Neutral soliton diffusion

Finally H NMR 77h in (mni -NT-PA is mentioned briefly. The anisotropy pattern of H NMR T h in (roti.s-S-PA has been reported to be basically understandable with neutral soliton diffusion [62]. Does it hold even for (ro .f-NT-PA The angular dependence of 7]]j is shown in Figure 6.14 [24]. It is noteworthy that the depression around > = 90° is deeper than that in b-atu-S-PA and the origin of such a depression cannot be expected firom the diffusion mechanism of the neutral solitons as depicted by the solid curve B in Figure 6.14. To account for such a depression by another mechanism, spectral diffusion and localized. spins as described in (6.18) (see, [8] p. 379) [8,83] it is... 

Figure 6.14. The H NMR 7]]) in trans-NT-PA as a function of the angle / at 50 MHz ]24]. The solid curve A is a total of the three parts, B, C and the isotropic one. B is the neutral soliton diffusion mechanism via modulation of the dipolar, the hyperfine and their cross-couplings. C is the localized spin -E spectral difrusion mechanism [8,83], The straight line is the isotropic contribution.

Problem (2) is that the spin-lattice relaxation rate of C NMR, 77c. should obey (6.14) as H NMR 77,j does, if the neutral soliton diffuses whole the sample. To investigate a role of spin flip/flop diffusion through H and C Scott and Clarke have measured 77] and 77c ll samples enriched by various ratios of C to D ( ) 98 0 (2) 90 98 and (3) 20 98 [152]. They observed ... 

Fig. 5. Temperature dependence of effective spin-lattice Tj and spin-spin Ti relaxation times calculated from Eqs. 8 as well as of intrachain and interchain diffusion rates calculated from Eq. 10 for neutral solitons diffusing in trans-PA along and between randomly oriented chains and chains oriented with their c-axis with respect to an external magnetic field by / = 90°, 60°, 30°, and 0°. [From Refs. 12(b) and 61 with permission.]...

Measurements of diffusion of neutral solitons have been made using the electron spin resonance (ESR) and the nuclear magnetic resonance (NMR). An unambiguous interpre-... 

On the dynamics of the neutral soliton, in 1980 Nechtschein and co-workers have demonstrated the evidence for the rapid diffusion of the neutral soliton along a one-dimensional chain from the observation of the pure Overhauser effect (OE) in trans-PA using a dynamic nuclear polarization (DNP) experiment and from the l/v frequency dependence of the H NMR spin-lattice relaxation rate 7Yil [143]. These observations give quantitative estimations that the pure OE implies the condition T (iJc 10" rad/s for the... 

Good evidence for the rapid motion and trapping of the soliton is demonstrated by a DNP experiment as shown in Figure 6.30 [146,173]. The dynamic nuclear polarization (DNP) experiments are carried out at 9 GHz between 1.5 and 300 K in cis [143,174] and irons PA [143,145,146,173,174]. At room temperature the pure Overhauser effect (OE) was observed in a -irans-PA without air or oxygen but a mixed solid state effect (SSE) together with OE was found in c/s-rich PA without air [143,174] and al -lrans-PA with air [146]. In particular, below 150 K, the mixed effect was observed, even in a -trans-PA without air [145,173], The OE is characteristic of dynamic interaction between nuclear spins and electron spins, with rapid motion, On the other hand, the SSE is of the static interaction between them. In other words, observation of the pure OE is clear evidence for the electron spin for motion with an inverse of the correlation time comparable with or larger than Wj,. At 300 K, the pure OE observed in all-/ra s-PA, is consistent with the conclusions for the neutral soliton to diffuse rapidly compared with 1/We as concluded from the ESR linewidth narrowed by motion [53] and the proton NMR 7Y ] [143] as a function of frequency over a broad range. 

Finally, they proposed a model to account for together with T h that only 1/30 of the chains carry one neutral soliton and that the nuclear magnetization flows to a cylindrical relaxation core due to a soliton diffusion, through the nuclear flip/flop diffusion. An interpretation of the above observations with this model is as follows. On H NMR T]]] it is reasonable that the non-deuterated sample (l)(98/0) shows /y/w dependence due to the soliton diffusion, but not for the fully deuterated samples (2)(90/98) and (3)(20/98) in which the flip/flop diffusion rate of H is slower than a sink rate of the relaxation cores. On C NMR a situation is more complicated. In the samples (l)(98/0) and (3)(20/98), the observed frequency independency... 

Kazama et ai reported that H NMR Tjj] as a function of frequency in poly(paraphenylene sulphide) (PPS) doped heavily with SO,", showed the inverse square law of frequency [371]. The diffusion coefficient estimated using (6.14) from such a frequency dependence was much smaller than that of the neutral soliton in trans-PA. 

The topic of neutral soliton dynamics has been controversial for many years. The reasons have been due to a lack of definite data and a lot of different interpretations from a variety of bases for many experimental data. In this review, we tried to explain most of the important experimental results in terms of a diffuse/trap model based on observations of the ESR linewidth as functions of temperature and frequency. Anomalous broadening observed only in (CH) but not in (CD), at frequencies lower than 6 MHz was explained in a clear-cut way by this model, giving a consistent value of the maximum spin density of the neutral soliton, 0.15-0.17 in comparison with 0.17 determined by the ENDOR technique. These successes represented in the finally obtained diffusion rates which are found to be consistent between NMR and ESR seem to settle the controversy. 

Several experiments have been carried out to confirm the physical properties of solitons in mns-polyacetylene [27]. Lately, this excitation has also been studied in another degenerate ground state conjugate polymer, poly(l,6-heptadiene) [28]. The onedimensional spin diffusion and associated spin dynamics are verified from electron magnetic resonance spectroscopy, nuclear magnetic resonance (NMR) spectroscopy and electron nuclear double resonance (ENDOR) measurements [13]. The density of neutral solitons has been estimated by Motsovoy and co-workers [29]. For more details on the physical properties of solitons, the reader is referred to a review article by Heeger and co-workers [13]. However, more theoretical and experimental work is... 

It appears that the stoichiometry of most doping reactions is such that two electrons are transferred, but as a Gedanken experiment Figure 1.25 is quite instructive. It demonstrates nicely that not single solitons but soliton-antisoliton pairs are generated. The charged soliton or antisoliton is likely to be electrostatically bound to the dopant ion, but a neutral soliton could diffuse freely at least towards the left. If... 

The presence of mobile spins, or neutral solitons, in undoped tr n5-(CHK was established unambiguously from the beginning by the observation of the Overhauser effect in DNP experiments. Further studies were then concerned with (I) quantitative estimates of the diffusion rate and of the anisotropy of the motion and (2) trapping of the diffusive solitons in the presence of defects or impurities or as the temperature is lowered. 

The analysis of the dielectric increment and the relaxation time in the dielectric relaxation spectra indicates that the HF relaxation is ascribable to a much more localized carrier transport along the polymer chain [175a]. The diffusion constant >h of the HF relaxation was about 2 x 10 m s with the transport range Lh of 50 nm. This Lh corresponds to approx. 100 thiophene rings in length, which may represent the effective size of the 7r-conjugation. Note that the value of Dh is of the same order as the diffusion constant of neutral solitons in polyacetylene determined by NMR [177] and ESR measurements [178] or that of polarons in polyaniline estimated by ESR [179], Thus, the diffusion constant of the MF relaxation... 

Furthermore, quantitative characterizations of the spin motion in trans-(CH) have been performed by measurements of the proton NMR relaxation time 7 and analysis of the ESR line width [70,71]. The spin motion can be described in terms of highly one-dimensional diffusion. The diffusion rate along the chains is very fast D[ 1013 rad/s (i.e., a diffusion coefficient of ca. 5 x 10 3 cm2/s), and the anisotropy is extremely high Z>j /Z>x > 105. The very high anisotropy is also an argument for the soliton picture neutral... 

---