Molecular design and architectonics towards film-based fluorescent sensing

The past few decades have witnessed encouraging progress in the development of high-performance film-based fluorescent sensors (FFSs) for detecting explosives, illicit drugs, chemical warfare agents (CWAs), and hazardous volatile organic chemicals (VOCs), among others. Several FFSs have transitioned from laboratory research to real-world applications, demonstrating their practical relevance. At the heart of FFS technology lies the sensing films, which play a crucial role in determining the analytes and the resulting signals. The selection of sensing fluorophores and the fabrication strategies employed in film construction are key factors that influence the fluorescence properties, active-layer structures, and overall sensing behaviors of these films. This review examines the progress and innovations in the research field of FFSs over the past two decades, focusing on advancements in fluorophore design and active-layer structural engineering. It underscores popular sensing fluorophore scaffolds and the dynamics of excited state processes. Additionally, it delves into six distinct categories of film fabrication technologies and strategies, providing insights into their advantages and limitations. This review further addresses important considerations such as photostability and substrate effects. Concluding with an overview of the field's challenges and prospects, it sheds light on the potential for further development in this burgeoning area.

Asymmetrical-Dendronized TADF Emitters for Efficient Non-doped Solution-Processed OLEDs by Eliminating Degenerate Excited States and Creating Solely Thermal Equilibrium Routes

The mechanism of thermally activated delayed fluorescence (TADF) in dendrimers is not clear. We report that fully‐conjugated or fully‐nonconjugated structures cause unwanted degenerate excited states due to multiple identical dendrons, which limit their TADF efficiency. We have synthesized asymmetrical “half‐dendronized” and “half‐dendronized‐half‐encapsulated” emitters. By eliminating degenerate excited states, the triplet locally excited state is ≥0.3 eV above the lowest triplet charge‐transfer state, assuring a solely thermal equilibrium route for an effective spin‐flip process. The isolated encapsulating tricarbazole unit can protect the TADF unit, reducing nonradiative decay and enhancing TADF performance. Non‐doped solution‐processed devices reach a high external quantum efficiency (EQE_max) of 24.0 % (65.9 cd A⁻¹, 59.2 lm W⁻¹) with CIE coordinates of (0.24, 0.45) with a low efficiency roll‐off and EQEs of 23.6 % and 21.3 % at 100 and 500 cd m⁻².

Solvophobicity-directed assembly of microporous molecular crystals

Dense packing is a universal tendency of organic molecules in the solid state. Typical porous crystals utilize reticular strong intermolecular bonding networks to overcome this principle. Here, we report a solvophobicity-based methodology for assembling discrete molecules into a porous form and succeed in synthesizing isostructural porous polymorphs of an amphiphilic aromatic molecule Py₆Mes. A computational analysis of the crystal structure reveals the major contribution of dispersion interaction as the driving force for assembling Py₆Mes into a columnar stacking while the columns are sterically salient and form nanopores between them. The porous packing is facilitated particularly in solvents with weak dispersion interaction due to the solvophobic effect. Conversely, solvents with strong dispersion interaction intercalate between Py₆Mes due to the solvophilic effect and provide non-porous inclusion crystals. The solvophobicity-directed polymorphism is further corroborated by the polymorphs of Py₆Mes-analogues, m-Py₆Mes and Ph₆Mes.

Sigmoidally hydrochromic molecular porous crystal with rotatable dendrons

Vapochromic behaviour of porous crystals is beneficial for facile and rapid detection of gaseous molecules without electricity. Toward this end, tailored molecular designs have been established for metal-organic, covalent-bonded and hydrogen-bonded frameworks. Here, we explore the hydrochromic chemistry of a van der Waals (VDW) porous crystal. The VDW porous crystal VPC-1 is formed from a novel aromatic dendrimer having a dibenzophenazine core and multibranched carbazole dendrons. Although the constituent molecules are connected via VDW forces, VPC-1 maintains its structural integrity even after desolvation. VPC-1 exhibits reversible colour changes upon uptake/release of water molecules due to the charge transfer character of the constituent dendrimer. Detailed structural analyses reveal that the outermost carbazole units alone are mobile in the crystal and twist simultaneously in response to water vapour. Thermodynamic analysis suggests that the sigmoidal water sorption is induced by the affinity alternation of the pore surface from hydrophobic to hydrophilic.

Aurophilicity-Mediated Construction of Emissive Porous Molecular Crystals as Versatile Hosts for Liquid and Solid Guests

The first examples of porous molecular crystals that are assembled through Au⋅⋅⋅Au interactions of gold complex 1 are here reported along with their exchange properties with respect to their guest components. Single-crystal X-ray diffraction (XRD) analyses indicate that the crystal structure of 1/CH₂ Cl₂ ⋅pentane is based on cyclic hexamers of 1, which are formed through six Au⋅⋅⋅Au interactions. The packing of these cyclic hexamers affords a porous architecture, in which the one-dimensional channel segment contains CH₂ Cl₂ and pentane as guests. These guests can be exchanged through operationally simple methods under retention of the host framework of 1, which furnished 1/guest complexes with 26 different guests. A single-crystal XRD analysis of 1/eicosane, which contains the long linear alkane eicosane (n-C₂₀ H₄₂ ), successfully provided its accurately modeled structure within the porous material. These host-guest complexes show chromic luminescence with both blue- and redshifted emissions. Moreover, this porous organometallic material can exhibit luminescent mechanochromism through release of guests.

Solvation-dependent switching of solid-state luminescence of a fluorinated aromatic tetrapyrazole

Creating stimuli-responsive materials with switchable solid-state luminescence remains a challenge. We report that the solvation of a novel organic fluorophore can be utilized to prepare such a material, which emits in the blue (442-446 nm) region when wet and in the green (497-503 nm) region when dry.

Self-assembly of lattices with high structural complexity from a geometrically simple molecule

Here we report an anomalous porous molecular crystal built of C-H···N-bonded double-layered roof-floor components and wall components of a segregatively interdigitated architecture. This complicated porous structure consists of only one type of fully aromatic multijoint molecule carrying three identical dipyridylphenyl wedges. Despite its high symmetry, this molecule accomplishes difficult tasks by using two of its three wedges for roof-floor formation and using its other wedge for wall formation. Although a C-H···N bond is extremely labile, the porous crystal maintains its porosity until thermal breakdown of the C-H···N bonds at 202°C occurs, affording a nonporous polymorph. Though this nonporous crystal survives even at 325°C, it can retrieve the parent porosity under acetonitrile vapor. These findings show how one can translate simplicity into ultrahigh complexity.

Anisotropic convergence of dendritic macromolecules facilitated by a heteroleptic metal-organic polyhedron scaffold

Anisotropic dendrimers with bipolar shapes were systematically obtained using a heteroleptic metal-organic polyhedron (MOP) as a robust core scaffold. The structure of one of these polyhedral shapes was unambiguously determined by single-crystal X-ray analysis, which revealed that the bulky dendrons converge to both axial positions of the heteroleptic MOP core.