Abstract
The structural origin of absorption and fluorescence anisotropy of the single crystal of the π-conjugated heterocyclic system 5,6,10b-tri-aza-acephenan-thrylene, TAAP, is presented in this study. X-ray analysis shows that the crystal framework in the space group P [Formula: see text] is formed by centrosymmetric dimers of face-to-face mutually oriented TAAP molecules joined by π-π non-covalent interactions. The conformation of the TAAP molecule is stabilized by intramolecular C-H⋯N(sp2), N(sp2)H⋯π(CN), and C-H⋯O(sp2) hydrogen bonds. The presence of weak π-π interactions is confirmed by quantum theory of atoms in molecules (QTAIM) and non-covalent interaction (NCI) analysis. The analysis of the optical spectra of TAAP in solution and in the solid state does not allow the specification of the aggregation type. DFT calculations for the dimer in the gas phase indicate that the lowest singlet excitation is forbidden by symmetry, suggesting H-type aggregation, even though the overall absorption spectrum is bathochromically shifted as for the J-type. The experimental determination of the permanent dipole moment of a TAAP molecule in 1,4-dioxane solution indicates the presence of the monomer form. The calculated absorption and emission spectra of the crystal in a simple approximation are consistent with the experimentally determined orientation of the absorption and emission transition dipole moments in TAAP single crystals. The electrostatic interaction between monomers with a permanent dipole moment (ca 4 D each) could result in the unusual spectroscopic JH-aggregate behaviour of the TAAP dimer.
Highlights
Optical properties of -conjugated crystalline materials depend on the packing arrangement in the crystal structure (Varghese & Das, 2011; Varughese, 2014)
Single-crystal X-ray diffraction analysis revealed that TAAP crystallizes from most of the solvents such as THF, CHCl3, toluene, CH3CN, CH3CN–water mixture and tetrachloroethane-d2, as triclinic crystals, with a centrosymmetric crystal structure in the space group P1 (Fig. 3 and Table S1.1)
In the low-lying excited states of the dimer, the angle between the transition dipole moment (TDM) and the line connecting the geometrical centres of the monomers is 54.22, which is very close to the critical angle of 54.7
Summary
Optical properties of -conjugated crystalline materials depend on the packing arrangement in the crystal structure (Varghese & Das, 2011; Varughese, 2014). As in the case of poly(3-hexylthiophene) aggregates (Brown et al, 2003), may lead to red shifts with the H-type geometry and nature Crystals that revealed such unusual spectral characteristics are classified as HJ- or JHtype according to the new paradigm for organic electronic materials design (Yamagata et al, 2014). The -conjugated TAAP planar chromophore exhibits a donor– acceptor architecture with a few different acceptor groups: carbonyl imide, nitrile, and two imines This specific electronic arrangement results in delocalization of the N5 and N10b lone electron pairs into the system, leading to the formation of zwitterions. We show the influence of face-to-face monomer – overlap in dimers on the aggregation type, single-crystal absorption and fluorescence anisotropy of the new heterocyclic system of TAAP. Based on intramolecular proton transfer, e.g. for alcohol vapour detection (Chen et al, 2016), as well as the multicolour fluorescence of hetero-Ln-MOF (where MOF denotes metal– organic framework), hierarchical single crystals, provide potential applications in miniaturized optoelectronic devices (Pan et al, 2017)
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