FRET and ligand related NON-FRET processes in single quantum dot-perylene bisimide assemblies.

Nanoassemblies are formed via self-assembly of ZnS capped CdSe quantum dots (QD) and perylene bisimide dyes (PBI). Upon assembly formation with functionalized dye molecules the QD photoluminescence (PL) is quenched. Quenching has been assigned partly to FRET (fluorescence resonance energy transfer) and NON-FRET processes. By means of time resolved single particle spectroscopy of immobilized QD-dye assemblies, it is demonstrated that NON-FRET processes are due to new non-radiative decay channels caused by the assembly formation process itself. Immobilized (single) assemblies exhibit the same processes as ensembles of assemblies in toluene solution. Only one dye molecule on a QD quenches the PL up to 50%, which is much stronger than is expected when replacing a volume related number of ligands. NON-FRET processes are distinct from photoinduced charge and/or energy transfer. A combination of a Stern-Volmer and FRET analysis of ensemble experiments supports the investigation of the dynamics of assembly formation at extremely low concentration ratios of PBI to QD. This allows us to distinguish between the effects of PBI and ligands on PL quenching on a single molecule level which is not possible in conventional ligand dynamic experiments.

Ultrafast energy-electron transfer cascade in a multichromophoric light-harvesting molecular square.

A molecular square with dimensions of about 4 nm, incorporating sixteen pyrene chromophores attached to four ditopic bay-functionalized perylene bisimide chromophores, has been synthesized by coordination to four Pt(II) phosphine corner units and fully characterized via NMR spectroscopy and ESI-FTICR mass spectrometry. Steady-state and time-resolved emission as well as femtosecond transient absorption studies reveal the presence of a highly efficient (>90%) and fast photoinduced energy transfer (k(en) approximately equal to 5.0 x 10(9) s(-1)) from the pyrene to the perylene bisimide chromophores and a very fast and efficient electron transfer (>94%, k(et) approximately equal to 5 x 10(11) up to 43 x 10(11) s(-1)). Spectrotemporal parametrization indicates upper excited-state electron-transfer processes, various energy and electron-transfer pathways, and chromophoric heterogeneity. Temperature-dependent time-resolved emission spectroscopy has shown that the acceptor emission lifetime increases with decreasing temperature from which an electron-transfer barrier is obtained. The extremely fast electron-transfer processes (substantially faster and more efficient than in the free ligand) that are normally only observed in solid materials, together with the closely packed structure of 20 chromophoric units, indicate that we can consider the molecular square as a monodisperse nanoaggregate: a molecularly defined ensemble of chromophores that partly behaves like a solid material.

Wavelength-dependent electron and energy transfer pathways in a side-to-face ruthenium porphyrin/perylene bisimide assembly.

A new side-to-face supramolecular array of chromophores, where a pyridyl-substituted perylene bisimide dye axially binds to two ruthenium porphyrin fragments, has been prepared by self-assembly. The array is formulated as DPyPBI[Ru(TPP)(CO)](2), where DPyPBI = N,N'-di(4-pyridyl)-1,6,7,12-tetra(4-tert-butylphenoxy)perylene-3,4:9,10-tetracarboxylic acid bisimide and TPP = 5,10,15,20-tetraphenylporphyrin. The photophysical behavior of DPyPBI[Ru(TPP)(CO)](2) has been studied by fast (nanoseconds) and ultrafast (femtoseconds) time-resolved techniques. The observed behavior sharply changes with excitation wavelength, depending on whether the DPyPBI or Ru(TPP)(CO) units are excited. After DPyPBI excitation, the strong fluorescence typical of this unit is completely quenched, and time-resolved spectroscopy reveals the occurrence of photoinduced electron transfer from the ruthenium porphyrin to the perylene bisimide dye (tau = 5.6 ps) followed by charge recombination (tau = 270 ps). Upon excitation of the Ru(TPP)(CO) fragments, on the other hand, ultrafast (tau < 1 ps) intersystem crossing is followed by triplet energy transfer from the ruthenium porphyrin to the perylene bisimide dye (tau = 720 ps). The perylene-based triplet state decays to the ground state on a longer time scale (tau = 9.8 micros). The photophysics of this supramolecular array provides remarkable examples of (i) wavelength-dependent behavior (a small change in excitation wavelength causes a sharp switch from electron to energy transfer) and (ii) intramolecular sensitization (the triplet state of the perylene bisimide, inaccessible in the free dye, is efficiently populated in the array).

Photoluminescent supramolecular polymers: metal-ion directed polymerization of terpyridine-functionalized perylene bisimide dyes.

Perylene bisimide dyes bearing one and two 2,2':6',2"-terpyridine receptor groups were prepared and the structural and optical properties of their Zn2+ complexes were investigated by 1H NMR and fluorescence experiments.