Vibronic Sidebands Research Articles

Advancing MR‐TADF OLED Toward True‐Blue CIE Value via Asymmetric Host Materials with Amorphous Morphology

AbstractThe new host materials are reported to boost the multiple resonance‐induced thermally activated delayed fluorescence (MR‐TADF) based pure deep‐blue organic light emitting diodes (OLEDs) toward further shortening the emission full width at the half maximum (FWHM). Based on an unusual asymmetric design concept, two new host compounds Bz2cb and Bz2cbz are synthesized. Despite possessing a compact and rigid molecular packing in crystal, Bz2cbz shows pure amorphous morphology via vapor deposition, as confirmed by the grazing‐incidence wide‐angle X‐ray scattering (GIWAXS) analysis. Via incorporating a promising MR‐TADF emitter, ν‐DABNA‐O‐Me, into Bz2cb and Bz2cbz films, highly pure deep‐blue OLEDs are successfully demonstrated. Remarkably, the Bz2cbz device exhibits a 464 nm electroluminescence (EL) with narrow FWHM of 22 nm, accompanied by the reduction of the 0–1 vibronic sideband, and a maximum external quantum efficiency (EQEmax) of 28.2%, which, overall, reaches the true‐blue Commission Internationale de l'Eclairage coordinates (CIE) of (0.13, 0.07) and the high blue index of 253. The amorphous film formation turns out to be an important factor that is previously unrecognized to externally fine‐tune the MR‐TADF OLEDs toward even higher color purity.

Comparison of Performance between Single- and Multiparameter Luminescence Thermometry Methods Based on the Mn5+ Near-Infrared Emission.

Herein, we investigate the performance of single- and multiparametric luminescence thermometry founded on the temperature-dependent spectral features of Ca6BaP4O17:Mn5+ near-infrared emission. The material was prepared by a conventional steady-state synthesis, and its photoluminescence emission was measured from 7500 to 10,000 cm-1 over the 293-373 K temperature range in 5 K increments. The spectra are composed of the emissions from 1E → 3A2 and 3T2 → 3A2 electronic transitions and Stokes and anti-Stokes vibronic sidebands at 320 cm-1 and 800 cm-1 from the maximum of 1E → 3A2 emission. Upon temperature increase, the 3T2 and Stokes bands gained in intensity while the maximum of 1E emission band is redshifted. We introduced the procedure for the linearization and feature scaling of input variables for linear multiparametric regression. Then, we experimentally determined accuracies and precisions of the luminescence thermometry based on luminescence intensity ratios between emissions from the 1E and 3T2 states, between Stokes and anti-Stokes emission sidebands, and at the 1E energy maximum. The multiparametric luminescence thermometry involving the same spectral features showed similar performance, comparable to the best single-parameter thermometry.

Спектры люминесценции и необычный температурный сдвиг бесфононной эмиссионной линии V-=SUP=-3+-=/SUP=- в SrTiO-=SUB=-3-=/SUB=-

The paper presents the first observation of photoluminescence spectra of V3+ impurity in SrTiO3. The broad band of the observed emission is located in the near IR region and at low temperatures consists of a pronounced zero-phonon line (ZPL) with a maximum at 1157.1 nm (8642 сm-1) at 77 K and developed vibronic sidebands extending to 1450 nm. The observed ZPL is associated with an intracenter 1T2g → 3T1g or a closely located 1Eg → 3T1g emission transition in V3+ (3d2) ions replacing Ti4+ ions. It was found that the temperature shift of zero-phonon line is unusually large and its frequency decreases upon lowering the temperature. The local configuration instability of V3+ ions in the 3T1g ground state caused by soft TO1 temperature-dependent phonon mode present in SrTiO3 is considered as a possible source of the observed unusual temperature shift.

Circularly Polarized Light Probes Excited-State Delocalization in Rectangular Ladder-type Pentaphenyl Helices.

Ladder-type pentaphenyl chromophores have a rigid, planar π-system and show bright fluorescence featuring pronounced vibrational structure. Such moieties are ideal for studying interchromophoric interactions and delocalization of electronic excitations. We report the synthesis of helical polymers with a rigid square structure based on spiro-linked ladder-type pentaphenyl units. The variation of circular dichroism with increasing chain length provides direct evidence for delocalization of electronic excitations over at least 10 monomeric units. The change in the degree of circular polarization of the fluorescence across the vibronic side bands shows that vibrational motion can localize the excitation dynamically to almost one single unit through breakdown of the Born-Oppenheimer approximation. The dynamic conversion between delocalized and localized excited states provides a new paradigm for interpreting circular dichroism in helical polymers such as proteins and polynucleic acids.

Circularly Polarized Light Probes Excited‐State Delocalization in Rectangular Ladder‐type Pentaphenyl Helices

AbstractLadder‐type pentaphenyl chromophores have a rigid, planar π‐system and show bright fluorescence featuring pronounced vibrational structure. Such moieties are ideal for studying interchromophoric interactions and delocalization of electronic excitations. We report the synthesis of helical polymers with a rigid square structure based on spiro‐linked ladder‐type pentaphenyl units. The variation of circular dichroism with increasing chain length provides direct evidence for delocalization of electronic excitations over at least 10 monomeric units. The change in the degree of circular polarization of the fluorescence across the vibronic side bands shows that vibrational motion can localize the excitation dynamically to almost one single unit through breakdown of the Born‐Oppenheimer approximation. The dynamic conversion between delocalized and localized excited states provides a new paradigm for interpreting circular dichroism in helical polymers such as proteins and polynucleic acids.

Intensity of cooperative vibronic transitions in the Eu(III), Gd(III) and Tb(III) formates spectra.

The luminescence and absorption spectra of the lanthanide ions in solids and coordination compounds are characterized by sharp pure electronic lines, which are accompanied by much weaker lines of vibronic transitions. The vibronic spectroscopy is a good probing tool for investigations of the properties of surrounding ion ligands. The lanthanides formates are efficient luminescent crystals and can be viewed as the elementary type in the whole class of the oxygen-containing lanthanide coordination compounds. The intensity of vibronic transitions in spectra of luminescence and excitation europium (5 D0 →7 F2 , 7 F0 →5 D2 ), terbium (7 F6 →5 D4 ), gadolinium (6 P7/2 →8 S7/2 ) in anhydrous formates of the type Ln(HCOO)3 (Ln = Eu, Tb, Gd) and Y(HCOO)3 .2H2 O doped with Eu3+ and Tb3+ (C ~1 mol%) are reported. Also, the infrared and Raman spectra were obtained for the same compounds. Related integral intensity vibronic sidebands depend on the type of electronic transition of the same ion and varies for the same electronic transitions in different crystals. The obtained experimental data referring to the rate constants of vibronic transitions and intensity distribution in vibronic spectra on normal vibrations of the formate groups are in agreement with the predictions based on the Stavola-Dexter theory of cooperative vibronic transitions.

Spectroscopic investigation of the different complexation and extraction properties of diastereomeric diglycolamide ligands

Abstract (2R,2′S)-2,2′-oxybis-(N,N-didecylpropanamide) (cis-mTDDGA) and (2R,2′R)-2,2′-oxybis-(N,N-didecylpropanamide) (trans-mTDDGA) were studied using time-resolved laser fluorescence spectroscopy (TRLFS), vibronic side-band spectroscopy (VSBS) and density functional theory calculations (DFT) to find reasons for their different extraction properties. Stability constants of the respective Cm(III) and Eu(III) complexes show cis-mTDDGA to be the superior ligand which is in agreement with results from extraction experiments. cis-mTDDGA extracts Cm(III) and Eu(III) as 1:3 complexes. In case of trans-mTDDGA, 1:2 complexes of the form [M(trans-mTDDGA)2(η1-NO3)(H2O)2]2+ (M = Cm, Eu) are extracted additionally to the 1:3 complexes. VSBS and DFT confirm the presence of inner-sphere nitrate in the 1:2 complex.

Disordered ensembles of strongly coupled single-molecule plasmonic picocavities as nonlinear optical metamaterials.

We propose to use molecular picocavity ensembles as macroscopic coherent nonlinear optical devices enabled by nanoscale strong coupling. For a generic picocavity model that includes molecular and photonic disorder, we derive theoretical performance bounds for coherent cross-phase modulation signals using weak classical fields of different frequencies. We show that strong coupling of the picocavity vacua with a specific vibronic sideband in the molecular emission spectrum results in a significant variation of the effective refractive index of the metamaterial relative to a molecule-free scenario due to a vacuum-induced Autler-Townes effect. For a realistic molecular disorder model, we demonstrate that cross-phase modulation of optical fields as weak as 10 kW/cm2 is feasible using dilute ensembles of molecular picocavities at room temperature, provided that the confined vacuum is not resonantly driven by the external probe field. Our work paves the way for the development of plasmonic metamaterials that exploit strong coupling for optical state preparation and quantum control.

Structural characterisation of hydrolysed Cm(III)-EDTA solution species under alkaline conditions: a TRLFS, vibronic side-band and quantum chemical study

We present a combined theoretical and experimental study of the Cm(III)-EDTA system in alkaline aqueous solutions. Time-resolved laser-fluorescence spectroscopy and vibronic side-band spectroscopy measurements are performed with , and . The evaluation of the TRLFS spectra and corresponding fluorescence lifetimes hints towards the predominance of Cm(EDTA) at pH = 7, with the subsequent formation of the hydrolysis species Cm(OH)(EDTA) with increasing pH. This speciation scheme is further supported by the excellent agreement obtained between experimental and calculated vibronic side bands. The relative stabilities of the complexes Cm(OH)(EDTA) with n = 0−2 and p = 1−2 are discussed on the basis of the equilibrium constants calculated from quantum chemical calculations.

Characterization of the red thermoluminescence emission of doped alumina: A case study for Al2O3:C and Al2O3:Cr,Ni

We herein report on the radiation effect on the red thermoluminescence (TL) emission of synthetic Al2O3:C (commercial TLD-500 supplied by Harshaw) and Al2O3:Cr,Ni (grown by the Verneuil method). The materials were previously characterized by cathodoluminescence (CL), displaying significant differences in the UV-IR spectral emission range related to the presence of dopants. Whilst the CL curve of C-doped alumina exhibits four wavebands peaked at 325 (due to F+ centers) 411 (F centers), 648 (Fe3+) and 785 nm (Ti3+), the Al2O3:Cr,Ni sample shows a narrow, sharp, high intensity maximum at 692 nm with two vibronic side bands of lower intensity at 679 nm and 713 nm attributed to the R-lines of Cr3+. The red TL emission at 650 nm of both materials indicates that they could potentially be used as dosimeters since (i) are sensitive to ionizing radiation at doses over 10 mGy; (ii) show a linear dose response in the range of 10–100 mGy with regression coefficients of 0.999 and (iii) can be reusable with negligible variation in the intensity (never higher than±0.8%) and no observed changes in the shape of the TL curves. The red glow emission intensity corresponding to the C-doped sample hardly changes with the storage time, however Al2O3:Cr,Ni fades significantly (ca 70%), up to ~700 h of storage following a behavior that can be fitted to a decreasing exponential function. Tmax-Tstop test suggests the coexistence of continuous and discrete trap systems in both compounds.

Extraordinary Photostability and Davydov Splitting in BN-Sandwiched Single-Layer Tetracene Molecular Crystals.

Two-dimensional molecular crystals have been beyond the reach of systematic investigation because of the lack or instability of their well-defined forms. Here, we demonstrate drastically enhanced photostability and Davydov splitting in single and few-layer tetracene (Tc) crystals sandwiched between inorganic 2D crystals of graphene or hexagonal BN. Molecular orientation and long-range order mapped with polarized wide-field photoluminescence imaging and optical second-harmonic generation revealed high crystallinity of the 2D Tc and its distinctive orientational registry with the 2D inorganic crystals, which were also verified with first-principles calculations. The reduced dielectric screening in 2D space was manifested by enlarged Davydov splitting and attenuated vibronic sidebands in the excitonic absorption and emission of monolayer Tc crystals. Photostable 2D molecular crystals and their size effects will lead to novel photophysical principles and photonic applications.

Engineering couplings for exciton transport using synthetic DNA scaffolds

Summary Control over excitons enables electronic energy to be harnessed and transported for light harvesting and molecular electronics. Such control requires nanoscale precision over the molecular components. Natural light-harvesting systems achieve this precision through sophisticated protein machinery, which is challenging to replicate synthetically. Here, we introduce a DNA-based platform that spatially organizes cyanine chromophores to construct tunable excitonic systems. We synthesized DNA-chromophore nanostructures and characterized them with ensemble ultrafast and single-molecule spectroscopy and structure-based modeling. This synthetic approach facilitated independent control over the coupling among the chromophores and between the chromophores and the environment. We demonstrated that the coupling between the chromophores and the environment could enhance exciton transport efficiency, highlighting the key role of the environment in driving exciton dynamics. Control over excitons, as reported here, offers a path toward the development of designer nanophotonic devices.

Absorption and Circular Dichroism Spectra of Molecular Aggregates With the Full Cumulant Expansion.

The exciton Hamiltonian of multichromophoric aggregates can be probed by spectroscopic techniques such as linear absorption and circular dichroism. To compare calculated Hamiltonians to experiments, a lineshape theory is needed, which takes into account the coupling of the excitons with inter- and intramolecular vibrations. This coupling is normally introduced in a perturbative way through the cumulant expansion formalism and further approximated by assuming a Markovian exciton dynamics, for example with the modified Redfield theory. Here, we present the implementation of the full cumulant expansion (FCE) formalism (J. Chem. Phys.142, 2015, 09410625747060) to efficiently compute absorption and circular dichroism spectra of molecular aggregates beyond the Markov approximation, without restrictions on the form of exciton–phonon coupling. By employing the LH2 system of purple bacteria as a challenging test case, we compare the FCE lineshapes with the Markovian lineshapes obtained with the modified Redfield theory, showing that the latter presents a less satisfying agreement with experiments. The FCE approach instead accurately describes the lineshapes, especially in the vibronic sideband of the B800 peak. We envision that the FCE approach will become a valuable tool for accurately comparing model exciton Hamiltonians with optical spectroscopy experiments.

Trivalent Actinide Ions Showing Tenfold Coordination in Solution.

Trivalent actinides generally exhibit ninefold coordination in solution. 2,6-Bis(5,6-dipropyl-1,2,4-triazin-3-yl)pyridine (nPr-BTP), a tridentate nitrogen donor ligand, is known to form ninefold coordinated 1:3 complexes, [An(nPr-BTP)3]3+ (An = U, Pu, Am, Cm) in solution. We report a Cm(III) complex with tenfold coordination in solution, [Cm(nPr-BTP)3(NO3)]2+. This species was identified using time-resolved laser fluorescence spectroscopy (TRLFS), vibronic side band spectroscopy (VSBS), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT). Adding nitrate to a solution of the [Cm(nPr-BTP)3]3+ complex in 2-propanol shifts the Cm(III) emission band from 613.1 to 617.3 nm. This bathochromic shift is due to a higher coordination number of the Cm(III) ion in solution, in agreement with the formation of the [Cm(nPr-BTP)3(NO3)]2+ complex. The formation of this complex exhibits slow kinetics in the range of 5 to 12 days, depending on the water content of the solvent. Formation of a complex [Cm(nPr-BTP)3(X)]2+ was not observed for anions other than nitrate (X- = NO2-, CN-, or OTf-). The formation of the [Cm(nPr-BTP)3(NO3)]2+ complex was studied as a function of NO3- and nPr-BTP concentrations, and slope analyses confirmed the addition of one nitrate anion to the [Cm(nPr-BTP)3]3+ complex. Experiments with varied nPr-BTP concentration show that [Cm(nPr-BTP)3(NO3)]2+ only forms at nPr-BTP concentrations below 10-4 mol/L whereas for concentrations greater than 10-4 mol/L the formation of the tenfold species is suppressed and [Cm(nPr-BTP)3]3+ is the only species present. The presence of the tenfold coordinated complex is supported by VSBS, XPS, and DFT calculations. The vibronic side band of the [Cm(nPr-BTP)3(NO3)]2+ complex exhibits a nitrate stretching mode not observed in the [Cm(nPr-BTP)3]3+ complex. Moreover, XPS on [M(nPr-BTP)3(NO3)](NO3)2 (M = Eu, Am) yields signals from both non-coordinated and coordinated nitrate. Finally, DFT calculations reveal that the energetically most favored structure is obtained if the nitrate is positioned on the C2 axis of the D3 symmetrical [Cm(nPr-BTP)3]3+ complex with a bond distance of 413 pm. Combining results from TRLFS, VSBS, XPS, and DFT provides sound evidence for a unique tenfold coordinated Cm(III) complex in solution-a novelty in An(III) solution chemistry.

Luminescence spectroscopy of Cr3+ ions in bulk single crystalline β-Ga2O3

Detailed investigations of the spectroscopic properties of Cr3+ ions in β-Ga2O3:0.05% Cr3+ single crystals grown by the floating zone technique have been performed in the temperature range 4.5–550 K. The luminescence of Cr3+ ions at low temperatures is due to narrow R-lines (2E → 4A2 transitions) and their vibronic sidebands, whereas the broad intense band (4T2 → 4A2 transition) dominates at temperatures above 200 K. The vibronic sidebands of R-lines with amplitude of less than 5% of the R1-line at a temperature of 4.5 K cover the region of 700–735 nm. The temperature dependences of the luminescence intensity and the decay time of Cr3+ ions indicate the same mechanism of quenching of R-line intensity and shortening of lifetime for 2E of Cr3+ ions. The temperature dependence of the R1-line decay time of β-Ga2O3:Cr3+ with maximal temperature coefficient |Δτ/ΔT| = 0.023 ms K−1 at 120 K and maximal specific sensivitity |(Δτ/ΔT)τ−1| = 0.017 K−1 at 160 K indicates an application potential of this phosphor for low-temperature fluorescence thermometry.

Observation of a 1.979-eV spectral line of a germanium-related color center in microdiamonds and nanodiamonds

Color centers in diamond are considered as a platform for quantum computing and communications, biomedical markers, and nanosensors. Negatively charged split-vacancy centers have outstanding properties due to bright and almost monochromatic luminescence, but they have poor spin coherence and relaxation times. This drawback is believed to be absent in the neutral charge state of the defects. So far only the neutral silicon-vacancy center has been observed in luminescence and absorption spectra. Here we report the observation of its germanium-based analog in luminescence spectra with the zero-phonon line (ZPL) at 1.979 eV in samples containing negatively charged ${\mathrm{GeV}}^{\ensuremath{-}}$ centers. The relationship between the center and Ge dopant is unambiguously confirmed by studies of $^{12}\mathrm{C}$ diamonds containing $^{70}\mathrm{Ge}$, $^{73}\mathrm{Ge}$, or $^{76}\mathrm{Ge}$ isotopes. The intensity ratio of the ZPL of these centers varies with the crystal size, and the intensity of the ${\mathrm{GeV}}^{0}$ centers reaches its maximum in samples 150 nm in size. In the vibronic sideband of this center the local vibrational mode with an energy of 23 meV was identified. The density functional theory calculations yield a ZPL energy value of the ${\mathrm{GeV}}^{0}$ center which is 150 meV higher than the experimentally observed one and matches the values of its relaxational energy as well as the local phonon mode frequency.

Large Spin-Orbit Splitting of Deep In-Gap Defect States of Engineered Sulfur Vacancies in Monolayer WS_{2}.

Structural defects in 2D materials offer an effective way to engineer new material functionalities beyond conventional doping. We report on the direct experimental correlation of the atomic and electronic structure of a sulfur vacancy in monolayer WS_{2} by a combination of CO-tip noncontact atomic force microscopy and scanning tunneling microscopy. Sulfur vacancies, which are absent in as-grown samples, were deliberately created by annealing in vacuum. Two energetically narrow unoccupied defect states followed by vibronic sidebands provide a unique fingerprint of this defect. Direct imaging of the defect orbitals, together with abinitio GW calculations, reveal that the large splitting of 252±4 meV between these defect states is induced by spin-orbit coupling.

Higher Order Vibronic Sidebands of Chlorophyll a and Bacteriochlorophyll a for Enhanced Excitation Energy Transfer and Light Harvesting.

Optical absorption and fluorescence spectra of molecules in condensed phases often show extensive sidebands. Originating from electron-vibrational and electron-phonon couplings, these spectral tails bear important information on the dynamics of electronic states and processes the molecules are involved in. The vibronic sidebands observed in conjugate Qy absorption and fluorescence spectra of chlorophyll a and bacteriochlorophyll a are relatively weak, characterized by the total Huang-Rhys factor which is less than one. Therefore, it is widely considered that only fundamental intramolecular modes are responsible for their formation. Here, we provide evidence for extra-long and structured fluorescence tails of chlorophyll a and bacteriochlorophyll a as far as 4000 cm-1 from respective spectral origins, far beyond the frequency range of fundamental modes. According to quantum chemical simulations, these sidebands extending to ∼960 nm in chlorophyll a and ∼1140 nm in bacteriochlorophyll a into the infrared part of the optical spectrum are mainly contributed to by vibrational overtones of the fundamental modes. Because energy transfer and relaxation processes generally depend on vibronic overlap integrals, these findings potentially contribute to better understanding of many vital photo-induced phenomena, including photosynthetic light harvesting.

Anti-Stokes fluorescence of Cr3+ ions doped crystals excited in one-phonon vibronic sidebands

The fluorescence of Cr3+ ions doped insulating crystals was studied under the laser excitation within the one-phonon vibronic sidebands of zero-phonon 4A2 - 2E transitions. The phonon-assisted anti-Stokes fluorescence spectra were observed at ambient and liquid nitrogen temperature. The spectroscopic results suggest that one-phonon vibronic sidebands of zero-phonon lines in impurity ions fluorescence spectra are of interest from the point of view of achieving optical refrigeration starting at ambient temperature as well as significantly below it.

Intense deep-red zero phonon line emission of Mn4+ in double perovskite La4Ti3O12.

Phosphors that emit in the deep-red spectral region are critical for plant cultivation light-emitting diodes. Herein, ultrabroadband deep-red luminescence of Mn4+ in La4Ti3O12 was studied, which showed intense zero phonon line emission. The double-perovskite structural La4Ti3O12 simultaneously contains two Ti4+ sites forming slightly- and highly-distorted TiO6 octahedra, respectively. The influence of octahedral distortion on the Mn4+ emission energy in the two distinct Ti4+ sites was studied both experimentally and theoretically. The spectral measurements indicated that Mn4+ in La4Ti3O12 showed intense zero phonon line emission (ZPL) at deep-red 710-740 nm under excitation of 400 nm charging the O2-→ Mn4+ charge transfer transition. The splitting of the ZPL of the Mn4+ 2Eg→4A2g transition as well as the intensity of ZPL relative to the vibronic phonon sideband emissions were found to be greatly influenced by the degree of octahedral distortion. The crystal-field strength and Racah parameters of Mn4+ in each Ti4+ site were also estimated. The Mn4+ 2Eg→4A2g luminescence exhibited severe thermal quenching, which was explained by the low-lying 4T2g level and charge-transfer state.