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Cite This: J. Phys. Chem. Lett. 2018, 9, 335−339

Twisted Molecular Structure on Tuning Ultralong Organic Phosphorescence Chen Sun,† Xueqin Ran,† Xuan Wang,† Zhichao Cheng,† Qi Wu,† Suzhi Cai,† Long Gu,† Nan Gan,† Huixian Shi,∥ Zhongfu An,*,† Huifang Shi,*,†,‡ and Wei Huang*,†,§

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†

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 211800, People’s Republic of China ‡ Key Laboratory for Organic Electronics & Information Displays (KLOEID), and Institute of Advanced Materials (IAM), Nanjing University of Posts and Telecommunications, Wenyuan Road 9, Nanjing 210023, China § Shaanxi Institute of Flexible Electronics (SIFE), Northwestern Polytechnical University (NPU), 127 West Youyi Road, Xi’an 710072, China ∥ School of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China S Supporting Information *

ABSTRACT: Compared to planar carbazole, the molecular conjugation of iminodibenzyl (Id) was destroyed by a C−C bond and a twisted structure was formed, which exhibited blue-shifted ultralong phosphorescence with a lifetime of 402 ms in a crystal under ambient conditions. For the presence of an oscillating C−C bond between the two benzene rings in Id, more than one molecular configuration in the crystal was discovered by X-ray singlecrystal analysis. Moreover, its ultralong phosphorescence color changed from blue to green by varying the excitation wavelength in solution at 77 K. Theoretical calculations also confirmed that different molecular configurations had certain impact on the phosphorescent photophysical properties. This result will allow a major step forward in expanding the scope of ultralong organic phosphorescent (UOP) materials, building a bridge to realize the relationship between molecular structure and UOP property.

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Adachi’s group achieved persistent RTP via a host−guest doping strategy and deuterated modifications by suppressing the nonradiative decay effectively.30 Our group proposed initially that H-aggregated molecules can stabilize the triplet excitons to achieve UOP under ambient conditions.16 After that, more and more studies on ultralong phosphorescence have been reported.26−39 Tang et al. introduced intense intermolecular interactions by a donor−acceptor (D−A) structure to realize the persistent RTP.40 In 2016, Chi et al. presented a novel mechanism and a molecular design strategy that the strong intermolecular electronic coupling with various excited-state configurations would promote the orbital overlap and thus hybrid ISC transitions for persistent RTP.41 Zhen Li and co-workers demonstrated the effect of molecular packing on the photophysical property of phosphorescent materials.42,43 Although great efforts have been exerted on how to realize UOP under ambient conditions, the relationship between a molecular configuration and ultralong phosphorescence property is rare to study.

etal-free organic room-temperature phosphorescence (RTP) has evoked considerable attention recently due to the long-lived triplet excitons,1 large Stokes shift,2 and relatively low cost in diverse applications, such as organic lightemitting diodes (OLEDs),3,4 molecular sensing5−10 and imaging,11,12 data encryption,13,14 and so on.15−20 However, triplet excitons of pure organic molecules are prone to nonradiative deactivation via molecular vibration and rotation,21 thermal and collisional processes, as well as oxygenmediated quenching.22 Furthermore, theoretically, a spinforbidden transition or weak spin−orbit coupling between singlet and triplet excited states also brings great challenges for RTP.23 Hence, more and more efforts have been devoted to enhancing metal-free organic phosphorescence under ambient conditions,24,25 e.g., enhancing the phosphorescence quantum efficiency (ΦP) by increasing the intersystem crossing (ISC)26−29 or decreasing the nonradiative decay30−34 or tuning the phosphorescent colors with different molecular structures.35 However, there exists limited research on lifetime tuning of metal-free organic phosphorescence under ambient conditions. Very recently, an amazing optical phenomenon, ultralong organic phosphorescence (UOP), namely persistent RTP or afterglow emission, was reported, which can last for seconds to hours even after the removal of the excitation light source. © 2018 American Chemical Society

Received: November 7, 2017 Accepted: January 3, 2018 Published: January 3, 2018 335

DOI: 10.1021/acs.jpclett.7b02953 J. Phys. Chem. Lett. 2018, 9, 335−339

Letter

The Journal of Physical Chemistry Letters Normally, multicolor fluorescence can be rationally tuned by certain ways to change the conjugated degrees of the luminophors, such as molecular structure twisting, a steric hindrance effect, introduction of a flexible fragment, etc.44 Inspired by this concept, we attempt to study the conjugation effect on ultralong phosphorescence by a twisted structure. A new organic compound, imidobenzyl (Id), containing a distorted part that breaks the connection of two benzene rings, showed bright green UOP with a lifetime of 402 ms under ambient conditions, as illustrated in Figure 1. In this

Interestingly, the major phosphorescence band of Id in dilute solution changed from 403 to 497 nm with the excitation from 290 to 370 nm at 77 K, as depicted in Figure 3a. However, no

Figure 1. Chemical structures and molecular configurations of carbazole and iminodibenzyl in a single crystal.

work, we found that a new phosphor with two methylene groups can break the original electron conjugated degree of carbazole (Cz) to tune its photophysical properties, e.g., its ultralong phosphorescent color and emission lifetime. Meanwhile, it also leads to different conformations whether in crystal or in solution, making it possible to adjust the emission color. Cz in the solid state is nearly a planar configuration with yellow ultralong phosphorescence ranging from 525 to 595 nm (Figure 1). With the introduction of two methylene groups, the molecular configuration of Id is twisted because the wiggled C− C bond in the flexible chain leads to destruction of the conjugated degree and thus a blue shift of phosphorescence from yellow to green. This result will provide a new design concept for ultralong organic phosphorescent materials. Photophysical properties of Id and Cz were fully studied in both solution and the solid state via UV−visible absorption, steady-state photoluminescence (PL), and time-resolved phosphorescence spectra. As shown in Figure 2a, it is obvious

Figure 3. Phosphorescence spectra under different excitation wavelengths from 290 to 370 for Id (a) and Cz (b) in 2-mTHF (1.5 × 10−5 M) at 77 K, respectively. Inset: corresponding UOP colors of Id and Cz with the excitation light source at 254 or 365 nm switched off. (c) Relationship between the phosphorescence wavelength and changed dihedral angles of Id and Cz by calculation. The enlarged blue points are the optimized initial molecular structures of Id and Cz. (d) Optimized structures of Id and Cz for simulation.

obvious changes for Cz were observed with a varied excitation wavelength (Figure 3b). We speculated that different twisted conformations in Id had various lowest triplet states at room temperature, which would be instantly fixed in solution at 77 K, forming single molecules at different energy levels. For Cz, the relatively nonobvious change of phosphorescence spectra compared with Id might stem from limited molecular conformation transformation caused by the small angle of 1.99° between two benzene rings in Cz molecules (Figure S1). In order to verify our speculation, the theoretical calculations were conducted by density function theory (DFT) on an optimized Id molecular structure, as shown in Figure 3c. We observed the optimized structure of Id with a dihedral angle of 34°. As expected, with the calculated dihedral angle changed from 24 to 44° (Figure 3d left), UOP emission of Id was redshifted around 50 nm. However, a small change of the UOP emission of Cz was observed when the simulated dihedral angle changed from −10 to 10° (Figure 3d right). It further proved that various molecular conformations in solution may lead to an emission spectral shift under different excitations. Moreover, the UOP color in dilute solution at 77 K for Id changed from blue to green with the removal of excitation light sources at 254 and 365 nm afterward, respectively, but there were no significant changes for Cz under the same conditions (inset images in Figure 3a,b). To further prove the influence of conformational torsion on the phosphorescent spectral change, not the solvent, both Id and Cz doped in poly(methyl methacrylate) (PMMA) thin films were prepared to simulate a rigid environment in the solid state and restrict the intermolecular interactions at 77 K (Figure S2). As expected, phosphorescence spectra of the Id-doped film changed similarly with Id in dilute solution at low temperature, while there was no obvious spectral shift for Cz, which further verified that the

Figure 2. (a) Normalized absorption (dash line) and PL (solid line) spectra of Id (red line) and Cz (black line) in 2-methyltetrahydrofuran (2-mTHF, 1.5 × 10−5 M) under ambient conditions. (b) PL spectra of Id (red line) and Cz (black line) excited at 290 and 300 nm in 2mTHF (1.5 × 10−5 M) at 77 K.

that the maximum absorbance of Id was blue-shifted to 290 nm compared to Cz, which was attributed to the reduced conjugated degree caused by the twisted structure of Id. Similarly, its emission band in dilute solution for Id was also blue-shifted compared to Cz (Figure 2a). At low temperature (77 K), two new emission bands in the range of 400−500 nm appeared for both Id and Cz, which were attributed to the phosphorescence (Figure 2b). The phosphorescence nature was further confirmed by the PL spectra with a 5 ms delay. 336

DOI: 10.1021/acs.jpclett.7b02953 J. Phys. Chem. Lett. 2018, 9, 335−339

Letter

The Journal of Physical Chemistry Letters

actions. As shown in Figure 5a,b, there are two configuration manners with different dihedral angles in crystal Id. As shown

excitation wavelength-dependent UOP was assigned to singlemolecule phosphorescence. Next, photophysical properties of Id and Cz in the crystal state were studied. Compared with Cz, the PL spectrum for Id showed a blue shift with split peaks at 346 and 361 nm under ambient conditions with short lifetimes (Figures 4a and S3).

Figure 5. Crystal packing models of Id (a,b) and Cz (c). (d) Id and Cz with H-aggregation formation in the aggregated molecules. (e) Schematic diagram of H-aggregated molecules stabilizing triplet excitons for UOP.

in Figure 5a, the central molecule is surrounded by five molecules with multiple intermolecular interactions, including N−H···π (2.614 Å), C−H···π (2.725, 2.782, and 2.857 Å), and N−H···H−C (2.693 Å) interactions. Relatively, the other type of molecular packing model has four types of N−H···π (2.614 and 2.88 Å) and two types of C−H···π (2.725 and 2.868 Å) intermolecular interactions (Figure 5b). Nevertheless, there are fewer intermolecular interactions for Cz with similar N−H···π (2.659 Å) and C−H···π (2.808 Å) interactions as the second stacking model of Id (Figure 5a). In a word, both Id and Cz are fixed by surrounded molecules, and thus, nonradiation transitions of triplet excitons are reduced effectively for UOP generation. The existence of a wiggled C−C bond in the Id molecule leads to looser molecular packing compared to that of Cz in the crystal state (Table S1). It is reasonably speculated that more nonradiative transitions occur in Id with a C−C bond, which may result in a relatively shorter phosphorescence lifetime for Id than Cz. As depicted in Figure 5d, the dipole moment angles calculated from single-crystal structures were 63.1° for Id and 82.8° for Cz, which are all larger than the theoretical value of 54.7°, the distinguished value of J- and Haggregations.13,16 Hence, H-aggregated molecules formed in the crystal states can stabilize the triplet excitons to realize the UOP for both compounds under ambient conditions, as illustrated in Figure 5e. In summary, a new carbazole analogue, imidobenzyl, showed UOP with a lifetime of 402 ms under ambient conditions. Its special twisted structure with two methylene groups can break the conjugated degree of the phosphor Id, resulting in an obvious blue-shifted UOP. More interestingly, we found that the ultralong phosphorescent color in solution was dependent on the excitation wavelength from 290 to 370 nm at 77 K, and the UOP color changed from blue to green after removal of the excitation light source, correspondingly. On the basis of both experimental and calculated results, this amazing phenomenon may be attributed to the multiple molecular conformations caused by the twisted structure in a rigid environment provided by the low temperature. Furthermore, H-aggregation formation of the phosphors in the crystal state can stabilize the triplet

Figure 4. (a) Normalized steady-state PL (dashed line) and phosphorescence spectra (solid line) for Id (red line) and Cz (black line) in a crystal excited at 300 and 330 nm, respectively. Insets: corresponding ultralong luminescent images after removing the excitation light source. (b) Lifetime decay profiles of emission bands of Id and Cz in a crystal excited at 365 nm and 300 K. (c,d) Excitation−phosphorescence emission mappings of Id and Cz, respectively.

Phosphorescence spectra of these two compounds exhibited similar profiles with two significant separated peaks. Also, the left peaks, 392 nm for Id and 421 nm for Cz, had large overlaps with their PL spectra (Figure 4a). From the relative long emission lifetimes of these blue peaks (Figure S4), we considered that these two peaks were from the delayed fluorescence (DF) by triplet−triplet annihilation (TTA).45 Similarly, the major phosphorescent band was blue-shifted by 40 nm for Id (520 nm) compared to that for Cz (560 nm). Moreover, their ultralong luminous colors changed from yellow to green after the 365 UV lamp switched off (inset image in Figure 4a). The ultralong lifetimes of 402 ms for Id at 525 nm and 910 ms for Cz at 555 nm further proved the UOP features of these two compounds (Figure 4b). This spectral blue shift was due to the reduced conjugated degree via introduction of a flexible chain with two methylene groups for Id, which was further confirmed by the higher T1 energy level of Id than that of Cz through theoretical calculation (Figure S5). UOP of Id can be efficiently excited from 315 to 385 nm, with the optimum excitation at approximately 330 nm in Figure 4c. As the excitation wavelength changed, UOP emission spectra were similar (Figure S6). To investigate the mechanism of UOP, both X-ray singlecrystal analysis and theoretical calculations were conducted. The crystal structure of Cz is nearly coplanar, while Id in the crystal has two chiral twisted configuration types with different larger dihedral angles. Such a distorted structure will be the main reason for a shifted ultralong phosphorescence. Moreover, their different crystal structures may contribute to various crystal stacking models and multiple intermolecular inter337

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excitons to realize UOP under ambient conditions. This study will provide a simple approach to adjust the luminescent color of ultralong organic phosphorescent materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b02953. Phosphorescence spectra, lifetime decay profiles of the fluorescence, single-crystal analysis, and structural data of Cz and Id single crystals (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Z.A.). *E-mail: [email protected] (H.S.). *E-mail: [email protected]/[email protected] (W.H.). ORCID

Wei Huang: 0000-0001-7004-6408 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (51673095, 61505078, and 21507098), National Basic Research Program of China (973 Program, No. 2015CB932200), the Natural Science Foundation (BK20150962), and the Natural Science Fund for Colleges and Universities (17KJB430020) and “High-Level Talents in Six Industries” (XCL-025) of Jiangsu Province.

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REFERENCES

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