Singlet Fission and 4 Electronic States: Amazing Overview

Singlet Fission Introduction:

In a photophysical process called singlet fission (SF), a singlet excited state converts or splits into two excited triplet states, and each triplet state has half as much energy as the singlet excited state. Initially, in the 1960s and 1970s, SF used theory to observe the delayed fluorescence in acene crystals. Interest in SF increased in the 2000s when it became clear that it could be used to improve the efficiency of photon-to-electricity conversion in organic photovoltaic systems.

Singlet fission can improve the organic photovoltaic system’s photon-to-electricity performance from 33 to 47 % by decreasing the thermal relaxation losses involved in the Shockley-Queisser limit. High-frequency photons are converted into lower-energy excitons during the SF phenomenon. The SF process is frequently explained as the consequence of two processes that follow one another, with the initial excited state being connected to the low excited singlet in a monomeric chromophore[1].

However, it is generally agreed that there are two main ways that SF can lead to the development of 1(TT): (a) After the first excitation, the multi-exciton triplet state can be produced through a direct pathway. (b) SF can also be proceeded through the two successive one electron processes through intermediate states having Charge transfer characteristics when the energy of states is not too high. The singlet ground states to excited state to triplet and then charge transfer state, these all states can be used to explain the different energy states of the system consist of two molecules.

The radical cation’s ground state, denoted by C+, and the radical anion’s ground state, denoted by A-, are both present in the charge transfer state. In another theory, due to initial excitation the lowest lying absorption state, charge transfer state and multi-exciton state are all coherently superimposed to each other. The multi-exciton state 1(TT) and quintet state 5(TT) created through spin evolution of the state 1(TT) and then the independent triplet (T1 + T1) states produced through either the 1(TT) and 5(TT) spin decoherence[2].

As a result of the process the coupled triplet-pair state with multiplicity = 1 is generated. Also, there are nine alternative excitation states in pair form that are spin eigenfunctions of Hamiltonian. Through the use of electron paramagnetic resonance (EPR) investigations, it has been discovered that the 1(TT) population relaxes into several states over the course of nanoseconds, such as the quintet 5(TT) state. As a result, it is still difficult to fully comprehend the underlying characteristics and features of the triplet pair state, m(TT)[3].

It is also very important to know that vibrational coupling and CT-state intermixing play a significant role in SF. However, the SF mechanism that is used will determine how much each factor is engaged. The electronically linked (electronically coherent) or electronically decoupled (electronically incoherent) triplet pair states can be formed in SF from the singlet excited state S1.

The formation of triplet pair states (electronically coherent) is explained by the coherent quantum and incoherent mechanisms that allows classical kinetic procedure. In incoherent process triplet pair state is formed through singlet state because of thermal assistance, as the electronic coupling between singlet and triplet state is weaker than the coupling with phonon bath. Contrarily, in coherent process the photoexcitation results in excited state that is superposition of the CT and 1TT state.

Recently proceeded research demonstrates that both mechanisms operate simultaneously at various timescales during the singlet fission process. Observing both the processes expecting due to the photoexcitation that produce a superposition of singlet, triplet, and CT intermediate state and how these species are formed through superposition depend on the energetics of excited state which in turn controlled by SF chromophores and their packing in a molecule.

The only purpose of using SF chromophores in photovoltaic systems is due to their high triplet diffusion coefficient,, and triplet diffusion is perceived as the conversion of individual triplet states into singlet states through triplet-triplet annihilation. Also, if the individual triplets contain singlet-triplet properties, the triplet diffusion is interpreted as coherent triplet pair diffusion[4] [5].

Due to the stringent conditions needed for a molecule to be able to demonstrate singlet fission, the number of organic compounds able to do so has remained limited despite multiple efforts. The fundamental condition for effective SF is that E(S1) ≥ 2E(T1), where E(S1) is the energy of S1 and T1 is the energy of the isolated triplets. This condition shrinks the range of SF chromophores that have very high activation barriers.

The second condition is that the coupling between chromophore must be adequate to produce a rate of SF in range of 1ns-1, to produce the high-quality triplets. The exoergic process is not good for SF as it slows down the speed of photophysical process and also heat up the system result in energy loss. That’s why pentacene system are found to have best energetics because they have S1 level high about twice the T1 energy level[6].

Electronic states present in SF:

The primary electronic states in SF, including the lowest singlet, individual triplets and multi-exciton states are those that characterize the photophysical process. Additionally, a few additional states are also participating in the whole process.

Triplet pair states:

This is an important state of singlet fission. On two adjacent chromophores, the 1(TT) state often refers to a state that develops during the initial stages of SF. The whole fission process occurs by an intermediate state which has multiexciton property and corresponds to the triplet pair state that coupled coherently as a spin singlet, which through valance bond theory described as strong correlation. These corelated triplet state called as 1TT in SF and also called as multiexciton state, doubly excited state and a dark state.

The triplet state as low energy than singlet state because of the slow intercrossing system and exchange interactions of three possible two spin triplet states. S1 exciton delocalization boosts SF rates and speeds up 1(TT) production, as shown by the investigations of acene and rylene oligomers. According to experiments using covalent pentacene, chromophores weak and strong coupling provide inter-interactions between chromophores to increase SF rate. But it is still unclear how m (T1T1) functions in SF rate[7].

Triplet state:

The independent triplet, the last state of SF, has a strong molecular feature and, roughly speaking, corresponds to the HOMO and LUMO of the chromophore occupying a single electron. Long-lived free triplets are formed through decorrelation, but before they can become decorrelated, they must go through electrical decoupling and change from 1(TT) to 5(TT)[8].

Charge transfer/resonance state:

In adiabatically electronic states of dimers, molecular solids and aggregates the localized single and multi-excitons have ability to mix with charge transfer or charge resonance configuration. The charge resonance is symmetric arrangement of chromophores which refer the charged configuration and describing the ionic nature of electronic wave function. On the other hand, charge transfer refers to the net charge displacement.

This process is totally system-dependent, and in acenes it is also rarely populated. It is also noticed that quicker SF is achieved by lowering the energy of the charge transfer state through increased solvent polarity and increasing mixing into low-lying excitonic states. However, in acenes, these functions are mostly dependent on structure[8].

Excimer:

In a ground state, when an excited monomer comes into contact with another monomer of the same atomic structure, an excimer is formed, which is an electronic state of a homodimer.  The excimer also refers to it as a self-trapping state due to the high stabilization energy. Its production results in photoluminescence depletion and a redshift of the fluorescent band. It is formed by nuclear relaxation, which occurs by intermolecular exciton interaction and charge resonance. But the function is still unclear in SF[9].

To learn more about the chemicals and processes, explore the deskchem.com website.

What is the mechanism of singlet fission?

It is generally agreed that there are two main ways that SF can lead to the development of 1 (TT): (a) After the first excitation, the multi-exciton triplet state can be produced through a direct pathway. (b) SF can also be proceeded through the two successive one-electron processes through intermediate states having Charge transfer characteristics when the energy of the states is not too high. The singlet ground states, excited states, triplets, and charge transfer states can all be used to explain the different energy states of the system consisting of two molecules.
The radical cation’s ground state, denoted by C+, and the radical anion’s ground state, denoted by A-, are both present in the charge transfer state. In another theory, due to initial excitation the lowest lying absorption state, charge transfer state, and multi-exciton state are all coherently superimposed to each other. The multi-exciton state 1(TT) and quintet state 5(TT) created through spin evolution of the state 1(TT) and then the independent triplet (T1 + T1) states produced through either the 1(TT) and 5(TT) spin decoherence

Leave a comment