Research

We are interested in the development of functional organic materials, such as organic semiconducting materials for photovoltaic cells and organic thin-film transistors (OTFT), highly fluorescent materials, etc. on the basis of organic synthesis. In particular, acenes and porphyrinoids are our current target compounds.

1. Photochemical synthesis of acene derivatives

Acenes are defined as polycyclic aromatic hydrocarbons (PAH) consisting of linearly fused benzene rings. We have succeeded to prepare pentacene from a soluble diketone precursor (PDK) by photoirradiation in solution and in film (see video below). This was the first pentacene precursor which can be converted to pentacene by photoirradiation.[1] We were also successful to fabricate the pentacene-based OTFT by solution process of PDK followed by photoirradiation. This device showed the hole mobility of 0.86 cm2 V–1 s–1. This method can be applied to anthracene-based semiconducting materials which are stable under ambient atmosphere.[2] This procedure, which we call "photoprecursor approach", is useful synthetic method for the preparation of unstable larger acenes and pentacene derivatives. We have reported the synthesis of 1,4,8,11-tetraarylpentacenes from α-diketone precursors by this method.[3]





The photoprecursor approach also enables layer-by-layer deposition of different materials by solution-based techniques, given the solubility of photoreaction product is sufficiently low. In addition, it allows for the control of film crystallinity or morphology through modulation of photoreaction conditions.[4] We have been applying this approach to the preparation of p–i–n-type active layers of organic photovoltaic cells (OPVs).[5]



α-Diketone moiety attached to highly fluorescent materials often makes them non-emissive. The diketone moiety can be removed by photoirradiation, then the fluorescence comes again. On the other hand, BCOD (bicyclo[2.2.2]octadiene) unit is converted to benzene by heating. Thus, BCOD-DK can show four different optical performances simply by irradiation and heating.[4]



[1] a) Tetrahedron Lett. 2005, 46, 1981–1983; b) Chem. Eur. J. 2005, 11, 6212–6220; [2] Chem. Commun. 2012, 48, 11136–11138; [3] Tetrahedron Lett. 2010, 51, 1397–1400; [4] Tetrahedron Lett. 2013, 54, 1790–1793.

2. Syntheses of functional π-conjugated molecules by new synthetic methods

A acenes such as tetracene and pentacene are as-yet difficult to be prepared and the selective introduction of functional groups to the acenes are not so easy due to their instability, low solubility and holding of several reactive sites. For the development and application of these acenes to organic devices, selective and facile synthetic methods are required. We have developed the original oxidative cleavage reaction from α-di-ol compounds to diformyl-substituted tetracene and pentacene. The diformylacenes were converted to dicyano[5] and bisimide-substituted[6] acenes which were used as n-type and ambipolar semiconducting materials, respectively.



[5] Org. Lett.. 2011, 13, 1454–1457; [6] Chem. Commun, 2011, 47, 10112–10114.

3. Novel porphyrinoids

Porphycene

Porphycenes, constitutional isomers of porphyrins, have lower symmetry than porphyrins and display intense absorption bands in the NIR regions due to the decreased LUMO level compared to porphyrin. In spite of the interesting electronic properties, the study of porphycenes has not been as popular as porphyrins. We have successfully synthesized benzo- and naphthoporphycenes by a retro Diels-Alder method from the corresponding bicyclo[2.2.2]octadine (BCOD) precursors and demonstrated the control of the fluorescence properties and the aromaticity by the substituents of the porphycene core[7]. In addition, free-base tetrabenzoporphycene showed herringbone structure in the crytal state, while its zinc complex made a hexahedral box-structure with six coordinated pyridine molecules inside. Dodeca-substituted porphycenes were also prepared for the first time[8]. These porphycenes can be applied to organic devices such as OTFTs and OSCs.



[14]Triphyrin(2.1.1)

Triphyrins, ring-contracted porphyrin analogues, consist of only three pyrrole and methine carbon atoms. Triphyrins are new family of porphyrin analogues. We have succeeded in preparation of the [14]triphyrins(2.1.1) by a modified Lindsey method[9] and an intramolecure McMurry coupling method[10]. The triphyrin has nearly planar structure with 14π aromatic system and was obtained as freebase form, while the previously reported triphyrins have mainly been coordinated with boron(III) atom and had bowl-shaped structure. [14]Triphyrins(2.1.1) can react with various metal ions to afford the manganese(I), rhenium(I), ruthenium(II), platinum (II) and platinum (IV) complexes. With coordination of the metal ions, the structures were changed from nearly planar to bowl-shaped structure and metal ions were placed on the top of macrocycles due to the small cavity. We have also synthesized thiatriphyrin (TTP), which includes one thiophene ring in place of one of three pyrrole rings. The neutral TTP was too unstable to be isolated and was highly reactive with alcohol to afford alkoxy-attached thiatriphyrin. Interestingly, it was converted to protonated TTP with 14π aromaticity by addition of acid to remove alcohol[11]. We have developed the synthetic methods, metal complexes and core-modification of the triphyrins to develpo the new structure and functionality.





[7] Chem. Eur. J. 2009, 15, 10060–10069; [8] Chem. Eur. J. 2011, 17, 3376–3383; [9] a)JACS. 2008, 130, 16478–16479; b)Chem. Eur. J. 2011, 17, 4396–4407; [10] Chem. Commun., 2011, 47, 722–724; [11] Angew. Chem. Int. Ed. , 2013, 52, 3360.

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