4.8.2 CPL-Active Hexahelicenic Structures
Hexahelicenic derivatives are prototypic helicenes exhibiting CPL activity. In
2018, our group showed that grafting diketopyrrolopyrrole (dpp) dyes onto a
carbo[6]helicene structure through ethynyl bridges (see 58a–c in Fig. 4.21) leads
to exciton coupling circular dichroism in the red region arising from the achiral -
red-absorbing DPP units in the helical environment [56]. Furthermore, red to nearinfrared circularly CPL was obtained. Indeed, the association of enantiopure [6]
helicene and dpp units provided helical π-conjugated molecules with strong ECD
signal in the visible region (~600 nm), intense red and near-infrared fluorescence
(ϕ F ~ 0.4), and CPL activity up to 650 nm with g lum found to increase from 1 Â 10
À4
to 6 Â 10
À4 then 9 Â 10
À4 with the increase of exciton coupling (i.e., through the
series 58a ! 58b ! 58c). The g abs values were also found to follow the same
increasing trend with the increasing exciton coupling. These results highlighted the
synergy between the chiral hexahelicene structure and the organic dye. Thus,
decorating carbohelicenes with dyes constitutes an appealing strategy of chemical
engineering of a π-helical platform to further improve the chiroptical responses.
In 2018, Tanaka and coworkers reported the enantioselective synthesis of fully
benzenoid single (59a,b) and double (59c,d) carbo[6]helicenes via efficient goldcatalyzed intramolecular hydroarylation (Fig. 4.22) [57]. Similar to the single
(12–14) and double azahelicenes (15–16) described in Sect. 4.2.3, the double
carbo[6]helicenes 59c,d exhibited relatively large CPL activities (up to
2.7 Â 10
À3 , see Table 4.10), as compared to the single carbo[6]helicenes 59a,b
whose CPL was below the limit of the apparatus.
In 2018, X-shaped and S-shaped pristine double hexahelicenes (60 and 61,
Fig. 4.23) were prepared and used as representative molecular models, and a
theory-guided, symmetry-based protocol was developed [23]. Compound 60 and
61 exhibited a strong increase in intensity of ECD and CPL. The enhanced
chiroptical responses were theoretically assigned to the electric (μ e ) and magnetic
(μ m ) transition dipole moments of component hexahelicenes aligned in the correct
symmetry. Indeed, 60 and 61, constructed by merging two hexahelicenes in D 2 and
C 2 symmetry, respectively, showed absorption dissymmetry factors per benzene unit
(g abs /n) for the
1 B b band that are larger by a factor of up to 1.5 than that of parent 10.
This enhancement was well rationalized by μ e and μ m and their relative angle (θ)
evaluated theoretically. In the double helicenes, μ e and μ m were parallel-aligned
(θ ¼ 0) to maximize the orientation factor (cos θ) up to unity, which was mere 0.24
(cos 76
) in 10, while |μ e | and |μ m | were comparable or only slightly improved.
Similarly, the luminescence dissymmetry factor per benzene unit (g lum /n) was up to
1.7-fold larger for the double helicenes than for 10, for which the increased |μ e | and θ
are responsible. The enhanced g abs /n and g lum /n values for double helicenes mean
that merging two helicenes is 50–70% more resource efficient than simply assembling them, in favor of the molecular, rather than supramolecular strategy for
constructing advanced chiroptical devices.
78
J. Crassous
Hexahelicenic derivatives are prototypic helicenes exhibiting CPL activity. In
2018, our group showed that grafting diketopyrrolopyrrole (dpp) dyes onto a
carbo[6]helicene structure through ethynyl bridges (see 58a–c in Fig. 4.21) leads
to exciton coupling circular dichroism in the red region arising from the achiral -
red-absorbing DPP units in the helical environment [56]. Furthermore, red to nearinfrared circularly CPL was obtained. Indeed, the association of enantiopure [6]
helicene and dpp units provided helical π-conjugated molecules with strong ECD
signal in the visible region (~600 nm), intense red and near-infrared fluorescence
(ϕ F ~ 0.4), and CPL activity up to 650 nm with g lum found to increase from 1 Â 10
À4
to 6 Â 10
À4 then 9 Â 10
À4 with the increase of exciton coupling (i.e., through the
series 58a ! 58b ! 58c). The g abs values were also found to follow the same
increasing trend with the increasing exciton coupling. These results highlighted the
synergy between the chiral hexahelicene structure and the organic dye. Thus,
decorating carbohelicenes with dyes constitutes an appealing strategy of chemical
engineering of a π-helical platform to further improve the chiroptical responses.
In 2018, Tanaka and coworkers reported the enantioselective synthesis of fully
benzenoid single (59a,b) and double (59c,d) carbo[6]helicenes via efficient goldcatalyzed intramolecular hydroarylation (Fig. 4.22) [57]. Similar to the single
(12–14) and double azahelicenes (15–16) described in Sect. 4.2.3, the double
carbo[6]helicenes 59c,d exhibited relatively large CPL activities (up to
2.7 Â 10
À3 , see Table 4.10), as compared to the single carbo[6]helicenes 59a,b
whose CPL was below the limit of the apparatus.
In 2018, X-shaped and S-shaped pristine double hexahelicenes (60 and 61,
Fig. 4.23) were prepared and used as representative molecular models, and a
theory-guided, symmetry-based protocol was developed [23]. Compound 60 and
61 exhibited a strong increase in intensity of ECD and CPL. The enhanced
chiroptical responses were theoretically assigned to the electric (μ e ) and magnetic
(μ m ) transition dipole moments of component hexahelicenes aligned in the correct
symmetry. Indeed, 60 and 61, constructed by merging two hexahelicenes in D 2 and
C 2 symmetry, respectively, showed absorption dissymmetry factors per benzene unit
(g abs /n) for the
1 B b band that are larger by a factor of up to 1.5 than that of parent 10.
This enhancement was well rationalized by μ e and μ m and their relative angle (θ)
evaluated theoretically. In the double helicenes, μ e and μ m were parallel-aligned
(θ ¼ 0) to maximize the orientation factor (cos θ) up to unity, which was mere 0.24
(cos 76
) in 10, while |μ e | and |μ m | were comparable or only slightly improved.
Similarly, the luminescence dissymmetry factor per benzene unit (g lum /n) was up to
1.7-fold larger for the double helicenes than for 10, for which the increased |μ e | and θ
are responsible. The enhanced g abs /n and g lum /n values for double helicenes mean
that merging two helicenes is 50–70% more resource efficient than simply assembling them, in favor of the molecular, rather than supramolecular strategy for
constructing advanced chiroptical devices.
78
J. Crassous