Investigation On The Photophysical Properties Of D-π-A Type Organic Oligomers

With the continuous development of society,optoelectronic materials have become a topic of great interest to people.Optoelectronic materials have a very wide range of applications,and people’s demand for optoelectronic materials is also increasing.In 2000,the Nobel Prize in Chemistry was awarded to American scientist Alan J.Heeger,American scientist Alan G.Mac Diarmid and Japanese scientist Hideki Shirakawa for their outstanding contributions to the conduct of polymers.Thanks to their discovery,organic materials have become an optoelectronic material that rivals inorganic materials.In the past two decades,organic optoelectronic materials have been considered as an alternative to inorganic materials for optoelectronic devices.They have the advantages of low cost,simple fabrication and easy processing on flexible substrates.In many fields,organic optoelectronic materials have very good application prospects.A number of organic optoelectronic molecules with short lifetimes and high photoluminescence quantum yields(PLQYs)of the donor–acceptor structures have recently been synthesized.Their nonlinear optical properties in monodisperse systems make them suitable for a wide range of applications in two–photon imaging,two–photon microscopy,and fine processing.If they are aggregated together,they can be used in the field of organic light–emitting diodes(OLEDs).Generally speaking,donor–acceptor organic molecules have strong light–harvesting ability,high photoluminescence quantum yield,low cost and excellent chemical stability.To expand their applications,further understanding of their intrinsic photophysical properties in monodisperse systems or aggregated states is needed to facilitate the rational design of organic optoelectronic materials.In this thesis,several organic materials were characterized using both steady–state spectroscopy techniques(steady–state absorption,steady–state emission)and transient spectroscopy(pump–probe,pump–dump–probe),and the photophysical properties of several quinoxaline derivatives were investigated by these spectroscopic techniques.Using time–correlated single photon counting(TCSPC)and emission tests under extreme conditions(low temperature,high pressure),the emission spectra and photoluminescence lifetimes of these materials under different conditions were studied,and the effects of pressure or low temperature on aggregated organic optoelectronic materials were analyzed,the influence of the intermolecular interactions existing in the molecules was analyzed,and the influence of the increase of conjugated branches on the intermolecular interactions was analyzed;The nonlinearity of the organic optoelectronic materials QTPA1 and QTPA2 materials was given by using two–photon photoluminescence and Z–scanning techniques,the two–photon absorption cross–sections of the two materials were compared;The application potential of several materials in organic light–emitting diodes was tested by using the color temperature test count.1.Through the tests of steady–state absorption and steady–state emission,we found that QDMA2 has a charge transfer state in a monodisperse system and has a solvochromic phenomenon.By fitting the Stokes shift to the solvent polarizability,we find that the Stokes shift increases linearly with increasing solvent polarity.Through pump–probe experiments,combined with the results obtained by global fitting,it is proved that the QDMA2 emission comes from an intramolecular charge–transfer state,and this charge–transfer state exists stably under low–polarity conditions.while under polar conditions,it will interact with polar solvents to form CT’ states with lower energy.By processing and fitting the kinetic data obtained from the pump–dump–probe experiment,the physical mechanism of QDMA2 molecule fluorescence generated by light excitation was elucidated.In addition to the monodisperse studies,we also fabricated an OLED based on polymerized QDMA2 molecules with a correlated color temperature(CCT)value of 1875 K,indicating their potential in organic display devices.By pressure–dependent PL measurements on aggregated QDMA2 molecules,it was demonstrated that the intermolecular forces strengthen due to the increase in pressure.These experiments suggest that donor–acceptor compounds with a high degree of charge transfer may be strong candidates for photovoltaic device applications.2.We compared the photophysical properties of QTPA1 and QTPA2,by a combination of steady–state absorption,steady–state emission spectroscopy techniques,and transient absorption techniques.Through steady–state absorption and steady–state emission,we found the presence of charge–transfer states in QTPA1 and QTPA2.At the same time,we also compared the fluorescence quantum yields of QTPA1 and QTPA2.Interestingly,we found that the increase of conjugated branches caused the unusual phenomenon of fluorescence weakening.By fitting the Stokes shift,we found that there are two distinct chromophore emission states in QTPA1,but in QTPA2,there is only one chromophore emission state.Using the conventional transient absorption system and an extended system of transient absorption(pump–dump/push–probe system),the excited state species generated during the excitation process of the two materials were analyzed.The analysis results showed that for QTPA1,there is a mixed state(HLCT state)in which LE and CT compete with each other.The LE state dominates the mixed state under low–polarity conditions,while CT dominates the mixed state under high–polarity conditions.For QTPA2,the stronger charge transfer ability brings stronger CT state characteristics,which makes the excited state component of QTPA2 only exhibit CT state.In addition,the two–photon fluorescence of the two materials was tested by means of nonlinear optics,and the difference between QTPA1 and QTPA2 caused by the increase of conjugated branches was compared.We found that QTPA2 has stronger two–photon fluorescence,which means that anomalies that existed during regular excitation disappeared during two–photon excitation.The nonlinear absorption coefficients and two–photon absorption cross–sections of QTPA1 and QTPA2 were calculated by Z–scan technique,and it was proved that the increase of conjugated branches would enhance the charge transfer state characteristics,enhance the dipole coupling,and increase the two–photon absorption cross–section.This is of great significance for finding excellent materials with large two–photon cross sections.3.We further compared the fluorescence characteristics of the organic material QTPA1 and the organic material QTPA2 to analyze the photophysical properties under the aggregated state.By testing the steady–state emission at low temperature and PLQY of QTPA1 and QTPA2 in the aggregated state,we found that the emission intensity of QTPA1 and QTPA2 both increased with the decrease of temperature,and the emission peaks of QTPA1 and QTPA2 both red–shifted with decreasing temperature.At the same time,the fluorescence peak of QTPA2 also red–shifted compared with that of QTPA1,it is due to the higher degree of conjugation in QTPA2.The photoluminescence characteristics in the aggregated state are different from those in the monodisperse system.With the increase of conjugated branches,the intermolecular distance decreases and the intermolecular interaction increases.The radiative lifetimes of QTPA1 and QTPA2 were analyzed by low–temperature TCSPC technology,and it was found that the radiative lifetimes of both decreased with the decrease of temperature,which was due to the fact that most of the non–radiative processes were inhibited at low temperature.Compared to QTPA1,the radiation lifetime of QTPA2 is longer at room temperature.This is because the increase of conjugated branches reduces the molecular distance and promotes the generation of non–radiative processes.When the non–radiative processes gradually disappear,the radiation rate gap is also reduced.The color temperature and color coordinates of QTPA1 and QTPA2 are tested separately.