The structure of a series of poly(amidoamine) dendrimers
Gn(C12) generated from a diaminododecane
core have been investigated using the photophysical properties of an
external dye, nile red. The modified
dendrimers Gn(C12) show the ability to host the hydrophobic dye,
nile red, in aqueous solution. The ability
of Gn(C12) to host nile red has been compared to corresponding
amino-core Gn(NH3) and diaminoethane-core Gn(C2) dendrimers of the same generation size. The
emission of nile red in aqueous media is significantly
enhanced in the presence of Gn(C12) and not at all for
Gn(NH3) and Gn(C2). These results imply a
strong
tendency for the nile red probe to associate with the long methylene
chain of the modified dendrimers in
aqueous solutions. Moreover, the interactions of these dendrimers
with anionic surfactants generate
supramolecular assemblies which greatly enhance their ability to
accomodate the nile red. Fluorescence
polarization and emission as a function of pH were also studied in an
effort to elucidate the interaction
of the nile red probe with the dendrimer−surfactant
assemblies.
The photophysics (2,4,6-trimethylbenzoyl)diphenylphosphine
oxide (1) and bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide (4) have been
investigated by fluorescence, phosphorescence, and low
temperature
time resolved electron spin resonance. Both 1 and
4 undergo α-cleavage to produce benzoyl and phosphorous
centered
radicals. The photochemistry of 1 and 4 has
been investigated by nanosecond laser flash photolysis,
picosecond
pump-probe spectroscopy, and steady-state photolysis. The singlet
states of 1 and 4 and the phosphorous
centered
radicals produced by α-cleavage were characterized directly by time
resolved absorption spectroscopy. The triplet
states of 1 and 4 were characterized indirectly
by quenching with 1-phenylnaphthalene as a selective triplet
quencher.
The use of 1-phenylnaphthalene indicates that α-cleavage occurs
mainly from the triplet states of 1 and 4.
However,
the observed rate of formation of phosphorous centered radicals derived
from picosecond investigations is
experimentally indistinguishable from the rate of disappearance of the
singlet states of 1 and 4. The results
are
compatible with mechanisms for which the rate of intersystem crossing
of the S1 states of 1 and 4 limits
the observed
rate of α-cleavage, because the rate of α-cleavage is of the same
order or faster than the rate of intersystem crossing.
This relatively uncommon situation appears to have an analogy in
the well investigated photochemistry of dibenzyl
ketone.
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