researchB

Jäkle Group
Research in Organometallic & Polymer Chemistry

Research

Project B. Main-Chain Functionalized Polymeric Lewis Acids
The incorporation of electron-deficient boron centers into conjugated polymer structures is of much interest because overlap between the empty p-orbital on boron and the pi-system of conjugated aromatic groups gives rise to unusual photoluminescent and electron-conducting properties. One of the key issues in the design of an efficient sensor system is whether individual recognition sites along the polymer backbone are acting independently or in a cooperative fashion.

X-ray structure courtesy of Prof. Rheingold				In order to address this question, we have synthesized new bifunctional conjugated organoboranes such as Th2B2Pf4 that represent a fragment of a conjugated organoboron polymer and thus allow us to study cooperative effects between adjacent boron centers. NMR spectroscopic and luminescence studies clearly indicate that the individual boron centers indeed strongly interact with each other.[1] Based on these observations we have prepared a new family of main chain polymeric Lewis acids (PTh-BAr) that contain Lewis acidic boron groups embedded into a polythiophene backbone. These organoboron polymers are formed under mild conditions through tin-boron exchange reaction (see C&EN, 09/13/2004 issue).[2,3] When aromatic groups such as phenyl and pentafluorophenyl groups are attached to boron, blue and green

luminescence is observed, respectively, while the attachment of ferrocenyl substituents leads to a characteristic red color. The incorporation of readily accessible highly Lewis acidic groups into the conjugated polymer backbone provides an opportunity for sensing of Lewis basic substrates. For instance, treatment of the polymers containing phenyl substituents on boron with pyridine leads to efficient quenching of the fluorescence, while polymers containing ferrocenyl groups change color from red to light orange.

Most recently we have succeeded in the preparation of a novel class of electronically interesting side-group borylated polythiophene via silicon-boron exchange of a silylated polythiophene precursor.[4] This new approach of lateral substitution of conjugated polymers with Lewis acidic boryl groups is highly promising because the conjugated polymer main chain remains uninterrupted, while the electronic structure is directly affected by the pendant boryl substituents.[5]

References:

(1) A. Sundararaman, K. Venkatasubbaiah, M. Victor, L. N. Zakharov, A. L. Rheingold, F. Jäkle, J. Am. Chem. Soc. 2006 , 128, 12879-12885; "Electronic Communication and Negative Binding Cooperativity in Diborylated Bithiophenes".

(2) This research has been covered in the 09/13/2004 issue of C&E News (Vol 82, issue 37, p 29): http://pubs.acs.org/email/cen/html/091404205912.html .

(3) A. Sundararaman, M. Victor, R. Varughese, F. Jäkle, J. Am. Chem. Soc. 2005, 127, 13748-13749; "A Family of Main Chain Polymeric Lewis Acids: Synthesis and Fluorescent Sensing Properties of Boron-modified Polythiophenes".

(4) H. Li, A. Sundararaman, K. Venkatasubbaiah, F. Jäkle, J. Am. Chem. Soc. 2007, 129, 5792-5793; "Organoborane Acceptor-Substituted Polythiophene via Side-Group Borylation ".

(5) This new approach has been further discussed in a recent Highlight Article in Angewandte Chemie: M. Elbing and G. C. Bazan Angew. Chem. Int .Ed. 2008, ASAP; “A New Design Strategy for Organic Optoelectronic Materials by Lateral Boryl Substitution”.

Research

Project A

Project C

Project D