arXiv subjects

Oskar Kefer

Publications and source records attributed to Oskar Kefer.

3 recordsLinked to original sources

arXiv2025

FRET-Enhanced Singlet Fission in Pentacene Derivatives

Conversion of solar energy with high quantum efficiencies is a key challenge in energy technologies. Excitation energy transfer (EET) mechanisms, such as F\"orster resonance energy transfer (FRET), play a crucial role in facilitating minimal energy loss in biological light-harvesting systems by directing excitation energy to conversion centers. Inspired by this, we show that singlet fission (SF) sensitizers are multi-exciton generation centers, to which surrounding molecules funnel excitation energy via FRET. We study the impact of such EET on SF using two structurally distinct yet optically similar pentacene derivatives: a stability-enhanced ''Gel\"ander'' pentacene, and the well-studied TIPS-pentacene. Transient absorption spectroscopy reveals a $R^{6}$ dependence of the SF rate on molecular separation $R$ in binary acene:polymethylmetacrylate thin film blends, which is typical for FRET. Optimizing FRET is a promising direction for future improvements in light harvesting using SF materials, inspired by natural light-harvesting complexes.

cond-mat.mtrl-sci↗

arXiv2024

Experimental Evidence of Direct Exchange Interaction Mediating Intramolecular Singlet Fission in Weakly-Coupled Dimers

The electronic interaction between an optically active singlet state ($S_1S_0$) and a dark state of singlet multiplicity, known as correlated triplet pair ($^1[TT]$), plays a crucial role in the effective transformation from $S_1S_0$ to $^1[TT]$ during intramolecular singlet fission (iSF). This process is understood through mechanisms such as direct exchange coupling and incoherent processes that involve super-exchange coupling through charge-transfer states. However, most insights into these mechanisms are derived from theoretical studies due to the difficulties in obtaining experimental evidence. In this study, we investigate the excited-state interactions between $S_1S_0$ and $^1[TT]$ in spiro-conjugated iSF sensitizers by employing transient two-dimensional electronic spectroscopy. This approach allows us to focus on the early stages of the conversion from $S_1S_0$ to $^1[TT]$. Upon optical excitation, a superposition of $S_1S_0$ and $^1[TT]$ is created, which gradually transitions to favor $^1[TT]$ within the characteristic time frames of iSF. The observed high-order signals indicate circular repopulation dynamic that effectively reinitiates the iSF process from higher energy electronic states. Our findings, supported by semi-quantum-mechanical simulations of the experimental data, suggest the presence of a direct iSF mechanism in the dimers, facilitated by weak non-adiabatic coupling between $S_1S_0$ and $^1[TT]$. This experiment provides new insights into the equilibrium between the two electronic states, a phenomenon previously understood primarily through theoretical models.

physics.chem-ph↗

arXiv2024

Dark excitons and hot electrons modulate exciton-photon strong coupling in metal-organic optical microcavities

Polaritons, formed as a result of strong hybridization of matter with light, are promising for important applications including organic solar cells, optical logic gates, and qubits. Owing to large binding energies of Frenkel excitons (matter), strong matter-light coupling phenomena are possible at room temperature, high exciton densities, and even with low-quality-factor microcavities. In such cases, due to polaritons' high degree of delocalization, simultaneous effects from dark excitons and hot electrons may affect performance of potential devices. Their understanding, therefore, is of paramount importance, but their disentanglement in optical spectroscopy, however, thus far remained unattainable. Here, we overcome this challenge by careful and systematic analysis of transient polaritonic spectra, supported by analytical models. In doing so, we conclude that dark excitons affect the strength of exciton-photon coupling and manifest themselves as Fano-like polaritonic gain-loss spectra. Free electrons add additional loss component to and imprint a two-temperature dynamics on the polaritonic response. The developed general methodology can be applied to a variety of other microcavity structures. Our findings are significant for distinguishing polaritons and other excitations in studies of polariton-electron and plasmon-electron coupling phenomena as well as photonic control over photophysical and photochemical processes.

physics.app-ph↗