Conformation-dependent Room-temperature Emission Spectra of Single MEH-PPV Chains in Different Polymer Matrices

Abstract

Emission spectra of single molecules of the conjugated polymer MEH-PPV in host matrices of ZEONEX Ò and polystyrene are reported. Depending on the matrix, the polymer chains attain different conformations, from compact defect cylinders to extended defect coils. The conformations are correlated with photophysical properties, such as emission intensity fluctuations, and with shapes and peaks of emission spectra. The observed bimodal distribution of the spectral peaks is related to distribution in conjugated segment lengths.Conjugated polymers are the subject of intense research as active materials for optical and electronic devices.1,2 Photophysical processes play crucial roles in the device function. However, photophysics of conjugated polymers depends strongly on the unique conformation of individual polymer chains. As a result, many questions on the relationship between structure and optical properties remain open. 3,4 The complexity of the study of conjugated polymer photophysics can be partially overcome by the use of single molecule spectroscopy. 5 Recently, we reported a novel fluorescence polarization microscopic method 6 and used it together with molecular dynamics simulations to reveal the conformation of individual polymer chains on a truly single-molecule level.7 This enabled us to study photophysical properties of MEH-PPV on the same single chains and to show that the conformational state is the determining factor for the photophysical processes. Here, we use the previously determined conformations of MEH-PPV in different host matrices and correlate the structural information with single chain emission spectra.Poly[2-methyloxy-5-(2-ethylhexyl)oxy-1,4-phenylenevinylene] (MEH-PPV, Aldrich, M w 200000, PDI 5) was dispersed by spin-coating in thin-film matrices of ZEONEX and of low-molecular weight polystyrene (PS). The film thickness was on the order of 120 nm. The experiments were carried out on an inverted fluorescence microscope 7 using the 488-nm line of Ar þ ion laser for excitation. Time evolving emission spectra were taken with an EM-CCD camera and an imaging spectrograph as a series of tens of images.Single MEH-PPV chains attain compact defect cylinder conformations in the poor-solvent matrix of ZEONEX (Figure 1). The resulting interchain interactions lead to localization of the exciton and to blinking in the emission time-traces (Figure 1a). Single-chain emission spectra measured for several tens of molecules were analyzed and their maxima were plotted in a histogram in Figure 1c. The histogram shows a single-mode Gaussian-like distribution with a center near 550 nm.In the good-solvent matrix of polystyrene, the MEH-PPV is present in two conformations, 7 a compact oblong defect coil (type 1) and an extended disc-like defect coil (type 2), as seen in Figure 2. The two conformations lead to different photophysical behavior. The compact defect coil causes partial localization of the exciton in domains on the chain, 8 resulting in blinking of the emission (Figure 2a). In contrast, the extended coil ...

Origin of the Red Sites and Energy Transfer Rates in Single MEH‐PPV Chains at Low Temperature

“…Single MEH-PPV chains (M W~2 00 kDa) have been studied by different techniques of single-molecule spectroscopy at a temperature of T = 1.2 K. We have found a bimodal distribution of emission maxima as reported before [19,24,[42][43][44][45][46][47] and have addressed the question as to the nature of the red subpopulation of emitters. The fact that no qualitative difference could be revealed in emission spectra, fluorescence excitation spectra and the fluorescence decay behaviour has led us to the conclusion that the red sites are to be attributed to longer chromophoric units rather than interactions between chromophores (supposedly resulting in excimers or aggregates).…”

“…Using a sufficiently short excitation wavelength, the distribution is characterised by the bimodal behaviour (Figure 1 a, l exc = 514 nm, 54 molecules studied), repeatedly reported in the literature for MEH-PPV. [19,24,[42][43][44][45][46][47] It was fitted with a combination of two Gaussians with maxima of 18 230 cm À1 and 16 570 cm À1 and full widths at half-maximum (fwhm) of 1100 cm À1 and 1020 cm À1 . An excitation wavelength of l exc = 568 nm resulted in a monomodal distribution of emission maxima, because only the red subpopulation was accessible under these conditions (255 molecules studied).…”

“…A bimodal distribution of emission maxima of single MEH-PPV molecules has been repeatedly reported in the literature. [19,24,[42][43][44][45][46][47] In principle, one might think of two possible origins of the red subpopulation: first, longer chromophoric units with a more extended p delocalisation (which on their part can also have several possible origins, see discussion below) and, second, electronic interactions between chromophores in close proximity as a consequence of chain backfolding. Such interactions might lead to excimers (resulting from the interaction of an excited with a ground-state chromophore) or aggregates (interacting both in ground and excited state), common phenomena in pristine films of conjugated polymers.…”

“…emission maxima has been repeatedly reported. [19,24,[42][43][44][45][46][47] The low-energy, red subpopulation, whose relative weight in different studies has been shown to depend on the molecular weight [42,43,45,46] of the sample or the host matrix in which the MEH-PPV chains are dispersed, [47] might be caused by either interchromophoric interactions delocalising the electronic excitation (excimers or aggregates) or by longer chromophoric units with a larger extent of p delocalisation favoured by characteristics of the chain conformation.…”

The red-phase of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV): A disordered HJ-aggregate

“…[28][29][30][31][32][33][34][35][36][37] Recent work by three of us 27 showed that MEH-PPV chains dissolved in methyltetrahydrofuran (MeTHF) undergo a phase transition at approximately 200 K. Above this temperature, the polymer exists in a disordered coil morphology referred to as the blue phase. As the solution is cooled through the transition temperature, the polymer assembles into aggregates (the "red-phase") in which polymer chains are substantially elongated causing a red shift of the absorption and fluorescence spectra relative to the spectra of more disordered chains.…”