2.6 Dependence of Circularly Polarized Luminescence
(CPL) on the Structure of the Tether Connecting
Binaphthyl and Fluorescent Units [25]
The binaphthyl family contains one of the best chirality-inducing building blocks
because binaphthyl units act as hinges that enable connections to a variety of
molecular components, in addition to being a fluorescence source. In this section,
control of the sign of the CPL of a chiral non-luminophoric binaphthyl-driven pyrene
excimer system is demonstrated with the help of freely rotating alkyl ether tethers.
Binaphthyl-pyrene 7 containing long oligoether tethers composed of ten single
bonds, and binaphthyl-pyrene 8 with shorter (six single bonds) alkyl ether tethers
were used (Fig. 2.14). In these systems, the binaphthyl moiety acts as a molecular
hinge.
The PL spectra of (R)-7 and (R)-8 in CHCl 3 solutions are provided in the lower
parts of Fig. 2.15. Both compounds exhibit almost identical PL maxima (λ PL :
465 and 480 nm for (R)-7 and 462 and 480 nm for (R)-8). Although these PL
bands are intense, the monomeric PL bands at ~380 nm are very weak. This feature
is ascribable to intramolecularly sterically constrained dimeric pyrenes in their S 0
states.
In contrast to their PLs, (R)-7 and (R)-8 exhibit opposite CPL signs in their S 1 -
state chiralities; they have the same binaphthyl (R)-chiralities as the S 1 state chirality:
a negative (À) sign for (R)-7 and a positive (+) sign for (R)-8. The excimeric origin
of the CPLs of (R)-7 and (R)-8 in CHCl 3 are attributable to long-distance intramolecular chiral interactions between the two remote pyrenes attached to the chiral
binaphthyl backbones through reorganization of the two photoexcited pyrenes in
(R)-7 and (R)-8. The g CPL values of (R)-8 (+1.2 Â 10
–3 at 462 nm and +1.0 Â 10
–3 at
480 nm) are larger than those of (R)-7 (À0.41 Â 10
–3 at 465 nm and À0.46 Â 10
–3 at
480 nm). In this case, not only is the axial chirality of the binaphthyl in the S 0 state a
critical factor that determines the CPL sign of the S 1 state, but so is the structure of
the tether.
O
O
7
O
O
O
O
O
O
O
O
8
Fig. 2.14 Chiral binaphthyl-pyrene fluorophores 7 and 8
2 Circularly Polarized Luminescence of Axially Chiral Binaphthyl Fluorophores
21
(CPL) on the Structure of the Tether Connecting
Binaphthyl and Fluorescent Units [25]
The binaphthyl family contains one of the best chirality-inducing building blocks
because binaphthyl units act as hinges that enable connections to a variety of
molecular components, in addition to being a fluorescence source. In this section,
control of the sign of the CPL of a chiral non-luminophoric binaphthyl-driven pyrene
excimer system is demonstrated with the help of freely rotating alkyl ether tethers.
Binaphthyl-pyrene 7 containing long oligoether tethers composed of ten single
bonds, and binaphthyl-pyrene 8 with shorter (six single bonds) alkyl ether tethers
were used (Fig. 2.14). In these systems, the binaphthyl moiety acts as a molecular
hinge.
The PL spectra of (R)-7 and (R)-8 in CHCl 3 solutions are provided in the lower
parts of Fig. 2.15. Both compounds exhibit almost identical PL maxima (λ PL :
465 and 480 nm for (R)-7 and 462 and 480 nm for (R)-8). Although these PL
bands are intense, the monomeric PL bands at ~380 nm are very weak. This feature
is ascribable to intramolecularly sterically constrained dimeric pyrenes in their S 0
states.
In contrast to their PLs, (R)-7 and (R)-8 exhibit opposite CPL signs in their S 1 -
state chiralities; they have the same binaphthyl (R)-chiralities as the S 1 state chirality:
a negative (À) sign for (R)-7 and a positive (+) sign for (R)-8. The excimeric origin
of the CPLs of (R)-7 and (R)-8 in CHCl 3 are attributable to long-distance intramolecular chiral interactions between the two remote pyrenes attached to the chiral
binaphthyl backbones through reorganization of the two photoexcited pyrenes in
(R)-7 and (R)-8. The g CPL values of (R)-8 (+1.2 Â 10
–3 at 462 nm and +1.0 Â 10
–3 at
480 nm) are larger than those of (R)-7 (À0.41 Â 10
–3 at 465 nm and À0.46 Â 10
–3 at
480 nm). In this case, not only is the axial chirality of the binaphthyl in the S 0 state a
critical factor that determines the CPL sign of the S 1 state, but so is the structure of
the tether.
O
O
7
O
O
O
O
O
O
O
O
8
Fig. 2.14 Chiral binaphthyl-pyrene fluorophores 7 and 8
2 Circularly Polarized Luminescence of Axially Chiral Binaphthyl Fluorophores
21