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Excited-State Characteristics of Tetracyanidonitridorhenium(V) and -technetium(V) Complexes with N‑Heteroaromatic Ligands
journal contribution
posted on 2013-06-03, 00:00 authored by Hayato Ikeda, Akitaka Ito, Eri Sakuda, Noboru Kitamura, Tsutomu Takayama, Tsutomu Sekine, Atsushi Shinohara, Takashi YoshimuraSix-coordinate
tetracyanidonitridorhenium(V) and -technetium(V)
with axial N-heteroaromatic ligands, (PPh4)2[MN(CN)4L] [M = Re, L = 4-(dimethylamino)pyridine (dmap),
3,5-lutidine (lut), 4-picoline (pic), 4-phenylpyridine (ppy), pyridine
(py), 3-benzoylpyridine (3bzpy), 4,4′-bipyridine (bpy), pyrazine
(pz), 4-cyanopyridine (cpy), or 4-benzoylpyridine (4bzpy); M = Tc,
L = dmap, lut, pic, py, pz, or cpy] were synthesized and characterized.
The crystal structures of 11 complexes were determined by single-crystal
X-ray analysis. All of the complexes showed photoluminescence in the
crystalline phase at room temperature. The emission maximum wavelengths
(λem) of the rhenium complexes with dmap, lut, pic,
ppy, or py were similar to one another with a quite high emission
quantum yield (Φem): λem = 539–545
nm, Φem = 0.39–0.93, and emission lifetime
(τem) = 10–45 μs at 296 K. The emission
spectra
at 77 K exhibited vibronic
progressions, and the emissive excited state is characterized as 3[(dxy)1(dπ*)1] (dπ* = dxz, dyz). On the other hand, the emission
maximum wavelength of the rhenium complex with 3bzpy, bpy, pz, cpy,
or 4bzpy was significantly dependent on the nature of the axial ligand
in the crystalline phase: λem = 564–669 nm,
Φem ≤ 0.01–0.36, and τem = 0.03–13.3
μs at 296 K. The emission spectra at 77 K in the crystalline
phase did not show vibronic progressions. The emissive excited state
of the rhenium complex with bpy, pz, cpy, or 4bzpy is assignable to
originate from the metal-to-N-heteroaromatic ligand charge-transfer
(MLCT)-type emission with a spin-triplet type. The change in the excited-state
characteristics of rhenium complexes by the N-heteroaromatic ligand
is a result of stabilization of the π* orbital of the N- heteroaromatic
ligand to a lower energy level than the dπ* orbitals.
The emission spectral shapes of technetium complexes were almost independent
of
the nature of the N-heteroaromatic ligand with λem = 574–581 nm at room temperature. The different emission
characteristics
between the pz and cpy coordinate rhenium complexes and the technetium
analogues would be due to stabilization of technetium-centered orbitals
compared with the rhenium ones in energy.