Two Highly Luminescent Concomitant Polymorphs of a Dinuclear Rhenium(I) Complex: Synthesis and Solid State Structural and Photophysical Characterization

A complex which exhibits a unique combination of unusual properties has been presented. The complex belongs to a new class of highly luminescent dinuclear rhenium(I)-diazine complexes recently reported by ourselves. Differently from many of the members of this family, this new derivative is moderately luminescent in solution, from 3MLCT states. However its emission shows an almost ten-fold enhancement on passing from the solution to the solid state, where the photoluminescence quantum yields become higher than 0.5. The complex therefore provides another remarkable example of aggregation induced emission, a phenomenon that is currently receiving a great deal of attention. Moreover, in solid the complex exists in two different crystal phases (yellow and orange), which can be either concomitantly or separately obtained, on varying the crystallization rate. Both the polymorphs are highly emitting (quantum yields 0.56 and 0.52, respectively), with a well different emission color. Another notable property of this system is the clean single-crystal-to-single-crystal irreversible conversion of the orange into the yellow phase, which occurs at 443 K, as revealed by variable temperature X-ray diffraction analysis.