and 5000 M
–1 , respectively. The effect of Ag is to silence the CPL reversibly, the
system without Ag exhibiting a remarkable g lum /g abs ratio, namely 1.15 in the first
case and 1 in the second. Besides, in the first case also g lum is particularly large
(0.012). Such large CPL values in principle provide a wide range for sensing quite
different quantities of Ag and thus make the two systems as optimal probes for
sensing silver. Compound (B), and the analogous compound (C) with longer lateral
arms presented in Fig. 10.5 (right panel), can interact also with other metals via
oxophilic mechanism through sulfoxide groups, reversible forming complexes, and
with no quenching of fluorescence. Many metals have been tested [47] giving
different CPL responses, some of them quite intense: compound (C) is particularly
interesting since it presents quite low CPL before complexation (CH 2 Cl 2 solution)
and CPL with different dissymmetry ratio depending on the associated metal.
An interesting sensing system, which we discuss below, regards the possibility of
probing Ag cation with a CPL feature, while recording in the same CPL spectrum a
second CPL band not sensitive to Ag, which thus can be used as internal reference
[41]. In Fig. 10.6 we report the data for the molecule that was designed and
synthesized to the scope by taking advantage of the property of high sensitivity to
silver of compound p,p-(1S,2S)-diol-o-OPE (the enantiomer of compound A of
Fig. 10.5), and substituting the H hydroxyl atoms with two other fluorescent groups,
namely two (CH 2 )-pyrene units. The CPL spectrum is composed of two bands, a
negative one centered at ca. 520 nm, the intensity of which is constant with addition
of Ag(I), and a positive one centered at ca. 410 nm, the intensity of which instead is
linearly dependent on the concentration of Ag(I); Ag has 6230 M
–1 binding constant
to this compound. This means that the ratio of the two CPL bands is linearly
dependent on Ag concentration, making the CPL band at 520 nm an internal constant
reference and the idea of measuring the concentration of Ag in absolute terms really
working. The lower part of Fig. 10.6 provides a possible theoretical justification to
the different roles of the pyrene 520 nm transition and of the o-OPE 410 nm
transition showing an indicative representation of molecular orbitals involved in
the transition localized on the pyrene moiety (calculations were run on the ground
state geometry).
10.2.2.3 Sensing pH
The final example we propose in this paragraph is the sensing of basicity conditions:
it was found that (1R,2R)- and (1S,2S)-trans-cyclohexane diesters and diamides
exhibit rather strong chiroptical ECD and vibrational CD (VCD) spectra, with
characteristic bisignate features, which were proven to be associated with excitons.
Such excitons, electronic and vibrational in the two cases, exhibit bathochromic and
hyperchromic behaviors with addition of NaOH, leaving the shape of the ECD
unaltered. Correspondingly, while in the neutral condition fluorescence is so low
that a CPL spectrum is unobservable, at high pH a CPL band is recorded. As
observed on other C 2 symmetric compounds presenting CD excitonic couplets
(see also the next paragraph), concordant with Kasha’s rule the sign of the CPL
10 Structural and Electronic Information Drawn. . .
229
–1 , respectively. The effect of Ag is to silence the CPL reversibly, the
system without Ag exhibiting a remarkable g lum /g abs ratio, namely 1.15 in the first
case and 1 in the second. Besides, in the first case also g lum is particularly large
(0.012). Such large CPL values in principle provide a wide range for sensing quite
different quantities of Ag and thus make the two systems as optimal probes for
sensing silver. Compound (B), and the analogous compound (C) with longer lateral
arms presented in Fig. 10.5 (right panel), can interact also with other metals via
oxophilic mechanism through sulfoxide groups, reversible forming complexes, and
with no quenching of fluorescence. Many metals have been tested [47] giving
different CPL responses, some of them quite intense: compound (C) is particularly
interesting since it presents quite low CPL before complexation (CH 2 Cl 2 solution)
and CPL with different dissymmetry ratio depending on the associated metal.
An interesting sensing system, which we discuss below, regards the possibility of
probing Ag cation with a CPL feature, while recording in the same CPL spectrum a
second CPL band not sensitive to Ag, which thus can be used as internal reference
[41]. In Fig. 10.6 we report the data for the molecule that was designed and
synthesized to the scope by taking advantage of the property of high sensitivity to
silver of compound p,p-(1S,2S)-diol-o-OPE (the enantiomer of compound A of
Fig. 10.5), and substituting the H hydroxyl atoms with two other fluorescent groups,
namely two (CH 2 )-pyrene units. The CPL spectrum is composed of two bands, a
negative one centered at ca. 520 nm, the intensity of which is constant with addition
of Ag(I), and a positive one centered at ca. 410 nm, the intensity of which instead is
linearly dependent on the concentration of Ag(I); Ag has 6230 M
–1 binding constant
to this compound. This means that the ratio of the two CPL bands is linearly
dependent on Ag concentration, making the CPL band at 520 nm an internal constant
reference and the idea of measuring the concentration of Ag in absolute terms really
working. The lower part of Fig. 10.6 provides a possible theoretical justification to
the different roles of the pyrene 520 nm transition and of the o-OPE 410 nm
transition showing an indicative representation of molecular orbitals involved in
the transition localized on the pyrene moiety (calculations were run on the ground
state geometry).
10.2.2.3 Sensing pH
The final example we propose in this paragraph is the sensing of basicity conditions:
it was found that (1R,2R)- and (1S,2S)-trans-cyclohexane diesters and diamides
exhibit rather strong chiroptical ECD and vibrational CD (VCD) spectra, with
characteristic bisignate features, which were proven to be associated with excitons.
Such excitons, electronic and vibrational in the two cases, exhibit bathochromic and
hyperchromic behaviors with addition of NaOH, leaving the shape of the ECD
unaltered. Correspondingly, while in the neutral condition fluorescence is so low
that a CPL spectrum is unobservable, at high pH a CPL band is recorded. As
observed on other C 2 symmetric compounds presenting CD excitonic couplets
(see also the next paragraph), concordant with Kasha’s rule the sign of the CPL
10 Structural and Electronic Information Drawn. . .
229