Chinese Chemical Letters  2016, Vol. 27 Issue (8): 1231-1240   PDF    
Room-temperature phosphorescence from purely organic materials
Liu Yang, Zhan Ge, Liu Zhi-Wei, Bian Zu-Qiang, Huang Chun-Hui     
Beijing National Laboratory for Molecular Sciences(BNLMS), State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China
Abstract: Room-temperature phosphorescence (RTP) materials have attracted great attention due to their involvement of excited triplet states and comparatively long decay lifetimes. In this short review, recent progress on enhancement of RTP from purely organic materials is summarized. According to the mechanism of phosphorescence emission, two principles are discussed to construct efficient RTP materials: one is promoting intersystem crossing (ISC) efficiency by using aromatic carbonyl, heavyatom, or/and heterocycle/heteroatom containing compounds; the other is suppressing intramolecular motion and intermolecular collision which can quench excited triplet states, including embedding phosphors into polymers and packing them tightly in crystals. With aforementioned strategies, RTP from purely organic materials was achieved both in fluid and rigid media.
Key words: Room-temperature phosphorescence     Purely organic material     Intersystem crossing     Non-radiation transition    
1. Introduction

In photophysics, phosphorescence arises from forbidden radiative transitions between the excited triplet state and ground state. Room-temperature phosphorescence (RTP) materials have attracted intensive interests due to their involvement of excited triplet states and comparatively long decay lifetimes. RTP materials have been applied in various fields, such as organic light-emitting diodes (OLEDs) [1], chemical and biological sensing [2], photovoltaic devices [3], bioimaging [4], and so on. In particular, RTP materials have a profound impact on the efficiency improvement of OLEDs, since they can utilize both triplet and singlet excitons generated during electrical injection and leading to a maximum internal quantum efficiency of 100%.

It has been extensively studied that some organometallic complexes, i.e. iridium complexes, are RTP materials owing to enhanced spin-orbit coupling induced by heavy atom effect. After a highly efficient non-doped OLEDs based on iridium complex has been fabricated [5], we mainly focused on improving device efficiency by designing efficient iridium complexes and optimizing device architecture [6]. Considering that noble metals present a large gap to commercialization, we have moved our interest to RTP materials with inexpensive, abundant and environmentally friendly properties, such as Cu(I) complexes [7] and purely organic materials.

Unlike organometallic complexes, most purely organic materials exhibited no RTP due to inefficient spin-orbit coupling and easy-quenched radiation relaxation process. Hence, the observation of phosphorescence from purely organic materials has been confined to cryogenic and inert conditions for a long time [8]. In 1977, Thomas et al. first reported that RTP could be observed inside micells after deoxygenation, which is known as micelle-stabilized RTP (MS-RTP) shortly afterwards [9]. Based on the initial report, cyclodextrin-induced RTP (CD-RTP) [10] and solid substrate-RTP (SS-RTP) [11] have been developed. In a protective ordered medium, excited triplet states of organic molecules are prone to radiative relaxation as non-radiactive relaxations are blocked by hindered molecular motions.

In this short review, recent progress on purely organic RTP materials is summarized after two basic principles for the phosphorescence emission have been analyzed. RTP were achieved both in fluid and rigid media from purely organic materials by promoting the ISC efficiency with specific compounds as well as suppressing non-radiative transition rates with phosphors confinement in polymers and crystals.

2. Basic principles for RTP

As shown in Fig. 1, the excited triplet state (T1) is populated by ISC from excited singlet state (S1), which is only possible through vibronic and spin-orbit coupling [12]. Thus, the first principle for phosphorescence observation is (i): kISC > 0, where kISC is the ISC rate. kISC is an intrinsic property of a molecule, which varies with the energy level and electronic configuration of the molecule. A small energy gap (ΔEST) between S1 and T1 levels facilitates ISC. It has been proved experimentally that whenΔEST is extremely small (-100 meV), both S1→T1 ISC and T1→S1 reverse ISC (RISC) are enhanced. In this case, triplet states become ‘dark’ and thermally activated delayed fluorescence (TADF) can be obtained [13]. As for electronic configuration, EI-Sayed has shown that spin-orbit coupling can be greatly promoted by mixing singlet and triplet states of different electronic configurations, such as (π, π*) and (n, π*) [9]. Moreover, heavy atom effect (i.e. bromine or iodine atom) has been used externally and internally to increase S1T1 conversion [14].

Figure 1. A general Jablonski diagram of organic emitters is presented. Basic principles of RTP are shown in the grey box.

The second criterion for phosphorescence needs to be met (ii) kP > knr, where kP and knr are radiative and non-radiative transition rate from excited triplet state to ground state, respectively. Nonradiative channels can be divided into intramolecular losses (kTS) and external losses caused by interaction with environment (kq). At room temperature, the non-radiative loss, with rate of knr~102- 106 s-1, surpasses the radiative transition [12]. Consequently, suppression of non-radiative relaxation is perhaps the most significant and challenging part for efficient RTP from purely organic materials.

3. Achievement of RTP in fluid medium

Based on aforementioned principles, RTP in fluid medium (i.e. solution), where intramolecular motion (rotation and vibration) and intermolecular collision are rampant and non-radiative relaxation is dominated, could be achieved mainly by promoting ISC efficiency. Non-bonding orbitals provided by a carbonyl or a heteroatom break the forbidden spin-orbit coupling between states of the same configuration in aromatic hydrocarbons. It is well known that S→Tn, π transitions in carbonyl compounds are much more probable (factor of 100-1000 times) than S→Tπ, π in aromatic hydrocarbons [15]. Consequently, increasing ISC efficiency is feasible via introducing aromatic carbonyl, heterocycle, or heavy atom, and RTP in fluid medium could be realized when bringing two or three components to one molecule. However, RTP observed in solution was rare, since non-radiative transitions in a fluid medium are rampant.

By using both heavy-atom effect and nitrogen heterocycle, RTP was first confirmed in solution by Gutie´ rrez et al., which is beyond traditional rigid conditions for RTP observation [16]. Until in 2013, Takeuchi et al. reported that a fluorene derivative (Br-FL-CHO), which has both bromo and formyl groups, presented a bright RTP in a common organic solvent with phosphorescence quantum yield (φp) of 5.9%. The emission at 500 nm appeared under Argon (Ar) and completely disappeared in the presence of dioxygen, which is repeatable by bubbling Ar gas and exposure to air in turn (Fig. 2a). With a decay lifetime (τ) of 335 μs, the emission is unambiguously generated from excited triplet state. Further experiments indicated that RTP can be observed only when both bromo and formyl were existed in the fluorene parent. It comes to a conclusion that RTP in Br-FL-CHO is promoted by synergy of bromo and formyl substituents [17].

Figure 2. (a) UV-vis (black line) and emission spectra of Br-FL-CHO (4.0×10-5 mol L-1) under air (red dashed line) and Ar (blue line) in CHCl3 at 298 K. Inset shows the chemical structure of Br-FL-CHO. (b) Images of Br-FL-CHO upon 365 nm excitation under Ar (left) and air (right) in CHCl3 at 298 K. Adapted with permission from ref. [17]. Copyright 2013, Royal Society of Chemistry.

In 2014, Taddei et al. reported another class of RTP in solution based on 1, 8-naphthalimide. Substituting a bromine atom at the 4- position enhances ISC through an internal heavy atom effect and prevents the generation of singlet charge transfer (1CT) state due to a weak electron-withdrawing effect. With a further heterocycle substitution, the compound N-((pyridin-4-yl)methyl)-4-bromo-1, 8-naphthalimide (2) displayed a phosphorescent emission around 600 nm (τ = 332 μs) with φp of 0.59% in solution at room temperature (Fig. 3), which is different to the compound 4- bromo-1, 8-naphthalimide (1), showing only fluorescence (Fig. 3a) [18].

Figure 3. (a) Emission spectra of 4-bromo-1, 8-naphthalimide (1, grey line) and N- ((pyridin-4-yl)methyl)-4-bromo-1, 8-naphthalimide (2, green line) in air-free toluene at room temperature. (b) An image of 2 in the air-free toluene solution upon UV irradiation (UVGL-55 lamp). Adapted with permission from ref. [18]. Copyright 2014, American Chemical Society.

4. Efficient RTP in rigid medium

Immobilizing molecules in a rigid medium is a promising way to prevent deactivation through intramolecular motion (i.e., rotation and vibration) and intermolecular collision, which leads to a suppression of non-radiative relaxation. By combination with promoted ISC efficiency, more and more RTP from purely organic materials was achieved in rigid media.

4.1. RTP in cocrystals assembled by halogen bonding

Halogen bonding is a weak electrostatic interaction between a positively polarized halogen atom and a nucleophilic atom, anion or π-electron system [19]. Because of its directionality, halogen bonding plays an increasingly important role in crystal engineering. Moreover, heavy halogen that participate in halogen bonding, such as iodine and bromine, can bring heavy atom effect closely to the phosphor, which enhances ISC efficiency between singlet and triplet states. In 2011, Kim et al. presented RTP from brominated aromatic aldehyde [20]. The strong halogen bonding between bromine and aldehyde reduces vibrational freedom in 2, 5- dihexyloxy-4-bromobenzaldehyde (Br6A) and promotes spinorbit coupling simultaneously. However, Br6A suffered greatly from aggregation-caused quenching with φp of only 2.9%. When diluting Br6A into a bi-brominated aromatic analogue, 2, 5- dihexyloxy-1, 4-dibromobenzene (Br6), the φp of Br6A/Br6 cocrystals was increased to 55% due to the same role that halogen bonding plays in the Br6A crystal (Fig. 4a). With this strategy, a series of efficient RTP materials with emission colors varied from blue to red was designed and synthesized by regulating the electron density of the chromophore. Further investigation proved that Br6A has been doped into the crystal lattice of Br6, hence the size of the guest material must match well with that of the host matrix to achieve high efficiency (Fig. 4b) [21].

Figure 4. (a) Schematic illustration of high phosphorescence efficiency in a mixed crystal. Adapted with permission from ref. [20]. Copyright 2011, Nature Publishing Group. (b) Chemical structures of emitters Br(5-8)A and hosts Br(5-8) (left), and emission images of dropcast crystals made from mixed aldehyde and host compound excited by 365 nm light (right). Adapted with permission from ref. [21]. Copyright 2014, American Chemical Society.

1, 4-Diiodotetrafluorobezene (1, 4-DITFB) is an excellent donor of halogen bonding owing to the incorporation of strongly electron withdrawing groups. In crystal engineering, 1, 4-DITFB is considered as a connective molecule. It can also be functionalized as a heavy atom probe to promote singlet-triplet conversion in the design of RTP cocrystal materials. In 2012, Jin et al. reported crystallographic and photophysical properties of a cocrystal assembled from pyrene and 1, 4-DITFB (Fig. 5a). Cocrystal with three dimensional architecture was constructed by C-I…I-C, C-I-…π halogen bonding together with π-π stacking and C-H…F interactions. The pyrene/1, 4-DITFB cocrystal displayed strong RTP with an average lifetime of 0.574 ms [22]. Subsequently, RTP cocrystals built with 1, 4-DITFBwere extended to polycyclic aromatic hydrocarbons, such as biphenyl (BP), naphthalene (Nap) and phenanthrene (Phe) [23], fluorene analogues [24], as well as conjugated hydrocarbons (diphenylacetylene (DPA), trans-stilbene(tS)) [25] (Fig. 5a). Taking Nap/1, 4-DITFB (cocrystal 4) and Phe/1, 4-DITFB (cocrystal 5) as examples [23], all cocrystals showed emission spectra with wellresolved vibrational fine-structure (Fig. 5b).

Figure 5. (a) Chemical structures of halogen bonding donor and acceptors involved herein. (b) Excitation and emission spectra of cocrystal 4 (Nap/1, 4-DITFB) and cocrystal 5 (Phe/1, 4-DITFB). Insets show the images of 4 and 5 excited by UV 365 nm. BNU: Beijing Normal University. Adapted with permission from ref. [23]. Copyright 2012, Royal Society of Chemistry.

As seen from aforementioned examples, efficient RTP can be obtained from purely organic materials via cocrystals assembled by halogen bonding. The host matrix with heavy atoms in this case serves a dual role as both a solid diluent to prevent aggregationcaused quenching and as a probe to enhance ISC through external heavy atom effect. It is noticeable that the emission of a RTP material in cocrystals is different from its monomer, which normally presented a red-shifted spectrum, due to interactions between emitter and host matrix that change radiative triplet energy and surroundings. Therefore, RTP in cocrystals can be modulated by tuning both molecular structure of the emitter and halogen bonding between the emitter and host matrix [24].

4.2. RTP in host-guest system

Aggregation-caused quenching is normally due to the formation of low energy quenching products such as excimers and exciplexes. Host-guest system has been exploited to isolate luminescent molecules to avoid aggregation-caused quenching. A common type of solid diluent is polymeric materials [10, 11], which is also a rigid matrix to suppress motion of the guest molecule.

In 2013, Kimet al. investigated RTP of Br6A by embedding it into a glassy poly(methyl methacrylate) (PMMA) matrix. Similar to thermal motion of molecule in solution, the β-relaxation in PMMA, which is caused by rotation of the ester side group, greatly affects vibrational dissipation of excited triplet states. The authors showed that φp climbed from0.7% to 7.5% as the relative amount of isotactic PMMA (iPMMA) in atactic PMMA (aPMMA) increased from 0 to 100% (Fig. 6a).Moreover, improved phosphorescence was observed by lowering the temperature (Fig. 6b). From these observations, it was hypothesized that φp is dependent on the degree of b-relaxation and which can be enhanced in a low isotacticity polymer and at low temperature. Based on this phenomenon, a temperature sensor by incorporating Br6A into a temperaturesensitive polymer matrix has been demonstrated [26].

Figure 6. (a) Phosphorescence quantum yields with various isotactic PMMA contents. Inset shows the chemical structure of Br6A. The excitation wavelength was 365 nm. (b) Emission images of Br6A embedded in (a) atactic PMMA (aPMMA), (b) syndiotactic PMMA (sPMMA), (c) isotactic PMMA (iPMMA), and (d) no polymer matrix at different temperatures. The excitation wavelength was 365 nm. Adapted with permission from ref. [26]. Copyright 2013, American Chemistry Society. (c) Schematic for illustrating water-induced phosphorescence to fluorescence switching. Inset shows the chemical structure of G1. Adapted with permission from ref. [27]. Copyright 2014, John Wiley and Sons. (d) Chemical structures of DA1 and PFMA and Diel-Alder reaction between them. Br6A plays a role as a reference. Adapted with permission from ref. [28]. Copyright 2015, Nature Publishing Group.

To further minimize motion of phosphor and polymer matrix, Kim et al. introduced strong intermolecular hydrogen bonds into the host-guest system. In a newly designed compound G1, the bromoaldehyde core is preserved as the RTP emitting center and the carboxylic acid periphery is functionalized as a structural basis for hydrogen bonds formation (Fig. 6c). Strong RTP was observed from G1-poly(vinyl alcohol) (PVA) blend film with a φp up to 24%, which is higher than that of Br6A-iPMMA amorphous film (φp = 7.5%). A systematic study revealed that hydrogen bonding between G1 and PVA plays a critical role in restricting the vibration and movement of G1 and PVA. The hydrogen bonds can be broken up by water molecules, showing as blue fluorescence displaces green phosphorescence at the spot where G1-PVA contacts with water. This unique system showed great potential in ratiometric water sensor [27].

Covalent cross-linking between phosphor and polymer matrix can also suppress molecular motion of the embedded phosphor. Kim’s et al. chose Diels-Alder click chemistry as the cross-linking method and the resulted DA1 doped poly(furfuryl methacrylate) (PFMA) system showed highly efficient RTP with φp up to 28%, which is ca. 2-5 times higher than that of the Br6A-polymer system (Fig. 6d) [28].

Despite polymeric material, RTP can be achieved in other matrices by elaborate design, and the system based on which usually demonstrated novel properties. In 2013, Adachi et al. developed efficient persistent RTP (pRTP, τ > 1s) with φp exceed 10% by minimizing non-radiative deactivation pathway of triplet excitons [29]. In details, a fully deuterated fluorene derivative (Dye 1) was designed to prevent triplet exciton quenching through C-H stretching vibration (Fig. 7a), and a mixed cholesterol and a, a, a′-tris(4-hydroxyphenyl)-1-ethyl-4-isopropylbenzene (THEB) amorphous film, which has a rigid structure and high T1 energy level, was used as the host to suppress nonradiative transition. Consequently, a mixture of 90 wt% cholesterol, 9.0 wt% THEB and 1.0 wt% Dye 1 showed a reversible thermoresponsive on-off phenomenon. When heating the mixture up to 90 ℃ (the crystallization temperature of blend host), crystallization of the host matrix induces aggregation of Dye 1, leading to strong emission quenching of RTP. The blue-green RTP could be completely recovered by heating the mixture above its melting point and then cooling it to room temperature (Fig. 7b). This novel thermoresponsive on-off material is a good candidate for highlyadvanced secret media [30].

Figure 7. (a) Material design for efficient pRTP in the air. Adapted with permission from ref. [29]. Copyright 2013, John Wiley and Sons. (b) Reversible thermal recording and erasing using the pRTP material, Dye 1. The recording material is composed of 1.0 wt% Dye 1, 9.0% wt% THEB, and 90 wt% cholesterol. The power and wavelength of excitation light were 0.3 mW cm-2 and 345 nm, respectively. All photographs were taken at room temperature in the air. Adapted with permission from ref. [30]. Copyright 2013, John Wiley and Sons.

In 2015, Yang et al. studied RTP of 3-bromoquinoline (3-BrQ) in supramolecular gels formed by self-assembly of 1, 3:2, 4-di-Obenzylidene- D-sorbitol (DBS). The RTP of 3-BrQ gels is nonquenchable in the presence of hydrophobic quenching reagent, which indicated that 3-BrQ has been entrapped in the hydrophobic 3D network structure of DBS gels. Thereby the RTP of 3-BrQ is enhanced by restricting the intramolecular motion and intermolecular collision of 3-BrQ in gels (Fig. 8a). The capability of collapse and reconstruction at gel-sol transition temperature enables the 3- BrQ/DBS gel system showing excellent reversible RTP “on-off” phenomenon between 80 ℃ and 10 ℃ (Fig. 8b) [31].

Figure 8. (a) Schematic illustration of 3-BrQ RTP induced by DBS supramolecular gels. (b) “On-off” reversible RTP of 3-BrQ in DBS gels (black line) and NaDC (Sodium deoxycholate) solutions (red line) at 10 and 80 ℃. The wavelengths of excitation and emission were 258 and 508 nm, respectively. Adapted with permission from ref. [31]. Copyright 2015, American Chemical Society.

Phosphors embedded in rigid medium such as polymer and self-assembled gels can suppress non-radiative transition channels and facilitate RTP under ambient conditions. The composite materials would be more promising for practical applications, such as OLEDs [32], secret recording substrates and sensors.

4.3. Crystallization-induced RTP

Crystalline state provides a rigid framework for phosphor so that non-radiation channels caused by molecular motion are blocked. In addition, strong intermolecular interactions in certain packing motifs lead to significant electronic couplings between two adjacent molecules. Consequently, efficient RTP from purely organic materials could be achieved in crystalline state.

In 2009, Tang et al. reported crystallization-induced phosphorescence (CIP) of a series of aromatic compounds [33], particularly of benzophenone derivatives [34]. An outstanding feature of CIP molecules is that RTP can only be observed in crystalline state, neither in solution nor in amorphous state (such as absorbing on TLC plate and doping into polymer). Compounds benzophenone (BP), 4, 4′-difluorobenzophenone (DFBP), 4, 4′-dibromobiphenyl (DBBP’) and diphenylacetylene (DPA) shown in Fig. 9 are known to be phosphorescent at low temperature. Surprisingly, they could emit blue or green-blue light with long lifetime in crystalline state at room temperature. The emission spectra of their crystals taken at room temperature resembled the phosphorescence spectra of their dilute solutions measured at 77 K [34], which demonstrated that excited triplet states in crystals were offered higher opportunity for radiative transition aided by restriction of intramolecular motions in crystal.

Figure 9. Images of phosphors with CIP emission. Insets show chemical structures, phosphorescence quantum yields (wp), average lifetimes (tA) and crystalline packing arrangements of (A) BP, (B) DFBP, (C) DBBP’ and (D) DPA. Photographs were taken under illumination of a handheld UV lamp. Adapted with permission from ref. [34]. Copyright 2010, American Chemical Society.

In 2015, Tang et al. discovered that two triplet-involved relaxations of delayed fluorescence (DF) and phosphorescence could be activated in crystals of a series of phthalic acid analogues. Theoretical calculations have been conducted to gain more insights into the photophysical process. Taking terephthalic acid (TPA) as an example, the energy gap between S1 and T2 decreased from 0.375 eV to 0.229 eV when single molecules in gas phase aggregate to crystal. Moreover, the spin-orbit coupling between singlet and triplet states increased upon crystallization (Fig. 10b). Computation results indicated that phosphors in crystal made a balance between ISC and RISC, thus DF and RTP were concomitant in condensed states of these aromatic acids or esters (Fig. 10c) [35]. Later, a new class of 1, 4-bis(aroyl)-2, 5-dibromobezenes were also found to exhibit CIP properties, and the RTP emission color can be tuned from blue to green, which depends on the nature of the aroyl group [36].

Figure 10. (a) Chemical structures of isophthalic acid (IPA) and TPA. (b) Calculated energy diagrams and spin-orbit coupling of TPA in the gas phase (left) and in the crystalline (right) state. (c) Schematic illustration of CIP phenomenon. Left: highly active molecular motions induced weak fluorescence in solution. Right: RIM processes boosted both fluorescence (prompt and delayed) and phosphorescence in crystal. Adapted with permission from ref. [35]. Reproduced from Ref. [35] with permission from the Royal Society of Chemistry.

It is widely known that intermolecular interactions result in a more extended electron distribution, thus molecules may show strong intermolecular electronic coupling between organic units when packing in a certain way, which greatly changes the energy level of excited molecules and may lead to a different type of RTP.

In 2014, Huang et al. presented a novel design rule for pRTP (up to 1.35 s) in crystal form based on effective stabilization of excited triplet states through strong coupling in H-aggregated molecules (Fig. 11a). The existence of H-aggregation was validated by the aggregation induced blue-shifted absorption spectra and the single crystal structure analysis. Time-dependent density functional theory (TD-DFT) investigations on a single molecule and a dimer aggregated in H-aggregation style indicated that ISC is enhanced due to more energy transition channels from S1→Tn. A newly formed triplet state was also stabilized by H-aggregation, which functionalized as an energy trap at a lower energy level, offering a long-lived excited state and pRTP. A series of phosphors (DEOPh, DECzT, DPhCzT, CzDCIT, DCzPhP) containing O, N or P atom capable of promoting ISC through n-π* transition were synthesized. And all organic compounds exhibited pRTP as anticipated (Fig. 11b) [37].

Figure 11. (a) Proposed mechanism for pRTP by constructing H-aggregates to stabilize the lowest excited triplet states. (b) The steady-state emission spectra (left) and pRTP spectra (right) of a series of specially designed molecules. Insets show the corresponding images taken before (left) and after (right) the excitation source was turned off. DEOPh was excited at 254 nm, whereas the other four compounds were excited at 365 nm. Adapted with permission from ref. [37]. Copyright 2015, Nature Publishing Group.

In 2015, Bryce et al. provided another mechanism to obtain pRTP in crystal form. Subunits with different excited-state configurations (i.e., nπ* and π* states) have a tendency to form close intermolecular stacking within crystals, which can result in significant interactions between their orbitals. A detailed study on 4-(9H-carbazol-9-yl)benzophenone (Cz-BP, φp = 0.3%) was conducted [38]. The Cz-BP molecule contains a carbonyl group as the n unit and a carbazole (Cz) group as thepunit. The distance between the two units is short, i.e. 3.373Åfor the oxygen atom and 3.561Åfor the carbon atom to the Cz plane, respectively (Fig. 12a). Thus, n and π units belong to two molecules couples with each other, which increase ISC channels. The couplings between n and p units were further verified in Cz-DPs and BCz-DPs, where sulfonyl group acts as the n unit (Fig. 12b). Furthermore, conjugating a heavy halogen atom into the n unit was found to enhance ISC rate of the intermolecular coupling. Very recently, Lu et al. synthesized two series of carbazole derivatives in which Br and carbazole are linked by flexible alkyl chains. With an intermolecular moderate heavy atom effect, the best material showed strong pRTP with φp up to 39.5% and decay lifetime as long as 200 ms in a crystal form [39].

Figure 12. (a) Cz-BP dimer in crystalline state (left) and the energy level diagram of isolated and coupled Cz-BP molecules. (b) Steady-state emission spectra (violet) and pRTP spectra (delay 25 ms; orange) of different crystal powder samples. Adapted with permission from ref. [38]. Copyright 2016, John Wiley and Sons.

Since crystallization-induced RTP arises from blocked molecular motion in crystal state that suppresses non-radiative transitions, these materials are expected to exhibit mechanoluminochromic properties. In 2015, Tang et al. studied reversible mechanochromism of Cz-BP, bromo-substituted BCz-BP and DBCz-BP, which were proved to be phosphorescent in crystal. As shown in Fig. 13, asprepared microcrystalline samples can transform into disordered amorphous state by grinding and the phase can reverse through heating or fuming. In the amorphous state, RTP disappeared and only fluorescence survived because crystallization suppressed nonradiactive transition has been collapsed [40]. Similarly, Xu et al. demonstrated a pure organic compound (ICz-DPS) with a room temperature fluorescent phosphorescent dual-emission property. By mixing different ratios of fluorescent (purple) and phosphorescent (yellow) emissions, a wide emission colors can be obtained simply by tuning mechanical grinding time (Fig. 14) [41].

Figure 13. (A-C) Photographs taken under 365 nm UV light, (D-F) prompt and delayed (td = 0.1 ms) emission spectra, and (G-I) XRD patterns of as prepared, ground, and dichloromethane fumed solids. (A, D, and G) for Cz-BP, B, E, and (H) for BCz-BP, and (C, F, and I) for DBCz-BP. Adapted with permission from ref. [40]. Copyright 2015, John Wiley and Sons.

Figure 14. (a) PL spectra of ICz-DPS in chloroform solution (10-5 mol L-1) and in crystal state. (b) PL spectra of ICz-DPS at different ground times. (c) Corresponding CIE chromaticity coordinates in CIE-1931 chromaticity diagram of the ICz-DPS samples at different ground times. (d) Corresponding photos of the ICz-DPS samples at different ground times under 365 nm UV irradiation. CIE chromaticity coordinates of three samples were indicated in (c): the original crystal (sample A), the sample after grinding for 120 s (sample B), and the ground sample with white emission (sample W). Copyright 2015, John Wiley and Sons.

5. Conclusion

In this short review, we have summarized recent progresses on RTP from purely organic materials. Basic principles for designing RTPmaterials are efficient ISC between singlet and triplet states and suppressed non-radiative transitions of triplet states. It has been widely investigated that aromatic carbonyl, heavy-atom or heterocycle/ heteroatom containing compounds have far higher intersystemcrossing efficiency than aromatic hydrocarbons.With enhanced S→T and T→S transitions, molecules with rigid core structure can exhibit RTP in fluid medium at an inert atmosphere. However, most RTP materials are non-emissive in solution where intramolecular motion (rotation and vibration) and intermolecular collision quench excited triplet states heavily. Therefore, many immobilization methods have been exploited for promoting triplet radiation transition and reducing triplet annihilation. Mixing phosphor with a host to form cocrystal and embedding them in a host matrix are natural ideas for prohibiting inter- or intramolecular motions, as well as preventing aggregation-caused quenching. Crystallizationinduced RTP is an abnormal phenomenon, which is explained by restriction of intramolecular motion, as well as that intermolecular electronic coupling between adjacent molecules enhances ISC transition and stabilizes triplet states. Recently, we are working on a novel system where microsecond-scale RTP can be obtained from purely organic materials via a triplet twisted intramolecular charge transfer excited state, which will definitely update traditional understanding for constructing efficient RTP materials.

Acknowledgment We gratefully acknowledge the financial support from The National Basic Research Program of China (No. 2014CB643802), Ministry of Science and Technology (No. 2016YFB0401001) and the State Key Laboratory of Advanced Technologies for Comprehensive Utilization of Platinum Metals.
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