LiquidBisazobenzenes as Molecular Solar Thermal Fuel with Enhanced Energy Density Masa-aki Morikawa,* 1,2 Yuta Yamanaka, 1 and Nobuo Kimizuka* 1,2 1 Department of Applied Chemistry, Graduate School of Engineering, Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan 2 Center for Molecular Systems (CMS), Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan E-mail: morikawa.masa-aki.973@m.kyushu-u.ac.jp (M. Morikawa), kimizuka.nobuo.763@m.kyushu-u.ac.jp (N. Kimizuka) Liquid molecular solar thermalfuels containing m-bisazo- benzene units are newly developed. Efficient E-Z photoisome- rization was observed both in solutions and in neat liquids, with the thermal stability of Z isomers comparable to those of azobenzene derivatives. The heat storage capacities of liquid m-bisazobenzenes (230262 J/g) are larger than that previously reported for a liquid azobenzene solar thermalfuel (168 J/g). This work provides a new design guideline for enhancing gravimetric energy density in condensed molecular solar thermal fuels. Keywords: Liquid bisazobenzene | Photoisomerization | Molecular solar thermal energy storage The development of advanced solar power utilization technology is becoming an increasingly important issue in order to solve global environmental and energy problems. 13 Among them, solar energy storage to photo-switchable molecules and on-demand release of heat energy, 47 which is referred to as molecular solar thermalfuels (MOST or STFs) in recent years, has drawn renewed attention. 814 In molecular STFs systems, photon energy absorbed by photoresponsive molecules is stored in metastable photoisomers as strain energy in chemical bonds. The stored energy is released upon their structuralisomerization to the thermally stable forms, which are triggered by external stimuli such as light and heat, occasionally in the presence of catalysts. Several photochromic pairs of isomers as exemplified by norbornadiene/quadricyclane (NBD/ QC), 79,1113,15 dihydroazulene/vinylheptafulvene, 16 and E-azo- benzene/Z-azobenzene 6,10,13,14,1719 have been used to demon- strate the performance as molecular STFs, in most cases in solutions. A familyof azobenzene has been intensively inves- tigated among these chromophores because of their photo- chemical stability and ease of synthesis. However, azobenzene molecules as molecular STFs have several drawbacks. First, the isomerization enthalpy, ¦H isom , (the energy difference between Z- and E-isomers) is limited to around ¹50 kJ/mol. 6,8 Second, their performance as molecular STFs has been investigated in solutions because Z-to-E photoisomerization is generallysig- nificantly suppressed in the condensed state such as crys- tals. 8,20,21 Accordingly, dilution with solvents is necessary to operate molecular STFs, which inevitably decreases the total energy density and diminishes their value. A solution to these criticalissues is to develop undiluted STFs that operate in the condensed state. We have recently solved this problem and developed room-temperature liquid azobenzene and arylazopyrazole derivatives that are liquefied at room temperature by introducing branched 2-ethylhexyl groups. 22,23 The liquid azobenzene showed photon energy storage properties under neat conditions and marked a ¦H isom of ¹52 kJ/mol, which is consistent with the isomerization enthalpy of unsubstituted azobenzenes. 22 To further overcome the heat capacity limitation of azobenzene chromophores, we pioneered a methodology to add latent heat by introducing phase transition phenomena, i.e., by realizing photo-induced liquefaction of E-isomer crystal and crystallization of Z-isomer in the course of Z-to-E back iso- merization. 24 By synchronizing the Z-to-E thermalisomerization and adding up the latent heat generated during crystallization of the E-isomer, a total heat storage capacity of ¹97 kJ/mol was achieved which is nearly twice as large as the isomerization enthalpy of the azobenzene unit. These studies solved the previous issues of molecular STFs and opened up a new research field of phase-changing STFs which directed subsequent research in this field. 2528 One of the key remaining issues is developing methodologies to enhance the gravimetric energy density, which is inversely proportional to the molecular weight and enhancing the mole fraction of Z-isomers at the photo- stationary state (PSS). A straightforward approach for the former issue is to design photoresponsive multichromophoric molecules that serve as molecular STFs in the condensed state while keeping the mole- cular weight as small as possible. It has already been reported that the dimeric and trimeric NBD/QC pairs show enhanced energy density and storage time in solution. 29 Photoisomerization of multi-azobenzene derivatives has been investigated both exper- imentally in solutions, 3034 and theoretically. 35,36 Although the immobilization of bisazobenzene derivatives on templates has been predicted to increase the energy density of hybrid STFs, 36 energy density determined for a Z-1,3,5-tris(arylazo)benzene powder was low (43.454.4 kJ/mol), which has been ascribed to the coexistence of various photoisomers. 37 There exist no examples of multi-azobenzene derivatives that show reversible E-to-Z photoisomerization in the solvent-free condensed phase, and a new molecular design principle is needed to go beyond the condensed-phase STFs, which is the most promising familyof molecular STFs. We herein report the development of liquidbisazobenzene derivatives (1 and 2,Figure 1a) and their excellent properties as solvent-free molecular STFs. Bis-azobenzene-containing chro- mophoric liquids 1, 2 were designed based on the melting point lowering effect of the branched 2-ethylhexyl group. 22,23,3840 Two azobenzene units are connected via a phenyl ring ina meta orientation, because each azobenzene chromophore in m-bisazobenzenes behaves nearly independently in solution and shows photoisomerization characteristics similar to mono- azobenzene derivatives. 31,33,34 Also, the bent m-bisazobenzene structure would be advantageous to lower the melting point compared with the para isomer. In compound 1, the 2- ethylhexyloxy group is introduced only in one of the azobenzene CL-210822 Received: December 26, 2021 | Accepted: January 24, 2022 | Web Released: February 1, 2022 402 | Chem. Lett. 2022, 51, 402–406 | doi:10.1246/cl.210822 © 2022 The Chemical Society of Japan