Introduction

Development of novel aromatic compounds have been continuously desired for their application to organic semiconducting devices, such as OLED, OPV, and OFET. Organic semiconductors are classified in p-type, n-type, and ambipolar materials. Among them, ambiploar semiconductivity is a distinctive character and such materials possess potential utility in a wide range of organic electronics. In organic light-emitting diodes (OLEDs), they are used as host materials, which function as conducting matrix of emitting dopants in emission layer. However, due to the paucity of synthetic accessibility to a new class of ambipolar materials, the current repertoire is quite limited. Designing wide gap ambipolar material is a challenging work, because increasing the band gap of the material to possess high singlet and triplet excited energies often adversely affects the charge transporting properties. Thus, during my Ph.D. course study, I demonstrated potential application of new ambipolar materials based on benzodifuran (BDF) skeleton for OLED devices and shed light on the structure-property relationship among synthesized BDF derivatives.

High mobility ambipolar material for full-color OLED devices

In my master course study, I developed a new and versatile synthetic method to construct a series of BDF derivatives. The robust BDF skeleton afforded the compounds with high thermal stability as well as excellent hole-transporting property. Being aware that steric congestion twists the 3- and the 7-aryl groups out of the BDF plane, I considered that installation of the electron transporting groups at 3- and 7-positions would make the molecule ambipolar and secure wide-gap character, which is requisite for full-color emission by various color dopants.

For this purpose, I chose carbazolyl group, and synthesized according to the zinc-mediated double cyclization protocol as shown in Scheme 1. Using thus synthesized p-PhCZBDF, I first verified its ambipolar and wide-gap character. Time-of-flight (TOF) mobility measurements using amorphous films of p-PhCZBDF indicated the generation of hole and electron photocurrent, and the carrier mobilities are 3.7×10-3 and 4.4×10-3cm2/Vs for hole and electron, respectively. Differential pulse voltammetry determined the oxidation and reduction potentials to be 0.72 V (in CH2Cl2) and -2.60 V (in THF), respectively (vs Fc/Fc+), which corresponds to HOMO and LUMO energy levels are -5.52 eV and -2.20 eV, respectively. These data indicate the feasible formation of both radical cation and anion as well as wide HOMO-LUMO gap character (3.32 eV), which is efficient for confinement of excitions within various color dopants.

Encouraged by these property measurements, OLED devices were fabricated and evaluated with heterojunction architecture (ITO/PEDOT:PSS/α-NPD/EML (emitting layer)/Alq3/Liq/Al, where p-PhCZBDF was used in the EML either as an emitting material by itself (undoped) or as a host material for various dopants), and found that p-PhCZBDF functions as both a blue-emitting dye by itself and a host material for various dopants. Figure 1 shows p-PhCZBDF serves as an effective emission host material for various dopants over the full range of visible region. From the viewpoint of device performances (Table 1), CZBDF-based device F showed superior performance and much longer device lifetime compared to the conventional host material, such as CBP (device G). In addition, taking advantage of the well-balanced high charge mobility of p-PhCZBDF, homojunction devices, which simply composed of a single organic matrix (p-PhCZBDF) doped with inorganic dopants and an emissive dye, were also developed.

Modulation of excited triplet energy level for phosphorescent emission

For high emission efficiency to reach over 5% external quantum efficiency (EQE) in OLED devices, phosphorescent emission is requisite. Although, host material employed must possess higher excited triplet energy level (ET) than those of phosphorescent dopants, the ET of CZBDF is not sufficient for blue and green phosphorescent emission. Thus, I designed a new host materials with high triplet energy level based on benzodifuran in terms of disrupting π-conjugation between BDF core and the substituents as well as tuning the furan fusion manner to construct MeCZBDF derivatives (Figure 2).

The phosphorescence spectra, taken in a frozen 2-methyl tetrahydrofuran matrix at 77 K, afforded the ET values: 2.64 and 2.77 eV for p-MeCZBDF and o-MeCZBDF, respectively, which are high enough to emit green and blue phosphorescent dyes, respectively. The origin of the higher ET value of o-MeCZBDF than that of p-MeCZBDF is attributed to the different ring fusion manner. This trend is quite similar to the relationship between phenanthrene and anthracene, a set of structural isomers of the carbon analogue. That is, p-BDF skeleton corresponds to anthracene-like acene system, whereas o-BDF to phenanthrene-like phenacene system, supported by X-ray single crystal analysis. TOF mobility measurements suggested that both p-MeCZBDF and o-MeCZBDF possess high charge carrier mobility for hole and electron over 10-3cm2/Vs. Utilizing o-MeCZBDF and p-MeCZBDF as hosts with phosphorescent dopants, phosphorescent blue and green OLED devices were realized, respectively, at over 5% EQE value.

Conclusion

I have developed the wide-gap ambipolar materials via zinc-mediated cyclization reaction. p-PhCZBDF possesses well-balanced hole and electron mobilities as high as greater than 10(-3)cm2/Vs. p-PhCZBDF serves as an effective emission host material for various dopants over the full range of visible region. Furthermore, to increase emission efficiency, higher ET MeCZBDF derivatives were also developed. Using these materials, phosphorescent blue and green emissions were also achieved.

Scheme 1.Synthesis of CZBDF.

Figure 1. EL spectra from CZBDF-based devices.

Table 1. Summary of OLED performance.

Figure 2. Strucrtures of phosphore scen host materials.

本論文は六章から構成されており,ベンゾジフラン誘導体の開発と有機半導体材料への応用について論じている.

第一章では,研究背景として,有機半導体素子ににおけるπ共役系化合物の重要性と歴史的背景が述べられている.π共役系化合物の物性制御を指向してヘテロ原子を導入するという手法がとられており,既に応用されている材料がある一方で,酸素原子を含むフラン誘導体は,合成的手段の制約と不安定性の問題から,これまで材料化学の分野ではほとんど応用されていなかった.筆者は,ベンゼン環とフラン環が縮環した「ベンゾジフラン」という構造で安定性を確保できると考え,修士課程において実際にベンゾジフラン骨格の簡便合成法を独自に見出し,p型半導体として優れた性質を見出している.これに対し本研究は,構造有機化学に立脚して新たに高性能両極性材料の設計・開発について言及している.

第二章では,両極性分子の開発を指向し,ベンゾジフランにカルバゾールを導入した材料(CZBDF)の開発とその応用について述べている.CZBDFは正孔・電子の移動度バランスに優れた両極性高移動度とワイドギャップ性を示すなど,有機ELのホスト材料として理想的な物性を有するという,前例のない化合物であることを物性測定から明らかにしている.この性質を活用し,CZBDFをホスト材料とした有機EL素子で単一ホストからのフルカラー発光を初めて実現するとともに,既存材料を凌ぐ高性能および長寿命化を達成した.

第三章では,CZBDFの優れた両極性特性に基づき,有機ELの新しい素子構造の開拓に成功した研究について述べられている.従前の有機EL素子はヘテロ接合型と呼ばれる多物質・多層構造であるのに対し,ここで述べられている「ホモ接合素子」は,両極性材料を唯一のマトリックスとした極めて単純化された構造のものである.CZBDFをマトリックスとしたホモ接合素子を作製・評価し,(1)蛍光EL・リン光ELともに適用可能であること,(2)適切な発光ドーパントを用いることで三原色発光が可能であること,(3)蛍光素子では外部量子効率が最高4.2%と,理論限界(5%)に迫る高効率が達成できることを明らかにしている.

第四章では,励起三重項エネルギー準位の制御と,この知見を活かした高効率リン光有機ELホスト材料の開発について論じている.アセン型とフェナセン型の二つのベンゾジフラン誘導体を新たに合成し,後者がより高い励起三重項エネルギーを有することを明らかとした.また,ベンゾジフランでは電気陰性度の高い酸素原子によってπ電子の非局在化が抑制され,高い励起エネルギー準位が実現されるという独特の性質が発現することについて考察している.また,フェナセン型化合物をホストとすることで,これまで実現が困難であった青色リン光ELを実現した.

第五章では,縮環位置が異なる種々のベンゾジフラン異性体の物性の検証から,フラン縮環系化合物の構造と電子物性の相関について述べている.ベンゾジフラン骨格を記述するモデルとしては,酸素架橋フェニレンビニレンモデルと,酸素置換アントラセンモデルが考えられるが,計算化学と実験結果とを総合することにより,ベンゾジフラン類の電子状態の記述には酸素置換アントラセンモデルが適切であることを明らかとした.

第六章は本研究の総括である.第二章から第五章の結果をもとに,ベンゾジフランが示す優れた性質を考察するとともに,今後の展望について述べている.

なお,本論文第二~五章は中村栄一博士および辻勇人博士と,第二,三,五章は佐藤佳晴博士との共同研究であるが,研究計画および検討の主体は論文提出者であり,論文提出者の寄与が十分であると認められる.

本研究は,有機合成化学,構造有機化学,元素科学を駆使した全く新しい機能性分子の開発という,学術的に非常に価値の高いものであるとともに,高効率有機EL材料への合目的的展開という社会的問題解決も指向したものである.したがって,本論文は博士(理学)の学位論文として価値のあるものと認める.