Lowest Electronic Transition Research Articles

Near-Infrared Room-Temperature Phosphorescence from Monocyclic Luminophores.

Compact luminophores with long emission wavelengths have aroused considerable theoretical and practical interest. Organics with room-temperature phosphorescence (RTP) are also desirable for their longer lifetimes and larger Stokes shifts than fluorescence. Utilizing the low electronic transition energy intrinsic to thiocarbonyl compounds, electron-withdrawing groups were attached to the 4H-pyran-4-thione core to further lower the excited state energies. The resulting mini-phosphors were doped into suitable polymer matrices. These purely organic, amorphous materials emitted near-infrared (NIR) RTP. Having a molar mass of only 162 g·mol-1, one of the phosphors emitted RTP that peaked at 750 nm, with a very large Stokes shift of 15485 cm-1 (403 nm). Thanks to the good processability of the polymer film, light-emitting diodes (LEDs) with NIR emission were easily fabricated by coating doped polymer on ultraviolet LEDs. This work provides an intriguing strategy to achieve NIR RTP using compact luminophores.

Near‐Infrared Room‐Temperature Phosphorescence from Monocyclic Luminophores

Compact luminophores with long emission wavelengths have aroused considerable theoretical and practical interest. Organics with room‐temperature phosphorescence (RTP) are also desirable for their longer lifetimes and larger Stokes shifts than fluorescence. Utilizing the low electronic transition energy intrinsic to thiocarbonyl compounds, electron‐withdrawing groups were attached to the 4H‐pyran‐4‐thione core to further lower the excited state energies. The resulting mini‐phosphors were doped into suitable polymer matrices. These purely organic, amorphous materials emitted near‐infrared (NIR) RTP. Having a molar mass of only 162 g·mol‐1, one of the phosphors emitted RTP that peaked at 750 nm, with a very large Stokes shift of 15485 cm‐1 (403 nm). Thanks to the good processability of the polymer film, light‐emitting diodes (LEDs) with NIR emission were easily fabricated by coating doped polymer on ultraviolet LEDs. This work provides an intriguing strategy to achieve NIR RTP using compact luminophores.

Phenylbenzothiazole-Based Platinum(II) and Diplatinum(II) and (III) Complexes with Pyrazolate Groups: Optical Properties and Photocatalysis.

Based on 2-phenylbenzothiazole (pbt) and 2-(4-dimethylaminophenyl)benzothiazole (Me2N-pbt), mononuclear [Pt(pbt)(R'2-pzH)2]PF6 (R'2-pzH = pzH 1a, 3,5-Me2pzH 1b, 3,5-iPr2pzH 1c) and diplatinum (PtII-PtII) [Pt(pbt)(μ-R'2pz)]2 (R'2-pz = pz 2a, 3,5-Me2pz 2b, 3,5-iPr2pz 2c) and [Pt(Me2N-pbt)(μ-pz)]2 (3a) complexes have been prepared. In the presence of sunlight, 2a and 3a evolve, in CHCl3 solution, to form the PtIII-PtIII complexes [Pt(R-pbt)(μ-pz)Cl]2 (R = H 4a, NMe2 5a). Experimental and computational studies reveal the negligible influence of the pyrazole or pyrazolate ligands on the optical properties of 1a-c and 2a,b, which exhibit a typical 3IL/3MLCT emission, whereas in 2c the emission has some 3MMLCT contribution. 3a displays unusual dual, fluorescence (1ILCT or 1MLCT/1LC), and phosphorescence (3ILCT) emissions depending on the excitation wavelength. The phosphorescence is lost in aerated solutions due to sensitization of 3O2 and formation of 1O2, whose determined quantum yield is also wavelength dependent. The phosphorescence can be reversibly photoinduced (365 nm, ∼ 15 min) in oxygenated THF and DMSO solutions. In 4a and 5a, the lowest electronic transitions (S1-S3) have mixed characters (LMMCT/LXCT/L'XCT 4a and LMMCT/LXCT/ILCT 5a) and they are weakly emissive in rigid media. The 1O2 generation property of complex 3a is successfully used for the photooxidation of p-bromothioanisol showing its potential application toward photocatalysis.

Shape fluctuations and radiation from thermally excited electronic states of boron clusters

The effect of thermal shape fluctuations on the recurrent fluorescence of boron-cluster cations, ${\mathrm{B}}_{N}{}^{+}$ $(N=9--14)$, has been investigated numerically, with a special emphasis on ${\mathrm{B}}_{13}{}^{+}$. For this cluster, the electronic structures of the ground state and the four lowest electronically excited states were calculated using time-dependent density-functional theory and sampled on molecular dynamics trajectories of the cluster calculated at an experimentally relevant excitation energy. The sampled optical transition matrix elements for ${\mathrm{B}}_{13}{}^{+}$ allowed us to construct its emission spectrum from the thermally populated electronically excited states. The spectrum was found to be broad, reaching down to at least 0.85 eV. This contrasts strongly with the static picture, where the lowest electronic transition happens at 2.3 eV. The low-lying electronic excitations produce a strong increase in the rates of recurrent fluorescence, calculated to peak at $4.6\ifmmode\times\else\texttimes\fi{}{10}^{4}\phantom{\rule{4pt}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$, with a time average of $8\ifmmode\times\else\texttimes\fi{}{10}^{3}\phantom{\rule{4pt}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$. The average value is one order of magnitude higher than the static result, approaching the measured radiation rate. Similar results were found for the other cluster sizes. The results indicate that the effect makes a significant contribution to the radiative cooling, even given the exploratory nature of the study. Furthermore, the radiationless crossing between the ground-state and first-electronic-excited-state surfaces of ${\mathrm{B}}_{13}{}^{+}$ was calculated, found to be very fast compared to experimental timescales, justifying the thermal population assumption.

General Design Rules for Bimetallic Platinum(II) Complexes.

A series of platinum(II) bimetallic complexes were studied to investigate the effects of ligands on both the geometric and electronic structure. Modulating the Pt-Pt distance through the bridging ligand architecture was found to dictate the nature of the lowest energy electronic transitions, localized in one-half of the molecule or delocalized across the entire molecule. By reducing the separation between the platinum atoms, the lowest energy electronic transitions will be dominated by the metal-metal-to-ligand charge transfer transition. Conversely, by increasing the distance between the platinum atoms, the lowest electronic transition will be largely localized metal-to-ligand charge transfer or ligand centered in nature. Additionally, the cyclometalating ligands were observed to have a noticeable stabilizing effect on the triplet excited states as the conjugation increased, arising from geometric reorientation and increased electron delocalization of the ligands. Such stabilization of the triplet state energy has been shown to alter the excited state potential energy landscape as well as the excited state trajectory.

Oxygen-vacancy and phosphate coordination triggered strain engineering of vanadium oxide for high-performance aqueous zinc ion storage

Rechargeable aqueous zinc ion batteries based on vanadium oxides have been increasingly studied because of the low cost and high safety. However, the performance of V-based cathodes is limited mainly due to the strong interaction between intercalated zinc ions and the host structure, low electronic conductivity, phase transitions and unstable architectures upon cycling. Here, strain engineering of vanadium oxide, through oxygen vacancy and phosphate group coordination, is developed. The layered structure of vanadium oxide distorts due to the localized strain to form a “cavity”, which contributes to the abundant zinc-ion storage sites and weakens the strong interaction. The introduced oxygen defects and phosphate groups also boost charge transfer and increase the electronic conductivity of the cathode. More importantly, due to the repulsive force between the phosphate groups and OH−, the phase transitions on the cathode surface during cycling is impeded, resulting in the stable architecture. Thus, the prepared cathode exhibits a high capacity of 161.8 mA h g−1 at 10 A g−1, a long cycle life (96.8% capacity retention after 3000 cycles) and an outstanding rate performance (124.3 mA h g−1 at 20 A g−1). This strain engineering may provide a rational construction to promote the capacity and durability of cathodes for other advanced batteries.

Vacuum-ultraviolet absorption and emission spectroscopy of gaseous, liquid, and supercritical xenon

Bose-Einstein condensation, an effect long known for material particles as cold atomic gases, has in recent years also been observed for photons in microscopic optical cavitites. Here, we report absorption and emission spectroscopic measurements on the lowest electronic transition ($5\text{p}^6 \rightarrow 5\text{p}^5 6\text{s}$) of xenon, motivated by the search for a thermalization medium for photon Bose-Einstein condensation in the vacuum-ultraviolet spectral regime. We have recorded pressure-broadened xenon spectra in the 135 nm to 190 nm wavelength regime at conditions near the critical point. The explored pressure and temperature range includes high pressure gaseous xenon below the critical pressure and supercritical xenon at room temperature, as well as liquid xenon close to the boiling point near the critical pressure.

Near Infrared Polyene Radical‐Cation Derived from 7,8‐Dihydrobenzo[c,d]Furo[2,3‐f]Indole: Synthesis, Spectra and Nature of Electron Transitions

AbstractThe stable polyene cation‐radical containing the 7,8‐dihydrobenzo[cd]furo[2,3–f]indol terminal groups was synthesized and the features of its lowest electron transitions were studied, in detail. The electronic structure and absorption spectra of the polyene cation‐radical obtained, was compared with the electronic and spectral properties of the corresponding neutral poleynes, dications and related polymethine dye having the same terminal groups. While polyene and its dication absorb in the relative short wavelength spectral region, the polymethine‐cation absorbs in the long wavelength region; it is shown that two splitting bands appear in the spectra of the polyenic cation‐radical: the low‐ intensive long wavelength band (≈ 1200 nm) and of the high‐ intensive short wavelength band (≈800 nm); they take correctly into consideration splitting of excited states, generated by α‐ and β‐electrons.

The Comparison of Optical Properties of Acridine Orange when Interact with Hybrids of Single-Walled Carbon Nanotubes and Single-Stranded DNA or DoubleStranded DNA

The optical properties (visible-ultraviolet emission and fluorescence) of acridine orange (AO) were compared and investigated to try to discuss the AO adsorption mechanisms when AO interacted with four aqueous dispersions, for example, single-stranded DNA (ssDNA), double-stranded DNA (dsDNA), hybrids of ssDNA and single-walled carbon nanotubes (SWNT-ssDNA), and hybrids of dsDNA and single-walled carbon nanotubes (SWNT-dsDNA). Molecule structure deformation and energy transfer maybe existed when AO attached to DNA chains or emerged into SWNT/DNA suspensions. DsDNA seemingly brought about an obvious structure deformation to AO; delocalized π-bond of AO/SWNT perhaps produce a little absorbance red-shift effect to AO, because of lower electronic transition energy for π-electron or n-electron; the energy transferred from DNA to AO, or from AO to SWNT. Most of AO molecules “prioritized” DNA chains, rather than SWNT surface; although AO molecules adsorbed to DNA chain and SWNT surface at the same time.

Electronic and spectral properties of phosphonium ylides-betaines, derivatives of 2 oxazoline-5-one with conjugated and non-conjugated substituents

The spectral and quantum-chemical study of phosphonium ylides derivatives of 2‑oxazoline-5-one with both conjugated and non-conjugated substituents were performed. It was shown that considerable positive charge is located at phosphorus atom, whereas the substantial negative charge is fixed at sulfur atom. It has been found from the calculations and 13 C NMR spectral data that introducing of the non-conjugated and conjugated substituents in the position 2 of the oxazole cycle in thiaphosphonium ylides causes only small change in the molecular equilibrium geometry and in charge distribution in oxazole moiety, whereas spectral characteristics of substituted derivatives are very sensitive to the nature of the lowest electron transitions which reflects in changes of their absorption maxima.

Mononuclear Sulfido-Tungsten(V) Complexes: Completing the Tp*MEXY (M = Mo, W; E = O, S) Series.

Orange Tp*WSCl2 has been synthesized from the reactions of Tp*WOCl2 with boron sulfide in refluxing toluene or Tp*WS2Cl with PPh3 in dichloromethane at room temperature. Mononuclear sulfido-tungsten(V) complexes, Tp*WSXY {X = Y = Cl, OPh, SPh, SePh; X = Cl, Y = OPh; XY = toluene-3,4-dithiolate (tdt), quinoxaline-2,3-dithiolate (qdt); and Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate} were prepared by metathesis of Tp*WSCl2 with the respective alkali metal salt of X-/XY2-, or [NHEt3]2(qdt). The complexes were characterized by microanalysis, mass spectrometry, electrochemistry, and infrared (IR), electron paramagnetic resonance (EPR) and electronic absorption spectroscopies. The molecular structures of Tp*WS(OPh)2, Tp*WS(SePh)2, and Tp*WS(tdt) have been determined by X-ray crystallography. The six-coordinate, distorted-octahedral W centers are coordinated by terminal sulfido (W≡S = 2.128(2) - 2.161(1) Å), terdentate facial Tp*, and monodentate/bidentate O/S/Se-donor ligands. The sulfido-W(V) complexes are characterized by lower energy electronic transitions, smaller giso, and larger Aiso(183W) values, and more positive reduction potentials compared with their oxo-W(V) counterparts. This series has been probed by sulfur K-edge X-ray absorption spectroscopy (XAS), the spectra being assigned by comparison to Tp*WOXY (X = Y = SPh; XY = tdt, qdt) and time-dependent density functional theoretical (TD-DFT) calculations. This study provides insight into the electronic nature and chemistry of the catalytically and biologically important sulfido-W unit.

Facile synthesis of graphene oxide sheet-immobilized perylene diimide radical anion salt and its optical response to different solvents and pH values

Graphene oxide/reduced perylene diimide radical anion hybrid (GO/rSFPDI) with high dispersibility in polar organic solvents has been flexibly developed using N,N′-diethylhexyl-1-bromo-7-pentafluorophenox-perylene diimide (SFPDI). The interaction of the GO with the SFPDI suggested the catalysis of perylene diimide (SFPDI) to reduced perylene diimide radical anion (rSFPDI). The successful preparation of GO/rSFPDI was confirmed via FT-IR, Raman, UV–Vis absorption spectrum, XRD, and SEM. Compared to the two neutral SFPDI and GO, GO/rSFPDI exhibited a novel lower energy electronic transition (max. at 700 nm) in addition to the solvatochromism and characteristics of spectral response to pH. Moreover, the fluorescence emission spectrum of GO/rSFPDI shows strong fluorescence quenching in dilute solution.

Perylene diimide dye/layered carbide charge transfer composite: Design, synthesis, and photophysical properties

Brominated perylene diimide dye (BrPDI) was found to intercalate into a host layer of two-dimensional OH-Ti3C2 to produce BrPDI–OTi3C2 nanocomposites that gave a stable dispersion in polar solvents. Compared to the parent BrPDI, BrPDI–OTi3C2 exhibited a novel lower energy electronic transition (max. at ~700nm) in addition to solvatochromism. Furthermore, molecular simulations (density functional theory) indicated that intramolecular charge transfer occurred from the perylene core units to the OH-Ti3C2 moiety in the HOMO–LUMO excitation process of BrPDI–OTi3C2.

Excitation of the 4 lowest electronic transitions in methanol by low-energy electrons

We report differential and integral cross sections for electronic excitation of methanol by low-energy electron impact. Cross sections were measured for the four lowest-lying excited states at incident electron energies from 9 to 20 eV and at scattering angles from 5° through 130°. The measured cross sections were normalized against previously reported elastic scattering data determined using the relative flow method. Corresponding cross-section calculations were carried out using the complex Kohn variational method within a 7-channel close-coupling scheme and using the Schwinger multichannel method within an 11-channel close-coupling scheme.

Rational design of long-wavelength absorbing and emitting carbostyrils aided by time-dependent density functional calculations

We have used computational chemistry methods to aid the rational design of long-wavelength absorbing and emitting organic materials. For this purpose, the vertical electronic transition energies of 3,4-dicyano carbostyrils (quinolone-2(1H)-ones) substituted by electron-donating substituents (methoxy, methylamino, dimethylamino) at positions 6 and 7, are calculated by time-dependent density functional theory (B3LYP) within the Tamm–Dancoff approximation. Bulk solvent effects (DMSO, CH3CN, H2O) were taken into account by the CPCM solvation model. Particular long-wavelength absorptions (∼540nm) are predicted for derivatives containing an amino group in position 6 irrespective of whether a 7-methoxy or a 7-amino group is present. In contrast, considerably shorter wavelength absorption (∼440nm) can be expected for 6-methoxy-7-amino substituted 3,4-dicyano carbostyrils. Optimization of the first excited singlet state of 6-dimethylamino substituted carbostyrils leads to a perpendicular arrangement of the (CH3)2N-group with respect to the heterocyclic ring system accompanied by an extremely low electronic transition energy (1000–1500nm) with vanishingly small intensity (oscillator strength f<0.000). 6-methoxy-7-amino substituted carbostyrils are predicted to emit at 490–520nm. An especially long-wavelength fluorescence (∼660nm) is calculated for 6-amino-7-methoxy- as well as 6,7-bis(methylamino)-3,4-dicyanocarbostyril.

Chromophores in Molecular Nanorings: When Is a Ring a Ring?

The topology of a conjugated molecule plays a significant role in controlling both the electronic properties and the conformational manifold that the molecule may explore. Fully π-conjugated molecular nanorings are of particular interest, as their lowest electronic transition may be strongly suppressed as a result of symmetry constraints. In contrast, the simple Kasha model predicts an enhancement in the radiative rate for corresponding linear oligomers. Here we investigate such effects in linear and cyclic conjugated molecules containing between 6 and 42 butadiyne-linked porphyrin units (corresponding to 600 C-C bonds) as pure monodisperse oligomers. We demonstrate that as the diameter of the nanorings increases beyond ∼10 nm, its electronic properties tend toward those of a similarly sized linear molecule as a result of excitation localization on a subsegment of the ring. However, significant differences persist in the nature of the emitting dipole polarization even beyond this limit, arising from variations in molecular curvature and conformation.

Thermo-selective Tm(x)Ti(1-x)O(2-x/2) nanoparticles: from Tm-doped anatase TiO2 to a rutile/pyrochlore Tm2Ti2O7 mixture. An experimental and theoretical study with a photocatalytic application.

This is an experimental and theoretical study of thulium doped TiO2 nanoparticles. From an experimental perspective, a method was used to synthesize thulium-doped TiO2 nanoparticles in which Tm(3+) replaces Ti(4+) in the lattice, which to our knowledge has neither been reported nor studied theoretically so far. Different proportions of anatase and rutile phases were obtained at different annealing temperatures, and XRD and Raman spectroscopy also revealed the presence of a pyrochlore phase (Tm2Ti2O7) at 1173 K. Thus, the structure of the Tm-doped nanoparticles was thermally-controlled. Furthermore, XPS showed the presence of Tm(3+) in the samples synthesized, which produces oxygen vacancies to maintain the local neutrality in the lattice. The presence of Tm(3+) in the samples led to changes in the UV-Vis absorption spectra, so they showed photoluminescence properties and new states in the band gap, which produce a new lower energy electronic transition than the main TiO2 one. Periodic DFT calculations were performed to understand the experimentally produced structures. The production of oxygen vacancies was analysed and the changes generated in the structure were fully detailed. The DOS and PDOS analyses confirmed the experimental results obtained using UV-Vis spectroscopy, and showed that the new electronic states in the band gap are due to interactions of the f state of Tm and the p state of O. Likewise, the charge study and the ELF analysis indicate that when Tm is introduced into the TiO2 structure, the Ti-O bond around the oxygen vacancy is strengthened. Finally, an example of a photocatalytic application was developed to show the high efficiency of the samples due to the heterojunction in the interfaces of the phases in the samples, which improved the charge separation and the good charge carrier mobility due to the presence of the pyrochlore phase, as was also shown theoretically.

Role of structural order at the P3HT/C60 heterojunction interface

The influence of structural order on the electronic and optical properties of C60/P3HT bulk heterojunctions (BHJs) was studied using first principles DFT and TDDFT methods. The electronic levels alignment between the two phases in the BHJs is mainly controlled by the interfacial dipole moment that shifts the P3HT electronic levels towards higher binding energies with respect to C60 levels. An increasing order translates into an increasing P3HT domains size, for which we considered different stacks of P3HT oligomers. A significant decrease of both the electronic (HOMOP3HT–LUMOC60) and the optical (HOMOP3HT–LUMOP3HT) band gap is observed with an increasing P3HT domain size. TDDFT approach was used to identify the orbitals involved in the electronic transitions, and to reveal that the reduction of the BHJ optical band gap cannot simply be predicted from the variation of the rrP3HT band gap. The lowest electronic transition in rrP3HT becomes optically forbidden due to the formation of H-aggregates.

Spectroscopic Study and Quantum Chemical Analysis of the Asymmetrical Cationic Polymethine Dye Versus Its Symmetrical Analogues

A detailed investigation of the steady-state and time-resolved spectral properties of new cationic asymmetrical trimethine cyanine dye with indolenine and benzoimidazole terminal groups has been performed in comparison with its symmetrical analogues. Steady-state measurements included absorption, fluorescence, excitation anisotropy in several solvents of different polarity and viscosity. Kerr-gated time-resolved study of the fluorescence properties has been performed using 1кHz femtosecond laser system. Experimental results are in agreement with DFT/6-31(d,p)/B3LYP quantum chemical calculations, allowing determination of the charge distribution, bond length alternation, permanent and transition dipole moments, energy levels and the leading molecular orbital configurations responsible for the lowest electronic transitions.

Selective Excitation of Atomic-Scale Dynamics by Coherent Exciton Motion in the Non-Born-Oppenheimer Regime.

Time-domain investigations of the nonadiabatic coupling between electronic and vibrational degrees of freedom have focused primarily on the formation of electronic superpositions induced by atomic motion. The effect of electronic nonstationary-state dynamics on atomic motion remains unexplored. Here, phase-coherent excitation of the two lowest electronic transitions in semiconducting single-walled carbon nanotubes by broadband <5-fs pulses directly triggers coherent exciton motion along the axis of the nanotubes. Optical pump-probe spectroscopy with sub-10-fs time resolution reveals that exciton motion selectively excites the high-frequency G mode coherent phonon, in good agreement with results obtained from time-domain ab initio simulations. This observed phenomenon arises from the direct modulation of the C-C interatomic potential by coherent exciton motion on a time scale that is commensurate with atomic motion. Our results suggest the possibility of employing light-field manipulation of electron densities in the non-Born-Oppenheimer regime to initiate selective atomic motion.