Synthesis and Fluorescence Characterization of MEHPPV Oligomers

Resolving conjugated polymer film morphology with polarised transmission and time-resolved emission microscopy

The alignment of chromophores plays a crucial role in determining the optoelectronic properties of materials. Such alignment can make interpretation of fluorescence anisotropy microscopy (FAM) images somewhat ambiguous. The time-resolved emission behaviour can also influence the fluorescence anisotropy. This is particularly the case when probing excitation energy migration between chromophores in a condensed phase. Ideally information concerning the chromophoric alignment, emission decay kinetics and fluorescence anisotropy can be recorded and correlated. We report on the use of polarised transmission imaging (PTI) coupled with both steady-state and time-resolved FAM to enable accurate identification of chromophoric alignment and morphology in thin films of a conjugated polydiarylfluorene. We show that the combination of these three imaging modes presents a comprehensive methodology for investigating the alignment and morphology of chromophores in thin films, particularly for accurately mapping the distribution of amorphous and crystalline phases within the thin films, offering valuable insights for the design and optimization of materials with enhanced optoelectronic performance.

Functional Catechol-Metal Polymers via Interfacial Polymerization for Applications in Water Purification

Phenols and polyphenols have been used as a scaffold for generating multidimensional molecular architectures via complexation with metal ions. Here, we report the synthesis and characterization of metallopolymer films from three catechol derivatives having different alkyl/aryl substituents via complexation with iron and copper ions at the organic-water interface. Such interfacial polymerization is instantaneous, one step to generate functional materials, and gives good control over the organization of repeating units along the film. The films were transferred to different substrates such as filter paper, cotton, or polyester fabrics. The films are superhydrophobic with a contact angle >160° which can be tuned by regulating the orientation of nonpolar groups at the interface during polymerization. In addition, the fabricated cloth membrane showed excellent oil/water separation efficiency of more than 99% even after 50 cycles. The polymers also showed good dye extraction capacity from aqueous solutions with fast kinetics data. Such metallopolymer networks can serve as a versatile material for applications in catalysis, protective coatings, drug delivery, water filtration membranes, and liquid separations.

Light losses from scattering in luminescent solar concentrator waveguides

The reductions in the transmission of emission originating from a fluorophore dissolved in a polymer matrix due to light scattering were compared in two forms of planar waveguides used as luminescent solar concentrators: a thin film of poly(methylmethacrylate) (PMMA) spin-coated on a glass plate and a solid PMMA plate of the same dimensions. The losses attributable to light scattering encountered in the waveguide consisting of the thin film of polymer coated on a glass plate were not detectable within experimental uncertainty, whereas the losses in the solid polymer plate were significant. The losses in the solid plate are interpreted as arising from light-scattering centers comprising minute bubbles of vapor/gas, incomplete polymerization or water clusters that are introduced during or after the thermally induced polymerization process.

Evolution of Nonmirror Image Fluorescence Spectra in Conjugated Polymers and Oligomers

The nonmirror image relationship between absorption and fluorescence spectra of conjugated polymers contrasts with most organic chromophores and is widely considered a signature of interchromopohore energy funneling. We apply broad-band ultrafast fluorescence spectroscopy to resolve the evolution of fluorescence spectra for dilute solutions of conjugated oligothiophenes, where no energy transfer is possible. Fluorescence spectra evolve from a mirror image of absorption, which lacks vibronic structure, toward a spectrally narrower and vibronically structured species on the hundreds of femtosecond to early picosecond time scale. Our analysis of this fluorescence spectral evolution shows that a broad distribution of torsional conformers is driven to rapidly planarize in the excited state, including in solid films, which is supported by Raman spectroscopy and quantum chemical modeling. Our data have important implications for understanding different energy-transfer regimes that are delineated by structural relaxation.

The photophysics of phenylenevinylene oligomers and self-absorption of their fluorescence in polymer films

The fluorescence spectra, quantum yields and lifetimes of a series of alkoxy-substituted phenylenevinylene molecules, which serve as short chain oligomer models for poly(p-phenylenevinylene), have been determined in fluid solvents and in a high viscosity polymer matrix. The effects of solvent polarity and a high viscosity molecular environment on the fluorescence yields and spectral shapes have been established. Alkoxy group substitution on the phenyl ring moieties of the molecules has an important effect on the vibronic structures and profiles of the absorption spectra. This was interpreted in terms of hot-band, ground to excited singlet state transitions from energetically closely-spaced torsional vibrational levels of the vinylene double bond in the ground state. The shapes of the absorption bands affect the overlaps of the absorption and fluorescence spectra. This has been quantified as the probability of fluorescence reabsorption in solid polymer films as a function of pathlength. This is an important determinant of the efficacies of these compounds for "harvesting" solar energy in luminescent solar concentrator systems. The reabsorption probabilities of these compounds are lower for all pathlengths than those determined in the same polymer film for the fluorophores, perylene and perylene diimide, which have been considered for concentrating spatially diffuse sunlight.

Excited state dynamics of organic semi-conducting materials

Time-resolved absorption and emission spectroscopy has been applied to investigate the dynamics of excited state processes in oligomer models for semi-conducting organic materials. Following the photo-excitation of a pentamer oligomer that is a model for the conjugated polymer MEH-PPV, an ultrafast component of a few picoseconds is observed for the decay of the initially formed transient species. Variable temperature absorption and emission spectra combined with X-ray crystallography and calculations support the assignment of this rapid relaxation process to an excited state conformational rearrangement from non-planar to more planar molecular configurations. The implications of the results for the overall photophysics of conjugated polymers are considered.

Energy transfer in PPV-based conjugated polymers: a defocused widefield fluorescence microscopy study

Both pendant and main chain conjugated MEH-PPV based polymers have been studied at the level of single chains using confocal and widefield fluorescence microscopy techniques. In particular, defocused widefield fluorescence is applied to reveal the extent of energy transfer in these polymers by identifying whether they act as single emitters. For main chain conjugated MEH-PPV, molecular weight and the surrounding matrix play a primary role in determining energy transport processes and whether single emitter behaviour is observed. Surprisingly in polymers with a saturated backbone but containing the same pendant MEH-PPV oligomer on each repeating unit, intra-chain energy transfer to a single emitter is also apparent. The results imply there is chromophore heterogeneity that can facilitate energy funneling to the emitting site. Both main chain conjugated and pendant MEH-PPV polymers exhibit changes in orientation of the emission dipole during a fluorescence trajectory of many seconds, whereas a model MEH-PPV oligomer does not. The results suggest that, in the polymers, the nature of the emitting chromophores can change during the time trajectory.

Conformational and photophysical changes in conjugated polymers exposed to Couette shear

Conjugated polymers in solution exhibit interesting photophysical behavior, which is dictated by their molecular conformation. The conformations and resulting photophysics can be altered by deformational flows such as simple shear. Solutions of poly[2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) in dimethylformamide (DMF) show large decreases in fluorescence intensity as a function of shear rate, combined with significant spectral shifts upon exposure to shear. The excitation and emission spectra shift toward shorter wavelengths, indicating a change in conformation with shortened conjugated segment lengths attributed to compressive hydrodynamic forces in flow. Addition of poly(methyl methacrylate) to the solutions is shown to alter the fluorescence emission spectral behavior, which we ascribe to energy transfer from the higher energy, short segments to a small population of lower energy conjugated segments. The measured fluorescence changes were not reversible upon cessation of shear, demonstrating that permanent conformational changes are induced by flow.