Designing dicationic organic salts and ionic liquids exhibiting high fluorescence in the solid state

Abstract

Dicationic ionic liquids (DILs) are emerging as a powerful, next-generation approach to designing applied ILs because of their superior physicochemical properties as well as their diverse complexity and tunability for task specific applications. DILs are scarce in the literature compared to monocationic ILs (MILs), and one of their main issues is their expected tendency to possess higher melting temperatures. A series of 1,4-bis[2-(4-pyridyl)ethenyl] benzene and 1,4-bis[2-(2-pyridyl)ethenyl]benzene quaternary salts (Q-BPEBs) with different counterions (bromide, tosylate, and triflimide) and carbon chain lengths (C₆, C₉, and C₁₂) have been synthesized for their potential as DILs with strong photoluminescent properties in the solid state. All Q-BPEB salts demonstrated robust thermal stabilities as determined by thermogravimetric analysis (TGA). The differential scanning calorimetry (DSC) thermograms for Q-BPEB tosylates and triflimides displayed crystalline polymorphisms before melting transitions as verified by polarizing optical microscopy (POM). The Q-BPEB bromide and tosylate salts all showed high melting points of above >170 °C because of their dicationic rigid structures and strong ionic interactions of their anions. Once the Q-BPEB tosylates were exchanged with triflimide ions, para- isomers 1aTf ₂ N, 1bTf ₂ N, and 1cTf ₂ N still possessed very high melting points (>225 °C), however, the ortho- isomers 2aTf ₂ N, 2bTf ₂ N, and 2cTf ₂ N exhibited melting points lower than 100 °C, classifying them as DILs. Their photoluminescent properties were also studied in methanol with the emission values of λ_em = 476-482 nm for the para- isomers and those of λ_em = 448-453 nm for the ortho- isomers. In the solid state, the Q-BPEB salts exhibited strong fluorescence with quantum yields of up to 50 %. The relatively simple synthesis of these fluorescent dicationic organic salts and ILs are pertinent towards the scarcity of these materials in the literature and provide a deeper insight on the design of fluorescent ILs containing more than one charge center.

Keywords: Fluorescence; Optoelectronics; Organic salts; Solid state fluorescence; Structural design; Synthesis.

Fig. 1.

TGA thermograms of 1a-1cOTs and 1a-1cTf₂N (left) and 2a-2cOTs and 2a-2cTf₂N (right) obtained at heating rate of 10 °C/min in nitrogen.

Fig. 2.

DSC thermograms of 1a-1cOTs and 1a-1cTf₂N obtained at both heating and cooling rates of 10 °C⋅min⁻¹ in nitrogen.

Fig. 3.

Photomicrographs of 1bTf₂N (left) acquired at 87 °C and 2cOTs (right) acquired at 138 °C under crossed polarizers exhibiting birefringent crystal phases at 400x magnification.

Fig. 4.

DSC thermograms of 2a-2cOTs and 2a-2cTf₂N obtained at both heating and cooling rates of 10 °C⋅min⁻¹ in nitrogen.

Fig. 5.

Room-temperature emission spectra (right) and excitation spectra (left) of 1aBr, 1aOTs, and 1aTf₂N (top) and 2aBr, 2aOTs, 2aTf₂N (bottom) dissolved in methanol (concentration 5.0 × 10⁻⁵ M). The emission spectra were measured by exciting the solutions around the lowest-energy absorption peaks. Excitation spectra were recorded by monitoring the fluorescence around 478 nm or 451 nm as the wavelength of the excitation wavelength was varied.

Fig. 6.

(a) Fluorescence of 1a and 2a Q-BPEB salts in the solid state using a handheld UV lamp (bottom) and appearance of those salts in ambient light (top).

Scheme 1.

Synthesis of α/γ-alkylpicolinium bromide salts 1a-1c and 2a-2c.

Scheme 2.

Synthesis of ortho- and para- Q-BPEB salts 1a-1cBr and 2a-2cBr.

Scheme 3.

Synthesis of para- Q-BPEB tosylates and triflimides 1a-1cOTs and 1a-1cTf₂N via metathesis reaction. Ortho- Q-BPEB salts 2a-2cOTs and 2a-2cTf₂N were synthesized according to the identical procedures adopted for the para- isomers.