Designing Efficient Solar-Thermal Fuels with [n

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Letter

Designing Efficient Solar-Thermal Fuels with [n.n] (9,10)Anthracene Cyclophanes: A Theoretical Perspective Gaurab Ganguly, Munia Sultana, and Ankan Paul J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b03170 • Publication Date (Web): 19 Dec 2017 Downloaded from http://pubs.acs.org on December 20, 2017

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The Journal of Physical Chemistry Letters

Designing Efficient Solar-Thermal Fuels with [n.n](9,10)Anthracene Cyclophanes: A Theoretical Perspective Gaurab Ganguly, Munia Sultana, and Ankan Paul∗ Raman Center for Atomic, Molecular and Optical Science, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, India. E-mail: [email protected] Phone: +91 33 2473 4971 (Ext. 1779)

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Abstract Molecular solar thermal storage (MOST) systems have been largely limited to three classes of molecular motifs: azo-benzene, norbornadiene, and transition metal based fulvalene-tetracarbonyl systems. Photodimerization of anthracene is known for a century, however, this photo-process has not been successfully exploited for MOST purposes due to its poor energy storage. Using well calibrated theoretical methods on a series of [n.n](9,10)bis-anthracene cyclophanes we have exposed that they can store solar energy into chemical bonds and can release in the form of heat energy on demand under mild conditions. The storage is mainly attributed to the strain in the rings formed by the alkyl linkers upon photo-excitation. Our results demonstrate that the gravimetric energy storage density for longer alkyl-chain linkers (n>3) are comparable to those for the best-known candidates, however, it lacks some of the deleterious attributes of known systems, thus making the proposed molecules desirable targets for MOST applications.

Graphical TOC Entry “While the Sun Shines”

[n.n]PI ∆Estorage(n) [n.n]BA

‘n’

The more the ‘n’ the merrier the‘ΔEstorage(n)’ in [n.n]Anthracenophane“Sloar-Thermal Fuels”

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The fervent drive for finding alternatives to global warming causing fossil fuels has led researchers to explore avenues for zero-carbon intensity (C-neutral) renewable energy sources. Solar energy is the most tangible and virtually infinite resource of C-neutral energy. 1 Molecular solar-thermal (MOST) storage device has emerged as a new approach for storing solar energy. 2–4 MOST systems, which are candidates for future renewable energy applications exploit photo-isomerization of molecules, triggered by UV-Vis radiation (300-700 nm) of the solar spectrum, between pair of photo-isomers. 2,3 The exposure of a low energy isomer to UV-Vis radiation must trigger its conversion to a higher energy metastable isomer (with an appreciable quantum yield, φ) for storing solar energy in chemical bonds. Most importantly, the reaction must have a large positive ground state enthalpy (∆Hstorage ) with an optimum kinetic barrier (Ea ) for preventing the thermal back reaction (TBR) thus ensuring the storage of energy for a reasonable period. However, the barrier must not be so high as to hamper the release of energy by gentle heating or by using a catalyst. Therefore, an ideal MOST material must possess an optimal mix of several unique attributes and in principle can be repeatedly used for infinite cycles of energy conversion and release. 2–4 The idea of storing solar energy in the chemical bonds of a molecule is not at all new. The two important photo-isomerization reactions, like E↔Z isomerization of azobenzene (AB), 5–10 and [2π+2π] cyclo-addition of norbornadiene (NBD) to yield quadricyclane (QC) 11–14 have been extensively investigated over the years in this regard. Transition metal containing fulvene tetra-carbonyl complexes have also been identified as a plausible candidate for solar-thermal storage. 15–17 However, all of the systems known till date have one or more deleterious property which possibly makes them non-ideal as a MOST candidate. Usual problems range from UV-Vis photo-activated back conversion, 8 low quantum yield (φ), 10 absorption spectra not falling in the visible range of the spectrum (e.g. unsubstituted NBD), or low energy storage density. 16 Tuning of absorption spectra to the visible range for triggering photo-isomerization has often been achieved at the expense of energy storage density. 11–14 In some cases, low quantum yield (φ) may arise simply due to

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the necessity of triggering too many photoisomerizations within a single MOST candidate. 10 Hence, it is important to realize that the number of known chemical motifs that can serve as ideal MOST systems are extremely limited, which necessitates identification of newer molecular motifs for better solar-thermal storage. The [4π+4π] cycloaddition of anthracene, at 9-9’C and 10-10’C position, triggered by absorption of UV-Vis radiation, is known for over a century. 18,19 Since this reaction is reversible with first order kinetics at reasonable temperature, hopes were raised among the researchers to store solar energy into anthracene based systems in the form of chemical energy as early as 1970’ies. 20–23 Unfortunately, anthracene based motifs did not show any tangible solar-thermal energy storage and interest in these molecules as a potential MOST system have waned. To the best of our knowledge, none of the anthracene-based motifs have been ever identified as a promising MOST system. 24,25 Herein, we present the results of a thorough study on a series of [n.n](9,10) bis-anthracene cyclophanes (henceforth referred as [n.n]BA, n=2-6), those congeners which are synthetically viable owing to advanced synthetic techniques, 26–28 using well calibrated density functional methods. Contrary to the existing consensus view we find that the [n.n]BA cyclophane candidates containing longer linker alkyl chains (large value of ‘n’) can serve as highly effective MOST systems with the highest gravimetric energy storage density reported till date (see Scheme 1). As stated earlier, for any solar energy-harvesting system to be effective, its photoabsorption spectrum must overlap significantly with the UV-Vis range of solar spectrum. Please note, for all [n.n]BA we observe the existence of a couple of near-energy degenerate conformers or tautomers depending upon the point group symmetry mostly C2h and D2 . Photo-absorption spectra of all the lowest energy [n.n]BA (n=2-6) conformers (among near degenerate conformers) were computed at TD-CAM-B3LYP/6-31++G(d,p) level of theory on M06-2X/6-31++G(d,p) optimized geometries using Gaussian 09 suite of programs. 29 For details of all the theoretical tools employed see Supplementary Discussion 1, in the Supporting Information (SI). Two distinct absorption peaks were observed for all the [n.n]BA

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and both correspond to π-π* excitation. However, we are only concerned about the absorption onset (λonset ) values those are listed in Table 1 along with the corresponding oscillator strengths (fosc ). The absorption onset (λonset > 350 nm) with strong oscillator strength (fosc > 0.1) in all cases, clearly indicates that all photodimerization reactions would be triggered by UV-Vis solar spectra. Full photo-absorption spectra, beyond simple vertical transition energies, were further simulated and are provided in Supplementary Discussion 2, in the SI. Here it should be noted that after [4π+4π] cyclo-addition four benzene rings are formed in the photo-isomer or photo-product ([n.n]PI) out of the two anthracene rings in the [n.n]BA (see Scheme 1). Therefore, the extended π-conjugation is lost in the photo-product, which results in the photo absorption of [n.n] PI below 300 nm. Hence, unlike AB based systems which undergo photosensitized back reaction (Z→E ) within the visible range of solar spectra (∼450 nm), 8 the [n.n]PI do not absorb in the UV-Vis region of the solar spectrum, which ensures that the photoproduct would not revert back to the parent form on exposure to sunlight. The quantity that dictates energy storage (∆Estorage ) is the difference in the total energies (E) between the [n.n]PI photo-product and the corresponding [n.n]BA or more strictly by the correponding difference in enthalpies (∆Hstorage ). Here we want to emphasize that the choice of a proper computational method, particularly in the case of anthracene dimerization, is extremely critical as it has been shown by Grimme et al. that several density functionals and even MP2 fail to accurately predict the exact gas phase dimerization energetics when the predicted values are compared to that of highly accurate and reliable FN-DMC method. 30 We found the M06-2X functional, in combination with Pople’s tripleζ 6-311++G(d,p) basis set (on M06-2X/6-31++G(d,p) optimized geometries), performed satisfactorily well in predicting the gas phase anthracene dimerization energy, -9.3 kcal/mol in comparison to the highly accurate FN-DMC numbers, -9±2 kcal/mol. Moreover, we also found that M06-2X/6-311++G(d,p) predictions (∆Hstorage =+8.6 kcal/mol in condensed phase) are in excellent agreement with even experimentally determined photo-isomerization

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enthalpies (∆Hstorage =+8.5 kcal/mol in condensed phase) for [2,2]BA. 20,21 For details of calibration please see Supplementary Discussion 3 in the SI. Please note, just like [n.n]BAs, for [n.n]PI also we observe the existence of a couple of near-energy degenerate conformers depending upon the point group symmetry mostly C2h and D2 . 25 However, to uniquely define the energy storage ∆Estorage or ∆Hstorage we have only taken into consideration the lowest energy [n.n]PI isomer and the corresponding lowest energy [n.n]BA, and ∆Estorage is defined as difference in energy between them (see Supplementary Discussion 4 in the SI). The gas phase ∆Estorage and the condensed phase ∆Hstorage (toluene as solvent) with their corresponding storage density are shown in Figure 1 with bar-diagram at M062x/6-311++G(d,p) level of theory. It is obvious that for any [n.n]BA two ‘(n+2)’ membered rings are being formed in the photo-product (see Scheme 1). Therefore, one may expect a huge ∆Estorage in [2.2]PI, as the two highly strained four-membered rings are being formed in the photo-product. However, the ∆Estorage for [2.2]PI turned out to be only +9.0 kcal/mol, which is also in satisfactory agreement with the previous findings. 20,21,24,25 For [3.3]PI, ∆Estorage drops to +1.0 kcal/mol. However, beyond [3.3]PI a sudden jump in ∆Estorage was observed (see Figure 1). For [4.4]PI we observed significantly high ∆Estorage of +33.7 kcal/mol. The ∆Estorage for [5.5]PI turned out to be +38.1 kcal/mol and ultimately for [6.6]PI we witnessed the ∆Estorage value as high as +51.3 kcal/mol. The corresponding gravimetric energy storage densities, which a much more practical quantity and obtained by dividing the ∆Estorage and ∆Hstorage by its molecular weight, are reported in parentheses in the unit of kcal/kg. The gravimetric storage densities also turned out to be consistent with the ∆Estorage and ∆Hstorage (see Figure 1). ∆Estorage and corresponding gravimetric energy storage densities predicted with other reliable functionals are provided in Supplementary Discussion 5 in the SI. To put the calculated ∆Estorage and gravimetric energy storage densities of in context, we have evaluated their ([n.n]PI, n>3) performance by comparing them with that of the most promising MOST systems reported in literature till date(see Table 2). The comparison clearly suggested that the proposed systems feature among the best candidates known so far. Please note that ∆Estorage and

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the corresponding storage densities are significantly sensitive to the theoretical techniques employed. The computed numbers shown in Table 2 are based on predictions made after proper calibration of density functionals (wherever possible) against very reliable ab-inito techniques like CCSD(T) with large basis sets or FN-DMC method. 31 However, for larger systems like FvRu2 CO4 and related systems, where functional calibration against CCSD(T) is impractical, we have used the reported literature values. 15,16 The density functional calibrations on NBD and AB systems are provided in the Supplementary Discussion 6 in the SI. The abrupt jump in ∆Estorage on moving along the series beyond [3.3]PI (see Figure 1) prompted us to understand the origin of this unusual feature. To rationalize the peculiar trend we resorted to the use of a systematic strain analysis to determine the strain in [n.n]BA intermediates (denoted as: ERS (INT)), and their corresponding [n.n]PI photoproducts (denoted as: ERS (PDT)) with the help of ‘homodesmotic approach’ 32,33 at M062X/6-311++G(d,p) level of theory. For the evaluation of ERS (INT) in [n.n]BA, and ERS (PDT) in [n.n]PI we have chosen (9,10)-dimethylanthracene and its photo-dimer as reference compounds respectively and it turned out that the photo-dimerized product is +2.0 kcal/mol energetically uphill compared to two free monomers. In this case we simply imagined [n.n]BA and [n.n]PI cyclophanes as a linear methylene-chain (−(CH2 )(n-2) −) connecting between two (9,10)-dimethylanthracenes and its photo-product. Therefore, the stored energy (∆Estorage ) in any [n.n]PI would essentially be given by: ∆Estorage = [(ERS (PDT) +2.0) - ERS (INT)] kcal/mol. The details of the homodesmotic scheme used in this work is presented in the Supplementary Discussion 7 in the SI. The computed ERS (INT) and ERS (PDT)) values are represented with a bar-chart in Figure 2. We found that with the increasing value of ‘n’, ERS (INT) gradually decreases, whereas ERS (PDT) gradually increases for n≥3. For small values of ‘n’ in [n.n]BA, the two anthracene rings are tightly held together with the shortlength alkyl linkers. Therefore, the planarity of both the rings are lost to some extent. However, from n=4 onward each anthracene ring regains their planar geometry which leads

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to decrease in the ERS (INT) close to zero. This is the reason why ERS (INT) decreases with increase in ‘n’. Essentially, the equilibrium molecular structures for [n.n]BA intermediates are a fine balance between mechanical strain and aromaticity of the two anthracene rings. 34 Conversely, in the [n.n]PI, there is a sharp drop in the ERS (PDT) from n=2 to n=3 because the five-membered ring is much more strain free compared to the four-membered ring. However, the sudden jump in ERS (PDT) in the lowest energy conformer of [4.4]PI is quite surprising and counter intuitive as the two six-membered rings are being formed in the photo-product, [4.4]PI. Here we want to emphasize that a six-membered cycloalkane or cyclohexane is known to be the least strained among all cycloalkanes. Conformational analysis of lowest energy photo-product of D2 symmetry [4.4]PI, surprisingly revealed that the dihedral angle between the four adjacent carbon atoms (φ(1-2-3-4)) of the two six-membered rings are 8.4◦ (see Figure 3). Therefore, the two six-membered rings are certainly not in the most preferred ‘Full Chair’ (FC) conformation (characterized by φ(1-2-3-4) 60.0◦ ) of a cyclohexane ring; rather it is closer to the higher energy ”twist chair” (TC) conformation characterized by the φ(1-2-3-4) 0.0◦ . TC is basically the transition state for ‘Full-Chair’ (FC) to ‘Twist-Boat’ (TB) interconversion and 11.2 kcal/mol higher than the corresponding FC conformation. Considering the presence of other linkages in the rings of [4.4]PI a more apt case for comparison would be with 1,1,2,2-tetramethylcyclohexane where the difference between ‘TC’ and ‘FC’ is 17.0 kcal/mol (due to the increased torsional strain owing to the presence of four vicinal −CH3 groups). Therefore, the origin of high value of ERS (PDT) in [4.4]PI is attributed to the presence of two 1,1,2,2-tetramethylcyclohexane rings are in ”TC” conformations. Hence, the net strain would be ∼17.0×2 ≈34.0 kcal/mol. In other [n.n]PI with high value of Estorage we also observe the same, i.e. four adjacent carbon atoms of the ‘(n+2) membered-ring (formed upon photo-isomerization of [n.n]BA) are in a near zero dihedral angle conformation. This creates the high strain in the photo-product. A more detailed analysis of conformational strain in high energy storing [n.n]PIs (n=4-6) are provided in the Supplementary Discussion 7 (Sch. S6, and Fig. S3-5) in the SI.

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As discussed earlier, along with higher energy storage densities, the shelf-life of a solar thermal fuel in its higher energy meta-stable conformation, which depends on the thermal back reaction (TBR) barrier, is paramount to its application as an ideal MOST system. The TBR for converting the [n.n]PI to its starting bis-anthracene intermediate,[n.n]BA may have two different PESs concerted (synchronous) and step-wise (asynchronous) C-C bond dissociations, as predicted by Kertesz et al. in a recent theoretical study. 25 The PESs for TBR (D2 [4.4]PI → D2 [4.4]BA) were generated via linear transit scan along the reaction coordinate of concerted and step-wise C-C bond cleavage in D2 [4.4]PI at UM06-2X/6-31++G(d,p) level of theory (see Figure 4). The C-C bond (d1 and d2 ) correlation diagram for concerted (blue) and step-wise (black) pathways are shown in the inset of Figure 4(a). The TBR via concerted C-C bond cleavage is predicted to proceed via a second-order saddle point (TSconc ) with a barrier height of 37 kcal/mol. However, we found that the TBR via step-wise C-C bond cleavage is predicted to happen via the formation of an open-shell singlet species, which is generated as an intermediate upon first C-C bond dissociation. This biradicaloid species, was treated with broken-symmetry (BS) unrestricted density functional theory (BS-UDFT), 35 named as BS-INT, and was predicted to be 21.2 kcal/mol higher than the corresponding D2 [4.4]PI. The spin-density of the BS-INT clearly shows that the radical sites are exclusively located on the carbon atoms of the broken C-C bond. (Figure 4(b)). The BS-INT is connected to the D2 [4.4]PI and the D2 [4.4]BA via two first order saddle points, TS1 and TS2 which lies 1.0 and 3.0 kcal/mol above the BS-INT respectively. The rate-determining barrier (RDB) via step-wise C-C bond breaking pathway turned out to be (21.2+3.0 =24.2) kcal/mol due to TS2 . The TBR barrier of 24 kcal/mol is likely to ensure a sufficiently long shelf-life (days or month) of the high energy meta-stable photo-isomer but can be transformed into the starting intermediate under application of mild heat. Furthermore, the RDB of the step-wise TBR mechanism for D2 [5.5]PI also turned out to be 24 kcal/mol suggesting it that could also be trapped for a reasonable period of time in their meta-stable photo-isomer. 36 As we have discussed in the first paragraph, quantum yield (φ) is another important

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parameter for any photochemical reaction to happen. In the condensed phase the quantum yield (φ) for anthracene photodimerization is basically diffusion controlled and depends on its concentration. 37 At lower concentration quantum yield is low (φ ∼0.12), 37 however, it becomes reasonably high (φ ∼0.3) at higher anthracene concentrations. 37 On the contrary, in case of [2.2]BA, the photodimerization reaction happens with a moderately high quantum yield (φ ∼0.36) at normal concentration due to its intramolecular nature. 20 It is to be noted that with the increase in the alkyl chain length two anthracene rings become distant. However, for [3.3]BA φ is reported to be ∼0.3. 28 Therefore, one can plausibly infer that increasing the length of alkyl chain in a cyclophane does not have any substantial negative impact on its quantum yield (φ). For practical applications of a MOST system, a decent solubility in common solvents can be a matter of concern. Grossman et al. have suggested simple yet elegant strategies for tackling solubility issues. 7,16 Admittedly, this is beyond the current scope of this work and can be best addressed experimentally. However, the presence of long methylene (−CH2 −) chains of [n.n]BA is expected to increase the solubility (compared to that of anthracene) without decreasing the storage density unlike FvRu2 CO4 (where the substitution of 1,1dimethyltridecyl groups increase the low solubility albeit including a huge decrease in overall gravimetric storage density). 16 Incorporation of hetero-atoms like ‘N’ in the long methylene chain might further increase the solubility due to the possibility of hydrogen bonding without decreasing the gravimetric storage density. This type of ‘N’ doped cyclophanes are reported in the literature and also known to absorb in the UV-Vis range of solar spectra. 38 Moreover, the lone pairs on the ‘N’ can be protonated to further increase its solubility in polar solvents. We found that the storage density of symmetrically ‘N’ substituted [5.5]PI (each linker= (−CH2 )2 −NH−(CH2 )2 −) and corresponding dicationic entity (each linker= (−CH2 )2 −NH2+ −(CH2 )2 −) has significantly high storage density (see Supplementary Discussion S9 in the SI). Additionally, incorporation of ‘N’ atom in these chains with adequate manipulation can be exploited to synthesize various derivatives and polymers or to tether

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these systems on nanotube surfaces by the use of highly active N-H hydrogen. 8 Probably polar solvent soluble species can be developed based on this strategy. Obviously, prudent substitutions on these motifs may enhance all desirable attributes and a high throughput computational screening strategy as was used by Grossman et al. for AB based systems 9 can be used to find better candidates. In summary, in-depth investigations with reliable theoretical methods on a series of revealed that [n.n]anthracenophane based systems are a fine blend of different unique attributes required for an ideal MOST system. Along with the favorable optical absorption properties and optimal thermal back reaction barrier this class of molecules possess a very high energy storage with an extraordinary gravimetric energy storage density. Length of alkyl chains can be tweaked to dramatically enhance ∆Hstorage in this class of molecules as the storage is mainly attributed to the ring strain in the higher energy photoisomer. Therefore, we believe that our findings bring back focus on hitherto ignored anthracene based systems as potential solar-thermal fuel. We also believe that the trends observed in case of bis-anthracene cyclophanes are not just limited to this particular class of systems but opens up several design possibilities for developing a new range of effective photo-switches.

Acknowledgement G. G. and M. S. acknowledge CSIR-SRF and CSIR-JRF fellowship, respectively. A. P. acknowledges funding from BRNS and technological research center (TRC).

Supporting Information Available General quantum chemical Informations; density functional calibrations; simulated spectra of lowest energy [n.n]BAs; detailed analysis of ring-strains; TBR for [n.n]PIs for n>4.

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[n.n]BA* 9 UV-Vis

10’

“Charging”

TBR

10

9’ [n.n]PI

9

10’

∆Estorage

9’ [n.n]BA

Scheme 1: Schematic representation of closed cycle photo-isomerization ([n.n]BA↔[n.n]PI) of [n.n](9,10)bis-Anthracene cyclophane based MOST systems.

Table 1: Vertical photo-absorption onset, λonset (in nm) and the corresponding oscillator strengths (fosc ) for lowest energy [n.n]BAs (n = 2-6) with their point group symmetry in parentheses at TD-CAM-B3LYP/6-31++G(d,p)level of theory. [n.n]BA

λonset

fosc

(nm) (D2 )[2.2]BA 379.2 0.10 (C2h )[3.3]BA 358.9 0.18 (D2 )[4.4]BA 361.9 0.19 (C2h )[5.5]BA 360.9 0.23 (D2 )[6.6]BA 362.0 0.23

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60

ΔE(storage)

ΔH(storage)

52.9 51.3 (101.8) (98.6)

(Storage Density in kcal/kg)

50 ∆Estorage and ∆Hstorage in kcal/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


40

33.7 32.7 (72.6) (70.5)

40.2 38.1 (81.7) (77.4)

30

20

10

9.0 9.1 (22.0) (22.2) 1.0 1.0 (2.3) (2.3)

0

[n.n]PI

Mol. Wt.(g)

n=2

n=3

n=4

n=5

n=6

408.3

436.3

464.3

492.3

520.3

Figure 1: ∆Estorage , and ∆Hstorage for [n.n](9,10)anthracene cyclophanes (n=2-6) at M062X/6-311++G(d,p) level of theory (in kcal/mol). Corresponding gravimetric energy storage densities (=∆Estorage /Mol. Wt. and ∆Hstorage /Mol. Wt.) are provided in parentheses (in the unit of kcal/kg).

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Table 2: Comparison of Estorage , and gravimetric storage density (kcal/kg) of different conventional solar thermal storage systems with [n.n]anthracenophane (n>3) based MOST systems.

systema

Computational Method

Estorage

Mol. Wt.

Storage Density

(kcal/mol)

(g)

(kcal/kg)

-

-

>125

Li-ion battery

-

NBD systemsa

B3LYP/6-311++G(d,p)

unsubstituted-NBDb

”

23.4

92.1

254.1

R1=Ph-NMe2 , R2=Ph-CF3

”

26.4

355.4

74.3

R1=Ph-OCH3 , R2=Ph-CN

”

26.2

299.4

87.5

AB systemsa

M06-2x/6-311++G(d,p)

AB

”

12.6

182.2

69.5

”

17.6

216.1

81.5

FvRu2 (CO)4

20.8

442.3

47.0

bis(1,1dimethyltridecyl)-

22.7

863.1

26.3

R1,R2 substituted NBDb

AB-derivatives 2,2’-(diazene-1,2-diyl) diphenolc FvRu2 (CO)4 systems

See Ref. 15,16

-FvRu2 (CO)4 d [n.n]anthracenophanesa

M06-2x/6-311++G(d,p)

D2 [4.4]PI

”

33.7

464.3

72.6

D2 [5.5]PI

”

38.1

492.3

77.4

D2 [6.6]PI

”

51.3

520.3

98.6

a

We found that B3LYP performs best for NBD-based systems, while M06-2X is most reliable for both AB based systems and [n.n]anthracenophane-based systems w.r.t CCSD(T) and FN-DMC numbers (see Supplementary Discussion 3 and 6 in the SI for details). For diruthenium-fulvene based systems we used the reported literature values. 15,16 b Unsubstituted NBD does not absorb the UV-Vis solar spectra at all, however, diaryl substitution (with one electron donating group and one electron withdrawing aryl group) is necessary to red-shift the absorption spectra albeit a decrease in gravimetric energy storage density. c Due to hydrogen bonding effects, the energy storage slightly increases in dihydroxy AB-based systems. 7,9 d Long chain alkyl group substitution in the fulvene ring increase the solubility of FvRu (CO) in 2 4 common organic solvents albeit a huge decrease in gravimetric energy storage density. 16

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ERS(INT) (kcal/mol)

25 20

20.0

15

9.8

10

2.9

5

0.4

-1.1 0

[n.n]BA

-5

P.G. Symm.

n=2 (D2)

n=3 (C2h)

n=4 (D2)

n=5 (C2h)

n=6 (D2)

(a) 60

49.7

50

ERS(PDT) (kcal/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


39.1

40

30.6 30

27.3

20

8.8

10

0

[n.n]PI P.G. Symm.

n=2 (D2)

n=3 (C2h)

n=4 (D2)

n=5 (D2)

n=6 (D2)

(b)

Figure 2: (a) calculated ring-strain in the [n.n]BA, ERS (INT) w.r.t the reference compound (9,10)dimethylanthracene monomer; and (b) the ring-strain in the corresponding photoproduct [n.n]PI, ERS (PDT) w.r.t. the reference compound (9,10)dimethylanthracene photodimer product at M06-2X/6-311++G(d,p) level of theory. See Supplementary Discussion 7 in the SI for details.

15



(5)

(1) (2)

(6)

(3)

(7) (8)

(4) [4.4]PI ϕ(1-2-3-4)=ϕ(5-6-7-8)=8.40

(a) (a) (3) (1)

(4) (2)

(3)

(1)

“TC” ϕ(1-2-3-4)=0.00

E (kcal/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

Page 16 of 22

(2)

(4)

(4) (1)

(2)

“TB” ϕ(1-2-3-4)=-59.60

(3) 17.0

“FC” ϕ(1-2-3-4)=50.40

7.8

Reaction Coordinate

(b) Figure 3: (a) Geometry optimized structure of [4.4]PI shows that the four adjacent carbon atoms of the two hexagonal rings are locked in a near zero dihedral angle (φ(1-2-3-4)=φ(5-67-8)=8.4◦ . This indicates that two cyclohexane analogous rings are geometrically very close to a high energy ‘Twist-Chair’ conformation; (b) Thermodynamics of conformational change in 1,1,2,2 tetramethyl cyclohehaxe, between ‘Full-Chair’ (FC) and ‘Twist-Boat’ (TB) conformations via a transition state (TS) of ‘Twist-Chair’ (TC). The ‘TC’ geometry is predicted to be 17.0 kcal/mol higher than the ‘FC’ geometry. (the hydrogen atoms are omitted for clarity)

16


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TSconc

TS2

TS1

BS-INT

(a)

BS-INT

(b)

Figure 4: (a) Total energy plot along reaction coordinate (linear transit scan) for concerted (blue) and step-wise(black) TBR pathway of D2 [4.4]PI→D2 [4.4]BA calculated at M06-2X/631++G(d,p) level of theory. (Inset) d1 (9C-9’C) and d2 (10C-10’C) bond distance correlation diagram for concerted (blue) and stepwise( black) paths. In D2 [4.4]PI, d1 =d2 =1.68 ˚ A and ˚ in D2 [4.4]BA, d1 =d2 =3.81 A. Please note, in step-wise mechanism, D2 [4.4]PI→BS-INT change in d2 is very negligible compared to d1 , and for BS-INT→D2 [4.4]PI the change in d1 is negligible compared to d2 ; (b) The spin-density of the BS-INT at M06-2X/6-31++G(d,p) level of theory (d1 =3.14 ˚ A, d2 = 1.70 ˚ A). The spin-density is located on the carbon atoms of the broken C-C bond.

17



References (1) Lewis, N.; Nocera, D. Powering the Planet: Chemical Challenges in Solar Energy Utilization. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 15729-15735. (2) 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. (3) Lennartson, A.; Roffey, A.; Moth-Poulsen, K. Designing photoswitches for molecular solar thermal energy storage. Tetrahedron Lett. 2015, 56, 1457-1465. (4) Scharf, H. D.; Fleischhauer, J.; Leismann, H.; Ressler, I.; Schleker, W.; Weitz, R. Criteria for the Efficiency, Stability, and Capacity of Abiotic Photochemical Solar Energy Storage Systems. Angew. Chem., Int. Ed. 1979, 18, 652-662. (5) Adamson, A. W.; Vogler, A.; Kunkely, H.; Wachter, R. Photocalorimetry. Enthalpies of Photolysis of transAzobenzene, Ferrioxalate and Cobaltioxalate Ions, Chromium Hexacarbonyl, and Dirhenium Decarbonyl. J. Am. Chem. Soc. 1978, 100, 1298-1300. (6) Taoda, H.; Hayakawa, K.; Kawase, K.; Yamakita, H. Photochemical Conversion and Storage of Solar Energy by Azobenzene. J. Chem. Eng. Jpn. 1987, 20, 265-270. (7) Kolpak, A. M.; Grossman, J. C. Azobenzene-Functionalized Carbon Nanotubes As High-Energy Density Solar Thermal Fuels. Nano Lett. 2011, 11, 3156-3162. (8) Kucharski, T. J.; Feralis, N.; Kolpak, A. M.; Zheng, J. O.; Nocera, D. G.; Grossman, J. C. Templated Assembly of Photoswitches Significantly Increases the Energy-Storage Capacity of Solar Thermal Fuels. Nat. Chem. 2014, 6, 441-447. (9) Lui, Y.; Grossman, J. C. Accelerating the Design of Solar Thermal Fuel Materials through High Throughput Simulations. Nano Lett. 2014, 14, 7046-7050. (10) Durgun, E.; Grossman, J. C. Photoswitchable Molecular Rings for Solar-Thermal Energy Storage. J. Phys. Chem. Lett. 2013, 4, 854-860. 18


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(11) Gray, V.; Lennartson, A.; Ratanalert, P.; Borjesson, K.; Moth-Poulsen, K. Diarylsubstituted Norbornadienes with Red-shifted Absorption for Molecular Solar Thermal Energy Storage. Chem. Commun. 2014, 50, 5330-5332. (12) Kuisma, M. J.; Lundin, A. M., Moth-Poulsen, K.; Hyldgaard, P.; Erhart, P. Comparative Ab-Initio Study of Substituted Norbornadiene-Quadricyclane Compounds for Solar Thermal Storage. J. Phys. Chem. C 2016, 120, 3635-3645. (13) Kusima, M. J.; Lundin, A.; Moth-Poulsen, K.; Erhart, P. Optimization of Norbornadiene Compounds for Solar Thermal Storage by First-Principles Calculations. ChemSusChem 2016, 9, 1786-1794. (14) Dreos, A.; B¨orjesson, K.; Wang, Z.; Roffey, A.; Norwood, Z.; Kushnird, D.; MothPoulsen, K. Exploring the Potential of a Hybrid Device Combining Solar Water Heating and Molecular Solar Thermal Energy Storage. Energy Environ. Sci. 2017, 10, 728-734. (15) kanai, Y.; Srinivasan, V.; Meier, S. K.; Vollhardt, P. C.; Grossman, J, C. Mechanism of Thermal Reversal of the (Fulvalene)tetracarbonyldiruthenium Photoisomerization: Toward Molecular Solar-Thermal Energy Storage. Angew. Chem. Int. Ed. 2010, 49, 8926-8929. (16) Moth-Poulsen, K.; Coso, D.; B¨orjesson, 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. (17) Harpham, M. R. et al. X-ray Transient Absorption and Picosecond IR Spectroscopy of Fulvalene(tetracarbonyl)diruthenium on Photoexcitation. Angew. Chem. Int. Ed. 2012, 51, 7692-7696. ¨ (18) Luther, R.; Weigert, F. Uber umkehrbare photochemische Reaktionen im homogenen System. Anthracen und Dianthracen. I. Zeitschrift f¨ ur Physikalische Chemie 1905, 51, 297-328. 19



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¨ (19) Luther, R.; Weigert, F. Uber Umkehrbare Photochemische Reaktionen im Homogenen System. Anthracen und Dianthracen. II. Zeitschrift f¨ ur Physikalische Chemie 1905, 53, 385-427. (20) Jones, G.; Reinhard, T. E.; Bergmark, W. R. Photon Energy Storage in Organic Materials-The Case of Linked Anthracenes. Sol. Energy 1978, 20, 241-248. (21) Mau, A. W. H. Mau, Albert WH. Solid state reversible reactions. Thermal behaviour of the photoisomer of bi[anthracene-9, 10-dimethylene]. J. Chem. Soc., Faraday Trans. I 1978, 74, 603-612. (22) Birks, J. B.; “Photophysics of Aromatic Molecules”. Wiley-Interscience, New York, N.Y., 1970, 319-323. (23) Stevens, B. Photoassociation in Aromatic Systems. Adv. Photochem. 1971, 8, 161. (24) Jezowski, S. R.; Zhu, L.; Wang, Y.; Rice, P. R.; Scott, G. W.; Bardeen, C. J.; Chronister, E. L. Pressure Catalyzed Bond Dissociation in an Anthracene Cyclophane Photodimer. J. Am. Chem. Soc. 2012, 134, 7459-7466. (25) Slepetz, B.; Kertesz, M. Volume Change during Thermal [4 + 4] Cycloaddition of [2.2] (9,10)Anthracenophane. J. Am. Chem. Soc. 2013, 135, 13720-13727. (26) Longone, D. T.; K¨ usefoglu, S. H.; Gladysz, J. A. A Convenient Synthesis of [3.3] Paracyclophane. J. Org. Chem. 1977, 42, 2787-2788. (27) Glans,

J.

H.;

Longone,

D.

T.

Synthesis

and

Polymerization

of

(E,E)-

[6.2](9,10)Anthracenophane-1,5-diene. J. Polym. Sci. A Polym. Chem. 1989, 27, 467474. (28) Tazuke, S.; Watanabe, H. Photoisomerization and Thermal Reversion of [3.3] (1,4)Naphthaleno (9,10)Anthracenophane Derivatives. Tetrahedron Lett. 1982, 23, 197200. 20


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(29) Gaussian 09, revision A.1; see Supporting Information for full reference. (30) Grimme, S.; Diedrich, C.; Korth, M. The Importance of Inter- and Intramolecular van der Waals Interactions in Organic Reactions: the Dimerization of Anthracene Revisited. Angew. Chem. Int. Ed. 2006, 45, 625-629. (31) We have mentioned that foranthracene based systems both M06-2X and B3PW91-D3 performs best in comparison to FN-DMC numbers (see the SI for calibration), however, B3PW91-D3/6-311++G(d,p) energy storage densitynumbers are a tad higher than that of M06-2X/6-311++G(d,p) numbers. Conversely, B3LYP functional performs best for NBD based systems while M06-2x functional performs best for AB based systems with respect to CCSD(T)/cc-pVTZ predictions (see the SI for calibration). (32) Khoury, P. R.; Goddard, J. D.; Tam, W. Ring Strain Energies: Substituted Rings, Norbornanes, Norbornenes and Norbornadienes. Tetrahedron 2004, 60, 8103-8112. (33) De Lio, A. M.; Durfey, B. L.; Gille, A. L.; Gilbert, T. M. A Semi-homodesmotic Approach for Estimating Ring Strain Energies (RSEs) of Highly Substituted Cyclopropanes That Minimizes Use of Acyclic References and Cancels Steric Interactions: RSEs for c-C3R6 that Make Sense. J. Phys. Chem. A 2014, 118, 6050-6059. (34) Plotnikov, N. V.; Martinez, T. J. Molecular Origin of Mechanical Sensitivity of the Reaction Rate in Anthracene Cyclophane Isomerization Reveals Structural Motifs for Rational Design of Mechanophores. J. Phys. Chem. C 2016, 120, 17898-17908. (35) Please note, BS-uDFT has been shown to perform with reasonable accuracy where high-level ab-initio multi-reference theory based calculations are impractical. Also note, in this particular case a multi-reference 2nd order perturbative technique CASPT2/NEVPT2 would not provide a reliable PES, as apart from the C-C bond breaking region, in [4.4]BA and [4.4]PI the system converges to a single reference system where a single-reference 2nd order perturbative technique (MP2) fails completely 21



(predicted by Grimme et al.), hence, it will introduce a significant error in the computed TBR. (36) Furthermore, the TBR for [5.5]PI → [5.5]BA was also computed. In the step-wise C-C bond cleavage, which turned out to be the lower energy pathway for TBR, the rate determining barrier turned out to be also ∼24 kcal/mol. This indicate that [5.5]PI also can be trapped for a legitimate period of time in its higher energy metastable isomer. Further details are provided in the Supplementary Discussion 8, in the SI. (37) Yang, N. C.; Shold, D. M.; Kim, B. Chemistry of Exciplexes. 5. Photochemistry of Anthracene in the Presence and Absence of Dimethylaniline. J. Am. Chem. Soc. 1976, 98, 6587-6596. (38) Usui, M.; Nishiwaki, T.; Anda, K.; Hida, M. Cyclophanes Containing Nitrogen Atoms in the Bridged Chains. Photochromism of N,N-bis(arylsulfonyl)-2,11diaza[3.3]paracyclo(9,10)anthracenophanes. Chem. Lett. 1984, 13, 1561-1564.

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Designing Efficient Solar-Thermal Fuels with [n

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