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Inorganic Chemistry Communications 10 (2007) 731–734

Structure and optical properties of new spirobisilole:2,3,3 0,4,4 05-hexaphenyl-1,1 0-siprobisilole

Haejin Lee, Jinho Kim, Youngjin Kang *

Division of Science Education and Department of Chemistry, Kangwon National University, 192-1 Hyoja-Dong, Chuncheon 200-701, Republic of Korea

Received 29 November 2006; accepted 20 March 2007Available online 27 March 2007

Abstract

Spiro-linked orthogonal bisilole, 2,3,3 04,4 05-hexaphenyl-1,1 0-spirobisilole, has been synthesized and characterized. Its crystal structureand optical properties were also investigated. This compound shows high thermal stability and strong bluish-green emission(kmax = 509 nm)with high PL efficiency (55 ± 5%).� 2007 Elsevier B.V. All rights reserved.

Keywords: Spirobisilole; X-ray structure; Optical properties

Organosilicon compounds have become an interestingsubject in materials science due to their varied applications,such as in conductors and OLEDs (organic light-emittingdiodes) [1,2]. Among these compounds, the five-memberedheterocycle containing a silicon atom, i.e., a silole (silacy-clopentadiene), has been shown to be an excellent electrontransporting material for OLEDs because it possesses alow-energy LUMO due to the interaction between the r*

orbital of its two exocyclic Si–C bonds and the p* orbitalsof the butadiene moiety [2]. Silole derivatives exhibit emis-sion from blue to orange-red by the incorporation of vari-ous substituents at the 2 and 5 positions of the silole ring,and photophysical properties and electronic structures of asilole significantly depend on the nature of the 2,5-arylgroups [3]. Moreover, silole derivatives have shown verygood efficiency in electroluminescence (EL) devices [4].Although silole compounds have a number of advantageswhen applied to OLEDs, a few pending problems stillremain to be solved. One of the problems is that silole com-pounds have a tendency to form exciplexes with N,N 0-bis(1-naphthyl)-N,N 0-diphenylbenzidine (NPB), which has beencommonly adopted as a hole-transport material in OLEDs

1387-7003/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.inoche.2007.03.010

* Corresponding author.E-mail address: kangy@kangwon.ac.kr (Y. Kang).

[5]. To overcome such phenomena, several approacheshave been introduced. These include the attachment ofbulky functional groups on the central silole ring and aspiro-type design of the molecule [6,7]. The developmentof spiro-type molecules have especially been studied toimprove the durability of OLEDs fabricated by small mol-ecules, reducing the tendency to crystallize and increasingthe glass transition temperature [8].

The investigation of molecular structure is of majorinterest when associated with the interpretation of opti-cal/photoluminescent characteristics, since these propertiesare generally related. For example, molecular structure ishelpful in obtaining information on excimer formationand/or photoluminescence (PL) quenching due to aggrega-tion in the solid state. However, there are few reports onthe molecular structure, the morphological properties,and the corresponding optical properties of spiro-bisilole.Only a few reports on the photoluminescence, thermal,and optical properties for spiro-silabifluorene derivatives,which are structurally similar spiro-bisilole anologues, havebeen published [9]. To our knowledge, our report is the firstexample of structural investigation with respect to unsym-metrical 2,3,4,5-functionalized spiro-bisilole [10]. Herein,we report in detail the crystal structure, thermal propertiesand optical characteristics of a new type of spiro-bisilole,2,3,3 0,4,4 05-hexaphenyl-1,1 0-siprobisilole.

Fig. 1. Molecular structure of 1 with atom labeling schemes and 50%thermal ellipsoids. Selected bond lengths (A) and angles (�): Si1–C11.867(4), Si1–C4 1.862(5), Si1–C29 1.851(5), Si1–C32 1.843(5), C1–C51.471(6), C4–C23 1.466(6), C1–Si1–C4 93.4(2), C29–Si1–C32 91.4(2);Dihedral angles: [C1–C4, Si1] with [C29–C32, Si1] 89.6(1), [C1–C4, Si1]with [C5–C10] 25.4(2), [C1–C4, Si1] with [C23–C28] 37.2(2).

732 H. Lee et al. / Inorganic Chemistry Communications 10 (2007) 731–734

The titled compound was obtained in 30–40% yields viaa one-pot synthesis that involved the intramolecular reduc-tive cyclization of 1,1-bis(phenylethynyl)-2,3,4,5-tetraphe-nylsilole, followed by the addition of 1 M HCl solution[11]. The significant factor in this reaction is the dropwiseaddition of 1,1-bis(phenylethynyl)-2,3,4,5-tetraphenylsilolesolution in THF into an excess amount of LiNp (lithium-naphthalenide). This addition should take longer than20 min and followed by reflux, since otherwise, the startingmaterial, 1,1-bis(phenylethynyl)-2,3,4,5-tetraphenylsilolewill be obtained as a major component. The optimum reac-tion condition is described in Scheme1. Compound 1 is sol-uble in common organic solvents. The structure of thecompound was confirmed by 1H, 13C, 29Si NMR, massspectroscopy, and elemental analysis including single crys-tal X-ray diffraction analysis. The 29Si NMR signal(�5.6 ppm) of 1 is compatible with the 29Si chemical shiftsreported previously for monocyclic silole compounds (�5to 8 ppm) and is shifted downfield considerably, comparedto that of starting material (�47.9 ppm), bis(phenylethy-nyl)diphenylsilane [12].

The bright green single crystals of 1 suitable for X-rayanalysis were obtained by the slow evaporation of CH2Cl2solution. The crystal structure of 1 is given in Fig. 1 withthe atomic numbering scheme. Selected bond lengths andbond angles of 1 are also shown in Fig. 1 [13]. In the crystalstructure of 1, a two-fold axis of symmetry lies along thevector linking Si1 and the midpoint of C2–C3 bond. Asshown in Fig. 2 (left), two mean planes of five-memberedrings around the Si atom are almost perpendicular to eachother with a dihedral angle of 89.66(12)�. The crystal struc-ture of 1 appears as a screw propeller-like arrangement ofthe benzene rings. The bond lengths Si1–C1 and C1–C5are 1.867(4) A, and 1.471(6) A, respectively, which are sim-ilar to those of previously reported silole compounds [10].The phenyl substituents at the 3,4-position of the centralsilole ring are twisted with torsional angels of 50.8� to74.3�. As shown in Fig. 1, the torsional angles betweenthe upper phenyl rings (C11–C16, C17–C22) at the 3,4-position and the silole ring are in the range of 64.3� to74.3�, while torsional angles of lower phenyl substituents(C33– C38, C39–C44) of the silole ring are in the rangeof 50.8� to 53.6�. These results can be attributed to the lackof 2,5-substituents. Notably, the dihedral angles between

S

Si

ClCl

i

Scheme 1. Reagents and conditions: (i) n-BuLi/phenyacetylene, THF, rt, 24

the phenyl rings at the 2,5-position and central silole ringare 25.3(2)� and 37.1(2)� for C5–C10 and C23–C28, respec-tively, indicating the presence of effective p conjugations.The compound has several intermolecular interactions thatappear to direct the extended packing of the solid-statestructure. Several weak face-to-edge C–H–p(arene) interac-tions between two adjacent molecules are observed. Incrystal packing, the adjacent molecules at (x, y, z,) and{(0.5 + x)�1, y, (0.5�z)�1} are connected via severalC–H–p(arene) interactions between H18 and the meanplane of the phenyl ring [C5A–C10A] with a distance of2.498(5) A. The details and bond lengths are given in thesupporting information. However, no direct intermolecularinteractions (e.g., p–p stacking) were evident in the crystal

SiHH

iii

1

h (85%) and (ii) Li/naphthalene, THF, reflux, 12 h, 1 M-HCl, (30–40%).

Fig. 2. Left: Stick model of 1 showing orthogonal arrangement of two silole ring; Right: Crystal packing diagram between two adjacent molecules,showing the presence of C–H–p(arene) interactions.

1.2

0.8

0.6

0.4

0.2

0

1

1.2

0.8

0.6

0.4

0.2

0

1

230 330 430 530 630Wavelength (nm)

Abs

(N

orm

aliz

ed)

Inte

nsity

(N

orm

aliz

ed)

AbsorptionEm-filmEm-solution

Fig. 3. Absorption and emission spectra of 1 in either CH2Cl2 or thin film.

H. Lee et al. / Inorganic Chemistry Communications 10 (2007) 731–734 733

packing. The above observation suggested that thestructural feature with the spiro segment hinders closepacking and intermolecular interactions. In addition, anon-planar molecular structure caused by the spiro centercould reduce the tendency of excimer formation, leadingto an enhancement in photoluminescence efficiency.

Thermal characterizations of 1 were accomplished byTGA (thermogravimetric analysis) and DSC (differentialscanning calorimetry). The TGA thermograms in a nitro-gen atmosphere showed that compound 1 has good ther-mal stability and no weight loss up to ca. 150 �C (seesupporting information). The weight loss of 1 is 2% uponheating to 280 �C. To our knowledge, this value is fairlyhigh in comparison with other silole analogues [1]. Theresults can be explained by the fact that the geometry of sil-ole, especially the spiro-configuration, ameliorates thermalstability. To investigate the glass transition temperature(Tg), DSC experiments were conducted in a nitrogen atmo-sphere. Two cycles of heating and cooling were performedin the temperature range of 20 �C up to the melting pointfor compound 1. This compound showed a consistentand reproducible DSC diagram, an indication that 1 is atleast thermally stable up to its melting transition. A glasstransition temperature of 73 �C was observed for the firstheating.

Fig. 3 shows the optical absorption and photolumines-cence spectra of 1 in both solution and thin film. As shownin Fig. 3, the two observed peak maximums at 248 and380 nm for 1 in the UV–vis spectrum are ascribed to thep–p* transition of the phenyl rings and the p–p* transitionof the silole ring, respectively. Based on previous reports onthe electronic transition of 2,5-functionalized silole, thehighest occupied molecular orbital (HOMO) included a porbital involving contributions from both the silole ringand the aromatic substituents at the 2,5-positions, whilethe lowest unoccupied molecular orbital (LUMO) wasnearly a pure p* orbital from the silole ring [14]. Zhuet al. reported a similar result with 2,3,4,5-tetraphenylsilole

[15]. The optical band-gap energies of 1, estimated fromextrapolation of the low-energy absorption spectra, wereapproximately 2.76 eV.

Compound 1 emitted strong bluish-green fluorescencewhen irradiated by UV light in the thin film. The PL spec-tra were recorded with an excitation wavelength corre-sponding to the absorption maximum wavelength of 1.When compared to hexaphenylsilole (HPS), kmax of 1 inthe emission spectrum of the solution lies in the longerwavelength region. The dihedral angles (47–62.4�) of thecentral silole ring and phenyl ring at 2,- and 5-position inHPS are much larger than those of 1. Thus, the resultobserved in the PL spectrum of 1 is apparently due tothe effective p-conjugation. It is noteworthy that com-pound 1 in the solid state shows higher PL quantum effi-ciency than in other silole analogues. The PL quantumefficiency (UPL) in the thin film is measured relative toAlq3 (32%); Alq3 = aluminum tris(8-hydroxyquinoline).The PL efficiency of 1 is 55 ± 5%, which is almost 1.5–2times larger than in Alq3. However, the emission of 1 insolution was hardly observable due to extremely low

734 H. Lee et al. / Inorganic Chemistry Communications 10 (2007) 731–734

quantum efficiency (0.05, as compared with 9,10-diphenyl-anthracene). These results can be explained by the correla-tion of rigidity and planarity of the silole. In solution, thefavorable rotation of phenyl rings may diminish the planar-ity, rigidity and effective p-conjugation between theattached phenyl rings and the central silole ring [16]. Inthe thin film, it exhibits a narrower emission band thanin solution. Full width at half maximum (FWHM) in thinfilm is 80 nm, while FWHM in solution is 85 nm. Mostluminescent compounds in solution, in general, displaywider emission bands than in solid state. This narrow emis-sion provided further support for a unique photophysicalproperty of siloles, called aggregation-induced emission(AIE), which is ascribed to the restriction of intramolecularrotation and enhancement of stacking forces in the solidstate. FWHM plays a key role in the determination of colorpurity in the sense that, with narrow FWHM, as color pur-ity increases significantly. Therefore, compound 1 would beexpected to be a good emitting material showing high colorpurity in an EL device. Indeed, the rigid structure inducedby the spiro arrangement around the Si atom renders goodsolubility, high thermal stability and improvement of PLefficiency.

In summary, new bluish-green light-emitting silole com-pounds, spiro-bisilole, possessing a rigid entity and advan-tages of thermal stability, have been synthesized andcharacterized. The synthesized compounds have good solu-bility and thermal stability. The crystal structure exhibitsnot only a propeller-like arrangement of the benzene rings,but also several weak face-to-edge C–H–p(arene) interac-tions between two adjacent molecules. The observed highPL efficiency and narrow emission band were attributedto the increased rigidity of spiro-linked orthogonal bisilolemoieties. The bright bluish-green emissions with high PLefficiency makes 1 a promising candidate for electron-transporting as well as emitting materials to be used inOLEDs. Further investigation on the electroluminescentcharacteristics of 1 is currently in progress.

Acknowledgement

This work was supported by Korea Research Founda-tion Grant funded by Korea Government (MOEHRD, Ba-sic Research Promotion Fund) (KRF-2005-015-C00232).

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at doi:10.1016/j.inoche.2007.03.010.

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