Density Functional Theory Study of the Solvent Effects on

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A DFT Study of the Solvent Effects on Systematically Substituted Dihydroazulene/Vinylheptafulvene Systems: Improving the Capability of Molecular Energy Storage Nicolai Ree, Mia Harring Hansen, Anders S. Gertsen, and Kurt V. Mikkelsen J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 05 May 2017 Downloaded from http://pubs.acs.org on May 7, 2017

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The Journal of Physical Chemistry

A DFT Study of the Solvent Effects on Systematically Substituted Dihydroazulene/Vinylheptafulvene Systems: Improving the Capability of Molecular Energy Storage Nicolai Ree, Mia Harring Hansen, Anders S. Gertsen, and Kurt V. Mikkelsen∗ Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen Ø, Denmark E-mail: [email protected]

1 ACS Paragon Plus Environment


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Abstract Former work has improved the energy storage capacity of the dihydroazulene/vinylheptafulvene photo-/thermoswitch by substitution with NH2 and NO2 in vacuum. This work extends the former by investigating the solvent effects systematically using cyclohexane, toluene, dichloromethane, ethanol, and acetonitrile and comparing them with the inclusion of vacuum calculations. The investigation includes more than 8000 calculations using density functional theory for comparison of energy storage capacities, activation energies for the thermal conversion of vinylheptafulvene to dihydroazulene, and UV-Vis absorption spectra. We thereby establish design and solvent guidelines in order to obtain an optimal performance of the dihydroazulene/vinylheptafulvene system for use in a solar energy harvesting and storing device.

Introduction The modern society and third world industrialization demand an ever rising need of energy, which urge for new innovative and sustainable ways of harvesting and storing energy to stop the catastrophic climate changes. 1 This work explores a molecular approach to solar thermal energy using the molecular photo/thermoswitch dihydroazulene/vinylheptafulvene (DHA/VHF) introduced by Daub et al. in 1984. 2 A photo-/thermoswitch which through a photoisomerization, resulting in a rearrangement of chemical bonds, can store solar energy as chemical energy in the molecules. The parent dicyano DHA/VHF system consists of three organic compounds (see Fig. 1). The yellow DHA isomer can undergo a photoinduced ring-opening to the red metastable s-cisVHF conformer through a conical intersection. The s-cis-VHF conformer can then undergo either a thermally induced ring-closure or a thermally equilibrated transformation to the red metastable s-trans-VHF conformer.


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Figure 1: The reaction scheme of dicyano DHA/VHF system, showing the photoinduced ring-opening of the DHA isomer to the s-cis-VHF conformer and the thermal equilibrium of the rotation that transforms the s-cis-VHF conformer into the s-trans-VHF conformer.

The DHA/VHF system is just one type of molecular photoswitches 3 that have attracted a lot of interest, 4–10 and experimental investigation of the parent DHA/VHF system in Fig. 1 have shown that the ring-opening reaction has a high quantum yield of ΦDHA→VHF = 0.55 in acetonitrile at room temperature. 11 Furthermore, the VHF has a half-life of 218 min in acetonitrile 5 and the DHA and VHF compounds have absorption maxima at 350 nm and 470 nm, respectively, resulting in almost no spectral overlap. 9 However, the DHA/VHF system needs further improvements for the use in a molecular solar thermal (MOST) system. 3,8 The ideal MOST system should imply a solar spectrum match of the DHA (a photon absorption in the region of maximum solar flux, λmax ∼ 500 nm), a high energy storage capacity, and the ability to contain the energy for a long period of time by means of a high thermal back-reaction barrier. Boulatov et al. have for photochromic compounds in real applications estimated an upper limit for the energy density of 1 MJ/kg, 8 which is comparable to second generation lithium-metal batteries. However, the advantage of the MOST systems are their ability to both harvest, store and release energy from the Sun in a closed-cycle. The main target of this work is therefore to use the power of computational chemistry for advanced modeling to effectively improve the DHA/VHF system. Systematic investigation in vacuum towards new molecular designs using an electron withdrawing group (NO2 ) and an electron donating group (NH2 ) have previously been performed. 12 Thus, demanding further investigations of these structures in solvents to examine the solvent effects for the use in practical applications. The applied solvents and their properties are shown in Table 1.



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Table 1: The investigated solvents with their static and dynamic dielectric properties. 13 Cyclohexane

Toluene

Dichloromethane

Ethanol

Acetonitrile

2.017 2.035

2.374 2.238

8.930 2.028

24.85 1.853

35.69 1.807

st op

Figure 2: The stereochemistry of the DHA structures with atom numbering. A monocyano RS stereoisomer is also investigated and all substitutions are examined, except position 2, 3a and 8a.

(a) Monocyano-(1R,8aR)-DHA

(b) Dicyano-(8aS )-DHA

Figure 3: The investigated monocyano VHF configurations and conformations.

(a) s-cis-E -VHF

(b) s-trans-E -VHF

(c) s-cis-Z -VHF

(d) s-trans-Z -VHF

Figs. 1, 2 and 3 depict the stereochemistry of the structures to which all substitutions with an electron donating group (NH2 ) and an electron withdrawing group (NO2 ) are examined, except for position 2, 3a and 8a. The monocyano DHA structure in Fig. 2(a) contains two stereocenters of which only the RR (syn-DHA) and RS (anti-DHA) stereoisomers are examined, since enantiomers have identical physical and chemical properties, besides their ability to rotate plan-polarized light. The dicyano DHA structure in Fig. 2(b) contains one stereocenter, and only the S stereoisomer is examined. All the monocyano VHF structures shown in Fig. 3 are examined. Furthermore, both the s-cis and the s-trans conformers of the dicyano VHF structures are considered (see Fig. 1).


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Figure 4: The energy level diagram of the dicyano DHA/VHF photo-/thermoswitch, showing the energy storage capacity (ΔGrel ) and the activation energy of the thermal back-reaction (ΔGTBR ).

The energy level diagram of the dicyano DHA/VHF system is shown in Fig. 4. The energy storage, ΔGrel , is calculated as the relative Gibbs free energy of the most stable DHA and the most stable VHF. Hence, storing a large amount of energy requires a large ΔGrel , which can be achieved by stabilizing DHA and/or destabilizing VHF. However, to maintain and control the energy storage, a large thermal back-reaction (TBR) barrier, ΔGTBR , is required. The TBR barrier is calculated as the relative free energy of the most stable s-cis-VHF and the most stable corresponding transition state structure. Reason being that only the s-cisVHF has the correct stereochemical conformation for the conversion to DHA. Another way to maintain the energy storage could therefore be to retard the conformational change of the s-trans-VHF to the s-cis-VHF.

Computational Approach The level of theory in this work is within density functional theory (DFT), which delivers a desirable balance between accuracy and computational cost for systems the size of the investigated compounds. 12 The calculations were performed in the electronic structure pro5 ACS Paragon Plus Environment


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gram Gaussian 09 13 using the functionals CAM-B3LYP, 14 M06-2X 15 and PBE0, 16,17 all in conjunction with the 6-311+G(d) basis set. 18–20 The solvents were included using the Integral Equation Formalism of the Polarizable Continuum Model (IEFPCM), initially devised by Tomasi et al. and Pascual-Ahuir et al. 21,22 These functional and basis set combinations were chosen due to former work on the DHA/VHF system, showing that they performed adequately in order to describe the desired properties such as the relative stability over a range of solvent polarities and optical properties including the absorption spectra. 9,12 Our previous work 9,12 has shown that the storage energy is best described by M06-2X, the back reaction barrier is best studied by PBE0 and the best method for obtaining excitation energies is given by CAM-B3LYP. A discussion of different functionals and basis sets can be found in SI.

Firstly, the DHA and VHF structures were geometry optimized and the harmonic frequencies were calculated. All solvent and vacuum calculations of free energies include the zero point vibrational energy and thermal contributions and are evaluated at 298.15 K and a pressure of 1.00 atm. Then the excited states and oscillator strengths of the transitions were obtained using time-dependent DFT (linear response) in order to construct UV-Vis spectra. The transition state structures were subsequently geometry optimized and confirmed by frequency analyses to be first-order saddle points with one imaginary frequency corresponding to the C1-C8a bond breaking/forming (see atom numbering in Fig. 2).

The transition state structures were obtained from the Woodward-Hoffmann rules and zwitterionic transition state mechanism. The photochemical and thermal electrocyclic reactions involve five π-electron pairs in the switching of the DHA/VHF system. This implies, in accordance with the Woodward-Hoffmann rules 23 and with the assumption of the DHA being fully conjugated, that the photochemical ring-opening of the DHA undergoes a conrotatory mechanism. On the other hand, the thermal ring-closure of the VHF undergoes a disrotatory


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mechanism. An alternative transition state mechanism is through the zwitterionic transition state, which has previously been predicted as the actual path of the TBR reaction for the parent dicyano DHA/VHF system. 5,6 This mechanism allows the rotation around the C3a-C3 bond of the acyclic VHF, resulting in the formation of two different DHA structures.

Results and Discussion This section contains a comparison of the substitutions and the employed solvents for the thermochemical properties including the energy storage capacities and the TBR barriers as well as the optical properties including the UV-Vis spectra. The section summarizes the key results, and the remainder, including the Gibbs free energies, can be found in SI. Energy Storage Capacity - ΔGrel In accordance with former work, that compares different functionals to DF-LCCSD(T)-F12a /VDZ//MP2/6-31+G(d), the most reliable functional for obtaining the energy storage capacities is the Minnesota functional M06-2X. 9 The remaining results for CAM-B3LYP and PBE0 can be found in SI. The energy storage capacities are presented relative to the parent DHA/VHF systems in vacuum, for which values are given in the caption of the figures. 12



Figure 5: The energy storage capacities of the NH2 monocyano structures relative to the parent system (ΔGrel diff ) in kJ/mol using M06-2X/6-311+G(d) level of theory. The x-axis indicates the position of the substituent. The results for the different solvents are presented by color code below. The energy storage of the parent monocyano system in vacuum is 57.70 kJ/mol. 12 Relative Energy Storage - Monocyano NH2 - M06-2X Cyclohexane

Toluene

Dichloromethane

Ethanol

Acetonitrile

10

-22.10 -22.40 -22.34 -22.76 -23.22 -23.32

−20

1

3

4

5

7

6

8

9

-3.27 -4.11 -4.20 -4.85 -3.98 -3.79

-7.57 -6.72 -6.51 -5.96 -6.65 -6.82

-9.14 -8.34 -8.03

-1.20 -3.48 -5.01 -11.54 -14.64 -15.26 -16.68 -16.56 -16.49

-22.60 -21.19 -21.17 -19.07 -17.62 -17.39

−10

-7.80 -8.50 -8.58 -8.81 -8.70 -8.66

-3.88 -5.56 -5.75 -7.37 -8.57 -8.78

0

-0.09 -1.62 -1.77 -1.54 -1.33 -1.32

12.57 11.79 11.39 9.13 9.04 8.71

Vacuum

ΔGrel diff relative to parent (kJ/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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10

11

Position of Substituent

Fig. 5 clearly shows that only the substitution with the donating NH2 group on position 3 increases the energy storage capacity relative to the parent system. Furthermore, a solvent medium with a low dielectric constant is preferable, except for the substitution on position 4. All structures experience higher stability with increasing solvent polarity, as seen in SI. Thus, the substitution on position 4 is the only structure, where DHA is more stabilized than VHF with increasing solvent polarity. The results also show that the phenyl meta (10) and para (11) positions have only a slight decrease in the energy storage capacity relative to the parent system and are relatively unaffected by the increasing polarity of the solvents. It is noteworthy that the substitutions on positions 6,7,8, and 10 experience decreasing energy storage from vacuum to dichloromethane and an increasing energy storage from dichloromethane to acetonitrile. This trend is opposite for the ortho (9) position of the phenyl group.


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Figure 6: The energy storage capacities of the NH2 dicyano structures relative to the parent system (ΔGrel diff ) in kJ/mol using M06-2X/6-311+G(d) level of theory. The x-axis indicates the position of the substituent. The results for the different solvents are presented by color code below. The energy storage of the parent dicyano system in vacuum is 27.67 kJ/mol. 12 Relative Energy Storage - Dicyano NH2 - M06-2X Cyclohexane

Toluene

Dichloromethane

Ethanol

Acetonitrile

30.86 29.33 29.11 27.49 26.66 26.47

Vacuum 40

20

5

-13.26 -14.93 -15.71 -19.91 -20.78 -21.48

-11.01 -38.12 -41.98 -42.84

10

11

-34.53 -39.08 -39.95

-23.12 -23.88

-15.71 -24.58 -26.88

7

6

-2.15 -6.95 -7.90 -12.81 -14.21 -14.46

4

-3.57 -7.53 -8.33 -13.41 -15.78 -16.24

3

-27.76 -30.67 -31.29

−40

-30.69 -32.91 -33.53

-15.75 -18.43

−20

-14.93 -17.82

-7.53

-6.86

0

-16.11 -18.19 -18.86 -23.42 -24.87 -25.12


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8

9


The results of the NH2 substitution on the dicyano structures in Fig. 6 show that the energy storage capacity for all positions decrease with increasing solvent polarity. Otherwise the tendency from the NH2 substituted monocyano structures is preserved, where only position 3 has a higher energy storage capacity than the parent system. However, the gain in energy storage is about three times higher for the dicyano structure compared to the monocyano structure with NH2 on position 3, and additionally it has an energy storage capacity twice as big as the parent system. Note that the VHF structures for the substitution on positions 5-8 in dichloromethane, ethanol, and acetonitrile are more stable than the DHA structures, resulting in negative energy storage capacities (ΔGrel parent = 27.67 kJ/mol).



Figure 7: The energy storage capacities of the NO2 monocyano structures relative to the parent system (ΔGrel diff ) in kJ/mol using M06-2X/6-311+G(d) level of theory. The x-axis indicates the position of the substituent. The results for the different solvents are presented by color code below. The energy storage of the parent monocyano system in vacuum is 57.70 kJ/mol. 12 Relative Energy Storage - Monocyano NO2 - M06-2X

3

4

5

7

6

0.34 -4.93 -5.08 -5.71 -8.31 -9.24 -9.52

1

-13.62 -16.15 -17.08 -19.33 -23.77 -23.96

−30

-14.53 -18.55 -19.27

−10

-12.65 -9.92 -9.22 -6.62 -5.20 -2.02

-2.65 -4.17

-1.94 -1.35 -0.78

0

−20

Acetonitrile

8

9

0.70 0.58 0.64 1.02 1.05 1.20

Ethanol

-3.06 -2.75 -2.74 -2.32 -0.44

3.43 5.27 5.51 8.27 9.75 10.02

7.73 9.34 10.28 11.39 11.19 11.38 3.72

10

Dichloromethane

7.54 9.31 9.74 12.67 14.66 15.01

Toluene

5.04 2.61 2.29

Cyclohexane

Vacuum 20


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10

11


The NO2 substituted monocyano structures in Fig. 7 have the highest number of options for increasing the energy storage capacities relative to the parent system. Furthermore, the largest energy storage of all the examined structures is given by position 6 in acetonitrile, which has an energy storage of 72.71 kJ/mol corresponding to an energy density of 0.26 MJ/kg. The results also show that the energy storage capacities for positions 4-8 and 10-11 are increased with increasing solvent polarity, however, the energy storage for position 7 is lowered for the solvents with polarities beyond dichloromethane.


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Figure 8: The energy storage capacities of the NO2 dicyano structures relative to the parent system (ΔGrel diff ) in kJ/mol using M06-2X/6-311+G(d) level of theory. The x-axis indicates the position of the substituent. The results for the different solvents are presented by color code below. The energy storage of the parent dicyano system in vacuum is 27.67 kJ/mol. 12 Relative Energy Storage - Dicyano NO2 - M06-2X Dichloromethane

Ethanol

Acetonitrile

0.04

10

Toluene

7.16 5.13 4.84 3.12 2.25 2.08

17.30 13.48 12.95 11.51 10.31 10.12

20

14.10 10.60 9.68 6.70 5.77 5.62

Cyclohexane

Vacuum

3

-21.01 -21.91 -21.54 -22.31 -22.66 -22.80

4

5

7

6

8

-12.55 -14.11 -14.32

-8.17 -10.26 -10.84

-3.22 -3.75

-1.37 -7.16 -7.66 -19.87 -22.65 -23.01

−30

-23.18 -25.76 -26.18

−20

-10.83 -12.01

−10

-3.41

-0.51 -3.67 -4.16 -6.34 -6.91 -7.03

0

-10.85 -13.87 -15.13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


9

10

11


The NO2 substituted dicyano structures in Fig. 8 have the same tendency as the NH2 substituted dicyano structures with decreasing energy storage capacities for increasing solvent polarity. The dicyano structure with NO2 on position 4 has the highest energy storage capacity relative to the parent dicyano system. However, also positions 5 and 6 show increased energy storage.



Thermal Back-Reaction Barrier - ΔGTBR In accordance with former work the most reliable functional for obtaining the TBR barriers is the PBE0 hybrid functional, 9 for which results will be presented. The results for CAMB3LYP and M06-2X can be found in SI. The TBR barriers of the parent systems in vacuum are given in the caption of the figures. 12 Figure 9: The NH2 substituted monocyano structures - Activation energies of the thermal back-reaction in units of kJ/mol using PBE0/6-311+G(d) level of theory. The x-axis indicates the position of the substituent. The results for the different solvents are presented by color code below. The TBR barrier of the parent monocyano system in vacuum is 112.10 kJ/mol. 12 Relative TBR Barrier - Monocyano NH2 - PBE0 Cyclohexane

20

Toluene

Dichloromethane

Ethanol

Acetonitrile

18.14 16.71 16.27 13.19 12.53 12.58

Vacuum

10

1

3

4

5

7

6

-2.13 -3.05 -3.29 -3.71 -3.27 -3.17

-2.79 -2.84

-9.22

-3.12 -4.14 -4.74

-20.98 -21.20 -21.21 -19.98 -24.56 -24.02

-18.58 -17.58 -17.36

-23.96 -24.69 -24.24 -24.14

−30

-7.53 -8.30 -8.39

-6.91

-12.34 -9.18 -8.93

-14.06 -18.47

-11.14 -11.92

-1.10 -3.20 -3.77

-3.73 -5.90

−20

-10.29 -11.70 -12.63 -12.75

−10

-0.94 -0.54 -0.49

0

-14.70 -16.77 -17.42 -13.00 -12.96 -13.31


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8

9

10

11


The overall tendency of the TBR barriers for the NH2 substituted monocyano structures (see Fig. 9) is a decrease in TBR barriers for increasing solvent polarity. This is due to the transition state structures being more stabilized when increasing the polarity of the solvent. Almost all of the investigated systems have decreased TBR barriers compared to the parent system. Only substitution on position 1 leads to higher TBR barriers than the parent system, where a large increase of over 16 kJ/mol (cyclohexane, toluene) and about 12 kJ/mol (other solvents), can be seen. 12 ACS Paragon Plus Environment

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Figure 10: The NH2 substituted dicyano structures - Activation energies of the thermal backreaction in units of kJ/mol using PBE0/6-311+G(d) level of theory. The x-axis indicates the position of the substituent. The results for the different solvents are presented by color code below. The TBR barrier of the parent dicyano system in vacuum is 91.82 kJ/mol. 12 Relative TBR Barrier - Dicyano NH2 - PBE0 Cyclohexane

Vacuum

Toluene

Dichloromethane

Ethanol

Acetonitrile

−40

-7.14 -9.68 -9.80

-5.15 -5.77 -9.29 -10.08 -10.20

3.22

4.07 -1.10 -2.03

2.18 -11.30 -10.06 -9.68 -18.15 -18.90 -16.81

-17.03 -18.26 -17.55 -14.95 -13.91 -13.67

-18.97 -32.92 -35.15 -35.55

−30

-25.55 -26.62

−20

-14.93 -15.01 -15.22 -13.58 -11.95 -11.59

−10

-3.31 -4.28 -7.33 -7.95 -8.09

0.31 -0.60 -1.33 -3.18 -0.02

-1.17 -0.42 -0.22

0

1.60

3.59 0.31 0.12

10


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3

4

5

7

6

8

9

10

11


Fig. 10, containing the results of the NH2 substituted dicyano structures, also shows the tendency of lowered TBR barriers when increasing the polarity of the solvent. However, substitutions on position 4 and 6 have slightly higher TBR barriers than the parent system for the low polarity solvents and high polarity solvents, respectively. Noteworthy is the large TBR barrier decrease of over 32 kJ/mol for the substitution on position 3 in dichloromethane, ethanol, and acetonitrile compared to the parent system.



Figure 11: The NO2 substituted monocyano structures - Activation energies of the thermal back-reaction in units of kJ/mol using PBE0/6-311+G(d) level of theory. The x-axis indicates the position of the substituent. The results for the different solvents are presented by color code below. The TBR barrier of the parent monocyano system in vacuum is 112.10 kJ/mol. 12 Relative TBR Barrier - Monocyano NO2 - PBE0 Cyclohexane

Toluene

Dichloromethane

Ethanol

Acetonitrile

-0.94 -8.78 -9.15 -10.86 -11.36 -11.46

-7.32 -8.46 -8.79 -10.55 -11.25 -11.39

0.79 -0.29 -0.50 -2.93 -4.11 -4.42

-4.27 -7.98 -8.76 -13.06 -14.48 -14.73

−20

-2.42 -3.27 -8.86 -11.24 -11.72

-16.65 -16.58 -16.53

-1.56 -2.66 -2.78 -3.47 -3.80 -3.80

-1.50 -3.57 -4.02

0

-0.05 -0.08

0.26

20

0.06 0.79 0.81 1.02

15.21 14.53 14.32 14.80 14.85 14.88

Vacuum

−40

−60

-53.38 -58.02 -58.11 -57.52 -56.40 -56.09


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1

3

4

5

7

6

8

9

10

11


The TBR barriers for the NO2 substituted monocyano structures show that substitution on position 3 is superior to the other positions and the parent monocyano system (see Fig. 11). Another trend to note is the decrease in TBR barriers for increasing solvent polarity. However, substitution on position 7 has slightly increased TBR barriers when increasing the polarity of the solvent. The results also show that the TBR barriers for the substitution on position 1 are half the height of the parent system with almost no variation when increasing the solvent polarity.


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Figure 12: The NO2 substituted dicyano structures - Activation energies of the thermal backreaction in units of kJ/mol using PBE0/6-311+G(d) level of theory. The x-axis indicates the position of the substituent. The results for the different solvents are presented by color code below. The TBR barrier of the parent dicyano system in vacuum is 91.82 kJ/mol. 12 Relative TBR Barrier - Dicyano NO2 - PBE0 Cyclohexane

Acetonitrile

0.16

3

4

5

7

6

8

-1.14

9

10

-11.23 -11.84 -11.96

-9.83 -10.09 -10.34

-6.37 -7.35

-6.18 -7.00

-3.39 -4.09 -6.17 -6.05 -5.35

-7.87 -9.81 -10.49

-13.07 -13.45 -13.24 -15.18 -15.54

-10.21 -11.51 -11.72

−10

-0.59

-0.23 -1.36 -6.48 -7.07

−5

-2.40

-2.21 -3.58 -3.80

0

−15

Ethanol

4.38

3.29 3.94 4.05

1.91 1.63

2.51

5

Dichloromethane

5.56

8.25 6.28 6.20 6.05 6.11 6.41

10

Toluene 10.49 8.19 7.74

Vacuum


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


11


Fig. 12 shows that the substitution of NO2 on position 3 or 7 for the dicyano structures has the highest TBR barriers and the substitution on position 4 has the lowest, especially in the most polar solvents. The results also show the same tendency as previously, with decreasing TBR barriers for increasing solvent polarity. Optical Properties The UV-Vis spectra are presented using the CAM-B3LYP/6-311+G(d) level of theory, 9 with a special focus on the region from 350 nm to 600 nm due to the high solar flux in this spectral range. The spectral analysis concerns only the structures with NH2 substituted on position 3 and NO2 substituted on position 4. Reason being that these structures showed the best energy storage capacities for both the di- and monocyano structures, thereby enabling a comparison of the effect of the cyano group on the system. The UV-Vis spectra of M06-2X and PBE0 can be found in SI.



The UV-Vis spectra are calculated from 15 vertical excitation for each structure by assuming Gaussian band shapes of the absorption peaks with the default Gaussian standard deviation (σ = 0.4 eV). The results show that UV-Vis spectra of cyclohexane and toluene merely differ in the location of the absorption peaks, which are shifted about 2 nm towards the spectra in dichloromethane. The UV-Vis spectra in toulene are therefore not presented. This also applies to the UV-Vis spectra of DHA and VHF in ethanol, which are completely identical to the spectra in acetonitrile. These spectra can be found in SI. Figure 13: UV-Vis spectra for the monocyano structure with NH2 substituted on position 3 for the DHA and VHF configurations in different solvents, using CAM-B3LYP/6-311+G(d) level of theory. anti-DHA-NH2 (3) syn-DHA-NH2 (3) 6

s-trans-E-VHF-NH2 (3) s-trans-Z-VHF-NH2 (3)

s-cis-E-VHF-NH2 (3) s-cis-Z-VHF-NH2 (3)

×104

Vacuum

5 4 3 2 1 0

Extinction coefficient ε (L mol−1 cm−1 )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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6

×104

Cyclohexane

5 4 3 2 1 0 6

×104

Dichloromethane

5 4 3 2 1 0 6

×104

Acetonitrile

5 4 3 2 1 0 200

250

300

350

400

450

500

Wavelength (nm)


550

600

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 13 depicts the UV-Vis spectra for the NH2 substituted monocyano structures. The results show that increasing the solvent polarity leads to increased absorption intensities and a red-shift of the absorption peaks. Furthermore, the absorption peaks of the DHA at 325 nm to 375 nm are more separated in the more polar solvents. This causes the absorption of the most stable DHA (syn-DHA) to overlap slightly more with the the absorption of the VHF structures. Otherwise, the spectral arrangement is favorable, but the DHA absorption is too far from the region of maximum solar flux. Note that the absorption of the E -VHF configurations are significantly larger than the absorption of the Z -VHF configurations. This results in a photostationary state favoring the Z -VHF configurations, which can cause minor changes (cf. SI) to the thermal- and photo-equilibrated energy storages.



Figure 14: UV-Vis spectra for the dicyano structure with NH2 substituted on position 3 for the DHA and VHF configurations in different solvents, using CAM-B3LYP/6-311+G(d) level of theory. S-DHA-NH2 (3)

s-trans-VHF-NH2 (3)

s-cis-VHF-NH2 (3)

×104

Vacuum

4 3 2 1 0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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×104

Cyclohexane

4 3 2 1 0 ×104

Dichloromethane

4 3 2 1 0 ×104

Acetonitrile

4 3 2 1 0 200

250

300

350

400

450

500

550

600

Wavelength (nm)

The UV-Vis spectra for the NH2 substituted dicyano structures in Fig. 14 show increasing absorption intensities for the DHA and the VHF conformers at around 200 nm and 500 nm, respectively, when increasing the solvent polarity. However, the absorption peaks for the VHF conformers at around 300 nm are decreased in intensity. The most stable VHF conformer was found to be the s-cis-VHF. Thus, the lowest spectral overlap with the DHA is observed in the least polar solvents. The results in cyclohexane show great spectral separation, making the NH2 substituted dicyano DHA/VHF photoswitch a good candidate for a MOST system. However, the highest 18 ACS Paragon Plus Environment

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wavelength for the DHA absorption is 329 nm in cyclohexane, which is far from the maximum solar flux. On the other hand, the highest wavelength for the s-cis-VHF absorption is well within the region of maximum solar flux, even though it is significantly displaced from 499 nm in vacuum to 556 nm in cyclohexane. Figure 15: UV-Vis spectra for the monocyano structure with NO2 substituted on position 4 for the DHA and VHF configurations in different solvents, using CAM-B3LYP/6-311+G(d) level of theory. anti-DHA-NO2 (4) syn-DHA-NO2 (4)

s-trans-E-VHF-NO2 (4) s-trans-Z -VHF-NO2 (4)

s-cis-E-VHF-NO2 (4) s-cis-Z -VHF-NO2 (4)

×104

Vacuum

5 4 3 2 1 0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


×104

Cyclohexane

5 4 3 2 1 0 ×104

Dichloromethane

5 4 3 2 1 0 ×104

Acetonitrile

5 4 3 2 1 0 200

250

300

350

400

450

500

550

600

Wavelength (nm)

The UV-Vis spectra for NO2 substituted monocyano structures seen in Fig. 15 are very similar for the different solvents. The acetonitrile results show that the DHA structures have maximum absorption at 387 nm (anti-DHA) and 388 nm (syn-DHA). Furthermore,



the maximum absorption of the s-trans-Z -VHF, which is the most stable VHF structure, is at 330 nm. Hence the arrangement of the absorption maxima are interchanged compared to the NH2 substituted structures. The DHA absorption peaks are still not in the region of maximum solar flux, but the spectral separation is favorable. As seen in the acetonitrile spectrum, the s-trans-Z -VHF has a small peak at 404 nm, but the oscillator strength is low (f = 0.079) and therefore almost negligible compared to the syn-DHA at 388 nm (f = 0.52), which is the most stable DHA structure. Figure 16: UV-Vis spectra for the dicyano structure with NO2 substituted on position 4 for the DHA and VHF configurations in different solvents, using CAM-B3LYP/6-311+G(d) level of theory. S-DHA-NO2 (4) 5

s-trans-VHF-NO2 (4)

s-cis-VHF-NO2 (4)

×104

Vacuum

4 3 2 1 0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 28

5

×104

Cyclohexane

4 3 2 1 0 5

×104

Dichloromethane

4 3 2 1 0 5

×104

Acetonitrile

4 3 2 1 0 200

250

300

350

400

450

500

Wavelength (nm)


550

600

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 16 depicts the UV-Vis spectra for the NO2 substituted dicyano structures. The results show that the absorption maxima for the DHA and the s-trans-VHF, which is the most stable VHF conformer, are increased in intensity and red-shifted for increasing solvent polarity. However, the larger solvent polarity results in a greater spectral overlap of the DHA and the s-trans-VHF, making the NO2 substituted dicyano structures poor for a MOST system, especially in conjunction with very polar solvents.

Concluding Remarks The solvent effects of cyclohexane, toluene, dichloromethane, ethanol, and acetonitrile on the dihydroazulene/vinylheptafulvene photo-/thermoswitch substituted with an electron donating (NH2 ) or an electron withdrawing group (NO2 ) were examined using electronic structure methods. We compared energy storage capacities, activation energies for the thermal conversion of s-cis-vinylheptafulvene to dihydroazulene, and UV-Vis absorption spectra to see the effects of using different solvents in combination with a systematic investigation of the photo-/thermoswitch.

The tendencies in the energy storage results for both the NH2 and NO2 substituted dicyano structures are that increasing solvent polarity leads to decreasing energy storage capacities. However, the monocyano structures do not posses this tendency, and some structures show the opposite tendency while others show varied energy storage capacities when increasing the polarity of the solvent. The results also showed relative stabilities with ΔGDHA < ΔGVHF , except for the dicyano structures with NH2 on position 5-8 in dichloromethane, ethanol and acetonitrile. Furthermore, the only position using the NH2 substituent that resulted in a greater energy storage capacity relative to the parent system was position 3. However, the structures with the NO2 substituent showed greater energy storage capacities relative to the parent system



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

for position 4, 5, and 6. The largest increase in energy storage was obtained in cyclohexane by the dicyano structure with NH2 on position 3. The highest energy storage was found to be 72.71 kJ/mol, which was obtained in acetonitrile for the monocyano structure with NO2 on position 6. This corresponds to an energy density of 0.26 MJ/kg, which is a quarter of the upper limit for photochromic compounds. 8 However, up till now the largest calculated energy storage of a DHA/VHF system is 0.54 MJ/kg. 24

The tendencies in the thermal back-reaction (TBR) barrier calculations are that increasing solvent polarity leads to decreasing TBR barriers, however, the effects were quite small. The results also showed that only very few structures had TBR barriers higher than the parent system. The TBR barrier for the parent dicyano system has previously been experimentally observed to be in the range of 75-88 kJ/mol depending on the solvent, 11 which is comparable to the calculated results. However, the TBR barriers of the monocyano structures were mainly between 90-110 kJ/mol. The experimental study also tested the dicyano structure with an NH2 group and a NO2 group on the para (11) position of the phenyl group in acetonitrile, which gave TBR barriers of 83.7 kJ/mol and 79.5 kJ/mol, respectively. Thus the calculated TBR barriers for exactly these structures are only 2.1 kJ/mol lower and 0.4 kJ/mol higher than the experimental results, respectively.

The optical results showed in general almost no variations in the UV-Vis spectra for the solvents with a polarity greater than dichloromethane. Furthermore, when going from vacuum to a solvent, the spectra were often red-shifted and the absorption intensities were increased. The structure with the best spectral arrangement was the monocyano structure with a NO2 group on position 4. The most stable DHA had a maximum absorption at 388 nm in acetonitrile and only a minor spectral overlap with the most stable VHF, having a maximum absorption at 330 nm.


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The results also showed that all the investigated DHA structures absorb light in the region below 400 nm, which is about 100 nm from the maximum solar flux. This is a problem that has to be solved in order to improve the performance of the DHA/VHF system. One way could be to include a photon upconversion technique in a practical device. 25 Another approach could involve molecular plasmonics by introducing metallic nanoclusters to control and manipulate light through collective electronic excitations. This would imply the use of QM/MM calculations in order to model the system. 26–29 We did not observe any changes in the design rules when going from vacuum to solvent and generally the larger dipole of VHF leads to decreases of the storage energies as the solvent becomes more polar. In our future work we will investigate how to provide a physical rational for the substitution patterns and compare the solvent effects from dielectric medium models with QM/MM-MD models.

Acknowledgement The authors thank the Center for Exploitation of Solar Energy, Department of Chemistry, University of Copenhagen, Denmark and the Danish e-Infrastructure Cooperation for making this work possible.

Supporting Information Available Supporting Information with additional material for the energy storage capacities, thermal back-reaction barriers and UV-Vis spectra are given in the annex to the article.

This

material is available free of charge via the Internet at http://pubs.acs.org/.

References (1) Treut, H. L.; Somerville, R.; Cubasch, U.; Ding, Y.; Mauritzen, C.; Mokssit, A.; Peterson, T.; Prather, M. Historical overview of climate change. In: Climate change 23 ACS Paragon Plus Environment


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2007: The physical science basis. Contribution of working group I to the fourth assessment report of the intergovernmental panel on climate change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)].; Cambridge University Press: Cambridge, United Kingdom and New York, NY, USA, 2007. (2) Daub, J.; Knöchel, T.; Mannschreck, A. Photosensitive dihydroazulenes with chromogenic properties. Angew. Chem., Int. Ed. Engl. 1984, 23, 960–961. (3) Lennartson, A.; Roffey, A.; Moth-Poulsen, K. Designing photoswitches for molecular solar thermal energy storage. Tetrahedron Lett. 2015, 56, 1457–1465. (4) Shahzad, N.; Nisa, R. U.; Ayub, K. Substituents effect on thermal electrocyclic reaction of dihydroazulene–vinylheptafulvene photoswitch: A DFT study to improve the photoswitch. Struct. Chem. 2013, 24, 2115–2126. (5) Broman, S. L.; Brand, S. L.; Parker, C. R.; Petersen, M. ˚ A.; Tortzen, C.; Kadziola, A.; Kils˚ a, K.; Nielsen, M. B. Optimized synthesis and detailed NMR spectroscopic characterization of the 1,8a-dihydroazulene-1,1-dicarbonitrile photoswitch. Arkivoc 2011, 2011, 51–67. (6) Broman, S. L.; Nielsen, M. B. Dihydroazulene: From controlling photochromism to molecular electronics devices. Phys. Chem. Chem. Phys. 2014, 16, 21172–21182. (7) Cacciarini, M.; Skov, A. B.; Jevric, M.; Hansen, A. S.; Elm, J.; Kjaergaard, H. G.; Mikkelsen, K. V.; Nielsen, M. B. Towards solar energy storage in the photochromic dihydroazulene-vinylheptafulvene system. Chem. Eur. J. 2015, 21, 7454–7461. (8) Kucharski, T. J.; Tian, Y.; Akbulatov, S.; Boulatov, R. Chemical solutions for the closed-cycle storage of solar energy. Energ. Environ. Sci. 2011, 4, 4449–4472.


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(9) Olsen, S. T.; Elm, J.; Storm, F. E.; Gejl, A. N.; Hansen, A. S.; Hansen, M. H.; Nikolajsen, J. R.; Nielsen, M. B.; Kjaergaard, H. G.; Mikkelsen, K. V. Computational methodology study of the optical and thermochemical properties of a molecular photoswitch. J. Phys. Chem. A 2015, 119, 896–904. (10) Gertsen, A. S.; Olsen, S. T.; Broman, S. L.; Nielsen, M. B.; Mikkelsen, K. V. A DFT study of multimode switching in a combined DHA/VHF-DTE/DHB system for use in solar heat batteries. J. Phys. Chem. C 2017, 121, 195–201. (11) Görner, H.; Fischer, C.; Gierisch, S.; Daub, J. Dihydroazulene/vinylheptafulvene photochromism: Effects of substituents, solvent, and temperature in the photorearrangement of dihydroazulenes to vinylheptafulvenes. J. Phys. Chem. 1993, 97, 4110–4117. (12) Hansen, M. H.; Elm, J.; Olsen, S. T.; Gejl, A. N.; Storm, F. E.; Frandsen, B. N.; Skov, A. B.; Nielsen, M. B.; Kjaergaard, H. G.; Mikkelsen, K. V. Theoretical investigation of substituent effects on the dihydroazulene/vinylheptafulvene photoswitch: Increasing the energy storage capacity. J. Phys. Chem. A 2016, 120, 9782–9793. (13) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A. et al. Gaussian 09 Revision E.01. Gaussian Inc. Wallingford CT 2009. (14) Yanai, T.; Tew, D. P.; Handy, N. C. A new hybrid exchange–correlation functional using the Coulomb-attenuating method (CAM-B3LYP). Chem. Phys. Lett. 2004, 393, 51–57. (15) Zhao, Y.; Truhlar, D. G. The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: Two new functionals and systematic testing of four M06-class functionals and 12 other functionals. Theor. Chem. Acc. 2007, 120, 215–241.



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of the dihydroazulene-vinylheptafulvene photochromic system. Chem. Eur. J. 2016, 22, 14567–14575. (25) Börjesson, K.; Dzebo, D.; Albinsson, B.; Moth-Poulsen, K. Photon upconversion facilitated molecular solar energy storage. J. Mater. Chem. A 2013, 1, 8521–8524. (26) Morton, S. M.; Jensen, L. A discrete interaction model/quantum mechanical method for describing response properties of molecules adsorbed on metal nanoparticles. J. Phys. Chem. 2010, 133, 074103. (27) Olsen, S. T.; Hansen, T.; Mikkelsen, K. V. Dark photoswitching induces Coulomb blockade diamond collapse. J. Phys. Chem. C 2015, 119, 14829–14833. (28) Olsen, S. T.; Arcisauskaite, V.; Hansen, T.; Kongsted, J.; Mikkelsen, K. V. Computational assignment of redox states to Coulomb blockade diamonds. Phys. Chem. Chem. Phys. 2014, 16, 17473–17478. (29) Kongsted, J.; Osted, A.; Mikkelsen, K. V.; Christiansen, O. Second harmonic generation second hyperpolarizability of water calculated using the combined coupled cluster dielectric continuum or different molecular mechanics methods. J. Chem. Phys. 2004, 120, 3787–3798.



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H

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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H CN O2N


Density Functional Theory Study of the Solvent Effects on

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