Influence of Packing Motifs on Photoluminescence Properties of Cocrystals of Energetic 3-Nitro-1 <i>H</i> -1,2,4-triazole with Crown Ethers

Seven cocrystals of the energetic material 3-nitro-1H-1,2,4-triazole (NT) with a series of 18- and 15-membered crown ethers (CE), including 18-crown-6 (18C6), cis–anti-cis–dicyclohexano-18-crown-6 (DCH18C6), benzo-18-crown-6 (B18C6), 4′-nitrobenzo-18-crown-6 (O2NB18C6), 4′-bromobenzo-18-crown-6(BrB18C6), 15-crown-5 (15C5), and 4′-nitrobenzo-15-crown-5 (O2NB15C5), were synthesized and crystallized. The reported compounds comprise the binary cocrystals 2NT·18C6, 2NT·DCH18C6, 2NT·B18C6, NT·15C5, and NT·O2NB15C5 with the NT:CE molar ratios of 2:1 and 1:1, and two monohydrates, NT·O2NB18C6·H2O and NT·BrB18C6·H2O, with a NT:CE:H2O molar ratio of 1:1:1. Compound NT·15C5 crystallizes in the noncentrosymmetric orthorhombic Pna21 space group, while all of the other compounds crystallize in the centrosymmetric triclinic P1̅ space group. The macrocyclic coformers significantly “dilute” the crystal structures in comparison with the pure NT form. The components are held together by the interplay of NH···O and CH···O hydrogen bonds and π–π stacking interactions. The NT–CE binding occurs via hydrogen bonds that combine the components in the H-bonded units NT-CE-NT· or NT-CE, depending on the size and rigidity of the macrocyclic coformer. In turn, the NT molecules are associated either in the homomeric stacking dimers NT-NT within the supramolecular chains -CE-NT-NT-CE- or tetrameric capsules CE-NT-NT-CE with different degrees of overlap between the NT skeletons or in heteromeric stacking patterns NT/XPh (X = H, NO2, Br) in cocrystals with aromatic CE coformers. The calculated CE-–NT interaction energies vary from −80.0 to −109.8 kJ·mol–1. The five contributors to the interaction energy were specified as electrostatic, dispersion, exchange, polarization, and repulsion components that provided insight into the binding nature between the NT energetic compound and CE coformer. The cocrystals’ photoluminescence (PL) profiles revealed turn-on/off effects originating from the CE’s nature and packing particularities. The dual fluorescence with maxima at 363 and 395 nm was registered for the NT·B15C5 cocrystal, attributed to the S2–S0 and S1–S0 energy transitions. The cocrystals 2NT·18C6, NT·O2NB18C6·H2O, 2NT·B18C6, NT·BrB18C6·H2O, and NT·O2NB15C5 revealed the gradually decreased fluorescence in the row with a maximum at 395 nm attributed to the S1–S0 transition. The variations in the PL intensities were attributed to different stacking patterns involving the NT quencher and different packing motifs. The broad emission for NT·BrB18C6·H2O was attributed to the heavy atom effect of the bromine substituent in the CE coformer, facilitating the S1–T1 transitions, as indicated by the peak at 650 nm. No PL signal was registered for 2NT·DCH18C6, which can be explained by the lack of favorable emission pathways in the case of the CE coformer with bulky aliphatic substituents.