Cyclic Conjugation Energy Effects in Polycyclic π-Electron Systems

The fact that cyclic arrangements of double bonds have a dramatic effect on the behavior of conjugated organic molecules is known since the 19th century. The fact that in monocyclic conjugated systems the size of the cycle and the number of -electrons involved is decisive for their stability (aromaticity) or lack of stability (antiaromaticity) is known since the 1930s. In polycyclic -electron systems several cyclic effects are present simultaneously and their separation became possible only recently. A molecular orbital method has been elaborated, by means of which the energy effects of individual cycles in polycyclic -electron systems can be estimated. This method is briefly outlined and illustrated by pertinent examples. An exhaustive bibliography of the topic considered is given.     

Total π-Electron Energy of Phenylenes: Bounds and Approximate Expressions

Summary.  The structure dependence of the total π-electron energy E (and therefore of the thermodynamic stability) of phenylenes is examined. Sharp and narrow upper and lower bounds for E are obtained. The gross part of E (above 99%) is determined by the size of the phenylene molecule, measured by the number h of hexagons. An approximate mathematical form of the dependence of E on h is established. Received March 12, 2001. Accepted April 13, 2001     

Alkali-Metal Aminotroponiminates: Selectivities and Equilibria in Reversible Radical Coupling of Delocalized π-Electron Systems

Aminotroponiminates (ATIs) have recently been shown to belong to the growing class of redox-active ligands. The choice of the metal center allowed to switch between reversible electron transfer (M=Rh) and reductively induced dimerization (M=Na). Here, we investigate if the reductively induced dimerization of ATIs is a more general phenomenon for their alkali-metal complexes. Lithium ATI complexes are shown to undergo reductively induced dimerizations, which are equilibrium reactions and chemically reversible. The choice of the metal center (Li vs. Na), the substitution pattern at the nitrogen atoms of the ATI ligands, and the solvent critically influence the regioselectivity and diastereoselectivity of the radical-dimerization reactions. Potassium ATIs are shown to be susceptible to side reactions, more specifically a reduction accompanied by hydrogen-atom transfer. Products and intermediates of the reductively induced dimerizations were characterized by techniques including NMR and EPR spectroscopy, cyclic voltammetry, DFT calculations, single-crystal X-ray diffraction, and mass spectrometry.     

Cationic Polymerization in the Presence of π-Electron Acceptors

The cationic polymerization of various vinyl and cyclic monomers was studied in the presence of strong electron acceptors (EA) such as tetracyanoethylene, chloranil, and syn-trinitrobenzene, added in quantities commensurate with the initiator concentrations. In all cases studied, the presence of EA brings about an increase both in the overall polymerization rate and in the molecular weights of the polymers obtained without changing the kinetic scheme of the reaction. The EA added also affects the monomer reactivities in the copolymerization reaction, the insertion of less reactive monomer being favored. The effect of EA was explained in terms of complexation with the counter-ion which causes a shift of the ionic equilibrium in the system.     

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.     

Dependence of Total π-Electron Energy on the Number of Non-Bonding Molecular Orbitals

Summary. In recent work [Gutman et al. (2004) Chem Phys Lett 383: 171] a method was developed by means of which the influence of non-bonding molecular orbitals (NBMOs) on the value of total -electron energy (E) can be separated from the multitude of other molecular-structure-dependent effects. We now extend this method and establish the relation between E and the number n ₀ of NBMOs. It is shown that E (when computed within the HMO approximation, and expressed in the units of the HMO resonance integral ) is a decreasing function of n ₀, and that the dependence of E on n ₀ is almost perfectly linear.     

Electron-Deficient Benzo[de]isoquinolino[1,8-gh]quinoline Diamide π-Electron Systems

Synthetically versatile electron-deficient π-electron systems are urgently needed for organic electronics, yet their design and synthesis are challenging due to the low reactivity from large electron affinities. In this work, we report a benzo[de]isoquinolino[1,8-gh]quinoline diamide (BQQDA) π-electron system. The electron-rich condensed amide as opposed to the generally-employed imide provides a suitable electronic feature for chemical versatility to tailor the BQQDA π-electron system for various electronic applications. We demonstrate an effective synthetic method to furnish the target BQQDA parent structure, and highly selective functionalization can be performed on bay positions of the π-skeleton. In addition, thionation of BQQDA can be accomplished under mild conditions. Fine-tuning of fundamental properties and supramolecular packing motifs are achieved via chemical modifications, and the cyanated BQQDA organic semiconductor demonstrates a high air-stable electron-carrier mobility.     

Construction of Helically Stacked π-Electron Systems in Poly(quinolylene-2,3-methylene) Stabilized by Intramolecular Hydrogen Bonds

π-Stacked polymers, which consist of layered π-electron systems in a polymer, can be expected to be used in molecular electronic devices. However, the construction of a stable π-stacked structure in a polymer is considerably challenging because it requires sophisticated designs and precise synthetic methods. Herein, we present a novel π-stacked architecture based on poly(quinolylene-2,3-methylene) bearing alanine derivatives as the side chain, obtained through the living cyclo-copolymerization of an o-allenylaryl isocyanide. In the resulting polymer, the neighboring quinoline rings of the main chain form a layered structure with π–π interactions, which is stabilized by intramolecular hydrogen bonds. The vicinal quinoline units form two independent helices and the whole molecule is a twisted-tape structure. This structure is established on the basis of UV/CD spectra, theoretical calculations, and atomic-force microscopy.     

Effects of Heavy Iodine Atoms and π-Expanded Conjugation on Charge Transfer Dynamics in Carboxylic Acid BODIPY Derivatives as Triplet Photosensitizers

Borondipyrromethene (BODIPY) chromophores are composed of a functional-COOH group at meso position with or without a biphenyl ring, and their compounds with heavy iodine atoms at −2, −6 positions of the BODIPY indacene core were synthesized. The photophysical properties of the compounds were studied with steady-state absorption and fluorescence measurements. It was observed that the absorption band is significantly red-shifted, and fluorescence signals are quenched in the presence of iodine atoms. In addition to that, it was indicated that the biphenyl ring does not affect the spectral shifting in the absorption as well as fluorescence spectra. In an attempt to investigate the effect of π-expanded biphenyl moieties and heavy iodine atoms on charge transfer dynamics, femtosecond transient absorption spectroscopy measurements were carried out in the environment of the tetrahydrofuran (THF) solution. Based on the performed ultrafast pump-probe spectroscopy, BODIPY compounds with iodine atoms lead to intersystem crossing (ISC) and ISC rates were determined as 150 ps and 180 ps for iodine BODIPY compounds with and without π-expanded biphenyl moieties, respectively. According to the theoretical results, the charge transfer in the investigated compounds mostly appears to be intrinsic local excitations, corresponding to high photoluminescence efficiency. These experimental findings are useful for the design and study of the fundamental photochemistry of organic triplet photosensitizers.     

Dithieno[a,e]pentalenes: Highly Antiaromatic Yet Stable π-Electron Systems without Bulky Substituents

Dithieno[a,e]penalenes (DTPs) with various substituents were synthesized as a class of antiaromatic compounds. Annulation of thiophene rings imparts the pentalene moiety with high thermal stability even without bulky substituents, while retaining antiaromaticity. The higher magnitude of antiaromaticity in DTPs, in addition to the differences in the electronic structures of the fused aromatic rings, resulted in a narrower HOMO–LUMO gap than that of the corresponding dibenzo[a,e]pentalene analog, giving rise to red-shifted electronic absorption that reaches the near-infrared region. Moreover, systematic investigations on the solid-state packing structure revealed that DTPs prefer offset face-to-face packing motifs rather than face-centered π–π stacking. In particular, the thienyl-substituted DTP bearing hydrophilic side chains exhibited thermochromic behavior in polar solvents, which was ascribed to the formation of aggregates.     

Photobase-Driven Excited-State Intramolecular Proton Transfer (ESIPT) in a Strapped π-Electron System

We report a new design strategy for an excited-state intramolecular proton transfer (ESIPT) fluorophore that can be used in acidic media. A photobasic pyridine-centered donor-acceptor-donor-type fluorophore is combined with a basic trialkylamine “strap”. In the presence of an acid, protonation occurs predominantly at the amine moiety in the ground state. A single-crystal X-ray diffraction analysis confirmed the formation of a pre-organized intramolecular hydrogen-bonded structure between the resulting ammonium moiety and the pyridine ring. Upon excitation, the intramolecular charge-transfer transition increases the basicity of the pyridine moiety in the excited state, resulting in proton transfer from the amine to the pyridine moiety. Consequently, the fluorophore takes on a polymethine-dye character in the ESIPT state, which gives rise to significantly red-shifted emission with an increased fluorescence quantum yield.     

Cyclic Conjugation in Benzo-Annelated Perylenes. How Empty is the “Empty” Ring?

Summary. Cyclic conjugation in benzo-annelated perylenes is studied by means of the energy-effects of their six-membered rings. Classical theoretical approaches (based on Kekulé structures, Clar formulae, or conjugated circuits) predict that the central ring in benzo-annelated perylenes is empty and thus negligibly contributes to cyclic conjugation. Our calculations show that this is true only to a limited degree. In particular, rings angularly annelated relative to the central ring significantly increase the extent of its cyclic conjugation.     

Cyclic Conjugation in Benzo-Annelated Perylenes. How Empty is the “Empty” Ring?

Cyclic conjugation in benzo-annelated perylenes is studied by means of the energy-effects of their six-membered rings. Classical theoretical approaches (based on Kekulé structures, Clar formulae, or conjugated circuits) predict that the central ring in benzo-annelated perylenes is empty and thus negligibly contributes to cyclic conjugation. Our calculations show that this is true only to a limited degree. In particular, rings angularly annelated relative to the central ring significantly increase the extent of its cyclic conjugation.     

Polycyclic π-electron system with boron at its center

We disclose a new planarized triarylborane in which the tri-coordinated boron atom is embedded in a fully fused polycyclic π-conjugated skeleton. The compound shows high stability toward oxygen, water, and silica gel, despite the absence of steric protection around the B atom. Reflecting the electron-donating character of the π-skeleton and the electron-accepting character of the B atom, this compound shows broad absorption bands that cover the entire visible region and a fluorescence in the visible/near-IR region. In addition, this compound shows dramatic property changes upon formation of a tetra-coordinated borate, such as thermochromic behavior in the presence of pyridine.     

Curved Polycyclic Aromatic Molecules That Are π-Isoelectronic to Hexabenzocoronene

Reported here are two types of curved π-molecules that are π-isoelectronic to planar hexabenzocoronene (HBC) but are forced out of planarity either by an embedded seven-membered ring or by atom crowding at the fjord region. Embedding a heptagon in HBC leads to a novel saddle-shaped molecule 1, whose π-backbone is slightly less curved than the previously reported [7]circulene in terms of the average Gauss curvature, but surprisingly much more rigid than [7]circulene. Overcrowded fjord regions in novel derivatives of hexabenzoperylene (HBP) 2a,b lead to both chiral twisted and antifolded conformers. The successful synthesis of 1 and 2a,b is related to introducing alkoxyl groups to unprecedented positions of hexaphenylbenzenes. It is found that the red twisted isomer of 2b isomerizes at elevated temperature to the yellow anti-folded conformer. This finding along with the study on the thermodynamics and kinetics of the thermal isomerization has improved the early understandings on the conformation of HBP. In the crystals, 1 lacks π-π interactions between neighboring molecules, while twisted-2a exhibits both face-to-face and edge-to-face π-π interactions. Twisted-2b is found to function as a p-type semiconductor in thin film transistors, but the thin films of 1 appear insulating presumably due to lacking π-π interactions. By exploring three different types of curvatures in 1 and the two isomers of 2b, this study has revealed that the curvature of π-face plays a role in determining the frontier molecular orbital energy levels and π-π interactions and thus needs to be considered when one designs new organic semiconductors.     

Flexibility versus Rigidity: How Conformational Constraint,Steric Hindrance and Solvent Impact Binding between π-Electron Poor Tweezer-Type Hosts and π-Electron Rich Guests

Tweezer-type receptors that form π−π stacked supramolecular complexes are important components in functional polymeric materials and molecular machines. Herein, we study how varying specific structural components of tweezer-receptors impacts their binding. A library of tweezer receptors, each containing two π-electron poor receptor residues and differing by the nature of the linking unit which was either a flexible 2,2′-(ethylenedioxy)bis(ethylamine) residue or a rigid 3,3’’-diamino-m-terphenyl diamine structure, were synthesised. Each tweezer formed 1 : 1 supramolecular complexes with π-electron rich residues (1,5-dihydroxynapthalene and pyrene) as confirmed by UV/Vis and ¹H NMR spectroscopic studies. Binding constants were determined to be between 2.3×10⁻⁵ and 71 M⁻¹ in organic solvents and were one magnitude greater in aqueous solvents for water soluble systems. The nature of the linker had variable effects on the binding constants, showing the design of tweezer type supramolecular receptors with targeted K_a values is non-trivial and requires structural optimisation supported by binding constant determination studies.     

Inter-Ligand Energy Transfer Process in an Ir-Complex with Expanding π-Conjugated Ligand

The inter-ligand energy transfer (ILET) process in heteroleptic iridium complex, [Ir(dfppy)₂(bpy-Im₂)]⁺ , where dfppy =2-(2,4-difluorophenyl)pyridine and bpy-Im₂ =4,4’-bis(1,2-diphenyl-1H-benzo[d]imidazole)-2,2’,-bipyridine, was investigated using a femtosecond transient absorption (fs-TA) spectroscopic technique. The photophysical properties of [Ir(dfppy)₂(bpy-Im₂)]⁺ with significantly expanding π-conjugated ligand are compared to those of [Ir(dfppy)₂(bpy)]⁺ ( bpy =2,2’-bipyridine) and a free bpy-Im₂ ligand. The emission spectrum of [Ir(dfppy)₂(bpy-Im₂)]⁺ shows no shift upon changing the solvent polarity, whereas the free ligand bpy-Im₂ showed bathochromic fluorescence shifts with increasing solvent polarity, which is attributed to intramolecular charge transfer (ICT). The unique photophysical properties of [Ir(dfppy)₂(bpy-Im₂)]⁺ are due to the fast ILET process from ³MLCT_dfppy to ³MLCT/³LC_bpy-Im2, resulting in the phosphorescence emission originating from ³MLCT/³LC_bpy-Im2. On the other hand, the TA bands of bpy-Im₂ are observed at 540 and 480 nm, corresponding to the singlet and triplet manifolds, respectively. In contrast, the TA spectrum of [Ir(dfppy)₂(bpy-Im₂)]⁺ showes broad bands centered at 420 and 600 nm, attributed to the transitions from ³MLCT_dfppy and ³MLCT/³LC_bpy-Im2, respectively. Time-resolved spectroscopic results confirm the efficient ILET dynamics from ³MLCT_dfppy to ³MLCT/³LC_bpy-Im2 in [Ir(dfppy)₂(bpy-Im₂)]⁺ . From the relaxation times determined by singular value decomposition analysis and simple sequential kinetic model, we infer that the ILET process from ³MLCT_dfppy to ³MLCT/³LC_bpy-Im2 occurs with a time constant of ca. 4 ps. The presented results in this study show that the introduction of an expanding π-conjugated ligand can lead to the efficient ILET dynamics for improving the OLED performance.     

Mini Review of Sulfur Heterocyclization from π-Electron Deficient Quinones via Charge-Transfer Interaction

Abstract

This survey is mainly concerned with selected reactions of 2,3-dichloro-1,4-naphthoquinone (DHNQ), 3,4,5,6-tetrachloro-1,2-benzoquinone (o-CHL), 2,3,5,6-tetrachloro-1,4-benzoquinone (p-CHL), and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) as π-deficient quinones that are used or offer potential use for sulfur heterocyclic synthesis. Reaction of various donors with the π-deficient quinones are studied, especially those via charge-transfer complex formation.     

Quantum Switching of π-Electron Rotations in a Nonplanar Chiral Molecule by Using Linearly Polarized UV Laser Pulses

Nonplanar chiral aromatic molecules are candidates for use as building blocks of multidimensional switching devices because the π electrons can generate ring currents with a variety of directions. We employed (P)-2,2'-biphenol because four patterns of π-electron rotations along the two phenol rings are possible and theoretically determine how quantum switching of the π-electron rotations can be realized. We found that each rotational pattern can be driven by a coherent excitation of two electronic states under two conditions: one is the symmetry of the electronic states and the other is their relative phase. On the basis of the results of quantum dynamics simulations, we propose a quantum control method for sequential switching among the four rotational patterns that can be performed by using ultrashort overlapped pump and dump pulses with properly selected relative phases and photon polarization directions. The results serve as a theoretical basis for the design of confined ultrafast switching of ring currents of nonplanar molecules and further current-induced magnetic fluxes of more sophisticated systems.