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Electronic coherence signatures can be directly identified in the time-frequency maps measured in 2 dimensional spectroscopy (2DES). We demonstrate the theory and discuss the advantages of this approach by a detailed application to the fast femtosecond beatings of a wide variety of electronic coherences in dimers of size-dispersed (8%) 3nm quantum dots (QDs). The observed and computed results can be consistently characterized directly in the time-frequency domain by probing the polarization in a 2DES set-up. Experimental and computed time-frequency maps are found in very good agreement and several electronic coherences are characterized at room temperature in solution before extensive dephasing due to the size-dispersion kicks in. As compared to the frequency-frequency maps that are commonly used in 2DES, the time-frequency maps allow for exploiting electronic coherences without additional post processing and with fewer 2DES measurements. Towards quantum technology applications, we also report on the modeling of the time-frequency photocurrent response of these electronic coherences, which opens the way to integrating QD devices with classical architectures thereby enhancing the quantum advantage of such technologies for parallel information processing at room temperature.

The advantages of the directly measured time-frequency maps are discussed as a useful representation of the coherent output in a 2 dimensional electronic spectroscopy (2DES). We demonstrate the theory by a detailed application to the fast femtosecond beatings of a wide variety of electronic coherences in dimers of size-dispersed (9%) 3nm quantum dots (QDs). The observed and computed results can be consistently characterized directly in the time-frequency domain by probing the polarization in a 2DES set-up. Experimental and computed time-frequency maps are found in very good agreement and several electronic coherences are characterized at room temperature in solution before extensive dephasing due to the size-dispersion kicks in. As compared to the frequency-frequency maps that are commonly used in 2DES, the time-frequency maps allow for exploiting electronic coherences without additional post processing and with fewer 2DES measurements of polarization. Towards quantum technology applications, we also report on the modeling of the time-frequency photocurrent response of these electronic coherences, which opens the way to integrating QD devices with classical architectures thereby enhancing the quantum advantage of such technologies for parallel information processing at room temperature.

This work reports on dye sensitized photoanodes for C-H activation in organic substrates, assembled by vacuum sublimation of a commercially available quinacridone (QNC) dye in the form of nanosized rods onto Fluorine doped tin oxide (FTO), TiO2 and SnO2 slides. The photoanodes display extended absorption in the visible (450-600 nm) and ultrafast photoinduced electron injection (1 ps, as revealed by transient absorption spectroscopy) of the QNC dye into the semiconductor. The proton-coupled electron-transfer reactivity of QNC is exploited for generating a nitrogen-based radical as its oxidized form, which is competent in C-H bond activation. In particular, the key reactivity parameter is the bond dissociation free energy (BDFE) associated with the N•/N-H couple in QNC of 80.5±2.3 kcalmol-1, which enables hydrogen atom abstraction from allylic or benzylic C-H moieties. A photoelectrochemical response is indeed observed for organic substrates characterized by C-H bonds with BDFE below the 80.5 kcalmol-1 threshold, such as γ-terpinene, xanthene, dihydroanthracene. This work opens a rationale and mechanistically oriented design of dye-sensitized photoelectrodes for selective organic transformations.

[Image: see text] 2D electronic spectroscopy (2DES) techniques have gained particular interest given their capability of following ultrafast coherent and noncoherent processes in real-time. Although the fame of 2DES is still majorly linked to the investigation of energy and charge transport in biological light-harvesting complexes, 2DES is now starting to be recognized as a particularly valuable tool for studying transport processes in artificial nanomaterials and nanodevices. Particularly meaningful is the possibility of assessing coherent mechanisms active in the transport of excitation energy in these materials toward possible quantum technology applications. The diverse nature of these new target samples poses significant challenges and calls for a critical rethinking of the technique and its different realizations. With the confluence of promising new applications and rapidly developing technical capabilities, the enormous potential of 2DES techniques to impact the field of nanosystems, quantum technologies, and quantum devices is here delineated.

By conducting in-depth mechanistic investigations to rationalize reaction manifolds, chemists can expand the generality of synthetic processes and discover novel reactivities. The challenge is to use the resulting information to control the selectivity of a given synthetic process. A particular challenge for organic chemists is to use an external stimulus to switch a reactivity on or off. Here, we mechanistically investigated light-driven [2+2] heterocycloadditions (Paternò-Büchi reactions) between indoles and ketones. We used ground-state UV-Vis absorption and transient absorption spectroscopy (TAS) at ns and fs timescale, together with DFT and TD-DFT calculations. We found that the reaction can proceed via an exciplex or electron-donor-acceptor (EDA) complex manifold, which are key intermediates in determining the reaction’s stereoselectivity. We used this discovery to control the reaction’s diastereoselectivity, gaining access to previously inaccessible diastereoisomeric variants. When moving from 370 to 456 nm irradiation, the EDA complex is increasingly favored, and the diastereomeric ratio dr (en-do:exo) moves from >99

The energy transfer (ET) from carotenoids (Cars) to chlorophylls (Chls) in photosynthetic complexes occurs with almost unitary efficiency thanks to the synergistic action of multiple finely tuned channels whose photophysics and dynamics are not fully elucidated yet. We investigated the energy flow from the Car peridinin (Per) to Chl a in the peridinin chlorophyll a protein (PCP) from marine algae Amphidinium carterae by using two-dimensional electronic spectroscopy (2DES) with a 10 fs temporal resolution. Recently debated hypotheses regarding the S2-to-S1 relaxation of the Car via a conical intersection and the involvement of possible intermediate states in the ET were examined. The comparison with an N89L mutant carrying the Per donor in a lower-polarity environment helped us unveil relevant details on the mechanisms through which excitation was transferred: the ET yield was conserved even when a mutation perturbed the optimization of the system thanks to the coexistence of multiple channels exploited during the process.

The biological light-harvesting process offers an unlimited source of inspiration. The high level of control, adaptation capability, and efficiency challenge humankind to create artificial biomimicking nanoarchitectures with the same performances to respond to our energy needs. Here, in the extensive search for design principles at the base of efficient artificial light harvesters, an approach based on self-assembly of pigment-peptide conjugates is proposed. The solvent-driven and controlled aggregation of the peptide moieties promotes the formation of a dense network of interacting pigments, giving rise to an excitonic network characterized by intense and spectrally wide absorption bands. The ultrafast dynamics of the nanosystems studied through two-dimensional electronic spectroscopy reveals that the excitation energy is funneled in an ultrafast time range (hundreds of femtoseconds) to a manifold of long-living dark states, thus suggesting the considerable potentiality of the systems as efficient harvesters.

Plexcitons, that is, mixed plasmon-exciton states, are currently gaining broad interest to control the flux of energy at the nanoscale. Several promising properties of plexcitonic materials have already been revealed, but the debate about their ultrafast dynamic properties is still vibrant. Here, pump-probe spectroscopy is used to characterize the ultrafast dynamics of colloidal nanohybrids prepared by coupling gold nanoparticles and porphyrin dyes, where one or two sets of plexcitonic resonances can be selectively activated. We found that these dynamics are strongly affected by the presence of a reservoir of states including plasmon resonances and dark states. The time constants regulating the plexciton relaxations are significantly longer than the typical values found in the literature and can be modulated over more than 1 order of magnitude, opening possible interesting perspectives to modify rates of chemically relevant molecular processes.

Harvesting the excess energy from absorbed above bandgap photons is a promising approach to overcome the detailed balance limit for higher solar cell efficiencies. However, this remains very challenging for 2D layered halide perovskites as the fast excess energy loss competes effectively with charge extraction. Herein, the authors engineer the energy cascade manifold of quantum well (QW) states in quasi-2D Ruddlesden–Popper perovskites by facile tuning of the organic spacer to decelerate the energy loss. The resulting excess energy loss rate is up to two orders slower compared to 3D perovskites, thus enabling efficient carrier extraction. 2D electronic spectroscopy reveals further insights into the structural and energetic disorder of these layered systems. Importantly, a judicious choice of the organic spacer holds the key to tailoring the coherent coupling between QWs that strongly influences the competition between the energy cascade and charge extraction. Ministry of Education (MOE) National Research Foundation (NRF) Accepted version The authors acknowledge Dr. Teddy Salim from the Facility for Analysis Characterization Testing and Simulation (FACTS), School of Material Science and Engineering, Nanyang Technological University, for the UPS measurement. This research/project was supported by the Ministry of Education under its AcRF Tier 2 grants (MOE2019-T2-1-006, MOE2019-T2-1-097, and MOE-T2EP50120-0004); and the National Research Foundation (NRF) Singapore under its NRF Investigatorship (NRF-NRFI-2018-04). E.C. acknowledges the “CQ-TECH” STARS Grant 2019 (2019-UNPD0Z9-0166571). E.F. acknowledges a Ph.D. fellowship from the Department of Excellence program “NExuS.

A wide variety of photoinduced electronic coherences are shown to be robust with respect to dephasing in ensembles of quasi-homodimers assembled with sub-nm ligands from colloidal 3 nm CdSe quantum dots (QDs) with controlled 9% size dispersion, both in solution and in solid-state. Coherence periods ranging from 40 to 300 fs are consistently characterized by multi-dimensional electronic spectroscopy in the Vis range in solution and solid-state samples. A theoretical model that includes size dispersion, spin orbit coupling, and crystal field splitting supports the assignment of electronic coherences. Further, this model provides a guide for optimizing the coherences by tuning the interplay between dimer electronic delocalization, optical activity and size dispersion. The experimental persistence of many QD electronic coherences at the level of the size dispersed ensemble in the solid-state and in solution opens the way for building versatile bottom-up materials well suited to quantum technology applications.

The performance of nonfullerene-acceptor-(NFA)-based organic solar cells is rapidly approaching the efficiency of inorganic cells. The chemical versatility of NFAs extends the light-harvesting range to the infrared, while preserving a considerably high open-circuit- voltage, crucial to achieve power-conversion efficiencies >17%. Such low voltage losses in the charge separation process have been attributed to a low-driving-force and efficient exciton dissociation. Here, we address the nature of the subpicosecond dynamics of electron/hole transfer in PM6/Y6 solar cells. While previous reports focused on active layers only, we developed a photocurrent-detected two-dimensional spectroscopy to follow the charge transfer in fully operating devices. Our measurements reveal an efficient hole-transfer from the Y6- acceptor to the PM6-donor on the subpicosecond time scale. On the contrary, at the same time scale, no electron-transfer is seen from the donor to the acceptor. These findings, putting ultrafast spectroscopy in action on operating optoelectronic devices, provide insight for further enhancing NFA solar cell performance. This research received funding from the Clean Planet Program supported by Fundació Joan Ribas Araquistain (FJRA) and the Ignite program (Q-SPET) supported by the Barcelona Institute of Science and Technology. N.F.v.H. acknowledges the financial support by the European Commission (ERC Advanced Grant 670949-LightNet and Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agree- ment No 713729), the Ministry of Science & Innovations (“Severo Ochoa” program for Centers of Excellence in R&D CEX2019-000910-S and Plan Nacional PGC2018-096875−B- I00), the Catalan AGAUR (2017SGR1369), Fundació Privada Cellex, Fundació Privada Mir-Puig and the Generalitat de Catalunya through the CERCA program. J.M. and F.B. acknowledge the financial support by the European Commis- sion (grant 951843), Spanish Ministry MINECO and FEDER (grant MAT2017-89522-R), the Severo Ochoa program (Grant SEV-2015-0522) and “Agencia Estatal de Investiga- ción” (Grant PRE2018-084881). E.C. acknowledges the financial support of the H2020 FET Project COPAC (766563).

The influence of hydrogen bonds (H-bonds) in the structure, dynamics, and functionality of biological and artificial complex systems is the subject of intense investigation. In this broad context, particular attention has recently been focused on the ultrafast H-bond dependent dynamical properties in the electronic excited state because of their potentially dramatic consequences on the mechanism, dynamics, and efficiency of photochemical reactions and photophysical processes of crucial importance for life and technology. Excited-state H-bond dynamics generally occur on ultrafast time scales of hundreds of femtoseconds or less, making the characterization of associated mechanisms particularly challenging with conventional time-resolved techniques. Here, 2D electronic spectroscopy is exploited to shed light on this still largely unexplored dynamic mechanism. An H-bonded molecular dimer prepared by self-assembly of two boron-dipyrromethene dyes has been specifically designed and synthesized for this aim. The obtained results confirm that upon formation of H-bonds and the dimer, a new ultrafast relaxation channel is activated in the ultrafast dynamics, mediated by the vibrational motions of the hydrogen donor and acceptor groups. This relaxation channel also involves, beyond intra-molecular relaxations, an inter-molecular transfer process. This is particularly significant considering the long distance between the centers of mass of the two molecules. These findings suggest that the design of H-bonded structures is a particularly powerful tool to drive the ultrafast dynamics in complex materials.

The synthetic procedures for the preparation of supramolecular BODIPY dimers decorated with complementary patterns able to induce the formation of a triple hydrogen bond through mutual interactions are here reported. The BODIPY and styryl-equipped BODIPY species have been suitably functionalized in meso position with 2,6-diacetamido-4-pyridyl and 1-butyl-6-uracyl moieties. Dimers and monomers have been subjected to computational and photophysical investigations in solvent media. Various peculiarities concerning the effects of the interaction geometry on the stability of the H-bonded systems have also been investigated. The combination of modelling and experimental data provides a paradigm for improving and refining the BODIPY synthetic pathway to have chromophoric architectures with a programmable supramolecular identity. Furthermore, the possibility of assembling dimers of different dyes through H-bonds could be appealing for a systematic investigation of the principal factors affecting the dynamics of the energy migration and possibly driving coherent transfer mechanisms. Our work highlights how the chemical versatility of these dyes can be exploited to design new BODIPY-based supramolecular architectures.

The non-radiative relaxation process within the Q-bands of chlorophylls represents a crucial preliminary step during the photosynthetic mechanism. Despite several experimental and theoretical efforts performed in order to clarify the complex dynamics characterizing this stage, a complete understanding of this mechanism is still far to be reached. In this study, non-adiabatic excited-state molecular dynamic simulations have been performed to model the non-radiative process within the Q-bands for a model system of chlorophylls. This system has been considered in the gas phase and then, to have a more representative picture of the environment, with implicit and mixed implicit-explicit solvation models. In the first part of this analysis, absorption spectra have been simulated for each model in order to guide the setup for the non-adiabatic excited-state molecular dynamic simulations. Then, non-adiabatic excited-state molecular dynamic simulations have been performed on a large set of independent trajectories and the population of the Qx and Qy states has been computed as the average of all the trajectories, estimating the rate constant for the process. Finally, with the aim of investigating the possible role played by the solvent in the Qx-Qy crossing mechanism, an essential dynamic analysis has been performed on the generated data, allowing one to find the most important motions during the simulated dynamics. The non-radiative relaxation process within the Q-bands of chlorophylls represents a crucial preliminary step during the photosynthetic mechanism. Despite several experimental and theoretical efforts performed in order to clarify the complex dynamics characterizing this stage, a complete understanding of this mechanism is still far to be reached. In this study, non-adiabatic excited-state molecular dynamic simulations have been performed to model the non-radiative process within the Q-bands for a model system of chlorophylls. This system has been considered in the gas phase and then, to have a more representative picture of the environment, with implicit and mixed implicit-explicit solvation models. In the first part of this analysis, absorption spectra have been simulated for each model in order to guide the setup for the non-adiabatic excited-state molecular dynamic simulations. Then, non-adiabatic excited-state molecular dynamic simulations have been performed on a large set of independent trajectories and the population of the Qx and Qy states has been computed as the average of all the trajectories, estimating the rate constant for the process. Finally, with the aim of investigating the possible role played by the solvent in the Qx-Qy crossing mechanism, an essential dynamic analysis has been performed on the generated data, allowing one to find the most important motions during the simulated dynamics.

We show in a joint experimental and theoretical study that ultrafast femto-second (fs) electronic coherences can be characterized in semi-conducting colloidal quantum dot (QD) assemblies at room temperature. The dynamics of the electronic response of ensembles of CdSe QDs in the solution and of QD dimers in the solid state is probed by a sequence of 3 fs laser pulses as in two-dimensional (2D) electronic spectroscopy. The quantum dynamics is computed using an excitonic model Hamiltonian based on the effective mass approximation. The Hamiltonian includes the Coulomb, spin–orbit, and crystal field interactions that give rise to the fine structure splittings. In the dimers studied, the interdot distance is sufficiently small to allow for an efficient interdot coupling and delocalization of the excitons over the two QDs of the dimer. To account for the inherent few percent size dispersion of colloidal QDs, the optical response is modeled by averaging over an ensemble of 2000 dimers. The size dispersion is responsible for an inhomogeneous broadening that limits the lifetimes of the excitonic coherences that can be probed to about 150 fs–200 fs. Simulations and experimental measurements in the solid state and in the solution demonstrate that during that time scale, a very rich electronic coherent dynamics takes place that involves several types of intradot and interdot (in the case of dimers) coherences. These electronic coherences exhibit a wide range of beating periods and provide a versatile basis for a quantum information processing device on a fs time scale at room temperature.

Coupling of molecular emitters to plasmon resonances in metal nanostructures has long been investigated to control the light– matter interaction at the nanoscale. The emergence of different coupling behaviors can be governed by the various combinations of emitters and plasmonic substrates, as well as the spatial arrangement of the individual components. Here colloidal assembly methods are exploited to prepare a responsive nanosystem where two sets of plexcitonic resonances in different coupling regimes can be selectively switched on and off, acting on external conditions such as concentration and presence of anions. The two sets of plexciton resonances are built exploiting the strong coupling between cationic gold nanoparticles and the same molecular moiety, an anionic porphyrin, in different aggregation states. When both plexciton resonances are simultaneously activated in the system, evidence for a plexciton relaxation cascade has been found in photoluminescence experiments. These findings have fundamental implications for achieving control over energy flow at the nanoscale.

The electronic state manifolds of carotenoids and their relaxation dynamics are the object of intense investigation because most of the subtle details regulating their photophysics are still unknown. In order to contribute to this quest, here, we present a solvent-dependent 2D Electronic Spectroscopy (2DES) characterization of fucoxanthin, a carbonyl carotenoid involved in the light-harvesting process of brown algae. The 2DES technique allows probing its ultrafast relaxation dynamics in the first 1000 fs after photoexcitation with a 10 fs time resolution. The obtained results help shed light on the dynamics of the first electronic state manifold and, in particular, on an intramolecular charge-transfer state (ICT), whose photophysical properties are particularly elusive given its (almost) dark nature.

The full blossoming of quantum technologies requires the availability of easy-to-prepare materials where quantum coherences can be effectively initiated, controlled and exploited, preferably at ambient conditions. Solid-state multilayers of colloidally grown quantum dots (QDs) are highly promising for this task because of the possibility of assembling networks of electronically coupled QDs through the modulation of sizes, inter-dot linkers and distances. To usefully probe coherence in these materials, the dynamical characterization of their collective quantum mechanically coupled states is needed. Here we explore by 2D electronic spectroscopy the coherent dynamics of solid-state multilayers of electronically coupled colloidally grown CdSe QDs and complement it by detailed computations. The time evolution of a coherent superposition of states delocalized over more than one QD was captured at ambient conditions. We thus provide important evidence for inter-dot coherences in such solid-state materials, opening up the effective exploitation of these materials towards quantum technologies.

A thorough characterization of the early time sub-100 fs relaxation dynamics of biologically relevant chromophores is of crucial importance for a complete understanding of the mechanisms regulating the ultrafast dynamics of the relaxation processes in more complex multichromophoric light-harvesting systems. While chlorophyll a has already been the object of several investigations, little has been reported on chlorophyll b, despite its pivotal role in many functionalities of photosynthetic proteins. Here the relaxation dynamics of chlorophyll b in the ultrafast regime have been characterized using 2D electronic spectroscopy. The comparison of experimental measurements performed at room temperature and 77 K allows the mechanisms and the dynamics of the sub-100 fs relaxation dynamics to be characterized, including spectral diffusion and fast internal conversion assisted by a specific set of vibrational modes.

Vibrational coherences and their time evolution are crucial for the correct description of the dynamical behavior of materials, also influencing the dynamics of electronic coherences and transport processes. For this reason, vibrational coherences are now becoming the subject of wider investigation, especially in the framework of 2D electronic spectroscopies. A correct interpretation of vibrational coherences in 2DES responses requires the comparison with Raman spectra. Here we propose a methodology that goes beyond the typical practice of merely looking for frequencies present in both signals. In particular, we discuss suitable experimental conditions and correction procedures that allows a direct comparison of Fourier spectra obtained from the analysis of the signal beating at specific coordinates in 2D maps with resonant and non-resonant Raman spectra, to clearly identify the vibrational modes more strongly coupled with the electronic transition. The advantages of this approach have been illustrated using Chl a chromophore as a case study.

The non-radiative relaxation of the excitation energy from higher energy states to the lowest energy state in chlorophylls is a crucial preliminary step for the process of photosynthesis. Despite the continuous theoretical and experimental efforts to clarify the ultrafast dynamics of this process, it still represents the object of an intense investigation because the ultrafast timescale and the congestion of the involved states makes its characterization particularly challenging. Here we exploit 2D electronic spectroscopy and recently developed data analysis tools to provide more detailed insights into the mechanism of internal conversion within the Q-bands of chlorophyll a. The measurements confirmed the timescale of the overall internal conversion rate (170 fs) and captured the presence of a previously unidentified ultrafast (40 fs) intermediate step, involving vibronic levels of the lowest excited state.

The interplay between active molecules and the protein environment in light-harvesting complexes tunes the photophysics and the dynamical properties of pigment–protein complexes in a subtle way, which is not fully understood. Here we characterized the photophysics and the ultrafast dynamics of four variants of the water-soluble chlorophyll protein (WSCP) as an ideal model system to study the behavior of strongly interacting chlorophylls. We found that when coordinated by the WSCP protein, the presence of the formyl group in chlorophyll b replacing the methyl group in chlorophyll a strongly affects the exciton energy and the dynamics of the system, opening up the possibility of tuning the photophysics and the transport properties of multichromophores by engineering specific interactions with the surroundings.

Efficient FRET systems are developed combining colloidal CdSe quantum dots (QDs) donors and BODIPY acceptors. To promote effective energy transfer in FRET architectures, the distance between the organic fluorophore and the QDs needs to be optimized by a careful system engineering. In this context, BODIPY dyes bearing amino-terminated functionalities are used in virtue of the high affinity of amine groups in coordinating the QD surface. A preliminary QD surface treatment with a short amine ligand is performed to favor the interaction with the organic fluorophores in solution. The successful coordination of the dye to the QD surface, accomplishing a short donor-acceptor distance, provides effective energy transfer already in solution, with efficiency of 76 %. The efficiency further increases in the solid state where the QDs and the dye are deposited as single coordinated units from solution, with a distance between the fluorophores down to 2.2 nm, demonstrating the effectiveness of the coupling strategy.

Computational spectroscopy is now a valuable tool to better understand experimental spectroscopic data. Recent advancements in computational methods allow the study of electronic excited states of medium-large molecular systems with greater accuracy, simulating spectra directly comparable with experiments. Besides the system of interest, proper modelling of the environment effects is essential to reach a sufficient level of accuracy. This is especially important for solvents with high polarity or those able to establish specific interactions with the solute. One of the most straightforward and fastest ways to deal with solvent effects is the use of implicit models, like the polarizable continuum model (PCM). However, these models cannot describe specific solute-solvent interactions. In this case, one possible solution is the adoption of a mixed model that includes the solute and few solvent molecules within a PCM cavity. With the aim of investigating the role played by specific solute-solvent interactions, the behaviour of styryl substituted BODIPY systems in methanol has been studied at a theoretical level, considering both implicit and mixed implicit-explicit models. In the first part of our analysis, vibrationally resolved electronic spectra for absorption and emission processes have been simulated, using several sets of coordinates to represent normal modes, and compared with experiments. Then, to verify if specific solute-solvent interactions play a crucial role during the ultrafast intraband relaxation processes, 2D electronic spectra were also simulated, which could provide valuable information on the complex ultrafast dynamics.

The Water-Soluble Chlorophyll Protein (WSCP) of Brassicaceae is a remarkably stable tetrapyrrole-binding protein that, by virtue of its simple design, is an exceptional model to investigate the interactions taking place between pigments and their protein scaffold and how they affect the photophysical properties and the functionality of the complexes. We investigated variants of WSCP from Lepidium virginicum (Lv) and Brassica oleracea (Bo), reconstituted with Chlorophyll (Chl) b, to determine the mechanisms by which the different Chl binding sites control their Chl a/b specificities. A combined Raman and crystallographic investigation has been employed, aimed to characterize in detail the hydrogen-bond network involving the formyl group of Chl b. The study revealed a variable degree of conformational freedom of the hydrogen bond networks among the WSCP variants, and an unexpected mixed presence of hydrogen-bonded and not hydrogen-bonded Chls b in the case of the L91P mutant of Lv WSCP. These findings helped to refine the description of the mechanisms underlying the different Chl a/b specificities of WSCP versions, highlighting the importance of the structural rigidity of the Chl binding site in the vicinity of the Chl b formyl group in granting a strong selectivity to binding sites.

The quantum properties of nanosystems present a new opportunity to enhance the power of classical computers, both for the parallelism of the computation and the speed of the optical operations. In this paper we present the COPAC project aiming at development of a ground-breaking nonlinear coherent spectroscopy combining optical addressing and spatially macroscopically resolved optical readout. The discrete structure of transitions between quantum levels provides a basis for implementation of logic functions even at room temperature. Exploiting the superposition of quantum states gives rise to the possibility of parallel computation by encoding different input values into transition frequencies. As an example of parallel single instruction multiple data calculation by a device developed during the COPAC project, we present a n-bit adder, showing that due to the properties of the system, the delay of this fundamental circuit can be reduced.

Elucidating the role of quantum coherences in energy migration within biological and artificial multichromophoric antenna systems is the subject of an intense debate. It is also a practical matter because of the decisive implications for understanding the biological processes and engineering artificial materials for solar energy harvesting. A supramolecular rhodamine heterodimer on a DNA scaffold was suitably engineered to mimic the basic donor-acceptor unit of light-harvesting antennas. Ultrafast 2D electronic spectroscopic measurements allowed identifying clear features attributable to a coherent superposition of dimer electronic and vibrational states contributing to the coherent electronic charge beating between the donor and the acceptor. The frequency of electronic charge beating is found to be 970 cm-1 (34 fs) and can be observed for 150 fs. Through the support of high level ab initio TD-DFT computations of the entire dimer, we established that the vibrational modes preferentially optically accessed do not drive subsequent coupling between the electronic states on the 600 fs of the experiment. It was thereby possible to characterize the time scales of the early time femtosecond dynamics of the electronic coherence built by the optical excitation in a large rigid supramolecular system at a room temperature in solution.

The intraband exciton dynamics of molecular aggregates is a crucial initial step to determine the possibly coherent nature of energy transfer and its implications for the ensuing interband relaxation pathways in strongly coupled excitonic systems. In this work, we fully characterize the intraband dynamics in linear J-aggregates of porphyrins, good model systems for multichromophoric assemblies in biological antenna complexes. Using different 2D electronic spectroscopy schemes together with Raman spectroscopy and theoretical modeling, we provide a full characterization of the inner structure of the main one-exciton band of the porphyrin aggregates. We find that the redistribution of population within the band occurs with a characteristic time of 280 fs and dominates the modulation of an electronic coherence. While we do not find that the coupling to vibrations significantly affects the dynamics of excitonic coherence, our results suggest that exciton fluctuations are nevertheless highly correlated.

The optical and transport properties of biological and artificial multichromophoric functional materials are strongly affected by the disorder and electron-vibration couplings. The conformational disorder in multichromophores becomes critical especially in the control of coherent dynamics. Indeed, microscopic details of the dephasing processes promoted by the disorder are not yet fully clarified. Here we applied 2D electronic spectroscopy to study the dynamics of vibrational coherences in porphyrin-functionalized polymer samples characterized by different conformational disorder. Distinct coherent dephasing behaviors have been found for low-frequency vibrational modes in the studied samples. The experimental results have been interpreted on the basis of molecular dynamics and quantum mechanical calculations, which allowed correlation of the trend in the dephasing times with different conformational distributions in the two polymers. © 2019 American Chemical Society.

A dianiline derivative of a symmetric donor-acceptor-donor diketopyrrolopyrrole-based dye is employed for the two-sided covalent functionalization of liquid exfoliated few layers graphene flakes, through a direct arylation reaction. The resulting nanohybrid features the properties of a polymeric species, being solution-processed into homogeneous thin films, featuring a pronounced red-shift of the main absorption band with respect to the model dye unit and energy levels comparable to those of common diketopyrrolopyrrole-based polymers. A good electrical conductivity and the absence of radical signals generated after intense white light illumination, as probed through electron paramagnetic resonance, suggest a possible future application of this composite material in the field of photoprotective, antistatic layers with tunable colors.

The urgent demand for low-cost green clean energy has stimulated the research for new materials for energy storage and conversion. Colloidal semiconductor nanocrystals (NCs) were demonstrated as promising materials for light harvesting applications, while nanostructured titanium dioxide (TiO2) has been successfully exploited in electrodes for photovoltaics (PV). In particular, NCs with controlled size and shape, coupled and/or functionalized with suitable organic chromophores proved efficient energy transfer (ET) and charge transfer (CT) processes [3]. Understanding the events occurring at organic/inorganic interfaces in both ET and CT processes is crucial for the direct application of the developed materials in real devices. In this work, CdSe NCs of selected size, synthesized by hot-injection methods [4], were coupled with an organic chromophore, based on boron-dipyrromethene (BODIPY) dye, properly functionalized with amino groups to improve its affinity with NC surface. The spectroscopic investigations on such hybrid nanosystems in solution revealed a FRET process occurring between the NCs acting as donor to the conjugated dye molecules. A remarkable increase of the FRET efficiency was obtained for the coupled nanosystems deposited on substrate, with respect to the layered thin films of NCs and dye, indicating a favourable influence of the reduced interparticle separation. In addition, TiO2 NCs and a zinc phthalocyanine functionalized with carboxylic acid groups (PcTC) were selected to investigate CT processes, due to proper band alignment. The hybrid composite was prepared in solution and in solid state, by exploiting both simple physisorption and effective chemical bond formation after activation of the carboxylic acid functionalities. The quenching of the PcTC fluorescence suggested the emergence of CT. In particular, in the solutionphase nano-hybrids, CT efficiency and stability were sensitive to the nature of the solvent, as well as to the preparation strategy.

Bacteriochlorophyll is the primary pigment in the light-harvesting pigment-protein complexes (PPCs) of the bacterial photosynthetic apparatus. 2D electronic spectroscopy (2DES) represents one of the most exploited and powerful techniques to characterize the ultrafast relaxation dynamics in PPCs, in particular, to assess the presence of coherent mechanisms during energy transport. The data reported in this work and the associated research article, “Characterization of the coherent dynamics of bacteriochlorophyll a in solution” [Meneghin et al., 2019] are an important contribution to the literature on coherent dynamics of light-harvesting complexes and can be useful in the interpretation of coherent motion in more complex systems with bacteriochlorophyll a (BChla) as a basic unit. The analysis of the provided data allows the identification of vibrational coherences associated with several Franck-Condon active modes and the characterization of their frequencies and dephasing times. Here we report additional data analysis and additional measures that complement the associated research article [Meneghin et al., 2019] and support its main conclusions. In particular, we compare vibrational coherences extracted from 2DES response with Raman modes detected for BChla powders at cryogenic temperature in resonant and non-resonant conditions. Finally, we show the time-resolved fluorescence decay of the chromophore to support the interpretation of non-coherent dynamics discussed in Ref. [Meneghin et al., 2019].

Disclosing the physical origin of quantum beatings in the early dynamics of biological light-harvesting pigment-protein complexes is one of the major challenges in the ultrafast spectroscopy community. 2D electronic spectroscopy (2DES) is a powerful tool for this purpose, but the complexity of the beating behavior in multichromophoric systems complicates the interpretation. For this reason, the availability of control datasets providing a full characterization of the response of isolated chromophores is highly desirable to untangle the features of intermolecular interactions from the properties of individual pigments. Here, a thorough 2DES characterization of the frequencies and dephasing times of intramolecular vibrational coherences of bacteriochlorophyll a in solution is provided. Several beating modes in the ground and excited state have been found and their dephasing times have been determined. The obtained results represent an essential interpretation key of the beating dynamics of pigment-protein complexes binding bacteriochlorophyll a.