transfer, restriction of intramolecular relaxation, and establish-
ment ofp–pinteractions between the cyclometalated ligands in
the solid state were arguments to explain the enhancement of
the emission ( 195 – 197 ).
It is worth to note that crystalline compounds sometimes
proved to be more efficient emitters than their amorphous
counterparts, showing the influence of molecular packing on the
solid-state emission(177,198), thus providing optimal conditions
to investigate their photophysical properties and the influence of
aggregation. The availability of different crystalline phases
(polymorphs) of a luminescent molecule is the best example for
studying the relationship between crystal packing and optical
properties (188,199– 205 ). In this respect, we recently reported
on two stable concomitant solid-state polymorphs (yellow and
orange) of the dinuclear complex [Re 2 (m-Cl) 2 (CO) 6 (m-4,5-
(Me 3 Si) 2 pyridazine)] (Fig. 11), both showing photoluminescence
quantum yields (PLQYs, 0.52 and 0.56), almost one order of
magnitude higher than those in solution ( 206 ).
The compound belongs to the recently reported class of
dinuclear, luminescent Re(I) complexes of general formula
[Re 2 (m-Cl) 2 (CO) 6 (m-1,2-diazine)] (207,208), which display intense,
broad, and featureless emission in fluid solution from excited
state that can be consistently ascribed to a^3 MLCT.
During the crystallization process, the concomitant formation
of two crystalline phases of the compound was observed. The con-
current existence of polymorphs provides a unique chance for
investigating the factors governing molecular packing. The emis-
sion spectra of the two crystalline phases for each single crystal
are shown inFig. 11. As can be seen even by naked eye, the emis-
sion has different energies and such a strong shift is most likely
due to different local packing of the independent molecules in the
FIG. 11. Left: chemical formula of the crystalline Re(I) complex; mid-
dle: images of the two polymorphs under the UV light; right: emission
spectra of the two crystalline phases upon excitation at 400 nm.
72 CRISTIAN A. STRASSERTet al.