Sequence Effects in Donor–Acceptor Oligomeric Semiconductors Comprising Benzothiadiazole and Phenylenevinylene Monomers

(E)-7-(4-(Diphenylamino)styryl)benzo[c][1,2,5]thiadiazole-4-carbonitrile

Donor-acceptor-donor (D-A-A) structures with 2,1,3-benzothiadiazole as an internal acceptor and the 4(7)-cyanogroup as anchor acceptor have been investigated for various photovoltaic applications such as dye-sensitized solar cells (DSSCs) and organic light emitting diodes (OLEDs). They are usually obtained by cyanation of 4(7)-bromo-2,1,3-benzothiadiazoles. In this communication, the reaction of (E)-4-(2-(7-bromobenzo[c][1,2,5]thiadiazol-4-yl)vinyl)-N,N-diphenylaniline with cyanating agents was studied and it was shown that the best yield of (E)-7-(4-(diphenylamino)styryl)benzo[c][1,2,5]thiadiazole-4-carbonitrile was achieved by heating with zinc cyanide in NMP at 120 °C in the presence of tetrakis(triphenylphosphine)palladium (0). The structure of newly synthesized compound was established by means of an elemental analysis, high resolution mass-spectrometry, 1H, 13C-NMR, IR, UV spectroscopy and mass-spectrometry.

A Novel Donor-Acceptor Thiophene-Containing Oligomer Comprising Dibenzothiophene-S,S-dioxide Units for Solution-Processable Organic Field Effect Transistor

A π-conjugated thiophene-containing oligomer with a D-A-D-A-D (D: donor, A: acceptor) architecture, namely, 2,6-bis{[4-(7-n-hexylthiophen-2-yl)thiophen-2-yl]-(dibenzothiophene-5,5-dioxide-3,3΄-diyl)}-bis((2-ethyl-hexyl)oxy)benzo[1,2-b:4,5-b’]dithiophen (BDT(DBTOTTH)₂), was synthesized by Stille coupling reactions. There are obvious shifts in the Ultraviolet-visible (UV-vis) and photoluminescence (PL) spectra of the thin film relative to its solution, indicating the existence of the π-π stacking in the solid state of the oligomer BDT(DBTOTTH)₂. The optical band gap of the oligomer determined from its absorption onset in UV-Vis spectra is 2.25 eV. It agrees with the value of 2.29 eV determined from the cyclic voltammetry (CV) measurement. Its highest occupied and lowest unoccupied molecular orbital (HOMO/LUMO) energy levels, which were calculated from its onset of oxidation and reduction waves in CV curve, are −5.51 and −3.22 eV, respectively. The oligomer is a P-type semiconductor material with a good thermal stability and solubility, which can be used to fabricate organic field effect transistors (OFETs) by the spin coating technique. The OFET with n-octadecanylltrichlorosilane (OTS)-modified SiO₂ dielectric layer exhibited a mobility of 1.6 × 10⁻³ cm²/Vs.

Selective solvent conditions influence sequence development and supramolecular assembly in step-growth copolymerization

Sequence control in synthetic copolymers remains a tantalizing objective in polymer science due to the influence of sequence on material properties and self-organization. A greater understanding of sequence development throughout the polymerization process will aid the design of simple, generalizable methods to control sequence and tune supramolecular assembly. In previous simulations of solution-based step-growth copolymerizations, we have shown that weak, non-bonding attractions between monomers of the same type can produce a microphase separation among the lengthening nascent oligomers and thereby alter sequence. This work explores the phenomenon further, examining how effective attractive interactions, mediated by a solvent selective for one of the reacting species, impact the development of sequence and the supramolecular assembly in a simple A-B copolymerization. We find that as the effective attractions between monomers increase, an emergent self-organization of the reactants causes a shift in reaction kinetics and sequence development. When the solvent-mediated interactions are selective enough, the simple mixture of A and B monomers oligomerize and self-assemble into structures characteristic of amphiphilic copolymers. The composition and morphology of these structures and the sequences of their chains are sensitive to the relative balance of affinities between the comonomer species. Our results demonstrate the impact of differing A-B monomer-solvent affinities on sequence development in solution-based copolymerizations and are of consequence to the informed design of synthetic methods for sequence controlled amphiphilic copolymers and their aggregates.

Mechanistic Insights into the Pressure-Induced Polymerization of Aryl/Perfluoroaryl Co-Crystals

Recently discovered diamond nanothreads offer a stiff, sp3-hybridized backbone unachievable in conventional polymer synthesis that is formed through the solid-state pressure-induced polymerization of simple aromatics. This method enables monomeric A-B...

Unraveling Sequence Effect on Glass Transition Temperatures of Discrete Unconjugated Oligomers

Sequence plays a critical role in enabling unique properties and functions of natural biomolecules, which has promoted the rapid advancement of synthetic sequence-defined polymers in recent decades. Particularly, investigation of short chain sequence-defined oligomers (also called discrete oligomers) on their properties has become a hot topic. However, most studies have focused on discrete oligomers with conjugated structures. In contrast, unconjugated oligomers remain relatively underexplored. In this study, three pairs of discrete oligomers with the same composition but different sequence for each pair are employed for investigating their glass transition temperatures (T_g s). The resultant T_g s of sequenced oligomers in each pair are found to be significantly different (up to 11.6 °C), attributable to variations in molecular packing as demonstrated by molecular dynamics and density function theory simulations. Intermolecular interaction is demonstrated to have less impact on T_g s than intramolecular interaction. The mechanistic investigation into two model dimers suggests that monomer sequence caused the difference in intramolecular rotational flexibility of the sequenced oligomers. In addition, despite having different monomer sequence and T_g s, the oligomers have very similar solubility parameters, which supports their potential use as effective oligomeric plasticizers to tune the T_g s of bulk polymer materials.

Electronically Governed ROMP: Expanding Sequence Control for Donor–Acceptor Conjugated Polymers

Controlling the primary sequence of synthetic polymers remains a grand challenge in chemistry. A variety of methods that exert control over monomer sequence have been realized wherein differential reactivity, pre-organization, and stimuli-response have been key factors in programming sequence. Whereas much has been established in nonconjugated systems, π-extended frameworks remain systems wherein subtle structural changes influence bulk properties. The recent introduction of electronically biased ring-opening metathesis polymerization (ROMP) extends the repertoire of feasible approaches to prescribe donor–acceptor sequences in conjugated polymers, by enabling a system to achieve both low dispersity and controlled polymer sequences. Herein, we discuss recent advances in obtaining well-defined (i.e., low dispersity) polymers featuring donor–acceptor sequence control, and present our design of an electronically ambiguous (4-methoxy-1-(2-ethylhexyloxy) and benzothiadiazole-(donor–acceptor-)based [2.2]paracyclophanediene monomer that undergoes electronically dictated ROMP. The resultant donor–acceptor polymers were well-defined (Đ = 1.2, Mn > 20 k) and exhibited lower energy excitation and emission in comparison to ‘sequence-ill-defined’ polymers. Electronically driven ROMP expands on prior synthetic methods to attain sequence control, while providing a promising platform for further interrogation of polymer sequence and resultant properties.

1 Introduction to Sequence Control

2 Sequence Control in Polymers

3 Multistep-Synthesis-Driven Sequence Control

4 Catalyst-Dictated Sequence Control

5 Electronically Governed Sequence Control

6 Conclusions

Well-Defined Conjugated Macromolecules Based on Oligo(Arylene Ethynylene)s in Sensing

Macromolecules with well-defined structures in terms of molar mass and monomer sequence became interesting building blocks for modern materials. The precision of the macromolecular structure makes fine-tuning of the properties of resulting materials possible. Conjugated macromolecules exhibit excellent optoelectronic properties that make them exceptional candidates for sensor construction. The importance of chain length and monomer sequence is particularly important in conjugated systems. The oligomer length, monomer sequence, and structural modification often influence the energy bang gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the molecules that reflect in their properties. Moreover, the supramolecular aggregation that is often observed in oligo-conjugated systems is usually strongly affected by even minor structural changes that are used for sensor designs. This review discusses the examples of well-defined conjugated macromolecules based on oligo(arylene ethynylene) skeleton used for sensor applications. Here, exclusively examples of uniform macromolecules are summarized. The sensing mechanisms and importance of uniformity of structure are deliberated.

Charge Transport in Sequence-Defined Conjugated Oligomers

A major challenge in synthetic polymers lies in understanding how primary monomer sequence affects materials properties. In this work, we show that charge transport in single molecule junctions of conjugated oligomers critically depends on the primary sequence of monomers. A series of sequence-defined oligomers ranging from two to seven units was synthesized by an iterative approach based on the van Leusen reaction, providing conjugated oligomers with backbones consisting of para-linked phenylenes connected to oxazole, imidazole, or nitro-substituted pyrrole. The charge transport properties of these materials were characterized using a scanning tunneling microscope-break junction (STM-BJ) technique, thereby enabling direct measurement of molecular conductance for sequence-defined dimers, trimers, pentamers, and a heptamer. Our results show that oligomers with specific monomer sequences exhibit unexpected and distinct charge transport pathways that enhance molecular conductance more than 10-fold. A systematic analysis using monomer substitution patterns established that sequence-defined pentamers containing imidazole or pyrrole groups in specific locations provide molecular attachment points on the backbone to the gold electrodes, thereby giving rise to multiple conductance pathways. These findings reveal the subtle but important role of molecular structure including steric hindrance and directionality of heterocycles in determining charge transport in these molecular junctions. This work brings new understanding for designing molecular electronic components.

Poly(arylenevinylene)s through Ring-Opening Metathesis Polymerization of an Unsymmetrical Donor-Acceptor Cyclophane

Reported are well-defined donor-acceptor alternating copolymers prepared using ring-opening metathesis polymerization (ROMP). Unsymmetrical cyclophanedienes comprising electron-donating (4-methoxy-1-(2-ethylhexyl)oxy)benzene (MEH) and electron-accepting benzothiadiazole (BT) rings were synthesized from the corresponding [3.3]dithiaparacyclophanes. ROMP of the strained unsymmetrical and "electronically-ambiguous" cyclophanedienes proceeded in a controlled manner in the presence of either Hoveyda-Grubbs II or Grubbs II initiator in wake of both steric and electronic encumbrance. The resulting polymers, comprising alternating BT and MEH-PPV units, are achieved in molecular weights exceeding 20k with Đ values ranging from 1.1-1.4. The living nature of the polymerization is verified through the formation of rod-coil and rod-rod block copolymers. Our strategy to develop previously unrealized polymers from functional building blocks featuring a locked-in D-A unit is significant in a field striving to achieve well-defined and sequence-specific materials.

Sequence-Controlled Polymers Through Entropy-Driven Ring-Opening Metathesis Polymerization: Theory, Molecular Weight Control, and Monomer Design

The bulk properties of a copolymer are directly affected by monomer sequence, yet efficient, scalable, and controllable syntheses of sequenced copolymers remain a defining challenge in polymer science. We have previously demonstrated, using polymers prepared by a step-growth synthesis, that hydrolytic degradation of poly(lactic- co-glycolic acid)s is dramatically affected by sequence. While much was learned, the step-growth mechanism gave no molecular weight control, unpredictable yields, and meager scalability. Herein, we describe the synthesis of closely related sequenced polyesters prepared by entropy-driven ring-opening metathesis polymerization (ED-ROMP) of strainless macromonomers with imbedded monomer sequences of lactic, glycolic, 6-hydroxy hexanoic, and syringic acids. The incorporation of ethylene glycol and metathesis linkers facilitated synthesis and provided the olefin functionality needed for ED-ROMP. Ring-closing to prepare the cyclic macromonomers was demonstrated using both ring-closing metathesis and macrolactonization reactions. Polymerization produced macromolecules with controlled molecular weights on a multigram scale. To further enhance molecular weight control, the macromonomers were prepared with cis-olefins in the metathesis-active segment. Under these selectivity-enhanced (SEED-ROMP) conditions, first-order kinetics and narrow dispersities were observed and the effect of catalyst initiation rate on the polymerization was investigated. Enhanced living character was further demonstrated through the preparation of block copolymers. Computational analysis suggested that the enhanced polymerization kinetics were due to the cis-macrocyclic olefin being less flexible and having a larger population of metathesis-reactive conformers. Although used for polyesters in this investigation, SEED-ROMP represents a general method for incorporation of sequenced segments into molecular weight-controlled polymers.

Property impact of common linker segments in sequence-controlled polyesters

Linker segments in sequence controlled polyester backbones significantly affect thermal, mechanical and degradation properties.