(Fig. 6.12) [41]. The required tetrahedral geometry of the central boron atom ensures
that the two aryl groups must twist to direct the two B-O bonds to opposite faces of
the BODIPY chromophore, inducing an overall helical chirality to the molecule
(Fig. 6.12). Although helically chiral BODIPY 16 was synthesized as a racemate,
Burgess et al. showed that the involved enantiomers can be detected by analytical
chiral HPLC (Pirkle covalent (S,S) whelk-01; 2-propanol/hexane). Unfortunately,
single enantiomers of BODIPY 16 could not be isolated and no further analysis of
their chiroptical properties was undertaken.
Several related helical N,N,O,O-boron-chelated BODIPYs and azaBODIPYs
were subsequently reported by a number of groups; however in each case researchers
focused on the red-shifting effect of such a substitution pattern rather than their
potential as CPL-SOMs [42–44].
Interestingly, only a single chelating ortho-phenolic moiety in the 3-position is
required in order to induce helicity in such BODIPY architectures. Thus, helically
chiral N,N,O-chelated BODIPYs such as 17 were first synthesized by Nabeshima
et al. through the chelation of the corresponding 3,5-diarylBODIPY with an appropriate arylboronic acid (Fig. 6.13) [45].
1 H NMR analysis of N,N,O-boron-chelated
BODIPY 17 in the presence of a chiral shift reagent (europium tris
[3-(heptafluoropropylhydroxymethylene)-(+)-camphorate]) showed splitting of the
methyl signal at 2.14 ppm into two, due to the formation of transient diastereotopic
complexes in solution, indicating stable helical chirality on the NMR timescale.
Related helically chiral N,N,O-boron-chelated BODIPYs have been reported by
the groups of both Harriman and Jiao, albeit with limited discussion of the potential
for CPL [46, 47]. Whilst in a more recent example, Nabeshima et al. reported the
N
N B
O
O
I
16
Fig. 6.12 Helically chiral
N,N,O,O-boron-chelated
BODIPY 16 (one
enantiomer shown) [adapted
with permission from [41],
published by the Royal
Society of Chemistry
N
N B
Ph
O
17
OH
Fig. 6.13 Helically chiral N,N,O-boron-chelated BODIPY 17 (one enantiomer shown) [adapted
with permission from [45], copyright 2009 Elsevier]
6 BODIPY Based Emitters of Circularly Polarized Luminescence
127
that the two aryl groups must twist to direct the two B-O bonds to opposite faces of
the BODIPY chromophore, inducing an overall helical chirality to the molecule
(Fig. 6.12). Although helically chiral BODIPY 16 was synthesized as a racemate,
Burgess et al. showed that the involved enantiomers can be detected by analytical
chiral HPLC (Pirkle covalent (S,S) whelk-01; 2-propanol/hexane). Unfortunately,
single enantiomers of BODIPY 16 could not be isolated and no further analysis of
their chiroptical properties was undertaken.
Several related helical N,N,O,O-boron-chelated BODIPYs and azaBODIPYs
were subsequently reported by a number of groups; however in each case researchers
focused on the red-shifting effect of such a substitution pattern rather than their
potential as CPL-SOMs [42–44].
Interestingly, only a single chelating ortho-phenolic moiety in the 3-position is
required in order to induce helicity in such BODIPY architectures. Thus, helically
chiral N,N,O-chelated BODIPYs such as 17 were first synthesized by Nabeshima
et al. through the chelation of the corresponding 3,5-diarylBODIPY with an appropriate arylboronic acid (Fig. 6.13) [45].
1 H NMR analysis of N,N,O-boron-chelated
BODIPY 17 in the presence of a chiral shift reagent (europium tris
[3-(heptafluoropropylhydroxymethylene)-(+)-camphorate]) showed splitting of the
methyl signal at 2.14 ppm into two, due to the formation of transient diastereotopic
complexes in solution, indicating stable helical chirality on the NMR timescale.
Related helically chiral N,N,O-boron-chelated BODIPYs have been reported by
the groups of both Harriman and Jiao, albeit with limited discussion of the potential
for CPL [46, 47]. Whilst in a more recent example, Nabeshima et al. reported the
N
N B
O
O
I
16
Fig. 6.12 Helically chiral
N,N,O,O-boron-chelated
BODIPY 16 (one
enantiomer shown) [adapted
with permission from [41],
published by the Royal
Society of Chemistry
N
N B
Ph
O
17
OH
Fig. 6.13 Helically chiral N,N,O-boron-chelated BODIPY 17 (one enantiomer shown) [adapted
with permission from [45], copyright 2009 Elsevier]
6 BODIPY Based Emitters of Circularly Polarized Luminescence
127