Enhancing the Solid-State Emission of Carbonyl-Containing Compounds by Means of Introducing a Bifuran Core

Abstract

Aromatic aldehydes and ketones are attractive as luminescent materials because they exhibit room temperature phosphorescence. However, an absence of significant luminescence in the solid state limits their practical application. This study investigates a series of bifuran dialdehydes and diketones (F1-F3) and their bithiophene analogues (T1-T3) and compares their photophysical properties in solution and the solid state. The incorporation of carbonyl groups into bifuran cores significantly enhances their solid-state fluorescence, with solid-state quantum yields reaching up to 35%, in contrast to low fluorescence for the thiophene-based analogues. Structural analysis via X-ray crystallography reveals that bifuran derivatives exhibit tighter packing and more rigid molecular backbones, which contributes to the observed aggregation-induced emission. The carbonyl group also stabilizes the furan core compared with unmodified bifurans.

Keywords: fluorescence; oligofurans; oligothiophenes; organic electronic materials; organic light-emitting diodes.

1. Structural Formulas of Bifurans F1–F3 and Bithiophenes T1–T3

2. General Synthesis and Yields of Bifurans F1–F3 and Bithiophenes T1–T3

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(a) Single-crystal molecular conformations (top) and packing (bottom). (b–d) Calculated rotational barriers for the (b) dimer core in aldehydes, (c) carbonyl moiety in aldehydes, and (d) phenyl moiety in phenyl ketones.

2

Normalized absorption (red line) and emission spectra in DCM (blue line) and in the solid state (green line). Excitation wavelength: in DCM, F1–F3 and T3 at 365 nm and T2 and T3 at 380 nm, and in the solid state, F1–F3 and T1 at 415 nm, T2 at 430 nm, and T3 at 370 nm. All measured in ambient conditions, except F1, which was measured under N₂.

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Fluorescence quantum yields (Φ_f) for biofuran-based (F1–F3) versus biothiophine-based (T1–T3) carbonyl-containing compounds in solution (blue), in the solid state (green), and embedded in PMMA (orange).

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X-ray structures of trimers of Fn (left) and Tn (right). For accuracy, the hydrogen atoms were optimized at the CAM-B3LYP/6-31-G­(d)/D3 level.