Effective Π-conjugation Length Research Articles

Light-Responsive Oligothiophenes Incorporating Photochromic Torsional Switches.

We present a quaterthiophene and sexithiophene that can reversibly change their effective π-conjugation length through photoexcitation. The reported compounds make use of light-responsive molecular actuators consisting of an azobenzene attached to a bithiophene unit by both direct and linker-assisted bonding. Upon exposure to 350 nm light, the azobenzene undergoes trans-to-cis isomerization, thus mechanically inducing the oligothiophene to assume a planar conformation (extended π-conjugation). Exposure to 254 nm wavelength promotes azobenzene cis-to-trans isomerization, forcing the thiophenic backbones to twist out of planarity (confined π-conjugation). Twisted conformations are also reached by cis-to-trans thermal relaxation at a rate that increases proportionally with the conjugation length of the oligothiophene moiety. The molecular conformations of quaterthiophene and sexithiophene were characterized by using steady-state UV-vis spectroscopy, X-ray crystallography and quantum-chemical modeling. Finally, we tested the proposed light-responsive oligothiophenes in field-effect transistors to probe the photo-induced tuning of their electronic properties.

Synthesis and properties of fluorene based small molecule acceptors containing aromatic malononitrile functionalities

Four novel small molecule acceptors (SMA) with fluorene-based cores were synthesized with terminal phenyl, thienyl, and furyl malononitrile groups in good yields. The products were obtained from a two-step reaction of 2,2′-(9,9-dioctyl-9H-fluorene-2,7-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) with m- or p-bromobenzaldehyde, 5-bromo-2-thiophenecarboxaldehyde and 5-bromo-2-furaldehyde under Suzuki conditions. Subsequent reaction with malononitrile yielded 9,9-dioctylfluorene −2,7 (1-phenyl-3(or 4)-methylene malononitrile) (m-FPM or p-FPM), 9,9-dioctylfluorene −2,7 (2-thienyl-5-methylene malononitrile) (FTM) and 9,9-dioctylfluorene −2,7 (2-furyl-5-methylene malononitrile) (FFM) respectively. Structural characterization of the compounds was performed using 1H and 13C NMR, and FT-IR. Optical and electronic characterization were done using, UV–visible and fluorescence spectroscopy, cyclic voltammetry, and DFT calculations. The compounds exhibit absorption bands ranging from 300 to 550 nm. m-FPM has the least coplanar structure while FFM displays the most coplanar structure, which increases the effective π-conjugation length resulting in a red shift of the absorption bands. The HOMO/LUMO levels were determined by CV and supported by DFT calculations, with optical band gaps of 2.70 eV and 2.41 eV for m- and p-FPM and 2.21 eV and 1.86 eV for FTM and FFM respectively. Fluorescence quenching was observed when poly-3-hexyl thiophene (P3HT) and each of the four SMAs were mixed. Films of P3HT and each SMA were cast in 1:1 and 1:4 ratios respectively. AFM indicated the formation of continuous films and the impact of small molecule structure on film morphology. These combined results indicated the capability of all of the new molecules to act as small molecule acceptors.

Spectroscopic and electrochemical properties of thiophene-phenylene based Shiff-bases with alkoxy side groups, towards photovoltaic applications

The influence of presence and elongation of alkoxy side chains in the π-conjugated Schiff-bases has been considered on the basis of UV–Vis absorption and photoluminescence spectra of model compounds and polymers solutions in chloroform and binary solvents of different polarity. The results of these investigations have been supported by electrochemical data. It has been demonstrated that introduction of electron donating methoxy side groups decreases the energy gap, however the elongation of alkyl chains only slightly affects the electronic structure of model compounds. In the case of polymers, such octyloxy side chains improves the solubility, enabling formation of longer polymer chains, with the enhanced effective π-conjugation length and narrower energy gap, however the intensity of emission band clearly decreased. Positive solvatochromism has been observed in both absorbance and photoluminescence spectra for all investigated compounds. As the concluding task, bulk-heterojunction (BHJ) photovoltaic (PV) structures, consisting of polyazomethines blended with the fullerene derivative, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) have been prepared and tested in the context of potential application in solar cells. All investigated polymers have shown the photovoltaic effect, but the best power conversion efficiency and other PV parameters have been obtained for polyazomthine with octyloxy side chains.

Morphological effects on the third-order nonlinear optical response of polydiacetylene nanofibers

Abstract

Effect of π-conjugation on electrochemical properties of poly(terthiophene)s 3′-substituted with fullerene C60

The series of copolymers based on terthiophene 3′-substituted with C60-pyrrolidine or styryl group, were electrochemically synthesized and characterized. The dense distribution of C60 side group caused the steric effects between them and the perturbation in the extent of π–electron delocalization of the poly(terthiophene) backbone. The ESR spectroscopy showed the presence of close-packed clusters especially in homopolymer: poly(terthiophene-C60). The oxidative electrochemical copolymerization of terthiophene units was performed to avoid this steric interactions and in results to extend the effective π-conjugation length in the polymer. The use of different ratio of the co-monomers contributed to the variable distribution of C60 unit and different effective π-conjugation of the resulting polymers. Those changes affected electrochemical and photocurrent-generating properties of these materials.

Synthesis of Polyaniline (PANI) in Nano-Reaction Field of Cellulose Nanofiber (CNF), and Carbonization.

Polymerization of aniline in the presence of cellulose nano-fiber (CNF) is carried out. We used dried CNF, CNF suspension, and CNF treated by enzyme and ultra-sonification to obtain polyaniline (PANI)/CNF as a synthetic polymer/natural nano-polymer composite. The polymerization proceeds on the surface of CNF as a nano-reaction field. Resultant composites show extended effective π-conjugation length because CNF as a reaction field in molecular level produced polymer with expanded coil structure with an aid of orientation effect of CNF. Possibility of PANI β-pleats structure in molecular level of PANI on the CNF is also discussed. SEM observation showed that fine structure is easily obtained by combining PANI with CNF. Carbonization of PANI/CNF allows production of nano-fine form with shape preserved carbonization (SPC).

Tailoring color emissions from N-doped graphene quantum dots for bioimaging applications

Unlike inorganic quantum dots, fluorescent graphene quantum dots (GQDs) display excitation-dependent multiple color emission. In this study, we report N-doped GQDs (N-GQDs) with tailored single color emission by tuning π-conjugation degree, which is comparable to the inorganic quantum dot. Starting from citric acid and diethylenetriamine, as prepared N-GQDs display blue, green, and yellow light emission by changing the reaction solvent from water, dimethylformamide (DMF), and solvent free. The X-ray photoelectron spectroscopy, ultraviolet-visible spectra results clearly show the N-GQDs with blue emission (N-GQDs-B) have relatively short effective conjugation length and more carboxyl group because H2O is a polar protic solvent, which tends to donate proton to the reagent to depress the H2O elimination reaction. On the other hand, the polar aprotic solvent (DMF) cannot donate hydrogen, the elimination of H2O is promoted and more nitrogen units enter GQD framework. With the increase of effective π-conjugation length and N content, the emission band of N-GQDS red-shifts to green and yellow. We also demonstrate that N-GQDs could be a potential great biomarker for fluorescent bioimaging. Nitrogen-doped graphene quantum dots emitting blue, green, or yellow light are promising biomarkers for fluorescence bioimaging. Dan Qu and colleagues in China tailored the emission color of the quantum dots by changing the reaction solvent used in their fabrication. Dots prepared from protic solvent-emitted blue (450 nm) light, those prepared with aprotic solvent emitted green (550 nm), and solvent-free dots emitted yellow (580 nm). The emission colors were pure and narrow band with full-width half-maximum values of 71, 86, and 111 nm and decay lifetimes of 14, 13, and 10 ns, respectively. Imaging experiments with a confocal fluorescence microscope and A549 cells labeled with the graphene dots demonstrated that the cells did indeed become brightly illuminated with visible colors when optically excited, suggesting that the graphene dots are suitable for bioimaging.

Dicopper(II) Metallacyclophanes with Oligo(p-phenylene-ethynylene) Spacers: Experimental Foundations and Theoretical Predictions on Potential Molecular Magnetic Wires

Two novel double-stranded dicopper(II) metallacyclophanes of formula (nBu4N)4[Cu2(dpeba)2]·4MeOH·2Et2O (1) and (nBu4N)4[Cu2(tpeba)2]·12H2O (2) have been prepared by the Cu(II)-mediated self-assembly of the rigid ('rod-like') bridging ligands N,N'-4,4'-diphenylethynebis(oxamate) (dpeba) and N,N'-1,4-di(4-phenylethynyl)phenylenebis(oxamate) (tpeba), respectively. Single crystal X-ray diffraction analysis of 1 confirms the presence of a dicopper(II)tetraaza[3.3]4,4'-diphenylethynophane metallacyclic structure featuring a very long intermetallic distance between the two square planar Cu(II) ions [r = 14.95(1) Å]. The overall parallel-displaced π-stacked conformation of the two nearly planar para substituted diphenylethyne spacers [dihedral angle (ψ) of 7.8(1)°] leads to important deviations from the perpendicular orientation of the copper mean basal planes with respect to the facing benzene planes [dihedral angles (φ) of 56.4(1) and 58.4(1)°]. X-band EPR spectra together with variable-temperature magnetic susceptibility and variable-field magnetization measurements of 1 and 2, both in solution and in the solid state, show the occurrence of a non-negligible, moderate to weak intramolecular antiferromagnetic coupling [-J = 3.9-4.1 (1) and 0.5-0.9 cm(-1) (2); H = -JS1·S2 with S1 = S2 = SCu = 1/2]. Density functional calculations on the BS singlet (S = 0) and triplet (S = 1) spin states of the model complexes 1 and 2 with an ideal orthogonal molecular geometry (ψ = 0° and φ = 90°) support the occurrence of a spin polarization mechanism for the propagation of the exchange interaction between the two unpaired electrons occupying the dxy orbital of each square planar Cu(II) ion through the predominantly π-type orbital pathway of the double p-diphenylethyne (1) and di(phenylethynyl)phenylene spacers (2). Time-dependent density functional calculations reproduce the observed bathochromic shift of the main intraligand (IL) π-π* transition in the electronic absorption spectra of 1 and 2 [λ1 = 308 (1) and 316 nm (2)]. In the series of orthogonal model complexes 1-5 with linear oligo(p-phenylene-ethynylene) (OPE) spacers, -C6H4(C≡CC6H4)n- (n = 1-5), a linear increase of the IL π-π* transition energy with the reciprocal of the intermetallic distance is theoretically predicted [νmax = 1.99 × 10(4) + 2.15 × 10(5) (1/r) (S = 0) or ν = 2.01 × 10(4) + 2.18 × 10(5) (1/r) (S = 1)], which clearly indicates that the effective π-conjugation length increases with the number of phenylethyne repeating units. This is accompanied by an exponential decay of the antiferromagnetic coupling with the intermetallic distance [-J = 1.08 × 10(3) exp(-0.31r)], which supports the ability of the extended π-conjugated OPEs to mediate the exchange interaction between the unpaired electrons of the two Cu(II) centers with intermetallic distances in the range of 1.5-4.3 nm. Further developments may be then envisaged for this new family of oxamato-based dicopper(II) oligo-p-phenylethynophanes on the basis of the unique ligand capacity to act as a molecular antiferromagnetic wire.

Polymer reaction of poly(p-phenylene–ethynylene) by addition of decaborane: modulation of luminescence and heat resistance

Polymer reactions of poly(p-phenylene–ethynylene) (PPE) based on the addition reaction of decaborane to the carbon–carbon triple bond were successfully carried out. The platform polymer was synthesized through Sonogashira–Hagihara coupling polymerization. The effective introduction of the boron cluster, and thus the subsequent formation of o-carborane, was supported by 1H and 11B nuclear magnetic resonance spectroscopies. The introduction rate of decaborane to the polymer main chain was proportional to the feed ratio of decaborane. Ultraviolet-visible absorption and photoluminescence studies illustrate the shortening of the effective π-conjugation length caused by the introduction of carborane clusters. Furthermore, the introduction of the boron cluster by the polymer reaction efficiently enhanced the heat resistance of PPE.

New organic semiconductors for thin-film transistors: Synthesis, characterization, and performance of 4 H-indeno[1,2- b]thiophene derivatives

Three new hybrid polycyclic aromatic 4 H-indeno[1,2- b]thiophene derivatives ( DIT-nT) were readily synthesized and characterized. Ultraviolet photoelectron spectroscopy (UPS) and inverse photoemission spectroscopy (IPES) study shows that the gap energies of these oligothiophene derivatives are dependent on the effective π-conjugation length of the molecules, with decreasing gap energies while the conjugated length is increased. More importantly, the hole injection barrier obtained from UPS and IPES studies decreases significantly at the metal-organic interface as the effective π-conjugation length of the molecules increases. Hence, the hole injection into DIT-2T is expected to be more facile than that into DIT-0T due to the smaller barrier. X-ray diffraction examination revealed that thin films are highly ordered, with the long axes of the molecules nearly perpendicular to the surface. The field-effect transistor (FET) devices based on the three oligomers show good p-type performance with high hole mobilities, and the highest value (0.014 cm 2 V −1 s −1) was obtained for the longest oligomer DIT-2T.

Photophysics of meso-β Doubly Linked Ni(II) Porphyrin Arrays: Large Two-Photon Absorption Cross-Section and Fast Energy Relaxation Dynamics

Two-photon absorption (TPA) properties and energy relaxation dynamics of meso-β doubly linked Ni(II) porphyrin arrays (DLNin, n = 2−5) in CHCl3 have been investigated. DLNin exhibits near-IR-extended absorption bands up to 1000 nm and enlarged TPA cross-section values without any one-photon contribution as the number of subunits increases mainly due to an elongation of effective π-conjugation length. Especially, in the case of pentamer, DLNi5 had a TPA cross-section value of 41 000 GM, which is the largest value per subunit (σ(2)/n ∼ 8000 GM) among the conjugated porphyrins ever studied. Fast energy relaxation processes within a few picoseconds were observed by femtosecond transient absorption measurements, indicating that various charge transfer and (d,d) states of the central Ni(II) ion are involved in deactivation pathways as the ladder states in DLNin.

Transfer of Photoenergy in π-Conjugated Polymers. Two Types of Photoluminescence that Involve Energy Transfer along a Polymer Chain

Two types of energy transfer in pi-conjugated polymers have been investigated using time-resolved photoluminescence (PL) techniques: type i, perpendicular-type energy transfer from the 2,3-di(p-tolyl)quinoxaline unit to the pi-conjugated main chain of poly[2,3-di(p-tolyl)quinoxaline-5,8-diyl], and type ii, parallel-type energy transfer from the oligo(pyridine-2,5-diyl) (O-Py) unit to the oligo(selenophene-2,5-diyl) (O-Se) unit in a block-type copolymer of O-Py and O-Se. Both types of energy transfer were very fast with a time constant shorter than approximately 0.1 ns; in particular, the type ii energy transfer took place with a time constant of approximately 5 ps. Both pi-conjugated polymers were considered to contain segments with various effective pi-conjugation lengths, and the energy transfer to the segment with a larger effective pi-conjugation length and a smaller pi-pi* transition energy required a longer transition time. A polarizing film was obtained by utilizing the perpendicular-type energy transfer.

Light emitting polyfluorene derivatives with three different structural configurations

We designed polyfluorene derivatives by connecting the spirobifluorene or 9,9-dibenzylfluorene moieties via two different link positions. By growing polymers via positions (2 and 7) and (2 and 2′) of spirobifluorene, we synthesized a linear-fully conjugated poly[(9,9′-spirobifluorene-2,7-diyl)- alt-co-(1,4-(2,5-di- n-dodecyloxy)phenylenevinylene)] and a kinked-locally conjugated poly[(9,9′-spirobifluorene-2,2′-diyl)- alt-co-(1,4-(2,5-di- n-dodecyloxy)phenylenevinylene)]. Their absorption and emission spectra were substantially red-shifted compared with the alternating copolymers of spirobifluorene and fluorene due to extended π-conjugation to the phenylenevinylene units. Especially, no long wavelength emission tail was observed in the EL spectra of the two polymers presumably because the spiro-annulated fluorene and the dodecyloxy moieties suppressed interchain interactions. By growing polymers via positions 2 and 7 of 9,9-dibenzylfluorene, we synthesized a linear-fully conjugated poly[(9,9-dibenzylfluorene-2,7-diyl)- alt-co-(9,9-di- n-hexylfluorene-2,7-diyl)]. This polymer showed electronic spectra almost identical to those of poly(9,9-di- n-hexylfluorene-2,7-diyl) and exhibited a long-wavelength tail in the EL spectrum. This indicates that the presence of the pendent dibenzyl group does not influence the electronic structure of polyfluorene backbone nor suppresses the interchain interactions. By growing polymers via para-positions of the two dibenzyl groups in 9,9-dibenzylfluorene, we synthesized a linear-locally conjugated poly[{9,9-bis(4-yl-benzyl)-fluorene}- alt-co-{9,9-di- n-heyxyl-fluorene-2,7-diyl}]. Its absorption and emission spectra were blue-shifted due to the shorter effective π-conjugation length and the emission tail was not observed probably due to reduction of interchain interactions by orthogonal orientation of the fluorene units to the main chain.

Molecular engineering versus energy level alignment: Interface formation between oligothiophene derivatives and a metal substrate studied with photoemission spectroscopy

Two series of thin films of oligothiophene derivatives grown on Ag substrates have been studied with photoelectron spectroscopy. The oligothiophenes were end-capped with either electron-deficient (dismesitylboryl) or electron-rich (diphenyltolylamine) moieties to create molecules with electron-accepting or -donating properties, respectively. The position of the highest occupied molecular orbital (HOMO) at the metal/organic interface is found to be strongly dependent on the effective π-conjugation length of the oligothiophenes capped with dimesitylboryl groups, whereas in the oligothiophenes capped with diphenyltolylamine, the position of the HOMO is independent of the molecular length. The difference in the observed HOMO characteristics is attributed to the different make-up of the frontier orbitals in the two molecular series. This will particularly affect the overall energy barrier for charge injection at the conductor/organic interface in a device structure, such as an organic light-emitting diode, utilizing the investigated molecules for carrier injection and transport.

Photocarrier transports related to the morphology of regioregular poly(3-hexylthiophene) films

Photocarrier transport has been investigated by means of time-of-flight (TOF) methods in regioregular poly(3-hexylthiophene) (HT–PAT6) films prepared by various casting and annealing processes. The absorption spectra and X-ray diffraction analyses suggest the electronic structure and the morphology change with the film preparation. The spin-coat films depicted the short effective π-conjugation length and amorphous, which contrasts to those of drop-cast films having a crystalline structure. The dispersive transient photocurrents were observed in the spin-coat films while the less dispersive in the drop-cast films. The negative field dependence of the TOF mobilities were observed at low fields, which can be explained by anisotropic transport. The results indicate that the ex situ preparation is one of the methods to modulate the transport characteristics in HT–PAT6 films.

Vibrational study of a well–barrier–well thiophene–based oligomer in relation to the effective π-conjugation length

We have studied the Fourier transform infrared (FTIR) and the Fourier transform Raman (FT-Raman) spectra of a new thiophene-based oligomer in neutral form. The compound has a well–barrier–well structure, where the well parts are two bithienyl end moieties and the barrier part is a vinylene fragment with a larger energy gap. Characteristic features are correlated with vibrational data of unsubstituted α-linked oligothiophenes and α,α′-dimethyl end-capped oligothiophenes to assess information about the effective conjugation length of this oligomer. The FTIR and FT-Raman spectra have also been theoretically calculated at the B3LYP/6-31G** and RHF/6-31G** levels, respectively, to propose a vibrational assignment for the main bands in the whole spectral range.