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Photochromic Molecules for Energy Storage

Postdoctoral Fellowship | 14/10/2016

One major challenge for efficient exploitation of solar energy is to be able to store the energy. This project has focused on developing photochromic molecules that undergo controlled cycles of light harvesting, energy storage, and energy release. Specifically, the current research is based on tuning the properties of the dihydroazulene-vinylheptafulvene (DHA/VHF) photo/thermoswitch. 

We are aiming at a high energy difference between the DHA and VHF isomers (storage capacity) and a long storage time (half-life of the metastable VHF). Preferably, the energy is stored for an infinitely long time and released on-demand by a trigger. The project is rooted in organic synthesis and a selection of new molecules has been prepared and studied. We are taking a systematic approach where the influence of each structural modification is investigated. The most important results are that we have increased the storage capacity by a factor close to four and managed to increase the lifetime of the metastable isomer significantly. These are important steps towards using photochromic molecules for solar energy storage although there is still a long way to actual applications.

Introduction

The Sun occupies the most abundant form of energy, but periods of supply do not always match periods of demand. Therefore finding solutions for storing solar energy is a major challenge. In this project we have taken a rather unconventional approach based on photochromic molecules that upon irradiation undergo a structural change to higher energy isomers – corresponding to light-harvesting. The high-energy isomer will ultimately return to the more stable isomer, releasing the energy as heat. This corresponds overall to a closed energy cycle with no chemical oxidation products being released, and which can be repeated over and over again – until photodegradation ultimately may occur. 

Factors to be Optimised for Molecular Solar Thermal Energy Storage Systems

•Photostability
•Solar spectrum match
•Quantum yield of photoisomerisation
•Photoactivity in only one direction (preferred)
•Energy storage capacity
•Lifetime of energy-releasing back-reaction
•Triggering of back-reaction by a specific stimulus

Several factors need to be optimised for such molecular systems as outlined in Box 1. Our focus points in this project have been to develop molecules that can store a sufficient amount of energy and stay in the high-energy form for a long period. Specifically, our goal has been to modify the dihydroazulene-vinylheptafulvene (DHA/VHF) couple shown in Figure 1. DHA undergoes a ring-opening reaction upon irradiation to form VHF, which in time returns to DHA in a ring-closing reaction. The quantum yield of photoisomerisation is quite high (ca. 55%) and DHA absorbs light at a wavelength of ca. 350 nm that can be further redshifted by functionalisation towards the solar spectrum. Yet, the DHA-VHF couple shown in Figure 1 only has an energy storage capacity of 0.1 MJ kg-1, while values close to 1 MJ kg-1 are desirable for real applications. In addition, the lifetime of the VHF isomer is rather short – ca. 3½ hours, but depending on the medium in which the compound is dissolved. Thus, two major challenges are:

  1. How is the energy storage capacity of the DHA-VHF couple increased?
  2. How is the rate of the VHF to DHA back-reaction decreased?

Figure 1. Dihydroazulene (DHA) undergoes upon irradiation a conversion to vinylheptafulvene (VHF), which in time returns to DHA, releasing the energy as heat.

Increasing the Energy Storage Capacity – Linking Photoisomerisation to Loss of Aromaticity Aromatic molecules like benzene are particularly stable so we reasoned that if we could combine the DHA to VHF conversion with a loss of an aromatic unit incorporated in the system, then we could possibly reach higher energy storage capacities. Calculations by our colleague Prof. Kurt V. Mikkelsen and his group in our Center for Exploitation of Solar Energy at University of Copenhagen showed that, indeed, the structure shown in Figure 2 would have an energy storage capacity close to four times that of the parent system. 

A synthetic route for achieving this molecule was developed and the compound was then subjected to switching studies. Unfortunately, it turned out that it did not undergo the desired photoisomerisation, but instead other photoisomerisation reactions. We managed nevertheless to achieve the VHF isomer by chemical means, but found that it underwent instantaneous ring-closure to the original DHA isomer, thereby regaining the favorable aromatic character.

Figure 2. Loss of aromaticity has a significant influence on the energy difference between DHA and VHF (Gibbs free energies).

If instead an aromatic unit is fused to the VHF system as shown in Figure 3, it becomes so stable that the ring-closure reaction is completely prohibited. In all, aromaticity is indeed a strong tool for controlling the properties, but what we gain in regard to energy storage capacity is lost in regard to optimum switching properties. It is clear that this structural modification has to be combined with others in order to make the right balance between energy storage capacity and forward and backward switchings. In our publication over this work in Chemistry – A European Journal (DOI: 10.1002/chem.201601190), a selection of refined target molecules is suggested.

Figure 3. A VHF incorporating an aromatic benzene ring does not undergo ring-closure to form the corresponding DHA.

Functionalisation at C1 of DHA – Tuning the Rate of the Energy-Releasing Back-Reaction 

In early 2015 we showed that exchanging one of the cyano groups at C1 of DHA for a hydrogen atom (Figure 4) had a remarkable consequence for the lifetime of the corresponding VHF – as soon as formed it would not return again to DHA and calculations predicted a half-life of years at ambient temperature. This modification was accompanied by a convenient doubling of the energy storage capacity, so it is possible to have the structural modifications to work in concert in regard to desired properties. We have in this project further investigated the influence of functionalization at the C1 position and made a selection of molecules, involving new synthetic protocols. In summary, we have found that the lifetime of the VHF can vary from seconds to years by changing the functional groups at C1 (Figure 4). This work was published in RSC Advances (DOI: 10.1039/C6RA06045E).

Figure 4. The rate of the energy-releasing VHF to DHA reaction depends strongly on the groups present at C1.

Macrocyclisation – Energy Release on Two Different Timescales

As a third structural change, we imagined that incorporating two DHA units in a macrocyclic motif could have consequences for the back-reactions. We managed to prepare the first such macrocycle (Figure 5) and found that not only are the two DHA units opened to VHF units in a stepwise manner upon irradiation, but the two VHFs also return to DHAs at two different timescales. Thus, the one VHF to DHA conversion occurs with a half-life around 1 hour in acetonitrile, while the second occurs with a half-life around four days. This behaviour could potentially meet demands for immediate and long-term storage. The work has been published in Chemistry – A European Journal (DOI: 10.1002/chem.201602512), and we are working now on the preparation of macrocycles of various sizes to investigate how the length of the bridging unit influences the properties. In addition, we are working on elucidating the thermodynamics associated with the conversions.

Figure 5. DHA-VHF conversions in a macrocyclic system – stepwise conversions are observed.

Conclusions

We have shown that it is possible to strongly tune the properties of the DHA-VHF photochromic system by various structural modifications. For an optimum molecule for solar energy storage, we may have to combine these various modifications, which sometimes work against each other and sometimes together in regard to desired properties. We are also working on finding catalysts for triggering the back-reaction.

Acknowledgements

The Carlsberg Foundation and the University of Copenhagen are gratefully acknowledged for supporting this work. By supporting the stay of Dr. Martina Cacciarini at University of Copenhagen, the grant from the Carlsberg Foundation has considerably strengthened our research on developing new ways for storing solar energy. We also thank all of the co-authors on the three publications that have resulted so far from the work.  

References

A. B. Skov, S. L. Broman, A. S. Gertsen, J. Elm, M. Jevric, M. Cacciarini, A. Kadziola, K. V. Mikkelsen, M. B. Nielsen, ”Aromaticity-Controlled Energy Storage Capacity of the Dihydroazulene-Vinylheptafulvene Photochromic System,” Chem. Eur. J., In press. DOI: 10.1002/chem.201601190

M. Cacciarini, M. Jevric, J. Elm, A. U. Petersen, K. V. Mikkelsen, M. B. Nielsen, “Fine-tuning the lifetimes and energy storage capacities of meta-stable vinylheptafulvenes via substitution at the vinyl position,” RSC Adv. 2016, 6, 49003-49010. DOI: 10.1039/C6RA06045E

A. Vlasceanu, S. L. Broman, A. S. Hansen, A. B. Skov, M. Cacciarini, A. Kadziola, H. G. Kjaergaard, K. V. Mikkelsen, M. B. Nielsen, ”Solar Thermal Energy Storage in a Photochromic Macrocycle,” Chem. Eur. J. 2016, 22, 10796-10800. DOI: 10.1002/chem.201602512