Excited state intramolecular proton transfer (ESIPT) of salicylideneaniline enol and subsequent isomerization of the proton transferred keto structure to the final long-lived photochromic product has been studied in different solvents using femtosecond transient absorption and picosecond fluorescence spectroscopy. Three different types of transient absorption have been identified, viz. (a) from initially excited enol form, (b) proton transferred keto form and (c) corresponding to the final photochromic product. The ESIPT rate was determined from the rise time of the characteristic transient absorption originated from the proton transferred keto form and was found to be in the order of (200-300 fs) À1 in most of the solvents. Photochromic product formation is completed within a few tens of picoseconds. The time dependent spectral change observed at the early time delay after excitation was considered to be due to cooling of the vibrationally '' hot '' proton transferred keto form towards the formation of the final photochromic product. The cooling process leads to the formation of some '' metastable '' state within a few hundred of femtoseconds in the ground state potential energy surface of the final trans product.

Intramolecular Förster-type excitation energy transfer (FRET) processes in a series of first-generation polyphenylene dendrimers substituted with spatially well-separated peryleneimide chromophores and a terryleneimide energy-trapping chromophore at the rim were investigated by steady-state and time-resolved fluorescence spectroscopy. Energy-hopping processes among the peryleneimide chromophores are revealed by anisotropy decay times of 50-80 ps consistent with a FRET rate constant of k hopp ) 4.6 ns -1 . If a terryleneimide chromophore is present at the rim of the dendrimer together with three peryleneimide chromophores, more than 95% of the energy harvested by the peryleneimide chromophores is transferred and trapped in the terryleneimide. The two decay times (τ 1 ) 52 ps and τ 2 ) 175 ps) found for the peryleneimide emission band are recovered as rise times at the terryleneimide emission band proving that the energy trapping of peryleneimide excitation energy by the terryleneimide acceptor occurs via two different, efficient pathways. Molecular-modeling-based structures tentatively indicate that the rotation of the terryleneimide acceptor group can lead to a much smaller distance to a single donor chromophore, which could explain the occurrence of two energy-trapping rate constants. All energy-transfer processes are quantitatively describable with Förster energy transfer theory, and the influence of the dipole orientation factor in the Förster equation is discussed.

in Polyphenylene Dendrimers with an Increasing Number of Peryleneimide Chromophores Maus, M.; Mitra, S.; Lor, M.; Hofkens, J.; Weil, T.; Herrmann, A.; Müllen, K.; Schryver, F.C. DeThe photophysical properties and the excitation energy transfer (EET) process of a series of first generation dendrimers having a rigid tetrahedral core and different numbers of peryleneimide chromophores at the rim were investigated by steady-state and time-resolved fluorescence spectroscopy in toluene. It was shown that the model dendrimer containing only one chromophore decays monoexponentially with a time constant of 4.2 ns. In the multichromophoric compounds an additional long decay time of 7.4 ns assigned to an "excimerlike" state was observed, and its contribution increases with the number of chromophores. The time-resolved polarization measurements resulted in a nanosecond relaxation component due to the rotation of the entire dendritic structure. However, for the compounds having more than one chromophore, an additional fast anisotropy decay time of 100-200 ps was observed which corresponds to Förster type EET among the chromophores. 3965 continuing support. S.M. and J.H. thank KUL and FWO, respectively, for postdoctoral fellowships. M.M. thanks the EC through TMR-Sisitomas for a postdoctoral fellowship.

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