Photo-generation of spins in trans-polyacetylene

• Published in: Synthetic metals 1987, Vol.17 (1), p.569-576
• Main Authors: Levey, C.G., Lang, D.V., Etemad, S., Baker, G.L., Orenstein, J.
• Format: Article
• Language: English
• ISSN: 0379-6779; 1879-3290
• DOI: 10.1016/0379-6779(87)90800-9

We report the observation of a photo-induced increase in the number of unpaired electron spins in trans -polyacetylene. This new effect is not predicted by the existing soliton theories, and is in contrast to previous reports and expectations. Previous light-induced ESR (LESR) measurements in trans -polyacetylene have been plagued with sample heating and the resulting apparent decrease in spins on illumination. We have solved this problem by chopping between equal powers of light at 647nm and 488nm. This eliminates the transient heating in our optically thick sample, while retaining sensitivity to effects with quantum efficiencies that do not scale linearly with photon energy. The resulting wavelength modulated LESR spectrum shows a single positive line, centered on the dark ESR signal (g=2), but with a linewidth of 3–4 Gauss (p-p) at 20K. This is about 2 3 of the dark ESR linewidth at this temperature. Under the proper conditions the effect is very large; up to one photo-generated spin in 10 3 carbons within the photoexcited region, or one in 10 4 total carbons in the sample. It is not affected by compensation with NH 3, and is not present in the cis form. We discuss the excitation wavelength, temperature, and temporal dependence of this effect. Comparison of the LESR signal with the photo-induced absorption (PA) results shows that the new signal is correlated with the 1.35 eV PA band due to an uncharged species. This suggests that it may correspond to photo-production and subsequent trapping of neutral solitons. In addition, the lack of an LESR signal which is correlated with the 0.45 eV PA band (due to a charged species) supports the previous conclusion of direct photo-production of charged solitons. However, the positive effect complicates the spectra and substantially weakens the limits one can set on other mechanisms which might contribute to the photo-induced charged species.