Photophysical Mechanism Research Articles

Recent Advances in Thermally Activated Delayed Fluorescence-Based Organic Afterglow Materials.

Thermally activated delayed fluorescence (TADF)-based materials are attracting widespread attention for different applications owing to their ability of harvesting both singlet and triplet excitons without noble metals in their structures. As compared to the conventional fluorescence and room-temperature phosphorescence pathways, TADF originates from the reverse intersystem crossing process from the excited triplet state (T1) to the singlet state (S1). Therefore, TADF emitters enabling activated and long lifetime T1 excitons are potential candidates for generating long-lived afterglow emission, an effect that can still be observed for a while by the naked eye after the removal of the excitation light source. Recently, TADF-based organic afterglow materials featuring high photoluminescence quantum yields and long lifetimes above 100 ms under ambient conditions, have emerged for advanced information security, high-contrast biological imaging, optoelectronic devices, and intelligent sensors, whereas the related systematic review is still lacking. Herein, the recent progress in TADF-based organic afterglow materials is summarized and an overview of the photophysical mechanism, design strategies, and the performances for relevant applications is given. In addition, the challenge and perspective of this area are given at the end of the review.

Tunable multimode emissions of Yb3+/Er3+-doped rare-earth-based Cs2NaYCl6 double perovskite crystals for versatile applications

Lead-free halide double perovskite materials are gaining recognition because of their excellent optoelectronic performance. However, achieving multimode luminescence with tunable emission colors in single-component double perovskites remains a significant challenge. Understanding the mechanism of trivalent lanthanide-ion doping could provide a valuable solution. Herein, we report the synthesis of rare-earth-based Cs2NaYCl6 double perovskites using a hydrothermal method. Efficient visible to near-infrared down-shifting (DS) photoluminescence and green up-conversion (UC) emissions were realized by incorporating Er3+ ions with abundant energy levels into the hosts. Notably, the DS and UC emission colors could both be adjusted by further introducing Yb3+ ions that acted as sensitizers, and the photoluminescence was significantly enhanced due to the strong absorption at 980 nm. Spectral analysis revealed that the high compatibility of lanthanide ions with the rare-earth-based double perovskites and cross-relaxation processes played a key role in achieving modulated emission and multiple excitation modes. This study aimed to understand the photophysical mechanism of lanthanide-ion-doped double perovskites, as well as to highlight their ability to modulate emissions. Furthermore, this study expands the versatile applications of lanthanide-ion-doped double perovskites in the fields of high-security anti-counterfeiting, tissue imaging, and night-vision systems.

Identifying Organic-Inorganic Interaction Sites Toward Emission Enhancement in Non-Hydrogen-Bonded Hybrid Perovskite via Pressure Engineering.

The interaction between organic and inorganic components in metal hybrid perovskites fundamentally determines the intrinsic optoelectronic performance. However, the underlying interaction sites have still remained elusive, especially for those non-hydrogen-bonded hybrid perovskites, thus largely impeding materials precise design with targeted properties. Herein, high pressure is utilized to elucidate the interaction mechanism between organic and inorganic components in the as-synthesized one-dimensional hybrid metal halide (DBU)PbBr3 (DBU = 1,8-diazabicyclo [5.4.0] undec-7-ene). The interaction sites are identified to be the N from DBU and the Br from inorganic framework by the indicative of enhanced Raman mode under high pressure. The change in interaction strength is indeed derived from the pressure modulation on both distance and spatial arrangement of the nearest Br and N, rather than traditional hydrogen-bonding effect. Furthermore, the enhanced interaction increased charge transfer, resulting in a cyan emission with photoluminescence quantum yields (PLQYs) of 86.6%. The enhanced cyan emission is particularly important for underwater communication due to the much less attenuation in water than at other wavelength emissions. This study provides deep insights into the underlying photophysical mechanism of non-hydrogen-bonded hybrid metal halides and is expected to impart innovative construction with superior performance.

Green Preparation of S, N Co-Doped Low-Dimensional C Nanoribbon/C Dot Composites and Their Optoelectronic Response Properties in the Visible and NIR Regions.

The green production of nanocomposites holds great potential for the development of new materials. Graphene is an important class of carbon-based materials. Despite its high carrier mobility, it has low light absorption and is a zero-bandgap material. In order to tune the bandgap and improve the light absorption, S, N co-doped low-dimensional C/C nanocomposites with polymer and graphene oxide nanoribbons (the graphene oxide nanoribbons were prepared by open zipping of carbon nanotubes in a previous study) were synthesized by one-pot carbonization through dimensional-interface and phase-interface tailoring of nanocomposites in this paper. The resulting C/C nanocomposites were coated on untreated A4 printing paper and the optoelectronic properties were investigated. The results showed that the S, N co-doped C/C nanoribbon/carbon dot hybrid exhibited enhanced photocurrent signals of the typical 650, 808, 980, and 1064 nm light sources and rapid interfacial charge transfer compared to the N-doped counterpart. These results can be attributed to the introduction of lone electron pairs of S, N elements, resulting in more transition energy and the defect passivation of carbon materials. In addition, the nanocomposite also exhibited some electrical switching response to the applied strain. The photophysical and doping mechanisms are discussed. This study provides a facile and green chemical approach to prepare hybrid materials with external stimuli response and multifunctionality. It provides some valuable information for the design of C/C functional nanocomposites through dimensional-interface and phase-interface tailoring and the interdisciplinary applications.

Pressure-Induced Changes in the Phase Distribution and Carrier Dynamics of Quasi-Two-Dimensional Ruddlesden-Popper Perovskites.

Quasi-two-dimensional (quasi-2D) perovskites hold significant potential for diverse design strategies due to their tunable structures, exceptional optical properties, and environmental stability. Due to the complexity of the structure and carrier dynamics, characterization methods such as photoluminescence and absorption spectroscopy can observe but cannot precisely distinguish or identify the phase distribution within quasi-2D perovskite films or correlate phases with carrier dynamics. In this study, we used pressure to modulate the intralayer and interlayer structures of (PEA)2Csn-1PbnBr3n+1 quasi-2D perovskite films, investigating charge carrier dynamics. Steady-state spectroscopy revealed phase transitions at 1.62, 3, and 8 GPa, with free excitons transforming into self-trapped excitons after 8 GPa. Transient absorption spectroscopy elucidated the structural evolution, energy transfer, and pressure-induced transition mechanisms. The results demonstrate that combining pressure and spectroscopy enables the precise identification of phase distribution and pressure response ranges and reveals photophysical mechanisms, providing new insights for optimizing optoelectronic materials.

Polymer-Gel-Derived PbS/C Composite Nanosheets and Their Photoelectronic Response Properties Studies in the NIR

Non-conjugated polymer-derived functional nanocomposites are one of the important ways to develop multifunctional hybrids. By increasing the degree of crosslinking, their photophysical properties can be improved. PbS is a class of narrow bandgap infrared active materials. To avoid aggregation and passivation of the surface defects of PbS nanomaterials, a large number of organic and inorganic ligands are usually used. In this study, PbS/C composite nanosheets were synthesized with Pb2+ ion-crosslinked sodium alginate gel by one-pot carbonization. The resulting nanosheets were coated on untreated A4 printing paper, and the electrodes were the graphite electrodes with 5B pencil drawings. The photocurrent signals of the products were measured using typical 650, 808, 980, and 1064 nm light sources. The results showed that the photocurrent switching signals were effectively extracted in the visible and near-infrared regions, which was attributed to the mutual passivation of defects during the in situ preparation of PbS and carbon nanomaterials. At the same time, the resulting nanocomposite exhibited electrical switching responses to the applied strain to a certain extent. The photophysical and defect passivation mechanisms were discussed based on the aggregation state of the carbon hybrid and the interfacial electron interaction. This material would have potential applications in broadband flexible photodetectors, tentacle sensors, or light harvesting interdisciplinary areas. This study provided a facile approach to prepare a low-cost hybrid with external stimulus response and multifunctionality. These results show that the interfacial charge transfer is the direct experimental evidence of interfacial interaction, and the regulation of interfacial interaction can improve the physical and chemical properties of nanocomposites, which can meet the interdisciplinary application. The interdisciplinary and application of more non-conjugated polymer systems in some frontier areas will be expanded upon.

Unraveling the Photocatalytic Mechanism of N2 Fixation on Single Ruthenium Sites.

Photocatalytic N2 fixation offers promise for ammonia synthesis, yet traditional photocatalysts encounter challenges such as low efficiency and short carrier lifetimes. Atomically precise ligand-metal nanoclusters emerge as a solution to address these issues, but the photophysical mechanism remains elusive. Inspired by the synthesis of Au4Ru2 NCs, we investigate the mechanism behind N2 activation on Au4Ru2, focusing on photoactivity and carrier dynamics. Our results reveal that vibration of the Ru-N bond in the low-frequency domain suppresses the deactivation process leading to a long lifetime of the excited N2. By the strategy of isoelectronic substitution, we identify the single Ru sites as the active sites for N2 activation. Furthermore, these ligand-protected M4Ru2 (M = Au, Ag, Cu) NCs show robust thermal stability in explicit solvation and decent photochemical activity for N2 activation and NH3 production. These findings have significant implications for the optimization of catalysts for sustainable ammonia synthesis.

Building New Structural Distortion Descriptors through Pressure Engineering toward Enhanced Violet Emission in 2D Hybrid Perovskite

AbstractBuilding an explicit structure‐property relationship for two‐dimensional (2D) hybrid perovskites is critical to guide the synthesis of highly luminescent materials. However, traditional studies are limited to the deviation of individual intra‐octahedron, which fails to well reflect collective lattice distortion. Here, a set of new structural distortion descriptors associated with inter‐octahedra is introduced for 2D perovskite (C7H10N)2PbBr4, that is,(PMA)2PbBr4 through pressure engineering. An experimental conclusion is reached that adjacent Pb displacement, as the inter‐octahedron distortion, is an effective parameter in determining the structural distortion and emission behavior of (PMA)2PbBr4 under high pressure. Under high pressure, 2D perovskite (PMA)2PbBr4 exhibits two emission behaviors, free excitons (FEs) emission and self‐trapped excitons (STEs) emission. Different types of four‐Pb‐shaped quadrilateral correspond to different emissions: square‐type (only FEs emission), parallelogram‐type (coexistence of FEs emission and STEs emission), kite‐type (only STEs emission). Moreover, an enhanced violet emission at ≈431 nm is achieved under a mild pressure of 1.0 GPa, which is a crucial light source of medical sterilization. The underlying structure‐property relationship is clarified fundamentally deepens the photophysical mechanism of (PMA)2PbBr4, thus facilitating the design of high‐efficiency perovskite luminophores.

Theoretical study the photophysical mechanism of fluorescent probe on detecting mitochondria-targeted hydrogen peroxide

The photophysical mechanism and dynamics behaviors of (E)-4-(2-(7-(diethylamino)-2-oxo-2H-chromen-3-yl)vinyl)-1-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzyl)pyridin-1-ium (Compound 1) on detecting hydrogen peroxide (H2O2) has been theoretically studied. The boron ester in Compound 1 is cleaved when H2O2 is added to the solution, resulting in the formation of (E)-7-(diethylamino)-3-(2-(pyridin-4-yl)vinyl)-2H-chromen-2-one (Compound 2). Theoretical calculations show that the fluorescence spectra of the compounds exhibit a significant blue shift from 616 nm of Compound 1 to 542 nm of Compound 2 before and after the reaction, which are in good agreement with the experimental results (640 nm → 535 nm). The calculated electron–hole transfer distance of Compound 1 (5.986 Å) is larger than that of Compound 2 (3.544 Å), and Compound 1 is demonstrated to be charge transfer excitation while the Compound 2 is localized excitation, which results in a blue shift of the fluorescence spectra. The analysis of molecular electrostatic potential demonstrates that compound 1 has the highest electrostatic potential (4.60 eV) at the pyridine position and the lowest (−0.30 eV) at the oxygen atom of the coumarin moiety, suggesting that compound 1 undergoes fragmentation at this position. This study provides a theoretical explanation for the reaction mechanism of molecular probes.

Photophysical mechanism study of photophysical properties of new red BODIPY chromophores with or without methyl substitution

The relationship between the photophysical properties and structure of BODIPY derivatives always has attracted much attention. Two new BODIPY derivative chromophores with phenyl group substitutions at meso-/6- positions, with or without methyl substitutions at 1/3/5/7 positions were investigated by ways of combination of multiple experimental techniques and theoretical methods. The chromophore without methyl substitutions has a larger Stokes shift and better viscosity sensitivity compared to the other one. The intrinsic reason is that considerable charge transfer between the BODIPY core and the 6- position group is achieved in chromophore. However, this charge transfer process is severely weakened, if 1-/3-/5-/7- methyl substitutions existing, via restricting rotation between the core and side groups. At the same time, the non-radiative process is amplified significantly in chromophore without methyl substitution and the fluorescence efficiency decrease from 51.7% (with methyl substitutions) to 1.84% (without methyl substitutions) in DMSO. Quantum calculations indicate the presence or absence of methyl substitution can change both the shape and height of the torsional potential energy curve of the side groups, thereby affecting the fluorescence quantum yield and viscosity response performance of chromophores. Our work can provide foundation and ideas for designing and optimizing BODIPY derivatives with objective photophysical properties.

Recent Advances in Fast-Decaying Metal Halide Perovskites Scintillators.

Fast-decaying scintillators show subnanoseconds or nanoseconds lifetime and high time resolution, making them important in nuclear physics, medical diagnostics, scientific research, and other fields. Metal halide perovskites (MHPs) show great potential for scintillator applications owing to their easy synthesis procedure and attractive optical properties. However, MHPs scintillators still need further improvement in decay lifetime. To optimize the decay lifetime, great progress has been achieved recently. In this Perspective, we first summarize the structural characteristics of MHPs in various dimensions, which brings different exciton behaviors. Then, recent advances in designing fast-decaying MHPs according to different exciton behaviors have been concluded, focusing on the photophysical mechanisms to achieve fast-decaying lifetimes. These advancements in decay lifetimes could facilitate the MHPs scintillators in advanced applications, such as time-of-flight positron emission tomography (TOF-PET), photon-counting computed tomography (PCCT), etc. Finally, the challenges and future opportunities are discussed to provide a roadmap for designing novel fast-decaying MHPs scintillators.

Thermal Enhanced Energy Transfer and High Thermal Sensitivity of Sb3+ Doped Cs2KYbCl6 Rare Earth Double Perovskites

AbstractRare‐earth based double perovskites (DPs) have attracted much attention due to stable, efficient, and unique luminescence, and wide applications in many optoelectronic fields. However, their weak near‐infrared (NIR) emission and poor anti‐thermal quenching severely limit the further applications. Herein, Sb3+‐doped Cs2KYbCl6 DP with cyan self‐trapped exciton (STE) and NIR emission is prepared by a solvothermal method, and the separate photoluminescence quantum yields from STE and NIR emission reached ≈26.1% and 43.8%, respectively. More importantly, Sb3+ not only contributes to the absorption and energy transfer but also helps to establish an effective thermally enhanced energy transfer channel through self‐trapping state, resulting in the NIR emission with superior anti‐thermal quenching resistance. The thermal sensitivity of luminescence intensity ratio (LIR) of I1010 nm/I505 nm (I is the intensity of PL) and time‐resolved temperature sensor reach 21.4 and 13.6% K−1, respectively, which are ahead of most temperature sensing materials. Furthermore, it is found that the material exhibited advanced multifunctional applications in LED lighting, flexible luminescent thin film, and NIR imaging. This work provides in‐depth understanding on photophysical mechanisms of rare‐earth luminescent materials and references for designing high‐performance materials.

Exploiting in-plane anisotropy in Ta2NiSe5 spanning near to mid-infrared photodetection

Miniaturized and stabilized polarization-sensitive mid-Infrared photodetectors at room temperature are indispensable in fields ranging from medical diagnostics to military surveillance in the next-generation on-chip polarimeters. Emerging two-dimensional materials offer a promising avenue to fulfill these requirements, facilitated by their ease of integration onto complex structures, inherent in-plane anisotropic crystal structures that enhance polarization sensitivity, and robust quantum confinement effects that enable superior photodetection performance at room temperature. Here, we report the systematic investigation of polarization-dependent infrared photoresponse based on Ta2NiSe5, revealing significant anisotropy photocurrent with excellent stability at room temperature. Significantly, a large anisotropic ratio of Ta2NiSe5 ensures the polarization sensitivity achieves a ratio of 1.23 at 1550 nm. Moreover, at 4.6 μm, the device exhibits a peak photocurrent response of 1.16 A/W along the armchair orientation, with an anisotropy ratio of approximately 3.3. These findings not only enhance our understanding of the photophysical mechanisms in two-dimensional materials but also guide the optimization of photodetector design for enhanced performance.

Improved Exciton Diffusion through Modulating Förster Resonance Energy Transfer for Efficient Organic Solar Cells

Förster resonance energy transfer (FRET) is commonly utilized in organic solar cells (OSCs) to enhance the power conversion efficiency (PCE) by promoting molecular interactions. The PCE is greatly affected by the number of photo‐generated excitons that effectively reach interfaces between the donor and acceptor materials of OSCs. However, the correlation between FRET and exciton diffusion has received limited attention. Therefore, it is crucial to understand and manipulate FRET process for investigating exciton dynamics in OSCs. Herein, BTA3 and its derivatives are chosen as the third components and energy donors to elucidate the intrinsic photophysical mechanism of FRET in OSCs by manipulating their efficiency. The results unambiguously demonstrate that the addition of guest F‐BTA3 exhibits the highest FRET efficiency, leading to a significant enhancement in the PCE of both PM6:Y6 and PM6:PY‐IT host systems. By correlating FRET process and exciton dynamics, this enhancement to remarkable increment in exciton diffusion length is attributed. Experimental results and theoretical simulations indicate high FRET efficiency arises from strong dipole–dipole interaction, and exhibits a positive correlation with exciton diffusion length. This study showcases the effective regulation of exciton diffusion in OSCs through modulation of FRET, offering a novel perspective for optimizing the performance of organic photovoltaic devices.

Deepening insights of AIE plus TICT activated fluorescent sensor mechanism in probe molecule DPA-CI

The DPA-CI molecule is a novel fluorescent sensing probe synthesized with TICT and AIE properties. It can detect phosgene, generating DPA-CI-PS. We investigated their photophysical mechanisms in THF solvent using DFT and TD-DFT methods. Large torsion angles and noticeable charge separation confirmed that the TICT occurs in DPA-CI. Due to the formation of a robust cyclic structure, the TICT is inhibited in DPA-CI-PS. Besides, based on NBO analysis, we demonstrated that DPA-CI-PS exhibits significant AIE characters. This work clearly illuminates the detection mechanism, which offers theoretical insights valuable for developing efficient fluorescent sensing probes.

Anisotropic Plasmon Resonance Enables Spatially Controlled Photothermal and Photochemical Effects in Hot Carrier‐Driven Catalysis

Comprehensive SummaryLocalized surface plasmon resonance has been demonstrated to provide effective photophysical enhancement mechanisms in plasmonic photocatalysis. However, it remains highly challenging for distinct mechanisms to function in synergy for a collective gain in catalysis due to the lack of spatiotemporal control of their effect. Herein, the anisotropic plasmon resonance nature of Au nanorods was exploited to achieve distinct functionality towards synergistic photocatalysis. Photothermal and photochemical effects were enabled by the longitudinal and transverse plasmon resonance modes, respectively, and were enhanced by partial coating of silica nanoshells and epitaxial growth of a reactor component. Resonant excitation leads to a synergistic gain in photothermal‐mediated hot carrier‐driven hydrogen evolution catalysis. Our approach provides important design principles for plasmonic photocatalysts in achieving spatiotemporal modulation of distinct photophysical enhancement mechanisms. It also effectively broadens the sunlight response range and increases the efficacy of distinct plasmonic enhancement pathways towards solar energy harvesting and conversion.

The Advent of Bodipy-based Chemosensors for Sensing Fluoride Ions: A Literature Review.

The detection of fluoride ions in water and other sources is crucial because they can harm human health if they exceed the safe limit of 1-1.5 ppm. BODIPY (boron dipyrromethene) dyes are promising fluorophores for chemosensors, and their design and modification have attracted a lot of attention. Their advantages include visible light excitation and emission, high molar absorption coefficients (ε) and fluorescence quantum yields [ϕ (λ)], and flexible scaffold manipulation for various applications. In this article, we review the progress of BODIPY-based sensors for fluoride ions from the early 2000s to the present. We focus on the different scaffold modifications of the sensors and their corresponding responses, as well as the underlying photophysical mechanisms and potential uses of each sensor.

Photophysical Properties of (E)-1-(4-(Diethyla-mino)-2-hydroxybenzylidene)-4,4-dimethylthiosemicarbazide Compound and Its Triple Fluorescence Emission Mechanism: A Theoretical Perspective.

In view of the application prospects in biomedicine of (E)-1-(4-(diethyla-mino)-2-hydroxybenzylidene)-4,4-dimethylthiosemicarbazide (DAHTS), the behavior of excited-state dynamics and photophysical properties were studied using the density functional theory/time-dependent density functional theory method. A series of studies indicated that the intramolecular hydrogen-bond (IHB) intensity of DAHTS was enhanced after photoexcitation. This was conducive to promoting the excited-state intramolecular proton-transfer (ESIPT) process. Combining the analysis of the IHB and hole-electron, it revealed that the molecule underwent both the ESIPT process and the twisted charge-transfer (TICT) process. Relying on exploration of the potential energy surface, it was proposed that the different competitive mechanisms between the ESIPT and TICT processes were regulated by solvent polarity. In acetonitrile (ACN) solvent, the ESIPT process occurred first, and the TICT process occurred later. In contrast, in the CYH solvent, the molecule first underwent the TICT process and then the ESIPT process. Furthermore, we raised the possibility that the TICT behavior was the cause of weak fluorescence emission for the DAHTS in CYH and ACN solvents. By the dimer correlation analysis, the corresponding components of triple fluorescence emission were clearly assigned, corresponding to the monomer, dimer, and ESIPT isomer in turn. Our work precisely elucidated the photophysical mechanism of DAHTS and the attribution of the triple fluorescence emission components, which provided valuable guidance for the development and regulation of bioactive fluorescence probes with multiband and multicolor emission characteristics.

Powders to Thin Films: Advances in Conjugated Microporous Polymer Chemical Sensors.

Chemical sensing of harmful species released either from natural or anthropogenic activities is critical to ensuring human safety and health. Over the last decade, conjugated microporous polymers (CMPs) have been proven to be potential sensor materials with the possibility of realizing sensing devices for practical applications. CMPs found to be unique among other porous materials such as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) due to their high chemical/thermal stability, high surface area, microporosity, efficient host-guest interactions with the analyte, efficient exciton migration along the π-conjugated chains, and tailorable structure to target specific analytes. Several CMP-based optical, electrochemical, colorimetric, and ratiometric sensors with excellent selectivity and sensing performancewere reported. This review comprehensively discusses the advances in CMP chemical sensors (powders and thin films) in the detection of nitroaromatic explosives, chemical warfare agents, anions, metal ions, biomolecules, iodine, and volatile organic compounds (VOCs), with simultaneous delineation of design strategy principles guiding the selectivity and sensitivity of CMP. Preceding this, various photophysical mechanisms responsible for chemical sensing are discussed in detail for convenience. Finally, future challenges to be addressed in the field of CMP chemical sensors are discussed.

Intra and interatomic energy contributions in the photophysical relaxation of small aromatic molecules

AbstractA theoretical study of the non‐radiative photophysical relaxation mechanisms of the first singlet excited state of benzene, cyclobutadiene and fulvene is presented. For these molecules, the calculation of the Minimum Energy Path (MEP) leading from the Franck–Condon region to the surface crossing with the ground state is carried out. Subsequently, the decomposition of the electronic energies into atomic and pair contributions is performed using the Interacting Quantum Atom (IQA) method. The IQA approach provides the important mechanistic information necessary to rationalise some relevant aspects of the processes, such as the components that explain the appearance of an energy barrier or that favour the crossing between potential energy surfaces (PES); it also allows to quantify the direct effect on the MEP due to the inclusion of a substituent. In particular, it is shown how the IQA energies allow measuring the extent to which the formation of biradicaloid structures affects the crossing of the PES. The analysis of electron density functions suggests that aromaticity is not a driving force on the relaxation processes. Overall, this work shows the potential of the IQA method as a useful tool for the detailed description of photophysical processes.