chromophore, maintains the atropisomeric nature of the system resulting in tris
(BODIPY) 11 (Fig. 6.8).
Resolution of the racemic mixtures of these axially chiral BODIPYs was again
achieved through chromatography on a chiral solid support (Chiralcel-OD, heptane/
2-propanol or ethanol) and their chiroptical properties studied. The enantiomers of
bis(BODIPY) 10 and tris(BODIPY) 11 both gave mirror image CD spectra with
large anisotropy factors (bis(BODIPY) 10, g abs % 1Á10
À1 ; tris(BODIPY) 11,
g abs % 5Á10
À2 (estimated from the reported CD spectra and extinction coefficients)
for the CD maxima relating to the S 0 -S 1 transition of the two BODIPY systems. Bis
(BODIPY) 10 gave a single long-wavelength signal corresponding to the S 0 -S 1
transition, whilst tris(BODIPY) 11 gave a long-wavelength asymmetric couplet
(Fig. 6.9).
A related set of atropisomeric bis(BODIPY)s (12 and 13), alongside an
atropisomeric bis(aza-BODIPY) (14), were prepared by Bruhn et al. based around
a direct 2,2
0 -linkage (Fig. 6.10) [39]. In these cases the lateral differentiation is
achieved for both BODIPY/aza-BODIPY chromophores via the 2,2
0 -linkage, whilst
rotational restriction is achieved through either a mutual double methylÀmethyl
clash (12 and 13) or mutual double phenylÀphenyl clash (14). Atropisomeric bis
Fig. 6.9 CD spectra of resolved enantiomers (red and blue lines) of both bis(BODIPY) 10 and tris
(BODIPY) 11 [adapted with permission from [38], copyright 2014 American Chemical Society]
N B
N
F F
12
N B
N
F F
N B
N
F F
13
N B
N
F F
N B
N
N
Ph
Ph
Ph
Ph
F F
14
N B
N
N
Ph
Ph
Ph
Ph
F F
Et
Et
Fig. 6.10 Cryptochiral atropisomeric bis(BODIPY)s 12 and 13 and cryptochiral atropisomeric bis
(azaBODIPY) 14
6 BODIPY Based Emitters of Circularly Polarized Luminescence
125