University of Groningen
Charge, energy and bond dynamics at molecular interfaces
Bakulin, Artem Alekseevich
IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to
cite from it. Please check the document version below.
Document Version
Publisher's PDF, also known as Version of record
Publication date:
2009
Link to publication in University of Groningen/UMCG research database
Citation for published version (APA):
Bakulin, A. A. (2009). Charge, energy and bond dynamics at molecular interfaces Groningen: s.n.
Copyright
Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the
author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).
Take-down policy
If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately
and investigate your claim.
Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the
number of authors shown on this cover page is limited to 10 maximum.
Download date: 17-06-2017
Summary
Numerous fascinating physical, chemical, and biochemical phenomena occur at
interfaces, i.e. in the region where molecules of different types meet. Even if the
interactions between the molecules are weak (much weaker than, for instance,
covalent or ionic bonds formed in chemical reactions) they can still substantially
change the properties of the molecules involved. For this reason, molecules at the
interface may behave different from those in the bulk. This leads to the formation
of specific ‘interfacial’ molecular layers bridging two phases of materials.
In this work, we study a number of specific properties of such interfaces in
different multi-phase (heterogeneous) systems. We also focus on the consequences
that interfaces impose on the behavior of the overall systems. The picture of
molecules at the interface cannot be a static one , it requires the additional
dimension of observing how these molecules behave in time, i.e. their dynamics.
As a tool we use ultrafast multicolor spectroscopy – a technique in which
molecules are selectively marked by an ultrashort (less then 10-13 s) optical pulse
and their evolution in time is being traced by a second optical pulse which arrives
somewhat later and usually has a different color. We applied this technique to two
quite different classes of systems: hydration shells of amphiphilic molecules
dissolved in water and blends of conjugated polymers with organic acceptors.
The majority of all complex biological and chemical processes going on in the
living cell occur in the hydration shells of amphiphilic (bio)molecules. Proton
transfer, biochemical reactions, protein folding, ligand binding to substrates, and
membrane formation – all these phenomena take place in an aqueous medium and
are to a large extent mediated by water. There are two limiting cases of
amphiphilic solvation depending on whether hydrophilic or hydrophobic groups
dominate in the solute-water interaction. Both types of solvation have been studied
in the present work.
We started with studying the dynamics of water molecules near the lipid
interface of reverse micelles, which is a typical example of solvation of highly polar
(ionic) hydrophilic groups in water. Using a polarization-sensitive pump-probe
spectroscopy technique (Chapter 2), we excited OH-stretch vibrations of nanoconfined water molecules and then monitored their reorientation and the
relaxation of vibrational energy. We observed that water in the core of the
encapsulated droplet is in its dynamical properties similar to the bulk phase of
liquid water. At the same time, the lipid wall of the reverse micelle influences the
behavior of approximately one molecular layer of water bound to the lipid heads.
Summary
Quite surprisingly, the vibration response of water in small micelles stayed
substantially anisotropic during the overall decay of the vibrational oscillations.
This shows that during the lifetime of the excited vibrations no intermolecular
transfer occurs, neither among the lipid-bound water molecules nor from the
hydration shell to the bulk-like core.
We studied the intermolecular interaction between water molecules in more
detail by looking at the frequency fluctuations of water OH-stretch vibrations with
2 dimensional infrared (2DIR) spectroscopy (Chapter 3). Our experiments showed
that the vibrational frequency of the water molecules near the lipid interface stays
almost constant on a, for water, exceptionally long timescale of a few picoseconds.
From this, we conclude that the lipid-bound water molecules do not exchange
energy and are, probably, decoupled from one another and from the bulk-water
core. Therefore, the generally strong hydrogen-bond-mediated interactions
between water molecules appear to be broken in the interfacial layer. Hence, the
hydration layer of a lipid interface does not represent a textbook ‘threedimensional mesh of water molecules tightly linked by hydrogen bonds’ but rather
a film of lipid-bound water. Strong water-lipid interaction in the layer creates an
almost “frozen” (at a 10-11 s timescale) OH-bond environment.
In the opposite case of water near hydrophobic (methyl) groups (Chapter 4), we
also observed a significant modification of the water dynamics. The same 2DIR
technique showed that changes in the vibrational frequency of OH oscillators occur
slower than in bulk water. This slowing down scaled linearly with the number of
methyl groups, indicating that this effect is due to the hydrogen-bond dynamics in
the hydrophobic hydration shell. Interestingly, the deceleration of changes in the
vibrational frequency of water correlates with the previously reported slowing
down of the orientational mobility of the water molecules in the hydrophobic
hydration shell. We concluded that the jump-like reorientations, typical for bulk
water, are suppressed near hydrophobic groups.
What our studies on water dynamics have shown is that water in the hydration
shells , both for hydrophobic and hydrophilic (ionic) groups, behaves substantially
different than bulk water. Truncation of hydrogen bonds together with specific
hydrophilic and hydrophobic interactions slow down the reorientation of the
water molecules and hamper the collective behaviour in an aqueous environment.
The highly flexible but tight network of hydrogen bound units no longer exists in
the solvation shell. Probably, water forms around membranes and proteins an
interfacial layer of molecules with a restricted mobility and a reduced
188 188
Summary
intermolecular coupling. This layer, on the one hand, stabilizes the structure of the
(bio)moiety, and, on the other hand, influences the surface bio-chemistry, heat and
charge transport, and other dynamical processes.
Another class of materials studied in this thesis are blends of conjugated
polymer and organic acceptors. Such blends have potential applications for plastic
solar cells, photodiodes, and non-volatile memory devices. It was recently
established that the polymer-acceptor interface in these materials is usually
modified by donor-acceptor interaction. Therefore, light-to-charge conversion in
phase-separated blends occurs mostly via an intermediate state(s) formed at the
polymer-acceptor interface. These intermediate states are associated with a partial
shift of electron density from donor to acceptor (called a charge-transfer complex
(CTC) formation). The presented study addressed the question – how a CTC of a
conjugated polymer influences the early stages of the light-to-charge conversion
process.
We demonstrate that the photophysics strongly depends on the donor-acceptor
interaction. In blends with a pronounced CTC formation (Chapter 5) the chargeseparated state is excited directly, i.e. no intra-polymer excitonic state is formed
and no energy transfer occurs before charge transfer. Moreover, the generated
charges are localized within parent donor-acceptor pairs, at least on a hundred-ps
timescale. Finally, the charges experience very fast and efficient geminate
recombination. For the acceptors studied, we observed a clear trend: the efficiency
of charge recombination increases with increasing oscillator strength of the CTC
transition.
On the basis of our studies of ternary (Chapter 6) and binary blends of
conjugated polymers with different organic acceptors, we concluded that CTC
states are responsible for the initial transformation of an excitation into a geminate
charge pair and, therefore, serve as an intermediate in charge photogeneration.
However, apart from the interfacial (CTC) manifold, an additional manifold of
electronic states is required for an efficient further separation of the geminate pair
into a pair of long-lived charges, as provided, for instance, by pure donor or/and
pure acceptor phases. For this reason, phase-separated blends of polymers with
fullerene derivatives display much higher yields of long-lived charges compared to
homogeneous blends of the same polymer with strong CTC acceptors. However,
the yield of long-lived charges drastically increases when an additional manifold
of electron-accepting states is introduced into the homogeneous CTC blend, for
example, by fullerene C60 doping.
189 189
Summary
We also performed a study of light-to-charge conversion in polymermethanofullerene blends (Chapter 7). In contrast to the previous studies, in which
usually the polymer was excited, we focussed on the dynamics of the hole transfer
after the methanofullerene excitation. Our experiments showed that the efficiency
(~unity) and timescale (~30 fs) of charge separation at the donor-acceptor interface
after excitation of the acceptor, are very similar to those previously reported for
donor excitation. These results serve as an additional indication that,
independently of which molecule was initially excited, charge separation occurs
through the same route, namely the CTC states at the donor-acceptor interface.
To summarize, a general conclusion that can be drawn from the contents of this
thesis is that an interface layer of molecules will have properties dramatically
different from those of the separate bulk materials. In particular, the strength and
nature of the intermolecular interactions at the interface have significant impact
not only on the structure but also on the dynamics of the molecular system.
190 190