Effect of Side-Chain Architecture on the Optical and Crystalline Properties of Two-Dimensional Polythiophenes

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Abstract

The present study reported here synthesis of three novel two-dimensional (2D) polythiophene derivatives with conjugated terthiophene–vinylene side chain—poly{3-(5″-hexyl-2,2′:5′,2″-terthiophenyl-5-vinyl)thiophene-alt-thiophene} (P1), poly{3-(5,5″-dihexyl-2,2′:5′,2″-terthiophenyl-3′-vinyl)thiophene-alt-thiophene} (P2), and poly{3-(4,4″-dihexyl-2,2:5′,2″-terthiophene-3′-vinyl)thiophene-alt-thiophene} (P3)—that were synthesized via stille coupling reaction. The terthiophene side chain with different conformations conjugated to the polythiophene main chain via vinyl linkage provided the ability to control the molecular organization, hence affecting the optoelectronic and electrochemical properties of 2D polymers. TD-DFT calculation with the B3LYP/6-31+g(d) function on electronic structures of the monomers was consistent with the experimental results. It suggested that the energetic states of HOMO and LUMO were highly dependent on the side-chain architectures. These polythiophene thin films fabricated by spin-casting show a broader absorption ranges from 300 to 700 nm which was significantly wider than the absorption of pure poly(3-hexylthiophene). When comparing the solid-state absorption spectra of these polymers before and after thermal annealing, P3 displayed the most red-shift in the wavelength range between 450 and 700 nm. It was presumably due to an extended conjugation length resulting from the linear conformation and preferred chain packing, as manifested in the X-ray diffraction. Molecular dynamics (MD) simulation on polymers with different side chains in isolated and packed states suggests planar conformation of the main chain was adopted and regulated by the side chains which were placed in parallel with the main-chain direction. Interestingly, P1 solution revealed an excitation-dependent emission property, suggesting a structural inhomogeneity in solution. Contrary to P1, the PL spectra of P2 and P3 showed only one emission peak at 460 nm, regardless of the excitation energy. Orientation and regiochemistry of the terthiophene side chain had a major impact on the overall optical and electronic properties of the polymer. Moreover, the HOMO and LUMO of these three polymers had been determined through cyclic voltammetry. HOMO of the three polymers were in the following order: P1 > P2 > P3. It implied that the energy level was regiochemistry dependent and directly associated with the linked position between backbone and conjugated side chain. Most importantly, through mesogen-jacketed-like design strategy employed in the present study, the improved packing of these two-dimensional polymers offered insights into structure design to enhance properties that have strong ties to the electronic devices.

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