Single chain versus single aggregate spectroscopy of conjugated polymers. Where is the border?

Abstract

Single chains of conjugated polymers e.g. MEH-PPV (poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene) have become interesting objects for single molecule spectroscopy (SMS) studies. However, most of the experiments so far were performed without full awareness of the isolation status of the polymer chains in host matrices. We used steady-state and time-resolved fluorescence methods and 2D polarization single molecule imaging technique to unravel the isolation/aggregation status of MEH-PPV in spin-coated films prepared at different conditions. It turned out that a sample showing isolated bright spots in fluorescence images could be obtained in a very broad concentration range of MEH-PPV when toluene was used as a solvent and PMMA as a matrix. If the MEH-PPV concentration was not sufficiently low, a substantial fraction of the fluorescence spots should be assigned to individual nano-aggregates rather than truly isolated chains of the polymer. Contrary to single aggregates, truly isolated MEH-PPV chains showed blue-shifted emission spectra, mono-exponential fluorescence decay dynamics with relatively long lifetimes (0.4-1.2 ns), and high polarization anisotropy. We argue that insufficient control of the concentration in the published SMS studies of MEH-PPV resulted in incorrect assigning of some spectroscopic properties of single aggregates to isolated MEH-PPV chains. We believe this to be the main origin of discrepancies among the published data in this field.

Optical probes of chain packing structure and exciton dynamics in polythiophene films, composites, and nanostructures

“…13,[45][46][47][48][49] Figure 9 shows an example of the photoluminescence measurements from isolated nanoparticles of different size and crystallinity in which the arrival time and polarization of each individual photon is recorded via routed time-tagged or time-resolved photon counting techniques. 45 This provides an interesting view into the interplay of amorphous and aggregated P3HT in confined geometries, although much more effort is needed both in synthesis (to minimize size and structural heterogeneities) and photophysical characterization.…”

Connection between the conformation and emission properties of poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] single molecules during thermal annealing

“…The blue shift of emission spectra upon dilution of MEH-PPV in PMMA has been observed by the previous works, 5,25,26 which can be explained by the cutting-off of the energy transfer to low energy traps and suppressing of generation of interchain excitons, and conformational changes of the polymer backbones. 27 Assuming that only intrachain excitons are responsible for the PL, the blue shift can be attributed to the thermally induced torsional motions shortening the conjugation lengths and differences in the HuangRhys coupling factors (S) for chains with distinct conjugation lengths, leading to differences in the relative intensities of the zero-and higher-order phonons, producing changes in the spectral profile. 28,29 The temperature dependent PL spectra have demonstrated that the thermally induced torsion and libration modes increase the conformation disorders to shorten the conjugation segments in the MEH-PPV with increasing temperature and thus reduce the extent of p -electron delocalization and increase the energy of the p -p* transition, leading to a blue shift in the PL spectra of MEH-PPV in solid matrix at high temperature.…”

Photoluminescence of Solvent-Selected Fluorescent Moieties in MEH-PPV Solutions and Films

“…7 In a previous work, 68 we have demonstrated that although MEH-PPV and PMMA are immiscible, there is some degree of miscibility enabling the interaction between branches of MEH-PPV segments and the PMMA matrix, in the distance of 65 Å. 68 Lin et al 25 presented fluorescence micrographs of distinct proportions of MEH-PPV embedded in PMMA matrices, obtaining distinct fluorescent emissions due to distinct emitting moieties. In particular, their system composed of 10 -5 m/m of MEH-PPV in PMMA showed a green colored (530 nm) fluorescence with some orange colored (580 nm) spots, which they related to the green emission of isolated MEH-PPV chains and the red-shifted emissions, due to other aggregates.…”

“…Lin et al 25 presented fluorescence micrographs of distinct proportions of MEH-PPV embedded in PMMA matrices, obtaining distinct fluorescent emissions due to distinct emitting moieties. In particular, their system composed of 10 -5 m/m of MEH-PPV in PMMA showed a green colored (530 nm) fluorescence with some orange colored (580 nm) spots, which they related to the green emission of isolated MEH-PPV chains and the red-shifted emissions, due to other aggregates.…”

“…An intermediate situation containing a mixture of undefined proportions of all the aforementioned species at distinct quantities is possible, and would be similar to the situation of having aggregates with a distribution of conjugation lengths in the system. 25 Theoretical calculations have been performed in many different systems to determine materials' properties such as geometry and electronic structure determinations, photoluminescent included, using a wide range of computational methods. Approximation hierarchies classify models into distinct complexity levels.…”