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Variation of Donor and Acceptor Strength in Analogues of Brooker’s Merocyanine and Generalization to Various Classes of Charge Transfer Compounds
journal contribution
posted on 2019-03-01, 00:00 authored by M. Dekhtyar, W. Rettig, A. Rothe, V. Kurdyukov, A. TolmachevDerivatives
of Brooker’s merocyanine (BM) have been investigated,
which possess different donors and acceptors and therefore vary their
donor–acceptor strength SDA. The
00 energies have been extracted from the spectra and compared. Under
basic conditions, where the neutral (merocyanine) form is present,
the absorption energies for all compounds are similar, whereas there
is a large difference for acidic conditions where the cationic (cyanine)
form is present. This behavior could be explained by a simple theoretical
model involving the dependence of the excitation energy ΔE01 on SDA. This
model can be generalized to describe in a consistent way two different
well-known classes of neutral chromophores with a certain degree of
charge separation, namely merocyanine I (TICT) and merocyanine II
(often betainic) compounds. Merocyanines I are characterized by a
medium polar aromatic ground state and a zwitterionic quinoid excited
state and hence positive solvatochromism, whereas merocyanines II
are formally characterized by a zwitterionic aromatic ground state
and a less polar quinoid excited state and, accordingly, by negative
solvatochromism. On increasing the donor–acceptor strength SDA sufficiently, merocyanines II can, however,
move to the so-called overcritical region with the excited state dominated
by the zwitterionic valence bond structure. For many of the merocyanine
II molecules investigated here, a weakly positive solvatochromic behavior
is observed indicating that the ground state contains less of the
zwitterionic valence bond wave function than the excited state and
that these compounds belong to the overcritical region. The fluorescence
spectra have been analyzed in terms of the Franck–Condon model
and confirm these conclusions.