Systematic Tuning of Electronic Ground and Excited States in Donor-Acceptor Dyes; Steps toward Designer Compounds for Modern Technologies

The vibrational and electronic properties of six systematically altered donor-acceptor dyes were investigated with density functional theory (DFT), spectroscopy, and electrochemical techniques. The dyes incorporated a carbazole donor connected to a dithieno[3,2-b:2′,3′-d]thiophene linker at either the C2 (m) or C3 (p) position. Indane-based acceptors contained either dimalononitrile (IndCN), ketone and malononitrile (InOCN) or diketone (IndO) electron accepting groups. Molecular geometries modeled by DFT using the BLYP functional and def2-TZVP basis set showed planar geometries containing large, extended π-systems and produced Raman spectra consistent with the experimental data. Electronic absorption spectra had transitions with π-π* character at wavelengths below 325 nm and a charge transfer (CT) transition region from 500 to 700 nm. The peak wavelength was dependent on the donor and acceptor architecture, with each modulating the HOMO and LUMO levels, respectively, supported by TD-DFT estimates using the LC-ωPBE* functional and 6-31g(d) basis set. The compounds showed emission in solution with quantum yields ranging from 0.004 to 0.6 and lifetimes of less than 2 ns. These were assigned to either π-π* or CT emissive states. Signals attributed to CT states exhibited positive solvatochromism and thermochromism. The spectral emission behavior of each compound trended with the acceptor unit moieties, where malononitrile units lead to greater π-π* character and ketones exhibited greater CT character.