IRAV mode

These formulae are key to the understanding of many of the properties of the IRAV modes. 

The first experimental observation of IRAV modes was on doped polyacetylene [115]. The spectra were correctly attributed to molecular vibrations made IR active by the added charge, and were considered as evidence for charged solitons. In a later experiment, the pinning mode of the soliton in doped polyacetylene was identified at 900 cm [125]. The pinned mode frequency for photoexcited solitons was found at a much lower frequency, -500 cm [126,127]. 

Analysis of the IRAV modes in terms of the amplitude mode and phase mode model [116,128-131] showed that the experimentally observed IRAV frequencies and relative intensities could be quantitatively understood and that... 

There are a number of other studies of the charged soliton induced IRAV modes both for doping induced and photoexcited solitons [132-134]. 

IRAV modes have been observed in many conjugated polymers and oligomers, introduced either by chemical doping and by photo-excitation. The IRAV modes accompany each polaron or bipolaron excitation. Typically, the IRAV modes are used as an identification mark for the nature of the excitation (whether charged or neutral). However, there is almost no attempt to quantify the mechanism of pinning. 

Figure 2. (a) Photoinduced IRAV features of YBaaCuaOy-S (5= 0.75) (b) Photoinduced IRAV modes and corresponding phonon bleachings of l 20u04-... 

The origin of the photoinduced IRAV modes in conducting polymers and their relation to resonant Raman active modes has been clearly establishedf. A one-to-one correspondence between photoinduced or doping induced IRAV modes and Raman-active modes arises because... 

Electronic Absorption at -Q.t3 9V Photoinduced Phonon Bleaching at 96 cm q Photoinduced IRAV mode at 20 cm ... 

Figure 3. Temperature dependence of the photoinduced electronic absorption at - 0.13 eV, the photoinduced bleaching at 598 cm-i, and the photoinduced IRAV mode at 520 cm-i.

Figure 4. Pump intensity dependence of the photoinduced absorption (a) the photoinduced eiectronic absorption centered at 0.5 eV (the measurement of the intensity dependence was done at 3000 cm-t) (b) the photoinduced IRAV mode at 486 cm-f.

S. Srinivasan, H. Neugebauer, and N. S. Saridftci. Electrochemically induced IRAV modes of BeCHA-PPV studied with in situ FTIR-ATR spectroscopy. Synth. Met., 84(l-3) 635-636, January 1997. 

Nevertheless, even for polyacetylene, the electronic structure is not that of a simple metal in which the bond-alternation and the tc-tc gap have gone to zero there are infrared active vibrational modes (IRAV) and a pseudo-gap. This is indicated by the spectra in Figure 2 which demonstrate the remarkable similarity between the doping-induced absorption found with heavily doped trans-(CH)x, and the photoinduced absorption spectrum observed in the pristine semiconductor containing a very few photoexcitations. Not only are the same IRAV mode spectral features observed, they have almost identical frequencies. 

The intensities of the IRAV modes increase linearly with the dopant concentration with essentially the same slope as observed at more dilute concentrations. This implies that all the doping-induced charges are involved and that the IRAV in the "metallic" state are not due to a small number of residual inhomogeneities (or nonuniformities) in the charge distribution. 

Since the IRAV mode frequencies are essentially identical with those observed with photoexcitation, the pinning of the 7c-electron charges which cause the IRAV (i.e. all the charges, see 2) has virtually disappeared. 

These three conclusions are not consistent with the excitation spectrum of the simple metal which would result if the Peierls gap had been reduced to zero (there would be no gap and no IRAV modes). 

As noted above, the free carrier contribution which extrapolates to the measured a(0) must be in the far-IR below 450 cm"1.15 Nevertheless, most of the 7C-electron oscillator strength remains in the broad absorption band above 0.2 eV. An alternative which appears to be in agreement with the essential experimental facts is that polyacetylene is an example of a polaronic metal. The polaron lattice with a half-filled polaron band is certainly consistent with the observed susceptibility, hi the case of a polaron lattice, the IRAV modes are expected, and would be r -shifted from the Raman modes provided that the pinning is weak. Although the intensity of the IRAV modes was initially csdculated to be much too weak, this calculation ignored the effect of the counter-ions the counter-ion Coulomb potentials may lead to sufficient nonuniformity in the charge density, to yield the observed IRAV mode intensities.For the polaron lattice, a((o) would have two contributions with a "gap" in between ... 

Although consistent with the photoinduced infrared active vibrational (IRAV) mode spectrum, this scheme must be verified by photoinduced electron spin resonance measurements, since the mechanism implies the photogeneration of unpaired spins via charge induced spin unpairing (i.e. direcdy analogous to the protoir-induced spin unpairing). 

Beginning in 1999, Kiebooms and Wudl reported the production of isothianaphthene analogs as shown in Scheme 12.29 [168]. After the removal of lower My, fractions by Soxhlet extraction with methanol and THF, polymer 124a was isolated as a purple solid that was found to be only soluble in 1,1,2,2-tetrachloroethane. It was found that the initially isolated material was partially doped, but could be dedoped by conventional hydrazine treatment. From the absorption onset of solvent cast films, the Eg was determined to be 1.2-1.5 eV [168,169]. Characterization of the IRAV modes correlated with the generation of charge within the doped polymer revealed sharp bands of quite low intensity, indicative of strongly localized charge carriers [98,169]. 

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