Highlights from Faraday Discussion on Ultrafast Photoinduced Energy and Charge Transfer, Ventura, CA, USA, April 2019

Abstract

Following a tradition more than 100 years old, a Faraday Discussion meeting was held on the topic of ‘Ultrafast Energy and Charge Transfer’ in April 2019. While these meetings are historically held in the United Kingdom, the Royal Society of Chemistry (who organises the conference and publishes its proceedings) recognises the importance of finding the space and opportunity for the chemical sciences international communities to connect and exchange knowledge. As such, the meeting hereby referred to took place in Ventura, California, USA. This conference report, produced by early career researchers, covers the highlights of this meeting, focusing on brief summaries of the papers discussed as well as particularly interesting or recurring topics of the ensuing discussion.

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Introduction

The Faraday Discussion on Ultrafast Photoinduced Energy and Charge Transfer took place in Ventura, California, between the 8th and the 10th of April, 2019. The meeting started with a clear message from the organising team, in particular co-chairs Mike Ashfold (University of Bristol, UK) and Steve Bradforth (University of Southern California, USA), who both opened the meeting, encouraging early career scientists to engage and contribute to the discussion. It is fortunate, therefore, that the task of collating these highlights has been given to a team of PhD students and early career researchers, since this dedicated effort may otherwise go unnoticed or not fully appreciated. The support of established academics is paramount to the professional advancement of early career researchers, and as such we are thankful for the explicit encouragement we received.

The group that gathered in Ventura for this Faraday Discussion was perfectly suited to the debating character of these meetings: there were no slow-paced moments, with plenty of questions and curious minds to make the discussion outlast its allocated time, with casual talk of science brimming over into leisurely time. Throughout the three days of this Faraday Discussion delegates displayed incredible willingness to engage in honest, open and collaborative discussion, offering suggestions and ideas, referencing each other's work and valuing each other's comments. There was a genuine atmosphere of collaboration and it was clear that many of the conversations that took place during this conference would lead to new experiments/simulations and new research directions once delegates returned to their respective laboratories. In what follows, we aim to highlight some of the science that was discussed and report on some of the moments which we found most interesting during the debate that ensued regarding the papers presented.

Opening lecture

Via video call from the École Polytechnique Fédérale de Lausanne, Switzerland, Majed Chergui delivered an excellent opening lecture which covered an array of topics to be discussed at the conference, setting the scene for the rest of the meeting. Firstly, Chergui provided an interesting outline of the history of the field, which was especially valuable owing to the wealth of early career scientists present at the meeting.

Chergui's talk revolved around the recognition of the diversity of questions, systems and methods that are encompassed by the topic of the meeting. The concepts identified as fundamental for the topic under discussion were coherence, excitons, interfacial charge transfer (particularly relevant for photovoltaics) and proton coupled energy transfer, all of which featured, to a greater or lesser extent, in all the conference sessions. Moreover, Chergui's talk deftly spanned multiple ultrafast techniques prevalent in the fields of both photochemistry and photophysics. The novel methods discussed in this talk were electronic 2-dimensional (2D) dichroism, deep ultraviolet (UV) circular dichroism, and new forms of ultrafast X-Ray spectroscopy. Chergui gave an excellent account of how 2D techniques can be used to map vibrational/electronic (vibronic) couplings in biochemical systems and, in addition, how augmenting these techniques with circular dichroism may also provide further information regarding racemic mixtures.

Having provided the outline of the field, exploring and exemplifying some of the key contemporary questions and how state-of-the-art and emerging techniques may be employed to answer these, Chergui closed by sharing his own ideas. Much discussion followed, with particular interest being given to time-resolved circular dichroism of DNA. The collaborative spirit of Chergui's conclusion would set the scene and the atmosphere for the remainder of the conference.

Session 1: Energy and charge-transfer in natural photosynthesis

Following the opening lecture, the first session got underway with the presentation by Alexandra Olaya-Castro (University College London, UK) on the synchronisation dynamics of a bio-inspired photosynthetic vibronic dimer (DOI: 10.1039/C9FD00006B). Olaya-Castro remarked that specific forms of decoherence channels and cross-interactions between quantum subsystems are essential for synchronization. Nevertheless, she noted that there are still unanswered questions as to how exactly synchronisation is regulated by coherent dynamics and decoherence, which Olaya-Castro and her co-workers attempted to address with their work. By implementing particular assumptions for a photosynthetic model, they demonstrated that positive and negative synchronisation can be achieved by interferences between vibronic coherences, and provided further insight into this mechanism. Stemming from these results, they posited that, under physiological temperatures, it is possible that localised quantum synchronisation occurs in some light transfer complexes during energy transfer, concurrently highlighting the importance of vibrational coherence in such systems (DOI: 10.1039/C9FD00006B).

Graham Leggett (University of Sheffield, UK) then presented work exploring the aforementioned concepts further, by focusing on the potential optimization of organic semiconductors for efficient long distance exciton transport (DOI: 10.1039/C8FD00241J). Also turning to quantum biology and natural photosynthetic systems for inspiration, Leggett and co-workers aimed to solve the issue of ineffective energy transfer across long distances. Leggett and co-workers presented a method for coherent transfer of energy via the coupling of excitons to collective surface electron oscillation modes (plasmons). Building on their previous work, where they effectively coupled a plasmon mode consisting of gold nanostructures to bacterial photosynthetic complexes, Leggett and co-workers demonstrate that there is strong coupling of gold nanoarrays to light harvesting complexes extracted from spinach leaves (DOI: 10.1039/C8FD00241J). They also showed that, according to the model they used to fit their data, energy is transferred coherently via a plasmon mode and one can control the energies of plexcitonic states by way of choosing the protein structure and the arrangement of the hybridised emitters. Leggett and co-workers conclude by emphasizing that in doing so, non-local coupling between excitons can be controllably manipulated, therefore enabling for coherent energy transport at a magnitude of more than 100s of nm (DOI: 10.1039/C8FD00241J).

The next speaker, Benjamin Fingerhut (Max Born Institute, Germany), reminded the audience of the challenges of attempting to faithfully model tightly integrated exciton energy transfer and charge transfer dynamics. Using data from the bacterial reaction centres as their basis, Fingerhut and co-workers demonstrate a detailed outline for the development of an excitonic donor–bridge–acceptor numerical simulation, where coherence dynamics and structure effects on spectral density function are benchmarked by the recently developed MACGIC-iQUAPI method (DOI: 10.1039/C8FD00189H). Based on their observations, they infer that transfer dynamics are considerably dependent on vibrational modes with a concomitant decoupling of both timescales and coherent exciton energy transfer and charge transfer dynamics. Furthermore, their results lend support to the working theory of intramolecular vibrations being essential for optimal charge transfer reactions. Fingerhut and co-workers conclude by providing further insight as to why, under optimum conditions, the impact of vibrational modes on transfer dynamics is near negligible.

Next, Yuan-Chung Cheng (National Taiwan University, Taiwan) discussed light harvesting pigment protein complexes as a way of mimicking photosynthetic pathways with a specific interest in the pigment protein complex “photosystem II supercomplex” (PSII) (DOI: 10.1039/C8FD00205C). Cheng discussed how the lack of theoretical insight into the dimer species (as compared to the monomer) fuelled the present study. Molecular dynamics simulations of the PSII dimer were utilized to obtain key parameters that govern excitation energy transfer and these were used to build an accurate model of light-harvesting. The simulations by Cheng and co-workers predict that dimerization significantly enhances the robustness of the energy transfer mechanism in the PSII system, leading them to conclude that dimerization was not evolutionarily selected simply for structural reasons (DOI: 10.1039/C8FD00205C). Cheng concluded by suggesting that the framework employed in the work presented at this Faraday Discussion should be applicable to general organic molecular aggregates.

The last presentation of session 1 was given by Graham Fleming (University of California Berkeley, USA). Along similar lines to the previous paper, Fleming discussed the study and modelling of photosynthetic pathways (DOI: 10.1039/C8FD00190A). Fleming and co-workers used two dimensional electronic vibrational spectroscopy (2DEV) to study the light-harvesting complex II. Whilst this technique is still relatively new and still being developed, Fleming suggests that it shows “considerable promise for the study of complex molecular dynamics such as energy, electron, coupled proton–electron transfer, as well as molecular dynamics involving large structural changes.” Fleming and co-workers went on to describe a methodology to calculate 2DEV spectra which is valid for strongly and weakly coupling molecular systems (DOI: 10.1039/C8FD00190A). Fleming closed by suggesting that, whilst their theoretical technique had been studied for weakly coupled species in the present work, it would be pertinent to apply the same approach to investigate its effectiveness with strongly-coupled species.

Following the format of any Faraday Discussion, after a brief presentation of the aforementioned scientific reports, the audience was invited to engage in discussion, for which most of session 1's time was reserved. Early in the discussion, the first explicit mention of Hush theory emerged – a reminder of how crucial the work of Noel Hush is to the topic of this Faraday Discussion. It is worth taking a moment here to pay homage to this Australian scientist who sadly passed recently (March 2019). Born in 1924 and a graduate of the University of Sydney, Hush worked as a researcher and lecturer at his alma mater, then at the University of Manchester and the University of Bristol before returning to Australia to found the Department of Theoretical Chemistry at the University of Sydney. Throughout his career, Hush worked to elucidate the mechanisms of chemical electron transfer.¹ His research culminated in the concept of adiabatic electron-transfer theory, which describes the rate of charge transfer within a donor–acceptor system, as well as the ability of such systems to absorb or emit light. These concepts and the now widely known Marcus–Hush theory^2,3 are paramount to the field of research discussed at this meeting and underpin much of the understanding of the work presented, perfectly demonstrating the enormous contribution of Noel Hush.

Both the topic of this session and the discussion that ensued proved Chergui's point regarding the central role of the concept of coherence in the field of energy and charge transfer. Much of the discussion in this session revolved around the complexities of coherence, particularly in terms of the difficulty in disentangling the contributions and interactions between electronic and vibrational states, both in isolation and when acting as vibronic states. Throughout the discussion, the audience made reference to the challenges in understanding and distinguishing between these complex interactions and the roles they play in energy and charge transfer, whilst also suggesting experimental methodologies to address these challenges. For example, Jeffrey Cina (University of Oregon, USA) suggested polarized fluorescence up-conversion measurements which Olaya-Castro recognised as a potentially direct experimental approach to confirm her team's predictions of negative and positive synchronisation. Moreover, referring to work presented in a poster at this meeting by Alexis Kiessling (University of Oregon, USA), Cina suggested the replotting of 2DEV data as “time versus time” as opposed to “frequency versus frequency” as this was found to help expose underlying nuclear wave-packet dynamics. Further to this, Jennifer Ogilvie (University of Michigan, USA) suggested using the above approach but with different frequency regimes, discussing the usage of 2D UV excitation, combined with vibrational probing. In addition, discussion also turned to how best to theoretically model these systems and their energy and charge transfer mechanisms. The audience suggested efforts would be required for improving currently employed computational methods, as well as to developing novel theoretical frameworks to best account for and disentangle the aforementioned contributions from vibrational and electronic manifolds to these mechanisms.

Finally, there was also time for debate surrounding the need for conceptual and experimental clarity on two main points. Firstly, Karen Morenz (University of Toronto, Canada) started by asking Olaya-Castro what the mathematical definition of synchronisation would be and how its extent could be quantified. This question sparked a discussion on the physical meaning of synchronisation – negative versus positive – as well as the thresholds for its occurrence and its evolution with time. Later, there were questions to the panel of speakers regarding whether it would be realistic to consider quantum phenomena such as coherence, the concept underpinning the discussion during this session, when referring to photosynthesis, a biological process that takes place under solar illumination and at a macroscopic scale. The audience agreed unanimously that this is an important question with no straight-forward answer, even though it seemed to be the consensus that current efforts in the field are crucial steps in the journey towards understanding both ‘real-life’ photosynthesis as well the relevance of coherence to the mechanism. Ogilvie emphasised the critical role of theory in “guiding the experiments that can test the functional relevance of electronic-vibrational resonance to photosynthetic energy transfer and charge separation”, while Fingerhut shared that, in order to demonstrate this relevance, spectroscopic measurements “should aim at successively expanding on the information content of the observed signals”. Olaya-Castro, on the other hand, while recognising the uniqueness of the coherent superpositions that are formed under laser illumination, emphasized the need to seek “a clear understanding of the mechanisms that support coherent dynamics because [they] will definitely play a role under incoherent illumination”.

For lack of time, and not lack of thought and willingness to share these, session 1 was closed to allow for the poster session to start and for dinner to be served (Fig. 1).

Fig. 1 The lively poster session, after session 1, happening in the horizon of this picture, justified the almost empty dinner tables. Poster presenters and other delegates were keen to network and learn about each other's work, especially after the tantalising flash presentations which introduced the topics of each poster.

Session 2: Photovoltaics and bio-inspired light harvesting

The next morning, session 2 of this Faraday Discussion continued the theme of natural light harvesting, this time applied to the context of photovoltaic technologies. This session began with the presentation by Emily Weiss (Northwestern University, USA) on improving the performance of photovoltaic devices for which the acceptors are lead chalcogenide (namely PbS) quantum dots. Specifically, Weiss and co-workers focused on analysing the rates of singlet fission of aggregates of the polyacene 5,13-diphenylpentacene (DPP) adsorbed to PbS quantum dots, a critical parameter in determining the performance of the photovoltaic devices (DOI: 10.1039/C8FD00157J). The team's research revealed that addition of quantum dots with specific sizes allowed for differing rates of singlet fission, as observed through transient electronic absorption spectroscopy. Although the addition of any sized quantum dots increased this rate, it was found that quantum dots of radius 2.2 nm produced a 5-fold increase compared to the pure aggregate of diphenylpentacene. Therefore, the authors conclude that their findings may inform the design of PbS quantum dot and polyacene structures which promote faster singlet fission, thus enhancing the photovoltaic devices in which they are used (DOI: 10.1039/C8FD00157J).

Focusing on technology to be applied to the antennae of artificial photosynthesis assemblies, Amanda Morris (Penn State University, USA) presented work on the synthesis and photophysical characterization of a water stable metal organic framework (MOF). Morris and co-workers suggest that this MOF, which contains multiple chromophore units, may serve as a model system to understand the energy transfer mechanisms in photosynthetic assemblies, which are highly sensitive to structural parameters (DOI: 10.1039/C8FD00194D). Morris and co-workers characterised the photophysical properties of their proposed MOF using both steady-state and ultrafast measurements. Their research revealed that inter-chromophore interactions exist in the weak coupling regime, allowing the authors to conclude that excitation energy transfer takes place along an incoherent hopping mechanism (DOI: 10.1039/C8FD00194D). Furthermore, Morris and co-workers employed a Förster energy transfer model to estimate the hopping rate; transient absorption experiments, on both the nanosecond and femtosecond timescales, demonstrate that the triplet state lifetime of the MOF is shorter than that of the ligands, possibly due to defects in the structure acting as trap states to quench the ligand triplet state. Nevertheless, Morris and co-workers found enhanced exciton migration distances in their proposed MOF structures, which lead them to propose potential applications of these MOFs as light-harvesters in various solar energy conversion devices (DOI: 10.1039/C8FD00194D).

Petter Persson (Lund University, Sweden) presented work aimed at developing more environmentally friendly and more sustainable photovoltaic devices. Persson suggested this could be achieved by swapping the currently popular second and third row transition metal complexes with first row transition metal complexes with equally good photophysical and photochemical properties. With this goal in mind, Persson presented the team's research on band selective dynamics of iron carbene complexes (DOI: 10.1039/C8FD00232K). The work presented demonstrated how different electronic excitations of an iron carbene complex, which access various metal–ligand charge transfer states, influences the excited state properties of the overall complex system. Persson and co-workers used ultrafast transient absorption and time dependent density functional theory to map the excited states of the complex and observed that, independently of photoexcitation energy (and thus independently of which state is accessed on the metal–ligand charge transfer manifold), an exceptionally long lived state is populated, possibly due to the strong σ-donation by hexa-carbene environments around the metal (DOI: 10.1039/C8FD00232K, Fig. 2).

Fig. 2 Ventura West Marina on a sunny California morning. Many Faraday Discussion delegates took advantage of the beautiful surroundings and enjoyed jogging around the marina before joining the conference proceedings.

Following a short coffee break, session 2 continued with a presentation by Andrew Marcus (University of Oregon, USA) on disorder of DNA fork junctions (DOI: 10.1039/C8FD00245B). In this work, Marcus and co-workers inserted a carbocyanine dye into strands of DNA and used it as a marker of the surrounding environment: either a single/double strand or a junction between the two. At these points, the backbone of DNA is conformationally flexible to allow for binding of genome regulatory proteins; the local conformations and the degree of conformational disorder are related to internal electronic-vibrational motions and the resonant electronic interaction that couples chromophores together. By employing absorbance, circular dichroism and two-dimensional fluorescence spectroscopies, the team determined that the conformation of the carbocyanine dye dimer at the fork junctions in the DNA deviated from the expected conformation of the Watson–Crick B form, which is consistent with DNA flexibility near junctions sites (DOI: 10.1039/C8FD00245B). Importantly, Marcus and co-workers also found that the Holstein-Frenkel Hamiltonian model they employed successfully predicts the spectroscopic data relating to this behaviour, leading them to conclude that this approach may be more widely applicable (DOI: 10.1039/C8FD00245B).

The penultimate speaker of the session was Eric Bittner (University of Houston, USA), who presented work on the relationship between charge separation and open-circuit voltage in organic photovoltaics (DOI: 10.1039/C8FD00182K). Specifically, Bittner and co-workers were interested in evaluating how certain parameters, such as entropy and configurational and energetic disorder, affect the open-circuit voltage of organic photovoltaics, the aim being shared with other speakers at the discussion: to inform the design of enhanced performance photovoltaic devices. The team developed a fully interacting lattice model to simulate how several parameters (such as density of states and entropy) affect open-circuit voltage, and compared the results to an analytical model (DOI: 10.1039/C8FD00182K). They found that energetic disorder is a significant factor in the resulting open-circuit voltage of the device, but that it is electron mobility in the direction perpendicular to the interface that determines the success of energy/charge transfer (DOI: 10.1039/C8FD00182K).

The final talk of session 2 was delivered by Jahan Dawlaty (University of Southern California, USA), who presented work on photobases, particularly aimed at exploring how ground state hydrogen bonding and solvation of photoproducts affect the extent of excited state intermolecular proton transfer (DOI: 10.1039/C8FD00215K). The absorption and fluorescence spectroscopy measurements by Dawlaty and co-workers revealed that appreciable ground state hydrogen bonding does not guarantee successful proton transfer in the excited state. Other factors were found to play a role in enabling excited state intermolecular proton transfer, such as the donor–acceptor concentration ratio, which must exceed 100 : 1 in order for a photoproduct solvation threshold to be reached, or the energy difference between the ground state and the excited state of interest, which would ideally be kept at a minimum. The team suggest that fine-tuning these factors may provide opportunities for the design of new photobasic molecules with potential applications in many chemical and biological phenomena (DOI: 10.1039/C8FD00215K).

When the discussion opened, the questions and comments reflected a topic that had already been central in session 1: the crucial role of theory to guide and interpret experiments, and the need for ever more powerful computational methods, particularly with respect to being able to track more than one reaction coordinate simultaneously. Another theme that emerged from the discussion, unsurprisingly, was that of structural effects on energy and charge transfer processes, which is also the recurring theme that underpinned much of the research presented in session 2. While there was much interest in knowing how changing certain molecular features would affect experimental observations, it was clear that the challenge oftentimes lies in the synthetic pathways to achieve the required structural changes and there was thus a call for renewed efforts towards improved molecular designing capabilities.

The audience were also particularly keen during this discussion to challenge each other to consider pushing through current conceptual barriers. For example, there were conversations during this session – reignited later in the conference – surrounding the nature of spin states and the appropriateness of spin labels (singlet, triplet, etc.), particularly (but not exclusively) in cases where the effects of strong spin–orbit coupling cannot be neglected. Highlighting experimental challenges in unravelling the femtosecond dynamics of such often mixed states, Persson also comments that this is a “rich area of current research where several groups have made contributions in the last few years to present convincing evidence for a much more elaborate view including considerable mixing of electronic and spin states”.

One final interesting point of discussion concerned the current understanding of proton transfer processes. Namely, and with regards to the work of Dawlaty and co-workers (DOI: 10.1039/C8FD00215K), Todd Martinez (Stanford University, USA) asked what experimental evidence there was for intermolecular proton transfer as opposed to hydrogen atom transfer, highlighting computational evidence for excited state intramolecular proton transfer first requiring structural rearrangement. Much debate followed as the audience explored when such transfer processes can be considered proton or electron transfer driven. Dawlaty added that excited state orbital symmetry may, in fact, necessitate that a hydrogen atom be transferred, rather than a proton. However, in the case of work by Dawlaty and co-workers, it would be incredibly difficult for ultrafast vibrational spectroscopies to distinguish between these different cases due to the role of solvent motion in the proton transfer process. There is, nevertheless, at least one example in the literature of a system for which experimental techniques were able to demonstrate that charge transfer occurs to the H-bonded solvent first (implying the electron moves first), with the movement then being neutralised with proton motion.⁴ The question of what moves first in a proton transfer process, and the discussion that took place at the conference with regards to it, is perhaps best summarised by the comment made by Sharon Hammes-Schiffer (Yale University, USA): “one picture will not cover all cases of photoinduced proton transfer or proton coupled electron transfer”. More research and further clarity on the topic are, therefore, still necessary.

Session 3: Photo-induced electron transfer

In the afternoon of the second day of this Faraday Discussion, session 3 began with a presentation by David Beratan (Duke University, USA), on developing a theoretical framework to explore the mechanism of energy transfer through a bridge between chromophores that change spin states during the process (DOI: 10.1039/C9FD00007K). The approach that Beratan and co-workers undertook to model these mechanisms differs from single electron or hole transfer frameworks in that it assumes the donor–bridge–acceptor energy transport involves two states that are non-adiabatically coupled which, in turn, results in quantum interferences among different energy coupling pathways. Following this scheme, the computational simulations by Beratan and co-workers showed effective coupling between pathways brought about by (sometimes dominating) two particle coupling interactions (DOI: 10.1039/C9FD00007K). These findings may allow for the design of donor–bridge–acceptor systems with enhanced energy transfer pathways, a much desired outcome as highlighted by previous sessions.

The topic of quantum coherence emerged once again with the presentation by Michael Wasielewski (Northwestern University, USA), this time applied in the context of electron transfer in two donor–acceptor molecular systems (DOI: 10.1039/C8FD00218E). By employing ultrafast transient absorption spectroscopic methods, Wasielewski and co-workers evaluated the electron transfer in two donor–acceptor molecular systems, each consisting of a p-(9-anthryl)-N,N-dimethylaniline chromophore (electron donor) and either one or two equivalents of naphthalene-1,8:4,5-bis(dicarboximide) (electron acceptor). These studies revealed that a significant enhancement of electron transfer rate is observed in the two-acceptor system when compared to the single-acceptor counterpart, suggesting a correlation between acceptors (DOI: 10.1039/C8FD00218E). This enhancement was temperature dependent, however, in such a way that lead Wasielewski and co-workers to conclude that the electron would be delocalized over both acceptors at low temperature but localized on a single acceptor at room temperature. Wasielewski and co-workers highlighted the importance of minimising bath fluctuations so as to preserve coherent interactions and thus enabling their targeted use (DOI: 10.1039/C8FD00218E).

Benjamin Schwartz (University of California, Los Angeles, USA) followed, also presenting ultrafast transient absorption spectroscopy results. Schwartz and co-workers focused on evaluating the relationship between chemical dopant concentrations and type of charge carrier present in semiconducting polymer films (DOI: 10.1039/C8FD00210J). Since different charge carriers – free polarons, trapped polarons and bipolarons – have different conductivities, the relative amounts of each charge carrier will determine the overall conductivity of the doped polymer film. Schwartz and co-workers found that the charge carriers in 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane-doped films of poly(3-hexylthiophene-2,5-diyl) are mainly free and trapped polarons, as suggested by the spectral and dynamic features they observed (DOI: 10.1039/C8FD00210J). Moreover, the authors found that higher doping levels result in an increased ratio of trapped to free polarons. Based on the information they collected, Schwartz and co-workers make a number of suggestions for designing new materials with improved carrier mobility, such as using dopants which avoid build-up of trapped polarons at high doping levels (DOI: 10.1039/C8FD00210J).

Next, Hammes-Schiffer presented another example of the importance of Noel Hush's work, which we briefly introduced earlier. Starting from the Marcus–Hush theory approach to photoinduced proton-coupled electron transfer (PCET),^1–3 Hammes-Schiffer and co-workers developed a framework in which the transferring proton and active electrons are treated quantum mechanically, with PCET being described as a non-adiabatic transition between mixed electron–proton vibronic states (DOI: 10.1039/C8FD00240A). The authors utilised their proposed approach to explore the cases for which the rate constant for electron transfer decreases as the driving force increases – the so called ‘inverted region’ of the potential energy surface of the reaction – which is a phenomenon useful for improving energy conversion efficiency that is often significantly suppressed or even non-existent in some cases. The simulations by Hammes-Schiffer and co-workers enable the modelling of this behaviour and can be employed in designing systems for which the ‘inverted region’ character is enhanced, providing new materials for more efficient energy conversion (DOI: 10.1039/C8FD00240A).

Next, Mark Thompson (University of Southern California, USA) introduced his and his co-workers’ efforts towards promoting excited state charge separation, which he suggested could be achieved by connecting two dye molecules into a weakly coupled non-polar symmetric pair (DOI: 10.1039/C8FD00201K). Thompson and co-workers used ultrafast transient absorption spectroscopy to monitor the kinetics of two dipyrrin-based bichromophoric systems in different solvents, having found that fine tuning of the solvent polarity can enhance symmetry breaking charge transfer (DOI: 10.1039/C8FD00201K). This is yet another parameter that can be exploited in designing materials and systems with improved ability for charge separation.

Up to this point in the conference, triplet states had only briefly been discussed, and often in the context of their role in decreased device performance. Nevertheless, Tom Penfold (Newcastle University, UK) presented his work on triplet–triplet energy transfer, focusing on the cases for which it can be beneficial, or its properties harnessed (DOI: 10.1039/C8FD00174J). Triplet states play crucial roles in both natural and artificial processes, and in particular have been under scrutiny owing to their potential applications in organic photovoltaics and organic light emitting diodes, which justifies the interest in exploring energy transfer processes amongst triplet states. The simulations by Penfold and co-workers of several triplet–triplet energy transfer scenarios allow for a rationalisation of previously reported experimental work and gleans further insight on the driving forces and fundamental rules which guide these processes (DOI: 10.1039/C8FD00174J). Nevertheless, given the complex nature of said processes and given that there are several, and often contradicting, factors at play, the authors suggest further understanding is required before approaches to controlling device performance in this context can be devised.

The final discussion paper in session 3 by Oliver Gessner (University of California, Berkeley) explored photoinduced changes in the oxygen K-edge X-ray absorption spectrum of a thin copper oxide film, which he and his co-workers monitored by employing picosecond time-resolved X-ray absorption spectroscopy and steady-state temperature dependent X-ray absorption spectroscopy (DOI: 10.1039/C8FD00236C). The work was aimed at further understanding and modelling the effects of laser induced heating and subsequent cooling. Given that these are non-negligible effects, Gessner and co-workers found that quantitative modelling of thermal effects in time-resolved X-ray absorption spectroscopy should be sought, in order to more accurately interpret the resulting data (DOI: 10.1039/C8FD00236C).

During the discussion, the audience rekindled the debate surrounding how best to address the breakdown of the singlet/triplet binary understanding and the potential effects of this breakdown on experimental observation and interpretation, following the discussion in session 2. Moreover, this was a session which sparked much discussion on novel experiments which not only help prove computational findings, but can also provide richer and, importantly, distinguishable information. From suggestions of ‘which-way’ experiments to probe coherent interference of coupling pathways in bridge-mediated energy transfer, to interdisciplinary efforts to realize novel double-slit quantum dynamics at the nanoscale, to molecular interferometers and transient reflectivity instead of transient absorption, this really was the session in which the audience gathered the collective mind to think of new and highly differential experimental methods to tackle the challenges raised during the presentations. Once the discussion session was finished, delegates were invited to join the conference dinner, during which the historical Loving Cup ceremony took place (Fig. 3).

Fig. 3 After session 3, delegates of the Faraday Discussion on ‘Ultrafast Photoinduced Energy and Charge Transfer’ were invited to dinner and, as tradition demands, the loving cup ceremony took place.

Session 4: Photoprotection and photodamage in natural systems

The final session of this Faraday Discussion diverged from the topics that had so far dominated the discussion (natural and artificial light harvesting) and considered the concepts of energy and charge transfer in the context of photoprotection and photodamage.

The first paper of session 4, presented by Tomáš Polívka (University of South Bohemia, Czech Republic), explored the effects of modifying fucoxanthin (a xanthophyll that contributes extensively to the production of carotenoids in nature) by adding an hexanoyl group, and monitoring the influence of this modification on the carotenoid–chlorophyll energy transfer (DOI: 10.1039/C8FD00193F). Polívka and co-workers showed that the presence of the hexanoyl group slows down the energy transfer process, which is interesting given that the hexanoyl group is not part of the conjugated backbone of the fucoxanthin. With the lack of conjugation, it would be expected that this group would have little impact on the fucoxanthin electronic state involved in the energy transfer; in fact, this was a question raised by Pavel Chabera (Lund University, Sweden) during the open discussion. In response, Polívka suggested that the hexanoyl group may be influencing the polarity of the environment which keto-carotenoids are sensitive to.

Still with respect to the work presented by Polívka, evidently of considerable interest, Collin Steen (University of California Berkeley, USA) presented spectroscopic data during the open discussion session. The data presented pertained to studies performed on carotenoid–chlorophyll to evaluate the energy transfer in the living cells of the algae nannochloropsis oceanica, rather than in an isolated solution. Steen's measurements showed that additional protein structures were required for the energy transfer to occur; while fucoxanthin is not present in the system studied by Steen, he pointed out that a similar protein is involved in the processes he was reporting on. This novel approach of using living cells in ultrafast laser spectroscopic measurements is of great interest to experimentalists interested in looking at living biological systems, and Steen's work highlights the possibility of ultrafast spectroscopic experiments being carried out in more real-world environments.

The second presentation of session 4 was delivered by Scott Habershon (University of Warwick, UK) and was concerned with developing improved simulation methods to model non-adiabatic dynamics of molecular systems, with a view to link chemical structure of a molecule (in particular photoprotective sunscreen molecules) with its photophysics (DOI: 10.1039/C8FD00228B). Specifically, Habershon and co-workers focused on creating an “on the fly” strategy for accurate wavefunction propagation to be used as a “black box” to provide information on ultrafast dynamics of photoinduced energy transfer processes. Habershon also presented more recent data not included in the initial submission, demonstrating how he and his co-workers’ new computational methods are increasingly more viable, while also pointing out some of the major challenges that still hinder its widespread use (DOI: 10.1039/C8FD00228B).

Heiko Lokstein (Charles University, Czech Republic) then followed with a presentation on two-photon absorption spectroscopy, a technique which allows for spectroscopic excitation of electronic states that are one-photon forbidden, of photosynthetic light-harvesting complexes and pigments (DOI: 10.1039/C8FD00198G). While much of the work presented at this Faraday Discussion revolved around photosynthesis, the work by Lokstein and co-workers targeted (mostly) understanding the photoprotective roles of xanthophylls and carotenoids within the photosynthetic machine. Understanding the energy transfer and non-chemical quenching mechanisms of carotenoids is not straight forward, as the authors point out, due to their lowest excited singlet state transition being typically one-photon excitation forbidden; which then necessitates the use of two-photon excitation spectroscopy (DOI: 10.1039/C8FD00198G). The results of Lokstein and co-workers suggest that, when photoexciting light-harvesting complex II, the observed two-photon absorption signal is due to chlorophylls and not carotenoids, in agreement with previous studies, which has significant implications for the carotenoid–chlorophyll energy transfer process (DOI: 10.1039/C8FD00198G). Understandably, this generated lively discussion with the audience at the conference.

After the morning break, session 4 resumed with a presentation by Spiridoula Matsika (Temple University, USA). The computational work presented demonstrated that one of the most important factors for the formation of cyclobutane pyrimidine dimers – mutagenic photolesions created in DNA upon interaction with ultraviolet radiation – may be the relative position of different base pairs which, in turn, influences the energy of charge transfer states. The accessibility of these states, determined by their relative energy in relation to each other and nearby triplet states, offers an alternative relaxation pathway which bypasses the formation of cyclobutane pyrimidine dimers (DOI: 10.1039/C8FD00184G). For example, guanine adjacent to thymine stabilises charge transfer states (lowering their energy), while guanine adjacent to guanine destabilizes these states (increasing their energy). Later, during the discussion, Shou-Ting Hsieh (National Taiwan University, Taiwan) raised a particularly interesting question regarding the application of Matsika's observations to determining the likelihood of an individual's DNA being susceptible to forming cyclobutane pyrimidine dimers, depending on the specific base pair sequence.

Finally, to close this session (Fig. 4), Christopher Grieco (Ohio State, USA) presented ultrafast spectroscopic measurements on the heterodimer formed by catechol and its related quinone, which Grieco and co-workers used as a model system for potential molecular interactions in the skin pigment eumelanin (DOI: 10.1039/C8FD00231B). The results from these experiments showed that photoexcitation of these heterodimers resulted in the formation of two semi-quinone radicals due to an excited state proton transfer from the catechol to the quinone, irrespective of which unit is photoexcited. Interestingly, upon excitation of the quinone, the system undergoes rapid intersystem crossing and the hydrogen transfer occurs due to a populated triplet state, which may also be an important relaxation (photoprotection) pathway in eumelanin (DOI: 10.1039/C8FD00231B).

Fig. 4 The audience at the Faraday Discussion, at the Four Points Hotel in Ventura, California, USA, attentively listen to the presentations and debates taking place.

The more general overview discussion that closed session 4 focused mainly on two-molecule interactions and on how these interactions impact the photodynamics of dimerised systems. These studies, and the discussion that ensued, serve to highlight the progress on photoprotection and photodamage by employing a ‘bottom-up’ approach to the study of the photochemistry and photophysics of natural systems – by firstly exploring the behaviour of simple systems and then expanding this knowledge by building up system complexity, exemplified through the study of monomers and dimers respectively. In addition, there were several remarks made during the discussion on how close these studies are to real-life systems, and how encouraging this is for potential future applications of this knowledge.

Concluding remarks

The summary and closing talk for this Faraday Discussion was delivered by Bern Kohler (Ohio State, USA), an important contributor to the field of photochemistry, especially in the context of photostability of biological systems. Kohler's light-hearted talk outlined the history of ultrafast chemistry and physics as well as presenting the history of the Faraday Discussion meetings themselves, with highly appropriate reference to the 1959 Faraday Discussion on energy transfer with special reference to biological systems. With this, Kohler provided an excellent insight on how the field has transformed over the years, with specific focus on the key players important to this change. Kohler also added, with reference to the participants of the 1959 Faraday Discussion, the sheer delight they would have in the technological advances which have undeniably transformed how both experiment and theory are performed today. Quoting a concluding sentence from one of the papers by the team responsible for the discovery of the double helix DNA structure, Kohler reminded the audience that “the best conclusions are not the ones that simply summarise what has been discussed, but the ones that bravely suggest what the future holds for the work presented”. Following this, Kohler added that, rather than looking to the past, we should consider the new generation of scientists, and then shared Haiku poetry written by his students.

Conclusions

Despite the many different perspectives, approaches and applications of the science presented, the delegates of this Faraday Discussion were undoubtedly captivated by the mysteries of energy and charge transfer. The collective interest span from mapping and understanding the fundamental laws that guide quantum coherence, to the observation of how the interactions that enable energy and charge transfer manifest in large molecules, such as DNA, or even living cells. The suggested applications of the research shared at this meeting were very clearly biased towards photovoltaics and artificial light harvesting, likely in response to society's ever-growing need for energy. Nevertheless, considering how much of life depends on energy and charge transfer processes – from solute–solvent dynamics to the functioning of the nervous system, as well as encompassing everything in between – it is undeniable that the knowledge gathered in this field has the potential for transformational impact. In fact, there is no telling what numerous future applications may benefit from this knowledge.

In addition, it is the opinion of the authors of this highlights report that the Faraday Discussion on Ultrafast Photoinduced Energy and Charge Transfer of April 2019 (Ventura, USA) was truly an example of what a scientific meeting should be. The atmosphere was collaborative throughout, with an engaged audience who were keen to both listen to and share thoughts and ideas. Discussion was effortless and truly interesting, spanning from advice on practical experimental logistics to much more abstract debate on the current boundaries of knowledge and understanding and how we propose to stretch and surpass these boundaries. The genuine collective interest in the topic at hand was evident even in social time, with scientific conversations spilling over from the conference room into coffee breaks and dinner. It was a lively and supportive environment, where there was as much appreciation for those who contributed to the current state-of-the-art as for those who will champion future innovation in the field. A very worthwhile experience and a thoroughly enjoyable and productive conference.

Acknowledgements

J. M. W. thanks the EPSRC and Newport Spectra-Physics Ltd for a joint doctoral studentship. K. M. K. thanks the EPSRC for a doctoral scholarship. M. A. P. T. thanks the EPSRC for a doctoral studentship through the EPSRC Centre for Doctoral Training in Molecular Analytical Science, Grant Number EP/L015307/1. M. D. H. thanks the Leverhulme Trust for postdoctoral funding. V. G. S. is grateful to the Royal Society and the Leverhulme Trust for a Royal Society Leverhulme Trust Senior Research Fellowship. N. d. N. R. wishes to acknowledge and thank the Royal Society of Chemistry for the award of a travel grant that allowed her to attend this Faraday Discussion. V. G. S. and N. d. N. R. also thank the HO2020 FET-OPEN Grant BoostCrop for financial support.

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