichroism (CD) and circularly polarized luminescence (CPL)
were successfully performed. In particular, HeliIm and MeO-HeliIm
have provide excellent circularly polarized luminescence (CPL) and
the values of the anisotropy factor glum were estimated to be ∼2.4 × 10–3 and
∼2.3 × 10–3, relatively. This is the
first observation of CPL in [5]­carbohelicene derivatives.