Spectroscopic properties of conducting TCNQ Langmuir-Blodgett films obtained via a homodoping process

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Spectroscopic properties of conducting TCNQ Langmuir-Blodgett films obtained via a homodoping

process

V. Yartsev, O. Fichet, J.-P. Bourgoin, P. Delhaès

To cite this version:

V. Yartsev, O. Fichet, J.-P. Bourgoin, P. Delhaès. Spectroscopic properties of conducting TCNQ

Langmuir-Blodgett films obtained via a homodoping process. Journal de Physique II, EDP Sciences,

1993, 3 (5), pp.647-660. �10.1051/jp2:1993158�. �jpa-00247862�

Classificafion

Physics

Abstracts 78.30J 78.65J

Spectroscopic properties of conducting TCNQ Langmuir.

Blodgett films obtained via

_a

homodoping _process

V. M. Yartsev

(I, *),

O. Fichet

(I),

J.-P.

Bourgoin (2)

and P. Delha~s

(I)

(I)

Centre de Recherche Paul Pascal, CNRS, _av. A. Schweitzer, 33600 Pessac, France (2) CEA-IRE-DPhG-SCM, CEN

Saclay,

91191 Gif-sur-Yvette Cedex, France

(Received 22 October J992, accepted in final form 9 Februaiy J993)

Abstract. We report ^the

preparation

and the

spectroscopic properties

of

Langmuir-Blodgett

films obtained

by mixing

in the _same

monolayer

_a

polar amphiphilic octadecyl tetracyanoquinodi-

methane

(Cj~TCNQ)

and _an ionic

semi-amphiphilic (Cj~s~dithiolium-TCNQ compound.

A

cluster approach is used for the

interpretation

of the electronic charge transfer band and of the vibronic features in infrared _{as a} result of electron-molecular vibration (EMV) linear

coupling.

Different

assumptions

used

previously

for an estimate of EMV

coupling

constants from infrared spectra are

analyzed

and _{a new} formula

relating positions

of vibronic bands in

absorption

and EMV

coupling

constants is

proposed. Experimental

and theoretical

absorption

_{curves are}

compared

and discussed. It allows _us to conclude in favor of the presence of mixed-valence dimers

which _are at the origin of the conduction process.

1. Introduction.

To build up ultrathin

organic

films via the

Langmuir-Blodgett (LB) technique

the

following

steps ^{have been}

recognized

_:

I) synthesis

of molecular blocks with both _an

amphiphilic

and _an electroactive character _;

it) monolayer feasibility

studies and the

thermodynamic stability

at the

gas-liquid

interface _;

iii)

formation of the LB film

following

a transfer of

monolayers

onto a substrate _to obtain _a ultrathin

organized

material.

For

obtaining

a

conducting film,

_a mixed-valence system ^from electroactive molecules

(which

_are either electron donors _or

acceptors)

must be realized

[Il. Among

the different

attempts ^{the works}

using tetracyanoquinodimethane (TCNQ)

salts and

complexes

have been the _most

developed

_ones with two main

approaches.

In the first _{one a} mixed-valence state is obtained

by post-doping

with iodine of

binary TCNQ

salts which _were

completely

ionized

during

the initial film

preparation [21.

(*) On leave from

Chelyabinsk

State

University,

454136

Chelyabinsk,

Russia.

The second way

(so-called homodoping strategy) employs

_a

binary TCNQ

salt which is

homogeneously

mixed in the

spreading

solution with _an

alkyl TCNQ (CIgTCNQ)

131, It appears that the latter

technique

is _{a more}

general approach

which allows _{one to} fumish _a mixed-valence state in many cases for different counter-ions _even when the iodine

doping

process does not work

[41. Furthermore,

it has been shown for the first time

[31

^that

using

the

homodoping strategy

one can obtain _{even a}

conducting monolayer.

Following

these tracks and

looking

for the influence of the counter-ion _we have worked with

a new cation

3-methylthio-4, 5-bis(octadecylthio)-1,

2-dithiolium abbreviated _as

(Cigs )~dithio- lium, giving

_a I _; I

TCNQ

salt

[51

which presents some

specific

features. In contrast to the

sulphonium

salts

[31,

where LB films of

Y-type

have been

prepared,

a

non-centrosymmetric

Z- type

multilayered

structure is obtained with

(CI~S)~dithiolium

cation. It has been shown that this characteristic is related with _a

specific monolayer organization

as shown

by

in situ

ellipsometric

^{and surface}

potential experiments [61.

After

checking

that this LB film remains

insulating

even after iodine

doping,

we have decided to extend the

homodoping technique

_to this ionic

semi-amphiphilic

salt

by mixing

the

liquid

^solution of

(Cigs)~dithiolium-TCNQ

with different amount of neutral

CIgTCNQ.

We

can vary the

stoichiometry

ratio [41 and also the number of

deposited layers

as two

primary independent parameters.

We

present

^{in the} next part ^the

experimental

conditions for the

preparation

of

(CI~S )~dithiolium-TCNQ

LB

homodoped

films

together

with their

optical absorption

_spectra.

The absorbance

spectra

^{of these LB films look} very much like the spectra ^of

quasi-ID

molecular

crystals

where the molecules _are

arranged

to form linear clusters of

2,

3 and 4 units.

It _means that the

optical properties

^{of both} LB films and

crystals

_are determined

by

rather small

molecular clusters in accordance with _a

higher degree

of electron localization in these

compounds compared

to «classic»

semiconductors,

such _as Ge _or Si.

Following

_our

experimental

results _we propose in the third section _a theoretical

approach

which is based upon the electron-molecular vibration

(EMV)

linear

coupling

model

[71

^for

TCNQ crystals.

We shall compare the EMV

coupling

constants found for

conducting

films and

charge

transfer

crystals

and discuss the local

description

ⁱⁿ the framework of _our model _versus the

general picture

extracted from the d.c,

experiments.

2.

Experimental.

2.I LB FILM PREPARATION. The salt

(Cj~S)~dithiolium-TCNQ

^{and the}

high purity

Cj~TCNQ

_are dissolved in chloroform

(c=10~~mol.l~')

_{and mixed}

_together

_before

spreading

at the air-water interface

(ATEMETA

LB 105

trough).

The

pressure-surface (w-A)

isotherms of these

spreading

solutions of the

composition (I

_{: n}

),

with _n

(0.5

w n w 3

denoting

the number of

CI~TCNQ

molecules per

(Cj~S)~dithiolium-TCNQ,

have been

recorded with _a

compression speed

of

0.5A~.mol.~'min~~

_{at T=19°C.} We observe

classical w-A _curves with _an almost

regular

increase of 20

A~ (at

_a

given pressure)

for each additional _n in

composition

which is due to the addition of _one

alkyl

chain. This observation allows _us to think that _{we are}

dealing

with

homogeneous monolayers,

the

only exception being

for the

(1

_: 3

)

mixture. In the latter _{case we} have detected _an inflexion

point

which could be the

onset of _some pressure

plateau

as

already

found in several

binary TCNQ

salts

[8].

The

samples

of LB films have been

prepared using

the vertical

dipping

^method on different

hydrophilic substrates, usually CaF~

and Znse for the

optical experiments.

^{In all} cases the

monolayer

transfer has been of

Z-type

with _a transfer ratio close _to

unity

_up to 90

deposits

_as

already

observed for the

pristine TCNQ

salt

[5].

This result is confirmed

by

low

angle X-ray

diffraction

technique

which indicates _an

interlayer spacing equal

to 36

A.

2.2 SPEcTRoscoPic PROPERTIES. The selected _room

temperature

spectra are

presented

in

figures

I and 2 which show the

absorption

spectra versus

respectively

the overall

stoichiometry (I

_:

n)

^{and the} number of

layers

N. These

experiments

have been carried out

using

an FT-IR interferometer NICOLET MXI in

particular

_we have checked

by

linear dichroism that these

isJa Viowmmher icm4)

Fig.

I. Infrared

absorption

spectra ^of

[(Cj~S )~dithiolium-TCNQ

Ii

ICIBTCNQ

_{In LB} films (50

layers)

for different stoichiometries I _{: n.}

g ^N=

.=

3

2-

«

N=40

N=5

Waenumb« (cm-i)

Fig.

2. infrared

absorption

_spectra of

[(Cj~S )~dithiolium-TCNQ

Ii

ICj~TCNQ12

LB films for different number of

deposited layers

N.

LB films appear as

optically isotropic

with the band intensities

being

constant as

long

as the electrical field lies in the

layers plane.

At first

glance

we can

distinguish

two kinds of

spectra

:

I)

for the mixture

(1:0.5)

_we observe _a

charge

transfer band at 8400 cm-' which

corresponds

to a «

B-type

band

»

[9]

associated with _a transition in _a

fully

^ionized cluster _as for

example

in _a

(TCNQ~

₎₂ dimer.

Indeed,

this spectrum ^is reminiscent of the

(octadecylphe- nanthrolinium++) (TCNQ~

₎₂ salt

[2]

which presents ^both a

charge

^transfer band around 9 000 cm~ ^' and several vibronic bands located at

575,

351 and 177 cm~ ^' for the

totally

symmetric (a~) ^TCNQ

^modes _{w~, w~} and w~,

respectively.

ii)

For the other

compositions

_{we see}

immediately

a «

A-type

_»

charge

^transfer band

[9]

characteristic of _a mixed-valence

system

^{associated with} several

strong

vibronic bands due _to excitation of intramolecular vibrations via the EMV

coupling.

Furthermore these films _are

conducting

_as demonstrated

by

d-c-

resistivity experiments [8]

which show _{a room}

temperature

value up ^to

10~~

_S.cm~ ' when the number of

deposited layers

reaches 10.

These

spectra

appear rather similar to those

already

observed

by Saclay's

_group

[3]

for

sulphonium

salts but

they

_are not

stoichiometry dependent

for _n

~ 0.5. It tums out that _{we are}

in the presence of _a fixed

type

^of mixed-valence cluster. So far we have _not

enough

experimental

evidence to conclude

definitively

what kind of molecular

organization

is

realized,

therefore _we shall

apply

the formulae valid for the

single charge

^{transfer excitation} band both in dimers and timers with its

charge

transfer excitation energy w~~r considered _{as an}

ajustable parameter.

Figure

2 shows the infrared

absorption spectra

^for the

stoichiometry (1

:2

)

with _a different number of

deposited layers

_on

CaF~

_or Znse substrates and

precoated

with 5

layers

of behenic acid. The fundamental observation is that _we do _{not see} any

large spectral changes

with either the film thickness _or the

type

^{of substrate. The observed} differences for N

= 5

layers

_are due

mainly

to the presence ^of

fatty acid,

in

particular

the band located at 700 cm~ associated

with -COOH groups.

Nevertheless,

_we have

integrated

the

absorption

of

symmetrical

and

antisymmetrical stretching

vibrations of

alkyl

chains between 2 800 and 3 000 cm~ and _we have found intensities values which _are almost

proportional

to

N,

the number of

Z-type deposited monolayers.

From these data it appears therefore that the

theory, already developed

for

crystalline

materials

[10]

_can be

applied

for

interpreting

LB films results _as well. We have selected _one

typical experiment (N

₌

50, stoichiometry 1:2)

for

applying

the _new

fitting procedure presented

in the _next section.

3. Electron-molecular vibration

coupling

model.

Vibrational bands

resulting

from the

coupling

of electronic

charge

transfer excitation to

totally symmetric (a~)

intramolecular vibrations _are characteristic features of low-dimensional

molecular

crystals [10, iii-

It has been

argued

that this

coupling

could be

responsible

for localized behaviour of

charge

carriers due to destruction of their coherent motion via the interaction with intermolecular vibrations

[10]

_; also it has been stressed that

strong

^electronic correlations

play

a crucial role for this localization process.

Two trends _can be noticed in the theoretical

interpretations

of the

optical properties

^of

quasi-

one-dimensional molecular

crystals.

On the _one hand the models become _more and _more

complex

in order to be

adequate

to these systems

(see,

_e-g-

[10, iii),

_on the other hand _a number of

simplified expressions

_are offered

[12-15]

for the determination of electron-

molecular vibration

(EMV) coupling

constants

directly

from the

positions

of vibronic bands in the infrared

spectra.

Careful examination shows that all

approaches

lead to more or less the

same

result,

but the

validity

of _numerous

approximations

has not been discussed. In this

part

^of

our paper we

analyze

the

assumptions

used in

[12-15]

and show the

possibility

and

necessity

of

quantitative

estimates to make the obtained

parameter

values reliable.

We start with the

already

known

expression

for the

complex conductivity

of

quasi-one-

dimensional

crystals [10, 16]

in the _case of _a molecular

organization

in clusters made up of dimers _or trimers _:

«(w)=-iwN~fi

i

,

(1)

4

j-D(w)

where _X

(w )

is the electronic

susceptibility

_term, D

(w ),

the propagator ^{function which}

couples

the electrons and the intramolecular

vibrations, N~

is the number of clusters per unit

volume,

a denotes the distance of

charge

transfer.

Equation (I)

is valid in the

frequency

_range from 100 to ca. 13 000 cm~

',

because below 100 cm~ ^'

coupling

to acoustic

phonons

must be included and above 13 000 cm~ ^' intramolecu- lar excitations _{are to} be considered

explicitly [10]. Throughout

this _{paper we} shall

widely

_use

the fact that the

general expression (I)

_can be

greatly simplified

^{in certain}

frequency

intervals.

For

example,

^if we are interested in the determination of EMV

coupling

constants from the

positions

of vibronic

bands, only

_one, the _so called dominant transition

[14],

_may be taken into account and the electronic

susceptibility

can be written in _a

simplified

form

(in

the

frequency

range w

w~).

2M~w~~

~~~~ wj~r-w~-iwr

^~~~

where w~~ denotes the

frequency

of the dominant electronic excitation with _a linewidth r and M is the associated transition moment

[7].

The

experimental absorption

coefficient is

equal

to

(with

_w

expressed

ⁱⁿ cm~

')

4 w Re

la (w ))

"

~*'~ (3)

cn(w

It should be noted that calculated spectra ^{of Re} «

(w )

and

a

(w )

differ

quantitatively

due to

frequency-dependent

refraction coefficient

n(w )

in

equation (3).

As it follows from

figure

3 where these spectra are

presented

for the _same set of parameter values, the

position

of the

electronic excitation band maximum in _a

(w )

^is shifted to

higher frequency

_w~~ =1.2

iJ~~

(see

also

[12]).

Here

am

denotes the

frequency

of the

absorption peak

in the absence of the EMV

coupling

shown

by

dashed line in

figure

3a and w~~ is the

frequency

of electronic

transition in

equation (2),

it

gives

the

position

of the

charge

transfer band in Rem

(w)

spectrum, ^{also in the absence of the EMV}

coupling.

If the EMV

coupling

is introduced the

charge

transfer bands in

a(w)

and Rem

(w)

_{spectra are} shifted _to

higher frequencies, i~c~

and

£l~~r, respectively.

The classical Drude-Lorentz oscillator model is used sometimes

[12]

to describe the

complex

dielectric function

(5)

in the absence of EMV

coupling

where the

plasma frequency w~

^{is related to the matrix element M introduced in} our

microscopic

model

(Eq. (2))

_:

w(

=

2

wN~

e~

a~ M~

wc~

(4)

The parameter

M~

is related to electronic excitation and _can be either calculated

theoretically

[10]

_or estimated

directly (see Eq. (5))

from

experimental data,

because its value determines

s

ticT j _jficT

-~ ,

~ ,

fl

_,

~j 1'

- /

~~' /

~

,'

' /

ucij joci

-~ '

/ '

' _'

~ ' '

u I '

f

tll ' '

~-' ' ^'

, ^'

~ '

.W _J

.3 '

~#u

'

CJ

imv axn <mv wx

w»mmb«

(cm-')

Fig.

3.

Absorption

(a) and

conductivity

(b) calculated from

equations

(1)-(6) for the

following

set of parameters _{w~ =} ^{3 000} cm~ _~, r

= 2 000 cm~

',

M

= I,

N~

₌ 1.12 _x 10~~' _{cm~ ~, a}

= 3.2

A,

w~ =

2 220 cm~

',

_w

~ ⁼ l 608 _cm

',

_w~

=

425 _cm

~',

w~ = 200 _cm

',

_g2

= 400 _cm

',

g~ 200 cm~

',

_g~

= 300 cm~

',

_g~ 300 cm~ ~, all y~ =

lo _cm~ ^' Dashed lines correspond to the absence of EMV

coupling

(all _g~

=

0).

the oscillator

strength

of the

charge

transfer

absorption

band. In the latter _{case we} note that the

peak

value _a

(Dc~)

^of the

charge

transfer band in the absorbance spectrum ^does not

depend appreciably

on the EMV

coupling,

_{so we} can

neglect

D

(w

in

equation (I)

for the

frequencies

w = wcT and find that

"

(~CT)

"

2

" "~CT ~

~

~

'~~CT j~~~ _~~~

^~

^(~)

T~

^{~ ~ ~}

Equation (5)

is valid for the _case of oriented clusters and extemal electric field

polarized along

the

charge

transfer

dipole

moment. In the _case of chaotic orientation of clusters and/or

unpolarized

extemal field one should introduce

corresponding

numerical factors in this

equation.

As _a result of EMV

coupling

the

position

of _a vibronic band

(for

a

given _a~

mode

a)

in Re _«

(w ) spectrum

^is shifted to a lower

frequency D~ compared

to the bare

frequency

w

~

recorded

by

Raman

scattering.

The

magnitude

of the shift

(w

~

£l

~

is related _to the value of EMV

coupling

constant g~ via

expressions

obtained in references

[12-15]

with the

following

approximations

:

(I)

the vibronic

frequency

_w

~

must be much smaller than the

frequency

of electronic

charge

transfer _mm

(it)

_« isolated vibronic band _»

assumption

is

made,

I-e- the influence of other vibronic modes _on the shift value is

disregarded

(iii)

^absorbance spectra are

used,

althouth the relations have been obtained for the real part of the

conductivity.

We shall discuss all these

assumptions

_one

by

_one.

3, I ESTIMATE _{OF THE} EMU _{COUPLING CONSTANT VIA THE POSITION OF THE} CORRESPONDING

VIBRONIC BAND IN THE coNDucTIviTY SPECTRUM. In this subsection the

approximation (it)

is used and

equation (I)

_may be

represented

in the form

~ ~~

~~

" ~~° " ~~° ^~

4 A

(w

iB

(w )

^~~~

where

~

tub w~ g$ w~(w( w~)

~~

2

M~

_wc~

(ml

w ~)~ +

(w

_{y~ )~} ^~~~

and

~ wr

g[

W« W y«

(°~

_"

~

~2

~

+

~~2

~ 2~2 ~

~~

_{~ ~2}

(8)

From

equation (6)

we see that IR bands in Re _«

(w )

_spectrum will be

given by

the _roots of

equation (11)

_as

already proposed [13].

^We obtain the value of EMV

coupling

constant

~ w~~ w~

~ n] ~ n]

~~~ ~

~fif2

^{~~ ~~}

^~~

°~« °~CT

The

physically meaningful

solutions of

equation (7)

for

A(w )

₌ 0 _{are :}

~~ 'CT~I

+

~

~~ _~~

W~ ^l12

~~~~ £~~)

" ~'CT

(lo)

and

12 ^{M~ g$}

w

~ wc~ ^{2 1/2}

fl~=W~

I-

(11)

(*'/T~ *'~)*'CT

_°'«

So,

the

frequency £lc~

of the maximum of the

charge

transfer band is shifted _to the

higher frequency

from _wc~

value,

and IR vibronic band goes down and the latter shift is

larger by

_a

factor of

(wc~/w~)~

_: this basic result is in

agreement

with

previous

authors

[14, 15].

3.2 INTERACTION _{BETWEEN VIBRONIC MODES VIA THE ELECTRONIC CHARGE TRANSFER.}

Now _{we tum} to the

investigation

of the role of interaction between two

a~

^modes

wj and w~ due _to their indirect

coupling

via electronic

charge

transfer. For the sake of

clarity

we assume that the relations

(I)

and the reasonable

assumption

^~'" ml

(for

2(N~ £l~)

w~ ^«

mm)

_are fulfilled for both modes and in this _case

equation (11)

takes the form

Wc~

g(

_W

j

g(

_W2

2M~ ml- w~~ ml- w~

^~~~~

The _roots of

equation (12)

_can be

easily

^found

12 ^{M~ g( (2 M~}

^)~

^{g( g(}

^{w~ ~~~}

jlj=Wj

1-~+

WjWCT

~ ~ ~

~M2

~ ~

*'CT*'l

*'l~*'2~fi(~l*'l~~2*'2)

CT

jl~

_{= W~}

~

~'~ _~~

_~~

~'~~~

_~~

~(

_W1

~~

°'2 *'CT

2 ~ ~ ~

Jf2 ⁽¹³⁾

°'CT *'2 °'1 W2

(g~

_Wi

g( _W~)

Wc~

We _see that the rust two terms inside the

curly

brackets

give equation (9) (in

the limit

£l~/wm ml) describing

the _case of isolated bands and the interaction between two

a~

^{modes is}

expressed by

the last term. In

figure

4 _we show

by

the dashed lines the _case of

« isolated vibronic bands

» calculated from

equation (6).

As _a result of interaction between _two

Qz

_uz

Qiui

Az

'~~

' ,

© # ,

# # _,

' # ,

~

i '

'~ / ' /

.>

' I

lj ' I

~ ' #

'O # #

< # #

O _#

CJ _{# '}#

# '

' '

'

"

,"'

_"' _-""' _--"""

-==zzCZ$--~"~'

__--

iav i~n i<m iso~

Yhvmum~r

(cm-')

Fig.

4.

Changes

in the real part ^of the

conductivity

due to the interaction of two vibrational modes via electronic

charge

transfer. Parameters for the electronic

subsystem

_are the _{same as} in

figure

3,

wj = 1454 _{cm~ ~, w~}

= 1390

cm~', _fly

= 1435

cm~', _fl~

=

344 cm~

'.

The _case of

non-interacting

modes is shown by dashed line :

fly

₌ 1421cm~ and

fl~

=

358 cm~'

a~

^{modes via electronic}

charge transfer,

the IR vibronic bands _are shifted apart

(shown by

solid line in

Fig. 4)

so that the band with lower

frequency

moves further down

by

_A~ and the

displacement

of the other _one is

partly compensated by Aj (see Fig. 4).

The values of these

displacements

_are related _:

Ai

=

A~(w~/wj ).

3.3 ESTIMATION OF EMU _{COUPLING CONSTANTS FROM ABSORPTION} SPECTRA. Now _we

consider the modifications which have to be introduced if the absorbance is

analyzed

instead of the

conductivity spectrum. Unfortunately,

no efficient transform from

absorption

data to

conductivity

has been found _so

far,

but _{one can}

manipulate

with

absorption

in order to

shnplify

the calculations.

Using equations (3)

^and

(6),

_we obtain

Nj~e~

a~

~~$~~~

~

°°~

F~

~

~2 F~~ _E~ ^~~~~'

^~~~~

where

F(w)=@, E(w)=@

(15)

The function

G(w )

has _no

specific physical meaning,

but it is

mathematically

very attractive

because _{we note} that in the

region

around the maxima of the function

G(w)

at

w =

ji~

one can

neglect

the second term in

equation (8)

and

E(w )

becomes constant. From the condition @G/@w

=

0,

_we obtain

~ w~~rw~

£l$ I£l$ ^rD~

~"

2

M~ ml w(~

^~

/ _wj~r

^~~~~

In

principle,

_one should

replot

the

experimental absorption

curve in

the

form of G

(w )

function and find D

~

as the

positions

of vibronic bands in G

(w )

_spectrum.

However,

for

typical _parameter

values the

position

of vibronic bands in

a(w )

and

G(w ) (see Fig. 5)

is

practically

the same, so we can

identify

the band

position

in

absorption spectrum i~~

with

£$~

ⁱⁿ

equation (16).

In

figure

5 _we compare the behaviour of

a

(w ), «(w )

and

G(w)

in the _case of

single

vibronic mode with the

frequency

_w~

^=1455cm~'

and g~ = 560 cm~ ^' The maximum of

conductivity

occurs at 241 cm~ ^' which differs consider-

ably

from the

corresponding

values of 1296 and 1298

cm~'

for

G(w)

and _a

(w), respectively.

With

D~ ^=1241cm~'

_we obtain from

equation (9)

_an estimated value g~ = 521 cm~ ^' Even _a better

estimate,

than that of

equation (9),

is

given by equation (16)

:

we obtain the values 550 and 546

cm~~

_for

g~ calculated from

equation (16)

with

ji~

=

296 _cm

(from

G

(w ) spectrum)

and

i~

~ ⁼

l 298 _cm ^'

(from

_a

(w ) spectrum), respectively.

4.

Absorption

spectrum ^of

[(C,~S)~dithiolium-TCNQ]1. [CI~TCNQI2

LB film.

In

figure

6a _we

present

^{the measured absorbance of} 50

layers

of

[(Cj~S)~dithiolium- TCNQl; lC18TCNQ12 deposited

_on a

CaF~

substrate. A broad band with _a maximum _ca.

2 700 cm~ ^' is attributed to the electronic

charge

transfer excitation described

by equation (2).

The bands w~ w~ are

assigned

to the excitation of intramolecular vibrations of

Cj~TCNQ molecule,

_even if _{we cannot} exclude the

participation

of the dithiolium

cation,

because their

frequencies

_are close to those of

totally symmetric _a~

vibrational modes of

TCNQ

alone and because of the

asymmetric shape

of these

bands, typical

for Fano _« anti-resonances _». We

m _-

Q« Q« Q«

j

,, _-_ ,

-

,~

, ,

u_ , ,

,~ _: , =.

, ; , :

,~ :. ',

,~ :.

, :.

, ,

, , :

, , :

,~ , :

, , ;

,, _{, :}

, , :

,~ :. _{, :}

,,olu)

_:~ ',

l

, ;. ,_,

:. ,_, i

:. ,

, :

:. _, :

,: , :

:. _, :

, :

, :

,:

,;

,;

Ian iitu i<w i%n

Havenumber

(cm~')

Fig.

5. Normalized spectra ^{of different functions F} (w )_: the

absorption

coefficient _a (w ) (solid line), the

conductivity

«(w ) (dashed line), and G(w ) function (dots) calculated from

equations

(Ii, (3) and (26) for w~ =

2 loo _cm r

= 2 000 cm~

',

_M

= I, _w~

= 455 cm~ ^' and g~ =

560 cm~

~

~

&~~ 4 ~~

Us 6~7

6~2

q g

~

~

WwenunAer cm-I

Fig.6.-Experimental

(a) and calculated 16, c)

absorption

spectra ^of

s01ayers

of

[(Cj~S)~dithiolium-TCNQ Ii1 ICj~TCNQI2

(see text). The _case of the absence of EMV

coupling

(all g~ = 0) is shown by ^{dashed line in} (b).

assume that the

replacement

of _one of the protons ⁱⁿ

TCNQ by

a

Cj~H~~

^{chain does} not affect

significantly

the mechanism of linear EMV

coupling

with the

TCNQ

modes which do not involve a motion of

hydrogen

atoms.

Indeed, experimental

and theoretical studies

[17]

of intramolecular vibrations in

Cj~TCNQ _support

this

assumption.

In table I we present some

intramolecular mode

frequencies

for

Cj~TCNQ

observed

experimentally

ⁱⁿ the Raman

spectrum ^{of neutral}

Cj~TCNQ

and calculated

theoretically.

The

frequencies

of the

analogous

modes for

TCNQ°

_are

_given

in the first column for

comparison.

Table I. Observed and calculated

frequencies of

_some

Cj~TCNQ

vibrations and EMV

coupling

constants in

[(Cigs)~dithiolium-TCNQ Ii ICj~TCNQI2

LB

film

and

MEM-TCNQ2 crystalline

salt.

TCNQ° Cj~TCNQ°

LB film

MEM-TCNQ~

w

~/cm~

^' _w

~/cm~

^' _w

~/cm~

^'

g~/cm~ g~/cm~

^'

g~/cm~

^'

Raman

[18]

Calc.

[17]

Raman

[17] Eq. (16)

fit in

Fig.

6

[7]

2 229 2 231 2 226 253 350 350

602 590 607 256 400 540

454 461 455 497 560 500

207 208 1214 414 280 300

948 950 961 80 85 85

711 679 106 100 190

Using equation (16)

_{we can} estimate the values of EMV

coupling

constants g~

given

in the

fourth column of table I.

Unfortunately,

these values _cannot be considered _{as a}

good

approximation

because of the effect of the interaction between vibronic modes via the electronic

charge

transfer discussed in section 3.3. So, _some

fitting procedure

is necessary

which results in the

absorption

_spectrum shown in

figure

6b. The

corresponding

EMV

coupling

constants are

given

in the fifth column of table I. The

frequencies

_are taken _as observed

experimentally

in the Raman spectrum ^of

Cj~TCNQ (column

3 of Tab.

I)

and the

damping

factor y~ is chosen for all modes

equal

lo

cm~'

_{The obtained EMV}

coupling

constants are close to those found in

TCNQ crystalline salts,

_e.g.

(methylethylmorpholinium)- TCNQ2

values

[7] reported

in the last column of table I.

To illustrate the role of interaction between several vibronic modes _we show in

figure

⁷

absorption

spectra ^{calculated in the} cases when the electronic

charge

transfer excites

only

_one

vibronic

mode,

shown

by

dashed _or dotted lines and the spectrum ^{when all these modes}

interact

indirectly

between themselves. With the values of parameters

typical

for

a~-modes

of

TCNQ

chosen

[7],

the tendencies

explored

in section 3.3 for _two vibronic modes is shown up in

figure

7. The role of interaction is _more

pronounced

when vibronic

frequencies

_are close _to each other

(a~-modes

w~, w~ and

w~)

^the

high-frequency

mode

(w~)

is

displaced

to

higher frequency

and its

intensity

is

lowered,

while the

low-frequency

mode

(w~)

is

amplified

and

shifted downwards.

The band at 255 cm~ ^' and fine structure of _w~ vibrational mode of anti-resonance type have been attributed

[19]

_to the excitation of vibrations inside the

alkyl

chain which _are

coupled

to w~ mode. If this

coupling

is taken into account, the

expression (I)

for

complex conductivity

is modified.

Namely,

the usual propagator ^function can be

changed

_to the

jouR~AL DE pHysiouE ii T 3. N. 5 MAY 199~ ?6

u2

Gl3 _:."

Gl4 .."

U5

,, _.."'

~-~ , , .. :

%

, ' ;.' I

H :' ,'

_.." I

~-' : _, :.

, ,

: , _.. _{_:} , ,' :

~ ^{, '} , :

< U6 :.' ;." " '

o ,' _;

-o :. , ;

#

,, , ,

~ , ,

_~

f ,_..

~Z _{_."}

, <f"

:, , ,

;, ^'

, '

, ,

, '

, #

, #

all tltn 22X 24tV

Havenumber

(cm-')

Fig.

7.

Absorption,

calculated in the _case of five vibronic modes (labelled _w~,

,

w~)

interacting

with electronic

charge

transfer (solid line) and the spectra

corresponding

to the _cases of

single

vibronic bands (dashes and dots) for the parameter ^values : w~ =

2 100 cm~

',

_r

=

2 000 cm~

',

_M

= I,

w~ =

2 226 cm~

',

_w~

= 607 _cm

',

w~ = 1455 cm~

',

w~ =

214 cm~ _~, w~ = 961 cm~ ^' g~ = 350 _{cm ~,} g~ =

400 _{cm ~, g4}

" 560 _{cm ~,} g~ = 280 _cm g~ = 85 _{cm ~,} all y~ = 10 _cm

modified form _:

D(W )

=

Z ^~~

^~°~

~

~ ~

(i?)

~

W~-W~-iwy~-£

~

~~

~~

^~

p VP -~°

-I~°Yfl

where _v

p denotes the intrachain vibrational

frequency coupled

to a-th a~ ^{mode of}

TCNQ

with

coupling

constant

k~p.

^{It follows from the} theoretical

description [19]

of intramolecular vibrations of the

TCNQ

molecule that the C-H

bending

motion

gives

the

major

contribution to w~

mode,

while for the other modes the influence of

hydrogen displacements

is

negligible.

Consequently,

we propose that

k~p

# 0

only

for _{a =} 5. The

spectrum

shown in

figure

6c is

calculated with this

«electron-TCNQ-Cj~

chain»

coupling

taken into account with the

parameter

values

given

in tableII. The parameters ^{of electron}

subsystem

_are:

mm = 2 loo cm~

',

r

=

2 000 _cm~

',

M

=

I. The

quality

of this fit

supports

_our

assumption

that mixed-valence clusters _are

organized

in

[(Cj~S)~dithiolium-TCNQ Ii ICj~TCNQI2

LB

film.

Comparing

the obtained values of EMV

coupling

constants g~ and matrix element

M _we find that these clusters _are dimers with _one radical electron. In order to discriminate

between dimers

(CI~TCNQ)~

and

(Cj~TCNQ, TCNQ)~

_we have

recently

carried out _a

generalized approach

of the

homodoping technique [20].

It has been shown that _a chemical

reaction, taking place only

at the air-water

interface, gives (Cj~TCNQb

mixed valence

dimers: this result confirms the

validity

of the above

assumptions

for

introducing

equation [17].

Table II. Parameters

for

the

TCNQ _a~

mode

C18H37

chain

coupling.

~

~P~Cm~

~5_P

YP~Cm~

l 1261 loo lo

2 190 15 lo

3 163 15 lo

4 1126 lo lo

5 106 15 lo

6 1028 30 lo

As evident from table

I,

the values of EMV

coupling

constants are close to those found in

TCNQ crystalline

salts. This

supports

our idea that vibronic modes which do _not involve

changes

of C-H bonds _{are not} modified

significantly

when H _atom is

replaced by Cj~H~~

chain.

Consequently,

the mechanism of EMV

coupling

is

preserved

in

spite

of the formal

lowering

of molecular

symmetry

when the

alkyl

chain is

grafted

on the

TCNQ cycle.

5. Conclusions.

We have shown that infrared spectroscopy can be used _{as a} tool to

investigate

the local molecular

organization through

the

analysis

of the electron-molecular vibration

coupling

in

conducting Langmuir-Blodgett

films. An extention of Rice _et al.

[7] theory

for the _case of LB films shows that mixed-valence systems are created for _a

variety

of

compositions

in the

layers

obtained via the

homodoping

_process. Because

polarized

reflectance measurements _are

impossible

for random

by

oriented LB

films,

_we focus our attention _on absorbance spectra.

Therefore,

_we have taken into account the difference of band

positions

in the

conductivity

and absorbance spectra.

Equation (16)

of this paper

gives

an estimate of the EMV

coupling

constant

directly

from

position

of the

corresponding a~-vibronic

band in absorbance. The role of the interaction between vibronic modes via _an electronic

charge

transfer is

investigated.

A

new

phenomenon

^of two-step

coupling

^of electrons with vibrations of

alkyl

chains indicates that _an energy transfer is realized between electroactive

part

^and the

alkyl

chain inside a

given monolayer.

An

interesting

observation is that the _same kind of mixed-valence dimers _are

organized

^for a

large

_range of chemical

composition (n

_~ 0.5

)

and almost

independently

of the number of

deposited layers.

Such dimers constitute the molecular blocks

prerequisite

for

obtaining

_a

conducting

LB film.

However,

_{one apparent}

descrepancy

remains to be clarified _:

although

the

spectroscopic

data in

TCNQ-based

LB films _are close to those of

TCNQ

ion-radical

crystalline salts,

the d.c. electrical

conductivity

in LB films is _a few orders of

magnitude

lower than in

conducting crystals [8].

In order to

explain

it, two factors have to be

envisaged

_:

(I)

the disorder effects due to limited size of

stacking

and/or defects between them inside _a

layer

with the presence of activation energy barriers between domains

(it)

the presence of

strong

^electronic correlations associated with _{a more or} less dimerized stack _as in _some

TCNQ

salts is relevant from _an Hubbard model

[21].

Indeed,

the steric hindrance of

alkyl

chains

plays

_a role which has _to be

quantified. Starting

from this molecular

organization

it will be necessary now to control the

in-plane long

_range order and associated

organization

to

really

obtain a metallic behaviour.

References

[Ii DELHALS P., YARTSEV V. M., Advances in

Spectroscopy,

vol. 23,

Chap.

5 R. J. H. Clark, R. E.

Hester Eds. (John

Wiley

& Sons, 1993) _pp. 199-289.

[2] VANDEVYVER M., Lower-dimensional

Systems

and Molecular Electronics, R. M. Metzger, ^P. Day, G. C.

Papavassiliou

Eds. (Plenum Press, 1990) _p. 503.

[3] RUAUDEL-TEIXIER A., VANDEVYVER M., ROULLIAY M., BouRGoiN J.-P., BARRAUD A., LEQUAN M., LEQUAN R. M., J.

Phys.

D _: Appl. Phys. 23 (1990) 987.

[4] BOURGOIN J.-P., Thesis,

University

of Paris XI (1991).

[5] GioNis V., FICHET O., Izumi M., AMIELL J., GARRioou-LAGRANGE C., PAPAvAssiLiou G. C., DELHALS P., Chem. Lett. (1991) 871.

[6] FICHET O., DUCHARME D., GioNis V., DELHAfIS P., LEBLANC R. M., Langmuir 9 (1993) 491.

[7] RicE M. J., YARTSEV V. M., JACOBSEN C. S., Phys. Rev. B 21 (1980) 3437.

[8] FICHET O., BouRGoiN J.-P., DELHALS P., DESBAT P., GioNis V., YARTSEV V., Mol.

Cryst.

Liq.

Cryst.

(to be

published

ⁱⁿ 1993).

[9] TORRANCE J. B., Acc. Chem. Res. 12 (1979) 79.

[10] YARTSEV V. M., SWIETLIK R., Rev. Solid State Sci. 4 (1990) 69.

I Ii Bono R., FEis A., PEDRON D., ZANON I., PECILE C., Lower-dimensional

Systems

and Molecular Electronics, R. M. Metzger, P. Day, G. C.

Papavassiliou

Eds. (Plenum Press, 1990) _p. 23.

[12] RICHARD J., DELHAfIS P., VANDEVYVER M., New J. Chem. 15 (1991) 137.

[13] PAINELLI A., GIRLANDO A., PECILE C., Solid State Commun. 52 (1984) 801.

[14] YARTSEV v. M., GRAJA A., J. Phys. _: Condens. Matter 2 (1990) 9631.

[15] WONG K. Y., Inorg. Chem. 23 (1984) 1285.

[16] RICE M. J., LIPARI N. O., STRASSLER S.,

Phys.

Rev. Lett. 39

(1977)

1359.

[17] FELTRE L., Thesis, Padova

university,

(1989).

[18] MENEGHETTI M., GIRLANDO A., PECILE C., J. Chem. Phys. 83 (1985) 3134.

[19] YARTSEV V. M., BouRGoiN J.-P., DELHALS P., VANDEVYVER M., BARRAUD A., Synth. Metals (1993) _to be

published.

[20] FICHET O., AGRICOLE B., AMIELL J., GioNis V., DELHALS P., C-R- Acad. Sci. Paris, 316 s6rie II (1993) 335.

[21] VAN SMAALEN S., KOMMANDEUR J.,

Phys.

Rev. B 31(1985) 8056.