inherently achiral BODIPY chromophore. The success of such a perturbation is
attributed to the following key factors: (1) axial chirality and C 2 symmetry provided
by such a chiral moiety; (2) an orthogonally fixed arrangement of the perturbing
moiety with respect to the perturbed one, which is imposed by the involved spiranic
geometry; and (3) electronic isolation of both chromophores, due to the aforementioned orthogonal arrangement and the involvement of boron at the spiranic junction
[25]. Additionally, synthetic access to this class of BODIPY based CPL-SOMs is
straightforward, allowing the direct preparation of enantiopure CPL-enabling
BODIPYs from two readily accessible and oft commercial available components:
(1) achiral F-BODIPY dyes and (2) enantiopure 1,1
0 -biphen-2-ols such as BINOL
and its derivatives.
The fluorescence behaviour of these 1,1
0 -biphen-2-ol-based spiranic O-BODIPYs
can be easily tuned by properly modulating electronic factors in both the BODIPY
moiety and the oxygenated biphen-2-ol connected to it, as de la Moya et al. have
demonstrated recently [51]. This approach adds significant value to the use of such a
C 2 -symmetric mono(BODIPY) design when developing CPL-SOMs. As an example,
the said photophysical tuning has allowed the successful preparation of 21 (Fig. 6.16)
exhibiting enhanced CPL activity (ϕÁ|g lum |) when compared to its analogue 20
[52]. This enhancement is achieved by simply introducing 3,3
0 -dibromoBINOL instead
of BINOL in the C 2 -symmetric mono(BODIPY) structure. Strikingly, the sign of the
g lum values exhibited by 20 and 21, having identical chiral absolute configuration, are
opposite (see Fig. 6.16). Besides, each individual enantiomer of 20 shows opposite
signs for its maximum g abs and g lum values [50], whereas the enantiomers of 21 do not
show such a striking g abs -versus-g lum sign reversal [52]. Interestingly, the significant
CPL activity of mono(BODIPY) 21 joined to its lasing capability under laser pumping
has also served to explore factors affecting CPL activity in dye lasers [52].
The aforementioned differential behaviour of 20 and 21 in relation to the sign of
the corresponding g lum values [50, 52], joined to the fact that both mono(BODIPY)s
involve highly similar BODIPY chromophores, but with very different capability to
populate emissive intramolecular charge transfer (ICT) states (higher for BINOLbased 20; undetectable for 3,3
0 -dibromoBINOL-based 21) [51], served de la Moya’s
group to establish a new strategy to manipulate the CPL sign in chiral emitters (i.e. to
change the handedness of the observed circularly polarized emission) by means of
modulating the promotion of luminescent ICT states [53]. To apply such a strategy,
it is not necessary that the emission from the ICT state surpass the one from the
locally excited state, since the ICT emission is expected to involve larger circular
polarization. However, both circularly polarized emissions do need to show opposite
handedness [53]. This is an unprecedented way to manipulate the CPL sign in chiral
emitters without changing absolute configurations, being specifically useful for rigid
emitters and emitters without capability to promote excimer emission [53]. The new
strategy was exemplified by de la Moya et al. by means of a set of CPL-enabling
1,1
0 -biphen-2-ol-based spiranic O-BODIPYs 20–25, where the BODIPY (in red in
Fig. 6.17) provides the functional visible light chromophore and the 1,1
0 -biphen-2-ol
moiety (in blue in Fig. 6.17) acts as the ICT-switching moiety depending on the
electronic nature of the BODIPY chromophore connected to it. Thus, in dyes with
130
M. J. Hall and S. de la Moya
attributed to the following key factors: (1) axial chirality and C 2 symmetry provided
by such a chiral moiety; (2) an orthogonally fixed arrangement of the perturbing
moiety with respect to the perturbed one, which is imposed by the involved spiranic
geometry; and (3) electronic isolation of both chromophores, due to the aforementioned orthogonal arrangement and the involvement of boron at the spiranic junction
[25]. Additionally, synthetic access to this class of BODIPY based CPL-SOMs is
straightforward, allowing the direct preparation of enantiopure CPL-enabling
BODIPYs from two readily accessible and oft commercial available components:
(1) achiral F-BODIPY dyes and (2) enantiopure 1,1
0 -biphen-2-ols such as BINOL
and its derivatives.
The fluorescence behaviour of these 1,1
0 -biphen-2-ol-based spiranic O-BODIPYs
can be easily tuned by properly modulating electronic factors in both the BODIPY
moiety and the oxygenated biphen-2-ol connected to it, as de la Moya et al. have
demonstrated recently [51]. This approach adds significant value to the use of such a
C 2 -symmetric mono(BODIPY) design when developing CPL-SOMs. As an example,
the said photophysical tuning has allowed the successful preparation of 21 (Fig. 6.16)
exhibiting enhanced CPL activity (ϕÁ|g lum |) when compared to its analogue 20
[52]. This enhancement is achieved by simply introducing 3,3
0 -dibromoBINOL instead
of BINOL in the C 2 -symmetric mono(BODIPY) structure. Strikingly, the sign of the
g lum values exhibited by 20 and 21, having identical chiral absolute configuration, are
opposite (see Fig. 6.16). Besides, each individual enantiomer of 20 shows opposite
signs for its maximum g abs and g lum values [50], whereas the enantiomers of 21 do not
show such a striking g abs -versus-g lum sign reversal [52]. Interestingly, the significant
CPL activity of mono(BODIPY) 21 joined to its lasing capability under laser pumping
has also served to explore factors affecting CPL activity in dye lasers [52].
The aforementioned differential behaviour of 20 and 21 in relation to the sign of
the corresponding g lum values [50, 52], joined to the fact that both mono(BODIPY)s
involve highly similar BODIPY chromophores, but with very different capability to
populate emissive intramolecular charge transfer (ICT) states (higher for BINOLbased 20; undetectable for 3,3
0 -dibromoBINOL-based 21) [51], served de la Moya’s
group to establish a new strategy to manipulate the CPL sign in chiral emitters (i.e. to
change the handedness of the observed circularly polarized emission) by means of
modulating the promotion of luminescent ICT states [53]. To apply such a strategy,
it is not necessary that the emission from the ICT state surpass the one from the
locally excited state, since the ICT emission is expected to involve larger circular
polarization. However, both circularly polarized emissions do need to show opposite
handedness [53]. This is an unprecedented way to manipulate the CPL sign in chiral
emitters without changing absolute configurations, being specifically useful for rigid
emitters and emitters without capability to promote excimer emission [53]. The new
strategy was exemplified by de la Moya et al. by means of a set of CPL-enabling
1,1
0 -biphen-2-ol-based spiranic O-BODIPYs 20–25, where the BODIPY (in red in
Fig. 6.17) provides the functional visible light chromophore and the 1,1
0 -biphen-2-ol
moiety (in blue in Fig. 6.17) acts as the ICT-switching moiety depending on the
electronic nature of the BODIPY chromophore connected to it. Thus, in dyes with
130
M. J. Hall and S. de la Moya