In the photoluminescence spectra of thin films made of singlet fission (SF) materials emission features that are red-shifted from the free exciton emission are of particular interest. They can be fingerprints of the correlated triplet-pair state and as such offer insights into the mechanisms of the multistep SF process. However, excimer formation or trap-state population can also cause such features and a clear disentanglement of the various contributions can be challenging. Here, we use blends of anthradithiophene (ADT) and weakly interacting organic semiconductors to control the polarizability of the molecular environment and, thus, to distinguish between excimer emission and emission from the correlated triplet-pair state. Using time-resolved photoluminescence spectroscopy measurements, we clarify the relation between excimer formation and SF in ADT and find that excimer formation constitutes a parallel relaxation channel for the exciton and neither mediates nor hinders SF.

The first step in photosynthesis is an extremely efficient energy transfer mechanism that led to the debate to which extent quantum coherence may be involved in the energy transfer between the photosynthetic pigments. In search of such a coherent behavior, we have embedded living cyanobacteria between the parallel mirrors of an optical microresonator irradiated with low intensity white light. As a consequence, we observe vacuum Rabi splitting in the transmission and fluorescence spectra as a result of strong light matter coupling of the chlorophyll a molecules in the photosystems (PSs) and the cavity modes. The Rabi-splitting scales with the number of the PSs chlorophyll a pigments involved in strong coupling indicating a delocalized polaritonic state. Our data provide evidence that a delocalized polaritonic state can be established between the chlorophyll a molecule of the PSs in living cyanobacterial cells at ambient conditions in a microcavity.

CdSe nanocrystals and aggregates of an aryleneethynylene derivative are assembled into a hybrid thin film with dual fluorescence from both fluorophores. Under continuous excitation, the nanocrystals and the molecules exhibit anti-correlated fluorescence intensity variations, which become periodic at low temperature. We attribute this to a structure-dependent aggregation induced emission of the aryleneethynylene derivative, which impacts the rate of excitation energy transfer between the molecules and nanocrystals. Energy transfer also affects the electric transport properties of the hybrid material under optical excitation. This work highlights that combining semiconductor nanocrystals with molecular aggregates, which exhibit aggregation induced emission, can result in unprecedented emerging optical properties.

Strong coupling has attracted much research interest motivated by the possibility to tune the energy levels of molecules enabling to control and modify chemical reactions. Strong coupling leads to the formation of new hybrid modes and is caused by coherent energy exchange between the individual constituents. Such a coherent energy exchange occurs when the coupling rate exceeds the damping rate of the individual components and has been observed for highly diverse systems. Here, we present a strongly coupled hybrid system consisting of a thin TDBC J-aggregate film inside an optical subwavelength microresonator coupled to a second microresonator. This hybrid structure combines strong coupling of purely optical modes with strong light–matter interaction. The coupling strength and damping sensitively depend on the position and concentration of the coupled molecules in the microresonator structure. Such a coupled system can be modeled by coupled damped oscillators, which allows to determine the coupling and damping constants. We show that the individual components making up the coupled hybrid system cannot be treated individually, but the coupled system needs to be considered as a whole. As a consequence, altering one parameter does influence the whole coupled system, and the individual components need to be carefully adapted to each other to achieve efficient coupling. These results can have important consequences for the field of optoelectronics or polaritonic chemistry.

Glioblastoma WHO IV belongs to a group of brain tumors that are still incurable. A promising treatment approach applies photodynamic therapy (PDT) with hypericin as a photosensitizer. To generate a comprehensive understanding of the photosensitizer-tumor interactions, the first part of our study is focused on investigating the distribution and penetration behavior of hypericin in glioma cell spheroids by fluorescence microscopy. In the second part, fluorescence lifetime imaging microscopy (FLIM) was used to correlate fluorescence lifetime (FLT) changes of hypericin to environmental effects inside the spheroids. In this context, 3D tumor spheroids are an excellent model system since they consider 3D cell–cell interactions and the extracellular matrix is similar to tumors in vivo. Our analytical approach considers hypericin as probe molecule for FLIM and as photosensitizer for PDT at the same time, making it possible to directly draw conclusions of the state and location of the drug in a biological system. The knowledge of both state and location of hypericin makes a fundamental understanding of the impact of hypericin PDT in brain tumors possible. Following different incubation conditions, the hypericin distribution in peripheral and central cryosections of the spheroids were analyzed. Both fluorescence microscopy and FLIM revealed a hypericin gradient towards the spheroid core for short incubation periods or small concentrations. On the other hand, a homogeneous hypericin distribution is observed for long incubation times and high concentrations. Especially, the observed FLT change is crucial for the PDT efficiency, since the triplet yield, and hence the O2 activation, is directly proportional to the FLT. Based on the FLT increase inside spheroids, an incubation time > 30 min is required to achieve most suitable conditions for an effective PDT. Graphical abstract

Photosynthesis is one the most important biological processes on earth, producing life-giving oxygen, and is the basis for a large variety of plant products. Measurable properties of photosynthesis provide information about its biophysical state, and in turn, the physiological conditions of a photoautotrophic organism. For instance, the chlorophyll fluorescence intensity of an intact photosystem is not constant as in the case of a single fluorescent dye in solution but shows temporal changes related to the quantum yield of the photosystem. Commercial photosystem analyzers already use the fluorescence kinetics characteristics of photosystems to infer the viability of organisms under investigation. Here, we provide a novel approach based on an optical Fabry–Pérot microcavity that enables the readout of photosynthetic properties and activity for an individual cyanobacterium. This approach offers a completely new dimension of information, which would normally be lost due to averaging in ensemble measurements obtained from a large population of bacteria.

Life as we know it cannot exist without photosynthesis, and even though the main photosynthetic mechanisms have been well investigated, some aspects are still unresolved. One example is the energy transfer to the reaction centers by accessory photosynthetic pigments after the absorption of photons.This process has an extremely high efficiency, which cannot be explained by a classical Foerster resonance energy transfer. However, a quantum mechanical process based on a coherent or wave-like energy transfer may provide an explanation for the high efficiency. In order to determine whether nature makes use of such a coherent process, we influence the potential coherence of photosynthetic pigments in vivo using an optical microresonator, which consists of two parallel silver mirrors separated only by the distance of a few wavelengths. The electromagnetic field inside such a microcavity is strongly confined, enabling coherent light-matter coupling. Here, we embedded living cyanobacteria of the species Synechococcus elongatus (strain PCC 7492) into the microresonator and exposed them to the confined electromagnetic field. The observation of vacuum Rabi splitting and anti-crossing observed in the transmission-and fluorescence spectra provides evidence of coherent coupling of the pigments with the resonator modes without harming the bacteria. Furthermore, we showed that not only some photosynthesis pigments are involved in this coupling, but all pigments in the excitation focus are coupled coherently. Our findings shed light on the function of quantum coherence in the evolution of photosynthetic organisms.

The application potential of singlet fission (SF), describing the spontaneous conversion of an excited singlet into two triplets, underlines the necessity to independently control SF rates, energetics and the optical band gap. Heterofission, whereby the singlet splits into triplets on chemically distinct chromophores, is a promising approach to control the above-mentioned parameters, but its details are not yet fully understood. Here, we investigate the photophysics of blends of two prototypical SF chromophores, tetracene (TET) and rubrene (RUB) using time-resolved photoluminescence spectroscopy and time-correlated single photon counting (TCSPC) to explore the potential for heterofission in combinations of endothermic SF chromophores.