Striking effects of halogen substituents on the glass-forming properties, glass- transition temperatures and stabilities of the glassy state of a new family of amorphous molecular materials, 1,3,5-tris (4-halogenophenylphenylamino)benzenes Hiroshi Kageyama, Koji Itano, Wataru Ishikawa and Yasuhiko Shirota" Department of Applied Chemistry, Faculty of Engineering, Osaka University, Yamadaoka, Suita, Osaka 565, Japan A new class of n-electron starburst molecules, 1,3,5-tris(4- halogenophenylphenylamino) benzenes, are synthesized for use as amorphous molecular materials. It was found that they readily form amorphous glasses, whereas the parent compound 1,3,5-tris(diphenylamino)benzene instantly crystallizes and that the ease of glass formation, glass- transition temperature, and stability of the glassy state are greatly affected by the type of halogen substituent.Amorphous molecular materials that readily form amorphous glasses with relatively high glass-transition temperatures are expected to constitute a novel class of organic functional materials with excellent processability, transparency, isotropic properties and homogeneous properties owing to the absence of grain boundaries. In addition, developing such amorphous molecular materials, which exhibit a glass transition usually associated with amorphous polymers, is of interest and signifi- cance from an academic viewpoint, opening up a new field of organic solid-state science that deals with molecular glasses. There are still few known examples.'-13 We have synthesized several novel families of organic x-electron systems, which we refer to as 'starburst' molecules, for use as amorphous molecu- lar materials.They include 1,3,5-tris(diphenylamino)triphenyl-amine and its derivative^,^*^,'*.^' 1,3,5-tris(diphenyl-amino) benzene and its derivative^,^-^*'^ and 1,3,5-tris(4-diphenylaminophenyl) benzene and its derivatives.' For the development of amorphous molecular materials, it is of impor- tance to establish guidelines for molecular design through the study of the correlation between molecular structure and glass- forming properties. We report here striking effects of halogen substituents on the glass-forming properties, glass-transition temperatures (Tg), and stabilities of the glassy state for 1,3,5-tris(4-halogenophen-ylphenylamino) benzenes, e.g.1,3,5-tris( 4-fluorophenylphenyl- amino) benzene (p-FTDAB), 1,3,5-tris( 4-chlorophenyl- phenylamino) benzene (p-ClTDAB), and 1,3,5-tris( 4-bromo- phenylphenylamino) benzene (p-BrTDAB). F CI Br p-FTDAB p-ClTDAB P-BrTDAB The new compounds, p-FTDAB, p-ClTDAB and p-BrTDAB, were synthesized by the Ullmann reaction of 1,3,5-tris(phenyl- amino)benzene (1.8 g, 5 mmol), prepared from phloroglucinol and aniline according to the method in the literat~re,'~ with p-fluoro-, p-chloro-, and p-bromo-iodobenzenes (5.0 g, 22.5 mmol), respectively, at 170°C for 30 h in the presence of copper powder (0.2 g, 3 mmol) and potassium hydroxide (1.1 g, 20 mmol) under a nitrogen atmosphere.The products were purified by column chromatography using silica gel, followed by recrystallization from benzene-hexane, and identified by various spectroscopies, mass spectrometry and elemental analysis. 1,3,5 -Tris(4- halogenophenylphenylamino) benzenes (p-XTDAB, X =F, C1, Br) were found to form readily amorphous glasses when the melt samples were cooled, whereas the parent compound 1,3,5-tris(diphenylamino)benzene (TDAB) instantly crystallized. Fig. 1 shows differential scanning calorimetry (DSC) curves of p-ClTDAB as an example. When a crystalline sample obtained by recrystallization from benzene-hexane was heated, an endothermic peak due to melting was observed at 181"C. When the resulting isotropic liquid was cooled on standing in air, it formed spontaneously an amorphous glass via a supercooled liquid.When the amorphous glass sample was again heated, glass transition took place at around 64"C, followed by crystallization at around 112 "C to give the same crystal as obtained by recrystallization from benzene-hexane, which melted at 181 "C. Similar DSC curves were obtained for p-FTDAB and p-BrTDAB; however, no crystallization was noticed for p-BrTDAB even when heated above the Tg. The formation of the amorphous glasses of p-FTDAB, p-ClTDAB and p-BrTDAB was also evidenced by X-ray diffraction and polarizing microscopy. Thus, p-XTDABs (X =F, C1, Br) consti- tute a new class of amorphous molecular materials. It is of note that the halogen substituent in p-XTDABs (X= F, C1, Br) strikingly affects the ease of glass formation, q,and stability of the glassy state. That is, while p-FTDAB requires rapid cooling of the melt sample with liquid nitrogen to form a glass, p-ClTDAB and p-BrTDAB readily produce amorphous glasses even on slow cooling of the melt samples at a cooling rate as slow as 1"C min-'.The glassy state of p-FTDAB is much less stable than those of p-ClTDAB and p-BrTDAB; 7 Tm 181 "C r, 64°C 112°C Tm 181 "C TI"C Fig. 1 DSC curves of p-ClTDAB; heating rate: 5 "C min-'. (a)-Crystalline sample obtained by recrystallization from benzene- hexane; (b)glass sample obtained by cooling the melt. J. Muter. Chem., 1996, 6(4), 675-676 675 Table 1 Glass-transition, crystallization and melting temperatures ($ ,T, and Tm)and thermodynamic parameters [specific heat jumps at $(AC,), enthalpy and entropy changes (AH and AS)] for p-XTDABs" ACp/ AHCI ' AHml ASmlql"c JK 'mol T,rc kJ mol Tm/"C kJ mol JK 'mol ' p-FTDAB 54 170 65 -16 228 51 102 p-CITDAB 64 220 112 -26 181 37 81 p-BrTDAB 72 220 -b -b 165' 41' 94' 'Glass samples were heated at a heating rate of 5 "C mm ' bNo crystallization 'Values for the crystalline sample obtained by recrystallization from benzene-hexane while the p-FTDAB glass tends to crystallize when allowed to stand for a few days at room temperature, the glasses of p-ClTDAB and p-BrTDAB do not The p-BrTDAB glass in particular exhibits no crystallization behaviour even on heating above the Tg, whereas the p-FTDAB and p-ClTDAB glasses crystallize on heating above the Tgs Thus, the ease of glass formation and stability of the resulting glass were found to increase in the order p-FTDAB <<p-ClTDAB <p-BrTDAB The G is also greatly affected by the type of halogen atom, increasing in the order p-FTDAB <p-ClTDAB <p-BrTDAB as shown in Table 1 It should be noted that the melting temperature (T,) decreases in this order In contrast to TDAB, which readily crystallizes even when the melt sample is rapidly cooled with liquid nitrogen, the facile glass formation for p-XTDABs as well as for the methyl- substituted TDABs' may be ascribed to the increase in the number of conformers due to the incorporation of the halogen substituent in TDAB Thus, increasing the number of confor- mers of nonplanar molecules could be a concept for molecular design of amorphous molecular materials The marked effects of the halogen substituent on the ease of glass formation and stability of the glassy state observed for p-XTDABs (X =F, C1, Br), 1 e easier glass formation and increasing stability of the resulting glass in the order F<<Cl<Br, can be explained in terms of the bulkiness and weight of the halogen atom That is, the more bulky and heavier halogen atom will prevent easy packing of molecules and easy diffusion of molecules in the process of crystal growth, and hence prevent crystallization Intermolecular interactions operative in p-XTDABs, e g n-n interactions, hydrogen bonding interactions between the hal- ogen substituent and aromatic C- H," and halogen-halogen T,and 7'' will increase in both attractions," cause an F <C1< Br may be attributed to poorer packing of molecules in the crystal due to the more bulky halogen substituent in this order, leading to smaller crystal lattice energy On the other hand, the increasing in the order F<Cl<Br is also ascribed mainly to the difference in the weight of the halogen atom The incorporation of a heavier halogen atom will tend to hinder translational, rotational, and vibrational motions of the molecule, leading to an increase in Tg Further work is necessary to clarify the correlation between Tg and T, The present study shows the striking effects of halogen substituents on the ease of glass formation, Tg and stability of the molecular glass, presenting an important guide for future development of amorphous molecular materials References 1 B Rosenberg, J Chem Phys, 1959,31,238 2 D J Plazek, J H Magill, J Chem Phys ,1966,45,3038 3 Y Shirota, T Kobata and N Noma, Chem Lett, 1989, 1145 4 A Higuchi, H Inada, T Kobata and Y Shirota, Adu Muter, 1991, 3,549 5 W Ishikawa, H Inada, H Nakano and Y Shirota, Chem Lett, 1991,1731 6 W Ishikawa, H Inada, H Nakano and Y Shirota, Mol Cryst Lzq Cryst, 1992,211,431 7 W Ishikawa, H Inada, H Nakano and Y Shirota, J Phys D Appl Phys ,1993,26, B94 8 H Inada and Y Shirota, J Muter Chem, 1993,3,319 9 K Naito and A Miura, J Phys Chem ,1993,97,6240 10 Y Kuwabara, H Ogawa, H Inada, N Noma and Y Shirota, Adu Muter, 1994,6,677 11 A Higuchi and Y Shirota, Mol Cryst Lzq Cryst, 1994,242,127 12 E Ueta, H Nakano and Y Shirota, Chem Lett, 1994,2397 13 Y H Kim and R Beckerbauer, Macromolecules, 1994,27,1968 14 Ng Ph Buu-Hois, J Chem Soc ,1952,4346However, the opposite trends in Tp and T, observed for p-15 J A R P Sarma and G R Desiraju, Acc Chem Res 1986,XTDABs, namely, increasing Tp but decreasing T, in the order 19.222 F<Cl<Br, may be ascribed to other factors operating more strongly in p-XTDABs That is, the decrease in T, in the order Paper 6/01 1lD, Received 5th January 1996 676 J Muter Chem, 1996,6(4), 675-676