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Electrochromism of merocyanine dyes as detector of internal charges in sandwich arrangements with molecular crystals
Volume 52, number 1
CHEMICAL PHYSICS LETTERS
ELECTROCHROMISM OF MEROCYANINE IN SANDWICH ARRANGEMENTS
15 November 1977
DYES AS DETECTOR OF INTERNAL CHARGES
WITH MOLECULAR
CRYSTALS
Hermann KILLESREITER Fachbereich Physikalische Mm-burg. Germany
Chemie der Phiiipps-Utdversitiit
Marbrcrg,
and Siegfried SCHNEIDER Institut fiir Physikalische Mtmich. Germany
Chernie und Theoretische
Chemie der Technischen
Universita‘t Munchen.
Received 22 December 1976 Revised manuscript received 9 August 1977 The photosensitized charge transfer reaction from a merocyanine dye laser to ap-chloranil single crystal is studied. It was found that the ma?timum of the action spectrum of the photocurrent shifts by several nanometers when the esternally applied voltage or the illuminating light level is increased. A model is proposed which interprets these observations as being . due to the induced electric field of a layer of ionized dye molecules at the very surface of the single crystal.
1. Introduction
By means of monolayer assemblies on the surface of a p-chloranil single crystal in a sandwich cell with evaporated aluminum electrodes it could be shown that the efficiency of the sensitized charge transfer reaction depends on the distance of the dye from the adjacent metal electrode [l] . In those experiments it furthermore has been ascertained that the regeneration mechanism of the dye is not the rate determining step since a linear dependence of the photocurrent on the light intensity could be measured. Nevertheless, this regeneration mechanism across the layer system can be of great interest, because (i) it depends on the internal field between the ionized dye molecules at the crystal surface and the metal electrode; (ii) the density of ionized dye molecules should also depend on the externally applied electric field that withdraws the injected electrons from the surface into the bulk. The present work aims for more information about the actual charge densities and field strengths at the
interface dye/crystal by making use of the electrochromism of merocyanine dyes. In case of a sensitized charge transfer reaction, the electric field affects not only the reactivity but also the electronic structure of the dye, i.e. the detector is the sensitizing molecule itseIf_ The dye’s response to an external electric perturbation, that is the spectral shifts in the actionspectra of the photocurrent, provides a new way of assessing internal charge distributions at the phase boundary.
2. Experimental In our experiment, an open chained merocyanine dye [2] with the formula Me2=N-(CH=CHf3CH=C=(CN)Z was brought onto the surface of apchloranil single crystal in a cell of sandwich type. In contrast to similar arrangements described earlier [3], the dye was not brought onto the surface by monolayer technique, but by slow evaporation in vacua at low temperature. Afterwards, 6 monolayers of pure arachidic acid were deposited by 191
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CHIILIICAL
PHYSICS
monolayer technique [4] onto the dye covered surface. The latter acted as a spacer bctwcen the dye molecules and the aluminum electrode. They show also the similarity to cells with dye monolayers and prevented the quenching of excitation energy [I] . The amount of dye molecules deposited was cstimated spectroscopically. A glass support that held the crystal during the procedure of evaporation had smultaneously been covered by dye molecules. Rfterwards it was dipped into a known volume of ethanol. By measuring the absorbance of this solution and taking into account the surface of tlic glass support, a coverage of the crystal surface with approximately IO dye layers could bc calculated. The experimental equipment consists of a tungsten lamp with a motor driven monochromator (spectral band width used: 4.8 nrn) for the illumination of the dye covered surface of the cell, and of a voltage supply and an clectrornctcr (sensitivity up to 1 O-l4 A) for the electrical circuitry. The action spectra of the photocurrent are plotted by an X-Y recorder and then corrected for constant light intensity (/(h)dh = constant).
3. Results The long wavelength band in the absorption spectrum of the dye used shows a strong solvatochromism, i.e. a bathochromic shift of about 2600 CITI-~ when one switches from a nonpolar (Me-cyclohexane) to a polar (methanol) solvent 121 . This is cnuscd by the different interaction of the dye’s dipole moments in the So and S, state, respectively, with the electrical reaction field of the solvent shell. Therefore it can be used as a sensitive riiolecular detector of internal clcctrical fields. Compared to the absorbance of the clye m a solution of methanol (cf. fig. 1) the absorbance of a vacuum deposited layer on a glass surface is not only shifted to longer wavelength but is very much broader too. This long wavelength shift and the broadening of the band appears also in the action spectra of the photocurrent, but there is an additional shift of its maximum of 7 111n (ZOO cm-‘) from 589 nm (16.98 kK) to 596 nm (16.78 kK) if the applied voltage across the sample is increased from 50 V to 300 V. As the thickness of the crystal sample has been 38 pm 192
LETTERS
15 November
1977
8 IOl0
-II-
wavelength
lnml
Fig. 1. Action spectra of the photoctment
at constant
illu-
mination (I = 6.6 x IO4 W cm- *), but different eutcrnally applied voltages (UI = 50 V, left scale; ---Uz = 300 V, rght SLIIC). The Asorbance of the dye III a solution of methanol (-.-.-) and of an evaporated dye ln5er (-..-._) is A0 shown an arbltr~ry units.
(a layer of IO dye molecules and 6 monolayers of arachidic acid with 150 A thickness give no significant contributions) the externally app!ied (uniform) clectric field could be calculated as 1.3 X lo4 V cm-! and 7.9 X IO4 V cm-l, respectively. The maxlmum in the action spectrum shows another shift of G m-n (170 cm-l) from 596 nm (16.78 kK) to 602 nm (16.61 kK) if at high externally apphed voltage the incident light Intensity increases from 6.6 X 10 -4 W cnims2 to 5.3 X 10m3 W cm-? (fig. 2). Current-voltage plots have been measured to ensure that not bulk but electrode processes are the rate determining step of the sensitized photocurrent. They show the usual transition from a space-charge limited r.
- . __,___ ___r_ ii
_____,
Fig. 2. nction spectra of the photocurrcnt at diffcrcnt tllumination (-It = 6.6 X IO4 W cm-‘, left scale; --f= = 5.3 x 10-3 W cm-2, right scale) but constant applied voltagz (U = 300 V).
Volume
52, number
1
CHEMICAL
PHYSICS
wavelength F bOlc f W?P % 593 0’ j:
S89-
15 November
LETTERS
shift of the observcti
nxq+itude.
1977
For the
explanation WC propose a model, according to which a strong internal’ficld is gencrdted across the layer system through the balance of charge carrier injection into the crystal and the subsequent regeneration mechanism. WC will show that plus field is sufficiently large, i.e. in the order of rnagnitudc of 106 V cm-I to pro-
I2
11
duce the observed effects in the action spectra of the pholocurrcnt. Chloranil is a strong electron acceptor. Thcrcforc, the process of sensitized charge carrier generation rc-
applied Fig. 3. Abovu:
VolLlgc
dcpcrttIcnt
peak at light intensities It m fig. 2 ,md below. Below: with broad hnd evcitatmn curve~havc been obtained trade-limited region (LLC)
voltage shift
(volts1 of’
the
pllotocurrcrlt
and 12 that correspond to values Typcal current -voltage plot at 600 nm. Points for lower from action spectrd in the elecand are cutrapolated ( ---) to
indicate the space-charge hmitcd current (SCLC).
current (SCLC) to an electrode limited region (ELC) at moderate external fields (fig. 3). At several points of the ELC a photccurrent spectrum has been rccorded and the shifts obtained for the example below are plotted above.
suits iri tile injection of an electron into the crystal. It is assumed, that only those excited dye molecules D* of the dye layer can contrlbute to the charge transfer that arc at the very surface of the crystal (cf. 17g. 4). Therefore, all the ionised dye molecules, D+, are located on the crystal surface. But as to electrons inside the crystal, ecA, they arc distributed over the whole volume of the crystal with thickness d. This dlstinction is important because m this cast only the ionized dye molecules contribute to an appreciable Internal electric field. In order to calculate lhc density of ~om/.cd dyr: moIccuIcs, [D+] , WC have to consider several competing processes: (i) The generation of excited dye molcculcs and their decay by intramolecular relaxation processes kx
D i hv+D*
(1)
.
1 orachrdic acid dye
chlornnll
crystal
i
4. Discussion The dependence of the dye’s excitation energy AZ&-, + S1) on an external perturbation (electrical field) as calculated in the well known Pariser-ParrPople approximation [2] suggesls that for the isolated n~oleculc a change in the perturbing electrical field (component parallel to the long axis of the molecule) of at least lo6 V cm-l is necessary to shift the S,, + S, absorption band by 200 cm-* -Therefore we can deduct that an electrical field of about 104 V cm-l, as it is calculated from the applied voltage under the assumption of uniform strength across the sandwich cell, is not strong enough to rationalize the
EWlt
hg. 4. Schcmntic cross section through the sandwich drrangcmcnt. For nn cuplanatmn of the symbols We text.
CIIL-MICAL PIIYSICS LEl-l-GRS
Volume 52, number 1
(Ii) The charge transfer reaction with a surface molecule of the crystal, CA, at an overall rate constant k-c3 D* +
CA+
kCT
D++e(:,
,
WI
15 Novemlxr
1977
dependent limiting currents the density of electrons inside the crystal, [e&l , with a mobility I*, decreases with increasing electrical field, Eekt according to
J = e [c&, 1 /.LE,,~ .
@I
Yet even in the case, when the second term in the deThe recombination of an ionized dye molecuic D+ with an injected electron c& (rate constant I&-) leads to a non cxcitcd dye n~olcculc, kb D+ -I-e&--D.
(2b)
Details about the formation of intermediate states (D’ ...e&) that may contrrbutc to internal fields but not to the photocurrent will be discussed later [5]. (iii)The regeneration of the ionized dye molecules via electrons c- that come from the metal electrode across the layer system
D+ + c-x-,,D From
_
(3)
the rate equations!
d[D*]ldr=k,,[Dj
-kd[D*]
-A-&X]
:D*]
(4)
and d[D+l/dt
=k,,[CAj
-k,[e-I
[D*] -k;,T[e&J
[U’J
[D’l ,
(5)
we can deduct the concentrations of excited and ionwed dye n~oleculcs, rcspectivcly, under cqui!!brium conditions:
[D*] =
u = EE,, IEI = 2 X 10M7 [A s cm-2]
km PI
=
k, + kcT [CA]
~OM~)IDl
k, + kcr [CA]
’
kc, iCAl D* 1 [D+] = -.__._ __ k&[ecAJ
+k,[c-j
’
w (7)
with U(V) = absorption cross section and I(v) = photon flux. [D] , [D*] and [D+] are introduced as surface concentratrons, whereas in this type of surface reaction the more formal quantities [CA] , [cc*] and [e-] can be inserted as volume concentrations. A discussion of ccl. (7) shows, that, firstly ID’] and concomitantly the electric field E,nt (cf. fig. 4) increases with an increasing externally applied voltage
194
nominator of eq. (7) is small, no simple inverse relatlonship between [D’J and the externally applied voltage U is expected. Thus is, because one has to assume that the rate constants in this kind of clectrochemical reactions depend on the electrical field. This connection with the injection kinetics explains also, why with respect to a current-voltage plot the larger changes of the wavelength shift are observed beyond the transition region from SCLC to ELC with an indicated saturation at high fields. On the other hand it is therefore not useful and possible to correlate directly wavenumber shifts or absorbance shifts at band edge with an applied ticld as it is usually done with results from electrochromic dyes in condenser arrangements with metal electrodes (e.g. ref. [4], p. 640). Secondly, the (induced) internal field depends on the density of excited molecules on the surface, [D’j , i.e. on the light intensity. This is also in agreement with our experimental findings. As mentioned above, a field of at least 1OG V/cm is necessary to induce the observed wavelength shift. if we assume a homogeneous field between the dye ions on the surface of the molecular crystal and the image charges on the surface of the metal electrode, the required charge density is
U. This is because
of the fact that in fkld
in-
0)
and, therefore, the density of ionized molecules [D’] = 1.2 X 1012 cms2. With an effective area of 250 A2 ftir a single molecule the density of surface molecuIes gets [D] = 4 X lo13 cm- 2. The comparison with the value of [D+] shows, that about 10% of the directly adsorbed dye molecules contribute to the internal field under steady state and limiting current conditions. The density of excited molecules [D*] should be low because of their short lifetime. An exact calculation is not possible up to now because the layered structure influences not only the lifetime, but allows also for the diffusion of excitation energy from the interior regions of the evaporated Iayer to dye molccules on the very surface of the crystal.
Voiumc
52, number 1
CHEMICAL
PlIYSICS
Because of the large intramolecular decay constant dye used (ground state recovery time measurements in ethanofic solution [6] fcad to a value ofkd = 5 X lO*O s-l ; the radiative decay constant is about 5 X IO8 s-l) WCcan, in a first approximation to eq. (6) neglect the influence of the chargetransfer reaction on the quasi stationary concentration of excited molecules [D*] _ _4ssurning k, = IO9 s-l , an incident photon flux of 2 X lOI quanta cm-* s-l, a transparency of 20% of the aluminum electrode, and an absorbance of 1% that is typical of a monolayer of dye molecules witfl an absorption coefficient of about 105 K mol-i cm-l, we get the result [D*] = 4 X I03 crnm2_ If we enter with [D’] = lOI cm 2 and [D*] = 4 X IO3 cm-* into cq. (7), kcr [CAJ /(k& [e&J + k&z-]) =3X IO8 fo Ifows. This seems to be rcasonable in that sense that k& [ecA ]
5. Concluding
remarks
Recently, the delayed ffuoresccnce lifetime as a fimction of the applied voltage has been Introduced as an optical detector in order to dcterminc the total excess charge carrier density in antflraccne single crystals [7] and the effect of space and polarization charge on the internal field within the bulk of a crystal has been investigated with related Stark experiments [8] _ The present method provides a new way not only to detect internal fields at phase boundaries in sandwich systems, but also to investigate rate constants for charge carrier injection processes via eq. (7). More information is expected from a prove of eq. (7) with respect to a more exact calculation of [D+j and, later on, by use or more defined structures at the surface of the molecular crystal, for instance monolayers of the merocyanine dye. Nevertheless, WC believe that the results confirm the hypothesis made above that in the layer system internally induced electrical fields exceed the ones which originate from the externally applied voltage by more than one order of magnitude. A more precise statement cannot be made since the observed band maxima are outside the calculated range of excitation energies [2] _The reason for this behaviour
LEITEKS
15 Novcrnbcr
1977
must be sought in the fact tflat in the dye faycr WC are not dealing wit11 isolated molcculc_s anymore. In that sense we also have to interpret the increase in tile width of the band in the absorption and action spectra of the dye layers compared to the one in the absorption spectra in solution by about 2000 cm-l (fwhm). Interestingly enougfl we observe a decrease in the full width half maximum of approximately 400 cm-l, wilCn tfle external applied voltage is increased from 50 to 300 V (fig_ 1). Both effects, tfle bathochromic shift of tflc band maxima, and the reduction m the half width upon application of an external electrical flcld resemble very much the beflaviour or the absorption band found upon cooling of a sample in solution [2] . In addition to changes in the ratio of cis and trans isomers and of monomers and dimers, respectively, efcctrical fields of more than IO6 V cm-I could very well influence the orientation of molecules wit11 a dipole moment as large as 15 debye. However, the rigid structure of adsorbed layers does not favour the motion and reorientation of dye molecufcs on the phase boundary. Finally, prcssurc induced absorption changes because of the charging of a capacitance have been considered in connection with the study of electrocllron+m of cyanine dyes in condenser arrangements with rnonolayer assernbfies betwcmi metal clcctrodes [VJ . With a field of 2.5 X 106 V cm-l there was estimated a negligible effect. The consequences of all the above effects on the band shape of the action spectra cannot be predicted without tflc knowledge of much more details.
Acknowledgement Both authors wish to thank the Deutschc Forschungsgemeinschaft for financial support. Free computer tlmc by the Lclbniz-Rechcnzentrum is also gratefully acknowledged.
References I?/13 (1976) 857. III H. Killcsreitcr, I. Lurnincsccncc P. Scheibc, S. Schneider, F. Dorr and E. Dal trozzo, Bcr. [21 Runscngcs.
Physik. Chern. 80 (1976) 630. Faraday Discussions Chern. SOC. 58 (1974)
(31 H. Krllcsrcltcr, 271.
19.5
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CHEMICAL
[4] 11. Kuhn, D. hfobius and H, Buchcr, in: Physical methods of chemistry. Vol. t, cds. A. Weissbergcr and B. Ro\sitcr (Whey, New York, 1972). [S] H. ICillcsrzitzr and S. Schncidcr, Ucr. Bunscnge\. Fhysik. Chcm., to bc submitted for pubhcation. [6] 1’. Wirth, Thesis, Tcchniscbc Univcrsit;i
PHYSICS
LETTERS
1977
[7] hi. Pope and W. Weston, Mol. Cryst. Liquid Cryst. 25 (1974) 205. [8] F.P. Chcn, S.J. Shcng and D.M. Hanson, Chem. Phys. 5 (1974) 60. [9] 11. Blicher. J. Wiegand, B.B. Snavely, K.H. Beck and H. Kuhn, Chem. Phys. Letters 3 (1969) 508.
ERRATUM P. Pyykko and J.P. Desclaux, Dirac---Fock one-ccntre calculations. The model systems TiH4, ZrH4, HfH, and ( 104)114, Chcrn. Phys. Letters 50 (1977) 503. There is an error in table 1 on page 504: in the left-most column Zrll,, lifH4 and (104)H4 should have been printed one line down.
196
15 November
CHEMICAL PHYSICS LETTERS
ELECTROCHROMISM OF MEROCYANINE IN SANDWICH ARRANGEMENTS
15 November 1977
DYES AS DETECTOR OF INTERNAL CHARGES
WITH MOLECULAR
CRYSTALS
Hermann KILLESREITER Fachbereich Physikalische Mm-burg. Germany
Chemie der Phiiipps-Utdversitiit
Marbrcrg,
and Siegfried SCHNEIDER Institut fiir Physikalische Mtmich. Germany
Chernie und Theoretische
Chemie der Technischen
Universita‘t Munchen.
Received 22 December 1976 Revised manuscript received 9 August 1977 The photosensitized charge transfer reaction from a merocyanine dye laser to ap-chloranil single crystal is studied. It was found that the ma?timum of the action spectrum of the photocurrent shifts by several nanometers when the esternally applied voltage or the illuminating light level is increased. A model is proposed which interprets these observations as being . due to the induced electric field of a layer of ionized dye molecules at the very surface of the single crystal.
1. Introduction
By means of monolayer assemblies on the surface of a p-chloranil single crystal in a sandwich cell with evaporated aluminum electrodes it could be shown that the efficiency of the sensitized charge transfer reaction depends on the distance of the dye from the adjacent metal electrode [l] . In those experiments it furthermore has been ascertained that the regeneration mechanism of the dye is not the rate determining step since a linear dependence of the photocurrent on the light intensity could be measured. Nevertheless, this regeneration mechanism across the layer system can be of great interest, because (i) it depends on the internal field between the ionized dye molecules at the crystal surface and the metal electrode; (ii) the density of ionized dye molecules should also depend on the externally applied electric field that withdraws the injected electrons from the surface into the bulk. The present work aims for more information about the actual charge densities and field strengths at the
interface dye/crystal by making use of the electrochromism of merocyanine dyes. In case of a sensitized charge transfer reaction, the electric field affects not only the reactivity but also the electronic structure of the dye, i.e. the detector is the sensitizing molecule itseIf_ The dye’s response to an external electric perturbation, that is the spectral shifts in the actionspectra of the photocurrent, provides a new way of assessing internal charge distributions at the phase boundary.
2. Experimental In our experiment, an open chained merocyanine dye [2] with the formula Me2=N-(CH=CHf3CH=C=(CN)Z was brought onto the surface of apchloranil single crystal in a cell of sandwich type. In contrast to similar arrangements described earlier [3], the dye was not brought onto the surface by monolayer technique, but by slow evaporation in vacua at low temperature. Afterwards, 6 monolayers of pure arachidic acid were deposited by 191
Volume
52, number
1
CHIILIICAL
PHYSICS
monolayer technique [4] onto the dye covered surface. The latter acted as a spacer bctwcen the dye molecules and the aluminum electrode. They show also the similarity to cells with dye monolayers and prevented the quenching of excitation energy [I] . The amount of dye molecules deposited was cstimated spectroscopically. A glass support that held the crystal during the procedure of evaporation had smultaneously been covered by dye molecules. Rfterwards it was dipped into a known volume of ethanol. By measuring the absorbance of this solution and taking into account the surface of tlic glass support, a coverage of the crystal surface with approximately IO dye layers could bc calculated. The experimental equipment consists of a tungsten lamp with a motor driven monochromator (spectral band width used: 4.8 nrn) for the illumination of the dye covered surface of the cell, and of a voltage supply and an clectrornctcr (sensitivity up to 1 O-l4 A) for the electrical circuitry. The action spectra of the photocurrent are plotted by an X-Y recorder and then corrected for constant light intensity (/(h)dh = constant).
3. Results The long wavelength band in the absorption spectrum of the dye used shows a strong solvatochromism, i.e. a bathochromic shift of about 2600 CITI-~ when one switches from a nonpolar (Me-cyclohexane) to a polar (methanol) solvent 121 . This is cnuscd by the different interaction of the dye’s dipole moments in the So and S, state, respectively, with the electrical reaction field of the solvent shell. Therefore it can be used as a sensitive riiolecular detector of internal clcctrical fields. Compared to the absorbance of the clye m a solution of methanol (cf. fig. 1) the absorbance of a vacuum deposited layer on a glass surface is not only shifted to longer wavelength but is very much broader too. This long wavelength shift and the broadening of the band appears also in the action spectra of the photocurrent, but there is an additional shift of its maximum of 7 111n (ZOO cm-‘) from 589 nm (16.98 kK) to 596 nm (16.78 kK) if the applied voltage across the sample is increased from 50 V to 300 V. As the thickness of the crystal sample has been 38 pm 192
LETTERS
15 November
1977
8 IOl0
-II-
wavelength
lnml
Fig. 1. Action spectra of the photoctment
at constant
illu-
mination (I = 6.6 x IO4 W cm- *), but different eutcrnally applied voltages (UI = 50 V, left scale; ---Uz = 300 V, rght SLIIC). The Asorbance of the dye III a solution of methanol (-.-.-) and of an evaporated dye ln5er (-..-._) is A0 shown an arbltr~ry units.
(a layer of IO dye molecules and 6 monolayers of arachidic acid with 150 A thickness give no significant contributions) the externally app!ied (uniform) clectric field could be calculated as 1.3 X lo4 V cm-! and 7.9 X IO4 V cm-l, respectively. The maxlmum in the action spectrum shows another shift of G m-n (170 cm-l) from 596 nm (16.78 kK) to 602 nm (16.61 kK) if at high externally apphed voltage the incident light Intensity increases from 6.6 X 10 -4 W cnims2 to 5.3 X 10m3 W cm-? (fig. 2). Current-voltage plots have been measured to ensure that not bulk but electrode processes are the rate determining step of the sensitized photocurrent. They show the usual transition from a space-charge limited r.
- . __,___ ___r_ ii
_____,
Fig. 2. nction spectra of the photocurrcnt at diffcrcnt tllumination (-It = 6.6 X IO4 W cm-‘, left scale; --f= = 5.3 x 10-3 W cm-2, right scale) but constant applied voltagz (U = 300 V).
Volume
52, number
1
CHEMICAL
PHYSICS
wavelength F bOlc f W?P % 593 0’ j:
S89-
15 November
LETTERS
shift of the observcti
nxq+itude.
1977
For the
explanation WC propose a model, according to which a strong internal’ficld is gencrdted across the layer system through the balance of charge carrier injection into the crystal and the subsequent regeneration mechanism. WC will show that plus field is sufficiently large, i.e. in the order of rnagnitudc of 106 V cm-I to pro-
I2
11
duce the observed effects in the action spectra of the pholocurrcnt. Chloranil is a strong electron acceptor. Thcrcforc, the process of sensitized charge carrier generation rc-
applied Fig. 3. Abovu:
VolLlgc
dcpcrttIcnt
peak at light intensities It m fig. 2 ,md below. Below: with broad hnd evcitatmn curve~havc been obtained trade-limited region (LLC)
voltage shift
(volts1 of’
the
pllotocurrcrlt
and 12 that correspond to values Typcal current -voltage plot at 600 nm. Points for lower from action spectrd in the elecand are cutrapolated ( ---) to
indicate the space-charge hmitcd current (SCLC).
current (SCLC) to an electrode limited region (ELC) at moderate external fields (fig. 3). At several points of the ELC a photccurrent spectrum has been rccorded and the shifts obtained for the example below are plotted above.
suits iri tile injection of an electron into the crystal. It is assumed, that only those excited dye molecules D* of the dye layer can contrlbute to the charge transfer that arc at the very surface of the crystal (cf. 17g. 4). Therefore, all the ionised dye molecules, D+, are located on the crystal surface. But as to electrons inside the crystal, ecA, they arc distributed over the whole volume of the crystal with thickness d. This dlstinction is important because m this cast only the ionized dye molecules contribute to an appreciable Internal electric field. In order to calculate lhc density of ~om/.cd dyr: moIccuIcs, [D+] , WC have to consider several competing processes: (i) The generation of excited dye molcculcs and their decay by intramolecular relaxation processes kx
D i hv+D*
(1)
.
1 orachrdic acid dye
chlornnll
crystal
i
4. Discussion The dependence of the dye’s excitation energy AZ&-, + S1) on an external perturbation (electrical field) as calculated in the well known Pariser-ParrPople approximation [2] suggesls that for the isolated n~oleculc a change in the perturbing electrical field (component parallel to the long axis of the molecule) of at least lo6 V cm-l is necessary to shift the S,, + S, absorption band by 200 cm-* -Therefore we can deduct that an electrical field of about 104 V cm-l, as it is calculated from the applied voltage under the assumption of uniform strength across the sandwich cell, is not strong enough to rationalize the
EWlt
hg. 4. Schcmntic cross section through the sandwich drrangcmcnt. For nn cuplanatmn of the symbols We text.
CIIL-MICAL PIIYSICS LEl-l-GRS
Volume 52, number 1
(Ii) The charge transfer reaction with a surface molecule of the crystal, CA, at an overall rate constant k-c3 D* +
CA+
kCT
D++e(:,
,
WI
15 Novemlxr
1977
dependent limiting currents the density of electrons inside the crystal, [e&l , with a mobility I*, decreases with increasing electrical field, Eekt according to
J = e [c&, 1 /.LE,,~ .
@I
Yet even in the case, when the second term in the deThe recombination of an ionized dye molecuic D+ with an injected electron c& (rate constant I&-) leads to a non cxcitcd dye n~olcculc, kb D+ -I-e&--D.
(2b)
Details about the formation of intermediate states (D’ ...e&) that may contrrbutc to internal fields but not to the photocurrent will be discussed later [5]. (iii)The regeneration of the ionized dye molecules via electrons c- that come from the metal electrode across the layer system
D+ + c-x-,,D From
_
(3)
the rate equations!
d[D*]ldr=k,,[Dj
-kd[D*]
-A-&X]
:D*]
(4)
and d[D+l/dt
=k,,[CAj
-k,[e-I
[D*] -k;,T[e&J
[U’J
[D’l ,
(5)
we can deduct the concentrations of excited and ionwed dye n~oleculcs, rcspectivcly, under cqui!!brium conditions:
[D*] =
u = EE,, IEI = 2 X 10M7 [A s cm-2]
km PI
=
k, + kcT [CA]
~OM~)IDl
k, + kcr [CA]
’
kc, iCAl D* 1 [D+] = -.__._ __ k&[ecAJ
+k,[c-j
’
w (7)
with U(V) = absorption cross section and I(v) = photon flux. [D] , [D*] and [D+] are introduced as surface concentratrons, whereas in this type of surface reaction the more formal quantities [CA] , [cc*] and [e-] can be inserted as volume concentrations. A discussion of ccl. (7) shows, that, firstly ID’] and concomitantly the electric field E,nt (cf. fig. 4) increases with an increasing externally applied voltage
194
nominator of eq. (7) is small, no simple inverse relatlonship between [D’J and the externally applied voltage U is expected. Thus is, because one has to assume that the rate constants in this kind of clectrochemical reactions depend on the electrical field. This connection with the injection kinetics explains also, why with respect to a current-voltage plot the larger changes of the wavelength shift are observed beyond the transition region from SCLC to ELC with an indicated saturation at high fields. On the other hand it is therefore not useful and possible to correlate directly wavenumber shifts or absorbance shifts at band edge with an applied ticld as it is usually done with results from electrochromic dyes in condenser arrangements with metal electrodes (e.g. ref. [4], p. 640). Secondly, the (induced) internal field depends on the density of excited molecules on the surface, [D’j , i.e. on the light intensity. This is also in agreement with our experimental findings. As mentioned above, a field of at least 1OG V/cm is necessary to induce the observed wavelength shift. if we assume a homogeneous field between the dye ions on the surface of the molecular crystal and the image charges on the surface of the metal electrode, the required charge density is
U. This is because
of the fact that in fkld
in-
0)
and, therefore, the density of ionized molecules [D’] = 1.2 X 1012 cms2. With an effective area of 250 A2 ftir a single molecule the density of surface molecuIes gets [D] = 4 X lo13 cm- 2. The comparison with the value of [D+] shows, that about 10% of the directly adsorbed dye molecules contribute to the internal field under steady state and limiting current conditions. The density of excited molecules [D*] should be low because of their short lifetime. An exact calculation is not possible up to now because the layered structure influences not only the lifetime, but allows also for the diffusion of excitation energy from the interior regions of the evaporated Iayer to dye molccules on the very surface of the crystal.
Voiumc
52, number 1
CHEMICAL
PlIYSICS
Because of the large intramolecular decay constant dye used (ground state recovery time measurements in ethanofic solution [6] fcad to a value ofkd = 5 X lO*O s-l ; the radiative decay constant is about 5 X IO8 s-l) WCcan, in a first approximation to eq. (6) neglect the influence of the chargetransfer reaction on the quasi stationary concentration of excited molecules [D*] _ _4ssurning k, = IO9 s-l , an incident photon flux of 2 X lOI quanta cm-* s-l, a transparency of 20% of the aluminum electrode, and an absorbance of 1% that is typical of a monolayer of dye molecules witfl an absorption coefficient of about 105 K mol-i cm-l, we get the result [D*] = 4 X I03 crnm2_ If we enter with [D’] = lOI cm 2 and [D*] = 4 X IO3 cm-* into cq. (7), kcr [CAJ /(k& [e&J + k&z-]) =3X IO8 fo Ifows. This seems to be rcasonable in that sense that k& [ecA ]
5. Concluding
remarks
Recently, the delayed ffuoresccnce lifetime as a fimction of the applied voltage has been Introduced as an optical detector in order to dcterminc the total excess charge carrier density in antflraccne single crystals [7] and the effect of space and polarization charge on the internal field within the bulk of a crystal has been investigated with related Stark experiments [8] _ The present method provides a new way not only to detect internal fields at phase boundaries in sandwich systems, but also to investigate rate constants for charge carrier injection processes via eq. (7). More information is expected from a prove of eq. (7) with respect to a more exact calculation of [D+j and, later on, by use or more defined structures at the surface of the molecular crystal, for instance monolayers of the merocyanine dye. Nevertheless, WC believe that the results confirm the hypothesis made above that in the layer system internally induced electrical fields exceed the ones which originate from the externally applied voltage by more than one order of magnitude. A more precise statement cannot be made since the observed band maxima are outside the calculated range of excitation energies [2] _The reason for this behaviour
LEITEKS
15 Novcrnbcr
1977
must be sought in the fact tflat in the dye faycr WC are not dealing wit11 isolated molcculc_s anymore. In that sense we also have to interpret the increase in tile width of the band in the absorption and action spectra of the dye layers compared to the one in the absorption spectra in solution by about 2000 cm-l (fwhm). Interestingly enougfl we observe a decrease in the full width half maximum of approximately 400 cm-l, wilCn tfle external applied voltage is increased from 50 to 300 V (fig_ 1). Both effects, tfle bathochromic shift of tflc band maxima, and the reduction m the half width upon application of an external electrical flcld resemble very much the beflaviour or the absorption band found upon cooling of a sample in solution [2] . In addition to changes in the ratio of cis and trans isomers and of monomers and dimers, respectively, efcctrical fields of more than IO6 V cm-I could very well influence the orientation of molecules wit11 a dipole moment as large as 15 debye. However, the rigid structure of adsorbed layers does not favour the motion and reorientation of dye molecufcs on the phase boundary. Finally, prcssurc induced absorption changes because of the charging of a capacitance have been considered in connection with the study of electrocllron+m of cyanine dyes in condenser arrangements with rnonolayer assernbfies betwcmi metal clcctrodes [VJ . With a field of 2.5 X 106 V cm-l there was estimated a negligible effect. The consequences of all the above effects on the band shape of the action spectra cannot be predicted without tflc knowledge of much more details.
Acknowledgement Both authors wish to thank the Deutschc Forschungsgemeinschaft for financial support. Free computer tlmc by the Lclbniz-Rechcnzentrum is also gratefully acknowledged.
References I?/13 (1976) 857. III H. Killcsreitcr, I. Lurnincsccncc P. Scheibc, S. Schneider, F. Dorr and E. Dal trozzo, Bcr. [21 Runscngcs.
Physik. Chern. 80 (1976) 630. Faraday Discussions Chern. SOC. 58 (1974)
(31 H. Krllcsrcltcr, 271.
19.5
Volume
52, number
1
CHEMICAL
[4] 11. Kuhn, D. hfobius and H, Buchcr, in: Physical methods of chemistry. Vol. t, cds. A. Weissbergcr and B. Ro\sitcr (Whey, New York, 1972). [S] H. ICillcsrzitzr and S. Schncidcr, Ucr. Bunscnge\. Fhysik. Chcm., to bc submitted for pubhcation. [6] 1’. Wirth, Thesis, Tcchniscbc Univcrsit;i
PHYSICS
LETTERS
1977
[7] hi. Pope and W. Weston, Mol. Cryst. Liquid Cryst. 25 (1974) 205. [8] F.P. Chcn, S.J. Shcng and D.M. Hanson, Chem. Phys. 5 (1974) 60. [9] 11. Blicher. J. Wiegand, B.B. Snavely, K.H. Beck and H. Kuhn, Chem. Phys. Letters 3 (1969) 508.
ERRATUM P. Pyykko and J.P. Desclaux, Dirac---Fock one-ccntre calculations. The model systems TiH4, ZrH4, HfH, and ( 104)114, Chcrn. Phys. Letters 50 (1977) 503. There is an error in table 1 on page 504: in the left-most column Zrll,, lifH4 and (104)H4 should have been printed one line down.
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15 November