Phosphorescence is a specific type of photoluminescence generated by spin-flipping from an excited singlet state to a triplet state, followed by radiative decay from the triplet state to the ground state. Materials with such property have been found to exhibit an array of high-technological application in optoelectronics devices, optical storage, sensors, photo-responsive switches and bio-imaging, and thus have attracted continuous attentions from multiple-discipline. Recently, several groups have employed different methodologies to develop pure organic room temperature phosphorescence (RTP) systems. These strategies tend to modulate the aggregation behaviors of organic phosphors to suppress their nonradiative decay pathways and thus can be classified as aggregation-induced RTP...[ Read more ]

Phosphorescence is a specific type of photoluminescence generated by spin-flipping from an excited singlet state to a triplet state, followed by radiative decay from the triplet state to the ground state. Materials with such property have been found to exhibit an array of high-technological application in optoelectronics devices, optical storage, sensors, photo-responsive switches and bio-imaging, and thus have attracted continuous attentions from multiple-discipline. Recently, several groups have employed different methodologies to develop pure organic room temperature phosphorescence (RTP) systems. These strategies tend to modulate the aggregation behaviors of organic phosphors to suppress their nonradiative decay pathways and thus can be classified as aggregation-induced RTP.

Despite of these exciting achievements, seldom examples of organic phosphors with both high efficiency and long lifetime are obtained. It is also quite difficult to explore further the strategies as a comprehensive investigation on the molecular structure-property relationship is lacking. After careful study of previous works about photophysics and photochemistry of phosphorescence, I have launched several programs directed towards the development of pure organic RTP materials.

In my work, a clear principle picture for comprehensive understanding the phosphorescence process of pure organic materials is presented. The principle, which involves the adjustment of the characteristics of molecular orbitals and the exciton configuration in the excited state by varying the molecular structure, provides a tool for modulating the intrinsic RTP performance. Theoretical investigations on the molecular orbitals reveal that the electronic configuration of the exciton is crucial to intersystem crossing and radiative phosphorescence decay. Thus, the molecular design of excited state with hybrid configurations (n,π*) and (π,π*) in appreciable proportion is desired to obtain efficient persistent RTP materials. By tuning the feature of molecular orbital and the energy level of the excited state through tailoring the aromatic subunits in arylphenones, a series of full-color pure organic phosphors with high efficiency and long lifetime are obtained. The key issue is the synergistic effect of aromatic subunits and ketones, which realizes remarkably fast k_ISC and slow k_P to afford balanced performance (Chapter II).

Based on our design principle for novel RTP materials, five new pure organic room temperature phosphors with dual RTP emission are design and synthesized (Chapter III). Experimental and theoretical investigations reveal that the dual RTP should arise from the low- and high-lying triplet states. Based on this, a new design strategy for achieving white light emission materials with pure phosphorescent was demonstrated for the first time. A single molecule white light emitter (ClBDBF) was explored with CIE 1931 coordinates of (0.33, 0.35). The design strategy gained from our experimental and theoretical discussions will allow for the exploration of novel white emission organic phosphors.

To decipher the effects of halogen atoms, a series of pure organic phosphor based aromatic phones are designed and synthesized (Chapter IV). Comprehensive investigations reveal that tailoring the substituted halogens in arylphenones can effectively tune the low- and high-lying triplet state. Our experimental data reveals that a series of pure organic phosphors with both long lifetime and efficiency can be realized under ambient conditions, demonstrating the validity of switching confugations of excited triplet states by rationally tuning the substituted halogens.

To comprehensively investigate the effects of packing modes in RTP materials, a pure organic room temperature phosphor with two types of crystals exhibit different persistent phosphorescence properties is presented (Chapter V). Experimental investigation and theoretical calculation analysis both reveal that molecular in different pack modes keep the same intersystem crossing and radiative decay process, but remarkably affect the nonradiative decay process of excited triplet state at room temperature due to the small changes in the intermolecular interaction, resulting in difference in the efficiency of persistent RTP in their two crystalline states. The results and conclusions gained from our experimental and theoretical investigations are conducive to comprehensively understand the phosphorescence process of pure organic materials, will allow for the exploration of novel organic phosphors.

Besides, a new design strategy for on-off mechanochromic luminescence (MCL) materials is proposed based on controlling of intersystem crossing of nitro substituted twisted fluorephores by tuning the conformations in solid states (Chapter VI). Theoretical calculation indicated that the change of torsion angle of nitrobenzene group is the key factor for on-off MCL through tuning intersystem crossing process during the conformation change in different solid states. Our experimental data and demonstration of applications revealed that the designed materials show high contrast on-off MCL properties, which offers several advantages, including stability, reversibility, high sensitivity and fast respondability. These results outline a fundamental principle to design organic materials with on-off MCL properties, providing a major step forward in expanding the scope of MCL applications.