Boron difluoride hydrazone (BODIHY) complexes: A new class of fluorescent molecular rotors

Abstract

Fluorescent dyes exhibiting luminescence in the solid state are crucial for the development of efficient solid-state lighting devices. Most fluorophores though do not emit in the solid state, mainly as a result of intermolecular interactions that cause emission quenching. Boron difluoride hydrazone (BODIHY) dyes are peculiar in this sense as their emission is drastically enhanced when transferred from solution to the solid state. The BODIHY dyes are easy to synthesize and derivatize, have tunable absorption and emission profiles and large Stokes shifts, which are all desirable characteristics from solid-state emitters. Here, we summarize the discovery and development of BODIHY dyes including the tuning of their photophysical properties through structure-property analyses.

Cooperative and Orthogonal Switching in the Solid State Enabled by Metal‐Organic Framework Confinement Leading to a Thermo‐Photochromic Platform

“…Very often different classes of commonly used moieties (e.g., azobenzene versus spiropyran) possess fundamentally different molecular structures. Furthermore, their photoisomerization could be accompanied with significant structural rearrangements as it was observed in the case of hydrazone or spiropyran derivatives or associated with significant changes in the dipole moment (spiropyran versus merocyanine forms) [2,3,17] . These complications (e.g., spatial constraints, π – π stacking, or electrostatic interactions) are especially pronounced upon transitioning from molecular systems in solution to material preparation in the solid state.…”

“…Furthermore, their photoisomerization could be accompanied with significant structural rearrangements as it was observed in the case of hydrazone or spiropyran derivatives or associated with significant changes in the dipole moment (spiropyran versus merocyanine forms). [2,3,17] These complications (e.g., spatial constraints, π-π stacking, or electrostatic interactions) are especially pronounced upon transitioning from molecular systems in solution to material preparation in the solid state. As shown previously, modular metal-organic frameworks (MOFs) could be considered not only as porous platforms promoting isomerization of sterically demanding photochromic units (i.e., possessing limited photoisomerization in the solid state [2,54] ), but they could also accommodate classes of stimuli-responsive molecules with different "needs".…”

Difluoro Dipyridomethene Boron Complexes: Synthesis, Characterization, and Ab Initio Calculations

“…Four-coordinated nitrogen-chelated boron complexes is an effective strategy to obtain photoluminescent compounds . Indeed boron complexation increases molecular rigidity and planarity of the aromatic core, leading to highly fluorescent dyes. ,− Among the four-coordinated boron-containing fluorescent dyes, − 4,4-difluoro-4-bora-3a,4a-diaza- s -indacenes (BODIPYs) are by far the most developed. − Their synthetic versatility, high extinction coefficient (ε) and often large fluorescence quantum yields (ϕ F ), good chemical robustness, and stability found a wide range of applications in various areas, e.g., biological sensors, imaging agents, lasers, electroluminescent devices, as well as solar cells. − However, their application as thin films in organic light-emitting diodes, in aqueous media for sensing and imaging is limited because of the highly planar structure and extended π-conjugation of BODIPYs inducing π–π stacking and aggregation, quenching the emissive properties. , Furthermore, they exhibit small Stokes shift values, leading to self-absorption processes and subsequent detrimental photobleaching . To overcome these limitations, the development of BODIPY analogues remains an active research area. , …”

Fluorescent Rotary Switches: Four- vs Three-Substituted Phthalimide Boron Difluoride Schiff Base Complexes

“…Phthalimide fluorophores find a broad range of applications in photonics and biomedical technology, and they exhibit very good fluorescence quantum yields in solution and solid state, covering the blue-green-red spectral region. − Due to the strong electron-accepting nature of the imide ring, they have a low-lying LUMO level, which allows tuning of the emission by environmental polarity, substitution pattern of the phthalimide ring, and nature of substituents. ,− Upon photoexcitation, electron-donating substituents at the 4- or 3-position cause photoinduced intramolecular or twisted charge transfer, resulting in broad-range emission. ,,,− Phthalimide-based molecular or supramolecular complexes are important functional materials in photonics, − ,,,, bioimaging, , photodynamic therapy, ,, organic light-emitting diodes, and sensor ,, technology. Until now, only a few papers have reported on the boron difluoride phthalimide-based complexes and their photoluminescence properties, which exhibit good quantum yields in the blue-green spectral region. ,− Boron dipyrromethene (BODIPY) − and boron difluoride hydrazone (BODIHY) dyes are attracting increasing research interest due to their very good photoluminescence properties coupled with an aggregation-induced emission phenomenon that allows tuning of absorption/emission bands covering all visible spectral range. Recently, they have been exploited as fluorescence molecular rotors − and configuration switches − with high photoconversion in the visible and NIR regions, making them promising candidates for photopharmacological and memory storage applications.…”

“…Until now, only a few papers have reported on the boron difluoride phthalimide-based complexes and their photoluminescence properties, which exhibit good quantum yields in the blue-green spectral region. 15,23−25 Boron dipyrromethene (BODIPY) 26−30 and boron difluoride hydrazone (BODIHY) 31 dyes are attracting increasing research interest due to their very good photoluminescence properties coupled with an aggregation-induced emission phenomenon that allows tuning of absorption/emission bands covering all visible spectral range. Recently, they have been exploited as fluorescence molecular rotors 32−37 and configuration switches 38−40 with high photoconversion in the visible and NIR regions, making them promising candidates for photopharmacological 41 and memory storage applications.…”