Light induced electron spin resonance in polyaniline

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

641

LIGHT INDUCED ELECTRON SPIN RESONANCE IN POLYANILINE

K. C R O M A C K

and A.J. E P S T E I N

Physics Department, The Ohio State University, Columbus, OH 43210 (U.S.A) J. M A S T E R S

, Y. SUN, and A.G. M A C D I A R M I D

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 attract 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 a n d / o r 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 photoinduced 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 preparation m e t h o d o{ the polyaniline samples is described elsewhere 5. For the present L E S R studies the samples were either mixed with KBr or cast as thin films on quartz slides. L E S R experiments were done on a Bruker E S P 300 system equipped with a TM102 9.48 GHz cavity with an Oxford 900 variable temperature cryostat installed. The samples were photoexcited using Ar + laser lines in the 2.41-2.7 eV range. Five E S R spectra were taken to record the background spin concentration before the sample was exposed to light. The p u m p b e a m was then allowed to illuminate the sample. The signal at the top of the derivative peak was monitored for 80 seconds. The total spectrum was then taken at intervals of 1 minute for up to 6 hours before the pump b e a m was blocked. After the b e a m was blocked the spectrum 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 m a x i m u m 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

duced E S R taken at ~ sity of 100 m W / c m 2.

pernlgranillne

base

(PNB)

dark

ESR

and

the

light

in-

60 K with a p u m p photon energy of 2.51 eV and intenIt is seen that the L E S R is positive; i.e.

the number of

spins is increased under illumination and that the L E S R is a single line.

In con-

trast, structure in the dark E S R signal is attributed to nitrogen hyperfine coupling.

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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

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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. L e t t , e4 (1990) 1184; Phys. Rev. B, 41 (1990) 10674. 2 M.C. dos Santos and J.L. Br~das, Synth. M e t , 29 (1989) E321; Phys. Rev. L e f 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. Astu~as, S.K. Manohax, J.G. Masters, E.M. Scherr, Y. Sun, and A.G. MacDiaxmld, these 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.