Impact of N-substitution on structural, electronic, optical, and vibrational properties of a thiophene-phenylene co-oligomer

Abstract

Properties of the organic semiconductors can be finely tuned via changes in their molecular structure. However, the relationship between the molecular structure, molecular packing, and (opto)electronic properties of the organic semiconductors to guide their smart design remains elusive. In this study, we address computationally and experimentally the impact of subtle modification of a thiophene-phenylene co-oligomer CF₃-PTTP-CF₃ on the molecular properties, crystal structure, charge transport, and optoelectronic properties. This modification consists in the substitution of two C-H atom pairs by N atoms in the thiophene units and hence converting them to thiazole units. A dramatic effect of the N-substitution on the crystal structure-the crossover from the herringbone packing motif to π-stacking-is attributed to significant changes in the molecular electrostatic potential. The changes in the molecular and crystal structure resulting from the N-substitution clearly reveal themselves in the Raman spectra. The increase of the calculated electron mobility in the corresponding crystals as a result of the N-substitution is rationalized in terms of the changes in the molecular and crystal structure. The charge transport, electroluminescence, and photoelectric properties are compared in thin-film organic field-effect transistors based on CF₃-PTTP-CF₃ and its N-substituted counterpart. An intriguing similarity between the effects of N-substitution in the thiophene rings and fluorination of the thiophene-phenylene oligomer is revealed, which is probably associated with a more general effect of electronegative substitution. The obtained results are anticipated to facilitate the rational design of organic semiconductors.

Fig. 1. Chemical structure of the compounds studied.

Fig. 2. Equilibrium geometries, HOMO (a and b) and LUMO (c and d) patterns, and electrostatic potential maps (e and f) for the compounds studied.

Fig. 3. Absorption and photoluminescence spectra of CF₃-PTTP-CF₃ and CF₃-PTzTzP-CF₃ in THF solution.

Fig. 4. Experimental Raman spectra of polycrystalline powders of CF₃-PTTP-CF₃ (black) and CF₃-PTzTzP-CF₃ (red) normalized to the maximal intensity in the high-frequency range. Insets show the low-frequency (LF) range. Dashed blue lines indicate the positions of selected Raman bands of CF₃-PTTP-CF₃.

Fig. 5. Crystal structures (two projections) for CF₃-PTTP-CF₃ (a and b) and CF₃-PTzTzP-CF₃ (c and d) and the corresponding unit cell parameters.

Fig. 6. Graphical representation of the total interaction energy in blue on panel in CF₃-PTTP-CF₃ (a) and CF₃-PTzTzP-CF₃ (b) crystals. The cylinders link molecular centroids, and their thickness is proportional to the magnitude of the energy; for clarity, the pairwise energies with magnitudes less than 5 kJ mol⁻¹ are not shown. Details are given in Table S1 and S2..

Fig. 7. Hirshfeld surfaces of CF₃-PTTP-CF₃ and CF₃-PTzTzP-CF₃ mapped with curvature C (a) and ESP (±65.6 kJ mol⁻¹ per unit charge) (b).

Fig. 8. Distribution of intermolecular contacts for CF₃-PTTP-CF₃ and CF₃-PTzTzP-CF₃ arranged by molecules on the basis of Hirshfeld surface analysis. “Conducting” contacts (see details in the text) are highlighted with the dashed frames.

Fig. 9. Charge transfer integrals, J, for CF₃-PTTP-CF₃ and CF₃-PTzTzP-CF₃; the J values are labeled. The thickness of the arrows illustrates the J magnitude.

Fig. 10. Typical transfer characteristics of OFETs based on CF₃-PTTP-CF₃ (a and c) and CF₃-PTzTzP-CF₃ (b and d) with HMDS (a and b) and PMMA (c and d) dielectric layers. Mobilities are shown for the backward direction of measurement, as their values are higher than for the forward direction.

Fig. 11. Maximal and average electron mobilities and average threshold voltages for all prepared devices with HMDS or PMMA layers based on thiophene-containing (CF₃-PTTP-CF₃) and thiazole-containing (CF₃-PTzTzP-CF₃) oligomers.