Photoexcitations in polyacetylene with controlled conjugation length

Synthetic Metals, 54 (1993) 331 336

331

Photoexcitations in polyacetylene with controlled conjugation length G. L a n z a n i a,h, A. Piaggi *'c, R. T u b i n o d, Z. V. V a r d e n y b and G. S. K a n n e r b ~ffstituto di Spettroscopia Molecolare del CNR, via dei Castagnoli 1, Bologna (Italy) bDepartment of Physics, University of Utah, Salt Lake City, UT 84112 (USA) "Dipartimento di Fisica 'A. Volta', Universitd di Pavia, via Bassi 6, Pavia (Italy) ¢~Istituto di Matematica e Fisica, Universit~t di Sassari, via Vienna 2, Sassari (Italy)

Abstract Photoexcitation of solitons has been studied by steady state and transient picosecond photomodulation spectroscopy in a form of soluble polyacetylene with controlled conjugation length. A systematic blue shift of the charged soliton band is observed by decreasing the average conjugation length from a few hundred to 20 CH units, showing that the chain segmentation has strong effects when the conjugation length becomes comparable or shorter than the soliton spatial extent. The linear dependence of the charged soliton band on the inverse of the average conjugation length is discussed, and an evaluation of the electron correlation energy is given. The effects of the decreased conjugation length have been also investigated by time-resolved photoinduced absorption. The transient electronic response in the time domain from 5 ps to 1 ns is interpreted in terms of soliton recombination associated with diffusion on chains distributed in length.

Introduction We r e p o r t on o u r s t udy of p h o t o e x c i t a t i o n s in samples of polya c e t y l e n e with c o n t r o l l e d c o n j u g a t i o n length. S t e a d y st at e and t r a n s i e n t p i c o s e c o n d p h o t o m o d u l a t i o n s p e c t r o s c o p y h a v e b e e n em pl oyed in o r d e r to clarify th e effects of t he s e g m e n t a t i o n w h e n t he a v e r a g e c o n j u g a t i o n l e n g t h becomes c o m p a r a b l e or s h o r t e r t h a n t he soliton ext ent , and t he finite c h a i n l e n g t h on t he p h o t o e x c i t e d solitons decay. Samples of soluble p o l y a c e t y l e n e (SCH) h a v e been o b t a i n e d by growing th e p o l y e n i c c ha i ns o n t o a c t i v a t e d sites of a flexible p o l y b u t a d i e n e c h a i n w h i c h acts as soluble c a r r i e r into c o m m o n o r g a n i c s o l v e n t [1]. T h e c o n j u g a t i o n l e n g t h of these samples, as d e t e c t e d by e l e c t r o n i c a b s o r p t i o n a n d R a m a n spectra, has been e s t i m a t e d to be of the o r d e r of a few h u n d r e d CH units. E l e c t r o n i c d e l o c a l i z a t i o n has been r e d u c e d by air e x p o s u r e of th e samples for a c o n t r o l l e d a m o u n t of time. T h i n films of SCH were o b t a i n e d by e v a p o r a t i o n in an i n e r t a t m o s p h e r e of soluble p o l y a c e t y l e n e on SiO2 s ubs t r a t e . *Author to whom correspondence should be addressed.

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332

The steady state photoinduced changes in transmission have been measured in the energy range from 0.4 to 2.2 eV by phase-sensitive detection technique. The sample was excited by the 488 nm line of an argon laser chopped mechanically at 70 Hz. Measurements were carried out with the cast film placed on a cold finger assembly of a Cryo-tip system. The sample temperature could not be properly controlled due to the poor heat conduction of the substrate and the insulating nature of the polybutadiene matrix. Picosecond photoinduced absorption was measured with a synchronously pumped dye laser consisting of a frequency-doubled mode-locked Nd:YAG laser (with 76 MHz repetition rate) synchronously pumping two dye lasers, one for the pump beam (with 580 nm wavelength) and the other for the probe beam (with wavelength between 580 and 1020 nm). The room temperature photoinduced changes in transmission were measured using 10 MHz modulation. The time resolution of the system was about 5 ps. Results and discussion

Steady state photoinduced absorption Linear absorption spectra taken on samples with different average conjugation length show a blue shift of the absorption maximum upon shortening the conjugation length. According to the Kuhn model [2], the energy E of the absorption maximum in polyenes is related to the conjugation length 'n' (n = number of conjugated double bonds) by B E= A +-n

(1)

where A and B were evaluated from the observed position of the absorption maxima in a series of polyenes with known conjugation lengths [3]. The average conjugation length n of our segmented samples was then estimated by eqn. (1). The photoinduced absorption spectra of four samples with different conjugation length are shown in Fig. 1. Strong oscillation is superimposed close to the isosbestic point spectral region; the oscillation disappears upon oxidation of the sample. A detailed analysis of this energy region, which exhibits significant differences from the Shirakawa polyacetylene, is reported in ref. 4. Here we discuss the l o w e n e r g y electronic transition correlated with the charged soliton (S +-) defect. We notice that, even for samples possessing the highest degree of delocalization, the S -+ peak position (0.61 + 0.05 eV) is somewhat higher than the values reported for Shirakawa [5] (0.49 eV) and Durham [6] (0.55 eV) polyacetylenes. This discrepancy could be related to temperature effects due to the poor temperature control of the sample. The S -+ sub-gap absorption band shows a significant blue shift with decreasing electron delocalization. In particular, as reported in Fig. 2, the S-+-band position is linearly dependent on the inverse of the conjugation length.

333 I

i

i

n=368

n=34

< n=26

n=10

[

\-J 0.4

0.8

1.2

1.6

2.0

ENERGY (eV/ Fig. 1. Liquid n i t r o g e n p h o t o i n d u c e d a b s o r p t i o n spectra of soluble polyacetylene with different c o n j u g a t i o n l e n g t h s 'n'.

0.9

~"

,

,

,

~

,

,

,

~

,

,

,

~

,

1 . 3

0.8

1.2

0.7

1.1

IJ..I •
1

> w

0.9

0.5 0.4

,,, 1.7

L,,, 1.9

t , , , 2.1

i 2.4

,

,

,

0.8 2.6

Energy (eV) Fig. 2. P h o t o i n d u c e d a b s o r p t i o n b a n d m a x i m u m of S -+ as a f u n c t i o n of the linear a b s o r p t i o n b a n d maximum. The s t r a i g h t line (referring to the r i g h t side axis) has been c a l c u l a t e d using eqn. (4) for the case U 0 = 0.

334

The SSH model predicts that solitons created on an infinitely long chain are associated with a mid-gap optical transition at a b o u t 0.8 eV. On the other hand, the p h o t o i n d u c e d absorption spectra for SCH, as well as those reported in the literature for polyacetylene [7], indicate that S ± absorption is significantly shifted from the mid-gap position. Moreover, a large splitting b e t w e e n charged and n e u t r a l soliton (S °) peaks has been observed. A model taking into a c c o u n t electronic correlation and Coulomb repulsion, not included in SSH theory, is therefore necessary. In the 'weak coupling p e r t u r b a t i o n limit' the effects of the electron electron correlation can be t r e a t e d as a p e r t u r b a t i o n of the one-electron Hamiltonian (including the e l e c t r o n - p h o n o n coupling) [8]. The introduction of the e l e c t r o n - e l e c t r o n repulsion breaks the degeneracy of the solitons states b e c a u s e the creation energy of S -+ is larger t h a n the energy required for S °. The shift of the energy levels with respect to the unpert u r b e d situation results in [9]

3E=~ (N-l)_+

(2)

where + ( - ) refer to charged (neutral) solitons, U0 is the on-site Coulomb repulsion parameter, and 2~a is the spatial extent of the soliton (2a = lattice constant) in a chain of N carbon atoms. The S -+ optical transition is now located at

u0

AEs± = A0 - - -

(3)

w h e r e the soliton width is related to A0, the energy gap parameter, by ~ W/(2A0) (W = bandwidth, estimated to be 10 12 eV). Finally one gets AEs± _~Ao(1 _ ~2U0~ /

(4)

Therefore, the transition energy for charged solitons is expected to scale linearly with the gap parameter, as confirmed by the data in Fig. 2, with a slope which depends on the on-site Coulomb repulsion. A linear leastsquares fit of our data yields an estimate of the on-site correlation energy U 0 ~ 9 - 1 1 eV. The e l e c t r o n - e l e c t r o n interaction leads to a splitting Ueff b e t w e e n S ± and S O transition energies Uefr = AEso - AEs_+ = 2U0/3{

(5)

The soliton spatial width 2{ for an infinite chain has been estimated to be a b o u t 14 CH units. By inserting this value in eqn. (5), Ueffm0.85-1.05 eV is obtained. An interesting point is the effect of the chain segmentation on solitons. This problem has been investigated by F6rner [10] who has shown that, within the f r a m e w o r k of the SSH model, a soliton shrinkage occurs upon shortening of the chain length. This b e h a v i o u r can possibly explain the reason w h y solitons can be supported also by very short

335

c o n j u g a t e d segments, as we have observed in the present work. We have estimated the soliton width in samples of different c o n j u g a t i o n length by the F6rner approach, then we have determined the effective Coulomb repulsion Uefffrom eqn. (5). As one could expect, Ueff increases upon chain shortening (U~ff=0.89, 1, 1.17 and 2.56 eV for n = 368, 34, 26 and 10, respectively) due to a stronger confinement of excess electronic density associated with the c o n j u g a t i o n defect.

Picosecond decay A typical room t e m p e r a t u r e decay of the p h o t o i n d u c e d absorption in SCH is shown in Fig. 3. The pump and probe w a v e l e n g t h s were 580 and 950 nm, respectively (no dependence of the transient signal on the probe w a v e l e n g t h was detected). The signal decays as t -°5 up to a b o u t 50 ps, then slows to t 0.1. A similar t r a n s i e n t b e h a v i o u r has been observed in S h i r a k a w a trans-polyacetylene [11, 12], w h e r e a longer crossover time tc b e t w e e n the two power-law decays and an higher e x p o n e n t ~ ( ~ 0 . 2 - 0 . 3 ) after tc were found. The initial t-°5 decay strongly suggests diffusion and geminate rec o m b i n a t i o n of the soliton antisoliton pairs along the chains. In fact, the survival probability of particles executing r a n d o m walks on an infinite chain is [13]

ua)

S(u, t) = er 2~2~t~1/2

(6)

where u is the distance of the two particles at t = 0 and D is the diffusion coefficient. In real samples chains are not infinite; for a finite chain, the t 0.5 decay continues until the particles diffuse to the ends of the chain. In

~i 10

10

100

1000

time (ps) Fig. 3. P i c o s e c o n d decay of the p h o t o i n d u c e d a b s o r p t i o n at room t e m p e r a t u r e . The pump and probe w a v e l e n g t h s are 580 and 950 nm, respectively. The lines are fits with power-law decays t ~ w i t h ~ =0.5 and =0.1, respectively.

336

this sense tc is significantly related to the average chain length. It is then reasonable that in SCH tc is lower t h a n in solid trans-(CH)x, due to the smaller chain lengths involved. In principle, the long time survival probability for carrier diffusing along a chain of length L is [13, 14]

S(t) cc exp( - ~2Dt/L 2)

(7)

Nevertheless, the measured decay t-~ calls for a model able to a c c o u n t for the slower and non-exponential behaviour. Weidman [15] proposed a model of diffusion-limited r e c o m b i n a t i o n in the presence of site disorder. The effect of a distribution of chain lengths q)(L) is important in determining the long time exponent a. Slow decays in p h o t o c o n d u c t i v i t y and photoluminescence of disordered materials have been explained in terms of a power-law distribution G(~) of carrier lifetimes [16, 17]. Starting from a distribution of chain lengths in our samples, a distribution of decay times develops naturally. We t a k e G(~) ~ ~ -(1 + ~), and if the survival probability in eqn. (7) is averaged over G(~), with ~ varying from the minimum time c o n s t a n t rml, that can be measured, i.e. the time resolution of our experimental set-up, and a maximum ~ma× determined by the pump m o d u l a t i o n frequency, the power-law t ~ is reproduced. The related distribution of chain lengths results in O(L) ~: L-(1 +2~) (with L greater t h a n the minimum l e n g t h Lmin). Smaller values of a are then c o n n e c t e d to b r o a d e r distributions of O(L), so that a broad distribution of lengths in SCH samples can be inferred.

References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

S. Destri, M. Catellani and A. Bolognesi, Makromol. Chem. Commun., 5 (1985) 353. H. Kuhn, Fortsch. Chem. Org. Naturst., 16 (1958) 169. L. S. Lichtmann, Ph.D. Thesis, Cornell University, 1981, unpublished. G. Lanzani, G. Kanner, S. Jeglinski and Z. V. Vardeny, Synth. Met., (1992) 50 (1992) 461. J. Orenstein, G. L. Baker and Z. V. Vardeny, J. Phys. (Paris) Colloq. 44C3 (1983) 429. P. D. Townsend and R. H. Friend, Phys. Rev. B, 40 (1989) 3112. G. B. Blanchet, C. R. Fincher, T. C. Chung and A. J. Heeger, Phys. Rev. Lett., 50 (1983) 1938. S. Kivelson and D. E. Heim, Phys. Rev. B, 26 (1982) 4278. D. Baeriswyl, D. K. Campbell and S. Mazumdar, Phys. Rev. Lett., 56 (1986) 1509. W. FSrner, Synth. Met., 30 (1989) 135. Z. V. Vardeny, J. Strait, D. Moses, T. C. Chung and A. J. Heeger, Phys. Rev. Lett., 22 (1982) 1657. G. S. Kanner, Ph.D. Thesis, University of Utah, 1991, unpublished. I. V. Zozulenko, Solid State Commun., 76 (1990) 1035. Y. Gaididei, A. I. Onipko and I. V. Zozulenko, Phys. Lett. A, 132 (1988) 329. D. L. Weidman, Ph.D. Thesis, Cornell University, 1987, unpublished. H. Scher and E. W. Montroll, Phys. Rev. B, 12 (1975) 2455. C. Tsang and R. A. Street, Phys. Rev. B, 19 (1979) 3027.