Published: May 25, 2011 r2011 American Chemical Society 5485 dx.doi.org/10.1021/ic200010q | Inorg. Chem. 2011, 50, 5485–5493 ARTICLE pubs.acs.org/IC Enhanced π-Conjugation and Emission via Icosahedral Carboranes: Synthetic and Spectroscopic Investigation Barada Prasanna Dash, † Rashmirekha Satapathy, † Elizabeth R. Gaillard, † Kathleen M. Norton, † John A. Maguire, ‡ Neha Chug, † and Narayan S. Hosmane* ,† † Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, Illinois 60115-2862, United States ‡ Department of Chemistry, Southern Methodist University, Dallas, Texas 75275-0314, United States b S Supporting Information ’ INTRODUCTION Boron clusters, namely boranes and carboranes, possess three- center, two-electron bonds, and subsequently, three-dimensional delocalization of skeletal electrons results in a highly polarizable σ-aromatic characteristic, often referred to as three-dimensional aromaticity. 1 The unusual thermal and chemical stability of icosahedral carboranes 2 and their ability to undergo benzenelike electrophilic substitution 3 and extend π-conjugation 4 are also attributed to their aromatic properties. 5 Theoretical evidence on the aromaticity of deltahedral boranes B n H n 2- (6 e n e 12) was given by Aihara, who found significant positive resonance energies for these clusters using a graph theoretical method. The highest resonance energy was observed for B 12 H 12 2- . 6 A number of other theoretical investigations point to the existence of aromaticity in the icosahedral carboranes (Figure 1; 1, 2, 3) which are isoelectronic to the B 12 H 12 2- cluster. 5 Icosahedral carboranes find a number of traditional applications such as in medicine 7 and organometallic chemistry, 8 and many new applica- tions of boron clusters in various other areas are also emerging. 9 The ability of icosahedral carboranes to extend π-conjugation has been exploited in synthesizing polymeric materials with improved photophysical properties. Recent reports showed that boron clusters, such as meta- and para-carboranes, are good candidates for the syntheses of light emitting polymeric materials. 4 Although the extension of π-conjugation and enhancement emission intensity via icosahedral carboranes has been reported, such properties of all three icosahedral carboranes have never been compared. Previously, we reported the synthesis and thermal and photophysical properties of o-carborane-appended phenylenes where the aromatic core was attached to o-carborane clusters via methylene moieties. 10 Thus, the enhancement of the emission intensity of the extended π-conjugated systems was attributed to the three-dimensional structure of the carborane clusters which is capable of preventing πÀπ stacking interactions. 10 In this paper, we report the synthesis and thermal and photophysical properties of ortho-, meta-, and para-carborane-appended 1,3,5-triphenylbenzenes (TBs) and 1,3,5-tris(biphenyl-4-yl)benzenes (TBBs) where the carborane clusters are directly attached to the π-conjugated core. Symmetrical 1,3,5-phenylenebenzene core based structures TB and Figure 1. Icosahedral carboranes. Received: January 3, 2011 ABSTRACT: The ability of ortho-, meta- and para-carboranes to enhance the emission intensity has been compared. For this purpose a series of carborane-appended 1,3,5-triphenylbenzene (TB) and 1,3,5- tris(biphenyl-4-yl)benzene (TBB) containing three ortho-, meta- and para-carborane clusters directly attached to the conjugated cores have been synthesized employing Suzuki, Heck, and trimerization reactions. The incorporation of the icosahedral carboranes was associated with a red shift in the UV absorption spectrum of up to 13 nm as well as enhancements of the emission intensities of up to 154%. The presence of ortho-carboranes showed the maximum red shift in the UV spectrum whereas the maximum enhancement of the emission intensity was observed in the presence of meta-carborane clusters. The order of π-conjugation extension is found to be ortho > meta ≈ para. A comparative thermal analysis indicated o-carborane-appended trimers to be the most thermally stable in the series. Proton NMR spectra of reported carborane-appended trimers indicated that ortho- and meta-carborane cages have benzenelike characteristics.