flexibility of the BODIPY core resulting in subtitle changes to the observed
photophysical properties [55].
Helically chiral mono(BODIPY)s 27–30 were synthesized as racemates and were
subsequently resolved by semi-preparative chiral HPLC (Chiralpak OB, toluene/nhexane 9:1 (27); Chiralcel OD-H, i-PrOH/n-hexane 1:4 (28–30)). Measurement of
the CD spectra of 27–30 gave mirror image spectra for the resolved P and
M enantiomers, with a strong Cotton effect apparent, corresponding to the S 0 –S 1
BODIPY transition. Assignment of absolute stereochemistry was performed through
comparison of the experimental and calculated CD spectra (TD-DFT). P and
M helically-chiral mono(BODIPY)s 27–30 showed red-shifted CPL upon irradiation, the CPL spectra giving maxima (|g lum | ¼ 4.7Á10
À3 , 3.3Á10
À3 , 4.3Á10
À3 and
4.2Á10
À3 for 27–30, respectively) corresponding to the BODIPY emission maxima
of each fluorophore. Helically chiral mono(BODIPY) 27 gave a marginally larger |
g lum | than 28–30, suggesting a subtle influence of the meso-substituent on the
conformation and thus chiroptical properties of these systems. X-ray crystal structures were obtained for helically-chiral mono(BODIPY)s 27–30 (albeit via crystals
containing a racemic mixture of the corresponding mono(BODIPY)). In all cases, a
significant twisting of the fluorophore was observed; the twist angle between the
planes as defined by the two pyrrolic rings being 11.2
, 9.0
and 9.8
for 27, 28 and
29, respectively. This provides some support for Gossauer’s proposition that for a
BODIPY to be CPL active, a twisting deformation of the planar fluorophore is
required, although crystallographically determined twist angles can be influenced by
crystal packing effects and would require confirmation through computational
prediction of solution conformations.
Subsequently, Nabeshima et al. examined the synthesis of a π-skewed helically
chiral mono(BODIPY) 31 (Fig. 6.20), formed through an oxidative annulation of the
corresponding 2,6-bis(biphenyl)BODIPY precursor [56]. Following separation from
the R,S meso form, the remaining R,R and S,S enantiomers of mono(BODIPY) 31
were separated by chiral HPLC (Chiralpak IA, chloroform/n-hexane 1:1). As previously, the twisting deformation of the fluorophore core was evaluated through
examination of the X-ray crystal structure, which displayed a twist angle between
N
N B
O
O
N
N B
O
O
N
N B
O
O
N
N B
O
O
N
lg lum l = 4.7·10 -3
f = 0.65
lg lum l = 3.3·10 -3
f = 0.73
lg lum l = 4.3·10 -3
f = 0.52
lg lum l = 4.2·10 -3
f = 0.28
27
28
29
30
Fig. 6.19 Helically chiral N,N,O,O-boron-chelated dipyrromethenes 27–30 and their visible CPL
signatures upon visible light irradiation in solution (one enantiomer shown in each case)
6 BODIPY Based Emitters of Circularly Polarized Luminescence
133
photophysical properties [55].
Helically chiral mono(BODIPY)s 27–30 were synthesized as racemates and were
subsequently resolved by semi-preparative chiral HPLC (Chiralpak OB, toluene/nhexane 9:1 (27); Chiralcel OD-H, i-PrOH/n-hexane 1:4 (28–30)). Measurement of
the CD spectra of 27–30 gave mirror image spectra for the resolved P and
M enantiomers, with a strong Cotton effect apparent, corresponding to the S 0 –S 1
BODIPY transition. Assignment of absolute stereochemistry was performed through
comparison of the experimental and calculated CD spectra (TD-DFT). P and
M helically-chiral mono(BODIPY)s 27–30 showed red-shifted CPL upon irradiation, the CPL spectra giving maxima (|g lum | ¼ 4.7Á10
À3 , 3.3Á10
À3 , 4.3Á10
À3 and
4.2Á10
À3 for 27–30, respectively) corresponding to the BODIPY emission maxima
of each fluorophore. Helically chiral mono(BODIPY) 27 gave a marginally larger |
g lum | than 28–30, suggesting a subtle influence of the meso-substituent on the
conformation and thus chiroptical properties of these systems. X-ray crystal structures were obtained for helically-chiral mono(BODIPY)s 27–30 (albeit via crystals
containing a racemic mixture of the corresponding mono(BODIPY)). In all cases, a
significant twisting of the fluorophore was observed; the twist angle between the
planes as defined by the two pyrrolic rings being 11.2
, 9.0
and 9.8
for 27, 28 and
29, respectively. This provides some support for Gossauer’s proposition that for a
BODIPY to be CPL active, a twisting deformation of the planar fluorophore is
required, although crystallographically determined twist angles can be influenced by
crystal packing effects and would require confirmation through computational
prediction of solution conformations.
Subsequently, Nabeshima et al. examined the synthesis of a π-skewed helically
chiral mono(BODIPY) 31 (Fig. 6.20), formed through an oxidative annulation of the
corresponding 2,6-bis(biphenyl)BODIPY precursor [56]. Following separation from
the R,S meso form, the remaining R,R and S,S enantiomers of mono(BODIPY) 31
were separated by chiral HPLC (Chiralpak IA, chloroform/n-hexane 1:1). As previously, the twisting deformation of the fluorophore core was evaluated through
examination of the X-ray crystal structure, which displayed a twist angle between
N
N B
O
O
N
N B
O
O
N
N B
O
O
N
N B
O
O
N
lg lum l = 4.7·10 -3
f = 0.65
lg lum l = 3.3·10 -3
f = 0.73
lg lum l = 4.3·10 -3
f = 0.52
lg lum l = 4.2·10 -3
f = 0.28
27
28
29
30
Fig. 6.19 Helically chiral N,N,O,O-boron-chelated dipyrromethenes 27–30 and their visible CPL
signatures upon visible light irradiation in solution (one enantiomer shown in each case)
6 BODIPY Based Emitters of Circularly Polarized Luminescence
133