Twisted Molecular Structure on Tuning Ultralong Organic

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A 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 J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b02953 • Publication Date (Web): 03 Jan 2018 Downloaded from http://pubs.acs.org on January 3, 2018

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The Journal of Physical Chemistry Letters

A Twisted Molecular Structure on Tuning Ultralong Organic Phosphorescence Chen Sun,a Xueqin Ran,a Xuan Wang,a Zhichao Cheng,a Qi Wu,a Suzhi Cai,a Long Gu,a Nan Gan,a Huixian Shi,d Zhongfu An,a* Huifang Shi,a,b* and Wei Huanga,c* 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 211800, P.R. China. b

. 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. c

. Shaanxi Institute of Flexible Electronics (SIFE), Northwestern Polytechnical University

(NPU), 127 West Youyi Road, Xi'an 710072, China. d

. School of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan

030024, China.

ABSTRACT

Compared to planar carbazole, the molecular conjugation of iminodibenzyl (Id) was destroyed by C-C bond and a twisted structure was formed, which exhibited blue-shifted ultralong phosphorescence with a lifetime of 402 ms in crystal under ambient conditions. For the presence of oscillating C-C bond between the two benzene rings in Id, more than one

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molecular configuration in crystal were discovered by the X-ray single crystal analysis. Moreover, its ultralong phosphorescence color changed from blue to green by varying 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 make a major step forward in expanding the scope of the ultralong organic phosphorescent materials, building a bridge to realize the relationship between molecular structure and UOP property.

TOC GRAPHICS

KEYWORDS Organic Semiconductor; Ultralong Organic Phosphorescence; H-aggregation; Twisted Molecular Structure; Molecular Crystal Engineering Metal-free organic room temperature phosphorescence (RTP) has evoked considerable attentions recently due to their long-lived triplet excitons,1 large Stokes shift2 and relatively low cost in diverse applications, such as organic light-emitting diodes (OLEDs),3,4 molecular sensing5-10 and imaging,11,12 data encryption13,14 and so on.15-20 However, triplet excitons of pure organic molecules are prone to non-radiative deactivation via molecular vibration and rotation,21 thermal and collisional processes as well as oxygen-mediated quenching.22 And theoretically, spin-forbidden transition or weak spin−orbit coupling between singlet and triplet excited states also brings great challenges for RTP23. Hence, more and more efforts


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have been devoted to enhancing the 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 non-radiative decay,30-34 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. Adachi’s group achieved persistent RTP via host-guest doping strategy and deuterated modifications by suppressing the non-radiative decay effectively.30 Our group proposed initially that Haggregated 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.


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Figure 1. Chemical structures and molecular configurations of carbazole and iminodibenzyl in single crystal. Normally, multicolor fluorescence can be rational tuned by certain ways to change the conjugated degrees of the luminophors, such as molecular structure twisting, steric-hindrance effect or 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 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 solid state is nearly a planar configuration with yellow ultralong phosphorescence ranged from 525 to 595 nm (Figure 1). With the introduction of two methylene groups, the molecular configuration of Id is twisted, because that the wiggled C-C bond in the flexible chain leads to a destruction of conjugated degree and thus a blue-shift of phosphorescence from yellow to green. This result will provide a new design concept for the ultralong organic phosphorescent materials.

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


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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. Photophysical properties of Id and Cz were fully studied in both solution and solid state via UV-visible

absorption,

steady-state

photoluminescence

(PL)

and

time-resolved

phosphorescence spectra. As shown in Figure 2a, it is obvious 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). In 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 5 ms delay.

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 nm or 365 nm switched off. (c) The 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.


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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 obvious changes for Cz were observed with excitation wavelength varied (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 non-obvious change of phosphorescence spectra compared with Id might stem from limited molecular conformation transformation caused by the small angle of 1.99o 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 optimized Id molecular structure, as showed 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 red-shifted around 50 nm. While a little change on 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 emission spectral shift under different excitations. Moreover, 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 afterwards, respectively, but there was no significant changes for Cz under the same conditions (insert images in Figure 3a and 3b). To further prove the influence of conformational torsion on 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 solid state and restrict the intermolecular interactions at 77 K (Figure S2). As expected, phosphorescence spectra of 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


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excitation-wavelength

dependent

UOP

was

assigned

to

the

single

molecular

phosphorescence.

Figure 4 (a) Normalized steady-state PL (dashed line) and phosphorescence spectra (solid line) for Id (red line) and Cz (black line) in 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 crystal excited at 365 nm and 300 K. (c), (d), Excitation-phosphorescence emission mappings of Id and Cz, respectively. Next, photophysical properties of Id and Cz in the crystal state were studied. Compared with Cz, PL spectrum for Id showed a blue-shift with split peaks at 346 and 361 nm under ambient conditions with short lifetimes (Figure 4a and Figure S3). Phosphorescence spectra of these two compounds exhibited similar profiles with two significant separated peaks. And the left peak, 392 nm of Id and 421 nm of Cz, had large overlaps with their PL spectra (Figure 4a). From the relative long emission lifetimes of these blue peaks (Figure S4), we considered these two peaks were from the delayed fluorescence (DF) by triplet-triplet annihilation (TTA).45 Similarly, the major phosphorescent band was blue-shifted of 40 nm for Id (520 nm) compared to Cz (560 nm). Moreover, their ultralong luminous colors changed


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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 owing to the reduced conjugated degree via introducing 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).

Figure 5 Crystal packing models of Id (a and 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 ultralong organic phosphorescence. To investigate the mechanism of ultralong organic phosphorescence, both X-ray single crystal analysis and theoretical calculations were conducted. The crystal structure of Cz is nearly coplanar; while Id in crystal has two chiral twisted configurations 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 interactions. As shown in Figure 5a and 5b, there are two configuration manners with different dihedral angles in crystal Id. As


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shown 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 less intermolecular interactions for Cz with the 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 non-radiation transitions of triplet excitons are reduced effectively for UOP generation. The existence of a wiggled C-C bond in Id molecule leads to a looser molecular packing compared to that of Cz in crystal state (Table S1). It is reasonably speculated that more non-radiative transitions occur in Id with C-C bond may result in a relative shorter phosphorescence lifetime of Id than Cz. As depicted in Figure 5d, the dipole moment angles calculated from single crystal structures were 63.1o for Id and 82.8o for Cz, which are all larger than the theoretical value of 54.7o, the distinguished value of J and H aggregations.10, 13 Hence, H-aggregated molecules formed in the crystal states can stabilize the triplet excitons to realize the ultralong organic phosphorescence for both compounds under ambient conditions, as illustrated in Figure 5e. In summary, a new carbazole analogue, imidobenzyl showed ultralong organic phosphorescence 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 UOP color changed from blue to green after the removal of the excitation light source, correspondingly. Based on both experimental and calculated results, this amazing phenomenon may be attributed to the multiple molecular conformations caused


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by the twisted structure in a rigid environment provided by the low temperature. Furthermore, an H-aggregation formation of the phosphors in the crystal state can stabilize the triplet excitons to realize UOP under ambient conditions. This study will provide a simple approach to adjust the luminescent color of ultralong organic phosphorescent materials. ASSOCIATED CONTENT Supporting Information Phosphorescence spectra, lifetime decay profiles of the fluorescent, single-crystal analysis and structure data of Cz and Id single crystals. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *[email protected], *[email protected], *[email protected]/[email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT 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. REFERENCES


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