Polyacetylene, photoinduced

In polythiophene and in the substituted polythiophenes, polarons were identified [135-137]. The data was analyzed using the amplitude mode and phase mode model, yielding an approximate value of 0.3 for the pinning parameter, for both the photoinduced and doping induced polarons (the IRAV appear at nearly the same frequencies). The larger pinning, relative to polyacetylene, is possibly related to the different electronic energy level structure of the polarons. 

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. 

The excellent agreement between the TSC and PIA results has two implications. First, since the TSC method probes the product of mobility and carrier density, while the PIA 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 rrany-polyacetylene [19, 36]. Second, photoconductivity (observed via the thermal release of photoexcited and trapped carriers) and photoinduced absorption probe the same charged entity [36, 37]. 

Keeping all these things in mind, it is intriguing to address the following two questions. (1) Why do all-frans-/3-carotene crystals give rise to photoconductivity that extends to the near-infrared region (2) Is it possible to find metastable states in dA -trans- -carotene crystals that corresponds to the solitons in fran -polyacetylene In order to answer these questions, we have applied photoinduced and time-resolved absorption spectroscopies to the aW-trans-/3-carotene single crystals (Hashimoto et al., 1998). 

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]. 

Graupner, W., Leising, G., Fischer, W., and Stelzer, F., Photoinduced absorption in polyacetylene polynorbomene block copolymers, Synth. Met., 58, 77-94 (1993). 

V. DOPING-INDUCED AND PHOTOINDUCED VIBRATIONAL SPECTRA OF POLYACETYLENE... 

Meanwhile, the charge excitations couple with the lattice vibrations and allow some symmetrical vibrational modes (Raman-active modes) to become infrared active by breaking the local symmetry [75]. This was first recognized in the doping and photoexcitation studies of rran -polyacetylene [62, 76, 77]. Moreover, the amplitude mode formalism developed by Horovitz [78] has been successful in explaining the one-to-one correspondence between the photoinduced and the doping-induced IR-active vibrational modes and their relationship to the Raman modes of the pristine polymer. 

Photoinduced difference spectra at 0.5 and 200 psec (a) for trans-polyacetylene and (b) for cw-polyacetylene, excited at 2.0 eV. After Reference [524], reproduced with permission. 

The resulting (CH)x exhibits a well defined fiber pattern and provides the opportunity of determining whether intrachain or interchain processes are most efficient for generating photoinduced carriers. Towsand s work on photoexcitation and charge transport in highly oriented Durham polyacetylene showed that there is a factor of 4 favoring interchain vs. intrachain motion. 

Absence of photoinduced electron transfer from polyacetylene and polydiacetylenes... 

Another important result from the photophysical studies on conjugated polymer/fullerene composites is the complete absence of any photoinduced interaction between fullerenes and either polyacetylene (PA) [54] or the polydiacetylenes (PDAs) [62], respectively. The results of absorption and emission spectroscopy, subpicosecond and millisecond photoinduced absorption spetroscopy, and picosecond transient and near steady state photoconductivity show that in contrast to the high quantum efficiency photoinduced electron transfer definitively established in the PPV and the P3AT composites with Cgo, photoinduced electron transfer from PA and PDAs onto Ceo is inhibited [62]. 

Some recent literature on photoinduced excitations in polyacetylene... 

Another physical phenomenon connected also with the change of occupation number of ground state and consequently with an incident power dependent refraction index variation (cf. Heeger et al. (I986)) is photoinduced absorption observed in polyacetylenes (Orenstein and Baker (1982)), polydiacetylenes (Orenstein et al. (198 )) and polythiophenes (Moraes et al. (198 )). In this process (cf. Fig. 20) with the pump at (o energy corresponding to the first allowed one photon transition one varies refractive index in both visible (o) ) and infrared frequencies. 

Figure 5. Photoinduced q>tical absoiption spectrum for unoriented Durtiam polyacetylene, (a) measured at 77 K with excitation at 488 nm [48], and (b) measured at 20 K with excitation at 457.9 nm [46].

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