Nanoparticles of conjugated polymers

  • Department of Chemistry
  (2024): Method Development for Pulsed EPR Distance Determination – Light-Induced Techniques and Model Based Analysis

Method Development for Pulsed EPR Distance Determination – Light-Induced Techniques and Model Based Analysis


The thesis opens up new measurement methods which are used to extract dynamics and distance information from biological or material science systems. For this purpose, pulsed electron paramagnetic resonance (EPR) was used, in particular dipolar spectroscopy for the determination of distance information. As a wellknown representative, double electron-electron resonance (DEER) was used and existing measurement and evaluation methods were further developed. Additionally, the recently developed distance determination method, LaserIMD, was further optimized and finally applied in spatially resolved EPR imaging.

A toolbox for distance determination within oligomer and nanoparticle research was established. It was demonstrated that precise conjugated oligomers can be described by a worm-like chain model with an additional Gaussian broadening model. The contour length and persistence length of the oligomers could be extracted from the double electron-electron resonance (DEER) measurements. Not only isolated precise oligomers can be studied, but also a mixture of oligomers from controlled Suzuki-Miyaura Coupling Polymerization (cSMCP) reaction. Here, not only the persistence length of the oligomers could be extracted in this mixture, but also the fractions of oligomers of different lengths. The proportions of the different oligomer fractions could be described as Poisson distribution, with an expectation parameter of = 5:4. In summary, the behavior as living polymerization of the cSMCP was proven. The method does not rely on prior determination of oligomer parameters, They can also be extracted directly from oligomer mixtures measurements. Furthermore, the application to other conjugated oligomers was demonstrated. Finally, the oligomers were incorporated into nanoparticles and successfully measured using DEER. While the incorporation of spin probes into oligomers and nanoparticles does not affect their structure for the most part, biological systems in particular can be disrupted in their function as a result. Laser-induced magnetic dipole spectroscopy (LaserIMD) was previously developed by our group and is an important building block for EPR distance determination, mostly in biological systems. If a chromophore is already present, LaserIMD and light-induced double electronelectron resonance (LiDEER) can be used, since only one additional spin probe is then required. In this work, the focus was set on the development and optimization of these two specialized EPR distance determination methods. The work presented has contributed to the regular use of both techniques in the scientific community today.182–187 For this work, a laser was used, with the capability to tune the wavelength freely between 405 nm and 709 nm. All optimizations were performed on two differently long model systems, bearing one porphyrin and nitroxide, named Short and Mid. The optimal excitation wavelength for the used porphyrin was determined to be 514 nm and different ways of light coupling were shown. Bleaching was found to play only a minor role in these measurements for the given measurement durations. With echo transient measurements, the optimal integration window was found for LaserIMD and LiDEER. A software suite, based in MATLAB was developed to allow zero time determination and separation of LaserIMD traces by symmetry criterion. Furthermore, the excitation scheme, i.e. the pulse positions within the resonator and EPR spectra as well as the pulse shapes were optimized for both LaserIMD and LiDEER. For LiDEER, symmetric excitation is more advantageous than asymmetric excitation. While for LaserIMD, chirped pulses did not bring any benefit in terms of modulation-to-noise ratio (MNR), LiDEER MNR improves with the use of chirped pump pulses. When comparing both techniques in terms of modulation-to-noise ratio (MNR), LiDEER has an advantage over LaserIMD for longer dipolar evolution times (19 % improvement). In contrast, for short dipolar evolution times (and distances), LaserIMD is favorable (67 % improvement). The reason for this is the faster phase memory time of the nitroxide (2 μs, LaserIMD) over the triplet (6 μs, LiDEER). Furthermore, the first spatially resolved distance determination experiment using LaserIMD was reported. This was possible because LaserIMD, unlike (Li)-DEER, uses only one m.w. excitation wavelength. It was shown that in a sample ontaining two different spatially separated model systems, both distances could

be resolved individually using pulsed EPR imaging with the LaserIMD experiment.

Origin (projects)

Funding sources
Name Finanzierungstyp Kategorie Project no.
SFB third-party funds research funding program 586/16
Further information
Period: 01.07.2016 – 30.06.2020