Design And Characterization Of Several Organic Room Temperature Phosphorescence/long Persistent Luminescence Systems

Organic room temperature phosphorescence(RTP)or organic long persistent luminescence(OLPL)have shown great potential in encryption or anti-counterfeiting,bio-imaging,white illumination,oxygen-water sensor,afterglow organic light emitting diode and circular polarized luminescence(CPL).It is crucial for RTP or OLPL systems to achieve simultaneously high phosphorescent lifetime and quantum efficiency.Within the past few decades,RTP and OLPL materials have grown robustly with many materials or systems developed.However,RTP and OLPL are still in their infancy,hence the design strategy of more RTP materials and elongation of afterglow duration is of vital importance.Based on current existing researches on RTP and OLPL materials,combined with CPL properties,the following design strategies for efficient RTP and OLPL systems are presented,hopefully would be beneficial for the development of this field.The main design strategies are presented in the order of chapter:1.A series of RTP compounds were designed based on paracyclophane with carbazole as electron donor unit and a series of electron acceptors,combining planar chirality and through-space charge transfer characteristics,resulting in the highest phosphorescent lifetime of 528.0 ms.Through comparison of four compounds with or without halogen incorporation,the strategy to enhanced phosphorescence efficiency and lifetime gradually based on these materials was proposed.In addition,four enantiomers based on two compounds were separated and characterized with excellent performances and decent dissymmetric factors,the maximum g value of one pair of enantiomers can even reach 10^-2,which is quite remarkable within small organic molecule categories.In this chapter,a new molecular skeleton that could potentially promote generation of RTP is introduced and corresponding design strategies are listed(Chapter 2);2.A pair of small fluorescent dyes based on diaminocyclohexane with charge transfer characteristics as guest and diphenylphosphine oxide substituted dibenzothiophene as host were introduced in a host-guest system.The system exhibited remarkable white luminescence under UV excitation and excellent phosphorescent quantum yield of45.9%.The resulting film displayed high transparency and extremely long afterglow lasting more than an hour.Through elaborate investigation of the photophysical properties it was found out that the origin of such long afterglow was due to two-photon ionization,a rarely reported mechanism.Therefore,in our system we are able to eliminate the harsh requirement of high excitation power and extremely low temperature,which greatly expands the design strategies of OLPL systems.Moreover,through modification of the CPL measurement method,the system exhibited excellent CPL with triple components of fluorescence/phosphorescence,room temperature phosphorescence and afterglow,all with remarkable dissymmetric factors.The g values of all three components are in the magnitude of 10^-2,which are almost 10 times that of corresponding toluene solutions.In this chapter,we reveal the great potential of two-photon ionization and its importance towards OLPL development(Chapter 3);3.Through trial with triphenylphosphine as host matrix,a series of guest molecules with appropriate singlet and triplet excited energy levels and reasonable room temperature phosphorescence performance was chosen as guest molecule based on their photophysical properties.Organic long persistent luminescence based on dual-photon ionization with standard white,warm white and sky-blue profiles for afterglow were obtained through modulation of contributions from singlet and triplet excited states.These materials were able to exhibit afterglow for around 20 to 40 minutes with excellent color stability.Through theoretical calculations with these molecules it was found that the spin-orbit coupling was mainly responsible for subsequent ratio of singlet/triplet contributions to the afterglow profiles(Chapter 4).4.Host-guest systems based on a series of TADF Cu(I)complexes as guests and triphenylphosphine as host were developed with more than 3 hours of OLPL afterglow,the longest one lasting even more than 10 hours.Through careful verification and demonstration,we were able to propose a new mechanism,wherein in-situ generated ionic pairs based on Cu(I)complexes were probably responsible for the OLPL.Besides,the counter ions determine the lifetime of afterglow due to different size and binding energy.In this chapter,a novel mechanism unlike any other reported ones is proposed which could help enrich the OLPL species(Chapter 5).