in which he sets out to clarify a misunderstanding
concerning a remark by
H. Shirakawa.
Prof. Dr. Hans Kuhn
23th July 2004
Ringoldswilstrasse 50
CH-3656 Tschingel, Switzerland
Fax (41) 33 2513379
concerns Internet site http://www.quantum-chemistry-history.com
Dear Dr. Anders,
As you know I was fascinated by doing research since my time as a
student. I did not defend priorities, so I let it go that the researchers
working on polyacetylene did not give citation of my work and that an
effect was called "Peierls Instability" that I had proposed and investigated
earlier at a time when It was generally assumed, based on Hückel
calculations, that long-chain polyenes show an equalisation
of C-C- bonds.
Professor Shirakawa in
his Nobel Lecture explicitly said that I did not
see this effect. I drew Shirakawa’s attention to his reversal of facts in a
letter of February 29th this year, but I did not get any response yet. At
this point, I think, the situation should be clarified.
In 1949 I published a theory on the light absorption of organic dyes
[1]. The fundamental difference in the structure of the π-electron systems
of cyanine dyes (well described by the free electron model) and polyenes
(free electron model entirely wrong as discussed below) was the heart of
that theory. It resulted in the equation
for polyenes (λmax = hc/∆E maximum
of absorption, m mass of electron,
h Planck’s constant, N number of π-electrons,
L length of π-electron
system). At that time polyenes with up to 15 double bonds had been
synthesized and an agreement of λmax
with theory was obtained when
setting V0 = 2eV
corresponding to a bond length alternation equal to that
of butadiene (135pm and 146pm). According to that theory the filled
orbitals and the empty orbitals respectively are arranged in bands
separated by the band gap of ∆E.
The oscillator strength resulting from the theory was in agreement
with experiment in this range [2].
The vibronic structure of the absorption band strongly depends on
bond length alternation. The quantum mechanical treatment was found to
be in agreement with experiment and this strongly supported the concept
of bond length alternation [3].
It was not clear to me in 1949 whether or not the theory could be
applied to infinitely long polyenes, but I saw this soon after. The essence
is given by the following figure
taken from [4].
In the case of the cyanine, the electron density resulting
from the free electron model is equal for each bond in contrast to the
polyene, where the π-electron density is larger at the "double bonds"
than at the "single bonds". This showed that the free electron model is
appropriate for the cyanines (equal bonds are the stable form), but
inappropriate for the polyenes. It was concluded that equal bonds are
unstable in the case of polyenes since the higher π-e1ectron
density at the
sites of the "double bonds" causes a decrease in bond length, which again
causes an increase in π-electron density etc. finally resulting in bonds
equally alternating as the bonds in butadiene.
This was later confirmed by
a quantitative approach showing that self-consistency of bond length and
π-electron density in the middle of the bonds is only reached, when
assuming single and double bonds alternating between 135pm and 146pm
[5].
Cyanine dyes up to 16 π-electrons have been synthesized and their
spectra agree with what follows from the free electron model. It was not
explicitly mentioned but very clearly seen at that time, that hypothetical
long-chain cyanines must show a disproportionation between a region
with equalized bonds and alternating single bonds and double bonds since
alternating bond lengths is the stable form for long chains:
The idea of a soliton was on hand. Recently the given theory has been
successfully applied to treat soliton dynamics [6].
Copies to diff. Professors - list cancelled on H. Kuhn′s request Dec. 2004
[1] H.Kuhn, J.Chem. Phys. 17, 1198-1212 (1949).
[2] H.Kuhn, J. Chem. Phys. 29, 958-959 (1958).
[3] H.Kuhn, W.Huber, F. Bär in "Calcul des Fonctions
d’onde moleculaires" ed. du Centre National
de la Recherche Scientifique, Paris (1958) p. 179-203.
[4] H.Kuhn, Chimia 4, 203-218 (1950), figure 9, p. 211.
[5] H.Kuhn, Angew. Chemie 69, 239 (1957).
H.Kuhn, Angew. Chemie 71, 93-101 (1959).
[6] C.Kuhn in "Handbook of Conducting Polymers" Second Edition,
Dekker Inc. (1998) p. 123-139.
