Chemistry Reference
In-Depth Information
continuous lattice distortion of back (nitrito-to-nitro) isomerization. Some of the
other representative examples studied more recently are linkage isomerization of
sulfur dioxide complexes of ruthenium [
60
,
61
], where the excited SO
2
molecule,
which is coordinated through the sulfur atom, changes its coordination mode to
bidentate, where it is coordinated through one sulfur and one oxygen atom. Similar
reactions can also proceed with larger ligands [
62
].
X-ray photodiffraction provides a unique possibility to peek into the geometric
changes that take place during molecular excitation, and to determine the accurate
geometries of long-lived (metastable) excited states. One necessary condition for
such an analysis is relatively long lifetime of the excited states, such as, for
example, unusually long-lived states or spin-trapped triplets. A second prerequisite
is that the structure of the excited state is sufficiently different from the ground-state
structure, which facilitates the analysis at small population ratios of the excited
species. One of the pioneering works in this direction was provided by structure
analysis of the metastable excited state of sodium nitroprusside, Na
2
[Fe(CN)
5
NO]·2H
2
O by Coppens et al. [
63
]. Upon excitation of the anion, Fe-N bond
stretches by 0.049(8)
˚
. More recently, more efforts have been focused on paddle-
wheel diplatinum complexes with long-lived luminescent excited states. By
employing in situ steady-state irradiation [
64
], for example, significant shortening
of the Pt-Pt bond was observed in a diplatinum paddlewheel complex with a
bridging diphosphine ligand (pop), [Pt
2
(H)
2
(pop)
4
]
2
. By using steady-state
X-ray diffraction in their equilibrium (photosteady) states the excited-state struc-
tures of similar anions [Pt
2
(pop)
4
]
4
and [Pt(pop)
2
(popH)
2
]
2
, isolated from each
other in the lattice by bulky quaternary ammonium cations, were analyzed [
65
]. The
results confirmed that significant decrease of the central Pt-Pt and Pt-P distances
occurs in the excited state. These experiments are important since they demonstrate
the simplicity of the experimental setup which could be employed for such analysis.
By the virtue of the steady-state X-ray photodiffraction, the group of Ohashi has
analyzed the excited-state structure of gold(I) complex, [AuCl(PPh
3
)
2
]·CHCl
3
[
66
].
It was demonstrated that due to excitation from an anti-bonding to a bonding
molecular orbital, in the excited state there is significant contraction of the Au-P
and Au-Cl bonds, which is expressed better at lower temperatures. By using a
laboratory-scale setup, recently the excited state of oxovanadium(IV) cation in its
acetylacetonato complex VO(acac)
2
was analyzed [
67
]. In the latter case, both
bonds V
O and V-O expand by excitation, which appears as a result of photoex-
cited
d
-
d
* transition.
Photoinduced spin-related phenomena are a particularly important field of the
solid-state photophysics, because fast spin switching is a prospective basis for
applications in the field of spintronics. An illustrative example is the production
of the metastable state of the iron propyltetrazole (ptz) complex [Fe(ptz)
6
](BF
4
)
2
by
laser light-induced excited spin-state trapping (LIESST) and the determination of
the resulting structure by steady-state X-ray photodiffraction [
68
]. In another
example, steady-state X-ray photodiffraction at cryogenic temperatures was suc-
cessfully utilized to study photoinduced phase transition due to spin crossover in the
tris(
¼
-picolylamine)iron(II) complex [
69
]. The phase transition is accompanied by
a
