VHF-DTE

Article pubs.acs.org/JPCC

A DFT Study of Multimode Switching in a Combined DHA/VHF-DTE/ DHB System for Use in Solar Heat Batteries Anders S. Gertsen, Stine T. Olsen,* Søren L. Broman, Mogens Brøndsted Nielsen, and Kurt V. Mikkelsen* Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen, Denmark S Supporting Information *

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.

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INTRODUCTION The exploitation of solar energy is an ever-emerging area of interest. Molecular photoswitches are an intriguing type of compound whose states can be controlled by photon absorption and thereby allowing e.g. the storage of thermal solar energy.1−5 Preceding work on the energy storage properties of one such system, namely the dihydroazulene/ vinylheptafulvene (DHA/VHF) photo-/thermoswitch and its derivatives, motivates further investigation of related structures with enhanced controllability and optimized photon absorption for use in solar heat batteries.6−8 A promising approach to obtain energy densities of these close to the proposed maximum of 1 MJ/kg is utilizing two or more photons in multimode or multifold switches.1,9,10 In addition, the approach of multimode switches might hold the key to controlled DHA/ VHF energy release, which until now has relied on balancing the thermodynamical equilibrium and/or catalyzing the thermal back-reaction.6,11 The diphotochromic photo-/thermoswitch addressed herein is shown in the middle panel of Figure 1. 1,2-Bis(2,5-dimethyl3-thienyl)-1,8a-dihydroazulene-1-carbonitrile (1), henceforth abbreviated DHA-“DTE”, serves as starting point. With this specific placement of the 2,5-dimethylthiophene substituents, a second photoswitch, namely dithienylethene/dihydrothienobenzothiophene (DTE/DHB),12,13 is introduced by photo© XXXX American Chemical Society

isomerization of 1 to its corresponding VHF compound 2. The combination of these two chromophores has previously been reported by placing dithienyl groups at positions 2 and 3 of DHA, providing the DHA-DTE shown in the top panel of Figure 1.14 This system underwent light-induced conversion of the DHA unit into a VHF and of the DTE unit into a DHB. The VHF unit thermally reverted to DHA, while the DHB unit returned to DTE upon irradiation.15 The new molecular design (cf. middle panel of Figure 1) is based on an alteration of the DHA unit to 1 that rules out the possibility of a four-step cyclic reaction pathway and allows for the formation of a thermally stable high-energy “VHF”-DHB isomer, 3, upon photoisomerization of VHF-DTE 2. The lack of a direct reaction pathway from 3 to 1 allows selective release of the stored energy by a light stimulus. Thus, when having absorbed two photons of a total energy of hν1 + hν2, “VHF”-DHB 3 will store a part of this until a back-reaction is triggered by a new photon of energy hν3. Upon relaxation to 2 and subsequently to 1, the stored energy will be released as thermal energy, thereby heating the solution. Received: October 26, 2016 Revised: December 6, 2016 Published: December 6, 2016 A

DOI: 10.1021/acs.jpcc.6b10786 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. Multimode switches combining the DHA/VHF and DTE/DHB photochromes. Note that the parts of the molecules assigned with quotation marks (“DHA”, “DTE”, and “VHF”) need the other parts of the respective molecules to isomerize to become true DHA, DTE, and VHF, respectively. E/Z isomers and s-cis/s-trans conformers of the VHF-DTE compound investigated in this work are shown in the bottom panel.

middle panel of Figure 1).20 Four of eight conformers of compound 2 exhibited antiparallel rotation with respect to the 2,5-dimethylthiophenes, while the rotary search of compound 3 gave rise to three conformers. It is important to stress that even with the geometry constraint of antiparallel 2,5-dimethylthiophenes, several conformers are relevant to this reaction. In pursuance of maintaining a general view of the reaction pathway, we limit ourselves to present results based on only one conformer of each compound 1 and 3 and two of 2. Since a thermodynamical equilibrium is present for the conformers of each compound, it is possible to drive the reaction toward 3. Therefore, the lowest energy conformer of 3 is chosen together with the conformer of 2 corresponding to the 6-π electrocyclic ring-opening of 3. A direct transition between these two was proven by an intrinsic reaction coordinate (IRC) calculation using this ring-opening/closure transition state (TS2) as a starting point. Furthermore, the s-cis of 2 was investigated (cf. Results and Discussion). Although the back-reaction 1 ← 2 is thermally induced and thereby relying on significant structural changes, the choice of conformer of 1 should still meet the following requirements: (i) the 2,5-dimethylthiophenes should be antiparallel, (ii) there should be no high-energetic transition states regarding mutual rotation of the 2,5-dimethylthiophenes in order to end up in 2, and (iii) the stereochemistry of 1 should match the one of 2. A transition state (TS1) corresponding to the 10-π electrocyclic ring-closure of the scis of 2 to a conformer of 1 fulfilling these requirements was determined, and a direct transition between these was also confirmed by an IRC calculation. This resulted in a suitable DHA-“DTE” conformer, yielding all in all four preliminary molecules representing the relevant energy minima of the reaction in Figure 1. These were geometry optimized at the

We here present a theoretical investigation of this not yet synthesized diphotochromic photo-/thermoswitch using density functional theory (DFT) which, with the correct choice of functional, has been shown to reproduce experimental data of various photoswitches quite accurately.7,16−18 Using the M062X functional,19 we substantiate the above predictions of the system’s qualitative behavior and calculate an energy storage capacity of 0.17 MJ/kgalmost a doubling compared to the one of the system synthesized by Mrozek and co-workers (top panel of Figure 1)14,15 which by Perrier and co-workers is calculated to have an energy storage capacity of 0.09 MJ/kg (between DHA-DTE and the metastable s-trans-VHF-“DTE”; cf. top panel of Figure 1).16 This new photo-/thermoswitch thereby represents significant improvements to both energy storage capacity and energy release control which strongly motivates future synthesis.

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COMPUTATIONAL DETAILS In order to determine and evaluate promising structure designs prior to the time-consuming synthesis of new compounds, quantum chemical computations prove their convenience. DFT calculations with the use of the M06-2X exchange-correlation functional19 have by a recent benchmark study been shown to provide rational results for the DHA/VHF photoswitch and its derivatives.7 Subsequent to a 3 kcal/mol cutoff rotary search yielding numerous conformers, seven of a total of 11 conformers of compound 1 were spatially suitable to the proposed reaction pathway in Figure 1 requiring antiparallel rotation of the 2,5dimethylthiophenes (i.e., that the two 2-methyl substituents point toward each other and the two 5-methyl substituents point away from each other as in the VHF-DTE structure in the B


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Figure 2. Overview of the ground state potential energy surface of the photo-/thermoswitch with M06-2X/6-311+G(d) structures in MeCN. The vertical axis indicates free energy relative to the energy of 1 which is set as zero point. The different compounds and their respective relative free energies are listed above and below the energy curve which serves as a guide for the eye. The bold arrows indicate the vibrational motion related to the ring-opening and ring-closure of TS1 and TS2, respectively.

M06-2X/6-311+G(d) level of theory in Gaussian 0921 and confirmed as minima by vibrational frequency analyses in both vacuum and a variety of solvents modeled by the polarizable continuum model in the integral equation formalism (IEFPCM) which is proven to describe solvent effects on organic molecules well.22−24 The optimized structures (see Figure 2 and Supporting Information Figures S1−S7) served as starting points for further calculations of which all presented herein are performed at the M06-2X/6-311+G(d) level of theory. The transition states are as usual determined as firstorder saddle points with one imaginary frequency corresponding to the vibration of the bond breakage/formation. The potential energy surfaces presented are results of intrinsic reaction coordinate (IRC) calculations using the transition state structures (TS1 and TS2) as starting points. A maximum of 100 points on each side of the transition states were determined along the reaction coordinates corresponding to ring-opening/closure after which the vertical excitation energies of each of the resulting geometries were calculated using the time-dependent self-consistent field method (TDSCF) within DFT as implemented in Gaussian 09.21 All thermochemical properties are calculated at a temperature of 298.15 K and a pressure of 1 atm.

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Table 1. Energy Storage Capacities of the System Calculated as the Thermal Free Energy Differences in kJ/mol and Energy Density of the System in MJ/kg ΔG1→2 ΔG2→3 ΔG1→3 energy density

vacuum

CH

toluene

MeCN

H2O

34.91 29.41 64.31 0.171

33.61 29.71 63.33 0.169

33.72 29.53 63.26 0.168

28.86 28.80 57.66 0.154

28.62 28.75 57.37 0.153

to vacuum.7 Consequently, the energy storage in the DHA/ VHF part (ΔG1→2) of this system is in MeCN a factor of 2 higher than that of the parent system, and including the energy storage capacity of the DTE/DHB part (ΔG2→3), a factor of 4 is achieved. The total energy storage capacity of this system is thereby comparable to some of the currently best performing synthesized single DHA/VHF systems and even norbornadiene/quadricyclane (NB/QC) systems.8,25−27 However, due to the larger molecular weight, the 0.17 MJ/kg energy density of this system is inferior to the 0.25 MJ/kg of the lighter monocyano derivative of the parent system8 and is furthermore only one-fourth that of the NB/QC switch (having a calculated energy density of 0.70 MJ/kg at the M06-2X/6-311+G(d) level of theory)26 which most likely constitutes the upper limit of energy storage in closed-cycle photochromic molecules1but it is important to emphasize that both the two sequential single photon absorptions and the possibility of controlling the energy release by simple light stimulus speaks greatly in favor of this system. The back-reaction 2 ← 3 is light-induced and will be discussed below. 1 ← 2 is, on the other hand, expected to be thermally induced from previous studies on DHA/VHF systems.28,29 As seen in Figure 2 or Figure S4, the equilibrium of 2 is shifted toward an s-trans conformation, but in order for the ring-closure 1 ← 2 to occur, 2 must be in an s-cis (10−20 kJ/mol higher in energy than s-trans depending on solvent; for geometry, see Figure 2 or Figure S3).30 The thermal backreaction barrier (TBR) is calculated as the energy difference

RESULTS AND DISCUSSION

Energy Storage Capacities and Thermal Back-Reaction Barriers. Having determined the relevant conformers (optimized geometries presented in Figures S1−S7), the energetics of the photo-/thermoswitch can be investigated. In Table 1, the amount of energy one would be able to store in the system is listed. As seen, the energy storage capacity is larger than 63 kJ/mol in vacuum, cyclohexane (CH), and toluene, while it decreases to approximately 57 kJ/mol in MeCN and H2O. A drop in energy storage capacity of only 6 kJ/mol in highly polar solvents is quite good compared to the parent DHA/VHF system (2-phenyl-1,8a-dihydroazulene-1,1-dicarbonitrile), which suffers from a 50% decrease in MeCN compared C


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however, not an (E) to (1R,8aS) transition, but a (Z) to (1R,8aR) (i.e., from s-cis-(Z)-VHF-DTE to another conformer of DHA-“DTE” than 1; see Figures S9 and S10). Hence, IRC calculations could not directly connect this TSX to neither the s-cis-2 nor 1, but most interestingly this opens for an s-cis-(Z)VHF-DTE to DHA-“DTE” transition, which presumably enables overall faster back-reactions. Despite the lower activation energy of this transition compared to the conrotatoryas predicted by the Woodward−Hoffmann rulesthe transition state (TS1), for which results are reported, is concluded to be the correct one for the chosen conformers of 1 and 2. However, as mentioned in the Computational Details, many conformers and hence transition states are relevant, and in order to keep focus on the important characteristics of this system, not all can be investigated in the present work. Photoactivity. The system can undergo the three different photoactivated reactions presented in Figures 1 and 2: the electrocyclic ring-opening 1 → 2, the electrocyclic ring-closure 2 → 3, and the electrocyclic ring-opening 2 ← 3. A fourth photoactivated reaction is however, possible, namely a E/Zisomerism of 2 which will be discussed below. The photoswitching of DHA/VHF systems can be monitored using different spectroscopies with UV−vis being the most common.9,28,29,32 From the calculated UV−vis absorption spectra (MeCN in Figure 3, vacuum, CH, toluene, and H2O in Figures S11−S14), trends similar to those of the individual DHA/VHF and DTE/DHB chromophores are observed (cf. Figure S15),6,7,12 but obviously the absorbance is red-shifted compared to the individual chromophores due to the enlargement of the π-system in the combined switch. In MeCN, the absorption peak of 1 at 320 nm is on the basis of this designated to the excitation triggering the DHA to VHF photoisomerization. Regarding the E/Z-isomerism of 2, the relative stability of strans-(Z)-VHF-DTE (Figure S5) and 2 (s-trans-(E)-VHFDTE) is negligible at 2 kJ/mol. We believe that the (E) to (Z) is triggered by photons of 405 nm wavelength and the (Z) to (E) by 400 nm in MeCN (cf. Figure 3 and Figures S11−S14 for vacuum and other solvents). When in an (E) configuration, the otherwise VHF characteristic peak of 2 at 400 nm is responsible for a significant absorption red-shift compared to the isolated DTE/DHB photoswitch which strongly indicates that the DTE to DHB photoisomerization is triggered by lower energy photons. The assumption of both photoisomerization reactions being triggered by photons of the same energy is substantiated by plots of the highest occupied and lowest

Table 2. Back-Reaction Barrier of the Thermal Conversion 1 ← 2 Calculated as the Thermal Free Energy Difference in kJ/mol ΔGTS1←2

vacuum

CH

toluene

MeCN

H2O

143.5

138.3

137.0

137.0

136.7

between the transition state of the ring-closure reaction 1 ← 2 (TS1) and the s-cis-(E)-VHF-DTE (s-cis-2). The resulting energies are listed in Table 2. They follow the general trend of decrease with increasing solvent polarity due to the more located charge distribution of the TS relative to the s-cis-2 but stay almost constant around 137 kJ/mol irrespective of solvent polarity. This is a favorable result since the expected trade-off between half-life and energy storage is not present, and one therefore can solvate in low-polarity solvents to maximize energy storage without compromising the desired low half-life. Regarding the size of the barrier, one should note that the thermally stable (and thereby energy storing) compound should be 3 and not 2 in this system; i.e. the TBR for the reaction 1 ← 2 should be as low as possible for the heat release to occur as fast as possible. The half-life of 2 can be estimated using the Eyring equation and the fact that the ring closure is a first-order reaction. This estimation (109 s < τ1/2 < 1011 s) indicates that the barriers listed in Table 2 are, however, still too large for a quick heat release at room temperature. It is though well-known that these barriers are overestimated at the M06-2X/6-311+G(d) level of theory compared to experimentwhy the TBR may well be overcome at elevated temperatures.7 If not, external stimuli from weak Lewis acids have shown to reduce half-lives of the VHF species in DHA/ VHF systemswhy catalysis of the TBR could provide an alternative solution.6 Furthermore, attachment of an electrondonating group on the seven-membered ring in DHA/VHF systems has been shown to reduce their half-lives11,31why there most likely are several options to promote the VHF to DHA conversion in this combined system, too. The thermally activated ring-closure 1 ← 2 is a 10-π electrocyclic reaction, and the energetically favorable pathway is therefore expected to be a disrotatory transition with respect to orbital symmetry according to the Woodward−Hoffmann rules. As seen in Figure 2, the s-cis-2 is an (E)-isomer, while the product of the back-reaction (1) is an (1R,8aR)-isomer; i.e., the reaction is conrotatory. To address this and determine a possible lower energy transition state, an additional one was investigated (TSX, see Figure S8). Although having a lower energy (27.05 kJ/mol lower than TS1 in a vacuum) and exhibiting a disrotatory ring-closure mechanism, it was,

Figure 3. Calculated UV−vis absorption spectra with oscillator strengths of 1, 2, and 3 in MeCN. Gaussian broadening functions with a 0.4 eV standard deviation are used. D


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Figure 4. Highest occupied and lowest unoccupied molecular orbitals of 2 (bottom and top, respectively). Arrows mark the parts relevant for the E/ Z-isomerization and for the electrocyclic ring closure. An isovalue (contour threshold) of 0.02 au is used in the visualization.

Figure 5. Calculated potential energy surfaces of the S0 state and the S1, S2, and S3 Franck−Condon states in the vicinity of TS1 and TS2 along the reaction coordinates corresponding to ring opening/closure in MeCN. Arrows indicate in which direction on the IRC surface the different isomers and conformers of the system lie.

though a part of the VHF-DTEs will end up in a (Z), this is still only metastable and can transfer to the DHA-“DTE” species upon thermal activation (cf. discussion in the previous subsection). The stored energy is therefore still extractable from (Z)-VHF-DTE which is a very positive results that might facilitate even higher energy release control as this backreaction through TSX happens on a different time scale than the one through TS1. The TS2 presented in Figure 2 and Figure S6 is according to the Woodward−Hoffmann rules the energetically lowest possible. Since the 2 ⇌ 3 isomerization is a 6-π electrocyclic reaction which is photoinduced both ways, a conrotatory transition is expected to be lower in energy. As 3 is an antiproduct with respect to the methyl substituents and 2 is in an (E), we conclude that TS2 is the correct transition state (cf. Figure 2 and Figures S4, S6, and S7). Unfortunately, no significant solvent dependence is seen in the UV−vis spectra of the four compounds. In order to efficiently exploit the solar spectrum, a red-shift of the absorption peak of 1 to >400 nm would have been ideal. In order to obtain this, enlargement of the π-system by conjugated substituents can be studied. The absorption peak of 2 at 400 nm, on the other hand, lies at a reasonable wavelength, and with an estimated absorption onset of >500 nm, most of the high-energy part of the solar spectrum is covered. It should be noted, however, that the absorption curves in the UV−vis spectra (Figure 3 and Figures S11−S14) are of Gaussians fitted to the oscillator strengths with an arbitrary standard deviation of 0.4 eV, why the estimated onset has no direct correspondence to an experimental onset.

unoccupied molecular orbitals (HOMO and LUMO, respectively) as seen in Figure 4. The electron densities of the two neighboring methyl-substituted carbons of the dimethylthiophenes are out of phase through space in the HOMO while they are in phase and in close proximity in the LUMO. This suggests that bond formation, i.e., the electrocyclic ring closure of 2 → 3, can occur. Looking at the E/Z-active double bond, the electron density around the bond is high in the HOMO while an extra nodal plane is introduced across the bond in the LUMO (which is the case for the s-trans-(Z)-VHF-DTE, too; cf. Figure S16). This suggests that the isomerization reaction can occur. Comprehensive theoretical studies of different derivatives of DTE/DHB switches conclude that photoinduced ring closure is possible even with low electron densities around the two reactive carbons,33 while experiments on crystalline derivatives of DTE/DHB switches show that the cyclization quantum yields are near unity for distances between the reactive carbons below 4.2 Å and practically zero above.34 With a distance of 3.2 Å between the reactive carbons in 2, one can be led to believe that the ring closure quantum yield is high for the DHA-“DTE” system, too, but it is important to note that the systems investigated in refs 33 and 34 do not have the possibility to undergo cis/trans isomerization, why it remains to be seen experimentally whether or not the larger MO amplitude around the cis/trans active double bond indeed is an indicator of the cis/trans isomerization being able to compete with the cyclization. However, since the electrocyclic ring closure locks the structure in 3, the reaction will eventually be shifted toward 3 although the quantum yield apparently is lower than hoped. An important note to this is that even E


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photoactive, tooa finding which most likely can help explain results recently published on the switching and storage of s-cis/ s-trans locked DHA/VHF systems.26 Investigation of this is ongoing in the group.

The “VHF”-DHB (3) has to undergo a photoactivated ringopening to be able to thermally reverse to 1. Looking at the spectrum for 3 in Figure 3, the absorption peak at 470 nm corresponds to the excitation responsible for this.14,15 The important observation is that the absorption peak corresponding to the ring opening is well separated from the other. This allows for a clear distinction between photons triggering the back-reaction and photons driving the reaction toward 3, and a selective filtration of incoming long wavelength solar photons is likely to shift the photostationary state in a mixture toward 3. With this knowledge, long-time energy storage in the thermally stable “VHF”-DHB compound might be possible. Potential Energy Surfaces. Using the method described in the Computational Details, the relative energetics of the ground and first three excited singlet states were investigated and are plotted in Figure 5. Besides providing an overview of the reaction pathways, qualitative data and tendencies can be extracted. First and foremost, the assumption of light-induced pathways only from 1 → 2 and 2 → 3 is substantiated, as the ground state thermal activation barriers of these reactions both exceed 150 kJ/mol and hence cannot be overcome at suitable temperatures. Comparing the UV−vis absorptions in Figure 3 to the energy differences between the ground state and the excited states in Figure 5, it is seen in the left panel that the 3.9 eV absorption of 1 at 320 nm is sufficient to an S1 excitation. The activation barrier of the 1 → 2 reaction is on the S1 surface significantly lower than on the ground state surface and do not exceed 80 kJ/mol, which allows for thermal activation. As expected, a conical intersection between 1 and s-cis-2 is indicated, and once the activation energy on the S1 surface of 1 is overcome, the reaction will proceed toward s-cis-2 through this. The S2 and S3 states are not expected to take part in the reaction due to the large energy differences between these and the S1; i.e., all photochemistry of the DHA/VHF part of the switch happens from the first excited state. The reaction 2 → 3 has an extremely low activation barrier on the S1 surface relative to the ground state, and furthermore a nice conical intersection is present in this right panel since the reaction coordinate plotted almost single handedly accounts for the ring opening/closure reaction. Comparing this to the UV− vis absorption spectra (Figure 3, middle and right panel), it is seen that the 3.1 eV absorption of 2 at 405 nm and the 2.6 eV absorption of 3 at 470 nm perfectly corresponds to the S0 → S1 transitions on the left and right side of TS2, respectively. With a very low activation energy on the S1 surface for the s-trans-2 ← 3 reaction, too, the photoisomerization of the DTE/DHB part of the switch will readily occur once the molecules are excited, while the vast activation barriers on the ground state surface prevents thermal switching as expected from previous studies.12 Regarding the thermal back-reaction 1 ← 2, it is seen on the left panel of Figure 5 that the reaction indeed is possible on the ground state surface as predicted. Interestingly one can note that a photoinduced back-reaction seemingly is possible from the s-cis-2, and although not particularly relevant for this work as the equilibrium is shifted greatly toward the s-trans, this result can have great impact on the understanding of DHA/ VHF switches in general. It underlines the importance of a fast s-cis/s-trans conformational change in these switches to ensure energy storage in a photoinactive and thermally metastable VHF-compound. The importance of this conformational change is greatly emphasized as locking the s-cis/s-trans conformational change will make the VHF compound

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CONCLUSIONS The multimode DHA-“DTE”/VHF-DTE/“VHF”-DHB switch is by a computational DFT study shown to possess great potential for thermal storage of solar energy. Its elegant molecular design is found to provide energy-release control beyond that of previous DHA/VHF systems; instead of having to balance thermodynamical equilibria, the thermal energy can in this switch be released by light stimulus. Furthermore, both absorption wavelengths and energies are calculated to be more or less constant in different solvents with only slight decreases in energy storage capacity in highly polar solvents. These energy storage capacities are determined to be ∼60 kJ/mol, corresponding to an energy density of 0.17 MJ/kg. This is not as high as was hoped, but due to the high tunability of DHA/ VHF systems, improvements are within reach. The focus should, however, be on the inclusion of the second photoswitch to ensure both two sequential photon absorptions, higher energy storage, and elegantly triggered heat release. This system is thus both a proof-of-concept and an important step on the way to efficient long-time energy storage in solar heat batteries.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b10786. Geometries for all molecules presented; UV−vis absorption spectra of the system in a vacuum and various solvents and of the individual chromophores in a vacuum; molecular orbitals (HOMO and LUMO) for (Z)-VHF-DTE (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.T.O.). *E-mail: [email protected] (K.V.M.). ORCID

Kurt V. Mikkelsen: 0000-0003-4090-7697 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank University of Copenhagen and Danish eInfrastructure Cooperation for financial support. REFERENCES

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