Highly Emitting Concomitant Polymorphic Crystals of a Dinuclear Rhenium Complex

The dinuclear complex [Re2(μ-Cl)2(CO)6(μ-4,5-(Me3Si)2-pyridazine)] gives in the solid state two polymorphs (yellow, 1Y,
and orange, 1O), which can be either concomitantly or separately
obtained on varying the crystallization rate. Both crystal phases exhibit intense photoluminescence from the lowest lying triplet metal-to-ligand charge transfer state, much stronger than in solution (quantum yields 0.56 and 0.52, for 1O and 1Y respectively, vs 0.06 in toluene), likely due to the restricted rotation of the Me3Si groups in the solid state. A clean, irreversible 1O ->
1Y single-crystal-to-single-crystal phase transition occurs at 443
K, as revealed by variable temperature X-ray diffraction analysis.
In spite of the absence of any strong intermolecular interactions
in both forms, 1O and 1Y show very different absorption and
emission maxima (λabs 370 and 393 nm, λem 534 and 570 nm, for
1Y and 1O, respectively). This behavior highlights the importance
of the local organization of molecular dipoles in perturbing the
photophysical properties of the molecule in the crystal.