The role of molecular packing on the UV-visible optical properties of [Re2Cl2(CO)64,5-(Me3Si)2pyridazine]

We report here on two stable polymorphs of the dinuclear complex [Re2(μ-Cl)2(CO)6(μ-4,5- (Me3Si)2pyridazine)]. The compound belongs to the recently reported class of dinuclear luminescent Re(I) complexes of general formula [Re2(μ-Cl)2(CO)6(μ-1,2-diazine)]. In the solid state, the complex exhibits a unique combination of unusual properties: (i) concomitant formation of two highly luminescent polymorphs, and single crystal- to-single-crystal conversion of one form into the other, (ii) remarkable differences in the absorption properties of the two polymorphs due to different redistribution of oscillator strength among the different excitons, and (iii) remarkable differences among the emission properties. In particular, a higher emission quantum yield was found in the solid state than in solution (measured to be 0.52 and 0.56 for the two polymorphs, almost one order of magnitude higher than that of the molecule in solution). Interest in luminescent materials able to efficiently emit in the solid state is continuously growing, because in most applications the dyes are used as solid films. Although rigid environments are expected to freeze rotovibrational relaxation pathways, luminescence efficiency often decreases in the solid state with respect to liquid solution due to concentration quenching, affecting both organic and organometallic emitters. However, an increasing number of molecular-based emitters exhibit enhanced solid state emission. Apart from the importance of bulky substituents in reducing concentration quenching effects, other intra- or inter-molecular phenomena have been invoked as well, such as conformational changes, π-π stacking, hydrogen bonds, or Jaggregates, which cause rearrangements of the energy levels and population. In these two polymorphs the enhancement of the emission with respect to the solution is most likely due to the restricted rotation of the Me3Si groups in the crystals, providing an interesting example of aggregation-induced emission effect (AIE). To provide more insight into the optical properties of the two polymorphs, we present a combined computational and experimental study in the framework of crystal optics in the aim to explore the role of molecular packing on the UV-visible absorption properties of the two known polymorphs of [Re2(μ- Cl)2(CO)6(μ-4,5-(Me3Si)2pyridazine)].