Quinoidal Oligo(9,10‐anthryl)s with Chain‐Length‐Dependent Ground States: A Balance between Aromatic Stabilization and Steric Strain Release

Quinoidal π‐conjugated polycyclic hydrocarbons have attracted intensive research interest due to their unique optical/electronic properties and possible magnetic activity, which arises from a thermally excited triplet state. However, there is still lack of fundamental understanding on the factors that determine the electronic ground states. Herein, by using quinoidal oligo(9,10‐anthryl)s, it is demonstrated that both aromatic stabilisation and steric strain release play balanced roles in determining the ground states. Oligomers with up to four anthryl units were synthesised and their ground states were investigated by electronic absorption and electron spin resonance (ESR) spectroscopy, assisted by density functional theory (DFT) calculations. The quinoidal 9,10‐anthryl dimer 1 has a closed‐shell ground state, whereas the tri‐ ( 2 ) and tetramers ( 3 ) both have an open‐shell diradical ground state with a small singlet–triplet gap. Such a difference results from competition between two driving forces: the large steric repulsion between the anthryl/phenyl units in the closed‐shell quinoidal form that drives the molecule to a flexible open‐shell diradical structure, and aromatic stabilisation due to the gain of more aromatic sextet rings in the closed‐shell form, which drives the molecule towards a contorted quinoidal structure. The ground states of these oligomers thus depend on the overall balance between these two driving forces and show chain‐length dependence.     

Quinoidal Oligothiophenes: Towards Biradical Ground‐State Species

A family of quinoidal oligothiophenes, from the dimer to the hexamer, with fused bis(butoxymethyl)cyclopentane groups has been extensively investigated by means of electronic and vibrational spectroscopy, electrochemical measurements, and density functional calculations. The latter predict that the electronic ground state always corresponds to a singlet state and that, for the longest oligomers, this state has biradical character that increases with increasing oligomer length. The shortest oligomers display closed‐shell quinoidal structures. Calculations also predict the existence of very low energy excited triplet states that can be populated at room temperature. Aromatization of the conjugated carbon backbone is the driving force that determines the increasing biradical character of the ground state and the appearance of low‐lying triplet states. UV/Vis, Raman, IR, and electrochemical experiments support the aromatic biradical structures predicted for the ground state of the longest oligomers and reveal that population of the low‐lying triplet state accounts for the magnetic activity displayed by these compounds.     

para‐Quinodimethane‐Bridged Perylene Dimers and Pericondensed Quaterrylenes: The Effect of the Fusion Mode on the Ground States and Physical Properties

Polycyclic hydrocarbon compounds with a singlet biradical ground state show unique physical properties and promising material applications; therefore, it is important to understand the fundamental structure/biradical character/physical properties relationships. In this study, para‐quinodimethane (p‐QDM)‐bridged quinoidal perylene dimers 4 and 5 with different fusion modes and their corresponding aromatic counterparts, the pericondensed quaterrylenes 6 and 7 , were synthesized. Their ground‐state electronic structures and physical properties were studied by using various experiments assisted with DFT calculations. The proaromatic p‐QDM‐bridged perylene monoimide dimer 4 has a singlet biradical ground state with a small singlet/triplet energy gap (?2.97 kcal mol^?1), whereas the antiaromatic s‐indacene‐bridged N‐annulated perylene dimer 5 exists as a closed‐shell quinoid with an obvious intramolecular charge‐transfer character. Both of these dimers showed shorter singlet excited‐state lifetimes, larger two‐photon‐absorption cross sections, and smaller energy gaps than the corresponding aromatic quaterrylene derivatives 6 and 7 , respectively. Our studies revealed how the fusion mode and aromaticity affect the ground state and, consequently, the photophysical properties and electronic properties of a series of extended polycyclic hydrocarbon compounds.     

Structural,Electronic and Optical Properties of Multifunctional Iridium(III) and Platinum(II) Metallophosphors for Organic Light‐Emitting Diodes

An elaborated theoretical investigation on the optical and electronic properties of three fluorene‐based platinum(II) and iridium(III) cyclometalated complexes Pt‐a , Ir‐a and Ir‐b is reported. The geometric and electronic structures of the complexes in the ground state are studied with density functional theory and Hartree Fock approaches, while the lowest triplet excited states are optimized by singles configuration interaction (CIS) methods. At the time‐dependent density functional theory (TD‐DFT) level, molecular absorption and emission properties were calculated on the basis of optimized ground‐ and excited‐state geometries, respectively. The computational results show that the appearance of triphenylamino (TPA) moiety at the 9‐position of fluorene ring favors the hole‐creation and leads to red‐shifts of absorption and emission spectra. Moreover, Pt‐a and Ir‐b are nice hole‐transporting materials whereas Ir‐a has good charge‐transfer balance, which render them useful for the realization of efficient OLEDs (Organic Light‐Emitting Diodes).     

The electron‐pair origin of antiaromaticity: Spectroscopic manifestations

It is shown that the antiaromatic character of certain conjugated cyclic hydrocarbons is due to the presence of an even number of distinct electron pairs in the system (such as, but not necessarily π electrons). In these systems, the ground state is constructed from an out‐of‐phase combination of two valence bond (VB) structures, and its equilibrium geometry is necessarily distorted along the coordinate that interchanges these structures. If a new symmetry element appears during the transition between the two structures, the ground electronic state at the symmetric point transforms as one of the nontotally symmetric irreducible representations of the point group. The conjugate excited state, formed from the in‐phase combination of the same two structures, transforms as the totally symmetric representation of the group and is strongly bound. Its structure is similar to that of the ground state at the symmetric point, and the energy separation between the two states is small compared to that of conjugated cyclic hydrocarbons having an odd number of distinct electron pairs. Motion along the “Kekulé‐type” vibrational mode on the excited‐state potential surface is very similar to motion along the reaction coordinate connecting the two distorted structures on the ground‐state surface. It is characterized by a significantly higher vibrational frequency compared to frequencies of similar modes in ground‐state molecules. These qualitative predictions are supported by quantum chemical calculations on cyclobutadiene, cyclooctatetraene, and pentalene. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 71: 133–145, 1999     

Singlet excitation energies of thiophene derivatives of fluorene: TD‐DFT study

Fluorene‐thiophene (FT)‐based oligomers and polymers and their derivatives are good candidates for organic blue light‐emitting diodes. In this work, the intrinsic properties of the ground and excited states of FT monomer and its derivatives are studied. The ground‐state optimized structures and energies are obtained using molecular orbital theory and density functional theory (DFT). The ground‐state potential energy curves or surfaces of FT and its derivatives are also obtained. All derivatives are nonplanar in their electronic ground states. The character and energy of the first 20 singlet–singlet electronic transitions are investigated by applying the time‐dependent density functional theory (TD‐DFT) approximations to the correspondingly optimized ground‐state geometries. The lowest singlet state is studied with the configuration interaction (singles) approach (CIS). Excitation energies are red shifted when the FT unit or its derivatives are extended longitudinally. CIS results suggest geometry relaxation in the first singlet excited state. When available, a comparison is made with experimental results. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Spiro[4.4]nonatetraene and its Positive and Negative Radical Ions: Molectronic Structure Investigations

The electronic structure of spiro[4.4]nonatetraene 1 as well as that of its radical anion and cation were studied by different spectroscopies. The electron‐energy‐loss spectrum in the gas phase revealed the lowest triplet state at 2.98 eV and a group of three overlapping triplet states in the 4.5 – 5.0 eV range, as well as a number of valence and Rydberg singlet excited states. Electron‐impact excitation functions of pure vibrational and triplet states identified various states of the negative ion, in particular the ground state with an attachment energy of 0.8 eV, an excited state corresponding to a temporary electron attachment to the 2b₁ MO at an attachment energy of 2.7 eV, and a core excited state at 4.0 eV. Electronic‐absorption spectroscopy in cryogenic matrices revealed several states of the positive ion, in particular a richly structured first band at 1.27 eV, and the first electronic transition of the radical anion. Vibrations of the ground state of the cation were probed by IR spectroscopy in a cryogenic matrix. The results are discussed on the basis of density‐functional and CASSCF/CASPT2 quantum‐chemical calculations. In their various forms, the calculations successfully rationalized the triplet and the singlet (valence and Rydberg) excitation energies of the neutral molecule, the excitation energies of the radical cation, its IR spectrum, the vibrations excited in the first electronic absorption band, and the energies of the ground and the first excited states of the anion. The difference of the anion excitation energies in the gas and condensed phases was rationalized by a calculation of the Jahn‐Teller distortion of the anion ground state. Contrary to expectations based on a single‐configuration model for the electronic states of 1 , it is found that the gap between the first two excited states is different in the singlet and the triplet manifold. This finding can be traced to the different importance of configuration interaction in the two multiplicity manifolds.     

Multi‐zinc‐expanded graphene patches: Tetraradical versus diradical character

Three classes of multi‐Zn‐expanded graphene patches in different shapes are computationally designed through introducing a Zn chain into the corresponding middle benzenoid chain. Both density functional theory and complete active space self‐consistent field calculations predict that molecules of nnn‐quasi‐linear and nnn‐slightly bent series have the open‐shell broken‐symmetry (BS) singlet diradical ground states, whereas those of n(n+1)n species possess quintet tetraradical as their ground state and become open‐shell BS singlet tetraradicals when they are in a higher energy state. These results offer the first theoretical attempt to introduce multi‐Zn into the small graphene patches to form Zn‐expanded graphene patches, leading them to polyradical structures. This work provides an executable strategy to yield molecules which have stable polyradicaloid character and enhanced electronic properties of multi‐Zn‐expanded graphene patches. © 2012 Wiley Periodicals, Inc.     

Time‐dependent density functional calculations on the electronic spectra of the neutral nickel complex [Ni(LISQ)2] (LISQ = 3,5‐di‐tert‐butyl‐o‐diiminobenzosemiquinonate(1−)) and its monoanion and dication

The electronic spectrum of the neutral nickel complex [Ni(L^ISQ)₂] (L^ISQ = 3,5‐di‐tert‐butyl‐o‐diiminobenzosemiquinonate(1^?)) and the spectra of its anion and dication have been calculated by means of time‐dependent density functional theory. The electronic ground state of the neutral complex exhibits an open shell singlet diradical character. The mandatory multireference problem for this electronic ground state has been treated approximately by using the unrestricted and spin symmetry broken Kohn‐Sham Slater determinant as the wave function for the noninteracting reference system in the time‐dependent density functional calculations. A reasonable agreement with observed transition energies and band intensities has been achieved. This holds also for the long wavelength transitions that are shown to be of charge transfer type. The charge distributions in the electronic ground state and the corresponding low lying excited states, however, are rather similar. Thus, the known failure of standard time‐dependent density functional theory to describe improperly long range charge transfer transitions is absent in this work. The applied computational scheme might be adequate for calculating electronic spectra of transition metal complexes with noninnocent ligands. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2009     

Nitrogen Analogues of o‐Quinodimethane with Unexpected non‐Kekulé Diradical Character

Two‐electron oxidations of three 1,2‐di(bisphenylamino)‐benzenes afforded a class of nitrogen analogues of o‐quinodimethane. Their electronic structures in the ground state were studied by spectroscopic techniques including EPR and UV‐vis absorption spectroscopy. They have open‐shell singlet ground states with thermally accessible triplet states. One of them ( 1 ²⁺) has been crystalized and isolated. SQUID measurements, single crystal X‐ray diffraction and theoretical calculations show 1 ²⁺ has unexpected non‐Kekulé diradical character, sharply different from o‐quinodimethane.     

Aggregation‐Induced Emission with Long‐Lived Room‐Temperature Phosphorescence from Methylene‐Linked Organic Donor–Acceptor Structures

Aggregation‐caused quenching (ACQ), where excited‐state and/or ground‐state electronic structures are altered to exhibit an increased proclivity for non‐radiative decay for the aggregates, is largely responsible for the lack of fluorescence and phosphorescence in molecular solids in general. Here we show that ACQ could be effectively circumvented by constructing an aromatic system with a methylene‐linker, where the system exhibits typical aggregation‐induced emission (AIE) with long‐lived room‐temperature phosphorescence, since the tetrahedral structure in the solid state may significantly reduce strong intermolecular interactions contributing to ACQ.     

Time‐dependent density functional theory study on the electronic excited‐state geometric structure,infrared spectra,and hydrogen bonding of a doubly hydrogen‐bonded complex

The geometric structures and infrared (IR) spectra in the electronically excited state of a novel doubly hydrogen‐bonded complex formed by fluorenone and alcohols, which has been observed by IR spectra in experimental study, are investigated by the time‐dependent density functional theory (TDDFT) method. The geometric structures and IR spectra in both ground state and the S₁ state of this doubly hydrogen‐bonded FN‐2MeOH complex are calculated using the DFT and TDDFT methods, respectively. Two intermolecular hydrogen bonds are formed between FN and methanol molecules in the doubly hydrogen‐bonded FN‐2MeOH complex. Moreover, the formation of the second intermolecular hydrogen bond can make the first intermolecular hydrogen bond become slightly weak. Furthermore, it is confirmed that the spectral shoulder at around 1700 cm^?1 observed in the IR spectra should be assigned as the doubly hydrogen‐bonded FN‐2MeOH complex from our calculated results. The electronic excited‐state hydrogen bonding dynamics is also studied by monitoring some vibraitonal modes related to the formation of hydrogen bonds in different electronic states. As a result, both the two intermolecular hydrogen bonds are significantly strengthened in the S₁ state of the doubly hydrogen‐bonded FN‐2MeOH complex. The hydrogen bond strengthening in the electronically excited state is similar to the previous study on the singly hydrogen‐bonded FN‐MeOH complex and play important role on the photophysics of fluorenone in solutions. © 2009 Wiley Periodicals, Inc. J Comput Chem 2009     

d8和d10配合物激发态性质和金属间弱相互作用的理论研究

随着量子化学理论方法和计算机技术的发展,量子理论模型已成为一种研究分子的高能、不稳定电子态-激发态的最有效手段.通过对一系列d8和d10配合物的激发态电子结构和电子激发前后金属相互作用的理论研究进行了评述.电子吸收和发射是极其复杂的微观过程,涉及到基态与激发态的电子结构性质、金属间弱相互作用、相对论效应等量子理论的基础问题,揭示配合物发光性质的规律性对新型光学材料的探索和设计具有重要指导意义.     

Ground and Electronically Excited Singlet‐State Structures of 5‐Fluoroindole Deduced from Rotationally Resolved Electronic Spectroscopy and ab Initio Theory

The structure and electronic properties of the electronic ground state and the lowest excited singlet state (S₁) of 5‐fluoroindole (5FI) were determined by using rotationally resolved spectroscopy of the vibration‐less electronic origin of 5FI. From the parameters of the axis reorientation Hamiltonian, the absolute orientation of the transition dipole moment in the molecular frame was determined and the character of the excited state was identified as L_b.     

Mechanistic Origin of the Vibrational Coherence Accompanying the Photoreaction of Biomimetic Molecular Switches

The coherent photoisomerization of a chromophore in condensed phase is a rare process in which light energy is funneled into specific molecular vibrations during electronic relaxation from the excited to the ground state. In this work, we employed ultrafast spectroscopy and computational methods to investigate the molecular origin of the coherent motion accompanying the photoisomerization of indanylidene–pyrroline (IP) molecular switches. UV/Vis femtosecond transient absorption gave evidence for an excited‐ and ground‐state vibrational wave packet, which appears as a general feature of the IP compounds investigated. In close resemblance to the coherent photoisomerization of rhodopsin, the sudden onset of a far‐red‐detuned and rapidly blue‐shifting photoproduct signature indicated that the population arriving on the electronic ground state after nonadiabatic decay through the conical intersection (CI) is still very focused in the form of a vibrational wave packet. Semiclassical trajectories were employed to investigate the reaction mechanism. Their analysis showed that coupled double‐bond twisting and ring inversions, already populated during the excited‐state reactive motion, induced periodic changes in π‐conjugation that modulate the ground‐state absorption after the non‐adiabatic decay. This prediction further supports that the observed ground‐state oscillation results from the reactive motion, which is in line with a biomimetic, coherent photoisomerization scenario. The IP compounds thus appear as a model system to investigate the mechanism of mode‐selective photomechanical energy transduction. The presented mechanism opens new perspectives for energy transduction at the molecular level, with applications to the design of efficient molecular devices.     

Quadrupolar Cyclopenta[hi]aceanthrylene‐Based Electron Donor‐Acceptor‐Donor Conjugates: Charge Transfer versus Charge Separation

Cyclopenta[hi]aceanthrylenes (CPAs) have been functionalized at two of the peripheral positions with electronically inert trimethylsilylethynyl ( 1 ), as well as with electron‐donating 4‐ethynyl‐N,N‐dimethylaniline ( 2 ), ethynyl Zn^IIphthalocyanine ( 3 ), and ethynyl Zn^IIporphyrin ( 4 ) units. Consistent with X‐ray crystal structures of 2 and 4 , analyses of absorption and fluorescence of 2 – 4 point to strong electronic communication between the CPA and the peripheral units, affording quadrupolar electron donor‐acceptor‐donor charge‐transfer conjugates. By virtue of their quadrupolar/dipolar charge‐transfer characters in the excited state, 2 – 4 exhibit fluoro‐solvatochromism. Transient absorption spectroscopy confirmed delocalized quadrupolar ground states and formation of weakly solvent stabilized quadrupolar singlet excited states. The latter transform into strongly stabilized dipolar excited states before deactivating to the ground state in 2 and give rise to a fully charge separated state in 3 and 4 .     

Coupled cluster investigation on the low-lying electronic states of CuCN and CuNC and the ground state barrier to isomerization

The observation of several metal cyanides and isocyanides in interstellar space has raised much interest these molecules. Optimum molecular structures, harmonic vibrational frequencies, and dipole moments of the ground electronic states (X1Sigma+), triplet excited states, and open shell singlet excited states of CuCN and CuNC were determined using different levels of nonrelativistic and scalar relativistic (Douglas-Kroll) [Ann. Phys. 82, 89 (1979)] coupled cluster theory in conjunction with atomic natural orbital basis sets and correlation consistent basis sets. For the relativistic computations the specially contracted correlation consistent Douglas-Kroll (DK) basis sets were used. Moreover, barriers to isomerization from CuCN to CuNC were computed. The predicted structures of the X1Sigma+ state for CuCN are re(Cu-C)=1.826 A and re(C-N)=1.167 A, at the most sophisticated level of theory, the scalar relativistic DK-CCSD(T)/cc-pVQZ(DK) method. These results are in excellent agreement with the experimentally determined Cu-C bond length of 1.829 A and C-N bond distance of 1.162 A. At the same level of theory, the zero-point corrected barrier to isomerization from CuCN to CuNC is estimated to be 14.7 kcal mol(-1), and the cyanide is more stable than the isocyanide by 11.5 kcal mol(-1). For both CuCN and CuNC the 3Sigma+ state is the lowest lying excited electronic state. At the DK-CCSD/cc-pVQZ(DK) level of theory, the energetic ordering of excited states of CuCN and CuNC is X1Sigma+相似文献   

Rich Athermal Ground‐State Chemistry Triggered by Dynamics through a Conical Intersection

A fundamental tenet of statistical rate theories (such as transition state theory and RRKM) is the rapidity of vibrational relaxation. Excited‐state reactions happen quite quickly (sub‐picosecond) and thus can exhibit nonstatistical behavior. However, it is often thought that any diversity of photoproducts results from different conical intersections connecting the excited and ground electronic states. It is also conceivable that the large energy of the photon, which is converted to vibrational energy after electronic transitions could lead to athermal hot ground state reactions and that these might be responsible for the diversity of photoproducts. Here we show that this is the case for sulfines, where a single conical intersection is implicated in the electronic transition but the excited state reaction leads to nine different products within less than a picosecond.     

Benzenoid Polycyclic Hydrocarbons with an Open–Shell Biradical Ground State

This Focus Review describes the recent progress, from both theoretical and experimental perspectives, on four types of benzenoid polycyclic hydrocarbons with an open–shell biradical ground state: 1) acenes, 2) periacenes and anthenes, 3) zethrenes, and 4) extended p‐quinodimethane derivatives. These interesting molecules have provided excellent platforms to investigate the electronic structures of nanographenes and represent promising candidates for the next generation of molecule‐based materials in the field of electronics, spintronics, and non‐linear optics. The focus of this article will be put on the structural significance, the physical properties relevant to the open–shell electronic configurations, and potential applications.     

Ab initio calculation of the electronic structures of the 3Phi ground and 5Phi excited states of CoH

The electronic structures and the spectroscopic constants of the electronic ground 3Phi and low-lying 5Phi electronic excited states of the CoH molecule were studied by multireference single and double excitation configuration interaction (MR-SDCI)+Davidson's correction (Q) calculations and size-consistent multireference coupled pair approximation (MRCPA) calculations. Calculations were performed under Cinfinityv symmetry using Slater-type basis functions. The electronic ground state was confirmed to be the 3Phi state. It was found that at least four reference configurations were needed to describe the ground 3Phi state correctly at the MR-SDCI+Q level, while the 5Phi state can be described well by one reference configuration, namely, the Hartree-Fock configuration. Larger dynamical electron correlation for the low-spin 3Phi state than that for the high-spin 5Phi state is discussed. Spectroscopic constants, i.e., equilibrium bond lengths (re), harmonic frequency (omegae), and excitation energy, obtained by the MR-SDCI+Q method showed good correspondence with experimental values. MRCPA calculations gave a slightly shorter value for re than experimental values, but improved omegae and the excitation energy bringing them very close to experimental values.