Results and Discussion

In order to examine the controlled nature of the polymerization time dependent GPC measurements were performed (figure 2-3). The PerInit 6 with 250 equivalents of styrene was heated at 125 °C.

Figure 2-3: A) Evolution of GPC elution curves for the polymerization of styrene (250 equiv) with 6 at 125 °C. B) Evolution of Mn (□) and conversion (●) for the same reaction. Mn was determined with GPC and the conversion with ¹H NMR.

In the beginning the conversion and Mn increased linear with time, but after one hour the increase became significantly lower. This behavior is due to the lower concentration of the monomer, but mainly due to the increase in viscosity. A series of different styrene or acrylate monomers were polymerized (table 2-1). Thus the properties of the resulting polymers could be tuned. For example, the glass transition temperature changes from -48°C for poly(n-butyl acrylate) to 133°C for poly(4-vinyltriphenylamine). As the polydispersities of the samples are

0 50 100 150 200 250 300 350 400

between 1.1 and 1.2, except for the polymerization of tert-butyl acrylate with a polydispersity of 1.3, the controlled nature of the polymerization is obvious.

Polymer monomer mol% 7 reaction time

Table 2-1: Polymerization conditions of different monomers, polymer data and thermal properties (from DSC and TGA) of the dye labeled polymers; polydispersity and Mn were determined by GPC with polystyrene standards using THF as solvent.*only the melting point at -17 °C could be detected.

The incorporation of initiators into a polymer can be investigated by UV/vis spectroscopy for dyes⁶⁸ or by potentiometric titration for amino terminated polymers⁶⁹. We used MALDI-TOF mass spectrometry as it is a very straightforward method and shows the complete composition

68 M. Rodlert, E. Harth, I. Rees, C. J. Hawker J. Polym. Sci. Part A: Polym. Chem. 2000, 38, 4749-4763.

69 C. J. Hawker, J. L. Hedrick Macromolecules 1995, 28, 2993-2995.

of the polymer. The spectra were measured in the reflectron mode giving the isotopic resolution of the polymer molecule (figure 2-4).

The MALDI-TOF mass spectra consist only of discrete peaks which have the distance of one repeating unit (here: styrene). A silver and a hydrogen atom is added from the matrix. The calculated spectrum matches with the measured spectrum of the 25mer and 26mer (figure 2-4B) with every polymer chain having one perylene bisimide acceptor and one alkoxyamine group. This is not only the proof for one perylene bisimide unit in each chain, but also for the controlled polymerization and the stability of the alkoxyamine initiator. The polymer is not fragmented at all in the workup or the ionization process.

Figure 2-4: A) MALDI-TOF-mass spectra of a low molecular perylene bisimide labeled polystyrene sample measured in reflectron mode. B) Magnification of two measured peaks of the 25mer and the 26mer and the corresponding simulated spectra (▼) of PerPS25 and PerPS₂₆. Recorded with DTCB as matrix and silver triflate.

Cyclic voltammetry was used for the determination of the HOMO (Highest Occupied M olecular Orbital) and LUM O (Lowest Unoccupied M olecular Orbital) level. A three-electrode assembly with a Ag/AgNO₃ electrode and CH₂Cl₂ containing 0.1 M tetrabutylammonium hexafluorophosphate as solvent was used. The perylene bisimide initiator PerInit 6 has two reversible reduction peaks and one oxidation peak. From these, the HOM O and LUM O can be calculated using ferrocene (HOM O = -4.80 eV) as internal standard. The LUMO is -3.74 eV and the HOMO is -6.01 eV with respect to zero energy level. The electronic gap is 2.27 eV.

2000 4000 6000 8000 10000

intensity

molecular weight [g/mol]

A

3660 3 680 3700 3 720 3740 3 760 [PerPS 26 + H + Ag]+

[PerPS25 + H + Ag]+

mo lecular weig ht [g/mol]

intensity

B

The optical and electro-optical properties were determined by UV/vis and fluorescence spectroscopy. In figure 2-5A the UV/vis absorption spectra of PerInit 6 and two triphenylamine substituted polymers 8A (Mn = 3050 g/mol) and 8B (Mn = 7510 g/mol) with different molecular weights are shown. The first vibronic transition of the electronic S0-S1

transition is shifted from 544 nm in 6 to 530 nm in 8A and 529 nm in 8B. The change in UV/vis spectra is caused by the π-π stacking of the perylene bisimide.

The aggregation of perylene bisimide dyes also change the intensity of the transitions⁷⁰. The quotient of the second and the first vibronic transitions is introduced as a parameter for the degree of order between the perylene bisimide moieties. This quotient changes from 1.68 for the PerInit 6 to 1.11 for 8A and 0.97 for 8B, thus indicating that the aggregation decreases from 6 to 8A and 8B.

Figure 2-5: A) UV/vis spectra of PerInit 6 and the polymers 8A and 8B in film. The UV/vis spectra are normalized. B) UV/vis and fluorescence spectra of the 8A and 9A in film.

The opto-electronic properties can also be tailored by using different monomers. In figure 2-5B the UV/vis and fluorescence spectra of a perylene bisimide labeled polystyrene 9 and a poly(4-vinyltriphenylamine) 8A are compared. PerPS 9A shows a strong red fluorescence in contrast to PerPvTPA 8A where the fluorescence is quenched completely. The triphenylamine quenches the fluorescence, most probably by an electron transfer. This is the same process as in block copolymer solar cells which will be discussed in the next chapters. By using polymers with a sufficient high molecular weight the stacking of the single perylene bisimide

70 B. A. Gregg J. Phys. Chem. 1996, 100, 852-859.

300 400 500 600 700 800

0

units can be avoided. The electron acceptor is then surrounded by electron donors which leads to a very fast fluorescence quenching.

Thus, perylene bisimide labeled initiators are suitable for the preparation of various dye labeled polymers. The polymerization has a controlled nature with low polydispersities and a complete incorporation of the perylene bisimide dye into the polymer chain is guaranteed.

The polymers can be viscous or semi-crystalline depending on the monomers used. As perylene bisimides are very strong fluorescent dyes the polymers can be used for single-molecule imaging⁷¹. If electron donors like triphenylamines are used, the fluorescence is quenched, therefore the polymers can be used as model systems for energy and electron transfer studies.

71 N. B. Bowden, K. A. Willets, W. E. Moerner, R. M. Waymouth Macromolecules 2002, 35, 8122-8125.