Excitation of phenanthridines in aqueous solution: Comparative theoretical analysis

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To elucidate the effect of side groups on the excitation of phenanthridines, six compounds were studied by DFT/TD-DFT: ethidium (E), dimidium (D), desphenylethidium (DE), desphenyldimidium (DD), dideaminoethidium (DDE), and dideaminodesphenylethidium (DDDE). Ten hybrid functionals were used with different contributions to the “exact” Hartree-Fock exchange, including those with long-range correction (B3LYP, APFD, PBE0, M06, M05, BMK, MN15, CAM-B3LYP, MN12SX, and ωB97XD). As a result, the best agreement with the experiment was given by the BMK functional in combination with the 6–311++G(d,p) basis set and the IEFPCM solvent model. The main structural change upon excitation of E, D, and DDE was a significant decrease in the dihedral φ between the phenyl ring and the chromophore. In the φ range from ≈75° to ≈105°, the rotation of the phenyl ring relative to the chromophore is almost unimpeded, but it becomes very difficult at |Δφ|>45°. HOMOs change significantly with the addition/removal of amino groups. At the same time, LUMOs are the same for all studied phenanthridines. HOMOs do not cover the side groups attached to the N5 atom. LUMOs cover these groups slightly. This feature explains the small differences in the absorption maxima λmax between E, D, DE, and DD, as well as between DDE and DDDE. The photoinduced shifts of electron densities have a complex form and quantitatively depend on the side groups. The dipole moments of dyes with phenyl rings increase upon excitation, while the lack of phenyl rings leads to a photoinduced decrease of dipole moments. From this feature, in particular, it follows that the negative solvatochromism of E is determined by hydrogen bonds with the solvent, and not by dipole–dipole interactions.