Time-correlated Single Photon Counting Measurements Research Articles

Morphology-transforming AuNPs-based fluorescent probe for ultra-low sensitive detection of levofloxacin in urine samples at pH 7.0 through excimer formation

Fluorescent metal nanoparticles (MNPs) have garnered considerable interest in the realm of healthcare applications. Within this domain, we present a novel fluorescent sensor: Pamoic acid (PA) functionalized excitation-dependent gold nanoparticles (PA@AuNPs), designed to ascertain levofloxacin (LF) concentrations in human urine samples under pH 7.0 conditions. The synthesis of PA@AuNPs was achieved via a chemical reduction method employing PA as both a reducing and stabilizing agent. Validation of the successful synthesis of PA@AuNPs was conducted utilizing a suite of analytical techniques including X-ray diffraction (XRD), ultraviolet–visible absorption spectroscopy (UV–Vis), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). Microscopic examination via field emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM) elucidated the spherical morphology of PA@AuNPs, with an average diameter ranging from 10 to 15 nm. Furthermore, atomic force microscopy (AFM) revealed an average surface roughness of 20.72 nm for PA@AuNPs. Thermal analysis results divulge that PA@AuNPs are highly stable up to 370 °C. The PA@AuNPs have shown more than 10-fold higher selectivity for LF in comparison with other commercially available antibiotics. The fluorescence emission intensity of PA@AuNPs (quantum yield (ΦF) 3.36 %) has shown enhanced redshift (436 → 497 nm; ∼61 nm) (λex 353 nm) upon the sequential addition of LF (0 → 100 µM) and achieved a lower detection limit (LoD = 36 nM; LoQ = 121 nM; Ka = 1.0457 × 104 M−1) with increased ΦF 18.96 %. This radiative process and fluorescence enhancement were confirmed by time-correlated single photon counting (TCSPC) measurement (4.89 → 5.36 ns). Interestingly, upon sensing LF, PA@AuNPs changed their morphology from spherical to octahedron and displayed an excimer formation through intermolecular π-π stacking. Furthermore, the stability of the complex, PA@AuNPs•LF which formed through coordination, by restoring the fluorescence intensity of PA@AuNPs•LF (turn on) in the presence of ethylenediamine tetraacetic acid (EDTA) (turn off) is demonstrated. The formation of the PA@AuNPs•LF complex was further confirmed by XRD analysis and FTIR spectroscopy. The HRTEM and FESEM validate that PA@AuNPs•LF have octahedral morphology with an average size of 20–30 nm, and the obtained moiré fringes in TEM images are due to the overlap of LF on the surface of PA@AuNPs. The AFM analysis divulges that PA@AuNPs•LF had a larger surface roughness (Sa = 26.91 nm). The experimentally attained zeta potential of PA@AuNPs (–12.6 mV) was further decreased (–17.6 mV) in the presence of LF, which confirms the interaction between PA@AuNPs and LF. The cell viability (82 %) in L929 Fibroblast cells augmented the biological applications of PA@AuNPs. The practicability of the fluorescent sensor probe was demonstrated in human urine samples with recovery ranges from 97.56 to 104.38 %.

Orange-Red-Emitting Carbon Dots for Bilirubin Detection and Its Antibacterial Activity Against Escherichia coli and Staphylococcus aureus.

In this study, orange-red-emitting carbon dots (OR-CDs) were prepared from p-phenylenediamine (p-PDA) and urea as starting precursors through the hydrothermal method. The OR-CDs exhibited bright orange-red fluorescence at 618 nm when excited at 480 nm. The obtained OR-CDs exhibited stable photophysical properties under different physiological conditions. The unique photophysical property of OR-CDs were then utilized for fluorometric determination of bilirubin. The fluorometric assay revealed that the fluorescence intensity of OR-CDs is gradually quenched upon the addition of bilirubin (1-20 μM). The mechanism of fluorescence quenching was evaluated by steady-state fluorescence analysis and time-correlated single photon counting measurements. The OR-CDs showed good selectivity and sensitivity toward bilirubin over other common interfering biomolecules. The present fluorometric assay showed a linear response toward bilirubin between 1 and 10 μM with a limit of detection of 4.80 nM. Further, a fluorescence test cotton swab-based detection probe has been successfully developed by incorporating OR-CDs for the point-of-care detection of bilirubin in biofluids. Furthermore, a light-emitting diode light that emits orange-red light was prepared by embedding the OR-CDs within the poly(vinyl alcohol) polymer matrix. Moreover, the antibacterial activity of OR-CDs was tested against Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus. The antibacterial efficacy of OR-CDs was demonstrated by various mechanisms, such as reactive oxygen species generation, destruction of cell structure, chemical binding to membrane, and surface wrapping. Interestingly, the survival assay against L929 fibroblast cells exhibits favorable biocompatibility and bioimaging.

Chiral-Induced Spin Selectivity Modulated Time-Correlated Single-Photon Counting for DNA Hybridization Detection.

The chiral-induced spin selectivity (CISS) effect can distinguish between the spin of electrons as they pass through chiral molecules by backscattering one of the spin components. Herein, we explore the role of the CISS effect in time-correlated single-photon counting measurements to detect DNA hybridization. We observe that the average lifetime of optical excited states of quantum dots attached to double-stranded DNA (dsDNA) varies with directions of the applied magnetic field. Specifically, the difference in the nonradiative average decay lifetime for the two orientations of the applied magnetic field is 2.21 ns in the case of hybridized strands, which is 130 times higher than that observed with quantum dots attached to single-strand DNA. Additionally, we investigate the application of Fourier transform infrared (FTIR) spectroscopy for detecting double-stranded DNA in the presence of a magnetic field, establishing a theoretical framework to substantiate the experimental evidence of magnetic field-dependent FTIR spectroscopy for dsDNA.

Characterization of the Reversible Intersystem Crossing Dynamics of Organic Photocatalysts Using Transient Absorption Spectroscopy and Time-Resolved Fluorescence Spectroscopy.

Thermally activated delayed fluorescence (TADF) emitters are molecules of interest as homogeneous organic photocatalysts (OPCs) for photoredox chemistry. Here, three classes of OPC candidates are studied in dichloromethane (DCM) or N,N-dimethylformamide (DMF) solutions, using transient absorption spectroscopy and time-resolved fluorescence spectroscopy. These OPCs are benzophenones with either carbazole (2Cz-BP and 2tCz-BP) or phenoxazine/phenothiazine (2PXZ-BP and 2PTZ-BP) appended groups and the dicyanobenzene derivative 4DP-IPN. Dual lifetimes of the S1 state populations are observed, consistent with reverse intersystem crossing (RISC) and TADF emission. Example fluorescence lifetimes in DCM are (5.18 ± 0.01) ns and (6.22 ± 1.27) μs for 2Cz-BP, (1.38 ± 0.01) ns and (0.32 ± 0.01) μs for 2PXZ-BP, and (2.97 ± 0.01) ns and (62.0 ± 5.8) μs for 4DP-IPN. From ground state bleach recoveries and time-correlated single photon counting measurements, triplet quantum yields in DCM are estimated to be 0.62 ± 0.16, 0.04 ± 0.01, and 0.83 ± 0.02 for 2Cz-BP, 2PXZ-BP, and 4DP-IPN, respectively. 4DP-IPN displays similar photophysical behavior to the previously studied OPC 4Cz-IPN. Independent of the choice of solvent, 4DP-IPN, 2Cz-BP, and 2tCz-BP are shown to be TADF emitters, whereas emission by 2PXZ-BP and 2PTZ-BP depends on the molecular environment, with TADF emission enhanced in aggregates compared to monomers. Behavior of this type is representative of aggregation-induced emission luminogens (AIEgens).

Linker-Assisted Assembly of Ligand-Bridged CdS/MoS2 Heterostructures: Tunable Light-Harvesting Properties and Ligand-Dependent Control of Charge-Transfer Dynamics and Photocatalytic Hydrogen Evolution.

We used linker-assisted assembly (LAA) to tether CdS quantum dots (QDs) to MoS2 nanosheets via L-cysteine (cys) or mercaptoalkanoic acids (MAAs) of varying lengths, yielding ligand-bridged CdS/MoS2 heterostructures for redox photocatalysis. LAA afforded precise control over the light-harvesting properties of QDs within heterostructures. Photoexcited CdS QDs transferred electrons to molecularly linked MoS2 nanosheets from both band-edge and trap states; the electron-transfer dynamics was tunable with the properties of bridging ligands. Rate constants of electron transfer, estimated from time-correlated single photon counting (TCSPC) measurements, ranged from (9.8 ± 3.8) × 106 s-1 for the extraction of electrons from trap states within heterostructures incorporating the longest MAAs to >5 × 109 s-1 for the extraction of electrons from band-edge or trap states in heterostructures with cys or 3-mercaptopropionic acid (3MPA) linkers. Ultrafast transient absorption measurements revealed that electrons were transferred within 0.5-2 ps or less for CdS-cys-MoS2 and CdS-3MPA-MoS2 heterostructures, corresponding to rate constants ≥5 × 109 s-1. Photoinduced CdS-to-MoS2 electron transfer could be exploited in photocatalytic hydrogen evolution reaction (HER) via the reduction of H+ to H2 in concert with the oxidation of lactic acid. CdS-L-MoS2-functionalized FTO electrodes promoted HER under oxidative conditions wherein H2 was evolved at a Pt counter electrode with Faradaic efficiencies of 90% or higher and under reductive conditions wherein H2 was evolved at the CdS-L-MoS2-heterostructure-functionalized working electrode with Faradaic efficiencies of 25-40%. Dispersed CdS-L-MoS2 heterostructures promoted photocatalytic HER (15.1 μmol h-1) under white-light illumination, whereas free cys-capped CdS QDs produced threefold less H2 and unfunctionalized MoS2 nanosheets produced no measurable H2. Charge separation across the CdS/MoS2 interface is thus pivotal for redox photocatalysis. Our results reveal that LAA affords tunability of the properties of constituent CdS QDs and MoS2 nanosheets and precise, programmable, ligand-dependent control over the assembly, interfacial structure, charge-transfer dynamics, and photocatalytic reactivity of CdS-L-MoS2 heterostructures.

Machine Learning-Directed Predictive Models: Deciphering Complex Energy Transfer in Mn-Doped CsPb(Cl1–yBry)3 Perovskite Nanocrystals

Lead halide perovskite nanocrystals with inclusion of a transition-metal dopant of Mn2+ offer a substantial degree of freedom to modulate the optoelectronic and magnetic properties owing to the introduced dopant in the host lattices. However, complexity as a result of the excited interactions between the exciton and dopant, involving dynamics of exciton recombination, competing forward and backward energy transfer (and vice versa), and Mn recombination, makes it difficult to understand and predict the Mn sensitization. Here, we have created machine learning-directed models using different nonlinear algorithms with initial 86 samples to decipher the complex energy transfer by navigating the reaction design space of various concentrations of Mn along with different halide compositions (band gap) in Mn-doped CsPb(Cl1–yBry)3 nanocrystals. K-nearest neighbor-based predictive models coupled with time-correlated single photon counting measurements allow for fully elucidating the complex and competing energy transfer kinetics occurring in two different Mn concentration regimes. Importantly, forward exciton-to-Mn energy transfer is more governed by the Mn concentration, while the backward Mn-to-exciton energy transfer is strongly dependent on the energy gap difference between the exciton and Mn energy state. This machine learning-guided approach and modeling can not only provide an efficient means for navigating the vast reaction design space but also provide significant insight into understanding and elucidating the complex physical phenomena throughout analyzing and predicting the dataset trend.

Orange Pomace-Derived Fluorescent Carbon Quantum Dots: Detection of Dual Analytes in the Nanomolar Range.

Green-emissive carbon quantum dots (CQDs) with exclusive chemosensing aspects were synthesized from orange pomace as a biomass-based precursor via a facile microwave method without using any chemicals. The synthesis of highly fluorescent CQDs with inherent nitrogen was confirmed through X-ray diffraction, X-ray photoelectron, Fourier transform infrared, Raman, and transmission electron microscopic techniques. The average size of the synthesized CQDs was found to be 7.5 nm. These fabricated CQDs displayed excellent photostability, water solubility, and outstanding fluorescent quantum yield, i.e., 54.26%. The synthesized CQDs showed promising results for the detection of Cr6+ ions and 4-nitrophenol (4-NP). The sensitivity of CQDs toward Cr6+ and 4-NP was found up to the nanomolar range with the limit of detection values of 59.6 and 14 nM, respectively. Several analytical performances were thoroughly studied for high precision of dual analytes of the proposed nanosensor. Various photophysical parameters of CQDs (quenching efficiency, binding constant, etc.) were analyzed in the presence of dual analytes to gain more insights into the sensing mechanism. The synthesized CQDs exhibited fluorescence quenching toward incrementing the quencher concentration, which was rationalized by the inner filter effect through time-correlated single-photon counting measurements. The CQDs fabricated in the current work exhibited a lower detection limit and a wide linear range through the simple, eco-friendly, and rapid detection of Cr6+ and 4-NP ions. To evaluate the feasibility of the detection approach, real sample analysis was conducted, demonstrating satisfactory recovery rates and relative standard deviations toward the developed probes. This research paves the way for developing CQDs with superior characteristics utilizing orange pomace (biowaste precursor).

Axial vs equatorial: Capturing the intramolecular charge transfer state geometry in conformational polymorphic crystals of a donor-bridge-acceptor dyad in nanosecond-time-scale.

Two conformational polymorphs of a donor-bridge-acceptor (D-B-A) dyad, p-(CH3)2N-C6H4-(CH2)2-(1-pyrenyl)/PyCHDMA, were studied, where the electron donor (D) moiety p-(CH3)2N-C6H4/DMA is connected through a bridging group (B), -CH2-CH2-, to the electron acceptor (A) moiety pyrene. Though molecular dyads like PyCHDMA have the potential to change solar energy into electrical current through the process of photoinduced intramolecular charge transfer (ICT), the major challenge is the real-time investigation of the photoinduced ICT process in crystals, necessary to design solid-state optoelectronic materials. The time-correlated single photon counting (TCSPC) measurements with the single crystals showed that the ICT state lifetime of the thermodynamic form, PyCHDMA1 (pyrene and DMA: axial), is ∼3ns, whereas, for the kinetic form, PyCHDMA20 (pyrene and DMA: equatorial), it is ∼7ns, while photoexcited with 375nm radiation. The polymorphic crystals were photo-excited and subsequently probed with a pink Laue x-ray beam in time-resolved x-ray diffraction (TRXRD) measurements. The TRXRD results suggest that in the ICT state, due to electron transfer from the tertiary N-atom in DMA moiety to the bridging group and pyrene moiety, a decreased repulsion between the lone-pair and the bond-pair at N-atom induces planarity in the C-N-(CH3)2 moiety, in both polymorphs. The Natural Bond Orbital calculations and partial atomic charge analysis by Hirshfeld partitioning also corroborated the same. Although the interfragment charge transfer (IFCT) analysis using the TDDFT results showed that for the charge transfer excitation in both conformers, the electrons were transferred from the DMA moiety to mostly the pyrene moiety, the bridging group has little role to play in that.

Evaluating strain and doping of Janus MoSSe from phonon mode shifts supported by ab initio DFT calculations.

With the study of Janus monolayer transition metal dichalcogenides, in which one of the two chalcogen layers is replaced by another type of chalcogen atom, research on two-dimensional materials is advancing into new areas. Yet only little is known about this new kind of material class, mainly due to the difficult synthesis. In this work, we synthesize MoSSe monolayers from exfoliated samples and compare their Raman signatures with density functional theory calculations of phonon modes that depend in a nontrivial way on doping and strain. With this as a tool, we can infer limits for the possible combinations of strain and doping levels. This reference data can be applied to all MoSSe Janus samples in order to quickly estimate their strain and doping, providing a reliable tool for future work. In order to narrow down the results for our samples further, we analyze the temperature-dependent photoluminescence spectra and time-correlated single-photon counting measurements. The lifetime of Janus MoSSe monolayers exhibits two decay processes with an average total lifetime of 1.57 ns. Moreover, we find a strong trion contribution to the photoluminescence spectra at low temperature which we attribute to excess charge carriers, corroborating our ab initio calculations.

Selective Luminescence Turn-On-Based Sensing of Phosphate in the Presence of Other Interfering Anions Using a Heterobimetallic (3d-4d) MOF with an Acidic Pocket.

A luminescent metal-organic framework with the molecular formula [YMn1.5(C7N1H3O5)3(H2O)6]·11H2O, 1 {where C7N1H3O5 = chelidamate}, was synthesized by a hydrothermal method by employing chelidamic acid as an organic ligand and Y(III) and Mn(II) as metal ions. A two-dimensional heterobimetallic structure with phenolic hydroxyl-functionalized pockets was revealed by single-crystal X-ray diffraction analysis of compound 1. PXRD, TGA, IR, BET analysis, and UV-vis spectroscopy were used for the thorough characterization of compound 1. Upon excitation at 280 nm, compound 1 shows bright blue emission, which was utilized for the selective and sensitive turn-on detection of the PO43- ion. Based on Bronsted-Lowry acid-base interactions, the photoluminescence of compound 1 was enhanced in the presence of very low concentrations of the aforementioned anion. The mechanism behind the detection of the phosphate ion has been explored in detail. It was seen that the PO43- anion entered the hydroxyl-functionalized pockets of compound 1 and stabilized the aromatic portion of compound 1 via molecular-level interactions through acid-base interactions. These molecular-level interactions are responsible for the enhancement of the photoluminescence intensity of compound 1 after the incorporation of phosphate ions by reducing the nonradiative transitions. These phenomena were also confirmed by time-correlated single photon counting (TCSPC) measurement, which shows that the excited-state lifetime increased with the increase in addition of phosphate anions. The calculated limit of detection (LOD) of 1 was 19.55 ppb for phosphate (PO43-), which was significantly lesser than the recommended level for the PO43-anion toward the human body. The luminescence enhancement coefficient, KSV, value was also much higher than those of other reported metal-organic frameworks.

Ultrafast transient absorption and solvation of a super-photoacid in acetoneous environments.

The phenomenon of photoacidity, i.e., an increase in acidity by several orders of magnitude upon electronic excitation, is frequently encountered in aromatic alcohols capable of transferring a proton to a suitable acceptor. A promising new class of neutral super-photoacids based on pyranine derivatives has been shown to exhibit pronounced solvatochromic effects. To disclose the underlying mechanisms contributing to excited-state proton transfer (ESPT) and the temporal characteristics of solvation and ESPT, we scrutinize the associated ultrafast dynamics of the strongest photoacid of this class, namely tris(1,1,1,3,3,3-hexafluoropropan-2-yl)8-hydroxypyrene-1,3,6-trisulfonate, in acetoneous environment, thereby finding experimental evidence for ESPT even under these adverse conditions for proton transfer. Juxtaposing results from time-correlated single-photon counting and femtosecond transient absorption measurements combined with a complete decomposition of all signal components, i.e., absorption of ground and excited states as well as stimulated emission, we disclose dynamics of solvation, rotational diffusion, and radiative relaxation processes in acetone and identify the relevant steps of ESPT along with the associated time scales.

Controlling the charge carrier dynamics by modulating the orientation diversity of perovskites

Photo-induced charge carrier dynamics over a wide time domain from sub-nanoseconds to microseconds, via time-correlated single photon counting and flash photolysis time-resolved microwave conductivity measurements.

Temperature-dependent excited state lifetimes of nitrogen vacancy centers in individual nanodiamonds

Nitrogen vacancy (NV) centers are fluorescent defects widely employed for thermometry, most commonly via temperature-dependent shifts of their optically detected magnetic resonance. Recently, all-optical approaches based on temperature-dependent features of the NV center fluorescence spectrum have also gained traction. Excited state lifetime thermometry is an all-optical technique that has been implemented using other fluorophores but has not previously been demonstrated for NV centers in individual nanodiamonds (NDs). Here, we report temperature-dependent excited state lifetime measurements of NV centers in individual NDs between 300 K and 500 K. We measure a 32 ± 7.0% and 35 ± 8.3% average decrease in the excited state lifetimes of individual NDs on silicon and glass substrates, respectively, over this temperature range. A linear approximation applicable to nearly all measured NDs yields temperature coefficients of −2000 ± 240 ppm/K and −2600 ± 280 ppm/K for NDs on silicon and glass, respectively. In addition to all-optical operation, single-ND excited state lifetime thermometry offers ∼100 ns temporal resolution and utilizes time-correlated single photon counting measurements ideally suited to low emission intensities, a limiting factor for other NV center thermometry techniques above 700 K. We demonstrate that atomic force microscope nanomanipulation can position individual NDs at critical locations on a sample of interest, enabling single-point temperature measurements that combine ∼100 ns temporal resolution and ∼100 nm spatial resolution. This work also has broader implications for other single-ND excited state lifetime sensing applications, where care is required to avoid conflating changes in temperature and other environmental parameters.

Hydrophobic and Hydrophilic Conjugated Polymer Dots as Binary Photocatalysts for Enhanced Visible-Light-Driven Hydrogen Evolution through Förster Resonance Energy Transfer.

Organic semiconducting polymers exhibited promising photocatalytic behavior for hydrogen (H2) evolution, especially when prepared in the form of polymer dots (Pdots). However, the Pdot structures were formed using common nonconjugated amphiphilic polymers, which have a negative effect on charge transfer between photocatalysts and reactants and are unable to participate in the photocatalytic reaction. This study presents a new strategy for constructing binary Pdot photocatalysts by replacing the nonconjugated amphiphilic polymer typically employed in the preparation of polymer nanoparticles (Pdots) with a low-molecular-weight conjugated polyelectrolyte. The as-prepared polyelectrolyte/hydrophobic polymer-based binary Pdots truly enhance the electron transfer between the Pt cocatalyst and the polymer photocatalyst with good water dispersibility. Moreover, unlike the nonconjugated amphiphilic polymer, the photophysics and mechanism of this photocatalytic system through time-correlated single-photon counting (TCSPC) and transient absorption (TA) measurements confirmed the Förster resonance energy transfer (FRET) between the polyelectrolyte as a donor and the hydrophobic polymer as an acceptor. As a result, the designated binary Pdot photocatalysts significantly enhanced the hydrogen evolution rate (HER) of 43 900 μmol g-1 h-1 (63.5 μmol h-1, at 420 nm) for PTTPA/PFTBTA Pdots under visible-light irradiation.

Electronic Relaxation Dynamics in 2-Quinolinones with Extended Conjugation.

The 2-quinolinone family of molecules, also known as carbostyrils, have been proposed as light absorbing donor molecules in energy transfer based sensing schemes and as possible photocatalysts. Both of these applications make use of electronic excited states, but the photophysics of 2-quinolinones have not yet been examined closely. This study applies static and dynamic spectroscopy, with supporting density functional theory calculations, to reveal the electronic relaxation dynamics of a family of five 2-quinolinones with extended conjugated rings. These modifications lead to red-shifted absorbance and emission maxima, relative to unmodified 2-quinolinone. Optical excitation of these molecules with near UV light resulted in transitions with strong π → π* and HOMO → LUMO character. Time-correlated single photon counting measurements yielded fluorescence lifetimes ranging from 849.3 (±0.6) ps to 4.586 (±0.002) ns. Transient absorption spectroscopy revealed relaxation dynamics of the S1 excited state formed by photoexcitation at 350 nm, along with formation of a long-lived signal assigned as excited state absorption by a triplet excited state. Vibrational relaxation in the S1 state was also characterized in some compounds. Overlapping signals of S1 decay and triplet growth in the transient absorption data set could not be fully disentangled. These results demonstrate a highly competitive relaxation scheme following multiple simultaneous pathways, a promising situation for establishing chemical control of electronic relaxation in the 2-quinolinone family.

Mechanistic Investigation of Molybdenum Disulfide Defect Photoluminescence Quenching by Adsorbed Metallophthalocyanines.

Lattice defects play an important role in determining the optical and electrical properties of monolayer semiconductors such as MoS2. Although the structures of various defects in monolayer MoS2 are well studied, little is known about the nature of the fluorescent defect species and their interaction with molecular adsorbates. In this study, the quenching of the low-temperature defect photoluminescence (PL) in MoS2 is investigated following the deposition of metallophthalocyanines (MPcs). The quenching is found to significantly depend on the identity of the phthalocyanine metal, with the quenching efficiency decreasing in the order CoPc > CuPc > ZnPc, and almost no quenching by metal-free H2Pc is observed. Time-correlated single photon counting (TCSPC) measurements corroborate the observed trend, indicating a decrease in the defect PL lifetime upon MPc adsorption, and the gate voltage-dependent PL reveals the suppression of the defect emission even at large Fermi level shifts. Density functional theory modeling argues that the MPc complexes stabilize dark negatively charged defects over luminescent neutral defects through an electrostatic local gating effect. These results demonstrate the control of defect-based excited-state decay pathways via molecular electronic structure tuning, which has broad implications for the design of mixed-dimensional optoelectronic devices.

Singlet Fission in Concentrated TIPS-Pentacene Solutions: The Role of Excimers and Aggregates.

The excited-state dynamics of 6,13-bis(triisopropylsilylethynyl)pentacene is investigated to determine the role of excimer and aggregate formation in singlet fission in high-concentration solutions. Photoluminescence spectra were measured by excitation with the evanescent wave in total internal reflection, in order to avoid reabsorption effects. The spectra over nearly two magnitudes of concentration were nearly identical, with no evidence for excimer emission. Time-correlated single-photon counting measurements confirm that the fluorescence lifetime shortens with concentration. The observed rate constant grows at high concentrations, and this effect is modeled in terms of the hard-sphere radial distribution function. NMR measurements confirm that aggregation takes place with a binding constant of between 0.14 and 0.43 M-1. Transient absorption measurements are consistent with a diffusive encounter mechanism for singlet fission, with hints of more rapid singlet fission in aggregates at the highest concentration measured. These data show that excimers do not play the role of an emissive intermediate in exothermic singlet fission in solution and that, while aggregation occurs at higher concentrations, the mechanism of singlet fission remains dominated by diffusive encounters.

Mixed bismuth‐antimony‐based double perovskite nanocrystals for solar cell application

Perovskite materials are third-generation optoelectronic materials and require a lot of research work to improve their stability. Recently, lead-free double perovskite materials have attracted significant attention due to their environmental friendliness, stability, and tunable optoelectronic properties. However, a limited number of double perovskite materials have been reported for optoelectronics applications. Therefore an effort is needed to explore more lead-free double perovskites and alter the properties for their further applications. In this manuscript, we report lead-free bismuth and bismuth/antimony-based perovskite materials [Cs2AgBiBr6, Cs2AgBi0.6Sb0.4Br6, and Cs2AgBi0.6Sb0.4(Br0.278I0.722)6] nanocrystals (NCs) synthesized using the hot injection method and benzoyl bromide has been used as a precursor and trimethyl-silyl iodide (TMSI) as the iodine source for anion-exchange (from Br to Br/I). The formation of NC has been confirmed using X-ray diffraction and X-ray fluorescence spectrometer studies for phase and elemental analysis of synthesized NCs. The transmission electron microscopy of Cs2AgBiBr6 and Cs2AgBi0.6Sb0.4Br6-based perovskite revealed cubic-shaped NCs with size 15.0 ± 4.5 nm and 14.0 ± 5.3 nm, respectively, which changed to hexagonal (Cs2AgBi0.6Sb0.4(Br0.278 I0.722)6 NCs) shaped NCs with size 30 to 70 nm (after anion exchange). The successful bismuth replacement by bismuth/antimony is carried out. The time-correlated single-photon counting measurements on Cs2AgBi0.6Sb0.4Br6 NCs is carried out. The calculated average lifetimes of toluene (T)-Cs2AgBi0.6Sb0.4Br6 and dichlorobenzene D- Cs2AgBi0.6Sb0.4Br6 NCs are 4.70 ns and 8.67 ns, respectively. After the anion exchange, the average lifetime value for T-Cs2AgBi0.6Sb0.4(Br0.278I0.722)6 and D- Cs2AgBi0.6Sb0.4(Br0.278 I0.722)6 NCs are 8.73 and 9.78 ns, respectively. Additionally, the effect of light on the current density-voltage characteristics of the Cs2AgBi0.6Sb0.4Br6 NCs device has been studied to ascertain its utility in solar cells. The device is fabricated using nickel oxide and PCBM as interface layers with power conversion efficiency 0.09%.

Optical Properties of Chiral Azo-Schiff Base Mn(II) and Zn(II) Complexes with Silver Nanoparticles

Herein we have originally designed chiral azo-salen Mn(II) and Zn(II) complexes for interacting silver nanoparticles (AgNPs) exhibiting localized surface plasmon resonance (LSPR). Understanding excited state and reaction intermediate during light irradiation to return to ground state may be important for such composite systems. Therefore, we investigated such optical properties for systems using time-resolved luminescence and transient absorption measurements. DMSO solutions of the four newly prepared and characterized complexes (MMn, MZn, CMn, and CZn) and ethanol solutions of the composite materials of each complex with AgNPs were served for optical measurements. The time-correlated single photon counting (TCSPC), the streak camera which is much shorter period of time than TCSPC and transient absorption measurement, was performed for the eight samples. The fluorescence lifetime of the sole complexes and the composite materials with AgNPs was derived from curve-fitting analysis of luminescence decay curves of TCSPC. Lifetime of the composite systems with AgNPs was longer than that of the corresponding sole metal complexes for three cases. It was revealed that composite systems may go through three reaction intermediates during relaxation from excited state to ground state.

Critical Role of Protons for Emission Quenching of Indoline Dyes in Solution and on Semiconductor Surfaces.

By combining time-correlated single photon counting (TCSPC) measurements, density functional theory (DFT), and time-dependent DFT (TD-DFT) calculations, we herein investigate the role of protons, in solutions and on semiconductor surfaces, for the emission quenching of indoline dyes. We show that the rhodanine acceptor moieties, and in particular the carbonyl oxygens, undergo protonation, leading to nonradiative excited-state deactivation. The presence of the carboxylic acid anchoring group, close to the rhodanine moiety, further facilitates the emission quenching, by establishing stable H-bond complexes with carboxylic acid quenchers, with high association constants, in both ground and excited states. This complexation favors the proton transfer process, at a low quencher concentration, in two ways: bringing close to the rhodanine unit the quencher and assisting the proton release from the acid by a partial-concerted proton donation from the close-by carboxylic group to the deprotonated acid. Esterification of the carboxylic group, indeed, inhibits the ground-state complex formation with carboxylic acids and thus the quenching at a low quencher concentration. However, the rhodanine moiety in the ester form can still be the source of emission quenching through dynamic quenching mechanism with higher concentrations of protic solvents or carboxylic acids. Investigating this quenching process on mesoporous ZrO2, for solar cell applications, also reveals the sensitivity of the adsorbed excited rhodanine dyes toward adsorbed protons on surfaces. This has been confirmed by using an organic base to remove surface protons and utilizing cynao-acrylic dye as a reference dye. Our study highlights the impact of selecting such acceptor group in the structural design of organic dyes for solar cell applications and the overlooked role of protons to quench the excited state for such chemical structures.