Photophysical Properties Of D-A Organic Photoelectric Materials

In recent years, due to its good nonlinear optical properties, ultra-fast opticalresponse, lower processing costs and other special optical and chemical properties, theelectron donor D (Donor)-electron acceptor A (Acceptor) conjugated photoelectricmaterial has been widely used in organic light-emitting diodes, field-effect transistors,and two-photon absorption materials. It has many advantages: the DA molecularstructure can achieve P-type or N-type doping through oxidized or reduced the πelectron, respectively; it can effectively regulate the HOMO and LUMO of the molecule;it can improve luminescence properties by changing the donor unit and the acceptor unit;it can regulate the band gap of molecules through charge transfer; it can broaden therange of light absorption, etc.This paper studies the structures and photophysical properties of D-A conjugatedphotoelectric materials in three different structures from theoretical and experimentalaspects based on their applications in the organic photoelectric field. The results areused to shed some light on the performance of the materials. The study consists of thefollowing:1. The photophysical properties of PFO and F8BT were studied with experimentaland theoretical methods. Quantum chemical calculations showed the electronictransition mechanism of PFO and F8BT and simulated the absorption spectra of the two.The results confirmed the delocalization of intrachain charge in F8BT of themono-disperse system. Femtosecond transient absorption measurement revealed thatintrachain excitons were dominant in the relaxation process of the excited PFO. Theintermediate state, which was absent in the delocalization of intrachain charge, appeared in the relaxation process of excited F8BT. Additionally, the intensity-based transientabsorption curve of PFO and F8BT located in the mono-disperse system showed thatthe interaction of intrachain excitons could not affect the relaxation properties of F8BTalthough they could affect those of PFO. The intensity-based transient absorption curveof PFO and F8BT located in solid membrane showed that the intermediate state mightdisappear from the relaxation process of F8BT. PFO and F8BT could show the intensityexciton-exciton quenching process under the condensed phase and high excitationintensity. Due to its good intrachain charge transfer characteristics, the relaxationprocess of PFO was longer than that of F8BT.2. Two fluorene-based oligomers (P-F-P and F-P-F-P-F) were synthesized byHeck reaction and in them the fluorene and the phenothiazine groups were arrangedalternately. Their photophysical properties were carefully studied with experimental andtheoretical methods. Of those methods, steady-state absorption and emission spectraindicated that: the spectral characteristics associated with the absorption peak, theemission peak, the band gap and the shift had been changed. Quantum chemicalcalculation provided the molecular structure and analyzed the electronic transitionmechanism. In addition, two-photon fluorescence and femtosecond open aperture Z-scanning technology confirmed: the two-photon fluorescence yield and the two-photonabsorption cross section of F-P-F-P-F were significantly higher than those of PFP.Intensity dependent transient absorption data showed that: compared with the excitationlifetime of P-F-P in interaction with non-aromatics-aromatics in solution, theexcitation lifetime of F-P-F-P-F was shortened. All improvements in the opticalproperties were derived from the extension of π-conjugated system.3. The optical properties of three linear conjugated oligomers (F-P, F-P-F andP-F-P-F-P) were carefully studied with experimental and theoretical methods.Steady-state spectroscopy measurement indicated that the absorption and emissionbands of these oligomers experienced red shift as the π-conjugated system enhanced while their other optical properties were modulated. Two-photon fluorescence andfemtosecond openings Z-scanning technology confirmed that the two-photonfluorescence yield and the two-photon absorption cross section of these oligomers wereenhanced with the improvement in the π-conjugated system. Time-resolvedfluorescence measurement manifested that the relaxation process would graduallyaccelerate, while the fitting result obtained from the continuous attenuation ratedistribution model indicated that the attenuation rate of oligomers would graduallyaccelerate, with the distribution width becoming smaller and smaller from F-P toP-F-P-F-P. Quantum chemical calculation provided the molecular structure andanalyzed the electronic transition mechanism, indicating that the improvement of TPAproperties should depend on the enhancements of π-conjugated system.