Journal ArticleThe optical, transport, and ESR properties of soluble conducting four-membered polydiethynylsilane  heterocycles are described. The undoped one-dimensional gap is 2.0 eV in both films and solutions; no photoluminescence is observed. I2 doping induces a single absorption band at -1.05 eV in solutions  and lightly doped films, but another at -0.55 eV in heavily doped films. Both are correlated with strong ir-active vibrations associated with known lines in Raman scattering. The doping-induced ESR of the solutions is linear in the 1.05-eV peak, but only ~2% of the carriers yield an ESR, which is motionally narrowed. No light-induced ESR is detected. The results are discussed in relation to solitons, polarons, and bipolarons in a degenerate ground-state system

Ni, Q.-X.

Vardeny, Zeev Valentine

English

journal articleThe optical, transport, and ESR properties of soluble conducting four-membered polydiethynylsilane  heterocycles are described. The undoped one-dimensional gap is 2.0 eV in both films and solutions; no photoluminescence is observed. I2 doping induces a single absorption band at -1.05 eV in solutions  and lightly doped films, but another at -0.55 eV in heavily doped films. Both are correlated with strong ir-active vibrations associated with known lines in Raman scattering. The doping-induced ESR of the solutions is linear in the 1.05-eV peak, but only ~2% of the carriers yield an ESR, which is motionally narrowed. No light-induced ESR is detected. The results are discussed in relation to solitons, polarons, and bipolarons in a degenerate ground-state system

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RAPID COMMUNICATIONS PHYSICAL REVIEW B VOLUME 44, NUMBER 11 15 SEPTEMBER 1991-1 Anomalous optical and ESR properties of doped polydiethynylsilanes Q.-X. Ni and J. Shinar Ames Laboratory and Physics Department, Iowa State University, Ames, Iowa 50011 Z. V. Vardeny Physics Department, University of Utah, Salt Lake City, Utah 84112 S. Grigoras Dow Corning Corporation, Midland, Michigan 48686 Y. Pang, S. Ijadi-Maghsoodi, and T. 1. Barton Ames Laboratory and Chemistry Department, Iowa State University, Ames, Iowa 5001 J (Received 29 April 1991) The optical, transport, and ESR properties of soluble conducting four-membered polydiethynylsilane heterocycles are described. The undoped one-dimensional gap is 2.0 eV in both films and solutions; no photoluminescence is observed. h doping induces a single absorption band at -1.05 eV in solutions and lightly doped films, but another at -0.55 eV in heavily doped films. Both are correlated with strong ir-active vibrations associated with known lines in Raman scattering. The doping-induced ESR of the solutions is linear in the 1.05-eV peak, but only -2% of the carriers yield an ESR, which is motionally narrowed. No light-induced ESR is detected. The results are discussed in relation to soli-tons, polarons, and bipolarons in a degenerate ground-state system. Various (semi)conducting polymers, notably polyace-tylene (CH)x, polydiacetylene (PDA), polyaniline, and poly thiophene (PT)-type five-membered polyheterocycles, have been studied extensively, I due to novel physical prop-erties 2 and promise in synthetic-metal, energy-storage, and nonlinear-optical (NLO) applications. 3 The ex-istence of solitons in the degenerate-ground-state trans-polyacetylene [t-(CH)xl, and polarons and bipolarons in nondegenerate systems is now well established. 2 To date, the doping-induced optical absorption signature of soli-tons has been accepted to be a single absorption band at or below midgap (Eg/2). Those of bipolarons and polarons are accepted to be two bands peaking at WI and W2, where WI +W2 -< Eg , and three bands peaking at WI. W2, and W3, where WI +W2 =W3, respectively.2 Among the charged ex-citations, only polarons have a spin. This paper describes anomalous doping-induced optical and ESR results on novel conducting poly(diethynylsi-lanes) (PDES); the strong ultrafast NLO response of these polymers has recently been described by Wong et al. 4 In solution, only a single Iz-doping-induced absorp-tion band is observed peaking at Eg/2 (1.05 eV), but an additional band appears in films at -0.55 eV. Both ab-sorption bands are correlated with strong ir active vibra-tions. The very slight doping-induced ESR is apparently motionally narrowed. Self-consistent ab initio Hartree-Fock (HF) calculations indicate that the splitting between the two dimerizations of the system is less than -4 meV, i.e., the idealized system has an essentially degenerate ground state. S The results are discussed not only in terms of electronic excitations in nearly degenerate polymers but also within the framework of degenerate-ground-state dia-tomic (AB) x polymers. 6 R2Si(C=CHh monomers, where R=phenyl (Ph), methyl (Me), butyl (Bu), or hexyl (Hex), were polymer-ized by MoCls or WC16 catalysis. 7,8 Weight-average molecular weights ranged from -100000 (-450 mon-omers/chain) with Ph to values in excess of 2 x lOs, or -1500 (-1200) monomers/chain, with Bu (Hex) substi-tuents. The polymers are soluble in various organic sol-vents, easily cast into films, and apparently quite stable in air. Detailed I3C and 29Si NMR measurements indicate the structure shown in Fig. I (a). 9 Acyclic segments prob-ably disrupt the chain conjugation,IO but since only -75% of the monomers become cyclic when polymerized via WCI6, it is suspected that one or two acyclic units do not disrupt the conjugation. The two isomers of the cyclic segments S are drawn in Fig. I (b). The absence of a solvatochromic effect,7 the strong dispersion in resonant Raman scattering, and the relatively low conductivity (-0.1 S/cm) of the heavily Iz-doped films (see below) is consistent with a relatively short average conjugation length. de conductivity measurements on pristine films cast from solution yielded 10 -9 -< <T -< 10 -7 S/cm. Roughly 3-wt. % Iz [one 13- group per 67 Bu2Si(C=CH)2 mono-mers] are absorbed during -I min exposure to -I torr Iz vapor, whereupon <T increases to -10 -2 S/cm. The iodine content increases to -6 wt. % following I h expo-sure, and <T increases to -0.1 S/cm. The Iz doping is re-versible. The undoped PDES display interband transitions peak-ing at 2.1 eV with an onset at 1.9 eV; we thus estimate the one-dimensional (ID) gap to be 2.0 eV. Only a single ab-sorption band at 1.05 eV (=Eg /2) is observed in a dilute CHCb solution at up to 50-wt. % Iz doping [Fig. 2(c)J. It grows at the expense of interband transitions. The optical response of films to Iz (or AsF s) doping [Fig. 2(b)] is 5939 © 1991 The American Physical Society RAPID COMMUNICATIONS 5940 Q.-X. NI et al. (a) z x y FIG. I. (a) The structure of polydiethynylsilanes (PDES), as determined by Parbhoo et a/. (Ref. 9). When polymerized via MoCls (WCI6), -42% (-75%) of the monomers are cyclic. (b) The two apparently degenerate dimerizations of the cyclic segments shown in (a) (Ref. 5). strikingly different from that of the solutions. While light ( < I wt. %) doping induces only the high-energy (HE) band at 1.05 eV as in solutions, heavier doping induces a second low-energy (LE) band peaking at -0.55 eV. The LE band is more intense at the highest doping levels (-6-wt. % h), but decreases more rapidly as the 12 evolves out of the film. Indeed, the intensity of the LE band is proportional to the square of the HE band in the BU2 PDES and to the fourth power of the HE band in the Hex2PDES. The doping efficiency of h in dilute solutions of BU2 PDES in benzene and acetonitrile (ACN) decreases from - 25% at to-wt. % 12 to -10% at 50-wt. % h II If it is similar in CHCh, this would correspond to one 13 in -80 monomers at IO-wt. % h and one h in -40 monomers at 50-wt. % h A 10% doping efficiency at 50-wt. % h in solution would then be similar to the maximal level of -6 wt. % observed in the films. The injection of mobile charge carriers onto the chains is also evident from the ir active vibrational modes in-duced by 12 or AsFs doping (Fig. 3). A typical room-temperature X-band ESR spectrum of pristine undoped films and solutions is shown in Fig. 4(a). The g value is 2.003; the derivative peak-to-peak linewidth !1Hpp is -10 G, and its integrated intensity in both films and solutions yields -I spin per -14000 monomers. The -3.6 G linewidth of deuterated samples [Fig. 4(b)] is sharply lower, indicating that the hyperfine interaction with neighboring protons dominates the linewidth. 12 dop-ing increases the (doubly integrated) ESR intensity by up to a factor of -8. !1Hpp initially decreases to 3.4 G in hy-drogenated and 2.9 G in deuterated films [Fig. 4(c)], but only to 7 G in 40-wt. % 12 in solutions of hydrogenated [(HC=ChSiBu2]x in benzene. Heavier doping slightly increases !1Hpp to 3.2 G in the deuterated films. Its dependence on T, broadening to 4.9 G at 7.5 K [Fig. 4(d)], suggests motional narrowing l2 with an activation 1.5 ,... 'in 1.0 ai o ~ g 0.5 -0 1.2 '" g.e 06 :il .D « 1.6 cu 1.2 u c 0 .0 0.8 5 .,. .0 « 0.4 0.0 (a) (b) (c) 4000 B 3.0 4.0 E(eV) A (C4 H2 SiBu2 )x B C D E 6000 8000 10000 wave number (cm-1) E D C B 6000 8000 10000 wave number (cm-1) FIG. 2. (a) The optical absorption of [(HC=ChSiBu2L films, during evolution of b, after exposure to - I-torr b for 15 min. A, undoped; B, after 4 min; C, 25 min; D, 65 min; E, 2\0 min. (b) Similar to (a): A, after 5 min; B, 18 min; C, 26 min; D, 40 min; E, 110 min. (c) b doping-induced absorption of di-lute solution of the same PDES in CHCb. energy of -3.7 meV, contrasting sharply with poly(3-hexylthiophene) solutions, where the linewidth increases (to -16 G upon 50-wt. % h doping). The weaker nar-rowing effect in solutions relative to films suggests that in-terchain hopping contributes significantly to the motional narrowing. As described above, at the highest doping level there is one 13 in -40 monomers, but only one spin in -1800 monomers. Yet the relation between the ESR spin density and the integrated intensity of the 1.05-e V band, during h doping of benzene solutions of [(HC=ChSiBu2]x is lin-ear. Thus, -98% of the carriers are spinless, independent of the doping level. The intensity of the light-induced ESR (LESR) is com-parable to the dark ESR. Yet the kinetics of the forma-tion and decay of the LESR are slow (several minutes to several hours) at room temperature, and slow down fur-ther upon cooling. They thus indicate that the LESR is due to metastable defect formation, rather than intrinsic RAPID COMMUNICATIONS ANOMALOUS OPTICAL AND ESR PROPERTIES OF DOPED ... 5941 ~ c 0.3 .8 j <1 ... u c: 0.1 i 0.4 <1 Hydrogenated [ BU2Si (C =CH)2]n (WCI6 ) 3000 2000 1400 1000 600 wave number (cm-1) Deuterated [BuzSi(C:CH)2]n (MaCI!!) 0.04000 3000 2000 1500 1000 500 wave number (cm-1) FIG. 3. The h doping-induced infrared active vibration modes in PDES films. The observed peaks are directly related to resonance Raman modes (see Ref. 13). photogene rated carriers. The absence of any double modulation LESR signal confirms this conclusion. 13 It thus appears that similarly to the doping-induced ESR, almost all of the photogene rated excitations are spinless. 14 Interpretation of the observed results in terms of con-ventional solitons, polarons, or bipolarons is problematic: (j) Charged solitons should not yield a doping-induced 4~-r--.-~--'-~---r--r--, .~ C :::J .e .!!'.. 3 Z (a) ~ oF-----------~------~~~ z (') -I U5 a: -2 [B -3 -4~~ __ ~~ __ ~~ __ ~ __ ~_J 3.32 333 334 3.36 MAGNETIC FIELD (kG) 100 r---,..-..,.--r:--,..-..,.---,r--,---, :§' 80 (e) .§ 60 .e 40 .!!'.. 20 -' « 0 (-====---------t-------:=----j ~ -20 U5 -40 ffi -60 W -80 -100 L....--'-_..l---'"--...l:J.._..l---''---'---' 3~ ~6 MAGNETIC FIELD (kG) ESR, nor two optical absorption bands (Fig. 2) in either films or solutions. (ii) Polarons should yield a much stronger ESR and three absorption bands. 2•15 (iii) If most of the polarons decay into bipolarons two doping-induced subgap bands should be observed even in solution. (iv) If the polarons decay into bipolarons only by interchain hop-ping, and are thus absent from dilute solutions, then the two bipolaron bands observed in the films should depend similarly on the doping level. In light of the foregoing problematic results, we note that they bear a striking qualitative resemblance to t-(CH)x: (j) There is only a single absorption band, at midgap, in solution. As in t-(CH)x, the (ii) photo-luminescence, (iii) LESR, and (iv) doping-induced ESR are very weak. (v) The results of ab initio HF calcula-tions indicate that the idealized system is essentially de-generate. It is therefore believed that the primary doping-induced excitations are spinless charged solitons. Yet if the degeneracy is slightly lifted (by -< 4 meV), then one would expect that doping would induce both po-larons and solitons. An alternative picture may involve the lifting of the de-generacy in short conjugated segments (i.e., chain-end effects). In such a case, the longer ground-state degen-erate segments would support solitons, and the shorter segments would yield bipolarons or two overlapping soli-tons. 16 The coincidence of the upper bipolaron transition at 1.0 eV with the soliton transition would then be very in-triguing. This picture, however, does not account for the absence of nondegenerate ground-state segments in solu-tion, unless the carriers can overcome the barriers be-tween the short segments in solution. The near degeneracy of the two dimerizations shown in UJ 8 .1§ 6 :::J -e 4 .!!'.. 2 ( b) ~ 0 I===::::::::.----+--~~====I z (') -2 U5 -4 a: [B -6 -8L....--'-_..l-_~~~-'-_"----'-~ 332 3.33 3.34 3.35 3.36 MAGNETIC FIELD (kG) ---,,-<:r-......., <3 0. 0.60 If" [BU2Si (C;; CH12]x I 12 doped .. :x: <l '2 8' ...J Eo ... 3.7 meV 0.50 ~~L--2LO-~--40"----''--' 103 /T(K-1) FIG. 4. (a) Room-temperature X-band ESR of pristine undoped hydrogenated PDES films and solutions. (b) ESR of deuterated films and solutions under similar conditions. (c) ESR of hydrogenated films following 3-5-wt. % h doping. (d) The logarithm of the linewidth of hydrogenated films vs I IT, indicating a motion ally narrowed ESR (see Ref. 12). , RAPID COMMUNICATIONS 5942 Q.-X. NI et al. Fig. 1 (to within - 4 me V) is surprising in view of the fact that in one form, all of the C=C double bonds are trans-like, whereas in the other they alternate between endocy-dic trans-like and exocyclic cis-like bonds. Since alter-nate C atoms of the backbone are bonded to Si, the on-site lattice potential a of the C p orbitals is expected to alter-nate in a manner similar to the hypothetical degenerate-ground-state diatomic (AB)x polymer. 5 The elementary excitations of such a system are two solitons SA and SB centered on the A and B atoms, respectively. Their levels are at ± a relative to midgap, respectively.4 In the lowest overall neutral configuration, SA is empty, and SB is dou-bly occupied. They are thus spin less, but carry fractional charges QA = (l - j)e, QB = - (l - j)e, where 0 <. j <. 1. However, the value of a in the PDES may be small, possi-bly due to nearly identical A and B species. The SA and SB levels would consequently be unresolved. Thus the relevance of the degenerate-ground-state (AB)x diatomic polymer may be marginal. I Proceedings of the International Conference on Synthetic Met-als, Santa Fe. /988, edited by M. Aldissi [Synth. Met. 27-29 (1989»). 2A. J. Heeger, S. Kivelson, J. R. Schrieffer, and W.-P. Su, Rev. Mod. Phys. 60, 781 (1988). 3Nonlinear Optical Properties of Polymers, edited by A. J. Heeger, J. Orenstein, and D. R. Ulrich, MRS Symposia Proceedings No. 109 (Materials Research Society, Pitts-burgh, 1988). 4K. S. Wong, S. G. Han, Z. V. Vardeny, J. Shinar, Y. Pang, S. Ijadi-Maghsoodi, T. J. Barton, S. Grigoras, and 8. Parbhoo, Appl. Phys. Lett. 58,1695 (199]). 5S. Grigoras and T. J. Barton (unpublished). 6M. J. Rice and E. J. Mele, Phys. Rev. Lett. 49,1455 (1982); D. K. Campbell, ibid. 50, 865 (1983). 7 J. Shinar, S. Ijadi-Maghsoodi, Q.-x. Ni, Y. Pang, and T. J. Barton, Synth. Met. 28, C593 (1989). 8y. Pang, Ph.D. thesis, Iowa State University, 1990 (unpub-In summary, the anomalous doping-induced optical ab-sorption in the electronic and ir spectral range and the ESR of PDES films and solutions were described and dis-cussed. The behavior is believed to result from generation of solitons in degenerate ground-state segments consisting of four-membered polyheterocycles attached to a vinylene unit. The striking anomalies are not clearly understood but may involve interchain excitations or a lifting of the degeneracy in short segments. We thank J. Hayes and G. J. Small for the generous use of their Bruker spectrophotometer. The work at the Uni-versity of Utah was partially supported by DOE Contract No. DE-FG02-89 ER 45409 and a grant from the Dow Corning Corporation. Ames Laboratory is operated by Iowa State University for the U.S. Department of Energy under Contract No. W-7405-Eng-82. The work at the Ames Laboratory was supported by the Director for Ener-gy Research, Basic Energy Sciences. Iished). 98. Parbhoo, Y. Pang, S. Ijadi-Maghsoodi, and T. J. Barton (unpublished). lOT. Masuda and T. Higashimura, Adv. Poly. Sci. 81, 121 (1986). IIQ._X. Ni, J. Shinar, D. Dobberpuhl, and D. Johnson (unpub-lished). 12C. P. Slichter, Principles of Magnetic Resonance, 2nd ed. (Springer, Berlin, 1978). 13L. X. Zheng and Z. V. Vardeny (unpublished). 14X. Wei, S. G. Han, K. S. Wong, 8. C. Hess, L. X. Zheng, Z. V. Vardeny, Q. X. Ni, J. Shinar, Y. Pang, S. Ijadi-Maghsoodi, T. J. Barton, and S. Grigoras, Synth. Met. 42, 1561 (1991). 15J. H. Kaufmann, N. Colaneri, J. C. Scott, and G. B. Street, Phys. Rev. Lett. 53, 1005 (1984). 16y. H. Kim, K. Choi, T. Masuda, and T. Higashimura, Solid State Commun. 77, 207 (1991). 

Anomalous optical and ESR properties of doped polydiethynylsilanes

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Anomalous optical and ESR properties of doped polydiethynylsilanes

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