Theoretical Insights into Branched and Fused Expanded Pyridiniums by the Means of Density Functional Theory

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With the aim of getting insights into the peculiar electronic, structural, and photophysical properties of four expanded pyridinium systems of potential use as electron acceptors in supramolecular architectures, their electronic and geometrical structures, at both the ground and the excited states, were investigated by the means of density functional theory (DFT) and time-dependent DFT (TD-DFT). Solvent effects were included by the means of a polarizable continuum model (PCM) at both the ground and the excited states. In particular, the computed photophysical behaviors (absorption and emission) of the fused architectures were compared to those of the respective branched precursors in order to clarify the origin(s) of (i) the extension of their electronic absorption toward the visible region and (ii) the increase of their luminescence quantum yields and red-shifted emission wavelengths experimentally observed. The theoretical insights gained allow for a clear-cut explanation of the different behavior of these systems of interest as electron acceptors and luminophores for more complex supramolecular architectures and opens the route for a joint experimental and theoretical design of new pyridinium-based acceptors.