Tsuyoshi Michinobu

A new class of organic donor-acceptor molecules with large third-order optical nonlinearities

Donor-acceptor molecules with 4-(dimethylamino)phenyl donor and 1,1,4,4-tetracyanobuta-1,3-diene acceptor moieties are readily prepared by short, high-yielding routes. The quite small chromophores are characterised by X-ray crystallography and feature intense intramolecular charge-transfer bands, substantial quinoid character in the donor rings, reversible electrochemical reductions and oxidations and powerful third-order optical nonlinearities.

Click-type reaction of aromatic polyamines for improvement of thermal and optoelectronic properties

A quantitative addition reaction between aromatic amino-substituted alkynes and tetracyanoethylene (TCNE), yielding donor-substituted tetracyanobutadiene chromophores, was for the first time employed as a click-type reaction to improve the thermal and optoelectronic properties of aromatic polyamines. The first reduction potentials or the LUMO levels of the aromatic polyamines were found to linearly decrease by the stepwise TCNE addition.

Highly efficient third-order optical nonlinearities in donor-substituted cyanoethynylethene molecules.

We investigated the third-order nonlinear optical properties of several donor-substituted cyanoethynylethene molecules in the zero-frequency limit. We observed nonlinearities that are extraordinarily large relative to the small molecular mass of these molecules and that are within a factor of 50 from the fundamental limit. At a wavelength of 1.5 microm, the rotational average of the third-order molecular polarizability is 53 +/- 13 x 10(-48) m5 V(-2) (3.8 x 10(-33) esu) for the best molecule, which corresponds to 1.7 x 10(-48) m5 V(-2) per delocalized electron. The high nonlinear efficiency of these molecules is due to the compact two-dimensional conjugated system and the effective donor-acceptor substitution patterns.

One-electron-reduced and -oxidized stages of donor-substituted 1,1,4,4-tetracyanobuta-1,3-dienes of different molecular architectures.

A series of monomeric and oligomeric donor-substituted 1,1,4,4-tetracyanobuta-1,3-dienes (TCBDs) with various topologies have been synthesized by means of thermal [2+2] cycloaddition between tetracyanoethylene (TCNE) and donor-substituted alkynes, followed by retro-electrocyclization. One-electron-reduced and -oxidized stages of the donor-substituted TCBDs were generated by chemical methods. The obtained radical anions and radical cations were studied by using electron paramagnetic resonance/electron nuclear double resonance (EPR/ENDOR) spectroscopy, supported by density functional theory (DFT) calculations. The extent of pi-electron delocalization in the paramagnetic species was investigated in terms of the EPR parameters. Despite favorable molecular orbital (MO) coefficients, the EPR results suggest that in radical anions the spin and charge are confined to the electron-withdrawing TCBD moieties on the hyperfine EPR timescale. The observed spin localization is presumably caused by an interplay between the nonplanarity of the studied pi systems, limited pi-electron conjugation, and very likely counterion effects. In radical cations, an analogous spin and charge localization confined to the electron-donating N,N-dialkylaniline moieties was found. In this case, an efficient electron delocalization is disabled by small MO coefficients at the joints between the donor and acceptor portions of the studied TCBDs.

Click synthesis of donor-acceptor-type aromatic polymers*

A high-yielding addition reaction between electron-rich alkynes and a small acceptor molecule, tetracyanoethylene (TCNE), was employed as a new click reaction to construct donor–acceptor chromophores in the polymer main chains and side chains. The donor–acceptor alternating conjugated polymers were prepared from the ferrocene-containing poly(aryleneethynylene)s in one step and atom-economic fashion. The energy levels and thermal properties of the aromatic polyamines substituted by electron-rich alkynes as a side chain can be tunable by the amount of the added TCNE. The resulting donor–acceptor-type polymers feature broad charge-transfer (CT) bands in the visible region, potent redox activities, and improved thermal properties.

Sequential double click reactions: a highly efficient post-functionalization method for optoelectronic polymers

A new post-functionalization method based on the Huisgens 1,3-dipolar cycloaddition between azides and alkynes, followed by the atom-economic addition reaction between electron-rich alkynes and tetracyanoethylene (TCNE) was developed to introduce the donor–acceptor chromophores into polystyrene derivatives in remarkably high yields.

Benzothiadiazole and its π-extended, heteroannulated derivatives: useful acceptor building blocks for high-performance donor–acceptor polymers in organic electronics

The past five years have witnessed significant achievements in the field of flexible, stretchable, and printable organic electronics, especially polymer-based organic photovoltaics (OPVs) and organic field-effect transistors (OFETs), which have become competitive to their inorganic counterparts. One of the main driving forces attributed to this remarkable progress is the rapid development of semiconducting polymeric materials. Therefore, the design and synthesis of new building blocks for efficient polymer semiconductors have attracted increasing attention from both the academic and industrial communities. This review attempts to critically summarize the recent advances with respect to the electron-deficient building blocks based on benzothiadiazole and its π-extended, heteroannulated derivatives, which have been mostly developed over the past five years for constructing π-conjugated polymers, particularly donor–acceptor (D–A) polymers. Semiconducting polymers containing these building blocks have demonstrated interesting properties and promising performances as active layers in OPVs and OFETs. The structural implications related to the performances of organic electronic devices are discussed.

Novel design of organic donor–acceptor dyes without carboxylic acid anchoring groups for dye-sensitized solar cells

Organic donor–acceptor dyes, formed by a high-yielding [2 + 2] cycloaddition–retroelectrocyclisation process between aniline-substituted alkynes and tetracyanoethylene (TCNE) or 7,7,8,8-tetracyanoquinodimethane (TCNQ), were employed as novel photosensitizers without carboxylic acid anchoring groups in dye-sensitized solar cells (DSSCs). The efficient adsorption of the donor–acceptor dyes onto TiO2 was confirmed by UV-vis and IR spectroscopies. The photovoltaic performances of the DSSCs suggested that the triphenylamine derivatives 3 and 4 provide higher current densities (Jsc) as compared to the corresponding dimethylaniline counter molecules 1 and 2. This was mainly due to the excellent charge-separation efficiencies and lower charge-recombination rates of the triphenylamine moieties. It was also found that the devices sensitized by the TCNQ-adducted dyes 2 and 4 display open-circuit voltages (Voc) higher than those of the TCNE-adducted counter dyes 1 and 3. All these results were reasonably explained by the J–V curve fitting based on the equivalent-circuit model as well as the comparison between the absorption and incident-photon-to-current-conversion efficiency (IPCE) spectra.

Digital Memory Versatility of Fully π-Conjugated Donor–Acceptor Hybrid Polymers

The fully π-conjugated donor-acceptor hybrid polymers Fl-TPA, Fl-TPA-TCNE, and Fl-TPA-TCNQ, which are composed of fluorene (Fl), triphenylamine (TPA), dimethylphenylamine, alkyne, alkyne-tetracyanoethylene (TCNE) adduct, and alkyne-7,7,8,8-tetracyanoquinodimethane (TCNQ) adduct, were synthesized. These polymers are completely amorphous in the solid film state and thermally stable up to 291-409 °C. Their molecular orbital levels and band gaps vary with their compositions. The TCNE and TCNQ units, despite their electron-acceptor characteristics, were found to enhance the π-conjugation lengths of Fl-TPA-TCNE and Fl-TPA-TCNQ (i.e., to produce red shifts in their absorption spectra and significant reductions in their band gaps). These changes are reflected in the electrical digital memory behavior of the polymers. Moreover, the TCNE and TCNQ units were found to diversify the digital memory modes and to widen the active polymer layer thickness window. In devices with aluminum top and bottom electrodes, the Fl-TPA polymer exhibits stable unipolar permanent memory behavior with high reliability. The Fl-TPA-TCNE and Fl-TPA-TCNQ devices exhibit stable unipolar permanent memory behavior as well as dynamic random access memory behavior with excellent reliability. These polymer devices were found to operate by either hole injection or hole injection along with electron injection, depending on the polymer composition. Overall, this study demonstrated that the incorporation of π-conjugated cyano moieties, which control both the π-conjugation length and electron-accepting power, is a sound approach for the design and synthesis of high-performance digital memory polymers. The TCNE and TCNQ polymers synthesized in this study are highly suitable active materials for the low-cost mass production of high-performance, polarity-free, programmable, volatile, and permanent memory devices that can be operated with very low power consumption, high ON/OFF current ratios, and high reliability.

Benzobisthiadiazole-based conjugated donor–acceptor polymers for organic thin film transistors: effects of π-conjugated bridges on ambipolar transport

A new series of benzobisthiadiazole (BBT)-based donor–acceptor copolymers, namely, PBBT-FT, PBBT-T-FT, and PBBT-Tz-FT, with different π-conjugated bridges have been developed for polymer thin film transistors (TFTs). It was found that inserting different π-conjugated bridges into the backbone of the polymer allowed tailoring of opto-electrical properties, molecular organizations, and accordingly, ambipolar transport of TFTs. The UV-vis-NIR spectra of all three polymers were similar with the low band gaps of around 1.1 eV. While the lowest unoccupied molecular orbital (LUMO) energy levels were also similar (around −3.8 eV), the highest occupied molecular orbital (HOMO) energy levels varied from −5.05 to −5.42 eV because of the different π-conjugated bridges; moreover, their TFTs exhibited different ambipolar transport. p-Type dominant TFT performances with the hole mobility (μh) reaching 0.13 cm2 V−1 s−1 were observed for the prototype polymer PBBT-FT. However, the device based on PBBT-T-FT with thiophene bridges displayed lower but more balanced hole (μh) and electron (μe) mobilities of 6.5 × 10−3 and 1.2 × 10−3 cm2 V−1 s−1, respectively. The device based on PBBT-Tz-FT with the thiazole units exhibited more evenly balanced hole and electron mobilities (μh/μe = 0.45) along with a significantly enhanced μe ∼0.02 cm2 V−1 s−1. These different semiconducting features were ascribed to different molecular orientations and film morphologies revealed by wide-angle X-ray scattering (WAXS) and atomic force microscopy (AFM).