600 nm at À75
C, the intensity of which increases slowly with decreasing temperature down to À120
C and leaps much more rapidly thereafter to achieve the largest
amplitude A of 193 M
À1 cm
À1 and anisotropy factor g abs of 2.3 Â 10
À3 at À120
C.
The apparent excitonic coupling at lower temperatures indicates that two (or more)
BODIPY units are positioned in close proximity under the low-temperature conditions employed (vide infra).
7.8 Supramolecular Behavior
In contrast to the two temperature domains (assignable to the propeller and toroidal
domains) found for HABs, three discrete temperature domains are shown to exist for
chiral BODIPY B7. The temperature-dependence behaviors of (chir)optical properties observed for B7 in the higher temperature domains are quite analogous to those
for H6. This allows us to assign the CD intensity of B7 gradually increasing with
decreasing temperature from T c ¼ +10
C to T d ¼ À70
C to the equilibrium shift of
the CC to C propeller (Fig. 7.16, top). At temperatures above T c , the rotation of
propeller blades becomes more vigorous to render the contribution of whizzing
toroids more significant. The much higher T c for B7 (+10
C) than for H6
(À50
C) is sensible in view of the larger inter-blade separation and the missing
aromatic blade at the boron atom. As such, higher energy is required to effectively
incorporate the quasi-toroidal interaction. The activation energy calculated for the
domino inversion of B7 propeller (3.8 kcal mol
À1 ) is appreciably higher than that of
H6 (2.1 kcal mol
À1 ), probably due to the irregular arrangement of propeller blades
on the BODIPY core.
The temperature domain below T d (À70
C), where the CD couplet rapidly
grows, is unique to heptaaryl-BODIPY B7, while HAB H6 does not show any
sign of such behavior at least down to À150
C. Concentration-dependence and
diffusion-ordered NMR spectroscopic studies have revealed the supramolecular
dimer formation of B7 facilitated particularly in this temperature domain
(Fig. 7.16, bottom). Theoretical investigation suggested the formation of a headto-tail dimer in a plus-screw alignment (φ % +15
), where two B7 molecules are
stacked to each other in a face-to-face distance of r ¼ 5.1 Å and a center-to-center
displacement of d ¼ 2.4 Å to avoid the sterical clashes yet mutually fill up the void
space around the boron. The right-twisted arrangement of two B7 should induce a
positive excitonic coupling in the CD spectrum, which is in good agreement with the
experimental observation (Fig. 7.15a). Covalently bonded chiral BODIPY dimers
have been enthusiastically developed recently, either by tethering BODIPYs to a
chiral scaffold or by directly connecting two BODIPY units to induce axial chirality
[80–85]. The BODIPY derivative B7 comprised from supramolecular dimer formation provides superior and well-defined chiroptical responses by the exciton coupled
transition, which can be easily modulated by the environmental factors such as
temperature. Moreover, the highly fluorescent nature of BODIPY core better serves
as a switchable CPL materials that emits polarized light in visible and near-infrared
region (vide infra).
7 Propeller Chirality: Circular Dichroism and Circularly Polarized Luminescence
167
C, the intensity of which increases slowly with decreasing temperature down to À120
C and leaps much more rapidly thereafter to achieve the largest
amplitude A of 193 M
À1 cm
À1 and anisotropy factor g abs of 2.3 Â 10
À3 at À120
C.
The apparent excitonic coupling at lower temperatures indicates that two (or more)
BODIPY units are positioned in close proximity under the low-temperature conditions employed (vide infra).
7.8 Supramolecular Behavior
In contrast to the two temperature domains (assignable to the propeller and toroidal
domains) found for HABs, three discrete temperature domains are shown to exist for
chiral BODIPY B7. The temperature-dependence behaviors of (chir)optical properties observed for B7 in the higher temperature domains are quite analogous to those
for H6. This allows us to assign the CD intensity of B7 gradually increasing with
decreasing temperature from T c ¼ +10
C to T d ¼ À70
C to the equilibrium shift of
the CC to C propeller (Fig. 7.16, top). At temperatures above T c , the rotation of
propeller blades becomes more vigorous to render the contribution of whizzing
toroids more significant. The much higher T c for B7 (+10
C) than for H6
(À50
C) is sensible in view of the larger inter-blade separation and the missing
aromatic blade at the boron atom. As such, higher energy is required to effectively
incorporate the quasi-toroidal interaction. The activation energy calculated for the
domino inversion of B7 propeller (3.8 kcal mol
À1 ) is appreciably higher than that of
H6 (2.1 kcal mol
À1 ), probably due to the irregular arrangement of propeller blades
on the BODIPY core.
The temperature domain below T d (À70
C), where the CD couplet rapidly
grows, is unique to heptaaryl-BODIPY B7, while HAB H6 does not show any
sign of such behavior at least down to À150
C. Concentration-dependence and
diffusion-ordered NMR spectroscopic studies have revealed the supramolecular
dimer formation of B7 facilitated particularly in this temperature domain
(Fig. 7.16, bottom). Theoretical investigation suggested the formation of a headto-tail dimer in a plus-screw alignment (φ % +15
), where two B7 molecules are
stacked to each other in a face-to-face distance of r ¼ 5.1 Å and a center-to-center
displacement of d ¼ 2.4 Å to avoid the sterical clashes yet mutually fill up the void
space around the boron. The right-twisted arrangement of two B7 should induce a
positive excitonic coupling in the CD spectrum, which is in good agreement with the
experimental observation (Fig. 7.15a). Covalently bonded chiral BODIPY dimers
have been enthusiastically developed recently, either by tethering BODIPYs to a
chiral scaffold or by directly connecting two BODIPY units to induce axial chirality
[80–85]. The BODIPY derivative B7 comprised from supramolecular dimer formation provides superior and well-defined chiroptical responses by the exciton coupled
transition, which can be easily modulated by the environmental factors such as
temperature. Moreover, the highly fluorescent nature of BODIPY core better serves
as a switchable CPL materials that emits polarized light in visible and near-infrared
region (vide infra).
7 Propeller Chirality: Circular Dichroism and Circularly Polarized Luminescence
167