TRANSIENT PHOTOLUMINESCENCE IN CIS-POLYACETYLENE

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TRANSIENT PHOTOLUMINESCENCE IN CIS-POLYACETYLENE

G. Fasol, R. Friend

To cite this version:

G. Fasol, R. Friend. TRANSIENT PHOTOLUMINESCENCE IN CIS-POLYACETYLENE. Journal

de Physique Colloques, 1983, 44 (C3), pp.C3-341-C3-343. �10.1051/jphyscol:1983368�. �jpa-00223029�

JOURNAL DE PHYSIQUE

Colloque C3, supplément au n°6, Tome 44, juin 1983 page 05-341

TRANSIENT PHOTOLUMINESCENCE IN CIS-POLYACETYLENE

G. F a s o l * a n d R.H. F r i e n d * *

*Max-Planck~Institut fur Festkorperforsehung, Stuttgart-Busnau, F. R. G.

**Cavendish Laboratory, Madingley Road, Cambridge, U.K.

Résumé - Nous avons étudié la photoluminescence du cis-polyacé'tylène préparé par la méthode de Luttinger. Les mesures ont été réalisées dans le domaine d'énergie compris entre 2,5 et 2,9 eV, avec un laser â colorant pompé par un laser à l'azote.Nous avons pu montrer que la durée de vie de la lumines- cence est inférieure à la limite de résolution du système de mesure (environ 5 ns). Cette courte durée de vie semble impliquer que l'excitation électron- ique responsable est localisée.

Abstract - We have observed photoluminescence from cis-polyacetylene prepared by the Luttinger route. We report a similar spectrum to that previously ob- served for Shirakawa material, with a broad photoluminescence band centred at 1.9 eV with superimposed structure attributed to resonance Raman scatter- ing. The lifetime of the luminescence is less than the measurement system response time, of 5 ns.

The photoexcited states of polyacetylene have attracted considerable interest recent- ly in view of the possibility of photoexcitation of mid-gap soliton states [1], Experimental studies include luminescence and photoconductivity [2], ultraviolet photoelectron spectroscopy [3] and photoinduced optical absorption changes [4,5].

We present here the first transient luminescence measurements on cis-polyacetylene, which show that the luminescence decays faster than our time resolution of 5 ns.

Samples studied in this work were produced using a Luttinger catalyst. The details of the sample preparation are discussed in ref. [6]. The samples were kept at low temperatures at all times to prevent the cis-trans isomerisation, and the lumines- cence measurements were performed with the samples immersed in liquid nitrogen. The Raman spectrum of the sample was measured with a conventional Raman spectrometer and c.w argon laser. The position of the two observed lines, at 1250 cm and 1542 cm

, agree well with previous reports for cis - (CH)

prepared by the Shira- kawa route [7,8]. There was no sign of the Tines associated with the trans isomer.

The transient luminescence measurements were performed with a tunable, pulse (6 ns pulses) dye laser and a boxcar detector, as described in ref. [9]. All spectra shown are corrected for the response of the detection system.

Figure 1 shows the luminexcence spectrum measured coincident with the pulsed excita- tion at 2.50 eV and 2.91 eV, with S - 1 and S - 20 photomultiplier tubes. Time- delayed spectra were below the system noise for the shortest delays possible, and, assuming an expontential decay, we place an upper limit of 5 ns for the luminescence lifetime. The spectra in Figure 1 are characteristic of those reported for Shira- kawa-route material, with a broad luminescence band centred at 1.9 eV, with super- imposed, sharp higher order Raman lines [2,8,10].

The rapid decay of the luminescence signal is consistent with the model developed in ref. [ 2 ] to account for the contrasting properties of the cis and trans isomers:

the trans isomer shows photoconductivity but no photoluminescence, whereas the cis isomer shows photoluminescence but no photoconductivity. In the trans isomer photo- production of soliton-antisoliton pairs gives rise to photoconductivity, since, by virtue of the symmetry of the trans isomer, the soliton defects are free to separate

and move along the chains. In contrast, similar photoexcited pairs of lattice

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983368

C3-342 JOURNAL DE PHYSIQUE

Figure 1 Luminescence spectra observed coincident with excitation pulse.

defects on the cis isomer are always bound, since the region in between the defects (trans-cisoid) is of higher energy than the usual cis-transoid configuration. The fact that the luminescence specta are Stokes shifted by as little as 0.15 eV requires in this model that the photoexcited pair be strongly localised. Radiative recombin- ation of this bound defect pair would be expected to be very rapid, in agreement with our measurements.

This simple picture is complicated by the recent observation of photoinduced optical absorption changes C4,51. If the sample is exposed to a short pulse of band-gap illumination, and its optical absorption spectrum is measured a short time later, it is found that the absorption rises very rapidly after illumination, at around 1.35 eV and 0.5 eV for the trans isomer, and at around 1.55 eV in the cis isomer.

However the absorption changes at around 1.55 eV in cis (CH), decay slowly, on a time scale of at least 10 us. This long decay time, in comparison to the lumines- cence decay may indicate that there is more than one type of excited state present, one responsible for the absorption changes, and another for luminescence.

We thank D.C. Bott, C.K. Chai, D. Gerrard and R.H. Weatherhead, at the B.P. Research Centre, Sunbury-on-Thames, for sample preparation and help with these measurements.

References

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