Sepas Setayesh

Interchain vs. intrachain energy transfer in acceptor-capped conjugated polymers.

The energy-transfer processes taking place in conjugated polymers are investigated by means of ultrafast spectroscopy and correlated quantum-chemical calculations applied to polyindenofluorenes end-capped with a perylene derivative. Comparison between the time-integrated luminescence and transient absorption spectra measured in solution and in films allows disentangling of the contributions arising from intrachain and from interchain energy-migration phenomena. Intrachain processes dominate in solution where photoexcitation of the polyindenofluorene units induces a rather slow energy transfer to the perylene end moieties. In films, close contacts between chains favors interchain transport of the excited singlet species (from the conjugated bridge of one chain to the perylene unit of a neighboring one); this process is characterized by a 1-order-of-magnitude increase in transfer rate with respect to solution. This description is supported fully by the results of quantum-chemical calculations that go beyond the usual point-dipole model approximation and account for geometric relaxation phenomena in the excited state before energy migration. The calculations indicate a two-step mechanism for intrachain energy transfer with hopping along the conjugated chains as the rate-limiting step; the higher efficiency of the interchain transfer process is mainly due to larger electronic coupling matrix elements between closely lying chains.

Optimisation of polyfluorenes for light emitting applications

The properties of polyfluorenes for use in luminescent devices have been improved by a variety of approaches. Their conjugation length may be extended by using more planar indenofluorene units. Formation of long wavelength emitting aggregates is suppressed by attachment of bulky dendron groups to give stable blue emission. The emission colour can be tuned across the visible spectrum by incorporation of perylene dye units on the main chain or as substituents. Rod-coil copolymers have been made in order to achieve control of the supramolecular order. Polyfluorenone prepared by a precursor route shows improved electron injection and transport properties.

Correlation Between Molecular Structure, Microscopic Morphology, and Optical Propertiesof Poly(tetraalkylindenofluorene)s

The emission properties of polyindenofluorenes with various proportions of straight- and branched-chain alkyl substituents have been compared. The polymer with straight octyl substituents shows green emission due to formation of aggregates, while the polymer with branched 2-ethylhexyl substituents shows blue emission. Studies of the film morphology show the presence of ordered structures due to π-stacking of the polymer chains for the octyl-substituted polymer, whereas the polymer with branched side-chains shows no such order. Copolymers show intermediate behavior. A clear correlation is established between the degree of straight-chain alkyl substitution, the formation of ordered structures in the films, and the amount of long wavelength emission in the solid-state spectra.

Excited-state absorption in luminescent conjugated polymer thin films: ultrafast studies of processable polyindenofluorene derivatives

Abstract Femtosecond transient absorption experiments on thin films of poly-2,8-indenofluorene, a processable step-ladder analogue of poly- p -phenylene, are presented. Probe wavelength and pump intensity dependence measurements allow the separation of the broadly overlapped singlet excited-state and charged excitation absorption bands. Morphological effects on the dynamics are investigated by modification of the aliphatic side chains in the monomer unit. Steric effects, controlled by the nature of the alkyl side groups, result in a group-dependent three-dimensional structure. The transient absorption dynamics are similar in all samples, in spite of distinct morphologies, indicating limited importance in the excited-state analysis.

Exciton migration to chain aggregates in conjugated polymers : influence of side-chain substitution

Abstract We have performed time-resolved photoluminescence (PL) measurements to study the transfer of excitons from non-interacting polymer chains to chain aggregates for thin films of polyindenofluorene with different attached side chains. The transfer time is shown to be fast (≈35 ps) and side-chain independent indicating that the substitution of different side chains does not affect the local concentration of aggregate sites within aggregate-containing domains, but rather the extent to which such domains are formed. We find the aggregate emission efficiency to be relatively high (≳20%) while still lower than the efficiency of emission from non-interacting chains.

A Comparative Study of the Photophysics in Polyfluorenes and Polyfluorenes with Polyphenylene Dendron Sidechains

The cw absorption, steady state photoluminescence (PL), photoinduced absorption (PA), PL-detected magnetic resonance (PLDMR), and the time resolved PL of a novel polyfluorene (PF) prepared with bulky polyphenylene dendrimer substituents are compared with those of (PF) with ethyl-hexyl substituents. We show that the dendronic sidechains suppress the contribution from unwanted low energetic emission, yielding a polymer with pure blue emission. The sidechains also strongly alter the dynamics of the excited entities. In particular, the time-resolved PL and temperature-dependence of the cw PL from 20-320 K reveal distinct singlet exciton (SE) dynamics in the polymer films, while the behavior in solution is essentially the same. However, the PA results show that the dynamics of polarons and triplet excitons (TEs) are similar, and the PLDMR shows that the interaction between the SEs and polarons are also similar.

Integrated complementary-like circuits based on organic ambipolar transistors

We report on an approach towards integrated complementary-like circuits based on organic ambipolar transistors. In particular, we show that ambipolar transport can be achieved within a single transistor channel using gold electrodes and a solution processable polymer-small molecule blend as the electroactive material. To demonstrate the suitability of these devices for practical utilisation in logic circuits we realise complementary-like voltage inverters comprised entirely of ambipolar transistors. Moreover, by integrating several such inverters we are able to demonstrate more complex circuits such as ring oscillators.