Synthetic Metals, 41-43 (1991) 641-644 641

LIGHT INDUCED ELECTRON SPIN RESONANCE IN POLYANILINE

K. CROMACK and A.J. EPSTEIN

Physics Department, The Ohio State University, Columbus, OH 43210 (U.S.A)

J. MASTERS , Y. SUN, and A.G. MACDIARMID

Department of Chemistry, University of Pennsylvania, Philadelpla, PA 19104 (U.S.A.)

ABSTRACT

Light induced electron spin resonance (LESR) is reported for members of the polyaniline

fam~|y of polymers. The LESR is composed of a single line at g,-~ 2 whose width and intensity

are dependent on the time of exposure and temperature. The LESR line intensity shows a

very long growth and decay time that can be fit to a Kohirausch stretched exponential type

law with coefllcients that follow a Vogal-Fulcher law. There is a direct dependence of the

linewidth of the photo-lnduced spins on the the concentration of induced spins suggesting

a strong interaction between induced defects and leading to the postulation of a phase

segregation of defect.

INTRODUCTION

Polyanilines have continued to at tract great interest in the last few years. The nature of

the chemically doped polymer and the correspondence to the photo-doped polymer are an

active areas of investigation. It is expected that the polyanilines support solitons, polarons,

and blpolarons stabilized by ring angle and/or bond length distortlons 1'2. Optical absorption

spectra and photo-induced absorption spectra3, 4 have shown there to be defects created in

these polymers, but have not been able to conclusively differentiate the type of defects

created.

In this paper, we report a systematic study of the spin associated with the defects photo-

induced in the polyaniline family of polymers. Our results show that polarons are created in

all forms of polyanillne and that these polarons are the defects responsible for the long time

photo-lnduced absorptions that were previously reported. Further, we report observation of

extremely long growth times of these polarons and analyze this growth and decay in terms

of very strongly interacting defects.

Elsevier Sequoia/Printed in The Netherlands

642

EXPERIMENT

The preparat ion method o{ the polyaniline samples is described elsewhere 5. For the

present LESR studies the samples were either mixed with KBr or cast as thin films on

quartz slides. LESR experiments were done on a Bruker ESP 300 system equipped with a

TM102 9.48 GHz cavity with an Oxford 900 variable tempera ture cryostat installed. The

samples were photoexci ted using Ar + laser lines in the 2.41-2.7 eV range. Five ESR spectra

were taken to record the background spin concentration before the sample was exposed to

light. The pump beam was then allowed to i l luminate the sample. The signal at the top of

the derivative peak was monitored for 80 seconds. The total spect rum was then taken at

intervals of 1 minute for up to 6 hours before the pump beam was blocked. After the beam

was blocked the spect rum was again measured at intervals for up to 24 hours. The individual

spectra were then integrated, the background was subtracted, and their full widths at half

maximum and center fields determined. The second integrals were calculated to obtain the

induced spin concentration. This procedure was done as a function of temperature , light

intensity, and uhoton energy.

RESULTS

Figure 1 shows the pernlgranillne base (PNB) dark ESR and the light in-

duced ESR taken at ~ 60 K with a pump photon energy of 2.51 eV and inten-

sity of 100 m W / c m 2. It is seen that the LESR is positive; i.e. the number of

spins is increased under i l lumination and that the LESR is a single line. In con-

trast, s t ructure in the dark ESR signal is a t t r ibuted to nitrogen hyperfine coupling.

IXlO~ . . . . ' 1 ' ' ' ' l . . . . [ . . . . [ . . . .

E = 2 . 5 4 e ¥

I = I ~ r o W / e r a *

5 X I O ~

- 5 × 1 0 ~

_ I X I O ~ . . . . I . . . . I . . . . I . . . . I . . . .

Fig. 1 Dark ESR signal (lower trace) and light induced ESR of PNB at 60K.

643

Figure 2a shows the growth of the derivative peak in the first 80 seconds after the laser

beam is turned on for a sample of PNB at 60K. Figure 2b, upper trace, shows the LESR

integrated intensity (which is proportional to the number of photoinduced spins) for longer

times. There is a continuous growth of the photo-induced spins over 6 orders of magnitude

of time for the temperatures studied.

~ 0 . . . . I . . . . 1 . . . . I . . . . I . . . . I S . . . . I . . . . I . . . . i . . . .

1.5 ~ t2

z.o ~ 8

<~ <~

0.5 4

0 . 0 . . . . 7 0 . . . . I . . . . I . . . . I . . . . 0 , , , , I . . . . I . . . . ] . . . .

40 60 80 100 0.0 5.0Xl0 I I.OXIO' 1.5X10" 2.0XI0' T I M E ( s ) T I M E ( s )

Fig. 2(a) The growth of derivative peak in the first 80s and (b)the longtime growth of

the integrated intensity for PNB at 60K and pump photon energy of 2.54 ev.

Figure 3 is the normalized spin decay versus time after the pump beam is blocked.

The decay time is extremely long and shows the asymetric growth and decay dynamics.

The ESR linewidth was studied as a function of time and concentration of induced spins.

The linewidth is strongly influenced by the number of induced spins whereas the g-value is

independent of spin concentration. 1 . 0 1 - ' ' ' • i . . . . I . . . . I . . . .

1.00 ~1~

~ o . 0 8

0.97

<:3 0.98

0.95

0.94 . . . . i , , , , I , , J , I . . . .

0.0 O.0X 10 = 1.0X 10" 1.5XI04 2.0X 10" TIME(s)

Fig. 3 The decay of the light-induced spins in PNB at 60K as a function of time after

the pump beam is blocked.

DISCUSSION

T h e long time growth of the induced spins in the polyaniline systems is suggestive of

the type of behavior seen in glassy systems. In many systems conventional Debye inde-

pendent relaxation is not applicable. Relaxation in complex, strongly interacting systems

644

often follows a Kohlrausch 8 stretched exponential form. This is found to be the case in the

growth of defects over many orders of magnltude in time in the polyanillnes. The coefficients

of the stretched exponential follow the Vogal-Fulcher law 7. Two usual circumstances that

lead to the Kohlrausch type behavior: a) simple statistical dlst~ibution of relaxation times

in the system (parallel relaxation) and b) presence of a hierarchy of constraints where the

relaxation of defect n ~ l depends on defect n (serial relaxation).

The behavior of the growth of induced spins in this system is not enough to distinguish

between these two explanations, but the correlation between the linewidth and the density

of induced spins is an added indication of strong interactions between induced defects. Also

the asymetry between the growth and decay dyn-mics leads to the conclusion that there is

a hierachy of constraints where the creation of a defect depends on the previously created

defects and that the interaction between these defects stabilize the excited state in the

system. This is in accord with the coordinated motion of the rings creating the free volume

necessary for relaxation of photo-lnduced defects. It is postulated, in consideration of the

linewidth dependence on the spin concentration, that the defects group together to form

regions with very high densities of defects. It is noted that PNB and emeraldine base at

low temeratures and leucoemeraldine base at room temperature have photo-induced triplet

states as detected by ESR in addition to the photo-lnduced doublet that is described in this

paper 8.

ACKNOWLEDGEMI~NT

This work is supported in part by DARPA through a contract monitored by US ONR.

REFERENCES

1 J.M. Ginder and A.J. Epstein, Phys. Rev. Le t t , e4 (1990) 1184; Phys. Rev. B, 41 (1990)

10674.

2 M.C. dos Santos and J.L. Br~das, Synth. Met , 29 (1989) E321; Phys. Rev. Lef t , 62

(1989) 2499; ibid., 64, (1990) 1185.

3 R.P. McCall, J.M. Ginder, J.M. Leng,H.J. Ye, A.J. Epstein, G.E. Asturias, S.K.

Manohar, J.G. Masters, and A.G. MacDiarmld, Phys. Rev. B,41 (1990) 5202;Phys. Rev. B,39

(1989) 10774.

4 R.P McCall, J.M. Ginder, J.M. Leng, K.A. Coplin, H.J. Ye, A.J. Epstein, G.E. As-

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proceedings.

5 A.G. MacDiarmld and A.J. Epstein, Faraday Discuss. Chem. Soc, 88 (1989) 317.

6 R. Kohirausch, Ann. Phys. (Leipzig) 12, (1847) 393

7 R.G. Palmer, D.L. Stein, E. Abrahams, and P.W. Anderson Phys. Rev. Left. 53, (1986)

958

8 K.R. Cromack, A.J. Epstein, J. Masters, Y. Sun, and A.G. MacDiarmid, to be published.