Non-equilibrium relaxation of hot states in organic semiconductors: Impact of mode-selective excitation on charge transfer

Abstract

The theoretical study of open quantum systems strongly coupled to a vibrational environment remains computationally challenging, due to the strongly non-Markovian character of the dynamics. We study this problem in the case of a molecular dimer of the organic semiconductor tetracene, the exciton states of which are strongly coupled to a few hundreds of molecular vibrations. To do so, we employ a previously developed tensor network approach, based on the formalism of matrix product states. By analysing the entanglement structure of the system wavefunction, we can expand it in a tree tensor network state, which allows us to perform a fully quantum mechanical time evolution of the exciton-vibrational system, including the effect of 156 molecular vibrations. We simulate the dynamics of hot states, i.e. states resulting from excess energy photoexcitation, by constructing various initial bath states, and show that the exciton system indeed has a memory of those initial configurations. In particular, the specific pathway of vibrational relaxation is shown to strongly affect the quantum coherence between exciton states in timescales relevant for the ultra-fast dynamics of application-relevant processes such as charge transfer. The preferential excitation of lowfrequency modes leads to a limited number of relaxation pathways, thus 'protecting' quantum coherence, and leading to a significant increase of the charge transfer yield in the dimer structure.

Cyclo[18]carbon including zero-point motion: ground state, first singlet and triplet excitations, and hole transfer

“…46,47 Finally, while our technique focuses on describing the effect of equilibrium nuclear configurations at a chosen temperature, excited state spectroscopy simulations have been implemented to provide information on the effect of vibrations that have been displaced due to electronic excitation, 48 or even on peculiarly displaced vibrational configurations that can lead to nonadiabatic transitions. 49,50 The integral of eqn ( 6) can by approximated using the Monte Carlo method by generating a sample of configurations u, according to the harmonic distribution at a specific temperature T, so that the expectation value of the observable O(T) can be computed as a simple average of the sample values. This method relies on no adjustable parameters, apart from the choice of DFT functional and basis set, which are fixed throughout the entire series of calculations.…”

“…46,47 Finally, while our technique focuses on describing the effect of equilibrium nuclear configurations at a chosen temperature, excited state spectroscopy simulations have been implemented to provide information on the effect of vibrations that have been displaced due to electronic excitation, 48 or even on peculiarly displaced vibrational configurations that can lead to non-adiabatic transitions. 49,50…”

Systematic improvement of molecular excited state calculations by inclusion of nuclear quantum motion: A mode-resolved picture and the effect of molecular size

“…While both the nuclear ensemble and Monte Carlo methods capture the renormalization of exciton energies due to quantum fluctuations, they do not provide information on the individual contribution of normal modes to this effect. For example, low-and high-frequency vibrations in organic systems are known to play different roles during vibrational relaxation 20 and electron-transfer reactions; 21 therefore, obtaining a mode-resolved picture for the exciton energy renormalization is critical to obtaining microscopic insights into the physics of these systems. Such mode-resolved information is available through the use of a quadratic approximation to the exciton-vibration coupling; 18 however, this comes at the cost of only capturing these interactions to third order.…”

Funneling dynamics in a phenylacetylene trimer: Coherent excitation of donor excitonic states and their superposition

“…However, the large coupling of some vibrational modes of this system imposes to work with non-perturbative, non-Markovian dynamical methods. Several methods are available to tackle such dynamics: discretization of the vibrational bath and ML-MCTDH (Multi Layer Multi Configuration Time Dependent Hartree) 61,62 , tensor network states 63 , the Davydov ansatz 64 , or the statistical Schrödinger equation 65,66 . Our choice for this work is the use of HEOM (Hierarchical Equations Of Motion) [67][68][69][70][71][72] .…”