Ajay Jha

Conical intersections (CIs) govern ultrafast radiationless relaxation in molecular excited states, but the dynamical consequences of the spatial extent of interstate coupling around a CI remain comparatively unexplored. Here, we investigate how confining the nonadiabatic coupling to a localized region around the CI modifies population transfer, vibrational coherence, and their signatures in two-dimensional electronic spectroscopy (2DES). The system is described by a three-state/two-mode vibronic Hamiltonian containing a ground state and two electronically excited states coupled to explicit tuning and coupling vibrational modes, together with dissipative Drude baths. A Gaussian modulation is introduced into the interstate coupling to compare delocalized, moderately localized, and strongly localized coupling scenarios. Quantum dissipative dynamics are propagated using the hierarchical equations of motion, while waiting-time-dependent 2DES is calculated through the equation-of-motion phase-matching formalism. Localization strongly reshapes the effective excited-state landscape and produces nonmonotonic population-transfer kinetics: intermediate localization yields the fastest transfer, whereas the delocalized model retains the strongest residual oscillations and strong localization suppresses them most efficiently. The calculated 2D spectra reveal pronounced redistribution of diagonal and off-diagonal features as the wave packet traverses the CI region, with localization generating distinct cross-peak growth, sign inversion, and vibronic progression patterns. Peak-resolved dynamics and Fourier analysis identify coherent components near 150, 500, and 1000 cm − 1 , while sliding-window transforms demonstrate that localized coupling selectively suppresses low-frequency coupling-mode coherence while preserving or enhancing tuning-mode vibrational signatures. These results establish the spatial extent of nonadiabatic coupling as an important control parameter in CI dynamics.

In this paper, we investigate the coherent control of nonadiabatic dynamics at a conical intersection (CI) using engineered ultrafast laser pulses. Within a model vibronic system, we tailor the pulse chirp and temporal profile and compute the resulting wave packet population and coherence dynamics using projections along the reaction coordinate. This approach allows us to resolve the detailed evolution of wave packets as they traverse the degeneracy region with a strong nonadiabatic coupling. By systematically varying pulse parameters, we demonstrate that both chirp and pulse duration modulate vibrational coherence and alter branching between competing pathways, leading to controlled changes in the quantum yield. The reported yield corresponds to the dynamical branching outcome following passage through the CI rather than the thermodynamic steady-state population of the open quantum system. Our results elucidate the dynamic mechanisms underlying pulse-shaped control near CIs and establish a general framework for manipulating ultrafast nonadiabatic processes.

For years, scientists have debated whether plants and algae use quantum coherence to enhance the transfer of light energy during photosynthesis. Some studies have suggested that wavelike quantum effects might help energy flow more efficiently between pigments. To investigate this idea, we studied allophycocyanin (APC), a light-harvesting protein complex found in cyanobacteria. Using a combination of ultrafast laser spectroscopic methods and rigorous hierarchical equations-of-motion simulations, we examined both the trimer protein complex and its monomer subunit. The experiments detected rapid oscillations in the optical signals that resembled quantum beats. However, temperature-dependent studies and further analysis showed that these signals were caused by vibrations rather than true quantum superpositions. The excitonic transitions of APC exhibit ultrafast dephasing, within ∼40 femtoseconds at low temperatures and ∼20 femtoseconds at room temperature. This is much shorter than the time required for energy to move between chromophores, which is around 344 femtoseconds. In contrast, certain vibrational motions lasted much longer and remained largely unaffected by temperature. Similar vibrations were observed in both the full protein complex and its isolated subunits, confirming that long-lived oscillations do not rely on interactions between pigments. These findings suggest that long-lived oscillations in APC are predominantly vibrational rather than functionally significant electronic coherence.

Photosystem I (PSI) converts light into chemical energy with near-unity quantum efficiency, yet its energy-transfer and charge-separation mechanisms remain debated. Evolution has diversified PSI architectures: Cyanobacterial PSI trimers confine red-shifted pigments to the core, whereas plant PSI-Light Harvesting Complex Isupercomplexes incorporate extensive peripheral red and charge-transfer states that reshape trapping. The unicellular red alga Cyanidioschyzon merolae exemplifies functional diversification across distinct evolutionary branches, combining a photosystem II and plant-like monomeric PSI core associated with a varying number of light harvesting antenna subunits, Light Harvesting Complexes from Red Lineage (LHCR). This hybrid organization functionally bridges mechanistic models across different lineages. We applied two-dimensional electronic spectroscopy at ultralow temperatures (8 and 80 K) to disentangle overlapping excitation pathways in C. merolae PSI. Cryogenic measurements suppressed thermal broadening, resolving five dynamical components: subpicosecond equilibration (0.3 to 0.8 ps) across the core–LHCR interface, subsequent population transfer (2.6 to 4 ps) into progressively lower-energy manifolds, and slower feeding (18 to 53 ps) into red pools distributed across both core and antenna. On the longest timescales (hundreds of ps), a persistent ground-state bleach signifies excitons stabilized in terminal sinks. Notably, comparison of 8 K and 80 K spectra reveals that excitations are heterogeneously partitioned among multiple sinks at low disorder, whereas modest thermal activation (kT ∼ 55 cm ⁻¹ ) promotes selective convergence into core-associated red chlorophylls. Atomistic excitonic modeling with time-nonlocal master equations supports these observations, revealing temperature-dependent energy redistribution. Overall, C. merolae PSI expands the kinetic funnel by distributing trapping sites, enhancing spectral coverage while maintaining high efficiency, which is an important functional diversification during evolution.

Efficient charge separation at donor-acceptor interfaces underpins organic photovoltaics and related optoelectronic technologies, yet the ultrafast vibronic processes enabling this remain incompletely understood. Charge-transfer (CT) excitons form and evolve on ultrafast timescales within a congested manifold of states strongly coupled to vibrational modes. Directly resolving such dynamics requires spectroscopies that correlate electronic and nuclear motion. Here, we present quantum-dynamical simulations of a donor-acceptor system with four CT states coupled to a high-frequency tuning mode. Using the Hierarchy Equations of Motion, we compute two-dimensional electronic spectroscopy (2DES) and two-dimensional electronic–vibrational spectroscopy (2DEV) signals. The 2DES maps show rapid exciton-to-CT transfer within ∼100-200 fs, accompanied by vibrational coherences near 940 cm−1, though spectral congestion obscures individual state contributions. By contrast, 2DEV disentangles these overlaps, revealing distinct cross-peaks that track vibronic wavepacket motion across the CT landscape. Oscillatory features at ∼1467 cm−1 highlight long-lived vibrational coherence persisting for several hundred femtoseconds. These results demonstrate that 2DEV provides vibrationally resolved fingerprints of CT dynamics inaccessible to 2DES alone. The predicted coherence signatures offer concrete experimental observables and suggest that targeted vibrational couplings can transiently promote charge separation. More broadly, this work establishes combined 2DES/2DEV approaches as powerful probes of vibronic dynamics and points to new design strategies for tuning vibrational couplings to optimise energy conversion in donor-acceptor systems.

Iron(II) complexes are increasingly recognized as sustainable, earth‐abundant alternatives to precious metal chromophores, though their use has been constrained by ultrafast excited‐state deactivation through low‐lying metal‐centered (MC) states. Here, we present a comprehensive theoretical investigation of the iron carbene complex [Fe(bmip) ₂ ] ²⁺ (bmi p = 2,6‐bis(3‐methyl‐imidazole‐1‐ylidene)pyridine), elucidating how vibronic coupling, spin–orbit coupling (SOC), and environmental reorganization 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 stabilizing 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 optimize excited‐state lifetimes and spin dynamics in next‐generation Fe‐based photosensitisers.

Chirality, the absence of mirror symmetry, is a fundamental molecular property with far-reaching consequences from chemistry to biology. Yet enantiosensitive optical responses are very weak. Here, we introduce a theoretical framework in which a chiral optical cavity under strong coupling directly lifts the degeneracy of opposite enantiomers at the electronic-dipole level. The cavity's parity-breaking field inside the cavity induces distinct site-energy shifts for left- versus right-handed molecules, producing robust enantioselective polariton states that overcome the weakness of traditional chiroptical effects. Using cavity quantum electrodynamics simulations, we show that strong light-matter coupling reshapes the polaritonic energy landscape and leads to enantiomer-specific coherence lifetimes and relaxation pathways. To reveal these dynamics, we propose ultrafast two-dimensional electronic spectroscopy (2DES) as a probe, capable of resolving polaritonic splittings on femtosecond timescales. Simulated 2DES spectra exhibit unambiguous enantioselective signatures of the cavity-induced asymmetry. These findings establish that chiral cavities provide a powerful platform for detecting and controlling molecular handedness beyond the limits of conventional optical methods.

Intersystem crossing (ISC) is a central nonradiative pathway in transition-metal complexes, critically shaping their behavior in photocatalysis and optoelectronics. In 3d metal systems, ligand-field (LF, d-d) excited states are especially important, yet the mechanisms governing their ISC remain obscure because LF transitions are commonly masked by intense metal-to-ligand charge-transfer (MLCT) bands. This knowledge gap is significant, as LF states frequently participate in deactivation channels and directly influence the photochemical reactivity. Here, we focus on unraveling the ultrafast ISC dynamics in a carefully chosen model system with a simplified electronic structure: cobalt(III)-acetylacetonate, ([Co(acac)₃]), a d⁶ low-spin complex. Upon selective ¹A₁ → ¹T₁ ligand-field excitation promoting a t_2g → e_g * electron, heterodyne-detected transient grating measurements with ∼10 fs pulses reveal vibrational coherences decaying in ∼50 fs, matching ISC dynamics. Fourier analysis reveals both low- and high-frequency vibrational modes associated with Co-O stretching and Co-O-C bending that actively mediate the spin-state transition. Complementary two-dimensional electronic spectroscopy (2DES) disentangles overlapping signals and localizes vibrational activity near the ¹T₁ excited-state absorption. Density functional theory (DFT) and GPU-accelerated hierarchy equation of motion (HEOM) calculations confirm that vibronic coupling, in concert with spin-orbit coupling (SOC), enables rapid singlet-to-triplet conversion via dynamic modulation of excited-state energies and reorganization along key nuclear coordinates. These results reveal that following LF excitation vibronic coupling plays an important role in reshaping excited-state potential energy surfaces and facilitating ISC in systems where SOC alone is weak. This work establishes a mechanistic foundation for understanding and controlling excited-state pathways in LF-dominated 3d transition-metal complexes.

This review critically explores quantum coherence and dissipation in photosynthetic protein complexes, revealing how system–bath interactions, rather than long-lived coherence, govern robust and efficient downhill energy transfer.

Intersystem crossing (ISC) is an important nonradiative process in transition metal complexes, governing their performance in photocatalysis and optoelectronic applications. Among the various excited states accessible in metal complexes, ligand-field (LF) or d-d excited states are particularly important in 3d transition metal systems due to their generality and fundamental role in dictating their photophysical properties. However, the mechanisms of ISC following direct LF excitation remain poorly understood, largely because these transitions are typically overshadowed by intense metal-to-ligand charge transfer (MLCT) bands in most complexes. This gap in our understanding is critical, as LF states are often involved in the deactivation pathways of photoexcited 3d transition metal complexes and directly influence their photochemical behaviour. In this study, we focus on unravelling the ultrafast ISC dynamics in a carefully chosen model system with a simplified electronic structure: cobalt(III)-acetylacetonate, ([Co(acac)3]), a d6 low-spin complex. Upon selective ligand-field excitation (1A1 → 1T1), one electron is promoted from bonding t2g orbital to antibonding eg∗ orbital. Using heterodyne-detected transient grating spectroscopy with ∼10 fs pulses, we observe vibrational coherences that decay on a ∼50 fs timescale, matching ISC kinetics. Fourier analysis reveals both low- and high-frequency vibrational modes associated with Co-O stretching and Co-O-C bending that actively mediate spin-state transition. Complementary two-dimensional electronic spectroscopy (2DES) disentangles overlapping signals and localizes vibrational activity near the 1T1 excited-state absorption. Density functional theory (DFT) and GPU-accelerated hierarchy equation of motion (HEOM) calculations confirm that vibronic coupling, in concert with spin-orbit coupling (SOC), enables rapid singlet-to-triplet conversion via dynamic modulation of excited-state energies and reorganization along key nuclear coordinates. These findings demonstrate that, upon LF excitation, vibronic coupling plays an important role in dynamically modulating the excited-state potential energy landscapes, thereby facilitating efficient ISC in regimes where SOC alone would be inadequate. By identifying vibronic coupling as an important factor driving ISC in LF-excited 3d metal complexes, this study provides a foundational understanding essential for probing and manipulating excited-state dynamics in systems characterized by intrinsically weak SOC. The resulting insights pave the way for the rational design of photoresponsive first-row transition metal complexes with potential applications in catalysis, solar energy conversion, and molecular photophysics.