Higher energy triplet-pair states in polyenes and their role in intramolecular singlet fission

TL;DR

This study uses advanced computational methods to identify higher-energy triplet-pair states in polyenes that facilitate intramolecular singlet fission, revealing new candidate states involved in the process.

Contribution

The paper demonstrates that higher-energy singlet states with triplet-pair character can enable singlet fission, expanding understanding of the states involved beyond the traditionally considered ones.

Findings

Higher-energy singlet states like $3^1A_g^-$ can undergo singlet fission.

States such as $1^1B_u^-$, $3^1A_g^-$, and $1^5A_g^-$ have triplet-pair character.

The $2^1A_g^-$ state cannot undergo singlet fission.

Abstract

Probing extended polyene systems with energy in excess of the bright state ($1^1B_u^+$/$S_2$) band edge generates triplets via singlet fission. This process is not thought to involve the $2^1A_g^-$/$S_1$ state, suggesting that other states play a role. Using density matrix renormalisation group (DMRG) calculations of the Pariser-Parr-Pople-Peierls Hamiltonian, we investigate candidate states that could be involved in singlet fission. We find that the relaxed $1^1B_u^-$, and $3^1A_g^-$ singlet states and $1^5A_g^-$ quintet state lie below the $S_2$ state. The $1^1B_u^-$, $3^1A_g^-$ and $1^5A_g^-$ states are all thought to have triplet-triplet character, which is confirmed by our calculations of bond dimerization, spin-spin correlation and wavefunction overlap with products of triplet states. We thus show that there is a family of singlet excitations(i.e., $2^1A_g^-$, $1^1B_u^-$, $3^1A_g^-$, $\cdots$), composed of both triplet-pair and electron-hole character, which are fundamentally the same excitation, but have different center-of-mass energies. The lowest energy member of this family, the $2^1A_g^-$ state, cannot undergo singlet fission. But higher energy members (e.g., the $3^1A_g^-$) state, owing to their increased kinetic energy and reduced electron-lattice relaxation, can undergo singlet fission for certain chain lengths.