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Solvent Dependent Self-Assembly and AIE in Zn(II) Complexes Containing Phenothiazine Based Terpyridine Ligand and Its Efficacy in Pyrophosphate Sensing Vishwa Deepak Singh, Roop Shikha Singh, Rajendra Prasad Paitandi, Bhupendra Kumar Dwivedi, Biswajit Maiti, and Daya Shankar Pandey J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00105 • Publication Date (Web): 09 Feb 2018 Downloaded from http://pubs.acs.org on February 11, 2018

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The Journal of Physical Chemistry

Solvent Dependent Self-Assembly and AIE in Zn(II) Complexes Containing Phenothiazine Based Terpyridine Ligand and Its Efficacy in Pyrophosphate Sensing Vishwa Deepak Singh, Roop Shikha Singh, Rajendra Prasad Paitandi, Bhupendra Kumar Dwivedi, Biswajit Maiti and Daya Shankar Pandey* Department of Chemistry, Institute of Science, Banaras Hindu University, Varanasi – 221 005 U.P. (India)

Taking into account the inherent hydrophilicity/hydrophobicity of solvents and compounds we describe solvent dependent self-assembly in phenothiazineterpyridine ligand (MTPY) and its Zn(II) complexes (C1 and C2). The role of solvent on morphology and emission characteristics of the self-assembled aggregates has been investigated by UV/Vis, emission and SEM studies. Complex C1 has been employed for detection of pyrophosphates (PPi) based on AIE.

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Solvent Dependent Self-Assembly and AIE in Zn(II) Complexes Containing Phenothiazine Based Terpyridine Ligand and Its Efficacy in Pyrophosphate Sensing Vishwa Deepak Singh, Roop Shikha Singh, Rajendra Prasad Paitandi, Bhupendra Kumar Dwivedi, Biswajit Maiti and Daya Shankar Pandey* Department of Chemistry, Institute of Science, Banaras Hindu University, Varanasi – 221 005 U.P. (India) Abstract Zn(II) complexes MTPYZnCl2 (C1) and MTPYZn(NO3)2 (C2) based on a new DA type ligand MTPY involving phenothiazine (PTZ) donor and terpyridine (TPY) acceptor units have been described. The ligand MTPY and complexes C1 and C2 display intramolecular charge transfer (ICT) and substantial solvatochromism. Solid state emission studies on MTPY further substantiated the occurrence of concentration induced emission in this molecule. As well, the complexes C1 and C2 displayed solvent dependent self-assembly which has been examined as a function of hydrophilic and hydrophobic nature of the solvent systems. The role of hydrophilicity/ hydrophobicity of solvent and compounds on morphology and emission characteristics of the self-assembled aggregates have been investigated by UV/Vis, emission and scanning electron microscopic (SEM) studies. In addition, it has been categorically shown that aggregation induced emission in C1 offers a simple, sensitive, and rapid means for detection of pyrophosphates (PPi) in aqueous medium. Job’s plot analysis suggested 3:1 binding stoichiometry between C1 and PPi which has been supported by ESI-MS and density function theory (DFT). Further, higher affinity of PPi towards C1 over C2 has also been rationalized by theoretical studies. *Author to whom all the correspondence should be addressed; E-mail ID: [email protected]


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Introduction Realization of solid state emission in organic luminophores have attracted the attention of many research groups due to its potential applications in diverse areas including biomedical and optoelectronics.13 Aggregation induced emission (AIE) has proved to be extremely useful in achieving significant emission in solid state by overcoming aggregation caused quenching (ACQ). The AIE luminogens display valuable photophysical properties with increasing concentration or upon aggregation4 as AIE alleviates close packing in the molecules via various pathways viz. conformational planarization, J–aggregate formation, E/Z isomerization, twisted intramolecular charge transfer (TICT), excited state intramolecular proton transfer (ESIPT)12,13 and restriction of intramolecular motion (RIM).14,15 An understanding of these processes may enable us to set a rationale between molecular structure and photophysical properties of the AIE luminogens. However, realization of a perfect balance between radiationless decay due to charge transfer and solid state emission has been challenging. 16,17 Likewise, donoracceptor (DA) systems have attracted enormous attention because facile tuning of their electronic states by appropriate groups may make it suitable for a variety of applications.1822 Cautiously designed DA systems may offer well separated HOMO and LUMO levels with low energy gap between the ground and excited states.23,24 Thus strategic modifications in DA systems can bestow them with distinctive emission properties in solution and high quantum efficiencies in the solid state. Further, intramolecular motion plays a vital role in designing AIE active luminogens and its importance in the photophysics of TICT active DA systems is well established. This revelation has paved a way for developing efficient AIE active luminogens based on a DA skeleton with outstanding properties.25 In this context, phenothiazine (PTZ) derivatives have drawn special attention due to their pharmaceutical properties and optoelectronic applications. Their non-planar butterfly conformation suppresses strong intermolecular interactions and creates highly fluorescent DA type self-assembled aggregates.22, 2832 In addition, 2,2’:6’,2”terpyridine (TPY) displays high chelation ability towards metal ions and have opened new avenues in the area of coordination and supramolecular chemistry.3336 Upon coordination these may alter the photoinduced charge transfer (PCT) and give rise to metal complexes with distinguish photophysical and electrochemical properties. Further, Zn(II) based complexes have shown great promise in high performance OLEDs due to high propensity of



Zn(II) to form complexes with a variety of fluorescent ligands and acquiring superior quantum yield relative to free ligands.37 Detailed mechanistic understanding of the AIE has proved to be highly beneficial toward development of terpyridine based fluorescent detectors for Zn(II) and ensuing complexes have been shown to selectively bind with various anions.40 In this direction, as continuation of our earlier studies and with an intent to develop red emitting Zn(II) complexes, a new PTZTPY based DA system MTPY has been designed and synthesized. Further, it has been employed in the synthesis of Zn(II) complexes MTPY-ZnCl2 (C1) and MTPY-Zn(NO3)2 (C2). The AIE and solid state emission in the ligand and complexes C1 and C2 have been investigated under the influence of inherent charge transfer processes of DA systems. This contribution deals with the synthesis and characterization of MTPY and Zn(II) complexes C1 and C2, their detailed photophysical properties in solution and solid/aggregated state. Notably, this work unveils ‘concentration induced emission’ in MTPY as an alluring analogue of AIE and implies the intervention of hydrophilicity and hydrophobicity of solvents in self-assembly of C1 and C2. In addition, the applicability of C1 for detection of pyrophosphates (PPi) in aqueous medium has also been described. Experimental Methods General Information Phenothiazine, iodomethane, and 2-acetylpyridine were purchased from Sigma Aldrich India. Common reagents, KOH/NaOH and the solvents dimethylsulphoxide (DMSO), dichloromethane (DCM), dimethylformamide (DMF) and methanol etc. were procured from Avra Chemicals Hyderabad, India and dried and distilled following standard literature procedures prior to their use.46 The synthetic manipulations have been performed under oxygen free nitrogen atmosphere and photophysical studies made using spectroscopic grade solvents. Elemental analyses for C, H, and N have been acquired on an Elementar Vario EL III Carlo Erba 1108 in the micro analytical laboratory of Sophisticated Analytical Instrumentation Facility (SAIF), Central Drug Research Institute (CDRI), Lucknow, India. Electronic absorption and fluorescence spectra have been acquired on a Shimadzu UV1601 and Perkin Elmer LS 55 fluorescence spectrometers at room temperature. 1H (500 MHz) and

13

C (125 MHz) NMR spectra have been acquired at room

temperature on a JEOL AL500 FT-NMR Spectrometer using tetramethylsilane [Si(CH3)4] as an internal reference. Electrospray Ionization Mass Spectrometric (ESI-MS) measurements have


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been made on a Bruker Daltonics Amazon SL ion trap mass spectrometer and Q-TOF LC/MS mass spectrometer of Agilent. Scanning electron microscopic images were captured on quanta 200 F microscope using silicon wafer. Dynamic light scattering (DLS) studies have been performed on a Horiba particle size analyzer SZ-100. Syntheses Synthesis of MTPY A basic methanolic solution (obtained by dissolving NaOH, 0.040 g, 1 mmol in 50 mL methanol) of 2 (0.241 g, 1 mmol) was treated with 2acetylpyridine (~2 mmol) and reaction mixture made ammoniacal by addition of concentrated NH4OH (10 mL). It was stirred for 1 h and subsequently refluxed for 36 h. Upon cooling to room temperature it gave greenish yellow precipitate which was filtered, washed with water and methanol. Yield: 79.6% (0.353g). M.P. = 192oC. 1H NMR (CDCl3, 500 MHz) 3.43 (s, 3H), 6.85 (d, J = 8.5 Hz, 1H), 6.90 (d, J = 8.5 Hz, 1H), 6.96 (t, J = 7.5 Hz, 1H), 7.19 – 7.17 (m, 2H), 7.36 – 7.33 (m, 2H), 7.75 – 7.72 (m, 2H), 7.85 – 7.89 (m, 2H), 8.67 (t, J = 7.5 Hz, 4H), 8.73 (d, J = 3.5 Hz, 2H). 13CNMR (CDCl3, 125 MHz)

35.5, 114.3, 118.0, 121.4, 122.8, 123.8, 125.7, 126.5, 127.3, 127.6, 136.9, 145.0, 146.8, 149.0, 149.1, 155.9, 156.3. IR (KBr pellet, cm-1): 1585, 1566, 1262, 1040, 623 cm-1. ESI-MS: m/z calcd for C28H20N4S [M+H]+: 445.1487; found: 445.1467. Elemental analyses calcd (%) for C28H20N4S: C 75.65, H 4.53, N 12.60; found: C 75.60, H 4.48, N 12.53. Synthesis of MTPYZnCl2 (C1) A solution of ZnCl2 (0.136 g, 1 mmol) dissolved in acetonitrile (30 mL) was added dropwise to a solution of MTPY (0.445 g, 1 mmol) in dichloromethane (15 mL)and the reaction mixture stirred further for 24 h at room temperature. Slowly, an orange-red solid started to separate from the reaction mixture which was filtered and washed with acetonitrile followed by dichloromethane, diethyl ether and then dried in air. Yield: 76% (0.441g). M.P. = 210oC. 1H NMR (DMSO-d6, 500 MHz) 3.36 (s, 3H), 6.95 – 7.02 (m, 3H), 7.20 (d, J = 7.5 Hz, 1H), 7.24 (t, J = 7.5 Hz, 1H), 7.73 (s, 2H), 8.04 (d, J = 8.0 Hz, 2H), 8.12 (d, J = 7.0 Hz, 2H), 8.75 (s, 4H), 8.83 (s, 2H). 13C NMR (DMSO-d6, 125 MHz)  35.9, 115.2, 119.1, 121.8, 122.9, 123.4, 123.7, 126.3, 127.4, 127.6, 128.3, 140.9, 144.8, 147.5, 149.1, 149.2, 153.1. IR (KBr pellet, cm-1): ν = 1600, 1574, 1253, 1025, 639 cm-1. ESI-MS: m/z calcd for C28H20Cl2N4SZn [M–Cl+CH3CN]+:



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584.0654; found 584.1913. Elemental analyses calcd (%) for C28H20Cl2N4SZn: C 57.90, H 3.47, N 9.65; found: C 57.85, H 3.42, N 9.60. Synthesis of MTPYZn(NO3)2 (C2) This compound was prepared following the above procedure for 1 using a methanolic solution (40 mL) of Zn(NO3)2.6H2O (0.297 g,1 mmol) in place of ZnCl2 and stirring the reaction mixture for 12 h. The red solid was collected by the filtration and washed with dichloromethane, methanol, and diethyl ether and dried in air. Yield: 79% (0.476g). M.P. = 218oC. 1H NMR (500 MHz, (DMSO-d6, 500 MHz)  3.45 (s, 3H), 7.03 (s, 2H), 7.06 (s, 1H), 7.24 (s, 2H), 7.45 (s, 2H), 7.90 (s, 2H), 8.24 (s, 2H) 8.29 (s, 2H), 9.11 (s, 2H), 9.30 (s, 2H).

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C NMR (DMSO-d6, 125

MHz)  36.0, 115.5, 115.7, 120.3, 121.8, 123.5, 123.6, 123.8, 127.2, 127.5, 128.1, 128.2, 128.5, 128.7, 141.7, 144.8, 148.2, 148.3, 149.8, 154.1. IR (KBr pellet, cm-1): ν = 1598, 1573, 1384, 1254, 1014, 638 cm-1. ESI-MS: m/z calcd for C28H20N6O6SZn [MNO3+CH3OH]+: 602.0840; found: 602.9929; Elemental analysis calcd (%) for C28H20N6O6SZn: C 53.05, H 3.18, N 13.26; found: C 53.19, H 3.13, N 13.22. Result and Discussion Synthesis and Characterization The

compounds

10methyl10Hphenothiazine

(1)

and

10methyl10Hpheno-

thiazine3carbaldehyde (2) have been synthesized following earlier procedures.30,47 Synthesis of the DA type ligand MTPY having PTZ donor and TPY acceptor units has been achieved by reacting 2 with 2-acetyl-pyridine in basic methanolic solution (obtained by using 1 equivalent of NaOH and making the reaction mixture ammoniacal by adding an excess of NH4OH). The ligand MTPY reacted with metal salts ZnCl2, and Zn(NO3)2.6H2O under stirring conditions at room temperature to afford C1 and C2 in reasonably good yield. A simple synthetic protocol showing preparation of the ligand and complexes has been depicted in Scheme 1. The ligand MTPY is highly soluble in hexane, benzene, toluene, chloroform, dichloromethane (DCM), tetrahydrofuran (THF), 1,4-dioxane, acetonitrile, dimethylformamide (DMF) and dimethylsulphoxide (DMSO), partially soluble in methanol and insoluble in water. On the other hand, complexes (C1 and C2) are soluble only in high boiling solvents like DMF or DMSO. These compounds have been thoroughly characterized by satisfactory elemental analyses, ESI-MS, 1H and 13C NMR spectroscopic studies.


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Scheme 1. Synthetic route to MTPY, C1, and C2 (a) ZnCl2 (1 equiv.), CH3CN, room temperature, stirring, 24 h (b) Zn(NO3)2.6H2O (1 equiv.), CH3OH, room temperature, stirring, 12 h. IR spectra of MTPY displayed vibrations due to pyridine ring at their usual positions.48 The formation of terpyridine moiety has also been supported by the loss of band due to C=O (1680 cm-1) in the IR spectrum of MTPY. Significant shift for the bands due to terpyridine moiety in the IR spectra of C1 and C2 clearly suggested the complexation of ligand with metal center Zn(II) (experimental section). IR spectra of C2 displayed sharp vibrations at 1384 cm-1 associated with NO3 group coordinated in monodentate fashion along with diagnostic bands due to coordinated MTPY (see Figure S7). 1

H and 13C spectral data are gathered in the experimental section and spectra shown through

Figure S1S5. The –NCH3 protons of MTPY resonated as a singlet at δ 3.43 ppm, while aromatic protons appeared as broad multiplet (δ ~ 6.858.73 ppm). Likewise, aromatic protons of C1 and C2 resonated in the range of δ 6.958.83 and 7.039.30 ppm, respectively. 13C NMR spectroscopic data for the ligand and complexes C1 and C2 further supported their formation and proposed structures. The ESI-MS strongly supported formation of MTPY and the complexes, C1, C2 and C1PPi. In its mass spectrum these compounds displayed molecular ion peak at m/z 445.1467 [M+H]+,

584.1913

[MCl+CH3CN]+,

602.9929


[MNO3+CH3OH]+,

1277.3540


[C58H42N10O7P2SZn3H+]+, respectively (see Figure S6 and S15) and strongly suggested their respective formulations. Photophysical Properties Photophysical properties of MTPY, C1 and C2 have been investigated using their dilute solution through UVvis and photoluminescence (PL) studies. In UVvis spectrum of MTPY (THF; c = 50 M) displayed two bands at 290 and 361 nm (Figure 1a). The band at lower wavelength has been assigned to * and the higher one to intra-ligand charge transfer transition (ILCT). On the other hand, UVvis spectra of C1 and C2 have been acquired in THF (c = 50 M) wherein major absorption band for the complexes red shifted (~ 40 nm) with respect to MTPY and appeared at 408 (C1) and 388 nm (C2), respectively. MTPY displayed a strong emission at 506 nm with a large Stokes shift of 145 nm when excited at 361 nm. Upon excitation at 408 nm, C1 showed emission maxima at 509 nm, while C2 emitted moderately at 511 nm when excited at 388 nm (Figure 1b).

Figure 1. (a) Absorption and (b) Emission spectra of MTPY, C1 and C2 at λex = 361, 408, and 388 nm, respectively in THF (c = 50 M). It has been observed that emission for C1 and C2 red shifted by 3 and 5 nm and displayed a reduced quantum yield [MTPY, (6.7%); C1, (0.36%); C2, (1.90%)] (Table S2). This may be related to variable extent of ICT in the ligand and respective complexes. Notably, TPY unit upon coordination with Zn(II) behaves as a better acceptor and produces more efficient ICT in the complexes which, in turn leads to red shift and emission quenching for the complexes. Distinct ICT processes in the ligand and complexes have further been assessed by solvatochromism. DA structure of the ligand and its complexes leads to substantial solvatochromism as shown in


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Figure 2a and 2b. An examination of the emission spectra of these compounds (MTPY, C1 and C2) revealed that an increase in polarity of solvents from extremely non-polar (hexane) to polar (methanol) with prominent decrease in intensity led to red shifts of 72, 108 and 75 nm, respectively. The emission spectra of these compounds in the solvents of high polarity displayed structureless feature, however in non-polar solvents distinct vibronic structures were observed (Figure 2 and Figure S8). Interestingly, the bathochromic shift was accompanied by broadening of the emission band, which is a characteristic of charge transfer emission. Over current range of solvent polarities, emission of these compounds lies in the range 465590 nm and is accompanied by visible emission colour change from blue to orange-red (ex, 365 nm, Figure S16). Noticeably, C1 involving chloride ion (Cl) shows prominent solvatochromism in halogenated solvents relative to non-halogenated ones. It was found that emission maxima for C1 in CH2Cl2 and CHCl3 bathochromically shifted by 80 and 85 nm, respectively as compared to that in THF, however no such changes were observable for C2. In halogenated solvents, possessing a DA scaffold, C1 experiences enhanced ICT due to presence of Cl as an anion and leads to red shifted emission (Table S1).49 Variation of the emission intensity with orientation polarizibility (f, solvent polarity parameter) displayed a downward curve suggesting vulnerability of the excited state to decay through nonradiative processes in polar solvents, leading to emission quenching. The plot of Stokes shift vs. f displays linear dependence of Stokes shift with a positive slope indicating positive solvatochromism (Figure S8 and Table S1).

Figure 2. Emission spectra of (a) MTPY and (b) C2 in solvents (c = 50 M) of varying polarity. Solid-state Emission Solid-state emission spectra of these compounds have also been acquired (Figure 3 and S17). Upon excitation at 361 nm, MTPY showed a broad band (425–650 nm) with the maxima



centered at ~ 529 nm. On the other hand, out of C1 and C2 only former showed emission in the solid state at ~ 610 nm (br., 550675 nm) when excited at 408 nm.

Figure 3. Solid-state emission spectrum of MTPY, C1 and C2 (λex = 361, 408 and 388 nm, respectively). Effect of Solvent Systems in Self-assembly Process Since, aggregation is a solvent mediated process the aggregation behavior of the molecules will strongly depend on the inherent hydrophilicity and hydrophobicity of both the solvents and molecules.54 Also, it has been established that during aggregation, alteration in relative distributions of the hydrophilic and hydrophobic moieties are responsible for specific aggregate morphologies. To envisage the effect of hydrophilicity and hydrophobicity of the solvents and compounds under study on self-assembly behavior, the photophysical properties of these molecules have been scrutinized in two sets of solvent/non-solvent systems; THF/water (hydrophilicity effect) and THF/hexane (hydrophobicity effect). The experiments have been carried out by monitoring emission profile of MTPY in THF/water by adding increasing concentration of MTPY. Noticeably, emission behavior of MTPY displayed strong concentration dependence in THF/water mixture. It was observed that as concentration of MTPY increases from 5400 µ emission intensity enhances with a red shift in the emission maxima from 459510 nm (λ, 51 nm). Upon increasing concentration to 600 µmaximum emission enhancement (~33 fold) occurred due to RIR with additional red shift (~58 nm). Further increase in the concentration from 6001000 µled to a red shift of emission maxima from 517527 nm however, emission intensity was significantly quenched. The red shift with enhanced emission spectra indicated formation of self-assembled aggregates while decrease in


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the emission intensity at higher concentrations may be attributed to reduced solubility and precipitation (Figure 4a). The enhanced lifetime from 0.65 (c = 50 M, THF) to 6.05 ns (c = 600 µTHF/wateralso supported aggregate formation (Figure S20). Further, solid state emission maxima for MTPY (529 nm) corresponded with the on generated by concentration induced aggregation emission (527 nm) and advocated the analogy of CIE to AIE and solid state emission.

Figure 4. (a) Emission spectrum and (b) Normalized emission spectrum of MTPY with varying concentrations (51000 M THF/water) displaying CIE. The concentration induced emission (CIE) is analogous to AIE for organic luminophores and represents the self-assembly in MTPY with increasing solvent hydrophilicity. To rationalize the effect of solvent hydrophobicity emission behavior of MTPY has been scrutinized in THF/hexane solvent system. Interestingly, upon increasing hexane fraction (fH) from 0100% the emission spectra displayed an appreciable blue shift of 46 nm (461507 nm), however intensity showed insignificant variations and remains virtually unaffected (Figure S9). At fH 100%, emission mainly arises due to the LE state (461 nm) whereas TICT diminished and appeared as a shoulder at ~ 485 nm. This behavior is consistent with aforementioned solvatochromic behavior of MTPY and implies that solvent hydrophobicity presents an insignificant effect on initiation or progression of the self-assembly process. Similar experiments have also been performed on the Zn(II) complexes C1 and C2. With increasing water fractions these displayed significant emission quenching in THF/water mixture along with a monotonous red shift in the emission maxima (Figure S10). These observations may lead to a conclusion that these Zn(II) complexes exhibit typical ACQ behavior, however this conclusion cannot be comprehensive enough to account for significant solid state emission of these complexes. Therefore, emission behavior of these compounds has been examined with



increasing hydrophobicity of the solvent system (THF/hexane mixtures) (Figure 5). Upon increasing hexane fraction (fH) in THF/hexane mixture, emission maxima for C1 (Figure 5a and 5b) and C2 (Figure 5c and 5d) displayed successive blue shifts. As the fH approaches 100%, both the complexes exhibited overall blue shifts of 27 and 46 nm, respectively.

Figure 5. (a) Emission spectrum of C1 in THF/hexane mixtures with different hexane volume fractions (c = 50 M). (b) Plot of maximum emission intensity and wavelength (em) of C1 versus fH. (c) Emission spectrum of C2 in THF/hexane mixtures with different hexane volume fractions (c = 50 M). (d) Plot of maximum emission intensity and wavelength (em) of C2 versus fH. Further, emission attained maximum value at fH 80% and a decrease in intensity occurred at fH 90 and 100%. It is presumed that at initial fH, emission enhancement is governed by the solvent polarity (TICT) and at high fH it is directed by formation of aggregates. At fH 90 and 100%, emission quenching may be ascribed to formation of insoluble aggregates of the complexes. Simultaneously under similar conditions, the fluorescence life time enhanced from 0.09 and 0.07 ns (fH 0%) to 5.94 and 5.64 ns (fH 80%), affirming aggregate formation in C1 and C2, respectively (Figure S20). In solution (THF), C1 and C2 displayed weak emission due to active intramolecular rotation which promotes decay in the excited state via nonradiative processes thus the molecules show a lower lifetime. With increasing hexane content these tend to form aggregates which restrict intramolecular rotation and prevent nonradiative decay and molecules stay longer in excited state. By comparing above results for MTPY and complexes C1 and C2, we have reached to a general understanding of the fact that hydrophilicity and


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hydrophobicity of the solvent largely influence aggregate formation leading to distinct photophysical properties. This assumption may be rationalized by considering inherent hydrophilicity/hydrophobicity of the molecules. PTZ being a tricyclic hydrophobic scaffold imparts significant hydrophobicity to MTPY, which in turn, enforces it to form aggregates only in hydrophilic solvents. This is consistent with the observed CIE in THF/water mixture and insignificant changes in THF/hexane. On the other hand, presence of hydrophobic (PTZ) and hydrophilic (MX2; X = Cl, NO3) units in the Zn(II) complexes enables them to form aggregates in both hydrophilic and hydrophobic solvent systems. However, photophysical properties of both the type of aggregates distinctly varied with the solvent system employed. Morphological Analysis To gain deep insight into the effect of solvent hydrophilicity/hydrophobicity on aggregate build up, morphology of the ensuing aggregates for MTPY and its complexes have been investigated in different solvent systems by scanning electron microscopy (SEM). The aggregates suspension of MTPY obtained at 600 µM solution of THF/water produced nanoballs aggregates of uniform size (Figure 6a), while no such aggregates could be observed in THF/hexane mixture with fH 100%. On the other hand, complexes C1 and C2 afforded aggregates of varying shapes and size depending upon the employed solvent system. C1 produced an entangled mass of densely packed fibers (Figure S11a) in hydrophilic solvent system (THF/water), however in hydrophobic solvent system it gave nanorod like structures of uniform thickness (THF/hexane) (Figure 6b). Further, C2 created flakes of irregular size and shape in hydrophobic solvent system (Figure 6c) while nearly spherical aggregates of welldefined size and shape in hydrophilic system (Figure S11b). It is believed that intermolecular interactions plays a decisive role in altering molecular packing and aggregate morphology through relative positioning of the hydrophilic and hydrophobic moieties in different solvents.16,54 These morphological differences are suggestive of marked effect of hydrophilic/hydrophobic interaction on aggregate build up and photophysical properties.



Figure 6. SEM images of the aggregates of MTPY (a) created in THF/water (c = 600 M), C1 (b) and C2 (c) formed in THF/hexane (fH 80%; c = 50 M). Effect of pH on Photophysical Properties During investigations on MTPY we encountered with CIE effect whose analogy with AIE has been evidenced by monitoring the effect of pH on its photophysical properties. The acidic and basic functionalities of TPY unit make it susceptible toward environmental pH changes.50,51 On the other hand, neutral and basic media promotes AIE in TPY derivatives, acidic media strongly quenches emission due to formation of water soluble pyridinium ions. Thus, addition of an acid to aqueous solution should quench emission associated with MTPY. It has been affirmed by monitoring emission in aqueous solution (MTPY, c = 600 M; pH = 7) by gradual addition of triflouroacetic acid (TFA). Generally, TFA is preferred over other acids due to its useful properties: volatility, miscibility in water as well as most of the organic solvents and its strength as an acid.58 Lowering the pH from 6 to 1 by successive addition of TFA to a solution of MTPY led to a decrease in fluorescence intensity for the band at 517 nm without any change in its position (Figure 7a). This observation is consistent with the formation of pyridinium ion under acidic conditions which, in turn, results the CIE behavior.

Figure 7. (a) Emission spectrum of MTPY (c = 600 M) in THF/water mixture with varying pH (17). (b) Plot of change in emission intensity at 517 nm with varying pH (17). Emergence of AIE Attribute in Hydrophilic Solvent System and Anion Detection The ACQ and possibility of AIE for metal complexes in THF/water mixture (hydrophilic solvent system) has been investigated by monitoring the effect of anion binding on Lewis acidic character of Zn(II).523 It has been presumed that C1 may act as a light-up probe for


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pyrophosphate (PPi, P2O74-) under the influence of anion triggered AIE. To explore the binding of PPi with C1 titration studies have been undertaken in THF/water at fw 60%. At this water fraction interference in emission due to self-assembly (vide-infra) is minimum and maximum emission enhancement may be realized (Figure S12). An inspection of the absorption profile of C1 in presence of a series of competitive anions [CO32, CH3COO, SO42, F, S2, HPO42PO43, H2PO4, HCO3, ATP (adenosine triphosphate), ADP (adenosine diphosphate), AMP (adenosine monophosphate)] revealed that the band at 408 nm red shifted upon addition of a small amount of PPi (c = 50 nM), however other tested anions at the higher concentration (c = 10 M) under exactly similar conditions could not show any appreciable change (Figure 8a). Likewise, fluorescence intensity of C1 remains unaffected upon addition of other anions (c = 10 M), whereas addition of PPi (c = 50 nM) led to a phenomenal increment (Figure 8b) which sustained to augment with gradual additions of PPi and attained saturation (~ 8 fold emission enhancement) at ~ 150 nM (Figure 8c).

Figure 8. (a) UV-vis spectra of C1 (c = 50 M) in THF/water mixture in presence of PPi (c = 50 nM) and other various anions (c = 10 M). (b) Emission spectra of C1 (c = 50 M) upon addition of PPi (c = 50 nM) and other anions (c = 10 M). (c) Fluorescence spectra of C1 (c = 50 nM) in presence of PPi (c = 20150 nM). (d) Dynamic light scattering for C1 and C1PPi in THF/water. All the measurements were carried out in THF/water mixtures (fw = 60%).



Job’s plot analysis indicated 1:3 binding stoichiometry between the PPi and C1 (Figure S13). LOD for PPi in this assay came out to be 2 nM and is comparable to that of recently reported fluorescence probes for PPi detection (Figure S14).40 The fluorescence turn on of C1 in presence of PPi may be explained as follows: Due to weak ICT effect, MTPY forms fluorescent aggregate in THF/water system, but C1 shows strong ICT leading a typical ACQ behavior. On addition of PPi, ACQ is overcome by its coordination with C1 as strong negative charge on PPi weaken the ICT effect of C1 leading to redemption of the fluorescence through AIE. Also, the complexation of PPi with C1 in 1:3 stoichiometry increases steric bulk of complex leading to formation of loosely packed aggregates. These effects can collectively lead to emission enhancement upon binding PPi with C1 in THF/water mixture. These assumptions have further been supported by DLS studies (Figure 8d). Expectedly, addition of PPi to C1 led to an increase in the average size of nanoaggregates [from 300 (C1) to 400 nm (C1PPi)]. It is worth mentioning that C2 (c = 50 M) displayed negligible changes toward anions relative to C1 which can be attributed to greater lability of the chloro group. Density Functional Theory Calculations Job’s plot analysis suggested 1:3 binding stoichiometry between the PPi and C1. To affirm this and to have an idea about the ultimate species resulting from interaction between PPi and C1 theoretical studies have been performed. In this direction structure of the complexes has been optimized using density functional theory (DFT) method. The geometry optimization for the complexes under study has been carried out using GAUSSIAN 0959 at the B3LYP level of theory. The metal center Zn(II) has been described by the LANL2DZ effective core potential (ECP) basis set while non-metal atoms by the 6-31G** basis set. We have performed frequency calculations to make sure that all the positive frequencies were obtained indicating a real minimum, not a transition state geometry. As shown in the Figure S15, for C1, the HOMO is only localized on phenothiazine ring, while LUMO is delocalized across the acceptor TPYZnCl2 unit. The HOMO to LUMO electronic transitions in the complex can be assigned as strong intramolecular charge transfer. In addition, band gap between HOMO (5.59 eV) and LUMO (2.42 eV) for C1 was calculated to be of 3.17 eV. The electron density of C1 in the HOMOLUMO was affected by the PPi. Due to the formation of C1PPi the band gap between HOMO (7.69 eV) and LUMO (5.73 eV) further decreased to 1.96 eV. The lower transition energy in the latter case suggests the red shift of absorption spectra after binding with PPi, which


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is qualitatively in agreement with the experimental results. The optimized structure of the C1PPi revealed that the C1 and PPi bind together in a ratio of 3:1 (which is in accordance with Job’s analysis) (Figure 9). The HOMO and LUMO in C2 were found to be at 5.71 eV and 2.71 eV, respectively with a gap of 3.00 eV. Based on the HOMOLUMO energy gap one can expect that both C1 and C2 would have similar donor-acceptor property. However, the experimental observation of the specific binding of PPi with C1 not with C2 can be anticipated as follows. In C2 the two nitrate anions are bonded to Zn through the center of the charge delocalized in the ONO unit which acts effectively as if a bidented ligand (Figure S18) making them much less labile compared to chloride in C1. As a matter of fact two chlorides are easily replaced and form 1:3 complexes with PPi when treated with C1 forming stronger ZnO bonds of equivalent number. If the nitrate anions are to be replaced when PPi is treated with C2 then it will be energetically unfavorable to replace effectively lesser number of ZnO bonds almost of same strength.

Figure 9. DFT optimized structure of C1PPi. Further, the time dependent density function theory (TD-DFT) also have been performed by using B3LYP/6-31G** for MTPY and B3LYP/LANL2DZ for C1 and C2. It was found that UV/Vis spectra obtained theoretically were comparable to those obtained experimentally (Figure S19). Conclusions Summarily, through this work we have successfully designed and synthesized a PTZ-TPY based DA system MTPY and Zn(II) complexes C1 and C2 derived from it. The DA construct in these compounds endows them with ICT dependent solvatochromism. Strategic investigations on



AIE and solid state emission in such DA systems unveiled analogous concentration induced emission (CIE) in MTPY which enabled us to decipher an exquisite balance between the selfassembly and solvent hydrophilicity/hydrophobicity. Increasing hydrophilicity (THF/water) causes CIE in MTPY however C1 and C2 displayed typical ACQ. Contrarily, increasing hydrophobicity (THF/hexane) exhibited in significant effect on MTPY and led to distinctive aggregation and emission enhancement for both the complexes. Further, emergence of AIE in C1 in presence of PPi in THF/water system enabled its rapid and efficient detection. More importantly, the DFT calculation supported the C1 is attached with PPi in 3:1 stoichiometry due to decrease in energy gap between HOMO and LUMO. These simple, yet significant results may be useful in controlled designing of nanostructures with valuable photophysical characteristics of DA systems. Acknowledgements Authors gratefully acknowledge the Science and Engineering Research Board (SERB), New Delhi, India for providing financial assistance through the scheme (EMR/2015/001535) and also to CSIR, New Delhi, India for award of a Junior Research Fellowship to VDS (Chem.09/013(0621)/2016-EMR-I). We are also thankful to the Head, Departments of Chemistry, Institute of Science, Banaras Hindu University, Varanasi (U.P.) India, for extending laboratory facilities. Supporting Information 1

H,

13

C NMR, ESIMS, IR, fluorescence spectra, SEM images, theoretical studies and Tables

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