Shortwave infrared (SWIR)-emitting small molecules are desirable for biological imaging applications. In this study, four novel pentamethine indolizine cyanine dyes were synthesized with N,N-dimethylaniline-based substituents on the indolizine periphery at varied substitution sites. The dyes are studied via computational chemistry and optical spectroscopy both in solution and when encapsulated. Dramatic spectral shifts in the absorption and emission spectrum wavelengths with added donor groups are observed. Significant absorption and emission with an emissive quantum yield as high as 3.6% in the SWIR region is possible through the addition of multiple donor groups per indolizine.

The interface between [6,6]-phenyl C61-butyric acid methyl ester (PC61BM) and the electrode has a critical effect on the performance of inverted perovskite solar cells (PSCs). Three organic cationic cyanine dye molecules with different highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) states are designed to passivate the PC61BM and Ag electrode interface to improve PSCs performance. The effects of energy-level alignment and the interfacial charge transfer resistance on the device performance are compared and studied. The dye interface passivation layer significantly reduces charge recombination. Moreover, the ClO4 – anions associated with the dye molecules improve the charge extraction and charge transport in the devices. Reduced interface charge recombination and improved charge transport are confirmed by photoluminescence (PL), time-resolved photoluminescence (TRPL), electrochemical impedance spectra (EIS), and charge-only device performance studies. The PSCs with one of the dyes as an interface passivation layer show an optimized power conversion efficiency (PCE) of 19.14% with an open-circuit voltage (V oc) of 1.09 V, a short-circuit current density (J sc) of 22.87 mA/cm2, and a fill factor (FF) of 76.81%. The devices maintain over 90% of the initial PCE for 120 h of storage under an ambient environment (25 °C and 30 ± 5% relative humidity (RH). The use of small dye molecules as an interface passivation layer to reduce charge recombination in PSCs represents a paradigm for improving the performance and stability of PSCs.

The design of shortwave infrared (SWIR) emissive small molecules with good stability in water remains an important challenge for fluorescence biological imaging applications. A series of four SWIR emissive rhodindolizine (RI) dyes were rationally designed and synthesized to probe the effects of nonconjugated substituents, conjugated donor groups, and nanoencapsulation in a water-soluble polymer on the stability and optical properties of the dyes. Steric protecting groups were added at the site of a significant LUMO presence to probe the effects on stability. Indolizine donor groups with added dimethylaniline groups were added to reduce the electrophilicity of the dyes toward nucleophiles such as water. All of the dyes were found to absorb (920–1096 nm peak values) and emit (1082–1256 nm peak values) within the SWIR region. Among xanthene-based emissive dyes, emission values >1200 nm are exceptional with 1256 nm peak emission being a longer emission than the recent record setting VIX-4 xanthene-based dye. Half-lives were improved from ∼5 to >24 h through the incorporation of either steric-based core protection groups or donors with increased donation strength. Importantly, the nanoencapsulation of the dyes in a water-soluble surfactant (Triton-X) allows for the use of these dyes in biological imaging applications.

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Fluorescence Quantum Yield allows scientists to both quantify spectroscopic properties of dyes and compare to literature references. With the growing interest in NIR-emissive dyes for biological imaging, it is of great importance to reliably measure the fluorescent quantum yield of these novel dyes. Using a broadband excitation source and liquid nitrogen cooled InGaAs detector, steady state emission of four novel pentamethine indolizine cyanine dyes synthesized with N,N-dimethylaniline-based substituents on the indolizine periphery at varied substitutions sites is recorded.