Theoretical Studies Of Photo-induced Charge Transfer Of Organic Photo-electronic Material

Photoinduced charge transfer (CT) is a primary step in photophysical, photochemical, life sciences as well as material sciences ea tl. CT process can be intermolecular CT in which an electron is transferred from electron-donating species to electron-accepting species, or intramolecular CT, involving charge redistribution in the excited molecules. The organic molecules with the CT character display some unique optical properties, which have attracted more and more people's attention. It is the basis of numerous present and possible future developments including dye sensitive solar cell (DSSC), light emitting diodes (LEDs) as well as nonlinear optical material ea tl. At present, theoretical simulations of organic molecules have becoming a very important research field. We can study the ground and excited state properties of CT compounds by means of the appropriate theoretical methods to further understand the microscopically stable structure, electron transition and interaction between molecules. Study of CT system can provide a certain theoretical basis for the application of new materials in the related field.In the thesis, organic photo-electronic materials including Donor-π-Acceptor organic molecule, doped organic dimer and chargedπdimer, have been studied to understand the excited state properties and the relation of the molecular structures and properties. Taking into account the contribution of all the possible molecular orbital to the electronic transition, real-space analysis have been used to reveal the CT mechanism. The thesis is organized as follows:(1) Dye molecules used as photo-sensitizer in DSSCs play the key role in light harvesting and the electron generation/transfer process. Recently, bis-dimethylfluoreneaniline moieties and its derivatives as donor have displayed promising properties in the development of photovoltaic devices. The ground and excited states of bis-dimethyl fluoreneaniline derivatives have been studied theoretically, demonstrating that CT of the dyes is from bis-dimethylfluoreneaniline moieties to cyanoacetic acid moieties, which enhances the electron coupling between the dyes and the semiconductor surface. With the introduction of ethylene, the absorption spectra make red-shifted, however, the dye restructures lead to the low of the driving force of electron injection.(2) Excited state properties of 1,3,5-triphenyl-2-pyrzoline (TPP) as an electron donor doped with orange dye of 4-(dicyanomethylene)-2-methyl-6-(p-dimethyl-aminostyryl)-4H-pyran (DCM) have been studied at the time-dependent fuctional theory level. There were some reports on photoinduced electron transfer in mixed coaggregates, but the calculation with currently orbital analysis is also limited to the mainly active molecular orbitals with high weight. With visualized analysis, we revealed that the absorption peak of mixed coaggregates in UV-visible spectrum is at 360 run, where the intermolecular CT occurs between TPP and DCN, and the intramolecuar CT occurs in TPP. During CT process, the #C unit of TPP not only serves as the electronic donor in the intramolecular CT, but also in the intermolecular CT process. Absorption spectrum at 681 nm is contributed to the first excited state, followed by intermolecular CT from TPP to DCN.(3) The ground state characters of the dianionic tetrocyanoethylene (TCNE) dimer and dicationic tetrathiafulvalene (TTF) dimer have been investigated theotetically, indicating that there exits the noval intermolecular covalent interactions that are contributed to the SOMOπ-stacking interactions between the monomers. The absorption spectra of TCNE dimer and TTF dimer have been calculated with the time-dependent density functional theory. It is found that absorption peak of TTF and TCNE dimers come from the seventh and the first excited states, and for the dianionic/dicationicπ-dimers system there exist two kinds of CT patterns, the mixture of intramolecular CT coupled with intermolecular CT and the intermolecular CT.(4) Retinal proteins (rhodopsins) are located in cell membrane and eye retina, which naturally exist in protonated Protonated Schiff base form. Through absorbing photon, the very first photoresponse of the chromophore takes place, and the isomerization time of the retinal chromophore is very fast in proteins (hundreds of femtoseconds). Previous report on gas-phase Protonated Schiff base retinal chromophores revealed that there are two excited states in visual region, and S2 excited states are very weak absorption (fis closed to zero). In this chapter, the S1 and S2 excited states of gas-phase Protonated Schiff base retinal chromophores in the one-and two-photon absorption (OPA and TPA) have been investigated with time-dependent density functional and response theory. We calculated the TPA cross section and revealed the excited states properties of above two states in OPA and TPA.