Polyyne electronic absorption spectra

FIGURE 8.9 (A) Electronic absorption spectrum of polyynes obtained in n-hexane after arcing with graphite electrodes. (B) Spectrum after polyyne precipitation from solution as acetylides. The residual absorption bands are due to PAHs formed as secondary products during arcing. The two spectra have been shifted for clarity. The absorbance units on the ordinate are a binary scale. 

The arc was conducted between graphite electrodes submerged in distilled water at 10 A. The electronic absorption spectrum of the crude aqueous solution of polyynes and other products showed the following absorption bands 200, 215, and 226 nm as the most intense. Other weaker bands were observed at 238, 246, 250, 259, 274, 284, 300, and 323 nm. 

Once we have verified that the polyynes in dilute solution are stable to air oxidation, we studied the effect of a more powerful oxidizing agent ozone. Figure 18.12(A) (bottom curve) shows the electronic absorption spectrum of the polyynes solution in DHNP. I he addition of O3 to this solution causes an increase in the intensity of the bands at 220 and 232 nm (Figure 18.12A, top curve) this increase in the absorption intensity is due to ozone dissolved in DHNP which has an absorption maximum just in this wavelength range. However, a reduction in intensity can be observed in Figure 18.12(A) for the other bands at 257, 245, 282, 303, and 322 nm as a consequence of the ozone treatment. This fact can be better appreciated by the spectrum difference... 

FIGURE 18.7 Electronic absorption spectra in n-hexane. (A) Polyyne solution as prepared. (B) Ene-ynes solution after 5 min reduction of polyynes with Zn/HCl. (C) Ene-yne solution after prolonged treatment with Zn/HCl. (D) Ene-yne solution left overnight in contact with Zn/HCl solution. (E) Difference spectrum between ene-yne solution and original polyyne solution. 

FIGURE 18.12 Electronic absorption spectra in DHNP. A study of the O3 (A). Bottom curve Pristine solution of polyynes. Top curve Solution of polyynes after the addition of ozone. (B) Difference spectrum. 

FIGURE 18.13 Electronic absorption spectra in CH3GN/H2O of polyynes photo-lyzed with a low pressure Hg lamp in air. (A) Pristine solution of polyynes. (B) Solution of polyynes after 90 min photolysis. (C) Difference spectrum. 

FIGURE 18.16 Electronic absorption spectra in //-hexane of polyynes photolyzed with a high pressure Hg lamp in N . (A) From top to bottom Absorption spectra taken before and after 26, 50, 75, and 160 min photolysis. (B to D) Difference spectra (with respect to the spectrum of the pristine solution) after 26, 50, 75, and 160 min photolysis. 

HPLC analysis also reveals additional molecular species which can hardly be discovered on the basis of the crude electronic spectrum of mixture of polyynes. As discussed previously [33—36], we have also identified polyynes with = 2 and 3, again on the basis of comparison with literature data. The detection of members of the polyynes series with = 1 and 2 is not very easy with HPLC analysis using the wavelengths selected and the spectral window available from our instrument (see Section 18.2.2). In fact, acetylene has its maximum absorption at 152nm followed by a moderate transition at 182 nm and by a very weak absorption at 220 nm. The first two bands are beyond the detection limits of our DAD, while the transition at 220 nm can be detected only if considerable concentrations of acetylene are present in solution, which is not the case. In the gas phase diacetylene also has its strongest absorption band at 165 nm, again beyond the detection limits of our instrument. We have used other weaker bands for identification of this molecule. 

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