Photo-induced charge transfer in Prussian blue analogues as detected by photoacoustic spectroscopy

Article abstract of DOI:10.1016/j.saa.2006.11.013

The photo-induced charge transfer in four series of Prussian blue (PB) analogues was studied from photoacoustic spectra. In cobalticyanides the observed signals were assigned to a metal-to-ligand charge transfer, which appears as a shoulder below 450 nm, and to d-d transitions for Co(II), Ni(II) and Cu(II) complex salts. No evidence of metal-to-metal charge transfer was observed for this series, which is probably due to the high stability of low spin cobalt(III) in the hexacyanide complex. Photoacoustic spectra for ferricyanides are broad bands, which result particularly intense up to 750 nm. Such features were attributed to the overlapping of contributions from metal-to-ligand (<600 nm) and metal-to-metal charge transfer transitions, with probably also a minor contribution from d-d transitions in the outer metal. The spectra for the ferrocyanides series are dominated by the metal-to-ligand charge transfer band below 550 nm, approximately 100 nm above this transition in cobalticyanides. Within the studied solids, the most intense and broad metal-to-metal charge transfer bands were found for a series of low spin Co(III) high spin Co(II) hexacyanoferrates(II,III) and with similar features also for ferric ferrocyanide (Prussian blue), assigned to Fe(II) → Co(III) and Fe(II) → Fe(III) photo-induced transition, respectively. The first of these transitions requires of more energetic photons to be observed, its maximum falls at 580 nm while for Prussian blue it is found at 670 nm. Prussian blue analogues are usually obtained as nanometric size particles and many of them have a microporous structure. The role of surface atoms on the observed charge transfer bands in the studied series of compounds is also discussed.

Full text of DOI:10.1016/j.saa.2006.11.013

Spectrochimica Acta Part A 68 (2007) 191–197
Photo-induced charge transfer in Prussian blue analogues
as detected by photoacoustic spectroscopy
E. Regueraa,b,∗, E. Marın , A. Calderon , J. Rodrıguez-Hernandez
a
a
b
´
´
´
´
a Centro de Investigacio´n en Ciencia Aplicada y Tecnolog´ıa Avanzada del IPN, Legaria 694,
Col. Irrigacio´n, Me´xico, D.F. C.P. 11500, Mexico
b Instituto de Ciencia y Tecnolog´ıa de Materiales, Universidad de La Habana, Cuba
Received 26 September 2006; received in revised form 7 November 2006; accepted 17 November 2006
Abstract
The photo-induced charge transfer in four series of Prussian blue (PB) analogues was studied from photoacoustic spectra. In cobalticyanides
the observed signals were assigned to a metal-to-ligand charge transfer, which appears as a shoulder below 450 nm, and to d–d transitions for
Co(II), Ni(II) and Cu(II) complex salts. No evidence of metal-to-metal charge transfer was observed for this series, which is probably due to the
high stability of low spin cobalt(III) in the hexacyanide complex. Photoacoustic spectra for ferricyanides are broad bands, which result particularly
intense up to 750 nm. Such features were attributed to the overlapping of contributions from metal-to-ligand (<600 nm) and metal-to-metal charge
transfer transitions, with probably also a minor contribution from d–d transitions in the outer metal. The spectra for the ferrocyanides series are
dominated by the metal-to-ligand charge transfer band below 550 nm, approximately 100 nm above this transition in cobalticyanides. Within the
studied solids, the most intense and broad metal-to-metal charge transfer bands were found for a series of low spin Co(III) high spin Co(II)
hexacyanoferrates(II,III) and with similar features also for ferric ferrocyanide (Prussian blue), assigned to Fe(II) → Co(III) and Fe(II) → Fe(III)
photo-induced transition, respectively. The first of these transitions requires of more energetic photons to be observed, its maximum falls at 580 nm
while for Prussian blue it is found at 670 nm. Prussian blue analogues are usually obtained as nanometric size particles and many of them have a
microporous structure. The role of surface atoms on the observed charge transfer bands in the studied series of compounds is also discussed.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Charge transfer; Molecular magnets; Photoacoustic; Photo-induced; Porous materials; Prussian blue
1. Introduction
but with a relatively large stability for the excited state to attain
magnetic ordering below certain critical temperature (Tc). The
MMꢀCT transitions in PB analogues have also been studied for
potential applications of these compounds as electrochromic
materials, e.g. smarts windows and electrochromic displays [6],
since the charge transfer transition leads to a color change. The
MMꢀCT effect in hexacyanometallates is closely related to the
natureoftheCNligand. Ithasalowenergy␲-antibondingorbital
at the C end which overlaps with the t2g orbitals of the inner
metal (Mꢀ), providing a low energy route for the electron move-
ment between the metal centers. The deep blue color of ferric
ferrocyanide (Prussian blue) has been ascribed to a MMꢀCT tran-
sition among iron atoms [7]. The existence of that low energy
␲*-orbitals also allows the occurrence of metal-to-ligand charge
transfer (MꢀLCT) in this family of compounds. The optical
absorption spectra of hexacyanometallates are mainly related
to these two charge transfer mechanisms together with the pos-
sibility of d–d transitions in the involved transition metals. Such
spectra are usually recorded by UV–vis spectroscopy.
Prussian blue (PB) analogues or hexacyametallates form a
family of coordination compounds that has received a notable
attention as prototype of molecular materials in the last decade.
The CN group is a strong bridge ligand that allows a pronounced
charge overlapping between the metal centers linked at its C and
N ends. From this fact, PB analogues show interesting prop-
erties as molecular magnets, among them, high temperature
of magnetic ordering [1,2], pole-inversion magnets [3], spin-
glass behavior [4] and photo-induced magnetism [5]. This last
effect is related to the photo-induced charge transfer between the
metalcenterschangingtheavailablepopulationofelectronswith
unpaired spins and, in consequence the material magnetic prop-
erties. This is a typical metal-to-metal charge transfer (MMꢀCT)
∗
Corresponding author. Tel.: +52 57296000x67774.
E-mail address: ereguera@yahoo.com (E. Reguera).
1386-1425/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2006.11.013

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E. Reguera et al. / Spectrochimica Acta Part A 68 (2007) 191–197
spectral range. PAS spectra were normalized using carbon black
Photoacoustic spectroscopy (PAS) is a versatile technique
as reference sample in the usual way. More details about PAS
can be found elsewhere [8,9].
with increasing applications to Chemistry and Physics. The pho-
toacoustic spectrum results from acoustic waves generated by
the sample heating during the absorption of intensity modulated
light [8,9]. From this fact, this technique appears ideally suited
for the study of opaque objects, such as many PB analogues.
However, the available information on applications of PAS to
this family of compounds is scarce [10,11]. In order to shed light
on the scope of PAS for the study of PB analogues, spectra for
ferrocyanides, ferricyanides and cobalticyanides, were obtained
and evaluated. For cobalt hexacyanoferrates, compounds closely
related to the above mentioned photo-induced magnetization, a
seriesofmixedvalencestates(MVS)ofhighspinCo(II)lowspin
Co(III)hexacyanoferrates(II,III)weregeneratedbyapartialheat
induced charge transfer in Co(II) ferricyanide(Co(II) → Fe(III))
[12] and the resulting compositions included in this study (the
fourth series). The obtained results indicate that from pho-
toacoustic spectra valuable information on the photo-induced
charge transfer transitions in PB analogues can be derived.
3. Results and discussion
3.1. On the nature of the studied samples
IR spectra of all the studied samples show only the absorption
bands reported for hexacyanometallates [14]. The frequency val-
ues found for these bands are available as Supplementary data.
According to the EDS analyses, the M:Fe and M:Co atomic
ratios obtained for ferricyanides and cobalticyanides were close
to 3:2. Such atomic ratio corresponds to complex salts with
the unit formula, M3[Mꢀ(CN)6]2·xH2O, with Mꢀ = Fe, Co. In
what follows, we will denote these compounds as M3Mꢀ2. For
ferrocyanides, mixed salts containing potassium were formed,
with unit formula, MK2[Fe(CN)6]·xH2O, except for Zn. For
this last metal, the Zn:Fe:K ratio (3:2:2) indicates formation
of Zn3K2[Fe(CN)6]2·xH2O. The prepared PB reference sample
was found to be practically free of K, which corresponds to the
insoluble modification (IPB), Fe4[Fe(CN)6]3·xH2O.
2. Experimental
XRD powder patterns of the studied M3Mꢀ2 solids, includ-
ing those resulting from the annealing of Co3Fe2 (MVS series),
correspond to a cubic unit cell. PB analogues usually crystallize
in the Fm-3m space group (cubic) [15]. In this structure both
metal centers remain octahedrally coordinated and the unit cell
edge corresponds to the M N C Mꢀ C N M chain length.
The charge balance (stoichiometry) forces the existence of 1/3
of vacant sites for the molecular octahedral block, [Mꢀ(CN)6],
in ferricyanides and cobalticyanides and of 1/4 in IPB. These
The studied complex salts were obtained mixing 0.01 mol/L
aqueous solutions of potassium hexacyanometallates and of the
appropriate metal (M) sulfate, with M = Mn(II), Co(II), Ni(II),
Cu(II), Zn(II) and Cd(II). The formed precipitates were then
isolated by filtration, followed by successive washing to obtain
a filtrate free of the accompanying anions and cations. The
reagents used were analytical grade from Sigma and Merck.
The resulting solids were air-dried until constant weight. By the
same procedure, a PB sample (M = Fe(III)) was prepared. The
nature and chemical composition of the obtained samples were
established from infrared (IR) spectra, energy dispersed X-ray
spectroscopy (EDS), X-ray diffraction (XRD) and thermo-
gravimetry (TG).
The studied samples of MVS were obtained heating cobalt
ferricyanide under a N2 flow (200 mL/min) during one hour at
80, 100, 120, 140 and 160 ◦C, and then allowing the sample
cooling until room temperature within the used furnace. The
following samples will be labeled MVS80, MVS100, MVS120,
MVS140 and MVS160, respectively. The characterization of
these MVS compositions according to their crystal and elec-
tronic structures is reported elsewhere [12].
IR spectra were recorded using the KBr pressed disk
technique except for ferricyanides. Ferricyanides reduce to fer-
rocyanide, through a mechanochemical reaction, during the
disk preparation for IR spectroscopy [13]. XRD powder pat-
terns were obtained in the Bragg-Brentano geometry using Cu
K␣ radiation. The TG curves were recorded under a nitro-
gen flow (800 mL/min) using a TA Instrument thermo-balance
(2950 model) operated in the high-resolution mode. PAS spec-
tra were recorded in the absorption mode using a conventional
cell and pressed disk-shaped samples. The light coming from
a halogen lamp (operated at 700 W) was modulated at a fixed
frequency of 17 Hz with a mechanical chopper after it passes
through a monochromator with 1200 lines/mm diffraction grat-
ing in order to obtain monochromatic light in the 400–1000 nm
˚
vacant sites lead to a network of pores of about 8.5 A (diam-
eter) which, in the as-synthesized samples, remain filled of
coordinated and zeolitic water molecules. The outer metal (M),
always found at the pore surface, has a mixed coordination
sphere, M(NC)4(H2O)2 for M3Mꢀ2 and Fe(NC)4.5(H2O)1.5 for
IPB. These coordinated waters stabilize the zeolitic ones through
hydrogen bonding bridges. The hydration degree for the studied
samples, as derived from the TG curves (not shown), is relatively
high, from 10 to 16 water molecules per unit formula, depend-
ing on the polarizing power of the metal (M) [16]. Ferrocyanides
were found to be orthorhombic (Mn, Co, Ni, Cu and Cd) and
rhombohedral (Zn). These results agree with the reported crystal
structures for these compounds [17–20]. Their structures are free
of vacancies and potassium is an extra-framework atom, playing
the role of a charge balancing cation. Additional structural infor-
mation on the studied samples is provided as Supplementary
data.
The CN group is a very strong ligand and the metal (Mꢀ) coor-
dinated at its C end is always found in low spin configuration.
Hexacyanometallates are only formed for transition metals (Mꢀ)
with a maximum of six electrons in the nd shell [21], which in
the low spin configuration are accommodated in the metal t2g
orbitals. At the N end the CN group behaves as a less covalent
ligand and the metal M is found in high spin state except for
Co(III). In the MVS series, for instance, Co(III) is found with a
low spin configuration, t62ge0g [12]. The reported photo-induced

E. Reguera et al. / Spectrochimica Acta Part A 68 (2007) 191–197
193
water molecules in their coordination sphere, as average. The
assignment of these signals as d–d transitions is supported by the
reported optical transitions for these metals in an octahedral lig-
and field. The reported PAS spectrum for nickel chloride shows
3
two sharp peaks at 605 and 640 nm assigned to a 3A2g ← T1g(F)
transition for Ni(II) [24]. For Co(II) the optical transitions
have been calculated to be at 503, 529 and 562 nm ascribed
4
4
to a T1(F) ← T1(P) transition [25] and for Cu(II) a triplet
around 670 nm (2Eg ← T2g) has been reported [26]. However,
2
in Co(II), Ni(II) and Cu(II) cobalticyanides the observed tran-
sitions appear as a broad band and not as resolved sharp peaks.
Such behavior was attributed to the random distribution of the
[Co(CN)6] vacancies within the material structure. This leads
to different local configurations of ligands for the metal at the
pore surface, M(NC)6−x(H2O)x, and also to certain shift for the
corresponding optical transitions. The net effect is the observed
broad peak.
3.2.2. Ferricyanides
Fig. 2 shows the recorded spectra for the studied ferri-
cyanides. For Zn and Cd two possible transitions are expected,
a d–d transition within the low spin iron(III) atom and a MꢀLCT
band from that last one towards the CN groups. According to
¨
the Mossbauer spectroscopy results [16,27], the energetic sepa-
Fig. 1. Photoacoustic spectra of divalent transition metal hexacyanocobal-
tate(III). These spectra are composed by a MꢀLCT transition, observed as a
shoulder below 450 nm and d–d transitions for the complex salts of Co(II),
Ni(II) and Cu(II).
ration between t2g orbitals in ferricyanides is of the order of kT
(at room temperature). From this fact, the d–d transition in the
iron(III) atom falls in the IR region. The pronounced shoulder
observed below 600 nm was interpreted as due to the MꢀLCT
band. Zn and Cd ferricyanides, and also potassium ferricyanide,
charge transfer from Co(III) → Fe(II) through the CN bridges to
form Co(II)–Fe(III) species also involves a spin crossover, from
low to high spin, for the cobalt atom [5,22].
3.2. Photoacoustic spectra
3.2.1. Cobalticyanides
Fig. 1 shows the obtained spectra for this series of hexacyano-
metallates. For Mn, Zn and Cd no evidence of photo-absorption
in the visible spectral region was obtained, except below 450 nm,
an expected result since these compounds are white powders. Zn
and Cd have 3d10 and 4d10 as electronic configuration, respec-
tively, without possibilities of d–d transitions. Mn(II) has five
unpaired electrons (3d5: t23ge2g) but this result suggests that a
d–d transition involving spin pairing has a low probability of
occurrence. A d–d transition at the cobalt(III) atom is practi-
cally forbidden due to the nature of the CN ligand. The observed
shoulder below 450 nm was attributed to a MꢀLCT transition
from the cobalt(III) atom towards the CN group. These tran-
sitions in hexacyanometallates are usually observed at short
wavelength, close to the ultraviolet region [23].
For Co, Ni and Cu spectra in addition to the MꢀLCT shoul-
der, signals assigned to d–d transitions were observed. These
samples are pink, greenish and pale blue (cyan) powders, respec-
tively, and their photoacoustic maxima were found to be at
512 nm (Co), 606 nm (Ni) and 722 nm (Cu) (Fig. 1). These are
the colors observed for many hydrated salts of these metals,
e.g. sulfates and chlorides. As mentioned above, in the studied
materials these metals are sited at the pore surface with two
Fig. 2. Photoacoustic spectra of divalent transition metal hexacyanoferrates(III).
ThesespectraarecomposedbyaMꢀLCTtransition, observedasashoulderbelow
600 nm, and MMꢀCT bands overlapped with d–d transitions for the complex salts
of Co(II), Ni(II) and Cu(II).

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E. Reguera et al. / Spectrochimica Acta Part A 68 (2007) 191–197
are yellow solids, in correspondence with the absorption of light
from the blue spectral region.
For Mn, Co, Ni and Cu the photoacoustic signal covers
a wide spectral range for the visible region. The peak shape
profiles assigned to d–d transitions in Co, Ni and Cu cobalti-
cyanides are not clearly observed for ferricyanides. These broad
signals (bands) were interpreted as resulting from the over-
lapping of MꢀLCT, MMꢀCT and d–d transitions. For Mn the
curve appears as two partially resolved contributions, at 537
and 659 nm (Fig. 2), probably due to the MꢀLCT band and to
the Mn(II) → Fe(III) charge transfer, respectively. In the sec-
ond band also certain contribution from a d–d transition in the
temporally formed Mn(III) species could be present. The exis-
tence of Mn(III) hexacyanoferrates, with sufficient stability to
¨
be observed by IR and Mossbauer spectroscopies, have been
reported for ozonized Mn(II) hexacyanoferrates(II,III) samples
[28].
Cu(II) atoms linked at the N end of CN groups in hex-
acyanometallates show unique bonding properties. The CN
group has certain ability to donate electron at its N end
through a 5␴ orbital which has a slight anti-bonding charac-
ter and the copper(II) atom is able to receive charge at its eg
hole to favor an electronic configuration close to 3d10. This
results in a particularly strong interaction of the copper atom
with the CN groups. Such strong interaction leads to a short
M N C Mꢀ C N M chain length for copper hexacyanomet-
allates, to a low hydration degree for these copper salts, and
Fig. 3. Photoacoustic spectra of divalent transition metal hexacyanoferrates(II).
ThesespectraarecomposedbyaMꢀLCTtransition, observedasashoulderbelow
550 nm, MMꢀCT bands overlapped with d–d transitions (Co,Cu). For these last
two metals a strong absorption in the near infrared region is observed.
¨
also to the lowest Mossbauer isomer shift value for copper
sition in Ni(II) ferricyanide. The occurrence of MMꢀCT in
ferric ferrocyanide (PB) is a well known and an accepted fact
[6], even when ferrous ferricyanide only exist is an unstable
species [31].
within transition metal ferricyanides [29]. These unique bonding
properties for copper(II) in hexacyanometallates could explain
the existence of a MMꢀCT in copper ferricyanide. The effec-
tive charge on the copper atom is close to that expected for
Cu(I) and this allows the Cu → Fe electron transfer by photon
absorption.
3.2.3. Ferrocyanides
Co(III) ferrocyanide can be obtained as a stable species,
which through light absorption transforms into Co(II) fer-
ricyanide [5]. The inverse process in Co(II) ferricyanide,
but induced by heating, leads to formation of Co(III) fer-
rocyanide [12]. These two processes suggest that in cobalt
hexacyanoferrates the Co(II)–Fe(III) and Co(III)–Fe(II) elec-
tronic configurations remain separated by a relatively low
energetic barrier. The observed photoacoustic curve for the
cobalt(II) ferricyanide sample (Fig. 2) indicates that in this
compound certain degree of Fe(II) → Co(III) charge transfer
is taking place by light absorption, particularly around the
blue spectral region. During the wet synthesis of Co(II) fer-
ricyanide always a small fraction of Co(III) ferrocyanide is
formed [12], probably favored by the reducing effect of the
ferricyanide anion in solution. That fraction of Co(III) ferro-
cyanide could be the responsible for that MMꢀCT band in the
sample of Co(II) ferricyanide. Such MMꢀCT band has been
reported at 510 nm for Co(III) ferrocyanide clusters [23]. For Ni
the photoacoustic curve suggests the occurrence of a MMꢀCT
transition (Ni → Fe). Ni(III) ferrocyanide has not been reported
as a stable species; its formation is not detected even under
ozonization of Ni(II) hexacyanoferrates [30]. However, this
last fact does not discard the occurrence of a MMꢀCT tran-
Fig. 3 shows the photoacoustic spectra obtained for the ferro-
cyanidesseries. Thepronounced shoulderbelow550 nmappears
to be independent of the involved M metal and was attributed
to the MꢀLCT transition in the iron atom environment. For the
cobalticyanides series, with also a 3d6e0 electronic configura-
tion for the inner metal (Mꢀ), that shoulder was observed to be
below 450 nm. Such difference of 50 nm corresponds to a lower
energy barrier, by about 0.2 eV, between the ground and excited
states for the iron complex.
For Mn, Zn and Cd only the MꢀLCT shoulder was observed.
That behavior for theses three metals is consistent with the
resultsdiscussedabovefortheircobalt(III)analogues. Cobalt(II)
ferrocyanide shows a broad peak at 450 nm, probably resulting
from the overlapping of the highly probable d–d transition in the
cobalt(II) atom and the MꢀLCT band. The photoacoustic band
observed for this compound around 612 nm was ascribed to a
MMꢀCT band of Fe(II) → Co(III) resulting from the existence
of a small fraction of Co(III) ferrocyanide in the studied sam-
ple. Such band is similar to that found in the Co(II) ferricyanide
sample, which was attributed to the Fe(II) → Co(III) transition.
The formation of at least a fraction of mixed Co(III)Co(II) ferro-
cyanide during wet synthesis of Co(II) hexacyanoferrates(II,III)
is an expected result due to the relatively high stability of that

E. Reguera et al. / Spectrochimica Acta Part A 68 (2007) 191–197
195
species. Such mixed ferrocyanide has the following unit formula
[18]:
(Co(II))1.5x(Co(III))1−xK[FeII(CN)6]·yH2O
(1)
The Fe(II) → Co(III) MMꢀCT transition in that mixed ferro-
cyanidehasbeenintensivelystudiedrelatedtothephoto-induced
magnetic ordering [32]. In the spectral region where the d–d
transitions for Ni(II) and Cu(II) are expected, only weak sig-
nals were observed. Unlike to the cobalticyanide analogues,
where the outer metal is found with a mixed coordination sphere,
M(NC)4(H2O)2, in the studied ferrocyanides series it remains
linked to six N ends of CN groups (see Supplementary data). In
this last coordination environment, a higher fraction of electrons
from the CN groups populates the eg orbitals of nickel(II) and
copper(II) atoms and probably this fact reduces the probability
of transition for the expected d–d transitions and their relative
intensity.
According to the obtained photoacoustic spectra for fer-
rocyanides and also from those for ferricyanides, the studied
materials show an intense light absorption in the near infrared
spectral region (Figs. 2 and 3). For Ni(II) and Cu(II) ferro-
cyanides such absorption begins at relatively low wavelength,
about 750 nm. From our point of view, the available spectral and
structural information is insufficient to provide an unequivocal
explanation to that behavior.
Fig. 4. Photoacoustic spectra of mixed valence state compounds,
(Co(II))3−x(Co(III))x[(Fe(III))2−x(Fe(II))x(CN)12]·xH2O. Indicated is the
temperature used to obtain the MVS samples. For comparison the spectrum of
the IPB sample has been included.
3.2.4. Mixed valence state compounds
By heating a MMꢀCT can also be induced in certain coor-
dination compounds. Such effect is observed, for instance,
in cobalt(II) ferricyanide to form the mixed valence states
compounds Co(II)Co(III) hexacyanoferrates(II,III), according
to [12]:
is observed. For the MVS samples obtained in the 80–140 ◦C
range of annealing temperature, the maximum light absorption
takes place around 580 nm. The reported UV–vis spectrum for
the mixed salt Rb0.66Co(III)0.84Co(II)0.41[Fe(CN)6] shows the
Co(III) → Fe(II) transition at 550 nm [33]. The sample annealed
at 160 ◦C shows an atypical behavior with a significant contribu-
tion from absorption in the red and near infrared spectral region.
The heating of Co(II) ferricyanide above 140 ◦C induces charge
transfer but also favors a partial sample decomposition and lost
of crystal ordering [12,16]. Probably these structural and com-
positional changes are responsible for the observed contribution
from the region of higher wavelengths in the MVS160 sample
spectrum.
The photoacoustic spectrum for ferric ferrocyanide has its
maximum around 670 nm, to a difference of about 90 nm related
to the MVS series. This is equivalent to a difference of 0.29 eV
for the energy barrier among the ground and excited states in
Co(III) ferrocyanide and Fe(III) ferrocyanide. Compared to the
studied series of MVS complexes, in PB the photo-induced
charge transfer requires of light of lower energy.
Co3[Fe(CN)6]2·xH2O
→ (CoII)3−x(CoIII)x[(FeIII)2−x(FeII)x(CN)12]·xH2O,
(0 ≤ x ≤ 2)
(2)
The inverse MMꢀCT process corresponds to that discussed
above by photon absorption.
Fig. 4 shows the photoacoustic spectra for the studied MVS
series. For comparison, the spectrum of the IPB sample has
been included in that figure. For all these compounds a broad
band in the 450–850 nm spectral range is observed, ascribed
to Fe(II) → Co(III) and Fe(II) → Fe(III) MMꢀCT transitions in
Co(II)Co(III) hexacyanoferrates(II,III) and ferric ferrocyanide
(PB), respectively.
The crystal structure for the MVS series is a 3D network of
Co(II) N C Fe(III) C N Co(II) and Co(III) N C Fe(II)
C N Co(III) chains in the form of a polymeric solid
solution. As the annealing temperature increases, a higher
amount of Co(II) N C Fe(III) chains participate in the
heat induced Co(II) → Fe(III) charge transfer [12] and more
Co(III) N C Fe(II) chains are available for the photo-induced
inverse process. The obtained spectra for this series show cer-
tain sensitivity to the annealing temperature (Fig. 4). As the
treatment temperature rises, a progressive signal broadening
3.3. Surface atoms and photo-induced charge transfer
The photo-induced charge transfer leads to a change in the
electronic configuration for the involved atoms and ligands.
For a given atom the crystal radius depends on its electronic
configuration [34]. These two facts suggest that the charge trans-
fer transitions are particularly favored for surface atoms where
the surface “absorbs” the change in the inter-atomic distances

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E. Reguera et al. / Spectrochimica Acta Part A 68 (2007) 191–197
among the atoms participating in the photon absorption. The
energy transferred to the solid during the photons absorption is
usually insufficient to produce a cooperative expansion or con-
traction of its structure. For porous and nanometric size solids a
large fraction of the atoms are sited at the surface and for such
materials these atoms are able to participate in photo-induced
charge transfer processes.
in the electronic configuration for both metals and ligands,
and since such change produces a variation in the inter-atomic
distance among the involved atoms, the photo-induced charge
transfer transitions must be favored for surface atoms. For such
atoms the surface “absorbs” all the local distortions generated
in the solid network.
Prussian blue analogues are usually obtained with a nanomet-
ric particle size and, in addition, many of them have a porous
structure [27,35]. For the studied cobalticyanides, ferricyanides
and MVS series, and also for the IPB sample, always the outer
metal (M) is found sited at the pore surface. These two features
of Prussian blue analogues and the strong charge overlapping
among the metal centers that the CN ligand allows, explain
the frequently observed charge transfer bands for this family
of molecular materials. For the photo-induced charge transfer
in Co(IIII) ferrocyanide to form Co(II) ferricyanide a variation
5. Supplementary material
Supplementary Information associated with this arti-
cle is available from the online version, and includes the
IR spectral data, the cell parameters and atomic pack-
ing within the unit cell for all the studied samples. The
calculated atomic positions, occupation and temperature
factors and the estimated inter-atomic distances and bond
angles derived from the structural Rietveld refinements
have been deposited at ICSD Fachinformationszentrum
Karlsruhe (FIZ) (email: crysdata@fiz-karlsruhe.de) with
CSD file numbers: 416745: Mn3[Co(CN)6]2·14H2O;
416742: Co3[Co(CN)6]2·14H2O; 416746: Ni3[Co(CN)6]2
·14H2O; 416743: Cu3[Co(CN)6]2·10H2O; 416744: Zn3
[Co(CN)6]2·13H2O; 416741: Cd3[Co(CN)6]2·13H2O; 416994:
Mn3[Fe(CN)6]2·14H2O; 416993: Co3[Fe(CN)6]2·14H2O;
416997: Ni3[Fe(CN)6]2·14H2O; 416995: Cu3[Fe(CN)6]2·
10H2O; 416998: Zn3[Fe(CN)6]2·12H2O; 416996: Cd3[Fe
(CN)6]2·14H2O; 417021: (Co(II))3−x(Co(III))x[(Fe(III))2−x
(Fe(II))x(CN)12]·12H2O; 416404: Co(II)K2[Fe(CN)6]·2H2O.
˚
of 0.2 A in the Co–N inter-atomic distance has been reported
[36,37]. During the annealing of Co(II) ferricyanide to obtain the
MVS compositions a progressive cell contraction was observed,
which amounts 2% of the cell volume reduction for the sample
annealed at 140 ◦C, where the ferrocyanide fraction formed is
close to 50 % of the sample weight [12].
The porosity of a given solid influences its thermal properties,
thus affecting the intensity of the corresponding photoacous-
tic signal. However, it is a very well known fact that one of
the advantages of PAS respecting other kinds of optical spec-
troscopy is that the position of the absorption bands in the signal
becomes almost insensitive to light scattering by the sample
[38,39], as taking place within porous samples [40], such as
our pressed ones. Therefore, the obtained information on the
photo-induced charge transfer processes was not affected by the
sample’s porosity. Moreover, it is worth to notice that all our
disk shaped samples were prepared under the same pressure
conditions
Acknowledgments
The authors thank Eng. M. Godinez for his help during the
PAS measurements. The partial support from the CLAF-ICTP
Small Grants Program and SIP-IPN Projects 20060097 and
20060062 is acknowledged.
Appendix A. Supplementary data
4. Conclusions
Supplementary data associated with this article can be found,
in the online version, at doi:10.1016/j.saa.2006.11.013.
The photoacoustic spectroscopy appears as versatile tech-
nique to detect photo-induced charge transfer transitions in
molecular materials based on the Prussian blue analogue. Many
of these materials are highly opaque solids, which absorb a
large fraction of the incident radiation. The absorbed energy
is then liberated in the form of heat, which is responsible
for an intense photoacoustic signal. For cobalticyanides, the
observed signals correspond to a metal-to-ligand charge trans-
fer at the cobalt atom, detected as a shoulder below 450 nm and
d–d transitions in Co(II), Ni(II) and Cu(II). In ferricyanides,
the spectra reveal light absorption in practically all the visible
region except for Zn and Cd. Such wide absorption region was
attributed to the overlapping of MꢀLCT, MMꢀCT and d–d tran-
sitions. The spectra for the ferrocyanides series are dominated
by the MꢀLCT transition except for cobalt and iron, where also
MMꢀCT bands were observed. The most intense MMꢀCT bands
were found for the MVS series and the IPB sample, attributed
to Fe(II) → Co(III) and Fe(II) → Fe(III) photo-induced electron
transfer. The MꢀLCT and MMꢀCT transitions lead to a change
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