The formation of exciplex and triplet-triplet transfer in organic room temperature phosphorescent guest-host materials

Organic materials typically do not phosphoresce at room temperature because both intersystem crossing (ISC) and phosphorescence back to the electronic ground state are slow, compared to the nonradiative decay processes. A group of organic guest-host molecules breaks this rule. Their phosphorescence at room temperature can last seconds with a quantum efficiency of over 10%. This extraordinary phenomenon is investigated with comprehensive static and transient spectroscopic techniques. Time-resolved vibrational and fluorescence spectral results suggest that a singlet guest-host exciplex quickly forms after excitation. The formation of exciplex reduces the singlet-triplet energy gap and helps facilitate charge separation that can further diffuse into the host matrix. The heavy atoms (P or As) of the host molecule can also help enhance the spin orbital coupling of the guest molecule. Both boost the rate of ISC. After the singlet exciplex transforms into the triplet exciplex through the ISC process, UV-visible transient absorption spectroscopic measurements support that the triplet exciplex quickly transforms into the guest molecule triplet state that is at a lower energy level, thereby reducing the reverse ISC-induced triplet population loss. Finally, the long-lasting separated charges that diffused into the host matrix can diffuse back to the guest hole to form new triplets, and the dilution effect of the host molecules can effectively reduce the triplet quenching. All these factors contribute to the dramatic enhancement of phosphorescence at room temperature.