Excitation Wavelength Dependence Research Articles

Anion Effect on the Excited-State Intramolecular Proton Transfer of 4'-N,N-Diethylamino-3-hydroxyflavone in Ionic Liquids.

The excited-state intramolecular proton transfer (ESIPT) reaction of 4'-N,N,-diethylamino-3-hydroxyflavone (C2HF) was studied using time-resolved fluorescence measurements in ionic liquids (ILs) of various anions with a fixed cation (1-ethyl-3-methylimidazolium [C2mim]+). C2HF showed an ESIPT reaction from the normal excited state (N*; keto form) to the tautomer excited state (T*; enol form) where both states are emissive. The ESIPT rate and yield were obtained by analyzing the time-resolved fluorescence spectra measured using the optical Kerr gate method. Both the ESIPT rate and yield decreased with increasing hydrogen-bond accepting ability of the anion. According to density functional theory calculations, the complex formation energy between C2HF and the anion became significantly negative with increasing the hydrogen-bond accepting ability of anion. The pseudoequilibrium constant between N* and T* ([T*]/[N*]) in the electronic excited state decreased with increasing hydrogen-bond accepting ability of the anion, while it increased with increasing the alkyl-chain length of alkyl sulfonate. The excitation wavelength dependence of the ESIPT rate and yield was studied for C2HF in [C2mim][C6H13SO3]. The ESIPT yield decreased by nearly a factor of 2 with increasing excitation wavelength from 360 to 425 nm, although the change in the ESIPT rate was small. The solvation heterogeneity due to the alkyl chain in the anion was considered to be the reason for the excitation wavelength dependence.

Circular Modulation Pathway of the Afterglow Color in Metal‐Organic Coordination Polymer

AbstractLong‐lived organic luminescence materials with color‐tunable afterglow have attracted considerable attention due to potential applications in display and information encryption. Nevertheless, the majority of these materials currently have limited capability for regulating the afterglow color in only one direction. Here, a metal‐organic coordination polymer (Zn‐PTZ‐O) is presented that demonstrates the ability to modulate its afterglow color in a circular modulation pathway. As the temperature increases from 77 to 400 K, the afterglow color red‐shifts from blue to green, and then blue‐shifts back to blue, achieving circular modulation. This outstanding phenomenon is attributed to the concurrence of long‐lived thermally activated delayed fluorescence and phosphorescence emission of Zn‐PTZ‐O. Furthermore, the afterglow color of Zn‐PTZ‐O also has forward and reverse excitation wavelength dependence at 77 K and room temperature, respectively. As the excitation wavelength increases from 340 to 460 nm at 77 K, the afterglow color of Zn‐PTZ‐O red‐shifts from blue to yellow. While as the excitation wavelength increases from 375 to 410 nm at room temperature, the afterglow color blue‐shifts from yellowish‐green to blue, almost forming another color‐changing circle. This work provides a good idea for the design of color‐tunable long‐lived organic materials.

Integrating Full-Color 2D Optical Waveguide and Heterojunction Engineering in Halide Microsheets for Multichannel Photonic Logical Gates.

Ensuring information security has emerged as a paramount concern in contemporary human society. Substantial advancements in this regard can be achieved by leveraging photonic signals as the primary information carriers, utilizing photonic logical gates capable of wavelength tunability across various time and spatial domains. However, the challenge remains in the rational design of materials possessing space-time-color multiple-resolution capabilities. In this work, a facile approach is proposed for crafting metal-organic halides (MOHs) that offer space-time-color resolution. These MOHs integrate time-resolved room temperature phosphorescence and color-resolved excitation wavelength dependencies with both space-resolved ex situ optical waveguides and in situ heterojunctions. Capitalizing on these multifaceted properties, MOHs-based two-dimensional (2D) optical waveguides and heterojunctions exhibit the ability to tune full-color emissions across the spectra from blue to red, operating within different spatial and temporal scales. Therefore, this work introduces an effective methodology for engineering space-time-color resolved MOH microstructures, holding significant promise for the development of high-density photonic logical devices.

Photoinduced Electron Transfer between Dipolar Reactants: Solvent and Excitation Wavelength Effects.

Electron transfer (ET) quenching in nonpolar media is not as well understood as in polar environments. Here, we investigate the effect of dipole-dipole interactions between the reactants using ultrafast broadband electronic spectroscopy combined with molecular dynamics simulations. We find that the quenching of the S1 state of two polar dyes, coumarin 152a and Nile red, by the polar N,N-dimethylaniline (DMA) in cyclohexane is faster by a factor up to 3 when exciting on the red edge rather than at the maximum of their S1 ← S0 absorption band. This originates from the inhomogeneous broadening of the band due to a distribution of the number of quencher molecules around the dyes. As a consequence, red-edge excitation photoselects dyes in a DMA-rich environment. Such broadening is not present in acetonitrile, and no excitation wavelength dependence of the ET dynamics is observed. The quenching of both dyes is markedly faster in nonpolar than polar solvents, independently of the excitation wavelength. According to molecular dynamics simulations, this is due to the preferential solvation of the dyes by DMA in cyclohexane. The opposite preferential solvation is predicted in acetonitrile. Consequently, close contact between the reactants in acetonitrile requires partial desolvation. By contrast, the recombination of the quenching product is slower in nonpolar than in polar solvents and exhibits much smaller dependence, if any, on the excitation wavelength.

Effect of dimensionality on the excitation wavelength dependence of the Fano-Raman line-shape: a brief review.

The already existing heterogeneity in nanomaterials makes it an intriguing yet complex system to study size effect vis-à-vis other external perturbations and thereby local modifications at the nanoscale, thus demanding an improved tool and analysis for the choice of study. The analysis of existential subtle perturbations and interactions in a wide class of materials using Raman spectromicroscopy has proved to be of utmost importance, and various phenomena such as quantum confinement and its interplay with Fano resonance have already been investigated in nanomaterials, including the role of various perturbations such as temperature, pressure, doping, bias, and excitation wavelength on Raman spectral line shape parameters. Amongst different perturbations that cause a change in the spectral profile of Fano resonance, the gray area of wavelength dependence of Fano Raman line shape profiles has been least analysed in the literature. Moreover, the true signature of Fano resonance in nanoscaled systems, which is the wavelength dependence of Fano interaction, remains the least discussed. This review summarises the wavelength dependent correlation of Fano resonance and its effect on the Raman spectral line-shape parameters in some bulk materials, nanomaterials, and molecular systems involving heavily doped p-type crystalline silicon, 2-D MoS2, graphene, WS2, single walled carbon nanotubes, etc. A brief overview of Fano resonance in metamaterials and photonic systems is also provided.

Yellow Emissive CsCu2I3 Nanocrystals Induced by Mn2+ for High-Resolution X-ray Imaging.

Recently, low-dimensional copper(I)-based perovskite or derivatives have gained extensive attention in scintillator applications because of their environmental friendliness and good stabilities. However, the unsatisfactory scintillation performance and complex fabrication processes hindered their practical applications. Herein, efficient yellow emissive CsCu2I3 nanocrystals (NCs) were successfully prepared via a simple Mn2+-assisted hot-injection method. The added Mn2+ effectively induced the phase transformation from Cs3Cu2I5 to CsCu2I3, leading to the preparation of single-phase CsCu2I3 NCs with few defects and a high fluorescence performance. The as-prepared "optimal CsCu2I3 NCs" exhibited superior photoluminescence (PL) performance with a record-high PL quantum yield (PLQY) of 61.9%. The excellent fluorescence originated from the radiative recombination of strongly localized one-dimension (1D) self-trapped excitons (STEs), which was systematically investigated via the wavelength-dependent PL excitation, PL emission, and temperature-dependent PL spectra. These CsCu2I3 NCs also exhibited outstanding X-ray scintillation properties with a high light yield (32000 photons MeV-1) and an ultralow detection limit (80.2 nGyair s-1). Eventually, the CsCu2I3 NCs scintillator film achieved an ultrahigh (16.6 lp mm-1) spatial resolution in X-ray imaging. The CsCu2I3 NCs also exhibited good stabilities against X-ray irradiation, heat, and environmental storage, indicating their great application potential in flexible X-ray detection and imaging.

Colors of entangled two-photon absorption

Multiphoton absorption of entangled photons offers ways for obtaining unique information about chemical and biological processes. Measurements with entangled photons may enable sensing biological signatures with high selectivity and at very low light levels to protect against photodamage. In this paper, we present a theoretical and experimental study of the excitation wavelength dependence of the entangled two-photon absorption (ETPA) process in a molecular system, which provides insights into how entanglement affects molecular spectra. We demonstrate that the ETPA excitation spectrum can be different from that of classical TPA as well as that for one-photon resonant absorption (OPA) with photons of doubled frequency. These results are modeled by assuming the ETPA cross-section is governed by a two-photon excited state radiative linewidth rather than by electron-phonon interactions, and this leads to excitation spectra that match the observed results. Further, we find that the two-photon-allowed states with highest TPA and ETPA intensities have high electronic entanglements, with ETPA especially favoring states with the longest radiative lifetimes. These results provide concepts for the development of quantum light-based spectroscopy and microscopy that will lead to much higher efficiency of ETPA sensors and low-intensity detection schemes.

Wavelength-Dependent Spin Excitation with Circularly Polarized Light in CdSe Nanoplatelets

Wavelength-Dependent Spin Excitation with Circularly Polarized Light in CdSe Nanoplatelets

Manipulation of clusteroluminescence in cholesterol-based liquid crystal polymers

Non-conjugated cluster luminescent materials have attracted much attention due to their great theoretical significance and potential applications in different areas. Despite great progress have already achieved, it is still challenging to regulate their photo-physical properties by manipulating aggregated structure. In this work, two cholesterol-based liquid crystal polymers PMJ0Chol and PMJ6Chol with different flexible spacer length were synthesized. The aggregated structures and photo-physical properties were systematically studied. Due to the different flexible spacer length, the coupling effect between the main chain and side groups varies, and the two polymers exhibit different liquid crystal phase structures. PMJ0Chol forms columnar nematic liquid crystal, while PMJ6Chol forms more compact and ordered smectic liquid crystal. PMJ0Chol and PMJ6Chol inherit the clustering-triggered emission characteristic of cholesterol and the luminescence behavior exhibits significant excitation wavelength dependence. However, the difference in aggregated structure also leads to changes in photo-physical properties. PMJ6Chol shows more red-shifted fluorescence luminescence, and room temperature phosphorescence with 293 ms long life time is observed.

Molecular Structures and Vibrational Spectra of trans- and cis-Polyacetylene and Their Oligoenes Revisited Using Density Functional Theory Calculations.

Polyacetylene, the most representative synthetic conducting polymer, has attracted much attention because it exhibits high conductivity upon doping. In this paper, molecular structures, electronic excitation energies, and Raman and infrared spectra were calculated using density functional theory for trans- and cis-oligoenes with various chain lengths up to the number of C═C bonds (n) of 100 and trans- and cis-polyacetylenes under one-dimensional periodic boundary condition. The harmonic vibrational frequencies obtained at the B3LYP/6-311G(d,p) level were scaled by the scaling factors determined with respect to the anharmonic vibrational frequencies using the B2PLYP method, in which the coefficients of the functional were optimized for trans-oligoenes. The calculated infrared and Raman frequencies reproduce reasonably well the observed frequencies for trans- and cis-polyacetylene. Based on the chain-length dependence of the calculated Raman spectra of trans-oligoenes, we proposed the possibility of longer conjugated trans-segments observed in the resonance Raman spectra of trans-polyacetylene excited at longer wavelengths of 647.1 and 1064 nm. We also elucidated the origin of the excitation-wavelength dependence of the resonance Raman spectra of trans-polyacetylene and the structure of isomerization intermediates from cis-form to trans-form. In addition, the previous assignments of Raman and infrared spectra of trans- and cis-polyacetylene were reexamined in the present study based on the chain-length dependence of the spectra.

Excitation wavelength dependent nonlinear absorption mechanisms and optical limiting properties of Bi12SiO20 single crystal

Nonlinear absorption mechanisms (NA), excitation wavelength dependence, and defect states of Bi12SiO20 (BSO) single crystal were investigated. The band gap and Urbach energies were found to be 2.51 and 0.4 eV from the absorption spectra. To evaluate the effect of excitation energy on the NA mechanism of the BSO single crystal, open aperture Z-scan experiment with 4 ns laser pulse at 532 and 1064 nm wavelengths with different intensities was performed. Obtained data were analyzed with a theoretical model considering the contributions of one photon absorption (OPA), two photon absorption (TPA) and free carrier absorption (FCA) to NA. The results indicated that the NA behavior decreased with increasing of the pump intensity as the defect states at around 2.32 eV by OPA at 532 nm, and TPA at 1064 nm excitations. The dominant NA mechanisms are OPA and sequential TPA at 532 nm as compared to the 1064 nm. A higher NA coefficient was obtained at 532 nm as compared to 1064 nm excitation. This observation was attributed to higher contribution of OPA at 532 nm even at lower input intensities compared to TPA contribution at 1064 nm. Onset optical limiting thresholds were found as 0.34 and 0.68 mJ/cm2 for 532 and 1064 nm input beams, respectively. In the light of the results, the BSO single crystal may be used as a saturable absorber or an optical limiter at convenient input intensity by effectively adjusting defect states and excitation wavelength.

Color tunable (Ba,Ca)ScO2F: Eu2+, Bi3+, K+ perovskite with dependence of excitation wavelength for advanced anti-counterfeiting application

Fluorescent materials are widely used for anti-counterfeiting of information security and commodities. However, the increasing requirements for fluorescent anti-counterfeiting technology urgently require the development of novel fluorescent materials with multicolor fluorescence, single component, and excellent stability. Herein, a color tunable Ba0·94Ca0.06ScO2F: Eu2+, Bi3+, K+ perovskite with dependence of excitation wavelength for high-level anti-counterfeiting is reported. The fluorescence spectra of Ba0·94Ca0.06ScO2F: Eu2+, Bi3+, K+ perovskite show the tunable emission band with the peak between 479 and 509 nm. It is worth noting that the peak intensity ratio at 479 and 509 nm changes with the dependence of the excitation wavelength. Meanwhile, the snowflake pattern is designed to demonstrate the potential applications in anti-counterfeiting, which obviously shows four different color patterns under the sunlight, 320 360, and 415 nm excitation, respectively.

Single-Particle Dynamics of ZnS Shelling Induced Replenishment of Carrier Diffusion for Individual Emission Centers in CuInS2 Quantum Dots.

Insights into blinking and photoactivation of aqueous copper indium sulfide (CIS) quantum dots have been obtained using fluorescence correlation spectroscopy (FCS) and fluorescence lifetime correlation spectroscopy (FLCS). An unusual excitation wavelength-dependence of photoactivation/photocorrosion is manifested in an increase in the initial correlation amplitude G(0) for λex = 532 nm, but a decrease for λex = 405 nm. This has been rationalized in terms of different contributions from surface-assisted recombination in the two cases. Blinking times obtained from the autocorrelation functions (ACFs) of the 100-200 ns lifetime component (core Cu-mediated recombination) are almost unaffected by shelling, but those from the ACF for the 10-30 ns lifetime (surface states) increase significantly. Absence of cross-correlation between the two recombinative states of bare CIS QDs and the emergence of an anticorrelation with the introduction of the ZnS shell are observed, indicating the diffusive nature of the two states for CIS-ZnS. The diffusion is inhibited in bare CIS QDs due to the preponderance of surface states.

Excitation-Wavelength-Dependent Charge-Carrier Lifetime in Hematite: An Insight from Nonadiabatic Molecular Dynamics.

Experiments have reported that the photoexcited carrier lifetime in α-Fe2O3 has a significant excitation-wavelength dependence but leave the physical mechanism unresolved. In this work, we rationalize the puzzling excitation-wavelength dependence of the photoexcited carrier dynamics in Fe2O3 by performing nonadiabatic molecular dynamics simulation based on the strongly constrained and appropriately normed functional, which accurately describes the electronic structure of Fe2O3. Photogenerated electrons with lower-energy excitation relax fast in the t2g conduction band within about 100 fs, while the photogenerated electrons with higher-energy excitation undergo first a slower interband relaxation from the eg lower state to the t2g upper state on a time scale of 135 ps, followed by the much faster t2g intraband relaxation. This study provides insight into the experimentally reported excitation-wavelength dependence of the carrier lifetime in Fe2O3 and a reference for regulating photogenerated carrier dynamics in transition-metal oxides through the light excitation wavelength.

Excitation-wavelength-dependent photochromic Bi3+/Ln3+(Eu3+, Er3+)-codoped phosphors for anticounterfeiting applications

Fluorescent powders exhibiting excitation dependence and luminescent discoloration are profoundly relevant for concealing information and enhancing optical antiforgery protection. In this study, Bi3+/Ln3+ (Eu3+, Er3+)-doped phosphors exhibiting multicolor luminescence and excitation wavelength dependence were synthesized. The Bi3+/Eu3+-codoped phosphors emitted blue, pink, lavender, and red lights under excitation from conventional ultraviolet (UV) sources. In addition to emitting blue light, the Bi3+/Er3+-codoped phosphors also transmitted upconversion-emission light from green to yellow under 980-nm laser excitation. Additionally, the luminous color of the Ca3Y2Ge3O12:0.01Bi3+, 0.2Er3+ phosphor changed from yellow to orange as the temperature increased. Combining the above luminous-color-change phenomena, the word-hiding experiment exhibited excellent realistic information hiding, with complex color transformation in the stamen and petal parts of the flower pattern. Furthermore, rapid multimodal and complex discoloration was achieved using easy-to-obtain light sources, confirming the potential of Bi3+/Ln3+ phosphors for anticounterfeiting applications.

Non-negligible roles of charge transfer excitons in ultrafast excitation energy transfer dynamics of a double-walled carbon nanotube.

Herein, we employed a developed linear response time dependent density functional theory-based nonadiabatic dynamics simulation method that explicitly takes into account the excitonic effects to investigate photoinduced excitation energy transfer dynamics of a double-walled carbon nanotube (CNT) model with different excitation energies. The E11 excitation of the outer CNT will generate a local excitation (LE) |out*〉 exciton due to its low energy, which does not induce any charge separation. In contrast, the E11 excitation of the inner CNT can generate four kinds of excitons with the LE exciton |in*〉 dominates. In the 500-fs dynamics simulation, the LE exciton |in*〉 and charge transfer (CT) excitons |out-in+〉 and |out+in-〉 are all gradually converted to the |out*〉 exciton, corresponding to a photoinduced excitation energy transfer, which is consistent with experimental studies. Finally, when the excitation energy is close to the E22 state of the outer CNT (∼1.05eV), a mixed population of different excitons, with the |out*〉 exciton dominated, is generated. Then, photoinduced energy transfer from the outer to inner CNTs occurs in the first 50fs, which is followed by an inner to outer excitation energy transfer that is completed in 400fs. The present work not only sheds important light on the mechanistic details of wavelength-dependent excitation energy transfer of a double-walled CNT model but also demonstrates the roles and importance of CT excitons in photoinduced excitation energy transfer. It also emphasized that explicitly including the excitonic effects in electronic structure calculations and nonadiabatic dynamics simulations is significant for correct understanding/rational design of optoelectronic properties of periodically extended systems.

Multifunctional Integrated Superhydrophobic Coatings with Unique Fluorescence and Micro/Micro/Nano-Hierarchical Structures Enabled by In Situ Self-Assembly

Conferring versatility to superhydrophobic materials is extremely desirable to advance their utility. Herein, we have developed a superhydrophobic material with montmorillonite as microskeleton supports and in situ grown ZIF-8 nanoparticles and loaded them with newly developed fluorescent carbon dots. In situ growth of the ZIF-8 on OMMT constructs a dense nanoscale rough structure and meanwhile self-assembly generates abundant microporous, thus forming unique hierarchical microporous/microsheet/nanoparticle tri-tier micro and nano structures. Then the multifunctional superhydrophobic coating is fabricated by a facile spraying technique using polydimethylsiloxane (PDMS) as a multifunctional polymer binder. The PDMS/RB-CDs/ZIF-8@OMMT exhibits superhydrophobicity with a water contact angle of 164.7° and a water sliding angle of 1.4°, which also possesses good self-cleaning performance. Moreover, novel carbon dots are developed in this work which can confer unique fluorescent properties and photothermal properties to materials. Fluorescence characterization reveals the multiple emission peaks among 300-800 nm and excitation wavelength dependence and independence. Photothermal experiments unveil an efficient light-to-heat conversion caused by the light traps and absorption wavelengths associated with photothermal heating. Benefiting from the dense microporous/microsheet/nanoparticle structures, the superhydrophobicity is still maintained after 120 cycles of abrasion. Moreover, electrochemical impedance spectroscopy (EIS) reveals a significant increase in impedance, which is associated with excellent corrosion resistance. The superhydrophobic coating also exhibits superior UV resistance and good thermal stability. Multifunctional fluorescent superhydrophobic materials will enable the development of various and potential applications in different fields.

Achieving excitation wavelength-power-dependent colorful luminescence via multiplexing of dual lanthanides in fluorine oxide particles for multilevel anticounterfeiting.

The optical characteristics of multimode luminescent materials like multimode luminescence (photoluminescence, afterglow, thermoluminescence) and a multi-excitation source (light, thermal, mechanical force) play crucial roles in optical data storage and readout, document security and anticounterfeiting. A higher level of advanced anticounterfeiting may rely on multimode anticounterfeiting materials that can realize multicolor luminescence. Here, a highly integrated multimode and multicolor Y7O6F9:Er3+,Eu3+ material is developed through multiplexing of dual lanthanides in fluorine oxide particles. In photoluminescence and photoluminescence/up-conversion luminescence modes, the material Y7O6F9:Er3+,Eu3+ has the characteristic of excitation wavelength and power dependence. In the photoluminescence mode, under excitation at 254 nm and 365 nm, Y7O6F9:Er3+ and Y7O6F9:Eu3+ showed bright red and green emissions, respectively. In the photoluminescence/up-conversion mode, under the increased excitation power from 0.2 to 2.0 W cm-2, the color of luminescence emission can be finely tuned from red to orange, yellow and green. Taking this unique excitation wavelength-power-dependent luminescence property into account, a multilevel anticounterfeiting device with the Lily pattern was designed. The device readily integrates the advantages of the excitation wavelength-dependent photoluminescence emissions and excitation power-dependent photoluminescence emissions in one overall device. These findings offer unique insight for designing highly integrated multimode, multicolor luminescence materials and advanced anticounterfeiting technology toward a wide variety of applications, particularly multilevel anticounterfeiting devices.

CO2-responsive tunable persistent luminescence in a hydrogen-bond organized two-component ionic crystal.

A reversible CO2-responsive luminescent material was constructed by a facile hydrogen-bond self-assembly of a two-component ionic crystal. The modification of CO2 on the ionic crystal not only alternates the green afterglow, but also endows the material with inverse excitation wavelength dependence for multicolor emission.

Giant enhancement of second-harmonic generation of indium selenide on planar Au.

Two-dimensional (2D) van der Waals layered γ-type indium selenide (γ-InSe) holds great promise for the development of ultrathin and low-energy-consumption nonlinear optical devices due to its broken inversion symmetry regardless of layer number. Nevertheless, the 2D InSe thin flakes still exhibit short light-matter interaction lengths, thus resulting in low efficiencies of nonlinear optical processes. In this work, we provide a facile 2D semiconductor-metal structure consisting of InSe thin flakes (thickness: 11-54 nm) on planar Au film, which exhibits great second-harmonic generation (SHG) enhancement by a factor of up to 1182. The SHG enhancement is attributed to the interference effect-induced strong electric field in highly absorbing InSe; meanwhile, the increase in reflectivity by Au film also plays an important role. Furthermore, the InSe thickness and excitation wavelength dependences of enhancement factors are revealed. This work provides a convenient approach to developing high-efficiency 2D nonlinear optical devices with ultrathin form.