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Article Cite This: ACS Omega 2019, 4, 5052−5063
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Positional Variation of Monopyridyl‑N in Unsymmetrical Anthracenyl π‑Conjugates: Difference between Solution- and Aggregate-State Emission Behavior Moghal Zubair Khalid Baig, Banchhanidhi Prusti, Durba Roy, and Manab Chakravarty* Department of Chemistry, Birla Institute of Technology and Science, Pilani-Hyderabad Campus, Jawahar nagar, Shamirpet Mandal, Hyderabad, Telangana 500078, India
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ABSTRACT: Fluorescence enhancement on aggregation for π-conjugates linked with pyridyl ring has been established as a part of widely studied smart organic functional materials. Therefore, the photophysical features in the solution and aggregate states for such compounds remain impressive. In this work, we synthesized three series of photostable unsymmetrical aryl-substituted anthracenyl π-conjugates linked to pyridyl ring with a variation of the position of a pyridyl-N atom and examined the difference in the photophysical properties preferably in the aggregate state. The so-called “aggregation-induced emission (AIE)” behavior was discernible for the 2- and 4-pyridyl- but not 3-pyridyl-10-p-tolyl or mesitylsubstituted π-conjugates. Curiously, a variation of the position of a pyridyl-N atom does not solely control the AIE phenomenon for 10-thiophenyl-substituted π-conjugates, where all of the isomers are found to be AIE-active. Hence, the dissimilarity in emission behavior in the aggregate state is governed by the position of N-atom for pyridine and also the substituent at the 10th position of the anthracyl ring. The mechanistic insight behind these observations is demonstrated by concentration-dependent fluorescence studies, time-resolved fluorescence, single-crystal X-ray diffraction studies (largely supportive to understand the molecular structure and packing in the aggregate), and average particle size measurement of the aggregates and partly by the density functional theory studies for a few representative molecules.
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INTRODUCTION The evolution in the discovery of organic small molecules as aggregation-induced emission active luminogens (AIEgens) has been continuing and being highlighted in many recent reviews.1 The tremendous applications of such AIEgens in numerous fields, such as optoelectronic materials, optical security, and fluorescence sensing, have made a steady and common platform for researchers from interdisciplinary areas.2 Among many such AIEgens, pyridyl-linked extensive πconjugates could acquire its place as promising AIEgens in the literature.3,4 There are typical and extensively reported AIEgens, such as tetraphenylethene (TPE), that are linked to pyridyl rings and established as potential smart materials and metal sensors.3a,b The significant attractions of pyridine core are mainly due to the freely available lone pair on nitrogen residing at sp2 orbital and strong N···H bonding in the aggregated state resulting many changes in the fluorescence behavior under different external stimuli, such as acid/base, pressure, and temperature.3,4 Apart from TPE, pyridine-linked anthracenyl π-conjugates are well established as useful AIEgens, as shown in Figure 1.4,5 The pyridyl π-conjugate A was blended with tyrosine polymer to afford highly emissive polymeric material5a and was also used as a ligand to make complexes with solvatochromic behavior along with significant photophysical studies.5b,c Further, the symmetrical di-styrylanthracene compound (B) © 2019 American Chemical Society
is quite well recognized as a valuable material, where the pyridyl core played a crucial role6 for the various applied field, as mentioned in Figure 1. Further, our literature search resulted in the molecule of type C that was reported as a patent related to the light-emitting device.7 Notably, the electrondonating substitution (such as −methyl) at the 10th position of the anthracyl ring for A had the advantageous effect for the charge transfer.5d However, the AIE studies were not investigated for such aryl-substituted A. In spite of many disputes, the restricted intramolecular motion in the aggregate state is well established and documented as the main cause of the AIE effect by Prof. Tang and others.8,1c Thus, the influence of large twisting on the system would be beneficial to restrict the intramolecular motion to impose enhanced emission. With this clue, we were earlier successful to generate distinct anthracenyl π-conjugates9 as new AIEgens, including these two pyridyl analogues ATh4P and AT4P9b (Figure 2), where AT4P was established as multiple metal-ion sensors.9c Meanwhile, the literature reports on the substitution10 and regioisomeric effect11 on the AIE behavior due to different molecular packing in the aggregate state and electronic structure prompted us to synthesize a variety of regioisomeric Received: January 6, 2019 Accepted: February 22, 2019 Published: March 8, 2019 5052
DOI: 10.1021/acsomega.9b00046 ACS Omega 2019, 4, 5052−5063
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Article
Figure 1. Reported pyridyl π-conjugates A−C and their applications.
Figure 2. Pyridyl π-conjugates linked to substituted anthracenyl skeleton.
Scheme 1. Synthesis of Anthracene-Based π-Conjugates Linked to Regioisomeric Pyridine
Scheme 2. Formation of Compound ATh2PR and Its Molecular Structure
favor aggregation-caused quenching (ACQ) behavior. Surprisingly, all thiophenyl-substituted pyridyl isomers including ATh3P are recognized as relatively better AIEgen under similar conditions. Such interesting features need explanation. To understand the observed photophysical behaviors, the molecular structures were determined for selected molecules by single-crystal X-ray diffraction studies to find the supramolecular interactions in the aggregates. The observed facts are explained through the concentration-dependent Fl. studies, measurement of excited-state lifetime, and aggregate particle size along with the electronic structure of selected molecules.
pyridyl compounds (Figure 2) and explore their photophysical behaviors in both solution and aggregate states. More importantly, a precise change in the position of pyridyl nitrogen atom by keeping the other part intact was expected to exert a substantial effect on the molecular conformation, crystal packing, electronic structure, and subsequently the optical properties. Thus, such molecules with the same molecular formulae can display different photophysical behavior just because of the different conformational flexibility within the molecule.10 At present, we focus on generating and studying the AIE properties for three series of regioisomeric pyridyl πconjugates that are linked with (hetero)aryl-substituted anthracene (Figure 2). The subtle change in the position of nitrogen for the pyridyl ring resulted in different absorption and emission behaviors on aggregation. In fact, the substitution effect at the 10th position of anthracenyl ring was also found to be exciting. The tolyl- and mesityl-substituted π-conjugates are AIE-active except 3-pyridyl isomers AT3P and AM3P that
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RESULTS AND DISCUSSION Synthesis of the Molecules. These compounds were designed based on our earlier observations,9b and particularly, the thiophene substitution made the system unique as AIEgen. All of these compounds were synthesized in high yields via simple but efficient Horner−Wadsworth−Emmons reactions 5053
DOI: 10.1021/acsomega.9b00046 ACS Omega 2019, 4, 5052−5063
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Figure 3. (a) Absorption spectra and (b) emission spectra of compound AT2P (10 μM; λex = 405 nm) at different f w’s in acetonitrile.
Figure 4. I versus I0 (I0: Fl. intensity before addition of water; I: Fl. intensity after addition of water) for (a) AT2P, (b) AT3P, and (c) AT4P. [Concentration of the probe: 10 μM, λex = 405 nm.] The image is taken at f w = 0 and 90% for all of these three compounds under 365 nm UV light.
relatively lower quantum yield compared to other solvents (Figure S1). Photophysical studies were carried out for all of the compounds in acetonitrile solution (10 μM) at room temperature. The AIE properties were examined by measuring the absorption and emission spectra for each compound in acetonitrile solution upon gradual addition of water fraction [a nonsolvent f w (v/v %)]. The absorption maxima peaked at ∼396 nm was profoundly observed in the UV−vis absorption spectra due to the familiar π−π* transitions of anthracene,13 and the peak had a red shift by 5−15 nm upon addition of water (Figure 3a), indicating the possibility of J-aggregate formation.14 However, red shift and enhanced fluorescence are not essentially limited to Jaggregates. The formation of nanoaggregate without J- or Haggregation may also cause such effect called the Mie scattering effect.14a In fact, the crystal packing of AT2P also reveals no parallel alignment of the molecules (H-aggregation), rather it is more closer to the shape of J-aggregation (head-to-tail directional packing).14b The compound AT2P was fairly emissive at λmax = 490 nm with a quantum yield (Φf) of 10%, which quenched gradually with the increase of water fraction f w ∼ 70% (Figures 3b and 4a) due to the polarity effect that can stabilize the excited state.11 Due to the presence of a pyridine ring, there could be a chance to generate the twisted intramolecular charge-transfer (TICT) state. However, the effect would be subtle, not like typical push−pull systems. To confirm the TICT effect, we have measured the emission spectra in solvents of different polarities, such as hexane, 1,4dioxane, and acetonitrile. The red shift (25 nm) was somewhat significant for both AT3P and AT4P, whereas for AT2P, the shift is only 12 nm (Figure S2). The quenching of Fl. intensity was observed for all of these compounds upon increasing the polarity. Thus, the presence of the TICT state can be anticipated, although the effect is not much significant. There
of phosphonates with the corresponding aldehydes in the presence of NaH or KOtBu (Scheme 1). All of the compounds were characterized using multinuclear NMR and mass spectroscopy. The trans-coupling was observed for some of these compounds; however, the molecular structures for few compounds were determined unequivocally by single-crystal X-ray diffraction studies. Initially, KOtBu was preferred as a base compared to NaH (60% dispersed in mineral oil) due to the operational simplicity and easy purification process. All of these πconjugates were synthesized using KOtBu except ATh2P as it could not be synthesized using KOtBu/tetrahydrofuran (THF) unexpectedly even after repetitive attempts. Instead, the reduced compound ATh2PR was obtained in 75% yield every time (Scheme 2). Nevertheless, ATh2P was prepared in 72% yield using NaH. The compound with the presence of two CH2’s was identified by 1H NMR and finally characterized by single-crystal X-ray diffraction (Scheme 2; right). Such reactions could be possible with KOtBu/THF due to the unusual effect in organic synthesis.12 However, such a compound is considered to be an analogue of C, as mentioned earlier (Figure 1). Due to the loss of conjugation with pyridyl ring and being quite different from our focus, we withhold ATh2PR for AIE studies. All of these πconjugates are soluble in most of the water-miscible solvents and significantly stable under photoexcitation in both the solution and solid states, whereas the photostability was an issue for the previously reported system with similar properties, in which two 9-vinylanthracene parts are attached to only 1,2positions of the benzene ring.10 AIE Studies. A solvent−nonsolvent system is typically preferred to study the difference in AIE behaviors. Among water-miscible solvents, including tetrahydrofuran, acetonitrile, and 1,4-dioxane, acetonitrile was preferred because of its 5054
DOI: 10.1021/acsomega.9b00046 ACS Omega 2019, 4, 5052−5063
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Article
Table 1. Photophysical Parameters Obtained from Absorption and Emission Studies comp. AT2P AT3P AT4P AM2P AM3P AM4P ATh2P ATh3P ATh4P
water fraction (f w %, average particle size in nm) Abs λmax (nm) Emi λmax (nm) relative QYa (%) 00% 90%, 00% 99%, 00% 90% 00% 80%, 00% 99%, 00% 90% 00% 90%, 00% 99%, 00% 99%
116 73
187 109
126 168
396 409 395 403 396 409 398 406 395 402 396 407 398 408 397 408 398 413
490 500 480 500 500 510 485 490 480 490 490 500 475 504 480 510 480 510
10 12 22 5 2 7 14 13 29 5 4 8
