Enhu He

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.

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.