π‑Truncation of a Parent Nanographene: Chirally Twisted Green Fluorophores

The
twist operation of the π-skeleton would induce left or
right handedness. Herein, we report two chirally twisted C64 nanographenes designed by formally erasing two fused benzenes from
a parent C76 nanographene. These C64 nanographenes
possess two [6]helical units implemented around a central pyrene core
in a different arrangement. The π-truncation pulls the emission
bands hypsochromically from the red (C76, λmax = 612 nm) to green (C64, λmax = 505
and 506 nm) region. Curiously, the +/– sign of circularly polarized
luminescence at the S1 → S0 transition
was reversed between the two C64 nanographenes despite
them possessing the same handedness. This is attributed to the slight
difference in the arrangement of the two transition dipole moments
fluctuating at around 90°, as suggested computationally.