enabled ICT (20 and 23), the CPL spectrum is inverted with respect to the CD one
(i.e. sign reversal for the corresponding visible light maximum-g abs and -g lum
values). However, in the cases for which ICT is not significantly promoted
(21–22, 24, and 25), the signs of the corresponding g abs and g lum values are equal
(Fig. 6.17).
Although the maximum |g lum | values reached for these spiranic C 2 -symmetric
mono(BODIPYs) are of the order 10
À3 , the aforementioned possibility of modulating their fluorescent signatures, including their CPL sign, has made this design
approach an exciting avenue for the development of highly functional CPL-SOMs.
Thus Cheng et al. have taken advantage of this in the creation of an interesting
enantiopure mono(BODIPY) dye capable of visible light CPL emission following
ultraviolet (UV) irradiation (340 nm) (26 in Fig. 6.18; λ max ca. 630 nm; maximum
|g lum | ¼ 2Á10
À3 , positive g lum for the S enantiomer) due to an efficient donor-acceptor
energy transfer resulting in a large pseudo-Stokes shifted CPL emission [54]. Besides
which CPL-enabling mono(BODIPY) 26 has been shown to aggregate in specific
solvent mixtures (dichloromethane and hexane) of low polarity, resulting in aggregation induced fluorescence (AIE), from ϕ ¼ 0.014 in pure dichloromethane
(emission from monomeric 26) to ϕ ¼ 0.582 in a less polar 9:1 mixture of hexane
and dichloromethane (emission from aggregated 26), without disturbing the CPL
activity (in terms of g lum value) [54].
6.3.1.2 Helically Chiral Mono(BODIPY)S
Inspired by the work of Burgess et al. [41], the group of Knight and Hall synthesized
a series of helically chiral N,N,O,O-boron-chelated dipyrromethenes 27–30
(Fig. 6.19), with a range of meso-substituents, for evaluation as potential
CPL-SOMs [30]. Helical chirality is induced in these BODIPYs through chelation
of the central boron atom by the oxygens of the 3,5-ortho-phenolic substituents. The
meso-substituents are electronically discrete from the BODIPY chromophore itself
(Fig. 6.19), and as such their direct electronic influence on the chromophore is
typically minimal; however they can sterically influence the conformational
N B
N
O
O
(R)-26
R
g lum = -2·10
-3
φ = 0.0014
Fig. 6.18 Energy transfer in a CPL mono(BODIPY) based on the spiranic O-BODIPY design.
Visible CPL is achieved by UV irradiation in dichloromethane
132
M. J. Hall and S. de la Moya
(i.e. sign reversal for the corresponding visible light maximum-g abs and -g lum
values). However, in the cases for which ICT is not significantly promoted
(21–22, 24, and 25), the signs of the corresponding g abs and g lum values are equal
(Fig. 6.17).
Although the maximum |g lum | values reached for these spiranic C 2 -symmetric
mono(BODIPYs) are of the order 10
À3 , the aforementioned possibility of modulating their fluorescent signatures, including their CPL sign, has made this design
approach an exciting avenue for the development of highly functional CPL-SOMs.
Thus Cheng et al. have taken advantage of this in the creation of an interesting
enantiopure mono(BODIPY) dye capable of visible light CPL emission following
ultraviolet (UV) irradiation (340 nm) (26 in Fig. 6.18; λ max ca. 630 nm; maximum
|g lum | ¼ 2Á10
À3 , positive g lum for the S enantiomer) due to an efficient donor-acceptor
energy transfer resulting in a large pseudo-Stokes shifted CPL emission [54]. Besides
which CPL-enabling mono(BODIPY) 26 has been shown to aggregate in specific
solvent mixtures (dichloromethane and hexane) of low polarity, resulting in aggregation induced fluorescence (AIE), from ϕ ¼ 0.014 in pure dichloromethane
(emission from monomeric 26) to ϕ ¼ 0.582 in a less polar 9:1 mixture of hexane
and dichloromethane (emission from aggregated 26), without disturbing the CPL
activity (in terms of g lum value) [54].
6.3.1.2 Helically Chiral Mono(BODIPY)S
Inspired by the work of Burgess et al. [41], the group of Knight and Hall synthesized
a series of helically chiral N,N,O,O-boron-chelated dipyrromethenes 27–30
(Fig. 6.19), with a range of meso-substituents, for evaluation as potential
CPL-SOMs [30]. Helical chirality is induced in these BODIPYs through chelation
of the central boron atom by the oxygens of the 3,5-ortho-phenolic substituents. The
meso-substituents are electronically discrete from the BODIPY chromophore itself
(Fig. 6.19), and as such their direct electronic influence on the chromophore is
typically minimal; however they can sterically influence the conformational
N B
N
O
O
(R)-26
R
g lum = -2·10
-3
φ = 0.0014
Fig. 6.18 Energy transfer in a CPL mono(BODIPY) based on the spiranic O-BODIPY design.
Visible CPL is achieved by UV irradiation in dichloromethane
132
M. J. Hall and S. de la Moya