Tuning energy transfer in switchable donor-acceptor systems

The synthesis and characterisation of a coumarin-dithienylcyclopentene-coumarin symmetric triad (CSC) and a perylene bisimide-dithienylcyclopentene-coumarin asymmetric triad (PSC) are reported. In both triads the switching function of the photochromic dithienylcyclopentene unit is retained. For CSC an overall 50% quenching of the coumarin fluorescence is observed upon ring-closure of the dithienylcyclopentene component, which, taken together with the low PSS (<70%), indicates that energy transfer quenching of the coumarin component by the dithienylcyclopentene in the closed state is efficient. Upon ring opening of the dithienylcyclopentene unit the coumarin emission is restored fully. The PSC triad shows efficient energy transfer from the coumarin to the perylene bisimide unit when the dithienylcyclopentene unit is in the open state. When the dithienylcyclopentene is in the closed ( PSS) state a 60% decrease in sensitized perylene bisimide emission intensity is observed due to competitive quenching of the coumarin excited state and partial quenching of the perylene excited state by the closed dithienylcyclopentene unit. This modulation of energy transfer is reversible over several cycles for both the symmetric and asymmetric tri-component systems

Optical Energy Transport and Interactions between the Excitations in a Coumarin-Perylene Bisimide Dendrimer

Energy transfer properties of novel coumarin-perylene bisimide dendrimer are studied by means of steady state and time-resolved UV/vis spectroscopy. At low donor excitation density fast (transfer rate ~10 ps-1) and efficient (quantum yield ~99.5%) donor-acceptor energy transfer is observed. The random distributions of donor-acceptor orientations and distances result in nonexponential energy transfer kinetics. The energy transfer remains independent of excitation density up to densities corresponding to one absorbed photon per 10 dendrimer molecules. At higher excitation densities the transfer rate is found to increase due to excitation of multiple donors per dendrimer. Control of the donor-acceptor energy transfer rate is achieved by pre-excitation of the acceptor and monitored by prepump-pump-probe experiments, which show that the energy transfer rate can be decreased by a factor of 2. The relative orientations of transition dipole moments in the donor and acceptor molecules are found to be one of the key factors determining the energy transfer dynamics at high excitation densities.

Intramolecular energy transfer in a tetra-coumarin perylene system: influence of solvent and bridging unit on electronic properties

The synthesis and characterisation of a novel coumarin donor–perylene bisimide acceptor light-harvesting system is reported, in which an energy-transfer efficiency of >99% is achieved. Comparison of the excited-state properties of the donor–acceptor system with model compounds revealed that although the photophysical properties of the perylene bisimide acceptor unit are affected considerably by the nature of the substituent at the imide positions and the solvent employed, through-bond interaction between the donor and acceptor units is negligible. Energy transfer in the present system can be described as occurring via a through-space energy-transfer mechanism. Careful consideration of the redox properties of the donor relative to the acceptor units allows for avoidance of potentially deleterious excited-state electron-transfer processes.

Photoswitchable Sexithiophene-Based Molecular Wires

Photochromic sexithiophenes were prepared by oxidative electrochemical coupling of terthiophenes. The redox properties in the open state are typical of sexithiophenes. Ring closure of both photochromic units leads to a decrease in the energy of the LUMO orbitals with little affect on the energy of the HOMO orbitals. The photochemical tuning of the conjugation of a molecular wire is achieved by combining dithienylethene units with a sexithiophene.