Emission band shape probes of the mixed-valence excited state properties of polypyridyl-bridged bis-ruthenium(II) complexes

The absorption spectra and emission spectral band shapes of several polypyridine-ligand (PP) bridged bis-ruthenium(II) complexes imply that the Ru(II)/Ru(III) electronic coupling is weak in their lowest energy metal to ligand charge transfer (MLCT) excited states. Many of these PP-bridging ligands contain pyrazine moieties and the weak electronic coupling of the excited states contrasts to the strong electronic coupling inferred for the correlated mixed-valence ground states. Although the bimetallic complexes emit at significantly lower energy than their monometallic analogs, the vibronic contributions to their 77 K emission spectra are much stronger than expected based on comparison to the monometallic analogs (around twofold in some complexes) and this feature is characteristic of bimetallic complexes in which the mixed-valence excited states are electronically localized. The weaker excited state than ground state donor/acceptor electronic coupling in this class of complexes is attributed to PP-mediated super-exchange coupling in which the mediating orbital of the bridging ligand (PP-LUMO) is partly occupied in the MLCT excited states, but is unoccupied in the ground states; therefore, the vertical Ru(III)-PP⁻ (MLCT) energy is larger and the mixing coefficient smaller in these excited states than is found for Ru(II)-PP in the corresponding ground states.     

Pressure-dependent Raman spectroscopy of metal-oxo multiple bonds in rhenium(V) and osmium(VI) complexes

The pressure dependence of stretching vibrations involving metal-oxo multiple bonds is reported and discussed. Metal–ligand vibrational frequencies involving double and triple bonds increase with pressure according to the nature of the metal center and ancillary ligands. Pressure-dependent luminescence spectra, at room temperature, of a trans-dioxo rhenium(V) complex exhibit resolved vibronic structure with a progression in a high frequency metal-oxo stretching mode that shows a change of the intensity distribution with pressure. Raman and luminescence spectra are used to rationalize the unusual decrease of the metal-oxo stretching frequency observed for two complexes at pressures over 30 kbar.     

Pressure-induced enhancements of luminescence intensities and lifetimes correlated with emitting-state distortions for thiocyanate and selenocyanate complexes of platinum(II) and palladium(II)

The luminescence properties of thiocyanate and selenocyanate platinum(II) and palladium(II) complexes show strong variations with temperature and pressure. The d-d luminescence band maxima for [Pt(SCN)(4)](PPh(4))(2) (1), [Pt(SCN)(4)](n-Bu(4)N)(2) (2), and [Pt(SeCN)(4)](n-Bu(4)N)(2) (4) complexes are centered at ca. 14500 cm(-1) whereas those of the [Pd(SCN)(4)](n-Bu(4)N)(2) (3) and [Pd(SeCN)(4)](n-Bu(4)N)(2) (5) complexes are approximately 2000 cm(-1) lower in energy. Low-temperature luminescence spectra from single-crystal samples have broad bands with highly resolved vibronic structure indicating large displacements of the emitting-state potential energy minimum along several metal-ligand normal coordinates. The largest displacements involve the totally symmetric (a(1g)) stretching modes with frequencies of 295 cm(-1) (1), 303 cm(-1) (2), 274 cm(-1) (3), 195 cm(-1) (4), and 185 cm(-1) (5). The lower frequencies of these dominant progression-forming modes for the selenocyanate complexes lead to luminescence bands that are narrower by ca. 500 cm(-1) (fwhm) than those observed from the thiocyanate complexes. Under external pressures, the room-temperature luminescence intensities and lifetimes show considerable enhancement at pressures up to 40 kbar. This effect is largest for the palladium(II) complexes with lifetimes increasing from approximately 350 ns at ambient pressure up to 62 micros at 30 kbar, an increase by more than 2 orders of magnitude. The platinum(II) complexes exhibit a significant, but noticeably lesser increase of luminescence lifetimes and intensities with increasing pressure. The temperature- and pressure-dependent luminescence decay behavior is rationalized using the emitting-state molecular geometry determined from the resolved low-temperature luminescence spectra combined with the strong-coupling limit of radiationless decay theory.     

Emitting-state displacements in ligand-centered vibrational modes in the trans-[OsO2(NCS)4]2- complex determined from near-infrared luminescence spectroscopy

Low-temperature luminescence spectra from three salts of the trans-[OsO(2)(NCS)(4)](2-) complex exhibit highly resolved vibronic structure in both metal-ligand high-frequency O=Os=O (885 cm(-1)) and lower-frequency Os-N(CS) (255 cm(-1)) symmetric stretching modes as well as in a ligand-centered CS stretching mode (858 cm(-1)). Band maxima range from 10000 to 12000 cm(-1), and spectra contain irregular frequency intervals that correspond to transitions from more than one origin and phonon sidebands. Experimental band shapes are distinctly different for all three compounds and are calculated assuming harmonic potential energy surfaces for both the ground and emitting states. Normal-coordinate offsets along all displaced vibrational modes are determined and compared for the three compounds. The analyses reveal emitting-state displacement of high-frequency ligand-centered (CS) and metal-ligand (O=Os=O) symmetric stretching modes, leading to observed high-frequency intervals (855-880 cm(-1)) that do not match any frequencies determined from ground-state Raman spectra. The values for the high-frequency normal-coordinate offsets, DeltaQ(O=Os=O) and DeltaQ(CS), were found to be on the order of 0.06 A. Offsets along the 255 cm(-1) Os-N mode varied noticeably between the three compounds and were largest for the compound with the largest value of DeltaQ(CS).     

Spectroscopic effects of excited-state coupling in a tetragonal chromium(III) complex

Detailed low-temperature single-crystal polarized absorption and luminescence spectra of Cs2[CrCl2(H2O)4]Cl3 are reported. The luminescence spectrum is a broad band with a maximum at 11,800 cm (-1), indicating that the trans-[CrCl2(H2O)4]+ complex emits from a quartet excited state. The resolved vibronic structure reveals a progression in a nontotally symmetric 445 cm (-1) b1g mode, a manifestation of a Jahn-Teller effect in the emitting state. The absorption spectrum shows completely linearly polarized, magnetic-dipole-allowed electronic origins, defining the tetragonal splitting of the states originating from 4T2g (Oh). An energy gap of approximately 800 cm (-1) is observed between the electronic origins of the emitting state and the onset of the pi-polarized absorption spectrum. Both Jahn-Teller and spin-orbit couplings in the orbitally degenerate 4Eg (D4h) state are necessary to account for the spectroscopic observations.     

Charge transfer vibronic transitions in uranyl tetrachloride compounds

The electronic and vibronic interactions of uranyl (UO(2))(2+) in three tetrachloride crystals have been investigated with spectroscopic experiments and theoretical modeling. Analysis and simulation of the absorption and photoluminescence spectra have resulted in a quantitative understanding of the charge transfer vibronic transitions of uranyl in the crystals. The spectra obtained at liquid helium temperature consist of extremely narrow zero-phonon lines (ZPL) and vibronic bands. The observed ZPLs are assigned to the first group of the excited states formed by electronic excitation from the 3σ ground state into the f(δ,?) orbitals of uranyl. The Huang-Rhys theory of vibronic coupling is modified successfully for simulating both the absorption and luminescence spectra. It is shown that only vibronic coupling to the axially symmetric stretching mode is Franck-Condon allowed, whereas other modes are involved through coupling with the symmetric stretching mode. The energies of electronic transitions, vibration frequencies of various local modes, and changes in the O═U═O bond length of uranyl in different electronic states and in different coordination geometries are evaluated in empirical simulations of the optical spectra. Multiple uranyl sites derived from the resolution of a superlattice at low temperature are resolved by crystallographic characterization and time- and energy-resolved spectroscopic studies. The present empirical simulation provides insights into fundamental understanding of uranyl electronic interactions and is useful for quantitative characterization of uranyl coordination.     

Theoretical study of the electronic nonadiabatic transitions in the photoelectron spectroscopy of F2O

The photoelectron spectrum of F2O pertaining to ionizations to the ground (X2B1) and low-lying excited electronic states (A2B2, B2A1, and C2A2) of F2O+ is investigated theoretically. The near equilibrium potential energy surfaces of the ground electronic state (X2B1) of F2O and the mentioned ground and excited electronic states of F2O+ reported by Wang et al. ( J. Chem. Phys. 2001, 114, 10682) for the C2v configuration are extended for the Cs geometry assuming a harmonic vibration along the asymmetric stretching mode. The vibronic interactions between the A2B2 and B2A1 electronic states of F2O+ are treated within a linear coupling approach, and the strength of the vibronic coupling parameter is calculated by an ab initio method. The nuclear dynamics is simulated by both time-independent quantum mechanical and time-dependent wave packet approaches. Although the first photoelectron band exhibits resolved vibrational progression along the symmetric stretching mode, the second one is highly overlapping. The latter is attributed to the nonadiabatic interactions among the energetically close A2B2, B2A1, and C2A2 electronic states of F2O+. The theoretical findings are in good accord with the available experimental results.     

含长链β-二酮环金属铂配合物的电子光谱和二阶非线性光学性质的理论研究

在B3LYP/LanL2DZ(6-31＋＋G**)理论水平对标题化合物进行结构优化和电子光谱与二阶非线性光学性质计算. 结果显示, 重金属的配合导致Pt原子与苯环, 吡啶环, β-二酮羰基环构成较大的共轭体系, 使得分子由基态到第一激发态的p→p*和n→p*跃迁伴随MLCT电荷转移, 对应的最大吸收波长在406 nm左右, 属于近紫外区, β-二酮碳链的长度对结构和电子光谱影响很小, 与实验结果一致. 长链β-二酮环金属铂配合物分子具有较好的非线性光学性质.     

Laser induced fluorescence and resonant two-photon ionization spectroscopy of jet-cooled 1-hydroxy-9,10-anthraquinone

We carried out laser induced fluorescence and resonance enhanced two-color two-photon ionization spectroscopy of jet-cooled 1-hydroxy-9,10-anthraquinone (1-HAQ). The 0-0 band transition to the lowest electronically excited state was found to be at 461.98 nm (21,646 cm(-1)). A well-resolved vibronic structure was observed up to 1100 cm(-1) above the 0-0 band, followed by a rather broad absorption band in the higher frequency region. Dispersed fluorescence spectra were also obtained. Single vibronic level emissions from the 0-0 band showed Stokes-shifted emission spectra. The peak at 2940 cm(-1) to the red of the origin in the emission spectra was assigned as the OH stretching vibration in the ground state, whose combination bands with the C=O bending and stretching vibrations were also seen in the emission spectra. In contrast to the excitation spectrum, no significant vibronic activity was found for low frequency fundamental vibrations of the ground state in the emission spectrum. The spectral features of the fluorescence excitation and emission spectra indicate that a significant change takes place in the intramolecular hydrogen bonding structure upon transition to the excited state, such as often seen in the excited state proton (or hydrogen) transfer. We suggest that the electronically excited state of interest has a double minimum potential of the 9,10-quinone and the 1,10-quinone forms, the latter of which, the proton-transferred form of 1-HAQ, is lower in energy. On the other hand, ab initio calculations at the B3LYP/6-31G(d,p) level predicted that the electronic ground state has a single minimum potential distorted along the reaction coordinate of tautomerization. The 9,10-quinone form of 1-HAQ is the lowest energy structure in the ground state, with the 1,10-quinone form lying approximately 5000 cm(-1) above it. The intramolecular hydrogen bond of the 9,10-quinone was found to be unusually strong, with an estimated bond energy of approximately 13 kcal/mol (approximately 4500 cm(-1)), probably due to the resonance-assisted nature of the hydrogen bonding involved.     

Theoretical studies of blue-emitting iridium complexes with different ancillary ligands

The structural and electronic properties of two heteroleptic iridium complexes Ir(dfppy)2(pic) (FIrpic) and Ir(dfppy)2(acac) (FIracac) have been investigated theoretically, where dfppy = 2-(2,4-difluorophenyl) pyridine, pic = picolinic acid, and acac = acetoylacetonate. The geometries of ground and excited states are optimized at PBE0/LANL2DZ and CIS/LANL2DZ levels, respectively. Time-dependent density functional theory (TDDFT) method is employed to explore the absorption and emission properties. In the ground state, the highest-occupied molecular orbital has a significant mixture of metal Ir(d) and dfppy(pi), the lowest-unoccupied orbital locates primarily on pi* of pic for FIrpic and pi* of dfppy for FIracac. The luminescence of each complex originates from the lowest triplet excited state, which is assigned to the mixing of metal-to-ligand charge transfer and intraligand charge transfer characters. The effects of ancillary ligands pic and acac on absorption and emission spectra are observed by analysis of TDDFT results. The connection between the nature of excited states and the behavior of the complexes with different ancillary ligands is elucidated.     

Effect of ancillary ligands on the electronic structures, DNA-binding and spectral properties of [Co(L)2(pip)] (L = phen, bpy, en, tap)

Studies on the electronic structures and related properties of a series of Co(Ш) complexes have been carried out, using the density functional theory (DFT) at the B3LYP/LanL2DZ level. The effect of the ancillary ligands on their electronic structures, DNA-binding affinities and spectra was revealed. The results show that an ancillary ligand has quite important effect on electronic structures and DNA-binding properties of these Co(Ш) complexes. The ancillary ligand possessing a great conjugated structure can effectively improve the DNA-binding affinity of the complex. Meanwhile, introducing a stronger electronegative N atom on the skeleton of ancillary ligand can obviously reduce the LUMO energy of the complex. Based on these findings, a designed complex 4 can be expected to have the greatest K_b value in complexes 1–4. So it may be able to control the interaction between the complex and DNA-base-pairs via varying ancillary ligands. In addition, the electronic absorption spectra of these complexes were calculated and simulated in aqueous solution using the time-dependent DFT (TDDFT) method and the effect of the ancillary ligands on the spectra was also explored. The calculated absorption spectra of these complexes in aqueous solution are in a satisfying agreement with the experimental results, and the properties of experimental absorption bands were theoretically explained in detail.     

Relativistic Jahn-Teller effects in the photoelectron spectra of tetrahedral P4, As4, Sb4, and Bi4

The group-V tetrahedral cluster cations P(4)(+), As(4)(+), Sb(4)(+), and Bi(4)(+) are known to exhibit exceptionally strong Jahn-Teller (JT) effects of electrostatic origin in their (2)E ground states and (2)T(2) excited states. It has been predicted that there exist, in addition, JT couplings of relativistic origin (arising from the spin-orbit (SO) operator) in (2)E and (2)T(2) states of tetrahedral systems, which should become relevant for the heavier elements. In the present work, the JT and SO couplings in the group-V tetramer cations have been analyzed with ab initio relativistic electronic structure calculations. The vibronic line spectra and the band shapes of the photoelectron spectra were simulated with time-dependent quantum wave-packet methods. The results provide insight into the interplay of electrostatic and relativistic JT couplings and SO splittings in the complex photoelectron spectra of these systems.     

Ytterbium(III) Porpholactones: β‐Lactonization of Porphyrin Ligands Enhances Sensitization Efficiency of Lanthanide Near‐Infrared Luminescence

The near‐infrared (NIR) luminescence efficiency of lanthanide complexes is largely dependent on the electronic and photophysical properties of antenna ligands. Although porphyrin ligands are efficient sensitizers of lanthanide NIR luminescence, non‐pyrrolic porphyrin analogues, which have unusual symmetry and electronic states, have been much less studied. In this work, we used porpholactones, a class of β‐pyrrolic‐modified porphyrins, as ligands and investigated the photophysical properties of lanthanide porpholactones Yb‐1 a – 5 a . Compared with Yb porphyrin complexes, the porpholactone complexes displayed remarkable enhancement of NIR emission (50–120 %). Estimating the triplet‐state levels of porphyrin and porpholactone in Gd complexes revealed that β‐lactonization of porphyrinic ligands lowers the ligand T₁ state and results in a narrow energy gap between this state and the lowest excited state of Yb³⁺. Transient absorption spectra showed that Yb^III porpholactone has a longer transient decay lifetime at the Soret band than the porphyrin analogue (30.8 versus 17.0 μs). Thus, the narrower energy gap and longer lifetime arising from β‐lactonization are assumed to enhance NIR emission of Yb porpholactones. To demonstrate the potential applications of Yb porpholactone, a water‐soluble Yb bioprobe was constructed by conjugating glucose to Yb ‐ 1 a . Interestingly, the NIR emission of this Yb porpholactone could be specifically switched on in the presence of glucose oxidase and then switched off by addition of glucose. This is the first demonstration that non‐pyrrolic porphyrin ligands enhance the sensitization efficiency of lanthanide luminescence and also display switchable NIR emission in the region of biological analytes (800–1400 nm).     

The influence of vibronic interactions on the chiroptical spectra of dissymmetric pseudo tetragonal metal complexes

The influence of vibronic interactions on the chiroptical spectra associated with a threesome of nearly degenerate electronic excited states in a dissymmetric molecular system is examined on a formal theoretical model. The model considers two vibrational modes to be effective in promoting pseudo Jahn-Teller (PJT) type interactions between the three closely spaced electronic excited states. Formal expressions are developed for the rotatory strengths of individual vibronic levels derived from the coupled electronic states. Two mode (vibrational)-three state (electronic) vibronic Hamiltonians are constructed (basis set size, 63–108, depending upon interaction parameters used) and diagonalized for a large number of different parameter sets representative of various vibronic coupling strengths, electronic energy level spacings, oscillator (vibrational mode) frequencies, and electronic rotatory strengths. Diagonalization of these vibronic Hamiltonians yields vibronic wave functions and energies which are then used to calculate rotatory strength spectra for the model system. The calculated results demonstrate the profound influence which vibronic interactions of the PJT type may have on the sign patterns and intensity distributions within the rotatory strength spectrum associated with a set of nearly degenerate electronic states. The implication of these results for the interpretation of circular dichroism spectra of chiral transition metal complexes with pseudo tetragonal symmetry are discussed.     

Systematic behavior of charge-transfer transitions and energy level variation in soft donor complexes of the trivalent lanthanides

The systematic behavior of the charge-transfer (CT) energies in mixed 2,2'-bipyridyl (bipy), N,N-diethyldithiocarbamate (Et2dtc-) complexes of the trivalent lanthanides, Ln(Et2dtc)3(bipy), is investigated to understand the electronic structure of f-element complexes containing soft donor ligands. The energies of ligand to Ln3+ CT are extremely low in this system, an effect attributed to the presence of the soft donor ligands. The lowest CT energy level for the Sm3+, Eu3+, and Yb3+ complexes falls into the visible range. In Eu(Et2dtc)(bipy), the Eu3+ ion becomes nonluminescent because the CT energy stretches below the metastable 5D0 electronic state, whereas luminescence from the CT state and the 4f13 (2)F(5/2) state are observed in the Yb compound. The variation in the energy of the lowest level CT transition for the entire Ln(Et2dtc)3(bipy) series has been evaluated using the experimentally determined CT levels of the Sm3+, Eu3+, and Yb3+ compounds based on the systematic behavior of the lanthanides, which is invariant with respect to the type of ligand. The energy difference between the ground electronic states of the lanthanide ions and the ligand-centered valence band may also be calculated from these results.     

Vibronic Structures in Absorption and Fluorescence Spectra of Firefly Oxyluciferin in Aqueous Solutions

To elucidate the factors determining the spectral shapes and widths of the absorption and fluorescence spectra for keto and enol oxyluciferin and their conjugate bases in aqueous solutions, the intensities of vibronic transitions between their ground and first electronic excited states were calculated for the first time via estimation of the vibrational Franck–Condon factors. The major normal modes, overtones and combination tones in absorption and fluorescence spectra are similar for all species. The theoretical full widths at half maximum of absorption spectra are 0.4–0.7 eV and those for the fluorescence spectra are 0.4–0.5 eV, except for phenolate‐keto that exhibits exceptionally sharp peak widths due to the dominance of the 0–0′ or 0′–0 band. These spectral shapes and widths explain many relevant features of the experimentally observed spectra.     

Ancillary Ligand Effects on Red-Emitting Cyclometalated Iridium Complexes

In this work, a series of ten new red-emitting heteroleptic iridium(III) complexes of the type Ir(C^N)₂(L^X) (C^N=cyclometalating ligand, L^X=monoanionic chelating ancillary ligand) is introduced. The suite of new complexes includes two different cyclometalating ligands and five different ancillary ligands, with the primary goal of investigating the effect of the ancillary ligand structure on the excited-state dynamics. The structural variety of the ancillary ligands permitted investigations of the effects of donor atom identity, chelate ring size, and substituents on the electronic structure and excited state properties. Electrochemical analysis showed that the ancillary ligand has a substantial effect on the energy of the HOMO, whereas the LUMO is left unperturbed. Photoluminescence spectra showed that the ancillary ligand can sometimes strongly influence the emission wavelength, but in all cases is an important determinant of the excited-state dynamics.     

Ab initio simulation of the vibrationally resolved photoelectron spectrum of Si3-

Electron photodetachment spectra provide a wealth of information about the electronic and vibrational level structures of neutral molecules that form stable anions. Experiments carried out for the smallest polyatomic silicon cluster anion (Si3-+hupsilon-->Si3*+e-) show vibrational progressions in six observed electronic bands (X-E) of the neutral species. The authors have performed ab initio calculations using the MRCI+D/aug-cc-pVQZ level for the corresponding electronic states followed by variational calculations of the vibronic levels associated with these adiabatic potential energy surfaces. In contrast to previous approaches, the authors treat the nonadiabatic dynamics on the potential energy surfaces, which allows for a vastly improved reproduction of the experimental level structure and a corrected assignment for band A.     

Vibronic interaction in europium nitrates Eu(NO3)3 x 4SOR2

Luminescence and excitation of luminescence vibronic spectra of europium nitrates Eu(NO3)3 x 4SOR2 containing sulphoxide derivatives were obtained and analysed. Some factors influencing the intensity distribution in vibronic sidebands are discussed. Significant variation of the intensity distribution in antiStokes sidebands of Eu3+ electronic transitions in series of nitrates results from the difference in effective charges on coordinated oxygen atoms of ligands. Another important detail of the vibronic spectra is a redistribution of intensity in the region of 5D0, 5D1-->7F2 transitions of luminescence spectra originated in overlap of different vibronic transitions. Mixing between the 7F2 electronic state of Eu3+ and vibronic satellites of 7F0 electronic state was studied both under conditions of resonance and in case of significant detuning.     

Wavepacket Dynamics on One-Dimensional Harmonic Potentials. An Intuitive Approach to the Understanding of Absorption and Luminescence Spectra

Absorption and luminescence spectra can be used to derive quantitative information about differences in molecular structure between the ground and excited electronic states. Examples illustrate the effect of the harmonic vibrational frequency and the offset of the potential energy minima along one normal coordinate on the observed spectrum. The time-dependent approach to spectroscopy is used, leading to intuitively appealing visual representations.