Excited mixed-valence states of symmetrical donor− acceptor− donor π systems

S Amthor, C Lambert, S Dümmler… - The Journal of …, 2006 - ACS Publications
S Amthor, C Lambert, S Dümmler, I Fischer, J Schelter
The Journal of Physical Chemistry A, 2006ACS Publications
We investigated the spectroscopic properties of a series of four bistriarylamine donor− π−
bridge-donor D− π− D compounds (dimers), composed of two asymmetric triarylamine
chromophores (monomers). UV/vis, fluorescence, and transient absorption spectra were
recorded and compared with those of the corresponding D-π monomers. Bilinear Lippert−
Mataga plots indicate a major molecular reorganization of the excited state in polar media
for all compounds. The excited states of the dimers are described as mixed-valence states …
We investigated the spectroscopic properties of a series of four bistriarylamine donor−π−bridge-donor D−π−D compounds (dimers), composed of two asymmetric triarylamine chromophores (monomers). UV/vis, fluorescence, and transient absorption spectra were recorded and compared with those of the corresponding D-π monomers. Bilinear Lippert−Mataga plots indicate a major molecular reorganization of the excited state in polar media for all compounds. The excited states of the dimers are described as mixed-valence states that show, depending on the chemical nature of the π bridge, a varying amount of interactions (couplings). We found that superradiant emission, that is, an enhancement of the fluorescence rate in the dimer, is observed only in the case of weak and medium coupling. Whether the first excited-state potential energy surface of the dimers is described by single minimum or a double minimum potential depends on the solvent polarity and the electronic coupling. In the latter case, the dimer relaxes in a symmetry broken CT state with partial positive charge at the triarylamine donor and negative charge at the π bridge. The [2.2]paracyclophane bridged dimer is an example of a weakly coupled system because the spectroscopic behavior is very similar to the corresponding p-xylene monomer. In contrast, anthracene as well as p-xylene bridges mediate a stronger coupling and reveal a significant cooperative influence on the optical properties.
ACS Publications
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