Vibrational Deactivation Research Articles

The 8‐Hydroxyquinolinium Cation as a Lead Structure for Efficient Color‐Tunable Ionic Small Molecule Emitting Materials

Albeit tris(8‐hydroxyquinolinato) aluminum (Alq3) and its derivatives are prominent emitter materials for organic lighting devices, and the optical transitions occur among ligand‐centered states, the use of metal‐free 8‐hydroxyquinoline is impractical as it suffers from strong nonradiative quenching, mainly through fast proton transfer. Herein, it is shown that the problem of rapid proton exchange and vibration quenching of light emission can be overcome not only by complexation, but also by organization of the 8‐hydroxyquinolinium cations into a solid rigid network with appropriate counter‐anions (here bis(trifluoromethanesulfonyl)imide). The resulting structure is stiffened by secondary bonding interactions such as π‐stacking and hydrogen bonds, which efficiently block rapid proton transfer quenching and reduce vibrational deactivation. Additionally, the optical properties are tuned through methyl substitution from deep blue (455 nm) to blue‐green (488 nm). Time‐dependent density functional theory (TDFT) calculations reveal the emission to occur from which an unexpectedly long‐lived S1 level, unusual for organic fluorophores. All compounds show comparable, even superior photoluminescence compared to Alq3 and related materials, both as solids and thin films with quantum yields (QYs) up to 40–50%. In addition, all compounds show appreciable thermal stability with decomposition temperatures above 310 °C.

Luminescent fac-[ReX(CO)3(phenyl-pyta)] (X = Cl, Br, I) complexes: influence of the halide ligand on the electronic properties in solution and in the solid state.

Tricarbonylrhenium(I) complexes that incorporate a chloride ligand are promising photoluminescent materials, but those incorporating a bromide or iodide ligand have received very little attention regarding their solid-state properties. In this work, three rhenium(I) complexes differing only by the nature of their halide ligand (X = Cl, Br, and I) were compared. They are based on a fac-[ReX(CO)3(N^N)] framework where the N^N bidentate ligand is a 3-(2-pyridyl)-1,2,4-triazole unit functionalized by an appended phenyl group. DFT calculations showed that the character of the lowest energy transitions progressively changes from Re → N^N ligand (MLCT) to X → N^N ligand (XLCT) when increasing the size of the halogen atom. Regarding the electrochemical behavior, the chloride and bromide complexes 1-Cl and 1-Br were similar, while the iodide complex 1-I exhibited a strikingly different electrochemical signature in oxidation. From a spectroscopic viewpoint, all three complexes emitted weak red-orange phosphorescence in dichloromethane solution. However, in the solid state, marked differences appeared. Not only was 1-Cl a good emitter of yellow light, but it had strong solid-state luminescence enhancement (SLE) properties. In comparison, 1-Br and 1-I were less emissiveand they showed better mechanoresponsive luminescence (MRL) properties, probably related to a loose molecular arrangement in the crystal packing and to the opening of vibrational non-radiative deactivation pathways. This study highlights for the first time how the nature of the halide ligand in this type of complex allows fine tuning of the solid-state optical properties, for potential applications either in bio-imaging or in the field of MRL-active materials.

Vibrational deactivation in O(3P) + N2 collisions: from an old problem towards its solution

In a recent communication [2021 Phys. Chem. Chem. Phys. 23 15475–79] we showed that the correct modelling of vibrational quenching events in O + N2(v) collisions, a fundamental process in air plasmas, requires the detailed representation of intermediate and asymptotic regions of the interaction and the inclusion of several types of processes as vibration to translation (V–T) and vibro-electronic (V–E) energy transfer. For the first time from the publication of experimental results in the 70’s, we obtained theoretical results in agreement with experiments, even at room temperature. In the present work we extend the approach to better describe non-adiabatic V–E deactivation and include the evaluation of the role of the higher excited singlet N2O surface, characterized by new high quality ab initio calculations, to that of the triplet Π and Σ ones. Within this framework, we calculate V–T, V–E and the corresponding total vibrational relaxation rate coefficients for initial vibrational N2(v) quantum numbers up to v = 10 in a wide temperature range (200–10 000 K). These data are of uttermost importance for the modelling of air plasmas, of earth’s and planetary atmospheres and for the design and construction of aircrafts and air-breathing propulsion systems for very low earth orbit (VLEO) satellites.

Boundary Conditions for Fluid-Dynamic Parameters of a Single-Component Gas Flow with Vibrational Deactivation on a Solid Wall

Boundary Conditions for Fluid-Dynamic Parameters of a Single-Component Gas Flow with Vibrational Deactivation on a Solid Wall

The case of [(bpm)Re(CO)3Br] and anti-[Br(CO)3Re(μ-bpm)Re(CO)3Br] (bpm: 2,2′-bipyrimidine) towards multi-metallic ReI species and their properties

The reaction of [Re(CO)3(THF)Br]2 and 2,2′-bipyrimidine (bpm) in dichloromethane at room temperature employing a stoichiometric 2:1 ratio leads to a mixture of [(bpm)Re(CO)3Br] (21%) and anti-[Br(CO)3Re(μ-bpm)Re(CO)3Br] (79%). When a 1:1 ratio is used, only anti-[Br(CO)3Re(μ-bpm)Re(CO)3Br] is obtained in a 89% yield. The reaction of [Re(CO)5Br] and (bpm) leads to [(bpm)Re(CO)3Br] as a single product. [(bpm)Re(CO)3Br] and anti-[Br(CO)3Re(μ-bpm)Re(CO)3Br] display one and two rhenium(I) centers of distorted octahedral coordination geometry respectively, each with a facial ligand correlation. The intramolecular metal to metal distance is 5.8508(16) Å in anti-[Br(CO)3Re(μ-bpm)Re(CO)3Br]. The HOMO-LUMO gap for [(bpm)Re(CO)3Br] results 0.74 eV larger (3.03 vs 2.29 eV) than the computed for anti-[Br(CO)3Re(μ-bpm)Re(CO)3Br]. In the same sense, the anti-isomer is computed to be 0.14 eV (13.4 kJ mol−1) more stable compared to the hypothetical syn-[Br(CO)3Re(μ-bpm)Re(CO)3Br]. Cyclic voltammetry shows that the difference between the ReI/Re0 reduction and its ReI/ReII oxidation in terms of potential is about 3.46 V for [(bpm)Re(CO)3Br], while this value diminishes to 2.62 V for anti-[Br(CO)3Re(μ-bpm)Re(CO)3Br]. Following this tendency, the MLCT band for [(bpm)Re(CO)3Br] is centered at 385 nm (DMF), while it is red-shifted almost 100 nm for anti-[Br(CO)3Re(μ-bpm)Re(CO)3Br]. While [(bpm)Re(CO)3Br] emits at 680 nm (excited at 380 nm), anti-[Br(CO)3Re(μ-bpm)Re(CO)3Br] remains non-emissive, which is attributed to a favored vibrational deactivation path.

Reconciling experimental and theoretical vibrational deactivation in low-energy O + N2 collisions.

Molecular dynamics calculations of inelastic collisions of atomic oxygen with molecular nitrogen are known to show orders of magnitude discrepancies with experimental results in the range from room temperature to many thousands of degrees Kelvin. In this work, we have achieved an unprecedented quantitative agreement with experiments even at low temperature, by including a non-adiabatic treatment involving vibronic states on newly developed potential energy surfaces. This result paves the way for the calculation of accurate and detailed databases of vibrational energy exchange rates for this collisional system. This is bound to have an impact on air plasma simulations under a wide range of conditions and on the development of Very Low Earth Orbit (VLEO) satellites, operating in the low thermosphere, objects of great technological interest due to their potential at a competitive cost.

Chiroptical property enhancement of chiral Eu(III) complex upon association with DNA-CTMA

DNA-based materials have attracted much attention due to their unique photo-functional properties and potential applications in various fields such as luminescent and biological systems, nanodevices, etc. In this study, the photophysical properties of a chiral Eu(III) complex, namely (Eu(D-facam)3), within DNA films were extensively investigated. The enhancement of photoluminescence (more than 25-folds increase of luminescence quantum yield) and degree of circularly polarization in luminescence (glum = − 0.6) was observed upon interaction with DNA. Various photophysical analyses suggested that the emission enhancement was mainly due to an increase of the sensitization efficiency (high ηsens) from the ligands to Eu(III) and suppression of the vibrational deactivation upon immobilization onto the DNA molecule. From CD and VCD measurements, it was suggested that the coordination structure of Eu(D-facam)3 was affected by the interaction with DNA, suggesting that the structural change of Eu(D-facam)3 contributed to the improvement of its luminescent properties.

Relaxation of Vibrationally Excited OH Radical by SO.

Molecular energy transfer in gas-phase collisions is of large interest in kinetics models. Vibrational excitation and deactivation processes can be concurrent with chemical reaction and/or sources of nascent excited species. In the present work, a full-dimension quasi-classical trajectory study of the vibrational deactivation of OH(v' = 1,5) in collisions with SO is presented. A global potential energy surface for the ground electronic state of HSO2, previously reported, is used to represent the interatomic interactions. The specific initial-state OH deactivation cross section and the corresponding SO activation cross sections are reported. Models for the maximum impact parameter and excitation function are discussed, while some details on the energy transfer mechanisms are also given. Specific initial-state deactivation thermal rate coefficients and vibrational average deactivation thermal rate coefficients are also presented and compared to previous results in the literature.

Solvent dependence of the emission intensities in photoluminescent mononuclear europium(III) complexes with tetradentate Schiff base ligands

Mononuclear tetradentate Schiff base–europium(III) complexes with different counter cations, X[Eu(3,5-Clsalen)2] {X = (C2H5)3NH+, (C2H5)4N+, K+; H2(3,5-Clsalen): N,N′-bis-3,5-dichlorosalicylideneethanediamine}, were prepared and photoluminescence properties of the europium(III) complexes in organic solvents (CH3CN, DMSO, DMF, (CH3)2CO, CH3OH, N-methylformamide (NMF), CH2Cl2) were investigated. All the emission spectra of the complexes displayed similar emission spectral patterns based on the f–f transitions by excitation with 365 nm in the solvents. In contrast, the emission intensities of the complexes varied greatly in the different solvents. All the complexes X[Eu(3,5-Clsalen)2] {X = (C2H5)3NH+, (C2H5)4N+, K+} displayed strong emission intensities in polar aprotic solvents (ϕ = 0.24–0.42 in CH3CN, DMF, DMSO, and (CH3)2CO), and weak emission intensities in polar protic solvents (ϕ = 0.052–0.10 in CH3OH and NMF) at 298 K. The emission intensities of (C2H5)3NH[Eu(3,5-Clsalen)2] and (C2H5)4N[Eu(3,5-Clsalen)2] in CH2Cl2 at 298 K are significantly different (ϕ = 0.016 for (C2H5)3NH[Eu(3,5-Clsalen)2] and ϕ = 0.19 for (C2H5)4N[Eu(3,5-Clsalen)2]). Mononuclear yttrium(III) and gadolinium(III) complexes X[Y(3,5-Clsalen)2] and X[Gd(3,5-Clsalen)2] {X = (C2H5)3NH+, (C2H5)4N+} were prepared. X-ray crystal structure analysis of (C2H5)3NH[Gd(3,5-Clsalen)2] was performed. Measurements of the 1H and 13C NMR spectra of the yttrium(III) complexes were performed to elucidate the structure of the complexes in the solvents. Measurement of the luminescence spectrum of the gadolinium(III) complex in ethanol/methanol (1:1) glass at 77 K was performed to estimate the energy level of the triplet state. The solvent effect on the emission intensities of the complexes is explained by the vibrational nonradiative deactivation caused by the OH, NH, CH vibrations of the solvent molecules, or alkylammonium cations, around the mononuclear europium(III) complex anion.

Pressure effects on the relaxation of an excited hydrogen peroxyl radical in an Argon bath

Molecular dynamics simulations were used to study the effect of pressure on the vibrational deactivation of HO2 embedded in an Ar bath gas at 800 K and at pressures ranging from 10 atm to 400 atm. The time dependent decay of vibrational energy is found to be poly-exponential for all of the simulated pressures. Plots of the relaxation rate constants as a function of density show deviation from the expected linear dependence at ∼250 atm. A combinatorial multi-bath-gas collisional model suggests this deviation is due to the breakdown in the isolated binary collision approximation. Comparisons to studies with similar findings and additional considerations for understanding this behavior are discussed.

Nonradiative Deactivation of Lanthanoid Excited States by Inner-Sphere Carboxylates.

The vibrational deactivation of metal-centered excited states is one of the fundamental processes that governs the luminescence of inorganic luminophores. In molecular lanthanoid luminescence, the most reliable way to modulate and systematically investigate these processes is deuteration of X-H stretching modes (X = O, N, C). Apart from the effect of these high-energy vibrational motifs, very little is known about the impact of other oscillator fragments present in lanthanoid complexes. We have developed a synthetic protocol to efficiently and selectively label the popular chelator motif "pyridine-2-carboxylic acid" with stable (13)C/(18)O isotope at the carboxylate group. The corresponding isotopologic lanthanoid complexes (Ln = Sm, Eu, Ho) show a decrease of the local-mode carbonyl stretching frequency of up to 5% after isotopic substitution. While this does not seems to have any effect on the luminescence of lanthanoids with medium- to high-energy gaps (Sm and Eu), we have found the first example of a quantifiable luminescence isotope effect for one of the near-IR transitions of holmium ((3)K8 → (5)I5) that only involves the isotopic editing of the vibrational environment at the four carbonyl oscillators.

Excited-state vibrational relaxation and deactivation dynamics of trans-4-(N,N-dimethylamino)-4′-nitrostilbene in nonpolar solvents studied by ultrafast time-resolved broadband fluorescence spectroscopy

Abstract The excited-state relaxation dynamics of trans -4-( N , N -dimethylamino)-4′-nitrostilbene excited to the first absorption band in nonpolar solvents (cyclohexane and n -hexadecane) has been studied with ultrafast time-resolved broadband fluorescence spectroscopy. The temporal evolution of the time-resolved fluorescence spectra and bandwidth narrowing reveal a multiple-stage vibrational relaxation within ∼15 ps followed by slower excited-state deactivation steps (80 and 950 ps in cyclohexane). In addition, an oscillatory fluorescence with a period of about 1.1 ps was observed and attributed to the coherent vibrational motion of the nitrophenyl/aminophenyl torsion that modulates the early-time fluorescence spectra. The decay of the total fluorescence intensity with cubic frequency correction was found to be consistent with a competitive triplet–single deactivation mechanism.

Metal-Centered Photoluminescence of Eu3+ and Tb3+ Coordination Polymers with Dianions of Camphoric and Tetrafluoroterephthalic Acids

It was shown that coordination polymers [Ln(Camph)(NO3)(MeOH)2]n (1Ln, Ln – Eu, Tb; Camph2––camphorate dianion) and [Ln2(tFbdc)3(DMF)2(H2O)2]n (2Ln, Ln – Eu, Tb; tFbdc2–– tetrafluoroterephtalate dianion) possess metal-centered emission with quantum yields up to 40% under UV-irradiation. It was found that tetrafluoroterephtalate dianion is better antenna ligand than camphorate dianion. It was assumed that for studied complexes the vibrational radiationless deactivation of excited states of ligands influence more significantly on the luminescence characteristics than energy of triplet states of ligands.

Monometallic europium, terbium, and neodymium complexes formed from the bridging ligand 2,3-bis(2-pyridyl)pyrazine: Crystal structure and temperature dependent luminescent properties

Lanthanide complexes of the form Ln(tdh)3dpp (where Ln=Nd(III), Eu(III) or Tb(III); tdh=1,1,1-trifluoro-5,5-dimethyl-2,4-hexanedione and dpp=2,3-bis(2-pyridyl)pyrazine) have been structurally characterized. X-ray quality crystals were grown by the slow evaporation of concentrated solutions of the complexes. Each metal is 8-coordinate with 6 oxygens from the tdh ligand, 1-pyridyl nitrogen, and 1-pyrazine nitrogen from the dpp ligand. Luminescent studies show that the Nd complex undergoes non-radiative relaxation through solvent vibrational deactivation. In contrast, the Eu and Tb complexes emit in the visible region of the spectrum when solutions of the complex are excited at 388nm associated with the π–π∗ transition of the tdh ligand. Emission lines corresponding to transitions from the 5D0 and 5D4 state to the 7FJ manifold of the Eu(III) and Tb(III), respectively, are observed. The intensity of these emissions decreases as temperature is increased.

Dynamics of ground and excited state vibrational relaxation and energy transfer in transition metal carbonyls.

Nonlinear vibrational spectroscopy provides insights into the dynamics of vibrational energy transfer in and between molecules, a crucial phenomenon in condensed phase physics, chemistry, and biology. Here we use frequency-domain 2-dimensional infrared (2DIR) spectroscopy to investigate the vibrational relaxation (VR) and vibrational energy transfer (VET) rates in different solvents in both the electronic ground and excited states of Re(Cl)(CO)3(4,4'-diethylester-2,2'-bipyridine), a prototypical transition metal carbonyl complex. The strong C≡O and ester C═O stretch infrared reporters, located on opposite sides of the molecule, were monitored in the 1600-2100 cm(-1) spectral region. VR in the lowest charge transfer triplet excited state ((3)CT) is found to be up to eight times faster than in the ground state. In the ground state, intramolecular anharmonic coupling may be solvent-assisted through solvent-induced frequency and charge fluctuations, and as such VR rates are solvent-dependent. In contrast, VR rates in the solvated (3)CT state are surprisingly solvent-insensitive, which suggests that predominantly intramolecular effects are responsible for the rapid vibrational deactivation. The increased VR rates in the excited state are discussed in terms of intramolecular electrostatic interactions helping overcome structural and thermodynamic barriers for this process in the vicinity of the central heavy atom, a feature which may be of significance to nonequilibrium photoinduced processes observed in transition metal complexes in general.

Plasma kinetics in molecular plasmas and modeling of reentry plasmas

State-to-state non-equilibrium plasma kinetics is widely used to characterize cold molecular and reentry plasmas. The approach requires a high level of dynamical information, and demands a large effort in the creation of complete databases of state-resolved cross sections and rate coefficients. Recent results, emphasizing the dependence of elementary process probability on both the vibrational and rotational energy content of the H2 molecule, are presented for those channels governing the microscopic collisional dynamics in non-equilibrium plasmas, i.e. electron-impact induced resonant processes, vibrational deactivation and dissociation in atom–diatom collisions and atomic recombination at the surface. Results for H2 plasmas, i.e. negative ion sources for neutral beam injection in fusion reactors, RF parallel-plate reactors for microelectronics, atmospheric discharges and the shock wave formed in the hypersonic entry of vehicles in planetary atmosphere for aerothermodynamics, are discussed.

Synthesis, luminescence properties, and theoretical insights of N-alkyl- or N, N-dialkyl-pyrene-1-carboxamide

We report the synthesis and photophysical properties of N-alkyl- or N, N-dialkyl-pyrene-1-carboxamide. These derivatives, as well as pyrene, exhibited blue emission. N-Alkyl-type derivatives exhibited strong fluorescence emission ( Φ fl = 0.61 in EtOH) in both nonpolar and polar solvents. On the other hand, N, N-dialkyl-type derivatives showed weak fluorescence emission ( Φ fl <0.01) due to vibrational deactivation. However, in highly viscous solvents such as glycerin, the quantum efficiencies of N-alkyl-type ( Φ fl = 0.91) and N, N-dialkyl-type ( Φ fl = 0.082) derivatives were increased. We also investigated the fluorescence mechanism of these compounds using time-dependent density-functional theory (TD-DFT). From these results, we find that highly fluorescent pyrene-1-carboxamide derivatives can be designed by introducing an appropriate functional group at the nitrogen atom of the amide. Thus, N, N-dialkyl-type pyrene-1-carboxamide has considerable potential for use in applications such as environmental response sensors and probes.

Vibrational and rotational cooling of NO+ in collisions with He

A quantum mechanical investigation of the vibrational and rotational deactivation of NO(+) in collisions with He atoms in the cold and ultracold regime is presented. Ab initio potential energy calculations are carried out at BCCD(T) level and a new global 3D potential energy surface (PES) is obtained by fitting ab initio points within the reproducing kernel Hilbert space method. As a first test of this PES the bound state energies of the (3)He-NO(+) and (4)He-NO(+) complexes are calculated and compared to previous rigid rotor calculations. The efficiency of the vibrational and the rotational cooling of this molecular ion using a buffer gas of helium is then investigated by performing close coupling scattering calculations for collision energy ranging from 10(-6) to 2000 cm(-1). The calculations are performed for the two isotopes (3)He and (4)He and the results are compared to the available experimental data.

Probing the local coordination environment and nuclearity of uranyl(vi) complexes in non-aqueous media by emission spectroscopy

We describe the synthesis, solid state and solution properties of two families of uranyl(VI) complexes that are ligated by neutral monodentate and anionic bidentate P=O, P=NH and As=O ligands bearing pendent phenyl chromophores. The uranyl(VI) ions in these complexes possess long-lived photoluminescent LMCT (3)Π(u) excited states, which can be exploited as a sensitive probe of electronic structure, bonding and aggregation behaviour in non-aqueous media. For a family of well defined complexes of given symmetry in trans-[UO(2)Cl(2)(L(2))] (L = Ph(3)PO (1), Ph(3)AsO (2) and Ph(3)PNH (3)), the emission spectral profiles in CH(2)Cl(2) are indicative of the strength of the donor atoms bound in the equatorial plane and the uranyl bond strength; the uranyl LMCT emission maxima are shifted to lower energy as the donor strength of L increases. The luminescence lifetimes in fluid solution mirror these observations (0.87-3.46 μs) and are particularly sensitive to vibrational and bimolecular deactivation. In a family of structurally well defined complexes of the related anion, tetraphenylimidodiphosphinate (TPIP), monometallic complexes, [UO(2)(TPIP)(thf)] (4), [UO(2)(TPIP)(Cy(3)PO)] 5), a bimetallic complex [UO(2)(TPIP)(2)](2) (6) and a previously known trimetallic complex, [UO(2)(TPIP)(2)](3) (7) can be isolated by variation of the synthetic procedure. Complex 7 differs from 6 as the central uranyl ion in 7 is orthogonally connected to the two peripheral ones via uranyl → uranium dative bonds. Each of these oligomers exhibits a characteristic optical fingerprint, where the emission maxima, the spectral shape and temporal decay profiles are unique for each structural form. Notably, excited state intermetallic quenching in the trimetallic complex 7 considerably reduces the luminescence lifetime with respect to the monometallic counterpart 5 (from 2.00 μs to 1.04 μs). This study demonstrates that time resolved and multi-parametric luminescence can be of value in ascertaining solution and structural forms of discrete uranyl(VI) complexes in non-aqueous solution.

Role of electronic friction during the scattering of vibrationally excited nitric oxide molecules from Au(111)

Some time ago, it has been observed that vibrationally highly excited $\text{NO}(v)$ molecules (with typical vibrational quantum numbers $v\ensuremath{\approx}15$) lose substantial amounts of vibrational energy when scattering off a Au(111) surface [H. Huang, C. Rettner, D. Auerbach, and A. Wodtke, Science 290, 111 (2000)]. This has been interpreted as a sign for the breakdown of the Born-Oppenheimer approximation due to vibration-electron coupling. It has been argued that this process cannot be understood on the basis of single-quantum transitions which are typical for ``electronic friction'' models based on a perturbative treatment of weak vibration-electron couplings. Rather, multiple-quanta transitions characteristic for strong nonadiabatic effects are needed according to recent classical surface hopping calculations involving multiple potential-energy surfaces and model Hamiltonians [N. Shenvi, S. Roy, and J. C. Tully, Science 326, 829 (2009)]. Here we address the importance and magnitude of electronic friction for $\text{NO}@\text{Au}(111)$ by using fully quantum-mechanical, parameter-free first-principles theories in reduced dimensions. Periodic density-functional theory calculations are performed to obtain a ground-state potential-energy surface along the desorption and NO-vibration coordinates, and coordinate-resolved, finite NO vibrational lifetimes due to vibration-electron coupling. Using this input, the scattering event is modeled by open-system density-matrix theory in the frame of the coupled-channel-density-matrix method, which allows for the inclusion of energy relaxation of the scattering NO molecules. It is found that within this model at least, electronic friction accounts for the observed vibrational deactivation of NO scattering from gold.