Effects of Crystalline Polymorphism on Luminescence

Polymorphs have been defined by McCrone as a solid crystalline phase of a given compound resulting from the possibility of at least two different ar- 

Two polymorphs of the salt [(C6HnNC)2AuI](PF6) have been found [38]. One of these is a yellow solid which displays a greenish emission, while the other forms colorless crystals that produce blue luminescence. Since both forms can be crystallized from a single solution, for example by adding diethyl ether to a dichloromethane solution of the complex, these are known as concomi- 

The structure of the yellow polymorph of [(C6HnNC)2AuI](PF6) is more complicated, as can be seen in Fig. 8. There are four cations in the asymmetric unit that are arranged to form a slightly helical chain. The Au Au contacts (2.9803(6), 2.9790(6), 2.9651(6), and 2.9643(6) A) between the cations are unusually short. Notice that these short contacts occur between cationic complexes where one might expect coulombic forces to keep the gold centers apart. 

The linear, two-coordinate (Me2PhP)AuICl complex also forms two polymorphs [39]. One polymorph involves self-association of two molecules while the other involves association of three molecules. The structures of the dimer and trimer are shown in Fig. 9. In the dimer the Au Au contact is 3.230(2) A, while in the trimer there are two slightly different, shorter Au Au separations, 3.091(2) and 3.120(2) A. 

In contrast, in the orthorhombic needles, there is only one molecule in the asymmetric unit and it has a different external shape. While the nine-membered ring at the core of this molecule is planar, one of the pentyl groups protrudes nearly vertically, as seen in Parts B and C of Fig. 10. The molecular packing involves a stair step arrangement of trimers with relatively long distances between them with the closest contact of 3.618(2) A. The absence of strong aurophilic interactions between trimers may be responsible for the non-emissive nature of this polymorph. 

The bonding within the extended chains of gold atoms in the two polymorphs of [(C6HiiNC)2Au ](PF6) can be understood in the context of Fig. 3. The overlap of the occupied gold orbitals (where z is the axis along the Au Au Au chain) produces a filled band comprised of the d orbitals, while overlap of the empty gold 6pz orbitals produces a corresponding unoc- 

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