configurationally-labile helically-chiral bis(BODIPY), and (3) two BODIPY moieties with intrinsic helical chirality are combined into a figure-of-eight motif.
Thus, this chapter aims to review the current state of the art with respect to the
design and synthesis of CPL-SOMs based on the BODIPY scaffold, including an
evaluation of how recent findings could inform chiral design strategies for the
development of future BODIPY based CPL-SOM with highly efficient CPL
emission.
6.2 Chirality in BODIPYs
Prior to the examination of BODIPYs as potential CPL-SOMs, a small number of
chiral BODIPYs had been described in the literature, either through their synthesis as
single enantiomers or through chiral resolution, thus allowing preliminary evaluation
of the chiroptical properties of BODIPYs, albeit not with a focus on CPL. We will
therefore discuss a number of these chiral BODIPYs to provide both context for the
discussion of current BODIPY CPL-SOMs and to underpin the conceptual basis for
future developments in the field [31].
6.2.1 BODIPYs with Covalently Attached Chiral Moieties
Early examples of enantiomerically pure BODIPYs were typically obtained through
the decoration of the central chromophore with asymmetric substituents. One of the
first examples of such systems was published by Gossauer et al. and involved the
preparation of two homochiral mono(BODIPY) architectures bearing asymmetric
phenyl substituents at either the meso- (1) or α-positions (2) [32]. Measurement of
the circular dichroism (CD) spectra of both 1 and 2 showed Cotton effects
corresponding to the S 0 –S 1 transition of the BODIPY fluorophores, with αfunctionalized BODIPY 2 showing a significantly stronger Cotton effect
(λ ¼ 520 nm, Δε max ¼ +12.5 Lmol
À1 cm
À1 ) than that of the meso-functionalized
BODIPY 1 (λ ¼ 538 nm, Δε max ¼ +2 Lmol
À1 cm
À1 ). Although no CPL were
reported for these compounds, these early observations led Gossauer et al. to
speculate that “. . .both a ‘chiral perturbation’ of an inherently planar dipyrrin
chromophore and a twisting deformation of the latter may give rise to high optical
activity of the corresponding derivatives...”. As we will see in later examples, the
requirement for a “twisted” fluorophore core remains a key design component in the
development of chiroptically active BODIPY CPL-SOMs (Fig. 6.2).
An alternate strategy for the synthesis of chirally substituted BODIPYs was
developed by Grimme et al. through the introduction of binaphthyl moieties at the
meso-position [33]. Mono(BODIPY) 3 and bis(BODIPY) 4 were prepared through
condensation of the appropriate enantiomerically pure 1,1
0 -binaphthalene
carbaldehydes with 2,4-dimethylpyrrole. The CD spectra of these two compounds
120
M. J. Hall and S. de la Moya
Thus, this chapter aims to review the current state of the art with respect to the
design and synthesis of CPL-SOMs based on the BODIPY scaffold, including an
evaluation of how recent findings could inform chiral design strategies for the
development of future BODIPY based CPL-SOM with highly efficient CPL
emission.
6.2 Chirality in BODIPYs
Prior to the examination of BODIPYs as potential CPL-SOMs, a small number of
chiral BODIPYs had been described in the literature, either through their synthesis as
single enantiomers or through chiral resolution, thus allowing preliminary evaluation
of the chiroptical properties of BODIPYs, albeit not with a focus on CPL. We will
therefore discuss a number of these chiral BODIPYs to provide both context for the
discussion of current BODIPY CPL-SOMs and to underpin the conceptual basis for
future developments in the field [31].
6.2.1 BODIPYs with Covalently Attached Chiral Moieties
Early examples of enantiomerically pure BODIPYs were typically obtained through
the decoration of the central chromophore with asymmetric substituents. One of the
first examples of such systems was published by Gossauer et al. and involved the
preparation of two homochiral mono(BODIPY) architectures bearing asymmetric
phenyl substituents at either the meso- (1) or α-positions (2) [32]. Measurement of
the circular dichroism (CD) spectra of both 1 and 2 showed Cotton effects
corresponding to the S 0 –S 1 transition of the BODIPY fluorophores, with αfunctionalized BODIPY 2 showing a significantly stronger Cotton effect
(λ ¼ 520 nm, Δε max ¼ +12.5 Lmol
À1 cm
À1 ) than that of the meso-functionalized
BODIPY 1 (λ ¼ 538 nm, Δε max ¼ +2 Lmol
À1 cm
À1 ). Although no CPL were
reported for these compounds, these early observations led Gossauer et al. to
speculate that “. . .both a ‘chiral perturbation’ of an inherently planar dipyrrin
chromophore and a twisting deformation of the latter may give rise to high optical
activity of the corresponding derivatives...”. As we will see in later examples, the
requirement for a “twisted” fluorophore core remains a key design component in the
development of chiroptically active BODIPY CPL-SOMs (Fig. 6.2).
An alternate strategy for the synthesis of chirally substituted BODIPYs was
developed by Grimme et al. through the introduction of binaphthyl moieties at the
meso-position [33]. Mono(BODIPY) 3 and bis(BODIPY) 4 were prepared through
condensation of the appropriate enantiomerically pure 1,1
0 -binaphthalene
carbaldehydes with 2,4-dimethylpyrrole. The CD spectra of these two compounds
120
M. J. Hall and S. de la Moya