geometry giving rise to different CD bands and possibly arriving to different S 1
structures. Also, in this case, one may have different components contributing to
emission; these components could be eventually discriminated by time resolved
fluorescence spectroscopy. An example of the last circumstance is represented by p,
m-(1R,2R)-OPE diol (Fig. 10.2). From ground state calculations [38] it is known that
different conformers with opposite CD contribute to the observed CD spectrum: in
this case, it has been possible to observe that different excitation wavelengths are
capable to evidence a minus–plus CPL couplet or a monosignated positive band.
Other interesting examples are present in the literature signaling a different geometry
between ground and excited states (see, for example, cryptochirality for the CD
technique, but manifesting in CPL [39, 40]). (4) A fourth case that is worthwhile
mentioning, when one considers relations between ground and emissive states, is
represented by systems bearing two different fluorophoric electronically independent groups, examples of which may consist of OPE backbone with pyrene pendant
groups, studied in Ref. [41] (a better description will be given later).
A similar but not identical aspect is worthwhile to mention: to understand the sign
of the observed signal, it is important to consider differences not only in structures,
but also in electronic properties, for example, due to a peripheral substituent with
electron-withdrawing or electron-donating character. An instructive example is an
azabora[5]helicene dye with different substituents [42] (see Fig. 10.3).
Depending on the electron-donor substituent one is dealing with, one observes for
the M-isomer: a positive band (lowest energy electronic CD [ECD] transition and
dominant CPL feature) for the first compound, and a negative one (first ECD and
CPL transition) for the other two compounds. Calculations suggest nearly superimposable structures for the three molecules; the origin of the sign change, accounted
for by the relative orientation of magnetic and electrical dipole transition moments,
Fig. 10.2 p,m-(1R,2R)-OPE diol: example of a molecule with several conformers presenting
opposite CD signals; this has a counterpart in S 1 states of different structures giving emission
CPL bands of different sign also depending on excitation wavelength (see Ref. [38])
224
G. Longhi and S. Abbate
structures. Also, in this case, one may have different components contributing to
emission; these components could be eventually discriminated by time resolved
fluorescence spectroscopy. An example of the last circumstance is represented by p,
m-(1R,2R)-OPE diol (Fig. 10.2). From ground state calculations [38] it is known that
different conformers with opposite CD contribute to the observed CD spectrum: in
this case, it has been possible to observe that different excitation wavelengths are
capable to evidence a minus–plus CPL couplet or a monosignated positive band.
Other interesting examples are present in the literature signaling a different geometry
between ground and excited states (see, for example, cryptochirality for the CD
technique, but manifesting in CPL [39, 40]). (4) A fourth case that is worthwhile
mentioning, when one considers relations between ground and emissive states, is
represented by systems bearing two different fluorophoric electronically independent groups, examples of which may consist of OPE backbone with pyrene pendant
groups, studied in Ref. [41] (a better description will be given later).
A similar but not identical aspect is worthwhile to mention: to understand the sign
of the observed signal, it is important to consider differences not only in structures,
but also in electronic properties, for example, due to a peripheral substituent with
electron-withdrawing or electron-donating character. An instructive example is an
azabora[5]helicene dye with different substituents [42] (see Fig. 10.3).
Depending on the electron-donor substituent one is dealing with, one observes for
the M-isomer: a positive band (lowest energy electronic CD [ECD] transition and
dominant CPL feature) for the first compound, and a negative one (first ECD and
CPL transition) for the other two compounds. Calculations suggest nearly superimposable structures for the three molecules; the origin of the sign change, accounted
for by the relative orientation of magnetic and electrical dipole transition moments,
Fig. 10.2 p,m-(1R,2R)-OPE diol: example of a molecule with several conformers presenting
opposite CD signals; this has a counterpart in S 1 states of different structures giving emission
CPL bands of different sign also depending on excitation wavelength (see Ref. [38])
224
G. Longhi and S. Abbate