Distinctive character of electronic and vibrational coherences in disordered molecular aggregates

Coherent dynamics of coupled molecules are effectively characterized by the two-dimensional (2D) electronic coherent spectroscopy. Depending on the coupling between electronic and vibrational states, oscillating signals of purely electronic, purely vibrational or mixed origin can be observed. Even in the "mixed" molecular systems two types of coherent beats having either electronic or vibrational character can be distinguished by analyzing oscillation Fourier maps, constructed from time-resolved 2D spectra. The amplitude of the beatings with the electronic character is heavily affected by the energetic disorder and consequently electronic coherences are quickly dephased. Beatings with the vibrational character depend weakly on the disorder, assuring their long-time survival. We show that detailed modeling of 2D spectroscopy signals of molecular aggregates providesdirect information on the origin of the coherent beatings.Comment: 7 pages, 4 figures, 1 tabl

Influence of the Carotenoid Composition on the Conformational Dynamics of Photosynthetic Light-Harvesting Complexes

Nonphotochemical quenching (NPQ) is the major self-regulatory mechanism of green plants, performed on a molecular level to protect them from an overexcitation during the direct sunlight. It is believed that NPQ becomes available due to conformational dynamics of the light-harvesting photosynthetic complexes and involves a direct participation of carotenoids. In this work, we perform a single-molecule microscopy on major light-harvesting complexes (LHCII) from different Arabidopsis thaliana mutants exhibiting various carotenoid composition. We show how the distinct carotenoids affect the dynamics of the conformational switching between multiple coexisting light-emitting states of LHCII and demonstrate that properties of the quenched conformation are not influenced by the particular carotenoids available in LHCII. We also discuss the possible origin of different conformational states and relate them to the fluorescence decay kinetics observed during the bulk measurements

Theory of exciton-charge transfer state coupled systems

Abstract We present a systematic density matrix theory of excitons interacting with charge transfer states in molecular systems subject to influence of a semiclassical bath. An excitonic dimer interacting non-linearly with an overdamped Brownian oscillator bath is studied, and the effect of eigenstate renormalization by interaction with the bath is shown to be essential in a correct description of the unusual temperature dependence of the absorption spectrum of the exciton-charge transfer state system. Ó 2006 Published by Elsevier B.V. Intermolecular charge transfer (CT) states, which occur in molecular aggregates with close packing of their building blocks, can strongly influence their exciton spectral properties [4] and references therein). Considerable effort has been invested into developing suitable theoretical models based on exciton-CT (EX-CT) state mixing that would allow for correct descriptions of absorption, Stark, hole burning and transient spectra of these systems Consider a dimer composed of two molecules, A and B. The electronic states of such an aggregate include excited states jEX A ae and jEX B ae representing excitation localized on the molecules A and B, respectively, while the other molecule is in its electronic ground state. Linear combinations of the states jEX A ae and jEX B ae form the usual excitonic states of the aggregate. Further, such a complex may display CT states jCT A ae and jCT B ae denoting transitions of an electron from the local excitation on molecule A to the molecule B and from B to A, respectively. W

Direct generation of optical vortices

A detailed scheme is established for the direct generation of optical vortices, signifying light endowed with orbital angular momentum. In contrast to common techniques based on the tailored conversion of the wave front in a conventional beam, this method provides for the direct spontaneous emission of photons with the requisite field structure. This form of optical emission results directly from the electronic relaxation of a delocalized exciton state that is supported by a ringlike array of three or more nanoscale chromophores. An analysis of the conditions leads to a general formulation revealing a requirement for the array structure to adhere to one of a restricted set of permissible symmetry groups. It is shown that the coupling between chromophores within each array leads to an energy level splitting of the exciton structure, thus providing for a specific linking of exciton phase and emission wavelength. For emission, arrays conforming to one of the given point-group families’ doubly degenerate excitons exhibit the specific phase characteristics necessary to support vortex emission. The highest order of exciton symmetry, corresponding to the maximum magnitude of electronic orbital angular momentum supported by the ring, provides for the most favored emission. The phase properties of the emission produced by the relaxation of such excitons are exhibited on plots which reveal the azimuthal phase progression around the ring, consistent with vortex emission. It is proven that emission of this kind produces electromagnetic fields that map with complete fidelity onto the phase structure of a Laguerre-Gaussian optical mode with the corresponding topological charge. The prospect of direct generation paves the way for practicable devices that need no longer rely on the modification of a conventional laser beam by a secondary optical element. Moreover, these principles hold promise for the development of a vortex laser, also based on nanoscale exciton decay, enabling the production of coherent radiation with a tailor-made helical wave front

Singlet-triplet annihilation in single LHCII complexes

In light harvesting complex II (LHCII) of higher plants and green algae, carotenoids (Cars) have an important function to quench chlorophyll (Chl) triplet states and therefore avoid the production of harmful singlet oxygen. The resulting Car triplet states lead to a non-linear self-quenching mechanism called singlet–triplet (S–T) annihilation that strongly depends on the excitation density. In this work we investigated the fluorescence decay kinetics of single immobilized LHCIIs at room temperature and found a two-exponential decay with a slow (3.5 ns) and a fast (35 ps) component. The relative amplitude fraction of the fast component increases with increasing excitation intensity, and the resulting decrease in the fluorescence quantum yield suggests annihilation effects. Modulation of the excitation pattern by means of an acousto-optic modulator (AOM) furthermore allowed us to resolve the time-dependent accumulation and decay rate (B7 ms) of the quenching species. Inspired by singlet–singlet (S–S) annihilation studies, we developed a stochastic model and then successfully applied it to describe and explain all the experimentally observed steady-state and time-dependent kinetics. That allowed us to distinctively identify the quenching mechanism as S–T annihilation. Quantitative fitting resulted in a conclusive set of parameters validating our interpretation of the experimental results. The obtained stochastic model can be generalized to describe S–T annihilation in small molecular aggregates where the equilibration time of excitations is much faster than the annihilation-free singlet excited state lifetime.VU University and by an Advanced Investigator grant from the European Research Council (no. 267333, PHOTPROT).Nederlandse Organisatie voor Wetenschappelijk Onderzoek, Council of Chemical Sciences (NWO-CW) via a TOP-grant (700.58.305), and by the EU FP7 project PAPETS (GA 323901).Academy Professor grant from the Netherlands Royal Academy of Sciences (KNAW). University of Pretoria's Research Development Programme (Grant No.A0W679) Research Council of Lithuania (LMT grant no. MIP-080/2015).http://www.rsc.orgpccp2016-08-31hb201