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micelles was elucidated by monitoring the Stern-Volmer quenching rates (K
SV
)of
tris(2,2
0
-bipyridyl)ruthenium(II) probes by methyl viologen in water (Gopidas
et al. 1991). A plot of the K
SV
versus dendrimer generation revealed an abrupt
increase in K
SV
going from generation 2.5 to 3.5, reminiscent of the CMC plot of
a micelle. At lower generations, the quenching kinetics were “bimolecular” in
nature, similar to that of small C-7/C-8 micelles. However, at generation 3.5 and
higher, the kinetic behavior was an “intramicellar” type similar to larger micelles
(C-9 and above). This behavior of the dendrimers was attributed to a cooperative
association of the terminal groups that occurs concomitant with a change from
a disklike conformation to a spherical topology. The conformational transition to a
dense-packed sphere was confirmed by electron paramagnetic resonance studies on
the corresponding copper complexes (Ottaviani et al. 1994, 1997). The copper com-
plexes exhibited a decrease in mobility with generation, consistent with a change in
dendrimer morphology.
11.2.2. Location of Terminal Groups
Theoretical Studies.
A majority of experimental (Mansfield and Klushin 1992;
Meltzer et al. 1992; Mourey et al. 1992; Wooley et al. 1997; Gorman et al. 1998)
and theoretical studies (De Gennes and Hervet 1983; Lescanec and Muthukumar
1990; Mansfield and Klushin 1993; Mansfield 1994; Boris and Rubinstein 1996;
Chen and Cui 1996; Murat and Grest 1996; Lue and Prausnitz 1997) indicate that
the terminal groups of nonamphiphilic dendrimers are distributed throughout the
molecular volume, resulting in a segmental density maximum near the core. In con-
trast, molecular simulation studies of dendrimers constructed with terminal groups
that interact with solvent differently compared with the internal monomers have
revealed a very different situation. These studies suggest that amphiphilic dendrimers
will adopt phase-segregated conformations placing the structural segments that inter-
act poorly with solvent near the core, regardless of whether the segment is located at
the periphery or at the interior of the dendrimer. Connolly et al. (2004) used
Metropolis Monte Carlo to vary the nature of the monomer-monomer interactions
in amphiphilic dendrimers. Two types of codendrimers were considered: “outer
H,” which consisted of hydrophobic (H) groups positioned at the periphery of an
otherwise polar (P) dendrimer; and an “inner H” system with the opposite
monomer polarity. Generally, they found that the inner H system formed well-
defined micellar structures at a lower generation, which projected the polar monomers
at the surface (Fig. 11.5a, c). In contrast, the outer H dendrimers adopted a “loopy
micelle” structure that backfolded the hydrophobic terminal groups toward the core
to simultaneously sequester the H monomers from solvent at the core while present-
ing the internal P monomers at the periphery (Fig. 11.5b, d). At higher generations,
both types of dendrimers formed elongated cigarlike cylindrical structures, which
presumably formed to balance the developing steric congestion at the periphery
with the need to segregate the P and H monomers (Fig. 11.5e, f ).
Giupponi and Buzza (2005) explored how the magnitude of the monomer-
solvent
interactions
impacted the dendrimer
structure using a lattice-based
