Light HarVesting across Aggregates in Polymers
J. Am. Chem. Soc., Vol. 123, No. 46, 2001 11389
intermolecularly mutual interaction between two or more
lumophores in the ground state by extending the delocalization
of π-electrons over these conjugated segments. Emission from
aggregates can result from direct excitation and/or energy
transfer from corresponding excited monomeric species, de-
pending on the wavelength of excitation.
The presence of intrachain or interchain interactions within
the aforementioned polymers would inevitably change their
emitting colors, which is usually not desirable for most optical
and optoelectronic applications. However, intentional change
of emitting color is frequently made by doping a small amount
of emitting guest material into host polymer matrix by physical
blending6 or by chemical linking as side groups.7 Both methods
involve an energy-transfer process between different emitting
species or groups. The light that finally evolves is almost or
entirely the emission from the lower-energy species. Such a
process is particularly important in PLEDs and OLEDs (small-
organic-molecule LEDs) because the device performance can
be significantly improved this way.8,9
Energy transfer is a process in which excitation energy is
released from one species to another. It plays an important role
in photosynthesis, by which almost all life activities are sustained
directly or indirectly. In a photosynthesis unit, energy is
harvested by antenna pigments and then transferred to reaction
centers where redox reactions then take place.10 Morphologi-
cally, the antenna molecules are arranged like shells of an onion,
in order of excitation energy, around the reaction center. As a
result, energy can be harvested and then transferred sequentially,
directionally, and efficiently (Figure 1a). Energy transfer in
artificial molecules and polymer systems has been extensively
investigated, but sometimes its process is not as clearly stated
as that in the photosynthesis.
Polymers with a high density of lumophores are known as
“light-harvesting” or “antenna” polymers.11 Common examples
involve polyvinylaromatics with pendant lumophores as side
chains.11 Such polymers are capable of efficient light collection
and long-range energy transfer. In most instances, energy is
transferred intramolecularly between different lumophores in
the direction of decreasing band gaps or from a lumophore to
an exciplex-forming site (Figure 1b).12 In conjugated polymers
such as PPVs, randomly distributed tetrahedral defects unavoid-
ably form during the Gilch-type polymerization13 or result from
incomplete elimination of the Wessling-type precursor poly-
mer.14 They act as conjugation interrupters in the polymer
chains, and energy is found to transfer from shorter to longer
Figure 1. Schematic diagrams of the environments for energy
transfer: (a) in a photosynthesis multipigment array; (b) in polymers
with different pendent chromphores; (c) in polymers with segments of
the same chemical structure but different conjugation lengths; (d) in
polymers capable of forming intramolecular associates between con-
jugated segments; (e) in polymers with intra- and intermolecular
interactions between chromphores; (f) in fully conjugated polymers with
oxadiazole side groups linked by long aliphatic spacers; (g) in
conjugated polymers with oriented segments in mesoporours silica
channels; and (h) in the polymer under investigation where aggregates
with various extents of interactions between lumophores are formed
sequentially and energy is transferred in a cascade from individual
conjugated segments to the most aligned, compact intermolecular
aggregates via intramolecular associates and less strongly interacted
aggregates. (The shell structure in (a) represents the array of antenna
pigments. The curved arrow represents the direction of energy transfer.
The square and circle in (b) represent chromophores with different
chemical structures. The squares with different lengths in (c) indicate
of the same chromophores but with different conjugation lengths. The
region defined by dashed lines stands for associates or aggregates. The
heavy lines in (f) and (h) represent the conjugated segements of
polymers, and their conjugation lengths are depicted as the length of
lines. The pentacycles and light lines between them and the conjugated
main chains in (f) stand for the oxadiazole groups and the aliphatic
spacers linked to main chains as pendant side chains. The hexagonal
tube in (g) represents the channel in the porous silica.)
conjugated segments (Figure 1c).14,15 The other example is poly-
(p-cyclophan-1-ene) and its derivitives,4b,16 in which “intra-
molecular” aggregates of lumophores form and energy is
transferred from independent lumophores to the aggregates
(Figure 1d).16c,17 Moreover, intermolecular aggregates in con-
jugated polymers could also act as energy acceptors (Figure
1e).3f,h We have reported a series of PPVs with pendant
oxadiazole side groups linked to the conjugated main chains
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(17) Lumophores may interact with one another within a single polymer
chain or between different polymer chains. We will define the former
“intramolecular associates” and the latter “intermolecular aggregates” to
make statements in the following text clear. The processes for such
interactions are termed “association” and “aggregation”, respectively.
However, interactions and the resulting species will be generlly described
as association and as associates for simplicity in the cases where doing so
does not cause misunderstanding.