Single-molecule spectroscopy is used to study the time-dependent spectral behavior of a short rodlike Poly͑phenylene vinylene͒ ͑PPV͒ derivative polymer spin cast in a polystyrene matrix. The fluorescent time trace is characterized by stepwise intensity emission with constant spectral composition, punctuated by abrupt intensity changes, which are usually accompanied by abrupt spectral changes. In contrast to coiled long chain polymers, defect-free rodlike polymers exhibit multiple-emission sites, each with its characteristic invariant spectrum. The distribution of spectral jumps in the emission spectrum reflects the distribution of the effective conjugation length. This implies the energy transfer ͑i.e., thermalized exciton migration͒ along the polymer backbone is inefficient. A static disorder induced conjugation length distribution model with limited energy transfer can be used in understanding the photophysics of an isolated polymer.
The energy landscape of single, isolated, short-and long-chain luminescent conjugated polymers was studied by photobleaching spectral shift analysis and interphoton time measurements. It has been found that the energy landscape of a polymer is dependent on both the conjugation length distribution as well as the arrangement of these conjugated segments. Energy funnels responsible for abrupt drops in the photoluminescence time trace intensity are found to be wide ͑in terms of the number of absorbing segments feeding energy into them͒ rather than deep ͑in terms of energy relative to the other emission sites in the polymer͒. Experimental results indicate that the energy landscape is influenced, but not dictated, by the solvent polarity. Moreover, it was observed that spectral blueshifts do not always accompany the quenching of a funnel, pointing to the existence of multiple independent regions of approximately the same energy distribution within an individual polymer.
Single molecule spectroscopy can uncover fluctuation averaged out in ensemble measurements. In addition, single molecule spectroscopy naturally implies ultra‐sensitive measurements, which should play important role in future proteomics and genomic research. Fluorescence method is the most commonly used single molecule detection. In this paper, we will use single conjugated polymer fluorescence spectroscopy to illustrate the power of this technique.
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