Beyond Chiral Organic (p-Block) Chromophores for Circularly Polarized Luminescence: The Success of d-Block and f-Block Chiral Complexes

Chiral molecules are essential for the development of advanced technological
applications in spintronic and photonic. The best systems should produce large circularly
polarized luminescence (CPL) as estimated by their dissymmetry factor (glum), which can
reach the maximum values of −2 ≤ glum ≤ 2 when either pure right- or left-handed
polarized light is emitted after standard excitation. For matching this requirement,
theoretical considerations indicate that optical transitions with large magnetic and weak
electric transition dipole moments represent the holy grail of CPL. Because of their
detrimental strong and allowed electric dipole transitions, popular chiral emissive organic
molecules display generally moderate dissymmetry factors (10−5 ≤ glum ≤ 10−3).
However, recent efforts in this field show that glum can be significantly enhanced when
the chiral organic activators are part of chiral supramolecular assemblies or of liquid
crystalline materials. At the other extreme, chiral EuIII- and SmIII-based complexes, which
possess intra-shell parity-forbidden electric but allowed magnetic dipole transitions, have
yielded the largest dissymmetry factor reported so far with glum ∼ 1.38. Consequently,
4f-based metal complexes with strong CPL are currently the best candidates for
potential technological applications. They however suffer from the need for highly pure
samples and from considerable production costs. In this context, chiral earth-abundant
and cheap d-block metal complexes benefit from a renewed interest according that
their CPL signal can be optimized despite the larger covalency displayed by d-block
cations compared with 4f-block analogs. This essay thus aims at providing a minimum
overview of the theoretical aspects rationalizing circularly polarized luminescence and
their exploitation for the design of chiral emissive metal complexes with strong CPL.
Beyond the corroboration that f–f transitions are ideal candidates for generating large
dissymmetry factors, a special attention is focused on the recent attempts to use chiral
CrIII-based complexes that reach values of glum up to 0.2. This could pave the way for
replacing high-cost rare earths with cheap transition metals for CPL applications.