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Aggregation Induced Emission or Hydrolysis by Water? The Case of Schiff Bases in Aqueous Organic Solvents Bapan Pramanik, and Debapratim Das J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12430 • Publication Date (Web): 29 Jan 2018 Downloaded from http://pubs.acs.org on January 29, 2018

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The Journal of Physical Chemistry

Aggregation Induced Emission or Hydrolysis by Water? Water? The Case of Schiff Bases ases in Aqueous Organic Solvents Bapan Pramanik and Debapratim Das* Department of Chemistry, Indian Institute of Technology Guwahati, North Guwahati, Assam 781039, India, Email: [email protected]

ABSTRACT: In the last two decades, several Schiff bases have been reported as AIEgens which remain non-emissive in organic solvents but show strong fluorescence in presence of water. A methodical analysis involving 21 Schiff bases, including some of the reported molecules, show that in presence of water, the Schiff bases hydrolyze to yield the corresponding starting aldehydes and amines. The observed emission in presence of water is found to be originated from the aggregation of the fluorogenic aldehydes and not of the original molecules. Thus, while the AIE effect is valid for these systems, certainly, these Schiff bases cannot be termed as AIEgens. Notably, the observation that these aldehydes can act as AIEgens through their excimer emission is an important phenomenon with respect to the current understanding of AIEgens.

non that may happen is hydrolysis of the imines. After several thorough searches, unfortunately, we could not find any detailed study on the actual mechanism of AIE in these Schiff base AIEgens. In general, most of these reports presented the photographic images of solutions showing enhanced emission with increase in water content. The photographs are accompanied by emission spectra with enhanced intensity as the water fraction increased. Unfortunately, no proper investigation could be found in order to prove the aggregation of the molecules in these systems. Dynamic light scattering (DLS) data along with microscopic images are provided in some cases, however, with no conclusive evidence in favor of aggregation.

INTRODUCTION Aggregation induced emission (AIE) has become a popular topic of research since it was described by Tang et al back in 2001.1 Since then, a plethora of AIEgens were developed and used for various applications.2-5 Different hypothesis have so far been proposed on AIE mechanisms, such as, the Restriction of Intramolecular Rotations (RIR)6, Restriction of Intramolecular Motion (RIM)7, Restriction of Intramolecular Vibrations (RIV)8, J-aggregates,9 excited state intramolecular proton transfer (ESIPT)10, twisted intramolecular charge transfer (TICT)11 etc.4 It is the enthusiastic effort of scientists from all over the world that produced a large list of AIEgens.12, 13 Among these reported molecules, majority of them fall under the category of RIR, RIV or RIM mediated AIEgens. One such group of AIEgens reported is the imines. A variety of imines (Schiff base) are reported where the molecules are non-fluorescent in organic solvents while the emission enhances in presence of water.14-27 In organic solvent, as proposed, the lone pair of the imine nitrogen is involved in photoinduced electron transfer (PET) process and thereby suppresses the emission. In presence of water, the PET is disturbed by the aggregation of molecules and therefore the molecules show AIE. To this end, imines are known for their instability in presence of water. With very high pKa values,28, 29 the imines are susceptible toward hydrolysis to give back the starting aldehyde/ketone and the corresponding amines. We were wondering how these reported imines show AIE in aqueous organic solvent mixtures where the most likely chemical phenome-

Scheme 1. Chemical structures of all the compounds synthesized and used for the study. Based on these observation on reported results, we were curious to understand the source of the enhanced emission in these cases. The primary objective of the investigation was to find out a) whether the molecules are aggregating at all or hydrolyzed in presence of water; b) if it is the case of hydrolysis, what is the source of the observed enhanced emission. In order to find the answers, we have prepared 21 Schiff bases (Scheme 1) containing either naphthalene, anthracene or pyrene as the main emissive core. The list of synthesized mole-

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cules contains both aldimines as well as hydrazones to cover all types of reported imine based AIEgens (in aqueous organic systems). Many of these compounds are already reported as AIEgens while the other part of the list contains new molecules to cover different structural possibilities.

METHODS General Information and Materials: All chemicals and reagents were purchased from Sigma-Aldrich (USA) and used without further purification. All solvents were procured from Merck (India). To prepare samples, Milli-Q water with a conductivity of less than 2 mScm-1 was used. UV-Visible spectra were recorded on a PerkinElmer Lambda 750 spectrometer, while fluorescence measurements were performed on a Cary Eclipse (Agilent) spectrophotometer. Standard 10 mm-path quartz cuvettes were used for all spectroscopic measurements. 1H NMR spectra were recorded with a Bruker Ascend 600 MHz (Bruker, Coventry, UK) spectrometer or an Oxford AS400 (Varian) spectrometer and referenced to deuterated solvents. ESI-MS was performed with a Q-tof-Micro Quadrupole mass spectrophotometer (Micromass). General synthesis of the Schiff derivatives: Aldehyde (1mmol) and the corresponding amine (1.1 mmol) were taken in dry MeOH and refluxed for 12 hours. The reaction mixture was cooled to room temperature and washed with MeOH several times to obtain the title compound as a solid. In case of compound 2, after completion of the reaction, the solvent was removed on a rotary evaporator and then subjected to column chromatography on a 60-120 mesh silica gel column using hexane as the eluent to obtain the desired compound as a brown solid. The spectroscopic characterization data for all the synthesized compounds are provided in the Supporting Information. Fourier Transformed Infrared Spectroscopy (FTIR): IR spectra were recorded on a Nicolet IS10 spectrometer using CaF2 windows. The baseline was subtracted from the obtained absorbance intensity in each case. UV−Visible and Fluorescence Spectroscopic Studies: Stock solutions of the Schiff derivatives were prepared in a 10 mL of volumetric flask by weighing appropriate amount of the compound and dissolving in the corresponding solvent. These stock solutions were diluted to the concentrations in the premixed solvent mixtures required for the experiments. Dynamic Light Scattering (DLS) Studies: The particle sizes of the samples were measured at 298 K using a 632.8 nm He − Ne laser using Zetasizer Nano-ZS90 (Malvern). The samples were prepared in the corresponding solvent and premixed solvent mixtures and filtered through appropriate filters to remove dust particles if present, and then allowed to settle for 12 h before the measurement. Luminescence certainly affect the light scattering experiments. However, the wavelength used for the DLS measurements is 632.8 nm and none of the investigated molecules show any emission at this wavelength. The λex range for these molecules is 290-450 nm. Thus any effect arising from the luminescence of these molecules is not possible.

least 12h) on a silicon wafer and air dried for at least 1 day. FESEM images were taken on a Gemini SEM 300 (Sigma Zeiss) instrument. Quantum Yield measurement: The fluorescence quantum yields of NapCHO, PyCHO and AntCHO in 80% water-ACN were determined by Parker-Rees method using quinine sulfate (for naphthalene and pyrene) and coumarin153 (for anthracene) as a standard fluorophore. The Parker-Rees equation30 is written as follows, ɸu = (AsFunu2/ AuFsns2) ɸs (1) where, ɸs = Quantum Yield of standard fluorophore (0.54 for quinine sulfate in 0.1(M) H2SO4, and = 0.58 for coumarin153 in EtOH), ɸu = Quantum Yield of unknown fluorophore, As = the absorbance of standard fluorophore at the excitation wavelength, Au = the absorbance of unknown fluorophore at the excitation wavelength, Fs = the area of integrated fluorescence intensity of the reference sample when excited at the same excitation wavelength, Fu = the total area of integrated fluorescence intensity for the unknown sample when excited at the same excitation wavelength, The refractive indices of the solvents for the unknown and the standard samples are denoted by nu and ns respectively. To minimize the reabsorption of the fluorescence light passing through the samples their absorption maximum was kept 0.1. RESULTS AND DISCUSSION In order to qualify as an AIEgen, any molecule has to show two phenomenon, 1) solid state emission and 2) aggregationinduced emission in mixture solution (water/MeCN, water/THF, water/EtOH, cyclohexane/CH2Cl2, or cyclohexane/toluene etc.).31 To begin with, the solid state emission of the synthesized molecules were checked under normal light as well as UV light. Only few molecules showed mild emission under UV light (Figure S1, Supporting Information). Importantly, none of these molecules could be solubilized in cyclohexane and thus, the AIE property in organic solvent mixtures like cyclohexane/CH2Cl2, or cyclohexane/toluene could be verified. However, as we were concerned about the possible hydrolysis of these Schiff bases in aqueous medium, we focused on the enhancement of emission in water-organic solvent binary mixtures. The molecules were solubilized in different water miscible organic solvents and their emission properties are checked (data not shown). The most suitable solvent was chosen based on the lowest emission intensity or the reported solvent system for the already described molecules. The effect of increasing amount of water was then investigated. Except in the case of 16, all the other molecules showed intense fluorescence when irradiated with UV light in presence of water (Fig. 1A and Fig. S2-21, Supporting Information). Moreover, the emission spectra of these samples also showed enhanced emission with increase in water content (Fig. 1B and Fig. S2-21, Supporting Information). For clarity, results obtained from compound 8 are presented in the main text. Observations from other molecules can be found in Supporting Information. Unless otherwise mentioned, the observations are very similar to that of compound 8 for similar experiments that have been carried out for the rest of the compounds.

Field Emission Scanning Electron Microscopy (FESEM): The FESEM samples with desired solvent compositions were prepared by casting a drop of dilute solution (incubated for at


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The Journal of Physical Chemistry The UV-visible absorption profile of 8 is recorded both in ACN and in presence of water. As can be seen in Figure 2A, the absorption maxima at 423 nm in ACN shifted to 368 nm in presence of 80% water. Interestingly, the absorption profile is found to be very similar when the parent aldehyde (ie, 1pyerenecarboxaldehyde (PyCHO), under similar experimental conditions) is analyzed in 80% water-ACN or even in only ACN. The hypsochromic shift of the absorption maxima for 8 thus could be due to the regeneration of the aldehyde in presence of water.

Figure 1. For compound 8 (10 µM), A) photographs (under UV light), B) emission spectra and C) DLS profile of solutions in ACN with varying amounts of water; D) FESEM image from solution of 8 in 80% water-ACN.

In order to evaluate the possible aggregation in presence of water, DLS are recorded for 8 in acetonitrile (ACN) with varying amount of water. As can be seen from Figure 1C, the particle size increases with water content and found to be highest in case of 80% water (96 nm) while no measurable distribution is found in ACN. Field Emission Scanning Electron microscopic (FESEM) image of the sample from ACN also do not show any particular morphology while that from 80% waterACN results in spherical particles with diameter of ~ 100 nm (Fig. 1D) which closely matches with the DLS observations.

Next, the fluorescence spectra of all the molecules in aqueous organic solvents were recorded where the emission is found maximum and compared them with that in the organic solvents (Figures 2B and S2-21, Supporting Information). Interestingly, upon addition of water, a significant shift (12-95 nm) in the emission maxima is observed compared to that in the organic solvent. In case of 8, the observed shift was bathochromic in nature (388 nm to 464 nm). The shift in λem is accompanied by significant enhancement of the emission intensity in all these cases. To further understand the situation, the emission property of the corresponding aldehydes as well as equimolar mixtures of the corresponding aldehydes and amines were also measured separately under similar experimental conditions. Interestingly, in cases of 1-15, both, the intensity as well as the emission maxima were observed to be similar to that of the water-organic systems of the molecules. Notably, in the cases of 16-21 the emission behavior of the parent aldehyde or aldehyde-amine mixtures are significantly different than that of the molecules. Among the list of the molecules, there are certainly two different groups, a) the bluebox compounds (Scheme 1): emission in aqueous organic system matches with that of the parent compounds; b) the black box compounds (Scheme 1): emission in aqueous organic system does not match with that of the parent compounds. It is worth mentioning here that though compound 16 was reported as an AIEgen, the observed results from our studies did not match with the report. Compound 16, even in 80% water-ACN (the reported solvent system) showed very small enhancement in its emission compared to that in ACN. However, the behavior of this compound also fall in the black-box group.

Figure 2. A) UV-Visible, B) emission spectra of different compositions of 8, PyCHO and DPA; λex = 340 nm.



spectra in aqueous organic solvent mixtures (Figure S2-S21, Supporting Information).

Figure 3. A) 1H NMR, spectra of different compositions of 8, PyCHO and DPA; B) Time dependent change in emission intensities recorded at 450 nm for 8 in 30% and 80% water-ACN, and time dependence of the ratio of integration areas of peaks for imine and aldehyde protons recorded for 8 in 30% D2O-CD3CN. 1H

To further elucidate the undergoing process, NMR was employed. In all cases, the NMR spectra were recorded and compared for the corresponding molecules in organic solvent and in 10% aqueous organic systems. For compounds 1-15, peaks corresponding to the aldehyde protons of the parent aldehydes appeared in aqueous system (Figure 3A and S2-21, Supporting Information). Notably, in organic solvents, those particular peaks were absent in all cases. In case of compound 8, a spike experiment by adding 1 equivalent of PyCHO resulted in increase in the peak area of the aldehyde peak confirming the fact that it is indeed arising from PyCHO. An equimolar mixture of PyCHO and DPA in 10% D2O-CD3CN also resulted in a very similar NMR spectrum as it was for 8 in the same solvent system. The appearance of the aldehyde peaks and lowering in the intensity of imine protons confirm the hydrolysis of the imines in cases of 1-15. In addition to that, a thin layer chromatographic analysis of 8 in 80% water-ACN and ACN showed the appearance of the starting materials and complete disappearance of compound 8 in 80% water-ACN whereas no such change was observed in ACN (Figure S22, Supporting Information). Additional confirmation about the hydrolysis was obtained from the IR analysis of 8 (Fig. S23, Supporting Information) where the sample from aqueous ACN showed presence of the aldehyde carbonyl stretching frequency at 1677 cm-1 which was absent in the sample in ACN. Additionally, the imine band at 1612 cm-1 disappeared completely in the aqueous sample confirming complete hydrolysis. In contrast to the blue-box compounds, the black-box molecules, 16-21, did not show the presence of aldehyde at all in the 1H NMR

Figure 4. For PyCHO (10 µM), A) photographs (under UV light), B) emission spectra (λex = 340 nm) of PyCHO in ACN with varying amounts of water.

Next, time dependent 1N NMR as well as fluorescence were recorded for compound 8 in 30% aqueous acetonitrile. Interestingly, both experiments provided similar saturation time of ~ 4 h (Figure 3B) indicating that the compound gets hydrolyzed completely within this period of time. When a similar experiment in 80% water-ACN is performed using fluorescence, the saturation is obtained within 1.5 h. These results indicate that the hydrolysis is a relatively fast process and its rate depends on the water content. In combination with the absorption, emission, IR studies, the results from 1H NMR experiments clearly demonstrate that in presence of water, compounds 1-15 get hydrolyzed to produce the corresponding aldehydes and amines. In contrast to compounds 1-15, the black-box molecules did not show any sign of hydrolysis. Based on molecular structures, this group of compounds can be classified into two groups, a) the red-box compounds (16-19, hydrazones), and b) the green box compounds (20-21, Schiff bases of salicylaldehyde). Compounds 16-19 are hydrazones and the hydrolytic stability of hydrazones in presence of water is already well discussed by Raines et.al.32 Hydrazones are hydrolyzed only in acidic condition. Thus in case of hydrozones, it can be concluded that the observed enhanced emission is due to AIE.


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The Journal of Physical Chemistry through aggregation but not of the Schiff base 8, but of PyCHO which is produced from the hydrolysis of 8. Next, the emission changes for the other aldehydes were also recorded at different water content in ACN. As expected, both, 2naphthaldehyde (NapCHO) and 9-anthracenecarboxaldehyde (AntCHO) showed AIE as in the case of PyCHO (Fig. S25-26, Supporting Information). The quantum yields for all three aldehydes were calculated in 80% water-ACN systems (6.63%, 7.96%, and 57.14% for NapCHO, AntCHO, and PyCHO respectively). Interestingly, the quantum yield of PyCHO was surprisingly high and this phenomenon needs further detailed analysis. The observed enhanced emission property of these aldehydes is an important finding from this study. This in turn, will certainly change the course of future research on AIE active molecules. A schematic presentation of the process is demonstrated in Scheme 2. The Schiff bases get hydrolyzed in presence of water and produce the respective aldehydes and amines. The fluorogenic aldehydes undergo aggregation process and in turn produces the aggregation induced emission. In the previously reported cases, this enhanced emissions were misunderstood and inaccurately assigned to the aggregation of the Schiff bases. Figure 5. For PyCHO (10 µM), A) DLS profile of solutions in ACN with varying amounts of water; B) FESEM image from solution in 80% water-ACN and C) 1H NMR spectra of PyCHO in CD3CN with varying amounts of D2O. Full range NMR spectra corresponding to (C) is provided in Figure S27, Supporting Information.

The green-box molecules (20-21) are of particular interest as they strictly fall under the category of aldimines like compounds 1-15 but the only difference is the presence of the ortho-hydroxy group on the corresponding aldehyde part (Salicylaldehyde) of the compounds. Importantly, several salicylaldehyde based Schiff bases are reported in literature which show their stability in aqueous medium.33-36 Most of these reported salicylaldehyde-based Schiff bases are AIEgens as well. The resistance of these molecules toward hydrolysis might come from the fact that the hydroxyl group can possibly form a six membered ring via hydrogen bonding with the imine nitrogen. Crystal structures of both 20 and 21 were previously reported.24, 37 Analyzing both the structures indeed showed strong hydrogen bonds between the hydroxyl and nitrogen of the imine (1.839-1.889 Å) in both cases. It can thus be speculated that the hydrogen bonds here play crucial role to provide enough resistance toward hydrolysis of the imines like the metal coordination by Schiff bases.38 It is clear from the above mentioned results that the observed emission enhancement in presence of water is not really AIE of the molecule but originated from the aldehydes in cases of 1-15. However, one question remained unanswered, why the emission enhanced with increase in water content. We investigated the emission of PyCHO in ACN with different amount of water and as can be seen in Figure 4A and B, the emission enhancement with increase in water fraction was very similar to that of 8. In ACN, it remained mostly nonemissive. Further investigation utilizing DLS and FESEM showed very similar particle sizes to that of 8 at different water content (~100 nm in 80% water-ACN for both, 8 and PyCHO, Figure 5A, B. Importantly, 1H NMR experiment shows significant up-field shift of the pyrene protons with increase in water content which clearly demonstrate its aggregation (Figure 5C). Importantly, the emission maxima appears at ~450 nm which is typical of pyrene-excimer.39 These results unequivocally suggest that the observed enhancement is indeed

Scheme 2. Schematic presentation of the hydrolysis of the blue-box molecules and AIE of the aldehyde generated from the hydrolysis. CONCLUSION In summary, it is now clearly established that the imine based systems, in general, cannot show aggregation induced emission unless there is any other stabilizing component which resist their hydrolysis in aqueous organic solvent mixture. The observed enhanced emission for these Schiff bases in presence of water is originated from the aggregation of the aldehydes generated from hydrolysis of the molecules by water. Though the term AIE is certainly true in these cases, but it is definitely not applicable to these Schiff-bases. Thus, prior to designing and reporting the AIE property of such systems, it is recommended to analyze the system properly to omit any possibility of hydrolysis. Moreover, the observation that the aldehydes can act as an AIEgen through their excimer emission and especially the extremely high quantum yield of PyCHO in 80% water-ACN will certainly open up a new window of future perspective of AIE research.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website.



Characterization of synthesized molecules and all other supporting data (PDF).

robust quantification of Zn(II) based on aggregation induced emission enhancement feature. ACS Sensors 2016, 1, 739-747.

AUTHOR INFORMATION

15. Kathiravan, A.; Sundaravel, K.; Jaccob, M.; Dhinagaran, G.; Rameshkumar, A.; Arul Ananth, D.; Sivasudha, T., Pyrene Schiff base: photophysics, aggregation induced emission, and antimicrobial properties. J. Phys. Chem. B 2014, 118, 13573-13581.

Corresponding Author [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT DD acknowledges financial support from SERB (EMR/2016/ 000857), India, UKIERI (DST/INT/UK/P-119/2016), Alexander von Humboldt Foundation, Germany, and DST-FIST program.

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