Synthesis And Photophysical Properties Of Covalently Linked Perylenetetracarboxylic Diimide And Tetracene Aggregates

Dye aggregates can be classified as H- and J-type on the basis of their spectroscopic changes with respect to their monomer. H-type aggregates usually present a blue-shifted absorption band and exhibit in most cases low or no fluorescence. The J-type aggregates usually have narrow red-shifted absorption and emission bands with small Stokes shift and relatively high fluorescence quantum yield. Following exciton theory, the molecules are arranged in a "face-to-face" manner in H-type aggregates, but in a "head-to-tail" manner in J-type aggregates. Dye aggregates have attracted considerable attention due to their various potential applications, such as sensitizers of the photographic process, organic semiconductors in field effect transistors, and fluorescence sensors in biological systems.For the purpose of developing novel functional materials, the structure-property relationship of dye aggregates need to be solved. Therefore, several supramolecular systems were synthetized through non-covalent bond interaction, such as hydrogen bond, π-π interaction, electrostatic interaction, dipole-dipole interaction, metal coordination bond. However, the identification of the structural detail of aggregate was difficult due to the flexible structure of supramolecular system. Moreover, the molecular photophysical property is sensitive to the structural changes. Therefore, the exact structure-property relationship of aggregates cannot be established in supramolecular systems. On the contrary, covalently linked aggregates with rigid structure have unique advantages owing to their resolved structures. Based on this, we design and synthesize a series of covalently linked perylenetetracarboxylic diimide (PDI) and tetracene aggregates with rigid structure for the purpose of developing the reliable structure-property relationship of aggregates. In this way, a good foundation can be laid for designing novel functional materials and resolving supramolecular structure by spectral method. My work was focused on the following aspects:In chapter 1, the research of perylene diimide derivatives, covalently linked PDI aggregates and singlet fission have been reviewed.In chapter 2, covalently linked PDI dimers and trimers with rigid structure are prepared and their molecular structures are characterized by 1H NMR, MALDI-TOF mass spectroscopy and elemental analysis. The minimized molecular structures of these compounds reveal that they are all "face-to-face" stacked aggregates with large longitudinal displacement. Compared to the absorption spectrum of monomer, their absorption spectra show redshifted bands, suggesting the presence of J-type excitonic coupling between the PDI subunits in these compounds. However, their steady state and time resolved fluorescence spectra revealed that the emission from "excimer-like" states dominates the fluorescence of these compounds, this is similar to that of "H-type" aggregates, In the fluorescence spectra of these compounds, a minor "J-type" emission can be identified for the compounds with a relatively large longitudinal displacement. An increase in the number of subunits in one aggregate from 2 to 3 brings about no distinctive changes in their photophysical properties, which can be ascribed to the changes in the stacking structure caused by the steric hindrance.In chapter 3, a series of PDI dimers with different substituents at the bay positions have been synthesized and the molecular structures are characterized by 1H NMR, MALDI-TOF and elemental analysis. The minimized molecular structures of these dimers reveal that they are all "J-type" aggregates. However, due to the different steric hindrance caused by the different substituents at the bay positions of the PDI ring, the PDI subunits in these dimers show "face-to-face" stacked structure but with different sideway slippage along the long and/or short molecular axises, different rotate displacements, and different dihedrals between the two PDI planes. These structural differences have caused significant differences on the absorption and emission spectra. The correlation between the photophysical properties and the structural parameters is discussed.In chapter 4, a covalently linked tetracene dimer with "face-to-face" stacked structure has been prepared. Its absorption spectrum differs significantly from that of the monomeric counterpart in solution, suggesting the presence of strong interactions between the two tetracene subunits. In solution, the fluorescence spectrum is dominated by a band at around 535 nm, due to an oxidative impurity. In the longer wavelength range, a short-lived lower energy emission can be identified as the intrinsic emission of the dimer. In a polystyrene matrix or at low temperatures, the lifetime of the lower energy emission lengthens and it becomes more prominent. Therefore, we suggest that the interactions between the two tetracene subunits produce a short-lived, lower energy "excimer-like" state. The fluorescence decays show no observable dependence on an applied magnetic field, and no obvious evidence of significant singlet fission (SF) is found in this dimer. This research suggests that even though there are strong electronic interactions between the tetracene subunits in the dimer, SF cannot be achieved efficiently, probably because the formation of "excimer-like" states competes effectively with SF.In chapter 5, a series of covalently linked linear tetracene dimer and trimer were synthesized. Due to the presence of benzene linkage, the neighbor tetracene subunits cannot take a coplanar configuration. This prevents the efficient through-bond electron delocalization. Small redshifts are observed for their absorption and fluorescence spectra with respect to that of monomer. This indicates that the ground state interaction between the tetracene subunits is weak. Delayed fluorescence can be identified from their fluorescence spectra, which suggests the presence of SF within these compounds. Strong triplet absorption at 490 nm in their transient absorption spectra can be identified and dynamic analysis revealed that the triplet states come from SF. The SVD analysis suggests that the generation rate of triplet in the trimer is faster than that in dimer, which reveals for the first time the importance of triplet diffusion for SF.