Multifunctional Self-Assembled Macrocycles with Enhanced Emission

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Multifunctional Self-Assembled Macrocycles with Enhanced Emission and Reversible Photochromic Behavior Soumalya Bhattacharyya,‡ Aniket Chowdhury,*,‡ Rupak Saha, and Partha Sarathi Mukherjee* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India

Inorg. Chem. Downloaded from pubs.acs.org by WEBSTER UNIV on 02/27/19. For personal use only.

S Supporting Information *

ABSTRACT: A series of self-assembled functional Pt(II) molecular hexagons (M1−M3) is reported. Hexagons M1 and M2 were designed employing aggregation induced emissive and photochromic building blocks, respectively, while macrocycle M3 is a bifunctional, containing both the kinds of building units. Hexagons M1 and M3 were found to inherit the enhanced emission with aggregate formation which was explored using UV−vis and fluorescence spectroscopy. The enhanced emission of macrocycle M3 compared to that of its building units was driven both by metal−ligand coordination and formation of nanoaggregates as evident from SEM, DLS and TEM analyses. Two of the macrocycles (M2 and M3) were also found to be photochromic due to the presence of spiropyran in the molecular backbone. Due to the virtue of protonation−deprotonation equilibrium of the spiropyran, these macrocycles (M2 and M3) showed reversible acidochromic behavior. Macrocycle M3 represents the first example of a self-assembled Pt(II) architecture which is multifunctional with aggregation-induced emission (AIE), photochromic, and acidochromic properties. This new generation macrocycle (M3) also showed coordination-driven enhanced emission and light-induced color change behavior compared to the starting building blocks. Our present approach of incorporating multiple functions into a single self-assembled structure with enhanced functionality compared to the starting building blocks via coordination self-assembly is noteworthy and has huge potential for the development of multifunctional materials.

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INTRODUCTION One of the main objectives of cutting-edge chemical research in recent times has been directed to improve and diversify the performance of specific target molecules by numerous approaches including the structural modulation by covalent/ noncovalent interactions,1 external stimuli,2 nanoconfinement,3 and so on. The molecules found their applications in various fields such as drug discovery, targeted drug delivery, functional materials,4 sensors,5 artificial enzymes,6 light harvesters,7 and others. New strategies are being implemented with attempts to incorporate different functionalities into a single molecular entity where their activity can be triggered using various external stimuli such as light,8 pH,9 structural distortions,10 acoustics,11 redox potential,12 magnetic field,13 and so on. Therefore, the demand for a new class of multifunctional complex molecular systems which can execute more than one type of functions is rapidly gaining momentum. The conventional approach to adjoin multiple functional groups with covalent attachment has been marred with several restrictions including multistep process leading to low final yields, time-consuming purification process, and most importantly the unpredictable behavior of the final compound. To achieve new multifunctional systems by combining different functionalities, diverse types of noncovalent but © XXXX American Chemical Society

stronger supramolecular interactions (e.g., metal ion−ligand coordination, hydrogen-bonding, etc.) have been utilized. Among them, supramolecular coordination complexes (SCC) formed by metal ion−ligand coordination self-assembly14 have drawn considerable attention due to several positive attributes like easy synthesis, high yield, desired shapes and sizes of the products, and the kinetic reversibility of the process that leads to the error correction and self-repairing of the final product(s).15 Various desired functional moieties can be included into the final coordination architectures by attaching proper functional groups to the starting building units.16 As the assembly is constructed by the spontaneous formation of metal−ligand bonds, the functional groups in the building blocks remain unaffected, thereby passing on the desirable properties to the final assembly.17 Apart from structural modifications, by tuning the photophysical properties of the building blocks, the luminescence behavior of the final assemblies has also been tuned to afford functional materials which are used as sensors,18 imaging agents,19 tunable and highly emissive materials,20 light-harvesting materials,21 and many more.22 Received: January 7, 2019

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DOI: 10.1021/acs.inorgchem.9b00039 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1. Structures of Building Blocks a1, a2, d1, and d2 Used in the Synthesis of the Macrocycles

Scheme 2. Self-Assembly of Macrocycles M1−M3 from Their Corresponding Building Blocks a1, a2, d1, and d2

drawbacks such as slow reversibility, structural fatigue, and most importantly generation of severe structural strain due to the direct inclusion of the photoactive moiety into the architectures’ backbone. We envisioned that if the photoactive group is attached to the periphery of the building block it will retain the activity without generating any geometrical distortions. As a photoactive unit, spiropyran is particularly important as it has been used as the photochromic unit in various smart materials including polymers, sensors, lightharvesting units, and so on.28 To incorporate dual functions in a single coordination architecture, we designed an AIE-active Pt(II) 120° acceptor and a spiropyran decorated triarylamine based dipyridyl donor with a complementary bite angle of 120° to match the coordination direction of the triphenylamine based AIE-active acceptor (Scheme 1). Herein we report the first example of a multifunctional selfassembled coordination architecture (M3) that shows

In recent times, Stang and others have reported a few selfassembled coordination architectures with AIE behavior.23 The incorporation of AIE units into molecular systems is reported to induce stimuli-responsive behavior in the final compound which has been exploited for various sophisticated applications,24 but such systems consist of a single kind of function. It will be fascinating to generate an AIE-active stimuli-responsive architecture incorporating a second functional unit that will lead to the generation of bifunctional advanced material. In this regard, light-triggered reversible transformation of coordination architectures has caught our attention due to the use of a benign external stimulus and most importantly the reversible structural transformation which is precisely controlled by photoirradiation.25 Almost all the photosensitive discrete coordination architectures have extensively used diarylethenes as the photoactive center,26,27 but the photoinduced reversible transformation of the architectures suffers from several B


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Figure 1. 1H NMR spectra of a2 (a), M3 (b), and donor d2 (c) in CDCl3 as stacked pattern.

analyses (Supporting Information). Ttriphenylamine (TPA)based acceptor a1 was treated separately with donors d1 and d2 in equimolar amount under an inert atmosphere in dry dichloromethane (DCM). The reaction mixtures were gently heated at 50 °C for 24 h, and the solvent was removed under reduced pressure. The crude was washed several times with cold diethyl ether to remove any impurities. Acceptor a1 upon treatment with donor d1 produced macrocycle M1; treatment with photochromic donor d2 generated macrocycle M2 (Scheme 2). Following the identical procedure, the targeted multifunctional macrocycle (M3) was obtained by selfassembly of AIE-active acceptor a2 with photochromic donor d2. All the building blocks have an inherent bite angle of 120° due to the presence of triarylamine core. The formation of a single type of product in the self-assembly reactions was verified using multinuclear NMR (1H, 13C, and 31P) and 2D diffusion-ordered spectroscopy (DOSY). In the 1H NMR spectrum of M3, the peaks corresponding to the pyridyl protons of building block d2 exhibited a prominent downfield shift (from 6.96 to 7.47 ppm) as compared to that of free ligand (Figure 1). The downfield shift of the pyridyl protons of the ligand was consistent with the electron delocalization from the donor to the platinum acceptor after metal−ligand bond formation. Similarly, in the 1H NMR of M2, the peaks corresponding to the pyridyl donor displayed a downfield shift (from 6.96 to 7.48 ppm) indicating dative bond formation by electron transfer from nitrogen donor to Pt acceptor a1 (Figure S16). The other macrocycles also exhibited a similar downfield shift in their respective donors after assembly formation (Supporting Information). Formation of a single product in all the cases was evident from the appearance of a single peak in 31P NMR spectra. The peak corresponding to free acceptor a2 was found to be at 20.07 ppm which was upfield-shifted significantly to 16.25 in M3 and the coupling constants of the 195Pt satellites also

coordination-driven enhanced emission in aggregates and light induced photoreversibility compared to the starting building units (Scheme 2). We also report two new analogous macrocycles M1 and M2 which contain only a single type of functional units like AIE29 and photochromic behavior, respectively. M1 and M3 exhibited brilliant solvatochromism and AIE activity. Also, M1 exhibited twisted intramolecular charge transfer (TICT), a property inherited by its donor building block. When exposed to UV and visible irradiation, M2 and M3 showed rapid reversible photochromic behavior that was shown to retain even after 10 cycles. Furthermore, M2 and M3 showed acidochromic behavior which stemmed out from the protonation−deprotonation equilibrium of the spiropyran unit. The enhanced photochromic and AIE properties of the multifunctional M3 compared to the starting building units were finally supported by theoretical study.

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RESULTS AND DISCUSSION Synthesis and Characterization. Building block a1 was synthesized and characterized following the reported literature procedure.30 AIE acceptor a2 was synthesized starting from the already reported triphenylamine di-iodo precursor 1 following a multistep synthesis (Scheme S1).31 The diiodo analogue (5) of the acceptor was transformed into the targeted acceptor a2 after treatment with AgNO3 in dry DCM. The aldehyde functionalized donor (d1) was prepared from precursor molecule 1 via conventional Sonogashira coupling reaction (Scheme S2). The photochromic donor (d2) was obtained from the coupling of N,N-bis(4-pyridyl)-4-iodoaniline with freshly prepared alkyne functionalized spiropyran under inert atmosphere in triethylamine in the presence of Pd(PPh3)2Cl2 as catalyst (Scheme S3).32 The crude product was purified by preparative TLC to yield the desired compound in a sufficient amount. All the building blocks were characterized using multinuclear NMR and ESI-MS C


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Inorganic Chemistry

Figure 2. 31P NMR spectra of acceptor a2 (a) and M3 (b) in CDCl3.

Figure 3. Calculated (top) and experimental (bottom) isotopic distribution patterns of the peaks corresponding to [M3 − 4NO3]4+ (m/z = 1440.0334) (right) and [M3 − 5NO3]5+ (m/z = 1139.6419) (left) fragments.

814.7839, respectively), and their isotopic distributions matched with the calculated values (Figures S6 and S7). Similarly, the ESI-MS analysis of M3 exhibited three well distinguished peaks [M3 − 4NO3]4+ (m/z = 1440.0334), [M3 − 5NO3]5+ (m/z = 1139.6400), and [M3 − 6NO3]6+ (m/z = 939.3665) which resembled the theoretically predicted values of 1440.0259, 1139.6231, and 939.3546, respectively, and their isotopic distribution patterns matched well with the calculated patterns (Figure 3). Multiple attempts to grow suitable single crystals of the macrocycles were unsuccessful. To explore the structural information of the macrocycles, their geometries were optimized using the PM6 method (Supporting Information). Although in the literature several hexagonal macrocycles are known to exhibit a 3D chair conformation,22b macrocycles M1−M3 turned out to be planar. The diameter of the

decreased from 3076 to 2872 Hz which was attributed to the electron migration from the ligand to the metal center (Figure 2). For M1 and M2, the changes in the coupling constants were found to be of ΔJ(Pt, P) = 236 and 204 Hz, respectively. The well-defined 1H and 31P NMR spectra indicated the formation of highly symmetric products. Although multinuclear NMR analysis indicated ligand to metal coordination and the formation of a single product, no information about the composition of the final assemblies could be obtained. The final stoichiometry of the building blocks in the macrocycles was undoubtedly verified from electrospray ionization mass spectrometric analysis. In the ESI-MS spectrum of M1, signals corresponding to the fragments [M1 −5NO3]5+ (m/z = 990.1397) and [M1 − 6NO3]6+ (m/z = 814.7788) were found. The signals were in good agreement with the calculated results (990.1321 and D


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Inorganic Chemistry

Figure 4. Optimized structures of macrocycles M1−M3. Color codes: carbon (gray), nitrogen (blue), oxygen (red), phosphorus (orange), platinum (light gray).

Figure 5. Normalized absorption and emission profiles of a1 (a), d1 (b), and M1 (c) in DCM.

optimized hexagon (M1) was calculated to be 5.6 nm. The optimized structures (Figure 4) were validated by comparing them with the experimental molecular size found from the 2D DOSY experiments (Supporting Information). Using Stokes− Einstein equation, the molecular size of M1 was found to be 6.3 nm which is slightly greater than the optimized geometry. The inconsistency can be attributed to the solvation that might increase the effective molecular size in the solution. Similarly, for M2 and M3 the experimental molecular

diameters were 7.2 and 6.1 nm which are comparable with the sizes obtained from DOSY analysis of 7.7 and 6.3 nm, respectively. Photophysical Properties. For all photophysical experiments, DCM was chosen as the solvent because in DCM all the building blocks as well as macrocycles showed high solubility. The UV−vis spectra of a1, d1, and M1 exhibited absorption bands centered around 357, 374, and 372 nm, indicating the fact that the main absorption peak of M1 at 372 E


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Figure 6. Absorption and emission of M1 in different solvents (λex = 370 nm). Bottom: images of M1 in different solvents kept in vials under the exposure of 365 nm UV. [Inset: toluene (Tol), hexane (Hex), chloroform (Chl), dioxane (Diox), acetonitrile (ACN), tetrahydrofuran (THF), ethyl acetate (EA), ethanol (EtOH), dimethyl sulfoxide (DMSO), dimethylformamide (DMF), dichloromethane (DCM)].

nm can be attributed to the π−π* transition of the donor moiety (Figure 5). The other two macrocycles, M2 and M3, also displayed absorption peaks at 331 and 335 nm, respectively, which are originating from the electronic transition of the donors. The fluorescence spectrum of M1 in DCM was centered at 500 nm, and a single narrow peak ruled out the formation of multiple species in the medium. The emission mainly occurred from aldehyde-functionalized donor d1, which also has an emission maximum at 495 nm (Figure 5). Macrocycle M1 was designed as a proof of concept to explore the influence of building blocks on the photophysical properties of the final assembly. Aldehyde donor d1 inherently possessed twisted intramolecular charge transfer (TICT) property due to the presence of electron-rich triarylamine and electron-deficient aldehyde group. When exposed to solvents of different polarity, the donor exhibited emission with a wide range of intensity and peak maxima (Figure S34). The emission intensity was maximum in low-polar solvents including hexane (444 nm) and toluene (435 nm), whereas the peaks gradually undergo a bathochromic shift in high polar solvents like dimethylformamide (DMF) (500 nm) and dimethyl sulfoxide (DMSO) (510 nm) with a gradual decrease in the emission intensity (Figure S34). It is well-known in the literature that with increasing solvent polarity the TICT state becomes more stabilized and the excitation energy mostly decays through nonradiative pathway leading to lowered intensity.33 To verify whether the TICT behavior was retained even after formation of the macrocycle, M1 was exposed to various solvents and displayed fluorescence with a varied range of intensity and peak maxima (Figure 6). In less polar solvents including toluene and hexane, the emission intensities were

high, and the peaks were centered at 445 and 450 nm and gradually red-shifted to 500 and 520 nm in DMF and DMSO, respectively. The large bathochromic shift was attributed to the TICT behavior of the donor moiety which was retained even after macrocycle formation. In the macrocycle the effective electron density on donor d1 decreased due to ligand to metal coordination; hence, the TICT behavior of the donor was more prominent in free donor form than in the M1. However, when M2 was dissolved in different solvents, it did not show discernible changes in emission due to electron deficiency of building blocks a1 and d2. In contrast with M1 and M2, M3 contained highly electron-rich AIE-active acceptor a2. Therefore, the fluorescence intensity was not affected by solvent polarity, and like conventional fluorophores, it showed a gradual bathochromic shift with increasing solvent polarity (480 nm in toluene to 525 nm in DMSO). Therefore, the macrocycles displayed a similar trend in their photophysical behavior as of their building blocks which prompted us to explore more critical photophysical behavior including AIE and photochromism embedded into the macrocycles from their corresponding building blocks. AIE Behavior of the Macrocycles. Acceptor a2 is designed in such a way that it contains an AIE-active triphenyl group on the periphery and is away from the platinum center so that after the ligand to metal coordinate bond formation the AIE activity of the group is not affected by the steric crowding. To validate our assumption, the AIE behavior of the acceptor unit was first explored in a solvent composition of DCM and hexane (Figure S44). Hexane is a poor solvent for the building block; hence, upon gradual increase of hexane fraction in the DCM, the molecules slowly agglomerated to form particles. In the aggregate state, the free rotational motion of the aryl group F


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Inorganic Chemistry

Figure 7. Change in absorbance and emission intensity of M3 with varying hexane fraction in DCM (λex = 340 nm). Below: images of M3 in different solvent fractions under UV radiation of 365 nm.

aggregate formation (Figures S50 and S51). However, M1 exhibited a sudden enhancement in fluorescence intensity on aggregation (Supporting Information). The emission intensity increased 2.5-fold from 10 to 90% hexane in DCM. When carefully inspected, it was observed that the emission maxima of the compound changed from 495 to 456 nm in 90% hexane in DCM. Such a large blueshift in emission spectra cannot be solely attributed to aggregate formation. As previously observed in the solvatochromic experiment, M1 revealed the TICT phenomenon. It is proposed that in pure DCM the molecule adopted twisted geometry which was stabilized due to the polarity of the solvent and hence the emission intensity was low. When hexane fraction was increased, the solvent environment changed to lower polarity and hence the twisted form transformed into in the aggregate state. In the aggregate form, due to close proximity, steric congestion as well as π−π* interaction increased, and the motion of the aromatic ring froze. Thus, the emission maxima blue-shifted due to this gradual change in the molecular environment which led to the transformation of the TICT state to the aggregate state. To check the role of coordination self-assembly on the AIE behavior of M3, change in fluoresnce intensity of macrocycle M3 and its building block a2 with hexane fraction was compared (Figure 8). It is evident from the plot that the AIE was much pronounced upon macrocycle formation presumably due to well-organized structure of the macrocycle that allows efficient packing during aggregate formation. Particle Size Analysis. The photophysical experiments helped us to explore and establish the solvatochromic and AIE behavior of the macrocycles. Due to change in solvent polarity, nanoparticles were formed after aggregation. It is important to properly characterize the morphology and the size of the particles as it is well-known that AIE behavior strongly correlates to these attributes of the particles. To explore the nature of the aggregates dynamic light-scattering analysis was

was restricted and the emission intensity of the solution drastically enhanced. It was found that with the addition of hexane to the DCM solution the emission intensity gradually dropped from 10 to 30% hexane fraction in DCM which was attributed to the poor solvent property of hexane. After the addition of more hexane, the emission intensity started to regain and surpassed the initial maxima at high fractions of 80 and 90%. The formation of emissive aggregates at high hexane fraction contributed to the emission enhancement. Following the similar protocol, the absorption spectra of M3 were recorded with variable hexane fraction in DCM. When compared together, the absorption spectra did not change considerably at lower hexane fraction up to 70% hexane in DCM. However, with further addition, the spectral width was broadened, and a long wavelength tail appeared. Such a long wavelength tail is presumably due to Mie scattering that is generated from the spherical particles present in the system having dimension comparable with the wavelength of light.34 In DCM, the emission intensity of the M3 at 505 nm was very low, which was attributed to the free rotation of the phenyl rings (Figure 7). Thus, the excitation energy dissipated through nonradiative pathway including free rotation, vibration etc. When hexane fraction increased more than 50%, due to poor solubility, M3 molecules started to form nanoaggregates. In the aggregate state, the rotation of the aryl rings was frozen due to steric congestion and the excitation energy was released via radiative fluorescence pathway leading to a gradual increase in fluorescence intensity and the peak maxima was at 500 nm. Therefore, even after macrocycle formation, the AIE activity of the building block was retained. When the hexane fraction was increased even further, the fluorescence intensity again increased but the peak maximum slightly blue-shifted to 470 nm. The emission intensity increased 5-fold and the quantum yield increased from 9.5 in 10% hexane in DCM to 33 in 90% hexane in DCM. Unlike M3, M2 does not contain any AIE-active precursor and hence it did not show any drastic change when subjected to the same G


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Inorganic Chemistry

morphology for prolong time. The average size of M2 particles changes from 180 to 270 nm with increment of hexane fraction from 80 to 90% (Figure 11). Similarly, M1 and M3 also displayed similar spherical morphology, and their particle size changed to ∼250 nm. Photochromic Experiments. Photochromic spiropyran is known to exist in nonpolar closed-ring form under room light and transform to polar ring-opened conformation in the presence of UV light. To arrest the less stable open form, polar solvent DMSO was chosen. No change in coloration was observed for donor d2 alone even after exposure to UV light for 10 min (Figure S86). Interestingly, when the solution of M3 was exposed to 365 nm light for 2 min in a closed chamber, the color of the solution changed from yellow to intense green indicating photochromism of M3 induced by metal−ligand coordination. The absorption maximum at 360 nm slightly shifted to 365 nm after UV irradiation, and a new intense peak at 625 nm appeared and is considered to be the main contributor for the change in coloration (Figure 12). When the solution was kept outside after UV−vis experiment, the color of the solution turned back to yellow in a minute. M2 also showed similar reversible photochromic behavior (Figure S87). To verify whether the photochromic behavior of the macrocycle could retain for multiple cycles, the M3 solution in DMSO was exposed to UV and visible light repeatedly, and their absorption spectra were recorded (Figure 13). From the cyclic reversibility experiment, it was found that the macrocycle retained its photochromic behavior even after 10 cycles. Theoretical Study. It is known that more electronwithdrawing group stabilizes the polar open merocyanine forms of the spiropyran derivatives. In our building block (d2), the inclusion of ethynyl group as well as triarylamine moiety increased the electron density in the ring. Hence, the opening of the ring is not favorable. To improve the photochromic behavior of the ligand it is necessary to reduce the electron density on the donor building block. As evident from multinuclear NMR analysis, the coordination of the donor to acceptor transferred electron density from the donor to the electron deficient Pt(II) center (Figure 1). As a result, under UV irradiation, after macrocycle formation, the N−Pt bond effectively acted as an electron withdrawing group and the photochromic behavior of the donor was pronounced significantly leading to a stable and reversible light-sensitive photochromic macrocycle (Figure 12). From the theoretical calculation, it was found that the coordination indeed lowered the total energy of the system from −46.862 keV in the

Figure 8. Comparative analysis of the change in fluorescence spectra of building block a2 and macrocycle M3 during aggregate formation.

performed on all the macrocycles. For particle size determination, high hexane fraction (80%) was chosen. For M1, spherical particles with the average diameter of 202 nm were obtained (Figure 9). From the distribution pattern, it was observed that particles with comparable sizes were formed into the system. For the macrocycles M2 and M3, the mean particles sizes were 177 and 168 nm, respectively (Figure 9 and Supporting Information). Although DLS experiment helped to predict the size of the particles, TEM analysis was performed to further explore the exact shape and dimension of the particles. Appropriate solutions (80 and 90%) of the macrocycles were drop-cast on copper grids and were slowly dried to afford the sample for TEM experiment. For M3, perfectly spherical and solid particles were obtained from both the solvent compositions. The particle size changed from 180 to 240 nm when the hexane fraction was further increased which supports our previous observation from DLS experiment (Figure 10). It was also observed that the distribution of the particle dimensions was very narrow leading to a uniform suspension. Macrocycles M1 and M2 exhibited the formation of similar spherical particles when their solutions were subjected to TEM analysis (Figures S84 and S85). Finally, to examine the morphology of the aggregates, SEM analysis was performed on all the macrocycles, and the same solutions (80 and 90%) were used for investigation. As observed in Figure 11, the macrocycles retained their spherical morphology even after drying, which indicates that once the particles are formed they are quite stable and could retain their

Figure 9. Particle size distribution of M1 (a) and M2 (b) with 80% hexane in DCM. H


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Inorganic Chemistry

Figure 10. TEM images of M3 with varying hexane fraction in DCM. 80% (a) and 90% (b).

Figure 11. SEM images of M2 with varying hexane fraction in DCM. 80% (a) and 90% (b).

Figure 13. Reversible photochromic behavior of M3 under UV and visible light exposure.

Figure 12. Reversible photochromic behavior of M3 in DMSO. λex = 365 nm and visible light.

was 0.054 hartree, and after ring opening, it increased to 0.01 hartree. The electron density on the oxygen atom in the free donor decreased from −0.012 to 0.054 hartree after metal coordination, which indicates migration of electron density from the ligand to the metal center. This observation holds true for the macrocycles as well. Hence, the open form is attainable and facile photochromism was observed. Acidochromism. In addition to light, proton source, or other metal ions may also arrest the merocyanine open form of the ligand in the macrocycle. Therefore, we added the equivalent amount of highly dilute nitric acid (2 M) solution to DMSO solutions of M2 and M3. After addition of nitric acid in the DMSO solutions in room light, no change was observed. When the mixture was exposed to 365 nm UV irradiation, the color of the solution changed from faint to intense yellow as the open form was arrested by protonation. Even after prolonged exposure to the visible light, no photoreversal was noticed. In the absorption spectrum of M3, apart from the main peak at 360 nm, a new peak was generated at 455 nm. To

noncoordinated form to −120.543 keV in coordinated form (Supporting Information). To further illustrate our proposition, the electron density mapping in the open and closed form of the free ligand and metal coordinated form were also explored (Figure 14). From Figure 14, it was observed that both in the ring closed and open forms (d2), the electron density mostly remained on the aromatic rings. The electron density on the oxygen atom of noncoordinated donor increased from −0.012 hartree (closed form) to −0.067 hartree (open form). Thus, the ring-closed form of free ligand d2 was more favorable, and photochromism did not occur. For the metal-coordinated model complex, however, the electron density mostly migrated to the electrondeficient Pt(II) and the electron mapping established that in the open form, the electron density moved from the hydroxyl group to the acceptor unit. In the metal coordinated model complex, the electron density on the oxygen in the closed form I


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Inorganic Chemistry

Figure 14. (a) Electron density mapping of the donor (open and closed forms) and (b) the model compound in closed and open forms.

acidochromism. Macrocycle M3 is unique not only due to its multifunctional nature but also it is noteworthy due to its coordination-induced enhanced AIE and the photochromism with respect to the starting building blocks. This enhanced photochromism in both M2 and M3 compared to that of the spiropyran functionalized building donor (d2) was supported by theoretical study. The strategy of incorporating multiple functions into a single self-assembled architecture with enhanced functionalities compared to that of the starting building blocks via coordination self-assembly may have potential for the development of multifunctional materials that can have extensive applications in display devices, memory devices, and drug delivery.

deprotonate the open form, very dilute solution of triethylamine was added to the mixture, and an immediate change in the color from intense yellow to bluish-green was observed for the system (Figure 15). The change indicated the formation of

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00039. Syntheses and characterization data (NMR, FTIR, ESIMS, TEM, SEM, DLS, Photoluminescence) (PDF)

Figure 15. Acidochromic behavior of the M3 solution and its reversibility. λex = 365 nm.

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AUTHOR INFORMATION

Corresponding Authors

deprotonated open form in the medium after neutralization. Upon exposure to the visible light, the color of the system changed from green to the original faint yellow color of the ring-closed macrocycle. Thus, the stability of the ring-opened form can easily be manipulated by using external protonating and deprotonating agents. Our current self-assembled macrocycle M3 turned out to be a 3-fold (AIE, photochromic, and acidochromic) multi-stimuli-responsive system.

*E-mail: [email protected] (P.S.M.). *E-mail: [email protected] (A.C.). ORCID

Partha Sarathi Mukherjee: 0000-0001-6891-6697 Author Contributions ‡

S.B. and A.C. contributed equally to this work.

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Notes

The authors declare no competing financial interest.

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CONCLUSION Three new hexagonal Pt(II) macrocycles (M1−M3) have been designed and synthesized by coordination self-assembly of 120° complementary building units. While M1 and M2 are aggregation-induced emissive and photochromic, respectively, M3 is a multifunctional architecture. The M1 contains aldehyde functionalized donor and is found to retain the TICT behavior from the parent donor and displayed brilliant solvatochromic effect and aggregation induced emission (AIE). M2 is photochromic in nature due to the presence of spiropyran functionalized donor. M3 represents the first example of a multifunctional self-assembled coordination architecture that shows AIE, reversible photochromism, and

ACKNOWLEDGMENTS P.S.M. is grateful to CSIR (New Delhi) for financial support. S.B. acknowledges the CSIR, New Delhi for the research fellowship. We sincerely thank Dr. Suman Ray (SSCU, IISc) for valuable inputs on mechanism of photochromism.

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REFERENCES

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