About

Professor Greg Engel leads the Engel Group at The University of Chicago, focusing on exploiting femtosecond dynamics to steer and control excited state reactivity. His research integrates ultrafast spectroscopy, theory, synthesis, and biophysics to understand and manipulate quantum effects in chemical and biological systems. The group investigates natural mechanisms, such as those found in photosynthesis, vision, and photoenzymes, to isolate new design principles for controlling quantum dynamics. Their work spans multiple research areas including quantum biology, ultrafast spectroscopy, quantum materials, photocatalysis, theory, coherent dynamics, light harvesting materials, ultrafast chiral response, and live cell studies. These efforts aim to reveal how biological systems harness quantum mechanical phenomena and to develop new materials and strategies for quantum communication, information processing, and light harvesting. The group also pioneers new spectroscopic tools to probe femtosecond electronic excitations in living cells, protein complexes, and small molecules, enabling unprecedented insight into ultrafast chemical dynamics and their control in biological contexts.

Research topics

• Physics
• Quantum mechanics
• Computer Science
• Artificial Intelligence
• Chemistry
• Atomic physics
• Biology
• Chemical physics
• Ecology
• Computational biology

Selected publications

• CCDC 2495125: Experimental Crystal Structure Determination

  Open MIND · 2026-02-06

  datasetOpen access

  An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

  OA PDFDOI

• Iminium-linked hyperporphyrin covalent organic framework mediates type I photodynamic therapy via a photoredox process

  Nature Communications · 2026-03-28

  articleOpen access

  Most photosensitizers (PSs) mediate type II photodynamic therapy (PDT) via energy transfer to produce singlet oxygen. However, this mechanism is oxygen-dependent and less effective in hypoxic tumors. Type I PDT, which generates radical reactive oxygen species such as superoxide through electron transfer, is more hypoxia-tolerant, yet molecular design strategies remain limited. Herein, we report an iminium-linked hyperporphyrin covalent organic framework (IH-COF) that facilitates efficient type I PDT via a photoredox process. In a one-pot synthesis, trimethyloxonium tetrafluoroborate simultaneously quaternizes imine bonds to introduce electron acceptors and protonates porphyrins, red-shifting the Q-band to 725 nm via the hyperporphyrin effect. Mechanistic studies reveal that photoinduced electron transfer from hyperporphyrin units to iminium ions generates α-amino radicals, which reduce oxygen to superoxide while regenerating iminium ions. The oxidized hyperporphyrins are then reduced by biomolecules such as 1,4-dihydronicotinamide adenine dinucleotide, sustaining the photocatalytic cycle. Consequently, IH-COF exhibits excellent PDT performance under both normoxic and hypoxic conditions and elicits potent antitumor efficacy in colorectal and triple-negative breast cancer models in female mice. This study highlights the potential of COFs as versatile and biocompatible platforms for synergistic photomedicine applications.

  PublisherOA PDFDOI

• Ultrafast Carrier Self-Trapping Driven by Strong Exciton–Phonon Coupling in 2D MA ₃ Bi ₂ I ₆ Cl ₃ Perovskite

  Journal of the American Chemical Society · 2026-02-05 · 1 citations

  article

  Two-dimensional (2D) hybrid bismuth halide perovskites have emerged as promising lead-free materials for optoelectronic applications due to their solution processability and tunable structures. Here, we investigate 2D layered hybrid perovskite MA3Bi2I6Cl3 using temperature-dependent photoluminescence (PL) and femtosecond transient absorption spectroscopy. Our results reveal strong coupling between excitons and phonons, evidenced by giant Huang–Rhys factors, coherent phonon oscillations, and ultrafast carrier self-trapping into small-polaron and self-trapped exciton (STE) states. These processes appear as time-dependent ground-state bleach and photoinduced absorption features, highlighting the influence of the lattice in carrier dynamics. Wavelength- and time-resolved measurements reveal that PL emission is dominated by STEs, while free exciton emission is weak and short-lived. By comparing 2D MA3Bi2I6Cl3 with 0D MA3Bi2I9, which contains molecularly isolated [BiI6]3− octahedra and 2D MA3Bi2Br9 perovskites, we demonstrate how halide composition and structural dimensionality influence the balance between free exciton populations and carrier localization. These insights uncover the intrinsic kinetic landscape of photoexcited states in MA3Bi2I6Cl3. Overall, our study contributes to a mechanistic understanding of exciton–phonon interactions in lead-free 2D perovskites.

  PublisherDOI

• Non-perturbative exciton transfer rate analysis of the Fenna–Matthews–Olson photosynthetic complex under reducing and oxidizing conditions

  The Journal of Chemical Physics · 2025-03-18 · 2 citations

  articleOpen access

  Two-dimensional optical spectroscopy experiments have examined photoprotective mechanisms in the Fenna-Matthews-Olson (FMO) photosynthetic complex, showing that exciton transfer pathways change significantly depending on the environmental redox conditions. Higgins et al. [Proc. Natl. Acad. Sci. U. S. A. 118(11), e2018240118 (2021)] have theoretically linked these observations to changes in a quantum vibronic coupling, whereby onsite energies are altered under oxidizing conditions such that exciton energy gaps are detuned from a specific vibrational motion of the bacteriochlorophyll a. These arguments rely on an analysis of exciton transfer rates within Redfield theory, which is known to provide an inaccurate description of the influence of the vibrational environment on the exciton dynamics in the FMO complex. Here, we use a memory kernel formulation of the hierarchical equations of motion to obtain non-perturbative estimations of exciton transfer rates, which yield a modified physical picture. Our findings indicate that onsite energy shifts alone do not reproduce the reported rate changes in the oxidative environment. We systematically examine a model that includes combined changes in both site energies and the frequency of a local vibration in the oxidized complex while maintaining consistency with absorption spectra and achieving qualitative, but not quantitative, agreement with the measured changes in transfer rates. Our analysis points to potential limitations of the FMO electronic Hamiltonian, which was originally derived by fitting spectra to perturbative theories. Overall, our work suggests that further experimental and theoretical analyses may be needed to understand the variations of exciton dynamics under different redox conditions.

  PublisherDOI

• Exciton-phonon coupling and phonon-assisted exciton relaxation dynamics in In1-xGaxP quantum dots

  Nature Communications · 2025-05-13 · 10 citations

  articleOpen accessSenior author

  Quantum dots leverage quantum confinement to modify the electronic structure of materials, separating electronic transitions from the composition of the corresponding bulk material. With ternary quantum dots, the composition may be varied continuously so that both composition and size may be used to tune the bandgap. As composition influences electron-phonon coupling which in turn governs relaxation dynamics, the composition of ternary quantum dots may be adjusted to change dynamics. Here, we show that exciton-phonon coupling and phonon-assisted exciton relaxation dynamics remain strongly correlated to material composition in ternary In0.62Ga0.38P/ZnS and In0.35Ga0.65P/ZnS quantum dots using both experimental two-dimensional electronic spectroscopy measurements and quantum dynamical simulations. Theoretical calculations show that alloyed In1-xGaxP quantum dots have more complex exciton level structure than parent InP quantum dots. We identify a slower hot exciton cooling rate in In0.62Ga0.38P/ZnS, attributed to the presence of ‘energy-retaining’ valley exciton states with strong exciton-phonon coupling. Experimental quantum beating maps reveal a more localized quantum beat pattern for In0.35Ga0.65P/ZnS quantum dots, which may relate to the increased number of ‘dim’ exciton levels with reduced spacings. These findings highlight that exciton relaxation dynamics and exciton-phonon coupling in an alloyed In1-xGaxP quantum dot system are composition-dependent. Excited state dynamics of alloyed quantum dots differ from that of binary quantum dots. Here, the authors use femtosecond spectroscopy and theoretical calculations to show that alloying tunes relaxation dynamics separately from traditional optical properties of quantum dots.

  PublisherOA PDFDOI

• Connectivity-Dependent Exciton–Phonon Coupling in Cesium Bismuth Halide Quantum Dots

  ACS Nano · 2025-03-06 · 4 citations

  articleSenior authorCorresponding

  Metal halide octahedra form the fundamental functional building blocks of metal halide perovskites, dictating their structures, optical properties, electronic structures, and dynamics. In this study, we show that the connectivity of bismuth halide octahedra in Cs3Bi2Br9 and Cs3Bi2I9 quantum dots (QDs) changes with different halide elements. We use first-principles calculations to reveal the key role of the connectivity of bismuth halide octahedra on the wave function symmetry, Huang–Rhys factor, and exciton–phonon interaction strength. Following QD synthesis via a ligand-mediated transport method, the effect of connectivity is verified with transient absorption spectroscopy, where we contrast Cs3Bi2Br9 and Cs3Bi2I9 QD exciton dynamics. In photoexcited Cs3Bi2I9 QDs, phonons related to the vibrational motions of face-sharing [BiI6]3– bioctahedra couple strongly to the electronic state and drive rapid carrier relaxation. Equivalent signals are not observed for photoexcited Cs3Bi2Br9 QDs, implying a lack of phonon involvement in band-edge absorption and subsequent exciton relaxation. Our findings suggest that structural engineering can effectively tune the exciton–phonon coupling and therefore influence exciton relaxation and recombination in perovskite nanomaterials.

  PublisherDOI

• Efficient up-conversion in CsPbBr3 nanocrystals via phonon-driven exciton-polaron formation

  Nature Communications · 2025-07-01 · 7 citations

  articleOpen access

  Lead halide perovskite nanocrystals demonstrate efficient up-conversion, although the precise mechanism remains a subject of active research. This study utilizes steady-state and time-resolved spectroscopy methods to unravel the mechanism driving the up-conversion process in CsPbBr3 nanocrystals. Employing above- and below-gap photoluminescence measurements, we extract a distinct phonon mode with an energy of ~7 meV and identify the Pb-Br-Pb bending mode as the phonon involved in the up-conversion process. This result was corroborated by Raman spectroscopy. We confirm an up-conversion efficiency reaching up to 75%. Transient absorption measurements under conditions of sub-gap excitation also unexpectedly reveal coherent phonons for the subset of nanocrystals undergoing up-conversion. This coherence implies that the up-conversion and subsequent relaxation is accompanied by a synchronized and phased lattice motion. This study reveals that efficient up-conversion in CsPbBr3 nanocrystals is powered by a unique interplay between the soft lattice structure, phonons, and excited states dynamics. Here, the Authors show that up-conversion in CsPbBr3 nanocrystals arises from an interaction between soft lattice vibrations and excited states, with a specific phonon mode enabling coherent and highly efficient up conversion process.

  PublisherOA PDFDOI

• Functional Connectivity of Red Chlorophylls in Cyanobacterial Photosystem I Revealed by Fluence-Dependent Transient Absorption

  The Journal of Physical Chemistry B · 2025-03-18 · 3 citations

  articleOpen access

  External stressors modulate the oligomerization state of photosystem I (PSI) in cyanobacteria. The number of red chlorophylls (Chls), pigments lower in energy than the P700 reaction center, depends on the oligomerization state of PSI. Here, we use ultrafast transient absorption spectroscopy to interrogate the effective connectivity of the red Chls in excitonic energy pathways in trimeric PSI in native thylakoid membranes of the model cyanobacterium Synechocystis sp. PCC 6803, including emergent dynamics, as red Chls increase in number and proximity. Fluence-dependent dynamics indicate singlet–singlet annihilation within energetically connected red Chl sites in the PSI antenna but not within bulk Chl sites on the picosecond time scale. These data support picosecond energy transfer between energetically connected red Chl sites as the physical basis of singlet–singlet annihilation. The time scale of this energy transfer is faster than predicted by Förster resonance energy transfer calculations, raising questions about the physical mechanism of the process. Our results indicate distinct strategies to steer excitations through the PSI antenna; the red Chls present a shallow reservoir that direct excitations away from P700, extending the time to trapping by the reaction center.

  PublisherOA PDFDOI

• Optically accessible long-lived electronic biexcitons at room temperature in strongly coupled H- aggregates

  Nature Communications · 2024-09-27 · 2 citations

  articleOpen accessSenior author

  Photon absorption is the first process in light harvesting. Upon absorption, the photon redistributes electrons in the materials to create a Coulombically bound electron-hole pair called an exciton. The exciton subsequently separates into free charges to conclude light harvesting. When two excitons are in each other's proximity, they can interact and undergo a two-particle process called exciton-exciton annihilation. In this process, one electron-hole pair spontaneously recombines: its energy is lost and cannot be harnessed for applications. In this work, we demonstrate the creation of two long-lived excitons on the same chromophore site (biexcitons) at room temperature in a strongly coupled H-aggregated zinc phthalocyanine material. We show that exciton-exciton annihilation is suppressed in these H- aggregated chromophores at fluences many orders of magnitudes higher than solar light. When we chemically connect the same aggregated chromophores to allow exciton diffusion, we observe that exciton-exciton annihilation is switched on. Our findings demonstrate a chemical strategy, to toggle on and off the exciton-exciton annihilation process that limits the dynamic range of photovoltaic devices.

  PublisherOA PDFDOI

• Vibronic Conical Intersection Trajectory Signatures in Wave Packet Coherences

  The Journal of Physical Chemistry Letters · 2024-12-13 · 7 citations

  articleSenior authorCorresponding

  Conical intersections are ubiquitous in the energy landscape of chemical systems, drive photochemical reactivity, and are extremely challenging to observe spectroscopically. Using two-dimensional electronic spectroscopy, we observe the nonadiabatic dynamics in Wurster's Blue after excitation to the lowest two vibronic excited states. The excited populations relax ballistically through a conical intersection in 55 fs to the electronic ground state potential energy surface as the molecule undergoes an intramolecular electron transfer. While the kinetics are identical on both vibronic energy surfaces, we observe different patterns of coherent oscillations after traversing the conical intersection indicating distinct nonadiabatic relaxation pathways through the conical energetic funnel. These coherences are not created directly by the excitation pulses but are the result of the dynamical trajectories projecting differently on the conical intersection vibrational space. Our spectroscopic data offers a fresh perspective into the complex conical intersection topology and dynamics that emphasizes the critical involvement of the intersection space in dictating the dynamics.

  PublisherDOI

Recent grants

• Chiral Rates and Dynamics: Time- and Frequency-Resolved Chiral Electronic Spectroscopy for Electronic Structure, Dynamics, and Quantum Optics

  NSF · $450k · 2019–2022

• Graduate Program in Biophysical Sciences at the University of Chicago

  NIH · $4.7M · 2009–2024

• QLCI-CI: NSF Quantum Leap Challenge Institute for Quantum Sensing in Biophysics and Bioengineering

  NSF · $26.9M · 2021–2027

Frequent coauthors

• Graham R. Fleming

  Lawrence Berkeley National Laboratory

  76 shared

• Elizabeth L. Read

  University of California, Irvine

  68 shared

• Marco A. Allodi

  University of Chicago

  62 shared

• Peter D. Dahlberg

  Stanford Synchrotron Radiation Lightsource

  61 shared

• Gabriela S. Schlau‐Cohen

  Massachusetts Institute of Technology

  53 shared

• Andrew F. Fidler

  Los Alamos National Laboratory

  49 shared

• Sabre Kais

  45 shared

• Justin R. Caram

  41 shared

Labs

• The Engel GroupPI

  The Engel Group at The University of Chicago studies excited state dynamics and ultrafast phenomena to understand and to reveal new design strategies for controlling excited state reactivity.

Awards & honors

• Coblentz Award
• National Security Science and Engineering Faculty Fellowship
• Vannevar Bush Fellowship
• Sloane Fellowship
• Searle Scholar Award