5696 J. Am. Chem. Soc., Vol. 122, No. 24, 2000
Wang et al.
chemical structure. Strategies for raising or lowering the HOMO
and LUMO levels include conjugation length control, as well
as the introduction of electron donating or withdrawing groups
to the parent chromophore.10 Regulating the HOMO and LUMO
energy levels permits fine-tuning of charge injection properties.
In emissive devices, the HOMO/LUMO energy difference
directly controls emission frequency. Organic materials also
offer the opportunity to adjust optical properties by taking
advantage of processes unique to the excited state, i.e., excimer
and exciplex formation.11
It is generally appreciated that the morphology of organic
films plays a fundamental role in defining the functional charac-
teristics of the material. However, studies that clearly relate
electroluminescence and charge transport properties with molec-
ular morphology remain scarce. The tendency of many small
molecules to spontaneously crystallize12 presents a limitation
for LED applications because crystal formation destroys film
homogeneity and crystal boundaries raise the resistance of the
sample, eventually leading to electric shorting.13 The thermal
stability of amorphous molecular solids as measured by the glass
transition temperature has been shown to directly correlate with
electroluminescence stability.14 It has been argued that in some
cases thermal cycling of an organic LED heterostructure device
above the glass transition temperature causes degradation result-
ing from disruption of the organic-organic interface rather than
crystallization.15 It is proposed on the basis of X-ray specular
reflectivity data that large thermal expansion of one of the
components associated with its glass transition causes cata-
strophic strain release at the hetero-interface between materials.
For transistor applications, proper alignment of chromophores
is desired because it enhances charge transport.16 On the other
hand, in the case of polymer LEDs, ordered regions result in
strong interchain coupling and lower emission quantum yields.17
Despite the obvious need to control the final arrangement of
individual molecules in the bulk a priori, a detailed understand-
ing of the relationship between chemical structure of a given
organic material with the resulting morphology is still lacking.
Strategies to minimize interchain contacts in conjugated
polymers have generally implemented the use of bulky side
groups on the polymer backbone. These attachments improve
solubility by limiting interchain contacts but are typically ali-
phatic in nature and therefore limit charge injection and migra-
tion across the solid sample.18 In response to these limitations,
considerable efforts have been dedicated to developing mole-
cules of intermediate molecular weight that minimize the
aliphatic content and at the same time resist crystallization.
Molecular shape is an important parameter in these efforts. Some
of the recent strategies rely on creating molecular shapes that,
from an intuitive perspective, can be considered “awkward” to
packing. Examples include “starburst”, dendritic, tetrahedral,
and spiro shaped molecules, as shown below.19
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