Triplet π→π* transitions in thiophene,furan and pyrrole by low-energy electron-impact spectroscopy

Low-energy (0–3 V above threshold) electron-impact excitation spectra and transmission spectra are presented for thiophene, furan and pyrrole. For each molecule two low-lying triplet states are reported. An additional new transition has been observed in thiophene at 6.5 eV. Assignments are suggested for some of the observed direct and resonance excitation processes.     

A non-empirical SCF and CI study of the electronic spectrum of pyrrole

Various electronically excited states of pyrrole have been studied by ab initio SCF and CI calculations including π → π^* and π → Rydberg excitations. Optically allowed valence type transitions are found at energies higher than 6.5 eV whereas all the lower singlet states are of Rydberg type. In addition to the experimentally known triplet states at 4.23 and 5.10 eV, several new triplet transitions with energies from 5.71 to 7.10 eV are predicted. In most cases good agreement with experimental data is found.     

Unsymmetric pyrrole, thiophene, and furan-conjugated comonomers prepared using azomethine connections: potential new monomers for alternating homocoupled products

Unsymmetric comonomers consisting of thiophene, pyrrole, and furan heterocycles were prepared using azomethine bonds. Photophysical investigation of the novel pi-donor-donor-donor segmented compounds revealed that their singlet excited state is only partially deactivated by internal conversion unlike their all-thiophene azomethine analogues. Temperature-dependent steady-state and time-resolved emission studies demonstrated that the unsymmetric compounds deactivated efficiently their singlet excited state by intersystem crossing to populate the triplet manifold. This lower energy state is rapidly deactivated by nonradiative self-quenching. The comonomers and their anodically prepared conjugated homocoupled products are both electrochemically active, resulting in new compounds that can be mutually oxidized and reduced. Meanwhile, the oxidation potentials of the coupled products are shifted by up to 400 mV to more cathodic potentials relative to their corresponding comonomers, confirming their increased degree of conjugation.     

Excited electronic states of the fluorobenzenes by variable angle electron impact spectroscopy

Electron-impact excitation spectra of benzene, fluorobenzene, o-difluorobenzene, 1,3,5-trifluorobenzene, 1,2,3,4-tetrafluorobenzene, pentafluorobenzene, and hexafluorobenzene have been measured at impact energies of 50 eV and either 25 eV or 30 eV, and scattering angles from 5° to 80°. Each molecule shows an absorption maximum at about 3.9 eV corresponding to a singlet → triplet, π → π^*, transition. In benzene, fluorobenzene, o-difluorobenzene, and 1,3,5-trifluorobenzene, an additional singlet → triplet transition was detected at about 5.6 eV. Three singlet → singlet transitions analogous to the 4.90, 6.20, and 6.95 eV transitions in benzene are seen in each of the fluorine-substituted molecules. The more highly substituted compounds exhibit an additional singlet → singlet transition that is most clearly observed in the hexafluorobenzene spectrum with a peak at 5.32 eV.     

The vibrational structures of furan, pyrrole, and thiophene cations studied by zero kinetic energy photoelectron spectroscopy

The vibrational structures of the electronic ground states ((approximately)X (2)A(2)) of furan, pyrrole, and thiophene cations have been studied by zero kinetic energy (ZEKE) photoelectron spectroscopic method. In addition to the strong excitations of the symmetric a(1) vibrational modes, other three symmetric vibrational modes (a(2), b(1), and b(2)) have been observed unambiguously. These results which cannot be explained by the Franck-Condon principle illustrate that the vibronic coupling and the Coriolis coupling may play important roles in understanding the vibrational structures of the five-membered heterocycle cations. The vibrationally resolved ZEKE spectra are assigned with the assistance of the density function theory calculations, and the fundamental frequencies for many vibrational modes have been determined for the first time. The first adiabatic ionization energies for furan, pyrrole, and thiophene were determined as 8.8863, 8.2099, and 8.8742 eV, respectively, with uncertainties of 0.0002 eV.     

Assignment of electronic transitions by geometry optimization

Adiabatic excitation energies, excited state geometries, excited state charges, bond orders and dipole moments have been obtained for HCN, CO₂,H₂CO, HFCO, F₂CO, ethylene, trans-butadiene, furan, pyrrole and uracil using the SINDO1 semi-empirical method with configuration interaction. Our results generally agree with those ofab initio calculations and experiment satisfactorily. Geometry optimization is found to mix configurations differing in their allowedness in vertical excitation from the ground state, which in turn helps in the assignment of spectral transitions. TheV excited singlet state of trans-butadiene and various excited states of furan, pyrrole and uracil have been found to be appreciably non-planar. The single and double CC bonds are found to exchange positions due to the lowest triplet and singlet transitions of furan and pyrrole. The first triplet and first singlet transitions of uracil have been found to be of π-π* and π-σ* types respectively in agreement with recent experimental findings. On leave of absence from the Department of Physics, Banaras Hindu University, Varanasi-221005, India     

Excited electronic states of 1,3,5-cycloheptatriene

The electron-impact energy-loss spectrum of 1,3,5-cycloheptatriene has been measured at impact energies of 30,50, and 75 eV, and scattering angles varying from 5° to 80 °. Singlet → triplet transitions were observed at 3.05 eV and 3.95 eV. No evidence for the weak transition at 2.1 eV previously reported on the basis of threshold electron-impact studies was found. Single → singlet transitions were observed at 4.85 eV and 6.40 eV in good agreement with the optical spectrum and semi-empirical calculations.     

Low-energy electron-impact excitation spectra of formaldehyde,acetaldehyde and acetone

Threshold electron-impact excitation spectra, transmission spectra and n→n^* excitation functions are presented for formaldehyde, acetaldehyde and acetone. Formation of temporary negative ion states and their decay-channels are discussed. The electronic transition near 4 eV is shown to be due both to triplet and singlet n→n^* excitation. The energy positions of the ³n→n^* and first triplet Rydberg transition are accurately located. In formaldehyde a new transition is reported at 8.50 eV. Assignments are given for the observed electronic excitation processes.     

The direction of the dipole moments of furan,thiophen, and pyrrole: A controversial question

The report that the dipole moment in the five-membered heterocyclic rings, furan, thiophene, and pyrrole has the positive pole on the heteroatom is contradicted with arguments based on reactivity data, theoretical calculations and moment values of substituted derivatives. In actuality, the dipole moment in furan and thiophene is directed from the ring (positive pole) to the heteroatom (negative pole).     

The Low‐Lying Excited States of 2,2′‐Bithiophene: A Theoretical Analysis

The low‐energy regions of the singlet→singlet, singlet→triplet, and triplet→triplet electronic spectra of 2,2′‐bithiophene are studied using multiconfigurational second‐order perturbation theory (CASPT2) and extended atomic natural orbitals (ANO) basis sets. The computed vertical, adiabatic, and emission transition energies are in agreement with the available experimental data. The two lowest singlet excited states, 1¹B_u and 2¹B_u, are computed to be degenerate, a novel feature of the system to be borne in mind during the rationalization of its photophysics. As regards the observed high triplet quantum yield of the molecule, it is concluded that the triplet states 2³A_g and 2³B_u, separated about 0.4 eV from the two lowest singlet excited states, can be populated by intersystem crossing from nonplanar singlet states.     

Palladium-catalyzed, one-pot carbonylation of heterocyclic compounds into their esters, in the presence of mercury salts.

One pot carbonylation of thiophene, furan, benzofuran and pyrrole in alcohol, to produce the corresponding esters, has been carried out at room temperature, using a PdCl₂/mercury salt/Copper(II) system.     

Spatial extension of the bipolaronic defect in highly conducting polymers based on five-membered aromatic rings

The molecular geometries of neutral and dicationic oligomers comprising 7, 10, 13 and 16 rings of thiophene, furan and pyrrole were determined using quantum mechanical calculations. The usual benzenoid structure was characterized for neutral oligomers, whereas a quinoid structural defect which extends over six rings was found for dications. Analysis of the transition region, where the quinoid structure changes to the benzenoid one, revealed structural differences between thiophene, furan and pyrrole derivatives.     

Photophysics of Push–Pull Distyrylfurans,Thiophenes and Pyridines by Fast and Ultrafast Techniques

Time‐resolved transient absorption and fluorescence spectroscopy with nano‐ and femtosecond time resolution were used to investigate the deactivation pathways of the excited states of distyrylfuran, thiophene and pyridine derivatives in several organic solvents of different polarity in detail. The rate constant of the main decay processes (fluorescence, singlet–triplet intersystem crossing, isomerisation and internal conversion) are strongly affected by the nature [locally excited (LE) or charge transfer (CT)] and selective position of the lowest excited singlet states. In particular, the heteroaromatic central ring significantly enhances the intramolecular charge‐transfer process, which is operative even in a non‐polar solvent. Both the thiophene and pyridine moieties enhance the S₁→T₁ rate with respect to the furan one. This is due to the heavy‐atom effect (thiophene compounds) and to the ¹(π,π)*→³(n,π)* transition (pyridine compounds), which enhance the spin‐orbit coupling. Moreover, the solvent polarity also plays a significant role in the photophysical properties of these push–pull compounds: in fact, a particularly fast ¹LE*→¹CT* process was found for dimethylamino derivatives in the most polar solvents (time constant, τ≤400 fs), while it takes place in tens of picoseconds in non‐polar solvents. It was also shown that the CT character of the lowest excited singlet state decreased by replacing the dimethylamino side group with a methoxy one. The latter causes a decrease in the emissive decay and an enhancement of triplet‐state formation. The photoisomerisation mechanism (singlet/triplet) is also discussed.     

Reactivity and regioselectivity of the pyrrole,furan, and thiophene radical cations in polymerization reactions

Within the framework of the delocalization model, based on the perturbation theory, the rate constants of the reactions of the pyrrole, furan, and thiophene radical cations with the initial compounds were compared. It was estabished that attack of a cation in the 2-position of the heterocycle is more probable in comparison with the 3-position. The results of a comparison of the reactivity of the radical cations and the corresponding molecules in the triplet state in these reactions are discussed in connection with the experimental data.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 28, Nos. 5–6, pp. 483–487, September–December, 1992.     

Alkylation of furan,thiophene, and pyrrole with furfuryl alcohol in the presence of the strongly acidic Amberlyst 15 sulfo cation-exchange resin

The corresponding 2-furylhetarylmethanes were obtained by the reaction of furan, thiophene, or pyrrole with furfuryl alcohol in the presence of the strongly acidic Amberlyst 15 cation-exchange resin in the H⁺ form. The alkylation of furan and thiophene takes place regiospecifically in the 2 position, whereas 2-furyl-2-pyrrolyl- and 2-furyl-3-pyrrolylmethane in a ratio of 6.21 are formed in the case of pyrrole.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 746–749, June, 1989.     

Theoretical fine spectroscopy with symmetry-adapted-cluster configuration-interaction method: outer- and inner-valence ionization spectra of furan, pyrrole, and thiophene

Theoretical fine spectroscopy has been performed for the valence ionization spectra of furan, pyrrole, and thiophene with the symmetry-adapted-cluster configuration-interaction general-R method. The present method described that the pi(1) state interacts with the pi(3) (-2)pi*, pi(2) (-2)pi*, and pi(2) (-1)pi(3) (-1)pi* shake-up states providing the split peaks and the outer-valence satellites, both of which are in agreement with the experiments. The intensity distributions were analyzed in detail for the inner-valence region. In particular, for furan, theoretical intensities were successfully compared with the intensity measured by the electron momentum spectroscopy. The interactions of the 3b(2) and 5a(1) states with the shake-up states were remarkable for furan and pyrrole, while the 4b(2) state of thiophene had relatively large intensity.     

Cyclopentadienes Annelated with Five-membered Heterocycles: Methods of Synthesis, Heteroorganic Derivatives, and Synthetic Precursors. (Review)

The review is devoted to cyclopentadienes annelated with five-membered heterocycles (furan, pyrrole, thiophene, and their benzo analogs) (hetarenocyclopentadienes), their heteroorganic derivatives, and the synthetic precursors (hetarenocyclopentanones).     

Ab initio calculations on urea

Multiconfiguration wave functions constructed from contracted Gaussian-lobe functions have been found for the ground and valence-excited states of urea. ICSCF molecular orbitals of the excited states were used as the parent configurations for the CI calculations except for the ¹A₁(π → π*) state. The ¹A₁(π → π*) state used as its parent configuration an orthogonal linear combination of natural orbitals obtained from the second root of a three-configuration SCF calculation. The lowest excited states are predicted to be the n π → π* and π → π* triplet states. The lowest singlet state is predicted to be the n π → π* state with an energy in good agreement with the one known UV band at 7.2 eV. The π → π* singlet state is predicted to be about 1.9 eV higher, contrary to several previous assignments which assumed the lowest band was a π → π* amide resonance band. The predicted ionization energy of 9.0 eV makes this and higher states autoionizing.     

Ab initio SCF CL study of the electronic spectrum of nitroso-methane

Configuration interaction studies of ground, n_ → π^*, n₊ → π^*, and π → π^* electronically excited states are reported for nitroso-methane in its eclipsed equilibrium geometry. The first (n_ → π^*) and the second (n⁺ → π^*) singlet states are calculated at 2.17 and 7.14 eV. it is shown that a significant delocalization of the nonbonding orbitals on the nitrogen and oxygen is responsible for the large energy gap between these two states. The two lowest triplet states occur at 1.29 and 5.39 eV and are of n_ → π^* and π → π^* origin.     

Chiral Calix[3]pyrrole Derivatives: Synthesis,Racemization Kinetics,and Ring Expansion to Calix[9]- and Calix[12]pyrrole Analogues

Chiral pyrrolic macrocycles continue to attract interest. However, their molecular design remains challenging. Here, we report a calixpyrrole-based chiral macrocyclic system, calix[1]furan[1]pyrrole[1]thiophene ( 1 ), synthesized from an oligoketone. Macrocycle 1 adopts a partial cone conformation in the solid state, and undergoes racemization via ring inversion. Molecular dynamics simulations revealed that inversion of the thiophene is the rate determining step. Pyrrole N-methylation suppressed racemization and permitted chiral resolution. Enantioselective N-methylation also occurred in the presence of a chiral ammonium salt, although the stereoselectivity is modest. A unique feature of 1 is that it acts as a useful synthetic precursor to yield several calix[n]furan[n]pyrrole[n]thiophene products (n=2–4), including a calix[12]pyrrole analogue that to our knowledge constitutes the largest calix[n]pyrrole-like species to be structurally characterized.