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International audienceno abstrac

In this paper, we report the interaction of the CB[7] molecular container with crown ether styryl and (bis)styryl dyes 1-6. The interaction of monostyryl dyes (1 and 2) with CB[7] results in the formation of 1:1 complexes where the CB[7] molecule is located on the region of the guest encompassing the pyridinium ring, C=C double bond, and a portion of the aryl ring of benzocrown ethers 1 and 2. For (bis)styryl dyes (3-5), the formation of two types of complexes with composition dye.CB[7].dye and CB[7].dye.CB[7] was confirmed by a combination of optical and electrospray ionization mass spectroscopy (ESI-MS) methods. In the case of (bis)styryl dye (6), both 2:1 and 1:1 compositions 6.CB[7].6 and CB[7].6 were formed. Complex formation is accompanied by substantial changes in the optical characteristics of the dyes and formation of long-lived excimer species. We tested the stimuli responsiveness of this system in response to metal ions. We find that the metal ions prefer to bind to the electrostatically negative ureidyl C=O portals of the CB[7] rather than with the crown ether moiety of the styryl dyes.

BF(2)-Azadipyrromethene dyes are a promising class of NIR emitter (nonhalogenated) and photosensitizer (halogenated). Spectroscopic studies on a benchmark example of each type, including absorption (one and two photon), time-resolved transient absorption (ps-ms) and fluorescence, are reported. Fast photodynamics reveal that intense nanosecond NIR fluorescence is quenched in a brominated analog, giving rise to a persistent (21 μs) transient absorption signature. Kinetics for these changes are determined and ascribed to the efficient population of a triplet state (72%), which can efficiently sensitize singlet oxygen formation (ca. 74%), directly observed by (1)Δ(g) luminescence. Photostability measurements reveal extremely high stability, notably for the nonhalogenated variant, which is at least 10(3)-times more stable (Φ(photodeg.) = < 10(-8)) than some representative BODIPY and fluorescein dyes.

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The photoinduced intramolecular charge transfer processes of three differently twisted 4-(dimethylamino)-4′-cyanobiphenyl derivatives (I-III) have been investigated using time-resolved transient absorption and gain spectroscopy in the subpicosecond range. Independent of twist angle and solvent polarity, the kinetics and spectral evolutions after excitation clearly reveal a precursor-sater relationship for the electron transfer from a less emissive state of mixed 1 L b /CT character to a highly emissive charge transfer ( 1 CT) state. Beside the occurrence of dual fluorescence gain, two transient absorption bands for the 1 CT state and one for the precursor state ( 1 FC) are observed. All bands are assigned to electronic transitions and correlated for all solvents and compounds. The band intensities are discussed with solvent polarity and twist angle controlled mixing between the charge transfer state 1 CT and the higher lying 1 L b and 1 L a states. In acetonitrile, the transient spectra of the pretwisted donor-acceptor biphenyl III, in contrast to the planar I and II, can be approximated by the sum of cation and anion spectra of the subunits demonstrating decoupled moieties. The kinetics of the CT processes are not dominated by solvation dynamics alone. As an example, in acetonitrile, (τ l ) 0.2 ps, τ s < 1 ps) the kinetics are slower than 2.5 ps. The involvement of a weak electronic coupling matrix element is favored as a source for the intramolecular control of the CT reactions. Furthermore, for the strongly twisted biphenyl derivative III, a secondary intramolecular process to a more relaxed species (CTR) occurs after the initial CT step, in agreement with fluorescence studies.

A series of photoactive triads have been synthesized and investigated in order to elucidate photoinduced electron transfer and hole migration mechanism across nanosized, rigid helical foldamers. The triads are comprised of a central helical oligoamide foldamer bridge with 9, 14, 18, 19, or 34 8-amino-2-quinolinecarboxylic acid repeat units, and of two chromophores, an N-terminal oligo(para-phenylenevinylene) electron donor and a C-terminal perylene bis-imide electron acceptor. Time-resolved fluorescence and transient absorption spectroscopic studies showed that, following photoexcitation of the electron acceptor, fast electron transfer occurs initially from the oligoquinoline bridge to the acceptor chromophore on the picosecond time scale. The oligo(para-phenylenevinylene) electron donor is oxidized after a time delay during which the hole migrates across the foldamer from the acceptor to the donor. The charge separated state that is finally generated was found to be remarkably long-lived (>80 μs). While the initial charge injection rate is largely invariant for all foldamer lengths (ca. 60 ps), the subsequent hole transfer to the donor varies from 1 × 10 s for the longest sequence to 17 × 10 s for the shortest. In all cases, charge transfer is very fast considering the foldamer length. Detailed analysis of the process in different media and at varying temperatures is consistent with a hopping mechanism of hole transport through the foldamer helix, with individual hops occurring on the subpicosecond time scale (k = 2.5 × 10 s in CHCl). This work demonstrates the possibility of fast long-range hole transfer over 300 Å (through bonds) across a synthetic modular bridge, an achievement that had been previously observed principally with DNA structures.