naphthylmagnesium bromide and subsequent selective oxidation of the 3-methyl
group with 2,3-dichloro-5,6-dicyanoquinone (DDQ) to form the corresponding
aldehyde. The loss of lateral symmetry on formation of B
à -BODIPY 8 was evident
through signal doubling in the
1 H NMR and further supported by the observation of
through space coupling of the formyl proton to the adjacent fluorine atom, arising
from the formation of an intramolecular NHÁÁÁF H-bond. Resolution of racemic
B
à -BODIPY 8 was achieved by chiral HPLC (Chiralcel-OD, hexane/2-propanol)
to give the corresponding enantiomers (absolute configurations were not determined). The enantiomeric nature of the thus resolved B
à -BODIPYs was confirmed
by the presence of mirror image CD spectra. A strong Cotton effect was observed at
the short-wavelength transitions associated with the carbonyl group; however only
weak signals were obtained corresponding to the S 0 -S 1 transition band of the
BODIPY chromophore (Fig. 6.6).
6.2.3 BODIPYs Displaying Axial Chirality
The aromatic structure of the BODIPY chromophore lends itself well to the introduction of axial chirality, via the formation of atropisomers containing rotationally
restricted aryl-aryl bonds. The formation of an atropisomeric BODIPY requires both
a laterally differentiated dipyrromethene core and a conformationally restricted
BODIPY-aryl bond. Hall et al. have demonstrated such an axially chiral BODIPY
through the preparation of 9. In this case lateral differentiation is achieved through
introduction of a substituent at the 2-position (through Heck coupling of the parent
2-bromo-BODIPY with ethyl acrylate), whilst the required conformationally
restricted BODIPY-aryl bond (methylÀmethyl clash) is introduced via the an
ortho-substituted aryl group in the meso-position (Fig. 6.7) [37]. The restricted
rotation of the meso-aryl substituent could be observed via the imposition of a
diastereotopic relationship on the two fluorine atoms of the chelating BF 2 moiety,
observable in the
19 F NMR spectrum (ABX coupling, each fluorine showing both
N B
N
F
I
O
H
8
Fig. 6.6 B
Ã
-BODIPY 8 (one enantiomer shown) and its associated CD spectra (racemic 8 (black);
firstly eluted (red) and secondly eluted (green) enantiomers by HPLC (Chiralcel-OD, hexane/2propanol)) [adapted with permission from [36], copyright 2010 American Chemical Society]
6 BODIPY Based Emitters of Circularly Polarized Luminescence
123
group with 2,3-dichloro-5,6-dicyanoquinone (DDQ) to form the corresponding
aldehyde. The loss of lateral symmetry on formation of B
à -BODIPY 8 was evident
through signal doubling in the
1 H NMR and further supported by the observation of
through space coupling of the formyl proton to the adjacent fluorine atom, arising
from the formation of an intramolecular NHÁÁÁF H-bond. Resolution of racemic
B
à -BODIPY 8 was achieved by chiral HPLC (Chiralcel-OD, hexane/2-propanol)
to give the corresponding enantiomers (absolute configurations were not determined). The enantiomeric nature of the thus resolved B
à -BODIPYs was confirmed
by the presence of mirror image CD spectra. A strong Cotton effect was observed at
the short-wavelength transitions associated with the carbonyl group; however only
weak signals were obtained corresponding to the S 0 -S 1 transition band of the
BODIPY chromophore (Fig. 6.6).
6.2.3 BODIPYs Displaying Axial Chirality
The aromatic structure of the BODIPY chromophore lends itself well to the introduction of axial chirality, via the formation of atropisomers containing rotationally
restricted aryl-aryl bonds. The formation of an atropisomeric BODIPY requires both
a laterally differentiated dipyrromethene core and a conformationally restricted
BODIPY-aryl bond. Hall et al. have demonstrated such an axially chiral BODIPY
through the preparation of 9. In this case lateral differentiation is achieved through
introduction of a substituent at the 2-position (through Heck coupling of the parent
2-bromo-BODIPY with ethyl acrylate), whilst the required conformationally
restricted BODIPY-aryl bond (methylÀmethyl clash) is introduced via the an
ortho-substituted aryl group in the meso-position (Fig. 6.7) [37]. The restricted
rotation of the meso-aryl substituent could be observed via the imposition of a
diastereotopic relationship on the two fluorine atoms of the chelating BF 2 moiety,
observable in the
19 F NMR spectrum (ABX coupling, each fluorine showing both
N B
N
F
I
O
H
8
Fig. 6.6 B
Ã
-BODIPY 8 (one enantiomer shown) and its associated CD spectra (racemic 8 (black);
firstly eluted (red) and secondly eluted (green) enantiomers by HPLC (Chiralcel-OD, hexane/2propanol)) [adapted with permission from [36], copyright 2010 American Chemical Society]
6 BODIPY Based Emitters of Circularly Polarized Luminescence
123