Carbon‐bridged Oligo(phenylenevinylene)s as Light‐harvesting Antenna for Porphyrins

The efficacy of carbon‐bridged oligo(phenylenevinylenes)s (COPVs) as light‐harvesting antenna for porphyrins is demonstrated using a series of 5,15‐di‐COPVn‐substituted free‐base and zinc porphyrins, COPVn‐MP‐COPVn (n=1–3, M=H₂, Zn). These molecules were synthesized by Suzuki–Miyaura cross‐coupling reactions of COPVn‐Bpin and Br‐H₂P‐Br . The absorption spectra of these compounds in solution show a significant expansion of the Soret band region together with a bathochromic shift of the Q band, suggesting a significant interaction between these chromophores in the ground state. The photoluminescence quantum yield of the porphyrin‐COPV conjugates is enhanced up to four times relative to the parent porphyrins. Theoretical calculations also indicated interactions between these chromophores in the HOMO, which suggests that the light‐harvesting ability stems from the expansion of the π‐electron‐conjugation system.     

Bis(merocyanine) Hetero-Folda-Dimers: Evaluation of Exciton Coupling between Different Types of π-Stacked Chromphores

Exciton coupling between different types of chromophores has been rarely investigated. Herein, a systematic study on the exciton coupling between merocyanine chromophores of different conjugation length with varying excited state energies is presented. In this work well-defined hetero-dimer stacks were obtained upon folding of bis(merocyanine) dyes in nonpolar solvents. They show distinctly different absorption properties in comparison with the spectra of the single chromophores, revealing a significant coupling between the different chromophores. The simulated absorption spectra obtained from time-dependent density functional theory (TD-DFT) calculations are in good agreement with the experimental spectra. Our theoretical analysis based on an extension of Kasha's exciton theory discloses strong coupling between the dyes’ transition dipole moments despite of an excited-state energy difference of 0.60 eV between the chromophores.     

Entropically Driven Self‐Assembly of Bolaamphiphilic Perylene Dyes in Water

The specific hydrophobic effect involved in the self‐assembly of a bolaamphiphilic perylene bisimide (PBI) dye bearing oligoethylene glycol (OEG) chains has been identified. In pure water, the self‐assembly is entropically driven and enthalpically disfavored, as explored by optical spectroscopy and isothermal titration calorimetry studies. Besides strong π–π interactions between the PBI units that are primarily of enthalpic nature, the major contribution to the self‐assembly is the gain of entropy by release of confined water molecules from the hydration shell of the hydrophilic OEG moieties. Both contributions favor self‐assembly, but their countervailing thermodynamic parameters are reflected in an uncommon temperature dependence, which can be inverted upon gradual addition of an organic cosolvent that makes the π–π interaction increasingly dominant.     

Covalent Organic Framework for Improving Near‐Infrared Light Induced Fluorescence Imaging through Two‐Photon Induction

Fluorescent materials exhibiting two‐photon induction (TPI) are used for nonlinear optics, bioimaging, and phototherapy. Polymerizations of molecular chromophores to form π‐conjugated structures were hindered by the lack of long‐range ordering in the structure and strong π–π stacking between the chromophores. Reported here is the rational design of a benzothiadiazole‐based covalent organic framework (COF) for promoting TPI and obtaining efficient two‐photon induced fluorescence emissions. Characterization and spectroscopic data revealed that the enhancement in TPI performance is attributed to the donor‐π‐acceptor‐π‐donor configuration and regular intervals of the chromophores, the large π‐conjugation domain, and the long‐range order of COF crystals. The crystalline structure of TPI‐COF attenuates the π–π stacking interactions between the layers, and overcomes aggregation‐caused emission quenching of the chromophores for improving near‐infrared two‐photon induced fluorescence imaging.     

Self‐Assembly of a Chiral Lipid Gelator Controlled by Solvent and Speed of Gelation

Glutamine derivative 1 with two‐photon absorbing units has been synthesized and was found to show gelation ability in some solvents. Its self‐assembly in the gel phase could be controlled by the solvent and speed of gelation. For example, in DMSO the organogelator self‐assembled into H‐aggregates with weak exciton coupling between the aromatic moieties. On the other hand, in DMSO/diphenyl ether (1:9, v/v) the molecules formed 1D aggregates, but with strong exciton coupling due to the small distance between the chromophores. Moreover, the formation of these two kinds of aggregates could be adjusted by the ratio of DMSO to diphenyl ether. In DMSO/toluene, DMSO/butanol, DMSO/butyl acetate, and DMSO/acetic acid systems similar results were observed. Therefore, conversion of the packing model occurs irrespective of the nature of the solvent. Notably, a unique sign inversion in the CD spectra could be realized by controlling the speed of gelation in the DMSO/diphenyl ether (1:9, v/v) system. It was found that a low speed of gelation induces the gelator to adopt a packing model with strong π–π interactions between the aromatic units. Moreover, the gels, when excited at 800 nm, emit strong green fluorescence and the quantum chemical calculations suggest that intramolecular charge transfer leads to two‐photon absorption of the gelator molecule.     

Localization/Delocalization of Charges in Bay‐Linked Perylene Bisimides

The copper‐mediated Ullmann coupling of 1,7‐dibromoperylene bisimides afforded structurally perfect singly‐linked perylene bisimide (PBI) arrays, whilst the homo‐coupling of 1,12‐dibromoperylene bisimides gave doubly‐linked and triply‐linked diperylene bisimides. The interactions of three bay‐linked diperylene bisimides that differed in their linkage (singly, doubly, and triply) were investigated in their neutral and reduced forms (mono‐anion to tetra‐anion). UV/Vis absorption and fluorescence spectroscopy revealed different degrees of interaction, which was explained by exciton coupling and conjugation effects. The electrochemical properties and spectroelectrochemistry also showed quite‐different degrees of PBI interactions in the reduced mixed‐valence species, which was apparent by the observation of CT bands. The interpretation of the experimental findings was supported by spin‐restricted and ‐unrestricted DFT and time‐dependent TD‐DFT calculations with the long‐range‐corrected CAM‐B3LYP functional. Accordingly, the degree of interaction in both the neutral and reduced forms of the bay‐linked PBIs was qualitatively in the order doubly linked<singly linked?triply linked, owing to the different degrees of twisting and flexibility between the two PBIs moieties. Only triply linked diPBI showed completely delocalized wavefunctions over the entire π‐system.     

Benzidine/Quinoidal‐Benzidine‐Linked,Superbenzene‐Based π‐Conjugated Chiral Macrocycles and Cyclophanes

Synthesis of fully conjugated cyclophanes containing large‐size polycyclic aromatics is challenging. Now, three benzidine‐linked, hexa‐peri‐hexabenzocoronene (superbenzene)‐based ortho‐, para‐, and meta‐cyclophanes are synthesized through intermolecular Yamamoto coupling reaction of structurally pre‐organized precursors. Subsequent oxidative dehydrogenation gave the corresponding quinoidal benzidine‐linked cyclophanes. Their geometries were confirmed by X‐ray crystallographic analysis and their electronic properties were investigated by electronic absorption, cyclic voltammetry, and DFT calculations. The quinoidal benzidine‐linked cyclophanes show thermally populated paramagnetic activity with a relatively large singlet‐triplet energy gap. Two enantiomers for the ortho‐cyclophanes ( 1‐NH and 1‐N ) were isolated and their chiral figure‐of‐eight macrocyclic structures were identified. The cage‐like cyclophanes 2‐NH and 3‐NH with concave surface can selectively encapsulate fullerene C₇₀.     

Photophysical properties of σ–π‐conjugated alternating oligothienylene–oligosilylene polymers

UV‐visible absorption and fluorescence properties of three series of σ–π‐conjugated polymers (copolymers of alternative oligothienylene and oligosilylene units) have been studied in dioxane solution. The energies of the absorption maximum, fluorescence maximum, and the 0–0 transition are found to be linearly dependent on the reciprocal of the number of thiophene rings in the repeating unit of the polymer chain, but almost independent of the silicon atom number. The σ–π‐conjugation in the polymers results in red shift in the absorption and fluorescence maxima, higher fluorescence quantum yields, and longer fluorescence lifetimes of the polymers, with respect to their corresponding analogous α‐oligothiophenes. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1873–1880, 1999     

Electron Delocalization in a Rigid Cofacial Naphthalene‐1,8:4,5‐bis(dicarboximide) Dimer

Investigating through‐space electronic communication between discrete cofacially oriented aromatic π‐systems is fundamental to understanding assemblies as diverse as double‐stranded DNA, organic photovoltaics and thin‐film transistors. A detailed understanding of the electronic interactions involved rests on making the appropriate molecular compounds with rigid covalent scaffolds and π–π distances in the range of ca. 3.5 Å. Reported herein is an enantiomeric pair of doubly‐bridged naphthalene‐1,8:4,5‐bis(dicarboximide) (NDI) cyclophanes and the characterization of four of their electronic states, namely 1) the ground state, 2) the exciton coupled singlet excited state, 3) the radical anion with strong through‐space interactions between the redox‐active NDI molecules, and 4) the diamagnetic diradical dianion using UV/Vis/NIR, EPR and ENDOR spectroscopies in addition to X‐ray crystallography. Despite the unfavorable Coulombic repulsion, the singlet diradical dianion dimer of NDI shows a more pronounced intramolecular π–π stacking interaction when compared with its neutral analog.     

A Rotaxane‐like Cage‐in‐Ring Structural Motif for a Metallosupramolecular Pd6L12 Aggregate

A BODIPY‐based bis(3‐pyridyl) ligand undergoes self‐assembly upon coordination to tetravalent palladium(II) cations to form a Pd₆L₁₂ metallosupramolecular assembly with an unprecedented structural motif that resembles a rotaxane‐like cage‐in‐ring arrangement. In this assembly the ligand adopts two different conformations—a C‐shaped one to form a Pd₂L₄ cage which is located in the center of a Pd₄L₈ ring consisting of ligands in a W‐shaped conformation. This assembly is not mechanically interlocked in the sense of catenation but it is stabilized only by attractive π‐stacking between the peripheral BODIPY chromophores and the ligands’ skeleton as well as attractive van der Waals interactions between the long alkoxy chains. As a result, the co‐arrangement of the two components leads to a very efficient space filling. The overall structure can be described as a rotaxane‐like assembly with a metallosupramolecular cage forming the axle in a metallosupramolecular ring. This unique structural motif could be characterized via ESI mass spectrometry, NMR spectroscopy, and X‐ray crystallography.     

Theoretical study on the second‐order nonlinear optical properties of nonconjugated D‐π‐A chromophores

Density functional theory calculations have been carried out on nonconjugated D‐π‐A chromophores to investigate the different electron donors and conjugated bridges effects on the molecular nonlinear optical response. The results show that the large second‐order polarizability values can be achieved through careful combination of available electron donors, conjugated bridges for our studied nonconjugated D‐π‐A chromophores. The calculations also provide a clear explanation for the second‐order polarizability changes from the standpoint of transition energies, oscillator strengths, electron density difference, and bond length alternation. Solvent effect has great influence on the second‐order polarizability and electronic absorption spectrum. It is hoped that the results presented in this article will give some hints to the interrelated studies. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Intramolecular π–π Interactions in Flexibly Linked Partially Fluorinated Bisarenes in the Gas Phase

Three compounds with phenyl and pentafluorophenyl rings bridged by (CH₂)₃ and (CH₂)₂SiMe₂ units were synthesized by hydrosilylation and C−C coupling reactions. Their solid‐state structures are dominated by intermolecular π stacking interactions, primarily leading to dimeric or chain‐type aggregates. Analysis of free molecules in the gas phase by electron diffraction revealed the most abundant conformer to be significantly stabilized by intramolecular π–π interactions. For the silicon compounds, structures characterized by σ–π interactions between methyl and pentafluorophenyl groups are second lowest in energy and cannot be excluded completely by the gas electron diffraction experiments. C₆H₅(CH₂)₃C₆F₅, in contrast, is present as a single conformer. The gas‐phase structures served as a reference for the evaluation of a series of (dispersion‐corrected) quantum‐chemical calculations.     

Novel π‐Electron Extension System via Chromophores Self‐Polymerization to Enhance the NLO Efficiency

A new strategy for the self‐polymerization of chromophores is investigated to develop a 2,7‐carbazole‐based nonlinear optical (NLO) conjugated polymer with an increasing conjugation length of chromophores. Elongation of the conjugation‐path length in chromophores has established engineering guidelines to enhance optical nonlinearity. Compared with the traditional synthesis of an NLO polymer, the chromophores should be well‐designed at a limited conjugation spacer, and then incorporated into a polymer matrix. In this research, the π‐conjugation spacer of chromophores extended perpendicularly to the dipole of chromophores during the polymerization process. Furthermore, this study marks the first research of integrating the π‐electrons of chromophores and conjugated polymers. These conjugated backbones promote a bulk‐polarization response, leading to large NLO coefficients.

     

Arsole‐Containing π‐Conjugated Polymer by the Post‐Element‐Transformation Technique

A synthetic method to obtain an arsole‐containing π‐conjugated polymer by the post‐transformation of the organotitanium polymer titanacyclopentadiene‐2,5‐diyl unit with an arsenic‐containing building block is described. The UV/Vis absorption maximum and onset of the polymer were observed at 517 nm and 612 nm, respectively. The polymer exhibits orange photoluminescence with an emission maximum (E_max) of 600 nm and the quantum yield (Φ) of 0.05. The polymer proved to exhibit a quasi‐reversible redox behavior in its cyclic voltammetric (CV) analysis. The energy levels of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) were estimated to be ?5.43 and ?3.24 eV, respectively, from the onsets for oxidation and reduction signals in the CV analysis. Further chemical modification of the arsole unit in the π‐conjugated polymer by complexation of gold(I) chloride occurred smoothly resulting in the bathochromic shift of the UV/Vis absorption and lowering of the LUMO energy level.     

Facile Fabrication of Solid‐state Electrochemiluminescence Sensor via Non‐covalent π‐π Stacking and Covalent Bonding on Graphite Electrode

Herein, a facile and efficient method was developed for fabrication of solid‐state electrochemiluminescence (ECL) sensor via non‐covalent π‐π stacking and covalent bonding on the graphite electrode (GE) surface. The electrode was firstly modified with 1‐aminopyrene via π‐π stacking between GE surface and the pyrene moiety. Thereafter a stable and efficient solid‐state ECL sensor was fabricated by covalent immobilization of ruthenium(II) onto the GE surface via amidation reaction between the 1‐aminopyrene and bis(2,2′‐bipyridyl)(4‐methyl‐4′‐carboxypropyl‐2,2′‐bipyridyl) ruthenium(II) bishexafluorophosphate. The sensor has been investigated using tripropylamine and tetracycline as representative analytes, and low detection limits of 0.7 nM and 3.5 nM (S/N=3) were reached, respectively.     

Ultrafast Exciton Self‐Trapping and Delocalization in Cycloparaphenylenes: The Role of Excited‐State Symmetry in Electron‐Vibrational Coupling

Upon photon absorption, π‐conjugated organics are apt to undergo ultrafast structural reorganization via electron‐vibrational coupling during non‐adiabatic transitions. Ultrafast nuclear motions modulate local planarity and quinoid/benzenoid characters within conjugated backbones, which control primary events in the excited states, such as localization, energy transfer, and so on. Femtosecond broadband fluorescence upconversion measurements were conducted to investigate exciton self‐trapping and delocalization in cycloparaphenylenes as ultrafast structural reorganizations are achieved via excited‐state symmetry‐dependent electron‐vibrational coupling. By accessing two high‐lying excited states, one‐photon and two‐photon allowed states, a clear discrepancy in the initial time‐resolved fluorescence spectra and the temporal dynamics/spectral evolution of fluorescence spectra were monitored. Combined with quantum chemical calculations, a novel insight into the effect of the excited‐state symmetry on ultrafast structural reorganization and exciton self‐trapping in the emerging class of π‐conjugated materials is provided.     

Synthesis of Thiophene‐Based π‐Conjugated Oligomers via Ligand‐Enabled Pd‐Catalyzed Suzuki–Miyaura Coupling of Haloterthienyls

A ligand‐enabled Pd‐catalyzed Suzuki–Miyaura coupling of haloterthienyls for the synthesis of various thiophene‐based π‐conjugated oligomers including quinquethiophenes is demonstrated. An indolyl phosphine ligand plays an important role in this transformation. Thiopheneboronic acids were well applied, which might open up a window for the application of thiopheneboronic acids in the synthesis of thiophene‐based π‐conjugated oligomers in materials chemistry.     

Theoretical insight into the photodeactivation pathway of the tetradentate Pt(II) complex: The π‐conjugation effect

In this work, density functional theory and time‐dependent density functional theory were used to investigate the effects of π‐conjugation of the ligand on the photophysical properties, radiative/nonradiative processes and phosphorescence quantum efficiency of tetradentate cyclometalated Pt (II) complex with carbazolyl‐pyridine ligands PtNON . By simulating the absorption spectra and emission wavelengths, increasing the π‐conjugation of the ligand could cause the absorption and emission wavelengths to red‐shift. The results of the computation of key parameters in the radiative decay process, such as singlet‐triplet splitting energy, transition dipole moment and spin‐coupled matrix element between the lowest triplet and singlet excited states, showed that the expansion of π‐conjugation on the carbazole ligand of PtNON resulted in reduction of these parameters, thereby reducing the radiation rate constant. The analyses of the PtNON nonradiative pathway also found that the high activation energy of PtNON made it one of the reasons for the high phosphorescence quantum yield. At the same time, enhancing the molecular orbital delocalization of the ligand further enlarged the energy barrier of the nonradiative pathway, and was conducive to the improvement of phosphorescence quantum yield.     

β‐Iminoenamine‐BF2 Complexes: Aggregation‐Induced Emission and Pronounced Effects of Aliphatic Rings on Radiationless Deactivation

The synthesis, photophysical, and electrochemical attributes of a novel class of boron difluorides containing an aromatic‐fused alicyclic/hetero‐alicyclic ring built on a β‐iminoenamine chromophoric backbone are reported. The compounds displayed large Stokes shifts (86–121 nm), and were emissive in the solid state. The quantum yields obtained in solution at room temperature were unusually lower by an order of magnitude compared to those in the solid state. Some of the tested compounds displayed aggregation‐induced emission (AIE). Single crystal XRD analyses revealed a lack of interplanar π–π interactions, which are presumed to be absent owing to non‐planarity of the alicyclic component in the molecule. For most of the studied compounds, time‐dependent DFT (TD‐DFT) calculations invariably reveal intramolecular charge transfer (π–π*) characteristics with the frontier orbitals concentrated on the boron–nitrogen heterocycle. The participation of boron and fluorine atoms was found to be negligible.     

The structure of cinnamic acid and cinnamoyl azides,a unique localized π system: The electronic spectra and DFT‐treatment

The electronic absorption spectra of cinnamic acid and some cinnamoyl azides have been recorded in absolute methanol and investigated to explore the structure of the titled compounds. Cinnamic acid and its derivatives have a double bond, ? C?C? , between the aromatic ring and the carboxyl group which disturbs the π electron system of the molecule and inhibits electron delocalization as compared with styrene or benzoic acid. The azide group is neither a strong electron donor nor a strong electron acceptor but it increases conjugation in the molecule. The observed spectra confirm that each of the cinnamic acid and cinnamoyl azide molecules is one of a kind of unique disturbed π‐system and not of different independent π systems, each on a fragment of the molecule as predicted by the quantum theory of atom in molecule calculations. The spectra of cinnamic acid and its derivatives are not the additive spectra of the different fragments of the molecule. The spectra are characterized by few number, low intensity, and high‐energy electronic transitions (absorption bands) in the UV‐vis region. Molecular orbital calculations confirmed the spectral observations. The optimized geometry of the ground state of the studied compounds is calculated using the DFT/B3LYP/6‐31G** level of theory and an explicit molecular orbital analysis is carried out. Excited states are calculated using the TD/DFT procedure as implemented by the Gamess 2009 package of programs. The correspondence between calculated and the observed transition energies is adequate. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012