DiaDEM Tutorial: Discovery of Singlet-Fission Materials

The discovery of singlet-fission materials can be achieved with the DiaDEM platform. Candidates can be purchased directly and tested for singlet-fission activity by the researcher.

General theory

Singlet fission can be described as the electronic transition where a singlet state (e.g. \(S_{1}\), typically generated by excitation from the ground state) is converted into two triplet excited states (\(T_{1}\)). This transition involves at least two molecules (or molecular fragments) and the two triplets in the final state are localized in different regions of space, e.g. on two different molecules. The transition is spin allowed, i.e. the state formed from the initial excited state is a multiexcitonic state where the two triplets are combined into an overall singlet (\(^{1}TT\)) that can later dissociate into separate triplets according to the scheme: \(S_{1}\) + \(S_{0}\) ⇌ \(^{1}TT\) ⇌ \(T_{1}\) + \(T_{1}\).

The enormous interest in singlet fission today is motivated by its potential applications in photovoltaic technologies. Through singlet fission a photon can generate two long lived excited states. One of the assumptions for the derivation of the thermodynamic limit of a single-junction solar cell, known as the Shockley–Queisser limit, is that a single photon can generate only one excited state (an electron–hole pair). A device able to generate two excited states per photon may be able to overcome this assumption and opens a path to increased efficiency, e.g. producing more energy under solar irradiation.

Requirements for singlet fission

Clearly, a requirement for singlet fission to take place is for the first singlet state energy to be more than double the first triplet state energy, e.g. \(E(S_{1})\) > 2x \(E(T_{1})\); something very rarely observed. Further conditions can be imposed, such as a bright \(S_{1}\) state by ensuring an efficiently large oscillator strength \(f(S_{1})\). Additionally, quenching through the reverse of singlet-fission, known as triplet-triplet annihilation, should be minimised by ensuring that the second triplet state be greater than twice that of the first singlet state: \(E(T_{2})\) > 2x \(E(T_{1})\). Additional criteria such as low flexibility to avoid conical intersections, and thus nonradiative decay, can be considered with the number of rotatable bonds (\(n_{rots}\)). All of the above values are present for the molecules in the DiaDEM database and can be used to find singlet-fission candidates directly.

Searching for candidates

Finding singlet-fission candidates based on the requirements detailed above can be achieved quite simply. To begin, filter the database using the main singlet-fission criterion. This can be performed using the Search functionality, found on the navigation bar. Begin by selecting the + Group button, then create three fields using + Rule. For the first Rule field choose the Property Name to be equal to \(E(S_{1})\). For the second Rule, select Value and set it to to be greater than \(E(T_{1})\) x2. For the third and final Rule, select Calculator equal to Default, then Submit Query.

This will search the DiaDEM database for all molecules which satisfy the main singlet-fission criterion. From this condition alone, known singlet-fission designs can be noticed such as structures related to para-quinone like TCNQ, pentacene derivatives, other polyacenes, polyenes, cumulenes, diketopyrrolopyrroles and indigo analogues. This helps to verify the search.

For additional constraints, return to the Search function and use the + Group button to add another condition block within the same query. Continue by choosing the oscillator strength for the first singlet transition \(f(S_{1})\) and set this Value to be greater than 0.05. This value can be modified; the greater the value, the brighter the \(f(S_{1})\) state is predicted to be, removing those entries with forbidden or low-intensity transitions. Once again, use Default for the Calculator, and Submit Query.

Use the + Group function multiple times in one search for additional constraints, e.g. for minimising triplet-triplet annihilation using the \(E(T_{2})\) > 2x \(E(T_{1})\) relation and/or nonradiative decay which is apparent in more flexible molecules and can, to a quick approximation, be mitigated with the number of rotatable bonds (\(n_{rots}\)).

Performing calculations

Although promising singlet-fission materials can be found simply by searching and purchasing materials directly for laboratory testing, additional properties to create a more specialist library of molecules may be desired. For example, a small excitonic reorganization energy for better singlet exciton dissociation is crucial e.g. for an efficient photovoltaic device. This can be computed on-demand with DiaDEM.

In the Search results, clicking on a molecular entry will open a new page with greater detail, including more properties, availability and price. From here, select Run on-demand calculation to add the molecule to a new Project, then choose the Calculate function on the navigation bar. Here, the calculation’s details can be chosen; for the above example, the Property to select is Exciton reorganization energy (\(S_{1}\)) with Default for the Calculator. On the top right, the runtime, CPU cost and cost is given and can be reviewed before hitting the Calculate button.

Once a calculation is finished, a Notification will appear which can be opened from the navigation bar. If email notifications are enabled, an automated email will be sent upon completion. This notification will lead to a calculation Results page where the newly computed property can be reviewed.

Purchasing materials

All of the molecules on the DiaDEM database are commercially available and can be purchased directly for laboratory testing; however, availability for a material at any given time can vary.

In the Search results, clicking on a molecular entry will open a new page with greater detail, including more properties, availability and price. You can add a material to cart by clicking Add to cart. The cart can be accessed at the top right of the banner. From here, the quantity of material can be modified. An Indicative price will be shown and a purchase request can be made by pressing Submit request. After processing, final pricing, delivery time and additional information will be provided, and a Notification will appear which can be opened from the navigation bar. If email notifications are enabled, an automated email will be sent.

Reference

Padula, D., Omar, Ö. H., Nematiaram, T. & Troisi, A. Singlet fission molecules among known compounds: Finding a few needles in a haystack. Energy Environ. Sci. 12, 2412–2416 (2019).