Thermosensitive probes with dual emission allow the ratiometric sensing of temperature with fluorescence measurements.

The equilibration of two interconverting species with resolved emission translates into ratiometric fluorescence response to temperature.

We report that supramolecular polymer films composed of a 2:1 mixture of monodiamidopyridine diketopyrrolopyrrole (DPP) electron donors and perylene bisdiimide (PDI) electron acceptors undergo photoinduced charge transfer in the solid state. Film formation is guided by complementary noncovalent interactions programmed into the molecular components, resulting in a film architecture comprised of polymer wires with order across the molecular-to-macroscopic continuum. Using ultrafast transient absorption spectroscopy, we show that recombination lifetimes increase 1000-fold compared to the same supramolecular polymers in solution. Supramolecular donor−acceptor polymer films, such as these, that are designed by considering structure and electron transfer dynamics synergistically could lead to breakthroughs in organic optoelectronics. ■ INTRODUCTIONSignificant efforts have been devoted toward improving the performance of small molecule organic photovoltaics (OPVs); 1−4 however, challenges still remain with regard to understanding, controlling, and ultimately optimizing the photon-to-current conversion within the photoactive layer. Natural photosynthetic processes that efficiently convert sunlight to fuel rely upon precisely positioned assemblies of photofunctional chromophores for light harvesting, charge separation, and catalysis. 5 Light conversion in the reaction center involves the synergistic combination of superstructure and ultrafast electron transfer to form long-lived radical pairs. 6,7 The near unity conversion of light to chemical energy in the reaction center is a compelling model for future solar technologies 8 and has inspired the design of organic materials for artificial photosynthesis. 9−11 Similar to the reaction center, the photoactive layers of OPVs require appropriately matched donor and acceptor frontier molecular orbital levels and electronic coupling that enable rapid charge generation, and the resulting films must also possess multilength scale order for charge migration through the films. 12,13 Approaches for designing these materials must consider ease of preparation, solar spectrum absorption, electron transfer dynamics, and film order together to achieve efficient charge generation. Supramolecular polymers are a promising class of materials that could potentially be used for preparing organized films that undergo photoinduced charge separation. Supramolecular polymers are macromolecules whose monomeric repeat units are held together by noncovalent bonds, and their noteworthy characteristics include assembly from easy-to-prepare small molecules, hierarchical structure, and stimuli responsiveness. 14−19 Several supramolecular polymer systems have been explored in the context of photoinduced charge separation in solution 20−23 and the solid-state, 24−27 including a system we developed composed of a monodiamidopyridine diketopyrrolopyrrole (mDPP) electron donor and a perylene bisimide (PDI) electron acceptor 28 that assembles into 2:1 mDPP:PDI helical supramolecular polymers ( Figure 1a) as a re...

A borondipyrromethene (BODIPY) chromophore is connected to a benzoxazole, benzothiazole, or nitrobenzothiazole heterocycle through an olefinic bridge with trans configuration. Rotation about the two [C−C] bonds flanking the olefinic bridge occurs with fast kinetics in solution, leading to the equilibration of four conformational isomers for each compound. Ensemble spectroscopic measurements in solutions fail to distinguish the coexisting isomers. They reveal instead averaged absorption and emission bands with dependence of the latter on the excitation wavelength. Using high-throughput single-molecule spectroscopy, two main populations of single molecules with distinct spectral centroids are observed for each compound on glass substrates. Computational analyses suggest the two populations of molecules to be conformational isomers with antiperiplanar and periplanar arrangements of the BODIPY chromophores about its [C−C] bond to the olefinic bridge. Thus, statistical analysis of multiple single-molecule emission spectra can discriminate stereoisomers that would otherwise be impossible to distinguish by ensemble measurements alone.

A coumarin fluorophore and an oxazine photochrome can be integrated within the same molecular skeleton and connected covalently to a secondary antibody. Illumination of the antibody−dyad conjugate at an appropriate activation wavelength opens the oxazine ring reversibly and shifts bathochromically the ground-state absorption of the coumarin component. Selective excitation of the photochemical product then produces significant fluorescence and allows the detection of activated bioconjugates at the single-molecule level. Such fluorescence activation events can be exploited to resolve temporally individual emitters and reconstruct images of immunolabeled cells with subdiffraction resolution. Relying on a similar conjugation protocol, a model compound, incorporating the same chromophore of the photochemical product, can also be connected covalently to a secondary antibody. Stimulated emission can be exploited to deplete the excited state of the bioconjugated chromophores and switch their fluorescence off. These operating principles for fluorescence switching also permit the imaging of immunolabeled cells with subdiffraction resolution. Thus, these photoswitchable molecules, in combination with the labeling ability of antibodies, can evolve into valuable probes for bioimaging with superresolution.