Theoretical Studies on the Electronic Structures and Optical Properties of Tri-aryl End-capped Terthiophene Derivatives

The relationships between electronic structures and spectra properties are investigated by DFT/TDDFT for terthiophene derivatives, BMA‐3T (tri‐aryl amine end‐capped terthiophene), BBA‐3T (tri‐aryl amine and tri‐aryl boron end‐capped terthiophene) and BPB‐3T (tri‐aryl boron end‐capped terthiophene). The calculated results show that BMA‐3T, BBA‐3T and BPB‐3T have higher HOMO energy level and lower ionization potentials (IPs) than 3T. BMA‐3T has good hole injection ability and hole‐transport property as reported in experiment. The designed molecule of BBA‐3T and BPB‐3T own lower LUMO level and higher electron affinities (EAs) than BMA‐3T, which facilitate electron injection and improve their electron‐transport properties. Surprisingly, BPB‐3T has preferable charge equilibrium property since its hole reorganization energy (λ_h) is close to electron reorganization energy (λ_e). The ΔE (HOMO−LUMO) and E_g of these three derivatives are narrower compared to 3T, and the absorption as well as emission spectrum exhibited red‐shifts.     

Energy Transfer from Locally Excited π* to Charge Transfer Ground States in a Silylene–π Hetero‐Junction Polymer

Summary: The silylene–π conjugating polymer, poly(di‐n‐hexylsilylenephenylene‐ethynylenephenylene) ( 1 ) adopted a fairly flexible coil‐like conformation due to the bent structure of silylene moiety and showed a unique photoexcited energy transfer behavior. The UV‐vis absorption and steady‐state/time‐resolved photoluminescence studies revealed the occurrence of an intramolecular photoexcited energy transfer (IET) between locally excited π* to charge transfer ground states as well as an intramolecular charge transfer (ICT).

The silylene–π conjugating polymer, poly(di‐n‐hexylsilylenephenylene‐ethynylenephenylene) showing a unique photoexcited energy transfer behavior.     

Ambipolar Field‐Effect Transistor (FET) and Redox Characteristics of a π‐Conjugated Thiophene/1,3,4‐Thiadiazole CT‐Type Copolymer

Summary: A π‐conjugated charge transfer‐type copolymer consisting of an electron‐donating thiophene and an electron‐accepting 1,3,4‐thiadiazole, P(ThdzTh), underwent facile electrochemical p‐ and n‐doping, as revealed by cyclic voltammetry. The copolymer gave a new ambipolar field‐effect transistor (FET), which showed typical I_DS (source–drain current)–V_DS (source–drain voltage) curves in both a p‐type working mode and an n‐type working mode. In the n‐type working mode, the polymer showed a carrier mobility of about 5 × 10⁻³ cm² · V⁻¹ · s⁻¹ and an on/off ratio of about 3 × 10⁴.

n‐Channel field‐effect transistor characteristics of P(ThdzTh).     

Preparation and Acid‐Responsive Photophysical Properties of T‐Shaped π‐Conjugated Molecules Containing a Benzimidazole Junction

T‐shaped π‐conjugated molecules with an N‐methyl‐benzimidazole junction have been synthesized and their acid‐responsive photophysical properties owing to the change in the π‐conjugation system are discussed. T‐shaped π‐conjugated molecules consist of two orthogonal π‐conjugated systems including a phenyl thiophene extended from the 2‐position and alkyl phenylenes connected through various π‐spacers from the 4,7‐positions of the N‐methyl‐benzimidazole junction. The π‐spacers, such as thiophene, ethyne, and ethane, have an effect on the acid response of photophysical properties in terms of changes in conformation, excited‐state energy and charge‐transfer (CT) characteristics. In particular, the π‐conjugated molecule with ethynyl spacers exhibited a marked redshift in the fluorescence spectrum with a large Stokes shift upon the addition of acid, whereas the other molecules showed substantial quenching. The redshift in emission was studied in detail by temperature‐dependent fluorescence measurements, which indicated the transition to a CT state over the finite activation energy at the excited state. The change in the frontier molecular orbitals upon acid addition was further discussed by means of DFT calculations.     

Efficient Intramolecular Charge Transfer in Oligoyne‐Linked Donor–π–Acceptor Molecules

Studies are reported on a series of triphenylamine–(C?C)_n–2,5‐diphenyl‐1,3,4‐oxadiazole dyad molecules (n=1–4, 1 , 2 , 3 and 4 , respectively) and the related triphenylamine‐C₆H₄–(C?C)₃–oxadiazole dyad 5 . The oligoyne‐linked D–π–A (D=electron donor, A=electron acceptor) dyad systems have been synthesised by palladium‐catalysed cross‐coupling of terminal alkynyl and butadiynyl synthons with the corresponding bromoalkynyl moieties. Cyclic voltammetric studies reveal a reduction in the HOMO–LUMO gap in the series of compounds 1 – 4 as the oligoyne chain length increases, which is consistent with extended conjugation through the elongated bridges. Photophysical studies provide new insights into conjugative effects in oligoyne molecular wires. In non‐polar solvents the emission from these dyad systems has two different origins: a locally excited (LE) state, which is responsible for a π*→π fluorescence, and an intramolecular charge transfer (ICT) state, which produces charge‐transfer emission. In polar solvents the LE state emission vanishes and only ICT emission is observed. This emission displays strong solvatochromism and analysis according to the Lippert–Mataga–Oshika formalism shows significant ICT for all the luminescent compounds with high efficiency even for the longer more conjugated systems. The excited‐state properties of the dyads in non‐polar solvents vary with the extent of conjugation. For more conjugated systems a fast non‐radiative route dominates the excited‐state decay and follows the Engelman–Jortner energy gap law. The data suggest that the non‐radiative decay is driven by the weak coupling limit.     

表面活性剂对共轭聚合物荧光性质及电荷转移的影响

探讨了表面活性剂存在下, 水溶性阴离子共轭聚合物聚[5-甲氧基-2-(3-磺酰化丙氧基)-1,4-苯撑乙烯](简写为MPS-PPV)的微环境变化对荧光性质及电荷转移的影响. 结果表明, 阳离子表面活性剂及非离子表面活性剂使MPS-PPV荧光增强, 阴离子表面活性剂使其荧光先增强后减弱; 在MPS-PPV/表面活性剂体系中加入电子接受体Pd^2＋, 发现非离子表面活性剂体系的荧光猝灭效率提高, 阴离子及阳离子表面活性剂体系荧光猝灭效率下降. 此研究对研制基于阴离子共聚物的新型生物化学传感器具有一定的指导意义.     

Spiral Intramolecular Charge Transfer and Large First Hyperpolarizability in Möbius Cyclacenes: New Insight into the Localized π Electrons

Much effort has been devoted to investigating the unusual properties of the π electrons in Möbius cyclacenes, which are localized in a special region. However, the localized π electrons are a disadvantage for applications in optoelectronics, because intramolecular charge transfer is limited. This raises the question of how the intramolecular charge transfer of a Möbius cyclacene with clearly localized π electrons can be enhanced. To this end, [8]Möbius cyclacene ([8]MC) is used as a conjugated bridge in a donor–π‐conjugated bridge–acceptor (D–π–A) system, and NH₂‐6‐[8]MC‐10‐NO₂ exhibits a fascinating spiral charge‐transfer transition character that results in a significant difference in dipole moments Δμ between the ground state and the crucial excited state. The Δμ value of 6.832 D for NH₂‐6‐[8]MC‐10‐NO₂ is clearly larger than that of 0.209 D for [8]MC. Correspondingly, the first hyperpolarizability of NH₂‐6‐[8]MC‐10‐NO₂ of 12 467 a.u. is dramatically larger than that of 261 a.u. for [8]MC. Thus, constructing a D–π–A framework is an effective strategy to induce greater spiral intramolecular charge transfer in MC although the π electrons are localized in a special region. This new insight into the properties of π electrons in Möbius cyclacenes may provide valuable information for their applications in optoelectronics.     

1-萘甲酰苯胺和2-萘甲酰苯胺的分子内电荷转移研究

A series of 1-naphthanilides (1) and 2-naphthanilides (2) with varied substituents at the para- or meta-position of anilino phenyl ring were prepared and their absorption and fluorescence spectra in a nonpolar solvent cyclohexane were investigated. An abnormal long wavelength emission assigned to the charge transfer (CT) state was found for all of the prepared naphthanilides in cyclohexane. A linear free energy correlation between the CT emission energies and the Hammett constants of the substituent was found within series 1 and 2. The value of the linear slope with 1 (0.42 eV) was higher than that with 2 (0.32 eV) being close to that of the substituted benzanilides 3 (0.31 eV) The higher slope value suggested higher charge separation extent in the CT state of 1 than that of 2. It was found that the corresponding linear slope of anilino-substituted benzanilides remained unchanged when para-, meta-, ortho-, or ortho, ortho-methyls were introduced into the anilino moiety, which ruled out the possible contribution of the difference in the steric effect and the electron accepting ability of the naphthoyl acceptor in 1 and 2. Compared with the early reported N-substituted-benzoyl-aminonaphthalene derivatives 4 and 5, it was considered that 1-naphthoyl enhanced the charge transfer in 1 and the proximity of its ^1La and ^1Lb states was suggested to be responsible. It was shown that 1- and/or 2-substituted naphthalene cores acting as either electron acceptor (naphthoyl) or electron donor (aminonaphthalene) were different in not only electron accepting (donating) ability but also shaping the charge transfer pathway.     

A Convenient Method for the Synthesis of α‐Ethynylphospholes and Modulation of Their π‐Conjugated Systems

Mind the (narrow) gap : α‐Ethynylphospholes generated in situ from the corresponding silyl‐capped precursors were converted into a series of α‐(arylethynyl) phospholes bearing functional substituents as well as an α,α′‐linked terphosphole (see scheme). The terphosphole has a narrow HOMO–LUMO gap owing to efficient π conjugation over the three phosphole rings.

     

Synthesis,Properties, and n‐Type Transistor Characteristics of π‐Conjugated Compounds Having a Carbonyl‐Bridged Thiazole‐Fused Polycyclic System

A series of electron‐deficient π‐conjugated systems with 4,9‐dihydro‐s‐indaceno[2,1‐d:6,5‐d′]dithiazole‐4,9‐dione‐based structures and fluorinated acyl groups as the terminal units have been designed and synthesized for application as organic field‐effect transistor (OFET) materials. The thermal, photophysical, and electrochemical properties and OFET performance of the synthesized compounds were investigated. OFET evaluation revealed that all compounds exhibited typical electron‐transporting characteristics, and electron mobilities up to 0.26 cm² V^?1 s^?1 could be achieved. The air stabilities of OFET operation were dependent on the nature of the compounds and were investigated by X‐ray diffraction and atomic force microscopy. The terminal units had a great influence not only on the molecular properties, but also on the film‐forming properties and OFET performance.     

Photophysical Properties of Intramolecular Charge Transfer in a Tribranched Donor–π–Acceptor Chromophore

The photophysical properties of intramolecular charge transfer (ICT) in a novel tribranched donor–π–acceptor chromophore, triphenoxazine‐2,4,6‐triphenyl‐1,3,5‐triazine (tri‐PXZ‐TRZ), with thermally activated delayed fluorescence character was investigated in different aprotic solvents by steady‐state spectroscopy and femtosecond and nanosecond transient absorption spectroscopy measurements. Increasing the solvent polarity led to a significant increase in the Stokes shift. The large Stokes shift in highly polar solvents was attributed to ICT properties upon excitation; this resulted in a strong interaction between the tri‐PXZ‐TRZ molecule and the surrounding solvent, which led to a strong solvation process. Quantum‐chemical calculations and changes in the dipole moment showed that this compound has a large degree of ICT. Furthermore, an apolar environment helped to preserve the symmetry of tri‐PXZ‐TRZ and to enhance its emission efficiency. The femtosecond and nanosecond transient absorption spectroscopy results indicated that the excited‐state dynamics of this push–pull molecule were strongly influenced by solvent polarity through the formation of a solvent‐stabilized ICT state. The excited‐state relaxation mechanism of tri‐PXZ‐TRZ was proposed by performing target model analysis on the femtosecond transient absorption spectra. In addition, the delayed fluorescence of tri‐PXZ‐TRZ was significantly modulated by a potential competition between solvation and intersystem crossing processes.     

Imidazole as a Donor/Acceptor Unit in Charge‐Transfer Chromophores with Extended π‐Linkers

Eleven new, stable, push–pull systems that feature 4,5‐bis[4‐(N,N‐dimethylamino)phenyl]imidazole and 4,5‐dicyanoimidazole as the donor and acceptor moieties and the systematically extended and varied π‐linker were prepared and investigated. Evaluation of the measured UV/Vis spectra, electrochemical data (cyclic voltammetry (CV), rotating‐disc voltammetry (RDV), and polarography) and calculated β and γ polarizabilities showed efficient charge transfer (CT) in biimidazole‐type chromophores. Push–pull system 27 , which features a planar thiophene‐derived π‐linker, was revealed to be the most efficient chromophore within the studied series. This chromophore possessed the most bathochromically shifted CT band, the lowest electrochemical gap, and highest β and γ polarizabilities. The CT transition was most significantly affected by structural features such as π‐linker length, planarity, conjugating arrangement, and the presence of olefinic/acetylenic or 1,4‐phenylene/thiophene subunits in the π‐linker.     

Theoretical Study on Electronic and Charge Transfer Properties of Oligo[8]thiophene and Its Circular,Hooped, and Helical Derivatives

The novel linear, circular, hooped, and helical molecules based on oligo[8]thio- phene were theoretically studied for the applications of charge transfer devices. To investigate the influence of topology for oligo[8]thiophene derivatives, the geometry structures, frontier molecular orbital （FMO） energies, charge transport properties, and stability property were predicted by density functional theory methods. The calculated results reported herein show that the oligo[8]thiophene derivative with linear structure has smaller energy gap, and fused oligo[8]thiophene derivative with circular structure has the smallest reorganization energy among the designed molecules. We have also studied the stability properties of the designed molecules, and oligo[8]thiophene derivatives are more stable tharJ the fused oligo[8]thiophene derivatives.     

Enhanced Electron‐Transfer Properties of Cofacial Porphyrin Dimers through π–π Interactions

π–π assisted : Photoinduced electron transfer from cofacial porphyrin dimers to electron acceptors is prominently accelerated, whereas the back electron transfer is decelerated, relative to the corresponding porphyrin monomer (see figure).

     

Synthesis,Crystal Structure,and Physical Property of Sterically Unprotected Thiophene/Phenylene Co‐Oligomer Radical Cations: A Conductive π–π Bonded Supermolecular meso‐Helix

Sterically unprotected thiophene/phenylene co‐oligomer radical cation salts BPnT^.+[Al(OR_F)₄]^? (OR_F=OC(CF₃)₃, n=1–3) have been successfully synthesized. These newly synthesized salts have been characterized by UV/Vis‐NIR absorption and EPR spectroscopy, and single‐crystal X‐ray diffraction analysis. Their conductivity increases with chain length. The formed meso‐helical stacking by cross‐overlapping radical cations of BP2T^.+ is distinct from previously reported face‐to‐face overlaps of sterically protected (co‐)oligomer radical cations.     

Tuning the Electronic Properties and Acid‐Response Behavior of N‐Heteroacene‐Based π‐Conjugated Liquids by Changing the Number of π‐Conjugated Substituents

We have designed and synthesized two room‐temperature‐fluorescent π‐conjugated liquids based on the N‐heteroacene framework ( 1 and 2 ). These two π‐conjugated liquids, which contained one and two thiophene rings, respectively, exhibited different electronic properties and rheology behaviors. Single‐crystal X‐ray analysis of dithiophene‐appended compound 4 revealed that two thiophene rings hindered the interactions of the imino N atoms with acids through the formation of interactions between the S atoms of the thiophene rings and the imino N atoms of the pyrazine group. On the other hand, monothiophene‐appended molecules 1 and 3 each contained an unhindered imino N atom on the opposite site to the thiophene ring. Upon dissolving various acids with different pK_a values in compounds 1 and 2 , these slight structural differences gave rise to marked differences in their acid‐response behaviors, thereby resulting in the emission of variously colored fluorescence in the liquid state. Furthermore, when acids with lower pK_a values was dissolved in compounds 1 and 2 , phase transition occurred from an isotropic liquid state to a self‐organized liquid‐crystalline phase.     

Lifetime Shortening and Fast Energy‐Tansfer Processes upon Dimerization of a A‐π‐D‐π‐A Molecule

Time‐resolved fluorescence and transient absorption experiments uncover a distinct change in the relaxation dynamics of the homo‐dimer formed by two 2,5‐bis[1‐(4‐N‐methylpyridinium)ethen‐2‐yl)]‐N‐methylpyrrole ditriflate ( M ) units linked by a short alkyl chain when compared to that of the monomer M . Fluorescence decay traces reveal characteristic decay times of 1.1 ns and 210 ps for M and the dimer, respectively. Transient absorption spectra in the spectral range of 425–1050 nm display similar spectral features for both systems, but strongly differ in the characteristic relaxation times gathered from a global fit of the experimental data. To rationalize the data we propose that after excitation of the dimer the energy localizes on one M branch and then decays to a dark state, peculiar only of the dimer. This dark state relaxes to the ground state within 210 ps through non‐radiative relaxation. The nature of the dark state is discussed in relation to different possible photophysical processes such as excimer formation and charge transfer between the two M units. Anisotropy decay traces of the probe‐beam differential transmittance of M and the dimer fall on complete different time scales as well. The anisotropy decay for M is satisfactorily ascribed to rotational diffusion in DMSO, whereas for the dimer it occurs on a faster time scale and is likely caused by energy‐transfer processes between the two monomer M units.     

Assessment of standard force field models against high‐quality ab initio potential curves for prototypes of π–π, CH/π, and SH/π interactions

Several popular force fields, namely, CHARMM, AMBER, OPLS‐AA, and MM3, have been tested for their ability to reproduce highly accurate quantum mechanical potential energy curves for noncovalent interactions in the benzene dimer, the benzene‐CH₄ complex, and the benzene‐H₂S complex. All of the force fields are semi‐quantitatively correct, but none of them is consistently reliable quantitatively. Re‐optimization of Lennard‐Jones parameters and symmetry‐adapted perturbation theory analysis for the benzene dimer suggests that better agreement cannot be expected unless more flexible functional forms (particularly for the electrostatic contributions) are employed for the empirical force fields. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2009