[Paper Review] Quantum dynamics simulation of intramolecular singlet fission in covalently linked tetracene dimer

Sam Mardazad, Yihe Xu+5 more|arXiv (Cornell University)|Jul 29, 2021

Spectroscopy and Quantum Chemical Studies77 references32 citations

TL;DR

This study presents a numerically unbiased quantum dynamics simulation of intramolecular singlet fission in a covalently linked tetracene dimer, using a tensor network method to model coupled excitonic and vibrational degrees of freedom from first principles. The key finding is a coherent dynamics time scale of t₀ ≈ 35 fs, during which most triplet yield is generated, with vibrational modes matching this frequency playing a dominant role in energy transfer and coherence control, while solvent polarity tuning can double the singlet fission yield without altering t₀.

ABSTRACT

In this work we study singlet fission in tetracene para-dimers, covalently linked by a phenyl group. In contrast to most previous works, we account for the full quantum dynamics of the combined excitonic and vibrational system. For our simulations we choose a numerically unbiased representation of the molecule's wave function enabling us to compare with experiments, exhibiting good agreement. Having access to the full wave function allows us to study in detail the post-quench dynamics of the excitons. Here, one of our main findings is the identification of a time scale $t_0 \approx 35 ext{fs}$ dominated by coherent dynamics. It is within this time scale that the larger fraction of the singlet fission yield is generated. We also report on a reduced number of phononic modes that play a crucial role for the energy transfer between excitonic and vibrational system. Notably, the oscillation frequency of these modes coincides with the observed electronic coherence time $t_0$. We extended our investigations by also studying the dependency of the dynamics on the excitonic energy levels that, for instance, can be experimentally tuned by means of the solvent polarity. Here, our findings indicate that the singlet fission yield can be doubled while the electronic coherence time $t_0$ is mainly unaffected.

Motivation & Objective

• To simulate the full quantum dynamics of intramolecular singlet fission in a covalently linked tetracene dimer, including both excitonic and vibrational degrees of freedom.
• To provide a numerically unbiased, first-principles description of the coupled exciton-phonon system, avoiding approximations common in prior methods.
• To identify the key vibrational modes responsible for energy transfer and coherence dynamics in singlet fission.
• To investigate how experimentally tunable parameters like solvent polarity affect the singlet fission yield and coherence time.

Proposed method

• A Frenkel-exciton-Hamiltonian with global coupling between non-local vibrational modes and excitonic states is used to model the system.
• Ab initio multireference quantum chemical calculations are employed to compute the spectral density and determine the coupling between electronic states and vibrational modes.
• A novel one-dimensional tensor network method is applied to simulate the real-time, non-perturbative quantum dynamics of the full vibronic wave function.
• The method dynamically selects relevant bosonic modes, enabling accurate simulation of large Hilbert spaces with high numerical control.
• A generalized Lang-Firsov transformation is used to analyze the renormalization of excitonic hopping integrals due to phonon coupling.
• The full wave function is used to compute time-evolving density matrices and identify coherence and decoherence dynamics.

Experimental results

Research questions

• RQ1 What is the role of coherent vibrational dynamics in driving intramolecular singlet fission in tetracene dimers?
• RQ2 Which specific vibrational modes dominate the energy transfer between electronic and vibrational subsystems?
• RQ3 How does the electronic coherence time t₀ relate to the frequencies of the dominant vibrational modes?
• RQ4 Can the singlet fission yield be enhanced by tuning solvent polarity, and how does this affect the coherence time?
• RQ5 What is the mechanism behind the formation of heavy exciton-phonon quasiparticles and their impact on coherence?

Key findings

• A coherent dynamics time scale of t₀ ≈ 34.725 ± 0.165 fs is identified, during which the majority of the singlet fission yield is generated.
• The oscillation frequency of the dominant vibrational modes matches the electronic coherence time t₀, indicating a direct dynamical link.
• The singlet fission yield ranges from 14% to 25% depending on solvent polarity, in good agreement with experimental measurements of ~21%.
• Solvent polarity tuning can double the singlet fission yield while leaving the electronic coherence time t₀ largely unchanged.
• The super-exchange path via charge-transfer states (CT1, CT2) is found to be the dominant channel for triplet state production, with direct LE→TT hopping being nearly negligible.
• The renormalization of excitonic hopping integrals shows a sign change at t₀, indicating a transition from enhanced to suppressed transition amplitudes into the triplet state, revealing a competition between coherence and quasiparticle formation.