Article pubs.acs.org/joc Influence of Conjugation Axis on the Optical and Electronic Properties of Aryl-Substituted Benzobisoxazoles † ‡ ‡ † † Brian C. Tlach, Aimeé L. Tomlinson, Alden G. Ryno, Dawn D. Knoble, Dana L. Drochner, † Kyle J. Krager, and Malika Jeffries-EL*, † Department of Chemistry, Iowa State University, Ames, Iowa 50010, United States ‡ Department of Chemistry, University of North Georgia, Dahlonega, Georgia 30597, United States *S Supporting Information ABSTRACT: Six different 2,6-diethyl-4,8-diarylbenzo[1,2-d:4,5-d′]bis(oxazoles) and four different 2,4,6,8-tetraarylbenzobisoxazoles were synthesized in two steps: a Lewis acid catalyzed orthoester cyclization followed by a Suzuki or Stille cross-coupling with various arenes. The influence of aryl group substitution and/or conjugation axis variation on the optical and electronic properties of these benzobis(oxazole) (BBO) compounds was evaluated. Structural modifications could be used to alter the HOMO, LUMO, and band gap over a range of 1.0, 0.5, and 0.5 eV, respectively. However, depending on the location and identity of the substituent, the HOMO level can be altered without significantly impacting the LUMO level. This is supported by the calculated frontier molecular orbitals. Our results indicate that the FMOs and band gaps of benzobisoxazoles can be readily modified either jointly or individually. ■ INTRODUCTION Among the aforementioned examples, the benzo[1,2-d:4,5- d′]bis(oxazole) (BBO)-based cruciforms are particularly During the past four decades, interest in the development of π- interesting, since these molecules have two different con- conjugated materials has increased due to their potential use as jugation pathways: 2,6-conjugation through the oxazole rings replacements for inorganic materials in a variety of semi- and 4,8-conjugation through the central benzene ring (Scheme conducting applications including field effect transistors − − 1). Since one pathway encompasses heterocyclic rings, the (FETs),1 5 organic light emitting diodes (OLEDs),6 8 and 9−12 optical and electronic properties can be altered as a function of photovoltaic cells (PVCs). In addition to the ability to be the substitution pattern around the central benzene ring. processed from solution, organic semiconductors can be Recently, our group24,25 and that of Miljanic13́have shown that modified at the molecular level to optimize the optical and fi the conjugation of BBOs is readily extended via Sonogashira electronic properties of the materials for speci c applications. cross-coupling reactions between 4,8-dibromoBBOs and π Since the characteristics of -conjugated systems are strongly various alkynes. The resulting materials have HOMO and fl in uenced by the highest occupied molecular orbital (HOMO) LUMO levels that could readily be tuned by substitution. and the lowest unoccupied molecular orbital (LUMO), However, the synthesized cruciforms that were reported feature synthetic strategies that can alter these parameters are of combinations of triple bonds between the arenes along the 4,8- paramount importance. Unfortunately, most chemical mod- axis and single bonds between the arenes along the 2,6-axis, so ifications of π-conjugated materials result in changes in the correlating optical and electronic properties to the conjugation position of both of the frontier molecular orbitals (FMOs). pathway was not possible.16,26,27 The Nuckolls group previously This is a result of the extensive delocalization of electrons synthesized tetraarylBBO cruciforms; however, since their within these systems, rendering selective modification of either interest was in developing rigid molecules for self-assembled the HOMO level or LUMO level difficult. One approach for molecular electronics, the optical and electronic properties of independently tuning FMOs within π-conjugated materials is these systems were not fully explored.15,28 through the synthesis of two-dimensional molecules that In order to understand how different substitution patterns feature two perpendicular π-conjugated linear “arms” connected will affect the optical and electronic properties of larger BBO − through a central aromatic core.13 23 These so-called “cruci- structures, we performed a more detailed structure−property forms” possess spatially segregated FMOs, enabling strategic relationship study. Modification of the aryl group has previously tuning of either the LUMO or the HOMO by varying the been used to vary properties through extension of the π system nature of the substituents and their arrangement around the and inductive effects of their electron-donating or electron- 16,26−32 central molecule. Representative examples include benzobis- withdrawing nature. Furthermore, aryl substituents are − (oxazole)-based cruciforms,13 15 distyrylbis(arylethynyl)- − − benzenes,16 18 tetrakis(arylethynyl)benzenes,19 22 and tetra- Received: April 24, 2013 styrylbenzenes.23 Published: June 25, 2013 © 2013 American Chemical Society 6570 dx.doi.org/10.1021/jo4007927 | J. Org. Chem. 2013, 78, 6570−6581 The Journal of Organic Chemistry Article Scheme 1. Proposed Sequential Functionalization of the 2,6- and 4,8-Axes of BBO robust, are easily modified, and can be installed through a Scheme 3. Synthesis of 2,6-diethylBBOs variety of methods. First, we evaluated the influence of different aryl groups at the 4- and 8-positions of the BBO moiety using a range of spectroscopic techniques along with computational modeling to gain further insight about the FMO shape and electron distribution. Then, we assessed the impact of conjugation pathway by comparing a pair of 2,6-diarylBBOs to the analogous 4,8-diarylBBOs. Finally, we compared four BBO cruciforms featuring two different aryl groups to study how properties change upon expanding to a two-dimensional π − system. These structure property studies provide basic insight are easier and safer to synthesize, the facile protodeboronation π 37 on BBO behavior in two-dimensional -systems as a function of that occurs with thien-2-ylboronic acids required the use of a aryl group selection and conjugation axis. Stille cross-coupling for the synthesis of 4,8-bis(5-dodecylthien- 2-yl)-2,6-diethylBBO 8. To our knowledge, this is the first ■ RESULTS AND DISCUSSION report of aryl−aryl cross-coupling of 4,8-dibromoBBOs. Synthesis of 4,8-diarylBBOs. The synthesis of compounds Synthesis of 2,6-diarylBBOs. The synthetic approach for 3−8 is shown in Scheme 2. Initially, we set out to synthesize the 2,6-diarylBBOs is shown in Scheme 4, and the synthesis of the requisite orthoesters is shown in Scheme 5. We prepared 5- a Scheme 2. Synthesis of 4,8-diaryl-2,6-diethylBBOs dodecyl-2-(triethoxymethyl)thiophene in one pot from 2- bromo-5-dodecylthiophene using the method reported by Tschitschibabin.36,38 The synthesis of 4-dodecyl-1- (triethoxymethyl)benzene proved more difficult, as the Tschitschibabin method failed to yield the desired orthoester. In a search for an alternative approach, we found that aryl orthoesters have been prepared from trithio orthoester intermediates.39,40 Since the corresponding aldehyde was more readily available, we decided to approach the synthesis of the trithio orthoester intermediate starting from 2-(4- bromophenyl)-1,3-dithiane. This decision proved beneficial, as the dithiane protecting group allowed for the addition of a solubilizing alkyl chain via a Kumada cross-coupling reaction, and the targeted orthoester was obtained in 84% yield. The 2,6- diarylBBOs 9 and 11 were obtained by the condensation of the corresponding orthoesters and 2,5-diamino-1,4-hydroquinone a − 36,41 Reaction conditions: (i, 3 7) Pd(dppf)Cl2,K2CO3, TBAB, boronic hydrochloride (DAHQ) in yields of 73% and 65%, o acid or ester, toluene/H2O (10/1); (ii, 8)Pd2(dba)3,P(tolyl)3,5- respectively. Due to their rigid-rod nature, compounds 9 and dodecyl-2-(trimethylstannyl)thiophene, toluene. 11 had noticeably higher melting points and decreased solubility in comparison to their respective 4,8-diarylBBOs 5 the 4,8-diarylBBOs using the Suzuki cross-coupling reaction of and 7. various arylboronic acids and 4,8-dibromo-2,6-dimethylBBO.24 Synthesis of 2,4,6,8-tetraarylBBOs. The synthetic Suzuki coupling was chosen, as boronic acids and esters do not approach for the 2,4,6,8-tetraarylBBOs is shown in Scheme 6. require the use of toxic reagents and are readily purified by The Nuckolls group accomplished the synthesis of their standard techniques. Unfortunately, this approach gave low tetraarylbenzo[1,2-d:4,5-d′]bis(oxazole) cruciforms via a dou- yields of 4,8-diarylBBOs. We hypothesized that the labile ble-Staudinger cyclization of substituted bis(azidoquinones).15 methyl protons were being deprotonated under the basic Although the reactions occurred in good yields, in their strategy reaction conditions, resulting in undesirable side reactions. The the substituents are introduced early in the reaction sequence. Hegedus group previously decreased the reactivity of the 2,6- Thus, several steps are repeated to make different molecules. A position of BBOs by substituting ethyl groups for methyl more versatile approach is to first synthesize 2,6-diaryl-4,8- groups.33 Therefore, we synthesized 2,6-diethyl BBOs 1 and 2 dibromoBBOs and then extend conjugation across the central via the Lewis acid catalyzed condensation reaction between the benzene ring via cross-coupling chemistry. This approach corresponding diamino hydroquinone and triethyl orthopropi- allows for the synthesis of several BBOs using common − onate, as shown in Scheme 3.34 36 The Suzuki cross-coupling intermediates. The condensation reaction of 3,6-diamino-2,5- reaction of 2 and the appropriate boronic acids or esters dibromo-1,4-hydroquinone (Br-DAHQ) with 5-dodecyl-2- afforded 3−7 in yields of 60−83%. Although arylboronic acids (triethoxymethyl)thiophene or 4-dodecyl-1-(triethoxymethyl)- 6571 dx.doi.org/10.1021/jo4007927 | J. Org. Chem. 2013, 78, 6570−6581 The Journal of Organic Chemistry Article Scheme 4. Synthesis of 2,6-diarylBBOs Scheme 5. Synthesis of Substituted Orthoesters 60−94% yield. Purification of these compounds was easily accomplished by first passing through a short silica gel plug to remove polar impurities and residual catalyst followed by recrystallization from an appropriate solvent.