Genotyping a second growth coast redwood forest : a high throughput methodology

The idea that excitonic (electronic) coherences are of fundamental importance to natural photosynthesis gained popularity when slowly dephasing quantum beats (QBs) were observed in the two-dimensional electronic spectra of the Fenna–Matthews–Olson (FMO) complex at 77 K. These were assigned to superpositions of excitonic states, a controversial interpretation, as the strong chromophore–environment interactions in the complex suggest fast dephasing. Although it has been pointed out that vibrational motion produces similar spectral signatures, a concrete assignment of these oscillatory signals to distinct physical processes is still lacking. Here we revisit the coherence dynamics of the FMO complex using polarization-controlled two-dimensional electronic spectroscopy, supported by theoretical modelling. We show that the long-lived QBs are exclusively vibrational in origin, whereas the dephasing of the electronic coherences is completed within 240 fs even at 77 K. We further find that specific vibrational coherences are produced via vibronically coupled excited states. The presence of such states suggests that vibronic coupling is relevant for photosynthetic energy transfer

Exciton relaxation in nanotubular TPPS4 aggregates in water solution and in polymeric matrix

Exciton properties in meso-Tetra (4-sulphonatophenyl)porphin e (TPPS4) aggregates in water solution and in PVA matrix were investigated at different temperatures by means of transient absorption and fluorescence spectroscopy. Analysis of the steady state and transient absorption spectra shows that excitons delocalized over about 10 TPPS molecules are formed under the aggregate excitation to the higher energy absorption band, however they localize on one or two molecules after relaxation to the lowest energy excited state. The transient absorption and time resolved fluorescence show different relaxation kinetics, which is interpreted in terms of competition between the exciton relaxation to the nonradiative state and quenching by the quenching centers reached during thermally activated exciton diffusion. Exciton-exciton annihilation starts at high excitation intensity when more than I exciton per 20 molecules is created indicating that excitations visit about 20 molecules during their lifetime. The exciton relaxation becomes faster in optically annealed samples, when optically destroyed molecules create additional quenching centers. (c) 2006 Elsevier B.V. All rights reserved

Quest for Order in Chaos: Hidden Repulsive Level Statistics in Disordered Quantum Nanoaggregates

The local distribution of exciton levels in disordered cyanine-dye-based molecular nanoaggregates has been elucidated using fluorescence line narrowing spectroscopy. The observation of a Wigner-Dyson-type level spacing distribution provides direct evidence of the existence of level repulsion of strongly overlapping states in the molecular wires, which is important for the understanding of the level statistics, and therefore the functional properties, of a large variety of nanoconfined systems