Modulation Of Thioxanthone Aggregation Structure And Room Temperature Phosphorescence Properties

Pure organic room temperature phosphorescent（RTP）materials has received considerable attention in the fields of light-emitting devices,optical sensors,photodynamic therapy,biological imaging etc.due to the unique photophysical properties（e.g.long lifetime,oxygen sensitivity and temperature dependence）.Generally,pure organic（without heavy atom）RTP is difficult to be observed under ambient conditions,because the spin orbital coupling（SOC）in pure organic system is very weak.On the one hand,the weak SOC leads to forbidden transition from singlet to triplet,which can not effectively form the triplet excitons.On the other hand,the weak SOC leads to the fact that phosphorescence radiative rate from the lowest triplet state（T₁）to ground state（S₀）is too slow to compete with many rapid non-radiative processes,such as vibrational quenching,oxygen quenching,etc.Therefore,the enhancement of SOC and the suppression of non-radiation are two main ways to achieve high efficiency pure organic RTP.The former mainly includes heavy atom effect,heteroatom,deuterium and conformation folding,while the latter is mainly realized by intermolecular aggregation,including crystallization,halogen bond,H-aggregation and host guest doping.Although it has been observed that intermolecular aggregation structure can seriously affect the RTP properties,the relationship between their structure and properties is not clear.Especially,the theory of aggregation triplet transition for pure organic RTP system is far from established.According to the classical Kasha model,the structure of organic molecular aggregation states,such as H-aggregation,J-aggregation and X-aggregation can be predicted accurately by using the absorption and emission spectra.In this model,the molecular packing in aggregates can be simplified as the packing of transition dipoles between adjacent molecules.The interaction between dipoles promotes the splitting of excited states energy levels and change the transition dipole moment,which eventually leads to the significant change of the spectral properties of aggregates.The theory can only be applied to the fluorescence molecular system of singlet exciton transition.However,the theory can not be applied to the triplet exciton transition system at all,because the transition dipole moment of triplet exciton is zero,which is the key theoretical basis for studying the RTP properties of organic molecules with different aggregation structures.In order to study the triplet exciton transition behaviors of organic molecular aggregates,we select the aggregation unit with RTP activity:thioxanthone（TX）.We designed and modified the side group of this planar rigid molecule TX to regulated the different stacking modes for studying the RTP properties of different aggregation structures in detail to establish the relationship between structure and properties and develop the triplet luminescence theory in organic aggregates.Based on the above research ideas,a series of TX derivatives were designed and synthesized.The photophysical properties of monomers and aggregates were characterized.Crystal engineering was used to analyze the different stacking modes.The characteristics of triplet excited states in organic aggregates and their photophysical kinetic processes were understood through the combination of experiment and theoretical calculation.The relationship between different aggregation behaviors of TX and RTP properties was revealed,which provided a theoretical guidance for the further design of high efficiency pure organic RTP materials.The main achievements of this paper are as follows:1.It is found that theπ-πstacking mode（relative to the"herringbone"stacking）is more conducive to achieve RTP.The reason is thatπ-πstacking leads to energy level splitting of excited states,which reduces the energy level difference between singlet and triplet states and increases intersystem crossing（ISC）channels for promoting the formation of triplet excitons.It is clear thatπ-πinteraction is an effective staking mode to realize organic RTP for TX.2.Halogen substitutions regulate the intermolecular stacking of TX to realize one-dimensionalπ-πH-aggregation of TX.The RTP with high efficiency（74.7%）and short lifetime was obtained,which is essentially different from H-aggregation of singlet excitons.In essence,the strongπ-πinteraction in the H-aggregate results in the splitting of the excited states energy levels,which generates more effective ISC channels and promotes more effective mixing of excited triplet and singlet states.Thus the radiative rate of phosphorescence emission was acclerated for the high efficiency RTP.3.Alkoxy substitutions regulate the intermolecular stacking of TX to realize one-dimensionalπ-πJ-aggregation of TX.Alkoxy groups induce the formation of multiple hydrogen bonds,which helps to inhibit the non-radiation of fluorescence and phosphorescence.J-aggregates also accelerate the radiation of both fluorescence and phosphorescence,and realize the high efficiency pure organic single molecular white light emission（SMWLE）with fluorescence phosphorescence dual emission.4.By using a guest molecule to regulate the intermolecular stacking of TX,the stacking of TX monomer,dimer and polymer with different aggragation sizes were obtained.The conversion of luminescence mechanism from pure thermally activated delayed fluorescence（TADF）,the co-existence of TADF and RTP to pure RTP emission is realized,which is beneficial to the maximum utilization of singlet and triplet excitons in the luminescence process.