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1. Acid-Triggered Switchable Near-Infrared/Shortwave Infrared Absorption and Emission of Indolizine-BODIPY Dyes

   https://doi.org/10.3390/molecules28031287  

   Saucier, Matthew A. ; Smith, Cameron ; Kruse, Nicholas A. ; Hammer, Nathan I. ; Delcamp, Jared H. ( February 2023 , Molecules)

   Fluorescent organic dyes that absorb and emit in the near-infrared (NIR, 700–1000 nm) and shortwave infrared (SWIR, 1000–1700 nm) regions have the potential to produce noninvasive high-contrast biological images and videos. BODIPY dyes are well known for their high quantum yields in the visible energy region. To tune these chromophores to the NIR region, fused nitrogen-based heterocyclic indolizine donors were added to a BODIPY scaffold. The indolizine BODIPY dyes were synthesized via microwave-assisted Knoevenagel condensation with indolizine aldehydes. The non-protonated dyes showed NIR absorption and emission at longer wavelengths than an aniline benchmark. Protonation of the dyes produced a dramatic 0.35 eV bathochromic shift (230 nm shift from 797 nm to 1027 nm) to give a SWIR absorption and emission (λmaxemis = 1061 nm). Deprotonation demonstrates that material emission is reversibly switchable between the NIR and SWIR. 

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2. Fluorescent Thienothiophene-Containing Squaraine Dyes and Threaded Supramolecular Complexes with Tunable Wavelengths between 600–800 nm

   https://doi.org/10.3390/molecules23092229  

   Liu, Wenqi ; McGarraugh, Hannah ; Smith, Bradley ( September 2018 , Molecules)

   A new family of fluorescent thiophene and thienothiophene-containing squaraine dyes is described with tunable wavelengths that cover the absorption/emission range of 600–800 nm. The deep-red and near-infrared fluorescent compounds were easily prepared by simple synthesis and purification methods. Spectral studies showed that each squaraine was rapidly encapsulated by a tetralactam macrocycle, with nanomolar affinity in water, to produce a threaded supramolecular complex with high chemical stability, increased fluorescence quantum yield, and decreased fluorescence quenching upon dye self-aggregation. Energy transfer within the supramolecular complex permitted multiplex emission. That is, two separate dyes with fluorescence emission bands that match the popular Cy5 and Cy7 channels, could be simultaneously excited with a beam of 375 nm light. A broad range of practical applications is envisioned in healthcare diagnostics, microscopy, molecular imaging, and fluorescence-guided surgery. 

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3. Flavylium Polymethine Fluorophores for Near‐ and Shortwave Infrared Imaging

   https://doi.org/10.1002/ange.201706974  

   Cosco, Emily D. ; Caram, Justin R. ; Bruns, Oliver T. ; Franke, Daniel ; Day, Rachael A. ; Farr, Erik P. ; Bawendi, Moungi G. ; Sletten, Ellen M. ( September 2017 , Angewandte Chemie)

   Bright fluorophores in the near‐infrared and shortwave infrared (SWIR) regions of the electromagnetic spectrum are essential for optical imaging in vivo. In this work, we utilized a 7‐dimethylamino flavylium heterocycle to construct a panel of novel red‐shifted polymethine dyes, with emission wavelengths from 680 to 1045 nm. Photophysical characterization revealed that the 1‐ and 3‐methine dyes display enhanced photostability and the 5‐ and 7‐methine dyes exhibit exceptional brightness for their respective spectral regions. A micelle formulation of the 7‐methine facilitated SWIR imaging in mice. This report presents the first polymethine dye designed and synthesized for SWIR in vivo imaging.

    

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4. Modeling selective narrowband light absorption in coaxial InAs-GaAs 0.1 Sb 0.9 nanowires with partial shell segment coverage

   https://doi.org/10.1088/2632-959X/accf27  

   Abrand, Alireza ; Anttu, Nicklas ; Mohseni, Parsian K. ( May 2023 , Nano Express)

   Vertical III-V nanowire (NW) arrays are promising candidates for infrared (IR) photodetection applications. Generally, NWs with large diameters are required for efficient absorption in the IR range. However, increasing the NW diameter results in a loss of spectral selectivity and an enhancement in the photodetector dark current. Here, we propose a nanophotonic engineering approach to achieving spectrally-selective light absorption while minimizing the volume of the absorbing medium. Based on simulations performed using rigorous coupled-wave analysis (RCWA) techniques, we demonstrate dramatic tunability of the short-wavelength infrared (SWIR) light absorption properties of InAs NWs with base segments embedded in a reflective backside Au layer and with partial GaAs_0.1Sb_0.9shell segment coverage. Use of a backside reflector results in the generation of a delocalized evanescent field around the NW core segment that can be selectively captured by the partially encapsulating GaAs_0.1Sb_0.9shell layer. By adjusting the core and shell dimensions, unity absorption can be selectively achieved in the 2 to 3μm wavelength range. Due to the transparency of the GaAs_0.1Sb_0.9shell segments, wavelength-selective absorption occurs only along the InAs core segments where they are partially encapsulated. The design presented in this work paves the path toward spectrally-selective and polarization-dependent NW array-based photodetectors, in which carrier collection efficiencies can be enhanced by positioning active junctions at the predefined locations of the partial shell segments.

    

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5. Going beyond the borders: pyrrolo[3,2- b ]pyrroles with deep red emission

   https://doi.org/10.1039/d1sc05007a  

   Tasior, Mariusz ; Kowalczyk, Paweł ; Przybył, Marta ; Czichy, Małgorzata ; Janasik, Patryk ; Bousquet, Manon H. ; Łapkowski, Mieczysław ; Rammo, Matt ; Rebane, Aleksander ; Jacquemin, Denis ; et al ( December 2021 , Chemical Science)

   A two-step route to strongly absorbing and efficiently orange to deep red fluorescent, doubly B/N-doped, ladder-type pyrrolo[3,2- b ]pyrroles has been developed. We synthesize and study a series of derivatives of these four-coordinate boron-containing, nominally quadrupolar materials, which mostly exhibit one-photon absorption in the 500–600 nm range with the peak molar extinction coefficients reaching 150 000, and emission in the 520–670 nm range with the fluorescence quantum yields reaching 0.90. Within the family of these ultrastable dyes even small structural changes lead to significant variations of the photophysical properties, in some cases attributed to reversal of energy ordering of alternate-parity excited electronic states. Effective preservation of ground-state inversion symmetry was evidenced by very weak two-photon absorption (2PA) at excitation wavelengths corresponding to the lowest-energy, strongly one-photon allowed purely electronic transition. π-Expanded derivatives and those possessing electron-donating groups showed the most red-shifted absorption- and emission spectra, while displaying remarkably high peak 2PA cross-section ( σ 2PA ) values reaching ∼2400 GM at around 760 nm, corresponding to a two-photon allowed higher-energy excited state. At the same time, derivatives lacking π-expansion were found to have a relatively weak 2PA peak centered at ca. 800–900 nm with the maximum σ 2PA ∼50–250 GM. Our findings are augmented by theoretical calculations performed using TD-DFT method, which reproduce the main experimental trends, including the 2PA, in a nearly quantitative manner. Electrochemical studies revealed that the HOMO of the new dyes is located at ca . −5.35 eV making them relatively electron rich in spite of the presence of two B − –N + dative bonds. These dyes undergo a fully reversible first oxidation, located on the diphenylpyrrolo[3,2- b ]pyrrole core, directly to the di(radical cation) stage. 

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