The linear electrochromic effect in metanitroaniline

J. Phys. Chem. Solids, 1973,Vol.34, pp. 235-239, PergamonPress. Printedin Great Britain

THE

LINEAR

ELECTROCHROMIC NITROANILINE

EFFECT

IN

META-

J. L. STEVENSON*, S. AYERS and M. M. FAKTOR Post Office Research Department, Dollis Hill, London, England (Received 14 February 1972)

Abstract--An electric field applied along the polar axis of the molecular crystal meta-nitroaniline causes a change in the optical transmission at the band edge between 505 and 540 rim. The size of the incremental change was found to vary linearly with applied field, and to reverse with field polarity. We have termed this 'the linear electrochromic effect'. We suggest here, that it arises from a shift in the transition energy of the visible-near u.v. electronic absorption band, which is implied by the observed linear electro-optic behaviour of this material. A theoretical model is proposed for the linear electrochroism of meta-nitroaniline, and from this a maximum band shift of 2.5 x 10-~a J per (kV mm -1) is predicted. 1. INTRODUCTION

MANY theoretical treatments of the linear electro-optic (Pockels) effect, use as a basis the electric field perturbation of dispersion controlling, electronic absorption bands. Kurtz and Robinson[l] used a model where such bands were contracted to a single frequency line spectrum. They were able to show that the linear electro-optic effect originates from a field reversible shift of this Sellmeier oscillator frequency. The physical processes leading to this shift were discussed by Garrett[2]. In most of the materials listed by Rez[3] as having large electro-optic coefficients (> 2 × 1 0 - n m V-l), the mechanism arises from an ionic polarisation process. This influences the control of refractive index dispersion by the u.v. absorption bands. Experimental observations of the linear electro-absorption effect implied by this argument, were reported by Kern [4] and Harbeke [5] for antimony sulphoidide(SbSI), and by Fridkin and Verkhovskaya [6] for barium titanate(BaTiO3). In this paper we describe the measurement of the same electro-absorption effect for a non-ferroelectric molecular solid; metanitroaniline(mNA). Since, in this case, the linear proportionality of the transmitted in*At present on study leave at Department of Electrical Engineering, Imperial College, London SW7 2BT.

tensity change to the applied field includes a reversal of sign with field polarity not seen in the ferroelectrics[4-6], we have termed it the linear electrochromic effect'. A theoretical model for this effect, in mNA, is proposed. 2. EXPERIMENTAL

We have previously reported the measurement of the linear electro-optic effect in tuNA [7]. It was largest when the field was applied along the z axis of the crystal, and the same field orientation was used in the electrochromic investigation. The crystals of tuNA were grown and prepared by standard techniques[7]. The optical apparatus consisted of a Hilger and Watts D330 single grating monochromator and a Mullard 1 5 0 A V P photomultiplier. A tungsten-halogen lamp (55 W), run from a stabilised d.c. supply, was used as a white light source. The output beam from the monochromator was focussed onto the[001] crystal face by a microscope objective; and limited in bandwidth (to approx. 1 nm) by a pin-hole aperture. Using this simple spectrophotometric system, optical transmission curves for the crystals of m N A could be recorded as shown in Fig. 1. The crystals were mounted between electrodes, as shown in Fig. 2. The longitudinal alignment of light and field was chosen so that

235

236

J . L . STEVENSON. S. AYERS and M. M. FAKTOR

a large field could be easily applied over a short optical path length. This geometry had also been used in the electro-optic work on mNA[7]. With the monochromator set at wavelengths between 505 and 540nm (i.e. on the band edge), application of a d.c.

voltage (from a Brandenburg generator) across the crystal gave a change in transmitted intensity. The change increased linearly with applied field strength and its sign was reversed when the field polarity, or the crystal, was inverted (see Fig. 3). No previous description

curve I, optical path length 1"93 mm

kL ~, I,

curve 2,

-

-

.

1.32 mm

"/. Intensity change

,/~2

/

uncorrected far reflectance 45

1

noise level

50

-I-5

-I.0

O-5

-OS

I'0

Applied Field t.5

kVmm"

15

2.45

2.40

2-35

or

crystal inverted-2

eV (electron volts)

Fig. I. Transmission curves-umpolarised beam along the z-axis.

Fig. 3. D.c. electrochromic response at 508 nm.

CHART

REco .0 R / I

y

EHT SUPPLY 4kV rms,50kHz

~-~z-axls

PHOTOMULTIPLIER

M:d~ated output Or'ginal input ~ beam {monochromatic,

unpolarised )

~.,~ " ~

._j._~ mNA crystal

Fig. 2. Optical apparatus and a.c. detection system.

THE LINEAR ELECTROCHROMIC E F F E C T

of this effect appears to have been reported. A non-synchronous a.c. detection system was chosen for further studies (Fig. 2). On application of a 50 k H z a.c. voltage to the crystal, a signal of the same frequency appeared on the oscilloscope. The signal disappeared on removal of the crystal. The system was calibrated, in terms of a percentage transmitted intensity change per unit applied field, by direct comparison with the d.c. measurements. From this basis, the electrochromic response to a constant field (1 kV mm -1) was measured over the wavelength range of the band edge. The experimentally determined wavelength dependence of the linear electrochromic effect is shown in Fig. 4. Electrochromic per

curve 1, optical path length 1.93mm

A

Tintensity change

curve 2.

-

1-32mm

kVmm"/per"unitfield flOE

2 !

237

obtained from solution spectra of mNA. This transition is described, for an isolated m N A molecule, as involving intramolecular charge transfer (i.e. its transition moment is derived from a combination of electronic donation by the amino-group to the aromatic ring, and electron withdrawal from the ring by the nitro-group) [9]. This in turn leads to a pronounced increase in the dipole moment upon excitation. Solvatochromic measurements [9] give a value of A/j. = 2"5 × 10-~a C m for the dipole moment increase in a m N A molecule. This description of the transition is oversimplified, even for an isolated molecule. It may become unacceptable when the absorption characteristics of the solid have to be considered. Nevertheless, we shall neglect excitation energy transfer and intensity borrowing processes in the solid state, and use a 'crystalline gas' of m N A as a model for the solid. Although we have not measured the dispersion of the dielectric constants in tuNA, we conclude from a determination of the low frequency constants [7]

004

el = 3.9

e~ = 4.2

ea = 4"6

(all at 20°C and 3 kHz) 00~

505

510

515

520

525

rml

Fig. 4. Variation of electrochroism with wavelength. 3. THEORETICAL DISCUSSION

(a) The physical nature o f the linear electrochromic effect The low energy electronic absorption bands of m N A have been studied by Lutski and Gorokhova[8], who list the first singlet-singlet transition as having a central absorption energy, Ftr, of 5.48 × 10-1°J and an oscillator strength, f, of 0.08. Both of these values were

JPC_~ VoL 34, No. 2 - G

that neither orientational, nor atomic polarisation play a major role in the electric field effects under consideration. The electronic polarisation will, in part, be derived from the absorption band described above. If Ea is the electric field applied along the z axis of the crystal (in the point group notation for m N A of mm2, this is the polar axis), then the change in energy of the first singletsinglet transition is given by [ 10]. F~,--Ftr=--(~:--lz3°)Ez--½E3 2

E (a~--t~)

J=1.2.3

(1) where /zs is the component of the dipole moment in the Z direction, o~3are components of the principal polarisabilities and the

J. L. STEVENSON, S. AYERS and M. M. F A K T O R

238

superscripts are e first singlet excited state g singlet ground state. With the values of/~e_/zg = A/~ = 2.5 × 10-29 C m given above, and assuming full alignment of the dipoles with the field direction, the first term in equation (1) gives an energy change of 2.5 x 10-~a J for an applied field of 1 kV mm -1. If the second term is to give a comparable energy change at this field strength, then the mean value of a e - au must be approx. 4.5 x 10-16 mm a. This is physically unrealistic. Measured values are 5 x 10 -z° mm s for molecules with the same type of lowest energy singiet transition[l 1]. Therefore, if the orientation of the m N A molecule in the crystalline solid is such as to make/~s ° and /zse sizeable components of/~" and/~e, there will be a transition energy change linear with applied field up to at least 1 kV mm -1 for the intra-molecular charge transfer band. The spectral absorption band will have considerable width due to the many transitions occurring between different vibrational energy levels in the ground and excited electronic states. Since the variation of/~ through these states will be small the transition energy change will be common to the whole absorption envelope[12]. So for any transition energy within the absorption band we can predict a frequency shift given by F ~ - - F~r = h(v E - v) = 2"5 X 10-2a J

(2)

i.e. Av ~< 40 G H z for an applied field of 1 kV mm -I. Clearly this shift, and its reversal with field polarity will give rise to a reversible change in the transmitted intensity exactly as described above. In fact, the measured linear electrochromic effect can be matched against this predicted source. T h e intensity change gained by shifting the transmission curves of Fig. 1 uniformly along the energy axis will be proportional to their slopes. Calculation of the latter

yields the predicted electrochromic response points plotted alongside the experimentally determined curves in Fig. 4. T h e predicted experimental values for two crystals of different optical path lengths are well matched if a transition energy change of 1.6 x 10-2aJ is applied to the static transmission curves. (b) T h e relation o f linear e l e c t r o c h r o i s m to the linear electro-optic effect We follow here the theory of the linear electro-optic effect proposed by Kurtz and Robinson[l]. This is based upon the electric field perturbation of an anharmonic dispersion electron oscillator, and gives as a result an expression for a perturbed, dispersion controlling absorption frequency. This can be rewritten as A v / f l E = v e / 1 6 rr4 m Vo3

(3)

where v = anharmonic force constant /3 = local field parameter Vo = unperturbed transition frequency Clearly A v / E is the linear electrochromic frequency shift per unit field, which is accessible from our experimental data. By taking /3 = 1 (as suggested by Kurtz[13]); v0 = 8.3 X 1014 s - l ; and with A v / E = 2"4 × 101° s -1 kV -1 mm, then v = 1.25 × 10a7 m -1 s -2 for m N A .

T h e maximum electro-optic coefficient, r, can be gained from this value by use of the Kurtz and Robinson model equation (7). r = (n 2 - 1)2fly/8 ~ran4Noevo 2

(4)

where n = 'refractive index' and No = density of dispersion electrons. Since we are dealing only with the lowest energy electronic transition, n a is obtained from a single-term Sellmeier dispersion equation (14).

THE LINEAR ELECTROCHROMIC EFFECT n 2 - 1 = N e 2 f / r r m ( v o 2 - v 2)

(5)

w h e r e N = m o l e c u l a r d e n s i t y ; No = N f u = observation frequency with and

N = 6 × 1018 m m -3

239

c i e n t a r i s i n g f r o m t h e e l e c t r o c h r o m i c shift o f m N A is c a l c u l a t e d (on t h e b a s i s o f a c r y s t a l l i n e g a s m o d e l f o r t h e m o l e c u l a r solid) to b e r = 6.2 × 10 -12 m V -1. T h e g o o d a g r e e m e n t o f this v a l u e with t h e e x p e r i m e n t a l r e s u l t s [7, 16], s e e m s p a r t l y f o r t u i t o u s o n c a r e f u l analysis of the assumption made.

v = 4"8 x 1014 s -1

(for o b s e r v a t i o n at t h e H e - N e 632.8 n m l a s e r line), e q u a t i o n (5) g i v e s n 2 = 1.084 w h i c h , b y s u b s t i t u t i o n into e q u a t i o n (4) g i v e s

Acknowledgements-We are grateful to Mr E. Bremner for skilful technical assistance, and to Mr W. G. I. Caughey and Mr D. Marr for e.xperimental aid. Acknowledgement is made to the Senior Director (Development) of the Post Office for permission to publish this work.

r = 6.2 x 10 -12 m V -1 REFERENCES

4. CONCLUSIONS It is f o u n d t h a t t h e e x p e r i m e n t a l a n d p r e dicted linear electrochromic response of t u N A a r e m a d e a l m o s t i d e n t i c a l if an o v e r a l l shift o f 1.6 × 10-2sJ in t h e t r a n s i t i o n e n e r g y o f a field o f 1 k V m m -1. T h i s v a l u e is well w i t h in t h e limit c a l c u l a t e d f r o m e q u a t i o n (1), b u t is s m a l l e r t h a n t h e v a l u e s f o u n d f o r S b S I a n d BaTiO3 both of which have maximum absorption e d g e shifts at t h e i r f e r r o e l e c t r i c C u r i e p o i n t s (2-4 × 10 -20 J a n d 3.6 × 10 -~° J r e s p e c t i v e l y f o r fields o f 1 k V m m -1) [5, 6]. N e i t h e r o f t h e s e shifts r e v e r s e s with field p o l a r i t y ; p r e s u m a b l y d u e to t h e s i m u l t a n e o u s r e v e r s a l o f c r y s t a l p o l a r i t y . T h e l i n e a r e l e c t r o c h r o i s m in m N A is, h o w e v e r , m u c h l a r g e r t h a n t h e q u a d r a t i c effect d e t e c t e d in As2Ses [15], a n d ( b e i n g e l e c t r o n i c in origin) s h o u l d p e r s i s t up to o p t i c a l f r e q u e n c i e s . T h e i s o t r o p i c l i n e a r e l e c t r o - o p t i c coeffi-

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