97 research outputs found
Photochemical Initiation of Polariton Propagation
Placing a material inside an optical cavity can enhance transport of
excitation energy by hybridizing excitons with confined light modes into
polaritons, which have a dispersion that provides these light-matter
quasi-particles with low effective masses and very high group velocities. While
in experiments polariton propagation is typically initiated with laser pulses,
tuned to be resonant either with the polaritonic branches that are delocalized
over many molecules, or with an uncoupled higher-energy electronic excited
state that is localized on a single molecule, practical implementations of
polariton-mediated exciton transport into devices would require operation under
low-intensity incoherent light conditions. Here, we propose to initiate
polaritonic exciton transport with a photo-acid, which upon absorption of a
photon in a spectral range not strongly reflected by the cavity mirrors,
undergoes ultra-fast excited-state proton transfer into a red-shifted
excited-state photo-product that can couple collectively with a large number of
suitable dye molecules to the modes of the cavity. By means of atomistic
molecular dynamics simulations we demonstrate that cascading energy from a
photo-excited donor into the strongly coupled acceptor-cavity states can indeed
induce long-range polariton-mediated exciton transport
Tuning the Coherent Propagation of Organic Exciton-Polaritons through the Cavity Q-factor
Transport of excitons in organic materials can be enhanced through polariton
formation when the interaction strength between these excitons and the confined
light modes of an optical resonator exceeds their decay rates. While the
polariton lifetime is determined by the Q(uality)-factor of the optical
resonator, the polariton group velocity is not. Instead, the latter is solely
determined by the polariton dispersion. Yet, experiments suggest that the
Q-factor also controls the polariton propagation velocity. To understand this
observation, we performed molecular dynamics simulations of Rhodamine
chromophores strongly coupled to Fabry-P\'erot cavities with various Q-factors.
Our results suggest that propagation in the aforementioned experiments is
initially dominated by ballistic motion of upper polariton states at their
group velocities, which leads to a rapid expansion of the wavepacket. Cavity
decay in combination with non-adiabatic population transfer into dark states,
rapidly depletes these bright states, causing the wavepacket to contract.
However, because population transfer is reversible, propagation continues, but
as a diffusion process, at lower velocity. By controlling the lifetime of
bright states, the Q-factor determines the duration of the ballistic phase and
the diffusion coefficient in the diffusive regime. Thus, polariton propagation
in organic microcavities can be effectively tuned through the Q-factor.Comment: arXiv admin note: text overlap with arXiv:2209.0730
The 'hidden side' of spin labeled oligonucleotides: Molecular Dynamics study focusing on the EPR-silent components of base pairing Corresponding authors
Enhanced Excitation Energy Transfer under Strong Light-Matter Coupling: Insights from Multi-Scale Molecular Dynamics Simulations
Transfer of excitation energy is a key step in light harvesting and hence of
technological relevance for solar energy conversion. In bare organic materials
energy transfer proceeds via incoherent hops, which restrict propagation
lengths to nanometers. In contrast, energy transport over several micrometers
has been observed in the strong coupling regime where excitations hybridise
with confined light modes to form polaritons. Because polaritons have group
velocity, their propagation should be ballistic and long-ranged. However,
experiments indicate that organic polaritons propagate in a diffusive manner
and more slowly than their group velocity. Here, we resolve this controversy by
means of molecular dynamics simulations of Rhodamine molecules in a
Fabry-P\'erot cavity. Our results suggest that polariton propagation is limited
by the cavity lifetime and appears diffusive due to reversible population
transfers between bright polaritonic states that propagate ballistically at
their group velocity, and dark states that are stationary. Furthermore, because
long-lived dark states transiently trap the excitation, propagation is observed
on timescales beyond the intrinsic polariton lifetime. These atomistic insights
not only help to better understand and interpret experimental observations, but
also pave the way towards rational design of molecule-cavity systems for
achieving coherent long-range energy transport
Chromophore Protonation State Controls Photoswitching of the Fluoroprotein asFP595
Fluorescent proteins have been widely used as genetically encodable fusion tags for biological imaging. Recently, a new class of fluorescent proteins was discovered that can be reversibly light-switched between a fluorescent and a non-fluorescent state. Such proteins can not only provide nanoscale resolution in far-field fluorescence optical microscopy much below the diffraction limit, but also hold promise for other nanotechnological applications, such as optical data storage. To systematically exploit the potential of such photoswitchable proteins and to enable rational improvements to their properties requires a detailed understanding of the molecular switching mechanism, which is currently unknown. Here, we have studied the photoswitching mechanism of the reversibly switchable fluoroprotein asFP595 at the atomic level by multiconfigurational ab initio (CASSCF) calculations and QM/MM excited state molecular dynamics simulations with explicit surface hopping. Our simulations explain measured quantum yields and excited state lifetimes, and also predict the structures of the hitherto unknown intermediates and of the irreversibly fluorescent state. Further, we find that the proton distribution in the active site of the asFP595 controls the photochemical conversion pathways of the chromophore in the protein matrix. Accordingly, changes in the protonation state of the chromophore and some proximal amino acids lead to different photochemical states, which all turn out to be essential for the photoswitching mechanism. These photochemical states are (i) a neutral chromophore, which can trans-cis photoisomerize, (ii) an anionic chromophore, which rapidly undergoes radiationless decay after excitation, and (iii) a putative fluorescent zwitterionic chromophore. The overall stability of the different protonation states is controlled by the isomeric state of the chromophore. We finally propose that radiation-induced decarboxylation of the glutamic acid Glu215 blocks the proton transfer pathways that enable the deactivation of the zwitterionic chromophore and thus leads to irreversible fluorescence. We have identified the tight coupling of trans-cis isomerization and proton transfers in photoswitchable proteins to be essential for their function and propose a detailed underlying mechanism, which provides a comprehensive picture that explains the available experimental data. The structural similarity between asFP595 and other fluoroproteins of interest for imaging suggests that this coupling is a quite general mechanism for photoswitchable proteins. These insights can guide the rational design and optimization of photoswitchable proteins
Conical intersections and photochemical mechanisms: Characterising the conical intersection hyperline using gradients, second-derivatives, and dynamics
The primary structural photoresponse of phytochrome proteins captured by a femtosecond X-ray laser
Phytochrome proteins control the growth, reproduction, and photosynthesis of plants, fungi, and bacteria. Light is detected by a bilin cofactor, but it remains elusive how this leads to activation of the protein through structural changes. We present serial femtosecond X-ray crystallographic data of the chromophore-binding domains of a bacterial phytochrome at delay times of 1 ps and 10 ps after photoexcitation. The data reveal a twist of the D-ring, which leads to partial detachment of the chromophore from the protein. Unexpectedly, the conserved so-called pyrrole water is photodissociated from the chromophore, concomitant with movement of the A-ring and a key signaling aspartate. The changes are wired together by ultrafast backbone and water movements around the chromophore, channeling them into signal transduction towards the output domains. We suggest that the observed collective changes are important for the phytochrome photoresponse, explaining the earliest steps of how plants, fungi and bacteria sense red light.Peer reviewe
The room temperature crystal structure of a bacterial phytochrome determined by serial femtosecond crystallography
Phytochromes are a family of photoreceptors that control light responses of plants, fungi and bacteria. A sequence of structural changes, which is not yet fully understood, leads to activation of an output domain. Time-resolved serial femtosecond crystallography (SFX) can potentially shine light on these conformational changes. Here we report the room temperature crystal structure of the chromophore-binding domains of the Deinococcus radiodurans phytochrome at 2.1 angstrom resolution. The structure was obtained by serial femtosecond X-ray crystallography from microcrystals at an X-ray free electron laser. We find overall good agreement compared to a crystal structure at 1.35 angstrom resolution derived from conventional crystallography at cryogenic temperatures, which we also report here. The thioether linkage between chromophore and protein is subject to positional ambiguity at the synchrotron, but is fully resolved with SFX. The study paves the way for time-resolved structural investigations of the phytochrome photocycle with time-resolved SFX.Peer reviewe
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