Universal Scaffold for Fluorescent Conjugated Organoborane Polymers

Abstract

Conjugated organoboron polymers represent an attractive class of new materials with diverse potential applications, including as nonlinear optical materials, conduction and emission layers for organic light-emitting devices, and chemosensors for anions and toxic small molecules. [1] Typical methods for their synthesis are based on hydroboration polymerization techniques [2] and polycondensation reactions, [3,4] including the metal-catalyzed C À C bond formation of suitable preformed organoboron building blocks. [5, 6] An alternative highly modular approach is to prepare a universal reactive polymer scaffold that can easily be transformed into a range of other polymers with widely varying properties and applications through straightforward polymer modification procedures. [4,7,8] For instance, in the case of polyolefins the reactive poly(4-dibromoboryl styrene) is readily converted to 1) a range of polymeric Lewis acids with variable degree of Lewis acidity, 2) luminescent sensor materials with pendant anion binding sites, 3) solutionprocessable polymeric organoboron analogues of the wellknown OLED device material tris(8-hydroxyquinolinato)aluminum (Alq 3 ). Even new ligand frameworks for redoxactive metallopolymers become available. [9,10] A universal fluorescent conjugated organoboron polymer scaffold would be very desirable owing to the interesting effects relating to the extended p p -p* conjugation [1] via the empty boron p orbital throughout the polymer main chain. Moreover, use of a single precursor polymer has the great advantage that the electronic and photophysical characteristics can more reliably be compared for polymer chains of equal length and distribution. We report herein the successful synthesis of such a novel polymer framework based on polyfluorene and demonstrate its utility as a universal scaffold for luminescent fluorene polymers that feature tricoordinate aryl borane and tetracoordinate organoboron quinolate functionalities.The main challenge in the preparation of a reactive polymer [ArB(X)] n (Ar = arylene bridge, X = labile substituent) is the need to achieve excellent control over the condensation reaction such that the (disubstituted) linear polymer forms selectively, without conversion of the remaining labile halide substituent X on boron. We chose the poly(fluorenylborane) framework [FlB(Br)] n (Fl = 9,9-dialkyl fluorenyl) because of the high selectivity with which this reactive polymer scaffold can be prepared from simple precursor components and the high luminescence quantum efficiency typically associated with fluorene polymers. [11,12] In a first attempt, we treated the bifunctional organotin species Fl(SnMe 3 ) 2 with an equimolar amount of BBr 3 . However, owing to the extremely high reactivity of BBr 3 , the polymerization was difficult to control. Instead, we then decided to treat the preformed (and less reactive) aryl dibromoborane species Fl(BBr 2 ) 2 with the aryl ditin reagent Fl(SnMe 3 ) 2 in an organometallic polycondensation reaction (Scheme 1). [13] The polymerizatio...

Synthesis of Functionalizable Boron‐Containing π‐Electron Materials that Incorporate Formally Aromatic Fused Borepin Rings

“…The homodimerization of 1 d that led to 7 was effected using a nickel catalyst (with stoichiometric zinc reductant). [28,29] Kumada-type couplings of aryl Grignard reagents onto 1 e using [Ni(dppp) 2 Cl 2 ] led to the aryl coupled products 8 a-b. As with the parent DBB, 7 and 8 were stable under ambient conditions and could be chromatographed with no apparent decomposition.…”

Doping the Backbone of π‐Conjugated Polymers with Tricoordinate Boron: Synthetic Strategies and Emerging Applications

“…Subsequently, Jäkle and co‐workers reported the Sn/B exchange polycondensation of bis‐stannyl fluorene 8 with the bis(dibromoboryl) fluorene derivative 9 (Scheme ) . This reaction sequence has the advantage that the B ‐substituents of the final products, 11 a , b , are introduced at the postpolymerization stage ( 10 → 11 ).…”

“…Jäkle and co‐workers applied the Sn/B exchange condensation approach to gain access to various organoborane polymers and oligomers . For example, a poly(thienylborane) was obtained by a polycondensation‐postmodification sequence similar to the one shown in Scheme .…”

Anorganische Chemie 2009

“…Modifizierbare konjugierte Polymere sind aus einem B‐Br‐verbrückten Fluorenpolymer erhältlich 1_66. Boranaloge Anthra‐, Naphtho‐ und Pentacene mit zentralen Borepinringen (30) zeigen blaue Fluoreszenz mit z. T. hohen Quantenausbeuten und akzeptabler Stabilität gegenüber Luft und Feuchtigkeit 1_67…”