To achieve effective RTP, two prerequisites shouldprimarily be considered: one is to obtain efficient spin-orbitcoupling for effectively populating triplet excitons throughfacilitating intersystem crossing (ISC) process, and this can begenerally realized by introducing transition metals, halogens,deuterated carbon, and aromatic carbonyl groups;[1, 15,18–20] theother is to stabilize excited triplet species, thus suppressingnon-radiative transitions. Such stabilized excited tripletspecies can usually be obtained by crystallization or embedding luminogens in appropriate matrices and is assisted byforming hydrogen bonds to restrict their vibration androtation.[2, 11,13–16, 21]Carbon dots (CDs), a newly emerged luminescent nanomaterial, have received widespread interest and have beensuccessfully employed in many fields of research thanks totheir superior optical properties, excellent biocompatibility,and cost-effective preparation.[22–25] Specifically, the RTP anddelayed fluorescence (FL) properties of CDs have beendiscovered recently, but are only observable by embedding orimmobilizing in certain matrices (e.g., poly(vinyl alcohol)(PVA), polyurethane, urea/biuret, or zeolites).[26–33] Inspiredby these findings and the general requirements for producingeffective RTP, we propose that it might be possible to prepareRTP CDs directly if the following conditions are compliedwith: 1) CDs should be mostly amorphous polymer-likestructures which hopefully will behave as matrices to embedand immobilize luminogens; 2) the interior of CDs shouldcontain functional groups that could form hydrogen bonds,for further stabilization of excited triplet species; and3) elements that favoring n!p* transitions (e.g., N, P, andhalogens) should be doped in CDs, to facilitate the ISCprocess and consequently to effectively populate tripletexcitons.[15, 19, 34]According to these considerations, we have found a onepot method for the facile and quick preparation of ultralongRTP (URTP) CDs. As shown in Figure 1 a, b, gram-scaleURTP CDs can be obtained in a few minutes via microwaveassisted heating of an ethanolamine and phosphoric acidaqueous solution, and subsequent dialysis purification andfreeze-drying (more details in Supporting Information). Thepurified CDs are a pale yellow powder and emit weak blue FLunder UV (365 nm) lamp irradiation. Importantly, the CDspresent the longest RTP lifetime to be 1.46 s (more than 10 sto naked eye, Figure 1 c and Video 1 in Supporting Information) among the reported CDs-based materials to date.[26–32]Note that the as-developed method features facile, quick andscalable merits, the obtained product possesses URTP life-time and could potentially be employed as securityink in anti-counterfeiting and information protection.
To achieve effective RTP, two prerequisites shouldprimarily be considered: one is to obtain efficient spin-orbitcoupling for effectively populating triplet excitons throughfacilitating intersystem crossing (ISC) process, and this can begenerally realized by introducing transition metals, halogens,deuterated carbon, and aromatic carbonyl groups;[1, 15,18–20] theother is to stabilize excited triplet species, thus suppressingnon-radiative transitions. Such stabilized excited tripletspecies can usually be obtained by crystallization or embedding luminogens in appropriate matrices and is assisted byforming hydrogen bonds to restrict their vibration androtation.[2, 11,13–16, 21]Carbon dots (CDs), a newly emerged luminescent nanomaterial, have received widespread interest and have beensuccessfully employed in many fields of research thanks totheir superior optical properties, excellent biocompatibility,and cost-effective preparation.[22–25] Specifically, the RTP anddelayed fluorescence (FL) properties of CDs have beendiscovered recently, but are only observable by embedding orimmobilizing in certain matrices (e.g., poly(vinyl alcohol)(PVA), polyurethane, urea/biuret, or zeolites).[26–33] Inspiredby these findings and the general requirements for producingeffective RTP, we propose that it might be possible to prepareRTP CDs directly if the following conditions are compliedwith: 1) CDs should be mostly amorphous polymer-likestructures which hopefully will behave as matrices to embedand immobilize luminogens; 2) the interior of CDs shouldcontain functional groups that could form hydrogen bonds,for further stabilization of excited triplet species; and3) elements that favoring n!p* transitions (e.g., N, P, andhalogens) should be doped in CDs, to facilitate the ISCprocess and consequently to effectively populate tripletexcitons.[15, 19, 34]According to these considerations, we have found a onepot method for the facile and quick preparation of ultralongRTP (URTP) CDs. As shown in Figure 1 a, b, gram-scaleURTP CDs can be obtained in a few minutes via microwaveassisted heating of an ethanolamine and phosphoric acidaqueous solution, and subsequent dialysis purification andfreeze-drying (more details in Supporting Information). Thepurified CDs are a pale yellow powder and emit weak blue FLunder UV (365 nm) lamp irradiation. Importantly, the CDspresent the longest RTP lifetime to be 1.46 s (more than 10 sto naked eye, Figure 1 c and Video 1 in Supporting Information) among the reported CDs-based materials to date.[26–32]Note that the as-developed method features facile, quick andscalable merits, the obtained product possesses URTP life-time and could potentially be employed as securityink in anti-counterfeiting and information protection.<br>
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