Fluorescence enhancement by symmetry breaking in a twisted triphenylene derivative.

1,4,5,8,9,12-hexamethyltriphenylene (HMTP) shows a high photoluminescence quantum yield (PLQY) of 31% in the solid state, making it of interest for luminescence applications. The detailed photophysical properties of HMTP have been investigated by using time-resolved and steady-state luminescence, PLQY, and molar absorption coefficient measurements. An enhancement of the transition dipole moment for fluorescence and absorption was demonstrated compared to the case of unsubstituted triphenylene, which resulted in a 20-fold increase in the radiative decay rate. This is attributed to a breaking of triphenylene symmetry as a result of the necessarily twisted structure induced by steric crowding. In addition, it was shown that HMTP shows similar photoluminescence energies in solution, powder, and film, indicating a reduced propensity for intermolecular π-stacking compared to the case of triphenylene, as a result of this twisted structure. This work also develops a method for calculating the photoluminescence quantum yield of powders by using a calibrated photodiode in combination with an uncalibrated CCD spectrometer.

1,4,5,8,9,12-Hexamethyltriphenylene. A molecule with a flipping twist.

The synthesis and characterization of 1,4,5,8,9,12-hexamethyltriphenylene (5) is described. In the solid state, X-ray crystallographic studies reveal that compound 5 presents a highly distorted C2 geometry with a 53 degrees end-to-end twist. In solution, variable-temperature 1H NMR studies and molecular modeling present a story of rapid dynamic conformational interconversions between two C2 enantiomers (with a low activation barrier) and a slower C2-D3 interconversion (with a relatively high barrier)--the first time clear evidence of conformational interchange for these hindered triphenylenes has been provided. Further studies have established that 5 is a fluorescent stable blue emitter, and that the compound undergoes an irreversible one-electron electrochemical oxidation. Calculations have predicted this to be a radical cation of C2 geometry with 60 degrees end-to-end twist.

Mechanically linked polycarbonate.

The synthesis, by solid-state copolymerization, and characterization of the first polycatenanes based on a commercial polymer are reported. Various amounts of a benzylic amide [2]catenane, the corresponding macrocycle, and a rigid bisphenol fluorene derivative have been quantitatively and homogeneously incorporated into bisphenol A polycarbonate. The resulting copolymers were characterized by size exclusion chromatography coupled with viscosimetry, (1)H NMR, differential scanning calorimetry, and dynamic mechanical analysis. The unexpectedly small influence of [2]catenane incorporation on the glass transition temperature of the copolymers points to remarkable internal mobility of the catenane comonomer rings. A new relaxation linked to the flexible catenane units is also observed. The studies represent a detailed structural characterization of a polymer containing small amounts of mechanical linkages in its backbone and demonstrate that significant effects can be induced by doping conventional polymers with small percentages (2-6% of repeat units) of flexible catenanes.

Specific interactions of complementary mono- and multivalent guests with recognition-induced polymersomes.

We have explored the interactions of mono- and multivalent guests with Recognition-Induced Polymersomes (RIPs) formed from complementary random copolymers featuring diamidopyridine and thymine functionality. Addition of monovalent guests featuring imide functionality to these RIPs induced a temporary swelling of the vesicles, followed by dissociation of the vesicles due to competitive binding of the guest. Conversely, multivalent thymine-functionalized nanoparticle guests were rapidly incorporated into the RIPs, inducing a contraction of RIP diameter over time. These mono- and multivalent interactions were extremely specific: highly analogous control systems showed no interaction with the RIP structures. Taken together, these studies demonstrate highly selective molecular "lock and key" control over higher-order assembly and recognition processes.

Supramolecular assembly on surfaces: manipulating conductance in noncovalently modified mesoscale structures.

Molecules capable of complementary hydrogen bonding were used to control the noncovalent self-assembly and electronic properties of a chemically well-defined surface mesostructure. In this work, we patterned a footprint region for molecular assembly on a surface and used moieties featuring complementary recognition to tune the current-voltage properties of the patterned region. With the appropriate functionalities on the complementary moieties, we were able to increase and decrease the observed conductance in surface-bound mesoscale structures imaged by scanning tunneling microscopy (STM).

Highly reactive heterogeneous Heck and hydrogenation catalysts constructed through 'bottom-up' nanoparticle self-assembly.

Polymer-mediated self-assembly of functionalized Pd and SiO2 nanoparticles provides highly active hydrogenation and Heck coupling catalysts.