2 research outputs found
Fluorescence Correlation Spectroscopy in Dilute Polymer Solutions: Effects of Molar Mass Dispersity and the Type of Fluorescent Labeling
Fluorescence correlation spectroscopy
(FCS) has become an important
tool in polymer science. Among various other applications the method
is often applied to measure the hydrodynamic radius and the degree
of fluorescent labeling of polymers in dilute solutions. Here we show
that such measurements can be strongly affected by the molar mass
dispersity of the studied polymers and the way of labeling. As model
systems we used polystyrene and polyÂ(methyl methacrylate) synthesized
by atom transfer radical polymerization or free-radical polymerization.
Thus, the polymers were either end-labeled bearing one fluorophore
per chain or side-labeled with a number of fluorophores per chain
proportional to the degree of polymerization.The experimentally measured
autocorrelation curves were fitted with a newly derived theoretical
model that uses the Schulz–Zimm distribution function to describe
the dispersity in the degree of polymerization. For end-labeled polymers
having a molecular weight distribution close to Schulz–Zimm,
the fits yield values of the number-average degree of polymerization
and the polydispersity index similar to those obtained by reference
gel permeation chromatography. However, for the side-labeled polymers
such fitting becomes unstable, especially for highly polydisperse
systems. Brownian dynamic simulations showed that the effect is due
to a mutual dependence between the fit parameters, namely, the polydispersity
index and the number-average molecular weight. As a consequence, an
increase of the polydispersity index can be easily misinterpreted
as an increase of the molecular weight when the FCS autocorrelation
curves are fitted with a standard single component model, as commonly
done in the community
Photophysical Investigation of Cyano-Substituted Terrylenediimide Derivatives
Two new terrylenediimide (TDI) chromophores
with cyano substituents
in the bay and core area (BCN-TDI and OCN-TDI, respectively) have
been characterized by a wide range of techniques, and their applicability
for stimulated emission depletion (STED) microscopy has been tested.
By cyano substitution an increase of the fluorescence quantum yield
and a decrease of the nonradiative rate constant is achieved and attributed
to a reduced charge-transfer character of the excited state due to
a lower electron density of the TDI core. For BCN-TDI, the substitution
in the bay area induces a strong torsional twist in the molecule which,
similar to phenoxy bay-perylenediimide (PDI), has a strong effect
on the fluorescence lifetime but appears to prevent the aggregation
that is observed for OCN-TDI. The single-molecule photobleaching stability
of BCN- and OCN-TDI is lower than that of a reference TDI without
cyano substitution (C7-TDI), although less so for OCN-TDI. The photophysical
properties of the excited singlet state are only slightly influenced
by the cyano groups. The observed intense stimulated emission, the
pump–dump–probe experiments, and STED single-molecule
imaging indicate that STED experiments with the cyano-substituted
TDIs are possible. However, because of aggregation and more efficient
photobleaching, the performance of BCN- and OCN-TDI is worse than
that of the reference compound without cyano groups (C7-TDI). Bay-substituted
TDIs are less suitable for STED microscopy