The photophysics of a dendrimer containing four chromophores are investigated at the single-molecule level. First, the multichromophoric character of single dendrimers' absorption is probed by modulating the linear polarization of the excitation beam. Subsequently, using circular polarization, the same dendrimers are excited, and their fluorescence transients are recorded. Using pulsed excitation in combination with the classical Hanbury-Brown and Twiss coincidence setup the presented data demonstrate that efficient singlet-singlet annihilation ensures that always only one photon is emitted even when several excitations are generated in an individual multichromophoric molecule.

(1)(2)
(1) Fluorescence transient for a single molecule using modulated linear polarized excitation (black). The gray curve shows the modulation in fluorescence intensity expected for a single chromophore.

(2) (A) Fluorescence intensity transient (black) and Nc/ ratio transient (gray) of the same dendrimer as in Figure 2 using circular polarized excitation (2.1 kW/cm2). (B) Interphoton distance (coincidence) histogram for the first intensity level (15 s) of the transient in (A).

Investigations of the fundamental aspects of energy-transfer processes are relevant for multichromophoric systems such as photosynthetic complexes and conjugated polymers.1 These systems are becoming increasingly accessible by single-molecule spectroscopy (SMS)2-5 which can provide detailed information on the spatial, conformational, and temporal inhomogeneity of populations that are otherwise lost due to averaging. One intriguing phenomenon that is resolved by SMS is collective intermittences in the fluorescence of multichromophoric systems such as conjugated polymers and multichromophoric dendrimers.3-5 Collective off-states are ascribed to the formation of nonfluorescent traps,3 either due to reversible reactions with oxygen4 or singlet-triplet annihilation.5 Dendrimers decorated with chromophores at the periphery are systems that possess a large absorption cross-section and show energy hopping of the exciton among the chromophores. They are easily tunable in size, geometry, and chromophore number, thus enabling the investigation of many photophysical phenomena. The multichromophoric system targeted in this communication is a shape-persistent polypheneylene dendrimer (1) with a tetrahedral core and four peryleneimide (PI) units at the rim (Figure 1).