Conformational structure of the unsymmetrical monomethine cyanine bearing 2-azaazulene and 2-benzothiazole residues as terminal groups: Experimental and quantum-chemical investigation

Conformational features of unsymmetrical monomethine cyanine dye 2-[(2-butyl-1,3-dimethylcyclohepta[c]pyrrol-6(2H)-ylidene)methyl]-3-ethyl-1,3-benzothiazol-3-ium iodide-perchlorate have been investigated in solution by means of NMR spectroscopy and in the solid state by X-ray diffraction. The possibility of molecule conformational transformations was proved by scanning of potential energy surface along torsion angels. The corresponding energy barriers values have been calculated by means of DFT (B3LYP and M05-2X) methods. A comparison of structural parameters obtained by means of both methods was reported. The isomerization process was investigated by dynamic NMR spectroscopy. A comparison of 1H NMR spectra recorded in different solvents was performed. It was shown that in solution, intramolecular rotation around the bond С(6)аzaazulene–Сmethyne decelerated in NMR time scale took place. The data on dynamic behavior of dye molecules have been compared with the experimental X-ray data. Quantum-chemical calculation results are in agreement with the experimental data.