we have in MeOH solution g lum /g abs % 1 and in DMSO solution g lum /g abs % 1.7.
Similar, though nonidentical, behaviors are observed for cyclohexane diamides. As
stated in Ref. [48], since only from the charged state does one observe the fluorescence and CPL phenomena, the latter may become a yes-or-no probe of the charged
state. Such effectiveness of the CPL technique may be related to the large Stokes
shift.
In conclusion, common motifs of the effectiveness of the CPL signal to sense
different sorts of phenomena, as in the three examples presented above, namely
aggregation, presence of ions, pH changes, appear large Stokes shifts and values of
g lum /g abs greater or equal to 1.
10.2.3 Do Inherently Dissymmetric Chromophores Exhibit
Large CPL?
Inherently dissymmetric chromophores may be defined as molecular moieties bearing groups exhibiting large transition amplitudes in the UV-vis and having a chiral
shape; generally “these compounds have in common a dissymmetrically twisted
π-system which essentially constitutes the entire chromophore” [49]. Such molecules have large specific optical rotations (ORs), of the order of several thousands;
for the ECD spectra, transitions with such large amplitude are both electrically and
magnetically allowed. The situation of the CPL spectra is more variegated. We
examine here three classes of molecules in which this variety is manifest.
10.2.3.1 C2-Symmetric Bi-chromophoric Systems
The first example is the class of C 2 -chromphores composed of two equal moieties
tied with a single bond, the two equal parts containing in the present examples
bithianaphthene units, with various pendants. In Fig. 10.8 we report the ECD and
CPL spectra of three such molecules, namely (S)-2,2’-bis(2,2’-bithiophene-5-yl)3,3’-bithianaphthene
(BT2-T4),
4H-cyclopenta[2,1-b3:4b’]dithiophene-3,3’-bithianaphthene
(BT2-CPDT2),
and
dithieno[3,3-b:2’,3’-d]pyrrole-3,3’-bithianaphthene (BT2-DTP2).
The values of g lum for the three inherently dissymmetric molecules BT2-T4,
BT2-CPDT2, and BT2-DTP2 are slightly smaller but close to 10
–2 , and the ratios
g lum /g abs are 4.7, 7.1, and 4.2, respectively (we considered the longest wavelength
transition in the ECD spectrum, in accordance to Kasha’s rule). In all cases such
ratios are pretty big, among the biggest in Mori et al. [25] classification, and indeed,
as pointed out by those Authors, these large values are associated with pretty large
values of the Stokes shifts (ca. 130 nm) indicating great excited state relaxation.
232
G. Longhi and S. Abbate
Similar, though nonidentical, behaviors are observed for cyclohexane diamides. As
stated in Ref. [48], since only from the charged state does one observe the fluorescence and CPL phenomena, the latter may become a yes-or-no probe of the charged
state. Such effectiveness of the CPL technique may be related to the large Stokes
shift.
In conclusion, common motifs of the effectiveness of the CPL signal to sense
different sorts of phenomena, as in the three examples presented above, namely
aggregation, presence of ions, pH changes, appear large Stokes shifts and values of
g lum /g abs greater or equal to 1.
10.2.3 Do Inherently Dissymmetric Chromophores Exhibit
Large CPL?
Inherently dissymmetric chromophores may be defined as molecular moieties bearing groups exhibiting large transition amplitudes in the UV-vis and having a chiral
shape; generally “these compounds have in common a dissymmetrically twisted
π-system which essentially constitutes the entire chromophore” [49]. Such molecules have large specific optical rotations (ORs), of the order of several thousands;
for the ECD spectra, transitions with such large amplitude are both electrically and
magnetically allowed. The situation of the CPL spectra is more variegated. We
examine here three classes of molecules in which this variety is manifest.
10.2.3.1 C2-Symmetric Bi-chromophoric Systems
The first example is the class of C 2 -chromphores composed of two equal moieties
tied with a single bond, the two equal parts containing in the present examples
bithianaphthene units, with various pendants. In Fig. 10.8 we report the ECD and
CPL spectra of three such molecules, namely (S)-2,2’-bis(2,2’-bithiophene-5-yl)3,3’-bithianaphthene
(BT2-T4),
4H-cyclopenta[2,1-b3:4b’]dithiophene-3,3’-bithianaphthene
(BT2-CPDT2),
and
dithieno[3,3-b:2’,3’-d]pyrrole-3,3’-bithianaphthene (BT2-DTP2).
The values of g lum for the three inherently dissymmetric molecules BT2-T4,
BT2-CPDT2, and BT2-DTP2 are slightly smaller but close to 10
–2 , and the ratios
g lum /g abs are 4.7, 7.1, and 4.2, respectively (we considered the longest wavelength
transition in the ECD spectrum, in accordance to Kasha’s rule). In all cases such
ratios are pretty big, among the biggest in Mori et al. [25] classification, and indeed,
as pointed out by those Authors, these large values are associated with pretty large
values of the Stokes shifts (ca. 130 nm) indicating great excited state relaxation.
232
G. Longhi and S. Abbate