About

Art Bragg is an Associate Professor in the Department of Chemistry at Johns Hopkins University. His research focuses on the characterization of light-driven chemical dynamics and material behaviors, utilizing ultrafast spectroscopic techniques to observe processes occurring on timescales shorter than 10^-13 seconds, which correspond to atomic and molecular motions. His work aims to determine fundamental structure-dynamics relationships that underlie molecular and material responses to light excitation. His specific research interests include the properties and dynamics of excited conjugated materials, which are significant for applications in flexible optoelectronics such as LEDs and solar cells. He investigates how molecular-level structural attributes influence the behavior of photoexcitations. Additionally, Bragg studies light-driven or switchable bond formation and isomerization processes, which are mechanisms for converting light energy into mechanical or chemical energy and for reversibly altering the photophysical properties of materials. His research also encompasses the photodynamics of plasmonic metal nanoparticles and assemblies, aiming to understand how their architecture affects their photoresponses and potential for light collection or harvesting. Throughout his work, he develops applications of photo-selective ultrafast time-resolved spectroscopies, contributing to the advancement of experimental physical chemistry.

Research topics

• Optics
• Physics
• Nanotechnology
• Materials science
• Atomic physics
• Photochemistry
• Molecular physics
• Composite material
• Optoelectronics
• Chemical physics
• Chemistry
• Quantum mechanics

Selected publications

• Engineering non-haem enzymes for nickel-catalysed C(sp2)‒S coupling via ligand-to-metal charge transfer photocatalysis

  Nature Synthesis · 2026-02-27 · 1 citations

  article
  PublisherDOI

• Singlet-Fission Dynamics Modified through Templated Organic Semiconductor Crystallization

  ChemRxiv · 2025-11-12

  articleSenior author

  Singlet fission (SF) is a process of multiexciton generation in molecular semiconductor materials that holds promise for improving light-to-charge conversion efficiencies in photovoltaic devices. Molecular packing structure is well-known to impact the electronic coupling that underlies triplet-pair generation, as well as pair separation and triplet transport that occur through singlet fission, ultimately affecting the potential to increase conversion efficiencies in light-harvesting devices. We previously demonstrated that templating the crystallization of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-Pn) films on a series of lead-halide perovskites provides a means to control intermolecular packing and alter the kinetics of triplet-pair separation and recombination. Here we contrast triplet transport with the ultrafast interconversion of the singlet exciton to the correlated triplet pair and the dependence of these processes on template-modified crystal packing. We observe that rate constants for the conversion of singlet excitons to correlated triplet pairs likewise are sensitive to the template structure but with a trend that is anticorrelated with those for triplet-pair separation and triplet-triplet annihilation. This observation is consistent with the divergent requisite frontier orbital overlap symmetries of adjacent chromophores that underlie the electronic coupling associated with the generation of triplet pairs and the subsequent triplet transport. We examine how the template-dependent packing structures, as determined by molecular dynamics simulations of a film-template interface, alter figures of merit for electronic interactions (estimated by frontier-orbital overlaps) that underlie each step of the SF mechanism. Our results demonstrate that templating is a general platform for tuning the relative rates of deactivation of correlated triplet pairs in SF-active materials with the potential to favor higher triplet-pair yields or for assisting singlet-mediated triplet-transfer mechanisms without structural modification of the SF-active chromophore.

  PublisherDOI

• Singlet-Fission Dynamics Modified through Templated Organic Semiconductor Crystallization

  ChemRxiv · 2025-10-15

  preprintSenior author

  Singlet fission (SF) is a process of multiexciton generation in molecular semiconductor materials that holds promise for improving light-to-charge conversion efficiencies in photovoltaic devices. Molecular packing structure is well-known to impact the electronic coupling that underlies triplet-pair generation, as well as pair separation and triplet transport that occur through singlet fission, ultimately affecting the potential to increase conversion efficiencies in light-harvesting devices. We previously demonstrated that templating the crystallization of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-Pn) films on a series of lead-halide perovskites provides a means to control intermolecular packing and alter the kinetics of triplet-pair separation and recombination. Here we contrast triplet transport with the ultrafast interconversion of the singlet exciton to the correlated triplet pair and the dependence of these processes on template-modified crystal packing. We observe that rate constants for the conversion of singlet excitons to correlated triplet pairs likewise are sensitive to the template structure but with a trend that is anticorrelated with those for triplet-pair separation and triplet-triplet annihilation. This observation is consistent with the divergent requisite frontier orbital overlap symmetries of adjacent chromophores that underlie the electronic coupling associated with the generation of triplet pairs and the subsequent triplet transport. We examine how the template-dependent packing structures, as determined by molecular dynamics simulations of a film-template interface, alter figures of merit for electronic interactions (estimated by frontier-orbital overlaps) that underlie each step of the SF mechanism. Our results demonstrate that templating is a general platform for tuning the relative rates of deactivation of correlated triplet pairs in SF-active materials with the potential to favor higher triplet-pair yields or for assisting singlet-mediated triplet-transfer mechanisms without structural modification of the SF-active chromophore.

  PublisherDOI

• Engineering Non-haem Enzymes for Nickel-Catalyzed C(sp2)‒S Coupling via Ligand-to-Metal Charge Transfer Photocatalysis

  ChemRxiv · 2025-08-19 · 2 citations

  articleOpen access

  Integrating new metal-catalysed transformations into enzymes is a key objective in biocatalysis. This study introduces photoinduced ligand-to-metal charge transfer (LMCT) as a new strategy for enabling abiotic cross-coupling reactions in metalloenzymes. By tailoring the primary coordination sphere to establish a 2-histidine metal binding site and replacing the iron center with nickel, the ethylene-forming enzyme from Pseudomonas savastanoi (PsEFE) was activated for nickel-catalysed C(sp2)‒S cross-coupling between aryl bromides and thiols. Directed evolution of PsEFE produced highly active variants capable of generating over 50 thioether products in up to 98% yield and 530 total turnover numbers. Mechanistic investigations suggest that this photoenzymatic reaction involves a Ni(II)/Ni(I)/Ni(III) catalytic cycle with generation of a reactive Ni(I) species and thiyl radical via photoinduced LMCT. We anticipate that these findings will inspire further exploration of integrating abiotic cross-coupling transformations into enzymatic catalysis.

  PublisherOA PDFDOI

• Structure-Dependent Photochemical Dynamics of Modified <i>ortho</i>-Terphenyls Studied with One and Two-Photon Probes

  The Journal of Physical Chemistry A · 2025-06-16

  articleSenior authorCorresponding

  We report on structure-dependent one- and two-photon induced photochemical dynamics of modified ortho-terphenyls (OTPs), model 6π electrocyclizing photoreactants that exhibit distinct branching between reactive and nonreactive nonradiative excited-state deactivation channels. Using both pump–probe transient absorption spectroscopy (TAS) and pump–repump–probe (PRP) TAS, we find an enhanced efficiency for cyclization over nonreactive deactivation with the addition of 4,4″ alkyl substitution on pendant rings. The nonreactive deactivation rate is largely unaffected by structural modifications, whereas the cyclization rate to form trans-4a,4b-dihydrotriphenylene (trans-DHT) decreases with increasing substituent size. Based on these observations, we conclude that the reactive and nonreactive deactivation channels involve distinctly different structural dynamics within low-lying electronic excited states to reach critical geometries (i.e., conical intersections). We show that the cis-DHT photoproduct yield obtained by resonant 1 + 1′ excitation is sensitive to dynamics in the low-lying excited-states of OTP that are in turn sensitive to structural modifications. Within the sub-ps time regime, PRP and TAS signals evolve on significantly different timescales, highlighting that these methods have different contrasts for probing dynamics on low-lying potential energy surfaces.

  PublisherDOI

• Spectroscopic, morphological, and dielectric distinctions between a conjugated polymer and its incorporated monomer in field-effect transistors and capacitors

  Journal of Applied Physics · 2025-06-04

  articleOpen access

  We investigate the effects of incorporating the monomer 2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene (BTTT) into thin films of its corresponding polymer, poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT). We examine how this incorporation influences the film's morphology, charge storage capabilities, and dielectric properties. In tri-layer dielectric organic field-effect transistor devices with pentacene as the semiconductor layer, the addition of BTTT to the PBTTT-polystyrene dielectric layer results in increased drain currents and unique threshold voltage shift behaviors, indicating enhanced charge storage capabilities. The key step in this mechanism is that a constant portion of charge-stabilizing entities is generated continuously with the presence of applied voltage. Capacitance measurements show a peak in charge storage at low BTTT concentrations, followed by a decrease at higher concentrations. Notably, dielectric strength analysis using Weibull statistics indicates that films with 20% BTTT content exhibit higher voltage tolerance compared to pure PBTTT or polystyrene films. Spectroscopy and x-ray diffraction analysis reveal that BTTT addition compromises the original ordering of the PBTTT, with higher concentrations leading to more significant disruption, even though distinguishable BTTT domains are formed. We propose a mechanism where BTTT/PBTTT clusters form charge-stabilizing entities, leading to improved charge storage capability and dielectric strength. These findings provide insights into the distinct contributions of monomers in conjugated polymer films and their potential applications in organic electronic devices.

  PublisherDOI

• Tuning Optical Properties of Plasmonic Aerosols through Ligand–Solvent Interactions

  The Journal of Physical Chemistry Letters · 2024-04-09 · 2 citations

  articleSenior authorCorresponding

  Plasmonic nanoparticles are highly tunable light-harvesting materials with a wide array of applications in photonics and catalysis. More recently, there has been interest in using aerosolized plasmonic nanoparticles for cloud formation, airborne photocatalysts, and molecular sensors, all of which take advantage of the large scattering cross sections and the ability of these particles to support intense local field enhancement ("hot spots"). While extensive research has investigated properties of plasmonic particles in the solution phase, surfaces, and films, aerosolized plasmonics are relatively unexplored. Here, we demonstrate how the capping ligand, suspension solvent, and atomization conditions used for aerosol generation control the steady-state optical properties of aerosolized Silica@Au plasmonic nanoshells. Our experimental results, supported with spectral simulations, illustrate that ligand coverage and atomization conditions control the degree of solvent retention and thus the spectral characteristics and potential access to surfaces for catalysis in the aerosol phase, opening a new regime for tunable applications of plasmonic metamaterials.

  PublisherDOI

• Exciton Transfer Between Extended Electronic States in Conjugated Inter-Polyelectrolyte Complexes

  ACS Applied Materials & Interfaces · 2024-01-30 · 2 citations

  reviewOpen accessCorresponding

  Artificial light harvesting, a process that involves converting sunlight into chemical potential energy, is considered to be a promising part of the overall solution to address urgent global energy challenges. Conjugated polyelectrolyte complexes (CPECs) are particularly attractive for this purpose due to their extended electronic states, tunable assembly thermodynamics, and sensitivity to their local environment. Importantly, ionically assembled complexes of conjugated polyelectrolytes can act as efficient donor-acceptor pairs for electronic energy transfer (EET). However, to be of use in material applications, we must understand how modifying the chemical structure of the CPE backbone alters the EET rate beyond spectral overlap considerations. In this report we investigate the dependence of the EET efficiency and rate on the electronic structure and excitonic wave function of the CPE backbone. To do so, we synthesized a series of alternating copolymers where the electronic states are systematically altered by introducing comonomers with electron withdrawing and electron-rich character while keeping the linear ionic charge density nearly fixed. We find evidence that the excitonic coupling may be significantly affected by the exciton delocalization radius, in accordance with analytical models based on the line-dipole approximation and quantum chemistry calculations. Our results imply that care should be taken when selecting CPE components for optimal CPEC EET. These results have implications for using CPECs as key components in water-based light-harvesting materials, either as standalone assemblies or as adsorbates on nanoparticles and thin films.

  PublisherOA PDFDOI

• Structure-Dependent Balance between Excited-State Deactivation Pathways in Cross-Conjugated Molecular Photoswitches

  The Journal of Physical Chemistry A · 2024-12-31 · 1 citations

  articleSenior authorCorresponding

  ]thiophenes (TT) are photoswitchable compounds that operate through reversible photoinduced cyclization/cycloreversion and have been designed specifically for integration within π-conjugated polymers to switchably manipulate polymer electronic properties. Here we report on how cross conjugating the central TT moiety impacts photocyclization dynamics as interrogated using transient absorption spectroscopy (TAS) for a series of switches built with electron-rich substituents that have various electronic interaction strengths with the TT core. For cross-conjugated structures exhibiting a propensity to switch in steady-state photoconversion experiments, ultrafast TAS reveals signatures of rapid dynamics (occurring within <1-10 ps) similar to those observed for unsubstituted switches and that are consistent with photocyclization. In contrast, TAS reveals comparatively slower spectral dynamics (∼100 ps) that are not consistent with cyclization for switches that are cross-conjugated with substituents that have greater electronic interaction with the TT core and that exhibit no propensity to photoswitch in photoconversion experiments. Microsecond TAS confirms that photoinduced cyclization occurs for the former and that a metastable triplet state localized on the conjugated backbone is generated with the latter. We find that the balance of these two deactivation pathways is sensitive to the interaction strength of the conjugated substituents with the core, with select structures exhibiting signatures of both. These findings are consistent with prior work demonstrating that the LUMO character is delocalized over the switch backbone when there are strong interactions with cross-conjugating groups and reveal that the competition between deactivation pathways can be controlled structurally by weakening π conjugation across the backbone.

  PublisherDOI

• Supramolecular Tuning of Exciton Transport in Pi-Peptide Assemblies

  The Journal of Physical Chemistry C · 2023-01-10 · 13 citations

  articleSenior authorCorresponding

  We demonstrate the use of sequence-dependent interactions between peptide-functionalized pi-conjugated pigments to tune exciton transport behavior in their supramolecular assemblies in aqueous solutions. Peptide sequences attached to a perylene diimide (PDI) core were selected based on their ability to foster a wide range of excitonic coupling strengths within organized assemblies, as reflected by experimentally measured steady-state absorption and emission spectra. Photoresponses of organized assemblies of weakly interacting PDIs closely resemble those observed for weakly assembled chromophores. In contrast, organized assemblies that support strong intermolecular coupling exhibit significant excitonic delocalization and transport, as observed through distinct transient signatures of fluence-dependent singlet–singlet annihilation and a short-lived biexcitonic state at high initial exciton densities. A one-dimensional (1D) diffusion model appropriately accounts for exciton–exciton encounters in assemblies, with effective exciton diffusion constants that scale with interchromophore coupling strength, ranging between 0 and 7 sites2/ps (0–1 nm2/ps or LD ranging from 0 to 40 nm). Variations in effective diffusion constant arise from peptide-tuned variations in interchromophore alignment and wavefunction overlap, with mixed Frenkel-CT exciton coupling identified as the key interaction that facilitates site-to-site exciton diffusion within PDI stacks. This work demonstrates the potential to use simple peptide sequence variation as a tool for rational engineering of exciton transport and other excited-state behaviors in supramolecular materials.

  PublisherDOI

Recent grants

• CAREER: Structural and solvent control of nonadiabatic bond formation probed with ultrafast time-resolved spectroscopies

  NSF · $677k · 2015–2022

Frequent coauthors

• Daniel M. Neumark

  University of California, Berkeley

  82 shared

• A. Kammrath

  University of Wisconsin–Madison

  54 shared

• Jan R. R. Verlet

  Durham University

  53 shared

• Ori Cheshnovsky

  Tel Aviv University

  33 shared

• Jiawang Zhou

  26 shared

• Alison V. Davis

  22 shared

• Roland Wester

  22 shared

• Howard E. Katz

  Johns Hopkins University

  15 shared

Education

• PhD, Chemistry

  University of California Berkeley

  2004

• B.A. (Chemistry & Physics)

  Albion College

  1999