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

Abstract

Single poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) molecules dispersed in thin poly(methylmethacrylate) films have been investigated by fluorescence emission, excitation and time-resolved spectroscopy at 1.2 K. For the molecular weight studied (∼200 kDa) a bimodal distribution of emission maxima is observed. Based on a comparison of the spectroscopic properties of blue and red sites and on polarisation-resolved measurements, we argue in agreement with recent quantum-chemical calculations that the red subpopulation most probably does not arise from interchromophoric excitation delocalisation but is to be attributed to longer chromophoric units originating from ordered regions of a polymer chain, where due to constraints on the chain conformation larger conjugation lengths can be realised. In excitation spectra within the red spectral region we can identify multiple chromophoric units, among them chromophores without correspondence in the emission spectrum-donors of the intramolecular energy transfer. Zero-phonon lines of donor chromophores proved to be significantly broadened, indicating fast excited-state population decay due to energy transfer. Thus, a distribution of energy transfer times within MEH-PPV chains could be determined from donor zero-phonon line widths, with an average value of 3.9 ps. Our study represents the first direct measurement of energy transfer times in conjugated polymers, parameters that are crucial for the performance of many technical applications based on this class of material.

Relaxation dynamics and exciton energy transfer in the low-temperature phase of MEH-PPV

“…22,24,26,27,30,59,60 It was suggested that photoluminescence decay in isolated chains is mono-exponential with characteristic time scales between 0.4 and 1.2 ns, while non-exponential relaxation was observed in presence of aggregation, 24,27 in films, 17,61,62 and in collapsed MEH-PPV chains. 23,63 All the aforementioned configurations are characterized by a strong reduction in the photoluminescence quantum yield, which is ascribed to the formation of non-emissive interchain species.…”

“…The red sites have been repeatedly ascribed to planarized segments characterized by an increased conjugation length compared to the blue sites. 25,26 It was also suggested that the formation of the red sites might be related to aggregation. 22,27 Recent studies showed that concentrated solutions of MEH-PPV in 2-methyltetrahydrofuran (MeTHF) undergo a phase transition from a coiled conformation ("blue phase"), characteristic of room temperature, to an aggregated and planarized phase at low temperatures ("red phase"), characterized by very long conjugation length.…”

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.…”

Impact of charge carrier injection on single-chain photophysics of conjugated polymers

“…The spectral blue-shift upon charge injection 11 suggests that formation of the quencher occurs predominantly, but not always, on low-energy chromophores. 29 The quenching of the low-energy chromophores also over-compensates a possible red-shift in emission of the remaining chromophores due to the Stark effect, which was shown to be in the order of 2nm for an electric field of 300kV/cm. 11 The electric field of a single charge at a distance of 5-10nm is approx.…”