The observation of electron motion along individual polydiacetylene chains in solution: The molecular wire

Synthetic Metals, 28 (1989) D569 D574

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THE OBSERVATION OF ELECTRON MOTION ALONG.!NDIVIDUAL POLYDIACETYLENE CHAINS IN SOLUTION: THE MOLECULAR WIRE.

K.d. DONOVAN and E.G.WILSON Department of Physics~ Queen Mary College, Mile End Rd. London El 4MS, England.

ABSTRACT A unique experiment using pulsed photoconductivity techniques demonstrates that photocarriers may be created, move along a polymer chain in solution~ and be detected as a current in an external c i r c u i t , provided that the polymer is in a chain extended conformation. For a random coil no signal is detected. Using the 3 and 4BCMU polydiacetylene urethane d e r i v a t i v e s i t

is possible~ i t has been argued,

to go from a random coil to r i g i d rod conformation~ this phase change being signalled by a colour change. Because an external c i r c u i t detecting charge motion is only sensitive to the real space distance t r a v e l l e d by the photocharge in the f i e l d direction~ i t

is to be expected that currents w i l l only be detected in the

chain extended conformation, as observed. Further~ i t

is seen that modifying the

theory of one dimensional Onsager geminate recombination to account for the case of f i n i t e chain lengths causes the p r o b a b i l i t y of avoiding

geminate recombination

to be d r a s t i c a l l y reduced.

INTRODUCTION In the case of single crystal polydiacetylenes i t has been shown that excess c a r r i e r s in the conduction band travel in a unique way. That i s , the c a r r i e r deforms the chain i t

is on and carries this deformation with i t as a S o l i t a r y Wave

Acoustic Polaron~ SWAP, (I~2). The d e t a i l s of SWAP motion are that i t moves at the v e l o c i t y of sound even at very low applied f i e l d s , as usual scattering mechanisms are very weak. Charge is thus transported r e a d i l y along these p a r t i c u l a r polymer chains. The opportunity of studying these same polymers in a solution environment offers a p o s s i b i l i t y of observing electron motion when the d i s t o r t i o n of the polaron is d e f i n i t e l y confined to one chain. Furthermore,

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the absence of long

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range order in a t r u l y one dimensional system may have implications for the motion of the SWAP. A further i n t e r e s t i n g difference between the crystal and solution s i t u a t i o n is that in the crystal i t travel away to i n f i n i t y

is possible for the electron to escape i t s twin hole and (discharging at the electrodes). For chains of f i n i t e

length in solution however the photocreated pair w i l l

always eventually recombine

and this effect can d r a s t i c a l l y l i m i t the observable photocurrent, I t is an established fact that the polydiacetylene 3 and 4BCNU d e r i v a t i v e s in solution undergo a phase change accompanied by a colour change; this phase change has been assigned to a change in conformation solvents, i t

from coil to rod, (3,4>.

In poor

is argued, the sidegroups of adjacent monomer units may hydrogen bond

and cause the chain to become ordered and chain extended. This effect is capitalised on to investigate the propagation of charge along individual polymer chains in the following experiments.

EXPERIMENT The solutions are contained in 2mm path length spectrometer c e l l s with spectrosil windows. In each cell there is a pair of c y l i n d r i c a l electrodes of Imm diameter spaced a distance 2mm apart. OV l i g h t at 337nm from a IOns pulsed nitrogen laser is focused onto the solution between the two electrodes. One electrode is connected to a fast high voltage step generator and a voltage switched on for 300~s. This is in order to avoid problems with the e l e c t r o p l a t i n g of polymer incurred with s DC f i e l d . The photocurrent induced by the laser, which f i r e s in the middle of the voltage pulse, is detected as a voltage drop across a r e s i s t o r , which is then amplified by a d i f f e r e n t i a l video amplifier. The second input to the amplifier is fed from a dummy cell alongside the test

cell.

In this

way radio frequency interference from the laser is eliminated. F i n a l l y the concentration of the polymer solution is chosen so that the l i g h t is absorbed within the c e l l , i . e . i t has an optical skin depth of 2mm. This e n t a i l s using -3 solutions of 10 m o l a r concentration. The 3 and 4BCMUs were dissolved in either chloroform or hot toluene (SO°C). The solutions were then yellow in colour, (random c o i l ) .

By adding ammounts of

hexane to the chloroform solutions or cooling the toluene solutions the phase change to extended chain was achieved, accompanied by a change in colour to blue, (3BCMU) or red,(4SCMU).

RESULTS The yellow solutions of 3 and 4BCMU did not give detectable photocurrents with an upper l i m i t of

0.33 ~A at a f i e l d , E, of 1.25 x106 V/m, a photon flux

x1013 photons/mm~

and a sample area of 20mm ?'. At the same f i e l d and l i g h t

of 1.7

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i n t e n s i t y the 3BCMU in the blue phase gave a peak photocurrent of 4.8 ~A and the 4BCMU in the red phase a photocurrent of 90 ~A ( F i g . l ) .

t-->

t'--~

I0 ns

20 ns

Fig.1. The photocurrents described in the t e x t for, 3BCMU and 4BCMU in the chain extended conformation. Also shown is the signal (horizontal line) for 3BCMU in the random c o i l .

The shape of the transient is instrumental, being set by an RC time constant; the important quantity is the charge under the curve which is the induced charge, Q . This is related to the photocreated charge,q, by equation ( I ) . Q = IpT~ /0.7

= q s/d;

(I)

where s is the real space distance t r a v e l l e d in the f i e l d d i r e c t i o n , peak current, ~ t h e

time for the current to f a l l

Ip is the

to half height = 20ns and d =

2mm, the electrode spacing. The photocharge q is given by equation (2). q

= e ~(E,s>N !

(2)

where N is the t o t a l number of photons absorbed, ~ the quantum e f f i c i e n c y for electron-hole pair generation and ~(E,s) the p r o b a b i l i t y of avoiding geminate recombination. The f i e l d dependence of Ip is shown in Fig.2 for 3 and 4BCMU in t h e i r chain extended conformation. If

it

is assumed that the c a r r i e r s travel the entire length L of the chain and

one equates s with L,(O.5~m), = 2.0 xlO4-for-- 4BCMU and ~

in equation 2 then at E= 1.5x 106 V/m

values of ~

= 9.3 x 1 ~ f o r L 3BCMU are deduced. At the same f i e l d

and wavelength in the single crystal polydiacetylenes ~

= 3.0 x10-3

; clearly

there is a considerable difference in these three values. A further assumption could be made that ~ the primary quantum e f f i c i e n c y is similar in the crystals and the solutions. Then the drastic difference in the three values is due to a difference in the Onsager p r o b a b i l i t y of avoiding geminate recombination, the p r o b a b i l i t y being vastly reduced in the case of f i n i t e polymer chains in solution. Limited data on 3BCMU with varying chain lengths reveal an increase in ~ factor of 30 with a mere factor 4 increase in average length.

by a

This lends support

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to the conclusion that the f i n i t e chain length holds the explanation to the radical d i f f e r e n c e in the

~

in s o l u t i o n as opposed to the value well established f o r

c r y s t a l s . Another i n d i c a t o r is the large d i f f e r e n c e in ~ b e t w e e n 3 and 4BCMU

at high f i e l d s ,

( F i g . 2 ) . This may be due to the f a c t that 4BCNU has a higher

proportion of longer chains. The consequences of t h i s are discussed below. Currently work is being c a r r i e d out in order to e s t a b l i s h the length dependence on a firmer basis.

/i~

I

j r,,,ii i

I I Ililll I

I

IIIII

4BCMU

3 < z]i. ii.iii

@

i

mm 3BCMU 1

........

2

I

, ~ll,,,,I

I

I

3 4 Log V (Volts)

IIII

5

Fig.2. Shown on a l o g / l o g p l o t are the f i e l d dependences of I p f o r 3BCMU and 4BCMU, in the chain extended conformation. The r e s u l t s are normalised to a photon f l u x of 3.25 x10;3/mm z .

ONSAGER THEORY ON FINITE CHAINS Clearly an electron hole p a i r at opposite ends of a f i n i t e eventually recombine.

chain w i l l

To describe t h i s s i t u a t i o n we extend a d e s c r i p t i o n of

Onsager geminate escape f i r s t

given by Habercorn and Michelle Beyerle (5).

Consider a current i o of electrons to thermalise a distance b from t h e i r twin holes. Then in an i n f i n i t e one dimensional chain there w i l l

be an outward current

i only provided a f i e l d E is present. The r a t i o i / i o is.eq~x:)), the Onsager geminate escape p r o b a b i l i t y . Let us now consider, as in f i g u r e 3, the t r a n s i e n t current i ( t )

on a f i n i t e

chain in response to a step of thermalising carriers. The

f a r end of the chain is assumed to have no bound state. The c h a r a c t e r i s t i c time f o r the current to maximise,~C~, is due to building up a density of carriers on the chain, and is of order the t r a n s i t time in the large coulomb f i e l d over the distance b. The t i m e ' L occurs when the f i n i t e chain, of length L, is s u f f i c i e n t l y

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populated by the outward current that an equal and opposite reverse current occurs. Then

""~=~_.exp(+eEL/kT)

(3)

This r e f l e c t s the f a c t that a c a r r i e r at the f a r end of the chain w i l l Boltzmann p r o b a b i l i t y of acquiring, by thermal

have a

f l u c t u a t i o n an energy eEL

s u f f i c i e n t to cause recombination,

i

.m

o

b

~

x

i

a)

L

t

b)

io]

>

0

0

t

~.

t

~T:L

c)

Fig. 3 a) A chain of l e n g t h L, with a c u r r e n t i of electrons thermalising at b; b) The current i o is turned on at time zero; c) The current at b along the chain.

From these considerations i t recombination, w i l l

f o l l o w s t h a t ~ L ( t ) , the chance of avoiding

depend on time.

Consider now a l i g h t pulse of duration T then the chain is e f f e c t i v e l y i n f i n i t e ,

~L(t)

= ~'(00);

. I f the pulse is s u f f i c i e n t l y short,

and

T ~L~

(4)

For a long l i g h t pulse however, most of the electrons reaching the f a r end w i l l have returned, and w i l l

~(T

) = 0 ;

T

have recombined; thus

~

(51

The main consequence of these considerations is that in a poly disperse c o l l e c t i o n of chains in s o l u t i o n there is an exponential dependance on chain length and e l e c t r i c f i e l d determining which chains c o n t r i b u t e to the photocurrent. In our experiments we envisage that only a few chains are long enough f o r the separated

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carriers not to have recombined in the l i g h t pulse duration of IOns! this is why the experimental values of ~

are less than that determined for single crystal

( i n f i n i t e chain) polydiacetylenes. This is also the reason for the rapid dependence of photocurrent on e l e c t r i c f i e l d . We can estimate ~[~as Ips (ie bA-5nm and a v e l o c i t y of 5 x 103 m/s). Then the condition for a chain to be f u l l y e f f e c t i v e in a IOns pulse is EL'~'-0.23 v o l t s . Thus at 106 V/m only chains with L~O.23~m f u l l y contribute. A f u l l e r discussion of this theory and i t s consequences is in preparation.

DISCUSSION Clearly then c a r r i e r s arm transported along the chains in the blue/red phase indicating that there must be chain extension in this case unlike in the yellow solutions. At the concentrations at which these experiments are carried out the volume/chain is found to be 4.0 x1~J-m3 while rods of length L = 0.5um would need a volume of L3 = 2.2 x1otqmS to be assured of being out of "contact" with neighbouring chains. Clearly then the chains are close to one another. But e l e c t r o n i c a l l y one would expect them to be isolated from neighbouring chains and the photocurrent observed may be considered a single chain phenomenom. The d i r e c t observation of c a r r i e r motion on a single macromolecule is unique and has many implications. One would not expect the single chain to possess a Bloch type conduction band as there is no long range order in one dimension and consequently lack of

t r a n s l a t i o n a l symmetry. This should lead to l o c a l i s a t i o n ;

however the photocurrents demonstrate that the polymer backbone behaves as a molecular wire acting as a conduit for charge c a r r i e r s over macroscopic distances

ACKNOWLEDSEMENT The authors would l i k e to thank Miss F.Moradi-Bidhendi for her help in c o l l e c t i n g the data.

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K.J.Donovan, P.D.Freeman and E.B.Wilson,

Springer Series in Solid State

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E.S.Nilmon, J. Phys. C., 16, (1983), ~739.

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G.N.Patel, R.R.Chancm and J.D.Witt, J. Chem. Phys., 70(9!,

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K.C.Lim, C.R.Fincher, J r . ,

(1979), 4387.

and A.J.Heeger, Phys. Rev. L e t t s . , 50 (24),

(1983),

1934. 5

R.Habercorn and M.E.Michel Beyerle, Chem. Phys. L e t t s . , 23, (1973), 128.