Theoretical Investigation of Substituent Effects on the Dihydroazulene

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Theoretical Investigation of Substituent Effects on the Dihydroazulene/ Vinylheptafulvene Photoswitch: Increasing the Energy Storage Capacity Mia Harring Hansen, Jonas Elm, Stine T. Olsen, Aske Nørskov Gejl, Freja Eilsø Storm, Benjamin Normann Frandsen, Anders B. Skov, Mogens Brøndsted Nielsen, Henrik G. Kjaergaard, and K. V. Mikkelsen J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b09646 • Publication Date (Web): 21 Nov 2016 Downloaded from http://pubs.acs.org on November 22, 2016

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The Journal of Physical Chemistry

Theoretical Investigation of Substituent E↵ects on the Dihydroazulene/Vinylheptafulvene Photoswitch: Increasing the Energy Storage Capacity Mia Harring Hansen,† Jonas Elm,‡ Stine T. Olsen,† Aske Nørskov Gejl,† Freja E. Storm,† Benjamin N. Frandsen,† Anders B. Skov,† Mogens Brøndsted Nielsen,† Henrik G. Kjaergaard,† and Kurt V. Mikkelsen⇤,† Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen Ø, Denmark, and Department of Physics, Helsinki University, P.O. Box 64, Finland E-mail: [email protected]

⇤

To whom correspondence should be addressed Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen Ø, Denmark ‡ Department of Physics, Helsinki University, P.O. Box 64, Finland †

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Abstract We have investigated the e↵ects of substituents on the properties of the dihydroazulene/vinylheptafulvene photoswitch. The focus is on the changes of the thermochemical properties by placing electron withdrawing and donating groups on the monocyano and dicyano structures of the parent dihydroazulene and vinylheptafulvene compounds. We wish to increase the energy storage capacity, that is, the energy di↵erence between the dihydroazulene and vinylheptafulvene isomers, of the photoswitch by computational molecular design and have performed over 9000 electronic structure calculations using density functional theory. Based on these calculations we obtain design rules for how to increase the energy storage capacity of the photoswitch. Furthermore, we have investigated how the activation energy for the thermally induced vinylheptafulvene to dihydroazulene conversion depends on the substitution pattern, and based on these results we have outlined molecular design considerations for obtaining new desired target structures exhibiting long energy storage times. Selected candidate systems have also been investigated in terms of optical properties to elucidate how sensitive the absorption maxima are to the functionalizations.

Keywords DHA/VHF, photoswitch, solar energy, energy storage, organic solar cell, subtituent e↵ects, density functional theory.

Introduction One of the greatest scientific challenges of our time is to meet the global demand for energy in a more sustainable manner. Innovation in technology is needed in order to solve this energy challenge the world is facing. Considerable progress in solar energy technology has already been made, for example photovoltaics for electricity generation, and solar water heating, but progress involving the ability to store solar energy has been limited. 1 2


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Solar heat batteries introduce long-term energy storage in closed-cycle systems, carbon neutral reactions, and possible mobile application. The molecules in question undergo a photoisomerization, which may result either in a rearrangement of chemical bonds or a cistrans isomerization. Some general principles should apply to such systems. The metastable isomer should be constrained in order to store the energy for a reasonable period of time, and the stable isomer should absorb photons, where the intensity of the solar flux is high. Furthermore, the absorption spectrum of the metastable isomer should not overlap with that of the stable isomer. Two important parameters for the solar heat battery systems are the energy density and the thermal back-reaction (TBR) barrier. The TBR rate should be slow in order to store the energy for a considerably amount of time. The energy density should be as high as possible to increase the energy output. Boulatov et al. 1 have for photochromic molecules estimated an upper limit for the energy density of 1 MJ/kg for real application. These closed-cycle systems involving organic molecules have recently attracted increasing interest, 1–4 but they need to be studied more systematically in order to guide the design of future systems. 1 An interesting photo/thermo-switch is the dihydroazulene (DHA) / vinylheptafulvene (VHF) pair, which was first introduced by Daub et al. in 1984. 5 The DHA/VHF switch can be in three di↵erent states. The yellow DHA compound can undergo a photo-induced ring-opening of the five-membered ring into the red metastable s-cis-VHF form, which can be thermally transformed into the red metastable s-trans-VHF conformer through thermal equilibrium (see Fig. 1). The back-reaction from s-cis-VHF to DHA is a thermally induced NC

NC

CN

CN

light CN

DHA

s-cis-VHF

CN s-trans-VHF

Figure 1: Photo-induced conversion of DHA into VHF. The initial product of the ringopening is the VHF in its s-cis conformation, from where it rotates into its more stable s-trans conformation through thermal equilibrium.

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process. The DHA/VHF photoswitch is particularly promising for the application in solar heat batteries, because of the visible absorption region of the DHA molecule. DHA has an absorption maximum around 350 nm, whereas VHF has an absorption maximum around 470 nm. 6 The ring opening reaction from DHA to VHF has a high quantum yield at room temperature (

DHA!VHF

= 0.55 in acetonitrile) and VHF has a half-life of 218 min in acetonitrile. 7,8

In Fig. 2, the energy levels of the di↵erent molecular conformations can be seen, where the relative stability ( Grel ) is the di↵erence between the DHA conformer of lowest free energy and either the s-cis- or s-trans-VHF, depending on which molecule has the lowest free energy (see Eq. 1). The activation energy ( GTBR ) for the thermal back-reaction (TBR) is the di↵erence between s-cis-VHF conformer with the lowest free energy and the lowest free energy transition state, to ensure the most reliable barrier (see Eq. 2). The experimental determined rates and activation energies are determined from the most stable VHF conformer, which for the parent system is the s-trans-VHF.

TS Photoinduced isomerization

Energy barrier for back reaction ∆GTBR

Energy

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s-cis-VHF NC

s-trans-VHF

CN

CN

DHA H

NC

CN CN

Energy stored

∆Grel

Reaction Coordinate

Figure 2: Energy level diagram of the DHA/VHF photoswitch. and GTBR is the activation barrier for the back-reaction.

4


Grel is the relative stability

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Grel = GTBR =

GVHF GTS

GDHA

(1)

Gs-cis-VHF

(2)

We have studied the e↵ects on both the thermochemical and optical properties of adding an electronic withdrawing or donating group to the DHA/VHF system. Rational design of new systems via substitution e↵ects, will greatly aid in identifying new target molecules with better applications as solar heat batteries. To investigate the isolated e↵ect of adding a substituent to the systems, N O2 and N H2 were chosen, such that there is no significant structural change in the compound from sterical e↵ects. The N O2 and N H2 groups represent some of the strongest electron withdrawing and donating groups, respectively, which allows for the investigation of the sheer electronic e↵ects. Both the monocyano and dicyano structures of the parent DHA and VHF are studied (see the DHA molecules in Fig. 3). The DHA/VHF photoswitch has already been attempted tuned computationally by substituents. 7,9–16 Shahzad and coworkers 14 have done a theoretical study of the activation barriers for the back-reaction, while we are mostly interested in the relative stabilities of the DHA/VHF systems, since this property sets the limit for how much energy can be stored in the solar heat batteries. Current synthetic methods have allowed regioselective substitution at position 2, 3, 7, 17–19 and nonregioselectively at the other positions. 20 Experimentally, it has been shown that an acceptor group on position 2 or a donor group on position 7 enhances the rate of the TBR. 21 Using a computational screening of all possible positions will elucidate whether it is worth attempting to synthesize compounds with substituents in the unexplored positions. This will greatly reduce the required time to obtain new target molecules with high output for solar energy storage applications.

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Computational Approach The calculations are performed using Density Functional Theory in the electronic structure program Gaussian 09. 22 All calculations are carried out in vacuum. From previous studies 6 of the DHA/VHF system, three exchange-correlation functionals (CAM-B3LYP, 23 M06-2X 24 and PBE0 25,26 ) in conjunction with the 6-311+G(d) basis set showed adequate performance in correctly predicting the relative stability, i.e

GDHA
1 kJ/mol to 40 kJ/mol). The results for the thermal back-reaction of the N H2 substituted systems show increase for position 1 in the monocyano system. The barriers are mostly lowered with substitutions on position 3, 5, 7, and 8, where position 8 shows the largest decrease. For the dicyano system, positions 9, 10, and 11 seem to slightly increase the barrier height, whereas positions 3, 5, 7, and 8 decrease the barrier height. The largest e↵ect is seen for position 7. The e↵ects are consistent for all methods. The results for the N O2 substituted systems show that for the monocyano system, a substituent on position 1 decreases the barrier height by a factor of two. The highest increase is seen for position 3.

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Table 1: Thermal back-reaction barriers for the single N H2 substituted systems using the functionals CAM-B3LYP, M06-2X, and PBE0. The thermal backreaction barrier is defined as the energy di↵erence between the lowest free energy s-cis-VHF structure and the corresponding lowest free energy transition state. The parent values for each functional are indicated in parentheses (see ESI). The units are in kJ/mol.

NH2 Monocyano Position

CAM-B3LYP (136.13)

M06-2X (133.02)

PBE0 (112.10)

1 3 4 5 6 7 8 9 10 11

154.93 126.33 130.23 119.07 132.30 122.68 112.91 134.87 133.61 129.49

150.71 120.55 124.37 113.62 132.81 119.60 110.02 131.39 134.85 131.36

130.24 97.40 108.37 98.04 111.00 94.55 91.12 111.16 111.65 109.97

Dicyano Position

CAM-B3LYP (107.86)

M06-2X (106.13)

PBE0 (91.82)

3 4 5 6 7 8 9 10 11

89.54 101.22 88.39 106.61 85.93 89.28 110.90 112.07 108.77

91.71 97.09 83.52 107.59 80.59 87.18 110.71 109.66 109.39

82.67 95.41 74.79 93.42 76.89 73.67 94.00 95.89 95.04

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Table 2: Thermal back-reaction barriers for the single N O2 substituted systems using the functionals CAM-B3LYP, M06-2X, and PBE0. The thermal backreaction barrier is defined as the energy di↵erence between the lowest free energy s-cis-VHF structure and the corresponding lowest free energy transition state. The parent values for each functional are indicated in parentheses (see ESI). The units are in kJ/mol. Some of the transition state calculations did not converge. NO2 Monocyano Position

CAM-B3LYP (136.13)

M06-2X (133.02)

PBE0 (112.10)

1 3 4 5 6 7 8 9 10 11

69.77 147.30 140.82 136.61 138.77 133.74 134.61 135.48 137.26 128.34

68.18 144.13 140.77 133.02 134.63 133.99 134.20 127.32 133.75 131.30

58.72 127.31 110.60 110.54 112.36 112.05 107.83 112.89 104.78 111.16

Dicyano Position

CAM-B3LYP (107.86)

M06-2X (106.13)

PBE0 (91.82)

3 4 5 6 7 8 9 10 11

115.13 121.61 118.58 111.19 117.12 104.68 104.96 104.67

113.86 120.03 116.63 110.02 120.54 113.35 104.19 104.10 103.03

100.07 94.33 97.38 89.42 102.31 96.20 91.23 90.68

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For the dicyano N O2 substituted systems, an increase in barrier height is seen for positions 3, 4, 5, 7, and 8. The degree of increase depends on the functional. The lowering of the barrier also depends on the functional. For CAM-B3LYP and M06-2X, position 9, 10, and 11 have the lowest barrier heights, while for PBE0 position 6 seems to be a↵ected the most. The electron donor and acceptor substituent e↵ects have earlier been studied experimentally for positions 2 and 7 of the dicyano system. 7,21,34 A donor or acceptor group was placed in the para position on the phenyl group on positions 7 and 2, where the rate of the back-reaction was examined. Their results showed that for substitution on position 2 (para-substituted phenyl), the rate was increased by a withdrawing group and decreased by a donating group. The opposite e↵ect was observed for the substitution on position 7 (para-substituted phenyl). A withdrawing group decreased the rate, and a donating group increased the rate for the back-reaction. The substituent on position 2 for the experimental investigation can be directly compared to the so-called position 11 in this study, while the substituent on position 7 can not be directly compared to the substituent on position 7 in this study. Nevertheless, the results for the present investigation show the same trends for the barrier heights (rate of reaction) as the mentioned experimental results for these two positions. Optical Properties The oscillator strengths and vertical excitation energies are obtained from TD-DFT calculations. The results are only presented for the functional CAM-B3LYP, since it has shown satisfactory results for di↵erent organic systems. 35,36 The functional, CAM-B3LYP, has also been successfully used to calculate diverse response properties such as polarizabilities 37 and absorption maxima. 38 Each transition has been convoluted with a Gaussian line shape with a FWHM of 0.94 eV ( = 0.4 eV, see ESI for further details). In Fig. 7, the spectra for the parent systems are presented for comparison. Only the lowest free energy structures for DHA

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and s-cis/trans-VHFs are displayed. In Fig. 7(b), we note that the long-wavelength band for s-cis-VHF is redshifted compared to s-trans-VHF, which is in agreement with previous studies using ultrafast time resolved spectroscopy. 39 UV-Vis Monocyano Parent cm 1) 1

4

3

2

1

0 200

250

300

350 400 450 Wavelength (nm)

UV-Vis Dicyano Parent

⇥104

syn-DHA s-cis-E-VHF s-trans-E-VHF

5

Extinction coefficient " (L mol

1

cm 1)

⇥104


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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500

550

4

3

2

1

0 200

600

S-DHA s-cis-VHF s-trans-VHF

5

250

300

(a)


500

550

600

(b)

Figure 7: UV-Vis spectra for the parent systems using the CAM-B3LYP/6-311+G(d) level of theory. Only the lowest free energy DHA and s-cis/trans-VHFs are shown. The optical investigation has been limited to only include molecules, which showed improved results for the storage energies. The oscillator strengths and wavelengths for the absorption peak at the longest wavelength, and spectra for M06-2X and PBE0 are all shown in ESI. The obtained results for the optical features will be compared to the parent spectra. It should be noted that the scale of the extinction coefficient axis is not the same for the spectra of the parent and substituted systems. In Fig. 8, the spectra for a single substitution of a donating group (N H2 ) on position 3 or a withdrawing group (N O2 ) on position 4 for both the mono- and dicyano systems are shown. Only the spectra for the lowest free energy structure of DHA and VHF are shown. The remaining results can be found in ESI. The spectra for the N H2 substituted monocyano system (Fig. 8(a)) shows a slight overlap of the DHA and VHF peak in the visible region, which would cause a lower absorption for the DHA and thus a more inefficient solar heat battery. On the other hand, the oscillator strength for DHA is more than twice that of VHF, for the visible transitions. 16


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UV-Vis Monocyano N H2 (3)

⇥104

4.0

s-cis-E-VHF N H2 (3)



3.0 2.5 2.0 1.5 1.0 0.5 0.0 200

S-DHA N H2 (3) s-cis-VHF N H2 (3)

3.5

s-trans-VHF N H2 (3)

250

300


500

550

3.0 2.5 2.0 1.5 1.0 0.5 0.0 200

600

(a) N H2 substituted monocyano system UV-Vis Monocyano N O2 (4)

⇥104

4.0

s-cis-E-VHF N O2 (4) syn-DHA N O2 (4)

300


500

550

600

UV-Vis Dicyano N O2 (4)

⇥104

S-DHA N O2 (4) s-cis-VHF N O2 (4)

3.5

s-trans-VHF N O2 (4)

1

1

3.5

250

(b) N H2 substituted dicyano system

cm 1)

4.0 cm 1)

UV-Vis Dicyano N H2 (3)

⇥104

1

cm 1)

syn-DHA N H2 (3)

3.5

1

cm 1)

4.0

3.0



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


2.5 2.0 1.5 1.0 0.5 0.0 200

250

300


500

550

3.0 2.5 2.0 1.5 1.0 0.5 0.0 200

600

(c) N O2 substituted monocyano system

250

300


500

550

600

(d) N O2 substituted dicyano system

Figure 8: UV-Vis spectra of the lowest free energy DHA and VHF structures for both the N H2 and N O2 substituted mono- and dicyano systems. In Fig. 8(b), the spectrum resulting from substitution of a donating group (N H2 ) on position 3 for the dicyano system is shown. The absorption peak in the visible part of the spectrum for DHA is blueshifted (⇠ 17 nm), and the intensity has decreased as well compared to the parent spectrum. The absorption peaks for the visible transitions for the VHFs are redshifted quite a bit (⇠ 100 nm), and the intensities have decreased considerably. The broad absorption observed in Fig. 7(b) and Fig. 8(b) is in good agreement with the spectral profile of heptafulvenes substituted with donor groups at the exocyclic carbon atom. 40,41 In Fig. 8(c), the spectrum for the substitution of a withdrawing group (N O2 ) on position 4 for the monocyano system can be seen. It shows barely no overlap of the DHA and VHF

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absorption peaks at long wavelengths, and the intensity of the visible absorption for DHA is comparable to the parent DHA, whereas the lowest free energy structure s-cis-E -VHF has a very low intensity at long wavelengths. This is a great feature for solar heat batteries, since the VHF molecules then do not steal any photons from the DHA molecules. In Fig. 8(d), the spectrum for the substitution of a withdrawing group (N O2 ) on position 4 for the dicyano system is shown. The visible transition peak for DHA is redshifted (⇠ 22 nm), and the intensity has not changed significantly compared to the parent spectrum. The absorption peaks for both the VHFs are blueshifted (⇠ 75 nm for s-cis-VHF, and ⇠ 50 nm for s-trans-VHF). The intensities have decreased for both s-cis- and s-trans-VHF, where the largest reduction is seen for s-trans-VHF. A large part of the absorption spectra for the VHFs overlap with the DHA spectrum at higher wavelengths, which is not ideal for solar heat batteries. As mentioned, the electron donor and acceptor substituent e↵ects have been studied experimentally for position 2 and 7 of the dicyano system. 7,21,34 This includes the optical properties. The absorption maximum of the VHF spectrum was redshifted (⇠ 14 nm) by electron withdrawing groups and blueshifted (⇠ 15 nm) by electron donating groups positioned in the five-membered ring (para-N H2 phenyl or para-N O2 phenyl in position 2) compared to only having a phenyl in position 2. The opposite behaviour was observed for substituents positioned in the seven-membered ring (position 7). A withdrawing group at position 7 (para-N O2 phenyl) caused a blueshift (⇠ 11 nm) and a donating group (paraOCH3 phenyl) caused a redshift (⇠ 15 nm). 7,9,21 The obtained optical features cannot be directly compared to the experimental results, due to the di↵erence in substituents. The phenyl group will by itself blueshift the absorption. 33 The results do, however, show that substitution on position 7 (para-N O2 phenyl, experimental) and substitution on position 4 (N O2 , theoretical) both cause a blueshift of the absorption maximum in the visible region of the VHF spectrum for the dicyano system. Substitution on the five-membered ring shows di↵erent experimental and theoretical results.

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Substitution on position 2 (para-N H2 phenyl, experimental) and position 3 (N H2 , theoretical) show a blueshift and redshift for the VHF absorption peak in the visible part of the spectrum, respectively. This indicates that the 2- and 3- positions on the five-membered ring behave di↵erently in terms of the optical properties. Single Substitution Results Summary When adding a donating substituent (N H2 ) to position 3 for both the mono- and dicyano systems, increases in both energy storage capacity and energy density were observed. The thermal back-reaction barriers are increased for position 1 (monocyano) and position 9, 10 and 11 (dicyano), while decreased for positions 3, 5, 7, and 8 for both monocyano and dicyano. The optical features for these systems had the same trends. When compared to the parent systems, the DHA peaks in the visible region were blueshifted, while the VHFs were redshifted, and the peak intensities were all reduced. Furthermore, the DHA and VHF spectra had a small spectral overlap. Thus, the substitution on position 3 with N H2 for both the mono- and dicyano systems, showed improved results for the energy storage and energy density, but not ideal optical features. Adding a withdrawing substituent to positions 4, 5, and 6 for both the mono- and dicyano systems show increases in storage capacities. The largest e↵ect was seen for position 4. The energy densities for substitution on positions 4, 5, and 6 were increased for the dicyano system, but not for the monocyano system. The thermal back-reaction barriers show no increase in barrier height for the monocyano systems. The barriers are decreased for position 3 (monocyano) and positions 9, 10, and 11 (dicyano). The optical features for substitution on position 4, also showed the same trends for the mono- and dicyano systems. When compared to the parent systems, the DHAs absorptions were redshifted, while the VHFs absorptions were blueshifted. The peak intensities for DHA were at the same level as the parent system, while the intensity for the VHF absorption peaks were decreased. For the monocyano system, no spectral overlap between the DHA and VHF spectrum occurred, while a large overlap

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was observed for the dicyano system. Thus, the substitution on position 4 with N O2 showed increased energy storage for both the mono- and dicyano systems. The optical features were promising, when it comes to location of the absorption wavelengths for visible transitions, the peak intensities (both mono- and dicyano) and the size of the spectral overlap between DHA and VHF in the visible part of the spectrum (monocyano).

E↵ects of Combined Substituents The results of the singly substituted systems are used to combine substituents in order to investigate whether the improved e↵ects are further enhanced or simply neutralized. The single substitutent will perturb the electron density in the molecule, and the addition of another substituent might not have the same e↵ect as for the singly substituted systems, since the electron density has been perturbed. To increase the energy storage, we place N H2 on position 3, and N O2 on position 4. Both the mono- and dicyano systems are investigated. The molecules are displayed in Fig. 9. The combined substituted systems concerning TBR barriers are randomly selected in order to get a broad investigation of the e↵ects. The new molecules can be seen in Fig. 10. H

H

CN

H

(R) (R)

NO2

NC

CN

(S)

NH2

NO2

NH2

Figure 9: Combined substituted DHA molecules based on storage energy results from the singly substituted systems. For the monocyano system, N O2 is substituted on position 1 and N H2 on position 8 (top left molecule in Fig. 10). The singly substituted systems both showed decreases in TBR barriers. For the dicyano system, N O2 is substituted on position 3 and N H2 on position 8 (top 20


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H2N

H2N H

O N CN H 2

NC

CN

(S)

(R) (S)

NO2 H

NC

CN

H2N

(S) NH2 H

CN

(S)

H2N H2N

H

NC

NC

CN

NH2 H2N

H

NC

CN

(S)

(S) H2N

H2N

NH2

Figure 10: Combined substituted DHA molecules that were used to investigate the TBR barrier. right molecule in Fig. 10). The singly substituted systems showed a decrease in TBR barrier for the N H2 (8) system, and an increase for the N O2 (3) system, where the number in parenthesis refers to the substituent position. The last studied molecules were di↵erent variations of positions 3, 5, and 7 for multiple electron donating groups on the dicyano system. Four molecules were designed: N H2 (5, 3), N H2 (7, 3), N H2 (7, 5), and N H2 (7, 5, 3) (4 bottom molecules in Fig. 10). All the singly substituted systems produced a lowering of the barrier height. Energy Storage Capacity -

Grel

The energy storage capacities for the combined substituted systems can be seen in ESI. In Fig. 11, the energy storage capacities are compared to the parent systems. Only the results for the M06-2X/6-311+G* method are shown. The first two bars from the left are monocyano systems, while the rest are dicyano systems. Only four systems show a gain in storage energy compared to the parent system, and no system show further improvement than the single N H2 (3) substituted systems.

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Page 22 of 34

The dicyano N O2 (4)N H2 (3) substituted system shows a larger energy storage than the parent dicyano, but the single N H2 (3) substituted system still has a 23 kJ/mol larger storage capacity. The dicyano N H2 (5, 3) system has the largest storage capacity of the combined substituents, but it only corresponds to storage capacity of the single N H2 (3) substituted system. The dicyano N H2 (7, 3) system has not gained any storage energy compared to the single N H2 (7) substituted system, and the storage energy is less than the single N H2 (3) substituted system. Even though the dicyano N H2 (7, 5, 3) substituted system has a larger storage energy than the parent system, the separate singly substituted systems have larger storage energies independently. There is no synergetic e↵ect for the combined substitutions. Since the energy storage capacities for the combined substituted systems did not improve further, the energy densities are not presented here, but can be found in ESI. The largest energy density for the combined substituted systems is obtained with the N H2 (5, 3) system (0.20 kJ/mol), which is not far from the single N H2 (3) substituted system (0.22 kJ/mol).

Figure 11: Bar plot showing the energy storage capacity for the combined substituted systems relative to the parent systems. The first two bars (N O2 (4)N H2 (3) and N O2 (1)N H2 (8)) are monocyano systems, whereas the rest are dicyano systems. The parent values are 57.7 kJ/mol and 27.67 kJ/mol for the monocyano and dicyano system, respectively.

22


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Thermal Back-Reaction Barrier -

GTBR

Table 3: Thermal back-reaction barriers for the combined substituted systems using the functionals CAM-B3LYP, M06-2X, and PBE0. The thermal backreaction barrier is defined as the energy di↵erence between the lowest free energy s-cis-VHF structure and the corresponding lowest free energy transition state. The parent values for each functional are indicated in parentheses (see ESI). The units are in kJ/mol. Combined substituent Monocyano N O2 (4)N H2 (3) N O2 (1)N H2 (8) Dicyano N O2 (4)N H2 (3) N O2 (3)N H2 (8) N H2 (5, 3) N H2 (7, 3) N H2 (7, 5, 3) N H2 (7, 5)

CAM-B3LYP (136.13)

M06-2X (133.02)

PBE0 (112.10)

139.00 55.57

134.53 49.07

104.75 42.66

CAM-B3LYP (107.86)

M06-2X (106.13)

PBE0 (91.82)

117.19 98.18 64.28* 63.96* 47.92* 77.49*

116.60 95.07 62.61 66.97 49.75* 71.45*

85.59 84.03 49.10 49.80* 49.61* 69.96*

* Only one transition state (zwitterionic or WoodwardHo↵mann) structure successfully determined.

In order to store the energy for a certain period of time, the thermal back reaction barrier should be high. In Table 3, the results for the TBR barriers are listed. The * in Table 3 emphasizes that only one of the transition states (zwitterionic or Woodward-Ho↵mann) was successfully optimized. The barrier heights are only slightly increased (CAM-B3LYP and M06-2X) for the N O2 (4)N H2 (3) substituted monocyano and dicyano systems. The rest of the systems show various decreases, and thus not an improvement regarding storage of solar energy. Since only one transition state was determined for some of these systems, the decreased barrier e↵ect could be even greater, if a lower transition state was determined.

23



Optical Properties We have limited the investigation of the optical properties to the four multisubstituted systems, where an improvement in the storage energy was seen compared to the parent system. Fig. 12(a) shows the spectrum for the N O2 (4)N H2 (3) substituted system. A slight redshift is seen for the DHA (⇠ 23 nm) for the peak at 350 nm, and the intensity is cut in half compared to the parent dicyano DHA. The VHF absorption peaks near 400 nm have oscillator strengths of ⇠ 0.04 and are thus weak transitions compared to the parent VHFs. A small spectral overlap of the VHF and DHA spectrum can be seen. UV-Vis Dicyano N O2 (4)N H2(3)

⇥104

4.0

S-DHA N O2 (4)N H2 (3)

cm 1)

s-cis-VHF N O2 (4)N H2 (3)

3.5 3.0



UV-Vis Dicyano N H2 (5, 3)

⇥104

S-DHA N H2 (5, 3) s-cis-VHF N H2 (5, 3)

3.5

s-trans-VHF N H2 (5, 3)

1

s-trans-VHF N O2 (4)N H2 (3)

1

cm 1)

4.0

2.5 2.0 1.5 1.0 0.5 0.0 200

250

300


500

550

3.0 2.5 2.0 1.5 1.0 0.5 0.0 200

600

250

300

(a) UV-Vis Dicyano N H2 (7, 3)

⇥104

4.0

S-DHA N H2 (7, 3)

cm 1)

s-cis-VHF N H2 (7, 3)

3.5


500

550

600

550

600

(b) UV-Vis Dicyano N H2(7, 5, 3)

⇥104

S-DHA N H2 (7, 5, 3) s-cis-VHF N H2 (7, 5, 3)

3.5

s-trans-VHF N H2 (7, 5, 3)

1

s-trans-VHF N H2 (7, 3)

1

cm 1)

4.0

3.0



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Page 24 of 34

2.5 2.0 1.5 1.0 0.5 0.0 200

250

300


500

550

3.0 2.5 2.0 1.5 1.0 0.5 0.0 200

600

250

300

(c)


500

(d)

Figure 12: UV-Vis spectra of the combined substituents. (a) is the combinations to increase storage capacity, while (b), (c), and (d) are combinations to tune the TBR barrier. 24


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Fig. 12(b) shows the spectrum for the N H2 (5, 3) substituted system. The DHA peak in the visible region is marginally redshifted (⇠ 10 nm) and the intensity is only half of the parent dicyano DHA. The VHF absorptions are strongly redshifted (⇠ 100 nm) and the intensities have decreased compared to the parent VHF. There are nearly no overlap between the spectra. In Fig. 12(c), the spectrum for the N H2 (7, 3) substituted system is shown. The DHA peak at ⇠ 350 nm has an oscillator strength of 0.06, and the intensity has thus decreased compared to the parent DHA. The VHF spectrum greatly overlaps the DHA spectrum, which is not ideal for a solar heat battery. Fig. 12(d) shows the spectrum for the N H2 (7, 5, 3) substituted system. The spectrum features resemble the N H2 (7, 3) substituted spectrum, where the DHA transition at 350 nm is a weak transition (f = 0.078), and the VHF peaks in the visible part of the spectrum have redshifted considerably compared to the parent VHFs. Combined Substitution Results Summary The combined substituted systems did not show any improvement over the singly substituted systems. The energy storage capacities were only on the same level or below the singly substituted storage energies, causing all the densities to be smaller. The barrier heights did not increase for any of the combined systems. The systems combining an electron donating group on positions 3, 5, and 7, all showed large decreases in TBR barriers for the dicyano system. The optical features did not improve, since the DHA absorption peak intensities are substantially decreased for the combined systems.

Conclusion Using electronic structure methods coupled with a thorough sampling of di↵erent molecular conformations, we have studied the e↵ects of placing electron donating and withdrawing

25



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groups on the dihydroazulene/vinylheptafulvene photoswitch. The focus has been on how the substituents change the relative stability of the DHA and VHF molecules, the activation barrier for the thermal back-reaction, and the absorption properties. Based on our investigations of placing one electron donating or withdrawing group on the photoswitch, we find that certain positions provide substantial e↵ects on the investigated properties. Adding an electron withdrawing N O2 group at position 4 yielded a large increase in the energy storage for both the monocyano and dicyano systems. For an electron donating N H2 group as a substituent it was observed, that only position 3 led to an improved change in the relative stability between DHA and VHF. The gain in energy storage capacity of having an electron donating substituent in position 3 has recently been shown also to be valid in the case of a weak donor such as a phenyl group. 33 The quality of photochromism of these promising compounds for solar thermal batteries should be tested in the future. Combinations of substituents that each led to improved energy storage e↵ects did not yield an additive e↵ect, indicating that each modification a↵ects the charge distribution of the system. The most optimal system in this study is still the singly N H2 (3) substituted system, if the energy storage is of highest priority. For most of the investigated systems, the thermal back-reaction barriers are decreased. In order to avoid immediate energy release, the barrier should be high, and ideally the energy should be released using some sort of trigger. These conclusions will aid the following synthesis of new DHA/VHF derivatives, since there is no need to carry out complicated and time-consuming synthetic work to obtain new molecules, which do not show an increase in the calculated energy storage capacity, unless they have other advantages. We will in future investigations focus on how the surrounding environment will change the storage energies and activation barriers as the solvents are changed from apolar to polar solvents. In these studies we will utilize some of our previous developed methods for understanding how the surrounding environments are able to change energies and molecular

26


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properties. 42–45 The DHA/VHF photoswitch performance as a solar heat battery might also be improved by other means than electronic substituent e↵ects. One way of tuning the energy storage capacity could be by introducing steric e↵ects, which destabilize the VHF compound, and thereby increasing the energy storage. The VHF compound can also be destabilized by introducing macrocyclic structures, 46 which could be an interesting route to further explore. Another approach could be to increase the light absorption by introducing metal nanoparticles, which can be excited at their surface plasmon resonance.

Acknowledgement The authors thank the Center for Exploitation of Solar Energy, Department of Chemistry, University of Copenhagen, Denmark and the Danish e-Infrastructure Cooperation. JE thanks the Carlsberg foundation for financial support.

Supporting Information Available In the Supporting Information, additional material for the parent systems can be found together with an overview of the substituted structures and tables including Gibbs free energies for all calculated structures. Furthermore, supplementary results, such as energy storage tables, energy density tables and equilibrium constants between s-cis-VHF and strans-VHF for all applied funtionals, are listed. Method for construction of simulated UV-Vis spectra, tables showing the

max

and corresponding oscillator strengths, and spectra using

the functionals M06-2X and PBE0 are also given. This material is available free of charge via the Internet at http://pubs.acs.org/.

27



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33



Graphical TOC Entry

Energy Storage (kJ/mol)

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Page 34 of 34

70.3 65.4 58.5 57.7

45.0

H H CN H H CN NH2 NO2

HNC CN

27.7 H H CN

HNC CN

NH2 NO2

34


HNC CN

Theoretical Investigation of Substituent Effects on the Dihydroazulene

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