Substantial fluctuations of the flourescence intensity have been detected for single clusters of poly(phenylenevinylene) containing more than 75 polymer chains or 30000 monomer units. To the best of our knowledge, this is the first time such fluctuations (which resemble the "blinking" effect in single-molecule flourescence) have been reported for such a large molecular ensemble containing several macromolecules. Together with the distinct jumps, smooth fluctuations of the flourescence intensity, with characteristic times from milliseconds to seconds, were observed. This fact distinguishes the flourescence behaviour of the polymer clusters from that of other multichromophoric systems such as the single chains of conjugated polymers reported in the literature. The consecutive or simultaneous switching of one or several emitting sites from the "on" to "off" state does not explain the character of the fluctuations observed. We suggest that the quenching of the light-emitting exciton by a long-lived species, such as, for example, polarons, plays an important role in these unusual fluctuations. Electric field induced fluorescence quenching differs significantly for different clusters. It is proposed that this flourescence was mainly quenched by polarons injected from the electrodes in the presence of an electric field. The specific behaviour of each cluster is explained by suggesting a different position of the clusters with respect to the electrodes.
