difference allows one to disentangle monomer and excimer contributions from the
overall CPL spectrum (g lum ~|10
À4 | for the monomer and ~|10
À3
| for the excimer).
On the contrary, these two contributions are not clearly separable in the fluorescence
spectra where the excimer is present as a broad tailing of the main monomer band
(Fig. 12.4). Upon decreasing the concentration of pyr-24 (from 8 Â 10
À3 M to
1 Â 10
À5 M in toluene, see Fig. 12.4), an increase in the monomer CPL g lum was
observed (from |3.3 Â 10
À4 | to |6.9 Â 10
À4 |), while excimer allied g lum showed only
minor variations (from |3.3 Â 10
À3 | to |3.9 Â 10
À3 |). This effect is probably due to
different contributions to total luminescence and CPL from intra- and intermolecular
excimer at different concentrations. On the other hand, ECD spectra at various
concentrations did not show any variation of shape or intensity (g abs ¼ |
6.2 Â 10
À4 |), indicating that the ground state remains unaffected.
Recently, Cuerva et al. reported ortho-oligo(phenylene)-ethynylenes [26] decorated with two pyrene moieties (pyr-25 and pyr-26, Scheme 12.6) [27]. These
systems displayed again a bisignated CPL spectra (C ¼ 10
À5 M in CH 2 Cl 2 /acetone
95:5): the spectrum was dominated by the CPL emission associated to both
phenylene-ethynylene moiety and to pyrene monomer at shorter wavelengths
(~400 nm), while longer wavelength region (~500 nm) showed CPL allied to pyrene
excimer emission with g lum ~|10
À2 |. Upon Ag
+ addition, the CPL profile is modified
and the relative intensities of shorter and longer wavelength components change. In
this way it was possible to quantify Ag
+ concentration, thanks to the determination
of the ratio between ΔI (I L –I R ) at 400 nm and ΔI at 500 nm. A CPL-based ratiometric
probe sensitive to Ag
+ was thus established. Moreover, considering the ECD of pyr26, the authors noticed that, upon Ag
+ addition, only the band related to oligo
(phenylene)-ethynylene scaffold was affected (345 nm), while the bands related to
pyrene absorption (387 nm) remained unaffected.
A different strategy was followed by the Lacour’s group [28]. Thanks to a
versatile synthetic approach in two steps only from common precursors, a family
of polyether and polyether-like macrocycles was prepared carrying a wealth of
different fluorophores (Scheme 12.7) [29]. In particular, 18C6, 18C4 and 16C4type scaffolds were decorated with pyrene units through amide bonds (pyr-27–pyr29, Scheme 12.7). These molecules displayed very intense excimer emission
O
O
O
X
O
X
X = CH 2
X = CO
pyr-25
pyr-26
Cuerva 2018
O
O
O
X
O
X
Ag
+
Ag
+
Scheme 12.6 Pyrene-decorated ortho-oligo(phenylene)-ethynylenes showing Ag-induced
chiroptical switch behavior
12 Circularly Polarized Luminescence from Intramolecular Excimers
283
overall CPL spectrum (g lum ~|10
À4 | for the monomer and ~|10
À3
| for the excimer).
On the contrary, these two contributions are not clearly separable in the fluorescence
spectra where the excimer is present as a broad tailing of the main monomer band
(Fig. 12.4). Upon decreasing the concentration of pyr-24 (from 8 Â 10
À3 M to
1 Â 10
À5 M in toluene, see Fig. 12.4), an increase in the monomer CPL g lum was
observed (from |3.3 Â 10
À4 | to |6.9 Â 10
À4 |), while excimer allied g lum showed only
minor variations (from |3.3 Â 10
À3 | to |3.9 Â 10
À3 |). This effect is probably due to
different contributions to total luminescence and CPL from intra- and intermolecular
excimer at different concentrations. On the other hand, ECD spectra at various
concentrations did not show any variation of shape or intensity (g abs ¼ |
6.2 Â 10
À4 |), indicating that the ground state remains unaffected.
Recently, Cuerva et al. reported ortho-oligo(phenylene)-ethynylenes [26] decorated with two pyrene moieties (pyr-25 and pyr-26, Scheme 12.6) [27]. These
systems displayed again a bisignated CPL spectra (C ¼ 10
À5 M in CH 2 Cl 2 /acetone
95:5): the spectrum was dominated by the CPL emission associated to both
phenylene-ethynylene moiety and to pyrene monomer at shorter wavelengths
(~400 nm), while longer wavelength region (~500 nm) showed CPL allied to pyrene
excimer emission with g lum ~|10
À2 |. Upon Ag
+ addition, the CPL profile is modified
and the relative intensities of shorter and longer wavelength components change. In
this way it was possible to quantify Ag
+ concentration, thanks to the determination
of the ratio between ΔI (I L –I R ) at 400 nm and ΔI at 500 nm. A CPL-based ratiometric
probe sensitive to Ag
+ was thus established. Moreover, considering the ECD of pyr26, the authors noticed that, upon Ag
+ addition, only the band related to oligo
(phenylene)-ethynylene scaffold was affected (345 nm), while the bands related to
pyrene absorption (387 nm) remained unaffected.
A different strategy was followed by the Lacour’s group [28]. Thanks to a
versatile synthetic approach in two steps only from common precursors, a family
of polyether and polyether-like macrocycles was prepared carrying a wealth of
different fluorophores (Scheme 12.7) [29]. In particular, 18C6, 18C4 and 16C4type scaffolds were decorated with pyrene units through amide bonds (pyr-27–pyr29, Scheme 12.7). These molecules displayed very intense excimer emission
O
O
O
X
O
X
X = CH 2
X = CO
pyr-25
pyr-26
Cuerva 2018
O
O
O
X
O
X
Ag
+
Ag
+
Scheme 12.6 Pyrene-decorated ortho-oligo(phenylene)-ethynylenes showing Ag-induced
chiroptical switch behavior
12 Circularly Polarized Luminescence from Intramolecular Excimers
283