Photoswitching of
Nitroprussides
irradiated part of the crystal
populated with SI and SII
V. Rusanov
Department of Atomic Physics, University of Sofia, Bulgaria
Sv. Stankov
ESRF, Grenoble, France
H. Paulsen, A. X. Trautwein
University of Lübeck, Germany
crystal in ground state
The cover page of the journal Laser und
Optoelektronik with visible information
written in single crystal cut of sodium
nitroprusside Na2[Fe(CN)5NO]2H2O, as a
congratulation to its 25th “birthday”
anniversary. Through a model the crystal
was first irradiated with light of
wavelength 457.9 nm and then for the
second time with light of wavelength 1064
nm. Expected population: GS 70 %, MSI
15% and MSII 15 %. Laser und
Optoelektronik, 25(6) 18 (1995).
The discovery !
On Optical Dispersion in Transparent Molecular Systems
Ulrich Hauser, Volker Oestreich,
and Heinz Dieter Rohrwecke
Physikalisches Institut, Universität
zu Köln, Köln, Germany
Received September 17, 1976
Mössbauer investigation of the FWHM of the metastable states in SNP
GS: QS=+1.716(3) mm/s
IS= 0.00 mm/s
MSI: Pop. 47.1(4) %
QS=+2.755(3) mm/s
IS=+0.178(3) mm/s
MSII: Pop. 35.5(5) %
QS=+2.862(3) mm/s
IS= 0.194(3) mm/s
Th. Woike, W. Kirchner, Hyung-sang Kim, S. Haussühl, V. Rusanov, V. Angelov,
S. Ormandjiev, Ts. Bonchev, A. N. F. Schroeder, Hyperfine Interactions 77, 265 (1993).
Wavelength range of transfers between GS, MSI and MSII
600-900 nm
MS I
350 580nm
GS
900 1200 nm
MS II
600-1200 nm
A maximum of about 45 % of the molecules can be transferred into the first metastable state, MSI, by
irradiation with blue light of 457.9 nm from an Ar+-laser and only a few percent into the second state, MSII.
The latter state can, however, be populated up to about 30 % when a crystal with the first state MSI already
populated at maximum is irradiated a second time with infrared light from a Nd-YAG-laser with wavelength
1064 nm. Both states can be completely and reversibly transferred to the ground state GS by irradiation with
light of the red spectral range, e.g. of 632.8 nm from a He-Ne-laser. A summary of the Mössbauer parameters
for all three states and of the population conditions can be found in: V. Rusanov, Sv. Stankov, and A. X.
Trautwein, Hyperfine Interactions, 144/145, (2002) 307.
G = guanidinium
l=450±35nm at 297 K
20 mm
Counts, 104
Irradiation of the GNP single crystals by Light-Emitting Diodes (LED)
120
MSI and MSII
GS
115
110
105
100
GS=74%
twelve lightemitting diodes
g-beam
95
c-cut, 435(5) m
MSI+MSII=26%
MSI and MSII
130
GS
125
single crystal cut
between two
mylar foils
population (%)
120
115
110
a-cut, 590(5) m
30
25
20
15
10
5
0
-2
-1
a-cut
0
10
20
30
40
50
60
70
irradiation time (h)
105
-3
c-cut
0
1
2
3
Velocity, mm/s
V. Rusanov, H. Winkler, C. Ober, and A. X. Trautwein, Eur. Phys. J. B 12, 191 (1999).
Energy
Explanation of the nature of MSI and MSII by strong structural
changes of the NP anion, proposed by M. Carducci et al.
MSII: Side-on bonding
MSI-Isonitrosyl bonding
GS of the NP anion
M. D. Carducci, M. R. Pressprich, P. Coppens, J. Am. Chem. Soc. 119, 2669 (1997).
LED
GNP
crystal
c
kapton
window
2 cm
temperature
control
NIS investigation of the MSI populated in GNP
synchrotron
beam
NFS
detector
b
NIS
detector
MSII
40
50
60
70
MSI
80
GS
90
a
100
Energy, meV
(a) Simulated NIS spectrum for a crystal populated with GS (75 %), MSI (18 %) and MSII (7 %). New Fe–O
isonitrosyl bond stretching vibrational mode of MSI at 72 meV has been predicted by DFT calculations ( ); (b) NIS
spectrum of the crystal before the illumination; (c) NIS spectrum of the crystal illuminated for 24 h by two light
emitting diodes (LED), 450 nm wavelength. A new line is observable in the NIS spectrum at 73 meV ( ). The inset
shows schematically the illumination cell. H. Paulsen, V. Rusanov, R. Benda, C. Herta, V. Schünemann, C. Janiak, T.
Dorn, A. I. Chumakov, H. Winkler, A. X. Trautwein, J. Am. Chem. Soc. 124, 3007 (2002).
Angle-dependent Mössbauer study of the metastable states in SNP
1.0
0.9
t1/2/t3/2
t1/2/t3/2
1.1
fit of the ratio t1/2/t3/2 for MSI
±3.6 limit for all fit parameters
experimental values
synthesized ratio t1/2/t3/2 for GS
 = 35.8(1)
 = 0.018(8)
 = 7(6)
0.7
0.6
-30
0
a
30
60

fit of the ratio t1/2/t3/2 for SII with 75 % GS
±3.6 limit for all fit parameters
experimental values
SI with 55 % GS
MS II
pure GS
o
 = 37.3(5)
GS with 45 % SI
1.0
MS I
o
0.8
1.1
f
a
f
b
f
c
o
= 0.70(1)
LM
LM
= 0.65(1)
LM
= 0.70(1)
90
b
Th. Woike, M. Imlau, V. Angelov, J. Schefer,
B. Delley, Phys. Rev. B 61, 18, 12249 (2000).
0.9
 = 0.17(5)
0.8
 = 2(7)
f
0.7
f
f
a
o
= 0.70(2)
LM
b
LM
c
LM
= 0.57(2)
= 0.74(3)
0.6
-30
0
a
30

60
90
b
V. Rusanov, Th. Woike, H. Kim, V. Angelov, Ts. Bonchev, S.
Haussühl, Solid State Commun. 126, 457 (2003).
Mössbauer parameters for MSI and MSII extracted from measured angle-dependent ratio of the left-hand to the righthand line effective thickness t1/2/t3/2. Three parameters for MSII given in bold differ substantially from those measured
for GS and MSI.
The dynamic properties of MSII
MSII
Transmission
MSII
GS
1.000
GS
1.000
0.975
0.975
0.950
0.950
0.925
0.925
Normalized area
14
0.900
0.875
12
GS
0.900
10
8
0.875
6
MSII
4
0.850
2
83 K
80
90
110
0.850
100
120
130
135 K
140
Temperature
0.825
0.825
-3
-2
-1
0
1
2
3
-3
-2
-1
0
1
2
3
Velocity, mm/s
Mössbauer spectra obtained at 83 K and 135 K from a SNP single crystal, b-cut, crystal thickness 490 m and natural
isotope abundance of iron, populated up to about 30 % with MSII. In the inset the normalized areas for GS and MSII
are given. In a temperature-dependent Mössbauer experiment new evidence in favour of the dynamic properties of
MSII is found. The normalized area of MSII is reduced by about a factor of three when going from 83 to 135 K. This
is not the result from a decay process of MSII to the GS (which occurs at about 150 K) because the normalized area of
GS does not change significantly. The change of the Lamb-Mössbauer factor fLM for MSII, however, is dramatic and
leads to a strong reduction of the normalized area.
Determination of the Mössbauer parameters of rare-earth
nitrosylpentacyanoferrates (RNP)
Counts, 10
4
e
138
III
Co
135
FeII
SNP
I
e
132
M S: 10.4%
129
FeII
CoII
CoIII
FeII
CoII
FeIII
126
-2
-1
0
1
2
3
680
670
LaNP
660
M S: 18%
650
-2
-1
0
1
2
3
430
GdNP
420
M S: 12.6%
410
-2
-1
0
1
Velocity, mm/s
2
3
A light-induced colour change, which is stable at
room temperature, has been observed in rareearth nitroprussides RNP. This internal
photochemical redox reaction has been
discovered by O. Sato, T. Iyoda, A. Fujishima,
K. Hashimoto, Science, 272,704 (1996) in
Prussian blue analogs K0.2Co1.4[Fe(CN)6]
·6.9H2O. The irradiation induced an electron
transfer from Fe to Co through the CN-bridges.
The iron and the rare-earth cations in RNP are
also connected via cyano bridges. The
photoinduced valency, spin state and molecular
magnetism can be observed because the rareearth cation has a vacant electron state and could
accept the electron transmitted via the cyano
bridges. This is not the case for SNP in which
this effect has not been observed.
V. Rusanov, Sv. Stankov, and A. X. Trautwein, Hyperfine Interactions, 144/145 307 (2002).
Conclusions
All nitroprussides can be switched by irradiation with blue light into two metastable states MSI and MSII,
which are stable below 200 and 150 K, respectively. The ground state GS and the metastable states MSI and
MSII are diamagnetic and, therefore, the photoswitching effects observed in nitroprussides can not be
explained by light-induced spin-crossover. The metastable state MSI corresponds to the linkage isomer of
the NO group, the so-called isonitrosyl configuration where the NO ligand is inverted and oxygen is bound
to the central iron atom. For the metastable state MSII was a side-on bonded configuration of NO proposed.
Mössbauer parameters of GS and MSI derived from Density Functional Theory calculations (DFT) are in
quantitative agreement with the measured values. For MSII such agreement is lacking. This discrepancy is
most likely due to the assumed static side-on configuration of the NO-group in the DFT calculations. The
experimental Mössbauer parameters of MSII could be reproduced by calculation when taking into account
the dynamic properties of the NO-group. In our opinion, in the liberation-rotation model of MSII, bending
vibration as well as rotation of the NO-group occur, the latter around the main molecular axis. Population
experiments were carried out on polycrystalline samples and confirmed the accessibility of light-induced
metastable states in the nitroprussides of rare-earth elements. The colour of these nitroprussides can be
changed (by light) and it remains stable at room temperature. Evidence for a new Light Induced Magnetic
Excited State (LIMES), observed for the first time in rare-earth nitroprussides, is provided. Photo-induced
changes in, structure, valency, spin and magnetization at low temperatures have been observed and are
discussed.
Acknowledgements
We would like to thank the Alexander von Humboldt Foundation, AvH (V. R. and A. X. T.), the
German Academic Exchange Serves, DAAD, (Sv. S.) and the German Research Foundation, DFG
(H. P. and A. X. T.) for supporting this research.