Density functional study on the structures and vibrational spectra of the radical anion and cation of biphenyl

Bimolecular electron transfers that deviate from the Sandros-Boltzmann dependence on free energy: steric effect

As we reported recently, endergonic to mildly exergonic electron transfer between neutral aromatics (benzenes and biphenyls) and their radical cations in acetonitrile follows a Sandros-Boltzmann (SB) dependency on the reaction free energy (ΔG); i.e., the rate constant is proportional to 1/[1 + exp(ΔG/RT)]. We now report deviations from this dependency when one reactant is sterically crowded: 1,4-di-tert-butylbenzene (C1), 1,3,5-tri-tert-butylbenzene (C2), or hexaethylbenzene (C3). Obvious deviation from SB behavior is observed with C1. Stronger deviation is observed with the more crowded C2 and C3, where steric hindrance increases the interplanar separation at contact by ~1 Å, significantly decreasing the π orbital overlap. Consequently, electron transfer (k(et)) within the contact pair becomes slower than diffusional separation (k(-d)), causing deviation from the SB dependency, especially near ΔG = 0. Fitting the data to a standard electron-transfer theory gives small matrix elements (~5-7 meV) and reasonable reorganization energies. A small systematic difference between reactions of C3 with benzenes vs biphenyls is rationalized in terms of small differences in the electron-transfer parameters that are consistent with previous data. The influence of solvent viscosity on the competition between k(et) and k(-d) was investigated by comparing reactions in acetonitrile and propylene carbonate.

A general measure of conjugation in biphenyls and their radical cations

Changes in the amount of conjugation in the biphenyl system upon torsion have been investigated with quantum chemistry computations. For the unsubstituted biphenyl and its radical cation, the change in the energy for the first electronic transition as determined from time-dependent density functional theory, specifically TD-B3LYP, upon an imposed change in torsion angle appears to provide a good general measure of the delocalization of the π-electrons. Upon substitution, though, a relationship could not be established between the excitation energy and the twist across the C–C bond connecting the two phenyl rings. In contrast, the π-interaction energy, i.e., the π–π interaction across the central C–C bond, determined from a natural bond orbital analysis, provides a uniform description of conjugation in the electronically different systems, for an imposed twist as well as upon substitution. Substituent effects are found to be larger and more varied for the biphenyl radical cations. Of particular interest is the introduction of the oxidizable substituents OCH3 (methoxyl) and SCH3 (methylthio), as this leads to the smallest changes in torsion angle in the biphenyl unit upon removal of an electron from the molecule. The findings of this work could ultimately prove useful for the rational design of conductive polymer materials.

Structures and energies of the radicals and anions generated from chlorpyrifos

The radicals and anions generated from chlorpyrifos by removing a hydrogen atom have been investigated using the hybrid density functional B3PW91 method. The results show that all the radicals have been classified as three groups and their stability order is methylene (radical 1, 3, 5, and 7) > methyl (radical 9, 11 and 13) > ring (15); the anions have the relative energetic order: methyl > methylene > ring. Moreover, some decomposition reactions are also reported. The large HOMO-LUMO gaps indicate that both radicals and anions are predicted to be high-kinetic stable molecules. We also find that radicals 9, 11 and 13 have the highest AEAs and anions 2, 4 and 6 have higher VDEs. Additionally, natural population analysis charges show that there is the lowest Deltaq (0.14) for the C7 and C9 atoms. We hope that our theoretical results may provide a reference for further experiment and practical application.

Conformationally gated photoinduced processes within photosensitizer-acceptor dyads based on osmium(II) complexes with triarylpyridinio-functionalized terpyridyl ligands: insights from experimental study

[(ttpy)Os(tpy-ph-TPH(3)(+))](3+) (2), [(ttpy)Os(tpy-xy-TPH(3)(+))](3+) (3), [(ttpy)Os(tpy-ph-TPH(2)(NO(2))(+))](3+) (4), and [(ttpy)Os(tpy-xy-TPH(2)(NO(2))(+))](3+) (5) are a series of dyads made of an Os(II) bis-tpy complex (tpy = 2,2':6',2"-terpyridine) as the photosensitizer (P) and 2,4,6-triarylpyridinium group (TP(+)) as the electron acceptor (A). These dyads were designed to form charge-separated states (CSS) upon light excitation. Together with analogous Ru(II) complexes (7-10), they have been synthesized and fully characterized. We describe herein how intramolecular photoinduced processes are affected when the electron-accepting strength of A (by nitro-derivatization of TP(+)) and/or the steric hindrance about intercomponent linkage (by replacing a phenyl spacer by a xylyl one) are changed. Electronic absorption and electrochemical behavior revealed that (i) chemical substitution of TP(+) (i.e., TP(+)-NO(2)) has no sizable influence on P-centered electronic features, (ii) reduction processes located on TP(+) depend on the intercomponent tilt angle. Concerning excited-state properties, photophysical investigation evidenced that phosphorescence of P is actually quenched in dyads 4 and 5 only. Ultrafast transient absorption (TA) experiments allowed attributing the quenching in conformationally locked dyad 5 to oxidative electron transfer (ET) from the (3)MLCT level to the TP(+)-NO(2) acceptor (k(el) = 1.1 x 10(9) s(-)(1)). For 4, geometrically unlocked, the (3)MLCT state was shown to first rapidly equilibrate (reversible energy transfer; k(eq) approximately 2 x 10(9) s(-)(1)) with a ligand centered triplet state before undergoing CSS formation. Thus, the pivotal role of conformation in driving excited-state decay pathways is demonstrated. Also, inner P structural planarization as a relaxation mode of the (3)MLCT states has been inferred from TA experiments.

Electronic structure of Li-intercalated oligopyridines: A comparative study by photoelectron spectroscopy

The role of nitrogen in the charge transfer and storage capacity of lithium-intercalated heterocyclic oligophenylenes was investigated using photoelectron spectroscopy. The development of new occupied states at low binding energies in the valence band region, as well as core level chemical shifts at both carbon and nitrogen sites, demonstrates partial charge transfer from lithium atoms to the organic component during formation of the intercalated compound. In small compounds, i.e., biphenyl and bipyridine derivatives, the position of the nitrogen heteroatom significantly affects the spacing between gap states in the Li-intercalated film; yet it has minimal effects on the charge storage capacity. In larger, branched systems, the presence of nitrogen in the aromatic system significantly enhances the charge storage capacity while the Li-N bond strength at high intercalation levels is significantly weakened relative to the nitrogen-free derivative. These observations have strong implications towards improved deintercalation processes in organic electrodes in lithium-ion batteries.

The Multichromophore Approach: Prolonged Room-Temperature Luminescence Lifetimes in RuII Complexes Based on Tridentate Polypyridine Ligands

A new family of ruthenium(II) complexes with multichromophoric properties was prepared based on a "chemistry-on-the-complex" synthetic approach. The new compounds are based on tridentate chelating sites (tpy-type ligands, tpy=2,2':6',2''-terpyridine) and most of them carry appended anthryl chromophores. Complexes 2 a and 2 b were synthesized through the Pd-catalyzed Suzuki coupling reaction between 9-anthrylboronic acid and the chloro ligands on the presursor species 1 a and 1 b, respectively. The monocoupling product 2 c was also synthesized as the starting complex for a dimetallic complex under optimized Suzuki coupling conditions. The palladium(0)-catalyzed homocoupling reaction on complexes 1 a and 2 c led to dimetallic Ru(II) species 2 d and 2 e, respectively. The solid structures of complexes 2 a and 2 b were characterized by X-ray diffraction. The absorption spectra, redox behavior, luminescence properties (both at room temperature and at 77 K), and transient absorption spectra and decays of 2 a-e were investigated. The absorption spectra of all new species are dominated by ligand-centered (LC) bands in the UV region and metal-to-ligand charge-transfer (MLCT) bands in the visible region. The new compounds undergo reversible metal-centered oxidation processes and several ligand-centered reduction processes, which have been assigned to specific sites. The complexes exhibit luminescence both at room temperature in fluid solution and at 77 K in rigid matrices; the emission was attributed to (3)MLCT states at room temperature and to the lowest-lying anthracene triplet ((3)An) at low temperature, except for 2 c, which does not contain any anthryl chromophore and whose low temperature emission is also of MLCT origin. The luminescence lifetimes of complexes 2 a-d showed that multichromophoric behavior occurs in these species, allowing the luminescence lifetime of the Ru(II)-based chromophores to be prolonged to the microsecond timescale, with the anthryl groups behaving as energy-storage elements for the repopulation of the (3)MLCT state. Nanosecond transient-absorption spectroscopy confirmed the equilibration process between the triplet MLCT and An levels at room temperature. Thermodynamic and kinetic factors governing the equilibration time and the lifetime of the equilibrated excited state are discussed.

Core Excitations of Biphenyl

High-resolution C(1s) near-edge X-ray absorption and X-ray photoionization spectra of the free biphenyl molecule are presented and theoretically analyzed in order to allow an assignment of the observed spectral features. Finite lifetime broadening, a high density of vibrational states, and a strong overlap of contributions from chemically different carbon atom sites only partially allow resolving the vibrational fine structure. However, the shape and width of the spectral profiles are strongly determined by both chemical shifts and vibronic effects. In particular, different from photoionization of valence levels, both types of core level spectra do not contain contributions from dihedral modes which are related to the twisting motion of the two phenyl rings. Contrary to naphthalene, C-H stretching modes are significantly enhanced in the core excitation spectra of biphenyl while the contributions from C-C stretching modes are reduced.

Femtosecond absorption spectroscopy of transition metal charge-transfer complexes

Our research is concerned with the application of femtosecond time-resolved absorption techniques to the study of the photophysics of transition metal complexes. The focus is to understand the events that characterize the process of excited-state evolution from the time a photon is absorbed by a molecule to the formation of the lowest-energy excited state of the system. This Account describes our initial observations in this area and includes examples detailing these dynamics as they occur in the charge-transfer excited states of transition-metal polypyridyl chromophores.

Radical cation and dication of fluorene fully annelated with bicyclo[2.2.2]octene units: importance of the quinoidal resonance structure in the cationic fluorene

Fluorene 1 fully annelated with bicyclo[2.2.2]octene units was newly synthesized and oxidized to stable cationic species. The structure of radical cation salt 1(.+)SbCl(6)(-) was determined by X-ray crystallography, while the first fluorene dication 1(2+) was characterized by (1)H and (13)C NMR at -80 degrees C. Combined with the results of theoretical calculations, an important contribution of a quinoidal structure to the resonance hybrid was demonstrated in both 1(.+) and 1(2+). [structure: see text]