Photophysical Tuning of Organic Ionic Crystals from Ultralong

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Spectroscopy and Photochemistry; General Theory

Photophysical Tuning of Organic Ionic Crystals from Ultralong Afterglow to Highly Efficient Phosphorescence by Variation of Halides Guilin Chen, Hui Feng, Feifei Feng, Pengfei Xu, Jing Xu, Saifei Pan, and Zhaosheng Qian J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b02742 • Publication Date (Web): 17 Oct 2018 Downloaded from http://pubs.acs.org on October 18, 2018

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The Journal of Physical Chemistry Letters

Photophysical Tuning of Organic Ionic Crystals from Ultralong Afterglow to Highly Efficient Phosphorescence by Variation of Halides Guilin Chen,† Hui Feng,† Feifei Feng, Pengfei Xu, Jing Xu, Saifei Pan 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].

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ABSTRACT: Manipulation of photophysical properties of pure organic materials via simple alteration is attractive but extremely challenging because of the lack of valid design strategies for achieving ultralong afterglow or efficient room-temperature phosphorescence. Herein, we report a first photophysical manipulation of organic ionic crystals from ultralong afterglow to highly efficient phosphorescence by variation of halides in the crystals. Crystal structural analysis reveals ultralong organic afterglow of tetraphenylphosphonium chloride (TPP Cl) is promoted by strong intermolecular electronic coupling in crystal, and theoretical analysis demonstrates that the tremendous boost of the phosphorescence of tetraphenylphosphonium iodide (TPP I) is caused by the coupling effects of significant heavy atom effect from iodine atoms and a small energy difference between the first singlet and triplet states. This work contributes to regulate long-lived emissive behaviors of pure organic ionic crystals in a controlled way, and will promote the development of optical switch controlled by external stimuli.

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Design and synthesis of efficient phosphorescent materials have been emerging as a central and urgent task due to their tremendous applications in energy and life sciences.1,2 Roomtemperature phosphorescence has been long considered as one exclusive feature of metalcontaining compounds owing to specific strong spin-orbit coupling.3 It is very challenging to achieve efficient room-temperature phosphorescence of pure organic molecules because it is generally forbidden for electronic transitions from excited singlet states to triplet states.4 Two dominant strategies were proposed to overcome this problem by enhancing intersystem crossing rate.5 One approach makes use of varying molecular structures to adjust the characteristics of molecular orbitals and the excitation configuration in the excited state,6 while another general principle is to incorporate bromine atoms into organic structures, and heavy atom effect thus facilitates intersystem crossing.7 In terms of the dependence of spin-orbit coupling on atomic number, the introduction of halogens with a larger atomic number would be more efficient to acquire bright phosphorescence of organic molecules. However, only very few examples with inclusion of iodine atoms have been reported till now despite it is a promising way to expand highly emissive organic phosphors.8 It is indispensable to stabilize triplet excitons reached by intersystem crossing to attain highly efficient phosphorescence emission.9 One major means is to embed the luminogens into a host material such as polymers, supermolecules and rigid alkanes, but emission duration of these materials is generally very short.10 In contrast to host-guest based strategy, crystallizationinduced phosphorescence (CIP) shows better performance in ultralong afterglow and wider choice of luminogens.11 Intermolecular electronic coupling in the crystals promotes the generation of ultralong phosphorescence, but most crystals show only observable afterglow with extremely low quantum yields except for a couple of exceptions.12 However, it is very



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challenging to switch ultralong afterglow to bright phosphorescence in a facile way due to their distinct generation mechanisms. Quaternary phosphonium salts are a fundamentally significant class of organophosphorus compounds, and have widely been exploited as valuable arylating reagents13 and powerful chiral phase-transfer catalysts.14 In this contribution, we discover distinct photoluminescence behaviours of tetraphenylphosphonium (TPP) halides depending on different halogens, and demonstrate photophysical turning of these organic ionic crystals from ultralong afterglow to highly efficient phosphorescence by halides. An observable ultralong-lived afterglow is found for TPP fluoride (TPP F) and TPP chloride (TPP Cl) while TPP iodide (TPP I) shows an ultrabright blue phosphorescence with a relatively short lifetime in microseconds. The underlying nature was investigated using structural analysis and theoretical computations. Ultralong-lived emissions for TPP F and TPP Cl are attributed to their strong intermolecular electronic coupling, and significant heavy atom effect from iodine atoms and a small energy gap between S1 and T1 are responsible for the highly efficient phosphorescence of TPP I. These principles contribute to a new valuable design strategy for controlling photoluminescence properties and achieving ultralong afterglow or efficient phosphorescence, and the specific ionic interaction-induced phosphorescence provides a controllable platform by external stimuli. TPP fluoride (TPP F) is prepared using the reaction of silver fluoride and TPP chloride (TPP Cl) in water, and characterized by 19F NMR (Figure S1). All the TPP halides are readily acquired in the crystalline state and carefully purified by multiple recrystallization. Purity analyses were further performed using high-performance liquid chromatography, and determined purity for each of them is above 99.0% (Figure S2), which ensures high purity of these crystals. TPP F, TPP Cl and TPP Br can dissolve well in many solvents such as water, ethanol, THF, acetonitrile,


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Figure 1. (a) PL spectra of tetraphenylphosphonium fluoride (TPP F) crystals. (b) PL spectra of tetraphenylphosphonium chloride (TPP Cl) crystals. (c) Time-resolved decay curves of TPP F crystal. (d) Time-resolved decay curves of TPP Cl crystal. (e) Ultralong afterglow of TPP Cl crystals after ceasing the UV light.

DMF and DMSO, but TPP I is only soluble in acetonitrile, DMF and DMSO. Their PL spectra in these solvents (Figure S3) show that all of them exhibit very weak emissions located at 400 nm in dissolved state at room temperature. Low-temperature experiments were further performed to



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explore their phosphorescence behaviors in the range of 77 K – 280 K (Figure S4). It is found that a new emission at 480 nm appears for TPP F, TPP Cl or TPP Br at low temperature, and this emission is gradually enhanced as the decrease of the temperature. A similar increase trend at 470 nm emission was also observed for TPP I, but its PL intensity is much brighter than the others. It is deduced from these observations that these new emissions belong to long-lived phosphorescence, and significant heavy atom effect from iodide boosts the phosphorescence of TPP I at low temperature. It is surprising that the four TPP halides show extremely distinct photoluminescence behaviors in the solid state depending on different halide ions when excited by UV light. Both TPP F and TPP Cl crystals possess observable afterglow phenomenon after ceasing the UV excitation, but their emission colors are apparently different. Figure 1a and 1b display PL spectra of TPP F and TPP Cl and their fluorescence images. Cyan emission is observed for TPP F crystals, which is consistent with its major emission band at 480 nm, but a small emission around 400 nm is also recorded. Time-resolved PL decay curves at 400 nm (Figure S5) and 480 nm (Figure 1c) suggest the two emissions belong to fluorescence and phosphorescence, respectively, in term of their huge difference in lifetime from the fact that 1.6 ns is for short-lived emission at 400 nm while 9.8 and 103.3 ms for long-lived emission at 480 nm. Time-delayed PL spectra of TPP F crystals in Figure S6 further confirm the great difference between the two emissions because the fluorescence at 400 nm completely disappears after 0.05 ms delay while the phosphorescence still remains even after 50.0 ms delay. The ultralong-lived component enables us to observe an instant afterglow for TPP F by naked eyes after ceasing the excitation light, but the green afterglow for TPP Cl can last for more than two seconds because of its longer triplet lifetime at 500 nm (253.2 ms) according to its time-resolved PL decay curves in Figure 1d and 1e. In contrast to TPP F, the component from the short-lived emission at 400


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Figure 2. (a) PL spectra of tetraphenylphosphonium bromide (TPP Br) crystals. Inset: fluorescence image. (b) Time-resolved decay curves of TPP Br crystal. (c) PL spectra of tetraphenylphosphonium iodide (TPP I) crystals. Inset: fluorescence image. (d) Time-resolved decay curves of TPP I crystal.

nm for TPP Cl (Figure S7) is largely lowered in comparison to the long- lived emission at 500 nm. Time-delayed PL spectra (Figure S8) indicate the appreciably different emission durations for short- and long-lived emissions of TPP Cl crystals and a longer afterglow compared to TPP F. As shown in Figure 2a, TPP Br exhibits a brighter green luminescence at 486 nm than TPP Cl, but its afterglow cannot be observed by naked eyes. The lifetimes of TPP Br from time-resolved PL decay curves in Figure 2b are significantly shortened to 1.5 ms (68%) and 15.7 ms (32%), and the short-lived emission almost completely disappears. Time-delayed PL spectra (Figure S9) further verify the absolutely dominant emission at 486 nm from TPP Br. Differing with the



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others, TPP I emits a bright blue emission at 430 nm under the excitation of UV light (Figure 2c). The most impressive for TPP I is its extremely bright blue phosphorescence, and its quantum yield is determined to be 0.42 using an absolute method (Table S1). Such a high phosphorescence emission efficiency is very rare, and only two examples including 1,2,3,4,5,6hexakis(arylthio)- benzenes12b and 1,4-dibenzoyl-2,5-bis(siloxy) -benzenes12d with higher quantum yields have been reported till now. In comparison with the others, the emission maximum of TPP I is appreciably blue-shifted to 430 nm, and its lifetime is largely shortened to 9.9 μs (Figure 2d). This short emission duration in microsecond is also supported by timedelayed PL spectra of TPP I in Figure S10. It is reasonable to attribute these precious photoluminescence behaviours to strong ionic interactions between tetraphenylphosphonium and halides because almost no PL emission can be detected for their solutions at room temperature. To

probe

the

underlying

nature

of

distinct

photoluminescence

behaviors

of

tetrephenylphosphonium halides, we first examined their crystal structures reported in the literature except TTP F.15 From their crystal structures in Figure 3, it is readily noted that the distances between two phosphorous atoms in tetraphenylphosphonium moieties are gradually increased from 7.72 Å to 9.16 Å as the change of anions in the order of Cl, Br and I, and the same increase trend is also found for the distance between two neighboring phenyl moieties in the crystals. The close proximity to each other in TPP Cl crystal implies a strong electronic coupling between two TTP Cl molecules, and such a significant electronic coupling is responsible for the generation of ultralong phosphorescence in seconds.16 For TTP Br and TPP I, the large atomic volumes of bromine and iodine atoms force the two adjacent TPP parts to be separated in comparison with TPP Cl, and consequently weaker intermolecular electronic coupling results in the decrease or disappearance of ultralong-lived emissions. It is generally


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Figure 3. Crystal structures of TPP Cl (CCDC No. 1146808), TPP Br (CCDC No. 1280681) and TPP I (CCDC No. 1255406).15

known that heavy atom effect greatly promotes intersystem crossing rate and boosts the phosphorescence emission due to a strong spin-orbit coupling from heavy atoms like bromine.4 Thus, apparent phosphorescence enhancement of TPP Br and TPP I relative to TPP Cl follows this general principle. Table S1 shows that the quantum yield of TPP Br (0.04) is doubled that of TPP Cl (0.02), but more than 20 times PL enhancement is accomplished for TPP I, demonstrating much greater heavy atom effect of iodine than bromine. To further support the aforementioned hypothesis for ultralong organic phosphorescence and efficient

room-temperature

phosphorescence,

we

investigated

another

series

of

methyltriphenylphosphonium (MTPP) halides with the replacement of a phenyl group by a



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methyl group. As shown in Figure S11, very similar photophysical behaviors of MTPP halides to TPP halides are observed with one exception for methyltriphenylphosphonium fluoride (MTPP F). MTPP F exists as an ionic liquid at room temperature, and only shows a bright blue fluorescence at 400 nm with a short lifetime of 1.5 ns (Figure S12). The disappearance of afterglow in liquid MTPP F relative to solid TPP F illustrates the requirement of solid state to retain the phosphorescence at room temperature. The emissions of MTPP Cl are composed of a singlet emission and an ultralong emission (Figure S13), which are almost same to those of TPP Cl. The change in lifetime from 65.0 ms to 3.4 μs for MTPP Br and MTPP I is also consistent with the variation trend for TPP Br and TPP I (Figure S14 and S15), and a much greater emission efficiency for MTPP I (0.26) is also observed. These consistent PL behaviours of MTPP halides to TPP halides further verify the origin of ultralong organic phosphorescence from close contact among molecules in the crystals and efficient RTP with the assistance of heavy atoms. To gain a deeper insight into their unique photoluminescence properties, density functional theory (DFT)17 and time-dependent density functional theory (TD-DFT)18 calculations were performed on TPP cation and TPP halides. NBO analysis of tetraphenylphosphonium cation in Figure S16 indicates that its HOMO is mainly composed of p orbitals (99.6%) of carbon atoms in four benzene moieties while its LUMO consists of p orbitals (7.4%) of the central phosphorous atom and p orbitals (80.4%) of carbon atoms, implying a weak charge transfer from benzene to phosphorous atom. Such a charge transfer trend during S0  S1 electron transition is clearly demonstrated from electron density difference between the first excited state S1 and ground state S0 of TTP cation in Figure 4a. In contrast to TPP cation, a significant charge transfer between TPP cation and halide anion is expected for each of tetraphenylphosphonium


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Figure 4. (a) Electron density difference map between S0 and S1 of TPP cation. (b) Electron density difference map between S0 and S1 of TPP Cl. (c) Electron density difference map between S0 and S1 of TPP Br. (d) Electron density difference map between S0 and S1 of TPP I. Isovalue for density is 0.001.

halides (TPP halides) from their HOMOs and LUMOs in Figure S17 because all HOMOs are mainly located at halide atoms while all LUMOs comprise atomic orbitals of TPP part. Electron density difference maps between S0 and S1 of TPP Cl, TPP Br and TPP I in Figure 4b-d also apparently demonstrate a marked charge transfer character for each TPP halide, and such an efficient charge transfer upon an S-T transition resembles the typical metal-to-ligand CT nature of heavy-metal complexes,19 which suggests halide atoms play an extremely important role to promote the triplet excitons and facilitate the occurring of long-lived phosphorescence.



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According to Fermi Golden Rule, intersystem crossing rate (kISC) can be expressed as the following equation20

k ISC 

2 1  H SO  3  2  FCWD 

where ħ is reduced Plank constant, ψ1 and ψ2 are wave functions of singlet and triplet states, HSO is the spin-orbit coupling (SOC) Hamiltonian, and FCWD represents the Franck-Condonweighted density of states. In terms of the dependence of SOC splitting on nuclear charge power four, a larger atomic number will lead to a greater intersystem crossing rate which interprets the highest phosphorescence quantum yield of TTP I. On the other aspect, we calculated energy gaps between S1 and T1 (∆EST) for TPP, TPP Cl, TPP Br and TPP I (1.566, 0.197, 0.026, and 0.015 eV) which was obtained from calculated excitation energies in Table S2, and found that they follow the opposite order of their atomic numbers. Because the FCWD term is inversely proportional to ∆EST,21 a low energy gap between S1 and T1 states substantially enhances intersystem crossing rate. The lowest ∆EST for TPP I among them further justifies its brightest emission from the triplet state, and such a design principle was also exploited to design ultralong room-temperature phosphorescent organic materials by structural isomerism in a recent paper.22 A combination of heavy atom effect and a small energy gap for ∆EST is first revealed to achieve ultrahigh phosphorescence quantum yields of TPP Cl and MTPP I, and this coupling approach as a general design strategy can greatly expand organic phosphors with bright triplet emission. In summary, distinct photoluminescence behaviours of tetraphenylphosphonium halides are revealed for the first time, and the shift between ultralong afterglow and bright phosphorescence is readily accomplished by varying halides. These results show a close proximity-induced intermolecular electronic coupling is required for the generation of ultralong organic afterglow while a significant heavy atom effect from iodine atoms coupled with a small energy gap 12 ACS Paragon Plus Environment

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between the first singlet and triplet states tremendously boost the phosphorescence. The key role of crystallization is also proved to attain the high emission efficiency from triplet excitons. These principles contribute to a new valuable design strategy for controlling photoluminescence properties and achieving ultralong afterglow or efficient phosphorescence, and the specific ionic interaction-induced phosphorescence provides a controllable platform by external stimuli. This work contributes to design and development of smart materials and phosphorescent sensors and bioimaging.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Zhaosheng Qian: 0000-0002-2134-8300 Hui Feng: 0000-0002-1906-0949 Author Contributions †

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 21705120), and Natural Science Foundation of Zhejiang Province (Grant Nos. LR18B050001 and LY17B050003).



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ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website. Experimental details, computational methods, NMR spectrum, NBO analysis, time-resolved and time-delayed PL spectra, and Cartesian coordinates. Videos showing ultralong phosphorescence of TPP Cl and MTPP Cl.

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