Abstract
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 π–π 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 π–π 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 (Φf = 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 (Φf = 3.80–4.00%) are typical for the centrosymmetric dimers present in crystalline (Z)-(N-pyridyl)enaminopyrrolo[2,3-b]quinoxaline and (E)-(N-ethylpyridyl)enamino-pyrrolo[2,3-b]quinoxaline. The fluorescence liftimes τ 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.
Highlights
Single crystals of π-conjugated fused heteroaromatic systems have become an important target of research aimed at determining the relationship between the structure and the fluorescence of crystalline molecular materials[1,2] as well as the charge transport in organic semiconductors.[3,4] The properties of the crystals are closely associated with both their molecular packing and the electronic properties of the parent molecules
The length of the linker (CH2)n (n = 0, 1, or 2) between the enamino-pyrroloquinoxaline system and the pyridyl group influences the general shape of assembling molecules and the packing of the molecules in the crystal structures
The reorientation of the molecular permanent dipole moment in some cases is caused by an additional intramolecular hydrogen bond interaction (N−H···nitrogen atom of the pyridyl substituent (Npy) apart from N−H···NQui) that affects the conformation of the molecules
Summary
Single crystals of π-conjugated fused heteroaromatic systems have become an important target of research aimed at determining the relationship between the structure and the fluorescence of crystalline molecular materials[1,2] as well as the charge transport in organic semiconductors.[3,4] The properties of the crystals are closely associated with both their molecular packing and the electronic properties of the parent molecules. Hydrogen bonds have a pivotal role in the stabilization of the crystal structures and influence the conformation of the parent molecules.[7] Intramolecular hydrogen bonds restrict the vibration and rotation of a molecule, making its conformation more rigid. These phenomena are crucial for molecular solid-state fluorescence enhancement.[8,9] Noncovalent interactions such as dispersion (London dispersion forces10) and van der Waals repulsion are responsible for π−π interactions. The interactions may be regarded as attractive interactions of electrical quadrupoles, which overpower the repulsion of π-electron clouds.[11]
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