Structure and spectal properties of cinnamoyl
pyrones and their vinylogs
Table 7. The influence of length and planarity of the polymethine chain on spectral properties of cinnamoyl pyrones*.
Compound
Solvent
DCM
AN
λabs
386
370
377
413
414
386
492
482
lg ε
3.81
3.58
3.65
3.55
3.88
3.57
4.01
3.70
λfl
515
-
ΔνSt
6490
-
φ
VIII
4.10
-
IX
X
DCM
AN
512
-
6990
-
1.00
-
DCM
AN
579
593
662
690
6880
9040
5220
6250
7.10
0.52
89.0
4.60
XI
DCM
AN
* – see λabs, lg ε, λfl, ΔνSt, φ in Table 6.
and explained by higher polarizability of π-electronic obvious that the emission of XI in acetonitrile is due to
systems in molecules with longer polymethine chains. the presence of an electron-donating substituent whose
However, in the case of cinnamoyl pyrone derivatives, electronic density transmits itself through polymethine
this effect may be of a great practical importance while chain into the pyrone moiety.
taking into account a very high quantum yield of IX and
Thus, it is possible to suppose that the fluorescence
the flattening (increase of rigidity) of the polymethine quenching in cinnamoyl pyrones is probably due to
chain in the excited state. The lengthening of the chain some processes taking place in the pyrone fragment in
would permit formation of dyes fluorescing in near infra- the excited state and does not originate from cis\trans
red region.
All the conclusions drawn above are based upon
isomerisation of cinnamoyl fragment.
Finally, it is worth noting that the solvatofluorochromy
the fluorescence spectra obtained in dichloromethane and dramatic fluorescence quenching in polar media
solutions. As observed from the data in Table 7, the permit the use of the substances VI,VII, X and XI as
emission of VIII and IX in polar acetonitrile is completely sensors in the examination of medium polarity in
absent. However, the lengthening of the polymethine biological and medical investigations.
chain up to n=3 (compound X) results in the appearance
of fluorescence. This spectral effect would not be
possible if the emission quenching was due to cis/trans
isomerizationintheexcitedstate.Thelongerchainisless
Acknowledgment
rigid, and its lengthening must promote the isomerization
process. Meanwhile, the longer polymethine chain is
more polarizable in the excited state, and it could act as
an electron-donating fragment for the pyrone moiety.
The quantum yield of compound XI in acetonitrile is
4.6%. It is higher than that of X, but 19.3 times weaker
than its emission in dichloromethane solution. It is
The financial support of this work from Science and
Technology Center in Ukraine (project STCU 3795) is
gratefully acknowledged.
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