Homo- and Copolymers Based on Carbon-Bridged Oligo(p-phenylenevinylene)s for Efficient Fluorescence over the Entire Visible Region

Radical Stitching Polymerization and Its Alternating Copolymerization

Polymers possessing saturated fused polycycles in the main chain repeating unit have been underexplored despite their potential utility based on their expected properties such as high rigidity, chemical resistance, transparency, and thermal stability. In this regard, herein, we developed a radical stitching polymerization of styryl vinyl ketones for the synthesis of polyketones possessing saturated fused bicyclic repeating units. The polymerization proceeded smoothly with a high degree of stitching efficiency in a chain-growth manner under free radical conditions. This method was further extended to the alternating copolymerization of styryl vinyl ketones and 1-styryl-2-vinylbenzenes, representing the first alternating stitching copolymerization of two different monomers. The obtained polymers were found to show promising thermal properties and high transparency in the visible light region.

Circularly polarized luminescence and high photoluminescence quantum yields from rigid 5,10-dihydroindeno[2,1-a]indene and 2,2'-dialkoxy-1,1'-binaphthyl conjugates and copolymers

5,5,10,10-Tetramethyl-5,10-dihydroindeno[2,1-a]indene (COPV1(Me)) was installed into either the 3,3'- or 6,6'-positions of chiral 2,2'-dioctyloxy-1,1'-binaphthyl to afford 2 : 1 conjugates (monomeric compounds) and 1 : 1 copolymers. These compounds showed high photoluminescence quantum yields of >0.95 whilst also exhibiting circular dichroism (CD) and circularly polarized luminescence (CPL). The dissymmetry factors of CPL (gCPL) for the 3,3'- and 6,6'-monomeric compounds in THF were 6.6 × 10-4 and 3.3 × 10-4, respectively. The 3,3'-isomer has a higher g value than the 6,6'-isomer, which was attributed to the difference in the extent of π-conjugation and the angle between electronic and magnetic transition moments. The gCPL values of the 3,3'-linked and 6,6'-linked copolymers were 1.1 × 10-3 and 6.8 × 10-4, respectively. The structural rigidity of the COPV units is beneficial to achieve relatively high g values whilst maintaining a photoluminescence quantum yield that is close to unity by using a single type of fluorophore.

Iron-Catalyzed Tandem Cyclization of Diarylacetylene to a Strained 1,4-Dihydropentalene Framework for Narrow-Band-Gap Materials

Carbon bridging in a form of a strained 1,4-dihydropentalene framework is an effective strategy for flattening and stabilizing oligophenylenevinylene systems for the development of optoelectronic materials. However, efficient and flexible methods for making such a strained ring system are lacking. We report herein a mild and versatile synthetic access to the 1,4-dihydropentalene framework enabled by iron-catalyzed single-pot tandem cyclization of a diarylacetylene using FeCl₂ and PPh₃ as catalyst, magnesium/LiCl as a reductant, and 1,2-dichloropropane as a mild oxidant. The new annulation method features two iron-catalyzed transformations used in tandem, a reductive acetylenic carboferration and an oxidation-induced ring contraction of a ferracycle under mild oxidative conditions. The new method provides access not only to a variety of substituted indeno[2,1-a]indenes but also to their thiophene congeners, 4,9-dihydrobenzo[4,5]pentaleno[1,2-b]thiophene (CPTV) and 4,8-dihydropentaleno[1,2-b:4,5-b']dithiophenes (CTV). With its high highest occupied molecular orbital level and narrow optical gap, CTV serves as a donor unit in a narrow-band-gap non-fullerene acceptor, which shows absorption extending over 1000 nm in the film state, and has found use in a near-infrared photodetector device that exhibited an external quantum efficiency of 72.4% at 940 nm.

Long-wavelength visible to near infrared photoluminescence from carbon-bridged styrylstilbene and thiadiazole conjugates in organic and aqueous media

Donor–acceptor–donor conjugates composed of electron-donating carbon-bridged styrylstilbene (COPV2) and electron-accepting thiadiazole derivatives equipped with carbazolyl (Cz) terminators, Cz-COPV2-A-COPV2-Cz (A = benzothiadiazole (BTz), naphthobis(thiadiazole) (NTz), or benzobis(thiadiazole) (BBTz)), were newly synthesized and found to serve as efficient and stable long-wavelength photoluminescent dyes in organic and aqueous media. In particular, Cz-COPV2-BBTz-COPV2-Cz showed photoluminescence in the near infrared region (895–927 nm) with a photoluminescence quantum yield (PLQY) of up to 0.19 in cyclohexane and of 0.02–0.03 in THF/water mixtures. Its analogues with weaker acceptors, Cz-COPV2-BTz-COPV2-Cz and Cz-COPV2-NTz-COPV2-Cz, showed yellow to deep-red emission in organic solvents, with PLQYs of up to 0.71 in organic solvents and 0.45 in THF/water mixtures.

Efficient long-wavelength visible to NIR-emitting materials have been synthesized by use of rigid planar styrylstilbene as a donor component.

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.

Unique Substitution Effect at 5,5'-Positions of Fused Azobenzene-Boron Complexes with a N=N π-Conjugated System

A recent report illustrated superior optical properties, such as near-infrared emission, of polymers connected at the 4,4'-positions to a fused azobenzene-boron complex (BAz). In this study, it is initially demonstrated that further narrowing of the band gap can be realized through the substituent effect with bromine groups at the 5,5'-positions of BAz compared with those at the 4,4'-positions. From a series of mechanistic studies, perturbation of the energy levels was rationally explained by the difference in contributions of the inductive effect and the variable resonance effect, which was correlated with the degree of electron distribution of molecular orbitals at the substituent positions. Moreover, it was found that unique electronic states, such as delocalized HOMOs and LUMOs, should appear on the main chains of the BAz-containing copolymers with fluorene and bithiophene units, according to the optical and electrochemical data and theoretical calculations. By taking advantage of property tunability and the dramatically low LUMO energy level (near -4.0 eV) of the BAz unit, it can be said that BAz should be a conjugated building block favorable for building advanced optoelectronic devices.

Rhodium-Catalyzed Stitching Polymerization of 1,5-Hexadiynes and Related Oligoalkynes

A new mode of polymerization, rhodium-catalyzed stitching polymerization, has been developed for the synthesis of π-conjugated polymers with bridged repeating units from nonconjugated 1,5-hexadiynes containing both terminal and internal alkyne moieties as monomers. The polymerization proceeded smoothly with a high degree of stitching efficiency under mild conditions, and 1,5,9-decatriyne and 1,5,9,13-tetradecatetrayne monomers could also be employed. The present polymerization strategy would be particularly beneficial for the synthesis of polymers consisting of a repeating unit that is difficult to prepare as a stable monomer because it does not require the use of a preformed bridged π-conjugated monomer.

Palladium-Catalyzed Double Carbonylative Cyclization of Benzoins: Synthesis and Photoluminescence of Bis-Ester-Bridged Stilbenes

A palladium-catalyzed double carbonylative cyclization of benzoins has been developed, which realizes the synthesis of bis-ester-bridged stilbenes just in two steps from aldehydes. Thus, the obtained fully fused tetracyclic π-systems have a pyrano[3,2- b]pyran-2,6-dione (PPD) core on their center, showing two reversible reductions at low potentials. In addition, their photoluminescence properties are strikingly affected by the aromatic rings fused to the PPD core; bis- thieno-fused PPDs are found to be excellent fluorophores with quantum yields up to 0.98.

Synthesis and Evaluation of a 1,3a,6a-Triazapentalene (TAP)-Bonded System

A method of synthesizing a directly connected 1,3a,6a-triazapentalene (TAP) ring system as a linearly bonded aromatic system with a planar form was established. Various TAP-dimers and a 2-alkyl-TAP-trimer were synthesized and their fluorescence properties were evaluated. Although the direct connection of the TAP ring with other TAP rings did not affect the fluorescence properties in diluted solvent, TAP-dimers showed unique fluorescence properties derived from the aggregation state under highly concentrated conditions. In particular, TAP-dimer 5 f showed aggregation-induced emission in highly concentrated solution, and 5 b showed typical mechanochromic fluorescence in the solid state despite their compact molecular size.