Synthesis and Photophysical Properties of Asymmetric Substituted Silafluorenes Erika Pusztai, † Irina S. Toulokhonova, † Nicole Temple, † Haley Albright, † Uzma I. Zakai, † Song Guo, ‡ Ilia A. Guzei, † Rongrong Hu, § and Robert West †, * † Organosilicon Research Center, Department of Chemistry, University of WisconsinMadison, 1101 University Avenue, Madison, Wisconsin 53706, United States ‡ Carbone Cancer Center, Wisconsin Institutes for Medical Research, 1111 Highland Avenue, Madison, Wisconsin 53705, United States § Department of Chemistry and State Key Laboratory of Molecular Neuroscience, Institute of Molecular Functional Materials, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, People’s Republic of China * S Supporting Information ABSTRACT: Several 1,3-diphenyl-substituted silafluorene compounds were synthesized and characterized as potential fluorescent materials for OLED fabrication and bioimaging. Introducing phenyl groups into the silafluorene ring at the 1- and 3-positions led to a red shift in the emission, resulting in blue light emitting compounds (λ max 368-375 nm in solution; λ max 362-371 and 482 nm in the solid state), and improved the quantum yield efficiency both in solution and as solids. Aggregation enhanced emission of the silafluorenes (AEE) was also investigated. Theoretical MO calculations were carried out to aid in understanding the optical properties of these molecules. Since these compounds might be useful in bioimaging, their toxicity was also investigated in skin fibroblast cells. All compounds were found to be nontoxic to the investigated cell cultures. ■ INTRODUCTION Dibenzoannulated analogues of silacyclopentadienes, or sila- fluorenes, have high electron affinities 1-3 and are promising potential materials as electron transporters and emitters for fabrication of organic light emitting diodes (OLEDs) 4 and photovoltaic cells. 5-9 Silafluorene conjugates are also antici- pated to be useful alternatives for expanding the repertoire of traditional fluorescent dyes in many biological assays and fluorescent imaging techniques. 10 Important features of useful fluorophores for such applications include high absorption, high quantum yield, high stability with respect to photobleaching, and compatibility with biological systems. Silafluorenes having 2-fold symmetric structures can be prepared by coupling of o,o′-dilithiobiphenyl with chlorosi- lanes 11 and by thermolysis of phenylchlorosilanes. 12 Substituted silafluorenes have been synthesized by [2 + 2 + 2] cycloaddition of Si-bridged 1,6-diynes with alkynes in the presence of an Ir(I)-phosphine catalyst, 13 by intramolecular sila-Friedel-Crafts cyclization, 14 by addition of a silyl group to an alkyne, 15 and by the cross-coupling reaction of silicon- bridged biaryls. 16 Asymmetric silafluorenes are of interest, as asymmetrically aryl-substituted 9,9-spiro-9-silabifluorene (SSF) derivatives prepared through the cyclization of the correspond- ing 2,2-dilithiobiphenyls with silicon tetrachloride have demonstrated remarkably high absolute photoluminescence quantum yields (1/4PL): 30-55%. 17 We were interested in synthesizing new blue light emitting silafluorenes. With molecular modification, the electronic and optical properties of π-conjugated compounds can be tuned; 2 thus, attaching conjugating substituents to the symmetric silafluorene ring can lead to red shifting, driving the emission maximum from the UV into the blue region. 3 Attaching different groups to the silicon atom in the silafluorene ring also influences the fluorescence quantum yield efficiency. 18 In this paper we provide a new route for synthesizing 1,3- diphenyl-9-silafluorene derivatives. We report X-ray structural analysis and photophysical properties, including AEE character- istics and solid-state fluorescence, for both previously synthesized and novel silafluorene derivatives. Also included are the cell toxicity studies of compounds 3, 4, 6, and 6′. ■ RESULTS AND DISCUSSION 2,2′-Dilithio-3,5-diphenylbiphenyl (1) was synthesized by the reaction of 1-bromo-2,4,6-triphenylbenzene with n-butyllithium in diethyl ether. 19,20 This reaction gave the desired product in over 90% yield in diethyl ether, but in THF or in hexane only 1- lithio-2,4,6-triphenylbenzene was obtained. Thus, it appears that the Li-coordinating diethyl ether molecules 21 are crucial for Received: November 12, 2012 Article pubs.acs.org/Organometallics © XXXX American Chemical Society A dx.doi.org/10.1021/om300891n | Organometallics XXXX, XXX, XXX-XXX