ABAB Homoleptic Bis(phthalocyaninato)lanthanide(III) Complexes: Original Octupolar Design Leading to Giant Quadratic Hyperpolarizability

The
octupolar cube, a Td symmetry
cube presenting alternating charges at its corners, is the generic
point charge template of any octupolar molecule. So far, transposition
into real molecular structures has yet to be achieved. We report here
a first step toward the elaboration of fully cubic octupolar architectures.
A series of octupolar bis­(2,3,16,17-tetra­(hexylthio)­phthalocyaninato)­lanthanide
double-decker complexes [Pc2Ln], Ln = Nd (1), Eu (2), Dy (3), Y (4),
and Lu (5), are described, whose original three-dimensional
structures display the required alternation of ABAB type for one face
of the cube and the delocalization between the two rings approximating
to the electronic interaction along the edges of the cube. Synthesis,
X-ray crystal structure, and study of the optical properties and of
the first molecular hyperpolarizability β are reported. The
size of the lanthanide (III) central ion modulates the ring-to-ring
distance and the degree of coupling between the two phthalocyanine
rings. As a consequence, the optical properties of these octupolar
chromophores and in particular the strong near-infrared absorption
due to the intervalence transition between the two rings also depend
on the central lanthanide (III) ion. The first oxidized and reduced
states of the complexes, while keeping a similar octupolar structure,
display considerably changed optical properties compared to the neutral
states. Second-order nonlinear properties were determined by nonpolarized
harmonic light scattering in solution at 1907 nm. Exceptionally large
dynamic molecular first hyperpolarizabilities ⟨βHLS2⟩1907, among the highest ever
reported, were found that showed a strong dependence on the number
of 4f electrons.