Correlated Fluctuations and Intraband Dynamics of J-Aggregates Revealed by Combination of 2DES Schemes

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

Global analysis of coherence and population dynamics in 2D electronic spectroscopy

2D electronic spectroscopy is a widely exploited tool to study excited state dynamics. A high density of information is enclosed in 2D spectra. A crucial challenge is to objectively disentangle all the features of the third order optical signal. We propose a global analysis method based on the variable projection algorithm, which is able to reproduce simultaneously coherence and population dynamics of rephasing and non-rephasing contributions. Test measures at room temperature on a standard dye are used to validate the procedure and to discuss the advantages of the proposed methodology with respect to the currently employed analysis procedures

Deciphering hot- and multi-exciton dynamics in core-shell QDs by 2D electronic spectroscopies

Although the harnessing of multiple and hot excitons is a prerequisite for many of the groundbreaking applications of semiconductor quantum dots (QDs), the characterization of their dynamics through conventional spectroscopic techniques is cumbersome. Here, we show how a careful analysis of 2DES maps acquired in different configurations (BOXCARS and pump–probe geometry) allows the tracking and visualization of intraband Auger relaxation mechanisms, driving the hot carrier cooling, and interband bi- and tri-exciton recombination dynamics. The results obtained on archetypal core– shell CdSe/ZnS QDs suggest that, given the global analysis of the resulting datasets, 2D electronic spectroscopy techniques can successfully and efficiently dispel the intertwined dynamics of fast and ultrafast recombination processes in nanomaterials. Hence, we propose this analysis scheme to be used in future research on novel quantum confined systems

Quantum Phenomena in Nanomaterials: Coherent Superpositions of Fine Structure States in CdSe Nanocrystals at Room Temperature

One of the most recent developments at the forefront of nanotechnology is the attempt to exploit quantum phenomena in nanometer scale materials, exploring novel applications of quantum effects. An effective exploitation of quantum phenomena must necessarily pass through a deep understanding of how to generate, manipulate, and characterize coherent superposition of quantum states in the nanosystems. However, despite the lively interest in this topic, the study of coherent effects in nanomaterials still represents relatively unexplored territory. Here we report an investigation on the ultrafast coherent dynamics of colloidal CdSe quantum dots (QDs) by the mean of two- dimensional electronic spectroscopy (2DES). The time evolution of specific coherent superpositions of fine structure levels in these nanomaterials is clearly demonstrated. The obtained results represent an important step forward toward a deeper understanding of quantum properties of nanomaterials

Photocurrent-detected 2D electronic spectroscopy reveals ultrafast hole transfer in operating PM6/Y6 organic solar cells

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).Peer ReviewedPostprint (published version

Ultrafast relaxation dynamics of multichromophoric systems through advanced optical spectroscopies

The light-matter interaction is one of the most interesting phenomena occurring in Nature. The interaction with light provides energy to the molecules, which is then released in the process of relaxation. This thesis deals with the investigation of relaxation mechanisms of molecules from a semi-quantum point of view. The use of quantum mechanics models is necessary to understand the operations and the results of advanced optical spectroscopies, such as 2D electronic spectroscopy, used precisely for the study of excitation and relaxation of molecules. The study and characterization of relaxation processes is of crucial importance to devise strategies to exploit and gain energy from them. Relaxation, as widely known, is strongly influenced by the surroundings and by the vibrations of the molecules. Therefore, we compared the relaxation dynamics of two systems with very different couplings with the environment. The first are aggregates of porphyrins that have weak coupling with the environment. However, it will be shown that the excitation is coupled with low frequency vibrations. The second is a quadrupolar system studied in its monomeric and dimeric form in different solvents. Surprisingly, a strong coupling between the dye transitions and a vibration of the solvent molecules was found. If confirmed, this effect could lead to new strategies for engineering light harvesting systems.L’interazione luce-materia è sicuramente uno dei più interessanti fenomeni in Natura. L’interazione con la luce fornisce energia alle molecole, che viene poi rilasciata nel processo di rilassamento. Questa tesi tratta meccanismi di rilassamento di molecole da un punto di vista semi-quantomeccanico. Questa trattazione è necessaria per comprendere il funzionamento e i risultati di spettroscopie ottiche avanzate, come la spettroscopia ottica 2D, utilizzate appunto per lo studio dell’eccitazione e del rilassamento delle molecole. Lo studio e la caratterizzazione di processi di rilassamento è di fondamentale importanza per ideare strategie per sfruttare il rilassamento al fine di ricavare parte dell’energia. Rilassamento che, come è noto, è fortemente influenzato dall'intorno e dalle vibrazioni delle molecole. Si sono quindi confrontate le dinamiche di rilassamento di due sistemi caratterizzati da accoppiamenti con l’intorno molto diversi. Il primo sistema considerato sono aggregati di porfirine che hanno uno scarso accoppiamento con l’intorno. Tuttavia si dimostrerà invece che tali sistemi sono caratterizzati da un forte accoppiamento con vibrazioni a bassa frequenza. Il secondo invece è un sistema quadrupolare del quale si studiano sia il monomero che il dimero in solventi diversi. Sorprendentemente si è trovato un forte accoppiamento delle molecole con una vibrazione del solvente, che se si dimostrerà vera, potrebbe aprire a nuove strategie per l’ingegnerizzazione di sistemi di light harvesting

Ultrafast relaxation dynamics of multichromophoric systems through advanced optical spectroscopies

The light-matter interaction is one of the most interesting phenomena occurring in Nature. The interaction with light provides energy to the molecules, which is then released in the process of relaxation. This thesis deals with the investigation of relaxation mechanisms of molecules from a semi-quantum point of view. The use of quantum mechanics models is necessary to understand the operations and the results of advanced optical spectroscopies, such as 2D electronic spectroscopy, used precisely for the study of excitation and relaxation of molecules. The study and characterization of relaxation processes is of crucial importance to devise strategies to exploit and gain energy from them. Relaxation, as widely known, is strongly influenced by the surroundings and by the vibrations of the molecules. Therefore, we compared the relaxation dynamics of two systems with very different couplings with the environment. The first are aggregates of porphyrins that have weak coupling with the environment. However, it will be shown that the excitation is coupled with low frequency vibrations. The second is a quadrupolar system studied in its monomeric and dimeric form in different solvents. Surprisingly, a strong coupling between the dye transitions and a vibration of the solvent molecules was found. If confirmed, this effect could lead to new strategies for engineering light harvesting systems.L’interazione luce-materia è sicuramente uno dei più interessanti fenomeni in Natura. L’interazione con la luce fornisce energia alle molecole, che viene poi rilasciata nel processo di rilassamento. Questa tesi tratta meccanismi di rilassamento di molecole da un punto di vista semi-quantomeccanico. Questa trattazione è necessaria per comprendere il funzionamento e i risultati di spettroscopie ottiche avanzate, come la spettroscopia ottica 2D, utilizzate appunto per lo studio dell’eccitazione e del rilassamento delle molecole. Lo studio e la caratterizzazione di processi di rilassamento è di fondamentale importanza per ideare strategie per sfruttare il rilassamento al fine di ricavare parte dell’energia. Rilassamento che, come è noto, è fortemente influenzato dall'intorno e dalle vibrazioni delle molecole. Si sono quindi confrontate le dinamiche di rilassamento di due sistemi caratterizzati da accoppiamenti con l’intorno molto diversi. Il primo sistema considerato sono aggregati di porfirine che hanno uno scarso accoppiamento con l’intorno. Tuttavia si dimostrerà invece che tali sistemi sono caratterizzati da un forte accoppiamento con vibrazioni a bassa frequenza. Il secondo invece è un sistema quadrupolare del quale si studiano sia il monomero che il dimero in solventi diversi. Sorprendentemente si è trovato un forte accoppiamento delle molecole con una vibrazione del solvente, che se si dimostrerà vera, potrebbe aprire a nuove strategie per l’ingegnerizzazione di sistemi di light harvesting

Unifying Nonlinear Response and Incoherent Mixing in Action-2D Electronic Spectroscopy

Action-detection has expanded the scope and applicability of 2D electronic spectroscopy, while posing new challenges for the unambiguous interpretation of spectral features. In this context, identifying the origin of cross-peaks at early waiting times is not trivial, and incoherent mixing is often invoked as an unwanted contribution masking the nonlinear signal. In this work, we elaborate on the relation between the nonlinear response and the incoherent mixing contribution by analyzing the action signal in terms of one- and two-particle observables. Considering a weakly interacting molecular dimer, we show how cross-peaks at early waiting times, reflecting exciton-exciton annihilation dynamics, can be equivalently interpreted as arising from incoherent mixing. This equivalence, on the one hand, highlights the information content of spectral features related to incoherent mixing and, on the other hand, provides an efficient numerical scheme to simulate the action response of weakly interacting systems

Delocalized triplet state in porphyrin J-aggregates revealed by EPR spectroscopy

In this work, the electronic structure of the triplet state of self-assembled J-aggregates of tetrakis(4-sulfonatophenyl)porphyrin (TPPS) has been characterized by means of time-resolved electron paramagnetic resonance spectroscopy. Several insights into the triplet properties of the aggregate have been gained through comparison with the corresponding monomeric unit in both free base and acidified forms. Molecular distortions in the monomeric acidified TPPS cause variation in its zero-field splitting parameters and a redirection of triplet spin sublevel activity. The aggregation process does not alter the mechanism of triplet state population compared to the acidified monomer but it is accompanied by a further reduction in the zero-field splitting parameter D, which is possibly indicative of the formation of a delocalized triplet state species. The detection of a long-lived spin-polarized radical species also proves polaron generation and movement to a trap site in the J-aggregates