Letter Cite This: J. Phys. Chem. Lett. 2018, 9, 3808−3813
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Biluminescence via Fluorescence and Persistent Phosphorescence in Amorphous Organic Donor(D4)−Acceptor(A) Conjugates and Application in Data Security Protection Harsh Bhatia, Indranil Bhattacharjee, and Debdas Ray* Department of Chemistry, Shiv Nadar University, NH-91, Tehsil Dadri, District Gautam Buddha Nagar, Uttar Pradesh 201314, India
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ABSTRACT: Purely organic biluminescent materials are of great interest due to the involvement of both singlet and long-lived triplet emissions, which have been rarely reported in bioimaging and organic light-emitting diodes. We show two molecules 3,4,5,6-tetraphenyloxy-phthalonitrile (POP) and 3,4,5,6-tetrakis-p-tolyloxy-phthalonitrile (TOP), in which POP was found to exhibit fluorescence and persistent roomtemperature green phosphorescence (pRTGP) in the amorphous powder and crystal states. Both POP and TOP show aggregation-induced emission in a tetrahydrofuran− water mixture. We found in single-crystal X-ray analysis that intra- and intermolecular lp(O)···π interactions along with π(C = C)···π(CN), hydrogen bond (H−B), and C−H···π interactions induce a head-to-tail slipped-stack arrangement in POP. In addition, the X-ray structure of TOP with a slipped-stack arrangement induced by only π(CC)···π(CN) and H−B interactions shows dim afterglow only in crystals. These indicate that more noncovalent interactions found in POP may reinforce relatively efficient intersystem crossing that leads to pRTGP. Given the unique green afterglow feature in amorphous powder of POP, document security protection application is achievable.
O
proximity (ΔES1−T1) of the singlet (S1) state and triplet manifolds (Tn) are required to observe the RTP feature.4,16,17 Because ΔES1−T1 is inversely proportional to the perturbation factor (δ) that describes SOC,21 it turns out that the δ increases with a greater value of the spin−orbit matrix element induced by heavy atoms5,14 and molecular distortion22,23 and lowering of ΔES1−T1. The ΔES1−T1, on the other hand, can be controlled by aggregation24 as well as angular orientation of the donor and acceptor.25,26 In recent studies, afterglow emission has been realized mainly through ordered packing in the crystals that favor exciton migration.27−29 Moreover, recently rigid-host matrixes15,30−32 that reduce nonradiative pathways due to oxygen diffusion and molecular vibrations have also been employed to observe persistent RTP (pRTP). However, these studies impede the practical utility due to difficult processing and the selective nature of the host materials. Therefore, if pure organic amorphous materials33 with fluorescence and RTP features could be obtained, the materials would create new opportunities in photophysical research due to the harvesting of both singlet and triplet excited states.1,4 Here, we report two donor(4)−acceptor (D4−A)-based molecular systems (3,4,5,6-tetraphenyloxy-phthalonitrile (POP) and 3,4,5,6-tetrakis-p-tolyloxy-phthalonitrile (TOP)) that focus on rigidity in accordance with the charge transfer (CT) state and aggregation for optimization of the energy gap
rganic light-emitting materials having biluminescence (BL) through fluorescence (singlet) and phosphorescence (triplet) appear to be of great importance for applications of display devices1−3 and imaging4 due to the involvement of both singlet and long-lived triplet emission. The potential advantages of having a large exciton dynamic range extended up to 9 orders of magnitude between nanosecond-lifetime and millisecond-lifetime and the low cost make BL materials promising as single-component light sources. It has been a long-held notion that dual emission through fluorescence5−9 has served as important photophysics for designing chemical systems optimized for sensors and white organic light-emitting diodes (WOLEDs). Recently, organic dual room-temperature phosphorescence (RTP)10 systems were developed by modifying π-extended dibenzothiophene with a carbonyl group (CO) and heavy halogens that could generate efficient white emission. On the other hand, to observe RTP, the control of triplet excitons is important to enhance their utility. For this purpose, several molecular design strategies including functionalization of an aromatic molecular backbone by aldehyde/carbonyl groups11−13 and a heavy halogen14 to reinforce spin−orbit coupling (SOC) and utilization of crystallization,12 deuterium substitution,15 organic host−guest systems,16,17 carbon dots dispersed into a matrix,18 and aggregates19,20 to suppress the nonradiative transition have been adopted. However, the key issue of the purely organic RTP emitters having an afterglow emission feature in the amorphous powder at ambient conditions is ill understood. Ideally, molecular rigidity as well as energetic close © 2018 American Chemical Society
Received: May 17, 2018 Accepted: June 25, 2018 Published: June 25, 2018 3808
DOI: 10.1021/acs.jpclett.8b01551 J. Phys. Chem. Lett. 2018, 9, 3808−3813
Letter
The Journal of Physical Chemistry Letters
crystal X-ray analysis of both compounds reveals that relatively stronger intramolecular lp(O)···π and intermolecular π(C C)···π(CN), lp(O)···π interactions in POP lead to an increase in rigidity that suppresses nonradiative pathways effectively. The pRTP feature of POP in amorphous powder is utlized to show the patterned security feature of 4, which is readily visualized by switching off the UV lamp (365 nm). Thus, we have developed new D4−A molecular systems with increased rigidity and simultaneous fluorescence and pRTGP characteristics in amorphous powder and crystals. The targeted compounds were synthesized by a nucleophilic aromatic substitution reaction of 3,4,5,6-tetrafluorophthalonitrile with sodium salt of phenol and cresol in dry DMF at room temperature (Scheme S1). We chose a nucleophilic aromatic substitution (SNR) reaction as the synthetic method due to its modular and inexpensive nature, leading to convenient preparation of the D4−A systems. Both compounds were charaterized by NMR spectroscopy, MALDI spectrometry, and X-ray analysis (see the Supporting Information). The absorption characteristics in dilute toluene solution (10−5 M) are very similar for both POP and TOP, with absorption at 280 and 320 nm, which could be assigned to the π−π* and ICT transitions,37 respectively (Figure S3). Photoluminescence (PL) spectra of both compounds (λex = 320 nm) in the same solvent show weakly emitting (PL quantum efficiency, PLQY = ∼1%) single broad peaks at 383 and 401 nm, respectively. However, in CH3CN, a new intense band at 468 nm along with the locally excited (LE) band (390 nm) is observed for POP (Figure S4a). Further increase in solvent polarity leads to a decrease in emission intensity of the λ468, suggesting intramolecular exciplex formation38−42 due to the absence of excimer formation seen for POP at the same concentration under identical conditions. In the case of TOP, a bathochromic shift with increasing solvent polarity indicates CT character of the excited state37(Figure S4b). In parallel, PL spectra were recorded at different THF− water fractions ( f w). Interestingly, the broad emission band at
between the singlet (S1) and lowest triplet (T1) state by covalently attaching four precisely oriented phenoxide donors (−OPh, −OPhMe) in an alternative array to a phthalonitrile acceptor to obtain a highly rigid geometry (Scheme 1). We Scheme 1. Molecular Structures of POP and TOP
hypothesized that incorporating a CT state into the molecular backbone could act as a mediator between the S1 and T1 to reduce the ΔEST for the observation of RTP. Unlike our earlier work34 with a D−A conjugate, which was focused on dual light emission through thermally activated delayed fluorescence and RTP, the new systems presented here are designed to explore new molecular architectures to create highly energy-efficient light sources via fluorescence and RTP. Photophysical measurements of POP show BL via fluorescence and persistent room-temperature green phosphorescence (pRTGP) with Commission Internationale de l’Eclairage (CIE) coordinates of (0.22, 0.30) in powder. In addition, the BL of POP is accompanied by dual fluorescence and dual phosphorescence features in both powder and crystals. Furthermore, intermolecular aggregation occurs in tetrahydrofuran−water mixtures (THF:H2O), leading to aggregate-induced emission (AIE)35,36 behavior in both compounds. Fluorescence and phosphorescence decay transient measurement of POP at RT shows BL with average lifetimes of 7.31 ns, 7.46 ns, 97.22 ms, and 123.64 ms in powder, while lifetimes of 3.93 ns and 56.62 ms were observed in amorphous powder of TOP. Single-
Figure 1. AIE of (a) POP and (b) TOP in THF−H2O mixtures at a concentration of 1 mM (λex = 350 nm). Steady-state and phosphorescence emissions of (c) POP and (d) TOP in amorphous powder and crystals at RT. Phosphorescence decays of POP in (e) amorphous and (f) crystal states. The inset shows average lifetimes of POP at λ461 in amorphous powder and crystals. 3809
DOI: 10.1021/acs.jpclett.8b01551 J. Phys. Chem. Lett. 2018, 9, 3808−3813
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The Journal of Physical Chemistry Letters Table 1. Photophysical Parameters of POP and TOP fluorescence sample POP (powder) POP (crystals) TOP (powder) TOP (crystals) POP (aggregates) TOP (aggregates)
λFl (nm)
phosphorescence
τavg (ns)
ΦF (%)
λPhos (nm)
± ± ± ± ± ±
9.66
461 520 461 520 473 416 473 -
457 500 457 500 437 457
7.31 7.46 7.16 7.40 3.93 3.64
0.01 0.02 0.01 0.06 0.13 0.12
402 453 440
1.09 ± 0.15 7.90 ± 0.07 4.43 ± 0.21
16.08 2.45 4.04 -
τavg (ms) 97.22 123.64 197.87 253.2 56.62 290.28 247.71 -
± ± ± ± ± ± ±
0.4 2.54 2.41 1.39 1.13 7.09 2.5
ΦP (%) 1.44 5.62 0.07 0.54 -
Phosphorescence analysis of POP in amorphous powder under ambient conditions (λex = 365 nm) shows two RTP peaks at 520 nm (τavg = 123.64 ms) and a broad peak at 415− 470 nm (τavg = 97.22 ms) (Table 1, Figure 1c,e). The extended lifetime confirms a pRTP feature that can be directly observed with the naked eye (web-enhanced object 1). Similar band features along with a relatively long green afterglow (λem ≈ 415−470 nm, τavg = 197.87 ms; λem = 520 nm, τavg = 253.2 ms) were measured in the crystals (PLQY = 5.62%) (Figure 1c,f, Tables 1 and S2). At 77 K, the vibrational feature of the λ520 is recorded (Figure S8a). However, the former band at ∼415− 470 nm is completely quenched. Temperature-dependent studies (77−273 K) indicate the thermally activated emission feature of the broad band (Figure S10). When we compared these results with the AIE behavior in the THF−H2O (90%, v/ v) mixture, we came to the conclusion that peaks at 457 nm in powders and crystals originate from the same species (exciplex) present in the solvent mixture. To test the origin of the λ520 in solids as well as in crystals, emission studies of drop-casted films (toluene, 0.4−20.0 mM) were undertaken. The appearance of the λ520 including other bands at 415−470 nm with increasing concentration of POP confirms that the origin of λ520 is a result of the inhomogeneity of the aggregates present in amorphous powder (Figure S11). Similarly, phosphorescence measurements of TOP in powder at RT show a broad emission feature (λem = 473, τavg = 56.62 ms) (Figures 1d and S12a, Tables 1 and S2). In crystals, the average lifetime at λ416 is found to be 290.28 ms, while at 473 nm, 247.71 ms is recorded, indicating an afterglow material due to rigid packing of the crystals (PLQY = 4.58%) (Table 1, Figure S12b,c). However, the lack of pRTP in the amorphous powder could be due to different molecular arrangement that activates efficient nonradiative pathways at RT. Moreover, low ΔEST values24−26 (0.03, 0.0098 eV) of POP (Figure S13) might play an important role to harvest both singlet and triplet states via BL (Figure 2). Single-crystal X-ray analysis (Figure 3a) of POP reveals that three adjacent phenyl rings lie below the plane with respect to the phthalonitrile core with torsions of −56.74, −55.29, and −14.64° when viewed along the C(3)−O(1)−C(9)−C(10), C(4)−O(2)−C(15)−C(16), and C(5)−O(3)−C(21)−C(22) atoms, respectively, while the fourth phenyl ring is oriented above the plane of the core moiety with torsion of 153.56° when viewed along the atoms of C(6)−O(4)−C(27)−C(28) (Figure S14a, Table S3). In addition, multiple intramolecular interactions (i.e, lp(O)···π, 2.71−3.03 Å) among the lone pairs of oxygen atoms (O1−O4) to the adjacent phenyl rings and respective nitrile groups were observed. These intramolecular
400 nm is bathochromically shifted to 462 nm (Figure 1a) with gradually reduced intensity through increasing concentration of water up to 50% (v/v) to the THF solution. Further increase of the water content (90%, v/v) leads to a ∼3.5-fold increase in emission intensity with a red-shifted broad-band feature (402 nm, τavg = 1.09 ns; 453 nm, τavg = 7.90 ns) (Figure S5a,b). At this stage both parent LE and exciplex bands appeared with increased intensity. The abrupt increase in the intensity of the ∼400 nm emission band at 90% water content is the result of restricted motion of the phenoxy groups due to aggregation.43 Interestingly, the lifetimes of the bands (∼400− 453 nm) increase from 3.63 to 7.90 ns (Figure S5, Table S1) with increasing water content (10−90%), which suggests that the excited states are stabilized due to aggregate formation.43 Likewise, TOP shows a similar AIE property without having dual emission features (λem = 439 nm, τavg = 4.43 ns) (Figures 1b and S5c). The initial bathochromic shift with increasing solvent polarity in both luminogens can be rationalized by stabilization of the CT excited states, while hypsochromic shifts of the bands are observed with higher water content (70%, v/v) that causes aggregation.34,35 The transmission electron microscopy (TEM) measurements show that the average sizes of the aggregates of POP and TOP were about 282 nm (Figure S6) and 12 μm (Figure S6), confirming aggregate formation that causes AIE. In addition, no diffraction pattern was observed in TEM analysis, which further ensures the amorphous nature of the aggregates. The steady-state PL of POP in the amorphous powder under ambient conditions shows dual peaks at 457 (τavg = 7.31 ns) and ∼500 nm (broad, τavg = 7.46 ns) (Figure S7) with CIE coordinates of (0.22, 0.30) and a PLQY of 11.1% (Figure 1c,e). These emission peaks can be assigned as CT (exciplex) and π−π* character due to the presence of the broad-band feature of the former peak and the characteristic vibrational feature of the latter peak in the low-temperature (77 K) measurements (Figure S8a). In crystals, PL measurements exhibit similar emission features with CIE coordinates of (0.24, 0.37), while a ∼5 nm red shift of the latter broad peak along with increased intensity and sharp band features are also seen, which indicates an ordered local environment due to molecular packing (Figure 1c). Likewise, in TOP, a single broad emission peak at 437 nm (τavg = 4.22 ns, Figure S7e) with a PLQY of 2.52% was observed in powder (Table 1, Figure 1d). Lowtemperature measurement shows a ∼40 nm bathochromic shift and broad-band feature of the λ473 nm (CT) with weakly intense LE nature of a new band at 416 nm (Figure S9). Similar features were also observed in the crystals (Figure S9). 3810
DOI: 10.1021/acs.jpclett.8b01551 J. Phys. Chem. Lett. 2018, 9, 3808−3813
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two dissimilar π(C2C3)···π(C8N) interactions (3.706(6), 3.635(6) Å) is found along the a-axis. Moreover, relatively longer intramolecular interactions among O1−O4 and adjacent phenyl and nitrile groups (2.747(5)−3.548(5) Å, Figure 3b) and the H−B interaction (N2···H10−C10, 2.742 Å, 171.15°; N1···H33−C33, 2.716 Å, 151.39°, Figure S15b) among adjacent molecules are also found. From comparison of the two X-ray structures, it is observed that shorter intramolecular lp(O)···π interactions and a supramolecular ordered slipped-stack arrangement via dissimilar intermolecular interactions (π(C = C)···π(C ≡ N), lp(O)···π) along with H-bonds in POP make it a special one that restricts nonradiative pathways due to increased rigidity. We anticipate that stronger and an increased number of n−π and π(C C)···π(CN) interactions may reinforce intersystem crossing (ISC) and stabilize the triplet states that lead to pRTP, which is in good agreement with the fact the pRTP emission is more intense for POP compared to that for TOP. The intermolecular π···π interactions between two core phthalonitriles (centroid to centroid) in POP were found to be 4.274 and 5.461 Å, respectively (Figure S16a), while TOP shows relatively even longer distance (5.724 Å) (Figure S16b), which remove the possibility of pRTP due to the effect of π···π interaction.44 The pRTP of the POP molecule is evident in its amorphous powder (Figure S17) in air, making it a promising smart material for use in document security protection (Figure 4, web-enhanced objects 1 and 2). The original security pattern of “8” was made of POP with pRTP and amorphous powder of CQ34 without the pRTP property (Figure 4a). After excitation by a UV lamp, a bluish white pattern “8” was observed. Then switching off the lamp, the green “4” pattern, encrypted by POP with pRTP, could be readily visualized, while the other parts disappeared due to the absence of the pRTP property of
Figure 2. Jablonski diagram for the BL process of POP.
noncovalent forces served as the increased rigidity that restricts intramolecular motion in solids, which is fully consistent with the hypothesis. In addition, these findings further substantiate the formation of an exciplex upon PL measurement due to the close proximity of the three adjacent phenyl rings that lie below the plane of the phthalonitrile backbone. Furthermore, two similar π(C1C6)···π(C7N) interactions (3.416(2) Å) form a slipped-stack packing with head-to-tail arrangement that further extends using two new n···π(phthalonitrile) interactions (lp(O2)···π, 3.256(2) Å). Because of these interactions, POP crystallizes into a rectangular structure through head-totail slipped-stack packing, as seen along the b-axis (Figure 3a). In addition, three intermolecular hydrogen bonds (H−B) (O1···H32−C32, 2.470 Å, 171.88°; N2···H30−C30, 2.749 Å, 137.05°; N1···H22−C22, 2.635 Å, 170.74°) among neighboring molecules were also seen (Figure S14b). In TOP, all of the phenyl rings are oriented alternatively below and above the plane of the core moiety with torsions of −32(6), 155.20(4), −15.58(5), and 137.76(4)° when viewed along the same atoms (Figure S15a, Table S3). The alternative arrangement of the phenyl rings leaves the TOP molecules far apart from each other, although a head-to-tail slipped-stack arrangement with
Figure 3. Thermal ellipsoid plots, intramolecular interactions, and intermolecular packing interactions of (a) POP and (b) TOP. The phenyl substituents and their protons are removed for the sake of clarity. 3811
DOI: 10.1021/acs.jpclett.8b01551 J. Phys. Chem. Lett. 2018, 9, 3808−3813
The Journal of Physical Chemistry Letters
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Letter
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Debdas Ray: 0000-0002-6169-8823 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS D.R. is grateful to the Science & Engineering Research Board (SERB) (File No: SB/EMEQ-004/2014 & YSS/2014/ 000923), DST, and Shiv Nadar University (SNU) for generous support. H.B. and I.B. thank SNU for a fellowship.
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Figure 4. Number “8” pattern (UV ON) made of (a) POP (powder) and CQ (powder) and (b) TOP (crystals) showing an afterglow “4” pattern when the UV lamp (365 nm) was switched off (web-enhanced objects for POP and TOP).
the CQ in amorphous powder. A similar pattern was achieved when pattern “8” was made of crystals and powder of TOP (Figures 4b and S17). In summary, two D−A conjugates of POP and TOP have been synthesized, which demonstrate different phosphorescence properties at RT in amorphous powder. Combining experimental and single-crystal analysis, we concluded that multiple effective intramolecular lp(O)···π interactions as well as intermolecular noncovalent forces (π(CC)···π(CN), lp(O)···π) between two neighboring molecules might reinforce SOC for highly efficient green afterglow phosphorescence of POP in amorphous powder. Our results demonstrate that the molecular rigidity and an imposed CT state inducing low ΔEST play crucial roles to observe BL. Given the pRTP property of POP in amorphous powder, a simple pattern for data encryption was demonstrated under a UV lamp. This study offers an avenue to design highly efficient BL materials for LEDs, sensors, data security, and bioimaging.
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REFERENCES
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.8b01551. Experimental procedures, NMR spectra, single-crystal Xray diffraction analysis, UV−vis absorption spectra, steady-state emission, time-correlated single-photon counting (TCSPC) measurement, and phosphorescence measurement (PDF) Crystallographic information for POP (CIF) Crystallographic information for TOP (CIF) W Web-Enhanced Features *
Web-enhanced objects for POP and TOP are available in the online version of the paper. 3812
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DOI: 10.1021/acs.jpclett.8b01551 J. Phys. Chem. Lett. 2018, 9, 3808−3813
