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Photoluminescence (PL) of Gd-doped silica in the ultraviolet (UV) is investigated. The efficient emission detected at 314 nm is due to the 6P 7/2 to 8S 7/2 transition of a 4f electron of the Gd3+ ion.

Artificial photosynthetic antenna systems have been synthesized based on carotenoid polyenes and polymer-polyenes covalently attached to tetrapyrroles. Absorption of light in the blue/green region of the spectra excites the polyenes to their S2 state, and ultrafast singlet energy transfer to the tetrapyrroles occurs when the chromophores are in partial conjugation. The additional participation of other excited states of the polyene in the energy-transfer process is a requirement for perfect antenna function. Analogs of photosynthetic reaction centers consisting of tetrapyrrole chromophores covalently linked to electron acceptors and donors have been prepared. Excitation of these constructs results in a cascade of energy transfer/electron transfer which, in selected cases, forms a final charge-separated state characterized by a giant dipole moment (>150 D), a quantum yield approaching unity, a significant fraction of the photon energy stored as chemical potential, and a lifetime sufficient for reaction with secondary electron donors and acceptors. A new antenna-reaction center complex is described in which a carotenoid moiety is located in partial conjugation with the tetrapyrrole π-system allowing fast energy transfer (

A model photosynthetic antenna system consisting of a carotenoid moiety covalently linked to a purpurin has been prepared to study singlet-singlet energy transfer from a carotenoid to a cyclic tetrapyrrole. Ultrafast fluorescence upconversion measurements of the carotenopurpurin dyad and an unlinked reference carotenoid demonstrate that the fluorescent S 2 excited state of the carotenoid model has a lifetime of 150 ′ 3 fs, whereas the corresponding excited state of the carotenoid in the carotenopurpurin dyad is quenched to 40 ′ 3 fs. This quenching is assigned to energy transfer from the S 2 state to the purpurin with a 73 ′ 6% efficiency, which is in accord with the 67 ′ 4% quantum yield obtained by steady-state fluorescence excitation measurements. Concomitant with the decay of the carotenoid S 2 excited state, a single-exponential rise of the excited S 1 state of the purpurin moiety was observed at 699 nm with a time constant of 64 fs. However, the decay of the fluorescence anisotropy was faster at this wavelength (40 fs) and isotropic rise times as short as 44 fs were determined at other emission wavelengths. The lifetime of the Si state of the carotenoid (7.8 ps) was the same in both the carotenoid model and the dyad. Taken together, these results unequivocally demonstrate that the S 2 state of the carotenoid moiety is the sole donor state in this efficient singlet-singlet energy transfer process. The simple dyad described in this work mimics the ultrafast energy transfer kinetics found in certain naturally occurring pigment protein complexes and is thus able to reproduce the high electronic coupling needed for efficient energy transfer from an extremely short-lived energy donor state.

Sensitive optical detection of nitroaromatic vapours with diketo-pyrrolopyrrole thin films is reported for the first time and the impact of thin film crystal structure and morphology on fluorescence quenching behaviour demonstrated.

The solvent dependence of the steady-state and time-resolved fluorescence emission from the S2 (11Bu) state of all-trans-β-carotene was investigated in a range of polar and nonpolar solvents and in hexane/carbon disulfide mixtures. The steady-state absorption and fluorescence emission maxima in both polar and nonpolar solvents showed parallel shifts with increasing solvent polarizability, indicating that the emitting state is the S2 (11Bu) state. The lifetime of the S2 state was determined using the fluorescence upconversion technique, and lifetimes varying from 120 fs in quinoline to 177 fs in hexane were found. An intramolecular relaxation process occurring on a similar time scale was observed as a broadening in the reconstructed time-dependent emission spectra in hexane and as emission wavelength-dependent lifetimes in all the solvents studied. Correlations were observed between the S2 lifetimes, the π* solvatochromic parameter, and the ratio of the absorption fine structure. A correlation between the ...

Two closely related, rigidly linked porphyrin−naphthoquinone dyads have been prepared and studied using time-resolved fluorescence and absorption methods. Dyad 1, whose quinone carbonyl groups are relatively close to the porphyrin macrocycle, exhibits photoinduced electron-transfer rate constants that are virtually independent of solvent dielectric constant and temperature within the range 77−295 K. Dyad 2, which has a similar donor−acceptor linkage but whose quinone carbonyl groups are ∼2 A farther from the porphyrin, features photoinduced electron-transfer rate constants that decrease with decreasing solvent dielectric constant. Electron transfer in this molecule ceases at low temperatures. Photoinduced electron transfer in dyad 2 exhibits the usual dependence on free energy change and solvent reorganization observed in many similar porphyrin−quinone systems. The behavior of 1 may be attributed at least in part to the smaller separation of the porphyrin radical cation and the quinone radical anion, whic...

Two approaches to eliciting photoelectrochemical reactions from carotenoids are being pursued. One uses LB films of amphipathic carotenoids deposited on semiconducting electrodes which are immersed in electrolytes containing electron donors or acceptors. Photocurrents are observed and their action spectra implicate the excited carotenoid pigment, possibly in an aggregated form, as the photoactive species. An example of the second approach is a molecular dyad consisting of a carotenoid covalently linked to Gjo. Excitation of the carotenoid moiety generates a charge-separated species in high yield. More complex systems include a molecular mad (C-P-Q) containing a porphyrin moiety (P) linked to a quinone (Q) and to a carotenoid (C). Triads of this type have been incorporated unidirectionally into lipid bilayer membranes. Time-resolved fluorescence experiments reveal that excitation of C-P-Q in the membrane leads to electron transfer from 1P to give C-P'+-Q". A subsequent electron transfer from C to P" yields C'+-P-Q'-. In this species the carotenoid pigment acts as a molecular wire to conduct charge across the membrane. In the presence of appropriate cofactors, this system translocates protons across the bilayer lipid membrane and generates proton motive force.

Time-resolved fluorescence and subpicosecond transient absorption experiments have been carried out on a triad molecular device (CPQ) comprising a porphyrin (P) covalently linked to an electron-donating carotenoid pigment (C) and a quinone (Q), which acts as an electron acceptor. Rate constants governing the intramolecular electron transfer processes originating from the excited singlet state of the porphyrin have been determined. Excitation of the porphyrin moiety of the triad in benzonitrile solution results in photoinduced electron transfer to give CP .+ Q .− ( k 1 =2.5 × 10 9 s −1 ) followed by rapid non-photochemical electron transfer to yield C .+  PQ .− ( k 3 ≈ 1 × 10 11 s −1 ). The intermediate CP .+ Q .− , also decays by charge recombination to the ground state with a rate constant k 2 of about 6 × 10 11 s −1 . Nanosecond laser flash experiments were used to measure a quantum yield of 0.13 for the long-lived (370 ns) charge-separated species C .+ PQ .− . Excitation of the carotenoid moiety of the triad results in singlet energy transfer to the attached porphyrin with a quantum yield of about 0.07.

Previously, the spatial arrangement of the carotenoid and bacteriochlorophyll molecules in the peripheral light-harvesting (LH2) complex from Rhodopseudomonas acidophila strain 10050 has been determined at high resolution. Here, we have time resolved the energy transfer steps that occur between the carotenoid's initial excited state and the lowest energy group of bacteriochlorophyll molecules in LH2. These kinetic data, together with the existing structural information, lay the foundation for understanding the detailed mechanisms of energy transfer involved in this fundamental, early reaction in photosynthesis. Remarkably, energy transfer from the rhodopin glucoside S2 state, which has an intrinsic lifetime of ~120 fs, is by far the dominant pathway, with only a minor contribution from the longer-lived S1 state., Swedish Natural Science Research Council. European Science Foundation. Gatsby Charitable Trust. Biochemical Society. Kempe Foundation. International Human Frontiers of Science Program. Biotechnology and Biological Sciences Research Council

We demonstrate that pulse shaping of a broadband Ti:sapphire laser can result in almost an order of magnitude increase in the sensitivity and signal to background ratio (SBR) of multiphoton total internal reflection fluorescence (TIRF) microscopy. We produced transform-limited pulses of 15 fs duration at the sample, and observed a 8-fold enhancement in the fluorescence of CdSe/ZnS quantum dots via two-photon objective-type TIRF excitation. There was a concomitant 6-fold increase of the SBR upon compression of the pulse duration. Enhancement of non-linear evanescent imaging has recently been demonstrated using surface-plasmons [Opt. Express 17, 5987 (2009)] and structured substrates [Opt. Express 18, 23218 (2010)]. Our approach of ultrafast pulse shaping could be used alone or combined with these new methods to offer significant gains in image quality.