Experimental and Theoretical Insights into the Optical Properties and Intermolecular Interactions in Push-Pull Bromide Salts.

Experimental and theoretical insights into the nature of intermolecular interactions and their effect on optical properties of 1-allyl-4-(1-cyano-2-(4-dialkylaminophenyl)vinyl)pyridin-1-ium bromide salts ( and ) are reported. A comparison of optical properties in solution and in the solid-state of the salts ( and ) with their precursors ( and ) is made. The experimental absorption maxima (λ) in CHCl is at 528 nm for and at 542 nm for , and a strong bathochromic shift of ∼110 nm is observed for salts and compared with their precursors. The absorption bands in solid-state at ∼627 nm for and at ∼615 nm for that are assigned to charge transfer (CT) effect. The optical properties and single crystal structural features of and are explored by experimental and computational tools. The calculated λ and the CT are in good agreement with the experimental results. The intermolecular interactions existing in the crystal structures and their energies are quantified for various dimers by PIXEL, QTAIM and DFT approaches. Three types of interactions, (i) the cation⋅⋅⋅cation interactions, (ii) cation⋅⋅⋅anion interactions and (iii) anion⋅⋅⋅anion interactions are observed. The cationic moiety is mainly destabilized by C-H⋅⋅⋅N/π and π⋅⋅⋅π interactions whereas the cation and anion moiety is predominantly stabilized by strong C-H⋅⋅⋅Br interactions in both structures. The existence of charge transfer between cation and anion moieties in these structures is established through NBO analysis.