Oligothiophene catenanes and knots: a theoretical study

Oligothiophene [2]catenanes and knots containing up to 28 thiophene units have been studied at the BHandHLYP/3-21G level of theory. Small knots (less than 22 thiophene units) and [2]catenanes (less than 18 thiophene units) are strained molecules. Larger knots and [2]catenanes are almost strain-free. [2]Catenanes and knots having less than 18 and 24 units, respectively, show transversal electronic coupling destroying one-dimensionality of molecules reflecting in smaller band gaps compared to larger knots and catenanes. Ionization potentials of knots and catenanes are always higher compared to that of lineal oligomers due to less effective conjugation. Polaron formation in catenanes is delocalized only over one ring, leaving another intact. In the case of a knot containing 22 thiophene units, estimated polaron delocalization is 8 to 9 repeating units.     

Tubular aggregates of cyclic oligothiophenes. A theoretical study

The geometries of neutral, mono-, and dioxidized tubular aggregates of cyclo[8]thiophenes containing up to 5 repeating units were fully optimized at the MPWB1K/3-21G* level of theory. Calculated interplane distances between macrocycles were found to be close to 3.1 A for neutral and charged aggregates. The binding energies between macrocycles in neutral intermediates were in the range of 40-45 kcal/mol, increasing for monocations and dropping strongly for dicationic species due to electrostatic repulsion between polarons. It was established that there exists a noticeable interaction between pi-orbitals of individual macrocycles in tubular aggregates as follows from decreasing of the band gap with a number of repeating units in aggregates and the polaron delocalization toward tube axes in oxidized species. A polaron pair is the most stable dicationic state for all studied molecules according to the calculations. A singlet polaron pair is more stable than a triplet one. The energy difference between singlet and triplet states is growing smaller with the size of the system, becoming zero for the pentamer corresponding to a completely dissociated bipolaron.     

Triplet-state and singlet oxygen formation in fluorene-based alternating copolymers

Data are reported on the triplet states of a series of fluorene-based A-alt-B type alternating copolymers based on pulse radiolysis-energy transfer and flash photolysis experiments. From the pulse radiolysis experiments, spectra are given for eight copolymers involving phenylene, thiophene, benzothiadiazole, and oligothienylenevinylene groups. Quantum yields for triplet-state formation (PhiT) have been obtained by flash photolysis following laser excitation and in one case by photoacoustic calorimetry. In addition, yields of sensitized formation of singlet oxygen have been determined by time-resolved phosphorescence and are, in general, in excellent agreement with the PhiT values. In all cases, the presence of thiophene units is seen to increase intersystem-crossing quantum yields, probably because of the presence of the heavy sulfur atom. However, with the poly[2,7-(9,9-bis(2'-ethylhexyl)fluorene)-alt-1,4-phenylene] (PFP), thiophene S,S-dioxide (PFTSO2) and benzothiadiazole (F8BT) copolymers, low yields of triplet formation are observed. With three of the copolymers, the energies of the triplet states have been determined. With PFP, the triplet energy is virtually identical to that of poly[2,7-(9,9-bis(2'-ethylhexyl)fluorene)]. In contrast, with fluorene-thiophene copolymers PFaT and PF3T, the triplet energies are closer to those of thiophene oligomers, indicating that there is significant conjugation between fluorene and thiophene units but also that there is a more localized triplet state than with the homopolymers.     

Structure and energetics of Fe2(CO)8 singlet and triplet electronic states

The singlet and triplet state potential energy surfaces (PES) of Fe2(CO)8 are explored by means of density functional theory (DFT) techniques. The two PES have different global mimima: the dibriged C(2v) isomer for the singlet and the unbridged D(2d) isomer for the triplet. The sign of the energy gap between singlet and triplet global minima depends on the type of adopted DFT functional: hybrid functionals predict the triplet is more stable than the singlet, but the opposite applies to generalized gradient approximated (GGA) functionals. The analysis of the computed CO stretching frequencies demonstrates that the experimental data for the unbridged form is compatible also with the unbridged triplet D(2d) isomer. Starting from these two facts, the electronic structure of unbridged D(2d) Fe2(CO)8 is discussed herein. Single-point energy computations at the coupled-cluster single and double (CCSD) level favor the D(2d) triplet state.     

Molecular engineering in symmetric end-substituted oligothiophene derivatives: analysis of condensed-phase photoemission spectra using semiempirical Hartree-Fock calculations

The electronic structures of two series of end-capped thiophene oligomers, one set containing the electron-deficient dimesitylboryl end-cap and one containing the electron-rich triaryl amine end-cap, have been modeled using semiempirical quantum chemical calculations and the results used to assign features in the photoemission spectra of the materials in the condensed phase. For the thiophene oligomers end-capped with the electron-deficient dimesitylboryl moieties, the energy of the occupied frontier orbitals is largely governed by pi-type orbitals of the thiophene repeat units in the oligothiophene main chain. Conversely, in oligomers end-capped with electron-rich triarylamine moieties, the occupied frontier orbital energies are largely governed by orbital states of heavily mixed character associated with thiophene pi-type systems and the low-lying nitrogen lone pairs of end capping groups.     

The Antiferromagnetic Spin Coupling in Non‐Kekulé Acenes—Impressive Polyradical Character Revealed by High‐Level Multireference Methods

Complete active space (CASSCF) and multireference (MR‐CISD(Q) and MR‐AQCC) calculations were performed for non‐Kekulé analogues of acenes, dimethylenepolycyclobutadienes, with lengths of up to eight cyclobutadiene (CBD) units. Multireference calculations predict that the most stable energy state of the system is either triplet (if there is an odd number of CBD units) or singlet (if there is an even number of CBD units) due to antiferromagnetic spin coupling, which thus violates Hund's rule in larger molecules. We also show an impressive polyradical character in the system that increases with the size of the molecule, as witnessed by more than eleven unpaired electrons in the singlet state of the molecule with eight CBD units. Together with the small energy gap between singlet and higher multiplicity energy states even above the triplet state, this demonstrates the exceptional polyradical properties of these π‐conjugated oligomeric chains.     

Theoretical study of long oligothiophene dications: bipolaron vs polaron pair vs triplet state

A series of oligothiophene dications (from the sexithiophene dication to the 50-mer oligothiophene dication, nT2+, n = 6-50) were studied. Density functional theory (DFT) at the B3LYP/6-31G(d) level and, in some cases, also at BLYP/6-31Gd, was applied to study the singlet and triplet states of the whole series. We found that the singlet state is the ground state for all oligothiophene dications up to the 20-mer, and that the singlet and triplet states are degenerate for longer oligomers. Thus, the triplet state is never a pure ground state for these dications. We found that, for short oligothiophenes dication (e.g., 6T2+), the bipolaron state is the more important state, with only a small contribution made by the polaron pair state. For medium size oligothiophene dications (e.g., 14T2+), both the bipolaron state and the polaron-pair state contribute to the electronic structure. Finally, in long oligothiophene dications, such as 30T2+ and 50T2+, the contribution from the polaron pair state becomes dominant, and these molecules can be considered as consisting of two independent cation radicals or a polaron pair. Results from isodesmic reactions show that the stability of oligothiophene cation radicals over dications is inversely proportional to chain length. Small oligothiophene dications (n = 6-12) were studied at the CASSCF(m,m)/6-31G(d) (m = 4, 6, and 10) level. The major conclusions of this paper regarding the relative energy of the singlet state versus the triplet state and regarding the relative stability of the bipolaron versus the polaron pair were also supported by CASSCF calculations.     

SiNP分子结构及稳定性的理论研究

采用B3LYP/6-311G(d),QCISD和CCSD(T)方法,对单重态和三重态SiNP体系的异构化进行了研究.在QCISD/6-311G(d)水平下,得到了7个稳定的异构体,它们由4个过渡态所连接,其中三重态线型异构体SiNP(31,3Σ1),单重态环状异构体SiNP(14,1A')和单重态线型异构体SiNP(11,1Σ)都具有较大的热力学及动力学稳定性.     

Stability and properties of oligomeric organoboron structures with six-coordinate carbon atoms at centers of hydrocarbon frameworks

Quantum chemical B3LYP/6-311G** calculations revealed that oligomers of a system containing a six-coordinate carbon atom at the center of a carbon framework composed of three 3,6-diboracyclohexa-1,4-diene rings annelated at double bonds remain energetically stable as the number of monomer units increases up to four. All oligomers have triplet ground states, the singlet states lying about 20 kcal mol^?1 higher in energy. Similar systems with B-B groups replaced by unsaturated (C=C) and saturated (HC-CH) hydrocarbon fragments lose their stability as the length of the oligomer chain increases.     

Theoretical study of singlet and triplet excitation energies in oligothiophenes

We have analyzed singlet and triplet excitation energies in oligothiophenes (up to five rings) using time-dependent density-functional theory (TD-DFT) with different exchange-correlation functionals and compared them with results from the approximate coupled-cluster singles and doubles model (CC2) and experimental data. The excitation energies have been calculated in geometries obtained by TD-DFT optimization of the lowest excited singlet state and in the ground-state geometries of the neutral and anionic systems. TD-DFT methods underestimate photoluminescence energies but the energy difference between singlet and triplet states shows trends with the chain-length similar to CC2. We find that the second triplet excited state is below the first singlet excited state for long oligomers in contrast with the previous assignment of Rentsch et al. (Phys.Chem. Chem. Phys. 1999, 1, 1707). Their photodetachment photoelectron spectroscopy measurements are better described by considering higher triplet excited states.     

Theoretical characterizations of HAsXH (X = N, P, As, Sb, and Bi) isomers in the singlet and triplet states

The lowest singlet and triplet potential energy surfaces for all group 15 HAsXH (X = N, P, As, Sb, and Bi) systems have been explored through ab initio calculations. The geometries of the various isomers were determined at the QCISD/LANL2DZdp level and confirmed to be minima by vibrational analysis. In the case of nitrogen, the global minimum is found to be a triplet H(2)NAs structure. For the phosphorus case, singlet trans-HAs==PH is found to be global minima surrounded by large activation barriers, so that it should be observable. For arsenic, theoretical investigations demonstrate that the stability of HAsAsH isomers decreases in the order singlet trans-HAs==AsH > triplet H(2)AsAs > singlet cis-HAs==AsH > triplet HAsAsH > singlet H(2)AsAs. For antimony and bismuth, the theoretical findings suggest that the stability of HAsXH (X = Sb and Bi) systems decreases in the order triplet H(2)AsX approximately singlet trans-HAs==XH > singlet cis-HAs==XH > triplet HAsXH > triplet H(2)XAs > singlet H(2)AsX > singlet H(2)XAs. Our model calculations indicate that the relativistic effect on heavier group 15 elements should play an important role in determining the geometries as well as the stability of HAsXH molecules. The results obtained are in good agreement with the available experimental data and allow a number of predictions to be made.     

Triplet energy studies of thiophene and para-phenylene based oligomers

A series of conjugated materials based on oligomers of the para-phenylene type and oligothiophenes was prepared, and their phosphorescence spectra were recorded at 77 K using a pulsed flash-lamp as a light source and gated detection. The triplet energies of the oligomers were estimated and correlated with their chemical structure. It was found that simple changes in the building block sequence in the thiophene-containing oligomers allowed for tuning the triplet energy from 1.86 to 2.35 eV (530-670 nm). Hypsochromic shifts and little variation of the triplet energy were obtained with increasing length of the pi-system for thiophene end-capped oligomers, contrary to the usual behavior of unsubstituted oligomers. The experimental results were supported with theoretical computations from density functional theory (B3LYP/6-31G*) calculations, which indicated that changes in the geometry and delocalization of the triplet excited state account for the trends in the triplet energy evolution.     

Triplets in extended nematic liquid crystals and polarons in their blends

Photoinduced absorption shows that triplets are the primary photoexcited species in a series of conjugated liquid crystals containing thiophene and fluorene groups. We find that the triplet generation rate can be varied substantially by molecular design. The introduction of extra thiophene groups into the elongated molecules changes the intersystem crossing rate by over two orders of magnitude, while modifying the singlet and triplet energies by only small amounts. This result is attributed to the high spin-orbit coupling constant of sulfur: An increase in the number of sulfur atoms increases the spin-orbit coupling between the singlet and triplet states. These results are relevant to the design of organic light emitting diodes, lasers, and other devices where triplet formation has a major impact on device performance. The molecules are shown to act as effective electron donors when blended with a perylene molecule which acts as an electron acceptor. The electron transfer rate is faster than the singlet lifetime so that the blend shows the efficient charge separation required for a photovoltaic device.     

High-resolution electronic spectra of ethylenedioxythiophene oligomers

The photophysical properties of a series of 3,4-ethylenedioxythiophene oligomers (OEDOT) with up to five repeat units are studied as function of conjugation length using absorption, fluorescence, phosphorescence, and triplet-triplet absorption spectroscopy at low temperature in a rigid matrix. At 80 K, a remarkably highly resolved vibrational fine structure can be observed in the all electronic spectra which reveals that the electronic structure of the oligomers strongly couples to two different vibrational modes (approximately 180 and approximately 50 meV). The energies of the 0-0 transitions in absorption, and fluorescence, phosphorescence, and triplet-triplet absorption all show a reciprocal dependence on the inverse number of repeat units. The triplet energies inferred from the phosphorescence spectra are accurately reproduced by quantum chemical DFT calculations using optimized geometries for the singlet ground state (S0) and first excited triplet state (T1). Using vibrational IR and Raman spectroscopy and quantum chemical DFT calculations for the normal modes in the ground state, we have been able to assign the vibrations that couple to the electronic structure to fully symmetric normal modes. The high-energy mode is associated with the well-known carbon-carbon bond stretch vibration, and the low-energy mode involves a deformation of the bond angles within the thiophene rings and a change of C-S bond lengths. Experimentally obtained Huang-Rhys parameters and theoretical normal mode deformations are used to analyze the geometry changes between T1 and S0 and to semiexperimentally predict the geometry in the S1 state for 2EDOT.     

Translational isomerism and dynamics in multi-hydroquinone derived porphyrin [2]- and [3]-catenanes

A series of porphyrins strapped with polyether chains containing two or three 1,4-dioxybenzene units has been synthesised with a view to the production of porphyrin-containing [2] and [3]catenanes, where the porphyrin is strapped between ortho-positions of 5,15-(meso)-diaryl groups, and is interlinked with the bipyridinium macrocycle cyclobis(paraquat-4,4'-biphenylene). The porphyrins were isolated as mixtures of atropisomers, where the linking strap spans across the face of the porphyrin (alpha,alpha-isomer), or 'twisted' around its side (alpha,beta-isomer). Their structures were determined by detailed 1H NMR spectroscopy. The bis-1,4-dioxybenzene-strapped derivatives were shown to undergo atropisomerisation on heating, to produce an equilibrium mixture. Catenation under high pressure conditions of the mixture, or of the individual isomers, produced only a single catenane, that of the alpha,alpha-isomer. Its structure was determined by mass spectral and dynamic NMR measurements. Rates were determined for: (i) translational motion or 'shuttling' between 1,4-dioxybenzenes; (ii) 'rotation' of the macrocycle around the 1,4-dioxybenzene axis; and (iii) 'rocking' of the 1,4-dioxybenzene within the macrocycle. The atropisomers of the strapped derivatives containing three 1,4-dioxybenzene units were also separated, and subjected to catenation. Both [2]- and [3]catenanes were isolated, and were shown to be stable to further atropisomerisation. Their solution structures were probed in detail by dynamic 1H NMR measurements. The rates for shuttling and rotation were obtained in certain cases, although the complexity of the spectra of the [3]catenanes prevented a more detailed investigation.     

Molecular structure and electronic properties of a series of oligoalkylthiophenes: A theoretical investigation

Molecular geometries and electronic properties of 3-alkylthiophenes (ATs) and their oligomers (OATs) are studied by the density functional theory (DFT). Calculations are performed on the oligomers formed by n repeating units, where n ranges from 1 to 6, using the B3LYP/6-31G** level of theory. The results obtained show that the doped oligomers have more satisfactory structural and electronic characteristics for the conducting polymers. The conjugated system in the doped oligomers has more aromaticity, with expanded and planar chains. The calculated energy gap values between the frontal orbitals and also the ionization potential values for the oligomers indicate that with increase in the oligomer chain length, the conductive band gap decreases. Furthermore, our calculations suggest that an electron-donating alkyl substituent at position 3 of the thiophene ring plays an important role in the structural and electronic properties of the polymers.     

Investigation of charge carriers in doped thiophene oligomers through theoretical modeling of their UV/Vis spectra

The nature of the charge carriers in conducting organic polymers (COPs) is a long standing problem. Polythiophene is one of the prototypes of COPs and intensively studied. Because doping leads to changes in UV/vis spectra that are characteristic of the absorbing species, UV/vis spectra of charged thiophene oligomers with up to 25 rings were calculated with time-dependent density functional theory. The credibility of the method was established by comparing the results with a variety of theoretical levels and with experiment. Effects due to counterions (Cl 3 (-)) and solvent (CH 2Cl 2) were examined. It was found that TDDFT employing hybrid functionals is accurate enough to distinguish the absorbing species. The findings offer an explanation for the experimentally observed difference in UV-spectra of medium-sized and long oligomers upon doping. As chain lengths of the oligomers increase and energy levels get closer, configuration interaction leads to additional absorption peaks in the high energy sub-band region (at around 1.5-2.5 eV). Thus, long oligomers do not behave differently from medium-sized ones upon doping, only their spectra are different. At low doping levels radical cations (polarons) are produced. At higher doping levels, dications that harbor weakly interacting polaron pairs are formed. Bipolarons are predicted only on very short chains or at high doping levels. There is no bipolaron binding energy and disproportionation of monocations into dications and neutral species is energetically unfavorable.     

Photophysical, crystallographic, and electrochemical characterization of symmetric and unsymmetric self-assembled conjugated thiopheno azomethines

Novel conjugated azomethines consisting uniquely of thiophene units are presented. The highly conjugated compounds were synthesized by simple condensation of a stable diamino thiophene (2) with its complementary thiophene aldehydes. These interesting nitrogen-containing thiophene units exhibit variable reactivity leading to controlled aldehyde addition. Because of the different amino reactivity, a one-pot synthesis of unsymmetric and symmetric conjugated azomethines with varying number of thiophene units was possible by judicious choice of solvent and careful control of reagent stoichiometry. The resulting covalent conjugated connections are both reductively and hydrolytically resistant. The thermodynamically E isomer is formed uniquely for all of the azomethines synthesized, which is confirmed by crystallographic studies. These also demonstrated that the azomethine bonds and the thiophene units are highly planar and linear. The fluorescence and phosphorescence of the thiopheno azomethines measured are similar to those of thiophene analogues currently used in functional devices, but with the advantage of low triplet formation and band-gaps as low as 1.9 eV. The time-resolved and steady-state temperature-dependent photophysics revealed the thiopheno azomethines do not populate extensively their triplet manifold by intersystem crossing. Rather, their excited-state energy is dissipated predominantly by nonradiative means of internal conversion. Quasi-reversible electrochemical radical cation formation of the thiophene units was found. These compounds further undergo electrochemically induced oxidative cross-coupling, resulting in conjugated products that also exhibit reversible radical cation formation.     

Cyclic boron clusters enclosing planar hypercoordinate cobalt, iron, and nickel

Planar cyclic boron clusters with cobalt, iron, and nickel atoms at their centerssinglet D(8h) CoB(8)(-), D(9h) FeB(9)(-), CoB(9), and NiB(9)(+)are computed to be stable minima at the BP86/TZVPP DFT level. Stochastic searches of the singlet and triplet potential energy surfaces show the planar hypercoordinate D(8h) CoB(8)(-) (1) and D(9h) FeB(9)(-) (2) singlet isomers to be the global minima. Their double aromatic character with 6 pi and 10 radial electrons is documented by detailed NICS(zz) grid and CMO-NICS(zz) analyses at PW91/TZVPP. These results encourage gas phase investigations of these two exotic anions. Although isoelectronic with D(9h) FeB(9)(-) (2), CoB(9) and NiB(9)(+) prefer nonplanar structures, triplet 3-aT for the former and singlet 4-a for the latter.     

Rationally Investigating the Influence of T1 Location on Electroluminescence Performance of Aryl Amine Modified Phosphine Oxide Materials

The correspondence between triplet location effect and host‐localized triplet–triplet annihilation and triplet–polaron quenching effects was performed on the basis of a series of naphthyldiphenylamine (DPNA)‐modified phosphine oxide hosts. The number and ratio of DPNA and diphenylphosphine oxide was adjusted to afford symmetrical and unsymmetrical molecular structures and different electronic environments. As designed, the first triplet (T₁) states were successfully localized on the specific DPNA chromophores. Owing to the meso‐ and multi‐insulating linkages, identical optical properties and comparable electrical performance was observed, including the same first singlet (S₁) and T₁ energy levels to support the similar singlet and triplet energy transfer and the close frontier molecular orbital energy levels. This established the basis of rational investigation on T₁ location effect without interference from other optoelectronic factors.