Photosystem II (PSII) reaction center (RC) is a unique complex that is capable of efficiently separating electronic charges across the membrane. The primary energy- and charge-transfer (CT) processes occur on comparable ultrafast timescales, which makes it extremely challenging to understand the fundamental mechanism responsible for the near-unity quantum efficiency of the transfer. Here, we elucidate the role of quantum coherences in the ultrafast energy and CT in the PSII RC by performing two-dimensional (2D) electronic spectroscopy at the cryogenic temperature of 20 kelvin, which captures the distinct underlying quantum coherences. Specifically, we uncover the electronic and vibrational coherences along with their lifetimes during the primary ultrafast processes of energy and CT. We construct an excitonic model that provides evidence for coherent energy and CT at low temperature in the 2D electronic spectra. The principles could provide valuable guidelines for creating artificial photosystems with exploitation of system-bath coupling and control of coherences to optimize the photon conversion efficiency to specific functions.

A reaction center is a unique biological system that performs the initial charge separation within a Photosystem II (PSII) multiunit enzyme, which eventually drives the catalytic water-splitting in plants and algae. The possible role of quantum coherences coinciding with the energy and charge transfer processes in PSII reaction center is one of the active areas of research. Here, we study these quantum coherences by using a numerically exact method on an excitonic dimer model, including linear vibronic coupling and employing optimal parameters from experimental two-dimensional coherent spectroscopic measurements. This enables us to precisely capture the excitonic interaction between pigments and the dissipation of the energy from electronic and charge-transfer (CT) states to the protein environment. We employ the time nonlocal (TNL) quantum master equation to calculate the population dynamics, which yields numerically reliable results. The calculated results show that, due to the strong dissipation, the lifetime of electronic coherence is too short to have direct participation in the charge transfer processes. However, there are long-lived vibrational coherences present in the system at frequencies close to the excitionic energy gap. These are strongly coupled with the electronic coherences, which makes the detection of the electronic coherences with conventional techniques very challenging. Additionally, we unravel the strong excitonic interaction of radical pair ($P_{D1}$ and $P_{D2}$) in the reaction center, which results in a long-lived electronic coherence of >100 fs, even at room temperature. Our work provide important physical insight to the charge separation process in PSII reaction center, which may be helpful for better understanding of photophysical processes in other natural and artificial light-harvesting systems.

Iron(II) complexes are increasingly recognised as sustainable, earth-abundant alternatives to precious metal chromophores, though their use has been constrained by ultrafast excited-state deactivation through low-lying metal-centred (MC) states. Here, we present a comprehensive theoretical investigation of the iron carbene complex [Fe(bmip)2]2+ (bmip = 2,6-bis(3-methyl-imidazole-1-ylidene)pyridine), elucidating how vibronic coupling, spin-orbit coupling (SOC), and environmental reorganisation collectively modulate spin-crossover (SCO) dynamics following metal-to-ligand charge transfer (MLCT) excitation. Using ab initio electronic structure calculations and hierarchical equations of motion simulations, we trace population transfer from the initially photoexcited singlet MLCT to triplet states. The strong σ-donor character of the carbene ligands raises the MC state energies, thereby stabilising the MLCT manifold and extending its lifetime to several picoseconds, around two orders of magnitude longer than in conventional Fe(II) polypyridines. The results reveal sub-100 fs intersystem crossing (ISC) from singlet to triplet MLCT states, followed by a slower decay that avoids rapid relaxation to MC levels. Importantly, ISC proceeds efficiently only via specific vibrational modes, identifying vibronic coupling as the principal gating mechanism. These findings provide a clear framework for tuning ligand fields and coupling strengths to optimise excited-state lifetimes and spin dynamics in next-generation Fe-based photosensitisers.

DNA exhibits remarkable photostability, largely attributed to ultrafast internal conversion pathways that dissipate absorbed UV photon energy through conical intersections (CIs). While isolated nucleobases undergo rapid deactivation on sub-picosecond timescales, their incorporation into stacked and paired DNA architectures introduces significant alterations to excited-state dynamics. Base stacking and Watson-Crick pairing give rise to delocalized excitonic and charge-transfer (CT) states that display extended lifetimes and complex relaxation behavior. In this work, we combine a linear vibronic coupling (LVC) Hamiltonian with an effective transformation to a structured system-bath representation, enabling explicit inclusion of key vibrational modes that drive nonadiabatic transitions. The hierarchical equations of motion (HEOM) method is employed to simulate population dynamics in systems ranging from adenine monomers to stacked heptamers, as well as A–T paired motifs and a combined (A–A)–T trimer. Our simulations show that increasing the number of stacked bases accelerates the initial decay rate (lifetimes decreasing from ∼300 fs in dimers to ∼230 fs in heptamers) but reduces the overall recovery to the ground state by up to ∼40%, indicating population trapping in long-lived excitonic states. Base-pairing effects are modeled via proton-coupled electron transfer (PCET) using a double Morse potential representation. For both A–T and C–G pairs, PCET was found to suppress direct ground-state recovery, further stabilizing excited-state populations. In the combined stacking–pairing configuration, PCET-mediated pairing reduced deactivation more effectively than stacking alone, especially in short oligomers. This vibrationally resolved framework captures the quantitative interplay between excitonic coupling, CIs, and PCET, providing mechanistic insight into how DNA structural motifs govern the fate and lifetime of photoexcited states.

Investigating exciton dynamics within DNA nucleobases is essential for comprehensively understanding how inherent photostability mechanisms function at the molecular level, particularly in the context of life’s resilience to solar radiation. In this paper, we introduce a mathematical model that effectively simulates the photoexcitation and deactivation dynamics of nucleobases within an ultrafast timeframe, particularly focusing on wave-packet dynamics under conditions of strong nonadiabatic coupling. Employing the hierarchy equation of motion, we simulate two-dimensional electronic spectra (2DES) and calibrate our model by comparing it with experimentally obtained spectra. This study also explores the effects of base stacking on the photo-deactivation dynamics in DNA. Our results demonstrate that, while strong excitonic interactions between nucleobases are present, they have a minimal impact on the deactivation dynamics of the wave packet in the electronic excited states. We further observe that the longevity of electronic excited states increases with additional base stacking and pairing, a phenomenon accurately depicted by our excitonic model. This model enables a detailed examination of the wave packet’s motion on electronic excited states and its rapid transition to the ground state. Additionally, using this model, we studied base stacks in DNA hairpins to effectively capture the primary exciton dynamics at a reasonable computational scale. Overall, this work provides a valuable framework for studying exciton dynamics from single nucleobases to complex structures such as DNA hairpins.