Locally Excited States Guided Enhancement in Reverse Intersystem Crossing Rate in Unconventional Acceptor-free Thermally Activated Delayed Emitters

Contrary to the conventional molecular design strategy for obtaining efficient thermally activated delayed fluorescent emitters, we report a couple of acceptor-free molecules (CZ2Bn, PhO2Bn, and PhS2Bn) that might provide a new design paradigm for the TADF molecules. The presence of locally available triplet states on the secondary donor units is found to be the most critical factor that controls the reverse intersystem crossing rate (krisc) and is explained in detail. Our approach to structural design was substituting the central carbazole unit with various secondary electron-rich aromatic units. The selection of the peripheral secondary donors contemplates their triplet-state energies, and in turn well-known aromatic units such as carbazole (CZ2Bn), phenoxazine (PhO2Bn), and phenothiazine (PhS2Bn) were chosen. The proof of concept of locally excited states facilitating the reverse intersystem crossing rate (krisc) is elaborated in the article. Despite having a significantly larger singlet–triplet energy gap (ΔEST) in the first derivative CZ2Bn, the large radiative rate of the singlet excited state and the availability of the isoenergetic local triplet states (3LE) led to the highest TADF efficiency. Detailed experimental outcomes and analysis are discussed in the present manuscript

Tuning Microstructures in Organogels: Gelation and Spectroscopic Properties of Mono- and Bis-cholesterol-Linked Diphenylbutadiene Derivatives

The gelation and photophysical properties of mono- and bis-cholesterol derivatives linked to diphenylbutadiene have been investigated. Scanning electron microscopy of xerogels of the monocholesterol derivatives indicated that these molecules self-assemble into 3D networks consisting of helically twisted fibers. In contrast, the morphology of xerogels of the bis-cholesterol derivatives indicated agglomerated spheres. In concentrated solutions (>10−4 M), the self-assembled superstructure of the monocholesterol derivatives consists of helically twisted fibers whereas that of the bis-cholesterol derivatives indicated clustered spheres. An investigation of spectroscopic properties suggests that the morphology of the superstructures formed in these systems may be correlated to the nature of the molecular aggregates formed. Absorption and emission spectral studies as a function of concentration and temperature suggested the formation of predominantly J-type aggregates in the monocholesterol and H-type aggregates in the bis-cholesterol derivatives. It is proposed that the slipped stack arrangement within the J aggregates of the monocholesterol derivatives resulted in the formation of helically twisted fibers, whereas the cofacial arrangement in the H aggregates of the bis-cholesterol derivatives could prevent such fiber formation, resulting in the formation of the agglomerated spheres

Tuning Microstructures in Organogels: Gelation and Spectroscopic Properties of Mono- and Bis-cholesterol-Linked Diphenylbutadiene Derivatives

The gelation and photophysical properties of mono- and bis-cholesterol derivatives linked to diphenylbutadiene have been investigated. Scanning electron microscopy of xerogels of the monocholesterol derivatives indicated that these molecules self-assemble into 3D networks consisting of helically twisted fibers. In contrast, the morphology of xerogels of the bis-cholesterol derivatives indicated agglomerated spheres. In concentrated solutions (>10−4 M), the self-assembled superstructure of the monocholesterol derivatives consists of helically twisted fibers whereas that of the bis-cholesterol derivatives indicated clustered spheres. An investigation of spectroscopic properties suggests that the morphology of the superstructures formed in these systems may be correlated to the nature of the molecular aggregates formed. Absorption and emission spectral studies as a function of concentration and temperature suggested the formation of predominantly J-type aggregates in the monocholesterol and H-type aggregates in the bis-cholesterol derivatives. It is proposed that the slipped stack arrangement within the J aggregates of the monocholesterol derivatives resulted in the formation of helically twisted fibers, whereas the cofacial arrangement in the H aggregates of the bis-cholesterol derivatives could prevent such fiber formation, resulting in the formation of the agglomerated spheres

Temperature-Regulated Dual Phosphorescence and Mechanical Strain-Induced Luminescence Modulation in a Plastically Bendable and Twistable Organic Crystal

A unique, thermally interchangeable, and tunable dual phosphorescence from a single-component organic crystal of DBC (3,6-dibromo-9-ethyl-9H-carbazole) is described here. Temperature-regulated relative intensities of a triple-channel radiative process, viz., singlet and dual triplets, enabled a near-white luminescence in a wide temperature range. Interestingly, the single crystals of DBC can also be plastically bent using external mechanical stress and allow the modulation of phosphorescence. The luminescence was completely singlet-mediated in the monomeric state, while the crystalline solid had three pathways of radiative processes. Strong n−π interactions mediated by non-covalent interactions significantly augment the initially forbidden n−π* transitions, which enabled a strong spin–orbit coupling to obtain large radiative triplet states in the solid state. Trapped higher energy triplets were the significant contributors to the emission at lower temperatures, whereas, at higher temperatures, the lower lying triplets dominated the process. A detailed mechanistic approach to unraveling the findings is summarized in the article

Phosphorescence or Delayed Fluorescence? Fate of Excitons in Core-Substituted Naphthalimide-Derived Emitters and Ratiometric Optical Oxygen Sensors

Competing rates of phosphorescence and thermally activated delayed fluorescence (TADF) need essential balance to yield efficient molecular emitters with color purity falling in either of the categories mentioned above. The large spin–orbit coupling (SOC) matrix element and the small singlet–triplet energy offset (ΔES–T) facilitate the reverse intersystem crossing (RISC) and efficient TADF. In contrast, the SOC constant and the radiative rate of triplets determine the efficiency of phosphorescence. Herein, we rationalize the above-mentioned radiative mechanisms in perspective of the chemical structure for a pair of naphthalimide derivatives (Br-NMI and NMI-Cz). With experimental evidence and computational support, we prove that the natural selection of these pathways is determined by the existence and nature of the higher-lying triplet states (Tn, n > 1) of suitable energy. Eventually, a highly radiative T1 state and a poor RISC rate led to room-temperature phosphorescence in Br-NMI. In contrast, the higher-lying 3LE facilitated fast RISC, leading to efficient TADF in NMI-Cz. Additionally, the Br-NMI-doped polymethyl methacrylate (PMMA) film (10 wt %), with enhanced phosphorescence quantum yield (ϕP) compared to its monomeric and pristine films, is also described. CO···Br noncovalent interactions between phosphors and the polymer matrix were responsible for the enhancement in ϕP and were proven by IR spectroscopic techniques. Interestingly, ϕP of the doped films was insensitive to the sample temperature (77–300 K) but highly susceptible to the sample O2 partial pressure. Utilizing these characteristics, we developed a self-referenced optical oxygen sensor with high sensitivity (KSV = 8.53 kPa–1) and reversibility

Temperature-Regulated Dual Phosphorescence and Mechanical Strain-Induced Luminescence Modulation in a Plastically Bendable and Twistable Organic Crystal

A unique, thermally interchangeable, and tunable dual phosphorescence from a single-component organic crystal of DBC (3,6-dibromo-9-ethyl-9H-carbazole) is described here. Temperature-regulated relative intensities of a triple-channel radiative process, viz., singlet and dual triplets, enabled a near-white luminescence in a wide temperature range. Interestingly, the single crystals of DBC can also be plastically bent using external mechanical stress and allow the modulation of phosphorescence. The luminescence was completely singlet-mediated in the monomeric state, while the crystalline solid had three pathways of radiative processes. Strong n−π interactions mediated by non-covalent interactions significantly augment the initially forbidden n−π* transitions, which enabled a strong spin–orbit coupling to obtain large radiative triplet states in the solid state. Trapped higher energy triplets were the significant contributors to the emission at lower temperatures, whereas, at higher temperatures, the lower lying triplets dominated the process. A detailed mechanistic approach to unraveling the findings is summarized in the article

Aromatic Ring Overlap Pedals the Nature of Exciton Coupling and Carrier Transport in a Series of Electron-Deficient Anthracenes

Planar aromatic semiconductors are excellent active layer materials for charge transport (mostly p-type) and light emission in various organic electronic devices. The exciton coupling is well known for controlling the optical traits of such materials in the bulk state. In terms of the carrier mobility, to achieve ambipolar transport in these classes of materials, the frontier molecular orbital energy levels should be appropriately engineered. Core functionalization using a moderately strong electron-withdrawing group (such as −CF3) could be a potential strategy to obtain appropriate energy levels. Herein, we have selected positional isomers of the bis–CF3 substituted derivatives as the possible group of molecules. In the bulk film state, depending on the spatial overlap of the aromatic core unit, distinct optical traits of the J- and H-type excitonically coupled dimeric states were evident in these series. Prominent enhancement of the radiative rate constant (and photoluminescence quantum yield) in J-type dimers (first pair of molecules) and substantially quenched excimer state emission for the H-type dimers was evident for the other pair in the solid-state samples. At the same time, in contradiction to the general perception of significant aromatic overlap for higher carrier mobility, the derivative with the highest spatial overlap had the least among the four products. An analogy could be drawn between the aromatic overlap in the solid-state dimers with their optical and the charge carrier mobility. Detailed experimental outcomes and the analysis are discussed in the present article

Exceptional Thermo-Mechano-Fluorochromism and Nanomechanical Analysis of Mechanically Responsive J and H Type Polymorphic Systems

Mechanically responsive organic crystals with thermo-mechano-fluorochromic behavior are relatively scarce. A polymorphic J and H aggregate system with such mechanosalient properties has potential importance for the understanding of structure-mechanochromic luminescence. Herein, we not only carefully investigated the nanomechanical properties of a polymorphic organic fluorophore of (biphenyl-4-ylmethylidene) propanedinitrile but also demonstrated their exceptional thermo-mechanochromic luminescence. Both the polymorphs were characterized using single crystal X-ray diffraction, solid-state UV, SSFL, nanoindentation, Hirshfeld surfaces, energy frameworks, powder X-ray diffraction, and differential scanning calorimetry thermograms. Relatively weak π–π stacking interactions present in the crystal structures govern the hypsochromic shift on mechanical grinding. Density functional theory calculations provide tremendous support to shed light on the origin of the distinct photophysical properties of the two polymorphs

Covalent Organic Framework Thin-Film Photodetectors from Solution-Processable Porous Nanospheres

The synthesis of homogeneous covalent organic framework (COF) thin films on a desired substrate with decent crystallinity, porosity, and uniform thickness has great potential for optoelectronic applications. We have used a solution-processable sphere transmutation process to synthesize 300 ± 20 nm uniform COF thin films on a 2 × 2 cm2 TiO2-coated fluorine-doped tin oxide (FTO) surface. This process controls the nucleation of COF crystallites and molecular morphology that helps the nanospheres to arrange periodically to form homogeneous COF thin films. We have synthesized four COF thin films (TpDPP, TpEtBt, TpTab, and TpTta) with different functional backbones. In a close agreement between the experiment and density functional theory, the TpEtBr COF film showed the lowest optical band gap (2.26 eV) and highest excited-state lifetime (8.52 ns) among all four COF films. Hence, the TpEtBr COF film can participate in efficient charge generation and separation. We constructed optoelectronic devices having a glass/FTO/TiO2/COF-film/Au architecture, which serves as a model system to study the optoelectronic charge transport properties of COF thin films under dark and illuminated conditions. Visible light with a calibrated intensity of 100 mW cm–2 was used for the excitation of COF thin films. All of the COF thin films exhibit significant photocurrent after illumination with visible light in comparison to the dark. Hence, all of the COF films behave as good photoactive substrates with minimal pinhole defects. The fabricated out-of-plane photodetector device based on the TpEtBr COF thin film exhibits high photocurrent density (2.65 ± 0.24 mA cm–2 at 0.5 V) and hole mobility (8.15 ± 0.64 ×10–3 cm2 V–1 S–1) compared to other as-synthesized films, indicating the best photoactive characteristics