Host Matrix Dependent Fluorescence Intensity Modulation by an Electric Field in Single Conjugated Polymer Chains

Abstract

An electric field oscillating at a frequency approximately 1 Hz is found to induce strong modulation of the fluorescence intensity of single poly[2-methoxy,5-(2'-ethyl-hexyloxy)-p-phenylene vinylene] (MEH-PPV) molecules (MW approximately 10(6)) embedded in a poly(methyl methacrylate) (PMMA) matrix. The MEH-PPV polymer chains are carefully isolated from the electrodes to avoid effects of injection. In a polystyrene matrix, fluorescence intensity modulations are on average much less pronounced. The difference in average modulation depth can be explained in terms of lower field-induced exciton dissociation rates in the MEH-PPV/polystyrene system compared to MEH-PPV/PMMA because of a lack of suitable acceptor sites. The observed electric field dependence of single-molecule fluorescence strongly suggests that energy transfer from singlet or even triplet excitons to long-living on-chain hole polarons contributes to the observed modulations. The observed large qualitative differences between the responses of different molecules probably reflect differences in chain topology and strongly anisotropic distributions of acceptor sites, while the hysteretic response of some molecules indicates conformational switching.

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

“…The typical reversible blinking behavior on the timescale of seconds can be observed. 1,13,27 Jumps in PL intensity to less than half (or more than double) of its former value were observed 10 in approximately 25% of the recorded transients. These jumps are distributed arbitrarily in time and are caused by reversible photoinduced radical formation, or by subsequent irreversible photo-oxidation.…”

“…Charges and their electric fields have a strong impact on the final device characteristics, e.g. exciton diffusion, 7 PL quantum yield [8][9][10] and PL emission wavelength. 4,11 The random formation of charges on a single conjugated polymer chain is responsible for the highly dynamic changes in PL intensity, i.e.…”

Detailed Investigation of Light Induced Charge Injection into a Single Conjugated Polymer Chain

“…Without this layer the observed fluorescence modulation is apparently due to a pure electric field effect rather than hole injection or in some cases, hole injection due to oxygen and/or multiphoton ionization. [42][43][44][45][46] As further evidence that quenching is the result of charge injection, insulating layers of either SiO 2 deposited by ebeam evaporation or poly(isobutyl methacrylate) spun cast from an orthogonal solvent were placed between the MEH-PPV and the CBP HTL. The incorporation of these layers completely arrests fluorescence modulation in the majority of the molecules (<5% of molecules modulate, data not shown), and the few molecules that do show fluorescence modulation in these control devices show erratic modulation behavior suggesting spurious charging and perhaps a pure electric field effect.…”

Single-Molecule Recognition and Dynamics with Pulsed Laser Excitation

“…Here, the applied electric current is proportional to the amplitude of applied external bias voltage, with the relation being I = ZU with Z = 0.87ohm -1 . The dye molecules embedded in the ITO film are distinct from the molecules in electric field experiments (Hania et al 2006). In the electric field experiments, a large electric field intensity of about 100 V/μm is needed, and dyes must be insulated from the electrode.…”