Nonvolatile liquid anthracenes for facile full-colour luminescence tuning at single blue-light excitation

Abstract

Nonvolatile room-temperature luminescent molecular liquids are a new generation of organic soft materials. They possess high stability, versatile optical properties, solvent-free fluid behaviour and can effectively accommodate dopant dye molecules. Here we introduce an approach to optimize anthracene-based liquid materials, focussing on enhanced stability, fluorescence quantum yield, colour tunability and processability, with a view to flexible electronic applications. Enveloping the anthracene core in low-viscosity branched aliphatic chains results in stable, nonvolatile, emissive liquid materials. Up to 96% efficient energy-transfer-assisted tunable emission is achieved by doping a minute amount of acceptor dye in the solvent-free state. Furthermore, we use a thermoresponsive dopant to impart thermally controllable luminescence colours. The introduced strategy leading to diverse luminescence colours at a single blue-light excitation can be an innovative replacement for currently used luminescent materials, providing useful continuous emissive layers in developing foldable devices.

Figure 1. Molecular structure of anthracenes and dopants.

The figure shows the chemical structure of anthracenes (1–4) and dopants (D1 and D2) used in this study.

Figure 2. Characterization of solvent-free room-temperature liquid anthracenes.

(a) DSC thermograms in the cooling trace showing the glass-transition temperatures (T_g). (b) XRD diagrams with small-angle X-ray scattering profiles as insets, variation of (c) storage modulus (G′)(circles) and loss modulus (G″) (squares), as well as (d) complex viscosity (η*), (diamonds) versus angular frequency on double logarithmic scale, of 1 (blue markers) and 2 (red markers).

Figure 3. Photostability studies.

Comparison of the variation of (a) fluorescence intensity at 430 nm in solvent-free states of 1 and 4 (Solvent-free, Film) supported on a quartz plate and in dichloromethane solution (Solution) (c=1 × 10⁻⁴ M, l=1 mm, λ_ex=375 nm) of 1 and 4. (b) Variation of the ¹H NMR peak area of aromatic protons of 1 (7.7 p.p.m.) and 4 (7.5 p.p.m.) in dichloromethane-d₂.

Figure 4. Optical features of liquid anthracenes.

Comparison of the normalized (a) absorption and (b) fluorescence spectra of 1 and 2 in a solvent-free bulk liquid state (solvent-free) supported on a quartz plate at room temperature and in dichloromethane solution (Solution) (c=5 × 10⁻⁵ M, l=1 mm, λ_ex=375 nm). Photographs of 2 under (c) visible and (d) ultraviolet light (365 nm).

Figure 5. Energy-transfer-assisted tunable emission.

(a) Normalized emission spectral changes of 1 upon increasing the concentration of D1 (λ_ex=350 nm). (b) Emission lifetime decay profile of 1 (blue squares) (λ_ex=377 nm) in the presence of 0.3 (orange squares) and 0.5 mol% (pink squares) of D1 monitored at 430 nm supported on quartz plate, and inset shows the decay profiles of D1 (0.3 mol%) in the matrix of 1 (red squares) and D1 alone (blue squares), monitored at 475 nm (λ_ex=377 nm). IRF corresponds to Instrument Response Function. (c) Photographs of the luminescent thin films of 1 (i) and 1 doped with D1 (ii–v), which are correlated to the emission spectra of (a).

Figure 6. Tunable emission colours from liquid composites.

(a) Photographs of the luminescence colour tunability and thermoresponsive feature of the composites of 1, D1 (0.5 mol%) and D2 (5 mol%). (b) Thermoreversible luminescence spectral changes of the composite of 1, D1 and D2, supported on a quartz plate (λ_ex=375 nm) upon heating; inset shows the corresponding emission changes from 600 to 640 nm. (c) Commission internationale de l′éclairage coordinate values of the overall colour tunability achieved by doping of 1 with D1 and/or D2, as well as with temperature. The Commission internationale de l′éclairage coordinate also includes 2. The indications of ii–v are from the composites of 1 and D1 (Fig. 5), vi–x are from the composites of 1 and D2 (Supplementary Fig. S14a) and xi–xv are from composites of 1, D1 and D2 (b).