Short Conjugation Length Research Articles

Surfactant Composition Dependent Evolution of Electronic/Structural Properties of P3HT Nanoparticle Colloids Synthesized via Mini-Emulsion Technique

: This study investigates the influence of surfactant (sodium dodecyl sulfate, SDS) concentration on the size and optoelectronic properties of poly(3-hexylthiophene) (P3HT) nanoparticles (NPs) synthesized via the mini-emulsion technique. P3HT NPs were fabricated with varying SDS concentrations. The size dependent optoelectronic properties, including the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels, were analyzed through photoelectron spectroscopy in air, UV-Vis absorption spectroscopy and cyclic voltammetry. A clear trend of decreasing NP size with increasing SDS concentration was observed. Higher SDS levels resulted in smaller NPs with enhanced pi-pi stacking and shorter conjugation lengths. This structural change led to a noticeable shift in HOMO and LUMO levels, indicating a direct correlation between surfactant concentration and the electronic properties of the P3HT NPs. The absorption spectra revealed a decrease in the A0-0/A0-1 ratio with smaller NP size, suggesting a transition towards more pronounced H-aggregate characteristics. As the size of the P3HT NPs decreases, there is a notable increase in H-aggregate formation. This increase can be attributed to enhanced interchain interactions between the polymer chains. In smaller NPs, the surface area-to-volume ratio is higher, leading to more significant interactions between adjacent polymer chains. These interactions promote the formation of H-aggregates, characterized by their interchain pi-pi stacking, which differs from the intrachain coupling observed in J-aggregates.

Anthracene and Pyrene Derivatives as Blue Fluorophores: Synthesis, Characterization and Photophysical Properties

Ant hracene and pyrene derivatives have been widely studied as emitting materials in optoelectronic devices due to their excellent fluorescence properties. In the present study, two novel blue fluorophores from anthracene and pyrene (PAP and PTATP) were successfully synthesized using a Suzuki-cross coupling reaction and their photophysical properties were elucidated. PTATP with an extended p-conjugation system had a fluorescence emission maximum in green-blue region at 475 nm, whereas PAP, with a shorter conjugation length, emitted pure-blue emission peak at 442 nm. The study of absorption and emission spectra in various solvents found that both compounds produced solvent-independent spectra due to their non-polar molecules. In addition, the aggregation-caused quenching phenomenon was observed, when the emission spectra were recorded in high water fraction solvent.

Conformation control of triplet state diffusion in platinum containing polyfluorene copolymers

AbstractThe spectral diffusion of singlet and triplet excitons in 9,9‐dioctylfluorene‐based conjugated copolymers is investigated using photoluminescence spectroscopy at both low (5 K) and room temperature (300 K). Inclusion of a N,N‐diphenyl‐4‐(pyridin‐2‐yl)aniline moiety into the polymer backbone allows subsequent cyclometalation with platinum acetylacetonate to increase the spin‐orbit coupling and yield radiative decay from the triplet state. For suitably low fractions (≤5%) of bulky ligand inclusion, cyclometalated or not, the resulting longer sequences of fluorene units are able to adopt the chain‐extended β‐phase conformation. Comparison between the phosphorescence spectral diffusion in glassy‐ and β‐phase Pt‐copolymer samples provide insight into the triplet exciton transfer in more‐ or less‐disordered conjugated polymer films. It is found in the glassy‐phase samples with shorter conjugation lengths that the triplet exciton relaxation becomes frustrated at low temperature due to a freezing out of the thermally activated hops required to move from one conjugated segment to another. In contrast, for films containing β‐phase chain segments, with increased conjugation lengths, this frustration is lifted as more hopping sites remain accessible through intra‐segment motion. This work demonstrates controlled use of changes in molecular conformation to optimize triplet diffusion properties in a member of the widely deployed fluorene‐based conjugated copolymers.

Ultrapure deep-blue aggregation-induced emission and thermally activated delayed fluorescence emitters for efficient OLEDs with CIEy < 0.1 and low efficiency roll-offs

Twisted rigid structures combined with short conjugation length building blocks provide an effective molecular design strategy for deep blue (CIEy < 0.1) aggregation-induced emission and thermally activated delayed fluorescence emitters.

The Red Phase of Polydiacetylene in Langmuir–Blodgett Film: The Short Conjugation Length of the Polymer Chain

The Red Phase of Polydiacetylene in Langmuir–Blodgett Film: The Short Conjugation Length of the Polymer Chain

Carbon-Bridged Oligo(phenylene vinylene)s: A de Novo Designed, Flat, Rigid, and Stable π-Conjugated System.

The modern history of conducting organic systems started with a fortuitous error in 1967 on acetylene polymerization, followed by a rational discovery in 1976 on the effects of doping that generates a polaron and, hence, dramatically increases conductivity. Not unexpectedly, however, the prototypical polyacetylene suffers many problems, including C-C single bond rotation, short effective conjugation length, radiationless deactivation, and instability of the polarons. Several strategies have been put in place to solve these problems. An early approach relied on partial rigidification of the polyene structure by conversion into polymers with thiophene, pyrrole, and benzene linkages. An oligo(phenylene vinylene) (OPV) is an all-carbon analogue of polyacetylene, where every other diene unit in the polyene chain is converted to a benzene unit, still leaving many C-C single bonds freely rotating in the molecule. We considered adding additional carbon bridges to rigidify the OPV skeleton entirely to create a carbon-bridged OPV (COPV). Making such a compound was an obvious challenge. This Account describes the authors' efforts to design and synthesize a series of COPV molecules, where the benzene rings in OPV are bridged by sp3 carbon atoms to form a bicyclo[3.3.0]octatriene framework bearing a tetrasubstituted olefin at the ring fusion. This olefinic bond is so strained that it resists further deformation or conversion to sp3 centers, and hence, it is chemically stable despite the strain. The sp3 carbon bridges can bear organic side chains that hinder intermolecular interactions, rendering the excited states stable and long-lived even in the solid state. They also increase solubility, a common problem among rigid molecular systems. With these structural features, the COPV molecules were found to be well behaved both at a single-molecule level and as a bulk material. We reported in 2009 a method for the synthesis of COPVs and have, since then, reported their structures and physicochemical properties, including basic photophysical properties of neutral and charged derivatives, thermal and photostability, and fast electron transfer. These properties have rendered the COPV molecules useful for electronic and photonic research, for example, lasers, solar cells, and molecular wire applications. Noteworthy discoveries in the area connecting chemistry and physics include inelastic tunneling and long-range resonance tunneling at ambient temperature, which were previously observed only for organic molecular wires placed under cryogenic conditions. Given the ready availability of the COPV skeleton bearing a wide variety of substituents, this class of molecules will serve as versatile building blocks for fundamental and applied research on physicochemical and materials chemistry.

3.2: Singlet‐Triplet Splitting Energy Management via Acceptor Substitution: Complanation Molecular Design for Deep‐Blue Thermally Activated Delayed Fluorescent Organic Light‐Emitting Diodes

Hardly reached balance between weak intramolecular‐charge‐transfer (ICT) and small singlet‐triplet splitting energy (ΔEST) for reverse intersystem crossing (RISC) from non‐emissive triplet state to radiative singlet state impedes the realization of deep‐blue thermally activated delayed fluorescent (TADF) materials. By discarding twisted‐ICT framework for flattened molecular backbone and introducing strong acceptor possessing n‐π* transition character, blue‐shifted color, large radiative rate (kF) and small ΔEST are achieved simultaneously. Six molecules with 9,9‐dimethyl‐10‐phenyl‐9,10‐dihydroacridine (i‐DMAc) donor were synthesized and investigated. Coinciding with time‐dependent density function theory (TD‐DFT), the reduced dihedral angles between donor (D) and acceptor (A) weaken ICT from dispersed charge density and capacitate large kF from increased FMOs overlap. Despite the separated highest occupied (HOMO) and lowest unoccupied molecular orbital (LUMO) population, the intercalation of phenyl‐bridges between D‐A increases kF but significantly lowers the locally triplet excited state, indicating small HOMO and LUMO overlap is not sufficient but necessary condition for reduced ΔEST. Integrating short conjugation length and carbonyl or 1,3,5‐triazine acceptors into the complanation molecules, deep‐blue TADF‐OLEDs demonstrated maximum external quantum efficiencies of 11.5% and 10.9% with Commission Internationale de l'Eclairage (CIE) coordinates of (0.16, 0.09) and (0.15, 0.11), respectively, which is quite close to the stringent National Television System Committee (NTSC) blue standard.

Molecular weight dependent structure and charge transport in MAPLE‐deposited poly(3‐hexylthiophene) thin films

ABSTRACTIn this work, poly(3‐hexylthiophene) (P3HT) films prepared using the matrix‐assisted pulsed laser evaporation (MAPLE) technique are shown to possess morphological structures that are dependent on molecular weight (MW). Specifically, the structures of low MW samples of MAPLE‐deposited film are composed of crystallites/aggregates embedded within highly disordered environments, whereas those of high MW samples are composed of aggregated domains connected by long polymer chains. Additionally, the crystallite size along the side‐chain (100) direction decreases, whereas the conjugation length increases with increasing molecular weight. This is qualitatively similar to the structure of spin‐cast films, though the MAPLE‐deposited films are more disordered. In‐plane carrier mobilities in the MAPLE‐deposited samples increase with MW, consistent with the notion that longer chains bridge adjacent aggregated domains thereby facilitating more effective charge transport. The carrier mobilities in the MAPLE‐deposited simples are consistently lower than those in the solvent‐cast samples for all molecular weights, consistent with the shorter conjugation length in samples prepared by this deposition technique. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 652–662

Factors affecting tacticity and aggregation of P3HT polymers in P3HT:PCBM blends

Optimising the performance of P3HT:PCBM (polymer–fullerene, P3HT – poly(3-hexylthiophene), PCBM – [6, 6]-phenyl-C61-butyric acid methyl ester) blends for solar cell applications is currently impeded by our lack of understanding of the effect of processing conditions on the resulting morphology of P3HT and hence its performance. The literature has generally overlooked studying the link between morphology and tacticity, which, in turn, affects charge mobility. The conformational change from trans to cis between adjacent thiophene rings in the P3HT backbone determines the tacticity of the polymer molecules. We used all-atom molecular dynamics simulations at 400 K, matching typical experimental processing temperatures, to unravel the molecular-level configuration changes that P3HT polymers undergo during processing, focusing here on temperature and shear. The effect of shear was included since it improves charge transport and polymer aggregation in polymer–fullerene blends. Our simulations implicate the side chains in initiating the polymer backbone twisting and curvature that we observe and, consequently, altering the trans-to-cis ratio (and hence mobility). Complementary density functional theory (DFT) calculations indicate that such tacticity conversions follow a rugged energy surface. The simulations also help explain the origin of disorder in the aggregation of polymer chains and shorter conjugation length, all of which are observed in experiments. Each individual polymer molecule’s response to local thermodynamic changes and the kinetics of neighbouring polymers guides its morphological evolution. Thus, we put forward the prominent role of tacticity in determining both kinetic and thermodynamic properties of this prototypical polymer–fullerene blend. As a result, we uncover two types of processing condition effects, one that directly affects polymer conformation (e.g. high temperature), and the other that directly changes the alignment between polymers (e.g. shear).

Influence of the nature of the anchoring group on electron injection processes at dye-titania interfaces.

We report (time-dependent) density functional theory calculations characterizing the changes in the electronic and optical properties of oligothiophene dyes when grafted on a titania surface via a carboxylic acid or catechol moiety as anchoring group, in relation to their use in dye-sensitized solar cells. The broadening of the LUMO level of the compounds upon adsorption has been extracted from the computed electronic structures and used to estimate electron injection times into the conduction band of the oxide. The strongly coupled carboxylic-containing dyes lead to faster electron injection times compared to catechol-substituted dyes. This difference is ascribed to the electron-donating character of the catechol moiety that polarizes the dye LUMO away from the dye@titania interface. The absorption spectra simulated at the TD-DFT level indicate that the grafted carboxylic-thiophene dyes undergo an indirect injection mechanism (type I) in which an intramolecular excitation is created before the charge is transferred to titania. In contrast, catechol dyes with a short conjugation length for the thiophene backbone are type II sensitizers exhibiting a direct injection mechanism leading to a direct photoexcitation from the dye HOMO to the titania conduction band. A mixed character prevails for the injection in the case of catechol dyes containing a longer oligothiophene chain.

Singlet–Triplet Splitting Energy Management via Acceptor Substitution: Complanation Molecular Design for Deep‐Blue Thermally Activated Delayed Fluorescence Emitters and Organic Light‐Emitting Diodes Application

A barely reached balance between weak intramolecular‐charge‐transfer (ICT) and small singlet–triplet splitting energy (ΔEST) for reverse intersystem crossing from non‐emissive triplet state to radiative singlet state impedes the realization of deep‐blue thermally activated delayed fluorescence (TADF) materials. By discarding the twisted‐ICT framework for a flattened molecular backbone and introducing a strong acceptor possessing n–π* transition character, hypsochromic color, a large radiative rate (kF), and small ΔEST are achieved simultaneously. Six molecules with a 9,9‐dimethyl‐10‐phenyl‐9,10‐dihydroacridine (i‐DMAc) donor are synthesized and investigated. Coinciding with time‐dependent density functional theory, the reduced dihedral angles between donor (D) and acceptor (A) weaken ICT from dispersed charge density and enable a large kF from increased frontier molecular orbitals overlap. Despite the separated highest occupied (HOMO) and lowest unoccupied molecular orbital (LUMO) population, the intercalation of phenyl bridges between D–A increases kF but significantly lowers the local triplet excited state, indicating small HOMO and LUMO overlap is not a sufficient, but necessary condition for reduced ΔEST. Integrating short conjugation length and carbonyl or triazine acceptors into the complanation molecules, deep‐blue TADF organic light‐emitting diodes demonstrate maximum external quantum efficiencies of 11.5% and 10.9% with Commission Internationale de l'Eclairage coordinates of (0.16, 0.09) and (0.15, 0.11), respectively, which is quite close to the stringent National Television System Committee blue standard.

D-A conjugated polymers containing substituted thiophene, 1,3,4-oxadiazole and non-conjugation linkers: Synthesis and study of optical and electrochemical properties

In this communication, we report synthesis and characterization of new D-A conjugated polymers (P1-P3) consisting of electron-donating (D) 3,4-didodecyloxythiophene, electron-accepting (A) 1,3,4-oxadiazole unit and non-conjugation linkers. The conjugated segment in P1-P3 contains only five aromatic rings resulting in short conjugation length, but has an alternate D-A arrangement which significantly enhances the intramolecular charge transfer (ICT) interaction within the segment. As a result, these polymers exhibited low optical band gap in the range 2.51–2.76 eV. Fluorescence emission studies revealed that the polymer thin films emit intense blue light with emission maxima in the wavelength rage 430–480 nm. All three polymers undergo both oxidation and reduction processes under electrochemical conditions. Further, these polymers (P1–P3) exhibit low-lying HOMO and LUMO levels as a result of D-A structure of the conjugated segment. Polymer light-emitting devices were fabricated using these polymers as emissive layer with a device configuration of ITO/MoO 3/polymer/LiF/Al. The test device based on P2 emitted blue light with a low threshold voltage of 5 V. Z-scan studies reveal that the polymers exhibit a strong optical limiting behavior. The value of the nonlinear absorption coefficient (β) of polymers is of the order 10 −11m/W which indicates that these materials may be accomplished for fabricating optical limiters.

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.

Synthesis of π-Conjugated Polymers Containing Dibenzosilepin Moieties with Pentacoordinate Silicon

Abstract Benzo[h]quinolyldibenzo[b,f]silepin containing pentacoordinate silicon was incorporated into π-conjugated polymers. Substituent effects were investigated from UV–vis absorption and photoluminescence spectra. The extension of π-conjugation involving pentacoordinate silicon was suggested from the characteristic absorption and emission bands through the polymer chain at meta-positions toward silicon in the silepin rings. The polymers containing the fluorinated silicon showed bathochromic shift compared to that from the methylated silicon-containing polymers. Polymerization at para-positions toward silicon resulted in the formation of short conjugation length. Finally, the emission band from the charge-transfer transition was observed in photoluminescence spectra of the fluorinated polymer.

Conductivity and long term stability of polypyrrole poly(styrene-co-methacrylic acid) core–shell particles at different polypyrrole loadings

Conductive core–shell particles were obtained by chemical polymerization of pyrrole over monodisperse poly(styrene-co-methacrylic acid) particles. The surface composition has been studied by elemental analysis, Fourier Transform Infrared Spectroscopy (FTIR), X-ray Photoelectron Spectroscopy (XPS), scanning electron microscopy and transmission electron microscopy techniques. XPS, elemental analysis and FTIR results allowed determining doping level and conjugation length of the polypyrrole (PPy) chain deposited on the latex surface. It is shown that the chain conjugation length, and not the doping level, is the principal factor that influences the conductivity. Samples with low PPy loading have short conjugation length and so low conductivity independently of their doping level. The experimental conductivity decay with time has been analysed following the electron hopping model, from this model the characteristic time (τ) of the conductivity degradation process has been determined.

Carotenoid–protein interaction alters the S1 energy of hydroxyechinenone in the Orange Carotenoid Protein

The Orange Carotenoid Protein (OCP) is a photoactive water soluble protein that is crucial for photoprotection in cyanobacteria. When activated by blue-green light, it triggers quenching of phycobilisome fluorescence and regulates energy flow from the phycobilisome to the reaction center. The OCP contains a single pigment, the carotenoid 3′-hydroxyechinenone (hECN). Binding to the OCP causes a conformational change in hECN leading to an extension of its effective conjugation length. We have determined the S1 energy of hECN in organic solvent and compared it with the S1 energy of hECN bound to the OCP. In methanol and n-hexane, hECN has an S1 energy of 14,300cm−1, slightly higher than carotenoids with shorter conjugation lengths such as zeaxanthin or β-carotene; this is consistent with the proposal that the presence of the conjugated carbonyl group in hECN increases its S1 energy. The S1 energy of hECN in organic solvent is independent of solvent polarity. Upon binding to the OCP, the S1 energy of hECN is further increased to 14,700cm−1, underscoring the importance of protein binding which twists the conjugated carbonyl group into s-trans conformation and enhances the effect of the carbonyl group. Activated OCP, however, has an S1 energy of 14,000cm−1, indicating that significant changes in the vicinity of the conjugated carbonyl group occur upon activation.

Fluorene‐Containing Organic Photosensitizers for Dye‐Sensitized Solar Cells

AbstractSeven new donor–spacer–acceptor metal‐free organic dyes containing a fluorene and/or thiophene unit as a conjugated spacer are prepared and their physicochemical properties as well as photovoltaic performance are studied. The electron transfer from donor to acceptor upon excitation of these dyes is sensitive to the conjugated spacers. If a stronger donor moiety such as (ethylenedioxy)thiophene is used as a spacer, such as in F‐E1 dye, the dye molecule has a better charge separation and therefore high efficiency. Furthermore, for the same donor and acceptor, fluorene is a better conjugated spacer than the bithiophene moiety because of its nonplanar conformation, which avoids the electron–hole recombination of dye upon photoexcitation. Short molecules with planar conformation aggregate easily, and the corresponding device reveals low open‐circuit voltage Voc. On the other hand, dye molecules with long alkyl side chains (large size) adsorb less on the TiO2 electrode, which results in a smaller short‐circuit current density Jsc. The difference in the efficiency of these dyes is small except for F‐1 (owing to its short conjugation length), and electron density difference map analyses reveal that photoexcitation transfer of electron density from the donating moieties toward the acceptor/anchoring units occurs for all molecules. The small difference in the efficiency is attributed partly to the degree of effective charge transfer upon excitation, not totally to the absorption profile of the dye molecules. Furthermore, time‐dependent DFT simulated absorption spectra are in excellent agreement with experimental spectra when the dyes are in their protonated states in THF solution. The transition probability calculated with the electron density difference analyses combined with the absorption data and photovoltaic performance provide useful information for designing organic sensitizers for dye‐sensitized solar cells.

The structural, electronic, and optical properties of ladder-type polyheterofluorenes: a theoretical study

The ladder-type polyheterofluorenes were investigated theoretically by using density functional theory (DFT) to reveal their optical and electronic properties for applications in organic optoelectronic devices. The incorporation of heteroatoms (B, Si, Ge, N, P, O, and S) into the ladder-type highly fused polyfluorene backbone can influence and modify the optoelectronic properties significantly. The functionalization on the heteroatoms allows for facile derivation and incorporation of substitutes to further tune the properties. Small geometry variations between the ground, anionic/cationic, the first excited singlet and triplet states were observed due to the very rigid ladder-type coplanar backbone. Ladder-type polycarbazole was predicted to have the highest HOMO and LUMO energy levels, polyphosphafluorene oxide have the lowest HOMO energy level, polyborafluorene have the lowest LUMO energy level and bandgap, and polysulfafluorene has the highest bandgap and triplet energy. The ladder-type carbazole and borafluorene show the highest hole and electron injection abilities respectively; while sulfafluorene has the highest electron transfer rate. Most ladder-type heterofluorenes show bipolar charge transport character suggested by the reorganization energy. All of them have significantly short effective conjugation length in comparison with linear conjugated polymers. Their absorption and emission spectra were also simulated and discussed. The diversified electronic and optical properties of the ladder-type polyheterofluorenes with the different incorporated heteroatom and the substituent on it indicate their broad potential applications in organoelectronics.

Novel redox‐active polycarbazole‐functionalized polycatechol network films produced by controlled electropolymerization

AbstractA novel precursor, 1,2‐bis[6‐(9H‐carbazol‐9‐yl)hexyloxy] benzene (BCHB), was successfully synthesized. Its polycarbazole‐functionalized polycatechol network films, poly{1,2‐bis[6‐(9H‐carbazol‐9‐yl)hexyloxy] benzene} (PBCHB), with good redox activity were formed by the direct anodic oxidation of BCHB in CH2Cl2 and boron trifluoride diethyl etherate binary solvent solution. Ultraviolet–visible spectroscopy, Fourier transform infrared spectroscopy, 1H‐NMR, and matrix‐assisted laser desorption ionization–time of flight mass spectrometry were used to characterize the polymers. The results indicate that the network polymers could be synthesized electrochemically with different polymerized units by controlled electropolymerization. The PBCHB films prepared at low potential were oligomers with short conjugation lengths and were soluble in common organic solvents, whereas the polymers with long conjugation lengths and hyperbranched network structures obtained at high potential were insoluble. The electrosynthesized polymers exhibited blue emission maxima around 450 nm and were much more redshifted than their monomer. The emissions were also brighter; this indicated the polymers are potential good blue‐light emitters. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Parent o-Phenylene Oligomers: Synthesis, Conformational Behavior, and Characterization

The o-phenylenes are an unusual class of conjugated polymer, defined largely by substantial steric twisting along their backbones. Consequently, they exhibit limited conjugation but also interesting conformational behavior: they have been shown to adopt well-defined helical secondary structures, both in the solid state and in solution. While several examples of functionalized o-phenylene oligomers have been reported, most of the basic properties of the parent compounds are unknown. Here we report the synthesis and characterization of the series of unsubstituted o-phenylene oligomers up to the octamer. Through a combination of NMR spectroscopy, including dynamic NMR (EXSY), and computational chemistry, we have found that these compounds adopt compact helical conformations in solution with three repeat units per turn. Although formally conjugated, the oligomers have a very short effective conjugation length of necl ≈ 4 (based on UV–vis spectra), significantly shorter than most other conjugated systems. Also, unlike other (substituted) o-phenylenes, no hypochromicity is observed in their UV–vis spectra. The fluorescence spectra of the series exhibit a systematic blue shift with increasing length. We believe this unusual property results from increased steric congestion in the longer oligomers, which are therefore less able to accommodate structural relaxation in the excited state.