Mechanism Study Of Long Persistent Luminescence In Organic Guest/Host System

Long persistent luminescent materials are promising for myriad applications in the field of bio-imaging,information technology,and sensors owing to the capability of energy storage for long-lived light emission.In organic luminogens,phosphorescence emission from triplet excited states has been widely adopted to extend the emission lifetime.In addition,a new strategy in virtue of the charge separated states has been implanted to extend the persistence to the scale of hours.Nevertheless,the mechanism for the long persistent luminescence,especially for the process of charge generation,remains poorly understood.In the few studies on this issue,it is generally assumed that the photo charge generation dynamics in organic photovoltaics blends is also applied here where the charge separation is enabled by dissociation of singlet charge-transfer states at the interfaces.To fully uncover the underlying mechanism,we developed transient absorption spectroscopy on time scales from femtosecond to second and observed the triplet channel for charge separation.The major findings are summarized in the following.1)We developed microsecond-resolved transient absorption spectroscopy with temporal window covering from sub-millisecond to 10 s of seconds.Persistent luminescence involves the dynamics of excited states of very different time scale from primary excitations on sub-picoseconds to the emissive states on seconds.In most available studies,time-resolved photoluminescence spectroscopy has been applied to probe the carrier dynamics,which,however,can monitor the emissive states.To capture the essential information about the dark excited states,we employ the transient absorption(TA)spectroscopy.We developed TA spectroscopy with micro-second temporal resolution that can probe the dynamics to the late stage up to 10 s of seconds.Combining the fs-and ns-resolved TA spectroscopy,we can access the excited states on very different time scale and probe the essential role of triplet intermediated states.2)We systematically investigated the carrier dynamics underlying the long persistent luminescence in the blend of TMB/PPT system.The experimental data suggest that the optically excited singlet charge-transfer states undergo the intersystem crossing to form the intermediate triplet states which are further dissociate into free charges.By combining the temperature-dependent transient absorption and photoluminescence measurements,electron spin resonance experiments and quantum chemical calculation,we identify the intermediate is a mixture of the locally excited triplet states at donor generated from intersystem crossing and the charge-transfer triplet states.Such a triplet state intermediated scenario is instrumental for searching donor/acceptor systems for persistent luminescence which may stimulate further efforts towards triplet state-based optoelectronic devices.