Dynamics of excitation transfer in dye doped Π-conjugated polymers

Electric-field modulated energy transfer in phosphorescent material- and fluorescent material-codoped polymer light-emitting diodes

The excited-state energy transfer widely exists in mixed-material systems and devices. The modulation of an electric field on the energy transfer in photoluminescence has been demonstrated. However, to date, no studies on the electric-field modulation of the excited-state energy transfer in organic optoelectronic devices have been reported. Herein, we investigate the effect of an electric field on the energy transfer in the poly(N-vinylcarbazole) (PVK) thin films doped with iridium(iii)[bis(4,6-difluorophenyl)pyridinato-N,C2']-tetrakis(1-pyrazolyl)borate (Fir6) and 5,6,11,12-tetraphenylnaphthacene (rubrene) (PVK:Fir6:rubrene) and the corresponding light-emitting diodes. Combined with the Onsager model describing electric-field enhanced exciton dissociation, we find that the electric field increases the rate of Dexter energy transfer from Fir6 to rubrene in the films and the diodes. The voltage-dependent color shift in the PVK:Fir6:rubrene light-emitting diodes can be explained by the electric-field enhanced Dexter energy transfer from Fir6 to rubrene. Our findings are important for the control of energy transfer process in organic optoelectronic devices by an electric field for desirable applications.

Time-resolved spectroscopy of radiative energy transfer between a conjugated oligomer and polymer in solution

We report a short investigation of the energy transfer process between the conjugated oligomer 1,4-bis(9-ethyl-3-carbazo-vinylene)-9,9-dihexyl-fluorene (BECV-DHF) and the conjugated polymer poly[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylene-vinylene] - end capped with DMP (MDMO-PPV). The radiative energy transfer (RET) process shows a time delay, and the formation of the excimer causes a further delay. All these processes were studied using time-resolved spectroscopy (TRS), which has three-dimensional (3D) features with wavelength, intensity and time (picosecond) as the X, Y and Z-axis, respectively. We observed a definitive delay (1 ns) in the fluorescence from MEDMO-PPV concerning the fluorescence of the oligomer, indicating the RET. The TRS of different relative concentrations and temperature effects on the energy transfer process was also studied. The quantum yield, critical distance, polarizability and change of MEDMO-PPV were calculated. The excimer of the MEDMO-PPV produces Amplified Spontaneous Emission (ASE) after a time delay of at least 0.5 ns, which was also observed in this study.

The Unusual Fluorescence Quenching of Coumarin 314 by β-Cyclodextrin and the Effect of β-Cyclodextrin on its Binding with Calf Thymus DNA

This paper discusses the binding of a laser dye, Coumarin 314 with β-cyclodextrin, studied mainly by UV-visible spectroscopy, 2D rotating-frame nuclear Overhauser effect spectroscopy (ROESY), steady-state spectroscopy and time-resolved fluorescence spectroscopy. The role of β-cyclodextrin on the binding of Coumarin 314 with calf thymus DNA was investigated. Coumarin 314 shows a hyperchromic shift of absorption and a quenching of fluorescence due to binding with β-cyclodextrin. The fluorescence quenching is non-linear and the reason for the non-linearity is discussed. The unusual fluorescence quenching on Coumarin 314–β-cyclodextrin binding is rationalised from the effect of acidity on absorption, fluorescence, and molecular modelling studies. Additional proof for the mode of binding is given by 2D ROESY. The capped and exposed portions of the Coumarin 314 molecule in the Coumarin 314–β-cyclodextrin complex when binding with calf thymus DNA were visualised based on spectral and molecular modelling studies.

Exciton migration by ultrafast Förster transfer in highly doped matrixes

The energy transfer between dye molecules and the mobility of the corresponding excitons are investigated in polymethyl methacrylate films highly doped with perylene bisimide dyes. The dynamics is measured by group delay corrected, femtosecond broad-band spectroscopy revealing the transfer route via absorption changes that are specific for the participating species. In films doped with 0.14 M perylene orange an ultrafast homotransfer between the dye molecules is found by analyzing the loss of the excitation-induced anisotropy. The process exhibits a stretched exponential time dependence which is characteristic for Förster energy transfer between immobilized molecules. The transfer time is 1.5 ps for an average transfer distance of 2.3 nm and results in a high mobility of the optically generated excitons. In addition, we find that the excitons move to perylene orange dimers, which have formed in low concentration during the sample preparation. The observed energy transfer time is slightly shorter than expected for a direct Förster transfer and indicates that exciton migration by multistep transfer between the monomers speeds up the transport to the dimers. In samples doped with perylene orange and perylene red heterotransfer to perylene red takes place with transfer times down to 600 fs. The mechanism is Förster transfer as demonstrated by the agreement with calculations assuming electric dipole interaction between immobilized and statistically distributed donor and acceptor units. The model predicts the correct time dependence and concentration scaling for highly doped as well as diluted samples. The results show that ultrafast exciton migration between dye molecules in highly doped matrixes is an attractive and efficient mechanism to transport and collect energy in molecular systems and organic electronic devices. Further optimization should lead to a loss-free transport over distances typical for the thickness of active layers in these systems.

An amphiphilic poly(phenylene ethynylene) as the structure-directing agent for periodic nanoscale silica composite materials

We have synthesized optically active nanostructured composite materials by using an amphiphilic semiconducting polymer, a poly(phenylene ethynylene) (PPE), and a conventional ammonium surfactant as the structure-directing agents. The PPE consists of phenylene units para-substituted with an octyloxy chain and a charged trimethylammoniumethoxy group, resulting in a surfactant-like structure that can assemble into cylindrical micelles. The resulting silica/organic composite material has a hexagonal honeycomb structure with a repeat distance of 45.3 A, as confirmed by low-angle X-ray diffraction and transmission electron microscopy. Scanning electron microscopy indicates that the larger scale particle size is on the order of micrometers. The incorporation of the polymer into the composite was confirmed by elemental analysis, photoluminescence spectroscopy, and fluorescence microscopy. The polymer retains its photophysical properties in the composite, showing luminescence similar to polymers in the solution phase. The polymer displays a high degree of luminescence polarization anisotropy, indicating that the polymer chains are straight and isolated from each other in the composite.

Well-Packed Chains and Aggregates in the Emission Mechanism of Conjugated Polymers

We synthesized dialkoxy-substituted poly[phenylene vinylene]s (dROPPV-1/1, 0.2/1, and 0/1) consisting of two repeating units with different side-chain lengths (methoxy and 3,7-dimethyloctyloxy). These polymers can serve as a model system to clarify roles of aggregates (the sites with ground-state interchain interactions) and the independent chain segments in the well-packed chains (the chain segments that are compactly packed without interaction) in the emission mechanism of conjugated polymers. Due to the packing of polymer chains, films of all of these polymers are accessible to interchain excitations, after which excitons can re-form to result in delayed luminescence. Besides, some chains form aggregates so that the delayed luminescence is no more the ordinary single-chain emission but red-shifted and less structured. Not only the re-formation of these indirect excitons but also the aggregation of chains are facilitated in the polymers with short methoxy side groups, revealing that both packing and aggregation of chain segments require a short spacing between polymer chains. However, the incorporation of other side chains such as the 3,7-dimethyloctyloxy group to dROPPVs is necessary for the formation of aggregates because these long branched side chains can reduce the intrachain order imposed by the short methoxy groups, which accounts for the absence of aggregate emission in the well-studied poly[2,5-dimethoxy-1,4-phenylene vinylene]. This study reveals that the well-packed chains do not necessarily form aggregates. We also show that the photophysical properties and the film morphology of conjugated polymers can be deliberately controlled by fine-tuning of the copolymer compositions, without altering the optical properties of single polymer chains (e.g., as in dilute solutions).

Control of energy transfer in oriented conjugated polymer-mesoporous silica composites

Nanoscale architecture was used to control energy transfer in semiconducting polymers embedded in the channels of oriented, hexagonal nanoporous silica. Polarized femtosecond spectroscopies show that excitations migrate unidirectionally from aggregated, randomly oriented polymer segments outside the pores to isolated, aligned polymer chains within the pores. Energy migration along the conjugated polymer backbone occurred more slowly than Forster energy transfer between polymer chains. The different intrachain and interchain energy transfer time scales explain the behavior of conjugated polymers in a range of solution environments. The results provide insights for optimizing nanostructured materials for use in optoelectronic devices.