Photoliquefaction and phase transition of m-bisazobenzenes give molecular solar thermal fuels with a high energy density

Abstract

A series of m-bisazobenzene chromophores modified with various alkoxy substituents (1; methoxy, 2; ethoxy, 3; butoxy, 4; neopentyloxy) were developed for solvent-free molecular solar thermal fuels (STFs). Compounds (E,E)-1-3 in the crystalline thin film state exhibited photoliquefaction, the first example of photo-liquefiable m-bisazobenzenes. Meanwhile, (E,E)-4 did not show photoliquefaction due to the pronounced rigidity of the interdigitated molecular packing indicated by X-ray crystallography. The m-bisazobenzenes 1-4 exhibited twice the Z-to-E isomerization enthalpy compared to monoazobenzene derivatives, and the latent heat associated with the liquid-solid phase change further enhanced their heat storage capacity. To observe both exothermic Z-to-E isomerization and crystallization in a single heat-up process, the temperature increase of differential scanning calorimetry (DSC) must occur at a rate that does not deviate from thermodynamic equilibrium. Bisazobenzene 1 showed an unprecedented gravimetric heat storage capacity of 392 J g^-1 that exceeds previous records for well-defined molecular STFs.

Scheme 1. Chemical structure of m-bisazobenzene derivatives (E,E)-1–4.

Fig. 1. Photoisomerization properties of bisazobenzene derivatives (1) in acetonitrile solution (concentration; 20 μM, 25 °C). (a) UV-vis absorption spectra of (E,E)-1 before (solid line) and after photoillumination at 365 nm (dashed line) and 520 nm (dotted line) for 3 min. (b) Changes in absorbance at 355 nm during repetitive photoillumination at 365 nm (open circle) and 520 nm (filled circle) for 3 min.

Fig. 2. (A) Optical microscopy images (top; bright field, bottom; under crossed polarizers) and (B) visible absorption spectra of drop cast film of 1 on a glass substrate. (a) As prepared, (b) after photoillumination with a mercury lamp (bandpass filter: 380–420 nm) for 5 s, (c) after photoillumination (bandpass filter: 510–560 nm) for 5 s, and (d) after standing for 5 min in the dark. (C) The crystal structure of (E,E)-1 along the b-axis.

Fig. 3. (A) DSC thermogram of 1 after UV light irradiation. The total percentage of Z% (86%) was determined by ¹H-NMR spectroscopy. Heating rate, 0.2 °C min⁻¹. The inset shows photographs of the DSC samples at each temperature. (B) Powder X-ray diffraction patterns obtained for 1 prepared on a glass substrate: (a) hot melted at 150 °C and allowed to stand at room temperature (total E%: 100%), (b) after UV irradiation at room temperature (total Z%: 83%), and (c) after standing at 60 °C overnight (total E%: 100%). A high-pressure mercury lamp was used for UV illumination (λ = 365 ± 10 nm, power density; ∼300 mW cm⁻²).

Fig. 4. (A) The total gravimetric energy density is shown as a sum of the isomerization enthalpy and the E-isomer crystallization enthalpy estimated from the exothermic peaks in the first heating process of DSC measurements. Compounds 1–4 and two reported compounds A1 and A2 are shown for comparison. (B) Schematic illustration of the thermally induced Z-to-E isomerization of m-bisazobenzenes and their crystallization processes. Both the Z-to-E isomerization enthalpy (ΔH_Z–E) and the latent heat of phase change (ΔH_L–S) are continuously obtained in a single heating step.