Communications
Information). As expected, the d8 platinum(II) ion adopts a
orbitals (Figure 1). In fact, the orbitals on the metal atom are
involved in many p-MOs (Figure 1; see the Supporting
Information for an excitation analysis in terms of MO-to-
MO contributions). These experimental and theoretical
results reveal the unique structural properties of 2a–2c, in
which a conjugated helicoidal system is intimately electroni-
cally coupled with an integrated transition metal center.
Metallahelicenes 2a–2c exhibit an emission behavior that
differs considerably from that of typical organic helicenes,
such as the free ligands 1a–1c. Compounds 1a–1c all have
blue fluorescence in dichloromethane solvent at room
temperature (see the Supporting Information). In a rigid
glass matrix at 77 K, the fluorescence is accompanied by long-
lived green phosphorescence. In contrast, even at room
temperature, platinum helicenes 2a–2c display phosphoresce
only, in the red region of the spectrum (Figure 1; see also the
Supporting Information); indeed, with luminescence quan-
tum yields of up to 10%, the metallahelicenes (2a–2c) are
efficient (N^C)platinum(II) red phosphors.[5] The absence of
fluorescence in platinum helicenes 2a–2c can be attributed to
the effective spin-orbit coupling associated with the platinum
center, favoring rapid intersystem crossing from the singlet to
the triplet state. The fact that the phosphorescence of the
complexes is substantially lower in energy than that of the
free ligands is testament to the involvement of the metal in
the emitting excited state. The influence of spin–orbit
coupling and the efficient mixing of metal orbitals and
ligand orbitals is also manifest in the much shorter lifetimes of
triplet emission in the complexes (see the Supporting
Information). Compounds 2a–2c are the first reported
helicene derivatives to exhibit strong phosphorescence at
room temperature,[10] a property induced by the incorporation
of a heavy metal atom within the helicene p-conjugated
framework. Indeed, the heavy metal plays a dual role, both
forming the [n]helicene skeleton upon ortho-metalation
(Scheme 1), and dramatically impacting on the optical
properties of the helicoidal p-conjugated system.
slightly distorted square-planar geometry; the metric data of
the platinum(II) coordination sphere is within the typical
range previously reported for other [(N^C)Pt(acac)] com-
plexes (acacH = 2,4-pentanedione).[5] The phenyl and pyri-
dine rings that are coordinated have a twist angle of 7.38, and
the consecutive twist angles between the fused aromatic rings
(24.78, 31.08, and 13.48) lead to a hc value of 52.38 in the
metalla[6]helicene-like skeleton (Scheme 1); these values are
similar to those reported in organic or heteroatomic [6]hel-
icene systems (hc ~ 508).[7b,9] Indeed, derivatives 2a–2c pos-
sess this targeted, inherently chiral, ortho-annulated p-
conjugated framework.
The electronic properties of these metallahelicenes were
investigated using UV/Vis spectroscopy and theoretical
calculations. The electronic absorption spectra of complexes
2a–2c displayed several intense absorption bands between
250 and 350 nm that were red-shifted compared to those of
the free ligands 1a–1c (lmax = 290 nm), and two weaker,
lower-energy broad bands below 450 nm (Figure 1). The
intense bands indicate the presence of an extended p-
conjugated system within the metallahelicenes, whilst the
lower energy bands are thought to arise from orbitals
involving the metal and the N^C-ligands. These assignments
are supported by time-dependent DFT calculations (BHLYP/
SV(P) level of theory), which accurately reproduced the
experimental spectra of 2a–2c after a modest red shift of
0.25 eV (Figure 1). For example, the long-wavelength band is
essentially
a HOMO–LUMO transition (3 eV, 89.2%;
Figure 1), involving two molecular orbitals (MOs) that consist
of a mixture of platinum orbitals and extended N^C p-
With these unusual systems in hand, it was crucial to
elucidate whether these organometallic helicenes would
possess the important chiroptical properties of [6]helicenes.
In other words, could they be resolved, and do they exhibit
large optical rotations and intense circular dichroism (CD)
bands? Derivatives 2a and 2b were selected for this study
because they are configurationally stable in solution at room
temperature,[11] thus allowing their enantiomers to be sepa-
rated by chiral stationary phase HPLC (ee values of 98%–
99.5%). They display very high specific and molar optical
rotations [(+)-2a:½aꢁD23 = 1300, ½ꢀꢁ2D3 = 8170 (ꢂ 5%) (c = 2.85 ꢀ
10ꢃ3, dichloromethane); (+)-2b: ½aꢁD23 = 1240, ½ꢀꢁ2D3 = 7420 (ꢂ
5%) (c = 1.8 ꢀ 10ꢃ3, dichloromethane)]. For comparison, the
23
molar optical rotations for P-[6]carbohelicene[2h] (½ꢀꢁD
=
11950) is of a similar order of magnitude; the calculated[2f,g]
gas phase values for P-2a (½ꢀꢁD = 10281, BHLYP/SV(P)) is in
the range of the experimental measurements. The mirror-
image CD spectra of complexes 2a and 2b are intense
(Figure 2a; also see the Supporting Information), and are
very similar for both complexes. The (+) enantiomers show
an intense negative band at 250 nm and a strong positive band
with several maxima tailing down to 410 nm. Together, these
Figure 1. a) Experimental spectrum (g) and calculated UV/Vis spec-
trum for 2a shifted by ꢃ0.25 eV (BHLYP/SV(P),c). b) Normalized
emission spectra of 2a at 298 K (dichloromethane; red line) and at
77 K (diethyl ether/isopentane/ethanol; 2:2:1 v/v; blue line;
lex =450 nm). The absorption (c) and excitation spectra
(lem =640 nm,g) at 298 K are shown on the same wavelength scale.
c) Views of selected MOs of 2a.
100
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Angew. Chem. Int. Ed. 2010, 49, 99 –102