Radiative Excited State Research Articles

Dimerization extends π‐conjugation of electron donor‐acceptor structures leading to phototheranostic properties beyond the sum of two monomers

AbstractNear‐infrared (NIR)‐II fluorescence imaging‐guided photothermal therapy (PTT) has attracted great research interest, and constructing donor‐acceptor (D‐A) electronic configurations has become an established approach to lower bandgap and realize NIR‐II emission. However, very few π‐conjugated phototheranostic agents can realize efficient NIR‐II guided PTT using a clinically safe laser power density, implying that sufficient photothermal performance is still desired. In addition to the continuously refreshed photothermal conversion efficiency levels, the strategies that focus on enhancing light absorptivity have been rarely discussed and endow a new direction for enhancing PTT. Herein, a dimerization π‐extension strategy is raised to synthesize π‐conjugated dimers with A‐D‐A monomers. We observe that the light absorptivity (ε) of the dimers is strengthened three times owing to the enhanced electronic coupling effect as a result of the π‐conjugation extension, thereby surpassing the 2‐fold increase in chromophore numbers from the monomer to dimers. Thanks to the enhancement in light absorption, the dimers could generate much more photothermal heat than the monomer in in vivo PTT treatments. Therefore, an efficient anti‐tumor outcome has been fulfilled by using dimers under a low laser power (0.3 W/cm2). Moreover, the dimers with extended π‐conjugation structures become more favorable to the radiative excited state decay, thus exhibiting a distinguishing improvement in NIR‐II imaging compared with monomer. Collectively, due to the improved light absorptivity, the dimers can gain superior NIR‐II fluorescence brightness and photothermal performance over the recently reported material, which goes beyond the monomer in double doses for in vivo applications. All these results prove that dimerization is an effective strategy for designing high‐performance phototheranostic materials.

Pt(II) Complexes with a Novel Pincer N^C^N Ligand: Synthesis, Characterization, and Photophysics

A series of new platinum square planar complexes [Pt(NCN)L]+/0 with the pincer N^C^N cyclometallated ligand (NC(H)N = 1,3-bis(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)benzene) containing the following L: Cl−, acetonitrile, pyridine, dimethylaminopyridine, 2,6-dimethylphenylisocyanide, has been synthesized. Application of bridging acetate ion as L ligand allowed obtaining a binuclear [Pt(NCN)]2OOCCH3 complex. The bulky and rigid structure of N^C^N-ligand provokes instability of its pincer coordination that makes possible transformation of the molecular architecture to give a heteronuclear complex with the Pt-Ag-Pt coordination core. The composition and structure of the obtained compounds were characterized in solution and in the solid state using ESI mass-spectrometry, NMR spectroscopy, elemental analysis, and single-crystal XRD crystallography. The complexes luminesce in solid state, solution, and in polymeric matrix demonstrating moderate to bright emission at ca. 550 nm with quantum yields up to 22% and lifetime of excited state up to 22 µs. TD DFT computational approach together with analysis of the photophysical properties in different media reveals the predominant ligand-centered 3IL nature of the radiative excited state localized at the N^C^N-ligand. The ancillary ligand L demonstrates a minor influence on the energy of emission but affects dramatically emission efficiency and lifetime. The chloride complex displays dual (fluorescence and phosphorescent) luminescence due to labile coordination of an N-coordinated functionality that produces a dangling aromatic fragment, which gives emission from a singlet excited state.

TADF and X-ray Radioluminescence of New Cu(I) Halide Complexes: Different Halide Effects on These Processes.

A series of complexes [Cu2X2(Pic3PO)2] (X = Cl, Br, I) based on tris(pyridin-2-ylmethyl)phosphine oxide (Pic3PO) has been synthesized. At 298 K, these compounds exhibit thermally activated delayed fluorescence (TADF) of 1(M+X)LCT type with λmax varying from 485 to 545 nm, and quantum efficiency up to 54%. In the TADF process, the halide effect appears as the emission intensification and bathochromic shift of λmax in the following order X = I < Br < Cl. Upon X-ray irradiation, the title compounds emit radioluminescence, the emission bands of which have the same shape as those at TADF, thereby meaning a similar radiative excited state. By contrast to TADF, the halide effect in the radioluminescence is reversed: its intensity grows in the order X = Cl < Br < I, since heavier atoms absorb X-rays more efficiently. These findings essentially contribute to our knowledge about the halide effect in the photo- and radioluminescent Cu(I) halide emitters.

Ag-Doped ZnInS/ZnS Core/Shell Quantum Dots for Display Applications

Development of nontoxic quantum dots (QDs) exhibiting superior photoluminescence properties is a major challenge in the commercialization of QDs-based solid-state lighting or display device applications. This work reports the synthesis of heavy-metal-free Ag:Zn–In–S/ZnS (Ag:ZIS/ZS) core/shell QDs via the heating-up method followed by a nonaqueous hot-injection route. Formation of the QDs, their growth, and structural qualities were studied by high-resolution transmission electron microscopy and XRD analysis. The optical properties of these QDs were investigated through absorption, photoluminescence, and time-resolved fluorescence decay spectroscopic measurements. Surface passivation of the core Ag:ZIS QDs was done by coating ZnS layers on them. The successful doping of Ag+ into the ZnInS host material changed the emission characteristics; the influence of Ag loading and the shell thickness on the optical features were studied. The shell thickness had a strong impact on the photophysics of the nanocrystal with enhanced radiative lifetime. By regulating the Ag concentration, the Ag:ZIS core QDs showed a wide ranging tunable emission from 392 to 547 nm with a prominent blue shift with the introduction of ZnS shell layers. The blue shift in the band gap energy and emission wavelength has been reported to be due to the cation-exchange process of Ag:ZIS/ZS core/shell QDs. The radiative excited state lifetime is prolonged with subsequent injection of shells with the highest reported value being 504.72 ns. These nontoxic core/shell QDs, endowed with notable spectral emission within the visible spectrum, can be used as a down-converting material by tailoring the dopant concentration and shell precursor injection. Furthermore, a nanocomposite film of PMMA–QDs was synthesized by the drop-casting method, and its overall luminous efficacy complements that of parental core/shell QDs without quenching. The transmittance plot delineates that the optical transparency of the film can be engineered over the entire visible spectrum by varying the dopant (Ag) concentration.

Manipulating exciton dynamics of thermally activated delayed fluorescence materials for tuning two-photon nanotheranostics.

Rational manipulation of energy utilization from excited-state radiation of theranostic agents with a donor–acceptor structure is relatively unexplored. Herein, we present an effective strategy to tune the exciton dynamics of radiative excited state decay for augmenting two-photon nanotheranostics. As a proof of concept, two thermally activated delayed fluorescence (TADF) molecules with different electron-donating segments are engineered, which possess donor–acceptor structures and strong emissions in the deep-red region with aggregation-induced emission characteristics. Molecular simulations demonstrate that change of the electron-donating sections could effectively regulate the singlet–triplet energy gap and oscillator strength, which promises efficient energy flow. A two-photon laser with great permeability is used to excite TADF NPs to perform as theranostic agents with singlet oxygen generation and fluorescence imaging. These unique performances enable the proposed TADF emitters to exhibit tailored balances between two-photon singlet oxygen generation and fluorescence emission. This result demonstrates that TADF emitters can be rationally designed as superior candidates for nanotheranostic agents by the custom controlling exciton dynamics.

Recent progress of phosphine electroluminescent materials and devices

Organic light emitting diodes (OLEDs) have been attracting much attention and have been studied extensively by scientists and engineers in both academic and industrial communities, due to their great advantages of self-emitting, fast response, wide view-angle, full color capacity, high efficiency, low power consumption, ultra-thinness, lightness and flexibility for the applications of new generation flat panel displays and solid-state lighting sources. With the full use of active-matrix OLED (AMOLED) screen in iPhone X, OLED technology has become the mainstream information display technology and has gained more than 40% of market share. In the era of information, liquid crystal display technology will be gradually replaced by OLED technology. However, OLEDs technology still faces many challenges, including low yield, high cost, short service life, environmentally unfriendly and so on, which urgently requires more in-depth and detailed research in material development and device design. Since the first OLED reported by CW Tang, OLED technology has developed from the first generation of fluorescence (FL) technology, the second generation of phosphorescence (PH) technology to the third generation of thermally activated delayed fluorescence (TADF) technology. In general, PH materials are heavy metal complexes, in which triplet excited state is the main radiative excited state. Since singlet exciton can translate into triplet exciton through intersystem crossing (ISC) process, therefore, in theory, 100% electrogenerated excitons can be used in phosphorescence materials. Compared with the first generation of fluorescence materials, which can only use 25% single state electrogenerated excitons, the electro-optic conversion efficiency is greatly improved in phosphorescence materials. However, heavy metal ions in PH materials not only increase the cost, but also bring some potential environmental pollution. TADF technology can realize the transformation of non-radiative triplet excitons into radiative singlet excitons through reverse ISC (RISC) process, and thus achieving 100% exciton utilization. TADF material is usually pure organic molecule based on a push-pull electron system, thus making up for the defects of phosphorescent material. Therefore, as a kind of molecular electroluminescent devices, innovation of material system is the foundation for the development of OLED technology. With the insulating effect, moderate electronic effect, large steric hindrance, multi-functionalizability and coordination effect, aryl phosphine (oxide) group is one of the few groups with multiple functions. Through comprehensively utilizing these functions, aryl phosphine (oxide) group reveal the unique advantage in selectively optimizing molecular optoelectronic properties, further realizing high-efficiency OLED, and providing a platform for selective investigation of the influence of single property variation on optoelectronic performance. Recently, phosphine optoelectronic materials have been gradually developed as one of the focuses in OLED field, whose excellent performance and clear structure-property relationship reveal the significant theoretical and application values regarding to enriching OLED material, suggesting the methods in material design and optimization and promoting the OLED technology innovation. Based on material design strategy, we developed phosphine host materials, emitters and electron transporting materials and deeply investigated the photophysical properties, excited-state characteristics, electrical properties and OLED performance. In this paper, the main works are summarized, with the expectation to provide reference for the subsequent study on phosphine optoelectronic materials.

New insights on ChlD1 function in Photosystem II from site-directed mutants of D1/T179 in Thermosynechococcus elongatus

The monomeric chlorophyll, ChlD1, which is located between the PD1PD2 chlorophyll pair and the pheophytin, PheoD1, is the longest wavelength chlorophyll in the heart of Photosystem II and is thought to be the primary electron donor. Its central Mg2+ is liganded to a water molecule that is H-bonded to D1/T179. Here, two site-directed mutants, D1/T179H and D1/T179V, were made in the thermophilic cyanobacterium, Thermosynechococcus elongatus, and characterized by a range of biophysical techniques. The Mn4CaO5 cluster in the water-splitting site is fully active in both mutants. Changes in thermoluminescence indicate that i) radiative recombination occurs via the repopulation of *ChlD1 itself; ii) non-radiative charge recombination reactions appeared to be faster in the T179H-PSII; and iii) the properties of PD1PD2 were unaffected by this mutation, and consequently iv) the immediate precursor state of the radiative excited state is the ChlD1+PheoD1− radical pair. Chlorophyll bleaching due to high intensity illumination correlated with the amount of 1O2 generated. Comparison of the bleaching spectra with the electrochromic shifts attributed to ChlD1 upon QA− formation, indicates that in the T179H-PSII and in the WT*3-PSII, the ChlD1 itself is the chlorophyll that is first damaged by 1O2, whereas in the T179V-PSII a more red chlorophyll is damaged, the identity of which is discussed. Thus, ChlD1 appears to be one of the primary damage site in recombination-mediated photoinhibition. Finally, changes in the absorption of ChlD1 very likely contribute to the well-known electrochromic shifts observed at ~430 nm during the S-state cycle.

Colloidal Gradated Alloyed (Cu)ZnInS/ZnS Core/Shell Nanocrystals with Tunable Optical Properties for Live Cell Optical Imaging

AbstractIn this work, Cd‐free, oleophilic (Cu)ZnInS(core)−ZnS(shell) core/shell quantum dots (QDs) with varying Cu content in the core and shell thickness were synthesized by heating‐up method (core) followed by hot‐injection route (shell). Effect of Cu ion doping (in core) and shell growth on the structural and optical properties of (Cu)ZnInS−ZnS core/shell QDs is reported. Incorporation of Cu ions causes the crystalline phase transformation of zinc blende ZnInS ternary alloy to wurtzite Cu−Zn−In−S quaternary alloy. The (Cu)ZnInS alloyed core QDs are encapsulated with a wider bandgap ZnS shell with gradient interface. The photoluminescence of (Cu)ZnInS core QDs exhibits wide tunable multicolor emissions from green (535 nm) to deep red (683 nm) with large Stokes shift. After the growth of ZnS layer, the PL peak position and absorption edge undergo a hypsochromic shift with enhanced photoluminescence quantum yield up to 47.31%, which is due to the partial cation exchange of Cu+ and In3+ with Zn2+, yielding a graded core/shell structure with smooth potential interface. The radiative excited state lifetime is lengthened from 22.91 ns to 291.11 ns. The mercaptoundecanoic acid (MUA) functionalized QDs have been successfully used for in vitro fluorescence cellular imaging of HEK‐293 and HeLa cells. These QDs are cell permeable and suitable for visible light induced multicolor fluorescence‐based cellular imaging.

Fluorescent carboxylic and phosphonic acids: comparative photophysics from solution to organic nanoparticles

Phosphonic and carboxylic fluorescent nanoparticles have been fabricated by direct reprecipitation in water. Their fluorescence properties strongly differ from those of the corresponding esters where strong H-bonding formation is prohibited. Comparative experiments between the two acid derivatives, differing only in their acid functions while keeping the same alkyl chain, have evidenced the peculiar behavior of the phosphonic acid derivative compared to its carboxylic analog. A dramatic emission quenching for the phosphonic acid in aprotic toluene could be observed while a fivefold increase in the fluorescence signal was observed for molecules assembled as nanoparticles. Such properties have been attributed on the theoretical basis to the formation of folded conformers in solution, leading to deactivation of the radiative excited state through intramolecular H-bonding. These studies evidence for the first time through time-resolved fluorescence measurements the stronger H-donating character of phosphonic acids compared to the carboxylic ones, and provide information on the degree of structural heterogeneity within the nanoparticles. They should pave the way for the rational fabrication of chelating acid fluorophores, able to complex metal oxides to yield stiff hybrid magnetofluorescent nanoparticles which are attracting considerable attention in the growing fields of bimodal imaging and vectorization applications.

Influence of Ge content on the optical properties ofXandWcenters in dilute Si-Ge alloys

Photoluminescence (PL) measurements, performed in Si and Si{sub 1-x}Ge{sub x} alloys (x= 0.0069 and 0.0125) irradiated with protons and annealed between 100 and 650 deg. C, are combined with first-principles calculations to assess the assignment of the W and X lines to the trigonal and tetragonal forms of the tri- and tetra-interstitial defects, denoted I{sub 3} and I{sub 4}, respectively. It is found that for the W line the annealing temperature that maximizes the PL intensity is independent of x, whereas for the X line a {approx}50 deg. C shift to higher temperatures was observed in the alloys when compared to pure Si. Analysis of the PL quenching mechanisms along with the calculations show that the radiative excited state for both defects comprises a pseudodonor state, where a trapped exciton combines a tightly bound hole and a diffuse electron. The broadening rate as a function of x for the X line is greater than that of the W line by about a factor of two. This is consistent with a stronger electronic localization in the X center than in the W center, as well as with the relative positions of the donor levels as obtained from the calculations for themore » I{sub 3} and I{sub 4} defects, respectively. The calculated change rates of donor levels with x are in qualitative agreement with the hole binding energy shifts obtained from the experiments. Our results support the assignment of I{sub 3} and I{sub 4} to the W and X centers, respectively.« less

Superradiance in ultracold Rydberg gases

Experiments in dense, ultracold gases of rubidium Rydberg atoms show a considerable decrease of the radiative excited state lifetimes compared to dilute gases. This accelerated decay is explained by collective and cooperative effects, leading to superradiance. A formalism to calculate effective decay times in a dense Rydberg gas shows that for these atoms the decay into nearby levels increases by up to three orders of magnitude. Excellent agreement between theory and experiment follows from this treatment of Rydberg decay behavior.

Temperature dependence of transient photoluminescence in tris(8-hydroxyquinoline) aluminum (Alq 3)

We have studied the temperature dependence of the dynamics of singlet excited states in pristine films of tris(8-hydroxyquinoline) aluminum (Alq 3). Measurements of the transient photoluminescence (PL) response revealed an increase in the radiative excited state lifetime and fluorescence quantum yield with decreasing temperature from 300 to 77 K. Bimolecular singlet exciton annihilation measurements show a decrease in the singlet exciton diffusion coefficient and diffusion length at 77 K. A singlet exciton trapping model is used to explain these results.

Dependence of the emission characteristics of magnetized surface-wave plasmas on the azimuthal configuration of the wave field

A 1D plasma-wave model initially developed for describing a plasma sustained by a surface wave in the azimuthally symmetric configuration (m=0) was modified for simulating surface waves in the dipolar mode (m=1). Using this model, it is shown that the maintenance limit for unmagnetized plasmas in the symmetric mode, which, as was previously determined, requires that the product of the wave frequency and the plasma radius be smaller than 2 GHz cm, is no longer valid for magnetized plasmas. Comparison of calculated and measured radial distributions of argon atoms in a radiative excited state shows that changes observed in this distribution when varying some of the operating conditions can be explained by a switching of the discharge from one mode to the other. Conditions for selection of the mode are derived from considerations on wave propagation and power dissipation along the plasma column.

Simultaneous measurement of the transient photoconductivity and photoluminescence in Alq 3

We present a unique and simple technique which allows one to measure the transient photoluminescence (PL) and transient photoconductivity (PC) of thin film materials simultaneously. This technique will be useful in characterizing thin film materials that have a relatively high PL quantum yield, such as those used in EL devices (i.e. metal chelates, polymers, dyepolymer composite, etc.). The photogeneration of charge carriers and radiative excited states in pristine tris-(8-hyaroxyquinoline) aluminum (Alq 3) were studied by measuring both its photoconductive and photoluminescence response simultaneously, with subnanosecond resolution. 25 psec laser pulses at 355nm were used for excitation of an Auston switch with a 2–4 μm thick film of Alq 3 vacuum-evaporated across the active region. Quantitative measurements of the dependence on laser intensity, and applied electric field were performed. A scheme for the photoprocess in Alq3 is presented.

Observation of the Temperature Dependence of the Dynamics of Photoexcited States in Pristine Tris(8-Hydroxyquinoline) Aluminum (Alq3)

AbstractWe have investigated the temperature dependence of the dynamics of radiative excited states in pristine thin films of tris(8-hydroxyquinoline) aluminum (Alq3). By measuring the transient photoluminescence (PL) response with subnanosecond resolution, our results revealed an increase in the radiative excited state lifetime and fluorescence quantum yield with decreasing temperature from 300K to 77K. At low temperature we observed a decrease in the bimolecular recombination rate constant, singlet exciton diffusion coefficient and diffusion length. A singlet exciton trapping model is used to explain these results.

Photooxidation effects on picosecond photoluminescence and photoconductivity in tris-(8-hydroxyquinoline) aluminum (Alq3).

The photogeneration of radiative excited states and charge carriers in pristine and photooxidized tris-(8-hydroxyquinoline) aluminum (Alq 3) was studied by measuring both its photoluminescence and photoconductive response simultaneously, with subnanosecond resolution. 25 psec laser pulses at 355nm were used for excitation of vacuum evaporated 2–4 μm thick films of Alq 3. We observed that photooxidation of Alq 3 with oxygen and UV light substantially decreases the fluorescence lifetime and quantum yield as well as the photoconductive response. This decrease is shown to be a result of quenching of singlet molecular excitations by products of photooxidation of Alq 3.

Vibrational distributions in a flowing nitrogen glow discharge

A spectroscopical investigation of a flowing DC nitrogen discharge is realized for determining the vibrational distributions of radiative excited states and . The distributions are found to be Boltzmannian and the characteristic vibrational temperatures increase with the discharge current but are not affected by the flow rate and pressure variations in the range 1 - 3 Torr. The vibrational distribution of the ground state is deduced from the distribution, following the Franck - Condon principle.

Effect of Photooxidation on the Transient Photoconductivity and Photoluminescence of Alq3

ABSTRACTPhotogeneration of charge carriers and radiative excited states in pristine and photooxidized tris-(8-hydroxyquinoline) aluminum (Alq3) were studied by simultaneously measuring the photoconductive response and the kinetics of photoluminescence with subnanosecond resolution. 25 ps laser pulses at 355 nm were used for the excitation of vacuum-evaporated films of Alq3 2–4 μm thick. At equal excitation intensities the time dependence of the photoconductive response was very similar to that of the photoluminescence, independent of whether the sample was pristine or partially oxidized. Photooxidation of Alq3 substantially decreases the magnitude and lifetime of the photoconductivity and the photoluminescence. The decrease is shown to be a result of quenching of singlet molecular excitations by product(s) of photooxidation of Alq3. Photogeneration of charge carriers is controlled by the mutual annihilation of singlet excitons.

Jahn-Teller effect in the Sb 3+ emission in zircon-structured phosphates

The luminescence of the Sb 3+ ion in YPO 4, LuPO 4 and ScPO 4 consists of two emission bands. The intensity ratio of these bands depends strongly on temperature. The observed phenomena are satisfactorily explained by assuming a dynamical Jahn-Teller effect to be operative in the radiative excited state of the Sb 3+ ion.