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Two-dimensional organic–inorganic hybrid perovskite (2D-OIHP) quantum wells exhibit a triplet of bright exciton fine structure states near the band edge, enabling the generation of transient macroscopic spin alignments with circularly polarized light. Here, we investigate the microscopic origin of photoinduced spin relaxation in 2D-OIHPs using multidimensional coherent spectroscopy together with a theoretical framework that combines time-dependent perturbation theory with the Fokker–Planck equation. Analysis of the spectral line shapes reveals highly correlated exciton fluctuations within the fine structure manifolds of a pair of 2D-OIHPs featuring different organic layer thicknesses and polaron binding energies. In particular, the Gaussian correlation coefficients determined for the two lead-iodide-based systems range from 0.67 to 0.80, while their polaron binding energies span 11.8–18.9 meV. Incorporating time-coincident solvation dynamics into a stochastic model shows that these energy level correlations reduce the exciton–bath couplings and extend dephasing times for spin-flip transitions, even in spectral broadening regimes governed by Marcus-like kinetics (which are typically considered incompatible with motional narrowing). Since photoexcitation occurs on the seam of intersection between the excited-state free energy surfaces, spin relaxation can proceed without an activation barrier, provided it outpaces energy dissipation into the environment. Overall, these results demonstrate that correlated exciton fluctuations play a central role in accelerating spin depolarization in 2D-OIHPs through motional narrowing of coherences between exciton states.
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Partially polaron-transformed quantum master equation for exciton and charge transport dynamics
Polaron-transformed quantum master equation (PQME) offers a unified framework to describe the dynamics of quantum systems in both limits of weak and strong couplings to environmental degrees of freedom. Thus, the PQME serves as an efficient method to describe charge and exciton transfer/transport dynamics for a broad range of parameters in condensed or complex environments. However, in some cases, the polaron transformation (PT) being employed in the formulation invokes an over-relaxation of slow modes and results in premature suppression of important coherence terms. A formal framework to address this issue is developed in the present work by employing a partial PT that has smaller weights for low frequency bath modes. It is shown here that a closed form expression of a second order time-local PQME including all the inhomogeneous terms can be derived for a general form of partial PT, although more complicated than that for the full PT. All the expressions needed for numerical calculation are derived in detail. Applications to a model of a two-level system coupled to a bath of harmonic oscillators, with test calculations focused on those due to homogeneous relaxation terms, demonstrate the feasibility and the utility of the present approach.
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- Award ID(s):
- 1900170
- PAR ID:
- 10440407
- Publisher / Repository:
- American Institute of Physics
- Date Published:
- Journal Name:
- The Journal of Chemical Physics
- Volume:
- 157
- Issue:
- 10
- ISSN:
- 0021-9606
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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