10.2.4 CPL of Transition Metal Complexes
Below we investigate how CPL reports information about the coordination of
organic ligands around two different transition metal atoms, namely Iridium and
Platinum, which sit close by in the periodic table.
The first cases are two types of complexes of organic ligands with Ir(III)
(Fig. 10.11). For the first one, which is commercially available, though not in
separated enantiomers, none of the organic ligands is chiral [57], while for the
second type one of the three ligands was chiral and we were able to obtain CPL
data for all diastereoisomers [58]. In the two cases, the three ligands are organized in
a chiral octahedral way around the Iridium ion, which is thus either Λ or Δ: the sign
of the CPL band is univocally determined by arrangement around the Ir(III) ion and
does not depend on the structure of the ligands. Even when the latter are chiral, their
configuration has no influence on the sign of the CPL band. There is a dependence
on the precise chemical nature of the substituents for the center wavelength of the
fluorescence and CPL band and for the band-shape, i.e., the vibronic features. The
observed CPL spectra are weak, being the g lum ratio of the order of 10
–3 , and the
g lum /g abs ratio of the order of 1; the presence of vibronic features can be also noticed.
In these systems contributions from triplet states and thus phosphorescence are
present, which makes these systems challenging from a computational point of
view [59].
The second example consists of Pt organometallic complexes. These complexes
are square-planar chiral systems with the Pt atom serving as stereo center, either (S)
or (R). In the first case [60] the spectra were measured in water solution, in the
second one [61] in CH 2 Cl 2 . The sign rule that the CPL sign should be the same as the
one for the longest wavelength CD feature is obeyed. CPL spectra are weak in both
cases: in the first case they are one order of magnitude lower than the Ir complexes
commented above, while in the second case they are of the same order of magnitude
(Fig. 10.12). Coming to the quantitative evaluation of the spectra, we had for
the [(ppy)Pt((R)-Campy)] complex g abs % 10
–3 , while g lum % 10
–4 . Thus
g lum /g abs % 10
–1 , and the Stokes shift is considerable (ca. 150 nm). For the dichelated
[PtL
CN
2 ] trans-2 complex we have g abs % 5 Â 10
–3 , while g lum % 10
–3 . Thus
g lum /g abs is slightly bigger, and once again the Stokes shift is large, ca. 100 nm.
The weakness of CPL is due to the square-planar arrangement of the complex. The
special arrangement of the square-planar coordination in the second type of compounds, whereby two different planar moieties coordinate at right angle to the central
ion, helps in boosting all chiroptical properties, from ECD to CPL. For either
complex, the luminescence is associated to a T 1 ! S 0 transition. (In the Supplementary information of Ref. [61], a calculation of the ECD spectra considering singlet
states is provided, which successfully compares to the experimental one.)
10 Structural and Electronic Information Drawn. . .
237
Below we investigate how CPL reports information about the coordination of
organic ligands around two different transition metal atoms, namely Iridium and
Platinum, which sit close by in the periodic table.
The first cases are two types of complexes of organic ligands with Ir(III)
(Fig. 10.11). For the first one, which is commercially available, though not in
separated enantiomers, none of the organic ligands is chiral [57], while for the
second type one of the three ligands was chiral and we were able to obtain CPL
data for all diastereoisomers [58]. In the two cases, the three ligands are organized in
a chiral octahedral way around the Iridium ion, which is thus either Λ or Δ: the sign
of the CPL band is univocally determined by arrangement around the Ir(III) ion and
does not depend on the structure of the ligands. Even when the latter are chiral, their
configuration has no influence on the sign of the CPL band. There is a dependence
on the precise chemical nature of the substituents for the center wavelength of the
fluorescence and CPL band and for the band-shape, i.e., the vibronic features. The
observed CPL spectra are weak, being the g lum ratio of the order of 10
–3 , and the
g lum /g abs ratio of the order of 1; the presence of vibronic features can be also noticed.
In these systems contributions from triplet states and thus phosphorescence are
present, which makes these systems challenging from a computational point of
view [59].
The second example consists of Pt organometallic complexes. These complexes
are square-planar chiral systems with the Pt atom serving as stereo center, either (S)
or (R). In the first case [60] the spectra were measured in water solution, in the
second one [61] in CH 2 Cl 2 . The sign rule that the CPL sign should be the same as the
one for the longest wavelength CD feature is obeyed. CPL spectra are weak in both
cases: in the first case they are one order of magnitude lower than the Ir complexes
commented above, while in the second case they are of the same order of magnitude
(Fig. 10.12). Coming to the quantitative evaluation of the spectra, we had for
the [(ppy)Pt((R)-Campy)] complex g abs % 10
–3 , while g lum % 10
–4 . Thus
g lum /g abs % 10
–1 , and the Stokes shift is considerable (ca. 150 nm). For the dichelated
[PtL
CN
2 ] trans-2 complex we have g abs % 5 Â 10
–3 , while g lum % 10
–3 . Thus
g lum /g abs is slightly bigger, and once again the Stokes shift is large, ca. 100 nm.
The weakness of CPL is due to the square-planar arrangement of the complex. The
special arrangement of the square-planar coordination in the second type of compounds, whereby two different planar moieties coordinate at right angle to the central
ion, helps in boosting all chiroptical properties, from ECD to CPL. For either
complex, the luminescence is associated to a T 1 ! S 0 transition. (In the Supplementary information of Ref. [61], a calculation of the ECD spectra considering singlet
states is provided, which successfully compares to the experimental one.)
10 Structural and Electronic Information Drawn. . .
237