π* ← n and π* ← π absorption spectra of aminopyrazine

π* ← n and π* ← π absorption spectra of aminopyrazine have been recorded and analysed assuming C_s symmetry for the molecule.     

Photoinduced π-π* band gap renormalization in graphite

As is well-known, the character of the π orbitals is of paramount importance for the chemical properties of the carbon allotropes and their derived compounds. While at equilibrium the nature of these orbitals is well understood, their photoinduced nonequilibrium behavior is under investigation. Here, we demonstrate that when a UV-laser pulse excites a carrier density larger than 10% of the π* density of state in graphite, a renormalization of the π-π* band gap takes place. This result has been achieved by detecting the transient reflectivity and the associated decay time of an infrared probe following the excitation of a UV pump pulse tuned across the π-π* absorption resonance. The pump photon energy at which both the transient reflectivity and the decay time are maximum is downshifted by 500 meV with respect to the relative absorption maximum at equilibrium. This finding is interpreted as a transient π-π* band gap shrinking of similar magnitude, near the M point of the Brillouin zone.     

Theoretical study of the valence π → π* excited states of polyacenes: anthracene and naphthacene

The valence π → π ^* excited states of anthracene and naphthacene are studied with multireference perturbation theory with complete active space self-consistent field reference functions. The predicted spectra provide a consistent assignment of all one- and two-photon spectra and T-T spectra of low-lying valence π → π ^* excited states of anthracene and naphthacene. The present theory predicts the valence π → π ^* excitation energies with an accuracy of 0.15 eV for anthracene and of 0.25 eV or better for naphthacene. The excited states of anthracene and naphthacene are compared with those of benzene and naphthalene studied previously. The present calculations predict that, going from anthracene to naphthacene, there is a symmetry reversal of the two lowest singlet state transitions, but not for the triplet, just as indicated by the experimental data. Some general trends of polyacene excited states are discussed based on the calculated results for benzene to naphthacene. Conclusive results obtained for anthracene and naphthacene can be used as a model for understanding the excited states of larger polyacenes. Received: 22 April 1998 / Accepted: 6 July 1998 / Published online: 28 September 1998     

Enhance photocatalytic properties of TiO2 using π-π* conjugate system

Abstract

A novel π-π* conjugate system photo-catalyst was successfully constructed using aniline, pyrrole and TiO₂. The structures of photo-catalyst were measured by XRD, SEM, BET, TEM, FT-IR, XPS and TGA in detail. The photocatalytic properties were investigated in rhodamine B and methyl orange solution, respectively. And the enhance mechanism of π-π* conjugate system was discussed in depth. The weight percentage of Ti weight about 47% in the compound and the π-π* conjugate system did not change the crystal structure of TiO₂. The photocatalytic degradation properties of the π-π* conjugate system photo-catalyst could get 99% for rhodamine B and methyl orange after 10 and 15?min, respectively. The introduction of the π-π* conjugate system into TiO₂ was beneficial to improve light harvest, photoelectric response and separation of electron-holes.     

Organometallic dithiolene complexes of benzenedithiolate analogues with π-coordinating and π-interacting Cp* ligand

Organometallic dithiolene complexes, which were formulated as [Cp*M(dcbdt)] and [Cp*M(dcdmp)] (M = Co, Rh, Ir; Cp* = η⁵-pentamethylcyclopentadienyl, dcbdt = 4,5-dicyanobenzene-1,2-dithiolate, dcdmp = 2,3-dicyano-5,6-dimercaptopyrazine) were prepared from a low valent Cp*Co^I or high valent Cp*M^III species (M^III = Co^III, Rh^III, Ir^III). The UV-Vis absorption spectral and electrochemical data of them were obtained. The lowest absorption (HOMO-LUMO) energies of them became redshift in order of the Co > Rh > Ir complexes. The reduction potentials suggested that the central metal modifies their LUMO levels. The molecular and crystal structures of [Cp*Co(dcbdt)] (3a), [Cp*Co(dcdmp)] (4a) and [Cp*Rh(dcdmp)] (4b) were determined by X-ray diffraction studies. The cobalt complexes 3a and 4a were monomeric, formally 16-electron complexes and have two-legged piano-stool geometries. The crystal structure of 3a indicated some plane-to-plane intermolecular interactions such as benzene?benzene interaction on the dcbdt ligand and two Cp*?benzene π-π stackings. 4a showed plane-to-plane interaction with a pseudo-4-fold-symmetry arrangement between the pyrazine moieties on the dcdmp ligand. The rhodium complex 4b was dimeric in the crystal to form a criss-cross arrangement and had a three-legged piano-stool geometry, but it was monomerized in solution. The dimer of 3b was observed in the oxidation process of the cyclic voltammogram.     

Experimental and theoretical studies on organic D-π-A systems containing three-coordinate boron moieties as both π-donor and π-acceptor

Four linear π-conjugated systems with 1,3-diethyl-1,3,2-benzodiazaborolyl [C(6)H(4)(NEt)(2)B] as a π-donor at one end and dimesitylboryl (BMes(2)) as a π-acceptor at the other end were synthesized. These unusual push-pull systems contain phenylene (-1,4-C(6)H(4)-; 1), biphenylene (-4,4'-(1,1'-C(6)H(4))(2)-; 2), thiophene (-2,5-C(4)H(2)S-; 3), and dithiophene (-5,5'-(2,2'-C(4)H(2)S)(2)-; 4) as π-conjugated bridges and different types of three-coordinate boron moieties serving as both π-donor and π-acceptor. Molecular structures of 2, 3, and 4 were determined by single-crystal X-ray diffraction. Photophysical studies on these systems reveal blue-green fluorescence in all compounds. The Stokes shifts for 1, 2, and 3 are notably large at 7820-9760 cm(-1) in THF and 5430-6210 cm(-1) in cyclohexane, whereas the Stokes shift for 4 is significantly smaller at 5510 cm(-1) in THF and 2450 cm(-1) in cyclohexane. Calculations on model systems 1'-4' show the HOMO to be mainly diazaborolyl in character and the LUMO to be dominated by the empty p orbital at the boron atom of the BMes(2) group. However, there are considerable dithiophene bridge contributions to both orbitals in 4'. From the experimental data and MO calculations, the π-electron-donating strength of the 1,3-diethyl-1,3,2-benzodiazaborolyl group was found to lie between that of methoxy and dimethylamino groups. TD-DFT calculations on 1'-4', using B3LYP and CAM-B3LYP functionals, provide insight into the absorption and emission processes. B3LYP predicts that both the absorption and emission processes have strong charge-transfer character. CAM-B3LYP which, unlike B3LYP, contains the physics necessary to describe charge-transfer excitations, predicts only a limited amount of charge transfer upon absorption, but somewhat more upon emission. The excited-state (S(1)) geometries show the borolyl group to be significantly altered compared to the ground-state (S(0)) geometries. This borolyl group reorganization in the excited state is believed to be responsible for the large Stokes shifts in organic systems containing benzodiazaborolyl groups in these and related compounds.     

Energies of π*- and π-states of the conduction band and valence band of chiral carbon nanotubes

Nanotubes (NTs) are mainly represented by (n,p) chiral NTs with chirality indices 0 < p < n delimited by (n,0) and (n,n) for achiral NTs. In (n,p) chiral NTs, the unit cell hexagons have a helical arrangement on the cylindrical surface of an NT and common angular and axial translations. An analytical formula was derived for calculation of the band structure of both chiral and achiral NTs with chirality indices 0 ≤ p ≤ n and band diagrams of some chiral NTs. Chiral NTs significantly extend the range of semiconducting NTs. An equation for the band gap width ΔЕ of semiconducting chiral and achiral NTs was derived: \(\frac{{\vartriangle E}}{{{\gamma _0}}} = \frac{{2\pi }}{{\sqrt {3{n^2} + 3np + 3{p^2}} }}\). Tables of the band structure parameters of metallic and semiconducting chiral NTs are presented.     

Intersystem crossing in the 1nπ* and 1ππ* states

Fast intersystem crossing is observed in the S(1)(1)nπ* state of N-heterocyclic aromatic hydrocarbons and carbonyl compounds. It is attributed to spin-orbit coupling with the (3)ππ* state in the same energy region. The strong singlet-triplet mixing was confirmed by large Zeeman splitting of rotational lines in a high-resolution spectrum. For the S(1)(1)ππ* state of aromatic hydrocarbons, the observed Zeeman splitting was found to be considerably small, and intersystem crossing was considered to be minor. These facts are in accordance with El-Sayed's rule, which states spin-orbit coupling is forbidden between the (1)ππ* and (3)ππ* states. The Zeeman splitting of several derivatives was also observed and the substitution effect on the intersystem crossing rate is discussed.     

Observation of Exceptionally Low-Lying π-π* Excited States in Oxidized Forms of Quadruple-Decker Phthalocyanine Complexes

Spectroscopic investigations have been performed for the oxidized forms of two quadruple-decker phthalocyanine complexes in order to clarify the electronic structures of multiply stacked π-systems. Up to three-electron-oxidized species were isolated by using phenoxathiin hexachloroantimonate as the oxidant. As the oxidations proceed, the Q-bands in the visible region shift bathochromically along with the clear isosbestic points. The one- and three-electron-oxidized species exhibited typical π-radical signals in the ESR spectra, while the neutral and two-electron oxidized species gave no indication of the presence of π-radicals. The electronic transitions observed for the oxidized species reach even into the so-called fingerprint region in IR spectroscopy (～1000 cm(-1)). With the aid of theoretical calculations, these bands can be assigned to the π-π* transitions. Our results provide new insights into π-electronic systems having exceptionally small MO energy gaps.     

C-H/π-interaction-guided self-assembly in π-conjugated oligomers

We report CH/π hydrogen-bond-driven self-assembly in π-conjugated skeletons based on oligophenylenevinylenes (OPVs) and trace the origin of interactions at the molecular level by using single-crystal structures. OPVs were designed with appropriate pendants in the aromatic core and varied by hydrocarbon or fluorocarbon tails along the molecular axis. The roles of aromatic π-stack, van der Waals forces, fluorophobic effect and CH/π interactions were investigated on the theromotropic liquid crystallinity of OPV molecules. Single-crystal structures of hydrocarbon OPVs provided direct evidence for the existence of CH/π interactions between the π-ring (H-bond acceptor) and alkyl C-H (H-bond donor). The four important crystallographic parameters, d(c-x)=3.79 ?, θ=21.49°, φ=150.25° and d(Hp-x)=0.73 ?, matched in accordance with typical CH/π interactions. The CH/π interactions facilitate the close-packing of mesogens in x-y planes, which were further protruded along the c axis producing a lamellar structure. In the absence of CH/π interactions, van der Waals interactions drove the assembly towards a Schlieren nematic texture. Fluorocarbon OPVs exhibited smectic liquid-crystalline textures that further underwent Smectic A (SmA) to Smectic C (SmC) phase transitions with shrinkage up to 11%. The orientation and translational ordering of mesogens in the liquid-crystalline (LC) phases induced H- and J-type molecular arrangements in fluorocarbon and hydrocarbon OPVs, respectively. Upon photoexcitation, the H- and J-type molecular arrangements were found to emit a blue or yellowish/green colour. Time-resolved fluorescence decay measurements confirmed longer lifetimes for H-type smectic OPVs relative to that of loosely packed one-dimensional nematic hydrocarbon-tailed OPVs.     

Nature of anion-templated π(+)-π(+) interactions

Interaction between positively charged aromatic groups (π(+)-π(+)) is characterized by anti-parallel, displaced-stacked structures in the presence of counteranions. Binding energies of pyridinium, N-methylpyridinium and N-methylimidazolium dimers are much larger than that of benzene-pyridine (π-π) and pyridinium-benzene (π(+)-π). Stabilization is attributed to attractive electrostatic interaction with significant dispersion contribution.     

π-Electron currents in fixed π-sextet aromatic benzenoids

In view of different patterns of π-electron density currents in benzenoid aromatic compounds it is of interest to investigate the pattern of ring currents in various classes of compounds. Recently such a study using a graph theoretical approach to calculating CC bond currents was reported for fully benzenoid hydrocarbons, that is, benzenoid hydrocarbons which have either π-sextets rings or “empty” rings in the terminology of Clar. In this contribution we consider π-electron currents in benzenoid hydrocarbons which have π-electron sextets and C=C bonds fully fixed. Our approach assumes that currents arise from contributions of individual conjugated circuits within the set of Kekulé valence structures of these molecules.     

Reversible and irreversible higher-order cycloaddition reactions of polyolefins with a multiple-bonded heavier group 13 alkene analogue: contrasting the behavior of systems with π-π, π-π*, and π-n+ frontier molecular orbital symmetry

The heavier group 13 element alkene analogue, digallene Ar(iPr(4))GaGaAr(iPr(4)) (1) [Ar(iPr(4)) = C(6)H(3)-2,6-(C(6)H(3)-2,6-(i)Pr(2))(2)], has been shown to react readily in [n + 2] (n = 6, 4, 2 + 2) cycloaddition reactions with norbornadiene and quadricyclane, 1,3,5,7-cyclooctatetraene, 1,3-cyclopentadiene, and 1,3,5-cycloheptatriene to afford the heavier element deltacyclane species Ar(iPr(4))Ga(C(7)H(8))GaAr(iPr(4)) (2), pseudoinverse sandwiches Ar(iPr(4))Ga(C(8)H(8))GaAr(iPr(4)) (3, 3(iso)), and polycyclic compounds Ar(iPr(4))Ga(C(5)H(6))GaAr(iPr(4)) (4) and Ar(iPr(4))Ga(C(7)H(8))GaAr(iPr(4)) (5, 5(iso)), respectively, under ambient conditions. These reactions are facile and may be contrasted with other all-carbon versions, which require transition-metal catalysis or forcing conditions (temperature, pressure), or with the reactions of the corresponding heavier group 14 species Ar(iPr(4))EEAr(iPr(4)) (E = Ge, Sn), which give very different product structures. We discuss several mechanistic possibilities, including radical- and non-radical-mediated cyclization pathways. These mechanisms are consistent with the improved energetic accessibility of the LUMO of the heavier group 13 element multiple bond in comparison with that of a simple alkene or alkyne. We show that the calculated frontier molecular orbitals (FMOs) of Ar(iPr(4))GaGaAr(iPr(4)) are of π-π symmetry, allowing this molecule to engage in a wider range of reactions than permitted by the usual π-π* FMOs of C-C π bonds or the π-n(+) FMOs of heavier group 14 alkyne analogues.     

Spectral shifts of the n → π* and π → π* transitions of uracil based on a modified form of solvent reorganization energy

According to our recent studies on the nonequilibrium solvation, the solvent reorganization energy is found to be the cost of maintaining the residual polarization P', which equilibrates with the extra electric field E(ex). On the basis of this solvent reorganization energy and the well-established equilibrium solvation energy, a novel and reasonable expression for the spectral shift of the electronic absorption spectra is proposed in this work. Furthermore, the two lowest transitions of uracil in aqueous solution are investigated as test cases with the TDDFT/6-311++G** method. The obtained spectral shift is 0.48 eV for n → π* transition and -0.14 eV for π → π* transition, agreeing well with available experimental results. The contributions to the shift are discussed and the electrostatic plus polarization components are found to be crucial for the electronic absorption spectra of uracil in aqueous solution.     

π-Depletion as a criterion to predict π-stacking ability

The screening of molecular targets benefits from design criteria, which can identify the most promising candidates. We demonstrate that π-depleted polyaromatic molecules present superior π-stacking ability. This realization is quantified using a computational criterion, LOLIPOP, that detects ideal π-conjugated frameworks. The utility of LOLIPOP is illustrated by identifying tailored chemosensors.     

π4s + π2s Cycloaddition: Synthesis and OXA-DI-π-Methane Rearrangement of Annulated Bicyclo (2.2.2) Octenones

A simple one step synthesis of tricyclo (2.2.2.0^2,6) tetradecadienones and 5-spiro tricyclo (2.2.2.0^2,6) undecadienone by trapping of 2,4-cyclohexa-dienone with various dienophiles followed by photochemical oxa-di-π-methane rearrangement of the resulting adducts to polyquinanes is described.     

Direct Observation of Singlet ππ* and nπ* Equilibrium State in 2-Amino-1,3,5-triazine Solution

1,3,5-Triazine molecules represent a class of molecules that may have been prebiotic information carriers in a primordial soup in early Earth and their excited state dynamics has received attention in recent years. In our previous study, one component with lifetime longer than 100 ps was discovered in 2-amino-1,3,5-trainzine (2-AT), but its nature has not been revealed. In this study, excited state dynamics of 2-AT is studied in different solvents by using femtosecond time-resolved transient absorption and fluorescence up-conversion spectroscopy. Interestingly, an equilibrium state consisting of the bright \begin{document}$ \pi\pi.* $\end{document} and dark n\begin{document}$ \pi.* $\end{document} states in 2-AT is directly observed in aqueous solution and its dynamics is solvent sensitive. The whole picture of the excited state deactivation mechanism of 2-AT is proposed based on our spectroscopy results.     

An n→π* interaction in aspirin: implications for structure and reactivity

Stereoelectronic effects modulate molecular structure, reactivity, and conformation. We find that the interaction between the ester and carboxyl moieties of aspirin has a previously unappreciated quantum mechanical character that arises from the delocalization of an electron pair (n) of a donor group into the antibonding orbital (π*) of an acceptor group. This interaction affects the physicochemical attributes of aspirin and could have implications for its pharmacology.     

Electronic structure and aromaticity of planar singlet π-electron monocyclic π2 carbenes

Induced -electron ring currents and relative diamagnetic susceptibilities of planar singlet -electron monocyclic ² carbenes are calculated in the coupled variant of perturbation theory in the CNO method. It is shown that the parity theorem holds for charged planar ² and ² carbenes formally generated by the detachment of a proton or a hydride ion from alternant hydrocarbons. This leads, in such carbenes, to the same energies of singlet-singlet and singlet-triplet transitions, electric polarizabilities, conjugation energies, -electron ring currents, diamagnetic susceptibilities and current contributions to chemical shifts, whereas the charge contributions to the latter are of opposite signs.Donbas State Academy of Building and Architecture, 1 Derzhavina St., 339023 Makeevka, Donetsk Region, Ukraine. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 32, No. 3, pp. 157–162, May–June, 1996. Original article submitted December 5, 1994.