[Methanol (MeOH)/DMF/N-methyl-2-pyrrolidone (NMP)/dimethylacetamide
(DMAc)], while no major changes occurred in the ECD spectra [18].
12.2.2 Ground State Cryptochirality and Excimer CPL
As the polarization effects on excimer bands can be higher than the ones in
absorption by orders of magnitude, it is possible in some cases to specifically detect
a CPL band even when the ECD is extremely small or non-detectable. This behavior
is clearly observed when the chirality is brought by stereogenic elements (centers or
axes) positioned at a distance from the chromophore/fluorophore units. The groundstate of such systems can be then considered as “cryptochiral,” for which the
chirality cannot be (fully) detected through absorption-based chiroptical techniques,
but the molecular asymmetry becomes evident upon emission polarization. In
principle, this peculiarity could open a door to applications in cryptography and
steganography [19].
Imai et al. prepared pyr-17 bearing two pyrene units linked to a core binaphthyl
moiety through ethoxyethyl chains and ester linkages (Scheme 12.2) [20]. When
dissolved in CHCl 3 (10
À3 M), this molecule displayed a g abs of |3.8 Â 10
À5 | at
344 nm while the excimer associated g lum value was |7.8 Â 10
À4 | (20.5 times
higher). The excimer CPL, in comparison with ECD, displayed thus a quite higher
sensitivity in revealing a long-distance chiral environment.
A similar behavior was observed for pyr-18–pyr-20 for which the two pyrene
moieties were linked to the central binaphthyl unit through amide, ether, and ester
bonds, respectively (Scheme 12.2) [19, 21]. In these examples, the measured g abs
values were around 10
À5 , while the excimer associated g lum factors were around
10
À3 (C ¼ 10
À5 M in CHCl 3 ), with g lum /g abs ratios ranging from 17.9 to 62.5.
Density functional theory (DFT) structures showed that the two pyrene moieties
assume a nearly achiral T-shaped arrangement in S 0 state, while in the excited state
O
O
O
O
O
O
O
O
Imai 2014
pyr-17
R
R
NH
O
O
O
O
R =
R =
R =
pyr-18
pyr-19
pyr-20
Imai 2016
Scheme 12.2 Structures of pyrene decorated binaphthyl systems
280
F. Zinna et al.
(DMAc)], while no major changes occurred in the ECD spectra [18].
12.2.2 Ground State Cryptochirality and Excimer CPL
As the polarization effects on excimer bands can be higher than the ones in
absorption by orders of magnitude, it is possible in some cases to specifically detect
a CPL band even when the ECD is extremely small or non-detectable. This behavior
is clearly observed when the chirality is brought by stereogenic elements (centers or
axes) positioned at a distance from the chromophore/fluorophore units. The groundstate of such systems can be then considered as “cryptochiral,” for which the
chirality cannot be (fully) detected through absorption-based chiroptical techniques,
but the molecular asymmetry becomes evident upon emission polarization. In
principle, this peculiarity could open a door to applications in cryptography and
steganography [19].
Imai et al. prepared pyr-17 bearing two pyrene units linked to a core binaphthyl
moiety through ethoxyethyl chains and ester linkages (Scheme 12.2) [20]. When
dissolved in CHCl 3 (10
À3 M), this molecule displayed a g abs of |3.8 Â 10
À5 | at
344 nm while the excimer associated g lum value was |7.8 Â 10
À4 | (20.5 times
higher). The excimer CPL, in comparison with ECD, displayed thus a quite higher
sensitivity in revealing a long-distance chiral environment.
A similar behavior was observed for pyr-18–pyr-20 for which the two pyrene
moieties were linked to the central binaphthyl unit through amide, ether, and ester
bonds, respectively (Scheme 12.2) [19, 21]. In these examples, the measured g abs
values were around 10
À5 , while the excimer associated g lum factors were around
10
À3 (C ¼ 10
À5 M in CHCl 3 ), with g lum /g abs ratios ranging from 17.9 to 62.5.
Density functional theory (DFT) structures showed that the two pyrene moieties
assume a nearly achiral T-shaped arrangement in S 0 state, while in the excited state
O
O
O
O
O
O
O
O
Imai 2014
pyr-17
R
R
NH
O
O
O
O
R =
R =
R =
pyr-18
pyr-19
pyr-20
Imai 2016
Scheme 12.2 Structures of pyrene decorated binaphthyl systems
280
F. Zinna et al.