This theoretical work rationalizes the absorption and fluorescence emission properties of conjugated dyes composed of dimethylamino flavylium heterocycles linked by a polymethine chain, which were recently reported to act as efficient shortwave infrared emitters. Density functional theory is used to characterize the electronic structure of the low-lying excited states as a function of the polymethine chain length. Decomposition of the computed excitations in terms of diabatic states is also performed to deconvolute the excited states wavefunction into charge-transfer intramolecular excitations. Based on these results, chemical substitution patterns consisting in enhancing the electron-withdrawing strength of the polymethine bridge and the electron-donating ability of the lateral flavylium fragments, are proposed to further redshift the photoluminescence of the fluorophores.
Keywords: Contrast agents; DFT calculations; Flavylium fluorophores; Polymethine dyes; Shortwave infrared.
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