Chemistry Reference
In-Depth Information
It is known that more effective stabilizers are substances, possessing the largest system of
conjugation. The whole complex of chemical and physical properties of polyconjugated
systems is caused by special electronic structure of polyconjugated systems and, first of all,
by delocalization of π - electrons along the units of conjugation chain. Necessary condition
providing the greatest degree of delocalization of π - electrons, and hence, efficiency of
conjugation, is coplanarity of fragments, composing conjugation system. That is more
effective conjugated systems have joints whose space construction of a molecule approaches
the plane.
While comparing structural formulae of modifiers and DTA, DTG, TG curves dyed by
these additives of PETP - fibres it has been seen that effect of the dye on PETP - fibre is in
direct dependence on structure and space configuration of dye molecules (hexaazocyclanes).
Disturbance of coplanarity of heteroatom ρ - electrons, taking part in conjugation with π -
electrons of the rest of the system, loads to the change in absorption spectra. At turning
around simple bond in conjugated system by the angle less or equal to 45ºC there occurs
decrease of intensity of maximum absorption band without considerable shift λ
max
. This effect
is observed in absorption spectra of HC-1 and HC-2 (Figure 2). In absorption spectrum of
HC-1 decrease of intensity of absorption band is observed comparing with absorption
spectrum of HC-2. This has given the possibility to make a conclusion that a molecule of HC-
2 has more plane structure than HC-1.
Computing of space configuration of hexaazocyclanes molecules (Figure 3.23 - 3.29)
shows that the structure of HC-2 molecule is more planar (Figure 3.24) than of HC-1 (Figure
3.23). So, HC-2 possesses greater system of conjugation than HC-1 and, therefore, has more
stabilizing effect. A great difference, being observed between values of A for HC-1 and HC-
2, despite small difference in their chemical structures, is described by the fact that HC-2
structure is more planar than HC-1.
Plane structure of HC-2 must facilitate molecules aggregation of light stabilizer, that
leads to very effective suppression of excited states. Computing of space configuration of
hexaazocyclanes molecules also explains difference of properties of PETP - fibres and DAC
films dyed by HC-3 and HC-4.
As it follows from abovementioned data, decrease of photosensiblizing HC action should
improve its protective effect. That is why the problem of the effect of HC - light stabilizers
structure on their photosensibilizing activity is very important.
Though molecule of HC-3 additive has the form close to the plane (Figure 3.25), its form
is not a completed cycle (Figure 3.1) and this facilitates decrease of conjugation system.
Molecule of HC-4 additive is not at all in two perpendicular planes (Figure 3.26), that is it is
not conjugated. That is why, stabilizing action of these compounds is not observed.
Space configurations of molecules of the remaining hexaazocyclanes have the form close
to plane (Figure 3.27 - 3.29).
From all hexaazocyclanes, being used, HC-2 has more plane molecule and, hence, greater
degree of conjugation. Proceeding from this, HC-2 additive must possess the best stabilizing
effect that is proved by complex thermogravimetric analysis and also by investigations of
light destruction of DAC films modified by HC.
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