Forbidden Electronic Transitions Research Articles

Current Development of Lanthanide Complexes for Biomedical Applications.

Luminescent molecule-based bioimaging system is widely used for precise localization and distinction of cancer/tumor cells. Luminescent lanthanide (Ln(III)) complexes offer long-lived (sub-millisecond time scale) and sharp (FWHM <10 nm) emission, arising from the forbidden 4f-4f electronic transitions. Luminescent Ln(III) complex-based bioimaging has emerged as a promising option for both in vitro and in vivo visualizations. In this mini-review, the historical development and recent significant progress of luminescent Ln(III) probes for bioapplications are introduced. The recent studies are mainly focused on three points: (i) the structural modifications of Ln(III) complexes in both macrocyclic and small ligands, (ii) the acquirement of high resolution luminescence images of cancer/tumor cells and (iii) the constructions of ratiometric biosensors. Furthermore, our recent study is explained as a new Cancer GPS (cancer grade probing for determining tumor grade through photophysical property analyses of intracellular Eu(III) complex.

Impact of Deposition Temperature on Physical Properties of MgO-SiO2 Preparer Based on Natural Resources (Rocks)

This search displays the effects of deposition temperature on the physical properties of MgO-SiO2 thin films. These thin films were deposited on glass substrates using the autoclave method (within 120min) and at temperature deposition range of within a deposition temperature range of 150 °C–300 °C. These films were diagnosed using X-ray diffraction (XRD) and scanning electronic microscopy (SEM) and UV–Vis spectrophotometer for structural, morphological and optical analysis respectively. Using energy dispersive spectroscopy (EDX), the compositional characterization analysis of the MgO-SiO2 films was also carried out. The XRD patterns reveal that the MgO-SiO2 films have a cubic and hexagonal phase polycrystalline composition with the preferential growth direction of (0 0 2) and (0 1 1) respectively. Granular size was found to decrease by increasing the deposition temperature except at 200 °C, The crystal and optical parameters are calculated and discussed in detail. Membranes were found to have an absorption coefficient below (104 Cm−1) which means indirect permissible and forbidden electronic transitions. The SEM observations show that the films have homogeneous surfaces with clear fullness, The experimental results reveal that MgO-SiO2 thin films may be used as an alternative material for environmentally friendly buffer layer.

Excitation of Forbidden Electronic Transitions in Atoms by Hermite–Gaussian Modes

AbstractPhotoexcitation of trapped ions by Hermite–Gaussian (HG) modes from guided beam structures is proposed and investigated theoretically. In particular, simple analytical expressions for the matrix elements of induced atomic transitions are derived that depend both on the parameters of HG beams and on the geometry of an experiment. By using these general expressions, the electric octupole (E3) transition is investigated in an Yb+ion, localized in the low–intensity center of the HG10and HG01beams. It is shown how the corresponding Rabi frequency can be enhanced by properly choosing the polarization of incident light and the orientation of an external magnetic field, which defines the quantization axis of a target ion. The calculations, performed for experimentally feasible beam parameters, indicate that the achieved Rabi frequencies can be comparable or even higher than those observed for the conventional Laguerre–Gaussian (LG) modes. Since HG‐like modes can be relatively straightforwardly generated with high purity and stability from integrated photonics, these results suggest that they may form a novel tool for investigating highly‐forbidden atomic transitions.

An Insight into nd10 Metal Cyanide-based Coordination Polymers Through ab-initio Calculations: Electronic Properties and Optical Response.

Semiconductors are essential for modern life since they are the basis of many current technologies that are related to better living standards. Some of them, characterized by the periodic assembling of metal cyanides with filled d-shell (nd10 ) constitute an interesting series of cyanide-based coordination polymers with physical properties such like anomalous anisotropic thermal expansion and quantum confinement effects related to the polymer's width that can be exploited for technological applications. Herein, the electronic structure of nd10 metal cyanide-based systems were studied both experimentally and through Density Functional Theory. The band gap found for one-dimensional (1D) -M-C≡N- (M=Cu, Ag, Au) and tetrahedral M-(C≡N)2 (M=Zn, Cd, Hg) systems can be attributed to Laporte-allowed π π* (Metal to Ligand Charge Transfer mechanism) combined with metal center (d s,p) electronic transitions. Aurophilic bonding was found on the AuCN structure, and a new forbidden electronic transition associated to its band gap is reported. Computed effective and reduced masses from carriers revealed that carrier mobility and quantum confinement effects are greater in 1D systems.

On-off-on Control of Molecular Inversion Symmetry via Multi-stage Protonation: Elucidating Vibronic Laporte Rule.

The Laporte rule dictates that one- and two-photon absorption spectra of inversion-symmetric molecules should display alternatively forbidden electronic transitions; however, for organic fluorophores, drawing clear distinction between the symmetric- and non-inversion symmetric two-photon spectra is often obscured due to prevalent vibronic interactions. We take advantage of consecutive single- and double-protonation to break and then reconstitute inversion symmetry in a nominally symmetric diketopyrrolopyrrole, causing large changes in two-photon absorption. By performing detailed one- and two-photon titration experiments, with supporting quantum-chemical model calculations, we explain how certain low-frequency vibrational modes may lead to apparent deviations from the strict Laporte rule. As a result, the system may be indeed considered as an on-off-on inversion symmetry switch, opening new avenues for two-photon sensing applications.

On‐off‐on Control of Molecular Inversion Symmetry via Multi‐stage Protonation: Elucidating Vibronic Laporte Rule

AbstractThe Laporte rule dictates that one‐ and two‐photon absorption spectra of inversion‐symmetric molecules should display alternatively forbidden electronic transitions; however, for organic fluorophores, drawing clear distinction between the symmetric‐ and non‐inversion symmetric two‐photon spectra is often obscured due to prevalent vibronic interactions. We take advantage of consecutive single‐ and double‐protonation to break and then reconstitute inversion symmetry in a nominally symmetric diketopyrrolopyrrole, causing large changes in two‐photon absorption. By performing detailed one‐ and two‐photon titration experiments, with supporting quantum‐chemical model calculations, we explain how certain low‐frequency vibrational modes may lead to apparent deviations from the strict Laporte rule. As a result, the system may be indeed considered as an on‐off‐on inversion symmetry switch, opening new avenues for two‐photon sensing applications.

Feasibility of p-Doped Molecular Crystals as Transparent Conductive Electrodes via Virtual Screening.

Transparent conducting materials are an essential component of optoelectronic devices. It is proven difficult, however, to develop high-performance materials that combine the often-incompatible properties of transparency and conductivity, especially for p-type-doped materials. In this work, we have employed a large set of molecular semiconductors extracted from the Cambridge Structural Database to evaluate the likelihood of transparent conducting material technology based on p-type-doped molecular crystals. Candidates are identified imposing the condition of high highest occupied molecular orbital (HOMO) energy level (for the material to be easily dopable), high charge carrier mobility (for the material to display large conductivity when doped), and a high threshold for energy absorption (for the material to absorb radiation only in the ultraviolet). The latest condition is found to be the most stringent criterion in a virtual screening protocol on a database composed of structures with sufficiently wide two-dimensional (2D) electronic bands. Calculation of excited-state energy is shown to be essential as the HOMO–lowest unoccupied molecular orbital (LUMO) gap cannot be reliably used to predict the transparency of this material class. Molecular semiconductors with desirable mobility are transparent because they display either forbidden electronic transition(s) to the lower excited states or small exchange energy between the frontier orbitals. Both features are difficult to design but can be found in a good number of compounds through virtual screening.

Influence the Addition (SiO2) Nanoparticles on Optical Properties for Methylene Blue Dye

Thin films from methylene blue dye dopant with silicon oxide nanoparticles (semiconducting) with percentages (0, 0.01, 0.03 and 0.05%) with a thickness of 0.001 nm, had been prepared in this research. The effect of silicon oxide impurities in methylene blue dye on the optical properties (absorption, absorbance coefficient, transmittance, energy band-gap for the forbidden and permissible electronic transitions, refractive index, extinction coefficient, dielectric constant with its real and imaginary particles, and photoconductivity) was investigated by Spectrophotometer route in the visible range 400-800 nm. Show that studying of pure and dopants samples with previous ratios increasing of absorption coefficient with increasing the doped ratios, in addition the optical conductivity, as for the values of the band gap for forbidden and permissible transitions, they decreased with the increase by the dopants percentage.

Segregated Array Tailoring Charge-Transfer Degree of Organic Cocrystal for the Efficient Near-Infrared Emission beyond 760nm.

Harvesting the narrow bandgap excitons of charge-transfer (CT) complexes for the achievement of near-infrared (NIR) emission has attracted intensive attention for its fundamental importance and practical application. Herein, the triphenylene (TP)-2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4 TCNQ) CT organic complex is designed and fabricated via the supramolecular self-assembly process, which demonstrates the NIR emission with a maximum peak of 770nm and a photoluminescence quantum yield (PLQY) of 5.4%. The segregated stacking mode of TP-F4 TCNQ CT complex based on the multiple types of intermolecular interaction has a low CT degree of 0.00103 and a small counter pitch angle of 40° between F4 TCNQ and TP molecules, which breaks the forbidden electronic transitions of CT state, resulting in the effective NIR emission. Acting as the promising candidates for the active optical waveguide in the NIR region beyond 760nm, the self-assembled TP-F4 TCNQ single-crystalline organic microwires display an ultralow optical-loss coefficient of 0.060dBµm-1 . This work holds considerable insights for the exploration of novel NIR-emissive organic materials via an universal "cocrystal engineering" strategy.

Enhanced Photoluminescence of Colloidal Lead‐Free Double Perovskite Cs2Ag1−xNaxInCl6 Nanocrystals Doped with Manganese

AbstractLead‐free halide double perovskite Cs2AgInCl6 has attracted great attention due to its highly efficient luminescence and excellent stability. However, the parity forbidden electronic transition limits further applications in lighting and display fields. In this work, Cs2AgInCl6 nanocrystals (NCs) are synthesized by injecting trimethyl chlorosilane into the dissolved precursors at 180 °C and the as‐synthesized Cs2AgInCl6 NCs exhibit a weak orange emission peak at 570 nm with a photoluminescence quantum yield (PLQY) of 1.5%. In order to break the parity forbidden transition, various amounts of Na are alloyed into Cs2AgInCl6 NCs, resulting a peak PLQY of 13.4% in Cs2Na0.6Ag0.4InCl6 NCs. Then Mn2+ doped Cs2NaxAg1−xInCl6 NCs are synthesized in the same approach, in which the highest PLQY can be enhanced up to 36%. As a result, a redshifted photoluminescence emission peak at 625 nm is observed, which derives from the 4T1→6A1 transition of Mn2+ that is related to the ultrafast energy transfer from self‐trapped exciton states to the 4T1 excited state of Mn2+.

Theoretical analysis of optically selective imaging in photoinduced force microscopy.

We present a theoretical study on the measurement of photoinduced force microscopy (PiFM) for composite molecular systems. Using discrete dipole approximation, we calculate the self-consistent response electric field of the entire system, including the PiFM tip, substrate, and composite molecules. We demonstrate a higher sensitivity for PiFM measurement on resonant molecules than the previously obtained tip-sample distance dependency, z-4, owing to multifold enhancement of the localized electric field induced at the tip-substrate nanogap and molecular polarization. The enhanced localized electric field in PiFM allows high-resolution observation of forbidden optical electronic transitions in dimer molecules. We investigate the wavelength dependence of PiFM for dimer molecules, obtaining images at incident light wavelengths corresponding to the allowed and forbidden transitions. We reveal that these PiFM images drastically change with the frequency-dependent spatial structures of the localized electric field vectors and resolve different types of nanoparticles beyond the resolution for the optically allowed transitions. This study demonstrates that PiFM yields multifaceted information based on microscopic interactions between nanomaterials and light.

Luminescence of Mn4+ activated Li4Ti5O12

This work demonstrates the luminescence of Mn4+ activated cubic spinel Li4Ti5O12 (LTO) powders. One-step solid-state method was used to obtain eleven samples with different quantities of Mn4+. The deep red emission centered at around 696 nm originates from 2Eg → 4A2g spin forbidden electronic transitions from 3 d3 electron configuration of Mn4+ ion. The concentration quenching of emission is also observed in values of excited-state lifetimes, ranging from 212 μs to 143 μs. Such emission in the red spectral region which can be excited by blue light is suitable for the plant growth LEDs. The exchange charge model (ECM) of crystal field was used to calculate the Mn4+ energy levels in the LTO host. The temperature dependence of the Mn4+ emission in LTO was measured in the 10–350 K temperature range. Excited-state lifetime strongly changes with temperature. A very large value of relative sensitivity of 2.6% K−1 is found at 330 K, which facilitates temperature measurements with the temperature resolution of about 0.26 K. A low concentration of Nb5+ co-doping sensitizes the Mn4+ optical center effectively showing a relative increment of emission intensity.

Cover Feature: Enhancing Light‐Absorption and Luminescent Properties of Non‐Emissive 1,3,4,6,9b‐Pentaazaphenalene through Perturbation of Forbidden Electronic Transition by Boron Complexation (Asian J. Org. Chem. 2/2020)

Cover Feature: Enhancing Light‐Absorption and Luminescent Properties of Non‐Emissive 1,3,4,6,9b‐Pentaazaphenalene through Perturbation of Forbidden Electronic Transition by Boron Complexation (Asian J. Org. Chem. 2/2020)

Enhancing Light‐Absorption and Luminescent Properties of Non‐Emissive 1,3,4,6,9b‐Pentaazaphenalene through Perturbation of Forbidden Electronic Transition by Boron Complexation

AbstractWe report an improvement of optical properties of the non‐emissive molecule through shuffling the molecular orbitals by boron complexation. The 1,3,4,6,9b‐pentaazaphenelene (5AP) derivative having a 2‐(dimesitylboryl)phenyl group was synthesized through a substitution reaction from a 2‐bromophenyl compound. The formation of the B−N dative bond was confirmed by 11B{1H} NMR spectroscopy and a single‐crystal X‐ray structure analysis. Most of the conventional 5AP derivatives hardly showed emission despite their planar and rigid molecular structures. The previous reports on 5AP derivatives ascribed the non‐luminescent behavior to the forbidden transition between frontier orbitals. On the other hand, the 5AP‐based boron complex in this work showed enhanced light absorption and luminescence from the HOMO−LUMO transition. Theoretical calculations suggested that the boron complexation should play a critical role in perturbing the forbidden character of the HOMO−LUMO transition. Because the nitrogen atom on the 5AP moiety formed the Lewis acid−base pair with the boron atom, the energy level of the HOMO of 5AP was downshifted. The character of the HOMO−LUMO transition of the boron complex was changed to afford the improved optical properties of the boron complex.

Rutile TiO2 films as electron transport layer in inverted organic solar cell

Titanium dioxide (TiO2) thin films were prepared by sol–gel spin coating method and deposited on ITO-coated glass substrates. The effects of different heat treatment annealing temperatures on the phase composition of TiO2 films and its effect on the optical band gap, morphological, structural as well as using these layers in P3HT:PCBM-based organic solar cell were examined. The results show the presence of rutile phases in the TiO2 films which were heat-treated for 2 h at different temperatures (200, 300, 400, 500 and 600 °C). The optical properties of the TiO2 films have altered by temperature with a slight decrease in the transmittance intensity in the visible region with increasing the temperature. The optical band gap values were found to be in the range of 3.28–3.59 eV for the forbidden direct electronic transition and 3.40–3.79 eV for the allowed direct transition. TiO2 layers were used as electron transport layer in inverted organic solar cells and resulted in a power conversion efficiency of 1.59% with short circuit current density of 6.64 mA cm−2 for TiO2 layer heat-treated at 600 °C.

Optical absorption and disorder in delafossites

We present compelling experimental results of the optical characteristics of transparent oxide CuGaO2 and related CuGa1-xFexO2 (with 0.00≤x≤0.05) alloys, whereby the forbidden electronic transitions for CuGaO2 become permissible in the presence of B-site (Ga sites) alloying with Fe. Our computational structural results imply a correlation between the global strain on the system and a decreased optical absorption edge. However, herein, we show that the relatively ordered CuGa1-xFexO2 (for 0.00≤x≤0.04) structures exhibit much weaker vis-absorption compared to the relatively disordered CuGa0.95Fe0.05O2.

Structural, optical and electrical properties of YbInSe thin films

In this work, the compositional, the structural, the vibrational, the optical and the electrical characterizations of the YbInSe compound are investigated by means of energy dispersion X-ray analysis, scanning electron microscopy and X-ray diffraction, Raman spectroscopy, ultraviolet -visible light spectrophotometry, impedance spectroscopy and temperature dependent electrical conductivity, respectively. The 300nm thick YbInSe films which were prepared by the co-evaporation of the source materials under a vacuum pressure of 10−5mbar, are observed to exhibit nanocrystalline clusters of size of 27nm regularly distributed among an amorphous structure. The most intensive Raman active lines are observed at 150 and 254cm−1. In addition, the optical analysis has shown that the films exhibit a direct forbidden electronic transitions type energy band gap of 1.07eV. The optical transitions are associated with interband tail states of width of 0.28eV. Moreover, the real and imaginary parts of dielectric spectra which were analyzed in the frequency range of 270–1000THz, were analyzed in accordance with the single oscillator and the Lorentz models, respectively. The modeling allowed determining the oscillator and dispersion energies, the terahertz free carrier scattering time, the free holes effective mass, the carrier density, the drift mobility and the reduced resonant frequency for the YbInSe films. In the electronic part of study, the temperature dependent dc electrical conductivity analysis, indicated the domination of the variable range hopping transport mechanism below 335K, the thermal excitation of charge carriers in the range of 337–390K and the extrinsic-intrinsic transition property at 390K. The ac conductivity spectra which were recorded in the frequency range of 10–1500MHz, revealed the domination of the correlated barrier hopping of free carriers between pairs of localized states at the Fermi level.

Communication: Proper treatment of classically forbidden electronic transitions significantly improves detailed balance in surface hopping.

Surface hopping is the most popular method for nonadiabatic molecular dynamics. Many have reported that it does not rigorously attain detailed balance at thermal equilibrium, but does so approximately. We show that convergence to the Boltzmann populations is significantly improved when the nuclear velocity is reversed after a classically forbidden hop. The proposed prescription significantly reduces the total number of classically forbidden hops encountered along a trajectory, suggesting that some randomization in nuclear velocity is needed when classically forbidden hops constitute a large fraction of attempted hops. Our results are verified computationally using two- and three-level quantum subsystems, coupled to a classical bath undergoing Langevin dynamics.

The excited-states intermolecular potential energy surfaces of the Ar–CS2 van der Waals complex: Ab initio study

The sixteen lowest potential energy surfaces (PESs) of the electronically excited Ar–CS2 van der Waals (vdW) complex are calculated in the singlet state. PESs are obtained based on the vertical excitation of the complex containing the linear geometry ground electronic state (1Σg+) of CS2 molecule to the valence excited electronic states. The computational method involves EOM-CCSD with the aug-cc-pVDZ basis set augmented with a set of midbond functions (3s3p2d1f1g). It was found that some forbidden electronic transitions like 1Σg+→1Πg in the CS2 monomer become possible due to the presence of the Ar atom. The interaction PESs with the visible well-depth for all of the configurations are globally fitted to an analytic function in order to estimate the isotropic dipole–dipole (C60) and dipole–quadrupole (C80) coefficients of the excited states of the complex.

Temperature Effect on Radiative Lifetimes: The Case of Singlet Oxygen in Liquid Solvents

A change in solvent can have an appreciable effect on the rate constant for the O2(a(1)Δg) → O2(X(3)Σg(-)) radiative transition at ~1275 nm. The data thus obtained have played an important role in understanding mechanisms by which environment-dependent perturbations can influence forbidden electronic transitions. We now report that the rate constant for O2(a(1)Δg) radiative deactivation, kr, also responds to changes in temperature. This result can have practical ramifications in experiments that use O2(a(1)Δg) phosphorescence to quantify yields of photosensitized O2(a(1)Δg) production. From a fundamental perspective, this result is significant, partly because there is little precedence for temperature-dependent changes in radiative rate constants. The data also require a re-evaluation of the current model by which oxygen is perturbed by solvent. Specifically, the evidence indicates that it is not appropriate to evaluate the interaction as a 1:1 complex between a given solvent molecule M and oxygen. Rather, one must consider an ensemble of solvent molecules surrounding oxygen.