Highly Efficient Ultralong Organic Phosphorescence through

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Highly Efficient Ultralong Organic Phosphorescence through Intramolecular Space Heavy Atom Effect Huifang Shi, Lulu Song, Huili Ma, Chen Sun, Kaiwei Huang, Anqi Lv, Wenpeng Ye, He Wang, Suzhi Cai, Wei Yao, Yujian Zhang, Zhongfu An, Wei Huang, and Ruilin Zheng J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b03712 • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on January 24, 2019

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The Journal of Physical Chemistry Letters

Highly Efficient Ultralong Organic Phosphorescence through Intramolecular Space Heavy Atom Effect Huifang Shia, Lulu Songa, Huili Maa, Chen Suna, Kaiwei Huanga, Anqi Lva, Wenpeng Ye a, He Wang a, Suzhi Caia, Wei Yaoa, Yujian Zhangb, Ruilin Zhengc, Zhongfu Ana*, Wei Huanga,d*

a. Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergistic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, China.

b. Department of Materials Chemistry, Huzhou University, East 2nd Ring Road. No.759, Huzhou 313000, China

c. College of Electronic and Optical Engineering, Nanjing University of Posts & Telecommunications, 9 Wenyuan Road, Nanjing 210023, China.

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d. Shaanxi Institute of Flexible Electronics (SIFE), Northwestern Polytechnical University (NPU), 127 West Youyi Road, Xi'an 710072, China.

*E-mail:

[email protected]; [email protected]

ABSTRACT

Metal-free organic phosphorescent materials have attracted considerable attention in the fields of organic electronics and bioelectronics. However, it remains a great challenge to achieve organic phosphors with high quantum efficiency in a singlecomponent system. In this work, we designed and synthesized two organic phosphors (PDCz and PDBCz) with ultralong organic phosphorescence (UOP) feature. Both molecules showed ultralong emission lifetime of over 200 ms. For PDBCz crystal, it was found that the absolute phosphorescence quantum efficiency


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reaches up to 38.1%. Combined the experimental and theoretical studies, the highly efficient UOP was mainly attributed to the intramolecular space heavy metal effect, which facilitates the spin-orbit coupling between singlet and triplet excited state to promote the intersystem crossing effectively. This study will provide a new platform to rationally design highly efficient UOP materials and show its potential in the field of flexible electronics.

TOC GRAPHICS

Ultralong organic phosphorescence (UOP), namely organic afterglow emission, which stores the excitation energy and shows persistent luminescence for several seconds, minutes or even hours after the removal of excitation light source, has aroused considerable interests due to their distinct photophysical properties


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recently.1-3 Taking advantage of their ultralong emission lifetime, these materials can be widely applied in data encryption4,5 and anti-counterfeiting.6 Ultralong organic phosphors will also be ideal agents in the highly sensitive sensing7-9 and imaging10-12 through a time-resolved emission technique to eliminate the short-lived background fluorescence interference effectively. Moreover, compared with the organometallic or inorganic compounds, metal-free organic phosphorescent materials have more merits, including low toxicity, low cost, and easy synthesis and modification. However, because of the weak spin-orbital coupling (SOC)13 between excited singlet and triplet states as well as fast non-radiative decay of triplet excitons via molecular motions14, annihilation from oxygen15 or moisture and so forth, highly efficient metal-free organic phosphorescence is normally difficult to obtain under ambient conditions (room temperature and normal pressure). Through great efforts, some feasible strategies, such as crystal engineering16-19, host-guest doping methods20,21, polymerization22,23, H-aggregation24-28, etc., have been gradually proposed for enhancing room temperature phosphorescence (RTP). Among these strategies, one way is constructing a relatively rigid environment to restrain the nonradiative decay of the triplet excitons, like construction of organic frameworks,


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crystalline inducement. The other is promoting the intersystem crossing (ISC) rate via the introduction of heavy atoms24,29, such as Br or I, aromatic carbonyl and heteroatoms with n electrons, which is beneficial to the SOC between excited singlet and triplet states.30 Organic phosphorescence efficiency can be greatly enhanced by introduction of halogen atoms due to heavy atom effect (HAE).31 Despite the great success in improving phosphorescence efficiency, however, most bright UOP was achieved through manipulation of intermolecular HAE (Fig.1a), which is not easily controllable owing to the unpredictable intermolecular stacking by non-covalent bonding in crystal. Recently, we found that UOP efficiency could be significantly improved by effective π-type halogen bonding between adjacent molecules.24 Compared with intermolecular HAE, introduction of heavy atoms in a molecule skeleton is much more controllable for manipulating π-type HAE.24,32 However, it is still quite rare to study the influence of intramolecular π-type HAE on ultralong organic phosphorescence in crystal under ambient conditions.


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Figure 1. Design concept (a) and chemical structure (b) for highly efficient ultralong organic phosphorescence

In this work, we proposed a valid strategy of intramolecular space heavy atom effect (IS-HAE) to improve phosphorescent efficiency (Fig.1b). A couple of molecules of PDCz and PDBCz with two Br atoms were designed and synthesized (Fig.1b). Two Br atoms can construct intense intramolecular interactions (C-Br∙∙∙N and CBr∙∙∙π) with N and C atoms on the carbazole unit. Both compounds showed ultralong phosphorescence with the lifetimes of over 200 ms. Unexpectedly, photoluminescence (PL) quantum efficiency (ΦPL) of PDBCz exceeded 40%, among which phosphorescence quantum efficiency (ΦPh) approached to 38.1%, occupying


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over 95% of the total ΦPL. So far as we know, this is one of the highest ΦPh in the reported single component UOP materials under ambient conditions. 30-31 The target molecules were easily prepared through a one-step nucleophilic substitution reaction (Scheme S1) and their chemical structures were fully characterized by NMR (Fig S1-S4), elemental and X-ray single crystal analyses. The photophysical properties were firstly studied by PL and phosphorescence spectra with 5 ms delay in solid state under ambient conditions. As shown in Figure 2a, in PL spectra, PDCz powder showed a strong blue emission with peaks at 366 (τa= 2.3 ns), 410 (τa= 10.2 ns) and 430 nm(τa= 9.2 ns) (Fig. S5a-S5c). Owing to the short emission lifetimes in scope of nanoseconds, these emission bands were assigned to fluorescence. After a delay time of 5 ms, an intense emission band appeared with a major peak at 558 nm and shoulders at 600 and 650 nm, which was ascribed to phosphorescence. Due to the introduction of Br atoms, however, PDBCz showed significantly different photophysical properties from that of PDCz. In solid state, it showed a bright yellow emission with a dominated band at around 550 nm in its PL spectrum. The area of fluorescence emission band before 500 nm with short lifetimes (0.5-0.9 ns) (Fig.S5d-S5e) in PDBCz was much weaker than that after 500


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nm which was considered as phosphorescence. Its phosphorescence spectrum showed similar profile with the corresponding PL spectrum after 500 nm. With the existence of carbazole unit, as we expected, both molecules showed ultralong emission with lifetimes of around 560.0 ms for PDCz and 217.3 ms for PDBCz (Fig. 2a inset, Fig. 2b and S6). The phosphorescence nature was further confirmed by the sensitivity of the ultralong luminescence to oxygen (Fig. S7). It is found that the intensity of phosphorescence was deceased to some extent after the phosphors was exposed to oxygen. The blue emission part of PL intensity of PDBCz decreased obviously compared with that of PDCz. It is estimated that the introduction of Br on the molecule enhanced the phosphorescence, but weakened the fluorescence due to the effective ISC caused by HAE. Meanwhile, the phosphorescence spectra of these two crystals further validated our prediction. Moreover, the phosphorescence and PL spectra of PDCz showed overlap at 420 nm that may be due to the triplet-triplet annihilation (TTA, Fig. S6 and Table. S1). Apparently, with the introduction of Br atom, PL quantum efficiency (ΦPL) of PDBCz (40%) was lower than that of


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Figure 2. Photophysical properties of PDBCz and PDCz in crystal under ambient condition. (a) Normalized steady-state photoluminescence (PL, black solid line) and phosphorescence spectra (Phos., red solid line) of PDBCz and PDCz. Insert: luminescent images under UV-lamp on (left) and off (right). (b) Lifetime decay profiles at 300 K. (c) The bar charts of phosphorescence quantum yields. (d) and (e) The excitation-phosphorescence emission mappings of PDBCz and PDCz crystals.


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PDCz (62%). Nevertheless, the ΦPh of PDBCz reached as high as 38.1%, over 15 times higher than that of PDCz, which is the highest phosphorescence quantum yield among the reported single component UOP materials so far as we know (Fig. S8). From the excitation-phosphorescence emission mappings shown in Fig. 2d and 2e, phosphorescence spectra of the two compounds remained the same emission position when the excitation wavelength changed from 220 to 420 nm. For PDCz, an emission from TTA was observed with the excited wavelength from 240 to 255 nm (Fig. 2e).


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Figure 3. (a), PL spectra of PDBCz (red line) and PDCz (black line) in TCM dilute solution excited at 310 under ambient conditions. (b) and (c), PL (black dashed line) and phosphorescence (red solid line) spectra of PDBCz and PDCz in TCM dilute solution excited at 320 and 300 nm at 77 K. (d) and (e), models selected as calculated SOC constant, calculated energy diagram and SOC (ξ) of PDBCz and PDCz in crystal.

To get insight into the high phosphorescence quantum yield of PDBCz, a set of experiments, along with theoretical calculation were conducted. Firstly, we studied PL spectra of the two phosphors in dilute trichloromethane solution (TCM, 2×10-5 M). Almost no obvious emission of PDBCz was obtained at the room temperature in TCM; while PDCz showed an intense emission at 355 nm under the same condition (Fig. 3a). It was supposed that the fluorescence of PDBCz in single molecular state was quenched by the introduction of heavy atom Br. Moreover, at 77 K, PL and phosphorescence spectra of PDBCz in TCM overlapped well as shown in Fig. 3b, which indicated that the IS-HAE played a vital role in enhancing phosphorescence. By comparison, the PL spectrum of PDCz had a sharp fluorescence emission


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peaked at 350 nm in dilute solution at 77 K (Fig. 3c). These spectral data of the dilute solution at both room and low temperature revealed that fluorescence quenching of PDBCz was the result of the strong intramolecular heavy atom interactions, which would promote SOC to enhance the phosphorescence efficiency. Compared with phosphorescence spectra of the phosphors in solid state, it is found that the phosphorescent spectra in dilute solution is blue shifted, indicating that the bright yellow phosphorescence is relevant to intermolecular interactions. To further explore the HAE on the phosphorescence, theoretical simulations were performed at the level of (TD) DFT/B3LYP/6-31G(d), as shown in Figure 3d and computational details in SI. When the heavy atom Br was introduced from PDCz to PDBCz, SOC constants (ξ) between S1 and Tn were hugely enhanced, for example, the ξ (S1, T3) was increased from 0.089 to 3.089 cm-1 and ξ (S1, T7) is increased from 0.489 to 4.226 cm-1, resulting in an enlargement of ISC process, thereby triggering a highly efficient phosphorescence. Unlike this, the IS-HAE has a tiny impact on the ISC process of T1→S0 because of the small change in ξ (S0, T1) (from 0.164 to 0.116 cm1),

which is responsible for the long phosphorescent lifetime in PDBCz. As a

comparison, the external HAE was also inspected in Figure S9, it showed that the


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external HAE plays a tiny role in the ISC process between singlet and triplet states due to the large C-Br···π distance. Therefore, the highly efficient UOP can be achieved by the intramolecular HAE through enhancement of S1→Tn ISC process.

Figure 4. Crystal packing models of PDBCz (a, b, c) and PDCz (d, e) with denoted both intramolecular and intermolecular interactions.

To gain the origin of ultralong phosphorescence of the two compounds, X-ray single-crystal diffraction analyses were performed to study their detailed molecular packing models and interactions (Fig. 4). For PDBCz, there existed three types of intramolecular interactions, including C-Br···π (3.591 Å), C-Br···N (3.13 Å) and CH···N (2.589 Å), as shown in Fig 4a. The strong C-Br···π and C-Br···N interactions


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effectively limited molecular rotations and then decreased the non-radiative decay. Intramolecular halogen bonds promoted SOC between excited singlet and triplet states as confirmed by the theoretical calculation discussed above. Moreover, we found a small overlap on carbazyl group with formed π···π stacking (3.438 Å) for a pair of PDBCz molecules in crystal (Fig. 4b). As PDBCz crystal is a centrally symmetric conformation, each molecule in crystal was surrounded by four types of intermolecular interactions shown in Fig 4c (C-Br···π, 3.546 Å). All these intramolecular and intermolecular interactions can restrain the molecular motions to further reduce the non-radiative decay of triplet excitons, and thus realizing ultralong phosphorescence. In contrast, there was only one type of intramolecular interaction in the monomer of PDCz with different distances (C-H···N, 2.586, 2.591, 2.613 and 2.62 Å, Fig. 4d), which is relatively weak van der Waals force. Each molecule was surrounded with two kinds of intermolecular interactions, C-H···N (2.586 to 2.62 Å) and C-H···C-H (2.388 Å) as revealed in Fig. 4e, which also suppressed the molecular motions to protect triplet excitons from non-radiative transition. A great deal of intermolecular and intramolecular interactions makes a relatively small radiative rate from T1 to S0 and obtained the ultralong lifetimes (Table S2). In particular, the H-


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aggregation formed for both compounds in crystal state, which can stabilize the triplet excitons for ultralong lifetime under ambient conditions (Fig. S10).2 As shown in Fig. S11, the T1 states in both PDCz and PDBCz were 3(π, π*) transition that localized on carbazole unit, such large molecular descriptor β =1 indicated a longlived lifetime in H-aggregation.34

Figure 5. Luminescent images of flexible and 3D patterns fabricated by PDBCz powder under room light or dark with the excitation UV lamp on and off.

Considering the highly efficient UOP feature of PDBCz, several small patterns were fabricated to exhibit its potential in flexible and 3D patterning. As shown in Figure 5, a thin film was prepared by PDBCz micro-crystals mixed with a commercial gel (epoxide resin) to show its transparency, flexibility and stretchability of this UOP


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material. Similarly, three-dimensional letters “UOP” and a solid ball with a diameter of 2 cm were also fabricated. Due to the high phosphorescence quantum efficiency, bright yellow luminescence of both flexible and 3D patterns can be observed even under room light with removal of the UV lamp. Especially for the solid ball, it shined as a night luminescent pearl with a persistent time of more than 3 seconds by naked eyes after the removal of the excitation light source (Figure S12). This highly efficient UOP materials will be promising in the flexible electronics. In summary, we have investigated the highly efficient pure organic phosphors by introducing intramolecular space heavy atom effect. Combining theoretical simulations and single crystal analysis, we concluded that the intramolecular heavy atom interactions are beneficial to promote phosphorescence quantum efficiency for PDBCz

(38.1%)

via

facilitating

spin-orbit

coupling.

Simultaneously,

the

phosphorescence lifetime of both phosphors in crystal were over 200 ms, which was the result of the stabilization of the excited triplet states by H-aggregation. So far as we know, this is one of the highest efficient UOP materials in the reported singlecomponent organic compounds with an emission lifetime over 200 ms. Moreover, 3D patterning was firstly fabricated by this highly efficient UOP material. This study will


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not only provide an effective strategy for the rational design of highly efficient UOP materials, but also expand their potential applications in 3D lighting and flexible electronics. Experimental section, computational methods, synthesis, additional single crystal analysis and photophysical measurement. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENT

This work is supported by the, National Natural Science Foundation of China (21875104, 51673095, 91833304, 91833302 and 61605074), National Basic Research Program of China (973 Program, No. 2015CB932200), Natural Science Fund for Distinguished Young Scholars of Jiangsu Province (BK20180037), the Natural Science Fund for Colleges and Universities (17KJB430020) and "High-Level Talents in Six Industries"(XCL-025) of Jiangsu Province, and NanjingTech Start-up Grant (3983500158, 3983500169 and 3983500201). We are grateful to the HighPerformance Computing Center of Nanjing Tech University for supporting the computational resources.


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Supporting Information Experimental section, computational methods, synthesis, additional single crystal analysis and photophysical measurements.

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Highly Efficient Ultralong Organic Phosphorescence through

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