Nanoscale π-conjugated ladders

It is challenging to increase the rigidity of a macromolecule while maintaining solubility. Established strategies rely on templating by dendrons, or by encapsulation in macrocycles, and exploit supramolecular arrangements with limited robustness. Covalently bonded structures have entailed intramolecular coupling of units to resemble the structure of an alternating tread ladder with rungs composed of a covalent bond. We introduce a versatile concept of rigidification in which two rigid-rod polymer chains are repeatedly covalently associated along their contour by stiff molecular connectors. This approach yields almost perfect ladder structures with two well-defined π-conjugated rails and discretely spaced nanoscale rungs, easily visualized by scanning tunnelling microscopy. The enhancement of molecular rigidity is confirmed by the fluorescence depolarization dynamics and complemented by molecular-dynamics simulations. The covalent templating of the rods leads to self-rigidification that gives rise to intramolecular electronic coupling, enhancing excitonic coherence. The molecules are characterized by unprecedented excitonic mobility, giving rise to excitonic interactions on length scales exceeding 100 nm. Such interactions lead to deterministic single-photon emission from these giant rigid macromolecules, with potential implications for energy conversion in optoelectronic devices.

Exciton Condensation in an Atomically-thin MoS2 Semiconductor

Condensation of a dilute Bose gas of excitons (coupled electron-hole pairs) in a direct bandgap semiconductor was first theoretically predicted in 19681. This exotic state of matter is expected to exhibit spectacular non-linear properties, such as superradiance and superfluidity. However, direct experimental observation of condensation of optically active excitons in conventional semiconductors has been hindered by their short lifetimes and weak collective excitonic interactions. Here, we have experimentally realized the condensation of short-lived excitons in a direct-bandgap, atomically-thin MoS2 semiconductor. The signature is the anomalous transport of the fast-expanding exciton density, originating from a thermalized dilute gas generated under the laser spot. Below the critical temperature Tc~150 K, the exciton liquid propagates over ultra-long distances (at least 60 micrometers) with record speed in a solid-state system of 1.8*10^7 m/s (~6% the speed of light), fuelled by the unconventionally strong repulsions among excitons. The condensation is controlled by many-body interactions in the gas mixture of excitons (bosons) and free-carriers (fermions) via an electrical backgate. Our results demonstrate electrostatic doping as a simple approach for the investigation of correlated states of matter at high-temperatures, excitonic circuitry and spin-valley Hall devices mediated by exciton superfluids in semiconducting monolayers.

Superradiance of bacteriochlorophyll c aggregates in chlorosomes of green photosynthetic bacteria

Chlorosomes are the main light-harvesting complexes of green photosynthetic bacteria that are adapted to a phototrophic life at low-light conditions. They contain a large number of bacteriochlorophyll c, d, or e molecules organized in self-assembling aggregates. Tight packing of the pigments results in strong excitonic interactions between the monomers, which leads to a redshift of the absorption spectra and excitation delocalization. Due to the large amount of disorder present in chlorosomes, the extent of delocalization is limited and further decreases in time after excitation. In this work we address the question whether the excitonic interactions between the bacteriochlorophyll c molecules are strong enough to maintain some extent of delocalization even after exciton relaxation. That would manifest itself by collective spontaneous emission, so-called superradiance. We show that despite a very low fluorescence quantum yield and short excited state lifetime, both caused by the aggregation, chlorosomes indeed exhibit superradiance. The emission occurs from states delocalized over at least two molecules. In other words, the dipole strength of the emissive states is larger than for a bacteriochlorophyll c monomer. This represents an important functional mechanism increasing the probability of excitation energy transfer that is vital at low-light conditions. Similar behaviour was observed also in one type of artificial aggregates, and this may be beneficial for their potential use in artificial photosynthesis.

First-principles studies of substituent effects on squaraine dyes

Squaraine dyes are candidates for DNA-templated excitonic interactions. This work presents substituent effects on the electronic and photophysicalproperties of squaraine dyes and a correlation between empirical Hammettconstant and those properties.

Dynamic polaronic screening for anomalous exciton spin relaxation in two-dimensional lead halide perovskites

Two-dimensional lead halide perovskites with confined excitons have shown exciting potentials in optoelectronic applications. It is intriguing but unclear how the soft and polar lattice redefines excitons in layered perovskites. Here, we reveal the intrinsic exciton properties by investigating exciton spin dynamics, which provides a sensitive probe to exciton coulomb interactions. Compared to transition metal dichalcogenides with comparable exciton binding energy, we observe orders of magnitude smaller exciton-exciton interaction and, counterintuitively, longer exciton spin lifetime at higher temperature. The anomalous spin dynamics implies that excitons exist as exciton polarons with substantially weakened inter- and intra-excitonic interactions by dynamic polaronic screening. The combination of strong light matter interaction from reduced dielectric screening and weakened inter-/intra-exciton interaction from dynamic polaronic screening explains their exceptional performance and provides new rules for quantum-confined optoelectronic and spintronic systems.

Splitting and reorientation of π-conjugation by an unprecedented photo-rearrangement reaction

π-Conjugation of a dodecahexaene was regioselectively cleaved by an unprecedented photo-rearrangement into an orthogonally oriented bis(hexatriene) in which two chromophores are arranged in a close proximity to exhibit excitonic interactions.

Coherent wavepackets in the Fenna-Matthews-Olson complex are robust to excitonic-structure perturbations caused by mutagenesis

The photosynthetic FMO complex is probed by femtosecond pump-probe spectroscopy and compared with a series of genetically modified mutants with distinct excitonic interactions. These experiments allow us to identify vibrational contributions to the coherence.

Excited states and excitonic interactions in prototypic polycyclic aromatic hydrocarbon dimers as models for graphitic interactions in carbon dots

The HOMO–LUMO transition in a stacked circum-1-coronene dimer as a model for excimer interactions in carbon dots.