Excitonic Frontiers

25 - 26 March 2024 09:00 - 17:00 The Royal Society Free Watch online

Scientific discussion meeting organised by Professor Sandrine Heutz, Professor Jenny Nelson FRS, Professor Garry Rumbles and Professor Martin Heeney.

Processes involving the photoexcited states of matter, or excitons, underpin some of today’s most important and diverse scientific advances, from solar cells, to quantum technologies via photosystems and catalysis. This meeting brings together world-leaders from traditionally separate fields to share views on the fundamental science, exploitation and characterisation of excitons, aiming to generate radical advances in exciton science and technology.

Meeting papers will be published in a future issue of Philosophical Transactions of the Royal Society A

Attending this event

This meeting is intended for researchers in relevant fields.

  • Free to attend.
  • Both in person and online attendance is available. Advance registration is essential. Please follow the link to register.
  • Lunch is available on both days of the meeting for an optional £25 per day. There are plenty of places to eat nearby if you would prefer to purchase food offsite. Participants are welcome to bring their own lunch to the meeting. 

Enquiries: contact the Scientific Programmes team

Organisers

  • Sandrine Heutz

    Professor Sandrine Heutz, Imperial College London, UK

    Sandrine Heutz is a Professor of Functional Molecular Materials at Imperial College London. She did her undergraduate degree at the University of Liege (Belgium) and Sherbrooke (Canada), and obtained her PhD in Chemistry from Imperial College London in 2002.  Following a Royal Society Dorothy Hodgkin Fellowship at University College London on molecular magnetic biosensors she returned to Imperial for a Lectureship in the Materials Department. She leads SPIN-Lab, a multiuser Facility that investigates the interaction of electronic spins and photons and is the Head of Department of Materials and the co-director of the London Centre for Nanotechnology.  Her current research is focused on the growth and characterisation of molecular thin films, with particular emphasis on understanding the relationship between molecular structure and optoelectronic and spintronic properties, as well as application of molecules in quantum technologies.  

  • Professor Jenny Nelson FRS, Imperial College London, UK

    Professor Jenny Nelson FRS, Imperial College London, UK

    Jenny Nelson is a Professor of Physics at Imperial College London, where she has researched novel varieties of material for use in solar cells since 1989. Her current research is focussed on understanding the properties of molecular and hybrid semiconductor materials and their application to solar energy conversion. She also works with the Grantham Institute for Climate Change at Imperial to explore the mitigation potential of renewable energy technologies. She is an ISI Highly Cited Researcher in Materials Science and has published over 250 articles and a book on the physics of solar cells. She was elected as a Fellow of the Royal Society in 2014.

  • Professor Garry Rumbles, The National Renewable Energy Laboratory and University of Colorado Boulder, USA

    Garry Rumbles is a Senior Research Fellow in the Chemistry and Nanoscience Center at the National Renewable Energy Laboratory (NREL) in Colorado, USA. He received his BSc in Chemistry with Electronics from the University of Southampton, UK in 1980. He spent 3 years in the Davy Faraday Research Laboratory of the Royal Institution, obtaining his PhD on the photophysics of synthetic polymers from the University of London in 1984. He spent over 3 years in the USA as a post-doctoral researcher first at the University of Arizona, and then the University of California, Irvine. He returned to the UK in 1987 and after a brief period as a research associate at the Royal Institution, he joined the chemistry department faculty at Imperial College London. He departed Imperial College in 2001 as a Reader moving to NREL as a Scientist in the Center for Basic Science and subsequently rising to his current position of Senior Research Fellow. He remains a visiting professor of Imperial College, and is also a professor adjoint in the department of chemistry at the University of Colorado Boulder. He is the Associate Director of Research for the Renewable and Sustainable Energy Institute, a joint institute of CU-Boulder and NREL. He is an associate editor of the RSC journal ‘Sustainable Energy and Fuels’. His current research interests are in the study of photo-induced electron transfer processes in molecular materials and the development of transient microwave conductivity. He has published over 220 journal articles, has over 13000 Google Scholar citations and an h-index of 64. He is the principal investigator of a $4M research program in solar photochemistry at NREL that is funded by the US Department of Energy, Office of Basic Energy Sciences.

  • Professor Martin Heeney, Imperial College London, UK

    Martin Heeney is a Professor of Organic Materials Chemistry and Royal Society Wolfson Fellow at Imperial College London. He is a graduate of the University of East Anglia and received his PhD from the same institution in 1999 under the supervision of Professor Michael Cook. Following eight years in industry, he joined the Materials Department at Queen Mary University of London as a senior lecturer in 2007 before moving to Imperial in 2009. His research interests include the design, synthesis and characterisation of solution processed materials for a variety of applications. He has published over 250 research papers, 5 book chapters and over 100 patents. In 2013 he was awarded the RSC Corday-Morgan Medal for most meritorious contributions to chemistry by a scientist under the age of 40. For the last five years, he has been named by Thomson Reuters as a HighlyCited researcher in the field of Materials Science.

Schedule

Chair

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Professor Jon Marangos, Imperial College London, UK

08:50-08:55 Welcome by the Royal Society and lead organiser
08:55-09:00 Chair’s Introduction
09:00-09:30 Exciton Delocalization and Large Coherent States

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

09:30-09:45 Discussion
09:45-10:15 Probing dynamics of chemical bonds in organic chromophores by X-ray spectroscopies

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

10:15-10:30 Discussion
10:30-11:00 Break
11:00-11:30 Exciton Transport and Interactions in the Quantum Regime
Dr Libai Huang, Purdue University, USA

Dr Libai Huang, Purdue University, USA

11:30-11:45 Discussion
11:45-12:15 New look at nanocrystal photophysics with spectator excitons

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

12:15-12:30 Discussion
12:30-13:30 Lunch

Chair

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Dr Jess Wade, Imperial College London, UK

01:30-13:35 Chair’s Introduction
13:35-14:05 Metal-to-Metal Charge Transfer in cyanido-bridged FeCo pairs

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

14:05-14:20 Discussion
14:20-14:50 Exciton and spin dynamics in luminescent molecular materials

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

14:50-15:05 Discussion
15:05-15:30 Break
15:30-16:00 Optical Gating of Molecular Spin Qubits

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

16:00-16:15 Discussion
16:15-16:45 Reassessing the role of exciton-vibration coupling in organic semiconductors

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

16:45-17:00 Discussion

Chair

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Professor Sir Richard Friend FREng FRS, University of Cambridge, UK

08:55-09:00 Chair’s Introduction
09:00-09:00 On the role of aggregates in the process of exciton dissociation for solar cells

The performance of efficient organic solar cells depends critically on the morphology of the active layer. The recent high efficiencies reported for blend cells made with PM6 and Y6 testify to this. Both, electron-hole separation and charge carrier transport to the electrodes benefit from the formation of suitable aggregates or crystallites. Anna Köhler will demonstrate how aggregates may be identified in organic solar cells. Using blends of PM6 with the non-fullerene acceptor Y6 as suitable model systems the author will illustrate the use of temperature dependent absorption and emission spectroscopy in solutions as necessary first step to obtain insight into the structures prevailing in a blend film. By combining the spectra obtained in solution with a careful Franck-Condon analysis of the thin film spectra, contribution from different aggregates are identified. Professor Köhler will also discuss the role of excited state delocalisation across aggregates versus the impact of static and dynamic disorder. Temperature-dependent measurements on solar cells as well as Monte-Carlo simulations are employed to determine the activation energy for exciton dissociation and to relate this to the film morphology and exciton binding energy. She will highlight the fact that for a high dissociation rate, the activation energy for photocurrent generation is not a measure for the exciton binding energy.

Professor Dr Anna Köhler FRSC, University of Bayreuth, Germany

Professor Dr Anna Köhler FRSC, University of Bayreuth, Germany

09:30-09:45 Discussion
09:45-10:15 Sub-gap and tail states in organic semiconductors

Excitons in organic semiconductors can decay radiatively or dissociate into free charge carriers at an interface, both with near unity quantum efficiencies. In these processes, electronic states within the optical gap of the material play an important role and often provide efficiency-limiting, non-radiative decay pathways for excitons and charge carriers. In this contribution, Koen Vandewal will describe their efforts to characterize these tail- and sub-gap states, including charge-transfer and triplet states, using highly sensitive emission and absorption spectroscopy. For organic semiconductor blends with strongly reduced non-radiative losses, time-resolved emission spectroscopy at nanosecond to millisecond timescale is further an excellent probe for the recombination dynamics and interplay between free carriers, charge-transfer states and triplet states. The author will discuss the molecular and morphological factors which are responsible for an efficient charge generation and reduced non-radiative recombination and provide guidelines for future high efficiency organic opto-electronic devices.

Professor Koen Vandewal, Hasselt University, Belgium

Professor Koen Vandewal, Hasselt University, Belgium

10:15-10:30 Discussion
10:30-11:00 Break
11:00-11:30 Charge Generation in Neat Non-Fullerene Acceptor Domains

Non-fullerene acceptors (NFAs) are exciting molecules allowing high efficiency in organic photovoltaic (OPV) blends with conjugated polymers. Interestingly, charges can also be generated by neat NFA films without additional donor. To understand the origins of exciton dissociation in neat NFAs, the authors have looked at the impact of aggregation, external electric field end non-linear effects. They used solvatochromism in order to gain insight on charge redistribution after excitation in isolated NFAs. The scientists found that unaggregated NFAs feature a more dipolar excited state, revealing intramolecular charge transfer (ICT) character. This ICT character, however, is not enough to generate separated charges. Aggregation is the key to exciton dissociation in neat NFAs, which we observe with TA of solutions and films of several different NFAs. To explore the impact of an electric field on exciton dissociation in neat NFA devices, the authors used bias-dependent external quantum efficiency (EQE) and transient absorption (TA) spectroscopy. Electromodulated differential absorption (EDA) measurements then allowed us to observe charge transport under bias. Excitation correlation spectroscopy and fluence-dependent TA finally revealed how non-linear effects can increase the charge yield. Lastly, they comment on whether the neat domain charge generation significantly affects the photophysics of blends or not. 

Professor Dr Natalie Banerji, University of Bern, Switzerland

Professor Dr Natalie Banerji, University of Bern, Switzerland

11:30-11:45 Discussion
11:45-12:15 Approaches to homojunction solar cells

Photocurrent generation in organic photovoltaic (OPV) devices is driven by the creation and subsequent separation of excitons into free electrons and holes. To do this effectively generally requires mixing one or more “donor” and “acceptor” materials with differing ionisation potentials and electron affinities to overcome the exciton binding energy. While efficient organic solar cells can be formed using this approach it can be difficult to scale laboratory efficiencies to cells with submodule dimensions. Furthermore, metastable blend morphologies that give rise to highly efficient devices may change over time leading to a decrease in performance. There would therefore be an advantage of having efficient homojunction devices composed of a single absorbing chromophore. Materials with low exciton binding energies such as silicon and three-dimensional perovskites generate free charge carriers directly upon photoexcitation at room temperature. The binding energy of an exciton is dependent on a number of factors, including being proportional to the inverse square of the dielectric constant. Hence, one strategy to develop homojunction organic solar cells is to increase the dielectric constant of the organic semiconducting materials. This presentation will describe the development of materials that have been engineered with the aim of understanding how to improve exciton dissociation. Approaches to be discussed include methods of increasing the dielectric constant to decrease the exciton binding energy. The synthesis of the materials and their optoelectronic properties will be described, with a particular focus on the factors that affect the dielectric constant.


Professor Paul Burn, University of Queensland, Australia

Professor Paul Burn, University of Queensland, Australia

12:15-12:30 Discussion
12:30-13:30 Lunch

Chair

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Professor Eric Bittner, University of Houston, USA.

13:30-13:35 Chair’s Introduction
13:35-14:05 Second-order photon cross correlations and signatures of quantum behaviour of excitonic systems
Professor Alexandra Olaya-Castro, University College London (UCL), UK

Professor Alexandra Olaya-Castro, University College London (UCL), UK

14:05-14:20 Discussion
14:20-14:50 Singlet fission and triplet-triplet annihilation in organic semiconductors and biological systems

The efficiency of single-junction solar cells is fundamentally limited to approximately 30%. To address this limitation, excitonic up and down conversion processes have emerged as a promising solution. Specifically, singlet exciton fission, where a spin-0 excited state produces two separate excitonic triplet states, can effectively generate two low-energy excitons per absorbed photon. Alternatively, triplet fusion or triplet-triplet annihilation combines two low-energy excitonic states into a high-energy excited state. Here, the author will present recent findings on the fundamental physics of singlet fission and triplet-triplet annihilation from their group's investigations in both organic semiconductor and biological systems, including discussing the impact of strong light-matter coupling (polariton formation).

Professor Jenny Clark, University of Sheffield, UK

Professor Jenny Clark, University of Sheffield, UK

14:50-15:05 Discussion
15:05-15:30 Break
15:30-16:00 Photosynthesis beyond the red limit
Professor Bill Rutherford FRS,  Imperial College London, UK

Professor Bill Rutherford FRS, Imperial College London, UK

16:15-17:00 Panel discussion