Synthesis, Morphology, and Optical Properties of Tetrahedral Oligo(phenylenevinylene) Materials
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Abstract
A novel topological strategy is described for designing amorphous molecular solids suitable for optoelectronic applications. In this approach, chromophores are attached to a tetrahdral point of convergence. Stilbenoid units were covalently linked to tetraphenylmethane, tetraphenyladamantane, or tetraphenylsilane cores using palladium catalyzed coupling methodology. Thus, reaction of E(C6H5X)4 (E = C and adamantane, X = I; E = Si, X = Br) with styrene or 4,4‘-tert-butylvinylstilbene under Heck coupling conditions yields the corresponding tetrakis(stilbenyl) (E(STB)4) and tetrakis(4-tert-butylstyrylstilbenyl) (E(tBuSSB)4) compounds. Similarly, reaction of 1,1-diphenyl-2-(4-dihydroxyboronphenyl)ethene or 2-(4-pinacolatoboronphenyl)-3,3-diphenylacrylonitrile with tetrakis(4-bromophenyl)methane using Suzuki coupling methodology gives tetrakis(4,4‘-(2,2-diphenyl-vinyl)-1,1‘-biphenyl)methane (C(DPVBi)4) or tetrakis(4,4‘-(3,3-diphenylacrylonitrile)-1,1‘-biphenyl)methane (C(DPAB)4), respectively, in good yields. Compounds with more extended conjugation can also be prepared. Thus, reaction of excess 1-(4‘-tert-butylstyryl)-4-(4‘-vinylstyryl)benzene with C(C6H4I)4 provides tetrakis(4-(4‘-(4‘ ‘-tert-butylstyryl)styryl)stilbenyl)methane (C(4R-tBu)4) in low yield (∼20%). The more soluble analogue, tetrakis(4-(4‘-(3‘ ‘,5‘ ‘-di-tert-butylstyryl)styryl)stilbenyl)methane (C(4R-2tBu)4) is prepared similarly using 1-(3‘,5‘-di-tert-butylstyryl)-4-(4‘-vinylstyryl)benzene and in better yield (∼80%). Alkoxy substituents can also be used to increase solubility. Tetrakis((4-(2‘,5‘-dioctyloxy-4‘-styryl)styryl)stilbenyl)methane, C(4R-(OC8H17)2)4, was prepared by treatment of C(C6H4I)4 with excess 2,5-dioctyloxy-1-styryl-4-(4‘-vinylstyryl)benzene (yield ∼ 73%). The simple stilbenyl-derivatives were found by DSC measurements and powder diffraction experiments to be crystalline compounds. Comparison of single-crystal X-ray diffraction data shows that C(STB)4 and Si(STB)4 form isomorphous crystals. The larger E(tBuSSB)4, C(DPVBi)4, and C(DPAB)4 compounds readily form amorphous glasses with elevated glass transition temperatures (Tg = 142−190 °C) in the absence of solvent. Extending the conjugation length of the arm leads to more stable glasses. For example, the glass transition temperature of C(4R-tBu)4 was measured at 230 °C. Solution phase optical spectroscopic data of E(tBuSSB)4 (E = C, adamantane, and Si) are characteristic of the parent distyrylbenzene chromophore. Films, however, show broad and significantly red-shifted emission spectra. In contrast, C(DPVBi)4 gives absorption and emission spectra which are nearly identical between dilute solution phase samples and neat solid films. The emission of C(DPAB)4 is broad and structureless, reminiscent of exciplex or excimer emission. Films of the tetramers with longer arms (C(4R-tBu)4, C(4R-2tBu)4, and C(4R-(OC8H17)2)4) show emission properties which are dependent on sample history. Annealing the sample at elevated temperature leads to red-shifted emission as a result of better interdigitation between the optically active fragments.
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