Naphthalonitriles featuring efficient emission in solution and in the solid state

Dual Emissive Zn(II) Naphthalocyanines: Synthesis, Structural and Photophysical Characterization with Theory-Supported Insights towards Soluble Coordination Compounds with Visible and Near-Infrared Emission

Metal phthalocyaninates and their higher homologues are recognized as deep-red luminophores emitting from their lowest excited singlet state. Herein, we report on the design, synthesis, and in-depth characterization of a new class of dual-emissive (visible and NIR) metal naphthalocyaninates. A 4-N,N-dimethylaminophen-4-yl-substituted naphthalocyaninato zinc(II) complex (Zn-NMe2Nc) and the derived water-soluble coordination compound (Zn-NMe3Nc) exhibit a near-infrared fluorescence from the lowest ligand-centered state, along with a unique push-pull-supported luminescence in the visible region of the electromagnetic spectrum. An unprecedentedly broad structural (2D-NMR spectroscopy and mass spectrometry) as well as photophysical characterization (steady-state state and time-resolved photoluminescence spectroscopy) is presented. The unique dual emission was assigned to two independent sets of singlet states related to the intrinsic Q-band of the macrocycle and to the push-pull substituents in the molecular periphery, respectively, as predicted by TD-DFT calculations. In general, the elusive chemical aspects of these macrocyclic compounds are addressed, involving both reaction conditions, thorough purification, and in-depth characterization. Besides the fundamental aspects that are investigated herein, the photoacoustic properties were exemplarily examined using phantom gels to assess their tomographic imaging capabilities. Finally, the robust luminescence in the visible range arising from the push-pull character of the peripheral moieties demonstrated a notable independence from aggregation and was exemplarily implemented for optical imaging (FLIM) through time-resolved multiphoton micro(spectro)scopy.

Design Concepts for Solution and Solid-State Emitters - A Modern Viewpoint on Classical and Non-Classical Approaches

For a long time, luminescence phenomena were strictly distinguished between the emission of isolated molecules in dilute solutions or close-packed structures such as in powders or aggregates. This changed with the breakthrough observation of dual-state efficient materials, which led to a rapid boost of publications examining the influence of structural features to achieve balanced emission with disregarded molecular surroundings. Some first general structural design concepts have already been proposed based on reoccurring patterns and pivotal motifs. However, we have found another way to classify these solution and solid-state emitters (SSSEs). Hence, this minireview aims to present an overview of published structural features of SSSEs while shining light on design concepts from a more generalized perspective. Since SSSEs are believed to bridge the gap of hitherto known aggregation-sensitive compound classes, we hope to give future scientists a versatile tool in hand to efficiently design novel luminescent materials.

Thermally activated delayed fluorescence (TADF) emitters: sensing and boosting spin-flipping by aggregation

Metal-free organic emitters with thermally activated delayed fluorescence (TADF) characteristics are emerging due to the potential applications in optoelectronic devices, time-resolved luminescence imaging, and solid-phase sensing. Herein, we synthesized two (4-bromobenzoyl)pyridine (BPy)-based donor–acceptor (D–A) compounds with varying donor size and strength: the emitter BPy-pTC with tert-butylcarbazole (TC) as the donor and BPy-p3C with bulky tricarbazole (3C) as the donor unit. Both BPy-pTC and BPy-p3C exhibited prominent emission with TADF properties in solution and in the solid phase. The stronger excited-state charge transfer was obtained for BPy-p3C due to the bulkier donor, leading to a more twisted D–A geometry than that of BPy-pTC. Hence, BPy-p3C exhibited aggregation-induced enhanced emission (AIEE) in a THF/water mixture. Interestingly, the singlet–triplet energy gap (ΔE_ST) was reduced for both compounds in the aggregated state as compared to toluene solution. Consequently, a faster reverse intersystem crossing rate (k_RISC) was obtained in the aggregated state, facilitating photon upconversion, leading to enhanced delayed fluorescence. Further, the lone-pair electrons of the pyridinyl nitrogen atom were found to be sensitive to acidic protons. Hence, the exposure to acid and base vapors using trifluoroacetic acid (TFA) and triethylamine (TEA) led to solid-phase fluorescence switching with fatigue resistance. The current study demonstrates the role of the donor strength and size in tuning ΔE_ST in the aggregated state as well as the relevance for fluorescence-based acid–base sensing.

Bridged Aromatic Oxo- and Thioethers with Intense Emission in Solution and the Solid State

In this contribution, we report on a class of emitters based on bridged oxo- and/or thioethers revealing striking photoluminescence properties in fluid solution and in the solid state. In total, nine compounds were investigated concerning their photophysical properties, which were interpreted by quantum chemical calculations. To our delight, we discovered compounds possessing nearly identical photoluminescence quantum yields (Φ_F ) in solution and in the solid state, which has been rarely reported so far. Besides these efforts, we shed light on the influence of polymorphism and solvent polarity on the emission properties. In addition, an in-depth X-ray diffractometric analysis was conducted to correlate molecular packing in the crystal with differences in the photophysical properties.

BOSHPY Fluorophores: BODIPY Analogues with Single Atom Controlled Aggregation

The reaction of diiminoisoindoline and iminoxoisoindoline with aminoazoles results in the formation of bidentate chelates that can be considered a semihemiporphyrazine. These chelates react with BF₃ to produce fluorescent compounds that are structurally analogous to the BODIPY dyes. These difluoroboron semihemiporphyrazines (BOSHPYs) aggregate, and the type of aggregation (H or J) is determined by a single atom at the periphery of the ligand (O or N). Notably, the imine terminated compounds remain fluorescent upon aggregation.