Thermal and Photo Control of the Linkage Isomerism of Bis(thiocyanato)(2,2‘-bipyridine)platinum(II)

Three linkage isomers, bis(thiocyanato-S)(2,2‘-bipyridine)platinum(II) ([Pt(SCN)2(bpy)]), (thiocyanato-S)(thiocyanato-N)(2,2‘-bipyridine)platinum(II) ([Pt(SCN)(NCS)(bpy)]), and bis(thiocyanato-N)(2,2‘-bipyridine)platinum(II) ([Pt(NCS)2(bpy)]) were isolated, and their structures were elucidated. The crystal data are as follows:  for [Pt(SCN)2(bpy)],
C12H8N4S2Pt, orthorhombic, P212121 (No. 19), a = 12.929(9) Å, b = 18.67(1) Å, c = 5.497(4) Å, Z = 4; for
[Pt(SCN)(NCS)(bpy)], C12H8N4S2Pt, monoclinic, P21/n (No. 14), a = 10.909(7) Å, b = 7.622(4) Å, c = 16.02(1) Å,
β = 102.323(7)°, Z = 4; for [Pt(NCS)2(bpy)], C12H8N4S2Pt, orthorhombic, Pbcm (No. 57), a = 10.3233(8) Å, b =
19.973(2) Å, c = 6.4540(5) Å, Z = 4. The stacking structures of the isomers were found to be different depending
on the coordination geometries based on the N- and S-coordination of the thiocyanato ligands, which control the
color and luminescence of the crystals sensitively. The isomerization behaviors of the complex have been investigated
both in solution and in the solid state. In solution, stepwise thermal isomerization from [Pt(SCN)2(bpy)] to [Pt(NCS)2(bpy)] by way of [Pt(SCN)(NCS)(bpy)] was observed using 1H NMR spectroscopy. Reverse isomerization,
from [Pt(NCS)2(bpy)] to [Pt(SCN)(NCS)(bpy)] and [Pt(SCN)2(bpy)], occurred when irradiated with near ultraviolet
(UV) light. In contrast, the [Pt(SCN)2(bpy)] yellow crystals exhibited thermal isomerization directly to red crystals of
[Pt(NCS)2(bpy)], as detected by changes in the emission spectrum, which indicates that the flip of two SCN-
ligands correlatively occurred in the solid state. The yellow crystals of [Pt(SCN)(NCS)(bpy)] were also converted to
the thermodynamically stable red crystal of [Pt(NCS)2(bpy)] though the reverse isomerization has never been observed
to occur by photoirradiation in the solid state.