Electronic energy transfer processes in {π-conjugated [pi-conjugated] polymers [Elektronische Ressource] / Panagiotis E. Keivanidis

Keivanid

Informations

• Publié par: johannes_gutenberg-universitat_mainz
• Publié le: 01 janvier 2005
• Nombre de lectures: 11
• Langue: English
• Poids de l'ouvrage: 11 Mo

Extrait

Electronic energy transfer processes in
π-conjugated polymers









Dissertation
Zur Erlangerung des Grades
“Doktor der Naturwissenschaften”
am Fachbereich Chemie und Pharmazie
der Johannes-Gutenberg-Universität Mainz



Vor- und Zunahme des Doktoranden


Panagiotis E. Keivanidis
geboren in Pireas, Attika




Mainz, 2004























To Venetia and Stathis


























3










































¨ Άν είσαι σοφός, πολέµ α στο κρα ν ίο, σκότωνε τις ιδέες , δηµ ιούργα καινούριες. Ο
Θεός κρύβεται µ έσα σε κάθ ε ιδέα, όπως µ έσα σε σάρκα. Σύντριψε την ιδέα,
λευτέρωσέ τον! ∆ώσ ε του µ ιαν άλλη ιδέα, πιό απλόχωρη, να κατοικήσει.¨
Nikos Kazntzakis

4 Currently π-conjugated polymers are considered as technologically interesting materials
to be used as functional building elements for the development of the new generation of
optoelectronic devices. More specifically during the last few years, poly-p-phenylene
materials have attracted considerable attention for their blue photoluminescence
properties. This Thesis deals with the optical properties of the most representative blue
light poly-p-phenylene emitters such as poly(fluorene), oligo(fluorene),
poly(indenofluorene) and ladder-type penta(phenylene) derivatives. In the present work,
laser induced photoluminescence spectroscopy is used as a major tool for the study of
the interdependence between the dynamics of the probed photoluminescence, the
molecular structures of the prepared polymeric films and the presence of chemical
defects. Complementary results obtained by two-dimensional wide-angle X-ray
diffraction are reported. These findings show that the different optical properties
observed are influenced by the intermolecular solid-state interactions that in turn are
controlled by the pendant groups of the polymer backbone. A significant feedback is
delivered regarding the positive impact of a new synthetic route for the preparation of a
poly(indenofluorene) derivative on the spectral purity of the compound. The energy
transfer mechanisms that operate in the studied systems are addressed by doping
experiments. After the evaluation of the structure/property interdependence, a new
optical excitation pathway is presented. An efficient photon low-energy up-conversion
that sensitises the blue emission of poly(fluorene) is demonstrated. The observed
phenomenon takes place in poly(fluorene) derivatives hosts doped with metallated
octaethyl porphyrins, after quasi-CW photoexcitation of intensities in the order of
2kW/cm. The up-conversion process is parameterised in terms of temperature,
wavelength excitation and central metal cation in the porphyrin ring. Additionally the
observation of the up-conversion is extended in a broad range of poly-p-phenylene blue
light-emitting hosts. The dependence of the detected up-conversion intensity on the
excitation intensity and doping concentration is reported. Furthermore the dynamics of
the up-conversion intensity are monitored as a function of the doping concentration.
These experimental results strongly suggest the existence of triplet-triplet annihilation
events into the porphyrin molecules that are subsequently followed by energy transfer to
the host. After confirming the occurrence of the up-conversion in solutions, cyclic
voltammetry is used in order to show that the up-conversion efficiency is partially
determined from the energetic alignment between the HOMO levels of the host and the
dopant.
5 Outline of this Thesis
This Thesis deals with the optical properties of phenylene-based π-conjugated polymers.
Currently these organic materials constitute an active field of research due to their optoelectronic
characteristics. The major experimental tool that was used for this Thesis was the laser induced
fluorescence spectroscopy. Both steady-state and time-resolved photoluminescence techniques
have been exploited.
The general principles of molecular spectroscopy and their applications in the field of π-
conjugated polymers are presented in Chapter 1. Particularly for this section of the Thesis, a
specialized bibliography has been used as a pool of figures and as a guideline of the presented
concepts. The reader may like to gain further details from the recommended literature that is
given at the end of this section.
Chapter 2 deals with the photophysical properties of poly- and oligo(fluorene) derivatives. The
importance of these compounds for their exploitation as potential blue light-emitting materials is
emphasized. Our results demonstrate the significant influence of the solid-state properties on the
optical properties of the studied compounds. We present two alternative approaches that aim in
the achievement of stable blue emission of fluorene derivatives; in the polymer regime we
explore the effect of dendron substitution onto the polymer backbone. By complementing our
results with previously measured X-ray data, we conclude that dendron substitution does not
hinder effectively the intermolecular interactions, as it was initially expected. Additionally we
show the positive impact of the thermal annealing on the spectral properties of an alkyl
derivatized poly(fluorene). The particular result becomes a representative example of how
positional disorder in a film influences the photophysical performance of a light-emitting
material. In the oligomer regime, the effect of pendant group that is attached onto the oligomer
backbone is addressed. More specifically, the photophysical performance of pentafluorenes as
films and as dilute solutions is evaluated as a function of the length of the side chain. After
concluding on the unique chemical purity of the studied oligo(fluorene)s, we discriminate two
different species to be responsible for the reduction of the blue light emissive performance: an
inherently driven excimer formation and an extrinsically driven fluorenone creation. The short
linear butyl side chain favors the former while the latter is found in all oligomers films after
thermal oxidation in air.
Chapter 3 introduces poly(indenofluorene)s, the successor molecules of poly(fluorene)s, in terms
of molecular architecture. Our interest in these materials arose due to technologically attractive
emission wavelength that is positioned in the blue spectral region where the human eye has
higher sensitivity. Four different poly(indenofluorene)s derivatives are studied. The structure-
6 property relationship is evaluated by complementing our experimental results that were obtained
by means of 2-dimensional wide-angle X-ray scattering of powder and oriented filaments and
optical spectroscopy. Through this work the positive impact of a new synthetic route on the
spectral stability of these materials is identified. Moreover, we exploit two different methods of
polymer doping: an extrinsic one and an intrinsic one. The former is based on mixing our host
polymer with a model monoindenofluorene-ketone that acts as photoluminescence quencher
whereas the latter one is achieved by endcapping the host polymer with a thiophene moiety.
Based on these results conclusions are drawn regarding the nature of photoluminescence
quenching of the host polymers and the mechanisms that are involved in this process. Finally
delayed fluorescence and phosphorescence are reported for the one of the four
poly(indenofluorene)s derivatives studied.
In Chapter 4 we present results of energy transfer in blends of poly(fluorene) and metal(II)-
octaethyl porphyrin (MeOEP). In the studied systems two alternative pathways of energy transfer
processes are investigated. Firstly an ordinary energy transfer that takes place upon direct
excitation of the polymer at 405 nm and leads to excitation energy transfer to MeOEP. Secondly,
for the first time an extraordinary energy transfer channel is identified. Upon quasi-cw laser
excitation of MeOEP at 532 nm the up-converted characteristic blue emission of PF2/6 is
2detected, for excitation intensities in the order of kW/cm . We further demonstrate that this
phenomenon occurs in a broad range of blue light-emitting polymers that are doped with PtOEP.
The photoluminescence dynamics of the up-converted host emission and PtOEP emission are
studied. We show that up-conversion takes place also in dilute solutions. By exploiting cyclic
voltammetry we deduce the HOMO and LUMO energy levels of polymer and MeOEP
components. This information in combination with our spectroscopic results enables a discussion
on the alternative mechanisms that may lead to the observed up-conversion process.