Abstract: | The spectra of azulene solutions in siloxane and heptadecane were studied. The VIS band (435–714 nm) associated with the color
blue was shown to transform reversibly as a result of a temperature increase and to assume a likeness to the less intensive
VIS band typical for solutions in polar ethanol and dibutyl phthalate. It was concluded that supramolecular dimers rather
than individual molecules are the chromogens of azulene, just as in the case of phthalocyanine, triphenylmethane, xanthene,
and thiazine dyes. Clar’s conclusions regarding the nature of the visible band of azulene absorption and the mechanism of
reversible thermal discoloration of azulene solutions in high-boiling-point hydrocarbons are consequently wrong. It was established
that the thermochromism of azulene solutions is actually not associated with the isomerization of azulene molecules into nonplanar
nonaromatic molecules but with the reversible isomerization of dimeric structures, accompanied by a change in the positions
of the aromatic molecules relative to one another. The corresponding equilibrium is somewhat endothermic: ΔH = 6.6 kJ/mol, ΔG
o ≈ − 1.45 kJ/mol. Based on these results, it was concluded that light absorption changes the energetic state of the dimeric
structures (transition S
D→ S
D*) without leading to electron transfer in single molecules according to the scheme S
0 →S
1. That is, the traditional idea that Kasha’s law is violated when azulene fluoresces is incorrect, since the radiation comes
from the S
1 level and not from the S
2 level, as is stated in the literature. |