Dimers, tetramers, and octamers of mono- and trimethyne thiacarbocyanine dyes. Structure, formation energy, and absorption band shifts

• We performed DFT-D modeling of an aggregation of thiacarbocyanine dyes.
• For dye dimers and tetramers, structures and formation energies were calculated.
• Tetramer energies decrease in the series brickwork > linear > ladder orientations.
• Driving forces of dye aggregation are π-stacking and electrostatic interaction.
• Brickwork and ladder tetramer absorptions are red and blue shifted, respectively.

Quantum-chemical calculations for monomers, dimers, and tetramers of 3-ethyl-2-[(1E,3E)-3-(3-ethyl-1,3-thiazol-2(3H)-ylidene)prop-1-en-1-yl]-1,3-thiazol-3-ium chloride and 3-ethyl-2-[(E)-(3-ethyl-1,3-benzothiazol-2(3H)-ylidene)methyl]-1,3-benzothiazol-3-ium chloride were performed. Both dyes were found capable of forming two kinds of π-stacking-dimers with various degree of overlap. Three kinds of tetramers: four-deck ladder, three-deck brickwork, and two-deck linear, can be formed without destruction of the dimers of the cyanine dyes. The energy of tetramer formation was found to decrease in the order brickwork > linear > ladder configurations. For dimers of the first dye and its tetramer (brickwork, ladder and linear) configurations, the energies of the first S0 → S1 singlet transitions, oscillator strengths, and structures of frontier MO have been calculated. The spectral shifts in dimers and tetramers were found to depend only on mutual orientation of components: brickwork and linear tetramers show red shift (+Δλ) while ladder tetramers, blue shift (–Δλ) of absorption bands (with respect to monomer).

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