Reversible Luminescence Switching of an Organic Solid: Controllable On–Off Persistent Room Temperature Phosphorescence and Stimulated Multiple Fluorescence Conversion

Abstract

wileyonlinelibrary.com COMMUNICATIONreported mechanochromic luminescent materials contain a single luminophore with two fl uorescent states, and thus their fl uorescence can be switched between two colors or two different intensities. [ 5a , 6 ] However, examples exhibiting mechanochromic phosphorescence are rather rarely seen in literature, except for a few metal complexes showing mechanochromic phosphorescence with short lifetimes. [ 7 ] It is known that room temperature phosphorescent (RTP) emissions often occur in metal complexes, but rare for pure organic luminogens. [ 8 ] The development of organic mechanochromic phosphorescence is of great importance from both a scientifi c and a practical point of view.Our group has observed RTP in organic crystals of nonplanar molecules containing carbonyl groups, while phosphorescence is absent in the corresponding amorphous aggregates. This phenomenon has been attributed to crystallization-induced phosphorescence (CIP). [ 9 ] Inspired if mechanochromic or thermochromic phosphorescence can be achieved by means of reversible morphology transformation between the crystalline and amorphous states of those luminescent organic materials. Unfortunately, quick crystallization of those RTP luminogens makes it diffi cult to obtain their amorphous aggregates by mechanical stimulus or thermal method. Therefore, to achieve mechanochromic phosphorescence, it is desirable to attenuate the crystallization ability of those luminegens, which can be realized through a suitable molecular design.In this paper, we present the unique photoluminescent (PL) properties of solid materials based on luminogen 1 ( Figure 1 ), where carbazolyl groups are introduced at the p -positions of phenyl rings in benzophenone molecule. In the molecular crystals of luminogen 1 , the fl uorescent processes can be significantly inhibited with the assistance of chloroform (TCM). However, luminogen 1 exhibiting RTP with ultralong lifetime and the RTP decay process is observable even with the naked eyes, which is unusual for organic luminogens. Remarkably, the persistent RTP of 1 can be repeatedly switched on and off facilely by repeating the fuming-heating or fuming-grinding process. Meanwhile, when phosphorescence was shut down, the solid materials can exhibit multiple fl orescent states depending on the morphologies. The different fl uorescent states can be reversibly switched between each other when vapor, thermal, and mechanical stimuli were applied. Owing to the versatile

Organic Nanocrystals with Bright Red Persistent Room‐Temperature Phosphorescence for Biological Applications

“…[1][2][3] As opposed to loosely bonded electrons in organometallic materials,t he highly bonded nature of electrons in pure organic materials restricts their ability to decay through radiative relaxation pathways from triplet states. [6][7][8][9][10][11][12][13] These discoveries sparked several attempts to find ag eneral strategy to design more of such materials for various applications.Itwas found that to yield efficient RTP, several key factors have to be considered. [4] In addition, the triplet states are easily quenched by atmospheric oxygen molecules through triplet-triplet energy transfer.…”

“…Therefore,triplet states in organic materials are prone to nonradiative relaxations through thermal and collisional processes. [6][7][8][9][10][11][12][13] These discoveries sparked several attempts to find ag eneral strategy to design more of such materials for various applications.Itwas found that to yield efficient RTP, several key factors have to be considered. As such, intensely phosphorescent materials are largely known to exist in inert gases and at very low temperatures,w hich in turn limits their practical applications.…”

Roles of Localized Electronic Structures Caused by π Degeneracy Due to Highly Symmetric Heavy Atom‐Free Conjugated Molecular Crystals Leading to Efficient Persistent Room‐Temperature Phosphorescence

“…[12] In the host-guest molecular materials, the highly rigid short conjugated matrix could suppress k q (RT) caused by endothermic triplet-triplet energy transfer from guest to host molecules and effectively protect triplet excited species from oxygen, contributing substantially to the appearance of persistent RTP. [56] However, for most heavy atom-free conjugated molecular crystals with persistent RTP, [9,[14][15][16]25,26,[46][47][48][49][50][51] the quantum yield of persistent RTP (Φ p (RT)) of conjugated molecular crystals with an RTP lifetime approaching to 1 s is often a few percent or less. [56] However, for most heavy atom-free conjugated molecular crystals with persistent RTP, [9,[14][15][16]25,26,[46][47][48][49][50][51] the quantum yield of persistent RTP (Φ p (RT)) of conjugated molecular crystals with an RTP lifetime approaching to 1 s is often a few percent or less.…”

“…Therefore, reports of RTP from heavy metal-free aromatic molecules under ambient conditions have been scarce. [19] Therefore, these materials are potentially useful for a variety of applications, such as thermometers, [20] security, [13,15,[21][22][23][24] stimuli sensors, [25,26] optical recording, [27][28][29] and bioimaging, [28,30,31] which are independent of autofluorescence. [11][12][13][14][15][16][17] Because heavy atom-free molecules with T 1 with strong ππ* characteristics have very small k p , [18] RTP from such conjugated structures exhibits persistent emission characteristics.…”

“…[11][12][13][14][15][16][17] Because heavy atom-free molecules with T 1 with strong ππ* characteristics have very small k p , [18] RTP from such conjugated structures exhibits persistent emission characteristics. [32][33][34][35][36][37][38] Except for a few reports before 2000, [9,10] persistent RTP characteristics under ambient conditions have been observed recently from heavy atom-free isolated conjugated molecules doped in a highly rigid amorphous host [12,20,21,[39][40][41] and crystalline host, [42,43] carbon nanodots, [13,[22][23][24]44,45] heavy atom-free aromatic crystals, [14][15][16]25,26,[46][47][48][49][50][51][52] metal-organic frameworks, [17,53] and nonconventional luminogens. [19] Therefore, these materials are potentially useful for a variety of applic...…”