Polymorphic Forms of a Gold(I) Arylacetylide Complex with Contrasting Phosphorescent Characteristics

The spectroscopic properties and crystal structures of the gold(I) arylacetylide complexes [(R3P)Au(C⋮CAr)] (R = Cy, Ar = 4-nitrophenyl, 1; 4-trifluoromethylphenyl, 2; pentafluorophenyl, 3; R = Ph, Ar
= 4-nitrophenyl, 4) have been examined. The dipole-allowed and -forbidden transitions of 1 (4 in
parentheses) at λmax 340 (336) and ca. 485 (470) nm in CH2Cl2 solution at 298 K are assigned to the
singlet and triplet intraligand charge transfer (ILCT) transitions of the 4-nitrophenylacetylide moiety, whereas
2 (3 in parentheses) shows localized singlet and triplet acetylenic ππ* transitions at λmax 287 (276) and 426
nm, respectively. Two polymorphs of 1 with contrasting phosphorescent characteristics have been identified.
At 298 K, the emissive form of 1, as well as 2−4, are highly phosphorescent with peak maximum at 504,
425, 521, and 495 nm, respectively; the other polymorph of 1 is nonemissive at 298 K but emission is
detected at 77 K with peak maximum at 486 nm. Crystallographic studies reveal that the major differences
between the emissive and nonemissive forms of 1 are the orientations of the molecular dipoles and the
dihedral angles between neighboring 4-nitrophenyl moieties. Crystal 2 is isostructural to the nonemissive
form of 1, but does not display polymorphism. The molecular planes of two neighboring lumophores are
coplanar in the emissive form of 1, parallel in 4, and nearly perpendicular (78.6°) to each other in the
nonemissive form of 1. Both the nature of the excited state and the dihedral angle between adjacent [Au(C⋮CAr)] moieties determine the phosphorescent properties of these molecular crystals.