Tailoring the excited-state energy landscape in supramolecular nanostructures

Nature's photosynthetic machinery uses precisely arranged pigment-protein complexes, often representing superstructures, for efficient light-harvesting and transport of excitation energy (excitons) during the initial steps of photosynthesis. This function is achieved by defined electronic Coulomb interactions between the conjugated molecules resulting in tailored excited-state energy landscapes. While such complex natural structures are synthetically difficult to achieve, supramolecular chemistry is now on its advent to realize defined artificial supramolecular nanostructures with tailored functionalities via controlled self-assembly processes of small molecules. In this review, we focus on recent work reporting photophysical studies on self-assembled and hierarchical nanostructures as well as complex superstructures. We discuss how the resulting excited-state energy landscapes influence energy transport. Progress in the field of supramolecular chemistry allows for the realization of distinct kinds of H- or J-aggregates with well-defined morphologies on the mesoscale. Advances in the field of optical spectroscopy and microscopy have permitted to resolve the incoherent/coherent dynamics of exciton transport in such systems down to the level of single nanostructures. Although outstanding diffusion lengths of up to several mu m were found in selected nanostructures, a full understanding of the underlying principles is still missing. In particular, the unavoidable structural and electronic disorder in these systems influences the excited-state energy landscapes and thus the transport characteristics, which can be exploited to refine the molecular design criteria of supramolecular nanostructures and complex superstructures. Despite the rapid progress in the field of functional supramolecular nanostructures, we believe that revealing the full potential of such systems is far from complete. In particular, criteria for tailored and optimized (hierarchical) supramolecular nanostructures in view of applications are not yet established. Finally, we outline current challenges and future perspectives for optical and optoelectronic applications of supramolecular nanostructures

Unusual Photophysical Properties of Porphyrin-Based Supramolecular Polymers Unveiled:The Role of Metal Ligands and Side Group Amide Connectivity

Supramolecular polymers based on porphyrins are an interesting model system, since the self-assembly and thus the photophysics can be modified by the chemical structure of the porphyrins, e.g., by a metal inserted in the ligand or by different (solubilizing) side groups. Here, we investigate the photophysical properties of supramolecular polymers based on free-base and Zn-centered porphyrins, each with different amide connectivity in the side chains, by absorption and (time-resolved) photoluminescence spectroscopy on solutions. We find that for all porphyrin derivatives the B-band absorption of supramolecular polymers is a superposition of H- and J-type aggregate spectra, while the Q-band absorption indicates only J-type aggregation. The emission of supramolecular polymers stems exclusively from the Q-band and shows only J-type behavior. For supramolecular polymers based on the free-base porphyrins, we identify only a single aggregate species, whereas for Zn-centered porphyrins, two distinct species coexist in solution, each with a (slightly) different arrangement of monomers. We rationalize this complex behavior by a slip-stacking of porphyrins along the direction of one of the two B-band transition dipole moments, resulting in simultaneous H- and J-type intermolecular coupling in the B-band. In the Q-band, with its transition dipole moments oriented 45° relative to the corresponding B-band moments, only J-type coupling is thus present. Our results demonstrate that the self-assembly and the photophysics of supramolecular polymers based on porphyrins can only be fully understood if spectral information from all bands is considered.</p

Conditional quantum nonlocality in dimeric and trimeric arrays of organic molecules

Arrays of covalently bound organic molecules possess potential for light-harvesting and energy transfer applications due to the strong coherent dipole-dipole coupling between the transition dipole moments of the molecules involved. Here, we show that such molecular systems, based on perylene-molecules, can be considered as arrays of qubits that are amenable for laser-driven quantum coherent control. The perylene monomers exhibit dephasing times longer than four orders of magnitude a typical gating time, thus allowing for the execution of a large number of gate operations on the sub-picosecond timescale. Specifically, we demonstrate quantum logic gates and entanglement in bipartite (dimer) and tripartite (trimer) systems of perylene-based arrays. In dimers, naturally entangled states with a tailored degree of entanglement can be produced. The nonlocality of the molecular trimer entanglement is demonstrated by testing Mermin's (Bell-like) inequality violation.Comment: 14 pages, 8 figures, comments are welcom

Unusual Photophysical Properties of Porphyrin-Based Supramolecular Polymers Unveiled: The Role of Metal Ligands and Side Group Amide Connectivity

Supramolecular polymers based on porphyrins are an interesting model system, since the self-assembly and thus the photophysics can be modified by the chemical structure of the porphyrins, e.g., by a metal inserted in the ligand or by different (solubilizing) side groups. Here, we investigate the photophysical properties of supramolecular polymers based on free-base and Zn-centered porphyrins, each with different amide connectivity in the side chains, by absorption and (time-resolved) photoluminescence spectroscopy on solutions. We find that for all porphyrin derivatives the B-band absorption of supramolecular polymers is a superposition of H- and J-type aggregate spectra, while the Q-band absorption indicates only J-type aggregation. The emission of supramolecular polymers stems exclusively from the Q-band and shows only J-type behavior. For supramolecular polymers based on the free-base porphyrins, we identify only a single aggregate species, whereas for Zn-centered porphyrins, two distinct species coexist in solution, each with a (slightly) different arrangement of monomers. We rationalize this complex behavior by a slip-stacking of porphyrins along the direction of one of the two B-band transition dipole moments, resulting in simultaneous H- and J-type intermolecular coupling in the B-band. In the Q-band, with its transition dipole moments oriented 45° relative to the corresponding B-band moments, only J-type coupling is thus present. Our results demonstrate that the self-assembly and the photophysics of supramolecular polymers based on porphyrins can only be fully understood if spectral information from all bands is considered

Two-photon induced ultrafast coherence decay of highly excited states in single molecules

Coherence is a key aspect of a large variety of processes, ranging from the coherent delocalisation of excitation energy, which is important for energy transfer in supramolecular nanostructures, to coherence between electronic states of a single quantum system, which is essential for quantum optical applications. Coherent control schemes exploit this quantum mechanical property by actively manipulating the outcome of dynamical processes. Moreover, this technique allows measuring dynamical processes under the influence of dephasing. However, going beyond the ensemble averaged situation, i.e. working on the level of single quantum systems, is highly challenging for quantum systems embedded in a solid matrix at elevated temperature. Since interactions between the quantum system and its specific local environment are a priori unknown, this requires a reliable approach to retrieve the relevant parameters governing the ultrafast coherent dynamics. Here, we present measurements of the ultrafast coherence decay of two-photon accessible excited states in single organic molecules embedded in a disordered environment at room temperature. Wecombine this experimental approach with a quantum dynamics identification procedure, which yields a minimum three-level model to describe the obtained data with very good agreement. In particular, we are able to retrieve the ultrafast (coherent) excited state dynamics in single molecules and demonstrate its sensitivity to the local nanoenvironment from molecule to molecule. This work provides a robust approach to measure and analyse ultrafast quantum dynamics in complex nanosystems

In situ investigation of light soaking in organolead halide perovskite films

Organolead halide perovskite solar cells (PSCs) have generated extensive attention recently with power conversion efficiency (PCE) exceeding 23%. However, these PSCs exhibit photoinduced instability in the course of their current-voltage measurements. In this work, we study the light-induced behavior in CH3NH3PbI3-xClx films in situ, by employing wide-field photoluminescence (PL) microscopy to obtain both the spatially and temporally resolved PL images simultaneously. Along with the increase in the PL intensity under continuous illumination, some areas render PL inactive. By characterizing the excitation energy dependent long-time PL decay behavior, we suggest that the PL quenching can be ascribed to a localized accumulation of iodide ions driven by the optical field. This ion localization leads to an enhancement of non-radiative recombination. The appearance of the PL inactive areas in the perovskite film impedes its photovoltaic device performance approaching the theoretical maximum PCE. Therefore, the herein presented real-time investigation of the light soaking of perovskite films is a versatile and adaptable method providing more details to improve the performance of PSCs.</p