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Electrochromism and Solvatochromism.

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proportional to l/[H+], [H202], and [Cu*+-Bipyj, but is
probably independent of the concentration of N H 2 0 H
(which is, however, consumed during the reaction). The value
of YO decreases with increasing [Bipy]. The following expression - analogous to (11) and (15) - was found for the
initial rate YO of disappearance of H202 in the same reaction
between H202 andNH2OH using free C P i o n as catalyst 190bl:
A reaction course as shown in equations (12) and (16) and
an active complex as shown in equations (13) and (17) can
again be formulated for this system [90a,bl.
If the metal ion undergoes a valency change during the catalysis in these reactions, it must be assumed that it occurs between CuI and Cu", and not Cu1II, since o n the one hand
Cull is reduced to yellow copper(]) oxide by both NH2NH2 [951
and NH2OH (95.961 in alkaline media, and on the other Cu1
can be reoxidized t o Cu" by H202 [971 (cf. also [ I l l ) .
6. Conclusion
The systems discussed here give an idea of the variety
of reactions that can occur on decomposition of H202,
and the examples in Section 5.3 and 5.4 also clearly
[95] 7'.R. Bhat, D . Radhamma, and J . Shanknr, J. inorg. nuclear
Chem. 27, 2641 (1965).
[96] J. H . Anderson, Analyst 91, 532 (1966); 89, 357 (1964).
1971 A . Zuberbuhler, Helv. chim. Acta 50, 466 (1967).
show the relation between catalase and peroxidase
activities. Similar active complexes, with their secondary reactions [see equations (13) and (17)], can be
formulated for the systems mentioned; one of the H202
molecules can be replaced by an H donor such as
HzN-NHz or NHzOH (cf. 190bI). The Cu2+-2,2'-bipyridyl ( 7 : l ) complex can thus catalyze both the catalase (1) and the peroxidase (2) reactions, depending on
the reaction conditions. The behavior of this catalyst
is thus similar to that found with enzymes (cf. r40,421).
A similar relation to that demonstrated in this survey
between the structures of Cu2+ complexes and their
catalytic activity is also to be expected for complexes
of other metal ions.
It may be assumed that the conditions that must be
satisfied by a Cu2f complex if it is to be capable of
catalyzing the catalase or peroxidase decomposition
of H202 are also applicable to enzymes containing
copper.
I am grateful to Pro$ Dr. H . Erlenmeyer for his
numerous suggestions. I thank Prof. Dr. S. Fallab,
Dr. B. Prijs, and Dr. R. Griesser for their readiness to
discuss the problems that arose. Thanks are due to the
Schweizerischer Nationalfonds zur Forderung der
wissenschaftlichen Forschung for support of our work.
Received: April 1, 1968
[A 683 IEl
German version: Angew. Chem. 81. 161 (1969)
Translated by Express Translation Service, London.
Electrochromism and Solvatochromism
By W. Liptayf*l
The position and the intensity of electronic bands are influenced by an electric fiefa'.
Pronounced changes in the position of absorption bands are mainly due to the dipole
moment of the molecule in the ground state and the change in the dipole moment during
the excitation process, and pronounced changes in intensity are due to the fieId dependence
of the transition moment, which can be described by the transition polarizability. The
efect of an external electric field on the optical absorption (electrochromism) of suitable
molecules can be used to determine the dipole moment in the ground state, the change in
dipole moment during the excitation process, the direction of the transition moment of
the electronic band, and certain components of the transition polarizability tensor. These
data largely determine the strong solvatochromism (solvent-dependence of the position
and intensity of electronic bands), which is observed in particular with molecules having
large dipole moments. Smaller contributions to solvatochromism result from dispersion
interactions, which predominate in the case of nonpolar molecules. The models developed
have been experimentally checked and verified by a combination of electro-optical absorption measurements (influence of' an external electric field on absorption) and investigation of the solvent-dependence of the electronic bands.
1. Introduction
The absorption or emission spectrum of a substance
in the vapor state can exhibit characteristic differences
in the positions and intensities of the bands from the
spectrum of the same substance in solution; these
Angew. Chem. internat. Edit.
1 Vol. 8 (1969) / No. 3
differences depend on the solvent 11-51. This solventdependence of electronic bands is known as solvatochromism.
.
['I
Prof. Dr. W. Liptay
Institut f i r Physikalische Chemie der Universitst
65 Mainz, Postfach 3980 (Germany)
[ I ] A . Kundt, Ann. physic. Chem. 4, 34 (1878).
177
The optical properties of molecules can also be influenced by an external electric field; this phenomenon
shall be called electrochromism. The fundamental
cause of strong solvatochromism, such as is observed
in polar dye molecules, is the same as that of electrochromism, i.e. the changes produced in the electronic
bands by an electric field.
2. Electrochromism
Changes in optical absorption due to the action of an
external electric field are basically attributable to three
effects, which will be described in the following
paragraphs.
2.1. Orientation Effect
As our example of a solute molecule, let us consider
p-nitroaniline, which exhibits an intense long-wave
absorption band (maximum at 28 900 cm-1, extinction
coefficient = 13000 lmole-1 cm-1, width at half height
= 6000 cm-1) in benzene.
The absorption curve can be recorded with natural,
unpolarized light or with linearly polarized light. In
an ordinary solution the molecules are distributed isotropically, and both methods consequently give the
same absorption curve, which can be represented as a
plot of the extinction coefficient E ~ O ~ ( G )against the
wave number ?. The extinction coefficient is a measure
of the intensity of the optical absorption at a given
wavelength. The measure of the intensity of an absorption band is the transition moment p","a'of the
dissolved molecule, which is related to the molar
decadic extinction coefficient
by the integral
absorption:
Band @-+a)
The integral should be extended over the whole of the
absorption band corresponding to the electronic excitation from the ground state g to a definite excited
state a. NA is Avogadro's number, h is Planck's constant, and e is the velocity of light. The transition
moment p g l is a vector, which can also be represented
by its absolute value p",",'
[ and a unit vector rn in the
direction of the transition moment.
I
For a single molecule or for a n assembly of molecules
having a definite orientation, the extinction coefficient
[2] S . E. Sheppard, Rev. mod. Physics 14, 303 (1942).
[3] K . Dimroth, S.-B. Ges. Beford. ges. Naturwiss. Marburg 76,
No. 3, 3 (1953).
[4] C . Reichardt, Angew. Chem. 77,. 30 (1965); Angew. Chem.
internat. Edit. 4, 29 (1965).
[S] N . Q. Chako, J. chem. Physics 2, 644 (1934).
178
&orient with light polarized in the direction of the unit
vector e is:
me is the scalar product of the unit vectors in and e,
and is therefore equal to the cosine of the angle y between them. For an assembly of molecules having a
definite orientation, the extinction coefficient depends
on y as indicated in eq. (3), i.e. on the polarization
direction of the incident light wave. The direction rn
of the transition moment of molecules having a known
orientation can therefore be determined from the
dependence of the extinction coefficient on the polarization direction of the light. Partial orientation of
molecules in solution can be achieved by means of an
external electric field Fa. The direction of the transition
moment can be found from the change in optical absorption due to this field.
The total electric dipole moment p: of a molecule in
the electronic ground state g in the electric field is
F
P,
=
P, + ag F,
(4)
p g is the permanent dipole moment, ug is the polarizability tensor, and Fe is the effective field strength at
the position of the molecule. According to eq. (4), the
dipole moment of a molecule is influenced by an electric field; the quantitative measure of the dependence
is the polarizability ag-For molecules having a sufficiently large permanent dipole moment, p g [
agFe
and the polarizability term in eq. (4) may be disregarded. In this approximation the energy EE of a
dissolved molecule in the electronic ground state in
the electric field is
I >I
1,
EP' is the energy of the dissolved molecule in the
ground state with no external field, and 9 is the angle
between the vectors p g and l i e . For molecules having
a small permanent dipole moment, on the other hand,
the term in eq. (4) that contains the polarizability ug
is no longer negligible. Eq. ( 5 ) then also contains an
additional term that depends on ug. For the sake of
simplicity, however, this will not be taken into account
here or in the following equations.
According to eq. ( 5 ) the energy E[ depends on the
direction of the dipole moment p g in relation to the
field Fe. The orientation distribution of the molecules
in a homogeneous field, e.g. between parallel electrodes, is therefore no longer isotropic. There will be
more molecules with their dipole moments parallel to
the field direction than with their dipole moments
opposing the field direction. The distribution of the
molecules over all possible orientations in the equilibrium state can be determined by means of MaxwellBoltzmann statistics. The extinction coefficient EF of
the dissolved molecules in the electric field is found
from eq. (3) by multiplication by the Boltzmann factor
C exp( -EE/kT) and integration over all orientations.
Angew. Chem. infernaf.Edit.
/ Vol. 8 (1969) J No. 3
The relative change in the extinction coefficient as a
result of the orientation effect in an electric field is
found to be
effect. This hinders the simple determination of the
directions of transition moments, but at the same time
it allows the determination of additional characteristic
molecular data.
2.2. Band Shift Effects
is the angle between the direction of the external
field Fa and the polarization direction e of the incident
light, k is the Boltzmann constant, and T is the absolute
temperature. The relative change in the extinction
coefficient depends on the square of the field strength,
the angle
and the angle y between pgaand pg,since
h p g = pg cos y. For the case in which the transition
moment pgaof a band is parallel to the dipole moment
pg of the molecule, and the polarization direction e of
the incident light is chosen parallel to the external
field Fa, it follows from eq. ( 6 ) that
x,
and for the case in which pgais perpendicular to pg
and e is again chosen parallel to Fa,we obtain
The extinction coefficient of an absorption band whose
transition moment is parallel to the dipole moment
increases in an electric field, while that of a band whose
transition moment is perpendicular to the dipole
moment decreases.
The numerical values given relate to p-nitroaniline
(pg = 6.3 D); the field strength used was Fa = 10s V/
cm, i.e. approximately the maximum external field
strength that can be achieved in solution, and the
temperature was T == 293 OK.
Extinction changes of the order of 10-4 due to the
orientation effect are thus to be expected in a strong
external electric field. To be able to measure the extinction changes with an error of = 1 %, an accuracy
of about 10-6 is necessary; this can be achieved with a
measuring set-up developed by Labhart L6.71. Measurements on the first intense band of p-nitroaniline showed an increase in the extinction coefficient in an electric field, i.e. the transition moment is parallel to the
dipole moment.
The orientation effect, which leads to dichroism of the
absorbing solutions in a n electric field, was described
by Werner Kuhri and his colleagues[sl, and was to be
used to determine the directions of the transition
moments of electronic bands. However, at least two
other effects are superimposed on the orientation
[6] H . Labhart, Chimia I S , 20 (1961).
[7] W . Liptay, W . Eberlein, H . Weidenberg, and 0 . Elf'lein, Ber.
Bunsenges. physik. Chem. 71, 548 (1967).
[8] W. Kuhn, H . Duhrkop, and H . Martin, 2. physik. Chem., Abt.
B 45, 121 (1940).
Angew. Chem. internat. Edit.
1
Vol. 8 (1969) 1 No. 3
The electronic excitation of a molecule may be associated with a change in the dipole moment p a - p g
(pa =: dipole moment in the excited electronic state in
question). The energy of a molecule in the ground
state in an electric field is given by eq. ( 5 ) . In the
excited state,
EF' is the energy of the dissolved molecule in the
excited state with no external field. In the second part
of eq. (7) and in the following equations, it is assumed
for simplicity that the dipole moment pa in the excited
state is parallel to the dipole moment pg in the ground
state; this is in fact so in p-nitroaniline and, in general,
in molecules having symmetry Czv. For a given transition having the absorption wave number Jp' in
g L
solution with no external field (he:?' = Eio' - Esol
the absorption wave number G: in an electric field is
given by
The change in the dipole moment (pa - pg) thus leads
to a field-dependent shift of the band, which also
depends on the angle 9. between the dipole moment
and the field direction, as illustrated in the term diagrams of Figure l .
a)
Fig. I .
b)
C)
d)
Dependence of the absorption wave number on the orientation
of the dipole moment with respect to the field direction (va ,!vg).
Figure l a shows the term diagram of the dissolved molecule
with no external field; the difference between the two levels is
proportional to G p I . If the dipole moment of the dissolved
molecule is oriented parallel to the field (Fig. lb), cos W = 1,
so that the energy in the ground state is reduced by pgFe and
that in the excited state by vaFe. In the example considered
here pa > vg; the distance between the levels EF and Eg" thus
decreases, and the absorption wave number G f t (parallel
orientation) < G p l . If the dipole moment is oriented perpendicular to the field (Fig. lc), cos W = 0 and the energy levels
will be the same as in the absence of an external field; thus G1
(perpendicular orientation) = "0'. Finally, if the dipole
moment is oriented antiparallel to the field (Fig. Id), i.e.
cos 0 = -1, the energy in the ground state will be increased by
! L ~ Fand
,
that in the excited state by paFe; in this particular
case, therefore G t 1 (antiparallel orientation) > GZOl.
179
cm-1. The width of the band at half its height, on the
other hand, is 6000 cm-1. The shift is therefore greatly
exaggerated in Figure 2, since for a 1 cm shift of the
band, the width at half height should be 6 m.
I
I
I
I
I
I
I
Fig. 2. Unsymmetrical broadening of an absorption band as a result
of the band shift.
Figure 2 shows the absorption spectrum ~sol(S)for the band
in the absence of an external field, with a maximum at S;ol.
For the molecules with p g parallel to Fe,Stt < S p l , i.e. the
f
at lower wavenumbers. For moleabsorption band ~ f lies
cules with pg perpendicular to Fe. the absorption band E~
appears at the same wavenumber as in the absence of an external field. Finally, for molecules with p g antiparallel to Fe,
the absorption band E T is~ situated at higher wavenumbers
than with no external field. The distribution of the molecules
over the various orientations is determined by Boltzmann
statistics. There are thus always more molecules with p g
parallel to Fe than with pg perpendicular to Fe, and more
with p g perpendicular to Fe than with p g antiparallel to Fe.
The absorption component E T is
~ therefore greater than the
,
this in turn is greater than E T J .
component E ~ and
In this discussion we have picked out three possible orientations. However, the molecules are distributed monotonically
over all orientations. The actual course of the absorption
curve EF@) in an external electric field is approximately as
shown by the broken curve.
The band displacement effect leads to broadening of
the absorption band in an external electric field. Since
this broadening is unsymmetrical, the maximum of the
band is also shifted. According t o eq. (8) the maximum
shift is Sy' - S t t = ( p a - pg)Fe/hc. In the example of
p-nitroaniline pa - pgw 8 D, and SO Sy' - S t t m 10
It can be seen that the shift of the maxima of the broad
absorption bands of molecules due to an external electric field, unlike the normal Stark effect in sharp spectral lines, cannot be observed directly. In the case of
a constant, very small shift, the field-dependent extinction at a given wave number depends an the slope
of the band, and increases with increasing steepness of
the band (Fig. 3). The change in extinction due to the
band shift can therefore be satisfactorily measured on
steep sides of the absorption band.
To calculate the relative change in the extinction coefficient in the external electric field as a result of the
band shift, it is necessary to start with equations (3)
and (8), and again to average over all orientations.
This gives
The slope of the band (d ln(ESOl/S)/d S ) must be taken
at the absorption wave number Ja in question; since
the slope is a function of F, the relative extinction
change also depends on the wave number. The relative
extinction change also varies with the square of the
field strength, the angle 1,the dipole moment in the
ground state, the change in the dipole moment, and the
direction o f the dipole moment with respect to the transition moment. In favorable cases therefore the change
in the dipole moment pa - pg and hence the dipole
moment pa of the molecule in the excited state can be
determined from the experimental quantity corresponding to the band shift. In the example of p-nitroaniline
For a steep flank of an absorption band, the derivative
of ln(Esol/S) with respect to S may be about 10-3, so
that [(EF-ESo')/ES0I]BS
may be about 5 x 10-3.
In the case of steep bands, the relative change in the
extinction coefficient caused by the band shift may be
more than ten times as great as that due to the orientation effect.
2.3. Direct Field-Dependence of the Transition Moment
y"Fig. 3. Change in extinction in an electric field due to the band shift.
absorption curve E S O I ( ~ ) of the dissolved molecule with no external
field, - - -: EF(?) in a field.
(EF - ESO~)G represents a large change in extinction in the region of a
steep rise, and (EF - E S O ~ ) K is a small change in the region of a gradual
rise.
-:
180
If the wave function of' a molecule is known, the dipole
moment can be calculated by an integration process;
for example the dipole moment p z of the molecule in
the ground state g in an electric field is
Angew. Chem. internat. Edit. J Vol. 8 (1969)/ No. 3
ri are the coordinates of the positions of the nuclei and
electrons of the molecule, their charges being ei, and
$: is the wave function in the electric field. The
integration must be carried out over the coordinates
of all the particles over the entire space.
Similarly, the electric dipole transition moment pza
for a transition between the ground state g and an
excited state a in an electric field is
Thus
F
P,
F
= Pgs
To find p:, it is formally only necessary to replace the
index a in eq. (11) by g, i.e. to use the wave function
@ of the ground state instead of the wave function
$: of the excited electronic state.
The dipole moment p: in an electric field can also be
expressed with the aid of the polarizability tensor ag
by eq. (4).A completely analogous expression can also
be given for the transition moment pza in an electric
field:
is the transition moment of a molecule in the
absence of an external field; it corresponds to the permanent dipole moment, and will therefore be referred
to as the permanent transition moment. The tensor aga
is a quantitative measure of the effect of an electric
field on the transition moment. Owing to the analogy
with polarizability, aga will be referred to as the
rransition polarizabiliry tensor. According to a quantum-mechanical perturbation calculation, its components (aga)ijare given by the following equation ‘91:
transition moment can be influenced by an electric
field [eq. (13)], with the result that the intensity of the
band also depends on the field. For an electronic excitation with a sufficiently large permanent transition
moment pga, pga
agaFe and the transition
polarizability tensor in eq. (13) may be neglected. In
excitations of this nature, the intensity of the absorption band is practically unaffected by an electric field,
as is frequently found e.g. in the case of p-nitroaniline.
However, there are also electronic excitations for
which the permanent transition moment is at least
approximately zero, though the excitation is not forbidden by symmetry, r.g. in fluorenone [*01. In cases of
this type the effect of an electric field on the absorption is easy to determine, even if the tensor components of agaare very small. Finally, some dyes exhibit
electronic excitations for which the tensor components
of a g a are so large that the intensity is strongly dependent on the field, despite the high value of the permanent transition moment pga19,111.
I
I>I
1,
To calculate the relative change in the extinction coefficient due to the direct field-dependence of the
transition moment, it is necessary to start with equations (3) and (13) and average over all orientations;
this gives (neglecting any change in dipole moment on
excitation):
pga
( p r g ) i and (p& are the i-th components of the permanent transition moments between the states r and g
and between r and a; E,, Ea, and Er are the energies
of the states. The components of the polarizability
tensor ag of a molecule can be expressed by an equation similar to eq. (14), since
ag = agg
(15 )
i.e. to determine the components (ag)ij of the polarizability, it is formally necessary only to replace the
indices a by g. According to equations (14) and (15)
the polarizability tensor is symmetrical: (a& =
(a&. In general, for the transition polarizability ten-
sor, (aga)ij
*
(aga)ji,
i.e. the tensor is not symmetrical [91.
According to eq. (l), the intensity of an absorption
band is determined by the transition moment. The
[9] W. Liplay, B. Dumbacher, and H . Weisenberger, 2. Naturforsch. 23a, 1601 (1968).
Angew. Chem. internat. Edit. / Val. 8 (1969) No. 3
The relative change in the extinction thus also depends
on the square of the field strength, the angle
the
dipole moment pg, and the transition moment pE’ of
the dissolved molecule, as well as on the tensor agaof
the transition polarizability. In all the molecules
studied so far, the terms of the type (mga)ij(aag)kl are
negligible in relation to the other terms.
x,
The relative change in the extinction coefficient due to
the field-dependence o f the transition moment may be
of the same order of magnitude as the changes due to
the orientation effect and the band shift.
2.4. Electro-Optical Absorption Measurements
The measurable extinction change in the absorption
spectrum of a molecule in an external electric field
results from the superposition of three effects, i.e. the
orientation effect, the band shift, and the direct effect
of an electric field on the transition moment. A complete theoretical treatment leads to relations that
[lo] W . Liptay, H . Weisenberger, F. Tiemann, W . Eberlein, and
G. Kunopka, 2. Naturforsch. Z3a, 3 1 1 (1968).
1111 W . Liptay, H.-J. Schlusser, B. Dumbacher, and S . Hiinig, Z.
Naturforsch. 23a, 1613 (1968).
181
permit the evaluation of experimental data ‘12-15~. To
obtain relations of this type it is necessary to calculate
the effective electric field F, at the position of the dissolved molecule with the aid of the external electric
field Fa,which can be determined in the case of parallel
electrodes from the applied voltage U and the distance
d between the electrodes by means of the expression
Fa = U/d. The effective field F, in equations ( 5 ) and
(6) can be expressed as the sum of a cavity field Fh and
a reaction field F R 116-171:
~
Fh and F R g can
be represented as functions of Fa by
means of approximations. In the approximations used,
the solvent is regarded as a homogeneous dielectric
continuum with the dissolved molecule situated in a
cavity. In the simplest approximation the cavity is
regarded as a sphere ‘14,151; however, comparison of the
electro-optical absorption measurements with dielectric measurements showed that this approximation is
unsatisfactory in the case of molecules whose shapes
are very different from spheres, and that it is necessary
to use ellipsoid-shaped cavities [9,1OJ. In the calculation
of the effective field Fe in eq. (7)and in the following
equations, it must be borne in mind that a change in
the dipole moment on excitation is associated with a
change in the reaction field, i.e. that the reaction field
of a molecule in the electronically excited state may be
different from that in the ground state. According to
the Franck-Condon principle, electronic excitation
leads to a change only in the electronic wave function,
whereas the positions of the nuclei in the FranckCondon excited state are, to a good approximation,
the same as in the ground state. To calculate the reaction field, therefore, the polarization of the solvent
must be separated into electron displacement polarization and atomic and orientation polarization, only
the first of these being different in the Franck-Condon
excited state from that in the ground state. Finally,
the anisotropic polarizability of the dissolved molecules and its change on excitation can also be taken
into account. Detailed calculation shows that the
change in the relative extinction coefficient in an external field for an isolated absorption band which is
not superimposed by another electronic band can be
represented in the following form [ l o , 14,151.
The quantity L;, which was given explicitly in earlier
publications [lo, 14,151, depends on the absorption
wavenumber Ga, the angle x between the polarization
1121 W. Liptay and J . Czekalla, Z . Naturforsch. 15a, 1072 (1960).
[13] W. Liptay and J . Czekalla, Z . Elektrochem., Ber. Bunsenges.
physik. Chem. 65, 721 (1961).
[14] W. Liptay, Z. Naturforsch. ZOa, 272 (1965).
[IS] W. Liptay: Modern Quantum Chemistry. Academic Press,
New York 1965, Vol. 3, p. 45.
[16] L. Onsager, J. Amer. chem. SOC. 58, 1486 (1936).
[17] C. J. F. Bottchrr: Theory of Electric Polarisation. Elsevier,
Amsterdam 1952.
182
direction of the light and the external field direction,
the dipole moment p g and change in dipole moment
pa-pg, the transition moment pga,and the transition
polarizability tensor aga.The polarizability tensor of
the dissolved molecule and its change on excitation
often play a minor role. For suitable molecules, particularly molecules having (approximately) symmetry
CzV,the dependence of L‘X(Ga) on the wave number
and on the angle x allows the evaluation of the results
of electro-optical absorption measurements and the
determination of the direction of the transition
moment, the dipole moments p p and pa. and certain
components of the tensor Uga. The dipole moment in
the ground state can naturally also be obtained from
dielectric measurements; comparison of the results of
the two methods provides a check in particular on the
electro-optical absorption measurements and the
models on which they are based (cf. Table 1).
So far we have been discussing the effect of an external
electric field on the optical absorption. The effect of an
external electric field on emission can be treated in a
similar manner [27J, except that the final lifetime of a
molecule in the excited state has also to be taken into
account in this case, so that the excited state need not
be identical with the equilibrium state.
0
Table 1 gives the dipole moments and the directions of
the transition moments that have been found so far
from electro-optical absorption and emission measurements. The dipole moments pg, the changes in dipole
moment pa-pg on excitation, and the components
(aga)==
or (aga)yzand (aga)zy
of the transition polarizability are responsible for the strong solvatochromism
of polar molecules (see Section 3). These quantities
allow a quantitative treatment of the problem.
Angew. Chem. internat. Edit. J Vol. 8 (1969) J No. 3
Table I .
Dipole moments and transition moment directions from electro-optical absorption and fluorescence measurements.
~
Solvent
Compound
[a1
i a .10-3 Y
(cm-1)
( ”)
Ibl
[cl
Ref.
[dl
rfl
le I
- __
__.
C
H
H
H
H
D
B
B
D
DME
B
H
B
DME
B
H
H
D
D
D
H
B
B
B
B
B
B
4-Dimethylamino-4’-nitrobiphenyl
B
B
4-Dimethylamino-4‘-cyanostilbene
D
D
4-Amino-4’-nitrostilbene
B
B
4-Dimethylamino-4’-nitrost~ibene
B
B
4-Dimeth~lamino-4’-nitroazobenzene
B
N-(4-Dimethylaminobenz~Iidene)-4-nitroaniline B
N-(4-Nitrobenz~lidene)-4-dimethylaminoaniline B
2-Amino-7-nitrofl uorene
B
B
9H-Carbazole
D
D
3,6-Dinitro3H-carbazole
D
9-Fluorenone
D
D
HP
1-Indanone
Xanthone
(1)
I71
C
C
D
D
D
D
D
D
D
D
D
D
D
D
26
36
31
35
42
23
29
29
28
27
26
44
26
26
25
36
36
25
30
25
28
24
19
31
25
25
18
24
19
25
21
24
18
23
18
28
24
22
25
20
29
34
28
23
31
31
31
39
34
29
27
28
31
19
19
20
19
17
18
12
0
90
~60-90
F90
F40
F 0
;r0
0
0
0
3.6
4.2
3.1
2.8
2.8
6.6
6.3
5.8
6.2
F O
6.9
6.9
F
90
0
0
F
F O
90
F
F O
00
0
90
F
F
F
F
F
F
F
O
O
O
O
O
O
O
F0
F0
F
F
F
F
F
F
F
F
F
O
O
O
O
O
O
O
O
O
small
small
0
90
90
90
90
90
90
90
F 10
0
90
90
0
0
0
0
F20
0
0
F30
3.5
%O
3.5
9
12.2
7.1
6.3
13
5.0
6.1
5.7
14
14
15
12
;rO
;rO
;rO
15
12
5.1
5.0
6.9
6.5
5.6
5.8
5.8
6.1
6.9
7.7
7.7
6.0
6.0
6.0
6.0
6.6
6.6
7.1
7.1
6.5
6.5
7.1
7.1
8.0
8.2
6.6
kI
-
5.0
5.2
5.8
;r0
13
12
13
13
13
12
16
12
14
18
%O
%0
-0
;rO
18
16
14
23
22
24
22
20
21
22
23
26
25
25
23
23
23
5.8
19
1.7
1.7
3.3
3.3
3.1
3.1
3.1
3.6
3.8
7.3
7.3
7.3
12.1
11.5
11.2
8.9
13.0
13.0
14.8
1.9
1.6
6.6
3.0
11.5
11.0
10.7
12.7
12.7
3.1
3. I
14
5.5
4.5
4.4
4.5
3.7
4.4
5
F O
F O
-0
F O
8
6
-0
F O
9
20
20
21
16
16
17
6
%O
3.0; 6.6
3.7; 6.2
3.5; 7.2
F O
F O
F O
41
37
38
28
120
%O
-25
[a1 H = hexane, H P = heptane, B = benzene, C = cyclohexane, D = 1,4-dioxane, D M E = dimethoxyethane. [bl Average value of the wavenumber
range evaluated. Icl Angle between the dipole moment in the ground state and the transition moment. [d] Dipole moment in the ground state from
dielectric measurements. [el Dipole moment in the ground state from electro-optical absorption measurements.
[I] Dipole moment in the excited
State from electro-optical absorption measurements. [gJ Dipole moment in the excited stale from electro-optical fluorescence measurements.
1181 H . Labhart and G. WagniZre, Helv. chim. Acta 46, 1314
(1963).
[I91 W. Liptay and B. Dumbacher, unpublished.
1201 J . Czekalla, W. Liptay, and K . - 0 . Meyer, Z. Elektrochem.,
Ber. Bunsenges. physik. Chem. 67, 465 (1963).
[21] J . Czekalla and G . Wick, Z. Elektrochem., Ber. Bunsenges.
physik. Chem. 65, 727 (1961).
Angew. Chem. internat.
Edit. / Vol. 8 (1969) / No. 3
[221 W. Liptay and H . Weidenberg, unpublished.
I231 H . Labhart, Helv. chim. Acta 44, 457 (1961).
1241 J . Czekalla and G. Wick, unpublished.
[25] W. Liptay and W. Eberlein, unpublished.
[261 W. Liptap
and H . Weisenberger,
[271 W. Liptay, Z . Naturforsch. 18a, 705 (1963).
183
3. Solvatochromism
molecule in the same way as an external electric field,
i.e. it is capable of causing a band shift and a change
It was shown in Section 2 that the position and intensity of an electronic band can be influenced by an
electric field. In any solution, molecules having a permanent dipole moment are located in a n electric field,
the reaction field of the dissolved molecule, as illustrated in Figure 4. The reaction field acts on the dissolved
b)
a)
in the transition moment, and hence in the intensity
of the band.
It can be seen in Figure 4 that the reaction field depends on the nature and arrangement of the surrounding solvent molecules. The accurate determination of
the (mean) reaction field, which is possible in principle
by averaging methods, presents difficulties that have
not yet been overcome. Useful approximations have
however been developed. In the simplest approximation the solvent is regarded as a homogeneous isotropic dielectric continuum having a dielectric constant EDC. The dissolved molecules are assumed to be
acccmmodated in this continuum in spherical cavities
having a radius a, or, in a refined version, in ellipsoidshaped cavities [9J. The dipole moment is represented
by a point dipole located in the center of the spherical
cavity. In this approximation, the reaction field F R
of a dissolved molecule in the electronic ground
state 116,171 is given by
p; is the total dipole moment (permanent plus induced
moment) of the dissolved molecule.
C)
Fig. 4.
Reaction field of a dissolved molecule:
(a) A molecule in the gas state with a dipole moment 8 , e.g. p-nitroaniline, causes an electric dipole field in its environment, the field lines
of which are shown broken.
(b) In solution, this dipole field acts o n the surrounding solvent molecules. If the solvent molecules have a permanent dipole moment, they
orient themselves with their dipole moment a s nearly a s possible parallel
t o the field lines. The orientation is opposed by thermal motion, so that
only some of the solvent molecules are ideally oriented at any time. If
the solvent molecules have no permanent dipole moment, a dipole
moment is induced in the dipole field of the dissolved molecule. In the
case of solvent molecules having a permanent dipole moment, the polarizability effect is superimposed on the orientation effect.
(c) If the electronic and nuclear configurations of all the solvent molecules are imagined t o be frozen and the dissolved molecule then removed,
a cavity surrounded by solvent molecules remains. Each of these solvent
molecules has a dipole moment, which is made up from the permanent
and the induced moment. Each of the dipole moments of the solvent
molecules produces a dipole field in its environment, the field lines of
which are shown broken. In the cavity, at the position of the dissolved
molecule. the dipole fields of the solvent molecules superimpose and produce a field having the same direction as the dipole moment of the original dissolved molecule; this field IS known as the reaction field PR.If
the dissolved molecule is now imagined to be back in the cavity, it is
clearly in an electric field.
184
The changes in the position and intensity of an electronic band depend not only on the reaction field in
the ground state [eq. (19)], but also on the reaction
field in the excited electronic state. When the FranckCondon principle for optical excitation processes is
taken into account, the expressions for the reaction
field in the excited state contain not only terms similar
to that in eq. (19), but also other similar terms that
depend on the refractive index n of the solution. The
effect of the field on the position and intensity of electronic bands is therefore determined by the dielectric
constant and the refractive index of the solution in
this approximation. The effect of the reaction field is
the main cause of strong solvatochromism, which is
observed in particular with molecules having large
dipole moments, and hence strong reaction fields. The
effect of the reaction field on the electronic bands is
much greater than that of an external field, for two
reasons.
The first is the much greater strength of the reaction
field. A molecule having an interaction radius a = 4 x
10-8 cm and a dipoIe moment of only & = 1 D, in a
nonpolar solvent having EDC = 2, has a reaction field
F Rw
~ 2 x 1 0 6 V/cm; in a polar solvent with EDC =
30, F R ~w 4.5 x 106 V/cm. The maximum field
strength that can be achieved with an external field, on
the other hand, is not much greater than 105 V/cm.
Secondly, the reaction field (on average) is always
parallel to the dipole moment of the dissolved molecule. In an external field on the other hand, the molecules are distributed over all possible orientations
with respect to the field; at the maximum possible
field strength the isotropic orientation distribution is
cnly slightly disturbed, i.e. there are relatively few
Angew. Chem. internat. Edit. 1 Vol. 8 (1969)
/ No. 3
~
more molecules with their dipole moments parallel to
the field than with their dipole moments perpendicular
or antiparallel to it. The contributions of the individual
molecules to the change in the positions and intensities
of the bands due to the external electric field thus largely cancel one another out in the average, so that the
observable effects are small.
3.1. Solvent-Dependence of the Position of an
Electronic Band
The solvent-dependence of the positions of electronic
bands was discussed in detail in an earlier publicationr281. For a molecule having a permanent dipole
moment p g - pgz in the electronic ground state (in
the z direction) and a moment pa = pa. parallel to
it in the excited state, the solvent-dependence of the
wave number Gy' of the absorption maximum is given
to a good approximation by the following equation
1 I
1 I
Gg is the wave number of the absorption maximum in
the free molecule (gas state). Eq. (20) corresponds to
eq. (8), except that it also contains a term that depends on a quantity D. This term describes the solventdependence of the position of the absorption band
due to the dispersion interactions between the dissolved molecule and the surrounding solvent molecules.
For the low electronic excitations D > 0, so that the
dispersion interactions always cause a red shift with
increasing refractive index of the solution. N o reliable
information is available for higher excitations. The
effective electric field FRM in eq. (20) is the mean of
the reaction fields of the dissolved molecule in the
ground state and in the Franck-Condon excited
state 1281, i.e. the primary state after the excitation
process. For the particular case in question the z component of FRM is:
EDK is the dielectric constant of the solution and n is
its refractive index. agz is the z component of the
polarizability tensor ag of the molecule in the ground
state, transformed to principal axes. Any polarizability
change on excitation is disregarded. Relations for
cases in which p g is not parallel to p a and in which the
polarizability change on excitation is taken into
account are given in 1281.
The solvent-dependence of the position of an electronic
band can, according to eq. (20) with the aid of eq. (21),
be represented by a function that depends on the
properties of the dissolved molecule (pgz,paz, agz,a,
[28] W. Lipray, Z. Naturforsch. 20a, 1441 (1965).
Angew. Chem. internat. Edit.
1 Vol. 8 (1969) 1 No. 3
D) and on the dielectric constant and the refractive
index of the solvent. Since there is no correlation between the dielectric constant and the refractive index
uf polar solvents, at least two parameters that depend
on the solvent are necessary for the general description
of the solvent - dependence of electronic bands.
A general description involving only one solventdependent parameter such as has repeatedly been
sought 141, is fundamentally impossible. In the case of
a dissolved nonpolar molecule, however, FRMdisappears, leaving only the term containing the refractive
index n in eq. (20), so that the solvent-dependence of
the wave number can now be described by a single
parameter (e.g. by n). For molecules having a large
dipole moment in the ground state and a large change
in moment, the solvent-dependence due to the term in
n is small, and the effect of the solvent on the wave
number approximately depends only on EDC.
The dipole moments in the ground and excited states
can be determined for suitable molecules from electrooptical absorption and dielectric measurements. The
polarizability agzmay be determined independently or
estimated, and the quantity D may be estimated (281 or
determined by matching of the experimental data.
Using an estimated value a and the values EDC and n,
it is then possible to calculate [ a 3 ( F ~ ~ ) Z ( p a z - p giz)
2 hcD(n2 - 1) / (2 d + l)] via eq. (21) for various
solvents. A plot of
as a function of this expression
should give a straight line, from which G g and a can be
obtained. This value a is then used to repeat the evaluation until a used for the calculation of the bracketed
expression in eq. (21) agrees, within the limits of accuracy, with the value of a obtained as the result.
Gr'
Corresponding experimental studies were carried out
on trans-4-dimethylamino-4'-ni trostil bene 1281, fluorenone 1101, and some dyes [111; the results satisfied the
linear relation with reasonable accuracy, and so confirmed the above models. The solvent-dependence of
the wave number of the absorption maxima of electronic bands can be described, both for an increase
and for a decrease in the dipole moment on excitation,
by eq. (20) with the aid of eq. (21). The solventdependence of the position of an absorption band is
determined mainly by the dipole moment of the dissolved molecule in the ground state and the change in
the dipole moment on excitation. An increase in the
dipole moment on electronic excitation leads to a red
shift, and a decrease in the dipole moment to a blue
shift of the band with increasing dielectric constant of
the solution. This change in position, which depends
on the dielectric constant, is superimposed on a shift
that depends on the refractive index. The latter shift is
due to dispersion interactions and the change in the
dipole moment on excitation, and, at least at low electronic excitations, always leads to a red shift with increasing refractive index of the solution. In the case of
apolar molecules with pg = pa =: 0 the reaction field
FRM=: 0 and the second term in eq. (20) disappears.
The solvent-dependence is then mainly due to the dispersion interactions. These interactions can cause a
strong red shift on transfer of the molecule from the
I85
gas state into solution, but because of the smaller
range of variation of the refractive index of the solvent,
only a weak red shift is generally observed with increasing refractive index of the solvent, e.g. in the case
of aromatic hydrocarbons 1281. For molecules having
only a small dipole moment in the ground state and a
largechange in dipolemoment onexcitation, thereaction
field FRMmay be largely determined by the term in eq.
(21) that depends on the refractive index. This will also
lead to a strong red shift of the absorption band on
transfer of the molecule from the gas state into solution,
and a weaker red shift with increasing refractive index
of the solvent, as is found e.g. in the case of electron
donor-acceptor complexes.
3.2. Solvent-Dependence of the Intensity of
Electronic Bands
In addition to the solvent-dependence of the extinction
coefficient, many molecules give a change in the width
of the absorption band, with the result that the total
intensity varies only slightly with fhe solvent [29,301.
Electro-optical absorption measurements have sometimes [9,101 shown a considerable field-dependence of
the transition moment. Owing to the solvent-dependence of the reaction field, the intensities of the absorption bands were also expected to depend on the solvent
in these cases, and this was in fact confirmed experimentally f10,111. A detailed treatment of the problem I311
showed that the intensity of an optical absorption can
be described with the aid of the integral absorption
(see eq. (1)); the solvent dependence of the integral
absorption is due to the solvent-dependence of the
transition moment:
pga and pgl are the transition moments in the free
molecule (permanent transition moment) and in the
dissolved molecule respectively; agais the transition
polarizability tensor, the components of which are
given by eq. (14). Eq. (22) corresponds closely to eq.
(13), except that it also contains a vector W,,, which
may be interpreted as the effect of the dispersion interaction on the transition moment. The effective electric
field FRM is again the mean of the reaction fields in
the ground state and in the Franck-Condon excited
state. For the special case of pg parallel to pa (in the
z direction), FRM = (FRM),is represented by eq.
(21). The vector W,, should generally be very small,
so that for molecules having a large reaction field FRM,
i.e. for molecules having a large dipole moment, the
last term in eq. (22) may be ignored; for molecules
having no dipole moment or having a small dipole
moment, p?' and hence also the integral absorption
should depend only to a small extent on the sol-
1
[29] R .
s.
I
Mulliken and C. A . Rieke, Rep. Progr. Physics 8, 231
(1941).
1301 L. E. Jacobs and J . R . Platt, J. chem. Physics 16,1137 (1948).
[31] W. Lipray, Z . Naturforsch. 21a, 1605 (1966).
186
vent [29,301. For molecules having a dipole moment,
the integral absorption depends on the reaction field,
which can be calculated for various solvents from the
data obtained in electro-optical and dielectric measurements (cf. Section 3.1). The theoretical models have
been checked and verified experimentally by means of
suitable plots L1O3111. It was found that some molecules
exhibited an increase and others a decrease in the
integral absorption with increasing field strength, and
hence with increasing reaction field 1111. The magnitude
of the increase or decrease in the integral absorption
is determined according to eq. (22) by the transition
polarizability tensor aga. The behavior of a given
electronic band cannot generally be predicted from
other simple molecular quantities. Electronic excitations associated with a very large increase in the dipole
moment may be expected to show an increase in the
integral absorption, while excitations associated with
a very large decrease in the dipole moment may be
expected to show a decrease in the integral absorption
with increasing field strength. However, this single
rule is not generally valid [111.
The possibility of simple observation of the solventdependence of the integral absorption of an electronic
band depends not only on the transition polarizability
Uga (Or more precisely the product ag&M
in eq.
(22)) but also on the permanent transition moment
pga.If pga is large, the solvent-dependence of the
integral absorption can be observed only if the components of the tensor a,, are sufficientIy Iarge. If on
the other hand pgais very small or zero, the solventdependence of the intensity of the absorption band is
clear even when the components of agaare small.
Particularly interesting relations are found in fluorenone [lo].
The direction of the dipole moment will be referred to as the
z axis. Solutions of this molecule give an absorption band at
about 31 000 cm-1 with a transition moment direction in the
molecular plane parallel to the y axis. The maximum extinction coefficient cmaxand the integral absorption of this band
are strongly solvent-dependent [dimethylformamide (EDC =
36.7): emax = 1320; ether (ZDC = 4.4):cmax = 685; heptane
(EDC = 1.97): cmax = 4101. The absorption band can no
longer be observed in the gas state, and cmax is certainly less
than 50. Fluorenone thus has an absorption band whose
intensity is almost zero in the free molecule. The intensity
recorded in solution is due to the perturbation of the molecule by its reaction field. The permanent transition moment,
and hence the integral absorption of the band in the free molecule, is not zero on symmetry grounds, but disappears purely
by chance. The appearance of a field-induced absorption
band with a transition moment in the y direction as a result
of a reaction field in the z direction will be explained in more
detail.
In the molecular orbital (MO) scheme the absorption band
in question can be interpreted, o n the basis of the experimentally observed transition moment directions and SCF-MO
approximation calculations, as resulting from the excitation
9 5 + 98 of an electron from the fifth (filled) r-MO 9 5 into the
eighth (empty) x-MO 9 s . 95 belongs to the irreducible representation A2 of the point group CzV, and 9s to the representation B1; the atomic orbital coefficients ~5~ and csp of
the MOs in the LCAO approximations are illustrated in
Figures 5a and 5b. I n the simple MO approximation the
transition moment in question p g a = e 1/? J 9 5 9 s r d T. In
the LCAO-MO approximation the equation simplifies further, the integral reducing to a sum: pga = e l/2
Angew. Chem. internat. Edit.
14
p= 1
C~,,CS~~,,
1 Vol. 8 (1969) 1 No. 3
where rp are the position vectors of the atoms p. T o
obtain the transition moment pga,it is necessary to form the
products cjpcgprpand carry out the summation over all the
atoms (p = I to 14). Similarly the components of the transition moment (pga)x,(pga)y, and (pga)zin the directions of
the three coordinates are obtained on replacement of the
position vectors rp by their components xp. yp, and zp.
2
2
parallel to the electric field, the atomic orbital coefficients of
atoms 1, 2, 11. 12, 13, and 14 will increase, while those of
atoms 4, 5 , 6 , 7, 8, and 9 will decrease. The contributions to
the integral (pga),,. i.e. the areas for the corresponding atoms
(Fig. 5d). will thus change. the sum of the positive areas
increasing and the sum of the negative areas decreasing, so
that a finite transition moment in the y direction arises in an
electric field in the z direction.
Similar variations of the transition moments and hence of the
integral absorptions of electronic bands have also been
found for some dye moleculesff11.
3.3. A Relation between the Solvent-Dependence of the
Position and the Solvent-Dependence of the Integral
Absorption of Electronic Bands
1168751
C)
d)
Fig. 5. Representation of the molecular orbitals ‘ p ~and ‘pa and of the
components (vga)z and ( w ~of~the
) transition
~
moment of fluorenone
(for practical reasons, the numbering of the skeletal atoms is different
from that normally used):
(a) Atomic orbital coefficients csp of the fifth x - M O ‘ps (representation
A2).
(b) Atomic orbital coefficients cgp of the eighth rr-MO ‘pa (representation B I ) .
( c ) Products cspc8pzp(representation B2).
(d) Products cspcgpyp (representation A,).
The areas of the circles are proportional to the magnitude of the orbital
CspCSpYp)
~ ~ Z for
~ .the atom
coefficient (csp, csp) or of the product ( C ~ ~ C
p in question. Continuous circles denote positive signs. and broken
circles negative signs.
The products c ~ ~ arec shown
~ ~ inz Figure
~
5c. Since the xz
plane of the molecule is a plane of symmetry, the magnitudes
of the coefficients qPand csP on equivalent atoms, e.g. o n
atoms 1 and 1 2 or 4 and 9, must be the same, and so also must
Z ~ signs
.
of the
the magnitudes of the products C ~ ~ C S ~The
product cspcgpzp for equivalent atoms are always dissimilar,
i.e. c54cS4z4 = -cs9csgzg. The sum, and hence the z component of the transition moment, must therefore be zero. The z
component of the transition p5 + ps is forbidden in the electric dipole approximation by the symmetry of the molecule
and the irreducible representation of the MOs involved. It
can similarly be shown that the x component of the permanent
transition moment is also zero.
Figure 5d shows the products cspCapYp.The y component of
the transition moment is proportional to the sum of the areas,
and will generally be finite. The optical electronic excitation
95 + ps is therefore generally allowed with a transition
moment direction parallel to the y axis. The y component of
the transition moment is zero only when the coefficients qP
and csp are just such that the sum of the negative terms (sum
of the negative areas in Fig. 5d; atoms 2 to 11) is equal to the
sum of the positive terms (sum of the positive areas in Fig.
5d; atoms 1 and 12). The optical electronic excitation q5 + ‘ps
is then forbidden, but this is accidental, and is not due to
symmetry relations. According to experimenlal studies, this
situation occurs, at least to a good approximation, in the case
of the fluorenone absorption in question in the free unperturbed molecule.
The electrons in the molecule are displaced by an external
electric field or by the reaction field of the dissolved molecule.
On orientation of the molecule with the dipole moment
Angew. Chem. internat. Edit. 1 Vol. 8 (1969) 1 No. 3
A plot of the wave number
of the absorption
maximum of an electronic band of a dissolved molecule as a function of [a3(F~h&(paz- pgJ + 2 hcD
(n2 - 1)/(2 n2 + l)] should, according to eq. (20) and
eq. (21), be linear. It is in fact found that the experimental points for the various solvents deviate more or
less strongly from linearity, and some solvents, such as
dioxane and benzene, are quite irregular. The reason
for the pronounced scatter is the fact that the reaction
field based on the continuum model is only a rough
approximation, which fails in particular in the cases
of dioxane and benzene 1101. The same deviations are
therefore to be expected, and are in fact found experimentally, in suitable plots of the integral absorption
as a function of the field FRMin accordance with
equations (l), (21), and (22). The relations (20) with
(21) and (1) with (21) and (22) can be checked only
with the aid of a large number of solvents, so that
individual deviations cancel out in the “mean”.
According to eq. (20) and equations (1) and (22), both
the position of the absorption band and the integral
absorption depend on the average value of the reaction
field FRM.The reaction field can be eliminated by
combination of equations (l),(22), and (20).
against a function of the integral abIn a plot of
sorption, which is possible in various cases, the deviations due to the poor approximation of the reaction
field should be eliminated, as has been confirmed experimentally 110,111.
The theory of the effect of a n external field on electronic bands can be checked and confirmed by comparison of the results of electro-optical absorption
measurements, electro-optical fluorescence measurements r16,201, and dielectric measurements 19,101. These
methods allow the measurement of the dipole moments
of suitable molecules in the ground and excited states
and certain components of the transition polarizability
tensor. These quantities are responsible for the strong
solvatochromism of polar molecules, and allow a
quantitative description o f the solvent-dependence of
both the positions and the intensities of absorption
bands. An independent check on the models was
provided by various plots of the absorption wave
number against functions of the integral absorption.
Another possible cause of a strong dependence of the
integral absorption on a n external field and on the
187
solvent would be an equilibrium between isomers having sufficiently different dipole moments, e.g. cis-trans
isomers. In such cases the relative quantities of the
isomers could be strongly field-dependent, and hence
solvent-dependent. Since the absorpt'on bands Of
isomers are usually different, the extinction of the solution would then depend on the field and o n the
solvent. Investigations on a number of dyes that exhibit an extremely strong solvent-dependence have
indicated that this possibility is very unlikely, though
it cannot be completely ruled out "11.
we are
to the Fonds der Chemisehen Industrie, the Deutsche Forschungsgemeinschaft, and the
StifLlng Volkswagenwerk for their support of' the experimental and theoretical strtdies.
Received: May 27, 1968
[A 687 IE]
German version: Angew. Chem. 81, 195 (1969)
Translated by Express Translation Service, London
Steps in Precipitation Reactions
By K. H. Lieser[*l
The combined application of various methods of investigation (e.g. nephelometry, conductivity measurements, electron microscopy, isotope exchange, BET surface area determinations, paper chromatography, coprecipitation) lead to a refinrd insight into the
course of precipitation reactions. The formation of a new solid phase within a solution
can, in the case of ionic crystals, be formally described as proceeding via a number of
steps - nucleation, growth, ripening, and recrystallization (aging) - which overlap in
time. The precipitation of hydroxides is a more complex process since additional chemical
reactions (e.g. hydrolysis, condensation reactions! take place within the newly formed
solid phase.
1. Introduction
Precipitation is a fundamental chemicai operation for
the separation and purification of substances. It is well
known that precipitations very often give the desired
result only if certain well-established procedures are
rigidly adhered to, though in many cases the basis of
these procedures, which have been evolved empirically,
are still far from clear.
The principal process in a precipitation reaction is the
formation of a new solid phase from a solution. It is
convenient to distinguish the following steps: nucleation, growth, ripening, and recrystallization (aging).
However, this well-established distinction is only
formal, since the various steps overlap in time.
The following discussion deals mainly with the precipitation of salts (particularly sparingly soluble salts).
Precipitation of hydroxides is a more complex process,
since chemical reactions (e.g. elimination of water,
conversion of basic salts into hydroxides) also occur
during formation of the solid phase, depending on
the pH value and time. Consequently, there are as yet
hardly any theoretical schemes for the general treatment of the precipitation of hydroxides; the principal
features of this process will be discussed in a separate
section.
[*I Prof. Dr. K. H. Lieser
Lehrstuhl fur Kernchemie der Technischen Hochschule
61 Darmstadt, Hochschulstrasse 4 (Germany)
188
2. Steps in a Precipitation Reaction
2.1. Nucleation
The formation of submicroscopic particles (nuclei) of
the new phase from a supersaturated solution is particularly interesting, but relatively difficult to observe
experimentally. It generally begins only when a certain
supersaturation has been reached, and proceeds very
rapidly. The questions that arise in connection with
the formation of a new phase have already been considered in detail by Tanzmann[", Ostwald[21, and
Volmer 131. The kinetics of precipitation reactions have
also been described by Nielsen [41.
Nucleation can be either homogeneous (spontaneous)
or heterogeneous. Homogeneous or spontaneous
nucleation occurs without the participation of other
substances, by combination of the dissolved ions
or molecules to form larger particles. Heterogeneous
nucleation begins on small particles of foreign
matter (seeds), on which ions or molecules are deposited (e.g. by adsorption) until a nucleus has been formed.
[I] G. Tammann: Kristallisieren und Schmelzen. J. A. Barth,
Leipzig 1903.
121 W. Osfwald Die wissenschaftlichen Grundlagen der analytischen Chemie. W. Engelmann, Leipzig 1897.
[3] M . Volmer: Kinetik der Phasenbildung. Th. Steinkopff,
Dresden 1939.
[4] A. E. Nielsen: Kinetics o f Precipitation. Pergamon Press,
Oxford 1964.
Angew. Chem. internat. Edit,1 VoI. 8 (1969)
No. 3
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