Anisotropic fluid with phototunable dielectric permittivity

Abstract

Dielectric permittivity, a measure of polarisability, is a fundamental parameter that dominates various physical phenomena and properties of materials. However, it remains a challenge to control the dielectric permittivity of materials reversibly over a large range. Herein, we report an anisotropic fluid with photoresponsive dielectric permittivity (200 < ε < 18,000) consisting of a fluorinated liquid-crystalline molecule (96 wt%) and an azobenzene-tethered phototrigger (4 wt%). The reversible trans-cis isomerisation of the phototrigger under blue and green light irradiation causes a switch between two liquid-crystalline phases that exhibit different dielectric permittivities, with a rapid response time (<30 s) and excellent reversibility (~100 cycles). This anisotropic fluid can be used as a flexible photovariable capacitor that, for example, allows the reversible modulation of the sound frequency over a wide range (100 < f < 8500 Hz) in a remote manner using blue and green wavelengths.

Fig. 1. Phototunable system with gigantic dielectric properties.

a Schematic illustration of a liquid-crystalline (LC) blend consisting of a ferronematogen (DIO) and an azobenzene-tethered phototrigger (Azo-H, Azo-Me or Azo-F) with phototunable dielectric permittivity. b–j UV–visible spectra of Azo-H (b, c), Azo-Me (e, f) and Azo-F (h, i) in MeCN after light irradiation to induce their cis states [λ_ex of Azo-H (b) and Azo-Me (e): 365 nm; λ_ex of Azo-F (h): 525 nm] and after light irradiation to induce their trans states [λ_ex of Azo-H (c) and Azo-Me (f): 450 nm; λ_ex of Azo-F (i): 415 nm]. Time-dependent absorbance profiles of the phototrigger molecules (cis states in dimethyl sulfoxide; 25 °C in dark) under excitation at a fixed wavelength [Azo-H (d): 355 nm; Azo-Me (g) and Azo-F (j): 340 nm]. Red lines indicate linear fits [R² = 1.00, 0.99, and 0.98 for panels (d), (g) and (j), respectively].

Fig. 2. Photoinduced transition between mesophase (M) and ferronematic phase (N_F).

a Temperature-dependent dielectric permittivity of liquid-crystalline (LC) blends ([Azo-F] = 0–8 wt%) in cis (orange plots) and trans (purple plots) states. The cis and trans states were realised by green light (GL; 525 nm) and blue light (BL; 415 nm) irradiation, respectively. b, c Phototunable permittivity range, (ε‘_max − ε‘_min); b and relative tunability [(ε‘_max − ε‘_min)/ε‘_max; c of the LC blends ([Azo-F] = 1–8 wt%), where ε‘_max and ε‘_min are the maximum permittivity in the trans state and the minimum permittivity in the cis state, respectively, at a fixed temperature. d Temperature-dependent phase behaviours of the LC blends ([Azo-F] = 0–8 wt%) in cis and trans states. e Polarised optical microscopy images under crossed Nicols of an LC blend ([Azo-F] = 4 wt%) in cis (lower) and trans (upper) states after passing through a long path filter (λ > 550 nm) at a constant temperature [left: 45 °C (cyan symbols in (d)); middle: 52 °C (green symbols in (d)); right: 60 °C (magenta symbols in (d))]. Scale: 100 μm. f–i Two-dimensional (2D; upper) and one-dimensional (1D; lower) X-ray diffraction profiles of an LC blend ([Azo-F] = 4 wt%) under a magnetic field (b; ~0.5 T): in a trans state (60 °C; f), in a trans state (52 °C; g), in a cis state after GL irradiation (52 °C; h), and in a trans state after subsequent BL irradiation (52 °C; i). Black double arrows indicate the direction of an applied magnetic field. Red lines in the 1D profiles are the best fits.

Fig. 3. Ultralarge-range phototunability of dielectric permittivity.

a, b Dielectric spectra [left: relative dielectric permittivity (ε′); right: dielectric loss (ε″)] of a liquid-crystalline (LC) blend ([Azo-F] = 4 wt%) as a function of frequency (f = 1–10⁶Hz) during green light (GL) (a) and blue light (BL) (b) irradiation for 30 s at 50 °C. c Dielectric permittivity changes (f = 1 kHz; 50 °C) during cis/trans isomerisation cycles (lower) and their enlarged profiles as a function of relative time (upper). Irradiation times for GL and BL were 20 and 40 s, respectively. d Stimuli-responsive tunability of dielectric permittivity of reported materials based on organic (circle), inorganic (square), and organic/inorganic (triangle) components. Stimuli-responsive permittivity range (ε′_max − ε′_min) and relative tunability [(ε′_max − ε′_min)/ε′_max] are shown. Photo-, thermo-, electro-, mechano- and chemo-responsive materials are represented by yellow, magenta, green, grey and cyan symbols, respectively.

Fig. 4. Demonstration of a photovariable capacitor operation.

a Schematic illustration of a sound-generating system with a photovariable capacitor consisting of a liquid-crystalline (LC) blend ([Azo-F] = 4 wt%). Sounds generated from an electric speaker were recorded using a microphone. b,c Real photoimages (upper) of a photovariable capacitor during blue light (BL) (b) and green light (GL) (c) irradiation and sound spectra (lower) obtained after BL (b) and GL (c) irradiation for 10 s. d Sound spectrogram generated by short-time Fourier transform of the audio recorded during alternate GL/BL irradiation for 10 s (Supplementary Audio 1). White dotted lines in the spectrogram represent the sound spectra in (b) and (c). White blurry regions (f < 1 kHz) denotes the resolution limit of a microphone.