Emiel Peeters

Singlet and triplet excitations of chiral dialkoxy-p-phenylene vinylene oligomers

The excited state properties of a series of α,ω-dimethyl-oligo{2,5-bis[2-(S)-methylbutoxy]-p-phenylene vinylene}s (OPVns, with n the number phenyl rings) are investigated for n=2–7 in solution at ambient temperature, under matrix-isolated conditions at low temperature, and as nanoaggregates using absorption (time-resolved), photoluminescence, photoinduced absorption, circular dichroism, and circular polarized luminescence spectroscopies. The singlet (S1←S0) and triplet (Tn←T1) transition energies decrease with conjugation length. For the S1 state of OPVn the lifetime strongly decreases with chain length due to enhanced nonradiative decay and radiative decay. The increase in the nonradiative decay rate constant is much more pronounced, and as a result the photoluminescence quantum yield is less for longer oligomers. Studies at low temperature afforded spectra with well-resolved vibronic fine structure. Under these conditions the Stokes’ shift is very small (⩽0.04 eV). The Huang–Rhys parameter and relaxatio...

Circular Polarization of the Fluorescence from Films of Poly(p‐phenylene vinylene) and Polythiophene with Chiral Side Chains

Circularly polarized photoluminescence has been observed from chiral -conjugated polymers (e.g., see Figure). It was found that the degree of circular polarization was greater in the absorption than in the emission, which is interpreted in terms of trap sites acting as luminescent centers. These results may be important for the fabrication of LEDs for producing circularly polarized light.

Chiroptical properties of highly ordered di[(S)-2-methylbutoxy] substituted polythiophene and poly(p-phenylene vinylene)

Circular dichroism (CD) spectroscopy is used to study the nature of the excited state of optically active di[(S)-2-methylbutoxy] substituted polythiophene (PDMBT) in small aggregates with a high degree of intrachain and interchain order. The observed CD effects indicate oscillator coupling between chains in the excited state. The photoluminescence of the PDMBT aggregates is characterized by a very small Stokes shift (< 0.09 eV) and is circularly polarized but less intense than of free PDMBT chains in solution. The decrease of luminescence efficiency in the aggregated phase can be explained by the exciton coupling observed in CD and results in a non-radiative low-energy state. Similar results are obtained for optically active poly(p-phenylene vinylene).

Photoinduced intermolecular electron transfer between oligo(p-phenylene vinylene)s and N-methylfulleropyrrolidine in a polar solvent

Abstract Photoinduced intermolecular electron transfer in mixtures of oligo( p -phenylene vinylene)s (OPV n s, with n =2–7, the number of phenyl rings) and N -methylfulleropyrrolidine (MP-C 60 ) is studied in o -dichlorobenzene. Photoinduced absorption (PIA) spectroscopy is used to show that photoexcitation of MP-C 60 generates the corresponding triplet state, which is quenched in a one-electron reduction reaction by OPV n for n >2. The electronic transitions of the photogenerated OPV n + radical cations evolve with conjugation length in a near linear relation with 1/ n . Charge transfer in these donor–acceptor systems for n >2 is in full agreement with the calculated change in free energy for charge separation.

Chiroptical properties of a chiral‐substituted poly(thienylene vinylene)

Poly(3.4-bis[(S)-2-methylbutoxy]-2,5-thienylene vinylene) (BMB-PTV), the first example of a PTV derivative with optically active side chains, was prepared using a TiCl 4 /Zn-induced coupling of the monomeric dialdehyde. The linear absorption spectra of BMB-PTV exhibit a strong solvatochromic effect: λ max = 601 nm in a good solvent and λ max = 570 nm in a poor solvent. In good solvents the polymer chains contain a high degree of intrachain order, while optical activity associated with the π-π * transition is absent. In contrast, the solvatochromic shift observed in poor solvents is accompanied by the emergence of a strong activity in circular dichroism spectroscopy, attributed to the formation of chiral superstructures of multiple chains. Chiral superstructures or microaggregates are also present in the solid state (spin-coated films) and their concentration can be altered by annealing.

Synthesis of liquid crystalline oligo(p-phenylene vinylene)

The synthesis of oligo(p-phenylene vinylene) with mesogenic side chains is described, with the objective to explore highly ordered n-conjugated materials for functional properties. In the case of dimeric and tetrameric (p-phenylene vinylene) only a melt transition was observed while in the case of hexameric (p-phenylene vinylene) a liquid crystalline phase was found under the polarization microscope.

Quadruple hydrogen bonded oligo(p-phenylene vinylene) dimers

Direct self-assembly of π-conjugated oligomers via self-complementary quadruple hydrogen bonding is achieved and the first steps towards supramolecular polymers with functional side chains are described.

Supramolecular fullerene architectures by quadruple hydrogen bonding

The synthesis and full characterization of a quadruple bonded fullerene dimer using self-complementary 2-ureido-4[1H]pyrimidinone units with high dimerization constants is described. The chemical integrity of the monomeric moiety in either compound is fully preserved, also with respect to its redox and UV-Vis behavior. Two novel supramolecular dyads consisting of an oligo(p-phenylene vinylene) (OPV) donor and fullerene (C-60) acceptor are created via quadruple hydrogen bonding upon mixing the fullerene dimer and an OPV dimer. In these supramolecular dyads, singlet-energy transfer from the excited OPV unit to the fullerene causes a strong quenching of the OPV fluorescence. The high association constant of the 2-ureido-4[1H]-pyrimidinone quadruple hydrogen-bonding unit results in high quenching factors (Q(max) greater than or equal to 90). The lower limit obtained for the rate constant for energy transfer (k(EN) greater than or equal to 6 x 10(10) s(-1)) is rationalized in terms of the Forster mechanism.

Oligomer-fullerene dyads and triads as model compounds for bulk-heterojunction PV cells

Covalent oligomer-fullerene donor-acceptor structures can serve as important model systems for plastic PV cells, based on interpenetrating networks of conjugated polymers and fullerene derivatives. Several series of [60]fullerene-oligomer dyads and triads were prepared. Photoinduced electron transfer phenomena were studied using photoinduced absorption spectroscopy. Measurements in solution and in the solid state reveal that the environment plays a crucial role in the formation and lifetime of the photoinduced charged states in these molecules.