Single-molecule FRET (smFRET) has become a mainstream technique for studying biomolecular structural dynamics. The rapid and wide adoption of smFRET experiments by an ever-increasing number of groups has generated significant progress in sample preparation, measurement procedures, data analysis, algorithms and documentation. Several labs that employ smFRET approaches have joined forces to inform the smFRET community about streamlining how to perform experiments and analyze results for obtaining quantitative information on biomolecular structure and dynamics. The recent efforts include blind tests to assess the accuracy and the precision of smFRET experiments among different labs using various procedures. These multi-lab studies have led to the development of smFRET procedures and documentation, which are important when submitting entries into the archiving system for integrative structure models, PDB-Dev. This position paper describes the current ‘state of the art’ from different perspectives, points to unresolved methodological issues for quantitative structural studies, provides a set of ‘soft recommendations’ about which an emerging consensus exists, and lists openly available resources for newcomers and seasoned practitioners. To make further progress, we strongly encourage ‘open science’ practices.

A series of four perylene diimide (PDI) chromophores were prepared with increasing steric bulk on the imide substituents with the aim of retarding the effect of concentration quenching on photoluminescence, commonly observed with these dyes. Spectroscopic investigations of the compounds in dilute solution confirmed that the photophysical properties of the PDI core chromophore were not perturbed by the bulky substituents. Solid film samples containing the PDI compounds at various concentrations dispersed in a poly(methyl methacrylate) (PMMA) matrix were examined and compared to amorphous neat films as well as crystalline samples. The PDI compounds containing di-tert-butylphenyl (bPDI-3) and trityl (bPDI-4) substituents showed near unity photoluminescence quantum yield (PLQY) up to 20 mM in PMMA compared to 10% PLQY for the reference compound (bPDI-1) without molecular insulation. Surprisingly, high concentrations (>40 mM) of a phenyl substituted PDI compound (bPDI-2) with moderate molecular insulation formed emissive aggregates that showed a higher PLQY compared to the PDI derivatives with greater steric bulk. By examining the molecular structure and solid state packing in conjunction with a series of photophysical measurements, new insights into designing highly fluorescent dyes, particularly in the solid state, were obtained. The trityl substituted PDI compound (bPDI-4) was used in a luminescent solar concentrator with optical quantum efficiency of 54%, flux gain of 6.4 and geometric gain of 45.

We demonstrate a systematic visualization of the unique photophysical and fluorescence anisotropic properties of polyfluorene coplanar conformation (β-conformation) using time-resolved scanning confocal fluorescence imaging (FLIM) and fluorescence anisotropy imaging microscopy (FAIM) measurements. We observe inhomogeneous morphologies and fluorescence decay profiles at various micrometer-sized regions within all types of polyfluorene β-conformational spin-coated films. Poly(9,9-dioctylfluorene-2,7-diyl) (PFO) and poly[4-(octyloxy)-9,9-diphenylfluoren-2,7-diyl]-co-[5-(octyloxy)-9,9-diphenylfluoren-2,7-diyl] (PODPF) β-domains both have shorter lifetime than those of the glassy conformation for the longer effective conjugated length and rigid chain structures. Besides, β-conformational regions have larger fluorescence anisotropy for the low molecular rotational motion and high chain orientation, while the low anisotropy in glassy conformational regions shows more rotational freedom of the chain and efficient energy migration from amorphous regions to β-conformation as a whole. Finally, ultrastable ASE threshold in the PODPF β-conformational films also confirms its potential application in organic lasers. In this regard, FLIM and FAIM measurements provide an effective platform to explore the fundamental photophysical process of conformational transitions in conjugated polymer.

Maintaining high incident light absorption while minimizing luminescence reabsorption is a key challenge for organic luminescent solar concentrators (LSCs). Energy migration and trapping using light-harvesting donors and a low-energy highly emitting acceptor is one strategy to reduce the reabsorption issue. However, concentration quenching and the potential formation of quenching aggregates is a limiting factor in realizing efficient devices. We describe the synthesis of a novel molecularly insulated perylene diimide that can resist luminescence quenching at concentrations in excess of 50 mM. Photophysical measurements show the insulated perylene diimide has an excitation energy migration diffusion length of 230 ± 10 Å at 60 mM in poly(methyl methacrylate). LSC devices using a mixture of the insulated perylene diimide light absorber and perylene red (LR305) as the low-energy trap emitter exhibit reduced reabsorption and a better current output than LR305 only devices. The results demonstrate that appropriately designed organic molecule dyes can potentially meet the stringent requirements required for efficient LSCs. S tationary light concentration without external cooling can be realized using luminescent solar concentrators (LSCs), making these devices attractive for building integrated photovoltaic devices. A typical LSC consists of large sheets of plastic or glass containing luminescent molecules that absorb the solar spectrum and then re-emit the absorbed energy into a waveguide mode that directs the luminescence to the thin edges of the concentrator. Edge-mounted solar cells can then harvest the concentrated luminescence for photoelectric conversion. Reabsorption of the emitted light in the waveguide limits the achievable light concentration contributing to parasitic losses such as re-emission into the escape cone or nonunity fluorescence quantum efficiency due to competing nonradiative processes. Separation of both the energy of the peak maxima of absorption and emission (the Stokes shift) and tail absorption of the chromophores in an LSC is crucial in reducing reabsorption. 1,2 Organic solar concentrators that use diluted organic chromophores in inert host matrices are lagging behind their inorganic counterparts due to their small Stokes shifts. 3 A key advantage of organic chromophores over inorganic materials is their solubility in inexpensive host matrices or waveguides, such as poly(methyl methacrylate) (PMMA) or glass, without any complicated processing and the ability to fine-tune their properties by simple molecular engineering approaches. This is particularly important for thin-film LSCs where the concentration required to achieve total light absorption over the wavelength range of the chromophore can be high ( Figure 1).Modification of dyes to achieve a large Stokes shift has achieved only limited success for LSC applications. In some cases, increasing the Stokes shift of the dye has led to a decreased quantum yield and there often remains the long tail absorption of the dye. 4 Energy transfer ...

As a hot topic in materials science and chemistry, molecular stereoisomerism in organic molecules is a key factor in precisely tuning their molecular arrangements, dominating their self-assembly behavior and optimizing their hierarchical condensed structures. We demonstrate a supramolecular chiral oligofluorenol (2O8-DPFOH-SFX) with a hierarchical uniform crystalline structure by precisely controlling molecular stereoisomerism. The resulting ordered supramolecular framework presented exceptional crystalline-enhanced emission and excellent spectral stability.