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RESEARCH Ultrafast Energy Transfer in Molecular Nanostructures
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One example where such an understanding has been, achieved to an enormous extent is the way bacteria. and higher plants use sunlight to gain energy Our, present day knowledge of all the involved processes. represents the results of decades of research work. done by biologists chemists and physicists just, giving an example of interdisciplinary work with theo. retical physicists involved Studies in this field are not. only concerned with phenomena taking place on frac. tions of nanometers but also on time scales extending. down to the femtosecond region 10 15 s It is this, and also the fact that these systems absorb sunlight. within the visible range that let come into play ultra. fast optical spectroscopy The correct interpretation of Fig 2. spectroscopic data and the improvement of our under Energy level scheme related to the monomeric part of the. standing of the functionality however asks for a prop FMO complex see Infobox A Every chlorophyll molecule. green is described by two electronic levels upper and lower. er description of the observations This can be at, edge of the purple stripes Excitation into the upper level.
tained by using methods and techniques of Quantum yellow arrow becomes possible by light absorption yellow. Physics which include the development of specific wavy line This excitation may move from one chlorophyll. models and the performance of numerical simulations molecule to the other by a de excitation excitation process. And possibly with all that in mind one may take a step blue arrows connected by a blue wavy line If the interaction. forward from such a basic research work into the field between different chlorophyll molecules is strong an energy. level scheme valid for the whole antenna complex is formed. of applied research Hence the attempt to understand. with collective excitations called excitons right part see also. the details of the excitation energy transfer in photo Infobox B Now photoabsorption directly leads to an excita. synthetic antenna systems may be considered proto tion of these spatially delocalized states right part yellow. typical for practical use of dynamic phenomena in arrow and yellow wavy line Since the chlorophyll molecules. nanosystems are embedded in proteins the whole structure of the antenna. fluctuates according to thermal fluctuations of the protein. II Theoretical Models for Photosynthetic Antenna This leads to a modulation of the energy level scheme with the. result that a certain portion of the excitation energy is dissi. pated into the surroundings, Long before any part of a photosynthetic antenna has. been known with atomic resolution it was believed, that an exciton mechanism was responsible for the try to develop a quantum mechanical model of the. excitation energy transport to the reaction center see energy level structure of the photosynthetic antenna. Infobox B Having data at hand which give a detailed as well as a quantum dynamical description of the. spatial arrangement of the chlorophyll molecules with exciton motion It is typical that these processes need. a resolution of few ngstr m 1 0 1 nm one may modern theoretical tools of quantum physics to be. Ultraschneller Energietransfer in molekularen gerufen durch die Anwendung u erer Felder aufzei. Nanostrukturen gen Der vorliegende Artikel berichtet ber theoreti. Die Nanotechnologie hat das Ziel Systeme zu schaffen sche Untersuchungen zu schnellen Energietransfer. deren Funktionseinheiten bis herab zu der Gr e ein prozessen in so genannten lichtsammelnden Komple. zelner Molek le miniaturisiert sind F r ein vollst ndi xen Dieser spezielle Typ von Nanosystemen findet. ges Verst ndnis all dessen was auf einer solchen L n sich im Photosyntheseapparat von Bakterien und. genskala im Experiment m glich ist sind theoretische h heren Pflanzen Da lichtsammelnde Komplexe ber. Untersuchungen unverzichtbar Numerische Simulatio eine sehr genau definierte Struktur verf gen und von. nen die Aufschluss ber Details der r umlichen Struk der Zellmembran isoliert werden k nnen stellen sie. tur eines Nanosystems geben nehmen dabei einen besonders geeignete Objekte f r das Studium zeitab. zentralen Platz ein Aber auch Rechnungen zur Elek h ngiger Ph nomene in Nanosystemen dar Und f r. tronenstruktur haben einen hohen Stellenwert Und die Prozesse des Energietransfers in ihnen ist typisch. h ufig werden auch Simulationen wichtig die zeitab dass sie eine konsequente quantenmechanische. h ngige Ph nomene eines Nanosystems etwa hervor Beschreibung erforderlich machen. HUMBOLDT SPEKTRUM 3 4 2004 83, Fig 3 results A particular type of such a model is explained. Absorption spectrum pro in Fig 2 It is applied here to a complex of seven chlo. portional to the absorbed rophyll molecules but more complex antenna systems. light intensity versus the, can be described in a similar way The result of the. light wavelength of the PS1, antenna complex cf Fig 1 parameter adjustment within such a model is demon.
and Infobox A The measured strated in Fig 3 for the PS1 antenna system cf Fig. as well as the calculated 1 It includes the calculation of the absorption spec. spectrum are shown which trum based on the known spatial structure of the PS1. are overlaid by a spectrum and a search for the optimal set of the 96 chlorophyll. of all dipole moments versus,excitation energies,absorbing exciton level. after 4 absorption above, 695 nm induced by red and The usefulness of such an exciton model for the under. yellow drawn chromophores standing of nonlinear optical experiments is described. of Fig 1 This part of the in more detail in the following section In Section IV a. figure not only displays the understood There have been attempts to compute the control of excitation energy motion in antenna com. distribution of all exciton electronic energy level structure for the complex of plexes by ultrafast laser pulses is suggested. levels but also gives a rough,chlorophyll molecules for example 2 3 However. impression of the measured, spectrum However only a such a quantum chemical approach has been success III Nonlinear Spectroscopy of Femtosecond Dynamics. combination with all mecha ful only in part since the consideration of the whole A number of excitation energy transfer processes in. nisms leading to a broa protein which carries the antenna is beyond present photosynthetic antenna systems take place on a time. dening of these sharp peaks day computational capabilities Moreover direct com scale below one picosecond 10 12 s Respective. results in a realistic descrip putation of the coupling of the electronic excitations techniques of ultrafast optical spectroscopy are neces. tion of the absorption excitons to vibrations of the chlorophyll molecules. and to the surrounding protein necessary for a, description of excitation energy dissipation also Fig 4.
seems to be hopeless at the moment Consequently an Transient absorption of the probe pulse versus delay time. approach has to be chosen which is based on addi measured in a pump probe experiment for the LH2 antenna. complex after 5 6 The right part shows the water wheel. tional assumptions mainly related to the energy level. like spatial structure of the LH2 the 27 chlorophyll molec. structure of the chlorophylls and their mutual inter ules are drawn in yellow and red the helical part of the. action Concrete parameter values are fixed by a com carrier protein in grey Besides the transient absorption the. parison of this model with different experimental left part also displays the shape of the pump pulse and the. total population of exciton levels one and two exciton states. Details of the excitation energy dynamics level populations. and cross correlations following the pump laser excitation. are shown in the middle panel for three different times they. are specified at the lower panel via the black lines for more. explanation see text,84 HUMBOLDT SPEKTRUM 3 4 2004. sary to follow them Pump probe spectroscopy repre,sents one commonly used method where one laser. pulse the pump pulse excites the system and the, other laser pulse the probe pulse probes the excited. state If the duration of both pulses are shorter than. the dynamics of excitation energy transfer one may. trace the dynamics by varying the time delay between. both pulses To achieve a proper interpretation of the. measured data accompanying calculations become, necessary Fig 4 displays respective results together. with the structure of the described antennae the LH2. of purple bacteria see Infobox A, In order to obtain a detailed understanding of such.
laser pulse induced ultrafast dynamics the model, introduced in Fig 2 has to be extended to the case. that the pump pulse may excite two excitons simulta. neously in the antenna complex formation of the two. exciton state Furthermore it is not sufficient to com. pute the probability for the excitation of a particular. exciton level Instead to get a correct quantum, mechanical description of the dynamics it is of basic. importance to include cross correlations between dif Fig 5. ferent energy levels via the so called density matrix Laser pulse control of excitation energy motion in the FMO. The complex temporal behavior of the cross correla complex cf Fig 2 and infobox A. The aim is the total localization of excitation energy in the. tions is shown in the upper left panel of Fig 4 The. central chlorophyll molecule target chromophore which has. various boxes with black background display dif to be achieved against the tendency of delocalization connec. ferent time steps during and after the pump pulse ted with the formation of exciton states The respective dis. excitation 0 stands for the ground state of the com tribution of excitation energy among the 7 chlorophyll mole. plex 1 for all exciton levels and 2 for the double cules at three different times specified at the lower panel. excitations two exciton levels The respective energy via the black lines is given at the top of the panel note the. color code for the excitation probability The time dependency. of all levels increases from the left to right as well. of the population spatial localized excitation probability of. from top to bottom The probability that an energy the chlorophyll molecules is drawn in the upper part of the. level has been excited is given by the spots on the diagram and the temporal shape of the exciting laser pulse. diagonal of the 1 1 and 2 2 box The contribu is shown in the lower part after 7. tions of the off diagonal regions in these boxes as well. as all contributions in boxes like 0 1 1 2 etc, are induced by the optical excitation and vanish mainly scale much shorter than the overall duration of the. due to the thermal fluctuations of the carrier protein pulse Then the quantum mechanical state may be. The scheme demonstrates the complex quantum dy excited and modified in such a fast way that it really. namical behavior of the antennae which is found when moves into the chosen target state cf 8 10. describing the spectroscopic data see lower panel of. Fig 4 The discussion related to laser pulse control of exciton. dynamics is somewhat speculative However when, IV Laser Pulse Control of Excitation Energy Motion putting this in a broader context it should be of huge. The idea of laser pulse control dates back to the interest to manipulate the transfer of excitation ener. eighties It aims at driving the motion of a molecule gy in nanostructures Indeed the structural robust. into a particular quantum mechanical state guided by ness of antenna systems favors these systems when. a respectively tailored laser pulse Since such quan compared for example with dye aggregates Fig 5. tum dynamics are strongly disturbed by thermal fluc shows the results of some computational studies car. tuations of the molecule itself and of its environment ried out for the FMO complex see Infobox It was the. the control should proceed in the femtosecond time aim of these studies to demonstrate the possibility to. region To tailor a laser pulse appropriately here localize the excitation energy on a single molecule. means to change its intensity and frequency on a time against the tendency of forming delocalized exciton. HUMBOLDT SPEKTRUM 3 4 2004 85, states drawn in the right part of Fig 2 In fact after.
600 femtoseconds nearly 80 of the overall excita, tion was concentrated on the chlorophyll molecule in. the center of the complex All other chromophores, were much less populated The laser pulse which real. ized such an excitation energy concentration followed. from the solution of an optimization problem and is. also shown in Fig 5,As already mentioned such studies are not mainly. undertaken to get a deeper insight into the biological. Dr Ben Br ggemann PD Dr habil Volkhard May function of antenna systems but rather to demonstrate. Born 1973 diploma in Phy Born 1954 diploma in Theo the basic possibility to manipulate transport pro. sics 1999 at the Freie Uni retical Physics in 1977 and cesses on an ngstr m length scale and a femto. versit t Berlin 1999 2004 PhD in 1981 at the Hum, second time scale This might be of huge importance. PhD student at the Hum boldt Universit t zu Berlin. boldt Universit t zu Berlin in Habilitation in Theoretical for any further development in the field of nano. the group of Volkhard May Physics at the College of systems either as a tool for investigation or as a strat. member of the collaborative Education G strow in 1987 egy to achieve a certain functionality. research center 450 Analy work at the Institute of. sis and Control of Ultrafast Molecular Biology Berlin References. Photoinduced Reactions Department of Biophysics 1 Jordan P Fromme P Witt H T Klukas O Saen. frequent research visits at from 1987 91 since 1992. ger W Krau N Nature 909 411 2001, the Vrije Universiteit senior researcher within the.
Amsterdam and Lund Uni chair Semiconductor Theo 2 Cory M G Zerner M C Hu X Schulten K J. versity within the framework ry at the Institute of Phys Phys Chem B 102 7640 1998. of the ESF ULTRA program ics of the Humboldt Univer 3 Scholes G D Gould I R Cogdell R J Fleming. 2004 Dr rer nat at the sit t zu Berlin research G R J Phys Chem B 103 2534 1999. Humboldt Universit t zu visits at USA and Russia 4 Br ggemann B Sznee K Novoderezhkin V van. Berlin since 2004 postdoc in project manager in the. Grondelle R May V J Phys Chem B 108 13536, the group of Prof Sundstr m collaborative research cen. at the Lund University ter 450 Analysis and Con, trol of Ultrafast Photoin 5 Br ggemann B Herek J L Sundstr m V Pulle. duced Reactions main rits T May V J Phys Chem B 105 11391 2001. Internet Contact research interest theory of 6 Br ggemann B May V J Chem Phys 120. www semic physik hu ber Lund University transfer phenomena in mo 2325 2004. lin de AG May Chemical Center lecular systems 7 Br ggemann B May V Gerald F Small Fest. Chemical Physics,schrift J Phys Chem B 108 10529 2004. 22100 Lund Sweden Contact 8 Rice S A Zhao M Optical Control of Molecular. E Mail ben brueggemann Humboldt Universit t Dynamics Wiley New York 2000. chemphys lu zu Berlin 9 Shapiro M Brumer P Principles of the Quan. Faculty of Mathematics tum Control of Molecular Processes Wiley New Jer. and Natural Sciences I sey 2003, Department of Physics 10 May V K hn O Charge and Energy Transfer. Newtonstr 15,Dynamics in Molecular Systems second edition.
D 12489 Berlin Adlershof,Phone 49 30 2093 4821 Wiley VCH Berlin 2004. Fax 49 30 2093 4725 11 van Amerongen H Valkunas L van Grondelle. E Mail may physik R Photosynthetic Excitons World Scientific Singapo. hu berlin de re 2000,86 HUMBOLDT SPEKTRUM 3 4 2004.

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