Probing the supramolecular features via π-π interaction of a di-iminopyrene-di-benzo-18-crown-6-ether compound: experimental and theoretical study

Abstract

A novel DPyDB-C[double bond, length as m-dash]N-18C6 compound was synthesised by linking a pyrene moiety to each phenyl group of dibenzo-18-crown-6-ether, the crown ether, through -HC[double bond, length as m-dash]N- bonds and characterized by FTIR, ¹H-NMR, ¹³C-NMR, TGA, and DSC techniques. The quantitative ¹³C-NMR analysis revealed the presence of two position isomers. The electronic structure of the DPyDB-C[double bond, length as m-dash]N-18C6 molecule was characterized by UV-vis and fluorescence spectroscopies in four solvents with different polarities to observe particular behavior of isomers, as well as to demonstrate a possible non-bonding chemical association (such as ground- and excited-state associations, namely, to probe if there were forming dimers/excimers). The interpretation of the electronic structure was realized through QM calculations. The TD-CAM-B3LYP functional, at the 6-311+G(d,p) basis set, indicated the presence of predominant π → π* and mixed π → π* + n → π* transitions, in line with the UV-vis experimental data. Even though DPyDB-C[double bond, length as m-dash]N-18C6 computational studies revealed a π-extended conjugation effect with predominantly π → π* transitions, thorough fluorescence analysis was observed a weak emission, as an effect of PET and ACQ. In particular, the WAXD analysis of powder and thin films obtained from n-hexane, 1,2-dichloroethane, and ethanol indicated an amorphous organization, whereas from toluene a smectic ordering was obtained. These results were correlated with MD simulation, and it was observed that the molecular geometry of DPyDB-C[double bond, length as m-dash]N-18C6 molecule played a defining role in the pyrene stacking arrangement.

Fig. 1. Structural formula of di-iminopyrene-di-benzo-18-crown-6-ether.

Scheme 1. Synthetic of the preparation of di-iminopyrene-dibenzo-18-crown-6-ether.

Fig. 2. Wide-angle X-ray diffraction (WAXD) diffractogram of the DPyDB–CN–18C6 film obtained from the toluene solution.

Fig. 3. Starting structures after the energy minimization for the DPyDB–CN–18C6 compound. In yellowish green is the i1 position isomer and in green is the i2 position isomer. Ethanol molecules are hidden, for clarity.

Fig. 4. (A) Final structure of the aggregate of DPyDB–CN–18C6 molecules, resulting after 800 ns of simulation. (B) Detail of a one-sided stack of pyrene (circled in red). Ethanol molecules are hidden, for clarity.

Fig. 5. UV-vis absorption spectra of di-iminopyrene-dibenzo-18-crown-6-ether in: (a) n-hexane, (b) toluene, (c) 1,2-dicholoroethane, and (d) ethanol.

Fig. 6. (a) Fluorescence emission spectra of DPyDB–CN–18C6 at λ_ex = 360 nm, in 1,2-dichloroethane, ethanol, toluene, and n-hexane. (b) Fluorescence emission spectra of DPyDB–CN–18C6 in 1,2-dichloroethane, at λ_ex = 360 nm, 370 nm, and 380 nm.

Fig. 7. Equilibrium geometries into the GS for the i1 isomer computed using the PBE0/6-311+G(d,p) method.

Fig. 8. Equilibrium geometries into the GS for the i2 isomer computed using the PBE0/6-311+G(d,p) method.

Fig. 9. Electronic absorption spectra of i1 and i2 position isomers, determined by the computational TD-CAM-B3LYP method in toluene (a) and ethanol (b), compared with the experimental UV-vis measurements.

Fig. 10. Molecular orbital representation of the i1 isomer using TD-CAMB3LYp/6-31+G(d,p) in the gas phase.

Fig. 11. Molecular orbital representation of the i2 isomer using TD-CAMB3LYp/6-31+G(d,p) in the gas phase.

Fig. 12. Example of a complex formed between DPyDB–CN–18C6 and a K⁺ ion. C atoms are shown in teal, O atoms in red, N atoms in blue, K⁺ atoms in orange and H atoms in white.