Aggregation-induced emission of 1-methyl-1,2,3,4,5-pentaphenylsilole

Charge-Transfer and Energy-Transfer Processes in π-Conjugated Oligomers and Polymers: A Molecular Picture

Today's technology is not possible without optoelectronic devices such as light-emitting diodes, transistors and solar cells. These basic units of modern electronic appliances may be made not only from traditional inorganic semiconductors, but also from organic semiconductors, i.e. hydrocarbon molecules that combine semiconducting properties with some mechanical properties such as easy processability and flexibility. The weak van der Waals forces that bind the molecules to a solid imply a low dielectric constant, so that coulomb and exchange interactions between electrons are significant. As a result, photoexcitation or electrical excitation results in strongly bound electron–hole pairs, so-called excitons. Depending on the relative orientation of the electron and hole spin, the exciton may be of a overall singlet or triplet spin state. While the fluorescent singlet state has been investigated intensively since the first reports of organic electroluminescence, research into the properties of the phosphorescent triplet state has intensified mainly during the last decade. In this review we give an overview on the photophysical processes associated with the formation of triplet states and their decay, as well as the energy levels and energy transfer processes of triplet states. We aim to give a careful introduction for those new to this particular research area as well as to highlight some of the current research issues and intriguing questions for those familiar with the field. The main focus of this review is on molecular assemblies and polymer films, though relevant work on molecular crystals is also included where it assists in forming a larger picture.

Triplet states in organic semiconductors

A carbazole homopolymer and carbazole copolymers based on 9,9′-dialkyl-[3,3′]-bicarbazolyl, 2,5-diphenyl-[1,3,4]-oxadiazole and 9,9-bis(4-[3,7-dimethyloctyloxy]phenyl)fluorene were synthesized and their electrical and photophysical properties were characterized with respect to their application as host in phosphorescent polymer light-emitting diodes. It is shown that the triplet energy of a polymer depends on the specific connections between its building blocks. Without changing the composition of the polymer, its triplet energy can be increased from 2.3 to 2.6 eV by changing the way in which the different building blocks are coupled together. For poly(9-vinylcarbazole) (PVK), a carbazole polymer often used as host for high-energy triplet emitters in polymer light-emitting diodes, a large hole-injection barrier of about 1 eV exists due to the low-lying HOMO level of PVK. For all carbazole polymers presented here, the HOMO levels are much closer to the Fermi level of a commonly used anode such as ITO and/or a commonly used hole-injection layer such as PEDOT:PSS. This makes high current densities and consequently high luminance levels possible at moderate applied voltages in polymer light-emitting diodes. A double-layer polymer light-emitting diode is constructed comprising a PEDOT:PSS layer as hole-injection layer and a carbazole-oxadiazole copolymer doped with a green triplet emitter as emissive layer that shows an efficacy of 23 cd/A independent of current density and light output.

Carbazole compounds as host materials for triplet emitters in organic light-emitting diodes: polymer hosts for high-efficiency light-emitting diodes.

A simple technique employing reverse bias measurements of photoluminescent efficiency is described to determine the excitonic singlet-triplet formation statistics of electroluminescent organic thin films. Using this method, the singlet fractions in thin films of two organic emissive materials commonly used in organic light emitting devices, tris(8-hydroxyquinoline) aluminum $({\mathrm{Alq}}_{3})$ and poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), are found to be $(20\ifmmode\pm\else\textpm\fi{}1)%$ and $(20\ifmmode\pm\else\textpm\fi{}4)%,$ respectively. Results are confirmed using a sensitive synchronous detection scheme. We discuss other measurements and the current understanding of exciton formation statistics in polymeric and small molecular weight organic electroluminescent materials.

Excitonic singlet-triplet ratios in molecular and polymeric organic materials

Using pulse radiolysis and triplet energy transfer has enabled us to
 measure the triplet energies in a broad range of different pi
 -conjugated polymers. In all eases we find that the 1(3)B(u) is Of
 order 0.6 to 1 eV below the 1 B-1(u), indicative of localized triplet
 states with strong electron-electron correlation. We also observe that
 the 1(1)A(g)-1(3)B(u), gap decreases linearly as the I(1)A(g)-1(1)B(u)
 gap decreases even though polymers with very different structure have
 been studied. This surprising result suggests that polymers with
 singlet gap <1.3 eV will have a triplet ground state.

/pdf/triplet-energies-of-pi-conjugated-polymers-3v81gy9d83.pdf

Triplet energies of pi-conjugated polymers.

https://estudogeral.sib.uc.pt/bitstream/10316/5231/1/file0159a5e145c74fe8a2dd96b84404459d.pdf

Triplet state dynamics in poly(2,5-pyridine diyl)

We have used pulsed radiolysis to elucidate the neutral triplet and charged excited states in soluble conjugated polymers, including poly(2-methoxy-5-(2'-ethyl-hexoxy)-p- phenylenevinylene), (MEH-PPV), poly(2,5-pyridinediyl) (PPY) and poly(4-hexyl-2,5-pyridinediyl) (HPPY) in various solvents. Using a range of triplet sensitizers we have determined the S0 - T1 energy separation in these polymers to be 1.27 plus or minus 0.07 eV, 2.4 plus or minus 0.1 eV and 2.5 plus or minus 0.1 eV respectively. Our experimental results confirm that photoinduced absorption features observed in the range 1 to 2 eV in these polymers, are due to triplet-triplet absorption. Triplet state lifetimes and molar absorption coefficients in solution have also been determined. This work not only demonstrates the versatility of the pulse radiolysis technique but also gives, for the first time, important and unambiguous assignment and detail on the various excited state species in conjugated polymers.

Measurement of triplet and charged states in conjugated polymers by pulse radiolysis

Photoinduced absorption experiments have been performed on solutions of emeraldine base in N-methyl-2-pyrrolidinone at various concentrations. In the energy range 0.5 eV - 1.7 eV photoinduced peak at 1.45 eV and photobleaching starting at 1.7 eV are observed, as is the case for solid films. The additional photoinduced peak at ca 0.9 eV which is seen in emeraldine thin films is not observed in solution, but a clearly resolvable induced feature is seen at 1.2 eV in the solutions. The 0.9 eV feature in films has been ascribed to an intra chain polaronic excitation, and the 1.45 eV feature to a long lived inter chain excitation. However the intensity of the 1.45 eV peak seen in the solutions scales linearly with concentration, clearly implying intra chain origin. The low energy feature of the thin films is either missing in the solution spectra implying it is an inter chain excitation or it substantially blue shifts in solution implying it too is an intra chain excitation but with a more highly dipolar nature. These results are suggestive of a strong role being played here by triplet excitons as well as residual polaronic species.

L. J. Hartwell

Papers

Triplet energies of pi-conjugated polymers.

Triplet state dynamics in poly(2,5-pyridine diyl)

Measurement of triplet and charged states in conjugated polymers by pulse radiolysis

Photoinduced absorption in polyaniline solutions

Photoinduced absorption measurements of triplet states in poly(p-pyridine)