Theoretical And Experimental Investigation On The Photophysical Properties Of Star-shaped Monodisperseoiigos Functionalized Truxenes

Compared with the inorganic photoelectric materials, the organic photoelectricmaterials have a lot ofcharacteristics,such as the low cost, the wide range of variety,easy to get large area of film and to optoelectronic integrated. Nowadays the organicphotoelectric materials have been widely used in the organic solar cells, organiclight-emitting diode, organic field-effect tube, organic photoconductor, photodynamic-therapy, three-dimensional fabrication and many other fields. As to be a new kind ofconjugated organic oligomers, Star-shaped oligomers get a widely attention for theirunique functions and advantages. The star-shaped oligomers are usually composed bya center-core and the linear oligomers arms around the core.So the star-shapedoligomersnotonlyhave the advantages of linear oligomers and dendritic macromole-cules, but own many new photoelectronic propertis and topology as well. Comparedwith the linear oligomers, the star-shaped oligomers can effectively reduce theintermolecular stacking and the effect of crosslinking. When being to be the film, thephotoelectronic propertis can get a large promotion. So during recent decades, peoplepay a lot of attention to the star-shaped oligomers. In this paper, we researched twostar-shaped oligomersTr-OFV1and Tr-OFV3, conducted a series of optical experiment,like UV-Vis absorption spectrum, one-photon fluorescence spectrum andtime-dependent transient absorption, then analyzedtheir photoelectronic properties,we get the following conclusion:Through the UV-Vis absorption spectrum and the one-photon fluorescencespectrum, we found that as the number of fluorene-vinylene (FV) units increases from1to3, the π-conjugated system in oligomers gradually enhances and all the peaks in absorptionemission spectra are red shifted together, meanwhile, the Stokes shiftdecreases and the bandgapbecomes narrower. Comprehensively analyzed theabsorption and emission spectra, we expected that the this two star-shaped oligomershave the similar energy level structure and Transition mechanism, and theenhancement of conjugated system could play an important role in the spectralfeatures of star-shaped oligomers based on truxene unit.For the time-dependent TA spectrum, we would see that an obvious fast decaybehavior appears in TA spectra of Tr-OFV3in the initial process after photoexcitation,and this fast decay behavior would disappear as the intensity decreased. But this fastdecy behavior didn’t appear in the Tr-OFV1solution with the same concentration andthe lower concentration Tr-OFV3solution, even though photoexcitation with a highintensity. This confirmed that the fast decay process of the Tr-OFV3solution afterphotoexcitation would attribute to the exciton-exciton annihilation (EEA) process.This EEA process was due to the arene packing which was caused by theself-associate in solution. However, intensity-dependent EEA process was notobserved in Tr-OFV1solution may becaused that the the π-conjugated system was tooweaken to have the arene-arene interaction.In the quantum chemistry calculations, after theoptimized geometry for these twooligomers, we found that both the truxene unit and the fluorene-vinylene (FV) unitshow a good planarity, but the twist angle between them became larger when the FVunit increased. Through analyzing the HOMO and LUMO of them,we expected thatthe difference between Tr-OFV1and Tr-OFV3in the spectral features should bemainly attributed to the change of electronic cloud distributed on the truxene unit andthe enhancement of π-conjugated system in branched arms of oligomers.