Novel polyfluorene based copolymers for optoelectronic applications [Elektronische Ressource] / von Frank Galbrecht

Novel Polyfluorene Based Copolymers
for Optoelectronic Applications


Dissertation

zur Erlangung des akademischen Grades

Doktor der Naturwissenschaften
(Doktor rerum naturalium)


Bergische Universität Wuppertal
Fachbereich C – Mathematik und Naturwissenschaften



von

Frank Galbrecht
aus Wuppertal

Wuppertal, 2008










„Alles Große in unserer Welt geschieht nur,
weil jemand mehr tut, als er muß.“

Hermann Gmeiner (1919-1986), österreichischer Sozialpädagoge,
Gründer der SOS-Kinderdörfer



„… von den sicheren Dingen
Das Sicherste ist der Zweifel.“

Bertolt Brecht (1898-1956), deutscher Dramatiker und Lyriker,
Begründer des epischen Theaters








Die Dissertation kann wie folgt zitiert werden:
urn:nbn:de:hbz:468-20080323
[http://nbn-resolving.de/urn/resolver.pl?urn=urn%3Anbn%3Ade%3Ahbz%3A468-20080323]

Die praktischen Arbeiten wurden in der Zeit von Juni 2004 bis August 2006 am Lehrstuhl
für Makromolekulare Chemie des Fachbereiches C – Mathematik und Naturwissenschaften
der Bergischen Universität Wuppertal unter Anleitung von Prof. Dr. Ullrich Scherf
durchgeführt.













Herrn Prof. Dr. Ullrich Scherf gilt mein besonderer Dank für die Überlassung des
interessanten Themas, seine stete Diskussionsbereitschaft, sowie seine vielfältige
persönliche Unterstützung. Die wissenschaftliche Freiheit die mir eingeräumt wurde habe
ich als überaus motivierend empfunden. Prof. Dr. Dieter Neher danke ich für gute
Zusammenarbeit und die Übernahme des Koreferats.




1. Gutachter: Prof. Dr. U. Scherf (Bergische Universität Wuppertal)
2. Gutachter: Prof. Dr. Dieter Neher (Universität Potsdam)
Eingereicht am: 14.02.2008









Für meine Familie,
Nicole und Ben

















Abstract

Organic light emitting diodes (OLEDs) have gained increasing attention because of there
remarkable properties and application potential. Therefore chemists are aiming for suitable
organic materials for optoelectronic applications. Prominent materials are semiconducting
polymers e.g. polyfluorenes. A major problem hereby is the colour purity of the blue-
emitting polyfluorene-type materials caused by degradation processes during device
operation. Besides the well-characterized keto defects in degraded polyfluorenes which emit
at a peak maximum of approximately 530-550 nm, we investigated an additional emission
feature localized at 485/515 nm. This particular green emission feature was attributed to
alkylidenefluorene defect structures. This proposal is supported with the synthesis of model
copolymers and their optical characterization as described in chapter 2.

One focus of material chemists is to design materials with increase of performance in
optoelectronic devices. There has been much interest in the application of cyclometalated
complexes as emitting components in such devices. The metal complexes may allow for the
efficient utilization of both singlet and triplet excitons generated upon electronic operation.
Consequently, internal quantum efficiencies approaching 100% may be achieved. Tuning of
the emission colour by manipulating the ligand sphere of the metal atom is also a very
attractive goal. Up to the present, there are still relatively few examples of
electrophosphorescent (co)polymers as single–component OLED materials. These examples
include semiconducting polyfluorenes with side-chain or main-chain iridium complexes,
ladder poly(para-phenylene)s with electrophosphorescent palladium centers, self-assembled

Schiff base polymers and platinum-based side chain copolymers. Transition metal (Co, Ni,
Zn) Schiff base polymers have been prepared by oxidative polymerisation, transesterification
and condensation of salen-type monomers but they have not been applied in OLEDs.
An often observed disadvantage of blend-based devices is a phase separation of polymer and
phosphorescent dye. One strategy to inhibit this phase separation is to incorporate the
electrophosphorescent dye into the polymer backbone. The approach presented in chapter 3
describes the covalent incorporation of phosphors into the backbone of a solution-
processable semiconducting copolymer. The synthesis of platinum(II) salen complexes and
the corresponding polyfluorene-based copolymers is reported in this chapter.
OLEDs were subsequently fabricated and showed promising efficiencies and also
demonstrated the potential of this stratagem. Further optimization of the copolymer
structure includes modification of the ligand sphere as well as a variation of the backbone
e.g. incorporation of suitable comonomers such as benzophenone which should allow for a
more efficient and directed energy transfer between the backbone polymer and the
chromophore.
Another remaining task is to improve the lifetime and the efficiency of these materials
during their application in OLEDs. The synthesis and characterization of new matrix
polymers for triplet emitters is presented in chapter 3.3. The incorporation of
benzophenone units into the main chain of polyfluorenes resulted in an improved lifetime
for operating OLEDs. Further investigations include the variation of the benzophenone
content and the incorporation of phosphorescent metal complexes into the main chain of
such copolymers.