A DFT Study of Multimode Switching in a Combined DHA/VHF-DTE/DHB System for Use in Solar Heat Batteries

Abstract

Development of photochromic molecules for solar energy storage has two major challenges: (i) to store a sufficient amount of energy in the metastable isomer and (ii) to control the energy-releasing step, i.e., setting it on hold until the energy is needed. Combining the dihydro­azulene/vinyl­heptafulvene (DHA/VHF) photo-/thermoswitch with the dithienyl­ethene/dihydro­thieno­benzothiophene (DTE/DHB) photoswitch could potentially meet these challenges. The combined multimode switch is studied by density functional theory in order to predict its energy storage properties and spectral behavior in various solvents before discussing its suitability for use in solar heat batteries. An energy storage capacity of 0.17 MJ/kg is calculated which corresponds to a specific energy of 46 Wh/kgslightly larger than that of a common lead–acid car battery (∼40 Wh/kg) but still only one-fourth of lithium-ion batteries (100–250 Wh/kg). The usual trend for 1,8a-dihydroazulene-1,1-dicarbonitrile and its derivatives is for their energy storage capacities to decrease dramatically as the solvent polarity is increased, but it is found that solvent effects are not as significant for the combined DHA–DTE system. Furthermore, the spectral data indeed imply a possibility of controlling the back-reaction from the energy-rich metastable isomer by light stimulus, thus enabling one to release the stored energy upon request. This work thereby represents important progress toward efficient long-time solar energy storage in photochromic closed-cycle molecular systems.

Excited‐State Topology Modifications of the Dihydroazulene Photoswitch Through Aromaticity

“…We note that the IRCs correspond to the minimum energy path on the S 1 PES, which is different from earlier mappings in which ground state IRCs were vertically projected onto the excited states . Therefore, the PES will look qualitatively different, and, as seen in Figure , no avoided crossing region is observed.…”

“…described the surface for model systems with no nitrile groups on the C1‐position, showing a conical intersection sloping on the DHA side. Other groups have estimated the surfaces, but a full description of the excited state surfaces for a non‐model system has yet to be completed.…”

Recent Implementations of Molecular Photoswitches into Smart Materials and Biological Systems

“…Due to its one‐way (DHA→VHF) photochromism (VHF←DHA isomerization can only be induced thermally), it is also a promising switch for solar thermal energy storage and solar heat batteries, particularly upon replacing the phenyl ring with pyridine (which allows for reversible protonation that influences the VHF lifetime), or by design of a multimode switch combining DHA/VHF with DAEs …”

Computational and Experimental Evidence of Two Competing Thermal Electrocyclization Pathways for Vinylheptafulvene

“…Earlier computational work includes a single TS, which we identify as the Disrot‐TS . Our identification of these two TSs has resulted in inclusion of these in the theoretical work in a number of recent publications …”

“…[9,25,26] Our identification of these two TSs has resulted in inclusion of these in the theoretical work in an umber of recent publications. [22,[27][28][29] The Gibbs energy differences between the lowest energy VHF conformer and TS, DG VHF!TS ,a re shown in Figure 2f or each investigated DHA/VHF system in vacuum and the four solvents (benzene, tetrahydrofuran, acetone, and acetonitrile). No diastereoisomerse xist for CN-DHA/CN-VHF and, therefore, CN-VHFs imply ring-closes to form CN-DHAv ia the lowestenergy TS, in this case the CN-Conrot-TS in all investigated solvents (Figure 2, left panel).…”