activity of helicenes and helicenoids is therefore being more and more investigated.
In this chapter, we present the helicene derivatives that have shown to display CPL.
Helically twisted acenes will not be considered here.
Theoretically, the luminescence dissymmetry factor g lum for the electronic transition
i ! j can be expressed by the following equation: g lum ¼ 4
μ ij
j j∙ m ji
j j∙ cosθμ, m
μ ij
j j
2 þ m ji
j j
2
,
where μ ij and m ij are, respectively, the electric and magnetic transition dipole
moment vectors, while θμ,m is the angle between them. In the case of an electronic
transition, the |m ji | term is usually small with respect to |μ ij | so the equation
becomes g lum ¼ 4
m ji
j j
μ ij
j j
cos θμ, m . High g lum values are therefore usually obtained
for magnetic dipole-allowed transitions. For this reason, CPL spectroscopy is
widely applied to chiral lanthanide(III) complexes: the magnetically allowed
intraconfigurational f ! f transitions of lanthanide metal ions often provide
extraordinary g lum values [11]. In general, chiral organic π-conjugated systems display
luminescence dissymmetry factors significantly lower than lanthanide(III) compounds,
due to their electronic transitions with strong electric dipole character. However,
their easy processability, the wide range of emission wavelengths accessible,
and good quantum yields of fluorescence, together with their propensity to selfassemble into chiral supramolecules or aggregates makes chiral π-conjugated
molecules appealing systems for improved materials with CPL activity [4] . Therefore,
there is a growing interest in the investigation of the CPL properties of chiral
π-conjugated systems.
In 2018, Mori et al. tried to see whether there was a correlation between excitation
and emission dissymmetry factors; they examined the experimental ratio g lum /g abs
(where g abs ¼ Δε/ε) for a series of chiral organic emissive molecules among which
helicenes. They found that this ratio significantly depended on the structure of the
helicenic molecule and varied between 0.16 and 28 [12].
This chapter is structured via the different types of CPL-active helicene
derivatives, i.e., the N-, O-, S-, B-, Si-, P-, and C-based helicenes and helicenoids,
together with transition metal complexes of helicenes exhibiting CPL.
4.2 CPL-Active N-Containing Helicenes
4.2.1 Helicenic Bridged Triarylamines
In 2003, Venkataraman et al. described the preparation of helical triarylamines.
These compounds were among the first helicenic structures displaying clear
CPL-activity [13]. The two diastereomers of (P,S)-1a and (M,S)-1b displayed
identical absorption spectra (Fig. 4.1a, b and Table 4.1) in the UV-vis region with
maximum absorption and emission occurring at 434 nm and 453 nm, respectively.
These pseudoenantiomeric compounds showed mirror-image ECD and CPL
spectra, revealing that the (1S)-camphanate substituent had no influence on the
54
J. Crassous
In this chapter, we present the helicene derivatives that have shown to display CPL.
Helically twisted acenes will not be considered here.
Theoretically, the luminescence dissymmetry factor g lum for the electronic transition
i ! j can be expressed by the following equation: g lum ¼ 4
μ ij
j j∙ m ji
j j∙ cosθμ, m
μ ij
j j
2 þ m ji
j j
2
,
where μ ij and m ij are, respectively, the electric and magnetic transition dipole
moment vectors, while θμ,m is the angle between them. In the case of an electronic
transition, the |m ji | term is usually small with respect to |μ ij | so the equation
becomes g lum ¼ 4
m ji
j j
μ ij
j j
cos θμ, m . High g lum values are therefore usually obtained
for magnetic dipole-allowed transitions. For this reason, CPL spectroscopy is
widely applied to chiral lanthanide(III) complexes: the magnetically allowed
intraconfigurational f ! f transitions of lanthanide metal ions often provide
extraordinary g lum values [11]. In general, chiral organic π-conjugated systems display
luminescence dissymmetry factors significantly lower than lanthanide(III) compounds,
due to their electronic transitions with strong electric dipole character. However,
their easy processability, the wide range of emission wavelengths accessible,
and good quantum yields of fluorescence, together with their propensity to selfassemble into chiral supramolecules or aggregates makes chiral π-conjugated
molecules appealing systems for improved materials with CPL activity [4] . Therefore,
there is a growing interest in the investigation of the CPL properties of chiral
π-conjugated systems.
In 2018, Mori et al. tried to see whether there was a correlation between excitation
and emission dissymmetry factors; they examined the experimental ratio g lum /g abs
(where g abs ¼ Δε/ε) for a series of chiral organic emissive molecules among which
helicenes. They found that this ratio significantly depended on the structure of the
helicenic molecule and varied between 0.16 and 28 [12].
This chapter is structured via the different types of CPL-active helicene
derivatives, i.e., the N-, O-, S-, B-, Si-, P-, and C-based helicenes and helicenoids,
together with transition metal complexes of helicenes exhibiting CPL.
4.2 CPL-Active N-Containing Helicenes
4.2.1 Helicenic Bridged Triarylamines
In 2003, Venkataraman et al. described the preparation of helical triarylamines.
These compounds were among the first helicenic structures displaying clear
CPL-activity [13]. The two diastereomers of (P,S)-1a and (M,S)-1b displayed
identical absorption spectra (Fig. 4.1a, b and Table 4.1) in the UV-vis region with
maximum absorption and emission occurring at 434 nm and 453 nm, respectively.
These pseudoenantiomeric compounds showed mirror-image ECD and CPL
spectra, revealing that the (1S)-camphanate substituent had no influence on the
54
J. Crassous