Theoretical Studies On The Two-dimensional Electronic Spectroscopy And Exciton Dynamics Of Molecular Aggregates

Molecular aggregate is the molecular system in which molecules are selfassembied into ordered structure and they are combined by the weak interaction, suchas hydrogen bonding, hydrophilic and hydrophobic forces, dipole-dipole interactionand van der Waals forces, etc. The functionality of molecular aggregates not onlydepends on the physics chemical properties of single molecule, but also depends onthe configuration of the molecular aggregates. The molecular aggregate exhibit thecollective optical response because there exist intermolecular excitation transferinteraction, which is different from that of isolated molecule. The deep understandingof the spectral behavior of molecular aggregate, such as the superradiance, amplifiedspontaneous emission, stimulated emission, play important role in developmentapplication of molecular aggregate material. The special spectroscopic properties ofmolecular aggregates included exchange narrowing of spectral lineshape,inhomogeneous broadening, homogeneous broadening phenomenon and so on.Investigation of spectroscopic and photophysical properties of molecular aggregatesare of a great importance for both fundamental research and practical applications. Toexplore the excitation behavior and physical nature of cooperative effect in molecularaggregate, the development of theory of exciton model is a very challenging issue.Two-dimensional electronic spectroscopy (2D ES) has emerged as a powerfulmethod for elucidating the structure function relationship in molecular aggregatesystems. This technology has been applied in different research fields, such as chemistry, physics and biology. The features of two-dimensional spectroscopy make itsuitable to address questions about the electronic structure, energy and charge transferprocesses in aggregate materials. Two-dimensional electronic coherence spectroscopymeasurements have been suggested as the high resolution means for the coherencebetween excited states and energy transfer. In this thesis, we provide theoreticalinsight into the spectral features and exciton dynamic of linear J aggregate bysimulation of2D electronic spectroscopy. Further, we investigate and analyze a realdimer molecular system to reveal the structure properties relationship. The paper isorganized as follows：In the first chapter, we introduce the conception of the molecular aggregate, thespectral properties, Frenkel exciton model, current research status and researchsignificance of the molecular aggregates. In addition, we introduce the process ofdevelopment and application of two-dimensional electronic spectroscopy.2D ESprovides the most effective approach to detect the exciton dynamics and structure ofmolecular aggregate.In chapter two, we start with the introduction of some basic knowledgements,including the density operator in Liouville space, time evolution of the densityoperator, the density operator in the interaction picture, the third order nonlinearoptical response theory and so on. These theories can be used for calculating two-dimensional electronic spectroscopy.The exciton delocalization length, which can be given as the number ofmolecules involved in a specific exciton, is thus the function of the disorder, and isclosely related with the exchange narrowing. The delocalization length has beensuggested to be measured from a pump probe experiment a while ago. However, theproposed relation implies that the homogeneous line width is much smaller than thestandard deviation of the diagonal disorder, hence it can be used at low temperature.The study of exciton properties is much more complicated at intermediate or hightemperatures when exciton dephasing is comparable to the whole exciton absorptionbandwidth. This case has not been extensively studied with respect to excitonproperties. However, it is commonly assumed that the two-dimensional spectroscopy has better parameter resolution than conventional pump probe or photon echo. Hence,in the third chapter，we study the lineshapes of the J band in the two dimensionalspectra of various types and suggest possible schemes to characterize the obtainedpeak shapes and to extract useful the exciton delocalization information.In chapter four, we consider a simplest model of an anharmonic oscillatordescribed by anharmonicity parameter of electric level and anharmonic value oftransition dipole. We analyse the two dimensional peak shapes of the linear Jaggregate and the anharmonicity oscillator in the two dimensional spectra of varioustypes, such as the rephrasing, the non-rephasing and the double quantum coherentspectra. The spectral characteristic of J aggregate is very similar to that in a simpleanharmonic oscillator signals. Our results show that the spectral behavior of theanharmonic oscillator is consistent with that of J aggregate. Furthermore, we havealso demonstrated the exciton bands are typical weakly anharmonic behavior.Intermolecular interactions between chromophores play a key role inmultichromophoric molecular assemblies, specially in determining theirphotophysical properties. Effectively control of photophysical properties is aprerequisite for fabricating photoelectronic function materials such as artificiallightharvesting systems, photovoltaic devices, organic lightemitting diodes, and solarcells. Theoretical studies on the relationship between structures and properties ofmolecular aggregates can improve material performance and develop understandingof the experimental phenomenon. Much attention has been paid to the fundamentaloptical properties of various molecular aggregates with the aim of revealing the natureof molecular interactions in relation to molecular structure, in particular, to thedistances and angles between adjacent molecule. So in chapter five, Two-dimensionalphoton echo spectroscopy have been applied to study Quinacridone dimer system. Wehave a deeply understanding of the relationship between the molecular interactions,molecular structure and optical properties. The main goal of this work is to dissect thetwo-dimensional spectrum of the dimer and give better understanding of how thedimeric configuration influence the spectral features.