| Intermolecular electronic energy transfer(EET)is a ubiquitous photophysical process and understanding the EET mechanism in donor-acceptor systems is important for engineering light harvesting in photosynthesis and photovoltaics.The intermolecular EET is consisting of incoherent energy transfer and coherent energy transfer.When the intermolecular interaction is weak,the molecular excitation energy is transferred from the donor molecule to the acceptor molecule in a hopping way,and the intermolecular EET is incoherent energy transfer which consists of Forster's resonance energy transfer(FRET)and Dexter energy transfer.However,when the intermolecular interaction is strong,according to theoretical predictions,the excitation energy will delocalize on the donor and acceptor molecules,forming a wave-like coherent energy transfer.The intermolecular EET mechanisms are crucially dependent on the intermolecular distances.For large intermolecular distances,based on Forster's resonance energy transfer theory(FRET)the excitation energy is transferred from the donor to the acceptor chromophores through dipolar interaction between two independent oscillating transition dipoles with a transfer rate following a d-6 distance dependency.With the decrease of intermolecular distances,the transfer would be dominated by Dexter mechanism due to the overlap of electronic densities,following an exponential distance dependency.Nevertheless,according to the theoretical prediction,at very small intermolecular distances,intermolecular EET processes can go beyond the conventional Forster-Dexter picture;the excited states of the donor and acceptor are entangled with each other,resulting in the delocalization of the excitation energy and the occurrence of quantum-coherent energy transfer.Nevertheless,such a quantum coherence is believed to be very fragile in the lossy and complicated molecular systems and it is not until 2007 that researchers reported to directly evidence such coherent energy transfer in light-harvesting systems through the quantum beating signals in ultrafast spectroscopy.However,the interpretation of these beating signals and the existence of quantum-coherent energy transfer in light harvesting complexes are still under extensive discussion,probably due to the challenges of excluding the"artificial" coherence induced by the coherent laser excitation as well as the ensemble averaging due to the diffraction limit in conventional far-field optics.Some reseatchers argued that the "quantum beats" were caused by molecular vibrations(the process of Raman scattering)and not by quantum coherence among excited electronic states.Some reseatchers argued that the phase of the exciton wavefunction involved in the coherent state in the photosynthetic system can be synchronized with the phase of the nuclear vibration through electron-vibration coupling,so as to resist the decoherence process caused by the environment and prolong the lifetime of the electron coherent state.Thus,to gain insight into the quantum-coherent energy transfer,it is highly desirable for the simple and manipulatable donor-acceptor model systems as well as the characterization ability at the level of individual molecules.Scanning tunneling microscope(STM)induced luminescence(STML),a technique combining STM with optical detection,offers an ideal platform for the investigation on the intermolecular EET on surfaces due to the sub-molecular resolved spectroscopic imaging ability beyond the far-field optics and can provide unprecedented opportunities for gaining insights into single-molecule optoelectronic phenomena occurring at the single-molecule level.In this thesis,two kinds of photoelectric molecules related to energy transfer with STML technology are studied,one is the pentacene molecule and the other is the phthalocyanine molecule.First,in order to understand the role of vibronic coupling in the energy transfer process,we investigate how the vibration affects the electron transition in a single molecule.We investigate the intramolecular vibronic coupling of a single pentacene molecule through sub-nanometer revolved spectroscopic imaging in real space,by exploiting the localized nanocavity plasmon(NCP)enhancement on a well-decoupled emitter.Furthermore,we visualize in real space the intermolecular energy transfer between pentacene and zinc phthalocyanine,as well as the intermolecular energy transfer between two different phthalocyanine molecules evolving from an incoherent hopping-type tranfer to a coherent wave-like transfer with the controlled decrease of intermolecular distances.Our findings provide the direct evidence for the existence of quantum-coherent energy transfer in molecular donor-acceptor systems first time.This thesis is mainly composed of four chapters,which are as follows:In chapter one,we mainly introduce the research background of this thesis.Firstly,we introduce the the basic concepts of incoherent energy transfer and coherent energy transfer,which provides a theoretical basis for indentifying the mechanism of energy transfer in subsequent experiments.Then,we introduce the basic concepts of surface plasmon and highlight the importance of localized surface plasmon to single molecule electroluminescence.Then,we introduce the basic principle and application of and the experimental techniques and a review of the history and current status of scanning tunneling microscope induced luminescence is also presented.At the end of this chapter,we describe the optical STM setup used in our experiments and and the main content of this thesis.In chapter two,in order to understand the role of vibronic coupling in the energy transfer process,we investigate how a vibration affects the electron transition in a single molecule.We investigate in real space the intramolecular vibronic coupling of a single pentacene molecule through sub-nanometer revolved spectroscopic imaging.The imaging patterns of FC term-dominated vibronic states are found to have the same orientation as that of the 0-0 peak,all along the short axis.However,the patterns of HT term-dominated vibronic states are found to be evidently different from that of the 0-0 peak,becoming rotated 90°along the long axis.Such a difference directly reflects a change in the transition dipole orientation,suggesting the occurrence of strong vibronic coupling associated with a large Herzberg-Teller contribution and going beyond the conventional Franck-Condon picture.By combining with theoretical calculations,the vibration-induced emission is found to occur on those non-total-symmetric molecular vibrations that can strongly perturb the electronic transition,especially through those atoms with large transition density populations.The strong and dynamic vibrational perturbation to these atoms leads to large vibration-induced transition charges oscillating in a different direction from the purely electronic transition.In this way,anisotropic vibronic-state imaging patterns offer a straightforward understanding on how molecular vibrations affect electronic transitions and related energy redistributions in real space.In addition,we have also investigated the effect of isotope substitution on molecular vibrations.The perdeuteration of pentacene is found to blue-shift both the electronic origin and vibronic emission due to the heavy-atom substitution effect,but vibronic peaks are shifted slightly stronger,yielding the expected frequency red-shifts in respective vibrational modes.Isotope substitution helps to make unambiguous assignments for vibrational modes,particularly for overtone vibrations.Anisotropic vibronic-state imaging for a combination mode composed of one FC-dominated and one HT-dominated fundamental mode indicates that the combined influence of these two simultaneously excited modes on the electronic transition is mainly determined by the HT-dominated mode.Our results provide a profound understanding on the microscopic picture of molecular spectroscopy,particularly on vibronic coupling,and open up new opportunities for real-space studies on the role of electron-vibration coupling in energy transfer processes at the individual molecular level.In chapter three,we visualize in real space the intermolecular energy transfer between a single pentacene molecule and a single ZnPc molecule.By controling the intermolecular distance and molecular orientation through manipulating molecules by STM,we investigate the influences of different configurations and intermolecular distances of molecular heterodimer on energy transfer.With the decrease of intermolecular distance,the aceptor(ZnPc)Q(0,0)peak become stronger accompanied by a decrease of donor(pentacene)Q(0,0)peak,which is due to a significant increase in the intermolecular energy transfer rate at a smaller inermolecular distance.The shifty of ZnPc Q(0,0)peak in the pentaphene-ZnPc dimer with two different configurations for smallest intermolecular distance has been observed.Especially when the 0-0 transition dipole direction of pentacene molecule along the dimer axial direction,ZnPc Q(0,0)peak is split whether exciting pentacene or ZnPc along the dimer axial direction,which is not only due to the increase of intermolecular interaction along the dimer axial direction,but also due to the possibility of the coherent energy transfer.In chaper four,we visualize in real space the intermolecular energy transfer between two different phthalocyanine molecules evolving from an incoherent hopping-type tranfer to a coherent wave-like transfer with the controlled decrease of intermolecular distances.For an intermolecular distance d?1.72 nm,judging from the peak energy position as well as the photon images,the intermolecular energy transfer obeys the FRET mechanism in a one-way manner with a roughly d-6 distance-dependency.The quantum-coherent energy transfer emerges when the intermolecular distance is about 1.52 nm.For a PtPc-ZnPc dimer at d=1.41 nm,the observed delocalized anti-bonding-like pattern of the new peak in spectroscopic imaging clearly suggests the directional quantum-coherent energy transfer along the dimer axial direction,probably due to the larger dipolar coupling strength in this direction over the dissipation of either the donor or the acceptor..These findings provide the direct evidence for the existence of quantum-coherent energy transfer in molecular donor-acceptor systems,which pave the way for the future investigation to explore at a new frontier the intermolecular energy transfers,such as the roles of vibronic coupling and the decoherence processes,as well as harness the coherence to enhance the functions of men-made light harvesting structures. |
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