Polyacetylene photoexcitation

The excellent agreement between the TSC and P1A results has two implications. First, since the TSC method probes the product of mobility and carrier density, while the P1A probes only the carrier density, there seems to be no dominant influence of temperature on the carrier mobility. This was also found in other conjugated polymers like /ra/ry-polyacetylene [19, 36]. Second, photoconductivity (observed via the thermal release of photoexcited and trapped earners) and photo-induced absorption probe the same charged entity [36, 37J. 

Figure 29 raises the question of how the energies of these two excited states evolve as one goes to longer polyene chains, in analogy to those found in polyacetylenes which become conductive upon oxidative doping (= ionization) or photoexcitation. 

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]. 

Electronic spectra of linear conjugated polyene radical cations are of interest for several reasons. Firstly, such species occur as intermediates in different processes of biological relevance, e.g. the protection of the photosynthetic reaction centre, the charge transfer processes in membranes or in model smdies for photoinduced charge separation. Secondly, they may be involved in the formation of solitons upon doping or photoexcitation of polyacetylene , and finally, they are of theoretical interest because their interpretation requires models which account for non-dynamic correlation. 

Upon photoexcitation of the film of tmns-polyacetylene, it gives rise to two induced-absorption bands in the gap state, i.e., a high-energy (HE) band at 1.4 eV and a low-energy band at 0.5 eV Based on time-resolved absorption measurements in the femtosecond to millisecond time regime, as well as EPR (electron paramagnetic resonance) measurements, it is now well established that the elementary excitations which give rise to the HE band are neutral... 

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. 

Photoinduced infrared absorption studies show that the photoinduced infixired modes are much weaker in intensity, than the photoinduced electronic transition [23], in contrast to the behavior of polyacetylene [51,52] and polythiophene [6]. Analysis, within the amplitude mode formalism [53], indicates that the polarons are massive, > or -60 m, while use of bond order [54] or Holstein [55] polaron formalisms leads to an even larger estimate of the mass of the polarons. Photoexcitation, into either the exciton peak or the 7C-to-7t peak of emeraldine base, produces essentially identical long-lived photoinduced infrared modes [29]. The long-lived photoinduced spectra of the leucoemeraldine base are much weaker. Similarly, the photoinduced infrared absorptions in the pemigraniline base are much weaker, than the photoinduced electronic transitions, again indicating massive photoinduced defects [30]. 

Fig. 6. V vs. for the normal modes of trans-polyacetylene. The experimental values are ( ) Raman shifts for different excitation wavelengths (a) infrared doping-induced bands and ( ) infrared bands induced by photoexcitation.

The primary photoexcitation dynamics in PDPA solutions and films in the fs to ps time domain using transient PM spectroscopy were extensively studied [182]. The PDPA polymer used was a disubstituted biphenyl derivative of frans-polyacetylene, where one of the hydrogen-substituted phenyl groups was attached to a butyl group, which is referred to as PDPA-mBu (Figure 22.25 inset) [181]. The polymer films were cast on sapphire substrates from a toluene solution the same solution was used for measuring the photoexcitation dynamics in a PDPA-mBu solution. 

CW experiments as well as time-resolved measurements down to picosecond resolution have been carried out for both PA and PC. Photo-induced absorption measurements yield information on the number of photo-excited particles (charged and neutral), whilst the photocurrent is due to the product of number of carriers and their mobility, and is sensitive only to charged excitations. Therefore comparison of the results of both types of experiment gives maximum information on photoexcitations in polyacetylene. 

Photoexcitation is one of the most powerful methods for the investigation of the nature of defects such as solitons and polarons in polyacetylene. For a consistent... 

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