Study On The Design,Synthesis And Excited State Charge Separation Properties Of Perylene Diimide Based Dyads

Organic conjugated molecules usually contain a number of conjugated ? electrons.The energy of the ???* electron transition is generally in the range of 1.5 eV to 4 eV,corresponding to the UV-visible photon energy.Therefore,these organic conjugated molecules are referred to as the chromophores.The interaction of chromophores in conjugated molecular aggregation state has an important influence on the photoelectric properties.However,the solid state is a macroscopic aggregate formed by a large number of microscopic molecules.Each molecule interacts with many surrounding molecules,and the intermolecular distance and orientation are totally diverse.Therefore,it is difficult to directly discuss the influence of molecular aggregation structure on photoelectric properties.We constructed dyad models by linking chromophore molecules with chemical bonds.It is an ideal strategy to study interactional mode of chromophores as well as the interactional relations of ground state and the excited state.Perylene diimide(PDI)is a type of important organic dyes with a large ? conjugate structure,and was also widely studied in the field of optoelectronics.In this paper,various PDI dyads molecules had been synthesized through covalent bond using identical and different PDI construction unit.Steady state and transient spectroscopies were performed to study the characteristics of the ground state and excited state of PDI dyads.Summarily,the molecular connections style,conformation,the energy level of donor and acceptor et al.,have a big impact on photo induced electron transfer.The first molecular class is the N-N linked PDI-dyad molecular system.We synthesized five kinds of molecules,1-1,2-2,3-3,1-3,2-3,which are composed by PDI 1,that PDI is substituted by four phenoxy groups,PDI 2,that PDI is substituted by two phenyl groups,and PDI 3,that PDI is not substituted.Steady state absorption spectra show absorption spectrum redshift,more than two times absorption coefficient increase,narrowing of half peak width,enhanced 0-0 transition visibly than that of 0-1 transition in 1-1,2-2,3-3 dyads compared with monomer molecules,which all indicate these homo-dyads are a typical J-type coupling.The fluorescence properties of the three homo-dyads are totally different in dichloromethane.The fluorescence efficiency of the homo-dyads with twisted structural such as PDI 1 units was significantly lower than that of the related monomer PDI,while the fluorescence efficiency of the homo-dyads with planar structural units such as PDI 3 units was close to that of related monomer PDI.Transient absorption spectra show that the difficulty of photoinduced symmetry-breaking charge separation(SB-CS)increases successively in 1-1,2-2 and 3-3.We found the molecular configuration vibration is helpful for SB-CS because it causes rotation of the chromophores and then strength the interaction of adjacent carbonyl groups,then electron transfer occurs through the carbonyl interaction.Therefore,asymmetric 1-3 in dichloromethane,due to greater free energy of CS and the presence of twist PDI 1,can achieve ultrafast charge separation of 14 ps.Meanwhile,the F?ster energy transfer efficiency of 1-3 in toluene from PDI 3 to PDI 1 is close to 100%,much higher than 60% of 2-3.The second molecular system is the bay region C-C linked PDI-dyads.We synthesized five kinds of materials: PDI-PDI,PhPDI-PD,PhPDI-PhPDI,RPDI-PDI,RPDI-RPDI which are composed of unsubstituted PDI,1-phenyl PDI(PhPDI),oxidative cyclization PhPDI.Steady state absorption spectra show absorption spectrum redshift,decrease of absorption coefficient,broad half peak width,in these dyads compared with related monomer molecules,indicates there is angle between intramolecular transition dipole moment.The steady state fluorescence spectra showed the large Stokes shift,red shift and the decrease of fluorescence efficiency depending on solvent polarity,which proves electrons transfer even occur in toluene and form charge transfer states.The arisen time of electrons transfer of PDI-PDI,PhPDI-PDI,PhPDI-PhPDI is 10 to 30 ps and the fluorescence efficiency is very low.While fluorescence efficiency of dyads(RPDI-PDI,RPDI-RPDI)are high and the efficiency are even close to 100% at the excitation of high energy state.Various experimental shows that RPDI molecule is more rigid,which is conducive to bay-bone rotation and then forms a new high-fluorescence configuration.Therefore,in the excited state of high energy state,it is more possible to overcome the energy barrier and completely transform to high-fluorescence configuration,so that the electron transfer state fluorescence efficiency can be close to 100%.In this dissertation,we have designed and synthesized two kinds of PDI-dyads with different intramolecular interactions.We have discussed the influence factor of photoelectric performance in detail about conformation,chromophores connection style,different molecular frontier orbital etc.Steady state and transient spectroscopy have been carried out to find the photoinduced electron transfer process and its regularity.These studies are contributed to have deep insight about the process of conjugated molecular excited states as well as a certain guiding significance of the further development of high-performance photoelectric functional material system.