Out on a limb: Sonogashira coupling of a suitable AB(2) monomer containing two iodine and one alkyne group forms a hyperbranched conjugated polymer that is studded with iodine end groups (see picture: I purple). These iodine groups are a perfect handle for convenient, efficient, and high-yielding post-functionalization to access hyperbranched, fluorescent poly(phenyleneethynylene)s.

In this contribution, the synthesis of two different hyperbranched poly(phenylene vinylenephenylene ethynylene) (HB-PPV-PPE, 3a-b) scaffolds are reported. The necessary AB 2 monomer 2 is obtained by an asymmetric Horner reaction and contains two iodine substituents and one ethynyl group. This AB 2 monomer provides, after Sonogashira polymerization, a hyperbranched conjugated polymer decorated with iodine groups, suitable to be further functionalized with terminal alkynes 4a-t by a Pd-catalyzed coupling. Elemental analysis of the postfunctionalized polymers 5a-t and 6a-q reveals nearly complete substitution of the iodine groups. As a consequence of the replacement, substantial increases in fluorescence quantum yields as well as variation in the optical response of the new polymers are observed. The spectroscopic properties of the postfunctionalized polymers were investigated in solution and in the solid state. The emission of the formed hyperbranched polymers shows a strong dependence on the functional groups attached to the conjugated structure and ranges from 510 nm for 5a to 602 nm for 6h. In all cases, solidstate emission is surprisingly strong and red-shifted compared with emission observed in solution.

Post-functionalization strategies allow functional tuning late in a reaction sequence, but classic polymer chemistry operates on a different principle, as each polymer starts from its own monomer. [1] Wecks universal polymer backbone, by which a simple polynorbornene carrying a supramolecular attachment site is reversibly occupied by complementary species, is an important step towards post-functionalization strategies. [2] Another is the 1,3-dipolar cycloaddition of alkynes and azides that allows functionalities to be clicked onto a polymer backbone. [3] In the case of conjugated polymers, post-functionalization schemes that allow the manipulation of the electronic structure by interposition of a suitable reagent are rare. End functionalization is possible, but problematic, owing to the low concentration of end groups. The synthesis of a hyperbranched conjugated polymer by polycondensation of an AB 2 monomer would circumvent this issue. Once formed, such a hyperbranched polymer would have one functional reactive B group per monomer. Such a polymer should-as long as the polycondensation reaction that forms the polymer is irreversible-allow the facile post-functionalization with any species that carries a complementary functional group.Linear and hyperbranched [4] poly(phenyleneethynylene)s and dendrimeric [5] species based upon 1,2,4-trisubstituted benzenes have been made, but their potential with respect to postfunctionalization schemes that manipulate electronic structure and potential binding affinities to metal ions or other potential analytes of interest has been surprisingly rarely exploited. [6] The most thorough investigation of hyperbranched PPEs was performed by Moore et al., [7] but their system, which is based on 1,3,5-triethynylbenzene units, was not designed nor intended to show enhanced electronic interactions, and they only reported post-functionalization of an insoluble hyperbranched polymer with 3,5-bis(tert-butyl)phenylacetylene to obtain a soluble material. Weder et al. [8] prepared truly conjugated branched PPEs with interesting optical properties. Herein, we introduce a dodecyloxy group into the monomer 4; the resulting polymers 5 and 9 are therefore soluble and processible, and 9 attains variable functionalities.Starting from 1, Horner reaction with 2 a furnishes 3, which, after a second Horner reaction with 2 b and subsequent deprotection, gives the monomer 4, which carries two iodine groups. Classic Sonogashira polymerization of 4 in a mixture of THF and piperidine with CuI as co-catalyst furnishes the hyperbranched polymer 5 in 87 % yield, with a molecular weight of 2.4 10 4 and a polydispersity index M w /M n of 2.0 (Scheme 1, Figure 1). In a similar fashion, the model compound 7 and the linear conjugated polymer 6 (M n = 2.5 10 4 , M w /M n = 2.5, Scheme 2) were prepared (see the Supporting Information). By coincidence, both polymers had a similar molecular weight, which allowed a comparison of their intrinsic viscosity in chloroform, namely [h] = 0.19 dL g À1 for 5 and [h] = 0.32 d...