BODIPY chromophores at the either end of said chiral helix [61]. In fact, the observed
bisignalization can be easily rationalized by the exciton coupling theory. Thus, if both
BODIPY units are connected by the R,R bridge, the preferred helical configuration is
anticipated to be P, because such a configuration places the exciton partners
(BODIPY chromophores) in an anticlockwise arrangement (negative exciton couplet; see Fig. 6.28), giving rise to the observed negative Cotton effect (plus-to-minus
pattern in the bisignalized CD signal of the R,R enantiomer with decreasing light
energy; see Fig. 6.27). Furthermore (R,R)-36 is computed to adopt a stable helical
conformation with P configuration (Fig. 6.28) [61], whereas (S,S)-36 is computed to
adopt preferably the helical M configuration. However, when an M configuration is
computationally enforced for (R,R)-36, or a (P) configuration enforced for (S,S)-36,
the corresponding helical conformation becomes unstable and a pleated (CD-silent)
conformation is shown to be the optimum geometry within these constraints [61].
The helically chiral geometry of (R,R)-36 and (S,S)-36 is not only able to perturb
efficiently the BODIPY absorption transitions but also the BODIPY emission,
giving place to CPL upon excitation with visible wavelength light in CHCl 3 solution
[62]. The recorded CPL spectra for (R,R)-36 and (S,S)-36 were virtually mirror
images, with maxima matching the maximum fluorescent emission of the involved
BODIPY chromophores (λ max ca. 570 nm). Although the obtained maximum g lum
values for each enantiomer are small (|g lum | ca. 1Á10
À3 , positive g lum for the R,R
enantiomer), they demonstrate that the involved helical architecture, despite its
conformational lability, is able to endow the involved BODIPY chromophores
with CPL activity in solution, since the emitted light is equally polarized with
opposite handedness for each labile enantiomer [62].
The feasibility of this helically labile design to construct CPL-enabled bis(BODIPY)s
was tested by the synthesis and study of (R,R)- and (S,S)-37 (Fig. 6.29). These new
bis(BODIPY)s are oxygenated analogues of (R,R)-36 and (S,S)-36, respectively, in
that 37 involve an oxygen-based bridge in place of the nitrogen-based bridge of 36.
These second-generation bis(BODIPY)s 37 were also obtained by nucleophilic
aromatic substitution reaction of the same 3,3
0 -dichloroBODIPY precursor as used
previously, but now using the corresponding commercially available enantiopure
diol as the nucleophile (chemical yield ca. 60%) [62].
An experimental and computational study of bis(BODIPY)s 37 demonstrated that
they too both adopt a preferred helical configuration in solution (similarly to 36, (P)
for the (R,R) enantiomer and (M ) for the (S,S) enantiomer), giving rise to similar,
opposite, and bisignalized CD spectra in CHCl 3 solution (again negative Cotton
Effect for the (R,R) enantiomer, positive for the (S,S) enantiomer), as well as
opposite CPL spectra under visible irradiation (maximum |g lum | ca. 1Á10
À3 ). However, whilst nitrogenated (R,R)-36 preferentially emits left circularly polarized light
(positive g lum value), oxygenated (R,R)-37 preferentially emits right circularly
polarized light, even though their absolute configuration is the same (Fig. 6.30).
The same reversal behaviour is also observed for the corresponding (S,S)
couple [62].
This striking behaviour, which is similar to that previously discussed for both 20
and 21 and related spiranic O-BODIPYs, was attributed to a differential ability for
140
M. J. Hall and S. de la Moya
bisignalization can be easily rationalized by the exciton coupling theory. Thus, if both
BODIPY units are connected by the R,R bridge, the preferred helical configuration is
anticipated to be P, because such a configuration places the exciton partners
(BODIPY chromophores) in an anticlockwise arrangement (negative exciton couplet; see Fig. 6.28), giving rise to the observed negative Cotton effect (plus-to-minus
pattern in the bisignalized CD signal of the R,R enantiomer with decreasing light
energy; see Fig. 6.27). Furthermore (R,R)-36 is computed to adopt a stable helical
conformation with P configuration (Fig. 6.28) [61], whereas (S,S)-36 is computed to
adopt preferably the helical M configuration. However, when an M configuration is
computationally enforced for (R,R)-36, or a (P) configuration enforced for (S,S)-36,
the corresponding helical conformation becomes unstable and a pleated (CD-silent)
conformation is shown to be the optimum geometry within these constraints [61].
The helically chiral geometry of (R,R)-36 and (S,S)-36 is not only able to perturb
efficiently the BODIPY absorption transitions but also the BODIPY emission,
giving place to CPL upon excitation with visible wavelength light in CHCl 3 solution
[62]. The recorded CPL spectra for (R,R)-36 and (S,S)-36 were virtually mirror
images, with maxima matching the maximum fluorescent emission of the involved
BODIPY chromophores (λ max ca. 570 nm). Although the obtained maximum g lum
values for each enantiomer are small (|g lum | ca. 1Á10
À3 , positive g lum for the R,R
enantiomer), they demonstrate that the involved helical architecture, despite its
conformational lability, is able to endow the involved BODIPY chromophores
with CPL activity in solution, since the emitted light is equally polarized with
opposite handedness for each labile enantiomer [62].
The feasibility of this helically labile design to construct CPL-enabled bis(BODIPY)s
was tested by the synthesis and study of (R,R)- and (S,S)-37 (Fig. 6.29). These new
bis(BODIPY)s are oxygenated analogues of (R,R)-36 and (S,S)-36, respectively, in
that 37 involve an oxygen-based bridge in place of the nitrogen-based bridge of 36.
These second-generation bis(BODIPY)s 37 were also obtained by nucleophilic
aromatic substitution reaction of the same 3,3
0 -dichloroBODIPY precursor as used
previously, but now using the corresponding commercially available enantiopure
diol as the nucleophile (chemical yield ca. 60%) [62].
An experimental and computational study of bis(BODIPY)s 37 demonstrated that
they too both adopt a preferred helical configuration in solution (similarly to 36, (P)
for the (R,R) enantiomer and (M ) for the (S,S) enantiomer), giving rise to similar,
opposite, and bisignalized CD spectra in CHCl 3 solution (again negative Cotton
Effect for the (R,R) enantiomer, positive for the (S,S) enantiomer), as well as
opposite CPL spectra under visible irradiation (maximum |g lum | ca. 1Á10
À3 ). However, whilst nitrogenated (R,R)-36 preferentially emits left circularly polarized light
(positive g lum value), oxygenated (R,R)-37 preferentially emits right circularly
polarized light, even though their absolute configuration is the same (Fig. 6.30).
The same reversal behaviour is also observed for the corresponding (S,S)
couple [62].
This striking behaviour, which is similar to that previously discussed for both 20
and 21 and related spiranic O-BODIPYs, was attributed to a differential ability for
140
M. J. Hall and S. de la Moya