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Industrial Application of the Semiconductor Properties of Dyes.

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Industrial Application of the Semiconductor Properties of Dyes[**]
By Hans Meier, Wolfgang Albrecht, and Ulrich Tschirwitz[*]
Effects of industrial interest have been detected in investigations on the dark conductivity
and photoconductivity of dyes. For example, it was found that organic dyes can be used in the
same way as inorganic photoconductors in vidicon television pickup tubes for the reproduction
of images. Among other things, pointers were obtained to the formation of rectifying and
photoelectrically sensitive pn junctions, to relatively high thermoelectromotive forces of
doped and polymeric dyes, to the possibility of use in reprography, and to a high catalytic
activity connected with the conductivity, which is already being utilized in fuel cells.
1. Introduction
IS
Though the semiconductor properties of organic compounds have been under investigation for a number of
years (cf. reviews
the question of an industrial use
of this new class of semiconductors is still in the discussion
stage. On the one hand, the dark conductivity and the
photoconductivity of organic compounds has long been
regarded merely as a side effect, so that the search for a
practical use did not seem very promising. On the other
hand, too much hope was placed on attempts to produce
organic semiconductor components for industrial use with
compounds that were unsuitable. The superiority of inorganic materials then started to lead to a certain amount
of pessimism ; however, there appears to be no justification
for this when one considers the decades that have gone into
the development of inorganic semiconductors.
What can be said about the present position in the industrial application of the semiconductor properties of
organic dyes and about future development possibilities?
2. Conduction Mechanism of Dark Conductivity
and Photoconductivity
Since the discussion of the above question is closely linked
to the mechanism of the dark conductivity and photoconductivity of dyes, some prefatory remarks on the
conduction mechanism are appropriate.
The conductivity of organic dyes, like that of inorganic
semiconductors, is attributed to electrons and defect
electrons (holes). Three processes act together to produce
the dark currents and the stationary photoelectric currents :
1. Generation process: For dyes with a difference ofless than
2 eV between the ground state and the excited state, it may
be assumed that, as in the case of inorganic intrinsic semiconductors, charge carriers are formed by thermal transfer
of an electron from the ground state to the excited state,
since measurements of the temperature dependence of the
dark conductivity 0 [Q- ' cm- '1, which follows eq. (1)
['I
Prof. Dr. H. Meier. Dr. W. Albrecht, and Dr. U. Tschirwitz
Staatliches Forschungsinstitut fur Geochemie
86 Bamberg, Postfach 4041 (Germany)
[**I Presented in part as a lecture to the 4th International Color
Symposium in Lindau on April 11, 1971.
Angew. Chem. infernat. Edit.
Val. 11 (1972) 1 No. 12
= u 0 e x p ( - A E/ 2 k T)
(1)
lead for many dyes to a thermal activation energy AE that
agrees with the optical excitation energy found from the
first absorption maximum[51.The possibility of the formation of charge carriers by an optical excitation process of
corresponding energy can also be seen from the agreement
of the photoconduction and absorption spectra and of the
thermal activation energy AE with the optical activation
energy (AE,,,) deduced from the long-wave limit of the
photoconductivity; for Cu-phthalocyanine (phthalocyaninatocopper(I1))for example, A E = 1.7 and AE,,, = 1.63 eV,
or for indigo AE=1.15 and AE0,,=1.79eV. In addition,
particularly where the difference between the electronic
excitation levels is greater than 2 eV, one may expect thermal
excitation of defect centers (impurities, adsorbed gases,
incorrectly placed counterions in the case of ionic dyes)
and injection of electrons or defect electrons from the
electrodes at high voltages, which leads to space charge
limited currents
(I=current intensity [A], U=voltage [V]). It is also
possible, particularly on excitation with high light intensities (laser light), that a detectable concentration of secondary charge carriers will be produced from excitons (by
ionization of defect centersL6!'I, by annihilation processed8,91).
2. Transport of charge carriers: As in the case of inorganic
semiconductors, many organic solids may be assumed to
have energy bands that arise from the discrete energy levels
of the molecules by electronic interaction and allow quasifree transport of electrons and defect electrons['0! On the
one hand this is confirmed by quantum-mechanical calculations of the energy eigenvalues belonging to the wave
functions of an excess electron or defect electron of the
organic solid, which give band widths of the order of kTor
lo-' eV['1,121,e.g. 0.47 kTfor metal-free phthalocyanine['31.
These band widths are characteristic of the narrow band
type that explains the transport of electronic charge carrier~['~],
as is also shown by the mobilities of 0.1 to 10 cm2
V - s- ' derived with the aid of approximations and by the
free paths corresponding to several lattice constants
(18-80 A for metal-free phthal~cyanine"~').
On the other
hand, the band model is confirmed for dyes and aromatic
hydrocarbons by experimental observations. These include
'
1051
the electron and defect electron mobilities of 0.1 to more
(7") and defect electrons ( T ~ take
)
into account the conthan 1oOcm2V-'s-' deduced from measurements of
centration of defect centers ( N , ) and their capture cross
transient currents['51, Hall
17], thermal EMF['*],
section (s) as follows :
and space charge limited currents["] ;adecrease in mobility
with rising temperature, which is characteristic of energy
band c o n d ~ c t i o nI' ~ ' ;~the
~ observation of an anomalous
Hall effect characteristic ofnarrow-band conductor^"^. 'I1;
and the detection of a charge carrier transport influenced
(thermal velocity v=107 cm/s). It can be seen from eq. (6)
by multiple capture in traps and quantitatively described
that changes in the defect center concentrations, which are
by the energy band theory (Shockley-Read m ~ d e l [ ~ * ] ) [ ~ ~ of
~ .the order of N , = 10'2-1018 ~ m - the
~ , capture cross
3. Recombination of the charge carriers: Two processes lead
to recombination. One is the direct recombination of the
electrons and defect electrons, which occurs in particular
at high concentrations of charge carriers, and the other is
the recombination via defect centers (recombination centers), which capture electrons or defect electrons in the first
step and allow them to recombine in the second step with
the defect electrons of the valence band or with electrons
of the conduction band. Since the parameters of the
recombination centers depend considerably on structural
effects and on the chemical purity, the recombination rate
and hence the lifetime T can vary within wide limits (up
to a factor of 10") in different samples of a compound.
The essential point is that the conductivity of organic dyes is
determined by the interplay of the elementary processes
discussed here; c~.I'~].
The generation of the charge carriers
is determined in the unilluminated system by the thermal
activation energy B E and in the illuminated system by the
primary quantum yield q (=number of excited charge
carriers per absorbed photon). The band width and the
concentration and energy of traps or defect centers are
important parameters to the transport of electrons and
defect electrons, which is expressed by the mobility
p [cm2 V-' s-']. On the other hand, the recombination
process fixes the lifetime and hence the steady-state concentration of the charge carriers.
These processes are generally contained in the equations
for the dark conductivity
sections ranging from s=1O-l2 to
cm', can strongly
influence the photoconductivity [eq. (4)] via eq. (5).
It should be pointed out that if the concentrations and
mobilities of the electrons and defect electrons are equal,
one speaks of intrinsic conduction. The predominant
migration of electrons in the conduction band is referred
to as n conduction, and the predominance of (positive)
defect electrons (or holes) as p conduction (cf. Fig. 1).
I
I
Intrinsic conduction
n -Conduction
p-Conduction
IP9011/
Fig. 1. Diagram of intrinsic conduction, n conduction, and p conduction. E,= Fermi level; E L =conduction band; E,= valence band.
3. Possibility of Influencing the Conduction
Behavior through Structure
In the examination of a possible industrial application,
the most important factors are the attainable orders of
magnitude of the dark conductivity and of the photoconductivity.
3.1. Dark Conductivity
and for the photoconductivity
n and p are the concentrations of electrons and of defect
electrons respectively in the unilluminated solid, pn and pp
are the mobilities of the electrons and defect electrons,
and An and Ap are the steady-state electron and defect
electron concentrations on illumination. An and Ap are
given by
where the generation rate g (=number of charge carriers
formed per cm3 and per second) takes into account the
primary quantum yield, and the lifetimes of the electrons
1052
Organic dyes generally have only a low dark conductivity,
of the order of lo-" to
[Q-' cm-'1, since only a
few charge carriers can be excited thermally because of the
relatively broad forbidden zone between the ground state
band and the conduction band. However, a high conductivity is desirable for some application problems, e. g. for the
production of thermoelectric components or rectifiers. This
can be achieved in dyes, as in other organic solids, by two
methods :
1. By increasing the number of delocalizable 7~ electrons.
This is possible for the following reasons. On the one hand
the activation energy of the dark conduction, as the
intramolecular excitation energy of the 7~ electrons,
decreases with increasing number of delocalizable 7~
electrons[2.241 in agreement with the electron gas theory.
On the other hand the conductivity within a given semiconductor type increases with decreasing activation
Angew. Chem. internat. Edit. / Vol. I 1 (1972) 1 No. 12
energy[25.261. These relationships are illustrated in Table 1
for polymethine dyes ( I a)-(I c ) that absorb at long
wavelengths.
o-chloranil[261.Figure 2 shows the increase in conductivity
that can be achieved by variation of the doping agent.
By means of the above methods, the dark conductivity of
p-conducting dyes can be displaced to the order of magnitude of the conductivity of inorganic materials.
3.2. Photoconductivity
( l a ) , n = 1; (Ib), n =
2;
(Ic), n = 3
Table 1. Relation between number of x electrons N , thermal activation
energy BE, and dark conductivity oD of polymethine dyes (after [27]).
Dye
Structure N
X,,
[nm]
Trimethinecyanine
(1 a)
Pentamethinecyanine ( I b)
Heptamethinecyanine ( I c)
8 605
10 710
12 817
BE
lev]
5D
[n-l cm-’1
1.8
2 x 10-l3
1.4-1.87 10-”-10-9
0.74.98 10-9-10-7
More highly polymeric dyes, such as the polymeric phthalocyanines, also become more conducting as the degree of
polymerization increases. Thus a Cu-phthalocyanine consisting of five units, in contrast with the monomer (AE=
1.7 eVr2I),has an activation energy of 0.12-0.26 eVrZS1,
the dark conductivity increasing from
to 0.30-’
cm-’.
2. Another possibility for increasing the conductivity is
offered by doping with special doping agents such as
o-chloranil, tetracyanoethylene, or tetracyanoquinodimethane126. 29.301 . The conductivity increases very rapidly
with the concentration of the doping agent. For example,
according to eq. (7)
OD =
a x
Under the influence of traps and recombination centers
the photoelectric current of organic dyes can become very
small, particularly when unsuitable measuring arrangements are used (e.g. with an excessively large distance
between the electrodes). This can be understood with the
aid of eq. (8),
c b
(7)
(C=concentration of the doping agent; a=constant), a p
value of 5.3 is found ofi doping of phthalocyanine with
(g’ = quanta absorbed per cm3 per second), which expresses
the relationship between the photoelectric current of a
sample (u= volume of sample, L=distance between electrodes, E = field strength) and the lifetime T and mobility p
of the charge carriers. A change in the defect concentration
affectingthe lifetimeT [according to eq. (611 and the decrease
in mobility p caused by temporary capture in traps must,
according to eq. (8), reduce the photoelectric current.
I,,,, can be increased by an increase in the field strength
and a decrease in the distance between the electrodes.
Above all, however, it is important that the photoconductivity can be reproducibly varied and the quantum yield G
[which can be deduced from eq. (8)] increased through the
elementary processes discussed in Section 2, i. e. by modification of q, p, and T.
- lp,,Je
G = number of detectable charge carriers -~
number of photons absorbed
E
or G = q x x p x - = q x I
L
T
do
(9)
(10)
L
where T = uE
(T=transit time of the charge carriers from one electrode
to the other).
This increase in G can be achieved as follows :
1. By addition of doping agents. It can be seen from Table 2
that the photoconductivity of dyes can be increased by
several orders of magnitude by doping.
The quantum yield G can even assume values >I
with
suitable doping agents[”]. Figure 3 shows how strongly
the photoconductivity can be influenced, as had already
been described by H 0 e g 1 [ ~in~ connection
]
with an electrophotographic application of organic semiconductors, for
phthalocyanine doped with o-chloranil.
For the doping mechanism cf.r15,26,301.
149012/
u PJI
Fig. 2. Dependence of the dark conductivity on doping; plot: log I,=
n log U.
1, iodine; 2, tetracyanoethylene; 3, o-chloranil; 4, tetracyanoquinodimethane; 5, p-bromanil; 6, p-chloranil.
Angew. Chem. internat. Edit. 1 Vol. I 1 (1972) 1 No. 12
2. By specific synthesis. As in the case of the dark conductivity, relationships exist between the photoconductivity and the chemical structure that suggest the possibility
of synthesizing good dye photoconductors. These structural
relationships include (cf. review inr141):
1053
degree of delocalization of the 7~ electrons. An increase in
the number of aromatic rings or methyl substitution increases the photoelectric sensitivity, whereas electronaccepting substituents decrease it.
This structural dependence is further illustrated in Table 3
by some IphoJID values.
Table 2. Change in the photoconductivity of dyes on doping.
Table 3. Dependence ofthe photoconductivity of pyrazines on structure
(after [34]).
Part B of ( 4 )
(2)
(3)
Phthalocyanine
o-Chloranil
Iodine
p-Chloranil
o-Chloranii
Tetracyanoethylene
o-Chloranil
60
1.7 x lo3
c311
6
150
~311
400
104
[26,321
[26,32]
a) In the triphenylmethane dyes, a proportionality between
the degree of methylation of the amino groups and the
photoconductivity and a decrease in sensitivity on replacement of a methyl group by other substituend’].
I Y
114073/
32
36
4.0
l4
L.O
5.2
IPh0J1dUk
7 xlos
7.2 x lo4
2.6 x lo4
4 XI02
The influence of the structure is particularly clear from
Figure 4,which shows the dependence of the photoelectric
current on the voltage for the case of two pyrazines ( 4 )
that differ by a methyl group. This example shows clearly
that systematic studies can be very important for the
clarification of relationships between structure and photoconductivity in the search for dye photoconductors suitable
for practical use.
5.6
l/T.103[K’lFig. 3. Dependence of the photoconductivity on doping; plot: log I,,,,
=f(l/T).
1, Phthalocyanine, undoped; 2, o-CA/Phth.=l.26 x lo-’; 3, o-CAI
Phth.=1.89 x lo-’; 4, o-CA/Phth.=3.15 x lo-’ (0-CA=o-chloranil).
b) In cyanines and merocyanines, an increase in the photoconductivity with the number of methine groups, i. e. with
lengthening of the conjugated
331.
0
c) On modification of part B of aceanthraquinoxaline ( 4 )
100
-
200
u [V]
300
400
Fig. 4. Dependence of the photoconductivity on the structure for aceanthraquinoxaline (4).
4. Possible Applications of the Dark Conductivity
a variation of the action of substituents on the photo-
What possible applications are presented by the conductivity of unilluminated organic dyes?
conductivity in the order[341:
4.1. Rectifiers
It can be concluded from this gradation that the photoconductivity is closely connected with the number and the
1054
Some dyes are p or n conductors in the unilluminated state,
particularly in the doped form. This is confirmed by measurements of the Hall e f f e ~ t t ~or
~ . of
~ ~the
] thermal
EMFr371;in agreement with the rules for extrinsic semiconductors, moreover, the Fermi levels are close to the
Angew. Chem. internat. Edit. 1 Vol. I 1 (1972) No. I 2
energy bands, e.y. close to the ground state band for
phthalocyanine doped with tetracyanoethylene, which is p
conducting (cf. Fig. 5).
t
-'I
contact and prove, with a ratio of forward current to reverse
current of 3000:1, to be suitable for the rectification of
alternating currents up to lo4 H z [ ~ ~cf.
] ;Figure 6.
4.2. Thermoelectric Components
-2
The observation of a thermoelectromotive force that is
high in comparison with that of inorganic semiconductors,
and which can reach an order of magnitude of almost
3 mV/degree for organic semiconductors[401also suggests
apossible use. Under theinfluenceofa temperaturegradient,
charge carriers can diffuse from the warm region to the
cold; on the other hand, cooling at one electrode and
warming at the other may be expected on passage of a
current. The practical possibility of using such an arrangement as a thermoelectric generator or electrothermal cooling element is favored :
-31-1
-4
/119075/
b) The formation of peak rectifiers, which are realized e. g.
in arrangements consisting of AgjphthalocyaninejAl point
2
4
6
TCNE [MOl-%l-
8
a) by a high differential thermoelectromotive force Q,
Fig. 5. Position of the Fermi level E , in phthalocyanine doped with
tetracyanoethylene (TCNE).
b) by a high conductivity 0,
c) by a low thermal conductivity K .
The use of organic dyes for the production of rectifiers
therefore seems attractive. There are two possibilities :
a) The formation of pn rectifiers by contact between n- and
p-conducting systems, an example being layers of p-conducting Cu-phthalocyanine and n-conducting C1In-chlorophthalocyanine pressed together[38!
Table 4 shows the quality factors or thermoelectric effectivities G,
of some systems.
Table 4. Thermoelectric effectivities GT of various systems.
a
[ W i cm-l]
System
Cooling system
Bi,Te,
Pyromeilitonitrile
Phthalocyanine (doped)
Cu-polyphthalocyanine
1000
10-7
0.3
Q
K
GT
[pV/deg]
[W/cm.deg]
[deg-'1
200
-150
lo00
3100
1.6 x lo-'
5 x lo-'
5x
5 x 10-3
6x
2 x 10-3
10-'2-10-8
10-10
5x
Ref.
~411
1411
[42]
1401
1431
Though pure phthalocyanines give a thermal EMF of up
to 2 mV/degree, the quality factor G, is very low because
of the low conductivity. However, the conductivity can be
lo
4
Fig. 6. Rectifier effect in the system Ag/phthalocyanine solid solution/
A1 point contact (after 1391).
Angew. Chem. internat. Edit. 1 Vol. 11 (1972)
1 No. 12
Fig. 7. Relation between thermoelectromotive force Q and conductivity
o ;plot: Q=C-(k/eloge)logo.
0 : iodine/phthalocyanine; x = TCNE/phthalocyanine; 0: o-chloranil/pol y-A'-vin ylcarbazole.
1055
increased by polymerization or doping (cf. Section 3) without
an excessive decrease in the thermoelectromotiveforce. This
is demonstrated in Figure 7 for the case of doped systems.
It can be seen from Figure 7 that when the conductivity is
increased to lo-’ K cm- thermoelectromotive forces
of up to 1 mV/degree are still obtained. It will be necessary
to look for systems having even higher conductivities,since
a Peltier refrigerator, for example, becomes competitive in
its power consumption only above a thermoelectric
effectivity of G,=6 x
deg-’. In the example of polyC u - p h t h a l o ~ y a n i n ethe
~ ~ ~quality
~,
factor should already
deghave a remarkably favorable value ofabout 5 x
’,
’.
4.3. Gas Detectors
5. Possible Applications of the Photoconductivity
The photoelectric sensitivity also offers possibilities for the
practical use of organic dyes.
5.1. Photoresistance Cells
It is possible to use organic dyes in photoresistance cells,
which are designed as surface cells (electric field normal to
the direction of the light) or sandwich cells (electric field
parallel to the direction of the light)[’,21.This possibility is
supported in particular by the fact that the characteristics
(dependence on intensity, voltage, etc.) largely agree with
those of the photoconductivity of inorganic photoconductors. Moreover, the high quantum yields found for some
dyes (G > 1) and the photoelectric current/dark current
ratio of up to 7 x lo4 (cf. Section 3) also suggest that a
systematic examination of this possibility would be worth-
Changes in the dark conductivity (and in the photoconductivity) of organic layers are observed on absorption
and desorption of gases[1.44*45!
Besides oxygen and hydrogen, other gases also have characteristic reversible effects on
the conductivity.As can be seen from Table 5, the conductivity changes of certain systems, e. g . of p - ~ a r o t e n e [ ~ ~ - ~ ~ ] ,
range over six orders of magnitude, depending on the type
of gas.
Table 5. Dependence of the dark conductivity of p-carotene on the
nature of an absorbed gas (after [48]).
Gas
NO
Butanol
Methyl acetate
Oxygen
so2
Acetone
Ethanol
NH3
Methanol
NO2
(Je.J(J..cLum
2
7
10
102
103
104
105
I 05
_ _ _ _ _ _ _ _ _ _ _OF_
106
106
0
m
As in the case of ZnO[497501,
it seems feasible to utilize this
effect for the detection of gases. According to Rosenbergr5”,
odorous substances have been qualitatively separated with
an instrument containing p-carotene.
2
4
6
u IVl
- - - - - - ------
-
8
10
12
14
Fig. 8. Influence of the cell arrangement (LF and QF: longitudinal and
transverse field cells respectively) on the magnitude of the photoelectric
current of the merocyanine dye FX 79 ( 5 ) .
End ot
exposure
J.
4.4. Electrets
Since the electronic charge carriers injected into organic
solids from ohmic contact^"^] can build up a stable space
charge by combination with deep traps, some dyes assume
the properties of electrets in the dark on application of an
electric field. According to Euler et al.[521,this effect can
be used for the storage of electrical energy. Thus with
nigrosine paste (using silver electrodes), a storage of 1.5
mAh/cm3 and, at a mean voltage of 0.9 V with a discharge
time of about 11 min, a volumetric storage capacity of
1.3mWh/cm3 were measured. The corresponding capacity
of a lead starter accumulator is 11 mAh/cm3 or 15 mWh/
cm3.
1056
0
20
40
60
80
100
120
1LO
t [sl+
Fig. 9. Influence of the cell arrangement (LF and QF : longitudinal and
transverse field cells respectively) on the inertia of the photoelectric
current of malachite green; plot: I,,,, = f (exposure time).
Angew. Chem. in’rernat.Edit. / Vol. I 1 (1972) / No. 12
while. With merocyanine dyes it was even possi_ble to
construct photoresistance cells that gave photoelectric
currents of up to
A with an external voltage of 100V
and a light intensity of 1 mW/cmz[']. It is important for
measuring cells of this type that the distance between the
electrodes be as small as possible. On the one hand, a short
distance between the electrodes can compensate for a low
mobility and a short charge-carrier lifetime due to high
defect concentrations in accordance with eq. (8) (cf. Fig. 8).
On the other hand, the effect of the traps in delaying the
start and the decay of the photoelectric current can be
largely eliminated['4], as is shown by a comparison of the
photoelectric currents measured for malachite green in the
longitudinal and transverse field arrangements (Fig. 9).
Since the photoelectric currents and no-load voltages can
be increased by doping, and there is evidence of the formation of pn junctions in homogeneous bodiesc6'], an
improvement on the outputs N,,, obtained so far seems
possible. It should also be mentioned that dyes can be used
in two-layer photocondensers (consisting e. g. of phthalocyanine and a ferroelectricceramic),which are very sensitive
to light and have a response time of
to
s[631.
5.3. Spectrally Sensitive Detectors
In addition to the photoelectric effects due to a pn boundary
layer or to a difference in the mobilities of the charge
carriers (Dember effect), the photovoltaic effects due to
metal/semiconductor or metal/gas/semiconductor inter5.2. Photoelectric Cells
faces could possibly also be of practical importance. The
photoelectric currents of these systems reach an order of
magnitude of only about 10- l o A/cmz, but they are characThe photoelectric sensitivity in conjunction with n and p
terized by the dependence of their direction on the waveconduction allows the production of photoelectric cells
length.
For example, the measurements in Figure 11 for
that give photoelectric currents and photoelectric voltages
Orthochrome T[641and a - c a r ~ t e n eshow
[~~~
that the photowith no applied EMF['* 53, 541. The essential arrangement
electric currents and voltages change their sign on.transition
here is the combination of an n-conducting dye such as
from short to long wavelengths. The approximate wavemalachite green or Rhodamine B with a p-conducting dye,
which may be e. g. a phthalocyanine or a mer~cyanine[~~]. length of the light falling on the dye can therefore be read
off simply from the direction of the photoelectric current.
The directional and intensity dependence of the photoelectric current (cf. Fig. 10) show that the photovoltaic effect
is due to a p-n junction between the compounds in contact.
I
0
16907101
I
20
40
I
I
60
80
B [%I
1
100
L
Fig. 10. Zphot =f(irradiation intensity B ) ; system: malachite green
(n)+merocyanine FX 79 (p) ( 5 ) (h=6200
100% corresponds to
3.7 x lo1* quanta/cm2s; cell dimensions: distance between electrodes
approx.
cm, illuminated area 1 cmZ).
A,
Ir81-
Fig. 11. Dependence of photoconductivity on wavelength.
1 : Orthochrome T; I,,,,=f(h) [64].
2: p-Carotene; U , =f(k) [65].
Exposureconditions:curvel:lo3 pW/cm2;curve2:3.9x 1015quanta/
cm2s.
An organic component can therefore also be replaced by
an inorganic photoconductor of the corresponding conduc-
tion type (e.g. Rhodamine B (n) by CdS (n)['5'24~56,571
1.
Table 6 shows the outputs obtained so far for some organic
It would be important to increase the sensitivity of the
pn photoelectric cells.
photoelectric effect.
Table 6. Outputs N,,, of organic pn photoelectric cells.
System
n
P
Malachite green
Phthalocyanine
CdS
Phthalocyanine
Triphenylmethane dyes
Phthaleins
Tetramethyl-p-phenyIenediamine
Mg-Phthalocyanine
Ion exchange membrane with thionine/ascorbic acid
Angew. Chem. internat. Edit. Vol. 11 (1972) No. 12
100
300
100
200
15
10-8
10-9
3x
O S X I O - ~ ~ X I O - ' ~ [SS]
3 x lo-''
[59,60]
1 . 6 ~ 1 0 - ~ 2xlO-*
[61]
lo-'
1057
5.4. Use in Vidicon Television Pickup Tubes
Because of the photoelectric sensitivity and the frequently
high I,,,JI, ratio, organic dyes of various classes can be
used as targets in vidicon television pickup tubes for image
reproductionr661.In principle, the dye is arranged in these
camera tubes as in a longitudinal field cell (cf. Fig. 12). In
the vidicon, however, one electrode is replaced by an
electron beam, which brings the dye layer to the potential
of the beam system cathode by scanning. Since the transparent electrode carrying the dye is slightly charged, light
falling on the dye produces a photoelectric current, which
can be measured in a galvanometer and recorded on the
television screen.
Fig. 12. Diagram of the vidicon measuring tube (for details see [66]).
1, Scanning beam (emerging from the beam system cathode with a
potential V=O volt); 2, gauze; 3, dye layer; 4, transparent electrode
(potential up to about +30 volt); 5 , amplifier; G, galvanometer.
Some characteristics of the organic vidicon are :
a)A sensitivity ofabout 10-100 pA/lm (or 100nA/lx)which
extends to the order of magnitude of inorganic vidicons.
b) A variation of the photoelectric current with voltage,
irradiation intensity, and beam current corrlsponding to
that of inorganic targets. Figure 13 shows this agreement
for the intensity dependence of the vidicon photoelectric
currents of phthalocyanine and of Sb,S,.
to test as many dyes as possible for use in television image
reproduction. The results are of interest e.g. for color
television, for medicinally important IR vidicons". 671,and
for the production of storage vidicons['].
5.5. Use in Reprography
There isa real chance for the use of organic photoconductors
on an industrial scale in the near future in the reproduction
methods based on photoelectric and electrostatic processes
(cf. review
Eyen in the general electrophotographic processes
(cf.". 14, 68- 701) organic systems offer advantages over
inorganic photoconductors. For example, the sensitivity of
polyvinylanthracene is higher than that of sensitized ZnO
materials by a factor of 1.7[711.Compounds having a dye
structure are also important. Thus the utilizability of photoconducting polymers with a phthalocyanine structure that
are easily applied to a substrate has been established1721,
and the feasibility of using photoconducting leuco bases
of triarylmethane dyes as components of transparent
electrophotographic recording materials, which allow e. g.
the electrophotographic production of microfilms, has
been described[731.
Above all, the development of new reproduction processes
based on the electrophotographic principle is made possible
by organic photoconductor systems (~f.['~]). Dyes play an
important role here, not only as spectral sensitizers of the
organic p h o t o c ~ n d u c t o r '7~4 3~751.
* Photoelectric effects of
the dye can also be utilized directly. Examples that could
be mentioned include :
a) the use of the memory effect observed e. g. for dyes after
exposure to light (persistent photoconductivity[763771),by
means of which several copies of equal quality can be
obtained from one original after exposure to light without
a corona discharge[781;
b) the use of the photoelectric sensitivity of dyes of the
triphenodioxazine class[791,of metal-free phthalocyanine
(a form), azo pigments, etc.f801in a polychromatic photoelectrophoretic image reproduction process.
6. Connections with Industrial Processes
6.1. Questions of Lightfastness
10
j1907131
B ["/.I
-
100
Fig. 13. Photoelectric currents of Sb,S, and phthalocyanine in the
vidicon arrangement; plot: log l,,,,=f(log irradiation intensity).
1, Sb,S,; 2, phthalocyanine (X=5500 A).
c) The fundamental possibility of reproducing several colors
with a single target containing photoconductor points that
absorb differently.
d) Appreciable storage in the case of layers that are rich
in traps. It should be the aim of further systematic studies
1058
In the discussion of the application of photoconduction
measurements to the lightfastness testing of dyes (cf.["]),
it must be noted that the photoconductivity is not due to
intermolecular or intramolecular bleaching processes. Only
the start of the photochemical decomposition of the dye
is recorded, since it is associated with a decrease in conductivity. For example, it was found for the triphenylmethane dyes used as standards in the I S 0 lightfastness
[*] We are grateful to AEG-Telefunkeh, Ulm, and particularly to
Dr. A . Bogenschiitr, for valuable technical support.
Angew. Chem. internat. Edit.1 Vol. 11 (1972)
No. 12
test method[' 'I that the conductivity decreases with the
photochemically induced transition of the dyes into the
leuco form1821.
It is not yet known to what extent negative
photoelectric effects1831
of colored fibers can be correlated
851. However, the relation found
with their lightfa~tness"~~
for the I S 0 standards between lightfastness and the thermal
activation energy of the photoconductivity (AEphoI)determined from eq. (13)
could be of importance for lightfastness measurements[821.
According to this relation, AE,,,, increases with the lightfastness for the standard dyes CI Acid Blue 104, 109, and
83 (see Table 7, and, for interpretation["]).
Table 7. Relation between AEpho,and time t , until the start of bleaching
(after [82]).
Dye
I,
[min]
CI Acid Blue 104 ( 6 )
CI Acid Blue 109
CI Acid Blue 83
9
2s
60
AE,,,,
rev1
0.39
0.48
0.58
6.3. Catalytic Effect
It can be concluded from various experiments that a
correlation exists between electrical and catalytic data f x
organic solids if the catalytically active substances to be
compared are very similar in their specific chemical and
structural properties['4*909 911. Thus in the oxidation of
acetaldehyde ethylene acetal to ethylene glycol monoacetate with Fe-M polyphthalocyanines (M: V, Cr, Cu,
etc.) as catalysts, a linear relation exists between catalytic
activity K , and the thermal activation energy of the dark
:
conductivity AE1herm[9Z1
Mention should also be made of the increase in the catalytic
activity of Cu-phthalocyanine samples for the decomposition of H,O, with increasing conductivity and decreasing
activation energy[93],and of the observed increase in the
catalytic activity, e.g. in the oxidation of cumene in the
presence of Cu-phthalocyanine, on transition from the
monomer to the polymer[941.The catalytic activity is also
related to the type of conduction. For example, the decomposition of HCOOH is catalyzed more strongly by
n-conducting poly-Cu-phthalocyanine than by p-conducting Cu-phthai~cyanine[~'~.
lnoue et al.'961found, moreover,
that dye photoconductors of the p type (eosin, fluorescein,
acridine, etc.) catalyze the photooxidation of isopropyl
alcohol with 0, to acetone, while semiconductors of the n
type (Crystal Violet, Rhodamine, etc.) are either inactive or
cause pinacol formation.
However, only structurally similar dyes can be used for
such comparisons.
6.2. Spectral Sensitization
In the case of an electron transfer mechanism for the spectral
sensitization of the photographic process and the photoconduction of inorganic semicondu~tors['~
24, 86-881 , a
relationship can obviously be assumed between the electric
behavior of a dye and its sensitizing effect. According to a
modified mechanism124* 891, the spectral sensitization is
regarded as a type of pn or nn' photoelectric effect, in which
two effects act together. One of these effects is the build-up
ofa space charge in the unilluminated state in the interfacial
zone between the dye and the semiconductor as a result of
an electron transfer leading to the balancing out of different
chemical potentials of the electrons. The second is a
separation under the action of the space charge field of the
electrons and defect electrons formed in the dye on exposure to light.
' ' 7
Photovoltaic effects in model arrangements of sensitized
systems are in agreement with this mechanism[241.However, it is not yet certain how far these findings can be
utilized for the production of particularly effective sensitizing dyes.
Angew. Chem. internat. Edit.
Vol. 11 (1972) 1 No. I 2
This reaction shows an important characteristic of organic
catalysts, i. e. their high selectivity.
The relations between structure, conductivity, and catalytic
activity are of interest particularly in connection with the
use of organic dyes for the reduction of oxygen in fuel cells.
Since Jasinski's discovery[97.981 that Co-phthalocyanine
can be used for the reduction of oxygen in fuel cell cathodes
instead of the usual platinum, which is uneconomical
because of its high price, several investigations have been
concerned with this question[99.1oo-1031.
Jahnke"03' was
able to show that the catalytic reduction of oxygen in
H 2 S 0 4 depends on the nature of the central atom and
particularly on the degree of polymerization. This last
finding (cf. also'991) probably suggests a relationship
similar to that between the conductivity and the catalytic
activity of the phthalocyanines. It should be stressed that
poly-Fe-phthalocyanines exhibit a higher activity than
platinum metal alloys in sulfuric acid-methanol cells1 031.
The stability of the phthalocyanine catalysts is probably
not yet ideal, but the results obtained so far should allow
'
1059
an optimistic prognosis regarding the possibility of using
fuel cells with high-conductivity dye catalysts, which could
be used e. g. in heart pacemakers['041.
We are grateful to the Deutsche Forschungsgemeinschaft and
to the Fonds der Chemischen Industrie for their support of this
work.
Received : August 2,1971 [A 907IE]
German version: Angew. Chem. 84,1077(1972)
Translated by Express Translation Service, London
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Biochemistry of the Pteridines
By Heinz Rembold and William L. Gyurer*]
Biochemical information about the ubiquitous pteridines has become generally available
only within the last fifteen years. This delay can be traced to the chemical lability of these
compounds and their small concentrations in organisms. New methods of isolation and
isotopic techniques have provided data on the biosynthesis, anabolism, and catabolism of
this class of compounds. Hydrogenated pteridines are recognized today as cofactors for
various mixed function oxygenases and are involved in cellular electron transport. Further
unknown catalytic functions for pteridines in cellular metabolism are indicated by their
physiological activity, negative redox potentials, and histoautoradiographic data.
1. Introduction
At the end of the last century Hopkins reported on a
crystalline pigment which he had extracted from the wings
of a butterfly“’. This finding formed the basis for later
investigationsby Wielandand hisco-workerswho succeeded
in isolating first a yellow121and then a white pigment”’.
After much experimental difficulty Purrmunn was finally
able in 1940 to determine the structure of these natural
pigments, which were named xanthopterin (35) and
leucopterin (37). At the same time he also elucidated the
structure of a related pigment called isoxanthopterin
(38)[41.
For the bicyclic ring system of these new butterfly pigments
Wielund chose the name “pteridine” while the designation
“pterin” was originally used as a general term for all insect
pigments. Today pyrazino[2,3-d] pyrimidine is called
pteridine (2). The naturally occurring pteridines are
commonly called pterins when they are derivatives of the
parent compound 2-amino-4-oxodihydropteridine( 3 ) and
lumazines when they are derivatives of 2,4-dioxotetrahydropteridine ( 4 ) .
As a simple approach pteridines (2) may be considered
related to the purines ( I ) to which they bear a structural
resemblance. Both classes of compounds have many
[*] Prof. Dr. H. Rembold and Dr. W. L. Gyure
(American Cancer Society Postdoctoral Fellow)“”
Max-Planck-Institut fur Biochemie
8033 Martinsried bei Miinchen (Germany)
[**I Present address:
University Hospital BB-203
University of Washington
Seattle, Washington 98105 (USA)
Angew. Chem. internat. Edit. / Vol. I1 (1972) / No. I 2
properties in common such as low solubility at neutral pH
values, well defined ultraviolet absorption spectra, ability
to form chelates with metal ions, and the property of
forming precipitates with mercury and silver salts. In
addition to the purine-like properties, fully oxidized, or
0
O
H
(4)
aromatic, pteridines exhibit strong blue to red fluorescence when irradiated with ultraviolet light. This fact aids
their purification from sources containing only trace
amounts of these compounds.
Reviews of the chemistry of the pteridines are availablec5.‘I.
One property of pteridines is especially important from
a biochemical standpoint and that is their ability to form
dihydro and tetrahydro derivatives. These reduced forms
are very reactive and not only can they serve as specific
reductants but they can also participate in electron transport reactions. It is precisely because these reduced forms
are so reactive that working with them is difficult. The
aromatic structures are already photolabile while the
reduced forms are even more photolabile and subject to
oxidative degradations. As an example of this lability,
1061
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