Excitonic Frontiers

Their recent work is motivated by the need for robust, large-scale coherent states that can play possible roles as quantum resources. A challenge is that large, complex systems tend to be fragile. However, emergent phenomena in classical systems tend to become more robust with scale. Do these classical systems inspire ways to think about robust quantum networks? This general question and its relationship to quantum coherence and exciton delocalisation will be discussed and the idea of exploiting emergent states will be examined. The talk comprises two parts.

First, they will report an experimental method to witness, with femtosecond time resolution, quantum correlations in a coherent state. They show how to obtain a ratio of relative dipole strength of transitions in transient absorption spectra to probe the time dependence of exciton coherence length in excitonic systems. They have demonstrated the method for a J-aggregate in solution, quantifying how the exciton localizes over a picosecond timescale from an initial delocalisation length of about 60 to an equilibrium of 15 molecular units.

Second, they will explore the question: is it worth looking for quantum effects in very complex, ‘messy’ systems? Many would argue in the negative. Indeed, the idea that quantum phenomena might underpin unforeseen function in life and other complex systems is viewed with skepticism—skepticism rooted in an influential monograph written several decades ago by Schrödinger. However, given the many relevant new ideas that have emerged over the past decades—including, for instance, more sophisticated understandings of networks, synchronisation phenomena, and advances in solving quantum dynamics for large and complicated systems, they will argue why the time is right to revisit this question. They will propose how the relevant states should differ significantly from typical qubits, and thus will function differently.

Professor Gregory Scholes FRS, Princeton University, USA

Professor Gregory Scholes FRS, Princeton University, USA

Greg Scholes is the William S Tod Professor of Chemistry at Princeton University. Originally from Melbourne, Australia, he later undertook postdoctoral training at Imperial College London and University of California Berkeley. He started his independent career at the University of Toronto (2000-2014) where he was the D J LeRoy Distinguished Professor. He was appointed Editor-in-Chief of the Journal of Physical Chemistry Letters in 2019. Dr Scholes was elected a Fellow of the Royal Society (London) in 2019 and a Fellow of the Royal Society of Canada in 2009. He served as Chair of Department at Princeton 2020-2023 and Director of an Energy Frontier Research Centre (BioLEC) since 2018.

Chemical bonding patterns fundamentally change when molecules dynamically evolve in electronically excited states created by optical excitations. These dynamics give rise to many useful properties and functionalities, which can be resolved in space and time at modern XFEL facilities. In this talk the author will overview some possible measurements that can be done with X-ray lasers suggested by computational investigations. In the first example, the scientists use dynamical simulations to compute X-ray Raman signals, which are able to monitor the coherence evolution in molecular photoswitches. Time-resolved X-ray diffraction can further probe key chemical features during the ultrafast dynamics. In the first example, X-ray Circular Dichroism (XCD) can exploit the localised and element-specific nature of X-ray electronic transitions. XCD therefore is more sensitive to local structures and the chirality probed with it can be referred to as local which in contrast to a conventional Optical Circular Dichroism probing the global molecular chirality. 

Professor Sergei Tretiak, Los Alamos National Laboratory, USA

Professor Sergei Tretiak, Los Alamos National Laboratory, USA

Sergei Tretiak is a Laboratory Fellow of Los Alamos National Laboratory (LANL). His research interests include development of electronic structure methods for molecular optical properties, non-adiabatic dynamics of electronically excited states, optical response of excitons in conjugated polymers, carbon nanotubes, semiconductor nanoparticles, mixed halide perovskites and molecular aggregates, and the use of Machine Learning for chemistry and materials.

The spectator exciton experiment follows stepwise changes in nanocrystal absorption with the accumulated number of excitons by comparing pump-probe spectra in pristine nanocrystal samples with that from samples uniformly excited with a progressive number of cold excitons. This approach reveals that unexpectedly 50% of hot electrons are blocked from cooling directly to the band edge in CdSe nano-dots already excited with a single cold exciton due to spin orientation conflicts. In addition this same approach quantifies elusive stimulated emission even if it is masked by much stronger overlapping excited state absorption. It will be shown how these previously undetected observables teach new things concerning band edge electronic structure in quantum confined nanocrystals.

Professor Sanford Ruhman, The Hebrew University of Jerusalem, Israel

Professor Sanford Ruhman, The Hebrew University of Jerusalem, Israel

Professor Sanford Ruhman is Professor emeritus in the Institute of Chemistry at the Hebrew University of Jerusalem where he has been applying cutting edge ultrafast spectroscopy to study photochemistry in condensed phases, photobiology of protein photoreceptors, and exciton dynamics in advanced materials such as nanocrystals and modern photovoltaics for the last three decades. He is a pioneer of utilising impulsive vibrational spectroscopy to reaction dynamics, and a recognized authority on the photochemical dynamics of retinal proteins. He served as the director of the Farkas Minerva centre for Light Induced Processes at the Hebrew university for several years and as the the chairman of the Institute of Chemistry. 

The rational design of molecules, which exhibit photoswitchable physical properties is a subject of the intense research activity. Chemists have investigated photoresponsive complexes through rational choices of cyanido-based building blocks. Various molecular architectures have been obtained with remarkable properties such as spin crossover, electron-transfer, and photoinduced magnetism. The understanding of the electronic mechanism during the photo-excitation requires the combined use of optical, magnetic and structural probes to better understand the photomagnetic mechanism.  In this presentation, the switching properties of dinuclear cyanido-bridged pairs will be analysed using a combination of magnetic, structural and X-ray Absorption Spectroscopy (XAS) measurements. The bulk and local structural measurements have shown the thermal- induced electron transfer.

[1] S. F. Jafri et al. J. Am. Chem. Soc., 2019, 141, 3470. 

[2] C. Mathonière et al. Chem. Comm., 2022, 58, 12098.

Corine Mathonière, Institute of Condensed Matter Chemistry of Bordeaux, University of Bordeaux

Corine Mathonière, Institute of Condensed Matter Chemistry of Bordeaux, University of Bordeaux

Corine Mathonière, born in 1968 in Montluçon (France), received her education in Chemistry at the University of Paris Pierre and Marie Curie, France. Her PhD work was devoted to the optical properties of molecular-based magnetic materials under the supervision of Professor O Kahn in 1993. After a post-doctoral stay in the group of Professor P Day (Royal Institution of Great Britain, London), she joined the University of Bordeaux as associate professor in 1994, then was promoted to professor in 2010. Since 1994, Dr Corine Mathonière has developed at the Institute of Condensed Matter Chemistry of Bordeaux (CNRS) research activities in “Switchable Molecules and Materials molecular” group interested in the synthesis and physical studies of switchable molecular materials. In this research field, she has published 145 articles with more than 6000 citations and has an h index of 44 (February 2020).

The spin of ground and excited levels in molecular materials dictates the exciton mechanisms for any photonic, optoelectronic and quantum technology applications. This talk explores the photo- and spin physics of excitons as revealed by optical and magnetic resonance studies. Where organic semiconductors operate via singlet (spin, S = 0) and triplet (S = 1) excitons, achieving higher luminescence efficiency from these states will generally lead to higher performance, for example in OLEDs. Studies of spin conversion and the orbital-nature of the triplet excitons that dictates luminescence are presented for novel series of fluorescent and phosphorescent emitters. Recent interest in organic radicals containing unpaired electrons has emerged from the design of new materials that undergo efficient light absorption and emission from transitions between doublet spin (S = 1/2) ground and excited levels. As well as being potential candidates for functional emitters in light-emitting devices, opportunities emerge to couple their optical, spin and magnetic properties in molecular excitons that could enable future quantum technology platforms.

Dr Emrys Evans, Swansea University, UK

Dr Emrys Evans, Swansea University, UK

Emrys completed a DPhil at University of Oxford in 2016 with Professor Christiane Timmel. His DPhil was on the photo- and spin physics of light-induced electron transfer reactions implicated in animal magnetoreception. Following this he worked with Professor Sir Richard Friend at the Cavendish Laboratory, University of Cambridge as a Postdoctoral Research Associate (2016-2019). There he began researching molecular semiconductors for optoelectronics. He received a Leverhulme Trust Early Career Fellowship in 2019. In 2020 he started a Royal Society University Research Fellowship at Swansea University. In 2021 he was awarded the Dillwyn Medal for STEMM from the Learned Society of Wales. The design, characterization, and exploitation of optical and magnetic properties in new molecular materials is the focus of his research. Emrys is fluent in English, Welsh and Thai.

The optical manipulation of spin quantum states provides an important strategy for quantum control with both temporal and spatial resolution for quantum computing, sensing and communications. While significant progress has been made in the discovery of molecular spin-based qubits with long decoherence times, current challenges are focused on molecular quantum sensors that have long decoherence times, (achieved by isolation from the bath) and strong coupling to the environment (achieved by strong coupling to the bath) required for sensing of external field or analytes. The scientists report here a strategy for coupling molecular spin-based qubits to the bath without sacrifice of decoherence times. Photochromic ligands that undergo isomerization in response to changes in solvation, electric field, temperature, or light can be used to modulate electron transfer-coupled spin transition processes, spin-orbit coupling, and ligand-metal electronic coupling in bound transition and lanthanide metal ions. Visible light irradiation (λexc = 550-600 nm) of a spirooxazine cobalt−dioxolene complex induces photoisomerization of the ligand that in turn triggers a reversible intramolecular charge-transfer coupled spin- transition process at the cobalt center between a low-spin Co(III)−semiquinone doublet and a high-spin Co(II)−bis-semiquinone sextet state. Determination of the spin relaxation and decoherence times of the low-spin Co(III)−semiquinone doublet state reveal slow spin dynamics and decoherence, and a change in the population of the SQ state (ms ± 1/2) qubit state with light modulation. Extension of this strategy to low-spin transition metal (S = 1/2) and lanthanide complexes leads to reversible changes in spin relaxation rate, decoherence time, and g-value with changes in photochromic ligand state, providing a robust strategy for quantum sensing in molecular spin-based qubits. The scientists report here a Cu(II) (N4) photochrome complex (S=1/2) that exhibits a shift in g-value at room temperature upon photoisomerization with a metastable state lifetime of 25-85 min at room temperature. Pulsed EPR measurements reveal a significant change in decoherence rates with isomerization, and Rabi oscillations with a spin-flip rate of 56 ns. Theory and pulsed EPR spectroscopy provides effective modelling of the phenomenon and long-term strategies to further modulate spin-based molecular systems for quantum sensing at the single molecule level. 

Professor Natia L Frank, University of Nevada-Reno, USA

Professor Natia L Frank, University of Nevada-Reno, USA

Professor Natia L Frank received her Bachelor’s degree with Honours from Bard College in 1987 (Chemistry, Math, Music), an M.Sc. in Inorganic Chemistry at the University of Wisconsin-Madison (1989), and PhD at the University of California-San Diego (1996, Organic Chemistry). She was a CNRS Postdoctoral Fellow with the late Professor Olivier Kahn at the University of Bordeaux, France (spin-based materials), and an NIH Postdoctoral Fellow (Biomaterials, Professor Thomas Meade/Professor Harry Gray) at Caltech. She began her independent career in 2000 as an Assistant Professor at the University of Washington-Seattle in the study of multifunctional magnetic materials for spintronics and biosensing. In 2005, she was recruited as a Canada Research Chair Tier II in Multifunctional Materials Chemistry at the University of Victoria where she developed optically switchable spin-based qubits for quantum science. In 2012, she was a Visiting Scholar  at Humbolt University (Physics), Berlin, Germany, and University of Rennes (Chemistry), France. In 2020, she joined the University of Nevada-Reno as Associate Professor of Chemistry. Her primary expertise is at the interface of organic chemistry, inorganic chemistry, spin-based materials and photochemistry/electron transfer theory which allows her to be well-situated to address current challenges in molecular quantum information science: the design of molecular qubits with long decoherence times, multiqubit arrays, and qubits/qudits that can respond to external stimuli for quantum computing and sensing. Professor Frank currently serves on two funded DOE EFRC advisory boards in quantum science, the ACS-PRF Advisory Board, and has served on numerous NSF funding panels in quantum relevant areas.

In the prevailing picture of organic semiconductors, molecular vibrations are thought to cause scattering and localisation of electronic wavefunctions. This results in the suppression of exciton and charge transport, leading to poor mobilities and diffusion lengths. At the same time, the coupling of excitons to high-frequency vibrational modes (1000 – 1600 cm-1) accelerates non-radiative decay dynamics, which is detrimental to the performance of all light emitters and photovoltaics based on molecular semiconductors. Since organic systems are primarily based on carbon-carbon bonds, vibrational coupling to high-frequency modes has been thought to be unavoidable. 

In this talk Professor Rao will present recent experimental ultrafast spectroscopy and ultrafast microscopy results which suggest that both these paradigms can be overcome. He will discuss a new mechanism for exciton diffusion, transient delocalisation, which can enable exciton diffusion constants up to 1cm2/s and diffusion lengths greater than 300nm. The author will also discuss results which suggest that it is possible to completely decouple excitons from high-frequency vibrational modes, thus greatly supressing non-radiative recombination and enabling high luminescence efficiencies in low-bandgap organic systems. Finally, the author will present exciting recent results on the most important exciton systems in the world, natural light harvesting complexes. Here they are now able to image in real-space and in real-time the flow of excitons from antenna complexes to the reaction centre within living photosynthetic bacterial, and what they find is quite surprising. 

Dr Akshay Rao, University of Cambridge, UK

Dr Akshay Rao, University of Cambridge, UK

Akshay is Professor of Physics at the Cavendish Laboratory, University of Cambridge. He obtained his BSc from St Stephen’s College, University of Delhi, in 2006 and MSc from the University of Sheffield in 2007. He completed his PhD from the University of Cambridge in 2011, following which he held a Junior Research Fellowship (JRF) at Corpus Christi College, before establishing his independent research group in 2014. His research interests lie in the study of energy materials, in particular to elucidate the fundamental electronic, structural and transport dynamics of these materials. He is co-founder of Cambridge Photon Technology and illumion.