Ground and first excited states of fractionally charged sodium atoms

The energy levels in sodium atoms with quarks attached to the nucleus are obtained using the configuration interaction method. It is found that the calculated 3p (²P) → 3s (²S) electronic transition in the Na^?1/3 quark–atom system is in agreement with that predicted by extrapolation of experimental data. Also, ionization potentials of fractionally charged sodium atoms are discussed. © 1993 John Wiley & Sons, Inc.     

Composite nanoparticles: the best of two worlds

Nanomaterials have rapidly moved into the mainstream for chemical and biological analysis. Nanoparticle probes enhance signal intensity, increase the chemical and physical stability of the probe, and facilitate surface modification for specific targeting. Unfortunately, common problems are encountered with many nanoparticle probes, e.g., poor solubility, poor biocompatibility, and leakage of encapsulated components, that severely restrict the application of probes to ex vivo samples under carefully controlled conditions. A wide range of recently developed multifunctional nanomaterials are poised to make significant contributions to molecular analysis of biological systems. Composite nanoparticle geometries, including composites, hybrids, and core–shell nanoparticles prepared using two or more materials, e.g., silica/inorganic, silica/polymer, or polymer/inorganic combinations, offer improved solubility, easier functionalization, and decreased toxicity compared with the related single-component materials. Furthermore, composite nanomaterials present substantial signal amplification, and improved multiplexing for higher-sensitivity and higher-resolution measurements. Further development and integration of composite nanomaterials into the quantitative sciences will play a key role in the future of functional probes for imaging, quantitative analysis, and biological manipulation.     

Highly oxidizing excited states of Re and Tc complexes

Like the Re analogue, the ligand-to-metal charge transfer (LMCT) excited-state of [Tc(dmpe)3]2+ (dmpe is bis-1,2-(dimethylphosphino)ethane) is luminescent in solution at room temperature. Surprisingly, both [M(dmpe)3]2+* species have extremely large excited-state potentials (ESPs) as oxidants-the highest for any simple coordination complex of a transition metal. Furthermore, this potential is available using a photon of visible light (calculated for M = Re(Tc); E1/2* = +2.61(2.52) V versus SCE; lambdamax = 526(585) nm). Using a Rehm-Weller analysis with a series of aromatic hydrocarbons as electron-transfer quenchers, E1/2(Re2+*/Re+) has been determined to be 2.58 V, in good agreement with the calculated value. Both [M(dmpe)3]2+* species are quenched by chloride ion and both can function as excited-state oxidants in water solution.     

Magnetic anisotropy in the excited states of low symmetry lanthanide complexes

Ab initio investigation of multiplet spectrum of lanthanides in archetypal coordination geometries shows an unexpected regular structure consisting of (i) mirror symmetry of anisotropic magnetic properties of doublet states, (ii) high magnetic axiality of low-lying and high-lying doublets, comparable to complexes with ideal axial symmetry, and (iii) the strong rotation of the anisotropy axes of individual doublets. The obtained high axiality of the ground doublet states explains the SMM behaviour of low-symmetry lanthanide complexes.     

Bonding in the ground state and excited states of copper-alkene complexes

The ground state and ¹B₂ excited state of Cu(C₂H₄)⁺ and of CuX(C₂H₄) (X  F, Cl) have been investigated by the Hartree-Fock-Slater (HFS) method. The main metal-ligand interactions in the ground state are ethene π → Cu 4s donation and Cu 3d_π → ethene π^* backdonation, which have comparable contributions to the metal-ligand bond strength. The excitation of CuX(C₂H₄) does not involve an alkene π → metal charge transfer (LMCT), but instead is metal 3d → alkene π^* charge transfer (MLCT) in character. The implications for the photochemistry of olefin-copper(I) complexes are discussed.     

Hydrogen bonding of excited states in supramolecular host-guest inclusion complexes

Host-guest inclusion complexes represent an important type of supramolecular structure, one which finds widespread applications in diverse areas including separations science, the food industry, molecular sensors and optical devices. There are several driving forces for the formation of such inclusion complexes in solution; one of the most important is hydrogen bonding between the guest and host molecules. The nature or strength of the hydrogen bonding may change upon electronic excitation of the guest, for example during fluorescence studies or when the inclusion complex is used as an optical sensor. In this Perspective article, the impact of hydrogen bonding between excited state guests and their hosts is examined in detail, in terms of the impact on the formation and stability of such excited state complexes, the effects on guest fluorescence, changes in the stability of ground state guest complexes upon electronic excitation, the application of inclusion complexes as fluorescent sensors and materials, and the use of fluorescence spectroscopy for their study.     

Vibronic coupling in excited charge-transfer states of solid-state charge-transfer complexes

Symmetry properties of CT excited states of some weak donor-acceptor complexes are discussed in the context of vibronic coupling with intermolecular vibrations. The results are applied to the analysis of electroabsorption spectra of anthracene-PMDA.     

Effect of charged and excited states on the decomposition of 1,1-diamino-2,2-dinitroethylene molecules

The authors have calculated the electronic structure of individual 1,1-diamino-2,2-dinitroethylene molecules (FOX-7) in the gas phase by means of density functional theory with the hybrid B3LYP functional and 6-31+G(d,p) basis set and considered their dissociation pathways. Positively and negatively charged states as well as the lowest excited states of the molecule were simulated. They found that charging and excitation can not only reduce the activation barriers for decomposition reactions but also change the dominating chemistry from endo- to exothermic type. In particular, they found that there are two competing primary initiation mechanisms of FOX-7 decomposition: C-NO2 bond fission and C-NO2 to CONO isomerization. Electronic excitation or charging of FOX-7 disfavors CONO formation and, thus, terminates this channel of decomposition. However, if CONO is formed from the neutral FOX-7 molecule, charge trapping and/or excitation results in spontaneous splitting of an NO group accompanied by the energy release. Intramolecular hydrogen transfer is found to be a rare event in FOX-7 unless free electrons are available in the vicinity of the molecule, in which case HONO formation is a feasible exothermic reaction with a relatively low energy barrier. The effect of charged and excited states on other possible reactions is also studied. Implications of the obtained results to FOX-7 decomposition in condensed state are discussed.     

Pump-probe spectroscopy of molecules driven by infrared field in both ground and excited electronic states

Pump-probe spectra of molecules driven by strong infrared (IR) field in both ground and excited states are studied theoretically. The role of the final state interaction becomes important when pump and probe pulses overlap, and the Rabi frequency is comparable with the lifetime broadening of the excited state and the duration of the pump pulse. Our theoretical approach is applied to x-ray absorption of nitrogen molecule and valence photoionization of carbon monoxide. It is shown that IR-x-ray pump-probe spectroscopy can directly evidence the delocalization of core hole.     

Direct tyrosine oxidation using the MLCT excited states of rhenium polypyridyl complexes

Rhenium(I) polypyridyl complexes have been designed for the intramolecular photogeneration of tyrosyl radical. Tyrosine (Y) and phenylalanine (F) have each been separately appended to a conventional Re(I)(bpy)(CO)(3)CN framework via an amide linkage to the bipyridine (bpy) ligand. Comparative time-resolved emission quenching and transient absorption spectra of Re(bpy-Y)(CO)(3)CN and Re(bpy-F)(CO)(3)CN show that Y is oxidized only upon its deprotonation at pH 12. In an effort to redirect electron transport so that it is more compatible with intramolecular Y oxidation, polypyridyl Re(I) complexes have been prepared with the amide bond functionality located on a pendant phosphine ligand. A [Re(phen)(PP-Bn)(CO)(2)](PF(6)) (PP = bis(diphenylphosphino)ethylene) complex has been synthesized and crystallographically characterized. Electrochemistry and phosphorescence measurements of this complex indicate a modest excited-state potential for tyrosine oxidation, similar to that for the (bpy)Re(I)(CO)(3)CN framework. The excited-state oxidation potential can be increased by introducing a monodentate phosphine to the Re(I)(NN)(CO)(3)(+) framework (NN = polypyridyl). In this case, Y is oxidized at all pHs when appended to the triphenylphosphine (P) of [Re(phen)(P-Y)(CO(3))](PF(6)). Analysis of the pH dependence of the rate constant for tyrosyl radical generation is consistent with a proton-coupled electron transfer (PCET) quenching mechanism.     

Self-assembly of heterobimetallic d-f hybrid complexes: sensitization of lanthanide luminescence by d-block metal-to-ligand charge-transfer excited states

Carboxylate-bearing d-transition metal bipyridyls form ternary complexes with seven-coordinate lanthanide centers. The neodymium,- ytterbium-, and erbium-containing complexes exhibit lanthanide-centered emissions sensitized by the MLCT states of the d-block components.     

The excited states of some organic charge transfer complexes studied by modulation excitation spectrophotometry

The excited states of some complexes between polycyclic aromatic hydrocarbons and chloranil dissolved in polymethylmethacrylate have been studied with a modulation excitation spectrophotometer. New bands similar to but slightly shifted in position as compared to the T₁–T₃ transition of the aromatic hydrocarbon are observed. Regressional analysis points to the new bands arising from transitions from a level in the chloranil to the T₃ level of the hydrocarbon. An enhancement of the T₁–T₃ bands of the hydrocarbon occurs in the complex. Other bands are also seen which are believed to be due to new levels of the complex which cannot be identified with local levels of the hydrocarbon or chloranil.     

Quenching of excited states of pyrene sulfonate solutions and their metal complexes by molecular oxygen

The results of study of fluorescence quenching, by molecular oxygen, of unsubstituted pyrene solutions within the framework of exchange complex are generalized. The oxygen quenching rate constants of the fluorescence and triplet state of pyrene monosulfonate and pyrene tetrasulfonate solutions and their aggregates and complexes (second and first coordination spheres) with REE ions in H₂O, DMSO, and DMFA are measured. The effect of the number of substituting sulfo groups, aggregates, complex formation, and the nature of the solvent on the efficiency of this quenching is investigated. It is shown that, in contrast to metalloporphyrins with the paramagnetic Cu(II) ion, the formation of pyrenesulfonates complexes with REE paramagnetic ions does not lead to an increase in oxygen quenching efficiency of the triplet state of organic ligands.Translated from Teoreticheskaya i Eksperimental'naya Khimiya, Vol. 23, No. 3, pp. 302–311, May–June, 1987.The authors express their deep gratitude to Prof. V. L. Ermmolaev for a fruitful discussion of the results.     

Lowest electronic excited states of platinum(II) diimine complexes

Absorption and emission spectra of Pt(diimine)L2 complexes (diimine = 2,2'-bipyridine (bpy) or 4,4'-dimethyl-2,2'-bipyridine (dmbpy); L = pyrazolate (pz-), 3,5-dimethylpyrazolate (dmpz-), or 3,4,5-trimethylpyrazolate (tmpz-)) have been measured. Solvent-sensitive absorption bands (370-440 nm) are attributed to spin-allowed metal-to-ligand charge-transfer (1MLCT) transitions. As solids and in 77 K glassy solution, Pt(bpy)(pz)2 and Pt(dmbpy)(pz)2 exhibit highly structured emission systems (lambda max approximately 494 nm) similar to those of the diprotonated forms of these complexes. The highly structured bands (spacings 1000-1400 cm-1) indicate that the transition originates in a diimine-centered 3(pi-->pi*) (3LL) excited state. The intense solid-state and 77 K glassy solution emissions from 3MLCT[d(Pt)-->pi*(bpy)] excited states of complexes with dmpz- and tmpz- ligands occur at longer wavelengths (lambda max = 500-610 nm), with much broader vibronic structure. These findings are consistent with increasing electron donation of the pyrazolate ligands, leading to a distinct crossover from a lowest 3LL to a 3MLCT excited state.     

Proton-coupled electron transfer originating from excited states of luminescent transition-metal complexes

Proton-coupled electron transfer (PCET) is of fundamental importance for small-molecule activation processes, such as water splitting, CO(2)-reduction, or nitrogen fixation. Ideally, energy-rich molecules such as H(2), CH(3)OH, or NH(3) could be generated artificially using (solar) light as an energy input. In this context, PCETs originating directly from electronically excited states play a crucial role. A variety of transition-metal complexes have been used recently for fundamental investigations of this important class of reactions, and the key findings of these studies are reviewed in this article. The present minireview differs from other reviews on the subject of PCET in that it focuses specifically on reactions occurring directly from electronically excited states.     

Photofunctional triplet excited states of cyclometalated Ir(III) complexes: beyond electroluminescence

The development of cyclometalated Ir(III) complexes has enabled important breakthroughs in electroluminescence because such complexes permit the efficient population of triplet excited states that give rise to luminescent transitions. The triplet states of Ir(III) complexes are advantageous over those of other transition metal complexes in that their electronic transitions and charge-transfer characteristics are tunable over wide ranges. These favorable properties suggest that Ir(III) complexes have significant potential in a variety of photofunctions other than electroluminescence. In this critical review, we describe recent photonic applications of novel Ir(III) complexes. Ir(III) complexes have been shown to affect the exciton statistics in the active layers of organic photovoltaic cells, thereby improving the photon-to-current conversion efficiencies. Nonlinear optical applications that take advantage of the strong charge-transfer properties of triplet transitions are also discussed. The tunability of the electrochemical potentials facilitates the development of efficient photocatalysis in the context of water photolysis or organic syntheses. The photoredox reactivities of Ir(III) complexes have been employed in studies of charge migration along DNA chains. The photoinduced cytotoxicity of Ir(III) complexes on live cells suggests that the complexes may be useful in photodynamic therapy. Potential biological applications of the complexes include phosphorescence labeling and sensing. Intriguing platforms based on cyclometalated Ir(III) complexes potentially provide novel protein tagging and ratiometric detection. We envision that future research into the photofunctionality of Ir(III) complexes will provide important breakthroughs in a variety of photonic applications.     

Merging the best of two worlds: artificial metalloenzymes for enantioselective catalysis

Artificial metalloenzymes result from combining a catalytically active organometallic moiety with a macromolecular host. The resulting hybrid catalysts combine attractive features of both homogeneous and enzymatic systems. Herein we summarize the recent progress in this emerging field and outline the challenges ahead.     

Characterization of the excited states of ethylene by MRCI

The excited states of ethylene are systematically analyzed and characterized according to the natural orbitals (NOs) resulting from multireference configuration interaction singles and doubles (MRCISD) calculations. By comparing the shapes and nodal structures of the NOs with those of hydrogen atomic orbitals, the Rydberg series can be classified. Two or three different types of Rydberg series appear within five excited states for each symmetry of D_2h. For example, in the ¹A_g symmetry there are three series having np and two nf hydrogen‐like atomic orbitals. Electronic correlation effects for the (π→π*) V state are also discussed on the basis of a complete active space self‐consistent field (CASSCF) calculation, showing that electron correlation effects merely within the valence space cannot explain contraction of the V state. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005