Directed Polymorphism and Mechanofluorochromism of Conjugated Materials through Weak Non-Covalent Control

Abstract

Understanding and manipulating crystal polymorphism can provide novel strategies for materials discovery in organic optoelectronics. In this paper, a series of seven ester-terminated three-ring phenylene ethynylenes (PEs) exhibit structure-dependent polymorphism wherein alkyl chain length modulates the propensity to form violet or green fluorescent solid phases, as well as tunable thermal and mechanofluorochromic (MFC) transitions. These compounds harness "soft" non-covalent control to achieve polymorphism: the electronic substituent effect of the ester groups weakens the fluoroarene-arene (ArF-ArH) interactions that typically direct crystal packing of this class of compounds, increasing competitiveness of other interactions. Small structural modifications tip this balance and shift the prevalence of violet- or green-emitting polymorphs. Compounds with short alkyl chain lengths show both violet and various green fluorescent polymorphs, while the violet fluorescent form dominates with alkyl lengths longer than butyl. Further, thermally induced green-to-violet fluorescent crystal-to-crystal transitions occur for single crystals of CO2-1 and CO2-3. Finally, the PEs show reversible violet-to-green mechanofluorochromism (MFC), with temperature required for reversion of this MFC decreasing with alkyl chain length. We therefore present this design of directional but weak interactions as a strategy to access polymorphs and tunable stimuli-responsive behavior in solids.

Figure 1.

Weak non-covalent control in ester-terminated PEs. a) Substituents in PE main chain affect the ArF-ArH stacking interaction that twists the backbone. When X = CO₂Me, aromatic stacking interactions between PEs become competitive with ArF-ArH programmed twisting, resulting in polymorphism. b) Lengths of alkyl chains alter polymorphism in ester-terminated PEs by tipping the balance of competing interactions; short chain lengths result in multiple polymorphs, while long chain lengths favor the violet fluorescent phase.

Figure 2.

(Top) Absorbance and emission spectra of CO2-R compounds in chloroform solution (ε ~ 35000 M⁻¹cm⁻¹ (315 nm) in all solvents). (Bottom) Excitation and emission spectra in thermally annealed thin films, λ_ex = 330 nm for solution and film spectra.

Figure 3.

Emission Spectra (left) and DSC thermograms (right) for polymorphs of CO2–1 and CO2–3 obtained by recrystallization. Solid lines used for polymorphs whose crystal structures have been solved, and dashed lines used to indicate absence of a corresponding solved crystal structure. λ_ex = 330 nm.

Figure 4.

a) Structures from violet emitting crystals of CO2–1 - CO2–4 showing twisted PEs. b) View of PEs along face of ArF ring, showing lateral displacements with respect to PE main chain. c) Intramolecular and intermolecular ArF-ArH stacking in CO2–3. d) Only intermolecular ArF-ArH stacking observed in CO2–4. Second PE highlighted magenta for clarity, with short contacts in ArF-ArH displayed. Hydrogens omitted for clarity.

Figure 5.

Structures of blue-green emitting crystals showing coplanar PE backbones and aggregated packing of PEs in a) CO2–3B and b) CO2–1C, which includes a CHCl₃ molecule. Hydrogens omitted for clarity, and aggregated PEs highlighted magenta.

Figure 6.

Thermal transition of CO2–3B. (Left) PXRD (λ = 1.54184 Å) shows patterns of green and violet emitting polymorphs at room temperature and after heating to 120 °C, with accompanying photos of green emissive CO2–3B before and after heating. Corresponding excitation and emission spectra (top and bottom right, respectively) of the polymorphs before and after heating indicate transformation to violet fluorescence after heating. λ_ex = 330 nm.

Figure 7.

Mechanofluorochromism in CO2-R films. (Left) Representative MFC response in emission spectra of CO2–1, grinding with a spatula and thermally annealing at 180 ᵒC. (Right) Plot of emission maxima of ground films while heating at 10 ᵒC increments to 100 ᵒC demonstrating how compounds with longer alkyl chain recover violet fluorescence across lower temperatures. See corresponding images and spectra, Fig S10. λ_ec = 330 nm.

Scheme 1.

Synthesis of ester-terminated PEs with varying alkyl chain lengths.