Polyynes: Possible Bulk Synthesis and Chemical Properties - Polyynes: Synthesis, Properties, and Applications - page 438

FIGURE 18.13 Electronic absorption spectra in CH 3 CN/H 2 O of polyynes photo-
lyzed with a low pressure Hg lamp in air. (A) Pristine solution of polyynes.
(B) Solution of polyynes after 90min photolysis. (C) Difference spectrum.
Completely analogous behavior was found in the photolysis of the poly-
ynes in n-hexane. Also, n-hexane is a solvent transparent to UV radiation.
In this solvent, the rapid and selective photolysis of the polyynes absorbing
at 239 and 251 nm can be observed, those absorbing closer to the emission
line of the lamp at 254 nm (see Figure 18.14 and the kinetic plot of Figure
18.15 ) . The spectra of Figure 18.14(A) were taken after 21, 47, and 83min of
photolysis; the respective difference spectra (versus the pristine spectrum)
are shown in Figure 18.14(B-D). It can be observed that the first to be
photolyzed are the compounds at 239 and 251 nm, but the polyynes
absorbing at longer wavelengths (namely at 262, 275, and 296) also follow
the same trend of the two mentioned bands. Instead, the bands at shorter
wavelength, those at 217 and 226 nm, are photolyzed only when all the
bands at 239 and 251 nm have been completely photolyzed. Thus, they need
a much longer exposure time to react, and the reaction starts only when the
''protection'' of the other species absorbing at longer wavelengths has
vanished. This can be easily appreciated in the kinetic graph of Figure 18.15.
This result opens the door to a simple technique for the selective degra-
dation of polyynes from a mixture. In other words, if the polyynes are or
will be produced in mixture through, for instance, the submerged electric
arc, some of them can be selectively photolyzed by a selected monochro-
matic radiation, leaving the other(s) almost untouched.