they acquire a π-stacked skewed geometry. The same properties are retained as well
by partially hydrogenated compound pyr-21 (Scheme 12.3), analogous to compound pyr-20 where the binaphthyl moiety is replaced by an octahydro-binaphthyl
moiety [22].
Imai’s group developed a dioxolane, bearing two stereogenic centers, from which
two pyrene moieties are connected through two –CH 2 OC(O)CH 2 – ester chains (pyr22, Scheme 12.4) [23]. The two pyrenes are thus located at the extremities of two
floppy arms that dispose from a rather large conformational freedom. As such, π–π
interactions between pyrene units could not be detected in the ground state; the
aromatic moieties assuming overall a T-shaped arrangement, as shown by DFT
calculations and hence non-detectable ECD. On the other hand, a weak but observable excimer CPL band was measured with a g lum ¼ |8.9 Â 10
À4 | (C ¼ 10
À4 M in
CHCl 3 ), demonstrating again that a chiroptically active geometry, bound by π–π
stacking of the aromatic subunits, is obtainable in the excited state.
12.2.3 Excimer CPL in Chiroptical Switches
Thanks to the exceptional CPL activity associated with pyrene excimers, researchers
have designed molecular systems that modulate their CPL response upon the
occurrence of an external stimulus, such as light or the addition of ions. These
systems are examples of so-called chiroptical switches.
R
R
O
O
R =
pyr-21
Imai 2018
Scheme 12.3 Structure of a pyrene-decorated octahydro-binaphthyl system
O
O
O
O
O
O
O
O
O
O
O
O
pyr-22
Imai 2015
ground state
excited state
Scheme 12.4 Structure of a pyrene-decorated dioxolane system
12 Circularly Polarized Luminescence from Intramolecular Excimers
281
by partially hydrogenated compound pyr-21 (Scheme 12.3), analogous to compound pyr-20 where the binaphthyl moiety is replaced by an octahydro-binaphthyl
moiety [22].
Imai’s group developed a dioxolane, bearing two stereogenic centers, from which
two pyrene moieties are connected through two –CH 2 OC(O)CH 2 – ester chains (pyr22, Scheme 12.4) [23]. The two pyrenes are thus located at the extremities of two
floppy arms that dispose from a rather large conformational freedom. As such, π–π
interactions between pyrene units could not be detected in the ground state; the
aromatic moieties assuming overall a T-shaped arrangement, as shown by DFT
calculations and hence non-detectable ECD. On the other hand, a weak but observable excimer CPL band was measured with a g lum ¼ |8.9 Â 10
À4 | (C ¼ 10
À4 M in
CHCl 3 ), demonstrating again that a chiroptically active geometry, bound by π–π
stacking of the aromatic subunits, is obtainable in the excited state.
12.2.3 Excimer CPL in Chiroptical Switches
Thanks to the exceptional CPL activity associated with pyrene excimers, researchers
have designed molecular systems that modulate their CPL response upon the
occurrence of an external stimulus, such as light or the addition of ions. These
systems are examples of so-called chiroptical switches.
R
R
O
O
R =
pyr-21
Imai 2018
Scheme 12.3 Structure of a pyrene-decorated octahydro-binaphthyl system
O
O
O
O
O
O
O
O
O
O
O
O
pyr-22
Imai 2015
ground state
excited state
Scheme 12.4 Structure of a pyrene-decorated dioxolane system
12 Circularly Polarized Luminescence from Intramolecular Excimers
281