Solvent Effects on the Absorption Profile, Kinetic Stability, and

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C: Energy Conversion and Storage; Energy and Charge Transport

Solvent Effects on the Absorption Profile, Kinetic Stability, and Photoisomerization Process of the Norbornadiene – Quadricyclanes System Maria Quant, Alice Hamrin, Anders Lennartson, Paul Erhart, and Kasper Moth-Poulsen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b02111 • Publication Date (Web): 06 Mar 2019 Downloaded from http://pubs.acs.org on March 12, 2019

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The Journal of Physical Chemistry

Solvent Effects on the Absorption Profile, Kinetic Stability, and Photoisomerization Process of the Norbornadiene – Quadricyclanes System Maria Quanta, Alice Hamrin b, Anders Lennartson a, Paul Erhart* b, Kasper Moth-Poulsen*a a

Department of Chemistry and Chemical Engineering, Chalmers University of Technology, Kemigården 4, SE-412 96 Gothenburg, Sweden, b

Department of Physics, Chalmers University of Technology, Sweden

*E-mail: [email protected], [email protected]

ABSTRACT Molecular photoswitches based on the norbornadiene-quadricyclane (NBD-QC) couple can be used to store solar energy and to release the stored energy as heat on demand. In this context, the energy storage time as well as the quantum yield of the energy storing reaction are important parameters. Here, we explore for the first time solvent effects on these processes for a series of four NBD-QC compounds in four different solvents with different polarity (acetonitrile, tetrahydrofuran, toluene, and hexane). We show that the energy storage time of the QC forms can vary by up to a factor of 2 when going from the most polar to the least polar solvent. Moreover, we show that for norbornadiene derivatives with an asymmetric 1,2 substitution pattern, the quantum yield of conversion is highly solvent dependent, whereas this is not the case for the symmetrically substituted compounds. The spectroscopic observations are further rationalized using classical molecular dynamics (MD) simulations and time-dependent density functional theory (TDDFT) calculations. These demonstrate the importance of vibrational and rotational excitations for obtaining broad-band absorption, which is a prerequisite for capturing a wide range of the solar spectrum.

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INTRODUCTION The expected increase in energy consumption in combination with the need to minimize the usage of fossil fuels implies that energy production has to shift increasingly toward sustainable energy sources, most notably solar energy.1 One of the key challenges in this regard pertains to managing variations in energy production and demand, which calls for load-leveling technologies. To this end, molecular solar thermal energy storage (MOST) systems, also known as solar thermal fuels, are receiving sustained attention as they have the potential to provide high storage densities and short delivery times in combination with portability and low cost.2-3 The approach relies on molecular photoswitches that can undergo reversible endergonic photoisomerization reactions.4 A prototypical and widely studied MOST system is based on the norbornadiene–quadricyclane

(NBD-QC)

cycle.5-8

Norbornadiene

(1)

undergoes

a

photoinduced [2+2] cycloaddition upon irradiation to its valence isomer quadricyclane (2),9-10 which is highly strained yet kinetically stable and therefore a large amount of energy can be stored and released as heat in the reverse reaction. 7 1

6 5

4

1

2 3

light heat 2

Scheme 1. The photoinduced isomerization and back conversion of the norbornadiene and quadricyclane system.

In order for the NBD-QC system to be efficient in MOST applications it has to be structurally modified.5, 11 To begin with, the absorption profile of the NBD species must match the solar spectrum while the absorption profile of the corresponding QC species should not overlap with the NBD spectrum. Secondly, the NBD-QC energy difference should be as large as possible while keeping the molecular mass per NBD unit as small as possible in order to maximize the energy storage density. Thirdly, the activation energy for the back reaction should be high (typically at least 100 to 110 kJ/mol) to enable long-term storage. Finally, the photoisomerization reaction to QC needs to proceed with high quantum yield and minimal side reactions.5 For bare (unsubstituted) NBD (1) a low quantum yield and an absorption profile with an onset below 300 nm imply that the isomerization cannot be driven directly with sunlight. Both solar spectrum match and quantum yield can be engineered by introducing substituents that red-shift 2 ACS Paragon Plus Environment

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the absorption profile by creating a push-pull (donor-acceptor) system,6-7, 12 albeit at the cost of a lowering of the storage density.12-14 Accordingly, the effect of various substitution patterns on the absorption profile, photoisomerization, storage density, and kinetic stability of the QC isomer has been studied rather extensively. 6, 8, 15-16 Since, to date, most NBD-QC systems require solvation, with very few exceptions,17 both absorption spectrum and quantum yield can be tuned by the choice of solvent. Yet, while solvent conditions are known to have an effect on the switching for other potential MOST candidates, such as azobenzene18-22, stilbene23-25, tetracarbonyl-fulvalene-dirutehium26 and dihydroazulene,27-28 to the best of our knowledge, the impact of solvation has not been systematically studied for NBD-QC systems. In the present study, we therefore analyze, both experimentally and computationally, a series of solvents that represent a range of dielectric environments with regard to absorption spectrum, QC half-life, and photoisomerization for several prototypical NBD-QC systems. Importantly, the experiments show that key parameters such as the energy storage time are very sensitive to the polarity of the solvent, whereas the impact on the quantum yield depends on the molecular structure such that stronger solvent effects are observed for donor-acceptor substituted systems compared to non-polar molecular variants. RESULT AND DISCUSSION The NBD variants studied here were selected to represent different structural motifs, in particular with respect to the link between the parent compound and the donor group (Figure 1). Compounds 3 and 4 feature 2,3 donor-acceptor substitution patterns combining a cyano acceptor

group

with

an

(phenylethynyl)norbornadiene

aryl

substituted

alkynyl

(3)

donor and

group:

(2-Cyano-32-Cyano-3-

((4(dimethylamino)phenyl)ethynyl)norbornadiene (4)). Compound 5 features a cyano acceptor group and an aromatic donor group attached directly to position 3 of the NBD system: (2Cyano-3-phenylnorbornadiene (5)). Finally, compound 6 bears a symmetric substitution pattern of phenyl groups in both position 2 and 3: (2,3-diphenylnorbornadiene (6)). NBDs 3-6 have been recently synthesized and evaluated as potential MOST candidates, using toluene as solvent.7, 12 In addition, 4 and 6 have been used successfully in a laboratory scale hybrid device combining water heating and solar thermal energy storage.29 Moreover, 6 has shown to be a very robust photoswitch, and can be photoconverted with a thermally induced back conversion for more than 127 cycles at 60 ºC in toluene with little or no degradation.29 For this study, some 3 ACS Paragon Plus Environment


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commonly used solvents were chosen that exhibit a range of polarities, specifically hexane, toluene, tetrahydrofuran (THF) and acetonitrile (MeCN).

Measured absorption profiles Toluene, THF, and MeCN mainly cause a rigid shift of the UV-Vis absorption spectra, with larger, more polar solvents leading to stronger red-shifts (Table 1, Figure 1). Hexane exhibits a more differentiated behavior, in particular for compound 4. While for 3, 4, and 5 there is a clear shift in absorption onset (defined as log(ɛ) = 2) in hexane compared to the other solvents, compound 6 is less affected. For 4 there is a major change in the shape of the peak in hexane and since this behavior is not obtained for 3 it can be attributed to the dimethylamino substituent on the donor group. It should be noted that the UV-Vis spectra of 4 in hexane were recorded at a range of temperatures from 10 to 55 ºC without affecting the shape of the spectrum (details can be found in the SI, S1). Table 1. Measured absorption maxima in nm for compounds 3 – 6 in hexane, toluene, THF and MeCN. The ET(30) polarity parameters are shown in brackets in units of kcal/mol.

NBD 3 4 5 6

Hexane (31.0) 327 387 301 304

Toluene (33.9) 330 397 305 307

THF (37.4) 328 394 304 305

MeCN (45.6) 325 392 304 303


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Figure 1. Measured UV/Vis absorption spectra for compound 3 – 6 in hexane, toluene, and MeCN.

Computationally obtained absorption spectra Atomic scale simulations were carried out for molecules 3 to 6 in order to understand the effect of solvation in greater detail. To this end, we generated statistically decorrelated configurations by MD simulations based on a classical force field and subsequently computed excitation spectra for these structures. We focused on the differences between different NBD compounds and considered only solvation in toluene. For compounds 3 and 4, which feature an ethynyl linker, the computed spectra are in good agreement with the experimental data both with regard to onset of absorption and intensity (Figure 2). In the case of 5 and 6, for which the aromatic groups are directly attached to the norbornadiene backbone, larger differences were observed. In both cases the calculated absorption spectra are blue-shifted and in the case of 5 the intensity is underestimated by a factor of two to three. The observed differences in the onset of absorption are comparable to the error that can be expected based on the documented performance of TDDFT calculations and the functional used 5 ACS Paragon Plus Environment


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here. In general, the calculated spectra appear to exhibit a few more features than the experimental data. At present, we attribute this behavior to the slow convergence of the spectra with the number of configurations included (undersampling). Further factors that affect the agreement include variations in the dielectric environment that should require an explicit representation of the solvation shell and the inaccurate representation of the molecular vibrations by the force field employed in the MD simulations. In the case of compound 5 the underestimation of the intensity relative to the experimental data is quite striking. Interestingly, the dipole strength obtained for the zero-K configuration is, however, in good agreement with experimental data.12 The dipole strength is rather sensitive to the degree of alignment of the conjugated pi-system in the sidegroup and the double bond. There are therefore several possible sources for the underestimation of the intensity in the calculations, including the representation of the dielectric environment and the force field used for sampling the rotational motion of the aryl group. In any case, the present calculations represent a major improvement compared to the static zero Kelvin configurations (indicated by vertical bars), which only yield a few discrete absorption lines. In previous work, we had therefore applied a Gaussian broadening function with an empirically adjusted width of 0.15 eV and 0.25 eV for systems with and without ethynyl linkers, respectively.12,34 The present simulation approach reproduces these peak widths without the need for empirical parameters, which is a non-trivial result that relies on an appropriate description of both optical and vibrational properties of these compounds.


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Figure 2. Computationally obtained absorption spectra for NBDs 3 to 6 in toluene along with the respective experimental spectra (dashed lines).

Figure 3. Contribution to the total absorption spectra of 6 in toluene from different electronic transitions, where the indices in brackets refer to the enumeration of KS orbitals relative to HOMO and LUMO, respectively. For example, (-3, 0) represents transitions from the third orbital below the HOMO to the LUMO.

It is now instructive to analyze the simulations further in order to provide a more in-depth understanding of the underlying microscopic features. Decomposing the absorption spectra into individual excitations shows that the onset of absorption is practically exclusively due to HOMO-LUMO transitions (Figure 3). Their excitation energies are approximately Gaussiandistributed with a width of approximately 0.15 eV. The transition dipole moments on the other 7 ACS Paragon Plus Environment


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hand are much more sensitive to the molecular geometry with a more complex distribution over several orders of magnitude. We have previously discussed the importance of HOMO-LUMO lineup for achieving large transition dipole moments.12, 30-31 This applies in particular to the conjugated π-system that extends from the double bond in the norbornadiene backbone to the substituents with aryl character. Rotations away from the optimal geometry reduce the alignment between HOMO and LUMO, which has a much more pronounced effect on the transition dipole moment than the excitation energies, providing an explanation for the larger variation in the former pointed out above. At the same time the softest modes in compounds 3 to 6 are tied to the rotational motion of substituents. A comprehensive statistical analysis of the MD trajectories demonstrates that side group rotations are very strong in particular for compounds with an ethynyl linker unit (Figure 4). In the case of 3 the dihedral distribution is practically flat indicating no resistance to rotation whereas for 4 it is centered around zero, reflecting the optimal orientation for the extended πsystem. In the case of compounds 5 and 6, which have aromatic substituents directly attached to the norbornadiene backbone, the average is close to 90 degrees due to the steric repulsion between the aryl substituents and the norbornadiene backbone.34 The much more facile rotation in 3 and 4 provides a simple rationale for the more pronounced broadening in these compounds that was already described above.

Figure 4. Gaussian fit of dihedral angular distributions of the ring side groups of the norbornadiene molecules in solid line. The angular distribution of dimethylamino group of 4 is shown by the dashed line. The angel of rotation is illustrated (gray line) with the structure of the compound.


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In order to evaluate the solvent effects on the thermally activated conversion of the quadricyclanes (QC) 3 and 4 to the respective norbornadienes (NBD), a kinetic study was performed. The norbornadienes were dissolved in different solvents and converted to the corresponding quadricyclanes by irradiation at 310 or 405 nm using a light emitting diode. The rate constants of the back-conversion from QC to NBD were measured at several temperatures. The half-lives at 25 °C were calculated for 3 and 4 by extrapolating Eyring plots (Table 2, details can be found in SI, S2). The Eyring plots for 5 and 6 could not be obtained by this method since the great thermal stability of 5 and 6 would have required impractically high temperatures for prolonged periods of time, resulting in partial degradation or prohibitive evaporation of solvents. Table 2. Half-lives (t1/2) in hours for quadricyclanes 3 and 4 at 25° C, calculated for by extrapolating Eyring plots

QC 3 4

Hexane 17 4.1

Toluene 18 4.7

THF 23 5.7

MeCN 29 8.8

The measurements show that changing the solvent has a large influence on the thermal stability of the quadricyclanes. For 3 and 4 back-conversion was faster in non-polar solvents than in polar ones. In fact, the half-life was as much as two times larger in acetonitrile than in hexane for 4. The rate constants for 3 and 4 were furthermore correlated to the polarity of the solvent by plotting lnk at 25°C against the empirical solvent parameters, ET(30) (Figure 5).32 These experiments show that the kinetic stability can be altered by changing the solvent and since there is a linear correlation between half-life and solvent polarity it is possible to predict, at least approximately, the half-life in different solvents for compounds structurally similar to 3 and 4.

Figure 5. Plot of lnk at 25°C against the empirical solvent parameters, ET(30) of the solvent for a) compound 3 and b) compound 4.



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Photoisomerization Since the solvent has such a large effect on the kinetic stability of quadricyclanes 3 and 4, it is instructive to evaluate if the solvents also affect the photoisomerization quantum yield. Solutions of norbornadiene 3 and 4 in CDCl3, Toluene-d6 and MeCN-d3 were prepared in NMR tubes to a concentration of 3.3 mg/mL and a volume of 3 mL. The solutions were irradiated with a 405 nm LED diode for 4 and a 310 nm LED diode for compound 3 and the formation of QC was monitored by NMR (details can be found in SI, S3). For compound 3 the obtained difference was relatively small between the solvents and due to the qualitative nature of this study the error in the method makes it hard to draw direct conclusions. Interestingly, for 4 there is a large difference in quantum yield between the most polar and least polar solvent (Figure 6). Using Toluene-d6 the reaction was completed in 7.5 minutes while no formation of the quadricyclane was detected in MeCN-d3 after 50 minutes of irradiation.

Figure 6. Formation of QC 4 versus irradiation time in toluene-d6, CDCl3, and MeCN-d3.

For a quantitative comparison of the impact of solvent polarity on quantum yield for the photoisomerization of NBD 4 to QC 4, the quantum yield in MeCN was determined.33 Potassium ferrioxalate was used as a chemical actinometer with a 365 nm light-emitting diode as irradiation source. The thus obtained quantum yield for 4 in MeCN was as low as 3% (3.2 ±0.1) compared to 28% (27.8±0.4) in toluene. Based on the observations of the solvent effects on the quantum yield of 3 and 4, we can speculate on the polarity of the excited states of the compounds. Compound 3 has a symmetric substitution pattern, and the quantum yield of photoisomerization is largely unaffected by 10 ACS Paragon Plus Environment

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solvent polarity. This is in stark contrast to compound 4, which is polar in its ground state but expected to be even more polar in the excited state due to charge transfer in the donor-acceptor structure. Interestingly, for the thermally activated conversion from QC to NBD, the relative changes in rates going from the most polar solvent (acetonitrile) to the least polar solvent (hexane) shows a similar trend for both compounds, with the compounds exhibiting the longest half-lives in the most polar solvent. This observation indicates that in polar solvents, in the thermally activated process, the QC isomers are stabilized relative to the transition state structure, suggestive of a concerted conversion mechanism.

CONCLUSION The photophysico-chemical properties of a series of NBD compounds have been investigated using a combination of computations and experiment. Considering the computational part of the work, it was found that a combination of classical MD simulations and TDDFT calculations provides a viable description of the broadening of the optical spectrum, which then enables a meaningful correlation between theory and experiment. If we turn our attention to the solvent effects, the studied series of solvents appears to have minor effects on the optical absorption of the compounds, with, at most, a 5 nm shift in absorption maxima. Surprisingly, the solvents had a dramatic effect on the storage time with at factor of 2 difference between the longest t1/2 found in MeCN compared to the less polar hexane for compound 4. The solvent dependence on the rate constant follows a linear correlation when plotted against ET(30) indicating an apolar transition state structure in the conversion from QC to NBD. Considering the conversion from NBD to QC, it was found that within experimental error, no effects were seen in the case of 3, whereas for the donor-acceptor compound 4 the choice of solvent had significant effects: in MeCN a quantum yield as low as 3% was observed whereas in toluene it was measured to be 28%. In conclusion, based on the studied series of compounds, when developing MOST systems for future applications, it is important not only to focus on the development of the molecular system itself as the local environment can have significant effects on the performance of the system. In



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order to fully describe these effects, not only the local medium, but also the conformational energy landscape of the systems needs to be taken into account. EXPERIMENTAL

General Norbornadienes 3 to 5 were synthesized according to published procedure in reference [12] and norbornadiene 6 was synthesized according to reference [7]. Anhydrous acetonitrile was used as received while tetrahydrofuran, toluene and hexane were dried using an MBraun MB SPS800 solvent purification system. Computational details System setup and control simulations: the system is set up in GROMACS34-35 using the GROMOS 54A7 force field.36 The norbornadiene molecules were constructed and subsequently converted to GROMACS topology files in PRODRG. The topology files were altered to represent an all-atom configuration. Geometries and topology files for solvent molecules were obtained from the Automated Topology Builder repository37-38 in all-atom representation. The solvent molecules were injected into the simulation box (5x5x5 nm) using gmx insert-molecules with a number of solvent molecules calculated from the solvents respective density in ambient conditions and the volume of the simulation box with the NBD molecule excluded. After solvating the NBD molecules all systems were relaxed using energy minimization with the steepest decent method. The minimization energy was plotted and convergence was confirmed. These structures were then used to equilibrate the system in NVT at 300 K for 500 ps. Density and temperature were used to confirm equilibration. An additional check via a NPT run for 100 ps at 1 atm was carrie out to ensure stability of the system. Here pressure and potential energy were monitored. The production run was started from the NVT equilibrated system in NVT. MD-simulations: The actual MD run was performed on the equilibrated conformations in NVT for 1000 ps at 300 K. The time step used was 2 fs and the simulation box was 5x5x5 nm in size with densities corresponding to tabulated densities for the different solvents at ambient conditions. From these runs 200 statistically uncorrelated snapshots were extracted for each system. These snapshots were used for further solvent interaction analysis and quantum chemical calculations. The dihedral angles between the NBD backbone and the different side 12 ACS Paragon Plus Environment

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groups were investigated by calculating the dihedral angles for each snapshot in VMD39 and presenting the statistical distribution over the angular spectrum in histograms to which Gaussian distributions could be fitted to. Quantum chemical calculations and root analysis: The snapshots from MD simulations were stripped from solvent molecules and unwrapped in VMD. Quantum chemical calculations were performed in NWChem40at the level of time dependent DFT (TDDFT) in order to obtain excitation energies. A 6-311+G* basis set41-42 and the B3LYP exchange-correlation functional43-44 were used as established in previous work.45 In NWChem the solvent molecules were replaced by the COSMO implicit solvent model46 based on the respective dielectric coefficients of the solvents. The calculated roots from each snapshot were combined into an average spectrum for each system. The excitation energies were broadened with a Gaussian shape with a HWHM of 0.1 eV to represent instrumental inaccuracy. The energies were also increased by a scissor shift of 0.3 eV since earlier studies45 have shown the DFT calculations to underestimate the band gaps of norbornadiene molecules by approximately this amount. The 200 spectra were then used to create a statistically averaged spectrum, and an additional 200 batches of 50 configurations were drawn in order to create error bars for the spectrum.

Kinetic study The norbornadienes were dissolved in the solvents in a cuvette and irradiated with a metalhalide-UV lamp (366 nm) for 2‒5 min, to obtain the corresponding quadricyclanes. Thereafter, the increase of the norbornadiene concentrations over time were measured by recording the increase in absorption at a fixed wavelenght with a Cary 100 UV/Vis-spectrophotometer. The measurements were performed under stiring at a varity of temperatures. An exponential fit of the Eyring equation was applied to the obtained data to determine the rate constants at the different temperatures for all compounds (details can be found in SI, S2). NMR study The samples were preperad in an NMR-tube to a concentration of 3.3 mg/ml in all solvents prior to the NMR experiment. The samples were irradiated directly in the NMR-tube with a 310 nm LED for compound 3 and a 405 nm LED for compound 4 and the conversion monitored by NMR. 1H-NMR of 4 were recorded at 800 MHz, and at 500 and 400 MHz for compound 3. The relaxation delays were set long enough to obtain quantitative integrals and other instruments 13 ACS Paragon Plus Environment


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setting (shiming, tuning, locking etc) were optimized for the first sampel prior to the irradiation, thus the resolutions of the spectra decreases during the experiment (details can be found in SI, S3). Quantum Yield Measurements Thee photonflux of the fiber-coupled LED (M365F1 (365 nm)) was determined by potassium ferrioxalate actinometry before the measurements were performed. The quantum yield measurments were carried out by irradiating a solution of 4 in MeCN and monitoring the decrease in absorption with a Cary 100 -UV-Visible spectrophotometer. The solutions were prepared and stored in the dark to avoid that any photoconversion occurred before measuremnts were executed. To ensure that all photons were absorbed, the concentrations of the solutions were high enough at the irradiation wavelenght to afford an absorbance greater than 2 at 365 nm). When all photons are absorbed, a linear dependence between the decrease in absorption and the irradiation time is obtained and the quantum yield were determined from the slope (details can be found in SI, S4).


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ACKNOWLEDGMENT The authors would like to thank the K&A Wallenberg foundation for financial support through Wallenberg academy fellow grants. SUPPORTING INFORATION DESCRIPTION S1, Absorption profiles in different solvents. S2, Kinetic study of the backconversion S3, NMR study of the photoisomerization S4, Quantum yield measurements REFERENCES 1. International Energy Agency, Technology Roadmap Solar Thermal Electricity, 2014. 2. Boerjesson, K.; Lennartson, A.; Moth-Poulsen, K., Efficiency Limit of Molecular Solar Thermal Energy Collecting Devices. ACS Sustainable Chem. Eng. 2013, 1, 585-590. 3. Kucharski, T. J.; Tian, Y.; Akbulatov, S.; Boulatov, R., Chemical Solutions for the Closed-Cycle Storage of Solar Energy. Energy Environ. Sci. 2011, 4, 4449-4472. 4. Moth-Poulsen, K.; Coso, D.; Boerjesson, K.; Vinokurov, N.; Meier, S. K.; Majumdar, A.; Vollhardt, K. P. C.; Segalman, R. A., Molecular Solar Thermal (Most) Energy Storage and Release System. Energy Environ. Sci. 2012, 5, 8534-8537. 5. Yoshida, Z., New Molecular Energy Storage Systems. J. Photochem. 1985, 29, 27-40. 6. Dubonosov, A. D.; Bren, V. A.; Chernoivanov, V. A., NorbornadieneQuadricyclane as an Abiotic System for the Storage of Solar Energy. Russ. Chem. Rev. 2002, 71, 917-927. 7. Gray, V.; Lennartson, A.; Ratanalert, P.; Boerjesson, K.; Moth-Poulsen, K., Diaryl-Substituted Norbornadienes with Red-Shifted Absorption for Molecular Solar Thermal Energy Storage. Chem. Commun. 2014, 50, 5330-5332. 8. Lennartson, A.; Roffey, A.; Moth-Poulsen, K., Designing Photoswitches for Molecular Solar Thermal Energy Storage. Tetrahedron Lett. 2015, 56, 1457-1465. 9. Hammond, G. S.; Turro, N. J.; Fischer, A., Photosensitized Cycloaddition Reactions. J. Am. Chem. Soc. 1961, 83, 4674-5. 10. Dauben, W. G.; Cargill, R. L., Photochemical Transformations. Viii. Isomerization of Bicyclo[2.2.1]Hepta-2,5-Diene to Quadricyclo[2.2.1.02.6.03,5]-Heptane (Quadricyclene). Tetrahedron 1961, 15, 197-201. 11. Moth-Poulsen, K., Organic Synthesis and Molecular Engineering. 2014, 179 196. 12. Quant, M.; Lennartson, A.; Dreos, A.; Borjesson, K.; Moth-Poulsen, K.; Kuisma, M.; Erhart, P.; Borjesson, K., Low Molecular Weight Norbornadiene Derivatives for Molecular Solar-Thermal Energy Storage. Chem. Eur. J. 2016, 22, 13265-74. 13. Kuisma, M.; Lundin, A.; Moth-Poulsen, K.; Hyldgaard, P.; Erhart, P., Optimization of Norbornadiene Compounds for Solar Thermal Storage by First-Principles Calculations. ChemSusChem 2016, 9, 1786-1794. 15 ACS Paragon Plus Environment


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TABLE OF CONTENS GRAPHIC


Solvent Effects on the Absorption Profile, Kinetic Stability, and

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