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Spectroscopy and Photochemistry; General Theory

Ultralong Room-Temperature Phosphorescence from Supramolecular Behavior via Intermolecular Electronic Coupling in Pure Organic Crystals Saifei Pan, Zhentian Chen, Xulian Zheng, Donghui Wu, Guilin Chen, Jing Xu, Hui Feng, and Zhaosheng Qian J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b01697 • Publication Date (Web): 02 Jul 2018 Downloaded from http://pubs.acs.org on July 3, 2018

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The Journal of Physical Chemistry Letters

Ultralong

Room-Temperature

Phosphorescence

from Supramolecular Behavior via Intermolecular Electronic Coupling in Pure Organic Crystals Saifei Pan,† Zhentian Chen,† Xulian Zheng, † Donghui Wu, Guilin Chen, Jing Xu, Hui Feng and Zhaosheng Qian* Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, College of Chemistry and Life Sciences, Zhejiang Normal University, Jinhua 321004, People’s Republic of China *Corresponding author. E-mail: [email protected]. Tel.: +86-579-82282269. Fax.: +86579-82282269.

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ABSTRACT: Ultralong room-temperature phosphorescence (RTP) of organic materials is extremely attractive for its tremendous potential use. However, the design of organic materials with ultralong and efficient RTP is very challenging due to the lack of general design principles. Herein, a new design principle for organic materials with ultralong room-temperature phosphorescence based on π-π dominated supramolecular aggregates in crystal is proposed, and strong intermolecular electronic coupling with specific molecular alignment is identified to be responsible for supramolecular behavior in persistent emission. Small substituents in molecular structure favor the formation of supramolecular aggregates in the crystal, thus facilitating the generation of ultralong RTP at ambient conditions. Our results also reveal that the introduction of heavy atoms into supramolecular aggregates as a general rule can be used to achieve efficient persistent phosphorescence.

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Persistent luminescence, referring to the luminescence that lasts for more than several seconds after ceasing the excitation source, has long dominated among inorganic materials containing specific rare elements.1 This precious feature attracted much attentions because of its advantageous merits in avoiding autofluorescence interference in biological systems.2 However, practical applications in bioimaging of inorganic nanoparticles with persistent luminescence are greatly hindered by the rarity of constituent elements and potential toxicity from transition metals, which forced scientists to explore organic counterparts as an alternative way. In general, isolated organic molecules only show short-lived singlet emission because they often possess weak spin-orbit coupling and the small population of triplet excitons are easily consumed by surroundings. Although great advances on room-temperature phosphorescence of pure organic materials have been made recently via a diversity of methodologies including polymer aggregation, crystallization and self-assembly,3,4 only very few examples and design principles have been demonstrated and exploited for ultralong room-temperature phosphorescence.5 One design strategy for organic materials with persistent luminescence is to embed phosphorescent organic molecules into a variety of hosts like polymers,6,7 supermolecules8 and rigid alkane compounds,9,10 and in such a way intramolecular vibrations and intermolecular motions can be effectively suppressed to enhance emissive triplet population. However, suitable host materials effectively protecting long-lived phosphorescence from guest molecules are very limited at present. As a result, most efforts have been devoted to crystallization-induced persistent phosphorescence via crystal engineering. A wide variety of molecules have been reported to show ultralong room-temperature phosphorescence in the crystal form via proper crystal packing, but no consistent explanations were proposed for this specific persistent luminescence behavior. First examples of organic crystals with ultralong-lived emission were



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reported by Tang group, and they attributed this phenomenon to locking the conformations of luminogens by physical constraints and multiple intermolecular interactions.11 However, more examples supported that such ultralong organic phosphorescence is as a result of the formation of H-aggregates in the crystal, which is easily deduced from a bunch of crystal structures.12-18 Differing from the preceding explanation, Yang et al.19 proposed a new mechanism that supports the key role of intermolecular coupling between n and π units in the crystals for the occurring of efficient persistent phosphorescence. This conclusion is very similar to that reported by Xie et al.20 who found that packing style of the compact face to face favors the long-lived phosphorescence due to interactions between dimers. A recent study also revealed interlayer n-π interactions is responsible for ultralong phosphorescence of hydrogen-bonded organic aromatic frameworks.21 From the previous advances, it is readily deduced that crystallization not only can constrain the luminogens in the crystal to suppress the non-radiative way, but also probably contributes to the formation of significant intermolecular interactions for such a long emission duration. However, the nature of intermolecular interactions is not very clear and thus design principles for ultralong organic phosphorescence is lacking now. We hypothesize that ultralong organic phosphorescence mainly originates supramolecular behavior of ordered aggregates with specific molecular arrangement in the crystal, and intermolecular electronic coupling by π-π interaction would dominate the photophysical behavior of formed supermolecules in the crystal. Small substitutes on conjugated structure could favor the close packing patterns through π electronic coupling among molecules in the crystal, whereas large bulk groups probably break down intermolecular π-π interaction due to significant steric effect leading to the lack of supramolecular behavior. In order to test this hypothesis, three acyl groups with different lengths of carbon chains consisting of acetyl, propionyl and butyryl groups


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were introduced into 2,3-dicyanohydroquinone respectively, and the photophysical properties of the resulting products were examined in detail. It is found that all the three compounds possess intense UV emissions from singlet exciton in soluble state, and a bright blue fluorescence can be attained when random aggregation takes place in organic solvents (Scheme 1). However, only 2,3-dicyano-1,4-phenylene diacetate (DCPDA) crystal demonstrates an ultralong green phosphorescence after ceasing the irradiation. Crystal structure analysis and computational results reveal that strong electronic coupling among DCPDA molecules packed in a specific pattern in the crystal mainly contributes to the ultralong luminescence, and these molecules behave like a supermolecule in photophysical properties. The design principle to achieve high efficient phosphorescence was also examined by incorporating heavy atoms like bromine into conjugate structure.

Scheme 1. Schematic Illustration of Different Packing Patterns with Distinct Emission Behaviors Mediated by π-π Interaction.



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Figure 1. (A) Excitation and emission spectra of DCPDA (compound 1) in CH3CN (80.0 μM). (B) Time-gated phosphorescence spectra of DCPDA in THF (80.0 μM) at different temperature from 170 K – 77 K. Delay time: 1 μs.

The starting material, 2,3-dicyano-1,4-benzenediol, was often used as an emission ratiometric probe of intracellular pH because of its intense visible emission.22 The introduction of acetyl group in 2,3-dicyano-1,4-phenylene diacetate (DCPDA, compound 1) excludes the deprotonation process in alkaline environment and hydrogen-bonding effect in solid state, facilitating intermolecular assembly dominated by π-π interaction. DCPDA can dissolve well in common organic solvents like THF and acetonitrile, but no visible emission can be observed when excited by UV light. An intense emission is located at UV light range with emission maximum of 350 nm, and only a very weak tail extends into visible range (Figure 1A). The lifetime of this UV emission is determined to be 2.9 ns from time-resolved fluorescence decay curve (Figure S1), and such a short lifetime is consistent with its very high quantum yield of 0.84. Low-temperature experiments were employed to investigate the phosphorescence of DCPDA in THF, and no apparent phosphorescence can be detected till 170 K. The time-gated phosphorescence spectra (Figure 1B) of DCPDA in THF versus temperature from 170 – 77 K clearly demonstrates the 6 ACS Paragon Plus Environment

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change of phosphorescence with temperature without the interference of its singlet emission, and it is noted that a new emission peak at 470 nm is gradually intensified as the decrease in temperature. Its long lifetime of 10.5 μs determined from time-resolved decay curve (Figure S2) suggests its nature from triplet emission. By comparing PL spectra and lifetimes among DCPDA, DCPDP and DCPDB (Figure S3 and S4), we found that the change in carbon chain of acyl group exerts no significant impact on photophysical properties of these molecules in solution.

Figure 2. (A) Emission spectrum of DCPDA in mixed CH3CN-water (9:1) solution (80.0 μM). (B) Fluorescence spectra of DCPDA in CH3CN solution versus the concentration from 0.1 μM to 10.0 mM.

It is interesting to find that a bright blue emission can be attained when a small amount of water was introduced into DCPDA solution in CH3CN. A new emission peak around 440 nm appears accompanying the emission peak at 350 nm from isolated molecule of DCPDA (Figure 2A). Time-resolved decay curve at 440 nm (Figure S5) clearly shows this emission duration is very short, and the lifetime is calculated to be 12.7 ns, indicating that this blue emission originates from singlet exciton. It is reasonable to deduce that the short-lived blue emission comes from DCPDA aggregates such as dimers or multimers of in mixed CH3CN/water solution because the introduction of water as a poor solvent for DCPDA molecule facilitates the 7 ACS Paragon Plus Environment


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formation of DCPDA aggregates and specific aggregation like J-aggregates generally enhance radiative decay rate.23 The generation of DCPDA aggregate was further confirmed by concentration-dependent experiment (Figure 2B), where the blue emission is gradually enhanced as the concentration of DCPDA in CH3CN is increased from 0.1 μM to 10.0 mM. The change in UV spectra was used to verify the assembly of DCPDA into J-dimers or multimers since Jaggregates exhibit a red-shifted absorption in comparison to monomer in solution.24 An appreciable visible absorption at 380 nm appears for DCPDA in water/acetonitrile solution in comparison to DCPDA in pure acetonitrile (Figure S6), and the large red-shift in absorption clearly demonstrates the generation of some amount of aggregates in the mixed solution. Similarly, a concentrated solution of DCPDA also shows a new broad absorption peak around 400 nm compared to a dilute solution (Figure S7). As a result, it is reasonably concluded that the intense blue fluorescence originates from DCPDA aggregates driven by π-π interaction. To further explore the key role of electronic coupling between two DCPDA molecules, we employed density functional theory calculation and natural bond orbital analysis. A dimer consisting of two DCPDA molecules in π-π packing manner was first fully optimized at M06/6311+G(d,p) level, and a very short interlayer distance of 3.25 Å suggests strong interaction between two DCPDA molecules. The HOMO and LUMO of the dimer (Figure S8) clearly show a large extent of orbit overlap from two molecules, and this close electron coupling dominates the photophysical behavior with exhibiting its own fluorescence. In comparison with DCPDA, DCPDP and DCPDB possess similar blue and short-lived emissions from their aggregates (Figure S9 and S10), illustrating that the difference in carbon chain of acyl groups brings no apparent influence on the formation of specific aggregates. The crystal of DCPDA was readily acquired because of its hydrophobic nature and strong


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Figure 3. (A) PL spectra of DCPDA in crystal form. (B) Time-resolved decay curve of DCPDA crystal at 354 nm. (C) PL spectra of crystal DCPDA at different decay times. (D) Time-resolved decay curve of DCPDA crystal at 526 nm.

intermolecular electronic coupling. It is found that an appreciable green emission can be observed after ceasing UV irradiation (Scheme 1), and this luminescence can last for 2 seconds without the continuous excitation of UV light. However, steady-state PL spectrum of DCPDA (Figure 3A) only shows one major emission peak located at UV light range, and its lifetime is determined to 2.0 ns according to the time-resolved decay curve (Figure 3B). The emission maximum and lifetime of crystal form for DCPDA are very close to those from isolated



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molecule in organic solvents, demonstrating that this emission originates from isolated molecule in crystal. However, its quantum yield for crystal form (0.12) is much lower than that in solution (0.84), which implies the generation of other forms like dimers or multimers in addition to single molecule in the crystal. The 3D mapping of PL spectra of DCPDA crystal reveals different decay time from 0.0 to 1.6 s after the excitation is stopped (Figure 3C). A very broad emission band ranging from 460 nm to 700 nm is recorded, and this green emission lasts for more than 1.6 s after ceasing the irradiation source. Its time-resolved decay curve (Figure 3D) indicates its ultralong lifetime of 0.42 s, and such a long emission duration is comparable to those of persistent luminescent organic crystals in the literature.12-21 However, the crystals of DCPDP and DCPDB do not exhibit such similar ultralong phosphorescence to DCPDA crystal despite of their almost identical emission maxima at 350 nm and short lifetimes (Figure S11 and S12). The distinct behaviors between DCPDA and the others in long-lived emission excludes the possibility that the ultralong phosphorescence is merely as result of confinement effect induced by crystallization,11 and also demonstrates the key role of small difference in carbon chain of acyl groups among them in crystal engineering.25 In such a series of compounds, weak π-π interaction would dominate the molecular self-assembly from isolated molecules to aggregates during the formation of crystal because of the lack of stronger intermolecular interactions like hydrogen bonding. On the other hand, the bulk effect of substitution might also exert appreciable impact on the molecular packing, and thus large substitutes probably break down the intermolecular π-π interaction dominating the packing pattern. We speculate that π-π interaction among DCPDA molecules mainly contributes to a specific packing pattern like H-aggregates with strong electronic coupling effect, and in such a manner the aggregates in the crystal behave like a supermolecule with unique photophysical properties such as ultralong phosphorescence. The


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Figure 4. Single-crystal structure and molecular packing of DCPDA (A), DCPDP (B) and DCPDB (C).

relatively bulk substitutes in DCPDP and DCPDB leads to another kind of molecular packing style with no significant intermolecular π-π interactions, and thus these molecules in the crystal behave like isolated molecule maintaining the photophysical properties of single molecule. To gain a deep insight into extremely different photophysical behaviors in ultralong emission among these molecules with structural similarity, single-crystal X-ray diffraction technique was exploited to analyze molecular packing patterns in the crystals. By comparing single-crystal structures and molecular packing patterns among DCPDA, DCPDP and DPDB (Figure 4), it is readily noticed that all of them adopt completely different packing patterns. A significant overlap of conjugated structure among DCPDA molecules occurs, and the intermolecular distance between conjugated benzene structures is very short (3.466 Å), providing an implication of strong intermolecular electron coupling for DCPDA. An obvious difference on packing pattern between dimers in solution and dimers in the crystal is that there is only partial overlap of conjugated structure between two close DCPDA molecules. This slight difference might lead to a distinct emission behavior from dimers in solution. In contrast to molecular alignment in DCPDA crystal, no ordered packing pattern is observed in DCPDP crystal due to bulk effect of its acyl group, and the distance between two nearest conjugated moieties is more than 4.3 Å.



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Similar to molecular alignment in DCPDP crystal, large bulk acyl groups in DCPDB molecules dominate the assembly pattern with no significant intermolecular π-π interaction concluded from large distance of 3.9 Å between two benzene parts. In order to further reveal supramolecular behavior of aggregates in DCPDA crystal, density functional theory (DFT) and time-dependent density functional (TD-DFT) calculations were utilized to analyze the photophysical properties of a single DCPDA molecule and two neighboring DCPDA molecules from crystal structure as the models. Natural transition orbitals for a single DCPDA molecule from S0 to S1 (Figure S13) indicate that the two orbitals mainly located on the same conjugated structure consisting of benzene and two cyano groups. Differing from isolated DCPDA molecule, the transition from S0 to S1 is forbidden for DCPDA dimer in H-aggregate manner from its calculated oscillator strength (0.000), and thus emissive transition from S1 to S0 is also forbidden, which is consistent with experimental observation of lacking fluorescent emission from aggregates in the crystal. This transition involves two couples of natural transition orbitals (Figure S14), and both molecules contribute to these natural transition orbitals behaving like a single molecule. The relatively high value of oscillator strength (0.0872) for S0S2 transition indicates the electron is allowed to promoted to S2 state, and provides the opportunity to release phosphorescence via intersystem crossing process. Two couples of natural transition orbitals are composed of molecular orbitals from two DCPDA molecules (Figure S15), and these overlaps in molecular orbitals represent strong electronic coupling between two molecules, allowing the two molecules functioning as a supermolecule. As a result, from the demonstration of the preceding two molecules as an example, it is reasonably deduced that ordered aggregates consisting with a specific number of single DCPDA molecule can behave like a supermolecule via strong electronic coupling among conjugated structures, and these supramolecular aggregates are


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Figure 5. (A) PL spectra of DBPDA in different solutions (80.0 μM). (B) PL spectra of DBPDA in crystal form. Inset: Luminescent image of a Br character consisting of DBPDA crystal when excited by UV light. (C) Time-resolved luminescence decay curve of DBPDA crystal. (D) Single-crystal structure of DBPDA.

responsible for the occurring of such an ultralong phosphorescence. Although DCPDA crystal shows ultralong phosphorescence phenomenon lasting for several seconds, its emission efficiency is extremely low, typically lower than 1%. In order to greatly promote phosphorescence quantum yield, we attempted to introduce heavy atoms into DCPDA structure, and thus we synthesized 2,5-dibromo-1,4-phenylene diacetate (DBPDA) crystal. DBPDA has a similar fluorescence emission at 350 nm in acetonitrile to that of DCPDA (Figure



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5A), and the emission from J-aggregates in mixed water-acetonitrile solution is also detected, but their quantum yields are very low in comparison with those of DCPDA. However, the crystal of DBPDA shows a very intense green phosphorescence with an emission maximum of 510 nm (Figure 5B), and the emission efficiency is determined to be 0.17, which is much greater than that of DCPDA. The phosphorescence duration is not so long that it is can readily observed by naked eyes, but its lifetime is also up to 9.0 ms determined from its time-resolved decay curve (Figure 5C). This observation is consistent with the previous finding that the introduction of heavy atoms generally lead to the increase of emission efficiency but the decline in phosphorescence lifetime due to the enhancing intersystem crossing rate. Through analyzing molecular packing pattern in DBPDA crystal, it is easily noted that DBPDA molecules are arranged in a typical H-aggregate packing pattern, and strong electronic coupling among DBPDA molecules takes place from the short interlayer distance of 3.71 Å (Figure 5D). The large difference in triplet lifetime and quantum yield between DCPDA and DBPDA is as the result of their different kinds of natural transition orbitals. Ultralong luminescence of DCPDA is due to low intersystem crossing rate induced by π  π* transition, whereas bright emission of DBPDA originates from high intersystem crossing rate caused by n  π* transition with the involvement of bromine atoms. In summary, a new design principle for achieving phosphorescent organic materials with ultralong emission via strong electronic coupling among conjugated structures in the crystal was proposed, and such an ultralong emission can be controlled by changing substituent in steric hindrance. Intermolecular π-π electronic coupling dominates the specific packing pattern in the crystal without the occurrence of bulk substituents, and is responsible for the occurring of ultralong phosphorescence. Such a strong electron coupling among molecules makes ordered


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aggregates in the crystal behave like a supermolecule exhibiting its specific photophysical properties. Different packing patterns between two identical molecules determine their distinct emission behaviours due to their contrasting supramolecular structures. The introduction of heavy atoms like bromine allows the emission efficiency of phosphorescence greatly improved due to heavy atom effect. This work demonstrates an example of pure organic materials with ultralong phosphorescence by tuning substituent, reveals the key role of strong intermolecular electronic coupling for persistent luminescence, and further clarifies the design principle of organic materials with persistent luminescence via crystallization. The proposal of this general design principle for ultralong organic phosphorescence will greatly promote the applications of these afterglow materials in newly-emerged technologies such as emergency lighting, displays, anti-counterfeiting and optical sensing and imaging.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website. PL spectra, NBO analysis, NMR spectra and MS spectra.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel: +86-579-82282269. Fax: +86-579-82282269. ORCID Zhaosheng Qian: 0000-0002-2134-8300 Hui Feng: 0000-0002-1906-0949 Author Contributions 15 ACS Paragon Plus Environment


†

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These authors contributed to this work equally.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT We gratefully acknowledge financial support from the National Natural Science Foundation of China (Grant Nos. 21675143, 21775139 and 21775138) and Natural Science Foundation of Zhejiang Province (Grant No. LR18B050001 and LY17B050003).

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