Increasing Conjugation Length Research Articles

Origin of the Bathochromic Shift of Astaxanthin in Lobster Protein: 2D Electronic Spectroscopy Investigation of β-Crustacyanin

We report on ultrafast spectroscopy study of β-crustacyanin, the carotenoprotein responsible for the coloration of the lobster shell. β-Crustacyanin is formed by two closely positioned astaxanthin molecules encapsulated in protein. The 2D electronic spectroscopy together with two-color pump-probe was applied to investigate the electronic structure, the excited-state dynamics, and the influence of the excitonic interaction between the two carotenoids in β-crustacyanin. By using the ∼20 fs laser pulses tuned to absorption bands of the S0-S2 and S1-Sn transitions of carotenoids, we were able to trace full excitation relaxation dynamics, starting with S2-S1 relaxation on the ∼30 fs time scale and finishing with the ground-state recovery of 3.2 ps. Superimposed on the relaxation dynamics in the 2D spectra, we observed long-lived beating signals at the characteristic frequencies of astaxanthin vibrational modes. We assign these oscillations to the ground-state vibrational wavepacket dynamics. All major features of the 2D spectra, including amplitude and phase maps of the long-lived oscillations, were reproduced by employing the exciton-vibronic model. Consistent modeling of all optical properties of β-crustacyanin (including absorption and circular dichroism spectra) points to the relatively weak coupling between the two astaxanthin molecules (∼250 cm(-1)). This implies that the excitonic coupling provides insignificant contribution to the bathochromic shift in β-crustacyanin. We discuss the origin of the shift and propose that it is caused by two major effects: conformational changes of astaxanthin molecules (increase in effective conjugation length) together with increased charge-transfer character of the S2 state. We put the bathochromic shift in the broad perspective of other "blue" carotenoids properties.

Tuning band structures of dyes for dye-sensitized solar cells: effect of different π-bridges on the performance of cells

Three organic dyes (LJ101, LJ102, and LJ103) with different π-bridges such as thiophene, 3-hexylthiophene and 3, 4-ethyldioxythiophene, were prepared and applied for the dye-sensitized solar cells (DSSCs). The effect of the different π-bridges on the photovoltaic performance of DSSCs were studied with the aid of quantum chemical calculation and electrochemical-impedance spectroscopy (EIS). The introduction of π-bridges to construct dyes makes the absorption spectra red shift and enhances the molar extinction coefficient compared to the parent structure of TH305 without π-conjugated bridge. However, the increasing conjugation length of the molecules decreases the performance of DSSCs, due to the two reduced gaps: one is between the LUMO levels of dyes and the conduction band edge of TiO2, the other one is the energy gap between the HOMO levels of dyes and the redox potential of iodide/triiodide. The conversion efficiencies of DSSCs based on LJ series dyes ranged from 6.1 to 6.4%, which are lower than that of TH305 (7.5%).

Investigation of oxygen-free Sonogashira step growth synthesis of mono-terminated di-tert-butyl-substituted oligo(phenylene ethynylene)s (OPEs)

A series of oligomers, the hetero-coupled mono-terminated di-tert-butyl-substituted phenylene ethynylene dimer, trimer, tetramer, and pentamer have been successfully synthesized in a stepwise process and characterized. Their electronic properties have been studied by UV–vis and fluorescence spectroscopy. Both UV absorption and FL emission maxima have red shift trends with increasing conjugation length of the oligomers. In addition, homo-coupling products inevitably complicate the hetero-coupling reaction. With increasing substituent group size on the terminal alkyne, the amount of homo-coupling product is increased.

Theoretical study of phenylene–thiophene oligomers: Structure–properties relationship

Theoretical results including geometrical characteristics, electronic structures, photophysical parameters (lowest excitation energies, electron affinities (EAs), ionization potentials (IPs), maximum of absorption and emission) and vibrational modes of some new oligomers, based on phenylene–thiophene motives, are investigated using Density Functional Theory DFT/B3LYP/6-31G(d) approach.The electronic and optical properties of phenylene-based derivatives can be tuned through the insertion of thiophene in the main oligomers backbone as well as the addition of alkoxy-substituent groups on 2 and 5 positions on phenylene groups. It can be noticed, that different conformational behaviors and steric effects take place. Then, an increase in conjugation length induces a decrease in the gaps energy and a bathochromic shift of absorption/emission spectra. Based on these computed results, which are consistent with the available experimental data, the correlation structure–properties is better understood, where these nanostructures show a great potential for opto-electronic devices.

Torsional Influences within Disordered Organic Electronic Materials Based upon Non-Benzenoid 1,6-Methano[10]annulene Rings

Conjugated polymers and small molecules containing the nonplanar aromatic 1,6-methano[10]annulene were synthesized in an effort to understand how torsional differences between planar and nonplanar π-electron components influence the electronic properties of π-conjugated materials. The polymers and small molecule model systems contain commonly employed aromatic subunits such as thiophene, diketopyrrolopyrrole, and 2,1,3-benzothiadiazole, leading to electron donor and donor–acceptor polymers. The curved geometry of 1,6-methano[10]annulene can lead to reduced local torsional strain in semiconducting polymers relative to large planar aromatics, potentially increasing intrapolymer conjugation. The relative amount of effective conjugation length increase granted by the annulene in each system of regioisomers was interrogated through the use of UV–vis and photoluminescence spectroscopy and electrochemistry, and it was found that 1,6-methano[10]annulene relieves some torsional strain associated with solubilizing alkyl chains clashing with aromatic rings along the polymer backbone. The polymers were also found to be highly disordered in thin films yet still provided reasonable hole mobilities (ca. 10–4 cm2/(V s)) in OFET devices. These results suggest that methano[10]annulene or other curved aromatics may prove useful in the future development of organic electronics.

Spectral Properties of Substituted Coumarins in Solution and Polymer Matrices

The absorption and fluorescence spectra of substituted coumarins (2-oxo-2H-chromenes) were investigated in solvents and in polymer matrices. The substitutions involved were: (1) by groups with varying electron donating ability such as CH3, OCH3 and N(CH3)2, mainly, but not exclusively, in positions 7 and (2), by either CHO or 4-PhNHCONHN=CH- in position 3. While the spectra of non-substituted coumarin-3-carbaldehyde has absorptions at approximately 305 and 350 nm, substitution at position 7 leads to remarkable changes in the shape of the absorption spectrum and shifts the absorption to a longer wavelength. Similarly, the replacement of the formyl group with a semicarbazide group substantially influences the shape of the absorption spectrum, and coumarins which have only N(CH3)2 in position 7 experience small changes. These changes are associated with the increasing intramolecular charge transfer (ICT) character and increasing conjugation length of the chromophoric system, respectively, in the studied molecules. The fluorescence is almost negligible for derivatives which have H in this position. With increasing electron donating ability, and the possibility of a positive mesomeric (+M) effect of the substituent in position 7 of the coumarin moiety, the fluorescence increases, and this increase is most intense when N(CH3)2 substitutes in this position, for both 3-substituted derivatives. Spectral measurements of the studied coumarins in polymer matrices revealed that the absorption and fluorescence maxima lay within the maxima for solvents, and that coumarins yield more intense fluorescence in polymer matrices than when they are in solution. The quantum yield of derivatives which have a dimethylamino group in position 7 in polymer matrices approaches 1, and the fluorescence lifetime is within the range of 0.5–4 ns. The high quantum yield of 7-dimethylamino derivatives qualifies them as laser dyes which have kF higher than knr in the given medium. This is caused by stiffening of the coumarin structure in polar polymer matrices, such as PMMA and PVC, due to higher micro-viscosity than in solution and intermolecular dipole-dipole interaction between chromophore (dopant) and matrix.

Grafting P3HT brushes on GO sheets: distinctive properties of the GO/P3HT composites due to different grafting approaches

In this study, we have grafted poly(3-hexylthiophene) (P3HT) brushes on GO sheets via a silylation reaction of the surfaces of GO sheets and a succeeding “click” reaction (GO(C)/P3HT composite). Compared with pure P3HT and the blend of P3HT and GO, the GO(C)/P3HT composite shows a red-shifted optical absorption maximum because of increased conjugation length of the grafted P3HT, which might be due to the crowding of the P3HT chains grafted on GO sheets. For comparison, P3HT chains were also grafted on GO sheets via the amidation reaction of the carboxylic acid groups (GO(A)/P3HT composite). However, the GO(A)/P3HT composite doesn't show a red shift in the UV-visible spectrum. The overall fluorescence quenching in the GO(C)/P3HT composite includes both dynamic quenching and forming a non-fluorescent ground-state complex. Additionally, thermogravimetric analysis and X-ray photoelectron spectroscopy results demonstrate that the efficiency of grafting P3HT on GO sheets via the “click” reaction approach is higher than via the amidation reaction approach. Atomic force microscopy and transmission electron microscopy observations show that the GO(C)/P3HT composite sheets are much thicker than the GO(A)/P3HT composite sheets, due to the higher grafting efficiency of the “click” reaction approach. Therefore, this work opens a novel avenue to tuning the optical properties of conjugated polymers via surface grafting chemistry.

A molecularly engineered fluorene-substituted Ru-complex for efficient mesoscopic dye-sensitized solar cells

A new high molar extinction coefficient ruthenium(II) bipyridyl complex ‘cis-Ru(L1)(2,2′-bipyridine-4,4′-dicarboxylic acid) (NCS)2, BDF’, where L1=4,4-bis(9,9-dibutyl-9H-fluorene-2-yl)-[2,2] bipyridine, has been synthesized and characterized by Fourier transform infrared (FTIR), hydrogen nuclear magnetic resonance (1 H-NMR) and electrospray ionization mass (ESI–MASS) spectroscopes. The dye, upon anchoring onto mesoporous nano-crystalline TiO 2 solar cells, exhibited a broader photocurrent action spectrum, with a solar-to-electric energy conversion efficiency (η) of 6.58% (J SC =14.66 mA cm −2, V OC =640 mV, fill factor=0.71) under sunlight at air mass (AM) 1.5, larger than the reference Z907 sensitized solar cell fabricated under similar conditions, which exhibited an η-value of 4.65% (J SC =11.52 mA cm −2, V OC =566 mV, fill factor=0.72). Absorption measurements and time-dependent density functional theory (TDDFT) calculations show that the increased conjugation length by introducing 9,9-dibutyl-9H-fluorene moiety relatively enhances the spectral response of the ancillary ligand and the corresponding BDF complex. The calculated dipole moments for BDF and Z907 are 17.71 and 16.34 Debye, respectively. The first three highest occupied molecular orbitals (HOMOs) of BDF have a t 2g character, as observed in Z907, while HOMO-4 and HOMO-5 have considerable sizable mixing from Ru-NCS with π-orbitals of L1.

Mesomorphic behaviour and photoisomerisation of phenylene and biphenylene-substituted azobenzenes

Azobenzene derivatives substituted with phenylene and biphenylene groups were synthesised and their liquid crystalline (LC) properties were studied by differential scanning calorimetry, polarising optical microscopy and X-ray diffraction. The effect of extra phenylene groups on the LC properties of the azobenzene was mainly reflected in the stability of mesophases as well as in the transition temperatures – both associated with the increasing cohesive forces between molecules as the conjugation length increases. Nematic and monolayered smectic-type mesophases with tilted molecules were observed for these compounds. Highly ordered mesophases (CrG and/or CrK/CrH) were particularly observed for the azocompounds bearing extra phenylene groups. The calculated layer spacing was correlated either to a parallel or to an anti-parallel arrangement of molecules in the mesophase, depending on the symmetry of the rigid core. On the other hand, the photoisomerisation was characterised (dilute solution) as part of a preliminary study of future investigations on photo-induced properties of these new azocompounds. High rates of photoisomerisation as well as trans–cis conversions higher than 80% in the photostationary state were determined for these compounds.

Self-Assembly of Thienylenevinylene Molecular Wires to Semiconducting Gels with Doped Metallic Conductivity

Oligo(thienylenevinylene) (OTV) based gelators with high conductivity are reported. When compared to OTV1, OTV2 having an increased conjugation length forms relatively strong gels with a metallic conductivity of 4.8 S/cm upon doping which is the highest value reported for an organogelator. This new class of conducting gels is expected to be useful for organic electronics and photonics application, particularly for bulk heterojunction devices.

Spectroscopic and electrical evaluation of poly(3-hexylthiophene) nanotubules made using template wetting nanofabrication

The unique properties of conjugated polymer nanostructures as compared to thin films of the same materials have fueled the investigation of a number of different polymer nanofabrication techniques. Here we examine the optoelectronic properties of poly(3-hexylthiophene)P3HT nanotubules made using one of the simplest of these techniques, template wetting nanofabrication. Comparison is made with results from our previous study in which in poly(2-methoxy-5-(2́-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV) nanotubules prepared in this manner were shown to have dramatically different properties in comparison with thin films due to increased molecular order in the nanostructures. With semicrystalline P3HT, the differences are much more subtle, we believe due to the greater inherent order in this material. In P3HT nanotubules, a small bathochromic shift (∼0.05 eV) in the onset of Uv–vis absorption was observed indicating increased long range order and a greater effective conjugation length in the polymer. Polarized FTIR spectroscopy indicated a small degree of preferential alignment of the inter-ring C–C bond with the axis of the nanotubules as the stretching vibration for this bond showed a dichroic ratio of 1.73. The degree of increased conjugation length and chain alignment in P3HT nanotubules indicated by both the UV–vis and polarized FTIR spectra were much smaller than that previously observed for MEH-PPV. No significant improvement in mobility as measured from the current voltage characteristics of simple single carrier devices based on the P3HT nanotubules was observed, in sharp contrast to previously studied MEH-PPV where more than an order of magnitude increase was seen. Unlike in amorphous MEH-PPV, the increase in molecular order suggested by spectral data was insufficient to significantly impact mobility of semicrystalline P3HT when using template wetting nanofabrication for either of the 100 or 200 nm nanotubules examined.

Investigation of third-order nonlinear optical properties of conjugated benzodioxal derivatives

An investigation of third-order nonlinear optical characterization of newly synthesized conjugated benzodioxal derivatives has been done by using nanosecond Z-scan technique at 532 nm. The molecules demonstrate self-defocusing effect with intensity dependent refractive index ( n 2) of the order of 10 −14 cm 2/W. The measured molecular TPA cross-section is ranging from 2.47 ×10 −47 cm 4 s/photon to 6.00 cm 4 s/photon. Their input-output curves indicate that there is a clear optical power limiting behavior with the limiting threshold in the range 125–181 μJ. The main factor to exhibit the observed nonlinearity in these molecules is the presence of charge donor and acceptor groups. The increased conjugation length increases the nonlinear refraction and increased electron density enhances the nonlinear absorption. The molecules exhibit good nonlinear optical properties, comparable to those of regular azoaromatic compounds. Therefore, the molecules investigated here are promising candidates for optical power limiting devices.

Enhanced optoelectronic properties of RNA–poly(o-methoxyaniline) hybrid containing monodispersed Au nanoparticles

In situ, gold nanoparticles are produced in poly(o-methoxy aniline) (POMA)–RNA (R) hybrids; POMA (P) acting both as reductant and stabilizer. The POMA (emeraldine base, EB) is oxidized into a mixture of POMA (emeraldine salt, ES) and POMA (pernigraniline base, PB) in the course of reaction as evident from FTIR and UV-vis spectra. The Au nanoparticles are of uniform size (diameter ∼ 14 nm) and the density of Au nanoparticles increases with increase in POMA (EB) concentration showing a maximum of 150 × 1010 particles per square centimetre in PRAu31 (the number indicates respective weight ratio). The electron diffraction pattern indicates the polycrystalline nature of the Au nanoparticles and CD spectra suggest a small conformational distortion of the A helix of RNA towards the B helix in the nanobiocomposite. The FTIR spectra indicate the presence of H-bonding, π–π and ionic interactions between POMA (ES) and RNA, and the Au nanoparticles are stabilized by complexation through nitrogen atoms of POMA (PB). The π band to polaron band transition peak of POMA shows a gradual red shift with aging time and the shift is greater in the case of POMA–RNA–Au nanobiocomposites than that of the POMA–RNA hybrid. The larger shift in the PRAu nanocomposites compared to the POMA–RNA hybrid is attributed to the more uncoiled state of POMA for stabilizing Au nanoparticles. The POMA–Au system generates new photoluminescence properties when excited at 500 nm and PL intensity increases abruptly with the addition of RNA in the hybrid. The excitation of plasmon electrons and the stabilization of excitons in the conjugated chain of POMA is attributed to the new PL property and its enhancement with addition of RNA is due to the increased conjugation length of POMA on the RNA surface. PRAu13 shows rectification property, while the other compositions do not exhibit such behavior. The rectification is explained from the band energy diagram, and its absence in PRAu11 and PRAu31 hybrids is attributed to the increased concentrations of Au and POMA causing a better conducting composite material.

3,4-Ethylenedioxythiophene (EDOT)-based π-conjugated oligomers: Facile synthesis and excited-state properties

A facile synthetic method to produce soluble 3,4-ethylenedioxythiophene (EDOT) oligomers was developed by using iron (III) nitrate nonahydrate as the oxidizing reagent. Two EDOT oligomers, i.e. octomers and octodecamers were obtained as the major products. Their UV–vis absorption, fluorescence at room temperature and 77K, and triplet transient difference absorption have been measured in CH2Cl2 solutions or glassy solutions. With increased conjugation length from EDOT octomers (A) to EDOT octodecamers (B), the UV–vis absorption, fluorescence and the triplet transient difference absorption band shift to longer wavelength. Both the oligomers also exhibit reasonably high efficiency to generate singlet oxygen.

The fluorescence characteristics of furfurylated wood studied by fluorescence spectroscopy and confocal laser scanning microscopy

The fluorescence of furfurylated wood was studied. Furfurylation of wood introduced fluorescent curing products into the cell walls. Fluorescent products were also found in the lumina if high loadings were used and/or if the wood contained cells with small radii. More intense fluorescence was obtained from lignin-rich parts of the compound cell wall (i.e., the middle lamella and the cell corners) than from the secondary cell wall if the wood was impregnated using furfuryl alcohol (FA), while impregnation using small poly(FA) oligomers showed no or only minor differences between different regions. The study also showed that when higher amounts of catalyst were added to the impregnation liquid, a red-shift in the fluorescence from the furfurylated wood was seen, corresponding to an increased conjugation length of conjugated poly(FA) formed within the wood cell wall. Longer conjugation length than within the cell wall was observed for the poly(FA) formed within the lumina. This indicates that the cell wall polymers restrict poly(FA) formation.

Donor–Acceptor‐Substituted Oligo(1,4‐phenylene)s

Abstractmagnified imageOligo(para‐phenylene)s (DAOPPs) 2a–2d (n=1–4) with terminal donor–acceptor substitution (D=C6H13O, A=NO2) were prepared by applying Suzuki cross‐couplings for chain extension and end capping. The push–pull effect induces short‐reaching polarizations of the chain consisting of conjugated but twisted benzene rings, which was studied by NMR measurements. Electron excitation from the ground‐state S0 to the more planar first‐excited singlet state S1 is combined with a strong intramolecular charge transfer (ICT), which is documented by the red shift of the long‐wavelength absorption (charge‐transfer band) for short chains (one or two repeat units, n = 1 or 2). The opposite influence of decreasing ICT and increasing conjugation length leads to a bathochromic series (λmax(n+1)≥λmax(n)) with a fast saturation of λmax (n). The effective conjugation length nECL=4 corresponds to λ∞ 349 nm. These results are discussed in the context of other oligo(para‐phenylene)s (OPPs).

Fluorene‐Based Copolymers Containing Dinaphtho‐s‐indacene as New Building Blocks for High‐Efficiency and Color‐Stable Blue LEDs

By incorporating a new building block, 7,7,15,15-tetraoctyldinaphtho-s-indacene (NSI), into the backbone of poly(9,9-dioctylfluorene) (PFO), a novel series of blue light-emitting copolymers (PFO-NSI) have been developed. The insertion of the NSI unit into the PFO backbone leads to the increase of local effective conjugation length, to form low-energy fluorene-NSI-fluorene (FNF) segments that serve as exciton trapping sites, to which the energy transfers from the high-energy PFO segments. This causes these copolymers to show red-shifted emissions compared with PFO, with a high efficiency and good color stability and purity. The best device performance with a luminance efficiency of 3.43 cd · A(-1) , a maximum brightness of 6 539 cd · m(-2) , and CIE coordinates of (0.152, 0.164) was achieved.

π-Conjugated oligothiophene–anthracene co-oligomers: synthesis, physical properties, and self-assembly

A new class of oligothiophene–anthracene co-oligomers, 9,10-bis(2-dodecyloligothienylvinyl)anthracene TnA (n = 1, 2 and 3), were synthesized and characterized. The photophysical and electrochemical properties of these oligomers demonstrated an increased conjugation length with increasing number of thiophene units. These oligomers also exhibited relatively larger ionization potential values compared with pentacene and sexithiophene and thus are expected to show improved stability. The strong aggregation tendency and excimer formation for T1A and obvious aggregation effect for T2A in solution were observed from the concentration and temperature-dependent fluorescence spectra. In contrast, no obvious aggregation behavior could be observed for T3A in solution. Thermal behavior and self-assembly of these co-oligomers in solid state were studied by a combination of TGA, DSC, powder X-ray diffraction and polarizing optical microscope. Liquid crystalline phases were observed in T2A and T3A at elevated temperature while only a crystalline phase was found for T1A. Single-crystal structures of T1A and T2A revealed a lamellar packing mode with ordered intermolecular anthracene-to-anthracene and oligothiophene-to-oligothiophene π–π stacking and these qualify them as potential semiconductors in electronic devices.

Cubic Nonlinear Optical Properties of Platinum-Terminated Polyynediyl Chains

The wavelength dependence of the cubic nonlinearity of ligated platinum-terminated polyynes trans, trans-{(p-MeC6H4)3P}2(p-MeC6H4)Pt(C[triple bond]C)n Pt(p-C6H4Me){P(p-C6H4Me)3}2 (n = 3-6, 8, 10, 12) has been examined by femtosecond Z-scan studies in the wavelength range 520-1500 nm and the results rationalized by density functional theory calculations on the model complexes trans, trans-(H3P)2(C6H5)Pt(C[triple bond]C)n Pt(C6H5)(PH3)2 (n = 2-8, 10, 12). Although the final states for one- and two-photon transitions are not the same in these centrosymmetric molecules, the Z-scan studies reveal coincidences in one-photon absorption with features in the frequency dependencies of both real and imaginary parts of the cubic hyperpolarizability, as well as inflections in the frequency dependencies of the real part of gamma that correspond to resonances in the imaginary part of gamma. The theoretical studies suggest that the linear absorption spectra are dominated by X(1)A g --> n(1)B(3u) transitions, with the first state of B(3u) symmetry playing a steadily diminishing role upon oligoyne chain lengthening. The theoretical studies also predict a red-shift of two-photon absorption (TPA) profile with increasing conjugation length, and a significant enhancement on proceeding from the shortest to the longest chromophore, trends that are observed experimentally. The experimental low-energy TPA maxima for these complexes can be approximated by a simple Gaussian profile. The sp-carbon chain-length dependence of linear and nonlinear absorption maxima enable an estimate (neglecting saturation) of 660 and 1000 nm for the infinite carbon chain, carbyne.

Highly polarized luminescence from aligned conjugated polymer electrospun nanofibers

In this contribution we show highly polarized photoluminescence (PL) from aligned polyethyleneoxide: polyphenylenevinylene derivative composite nanofibers. We demonstrate PL polarization ratios (parallel to perpendicular) greater than 13. This ratio is further increased (up to ∼25) by stretching the nanofibers. Stretching also results in an increase in conjugation length, fiber density, and PL lifetime. We argue that the effect of stretching is equivalent to applying a permanent and strong pressure. Our results open up the possibility for new optoelectronic devices and fundamental science studies based on polymer nanofibers.