Organic Ultralong Room-temperature Phosphorescence Materials: Design,Preparation And Performance Study

Organic ultralong room-temperature phosphorescence of purely organic molecules has boomed recent advances of organic optoelectronics and breaks through the traditional understandings of luminance from molecular aggregated structures of π-conjugated materials,and facile intersystem crossing and strong coupling aggregation have been demonstrated as two key factors to make it come true.However,it still remains challenging in purely organic molecules of realizing the two factors together.Contrary to the widely accepted singlet-state emission feature of fluorescence and tripletstate radiative decay mechanism of phosphorescence,the exact origin of organic afterglow is still unclear,especially for the relation between property and packing structure.And organic afterglow suffers low intensity and efficiency and generally needs to be excited by UV-light owing to its spinforbidden phosphorescent nature that essentially requires facile intersystem crossing.Besides,there is lack of strategy to convert traditional high-performance organic luminophor to afterglow material and subject to stokes shift caused by close molecular packing of ordered crystal lattice,rare blue organic afterglow has been reported.Last but not least,there is lack of alternative methods to populate triplet excited states through physical method.Here,we have proposed several strategies to build efficient organic afterglow materials as follow:1.Organic afterglow still remains challenging in purely organic molecules of realizing the facile intersystem crossing and strong coupling aggregation together.Here,we proposed a simple strategy to enhance the intersystem crossing as well as the intermolecular interaction of purely organic optoelectronic molecules by introducing pseudohalogen.Based on this principle,a series of cyanophenyl derivatives have been carefully designed and all they show extraordinary OURTP with lifetime up to 0.51 s.Experimental and theory studies reveal that the pseudohalogen activated n-π* transition capable of promoting spin-flipping while substituent effect reduces the electron cloud density favoring strong face to face π-π stacking.2.Molecular aggregation plays an important role in the luminescent processes of organic π-conjugated materials and the exact relations between aggregated structures and emission behaviors are still unclear.Here we designed a series of CN-substituted phenylcarbazole isomers,which show varied aggregated structures and extraordinary organic ultralong room-temperature phosphorescence(OURTP)with lifetime up to 0.92 s.Systematic experimental and theoretical studies reveal that OURTP properties are closely related to the splitting energy(Δε)of H-aggregation.Specifically,the Δε-controlled thermally activated reversed phase transformation from the low-lying dark state to the high-lying transition-allowed emission state of H-aggregation was identified,for the first time,as the key process in OURTP: larger Δε leads to longer lifetime,while smaller Δε results in short lifetime but higher OURTP efficiency.3.Organic afterglow suffers low intensity and efficiency and generally needs to be excited by UVlight owing to its spin-forbidden phosphorescent nature that essentially requires facile intersystem crossing(ISC).We propose a strategy to bypass the traditional ISC through facilitating singlettriplet transition to directly populate triplet excited states from the ground state by combining synergetic effects of both heavy/hetero-atom incorporations and aromatic aggregations.Verified by systematic experimental and computational investigations,this unique singlet-to-triplet absorption results in much improved organic afterglow quantum efficiency up to 9.5% with elongated lifetime of 0.25 s under visible-light irradiation.4.Invoking efficient afterglow in metal-free organic molecules represents an important material advance.However,organic afterglow suffers low intensity and efficiency,and lack of strategy to convert traditional high-performance luminophor to OURTP materials.Here,we designed a general strategy for amorphous organic afterglow materials through host-guest system which has successfully converted traditional thermal active delay fluorescent(TADF)molecular to OURTP materials,and shows extremely high PLQY.The OURTP emission was preliminarily demonstrated to come from the reversed intersystem crossing(RISC)process controlled delay fluorescent(DF)emission.5.Since promoting intersystem crossing(ISC)in purely organic optoelectronic materials with large singlet and triplet energie(ΔEST)and weak spin-orbit coupling(SOC)between S1 and T1 for organic afterglow is not an easy task,instead of attempting to promote the traditional S1→Tn ISC through chemical modification,we decide to explore alternative methods to populate triplet excited states through physical process.A facile route for enhancing organic afterglow property through doping with phosphorescence/TADF sensitizer guest in organic microcrystal with weak ISC process has been developed.This unique method results in much enhanced organic afterglow quantum efficiency up to 14.5% with elongated lifetime of 0.45 s.