OPTICAL ABSORPTION OF MOLECULES IN THE ELECTRIC FIELD OF THE DOUBLE LAYER

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OPTICAL ABSORPTION OF MOLECULES IN THE ELECTRIC FIELD OF THE DOUBLE LAYER

P. Schmidt, W. Plieth

To cite this version:

P. Schmidt, W. Plieth. OPTICAL ABSORPTION OF MOLECULES IN THE ELECTRIC FIELD OF THE DOUBLE LAYER. Journal de Physique Colloques, 1983, 44 (C10), pp.C10-175-C10-178.

�10.1051/jphyscol:19831035�. �jpa-00223492�

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30URNAL DE PHYSIQUE

Colloque CIO, supplément au n°12, Tome W, décembre 1983 page C10-175

OPTICAL ABSORPTION OF MOLECULES IN THE ELECTRIC FIELD OF THE DOUBLE LAYER

P . Schmidt and W. P l i e t h

Vreie Universitdt Berlin, Institut fur Physikalisehe Chemie, Takustrasse 3, D-1000 Berlin 32, F.B.G.

Résumé - Les s t r u c t u r e s e t les p r o p r i é t é s optiques de monocouches orga- niques adsorbées sur une é l e c t r o d e de p l a t i n e ont été étudiées par r é f l e c - t i v i t é d i f f é r e n t i e l l e . Nous observons des e f f e t s électrochromiques pour des monocouches de p-amino-nitrobenz'ene, de p-diméthylamino-nitrosobenzène e t de p - d i m ë t h y l a m i n o - n i t r o s t i l b è n e . Les mesures donnent des i n f o r m a t i o n s sur l ' o r i e n t a t i o n des molécules adsorbées sur l ' é l e c t r o d e . Les e f f e t s électrochromiques peuvent ê t r e employés pour c a l c u l e r l e champ é l e c t r i q u e dans l a couche double. Finalement, les spectres de 1'anthracène adsorbé sur une é l e c t r o d e de p l a t i n e sont présentés.

Abstract - The s t r u c t u r e s and p r o p e r t i e s o f organic monolayers adsorbed on a polished platinum e l e c t r o d e were s t u d i e d w i t h the help of modulation r e - f l e c t a n c e spectroscopy. The monolayers o f p-amino-nitrobenzene, p - d i m e t h y l - amino-nitrosobenzene, and p - d i m e t h y l a m i n o - n i t r o s t i l b e n e show electrochromic e f f e c t s what allows the d e t e r m i n a t i o n o f the o r i e n t a t i o n o f the adsorbed molecules and f i n a l l y the c a l c u l a t i o n o f the double l a y e r f i e l d s t r e n g t h . In a d d i t i o n to t h a t , the MR spectrum of anthracene adsorbed on a p l a t i n u m e l e c t r o d e was also detected.

1 . I n t r o d u c t i o n - A strong external e l e c t r o s t a t i c f i e l d (E > 10' V m"1) changes the absorption spectrum of a molecule. This phenomenon i s known as electrochromism / 1 , 2 , 3 / . The molecules adsorbed on the surface o f an e l e c t r o d e should show the e l e c - trochromic e f f e c t s w i t h i n the f i e l d o f the electrochemical double l a y e r (EQ[_=109 V m " l ) . The modulation o f the double l a y e r f i e l d causes a modulation of the e l e c t r o - chromic e f f e c t s which can be observed by a modulation r e f l e c t a n c e (MR) spectrum / 4 / . S t r u c t u r e s and p r o p e r t i e s o f the organic adsorption l a y e r can be determined by means of the dependence o f the MR spectra on various experimental parameters / 5 , 6 / . The f i e l d s t r e n g t h i n the double l a y e r i s also c a l c u l a t e d from the MR s p e c t r a .

2. Experimental - A conventional MR spectrometer was used t o d e t e c t the MR spectra / 4 / . The s p e c t r a l range was 370 nm - 830 nm. The angle of incidence was 65 ° . P a r a l l e l ( II ) and perpendicular ( _L ) p o l a r i z e d l i g h t was a p p l i e d . E l e c t r o d e s : working e l e c t r o d e : Pt p l a t e (99.99 %), mechanically polished and cleaned w i t h hot HN03 (65 %); counter e l e c t r o d e : Pt n e t ; reference e l e c t r o d e : H g / l ^ S O ^ , K2SO4

( s a t ' d ) . The anodic voltage range o f the e l e c t r o d e sweeped only the double l a y e r range o f the working e l e c t r o d e (i=2 yA cm"2 = c o n s t . ) which i s covered w i t h an organic monolayer. A s i n u s o i d a l reference voltage was a p p l i e d w i t h a modulation a m p l i - tude o f ±50 mV and a modulation frequency o f 33 Hz.

The organic substances w i t h p.a. q u a l i t y and a cone, o f 10"6 M - 10"4 M were used.

The p u r i t i e s o f the organic substances were c o n t r o l l e d by chromatography. The basic e l e c t r o l y t e s were 0.5 M ^ S O * and 1 N H2SO4 and were deaerated w i t h argon (99.99 %).

The i n v e s t i g a t e d organic substances can be taken i n the f o l l o w i n g t a b l e :

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19831035

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JOURNAL DE PHYSIQUE

p-amino-ni trobenzene(1) :

O ~ N O N H ~

pi = 6.3 D; p f = 14 D; Au = 7.7 D 131; ,,,A = 380 nm; water p-dimethylami no-ni trosobenrene(1I) : O N ~ N ( C H ~ ) ~

pi = 6.9 D; p f = 13 D; AII = 6.1 D 131; ^Amax ⁼439 nm; water p-dimethylamino-ni t r o s t i l b e n e ( I I 1 ) : O ~ N Q C H = C H Q N ( C H ~ ) ~ pi = 7.1 0; p f = 26 D; ALI = 18.9 D 131; ,,,A = 380 nm; > 1 N H2S04

u., uf denote t h e d i p o l e moment values o f t h e ground s t a t e and o f the f i r s t e x c i t e d s t a t e , r e s p e c t i v e l y .

+ + +

aU=lpi - p f

1

i s the value o f the d i f f e r e n c e between

zf

^and^{u i .}

Amax denotes t h e wavelength o f t h e a b s o r p t i o n band o f t h e e l e c t r o n i c t r a n s i t i o n i

--

^fi n the above mentioned s o l u t i o n .

The d i p o l e moment as w e l l as t h e t r a n s i t i o n moment a r e p a r a l l e l w i t h t h e a x i s o f t h e molecule ( I , 11, 111) 131. Molecule I 1 1 i s d i s s o l v e d i n i t s protonated form which

i s predominantly i n the t r a n s c o n f i g u r a t i o n 171. For anthracene t h e t r a n s i t i o n moment l i e s i n t h e conjugated r i n g system. As t h e molecules show r e l a t i v e l y h i g h

Ap values a measurable band s h i f t e f f e c t i s expected.

3. Results and Discussion

-

^MRspectra obtained f o r I, 11, and I 1 1 ( F i g . 1-3) show a "two peakt' s t r u c t u r e f o r

II

p o l a r i z e d l i g h t b u t no peaks f o r l p o l a r i z e d l i g h t a r e observed. Due t o t h i s s t r o n g a n i s o t r o p y we conclude t h a t t h e molecules are o r i e n t a - t e d w i t h t h e i r d i p o l e moments perpendicular t o t h e p l a t i n u m surface.

Peak-!

i s explained by a modulation o f t h e main a b s o r p t i o n band o f t h e molecules.

For the molecule I i n n e u t r a l e l e c t r o l y t e s , t h i s modulation i s caused by t h e f i e l d dependence o f t h e resonance frequency (band s h i f t e f f e c t ) . Considering a l l e x p e r i - mental f a c t s t h e r e i s a d i f f e r e n t explanation o f t h e modulation f o r molecule I 1 1 which i s s o l u b l e o n l y i n i t s protonated form i n s u l f u r i c a c i d . I n t h i s case we be- l i e v e t h a t t h e e q u i l i b r i u m between the protonated and t h e deprotonated form o f t h e molecule i s f i e l d depending. Therefore t h e modulation o f t h i s e q u i l i b r i u m i s observed. For molecule I 1 the o r i g i n o f the modulation i s a process which c o u l d n o t be i d e n t i f i e d so f a r , f o r molecule I 1 was i n v e s t i g a t e d l e s s i n t e n s i v e l y .

Peak 2 i s assumed t o be caused by a c l u s t e r i n g o f the admolecules on t h e surface

---

g i v i n g r i s e t o a band extending i n t o the r e d r e g i o n o f t h e spectrum.-According t o the equation ( x ) t h e peak h e i g h t s depend on the modulation amplitude AU as w e l l as on t h e s t a t i o n a r y e l e c t r o d e p o t e n t i a l Uo.

The l i n e a r dependence on AU was o n l y observed i n l i m i t e d regions f o r a l l t h r e e types o f t h e molecules. But the dependence on U i s d i f f e r e n t f o r each type. For I i t i s v a l i d Bk = 0 (independence), f o r 11, 111

Ek

⁼constk > 0, r e s p e c t i v e l y . I n the case o f molecule I11 the peaks were o n l y observed above the p o t e n t i a l Uo > Ucrit

= 590 mV (NHE). For Uo < Ucri the protonated c o l o r l e s s form o f t h e molecule dopinates on t h e surface and t h e r e f o r e no s i g n a l i s observed. For Uo > UCri an i n - c r e a s i n g surface area i s covered by the deprotonated c o l o r e d form and t h l s process causes t h e l i n e a r p o t e n t i a l dependence o f the peak h e i g h t s .

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I

^{FIG. 3}

Modulated reflectance spectra of P t (change of reflectance A R versus wave- length

x)

FIG. 1

-

0.5 M Na2S04, 5 . 1 0 - ~ M ( I ) , Uo = 440 mV viz NHE

FIG. 2

-

0.5 M Na2S04, M (11), with increasing peak height, Uo = 340 mV, Uo = 440 mV, Uo = 540 mV viz NHE

FIG. 3 - 2 M H2S04. 5 . 1 0 - ~ M (111). Uo = 840 mV viz NHE FIG. 4

-

1 N H2S04, (IV) saturated

Modulation amplitude AU = +50 mV; modulation frequency v = 33 Hz, angle of incidence a i n = 65 O . All normed AR/R spectra have value!! of the order

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CIO-178 JOURNAL DE PHYSIQUE

F o r a l l t h r e e t y p e s o f molecules peak 1 s h i f t s l i n e a r l y t o s h o r t e r wavelengths b y i n c r e a s i n g anodic e l e c t r o d e p o t e n t i a l Uo. We conclude t h a t t h e s e s h i f t s r e p r e s e n t t h e f i e l d dependence o f t h e resonance f r e q u e n c y . C o n s i d e r i n g t h i s band s h i f t t h e o r i e n t a t i o n o f t h e molecule i n t h e double l a y e r f i e l d s h o u l d be w i t h t h e NR2-group on t h e e l e c t r o d e s u r f a c e and t h e NO2-group p o i n t i n g towards t h e e l e c t r o l y t e . We suggest t h a t t h e i n t e r a c t i o n o f t h e NO2-group w i t h t h e w a t e r m i g h t be r e s p o n s i b l e f o r t h i s s u r p r i s i n g r e s u l t . From t h e resonance frequency/Uo-dependence t h e f i e l d s t r e n g t h i n t h e double l a y e r can b e c a l c u l a t e d i f t h e dip01 moment d i f f e r e n c e rill o f a m o l e c u l e i s g i v e n . The r e s u l t s o f t h i s c a l c u l a t i o n a r e 0 . 8 . 1 0 ~ V m - I (mole- c u l e I ) and 0.3.108 V m-1 ( m o l e c u l e 111) f o r one v o l t e l e c t r o d e p o t e n t i a l .

A q u i t e d i f f e r e n t t y p e o f m o l e c u l e i s anthracene. The MR spectrum o f t h i s m o l e c u l e i s shown i n F i g . 4. The spectrum shows a t y p i c a l m o d u l a t i o n s t r u c t u r e f o r II po- l a r i z e d l i g h t , w h i l e no s t r u c t u r e appears f o r

I

p o l a r i z e d l i g h t . The t r i p l e t e x c i - t o n e x c i t a t i o n i s n e a r t o t h e observed s t r u c t u r e /8/. T h i s t r a n s i t i o n m i g h t be s t r o n g l y enhanced i n t h e v i c i n i t y o f t h e e l e c t r o d e . I n t h i s c o n n e c t i o n i t s h o u l d be mentioned t h a t a d i s p e r s i o n was found f o r peak 2 o f m o l e c u l e I . These p r e l i m i n a r y r e s u l t s open up t h e p o s s i b i l i t y of o b s e r v i n g n o n l o c a l e f f e c t s i n o r g a n i c a d s o r p t i o n 1 ayers.

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