Long Lifetime Room Temperature Phosphorescent Materials Based On Morpholine Derivatives

Room temperature phosphorescence（RTP）has a widely promising application in the fields of data anti-counterfeiting,bio-imaging,optical display,optoelectronic devices and photodynamic therapy attributed to its triplet-state luminescence properties.Early phosphorescent materials are usually composed of inorganic metallic elements,which are capable of producing phosphorescence at room temperature with the heavy atom effect of metals enhancing the spin orbit coupling（SOC）ability.However,these materials are not suitable for large scale production because of the high cost,serious environmental pollution,high toxicity,difficulty in synthesis and processing.However,pure organic materials have shown a lot of advantages,such as high modifiability,easy processing,low cost,and environmental friendliness.In practical applications,long lifetime RTP materials tend to reflect changes in specific environments more visually and exhibit better application performance.Phosphorescence in organic materials is difficult to observe at room temperature due to the weak SOC,which makes it not easy to generate triplet state excitons through intersystem crossing（ISC）.Meanwhile the radiative transition of phosphorescence is also difficult because the triplet state back to the ground state is likewise involving a change in the spin multiplet.The slower rate of radiative transition makes its energy often consumed through molecular vibrations,oxygen quenching,etc.Currently,many RTP materials are used to achieve long lifetime through host-guest doping,co-crystal,polymer doping,and so on,to enhance the production of triplet excitons or to construct a rigid environment to suppress the nonradiative transition.Compared with these materials,the simple preparation and clear mechanism of single-component phosphors show tremendous advantages.It is of great importance to develop new functional groups to enrich phosphorescent material systems.1-Oxa-4-azacyclohexane,also known as morpholine,is a six-membered heterocycle formed by replacing the carbon atoms at the 1 and 4 positions of cyclohexane with oxygen and nitrogen atoms,respectively.It is commonly used as an analytical reagent and a solvent for resins,waxes,gum,etc.with a wide range of industrial applications.Morpholine derivatives are also frequently used in the production of sedatives,antimalarials,fungicides and other drugs,but as RTP building blocks are rarely reported.In this paper,we use morpholine as functional group for constructing room temperature phosphorescent materials for the first time.By connecting morpholine to theπ-conjugated system,the lone pair of electrons of the nitrogen atom can effectively produce n-π^*transition to enhance the ISC process.The oxygen atom and methylene can form a variety of supramolecular interactions such as C-H...O and C-H...πto suppress nonradiative transition,and the chair conformation of the six-membered ring is capable of avoiding aggregation-induced quenching.The synergistic effect of multiple aspects will have a significant impact on the room temperature phosphorescence properties.Combining the above-mentioned group properties,we attached morpholine groups to benzene rings to enhance ISC and inhibit nonradiative transition in various ways,as well as to improve phosphorescence quantum yield and lifetime.Exploiting the potential of morpholine in the field of room temperature phosphorescence.The experimental results are combined with theoretical calculation data to deeply analyze the excited state characteristics,which provide new ideas for the development of novel pure organic long lifetime RTP materials.The main contents of this paper are as follows:1.Halogen bonding is a kind of non-covalent bonding interaction between a halogen atom and a neutral or negatively charged Lewis base,which is comparable to hydrogen bonding in strength.It can improve the SOC while suppressing nonradiative transition such as molecular vibration and rotation,which effectively modulate intermolecular interactions and RTP properties.We have designed and synthesized a series of 4-（4-halophenyl）morpholine compounds by introducing halogen atoms（F,Cl,Br,I）into the 4-phenylmorpholine system.Among them,4-（4-chlorophenyl）morpholine（MPh-Cl）has a 10-fold higher phosphorescence quantum yield of 6.8%compared to the halogen-free substituted 4-phenylmorpholine,and the lifetime is208.53 ms.In contrast,4-（4-fluorophenyl）morpholine（MPh-F）,which does not contain halogen bonds,exhibited only single fluorescence emission.Experimental data,theoretical calculations and electrostatic potential distributions reveal the influence of halogens on the properties of phosphorescence.The results show that the tight molecular stacking due to halogen bonding is conducive to the generation of room temperature phosphorescence,which can effectively enhance ISC and improve the quantum yield of phosphorescence.In addition,the difference in lifetimes of MPh,MPh-Cl and MPh-Br was utilized to show good display effects in patterning applications.2.The energy levels of the n-π^*configuration usually have smaller singlet-triplet splitting energies,while the S₁ state exhibiting the n-π^*transition feature can effectively couple with the triplet state energy level of theπ-π^*transition characteristic according to the El-Sayed rule.Two molecules o MPh and p MPh were synthesized by introducing two morpholine substituents at the benzene ring o-and para-positions.The double delayed emission from different sources was achieved by modulating the molecular orbital properties through positional isomerism and changing the degree of conjugation as well as the charge transfer properties.The spatial resistance of the hexagonal ring structure substituents can have a significant effect on the molecular conformation.In which,the more distorted structure of o MPh exhibits stronger charge transfer property,the smaller single and triplet state energy differences enable it to exhibit dual delayed emission of thermally activated delayed fluorescence and room-temperature phosphorescence.Relatively planar p MPh has a larger degree of conjugation and more intermolecular interactions,achieving double phosphorescence emission from both the monomeric and aggregated states.3.The triplet state excitons are susceptible to quenching by the thermal motion of the molecule,and hydrogen bonding as a strong supramolecular interaction can effectively limit the deactivation of this nonradiative pathway.Three identical substituents were introduced at positions 1,3 and 5 of benzene,including piperidine,methylpiperazine,morpholine,and thiomorpholine,with the aim to explore the effect of the morpholine 1-position substituent outside the conjugated system on the room-temperature phosphorescent properties.The three nitrogen atoms on these substituents are directly connected to the benzene ring,which can effectively produce n-π^*transition and generate more ISC channels in the aggregated state,facilitating the ISC process.With the gradual enhancement of intermolecular hydrogen bonding interactions,the non-radiative transition is gradually suppressed and the phosphorescence lifetime of the material increases significantly.The lifetime of 1,3,5-tris（4-methylpiperazin-1-yl）benzene is 800.03 ms.1,3,5-trimorpholinylbenzene with stronger hydrogen bonding interactions exhibits a double phosphorescence with an aggregated phosphorescence of up to 1.03 s and afterglow lasting up to 10 s.Its ultralong afterglow property shows potential application in the prepared data anti-counterfeiting model.