We review our recent experiments by linear optical spectroscopy in the visible spectral range on interface controlled monolayers and thin films of π-conjugated organic molecules on well-defined surfaces. In particular, cw fluorescence and fluorescence excitation spectroscopy are considered.We discuss two interesting aspects: the possibility to prepare interface controlled films with non-bulk like structures, and consequently different and possibly superior optical properties, and the use of optical spectroscopy as an analytical tool to investigate properties of interfacial organic films.
1 Introduction
The optical properties of π-conjugated organic molecules in the condensed state have been a subject of major interest over the last years under both fundamental [1] and applied aspects [2]. The latter are in particular motivated from the development of organic light emitting devices (OLEDs) and organic photovoltaic cells (OPVCs) on the basis of thin semiconducting organic films prepared by vapour deposition onto solid substrates. Very important in this context is the strong correlation of resulting electronic and optical properties
with structural and morphological properties in organic thin films. Since thin films can be structurally very different from the respective molecular bulk crystals, they can also exhibit different and novel optical properties. These may be decisive for their operation as a part of an active optical device, e.g., an OLED. This aspect has indeed motivated experiments on the optical properties of thin films with a range in thickness from several nanometers to a single monolayer of molecules or even to the limit of isolated molecules deposited onto a solid substrate surface. Obviously, the thinner the films are the more important becomes the interface to the substrate surface. Hence, meaningful experiments on very thin organic films require a control and a high definition of the substrate surface by the use of surface science methods. An important aspect of optical excitation in condensed molecular systems is delocalization. The theoretical description of delocalized optical excitations in organic condensed matter, in particular in the crystalline state, is already rather well established and is based on the concept of Frenkel excitons. It was introduced by Davydov in the 1960s [3].
The Coulomb coupling of the excited molecular state on a molecule to other molecules in the solid, that is often described in the approximation of a dipole–dipole coupling of the respective transition dipoles, leads to delocalization of the excitation and formation of exciton bands [4]. The properties of these bands, for instance, the width of the band and the position of the energetic minimum in k-space, depend on the detailed structural arrangement of the molecules within the crystal [4, 5]. They are decisive for the resulting optical properties, e.g., the fluorescence yield. As a consequence, for organic molecular crystals, there exists a relationship between
the structural and the optical properties. One of the
first experiments which tested this relationship by a deliberate
change of the structural arrangement was that of Kirstein
et al. [6, 7]. The authors followed the variation in the absorption
spectra of a Langmuir film that was compressed on the
water surface in a trough.
Today, the optical experiments on organic molecules
close to an interface to a substrate can be divided on the baM.
Müller et al.
sis of the thickness of the molecular films into three groups:
(a) experiments on thin epitaxial films (with a thickness of
several molecular layers), in particular, on such films which
exhibit a structure different to that of the known bulk crystals
of the respective molecules, (b) experiments on molecular
monolayers which resemble true two-dimensional systems
and hence have optical properties different to bulk crystals,
and (c) experiments on isolated molecules on surfaces, for
which the intermolecular coupling effects are negligible.
The aspects of interests in these experiments are numerous.
Roughly speaking, one can identify two categories. The
first comprises those experiments which investigate the implications
of the substrate surface on the optical properties.
Hereby, one aspect is to understand the interplay of the interface
controlled structural arrangement and the resulting optical
properties. Most appealing in this context is the use of the
interface to the substrate surface to induce molecular structures
which have optimal or special optical properties, e.g.,
a strong fluorescence due to a J-aggregate-like intermolecular
coupling [8], resulting from a surface induced parallel
arrangement of the molecular transition dipoles. In addition,
molecular layers on surfaces allow very controlled investigations
of the relation between the quality of their structural
order, e.g., the size of ordered domains, and the resulting
line widths in optical spectra. Another aspect of interest is
charge transfer across the interface [9, 10] or rapid quenching
of the fluorescence by the interface to the substrate surface
[11]. This latter aspect is in particular relevant, when
fluorescence spectroscopy of molecules directly bonded to
the substrate surface is considered.
The second category contains experiments which use
optical spectroscopy as an analytical method to study
molecules on surfaces. This type of experiments aims at information
on various aspects, e.g., the structures of molecular
layers, phase transitions between different structural
phases, and kinetic aspects of molecules, as for instance
diffusion and aggregation into ordered domains. Evidently,
the number of experiments in this category is presently still
very small, since optical spectroscopy in the visible spectral
range on molecules on surfaces is a rather novel field
of research. In particular, experiments on well-defined surfaces,
i.e., single crystal surfaces that are prepared and controlled
by state of the art surface science analytic techniques
and on which the molecules are adsorbed on chemically
and structurally well defined sites, and hence lead to clear
spectroscopic features are rare. The interpretations of earlier
experiments which have been performed for instance on
dyes deposited from solution on solid substrates, often suffer
from the absence of a well-defined structural situation of
the molecules on the surface [12–15].
In this review, we report on some sample systems which
we have studied over the last years under the aim to establish
optical spectroscopy as a tool to study molecules on welldefined
single crystal surfaces.We have concentrated on fluorescence
spectroscopy in the visible spectral range. This
had the implication that for investigations on monolayers in
direct contact to the substrate interface we had to find sample
systems where fluorescence is present, and not quenched
as it occurs on a metallic substrate. However, since we also
wanted to use electron based surface science techniques, in
particular low energy electron diffraction, for structural control,
the samples had to allow for some charge conduction
at the same time. The response to these two conflicting demands
turned out to be difficult, as we will report below.We
will discuss our results also in the context of important findings
of other groups, although we will not be able to give a
complete review of the field here.
