Control of particle-particle spacing is a key determinant of optical, electronic, and magnetic properties of nanocomposite materials. We have used poly(amidoamine) (PAMAM) dendrimers to assemble carboxylic acid-functionalized mixed monolayer protected clusters (MMPCs) through acid/base chemistry between the particle and dendrimer. Small angle X-ray scattering was then used to establish average inter-MMPC distances. Five generations of PAMAM dendrimer (0, 1, 2, 4, 6) were investigated, with a monotonic increase in interparticle spacing from 4.1 to 6.1 nm observed with increasing generation.
Polystyrene-based diblock copolymers, featuring diaminotriazine functionality on one of the blocks were used to assemble complementary uracil-functionalized nanoparticles into micellar aggregates. The size of these self-assembled aggregates was controlled by block length, as determined in solution (using dynamic light scattering), and in thin films (using transmission electron microscopy).
The syntheses of three new bis(semiquinone)s (2
••2-, 3
••2-, and 5
••2-) linked through carbon−carbon
double bonds in a geminal fashion (2
••2- and 3
••2-), and through an sp3 carbon (5
••2-), are presented,
as well as the results of variable-temperature EPR (VT-EPR) spectroscopy on these biradicals and
two previously reported bis(semiquinone)s, 1
••2- and 4
••2-. We suggest that the potential difference
in redox couples associated with a biradical is useful for qualitatively assessing changes in the
exchange parameter within an isostructural series. The zero-field-splitting parameters for 1
••2-−5
••2- are consistent with their electronic structures: biradicals 1
••2-−3
••2- which have conjugating
groups attached to the semiquinone rings have D-values less than 5
••2-, a bis(semiquinone) that
lacks such a conjugating group. Also, the D-value of 3
••2- is significantly less than those of 1
••2-
and 2
••2-, in agreement with larger interelectron separation in 3
••2-, a biradical with quinone-methide
π-system delocalization. Changes in counterion Lewis acidity are not manifested in the D-values
of the biradical dianion 4
••2-. The EPR spectrum of biradical 5
••2- is consistent with the existence
of at least two rotamers having different zero-field-splitting parameters. Biradicals 1
••2-−4
••2- gave
linear Curie plots, consistent with J > 0 (ferromagnetic coupling) or J = 0. The temperature-dependent intensity of EPR signals of 5
••2- are characteristic of antiferromagnetic coupling. Best
fit results give J = −114 ± 6 cal/mol for the |D/hc| = 0.01309 cm-1 rotamer of 5
••2-, and J = −76
± 3 cal/mol for the |D/hc| = 0.01026 cm-1 rotamer of 5
••2-.
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