Influence of Hydrogen Bonds and π-π Interactions on the Fluorescence of Crystalline (N-Alkylpyridyl)enamino-pyrrolo[2,3-b]quinoxalin-2-one Derivatives

This paper presents the relationship between the fluorescent properties of (E/Z)-(N-alkylpyridyl)enamino-pyrrolo[2,3-b]quinoxalin-2-one derivatives in the crystalline state and the molecular packing governed by the conformation of the molecules, hydrogen bonding, and p-p interactions. In particular, the type of 2-pyridyl alkyl chain (2-Py(CH2)(n) with n = 0,1,2) is responsible for the molecular packing and influences the fluorescence properties of crystalline pyrrolo[2,3-b]quinoxalines. The molecules studied exhibit permanent dipole moments that, in the crystalline phases, promote either the formation of dimers with an antiparallel orientation or stacks with a parallel dipole moment orientation. In dimers and stacks, the main observed interaction is the pi-pi type. The conformations of both (E)- and (Z)-enamines are stabilized by intramolecular hydrogen bonds from the NH group of the enamine to either the nitrogen atom N4 of the quinoxaline (E)-diastereoisomer or the oxygen atom of the amide carbonyl group in the (Z)-diastereoisomer. When an additional intramolecular hydrogen bond from the enamine NH group to the pyridyl nitrogen atom forms, it affects the conformation of the molecules, causing the reorientation of the molecular permanent dipole moment. Density functional theory (DFT) calculations performed for two neighboring molecules in dimers or stacks indicate two charge transfer mechanisms: intra- and intermolecular mechanisms. For centrosymmetric dimers, charge transfer occurs within each component molecule (intramolecular charge transfer). For stacks with molecules arranged by translation and noncentrosymmetric dimers, charge transfer is of an intermolecular nature. Higher absolute fluorescence quantum yields (Phi(f) = 12.06-13.77%) are exhibited by the (E-diastereoisomers, which contain methylene or ethylene chains and form either translational stacks of the molecules or noncentrosymmetric dimers. The lowest absolute fluorescence quantum yields (Phi(f) = 3.80-4.00%) are typical for the centrosymmetric dimers present in crystalline (Z)-(N-pyridyl)enaminopyrrolo[2,3-b]quinoxaline and (E)-(Nethylpyridyl)enamino-pyrrolo[2,3-b]quinoxaline. The fluorescence liftimes tau were determined for single crystals of the studied phases (from 13.3 to 15.6 ns) and revealed one single process of radiative energy transfer.

Automated summary

This study reveals the relationship between the fluorescent properties of (E/Z)-(N-alkylpyridyl)enamino-pyrrolo[2,3-b]quinoxalin-2-one derivatives in the crystalline state and the molecular packing governed by the conformation of the molecules, hydrogen bonding, and p-p interactions. The type of 2-pyridyl alkyl chain influences the molecular packing and fluorescence properties. The conformations of the molecules are stabilized by intramolecular hydrogen bonds, and the additional hydrogen bond affects the conformation and orientation of the molecular permanent dipole moment. The fluorescence lifetimes indicate a single process of radiative energy transfer.