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Intersystem crossing in Fe(II) coordination compounds
275
C~rd~~t~n ~~~~t~ Reviews, 111 (1991) 275-290 Eisevier Science Publishers B.V., Amsterdam
Andreas Hauser, Institut fiir Anorganische Staudingerweg
Chemie , Johannes-Gutenberg-UniversitXt,
9, D-6500 Mainz, FRG
Abstract Due to the fact that for d6-systems there are a number of low-lying ligand field (LF1 states the relaxation from excited states of Fe(D) coordination compounds is, in general, a very Fast and radiationless process. In Fe(%) spin-crossover systems, however, the zero point energy difference between the two lowest states, namely the lowspin &Si ‘A, and the high-spin (HSi ?a state, is of the order of k,T, and some systems can be converted quantitatively to the HS state well below the thermal transition temperature by irradiating either into MLCT or LF absorption bands of the LS species, with HS+LS relaxation rates as small as I@ s-l at -10 K. It is also possible to achieve a light-induced transient population of a HS state in Fe(%) LS compounds, but in this case the HS+LS relaxation rates can be larger than IO6 s-l even at law temperatures. The HS+LS relaxation rates show strong deviations from Arrhenius kinetics with nearly temperature independent tunnelling below * 70 K and a thermally activated behaviour above -100 K. The range of 12 orders of magnitude in the low temperature tunnelling rate can be understood in terms of nonadiabatic multiphonon relaxation, where in the strong coupling iimit, with the Huang-Rhys parameter S much larger than the reduced energy gap p, an inverse energy gap Iaw holds.
1. INTRODUCTION Compared to the coordination photophysical
and photochemical
received very little attention. chemistry
“Concepts
with 6 d-electrons due to the fact iuminescence, a very efficient timescale.
of most other transition
properties
of Fe(%) coordination
in Inorganic Photochemistry” without
metal ions the
compounds
In fact one of the standard works of inorganic even mentioning
that Fe(%) coordination
devotes
two sections
Fe(%) compounds
compounds,
because of a number of low-lying
have photo-
to systems
HI. This is basically
in general,
do not show
any
ligand field (LF) states which provide
nonradiative decay path back to the ground state on the subnanosecond
Thus the important
spectroscopy,
compounds
experimental
with which quantum
internal conversion
Wf processes
metal ions have been determined,
method
efficiencies
in coordination
of time resolved
for intersystem compounds
luminescence
crossing
(IX)
and
of most other transition
cannot be applied to Fe(%) compounds.
OO~O-~~5/91/$05.~ 0 1991 Elsevier Science Publishers B.V. All rights reserved,
That despite this efficient might possess Decurtins
nonradiative relaxation
some unique photophysical
Fe(B) coordination
properties
et al. CZ,31 showed that in a number of Fe(H) spin-crossover
quantitative
light-induced
ratures well below
IA, (LSl+STz
(HSl conversion
the thermal transition temperature,
the visible either into the spin allowed species.
In the spin-crossover
for instance,
d-d
system
the HS++LS relaxation
compounds
emerged some years ago, when compounds
a
could be achieved at tempeby irradiating the sample in
or MLCT absorption
IFe(ptzl,l(BF,l,
(ptz
bands
of
the LS
= l-n-propyltetrazolel,
rate is less than lob6 sWi at 10 K, and thus the
light-induced metastable state could unambiguously be identified as the ‘T, state. The mechanism for the light-induced LS+HS conversion, subsequently called “LightInduced
Excited
involving
Spin State Trapping
(LIESST)“, proposed
Since the discovery of LIESST long-lived does the light-induced low temperatures. function
metastable
> 100 K, but below
-70
sulfonate.
CFe(pic131C12.MeOH (pit
q
possible
compound
Z-picolylaminel
tunnelling
spin-crossover
to create alight-induced
tion is considerably
faster even at low temperatures
The crucial other.
parameters The theory
s-l.
For the
system a similar behaviour s-l C3, 61.
state
in LS systems
171. of the metastable
of the LS and HS potential
nonadiabatic
multiphonon
relaxation
HS state wells
are
relative
C83, as first
by Buhks et al. C91, can be used to explain
the
variation of the low T tunnelling rates over 12 orders of magnitude.
The discussion constiute
of
ISC processes
-lo4
for T
kinetics
, but in these systems the HS+LS relaxa-
the lifetime
and vertical displacements
to HS+LS
observed
governing
rate of
metastableHS
(bipy = 2,2’-bipyridinel
at
as a
CFe(mepylz(pyl(tren11(PF6)2
T tunnelling rate of *lo-’
such as [Fe (bipyl,l’+
the horizontal
relaxation
a strong deviation from Arrhenius
independent
but in this case with alow
It is also
long lifetime
the HS+LS
They found a thermally activated process
K they observed
with a more or less temperature is observed,
CSI investigated
of the spin-crossover
in poly-styrene
C41.
systems. However, not for all compounds
state have such an astonishingly
Xie and Hendrickson
of temperature
embedded
to each
et al. 131,
metastable HS states have been found for
an increasing number of Fe(D) spin-crossover
applied
by Decurtins
two ISC steps with AS = 1, has recently been proved to be correct
of
the
the two central
mechanism
of
LIESST and of
the HS+LS
relaxation
themes of this paper.
2. LIGAND FIELD THEORITI-ICAL CONSIDERATIONS According d-electrons
to the Tanabe-Sugano
(Fig. I) octahedral
diagram Cl01 for a transition
Fe(D) complexes
metal ion with 6
can either have a ‘T,
(HS) ground
state for ligands exerting but a weak LF such as H,O, or a IA, (I...% ground state for ligands exerting a strong LF such as CN-. If for a given ligand the LF strength
happens
211
to be In the vicinity of the crossover In order to actually
understand
point Acrlt, a thermal spin transition
this, it 1s not sufficient
to just look at the Tanabe-
Sugano diagram. Such a diagram only depicts the electronic states relative to the respective
ground state as a function
The LF strength is not only determined by the properties function
of the metal-ligand
may occur.
energies
of the excited
of the LF strength
1ODq.
of the ligand but it is also a
distance r. For neutral ligands iODq (i-1 _ l/r6 Clil. Thus
If for a given ligand the LF strength at the equilibrium
distance of say the LS state
10DqLS = 10Dq(rLsI is known from experiment, lODq(r1 for an arbitrary value of r can be estimated
for this ligand, and assuming an appropriate
potential
surface for the IS
state along the totally symmetric normalcoordinate,
the energies of all the
other LF states along this coordinate can then be calculated. state,
for
instance,
potential
surface
displaced
For the HS the
resulting always
be
towards a substantially
lar-
ger metal-ligand
will
distance irrespective
of the ligand (Fig. 2i.This is not surprising, since in the ‘A, state the 6 d-electrons bonding
occupy the basically non-
t2-orbitals,
whereas
in the
sT, state two of those electrons promoted
to the antibonding
tals. The vertical displacement HS surface,
of the
on the other hand, de-
pends strongly HS system
get
e-orbi-
it
Ds cm-’
upon the ligand. In a will
of
course
be
Fig,l:Tenabe-Sugano
dlagramforad6-system.
:
Fig. 2 for
Potential
the
state
HS
of
end
wells the
LS
Fe'e(II)In octa-
hcdralcoordinatlonrlong the
totally
normal
Normal
Coordinate
Q,,
symmetric
coordinate.
218 displaced
towards
a thermal
lower
positive
and of
populated entropy
driven
electronic
population
degeneracy
ligand
Fe-L
bondlengths
spin-crossover tionality
a factor
the crossover
The
CFe(ptz),l(BF,),
version but
discovered
it is with
mechanism whole
to just
The thermal It manifests purple tion
spectra
of
transition
LS state
with
has
scheme
for
change
rHs
- rLs
from
in Fe(E)
the propor-
is expected So in fact
a very
to
increase
for
a Fe(E)
to the right
important
role
of the light-induced properties been
LIESST.
This
well
in
LS +HS
the
con-
of CFe(ptz),l(BF,),.
possible
has been
in colour
from
to
justifies
discussed
colourless
In Figs. 3a and b the corresponding Referring
prove
the
devoting
a
of
the
HS species
of almost
experimentally.
token
with
- E(‘A,) a factor
+ (E(iT,)
- E(‘T,)
of 2 on going
absorp-
diagram
(Fig.
spin
= E(‘El
bands
transitions
C3, 131.
crystal
to the
10DqHS
the two
to the spin allowed
with 10DqLS * E(lT,) increase
single
the 293 K spectrum
By the same
before
at 293 K to deep
once again to the Tanabe-Sugano
in the NIR of
can be assigned
the predicted
never
(LIESST)
eventually
in CFe(ptz),l(BF,l,
characteristic
is confirmed
plays
of the optical
it
a dramatic
the band
and ‘AI+rTZ,
can
a
are
one system.
is straightforward.
the 20 K spectrum C141. Thus
this
are shown.
= 11760 cm-’
a full
temperature.
the assignment
rAr+rT,
system
spin transition
itself
at low
‘T,qSE
spin-crossover
Not only was the phenomenon that
for
-1.96 - 2.00 A and the
ArHL=
TRAPPING
system
and establish
section
that
configuration
LF strength
spln-crossover
compound
1S-fold
states.
diagram.
SPIN STATE
during an investigation
this
the
means,
- 0.21 A C121. Thus
the
be
state.
the HS to the LS state.
Tanabe-Sugano
EXCITED
of LIESST.
nuclear
rLs are characteristically
from
of
of vibrational
10DqHS will be well to the left and 10DqLS well
3.1 The CFe(ptz),l(BF,),
discussion
this
will
quantitative
the HS and the LS state
of the ground
= (r HS/rLS)6,
in the
LIGHT-INDUCED
that
because
for
has to be
the LS state
now simply
which
In order
AE&_
an almost
place,
the HS and the LS state
of 2 on going point
only
density
diagram
is in the range -0.16
compound
case
take
for
configuration
between
compounds
spin-crossover
3.
it is clear
in the LS state
( 10DqLS/ 10DqHS)
by almost of
but
difference
will
energy.
difference
temperatures
and its higher
configuration
higher
energy
21, in which
at higher
in the Tanabe-Sugano
is a nuclear
towards
point
the HS state
to the equilibrium
bondlength
(Fig. but
of
degenerate,
correspond
k,T
the zero
of the HS state
point
there
accidentally
of
temperatures,
The crossover given
to occur,
the order
at low
in a LS system
energy,
spin transition
in the of
-
Ec5T,)
visible
the IS
11,
allowed of
species
) /4 = 20200 cm-’ from
the HS to the
279
Fig.3: Single crystalabsorptionspectraofCFe(ptz),ll(BF,), K. b) at20
afat
K.c)
after
Irredlat)onatS¶4.5nm,dlaftcr clt820nm,forcomparfsonthe 20 K
normal sow. 1cml. fin.
1
6”
c)
2woo. 25wo.3MoQ
J5ow.
‘Thermal Spin Crossover 128 K 135 K t
tight-
Included, at 980
nm(from
In Fig. 4 the transition curve for the thermal spin transition of effects
crystallographic
irradiation ref. C41).
~0’ 4
as derived from the temperature dependence of the ‘Al+tT, cooperative
spectrum f--f is
e) after
CFe(ptz)61(BF,),
band is shown. Due to
the spin transition in the neat material
is accompanied
by a
phase transition with a hysteresis of 7 K (T: = 128 K, T,$ = 13.5Kf.
280 The
transition
curve
mixed crystal
for
CZnr_,Fex(ptz161(BF~)2
(x 23 0.11, included more
gradual,
simple tion the
in
spin equilibrium. temperature
fraction
yHs
system
is -9s
the
laser,
Ti,z,
after
the typical cally HS+LS
line
the
‘T,+‘E
3T 1 state.
an Ar+
‘A,+‘T,
band,
band
lAi+
sT,
Fig. 4 : The
HS
the thermal
spin transition
CD) and
by Decurtins
if
that
optically
pumped
the
temperature
being
However,
repeated
irradiation
reached.
The
Indeed
with
a practithermal
is depicted
in Fig. 5.
be
of
are two
weak
the spin forbidden these
weak
bands
with the spectrum
crystal
value for
with
bands
the can
‘A,+ST, low
‘AIa3T,
the
unity,
be,
state
also band,
be the
LS spectrum
complete.
that
band
‘E
the absorption
sT,+SE
is not
the
yLs
After = 0.9 is
LS species
in such a way,
that
conversion. energy
side
and iA,+3T2
to CFe(ptz),l(BF,),
of CZn(ptzl,l(BF,),.
below
the typical
would
the ‘T,
not
system
the LS fraction
ST,+SE
on the
transitions
for
this the
are
Fig 3d shows Again
into
of the energy
to the HS state.
the
conversion
to the
A in metal-ligand
to just
indeed
the cycle.
ST,+‘A,
are intrinsic
IT, state
steps
at 820 nm, i. e., into
in a light-induced
very
-0.18
various
3T, state And
reason
overlapping
results
of
the
(reverse-LIESST).
at 20 K all through
straightforward
in turn
trapped,
the whole
reversible.
irradiation
excited
can drop
convert
the light-induced
band
LIESST
the initially
because
at 820 nm a “saturation”
most
to which 6). That
comparison
kept
absorption
there
(a).
a noticeable
AS = 1, the system
puts the low-lying
should
a subsequent
absorption
of
(x in 0.1)
it remains
efficiencies
back to the IS state
is observed.
has a weak
does
due to the difference
eventually
LF theory
from
again with
quantum
will
LIESST
after
as function
In CFe(ptz)bl(BF4)s
is metastable
-5OK
et al. I31 for
low temperature
LS and HS state
that
yHs
CZn 1_xFex(ptz)61(BF4),
above
the system
ISC step,
excitation
fact
fraction
HS state
at temperatures
at sufficiently
suggests,
250.0 T [Kj
with
proposed
Even
spectrum
200.0
,
set in C3, 1.51.
between
continuous
150.0
..
in the NIB,
Only
In a second
bondlength.
(Fig.
at 20 K
of
lifetime.
where
I
as
ISC step with AS = 1 leads
The
100.0
I’
dilute
the irradiation,
The mechanism
barrier
50.0
‘.
HS
The absorption
relaxation
A first
I
in Fig. 3c. At 20 K the light-induced
infinite
state,
the
the crystal
(LIESST).
is shown
a
the
leads to a complete
spectrum
is
for
I “O”
K.
i. e., into
quickly
4,
defined
= O.S, for
S14.S nm
conversion
3
The transi-
at which
Irradiating with
band,
Fig.
as is expected
temperature
this
the dilute
of
‘A,+‘T,
can be assigned
is borne
In this spectrum
the out
by the
the comparatively
281 sharp lines corresponding tional overtones tions
are
identical
CFe(ptz),l(BF,),, bands
to
those
in
but the two broad
are missing.
triplet
to vibra-
of the ligand vibra-
states
Since
originate
the two
from
the
same configuration
as the two exci-
ted singlet
namely the the
states,
(t; e) configuration,
the absorption
bands of the spin forbidden
transi-
tions are just as broad as those of the spin allowed
ones.
The maximum
of the ‘A1+3T,
band at 10280 cm-’
t-980
nm) is
just to the low energy side of the ‘T, e5E band. Starting fraction
of
yLs
irradiating
1 at
the crystal
again results
20 K, and at 980 nm
in a complete
induced ‘A,+*T,
I~~a~io~ at S14.Snm -
from a LS
980.0nm -
light-
conversion
Irradiation al 820.0nm
(Fig 3e).
A direct irradiation into the 1A,+3T, band
therefore
results
in LIESST
too, proving two things : The two weak bands are not due to impurities
Nuclear Coordinate
and therefore the assignment to spin forbidden and,
more
that
the
transitions
is
importantly, mechanism
justified it
Fig. S : Potential state
shows
For LIESST
with the 3T, state as intermediate state is correct.
wavelength
excited
spin-crossover
system.
the mechanism
of
k
ground “T2 state4
LF states
Arrows
of
LIESST.
the
value for yLs, since this absorption band, whereas
although not zero, is minima1 at this wavelength.
and the buildup of the IS state in LIESST and reverse-LIESST
quantitatively.
The quantum efficiencies
can
of the first ISC step either from
the IA, state far LIESST or the SE state for reverse-LIESST tively. The branching
a d’
indicate
are equal but not zero. The value of 0.9 reached
steady state to the maximum of the ‘T,+%
of the IS species,
The depletion be followed
higher
at 820 nm is the maximum
corresponds
the absorption
the
the
accessible
be called steady state value, since For this value of yrs
rates for LIESST and reverse-LIESST by irradiating
as wall
for
value of yLs = 0.9 reached after long irradiation
In view of this the “saturation” at 820 nm had better
wells
and the thermally
are ... 1 and +,0.7s. respec-
ratio from the 3T, state either to the ‘A, ground
state or the
282 metastable
ST,
state
processes,
except
is
1: 4 C41. The
of course
actual
the ‘T,+‘A,
rate
relaxation,
constants
for
are z 10”
s-r.
the
Fig.6: Blow energy trum
up
den
I ., 10000.
5000.
LIESST
In other
LIESST
is quite
it is basically states only
of
CFe(pta),XBF,),
neat
In most resulting
larger
and irradiation In a certain
Ci61, but
however, overlap
the MLCT
sense
the
LIESST
relaxation
rates
number
dilute
in polymer
MLCT
laser
and Lawthers
irradiation
of the order
states
general
of lo7 s-l
3T,. Street
CFe(phen).$*+
for this state
ef aL Ulal
at 2.93 K in solution
basically
either
tentatively
agreeing
with
a 3MLCT
identify
state
this
last
HS states Prior
point
time-differential
and the LS state,
too
C3, 171.
depopulation
of
compounds,
and at ambient
tempera-
Even for the LS systems
found.
state Kirk
or the low-lying
witha
is the
Mijssbauer
life-
et al. CZOI list LF states
and Creutz
out that
suggestion
by low-
to the discovery
a rapid
a transient was
in the
spin-crossover
it with the 3T, state
this assignment,
‘T 2 state. With hindsight of course It is furthermore supported by the
intense
short-lived.
ns CZOI and 0.71 ns 1211, respectively
not
171, and
are obscured
than that. Fe(B)
systems, Cl&
in the LS state
long-lived
in solution
were comparatively
and HS -
Cl81.
1191 noticed
for some
compounds
crystals
are more
and
although
(from
metastable
spin-crossover
mixed matrices
bands
time
as possibilities
Long-lived
Fe-L bondlength
CFe(bipy)a12+ 0.83
spin-crossover
of Fe(B)
for
is even more
upon pulsed
the light-induced
Fe(B)
molecule.
band at 10 K produces
et al. C&31 McGarvey
but with
for
of the shorter
the ‘A,
of
transitlons
. I 20000.
the d-d bands of the LS species
of Decurtins state
cm
also
embedded
But because
orbital into
.
-,
complex
for an increasing
complexes
bands.
.
phenomenon
of the single
compounds
systems,
MLCT
I . 15000.
a common
a property
for spin-crossover
tures
d-d
at
the spin forbid-
systems
have been found for
lying
of the low
ref. C41).
0.0 -
3.2
ISC
part of the LSspec-
20 K with
0.5 -
various
it could most
‘T,
also
be the
probable
emission
or
et aL CZlbl,
study
one. of
283 Grimm et
l221, who observed
al.
anomalous
HS states
nuclear decay in Cs7Co/Co(phen133(C10J~~2H,0. Mirssbauer consists
emission
spectra
at 40 and f72
of just the same LS doublet
is observed spectrum
in a Massbauer
indicating process
HS
the
The 172 K spectrum
as
states
cha-
appear,
that after the 57Co (EC) s7Fe at least
complexes
of
some
the
Fe(E)
end up in a HS state with a
lifetime on the Mijssbauer timescale
of
IO”? s. Grimm et si. E221 determined lifetime
reproduced.
are
of
integral
but at
40 K additional quadrupol doublets for
effect
absorption
of [Fe(phen)~l(ClO,~z,
racteristic
K
as an after
In Fig. 7 the corresponding
a
of 205 ns at 47 K for the HS
state marked with HSl in Fig 7b. This compares found
well with the 188
for
at -SO K
ns
the light-induced
transient
state for CFe(phenl,12* embedded Nafion polymer
in a
0
foil Ci71.
0
-2
-4
2
4
v/mms-’
The
quantum
light-induced stable
HS
efficiency
population state
in the
CZnr-xFex(bipy),l(PF,)z is with -0.6
for LS
Fig. 7
system
:
Integral
spectra
(x = 0.00011
of
“Co
MiSssbauer
emission
CS7Co/Co(phen),l(C10~)2.
at a) 40 K md
b)
172 K (from
2H,O ref.
C233).
Cl21 of the same order of
magnitude as the one determined except
the
of the meta-
for the lifetime
the photophysical
for the CFe(ptzl,,l(BF,l,
of the ligh-induced
HS state,
spin-crossover
system.
So,
there is no big difference
in
behaviour of Fe(H) Ls and spin-crossover
In analogy to reverse-LIESST stable LS states in HS compounds
compounds.
it should in principle be possible
to create
HL< 0 by irradiating into the ‘T,+‘E with AE”
tion band. However, in order to have an nonvanishing quantum efficiency
metaabsorp-
for the first
ISC step, the ‘E state must be higher in energy than the 3T, state, and this is only the case for LF strengths substituted system
CFe(mtzl,l(BF,l,
the complex
fairly close
to the spin-crossover
such a LS+HS conversion
molecules
sit on two different
on site A show a thermal spin transition
systems.
In the methyl
is indeed possible
crystallographic
C241. In this
sites.
The ones
with Ti,z 4 72 K and behave pretty
much
the same way as the CFe(ptz)61(BF,lz system. The ones on site B remain in the HS state at all temperatures, but can be converted to the LS state by irradiating in the NIR. At 10 K the lifetime of this now metastable LS state is again practically infinite, only above -45
K does a noticeable LS+HS relaxation set in.
284 THE HS +LS
4.
RELAXATION
Work on the LS *HS ISC dynamics in Fe(H) spin-crossover
compounds
has mostly
been performed in solution at ambient temperatures CZSI. Over the limited temperature interval available in solution 1nCkHLl vs. l/T plots Mrrhenius plots) usually result in straight
lines, and consequently
of absolute
the results have generally been discussed
in terms
rate theory. But as had already been pointed out by Buhks et al. C91 from
theoretical
considerations
and was confirmed
C51 the HS +LS relaxation which classical
concepts
Fig. 8 shows
is a highly nonadiabatic
by Xie und Hendrickson
process,
for the description
the Arrhenius plots of the HS+LS relaxation
pulsed laser excitation.
compounds
determined
rate constants
in transient
for a
absorption
after
For the dilute mixed crystals studied the determination
from the relaxation
slngle exponential
of
are not really adequate.
number of Fe011 coordination rate constants
experimentally
cur ‘ves is straight
forward
of the
as the relaxation
is
El?]. Ail five systems show strong deviations from simple Arrhenius
behaviour. At elevated temperatures the
relaxation process is thermally -
activated,
but at low
the relaxation
rates
temperature towards tunnelling
temperatures are much less
dependent,
a temperature
going
20.0
2 s
15.0 10.0
independent
5.0
rate for T + 0. In table I
the low temperature
tunnelling
constants
and the activa-
tion
z
k,,fT+O)
parameters
Ea and A for
high temperature
region
rate
0.0 -5.0
the
T 2 100 K
-10.0 I
are listed. in addition the low T tunneling rate constants systems
l l
for some other
-15.0
are included in table I. The
[~n:f+-+(~~)&~F,),
f....l....f....f
0.00
..‘.I
0.02
0.01
0.03
low T tunnelling rates are spread over
l/T
more than twelve orders of magnitude from
< lob6 s-l
over
system
(X *
IZnt_xFex(ptz)63tBF,)2
0.1) to 1) > 106 s-* for the LS sys-
tern CFe(phen),12+embedded
different
occurring
simple
into vibrational model
plot of the experimental
rate constants
k,&I’,
mixed
systems
crystal
for
same
(from
dilute
FeCIX)
ref. E171).
multiphonon
relaxation
the HS +LS ISC is described
between two distinct zero order spin states characterised
nuclear configurations,
is transformed
: Arrhenius
[K-l1 _ _
in Nafion.
In the theory of nonadiabatic as a process
most
Fig. 8
for the spin-cross-
0.05
0.04
treats
space, with the potentials
during which the electronic
energy of the final state in a nonadiabatic
this problem
in single configurational
displaced by AQ,,
by
energy of the initial state step. The
coordinate
(SCC)
along one, namely the totally symmetric,
285
Table I : Transition temperature A for the high temperature
T,,z, activation energy
region, and low T tunnelling
Ea and frequency rate constant
factor
kHL(T+O)
for some Fe(H) coordination estimated
compounds. The reduced energy gap p has been to the “inverse energy gap law”.
according
T112
Ea
CKI
[cm-‘1
CZni_xFex(ptz),l(BF~I,
9.5
CMnr_xFex(pic)al Cl; EtOH
a)
kH,_(T+O)
c s-11
1100
cs-‘3
5x107
*5x10-7 6 x lO-6
95
CMn1_xFex(pic)31CI,*MeOH CZn,_xFex(pic),lC1z~MeOH
P
76
a)
CZnr_xFex(pic)al Cl;EtOH
A
2.5x10-s
118 a)
907
2x10s
-1-Z
2.5x10-3
140
9 x 10-a
CZni_xFex(mepy)a(tren)l(PF6)2
210
CFe(mepy)z(py)(tren)12+in
PMMAb’
270
~SXlO’
CFe(mepy)(py)z(tren)l*+in
PMMAb)
370
-2x102
CZni_xFex(py)3(tren)I(PFJz
sx1os
a37
-3-4
1.4x10-’
> 400
640
1 xl09
-7-8
4x102
CZnr-xFex(bipy)sl(PF,)2
low-spin
364
2x109
-11-13
6~10~
CFe(phen1312* in Nafion b,
low-spin
a)
from ref. C261
normal coordinate
-5x106
b’ Teff 4 SO K (see ref. CIZI)
only. With AS = 2 only higher order spin-orbit
coupling
can mix
the two states. Starting from Fermi’s Golden Rule and using the Condon approximation the HS+I_S relaxation
rate constant k,,(T)
where
the electronic
strengths
=
&
2s
matrixelement
near the spin-crossover
is given by Cal : F,(T)
pH,_ = <0= point
(4.1)
,
I H,, I@,,,>
C91. For harmonic
w 150 cm-’ potentials
for
LF
with equal
force constants
f and vibrational frequencies o, the thermally averaged Franck-Condon
factor
given
F,(T)
is
by :
2 I12. F,(T)
=
e-mno’kT
-mho/kT Lie
(4.2)
286 where te sum goes over all the vibrational levels rn of the initial state, and n = m + p (the reduced energy gap p = AE&/hwl T + 0 F, simplifies
Fp(T+o)
where the Huang-Rhys mean force
in order to ensure energy conservation.
constant
= ii~
factor
tunnelling
for
with 6-fold
N-coordination
is the reduced
S will not vary to
energy gap p. In Fig. 9a
for is plotted
i/T
for
and the thermally activated region, are well reproduced.
The low T
rate k,, (T+O)
features of the experimental
against
namely the
is predicted
to increase
Tl,z is only a crude
measure
exponentially for
AI&_,
with increasing
the observed
system CZn,_xFe,(ptzl,l(BF,),
for the LS systems
-15.0~
0.01
Fig. 9 : af InCkHL(T)3 p as
vs. p with
of the extremely
This increase
0.02
hw = 250 cm-”
limit.
. . . . . . . ..t
0.03
vs. l/T
parameter.
of
large value of
“inverse energy gap law” in the strong vibronic coupling
5 . . . . . ‘....,
0.00
T for
(x a 0.11 with T1,2 = 95 K to 2 f06 s-l
with Tl,e much larger than room temperature.
more than 12 orders of magnitude is a consequence S and the resulting
p
low
rates do indeed increase with increasing T,,e (Fig. 10) from i low6 s-l
the spin-crossover
energy
* 0.5 A\, a dyn/cm
curves,
(Fig. 9b). Although tunnelling
= &ArHL
a value for S of 40 - 50 is estimated.
with S = 45 and ho = 250 cm-’
various values of p. The characteristic low T tunnelling
With AQ,,
for CFe(CN),I*+ C27bll and a typical vibrational
What will vary though
lnCkH,(TIJ calculated
(4.31
)
(between the value of l.SxlO*
hw in the range of 200 to 300 cm-‘, extent.
SPevS
-
S = 1 f AQ&/ho.
Within the series of Fe(E) compounds any great
L
f a 2.0 x IO’ dyn/cm
CFe(HzO1,lzC C27a3 and 2.7x iOs dyn/cm frequency
For
to I81 :
0.04 0.05 ‘/r @-‘I
calculated
b) Calculated and
reduced energy gap
with low
S = 45 and hw = 250 cm-’ Ttunneling
S as parameter
(from
rate ref.
constants
fi71).
and the reduced lnCkHL(T-+O)
P
Fig.
10 : Experimental
reture
tunnemng
lnCkHL(T+0)3 sltion
low rate
tempe-
vs. the thermal
temperature
spin-crosssystems
T,,*
c
constants
a
tran-
0
for the FeUI)
from
table
l
I.
a
-15.0-
0. What about other spin-crossover or Co(D) with a 2E = ‘T, successfully
applied pulsed
temperature.
In principle
relaxation
100. 200.
Lawthers
ArHL is typically
-0.11 A, and for the resulting
at room
of the HS+LS
But because
for FeUII)
value of S of _ 17 is much
rates of IO’ - 10’ s-i can be estimated for small energy gaps p. For Co(III1 systems predictions are more difficult, as the
SCC model is most certainly a very bad approximation Teller
500.
1281 have
in solution
dependence
rates as for the Fe(D) systems should be observed.
and low T tunnelling
400.
and McGarvey
to FeUII) systems
the same type of temperature
systems
300.
for instance of FeUIII with a ‘T, *‘Ai
Well,
laser excitation
smaller reduced
compounds,
spin transition?
h/z WI 1....1....,....,....,
0
effect
metastable
states of the Fe(D) spin-crossover
S. PHOTORBFRACTIVB
of the strong
in the Fe-L bondlength
index of an Fe(D) spin-crossover
tially different
between
compound
Jahn-
long lived
systems is not to be expected
PROPERTIES OF PB(ID SPIN-CROSSOVER
Due to the large difference the refractive
either.
COMPOUNDS
the LS and HS state
is expected
to be substan-
in the two states. This effect can be used to create phase holograms.
The most simple
hologram
two
of the same wavelength
laser beams
experiment
because
in the 2E state. But LIESST in the sense of the extremely
is an excitation
a third beam can be diffracted
intensity I of the diffracted
grating produced on the sample. at this
by theinterference In a four
excitation
beam is a measure of the modulation
For a phase grating the diffraction
q = I/I,
efficiency
grating
C291. The
depth of the grating.
is given by 1291 :
= (rrAnd/X)2,
of
wave mixing
6.1)
288 where
An is the modulation
and X the wavelength If the grating the diffracted diffracted
of
decays
beam
beam
after
falls
An is proportional
crystal
tG is expected tranfer
from
absorption
this
mixed
Fig. 11: Decay with HS
rG = 9.5 ms and state
with
of
the grating
wave
modulation
relation
the intensity intensity
mixing
geometry
with
x 5 0.0s
the lifetime
At 106 K with
is confirmed
tG
of
in the
C291 with
X =
to An’,
the
excited
the state
9.5 ms and T = 22.2 ms
q
within
of
at 106 K. Since
AN and q is proportional
to half
processes.
beams,
the relative
experimental
error
for
crystal.
of the transient
... experimental
grating
-
the light-induced
r = 22.2 ms
absorption
in transient
four
to be equal
of energy
transient
exciting
CZni_xFex(ptz),l(BF,)z
T:in the absence the dilute
the two
Fig. 11 shows
degenerate
to the population
lifetime
off
accordingly. of
d the thickness
beam.
switching
off
in so called
514.5 nm in a mixed grating
of the index of refraction, the probing
CZnl_xFex(ptr),l(BF,)?
exponential fit
determined
at 106 K for (x = 0.05).
I . . . . . . . ..I 0.025
0.000
0.050
0.075
t [sl
From An/n
of
the diffraction -SX~O-~
_ 5x10’*!m3. for
This is a factor
achieved make
AN by an order diffraction
for
them
11of
5~10~~ in the above
at an estimated
of 100 larger of
efficiencies
magnitude
for
more
0.1 at 95 K),
in real time
a value
modulation of CrUII)
for
AN of
in ruby C301
it should be possible
concentrated
10% (in a preliminary
x *
with
application
compounds
by using
of some
CZn i_xFex(ptz)~l(BF~)2 attractive
experiment
population
than for the 2E state
of AN. For Fe(H) spin-crossover
the same value
to increase to achieve
efficiency
can be calculated
which
crystals
experiment in principle
and
8% were would
holography.
6. CONCLUSIONS The firmly over
established
compounds
and quintet
states
with
scheme the
large
for LIESST
and reverse-LIESST
number
ISC
is in its complexity
of
outstanding
processes among
in Fe(H)
between coordination
spin-cross-
singlet,
triplet
compounds.
289 Fe(D)
coordination
compounds
of one of the most there
simple
are no luminescent
possible
to accurately
ture range it is well
determine
feature
Future
work
anharmonicity active
should
force
should
include
should
be
and
the
with
namely relaxation
rate constants From
lifetimes
vibrational
taken
into
coupling
low
of several
too.
SCC in
The
tempera-
days at low of S of
model
the
influence
lattice
(or
results process.
two of
T are a
m 40 - 50,
of any coordination
of the simple
frequency
it has been
a large
nonadiabatic
due the value
frequencies
account
to
over
ISC. Because
the experimental
is a highly
compounds,
the extension
and
the dynamics
the HS+LS
value found for th LF states
constants
vibration
states
for studying
in chemistry,
with
relaxation
spin-crossover
system
of magnitude.
the HS+LS
which is by far the largest
different
processes
the relaxation
metastable
of Fe(D)
a model
competing
than 12 orders
that
The light-induced unique
radiationless processes
and over more established
constitute
compound. to allow
for
states,
and
more
solvent)
than
one
vibrations
be investigated.
Acknowledgement I thank A. Vef, his generous fUr Forschung for
P. Adler
support.
and H. Spiering
The work
und Technologie
experimental
results
for helpful
was financially
and the Schweizerische
made available
discussolons
supported
prior
and P. Giitlich
for
by the Bundesministerium
Nationalfonds.
I thank
A. Vef
to publication.
References 1
al E. Zinato
in “Concepts
Fleischauer 2
J. D. Peterson, S. Decurtins, L&t.
3
in Inorganic
eds.1 John Wiley,
New
Photochemistry”
York,
ibid., p. 203. P. Giitlich, C. P. Kiihler,
(A. W. Adamson,
1975, p. 143. b) P. C. Ford, H. Spiering,
A. Hauser,
P. D.
R. E. Hinzer,
Chem.
Phys.
139 (1984) 1.
S. Decurtins, Chem.
P. Giitlich,
K. M. Hasselbach,
H. Spiering,
A. Hauser,
Inorg.
24 (198.51 2174.
4
A. Hauser,
5 6
C.-L. Xie, D. N. Hendrickson, J. Amer. Chem. Sot. 109 (19871 6981. P. Adler, A. Hauser, A. Vef, H. Spiering, P. Giitlich, Hyperfine Interactions
J. Chem.
Phys.
94 (1991) 2741. 47
(19891 343. 7 8
A. Hauser, Chem. Phys. Lett. 173 (1990) S07. a) P. Avouris, W. M. Gelbart, M. A. El-Sayed, b) C. J. Donnelly, Publ. Corp.,
G. F. Imbush, New York
1990.
Nato
Chemical
AS1 B in press
Reviews
77 (1977) 793.
fed. B. DiBartolo)
Plenum
290 9 10 11 12
E. Buhks, G. Navon, M. Bixon, J. Jortner, J. Amer. Chem. Sot. 102 (1980) ‘2918. “Multiplets of Transition Metal S. Sugano, Y. Tanabe, H. Kamimura, Ions”, Pure and Applied Physics 33, Acad. Press, New York, 1970. H. L. SchlPfer, G. Gliemann, “Einfilhrung in die Ligandenfeldtheorie”, Akad. Verlagsgesellschaft, Wiesbaden, 1980, p. 462. a) M. A. Hoselton, L. J. Wilson, R. S. Drago, J. Amer. Chem. Sot. 97 (1975) 1722. bl M. Mikami-Kido, Y. Saito, Acta Cryst. B38 (1982) 4S2. c) L. Wiehl, C. P. Kohler, H. Spiering, P. Giltlich, Inorg. Chem. 25 (1986) 1565.
I3
P. L. Franke,
14
A. B. P. Lever, “Inorganic Electronic Spectroscopy”, Studies in Physical and Theoretical Chemistry 33, Elsevier, Amsterdam, 1984. A. Hauser, P. Giitlich, H. Spiering. Inorg. Chem. 2.5 (1986) 4345. a) S. Decurtins, P. Giltlich, C. P. KBhler, H. Spiering, J. Chem. Sot. Chem.Commun
15 I6
J. G. Haasnoot,
A. P. Zuur,
Inorg.
Chem.
Acta
G.Kiel,
S9 (1982) 5.
1985, 430. bl P. Poganiuch, P. Gtltlich, Inorg. Chem 26 (1987) 4.55. c) R. H. Herber, Inorg. Chem. 26 (1987) 173. dl A. W. Addlson, S. Burman, C. G. Wahlgren, 0. A. Rajan,
T. M. Rowe,
R. H. Herber, 17 18
A. Hauser, A. Vef, a) K.-U. Baldenius, 295. b) A. Hauser,
19 20
E. Sinn, J. Chem.
Hyperfine
Sot.
Interactions
P. Adler, submitted A. K. Campen, H.-D. J. Adler,
Dalton
Trans.
1987, 2621. e) D. C. Figg,
62 (1990) 99.
P. Glltlich,
to J. Chem. Phys. Honk, A. J. Rest, J. Mol. Chem.
Phys.
Lett.
J. J. McGarvey, I. Lawthers, J. Chem. Sot. Chem. Comm. (1990) 252. A. D. Kirk. P. E. Hoggard, G. B. Porter, M. W. Windsor, (1976)
Struct.lS7
152 (1988)
(1987)
468.
1982, 906. Chem. Chem.
Phys.
29
Lett.
37
199.
21
a) A. J. Street, D. M. Goodall, R. C. Greenhow, Chem. Phys. Lett. 56 (1978) 326. b) C. Creutz, M. Chou, T. L. Netzel, M. Okumura, N. Sutin, J. Amer. Chem. Sot. 102 (1980) 1309.
22 23
R. Grimm, P. Giitlich, E. Kankeleit, R. Link, J. Chem. Phys. 67 (1977) 5491. C. Hennen, Dissertation, Universit’at Mainz, 1990.
24 25
P. Poganiuch, J. K. Beattie,
26 27 28
A. Vef, unpublished results, Mainz 1991. a) J. T. Hupp, M. J. Weaver, J. Phys. Chem. 89 (198s) 2795. b) E. Heller, Ahsbahs, G. Dehnicke, K. Dehnicke, Naturwissenschaften, 61 (1974) 502. I. Lawthers, J. J. McGarvey, J. Amer. Chem. Sot. 106 (1984) 4280.
29
H. J. Eichler, Series
30
S. Decurtins, Adv. Inorg.
P. Glnter, in Optical
D. W. Pohl,
Physics
P. GUtlich, J. Amer. Chem. Sot. 112 (1990) 3270. Chem. 32 (1988) 1.
D. W. Pohl, Sciences Lett.
“Laser
SO, Springer
26A (1968) 37s.
Induced Verlag,
Dynamic Berlin,
Gratings”, 1986.
H.
Springer
C~rd~~t~n ~~~~t~ Reviews, 111 (1991) 275-290 Eisevier Science Publishers B.V., Amsterdam
Andreas Hauser, Institut fiir Anorganische Staudingerweg
Chemie , Johannes-Gutenberg-UniversitXt,
9, D-6500 Mainz, FRG
Abstract Due to the fact that for d6-systems there are a number of low-lying ligand field (LF1 states the relaxation from excited states of Fe(D) coordination compounds is, in general, a very Fast and radiationless process. In Fe(%) spin-crossover systems, however, the zero point energy difference between the two lowest states, namely the lowspin &Si ‘A, and the high-spin (HSi ?a state, is of the order of k,T, and some systems can be converted quantitatively to the HS state well below the thermal transition temperature by irradiating either into MLCT or LF absorption bands of the LS species, with HS+LS relaxation rates as small as I@ s-l at -10 K. It is also possible to achieve a light-induced transient population of a HS state in Fe(%) LS compounds, but in this case the HS+LS relaxation rates can be larger than IO6 s-l even at law temperatures. The HS+LS relaxation rates show strong deviations from Arrhenius kinetics with nearly temperature independent tunnelling below * 70 K and a thermally activated behaviour above -100 K. The range of 12 orders of magnitude in the low temperature tunnelling rate can be understood in terms of nonadiabatic multiphonon relaxation, where in the strong coupling iimit, with the Huang-Rhys parameter S much larger than the reduced energy gap p, an inverse energy gap Iaw holds.
1. INTRODUCTION Compared to the coordination photophysical
and photochemical
received very little attention. chemistry
“Concepts
with 6 d-electrons due to the fact iuminescence, a very efficient timescale.
of most other transition
properties
of Fe(%) coordination
in Inorganic Photochemistry” without
metal ions the
compounds
In fact one of the standard works of inorganic even mentioning
that Fe(%) coordination
devotes
two sections
Fe(%) compounds
compounds,
because of a number of low-lying
have photo-
to systems
HI. This is basically
in general,
do not show
any
ligand field (LF) states which provide
nonradiative decay path back to the ground state on the subnanosecond
Thus the important
spectroscopy,
compounds
experimental
with which quantum
internal conversion
Wf processes
metal ions have been determined,
method
efficiencies
in coordination
of time resolved
for intersystem compounds
luminescence
crossing
(IX)
and
of most other transition
cannot be applied to Fe(%) compounds.
OO~O-~~5/91/$05.~ 0 1991 Elsevier Science Publishers B.V. All rights reserved,
That despite this efficient might possess Decurtins
nonradiative relaxation
some unique photophysical
Fe(B) coordination
properties
et al. CZ,31 showed that in a number of Fe(H) spin-crossover
quantitative
light-induced
ratures well below
IA, (LSl+STz
(HSl conversion
the thermal transition temperature,
the visible either into the spin allowed species.
In the spin-crossover
for instance,
d-d
system
the HS++LS relaxation
compounds
emerged some years ago, when compounds
a
could be achieved at tempeby irradiating the sample in
or MLCT absorption
IFe(ptzl,l(BF,l,
(ptz
bands
of
the LS
= l-n-propyltetrazolel,
rate is less than lob6 sWi at 10 K, and thus the
light-induced metastable state could unambiguously be identified as the ‘T, state. The mechanism for the light-induced LS+HS conversion, subsequently called “LightInduced
Excited
involving
Spin State Trapping
(LIESST)“, proposed
Since the discovery of LIESST long-lived does the light-induced low temperatures. function
metastable
> 100 K, but below
-70
sulfonate.
CFe(pic131C12.MeOH (pit
q
possible
compound
Z-picolylaminel
tunnelling
spin-crossover
to create alight-induced
tion is considerably
faster even at low temperatures
The crucial other.
parameters The theory
s-l.
For the
system a similar behaviour s-l C3, 61.
state
in LS systems
171. of the metastable
of the LS and HS potential
nonadiabatic
multiphonon
relaxation
HS state wells
are
relative
C83, as first
by Buhks et al. C91, can be used to explain
the
variation of the low T tunnelling rates over 12 orders of magnitude.
The discussion constiute
of
ISC processes
-lo4
for T
kinetics
, but in these systems the HS+LS relaxa-
the lifetime
and vertical displacements
to HS+LS
observed
governing
rate of
metastableHS
(bipy = 2,2’-bipyridinel
at
as a
CFe(mepylz(pyl(tren11(PF6)2
T tunnelling rate of *lo-’
such as [Fe (bipyl,l’+
the horizontal
relaxation
a strong deviation from Arrhenius
independent
but in this case with alow
It is also
long lifetime
the HS+LS
They found a thermally activated process
K they observed
with a more or less temperature is observed,
CSI investigated
of the spin-crossover
in poly-styrene
C41.
systems. However, not for all compounds
state have such an astonishingly
Xie and Hendrickson
of temperature
embedded
to each
et al. 131,
metastable HS states have been found for
an increasing number of Fe(D) spin-crossover
applied
by Decurtins
two ISC steps with AS = 1, has recently been proved to be correct
of
the
the two central
mechanism
of
LIESST and of
the HS+LS
relaxation
themes of this paper.
2. LIGAND FIELD THEORITI-ICAL CONSIDERATIONS According d-electrons
to the Tanabe-Sugano
(Fig. I) octahedral
diagram Cl01 for a transition
Fe(D) complexes
metal ion with 6
can either have a ‘T,
(HS) ground
state for ligands exerting but a weak LF such as H,O, or a IA, (I...% ground state for ligands exerting a strong LF such as CN-. If for a given ligand the LF strength
happens
211
to be In the vicinity of the crossover In order to actually
understand
point Acrlt, a thermal spin transition
this, it 1s not sufficient
to just look at the Tanabe-
Sugano diagram. Such a diagram only depicts the electronic states relative to the respective
ground state as a function
The LF strength is not only determined by the properties function
of the metal-ligand
may occur.
energies
of the excited
of the LF strength
1ODq.
of the ligand but it is also a
distance r. For neutral ligands iODq (i-1 _ l/r6 Clil. Thus
If for a given ligand the LF strength at the equilibrium
distance of say the LS state
10DqLS = 10Dq(rLsI is known from experiment, lODq(r1 for an arbitrary value of r can be estimated
for this ligand, and assuming an appropriate
potential
surface for the IS
state along the totally symmetric normalcoordinate,
the energies of all the
other LF states along this coordinate can then be calculated. state,
for
instance,
potential
surface
displaced
For the HS the
resulting always
be
towards a substantially
lar-
ger metal-ligand
will
distance irrespective
of the ligand (Fig. 2i.This is not surprising, since in the ‘A, state the 6 d-electrons bonding
occupy the basically non-
t2-orbitals,
whereas
in the
sT, state two of those electrons promoted
to the antibonding
tals. The vertical displacement HS surface,
of the
on the other hand, de-
pends strongly HS system
get
e-orbi-
it
Ds cm-’
upon the ligand. In a will
of
course
be
Fig,l:Tenabe-Sugano
dlagramforad6-system.
:
Fig. 2 for
Potential
the
state
HS
of
end
wells the
LS
Fe'e(II)In octa-
hcdralcoordinatlonrlong the
totally
normal
Normal
Coordinate
Q,,
symmetric
coordinate.
218 displaced
towards
a thermal
lower
positive
and of
populated entropy
driven
electronic
population
degeneracy
ligand
Fe-L
bondlengths
spin-crossover tionality
a factor
the crossover
The
CFe(ptz),l(BF,),
version but
discovered
it is with
mechanism whole
to just
The thermal It manifests purple tion
spectra
of
transition
LS state
with
has
scheme
for
change
rHs
- rLs
from
in Fe(E)
the propor-
is expected So in fact
a very
to
increase
for
a Fe(E)
to the right
important
role
of the light-induced properties been
LIESST.
This
well
in
LS +HS
the
con-
of CFe(ptz),l(BF,),.
possible
has been
in colour
from
to
justifies
discussed
colourless
In Figs. 3a and b the corresponding Referring
prove
the
devoting
a
of
the
HS species
of almost
experimentally.
token
with
- E(‘A,) a factor
+ (E(iT,)
- E(‘T,)
of 2 on going
absorp-
diagram
(Fig.
spin
= E(‘El
bands
transitions
C3, 131.
crystal
to the
10DqHS
the two
to the spin allowed
with 10DqLS * E(lT,) increase
single
the 293 K spectrum
By the same
before
at 293 K to deep
once again to the Tanabe-Sugano
in the NIR of
can be assigned
the predicted
never
(LIESST)
eventually
in CFe(ptz),l(BF,l,
characteristic
is confirmed
plays
of the optical
it
a dramatic
the band
and ‘AI+rTZ,
can
a
are
one system.
is straightforward.
the 20 K spectrum C141. Thus
this
are shown.
= 11760 cm-’
a full
temperature.
the assignment
rAr+rT,
system
spin transition
itself
at low
‘T,qSE
spin-crossover
Not only was the phenomenon that
for
-1.96 - 2.00 A and the
ArHL=
TRAPPING
system
and establish
section
that
configuration
LF strength
spln-crossover
compound
1S-fold
states.
diagram.
SPIN STATE
during an investigation
this
the
means,
- 0.21 A C121. Thus
the
be
state.
the HS to the LS state.
Tanabe-Sugano
EXCITED
of LIESST.
nuclear
rLs are characteristically
from
of
of vibrational
10DqHS will be well to the left and 10DqLS well
3.1 The CFe(ptz),l(BF,),
discussion
this
will
quantitative
the HS and the LS state
of the ground
= (r HS/rLS)6,
in the
LIGHT-INDUCED
that
because
for
has to be
the LS state
now simply
which
In order
AE&_
an almost
place,
the HS and the LS state
of 2 on going point
only
density
diagram
is in the range -0.16
compound
case
take
for
configuration
between
compounds
spin-crossover
3.
it is clear
in the LS state
( 10DqLS/ 10DqHS)
by almost of
but
difference
will
energy.
difference
temperatures
and its higher
configuration
higher
energy
21, in which
at higher
in the Tanabe-Sugano
is a nuclear
towards
point
the HS state
to the equilibrium
bondlength
(Fig. but
of
degenerate,
correspond
k,T
the zero
of the HS state
point
there
accidentally
of
temperatures,
The crossover given
to occur,
the order
at low
in a LS system
energy,
spin transition
in the of
-
Ec5T,)
visible
the IS
11,
allowed of
species
) /4 = 20200 cm-’ from
the HS to the
279
Fig.3: Single crystalabsorptionspectraofCFe(ptz),ll(BF,), K. b) at20
afat
K.c)
after
Irredlat)onatS¶4.5nm,dlaftcr clt820nm,forcomparfsonthe 20 K
normal sow. 1cml. fin.
1
6”
c)
2woo. 25wo.3MoQ
J5ow.
‘Thermal Spin Crossover 128 K 135 K t
tight-
Included, at 980
nm(from
In Fig. 4 the transition curve for the thermal spin transition of effects
crystallographic
irradiation ref. C41).
~0’ 4
as derived from the temperature dependence of the ‘Al+tT, cooperative
spectrum f--f is
e) after
CFe(ptz)61(BF,),
band is shown. Due to
the spin transition in the neat material
is accompanied
by a
phase transition with a hysteresis of 7 K (T: = 128 K, T,$ = 13.5Kf.
280 The
transition
curve
mixed crystal
for
CZnr_,Fex(ptz161(BF~)2
(x 23 0.11, included more
gradual,
simple tion the
in
spin equilibrium. temperature
fraction
yHs
system
is -9s
the
laser,
Ti,z,
after
the typical cally HS+LS
line
the
‘T,+‘E
3T 1 state.
an Ar+
‘A,+‘T,
band,
band
lAi+
sT,
Fig. 4 : The
HS
the thermal
spin transition
CD) and
by Decurtins
if
that
optically
pumped
the
temperature
being
However,
repeated
irradiation
reached.
The
Indeed
with
a practithermal
is depicted
in Fig. 5.
be
of
are two
weak
the spin forbidden these
weak
bands
with the spectrum
crystal
value for
with
bands
the can
‘A,+ST, low
‘AIa3T,
the
unity,
be,
state
also band,
be the
LS spectrum
complete.
that
band
‘E
the absorption
sT,+SE
is not
the
yLs
After = 0.9 is
LS species
in such a way,
that
conversion. energy
side
and iA,+3T2
to CFe(ptz),l(BF,),
of CZn(ptzl,l(BF,),.
below
the typical
would
the ‘T,
not
system
the LS fraction
ST,+SE
on the
transitions
for
this the
are
Fig 3d shows Again
into
of the energy
to the HS state.
the
conversion
to the
A in metal-ligand
to just
indeed
the cycle.
ST,+‘A,
are intrinsic
IT, state
steps
at 820 nm, i. e., into
in a light-induced
very
-0.18
various
3T, state And
reason
overlapping
results
of
the
(reverse-LIESST).
at 20 K all through
straightforward
in turn
trapped,
the whole
reversible.
irradiation
excited
can drop
convert
the light-induced
band
LIESST
the initially
because
at 820 nm a “saturation”
most
to which 6). That
comparison
kept
absorption
there
(a).
a noticeable
AS = 1, the system
puts the low-lying
should
a subsequent
absorption
of
(x in 0.1)
it remains
efficiencies
back to the IS state
is observed.
has a weak
does
due to the difference
eventually
LF theory
from
again with
quantum
will
LIESST
after
as function
In CFe(ptz)bl(BF4)s
is metastable
-5OK
et al. I31 for
low temperature
LS and HS state
that
yHs
CZn 1_xFex(ptz)61(BF4),
above
the system
ISC step,
excitation
fact
fraction
HS state
at temperatures
at sufficiently
suggests,
250.0 T [Kj
with
proposed
Even
spectrum
200.0
,
set in C3, 1.51.
between
continuous
150.0
..
in the NIB,
Only
In a second
bondlength.
(Fig.
at 20 K
of
lifetime.
where
I
as
ISC step with AS = 1 leads
The
100.0
I’
dilute
the irradiation,
The mechanism
barrier
50.0
‘.
HS
The absorption
relaxation
A first
I
in Fig. 3c. At 20 K the light-induced
infinite
state,
the
the crystal
(LIESST).
is shown
a
the
leads to a complete
spectrum
is
for
I “O”
K.
i. e., into
quickly
4,
defined
= O.S, for
S14.S nm
conversion
3
The transi-
at which
Irradiating with
band,
Fig.
as is expected
temperature
this
the dilute
of
‘A,+‘T,
can be assigned
is borne
In this spectrum
the out
by the
the comparatively
281 sharp lines corresponding tional overtones tions
are
identical
CFe(ptz),l(BF,),, bands
to
those
in
but the two broad
are missing.
triplet
to vibra-
of the ligand vibra-
states
Since
originate
the two
from
the
same configuration
as the two exci-
ted singlet
namely the the
states,
(t; e) configuration,
the absorption
bands of the spin forbidden
transi-
tions are just as broad as those of the spin allowed
ones.
The maximum
of the ‘A1+3T,
band at 10280 cm-’
t-980
nm) is
just to the low energy side of the ‘T, e5E band. Starting fraction
of
yLs
irradiating
1 at
the crystal
again results
20 K, and at 980 nm
in a complete
induced ‘A,+*T,
I~~a~io~ at S14.Snm -
from a LS
980.0nm -
light-
conversion
Irradiation al 820.0nm
(Fig 3e).
A direct irradiation into the 1A,+3T, band
therefore
results
in LIESST
too, proving two things : The two weak bands are not due to impurities
Nuclear Coordinate
and therefore the assignment to spin forbidden and,
more
that
the
transitions
is
importantly, mechanism
justified it
Fig. S : Potential state
shows
For LIESST
with the 3T, state as intermediate state is correct.
wavelength
excited
spin-crossover
system.
the mechanism
of
k
ground “T2 state4
LF states
Arrows
of
LIESST.
the
value for yLs, since this absorption band, whereas
although not zero, is minima1 at this wavelength.
and the buildup of the IS state in LIESST and reverse-LIESST
quantitatively.
The quantum efficiencies
can
of the first ISC step either from
the IA, state far LIESST or the SE state for reverse-LIESST tively. The branching
a d’
indicate
are equal but not zero. The value of 0.9 reached
steady state to the maximum of the ‘T,+%
of the IS species,
The depletion be followed
higher
at 820 nm is the maximum
corresponds
the absorption
the
the
accessible
be called steady state value, since For this value of yrs
rates for LIESST and reverse-LIESST by irradiating
as wall
for
value of yLs = 0.9 reached after long irradiation
In view of this the “saturation” at 820 nm had better
wells
and the thermally
are ... 1 and +,0.7s. respec-
ratio from the 3T, state either to the ‘A, ground
state or the
282 metastable
ST,
state
processes,
except
is
1: 4 C41. The
of course
actual
the ‘T,+‘A,
rate
relaxation,
constants
for
are z 10”
s-r.
the
Fig.6: Blow energy trum
up
den
I ., 10000.
5000.
LIESST
In other
LIESST
is quite
it is basically states only
of
CFe(pta),XBF,),
neat
In most resulting
larger
and irradiation In a certain
Ci61, but
however, overlap
the MLCT
sense
the
LIESST
relaxation
rates
number
dilute
in polymer
MLCT
laser
and Lawthers
irradiation
of the order
states
general
of lo7 s-l
3T,. Street
CFe(phen).$*+
for this state
ef aL Ulal
at 2.93 K in solution
basically
either
tentatively
agreeing
with
a 3MLCT
identify
state
this
last
HS states Prior
point
time-differential
and the LS state,
too
C3, 171.
depopulation
of
compounds,
and at ambient
tempera-
Even for the LS systems
found.
state Kirk
or the low-lying
witha
is the
Mijssbauer
life-
et al. CZOI list LF states
and Creutz
out that
suggestion
by low-
to the discovery
a rapid
a transient was
in the
spin-crossover
it with the 3T, state
this assignment,
‘T 2 state. With hindsight of course It is furthermore supported by the
intense
short-lived.
ns CZOI and 0.71 ns 1211, respectively
not
171, and
are obscured
than that. Fe(B)
systems, Cl&
in the LS state
long-lived
in solution
were comparatively
and HS -
Cl81.
1191 noticed
for some
compounds
crystals
are more
and
although
(from
metastable
spin-crossover
mixed matrices
bands
time
as possibilities
Long-lived
Fe-L bondlength
CFe(bipy)a12+ 0.83
spin-crossover
of Fe(B)
for
is even more
upon pulsed
the light-induced
Fe(B)
molecule.
band at 10 K produces
et al. C&31 McGarvey
but with
for
of the shorter
the ‘A,
of
transitlons
. I 20000.
the d-d bands of the LS species
of Decurtins state
cm
also
embedded
But because
orbital into
.
-,
complex
for an increasing
complexes
bands.
.
phenomenon
of the single
compounds
systems,
MLCT
I . 15000.
a common
a property
for spin-crossover
tures
d-d
at
the spin forbid-
systems
have been found for
lying
of the low
ref. C41).
0.0 -
3.2
ISC
part of the LSspec-
20 K with
0.5 -
various
it could most
‘T,
also
be the
probable
emission
or
et aL CZlbl,
study
one. of
283 Grimm et
l221, who observed
al.
anomalous
HS states
nuclear decay in Cs7Co/Co(phen133(C10J~~2H,0. Mirssbauer consists
emission
spectra
at 40 and f72
of just the same LS doublet
is observed spectrum
in a Massbauer
indicating process
HS
the
The 172 K spectrum
as
states
cha-
appear,
that after the 57Co (EC) s7Fe at least
complexes
of
some
the
Fe(E)
end up in a HS state with a
lifetime on the Mijssbauer timescale
of
IO”? s. Grimm et si. E221 determined lifetime
reproduced.
are
of
integral
but at
40 K additional quadrupol doublets for
effect
absorption
of [Fe(phen)~l(ClO,~z,
racteristic
K
as an after
In Fig. 7 the corresponding
a
of 205 ns at 47 K for the HS
state marked with HSl in Fig 7b. This compares found
well with the 188
for
at -SO K
ns
the light-induced
transient
state for CFe(phenl,12* embedded Nafion polymer
in a
0
foil Ci71.
0
-2
-4
2
4
v/mms-’
The
quantum
light-induced stable
HS
efficiency
population state
in the
CZnr-xFex(bipy),l(PF,)z is with -0.6
for LS
Fig. 7
system
:
Integral
spectra
(x = 0.00011
of
“Co
MiSssbauer
emission
CS7Co/Co(phen),l(C10~)2.
at a) 40 K md
b)
172 K (from
2H,O ref.
C233).
Cl21 of the same order of
magnitude as the one determined except
the
of the meta-
for the lifetime
the photophysical
for the CFe(ptzl,,l(BF,l,
of the ligh-induced
HS state,
spin-crossover
system.
So,
there is no big difference
in
behaviour of Fe(H) Ls and spin-crossover
In analogy to reverse-LIESST stable LS states in HS compounds
compounds.
it should in principle be possible
to create
HL< 0 by irradiating into the ‘T,+‘E with AE”
tion band. However, in order to have an nonvanishing quantum efficiency
metaabsorp-
for the first
ISC step, the ‘E state must be higher in energy than the 3T, state, and this is only the case for LF strengths substituted system
CFe(mtzl,l(BF,l,
the complex
fairly close
to the spin-crossover
such a LS+HS conversion
molecules
sit on two different
on site A show a thermal spin transition
systems.
In the methyl
is indeed possible
crystallographic
C241. In this
sites.
The ones
with Ti,z 4 72 K and behave pretty
much
the same way as the CFe(ptz)61(BF,lz system. The ones on site B remain in the HS state at all temperatures, but can be converted to the LS state by irradiating in the NIR. At 10 K the lifetime of this now metastable LS state is again practically infinite, only above -45
K does a noticeable LS+HS relaxation set in.
284 THE HS +LS
4.
RELAXATION
Work on the LS *HS ISC dynamics in Fe(H) spin-crossover
compounds
has mostly
been performed in solution at ambient temperatures CZSI. Over the limited temperature interval available in solution 1nCkHLl vs. l/T plots Mrrhenius plots) usually result in straight
lines, and consequently
of absolute
the results have generally been discussed
in terms
rate theory. But as had already been pointed out by Buhks et al. C91 from
theoretical
considerations
and was confirmed
C51 the HS +LS relaxation which classical
concepts
Fig. 8 shows
is a highly nonadiabatic
by Xie und Hendrickson
process,
for the description
the Arrhenius plots of the HS+LS relaxation
pulsed laser excitation.
compounds
determined
rate constants
in transient
for a
absorption
after
For the dilute mixed crystals studied the determination
from the relaxation
slngle exponential
of
are not really adequate.
number of Fe011 coordination rate constants
experimentally
cur ‘ves is straight
forward
of the
as the relaxation
is
El?]. Ail five systems show strong deviations from simple Arrhenius
behaviour. At elevated temperatures the
relaxation process is thermally -
activated,
but at low
the relaxation
rates
temperature towards tunnelling
temperatures are much less
dependent,
a temperature
going
20.0
2 s
15.0 10.0
independent
5.0
rate for T + 0. In table I
the low temperature
tunnelling
constants
and the activa-
tion
z
k,,fT+O)
parameters
Ea and A for
high temperature
region
rate
0.0 -5.0
the
T 2 100 K
-10.0 I
are listed. in addition the low T tunneling rate constants systems
l l
for some other
-15.0
are included in table I. The
[~n:f+-+(~~)&~F,),
f....l....f....f
0.00
..‘.I
0.02
0.01
0.03
low T tunnelling rates are spread over
l/T
more than twelve orders of magnitude from
< lob6 s-l
over
system
(X *
IZnt_xFex(ptz)63tBF,)2
0.1) to 1) > 106 s-* for the LS sys-
tern CFe(phen),12+embedded
different
occurring
simple
into vibrational model
plot of the experimental
rate constants
k,&I’,
mixed
systems
crystal
for
same
(from
dilute
FeCIX)
ref. E171).
multiphonon
relaxation
the HS +LS ISC is described
between two distinct zero order spin states characterised
nuclear configurations,
is transformed
: Arrhenius
[K-l1 _ _
in Nafion.
In the theory of nonadiabatic as a process
most
Fig. 8
for the spin-cross-
0.05
0.04
treats
space, with the potentials
during which the electronic
energy of the final state in a nonadiabatic
this problem
in single configurational
displaced by AQ,,
by
energy of the initial state step. The
coordinate
(SCC)
along one, namely the totally symmetric,
285
Table I : Transition temperature A for the high temperature
T,,z, activation energy
region, and low T tunnelling
Ea and frequency rate constant
factor
kHL(T+O)
for some Fe(H) coordination estimated
compounds. The reduced energy gap p has been to the “inverse energy gap law”.
according
T112
Ea
CKI
[cm-‘1
CZni_xFex(ptz),l(BF~I,
9.5
CMnr_xFex(pic)al Cl; EtOH
a)
kH,_(T+O)
c s-11
1100
cs-‘3
5x107
*5x10-7 6 x lO-6
95
CMn1_xFex(pic)31CI,*MeOH CZn,_xFex(pic),lC1z~MeOH
P
76
a)
CZnr_xFex(pic)al Cl;EtOH
A
2.5x10-s
118 a)
907
2x10s
-1-Z
2.5x10-3
140
9 x 10-a
CZni_xFex(mepy)a(tren)l(PF6)2
210
CFe(mepy)z(py)(tren)12+in
PMMAb’
270
~SXlO’
CFe(mepy)(py)z(tren)l*+in
PMMAb)
370
-2x102
CZni_xFex(py)3(tren)I(PFJz
sx1os
a37
-3-4
1.4x10-’
> 400
640
1 xl09
-7-8
4x102
CZnr-xFex(bipy)sl(PF,)2
low-spin
364
2x109
-11-13
6~10~
CFe(phen1312* in Nafion b,
low-spin
a)
from ref. C261
normal coordinate
-5x106
b’ Teff 4 SO K (see ref. CIZI)
only. With AS = 2 only higher order spin-orbit
coupling
can mix
the two states. Starting from Fermi’s Golden Rule and using the Condon approximation the HS+I_S relaxation
rate constant k,,(T)
where
the electronic
strengths
=
&
2s
matrixelement
near the spin-crossover
is given by Cal : F,(T)
pH,_ = <0= point
(4.1)
,
I H,, I@,,,>
C91. For harmonic
w 150 cm-’ potentials
for
LF
with equal
force constants
f and vibrational frequencies o, the thermally averaged Franck-Condon
factor
given
F,(T)
is
by :
2 I
=
e-mno’kT
-mho/kT Lie
(4.2)
286 where te sum goes over all the vibrational levels rn of the initial state, and n = m + p (the reduced energy gap p = AE&/hwl T + 0 F, simplifies
Fp(T+o)
where the Huang-Rhys mean force
in order to ensure energy conservation.
constant
= i
factor
tunnelling
for
with 6-fold
N-coordination
is the reduced
S will not vary to
energy gap p. In Fig. 9a
for is plotted
i/T
for
and the thermally activated region, are well reproduced.
The low T
rate k,, (T+O)
features of the experimental
against
namely the
is predicted
to increase
Tl,z is only a crude
measure
exponentially for
AI&_,
with increasing
the observed
system CZn,_xFe,(ptzl,l(BF,),
for the LS systems
-15.0~
0.01
Fig. 9 : af InCkHL(T)3 p as
vs. p with
of the extremely
This increase
0.02
hw = 250 cm-”
limit.
. . . . . . . ..t
0.03
vs. l/T
parameter.
of
large value of
“inverse energy gap law” in the strong vibronic coupling
5 . . . . . ‘....,
0.00
T for
(x a 0.11 with T1,2 = 95 K to 2 f06 s-l
with Tl,e much larger than room temperature.
more than 12 orders of magnitude is a consequence S and the resulting
p
low
rates do indeed increase with increasing T,,e (Fig. 10) from i low6 s-l
the spin-crossover
energy
* 0.5 A\, a dyn/cm
curves,
(Fig. 9b). Although tunnelling
= &ArHL
a value for S of 40 - 50 is estimated.
with S = 45 and ho = 250 cm-’
various values of p. The characteristic low T tunnelling
With AQ,,
for CFe(CN),I*+ C27bll and a typical vibrational
What will vary though
lnCkH,(TIJ calculated
(4.31
)
(between the value of l.SxlO*
hw in the range of 200 to 300 cm-‘, extent.
SPevS
-
S = 1 f AQ&/ho.
Within the series of Fe(E) compounds any great
L
f a 2.0 x IO’ dyn/cm
CFe(HzO1,lzC C27a3 and 2.7x iOs dyn/cm frequency
For
to I81 :
0.04 0.05 ‘/r @-‘I
calculated
b) Calculated and
reduced energy gap
with low
S = 45 and hw = 250 cm-’ Ttunneling
S as parameter
(from
rate ref.
constants
fi71).
and the reduced lnCkHL(T-+O)
P
Fig.
10 : Experimental
reture
tunnemng
lnCkHL(T+0)3 sltion
low rate
tempe-
vs. the thermal
temperature
spin-crosssystems
T,,*
c
constants
a
tran-
0
for the FeUI)
from
table
l
I.
a
-15.0-
0. What about other spin-crossover or Co(D) with a 2E = ‘T, successfully
applied pulsed
temperature.
In principle
relaxation
100. 200.
Lawthers
ArHL is typically
-0.11 A, and for the resulting
at room
of the HS+LS
But because
for FeUII)
value of S of _ 17 is much
rates of IO’ - 10’ s-i can be estimated for small energy gaps p. For Co(III1 systems predictions are more difficult, as the
SCC model is most certainly a very bad approximation Teller
500.
1281 have
in solution
dependence
rates as for the Fe(D) systems should be observed.
and low T tunnelling
400.
and McGarvey
to FeUII) systems
the same type of temperature
systems
300.
for instance of FeUIII with a ‘T, *‘Ai
Well,
laser excitation
smaller reduced
compounds,
spin transition?
h/z WI 1....1....,....,....,
0
effect
metastable
states of the Fe(D) spin-crossover
S. PHOTORBFRACTIVB
of the strong
in the Fe-L bondlength
index of an Fe(D) spin-crossover
tially different
between
compound
Jahn-
long lived
systems is not to be expected
PROPERTIES OF PB(ID SPIN-CROSSOVER
Due to the large difference the refractive
either.
COMPOUNDS
the LS and HS state
is expected
to be substan-
in the two states. This effect can be used to create phase holograms.
The most simple
hologram
two
of the same wavelength
laser beams
experiment
because
in the 2E state. But LIESST in the sense of the extremely
is an excitation
a third beam can be diffracted
intensity I of the diffracted
grating produced on the sample. at this
by theinterference In a four
excitation
beam is a measure of the modulation
For a phase grating the diffraction
q = I/I,
efficiency
grating
C291. The
depth of the grating.
is given by 1291 :
= (rrAnd/X)2,
of
wave mixing
6.1)
288 where
An is the modulation
and X the wavelength If the grating the diffracted diffracted
of
decays
beam
beam
after
falls
An is proportional
crystal
tG is expected tranfer
from
absorption
this
mixed
Fig. 11: Decay with HS
rG = 9.5 ms and state
with
of
the grating
wave
modulation
relation
the intensity intensity
mixing
geometry
with
x 5 0.0s
the lifetime
At 106 K with
is confirmed
tG
of
in the
C291 with
X =
to An’,
the
excited
the state
9.5 ms and T = 22.2 ms
q
within
of
at 106 K. Since
AN and q is proportional
to half
processes.
beams,
the relative
experimental
error
for
crystal.
of the transient
... experimental
grating
-
the light-induced
r = 22.2 ms
absorption
in transient
four
to be equal
of energy
transient
exciting
CZni_xFex(ptz),l(BF,)z
T:in the absence the dilute
the two
Fig. 11 shows
degenerate
to the population
lifetime
off
accordingly. of
d the thickness
beam.
switching
off
in so called
514.5 nm in a mixed grating
of the index of refraction, the probing
CZnl_xFex(ptr),l(BF,)?
exponential fit
determined
at 106 K for (x = 0.05).
I . . . . . . . ..I 0.025
0.000
0.050
0.075
t [sl
From An/n
of
the diffraction -SX~O-~
_ 5x10’*!m3. for
This is a factor
achieved make
AN by an order diffraction
for
them
11of
5~10~~ in the above
at an estimated
of 100 larger of
efficiencies
magnitude
for
more
0.1 at 95 K),
in real time
a value
modulation of CrUII)
for
AN of
in ruby C301
it should be possible
concentrated
10% (in a preliminary
x *
with
application
compounds
by using
of some
CZn i_xFex(ptz)~l(BF~)2 attractive
experiment
population
than for the 2E state
of AN. For Fe(H) spin-crossover
the same value
to increase to achieve
efficiency
can be calculated
which
crystals
experiment in principle
and
8% were would
holography.
6. CONCLUSIONS The firmly over
established
compounds
and quintet
states
with
scheme the
large
for LIESST
and reverse-LIESST
number
ISC
is in its complexity
of
outstanding
processes among
in Fe(H)
between coordination
spin-cross-
singlet,
triplet
compounds.
289 Fe(D)
coordination
compounds
of one of the most there
simple
are no luminescent
possible
to accurately
ture range it is well
determine
feature
Future
work
anharmonicity active
should
force
should
include
should
be
and
the
with
namely relaxation
rate constants From
lifetimes
vibrational
taken
into
coupling
low
of several
too.
SCC in
The
tempera-
days at low of S of
model
the
influence
lattice
(or
results process.
two of
T are a
m 40 - 50,
of any coordination
of the simple
frequency
it has been
a large
nonadiabatic
due the value
frequencies
account
to
over
ISC. Because
the experimental
is a highly
compounds,
the extension
and
the dynamics
the HS+LS
value found for th LF states
constants
vibration
states
for studying
in chemistry,
with
relaxation
spin-crossover
system
of magnitude.
the HS+LS
which is by far the largest
different
processes
the relaxation
metastable
of Fe(D)
a model
competing
than 12 orders
that
The light-induced unique
radiationless processes
and over more established
constitute
compound. to allow
for
states,
and
more
solvent)
than
one
vibrations
be investigated.
Acknowledgement I thank A. Vef, his generous fUr Forschung for
P. Adler
support.
and H. Spiering
The work
und Technologie
experimental
results
for helpful
was financially
and the Schweizerische
made available
discussolons
supported
prior
and P. Giitlich
for
by the Bundesministerium
Nationalfonds.
I thank
A. Vef
to publication.
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