Dicyanorhodanine-Pyrrole Conjugates for Visible Light-Driven Quantitative Photoswitching in Solution and the Solid State.

Small molecule photoswitches capable of toggling between two distinct molecular states in response to light are versatile tools to monitor biological processes, control photochemistry, and design smart materials. In this work, six novel dicyanorhodanine-based pyrrole-containing photoswitches are reported. The molecular design avails both the Z and E isomers from synthesis, where each can be isolated using chromatographic techniques. Inter- and intramolecular hydrogen bonding (H-bonding) interactions available to the E and Z isomers, respectively, uniquely impart thermal stability to each isomer over long time periods. Photoisomerization could be assessed by solution NMR and UV-vis spectroscopic techniques along with complementary ground- and excited-state computational studies, which show good agreement. Quantitative E → Z isomerization occurs upon 523 nm irradiation of the parent compound (where R = H) in solution, whereas Z → E isomerization using 404 nm irradiation offers a photostationary state (PSS) ratio of 84/16 (E/Z). Extending the π-conjugation of the pyrrole unit (where R = p-C6H4-OMe) pushes the maximum absorption to the yellow-orange region of the visible spectrum and allows bidirectional quantitative isomerization with 404 and 595 nm excitation. Comparator molecules have been prepared to report how the presence or absence of H-bonding affects the photoswitching behavior. Finally, studies of the photoswitches in neat films and photoinactive polymer matrices reveal distinctive structural and optical properties of the Z and E isomers and ultimately afford reversible photoswitching to spectrally unique PSSs using visible light sources including the Sun.

Synthesis and Optical Characterization of a Rhodamine B Spirolactam Dimer.

Amide derivatives of xanthene dyes such as rhodamine B are useful in a variety of sensing applications due to their colorimetric responses to stimuli such as acidity changes and UV light. The optical properties of these molecules can be influenced by intermolecular associations into dimeric structures, but the exact impact can be hard to predict. We have designed a covalently linked intramolecular dimer of the dye rhodamine B utilizing p-phenylenediamine to link the two dyes via amide bonds. The doubly closed spirolactam version of this dimer, RSL2, is isolated as a colorless solid. Under acidic conditions or UV exposure, RSL2 solutions develop a pink color that is expected for the ring-opened form of the molecule. However, nuclear magnetic resonance (NMR) and single-crystal diffraction data show that the equilibrium still prefers the closed dimer state. Interestingly, the emission profile of RSL2 shows solvatochromic blue fluorescence. Control studies of model compounds with similar structural motifs do not display similar blue fluorescence, indicating that this optical behavior is unique to the dimeric form. This behavior may lend itself to applications of such xanthene dimers to more sophisticated sensors beyond those with traditional binary on/off fluorescence profiles.

Thin-Film Morphology and Optical Properties of Photoisomerizable Donor-Acceptor Oligothiophenes.

It was recently reported that the most popular electron-accepting units introduced to π-conjugated oligomers studied for organic photovoltaic applications are susceptible to unwanted and even destructive photochemical reactions. The consequences of Z/E photoisomerization of the popular 2-(1,1-dicyanomethylene)rhodanine (RCN) unit on the optical and morphological properties of a homologous series of RCN-functionalized oligothiophenes are studied here. Oligomers consisting of one, two, or three thiophene units were studied as pure Z isomers and with E isomer compositions of 25, 53, and 45%, respectively, for Z/E mixtures. Solutions of Z isomers and Z/E mixtures were characterized by UV-vis and photoluminescence spectroscopy, wherein changes to optical properties were evaluated on the basis of E isomer content. X-ray diffraction of thin-film Z/E mixtures reveals crystalline domains of both Z and E forms after thermal annealing for mono- and bithiophene oligomers, with greater interplanar spacing for E crystalline domains than the Z counterparts along the substrate normal direction. The surface morphology viewed by atomic force microscopy also shows fiberlike structures for the E form with a much larger aspect ratio than for the Z domains in the bithiophene oligomer. Optical characterization reveals drastic changes in the solid state upon introduction of the E form for the mono- and bithiophene derivatives, whereas subtle consequences are noted for the terthiophene analogue. Most notably, a 132 nm redshift in maximum absorption occurs for the bithiophene oligomer films containing 53% E isomer compared to the pure Z counterpart. Finally, although solid-state photoisomerization experiments find no evidence of Z → E isomerization in polycrystalline Z films, E → Z isomerization is observed and becomes more restrictive in films with higher crystallinity (i.e., after thermal annealing). This structure-property study, which elucidates the consequences of the RCN configuration on solid-state packing and optical properties, is expected to guide the development of more efficient and stable organic optoelectronic devices.