Emission Spectral Fitting Research Articles

High-Throughput Screening and Automated Data-Driven Analysis of the Triplet Photophysical Properties of Structurally Diverse, Heteroleptic Iridium(III) Complexes.

Steady state emission spectra and excited state lifetimes were measured for 1440 distinct heteroleptic [Ir(C^N)2(N^N)]+ complexes prepared via combinatorial parallelized synthesis; 72% of the complexes were found to be luminescent, and the emission maxima of the library spanned the visible spectrum (652-459 nm). Spectral profiles ranged from broad structureless bands to narrow emissions exhibiting vibrational substructure. Measured excited state lifetimes ranged between ∼0.1-14 μs. Automated emission spectral fitting with successive Gaussian functions revealed four distinct measured classes of excited states; in addition to well understood metal-ligand to ligand-charge transfer (3MLLCT) and ligand-centered (3LC) excited states, our classification also identified photophysical characteristics of less explored mixed 3MLLCT/3LC states. Electronic structure features obtained from DFT calculations performed on a large subset of these Ir(III) chromophores offered clear insights into the excited state properties and allowed the prediction of structure/luminescence relationships in this class of commonly used photocatalysts. Models with high prediction accuracy (R2 = 0.89) for emission color were developed on the basis of experimental data. Furthermore, different degrees of nuclear reorganization in the excited state were shown to significantly impact emission energy and excited state lifetimes.

Bright Luminescent Platinum(II)-Biaryl Emitters Synthesized Without Air-Sensitive Reagents.

Transition-metal complexes bearing biaryl-2,2'-diyl ligands tend to show intense luminescence. However, difficulties in synthesis have prevented their further functionalization and practical applications. Herein, a series of platinum(II) complexes bearing biaryl-2,2'-diyl ligands, which have never been prepared in air, were synthesized through transmetalation and successive cyclometalation of biarylboronic acids. This approach does not require any air- or moisture-sensitive reagents and features a simple synthesis even in air. The resulting (Et4 N)2 [Pt(m,n-F2 bph)(CN)2 ] (m,n-F2 bph=m,n-difluorobiphenyl-2,2'-diyl) complexes exhibit intense green emissions with high quantum efficiencies of up to 0.80 at 298 K. The emission spectral fitting and variable-temperature emission lifetime measurements indicate that the high quantum efficiency was achieved because of the tight packing structure and strong σ-donating ability of bph.

Characterization of pulsed metallic hydride vacuum arc discharge plasmas by optical emission spectroscopy

The characteristics of plasmas in a titanium hydride vacuum arc ion source were experimentally investigated by a temporally- and spatially-integrated optical emission spectroscopy method. A plasma emission spectral fitting model was developed to calculate the plasmas temperature and relative density of each particle component, assuming plasmas were in local thermodynamic equilibrium state and optical thin in this study. The good agreement was founded between the predicted and measured spectra in the interesting regions of 330–340 nm and 498–503 nm for Ti+ ion and Ti atom respectively, while varying the plasma temperature and density. Compared with conventional Boltzmann plot method, this method, therefore, made a significant improvement on the plasma diagnosis in dealing with the spectral profile with many lines overlapped. At the same time, to understand the mechanism of the occluded-gas vacuum arc discharge plasmas, the plasmas emission spectra, ion relative density, and temperature with different discharge conditions were studied. The results indicated that the rate of Ti metal evaporation and H desorption from the electrode would be enhanced with arc current, and the ionization temperature increased with the feed-in power of arc discharge, leading more H+ and Ti+ ions, but reducing the H+ proportion in arc discharged plasmas.

Distance Dependence of Intrahelix RuII* to OsII Polypyridyl Excited-State Energy Transfer in Oligoproline Assemblies

Energy transfer between the metal-to-ligand charge transfer (MLCT) excited states of [Pra [M(II)(bpy)2(4-Me-4'(-N(H)CO)bpy)](PF6)2 units ([Pra(M(II)bpy2(mbpy)](2+): M(II) = Ru(II) or Os(II), bpy = 2,2'-bipyridine, mbpy = 4'-methyl-2,2'-bipyridine-4-carboxamido, Pra = 4-M(II)-L-proline) linked covalently to oligoproline assemblies in room temperature acetonitrile occurs on the picosecond-nanosecond time scale and has been time-resolved by transient emission measurements. Three derivatized oligoprolines, [CH3-CO-Pro6-Pra[Os(II)(bpy)2(mbpy)](2+)-Pro2-Pra[Ru(II)(bpy)2(mbpy)](2+)-Pro2-Pra[Ru(II)(bpy)2(mbpy)](2+)-Pro6-Glu-NH2](6+) (ORR-2, Pro = L-proline and Glu = glutamic acid); [CH3-CO-Pro6-Pra[Os(II)(bpy)2(mbpy)](2+)-Pro3-Pra[Ru(II)(bpy)2(mbpy)](2+)-Pro3-Pra[Ru(II)(bpy)2(mbpy)](2+)-Pro6-Glu-NH2](6+) (ORR-3); and CH3-CO-Pro6-Pra[Os(II)(bpy)2(mbpy)](2+)-Pro5-Pra[Ru(II)(bpy)2(mbpy)](2+)-Pro5-Pra[Ru(II)(bpy)2(mbpy)](2+)Pro6-Glu2-NH2](6+) (ORR-5), were prepared by using solid-phase peptide synthesis. Given the helical nature of the resulting assemblies and the nature of the synthesis, composition, length, and loading pattern are precisely controlled in the assemblies. In acetonitrile, they adopt a proline I helical secondary structure, confirmed by circular dichroism, in which the appended chromophores are ordered in well-defined orientations and internuclear separation distances although helix formation for ORR-2 is incomplete. Quantitative comparison of oligoproline ground-state absorption and steady-state emission spectra to those for the constituents, [Boc-Pra[M(II)(bpy)2(mbpy)](2+)-OH](PF6)2 (Boc = N(α)-(1,1-dimethylethoxycarbonyl), shows that following Ru(II) light absorption, Ru(II)* undergoes facile energy transfer resulting in sensitization of Os(II). Sensitization efficiencies are 93% for ORR-2, 77% for ORR-3, and 73% for ORR-5. Picosecond-resolved emission measurements reveal complex, coupled dynamics that arise from excited-state decay and kinetically competitive -Ru(II)*-Ru(II)- → -Ru(II)-Ru(II)*- energy transfer migration/exchange and downhill -Ru(II)*-Os(II) → -Ru(II)-Os(II)* energy transfer. These processes were modeled simultaneously to extract rate constants for Ru(II)* → Ru(II) energy-transfer migration, k(Ru*-Ru), and Ru(II)* → Os(II) energy transfer, k(Ru*-Os). For ORR-2, k(Ru*-Ru) = 2.9 × 10(7) s(-1) and k(Ru*-Os) = 3.4 × 10(8) s(-1). For ORR-3, k(Ru*-Ru) = 1.2 × 10(7) s(-1) and k(Ru*-Os) = 1.3 × 10(8) s(-1). For ORR-5, k(Ru*-Ru) = 3.6 × 10(6) s(-1) and k(Ru*-Os) = 5.8 × 10(7) s(-1), all in acetonitrile at 22 °C. The data were analyzed by assuming Dexter energy transfer with the Franck-Condon factors arising from intramolecular structural and medium changes evaluated by use of an emission spectral fitting procedure. Fits of the data to the Dexter mechanism were consistent with the predicted distance dependence of energy transfer.

Influence of the Fluid-to-Film Transition on Photophysical Properties of MLCT Excited States in a Polymerizable Dimethacrylate Fluid

Photophysical properties of the salts [Ru(bpy)(3)](p-Tos)(2), [Ru(dmb)(3)](PF(6))(2), [Ru(vbpy)(3)](PF(6))(2), and [Ru(phen)(3)](p-Tos)(2) (bpy = 2,2'-bipyridine, dmb = 4,4'-dimethyl-2,2'-bipyridine, vbpy = 4-methyl-4'-vinyl-2,2'-bipyridine, phen = 1,10-phenanthroline, and p-Tos = p-toluene sulfonate) in fluid and film polyethylene glycol dimethacrylate containing nine ethylene glycol spacers (PEG-DMA550) are reported. MLCT absorption energies and bandshapes are similar in fluid and film PEG-DMA550 pointing to similar local dielectric environments, presumably dominated by the polar acrylate groups. Emission energies and excited-to-ground state 0-0 energy gaps (E(0)), determined by emission spectral fitting, are blue-shifted, and band-widths-at-half height (Δv(0,1/2)) decreased, due to an expected "rigid medium effect" in PEG-DMA550 film. The extent of loss of medium dipole reorientation in the rigid environment, and the increased emission energies in the film, resulted in enhanced emission quantum yields and excited state lifetimes in accordance with the energy gap law. The "rigid medium effect" in PEG-DMA550 is less pronounced than in films of poly(methyl methacrylate) (PMMA) pointing to a more fluid-like local environment presumably arising from the ethylene glycol linker spacers in PEG-DMA550. Comparison of the absorption, emission, emission spectral fitting, and emission lifetime results for [Ru(dmb)(3)](PF(6))(2) and [Ru(vbpy)(3)](PF(6))(2) shows that the vinyl groups of vbpy copolymerize with PEG-DMA550 covalently incorporating Ru(vbpy)(3)(2+) as a cross-linker into the polymer network. The most dramatic effect of the fluid-to-film transition is seen in the emission lifetime data for [Ru(phen)(3)](p-Tos)(2), with an increase of ~3 in the PEG-DMA550 film. Ru(phen)(3)(2+) cations appear to occupy a low symmetry site in the films probably close to the polar acrylate groups in a structurally confined environment.

Rigid Medium Stabilization of Metal-to-Ligand Charge Transfer Excited States

The salts [Ru(bpy)3](PF6)2, cis-[Ru(bpy)2(py)2](PF6)2, trans-[Ru(bpy)2(4-Etpy)2](PF6)2, [Ru(tpy)2](PF6)2, and [Re(bpy)(CO)3(4-Etpy)](PF6) (bpy=2,2'-bipyridine, py=pyridine, 4-Etpy=4-ethylpyridine, and tpy=2,2':6',2-terpyridine) have been incorporated into poly(methyl methacrylate) (PMMA) films and their photophysical properties examined by both steady-state and time-resolved absorption and emission measurements. Excited-state lifetimes for the metal salts incorporated in PMMA are longer and emission energies enhanced due to a rigid medium effect when compared to fluid CH3CN solution. In PMMA part of the fluid medium reorganization energy, lambdaoo, contributes to the energy gap with lambdaoo approximately 700 cm-1 for [Ru(bpy)3](PF6)2 from emission measurements. Enhanced lifetimes can be explained by the energy gap law and the influence of the excited-to-ground state energy gap, Eo, on nonradiative decay. From the results of emission spectral fitting on [Ru(bpy)3](PF6)2* in PMMA, Eo is temperature dependent above 200 K with partial differentialEo/ partial differentialT=2.8 cm-1/deg. cis-[Ru(bpy)2(py)2](PF6)2 and trans-[Ru(bpy)2(4-Etpy)2](PF6)2 are nonemissive in CH3CN and undergo photochemical ligand loss. Both emit in PMMA and are stable toward ligand loss even for extended photolysis periods. The lifetime of cis-[Ru(bpy)2(py)2](PF6)2* in PMMA is temperature dependent, consistent with a contribution to excited-state decay from thermal population and decay through a low-lying dd state or states. At temperatures above 190 K, coinciding with the onset of the temperature dependence of Eo for [Ru(bpy)3](PF6)2*, lifetimes become significantly nonexponential. The nonexponential behavior is attributed to dynamic coupling between MLCT and dd states, with the lifetime of the latter greatly enhanced in PMMA with tau approximately 3 ns. On the basis of these data and data in 4:1 (v/v) EtOH/MeOH, the energy gap between the MLCT and dd states is decreased by approximately 700 cm-1 in PMMA with the dd state at higher energy by DeltaH0 approximately 1000 cm-1. The "rigid medium stabilization effect" for cis-[Ru(bpy)2(py)2](PF6)2* in PMMA is attributed to inhibition of metal-ligand bond breaking and a photochemical cage effect.

Calculation of Electron Transfer Rate Constants from Emission Spectra in Re(I) Chromophore−Quencher Complexes

The photophysical properties of the chromophore−quencher complexes, fac-[(4,4‘-R2bpy)ReI(CO)3(LA)]n+ (4,4‘-R2bpy = 4,4‘-R2-2,2‘-bipyridine, R = Me or tBu, and LA = the quinone acceptor ligands, benz[g]isoquinoline-5,10-dione (BIQD), 2-oxy-1,4-naphthoquinone anion (ONQ-), 1/2 2,6-dihydroxyanthraquinone dianion (AFA2-), or the pyridinium acceptor, 1-methyl-6-oxyquinoline (OQD)) in 1,2-dichloroethane are described. Following ReI → 4,4‘-R2bpy metal-to-ligand charge transfer (MLCT) excitation, intramolecular electron transfer leads to the transients, fac-[(4,4‘-R2bpy)ReII(CO)3(LA•-)]n+. They have been characterized by emission spectral fitting and transient absorption measurements and, for LA = OQD, by transient infrared measurements. As shown by analysis of excited-state emission, there is weak-to-moderate electronic coupling between the electron donor and acceptor sites in the transients with HDA varying from 153 cm-1 for the BIQD complex to 3.9 cm-1 for the OQD complex. The transients are redox-separated (R...

Energy Transfer between Ru(II) and Os(II) Polypyridyl Complexes Linked to Polystyrene

Energy transfer between RuII and OsII polypyridyl complexes covalently attached to polystyrene has been in studied in CH3CN. The polymer is a 1:1 styrene-p-aminomethylstyrene copolymer derivatized by amide coupling with the acid-functionalized metal complexes [MII(bpy)2(bpy-COOH)](PF6)2 (MII = RuII, OsII; bpy is 2,2‘-bipyridine and bpy-COOH is 4‘-methyl-2,2‘-bipyridine-4-carboxylic acid). In the resulting polymer [co-PS−CH2NHCO−(RuII11OsII5)](PF6)32, 11 of, on the average, 16 polymer sites are derivatized by RuII and five by OsII. Photophysical properties compared to the homopolymers [co-PS−CH2NHCO−(RuII16)](PF6)32 and [co-PS−CH2NHCO−(OsII16)](PF6)32 reveal that excitation at RuII is followed by efficient energy transfer to the lower energy OsII sites with near unit efficiency (95%). Time-correlated single photon counting measurements with picosecond time resolution reveal that quenching of RuII* produced adjacent to an OsII trap site is quenched with an average rate constant 〈ken〉 = 4.2 × 108 s-1. RuII* decay and OsII* sensitization kinetics are complex because the polymer sample consists of a distribution of individual strands varying in chain length, loading pattern, and number of styryl spacers. The kinetics are further complicated by a contribution from random walk energy migration. An average energy transfer matrix element of 〈Ven〉 ∼ 2 cm-1 for RuII* → OsII energy transfer has been estimated by using emission spectral fitting parameters to calculate the energy transfer barrier.

Excited state electronic properties of the [(phen)(OC)3ReI(NC)RuII(bpy)2(CN)]PF6 and [(phen)(OC)3ReI(CN)RuII(bpy)2(CN)]PF6 linkage isomers †

The results of temperature dependent emission lifetimes, emission spectral fitting and excited state transient absorption spectroscopy of the two hetero-dinuclear [(phen)(OC)3ReI(CN)RuII(bpy)2(CN)]+ and [(phen)(OC)3ReI(NC)RuII(bpy)2(CN)]+ linkage isomers (abbreviated Re–CN–Ru and Re–NC–Ru) have been examined. In both dinuclear species efficient intramolecular Re*→Ru energy transfer occurs, leading to population of emitting dπ(Ru)–π*(bpy), 3MLCT states. The temperature dependent emission data suggest that the barrier for the MLCT–dd surface crossing is smaller in magnitude when the “[(phen)(OC)3Re(NC)]” moiety is bound to the Ru(bpy)2(CN) emitting unit. This is interpreted as the result of its enhanced ability to backbond from RuII which increases the energy of the MLCT relative to the dd states.

A displaced and low-frequency vibration of phosphorescent state of trans-[Rh(ethylenediamine)2Cl2]PF6 in a range of 5–497 K

The lowest (dπ-dσ*)3 excited states of both cis- and trans-isomers of [Rh(en)2Cl2]X (en=ethylenediamine; X=PF6−, NO3−) and the deuteriated crystal of trans-[Rh(en-d4)2Cl2]PF6 have been investigated in the solid state and in a wide temperature range of 5–497 K by means of emission spectra, lifetime and quantum yield measurements. Emission spectral simulation of trans-[Rh(en)2Cl2]PF6 shows that the emission from the lowest (dπ-dσ*)3 excited state exhibits a progression of a low-frequency metal-chloride stretching vibration (250 cm−1) with a large Huang-Rhys factor (S) of 21 and a progression of a high-frequency N–H stretching vibration (3000 cm−1). The increasing full-width at half maximum (2200 cm−1→4400 cm−1) with increasing temperature (77 K→468 K) is ascribed to hot bands from the excited levels of low-frequency vibration. The luminescence quantum yields of the crystal samples are determined to 0.0008 at 298 K and 0.003 at 80 K for trans-[Rh(en)2Cl2]PF6 and 0.18 at 298 K and 0.40 at 80 K for trans-[Rh(en-d4)2Cl2]PF6. From a combination of lifetime and emission quantum yield measurements, values for kr and knr have been obtained. The observed temperature dependence of nonradiative decay rates of trans-[Rh(en-d4)2Cl2]PF6 in a low-temperature region (<300 K) is possible to reconstitute by using the emission spectral fitting parameters and assuming nuclear tunneling mechanism. The temperature effect and deuteriation effect on the nonradiative rate definitively establishes that the dominant “accepting” modes in the nonradiative transition are a highly displaced (S=21) vibrational mode of low-frequency Cl–Rh–Cl stretching and a weakly displaced (S=0.1) vibrational mode of high-frequency N–D stretching. The nonradiative transition in a high-temperature region occurs via barrier passing along a displaced coordinate of Cl–Rh–Cl vibration with a pre-exponential factor of 1011 s−1 and is relatively insensitive to the high-frequency vibrational mode. The crystal of cis-[Rh(en)2Cl2]NO3 shows a red shift of the emission peak energy and an increase in the full-width at half maximum with increasing temperature. The results of temperature-dependent decay and spectra of emission can be interpreted in terms of two (dπ-dσ*)3 emitting states model.

Inductively coupled plasma–atomic emission spectral fitting analysis of rare-earth elements in ion-adsorption type ores

Inductively coupled plasma–atomic emission (ICP–AE) spectral fitting analysis was applied to the determination of rare-earth elements (REEs) in ion-adsorption type ores. An analytical window for each REE with strong emission (1 nm) was simulated based on a linear combination of reference spectra of REEs using our own developed software. The analytical results obtained were in good agreement with those obtained by a coupled analytical method, i.e. isotachophoresis-particle induced X-ray emission (ITP–PIXE).

Formation and dynamics of intermolecular charge transfer (CT) intermediates. A study with photo-excited all- trans-1,6-diphenylhexa-1,3,5-triene and p-dicyanobenzene

Photoinduced electron transfer reactions between singlet excited all- trans-1,6-diphenylhexa-1,3,5-triene ( IDPH ∗) and p-dicyanobenzene ( p-DCB) were investigated by electrochemical methods, stationary and time-resolved fluorescence and absorption measurements and charge transfer (CT) emission spectral fitting in five solvents of different polarity. Rate constants of triplet DPH ( 3DPH ∗) formation, fluorescence (CT emission), and radiationless deactivation (charge recombination, CR) of the respective CT intermediates (solvent-separated ion pairs or exciplexes) and the free energy changes of the corresponding processes were determined. The S 1-S n absorption spectra of the CT intermediates in the less polar solvents provided solid evidence that the CT intermediates can be approximately described by pure CT states with negligible contributions of locally excited states. A bandshape analysis of the CT emission spectra (CT emission spectral fitting) was employed to determine the Franck-Condon factors which depend, among other parameters, on the vibrational and solvent reorganization energies ( λ i, λ 0), on the electronic matrix coupling element ( V CR) and on the free energy change ( ΔG CR G). With the experimental values for λ i, λ 0, δG CR G, and V CR, rate constants of the charge recombination reaction of the CT intermediates were calculated in the framework of non-adiabatic electron transfer theory, and the results were found to be in good agreement with experimental rate constants.

Enhancements in the Livermore spectrum analysis package PIXEF

A number of enhancements have been incorporated into the Livermore particle-induced X-ray emission (PIXE) spectral fitting program PIXEF (for PIXE-Fit) in order to improve our quantitative analysis capability for data collected with Si(Li) detectors. Some of the new features include an improved description for the background which takes into account X-ray absorption effects, a better representation of the line shapes using Voigtian distributions, and a better treatment of peak fitting in regions of poor counting statistics. The implementation of these improvements into PIXEF is described.

Energy Transfer in the Inverted Region: Calculation of Relative Rate Constants by Emission Spectral Fitting

Energy Transfer in the Inverted Region: Calculation of Relative Rate Constants by Emission Spectral Fitting

Photophysical properties of polypyridyl carbonyl complexes of rhenium(I)

The photophysical properties of the metal to ligand charge transfer (m.l.c.t.) excited states of the complexes [Re(4,4′-X2-bipy)(CO)3Cl](X = NH2, NEt2, NHCOCH3, OCH3, CH3, H, Ph, Cl, CO2Et or NO2; bipy = bipyridine) vary systematically as the substituent X is varied. For the cases where m.l.c.t. states are lowest lying a quantitative correlation exists between ln (knr× 1 s)(knr is the rate constant for non-radiative decay) and the Franck–Condon factor calculated from parameters obtained by emission spectral fitting. The solvent reorganizational energy for [Re(bipy)(CO)3Cl] has been determined to be 1100 cm–1 in EtOH–MeOH (4:1 v/v) and 650 cm–1 in 2-methyltetrahydrofuran by a temperature dependent bandwidth study. Based on a comparative analysis of properties with related polypyridyl complexes of RuII and OsII it has been concluded that: (1) the extent of distortion at the 4,4′-X2-bipy acceptor ligand correlates with the energy gap between the excited and ground states; these results are in agreement with an earlier correlation found for polypyridyl complexes of OsII; (2) the unusually large Stokes shift and the broadening of the vibronic components in absorption and emission spectra arise from a combination of increased solvent reorganizational energies and greater distortions in the low-frequency modes between the excited and ground states; and (3) the relatively short lifetimes for the complexes of ReI have as a major contributing factor the participation of a ν(CO) mode at ca. 2020–2040 cm–1 as an energy acceptor in non-radiative decay.

Effect of the glass-to-fluid transition on excited-state decay. Application of the energy gap law

As observed earlier for related excited states in 4:1 (v/v) ethanol/methanol from 90 to 170 K, emission energies for the metal-to-ligand charge transfer (MLCT) excited states of (Os(bpy)/sub 3/)/sup 2 +/ and (Os(bpy)/sub 2/(CO)(py))/sup 2 +/ shift to lower energies while the lifetimes of the excited states decrease with increasing temperature. From temperature-dependent emission spectral fitting and lifetime data it is concluded that the origin of the effect is in the response of solvent dipole orientations to the changes in electronic configuration between ground and excited states and how it affects the energy gap between the states. For (Os(bpy)/sub 2/(CO)(py))/sup 2 +/, the nonradiative rate constant (k/sub nr/) varies with the energy gap, ln (k/sub nr/) proportional to -E, as predicted by the energy gap law, while for (Os(bpy)/sub 3/)/sup 2 +/ additional complications appear to exist because of contributions to excited-state decay from more than one low-lying MLCT state.