Covalent Functionalization of Organic Nanoparticles Using Aryl

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Covalent Functionalization of Organic Nanoparticles by Aryl Diazonium Chemistry and their Solvent-dependent Self-assembly Sreedevi Krishnakumar, and Karical R. Gopidas Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03269 • Publication Date (Web): 06 Jan 2017 Downloaded from http://pubs.acs.org on January 8, 2017

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Covalent Functionalization of Organic Nanoparticles by Aryl Diazonium Chemistry and their Solvent-dependent Self-assembly Sreedevi Krishnakumar† and Karical R. Gopidas†‡* †

Photosciences and Photonics, Chemical Sciences and Technology Division, CSIR-National

Institute for Interdisciplinary Science and Technology, Council of Scientific and Industrial Research (CSIR), Trivandrum 695019, India. ‡

Academy of Scientific and Innovative Research (AcSIR), CSIR-NIIST Campus, Thiruvananthapuram

KEYWORDS. Covalent functionalization, Organic nanoparticle, Diazonium salt, Self-assembly, Ruthenium(II) bipyridine complex. ABSTRACT. A simple method for covalent functionalization of Fréchet-type dendron nanoparticles (FDN) by tris-bipyridylruthenium(II) is described. Covalent functionalization is achieved by chemically reducing the diazo derivative of a ruthenium(II) bipyridine complex in the presence of FDNs wherein the radical species generated gets covalently linked to the nanoparticle surface. Simplicity, rapidity and robustness are the advantages offered by the present approach. The nanoparticles, post functionalization, were characterized by TEM, EDX, TGA, and IR, UV-visible, and NMR spectroscopic techniques. Depending on the solvent the

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ruthenium complex-linked FDN displays a range of morphologies, including nanoparticles, fibernetworks and nanocapsules. In nanocapsules and fiber-networks observed in organic solvents, the ruthenium complex is confined within the interior domain of the aggregate, whereas in the nanoparticles observed in water, they are present on the periphery. The formation of predictable morphologies in different solvents plays a key role in using such self-assembled structures for various applications such as sensing, catalysis and light harvesting. The characterization of these nano-aggregates using different spectroscopic and microscopic techniques is also described. 1. INTRODUCTION Surface modification and functionalization of nanoparticles are areas of crucial importance in the emerging field of nanotechnology. Functionalization of nanoparticles with different moieties extends the potential of these materials in different fields.1-3 Surface functionalization enables us to alter the wetting or adhesion characteristics of nanoparticles, thereby improving their dispersion in the required medium.4-5 Creation of specific sites on the nanoparticle surface for selective molecular attachment is considered to be a promising approach for their applications in various areas such as nanofabrication, self-assembly, bio-labels, drug delivery, nanosensors, catalysis, etc.6-9 Three basic strategies have been employed for the functionalization of organic nanoparticles.10 In the first method, the starting materials themselves possess specific functional groups, such as alkoxyl chains, amino or carboxyl groups and targeted biomolecules.11-13 In the second strategy, naked organic nanoparticles are functionalized through electrostatic or hydrophobic interactions between the recognition groups and the nanoparticles.14-16 The third design type involves covalent attachment of specific functional groups to the nanoparticle.17-20 The surface of the nanoparticles are initially covered with carboxyl or amino groups according to


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the second or first strategy. The specific functional groups like peptides, sugar, protein, antibody etc. are then covalently linked to these surface functional groups (–COOH, –NH2, and –N3) by condensation or click reactions. The covalent strategy has an upper hand over the other strategies as it prevents the leakage of the functional elements from the nanoparticle and the particles formed are more stable towards different conditions. However, the covalent strategy described above has its own shortcomings. This strategy is applicable only if the nanoparticle contains functional groups which are active towards condensation or click reactions. If the nanoparticles do not possess any such groups, it has to be surface modified to incorporate reactive groups such as –COOH, -NH2 or -N3. In this paper, we report a simple one-step reaction for the covalent functionalization of nanoparticles by chemical reduction of an aryl diazonium salt. Owing to the strong electron withdrawing nature of the N2+ group the diazonium salts accept an electron very easily and then decompose spontaneously to give aryl radicals and dinitrogen. The aryl radicals thus formed are highly reactive and exhibit a tendency to get chemically attached to the nearby substrate.21,22 This reaction has been exploited for the modification of various substrates including metals,23-25 semiconductors26,27 and different forms of carbon.28-30 This method has also been used for the preparation of stable monolayer protected nanoclusters31,32 as well as nanoparticle cored dendrimers, where the organic moieties are linked to the nanoparticle through metal-carbon bonds.33,34 The appealing features of using aryl diazonium salts include their ease of preparation, rapid reduction, large choice of functional groups and strong aryl–substrate covalent bonding. Ruthenium polypyridine complexes have been studied extensively due to their interesting photophysical and electrochemical properties. These complexes absorb in the visible region, have a large Stokes shift, exhibit long luminescence lifetimes and are observed to be relatively


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stable. They readily undergo energy and electron transfer reactions. Because of these properties, ruthenium polypyridine-based complexes have been incorporated into different substrates (e.g. polymers,35 carbon nanotubes,36 quantum dots,37 gold nanoparticles,38 silica nanoparticles39 etc.) for a number of applications, such as light-emitting materials,40 light-harvesting devices,41 photovoltaic cells,42 bioimaging43 and sensors.44 Bidan and co-workers reported the synthesis of a diazonium salt of ruthenium bipyridine complex RuN2+ (Chemical structure of the complex is given in Figure S1, ESI).45 They further demonstrated the electro-grafting of this metal complex onto carbon surfaces by reduction of the diazonium ion to a radical. Herein, we have extended this strategy to covalently modify organic nanoparticles using ruthenium bipyridine (Ru(bpy)32+) complex. Although, diazonium chemistry has been utilized for the modification of many substrates, to the best of our knowledge this is the first report of using this technique for the modification of Fréchet-type dendron nanoparticles (FDN). In a recent publication we have reported the synthesis and characterization of FDNs.46 Preparation of these nanoparticles involved the dissolution of the gold core in a first generation gold nanoparticle-cored dendrimer (AuG1). This results in the generation of a large number of dendron radicals in a confined space, which then undergo fast coupling and addition reactions to form the FDNs. In the present paper we describe covalent functionalization of FDN by a Ru(bpy)2(bpy-ph)2+ moiety as shown in Scheme 1. NaBH4 reduction of RuN2+ gave the aryl radical of the ruthenium complex (Ru•, Scheme 1) which gets attached onto the surface of FDN present in the solution resulting in the formation of tris-bipyridyl ruthenium(II)-functionalized FDN or RuFDN.


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Scheme 1.Scheme for the synthesis of tris-bipyridylruthenium(II)-functionalized FDN

2. Experimental section 2.1 Materials RuN2+ was synthesized using a reported procedure.45 FDN was synthesized as reported previously by us.46 Reagents and deuterated solvents were purchased from Sigma-Aldrich and Alfa Aesar. Solvents such as acetone, toluene, dichloromethane, tetrahydrofuran etc. were obtained from MERCK and used as received. For the spectroscopic studies, spectroscopic grade solvents from MERCK were used. 2.2 Synthesis of RuFDN To a stirred solution of FDN (50 mg, 0.01 mmol) and RuN2+ (98 mg, 0.12mmol) in acetonitrile (5 mL), an aqueous solution of NaBH4 (5 mg in 1 mL, 0.13 mmol) was added drop wise at -40 ºC. The reaction mixture was then allowed to stir at the same temperature for another 1 h. The reaction mixture was filtered, acetonitrile was evaporated off and the residue obtained (52 mg) was taken in water (50 mL) and sonicated for 30 min. It was allowed to settle and water


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was decanted. Sonication with water and decantation were continued till the water layer was colorless. This procedure was carried out to remove any free ruthenium complexes present in the product. The same procedure was repeated by sonicating the solid product in toluene (50 mL) to remove any unfunctionalized FDNs. The toluene was decanted and product dried in air and then under vacuum for 2 days. The product obtained was orange-brown powder, soluble in organic solvents like tetrahydrofuran, acetone and acetonitrile and partially soluble in solvents like dichloromethane and chloroform. IR (KBr)

max

(cm-1): 841.86, 1029.50, 1082.55, 1106.42,

1160.30, 1242.02, 1262.00, 1294.87, 1340.78, 1383.17, 1453.44, 1465.71, 1497.34, 1507.93, 1600.34, 1731.40, 2960.24, 2960.24, 2979.15, 3011.38, 3038.34, 3115.01, 3141.89. 1H NMR (500 MHz, TMS, CD3CN, δ): 4.96-5.18 (broad), 6.53-6.59 (broad), 6.66-6.76 (broad), 7.20-7.49 (broad), 7.71–7.83 (broad), 8.02–8.09 (broad), 8.48–8.54 (broad). For studies carried out in water, hexafluorophosphate (PF6) salt of RuFDN was converted to the corresponding bromide salt. For this purpose, the PF6 salt of RuFDN was dissolved in minimum amount of acetonitrile, and the bromide salt was allowed to precipitate upon addition of a saturated tetrabutylammonium bromide solution in acetonitrile. The bromide salt of the RuFDN was then collected by filtration and dried in vacuum. 2.3 Characterization Methods FTIR spectra were recorded with a Shimadzu IR Prestige 21 spectrometer. 1H NMR data were collected at 500 MHz, on a Bruker Avance DPX spectrometer. Thermogravimetric analysis (TGA) experiments were performed using a Perkin-Elmer Pyris Diamond TG/DTA analyzer under nitrogen atmosphere at a heating rate of 10 ᵒC min-1. High resolution transmission electron microscopy (HRTEM) images were obtained with a 300 kV FEI Tecnai 30G2S-Twin transmission electron microscope with energy dispersive X-ray analysis (EDX). Samples for


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TEM analysis were prepared by drop-casting solutions of RuFDN onto standard carbon-coated Formvar films on copper grids (300 mesh) and drying in air for 2 days. Atomic Force Microscopy (AFM) images were recorded under ambient conditions using a NTEGRA (NTMDT) operating with a tapping mode regime. Micro-fabricated TiN cantilever tips (NSG10) with a resonance frequency of 299 kHz and a spring constant of 20-80 Nm-1 were used. AFM section analysis was done offline. Samples for the imaging were prepared by drop-casting the solution on freshly cleaved mica surface at the required concentrations at ambient conditions. Confocal laser scanning microscopy (CLSM) images were obtained on a Leica SP8 Spectral Confocal microscope by collecting the emission in the 550-610 nm region. Samples for CSLM were prepared by drop casting the above solution on a glass slide followed by slow evaporation. For scanning electron microscopic (SEM) measurements, samples were drop-cast on silicon wafers and air-dried on flat surface of cylindrical brass stubs and subjected to thin gold coating using JEOL JFC-1200 fine coater. The probe was inserted into JEOL JSM-5600 LV scanning electron microscope for taking photographs. Zeta potentials were measured using Zetasizer nanoseries (Zeta Nano–ZS, Ms. Malvern Instruments). A minimum of seven measurements for each sample were taken to ensure statistical significance. Absorption spectra were obtained using a Shimadzu 3101PC UV-Vis-NIR scanning spectrophotometer. Emission spectra were recorded on a SPEX-Fluorolog-3 spectrofluorimeter. 2.4 Calculation of number of ruthenium complex per FDN from EDX and TGA The average number of ruthenium (II) complexes attached to FDN was calculated in two ways. In the first method we used the atomic percentages obtained by quantitative EDX analysis (Table ST1, SI). The values obtained were 2.1 and 0.1, respectively, for oxygen and ruthenium. On the average the FDN contains 36 oxygen atoms. If 2.1% correspond to 36 atoms, 0.1% would


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correspond to 1.7 (or ∼2) Ru atoms. This calculation gave ∼2 Ru atoms (or two Ru(bpy)2(bpyph)2+ complexes) per FDN. Alternatively, the average number of ruthenium (II) complexes/FDN can be calculated from TGA data, which is given in Figure S5 (SI). The char yields obtained were 10% and 13% for FDN and RuFDN, respectively. The additional 3% char yield observed for RuFDN can be taken as the weight percentage of Ru in RuFDN. The following equation was then used

(AWRu × NRu) ×

% Ru in RuFDN = 3 =

100

MWFDN + (NRu(bpy)2(bpy–ph) × MW Ru(bpy)2(bpy–ph)) where, AWRu is the atomic weight of Ru (= 101.07), NRu is the number of Ru atoms in FDN, MWFDN is the molecular weight of FDN (= 4730, from previous study), NRu(bpy)2(bpy-ph) = NRu is the number of Ru complexes attached to FDN and MWRu(bpy)2(bpy-ph) is the molecular weight of the Ru complex (= 934.64). Substituting we get 3=

101.07×NRu×100 4730 + (NRu × 934.64)

Solving the equation we get NRu = 1.94 ∼ 2. Both the methods gave same value for NRu. 3. Results and discussion 3.1 Synthesis of RuFDN Diazonium derivative of the ruthenium complex, [Ru(bpy)2(bpy-ph-N2+)][PF6]3 (RuN2+, Figure S1, ESI), was synthesized by a reported procedure.45 Functionalization of the organic nanoparticles with the dye was carried out by reducing RuN2+ in the presence of FDNs. The ruthenium dye is soluble in water and FDN is soluble in toluene. However, RuFDN (PF6 salt) was insoluble in both water and toluene. RuFDN thus prepared was obtained as orange-brown


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powder, stable in the solid form as well as in solution. They were soluble in organic solvents such as tetrahydrofuran, acetone and acetonitrile and partially soluble in solvents like dichloromethane and chloroform. The PF6 salt of RuFDN was insoluble in water but can be easily converted to the soluble bromide salt with the aid of tetrabutylammonium bromide. The RuFDN thus synthesized was analyzed by UV-Visible, fluorescence, IR and NMR spectroscopy. Information about the size and elemental composition of the RuFDN was obtained using TEM and EDX analysis, respectively. Information about the number of ruthenium complexes per FDN was obtained by quantitative EDX analysis as well as TGA. 3.2. Characterization of RuFDN RuFDN was analyzed using UV-visible absorption and fluorescence spectroscopy to confirm the attachment of the ruthenium complex to the FDN. The absorption and emission spectra of the RuFDN in acetonitrile are given in Figure S2 a, b (ESI), which shows the characteristic absorption features of the FDN moiety as well as the characteristic absorption bands of the ruthenium(II) complex at 287 nm (ligand-centered π–π* transition) and 457 nm (d (Ru2+)–π* metal-to-ligand charge-transfer (1MLCT) transition). The absorption spectrum of RuFDN matches the profile obtained by summing up the spectra of the component units, indicating that there are no significant ground state interactions between the two moieties. For a comparison, the absorption spectra of FDN and tris(bipyridine)ruthenium(II) chloride (Rubpy) are given in Figure S2 c (ESI). The absorption spectrum of FDNs features a strong optical absorption in the UV region, with a broad shoulder around 275 nm. This broad band can be ascribed to the π–π* transitions in the FDN. The tail of the absorption spectrum extended into the visible region, which can be attributed to the Mie effect, indicating the presence of nanoparticles in solution. Emission spectrum of RuFDN in acetonitrile showed maximum around 624 nm,


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which is characteristic of tris(2,2′-bipyridine)ruthenium(II), caused by radiative process from the 3

MLCT state to the ground state. The emission spectrum of RuFDN exhibited a broad shoulder

around 527 nm. This broad band around 527 nm corresponds to the FDN unit of RuFDN. Emission spectrum of FDN is given in Figure S2 d (ESI). The above data confirms the grafting of the ruthenium complex on to the FDN. The FT-IR spectrum of RuN2+ is compared with that of RuFDN in Figure S3 (ESI). FTIR spectrum of RuN2+ clearly showed vibrational stretching of diazo group at 2278 cm-1.45 The diazo group decomposes during the reaction and hence this peak is absent in the IR spectrum of RuFDN. This shows the absence of any diazo precursor in the RuFDN. IR spectrum of RuFDN was similar to that of FDN and featured methylene C-H stretching modes at 2960 and 2979 cm-1 and aromatic C-H vibration modes at 3011 and 3038 cm-1. The peak at 3115 cm-1 and 3141 cm-1 corresponds to the aromatic C-H vibration modes of the ruthenium complex.45 Signals corresponding to the bipyridine ligands appear at ca. 1600–1350 cm−1. These peaks are indistinguishable in the IR spectrum of the RuFDN as they are merged with the IR bands of the organic nanoparticles. In Figure 1 1H NMR spectra of RuN2+45, FDN46 and RuFDN are compared. The 1H NMR spectrum of RuN2+ (Figure 1, top) features peaks at δ 8.62(m), 8.49 (m), 8.05(m), 7.81(d), 7.73 (m), 7.67(d), 7.54(m) and 7.39 (m). The 1H NMR spectrum of RuFDN also displayed peaks in the same region at ca. δ 8.51, 8.06 and 7.74 (shown by arrows). The peaks in the δ 7.56 -7.36 region of the ruthenium complex are merged along with the NMR signals of the FDN. The multiplet at δ 8.62 ppm in the 1H NMR of RuN2+ can be assigned to the protons belonging to the diazo-bearing phenyl ring. In the 1H NMR spectrum of RuFDN, the diazophenyl proton signals have vanished. The peaks around δ 8.51, 8.06 and 7.74 are absent in the 1H NMR spectrum of


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FDN (Figure 1, bottom) and can be assigned to the ruthenium complex linked to FDN. However, the peaks corresponding to the ruthenium complex appeared to be broadened in the 1H NMR spectrum of RuFDN. The radical of the ruthenium complex, formed by the reduction of the diazo derivative (as shown in Scheme 1), can attack at different positions on the FDN. This can lead to the formation of a number of position isomers for RuFDN, each of which produces NMR signals, resulting in the broadening of the signals.

Figure 1.1H NMR spectra of RuN2+, RuFDN and FDN in CD3CN.

The TEM image of the RuFDN and the corresponding core-size histogram is shown in Figure 2. The TEM image of RuFDN displayed particles in the size range of 2-4 nm. The size of the nanoparticles did not show any significant change, post functionalization. However, the TEM image of the RuFDN exhibited more contrast compared to the precursor FDN, most probably due to the presence of electron dense ruthenium.43,47


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Figure 2.TEM image (a) and core-size histogram (b) of RuFDN. The EDX analysis of RuFDN (Figure S4, ESI) showed peaks corresponding to ruthenium, in addition to the peaks of carbon, oxygen and copper (from the grid). This further confirms the linking of the ruthenium complex to the organic nanoparticle. EDX also gives the quantitative measurement of the elemental composition in the inspected region in terms of atomic percent and the values are given in Table ST1 (ESI). The value of carbon cannot be taken into account for quantitative purposes due to the interference from the carbon-coated Formvar films on the copper grids used. Hence, we compared the atomic percentages of oxygen and ruthenium in RuFDN in order to estimate the number of Ru atoms per FDN. Our calculation (see Experimental section for calculation) suggested that on an average the RuFDN contains two ruthenium(II) polypyridyl complexes. Figure S5a (ESI) shows the thermogram of RuFDN. RuFDN was found to be stable up to 130 °C, but decomposed in the 130– 650 °C range leaving a char yield of 13%. The char yield of FDN was observed to be 10% (Figure S5b, ESI). We assumed that the remaining 3% corresponds to the weight percentage of ruthenium in RuFDN. Using this data, the number of ruthenium per organic nanoparticle or the number of ruthenium complexes per organic nanoparticle was calculated to be approximately 2 (See Experimental section).


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UV-Visible, IR and EDX analysis clearly show that Ruthenium complex has been attached to the FDN. From the quantitative EDX and TGA measurements, it is determined that, on an average two molecules of the ruthenium complex are present per FDN (Scheme 2). Scheme 2. Schematic representation of RuFDN.

Reduction of diazonium salt can lead to multiple layers.22,48 A large number of polymeric structures are possible, all of which arises by addition of radicals ([Ru•], Scheme 1) to the bipyridine rings of the Ru complex. If multiple layering is occurring in the present case, we would get structures similar to the ones shown in Figure S6 (ESI). All these structures have binuclear or polynuclear Ru(II) complexes linked through ligands. Several binuclear and polynuclear Ru(II) complexes are reported in the literature.49-54 The absorption and emission spectra of these species are red-shifted compared to mono-nuclear Ru(II) polypyridine complexes. The emission lifetimes will also be different. In the case of RuFDN, the absorption, emission and emission lifetimes corresponding to the ruthenium complex part were nearly identical to Ru(bpy)32+ (Figure S7, Table ST2). This actually rules out the possibility of multiple layering in our case. We have used excess of the ruthenium precursor for carrying out the FDN fictionalization. Some of the [Ru•] formed will be reacting with each other to form dimers and oligomers. These will be soluble in water and hence removed during the isolation and


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purification of RuFDN. Only few of the radicals get attached to the FDN. We may be able to control the number of ruthenium complexes getting attached to the FDN by adjusting the reaction parameters like concentration of the substrates, reaction temperature and time. Research in this direction is going on in our laboratory. 3.3. Self-assembly of RuFDNs In RuFDN, the FDN part is hydrophobic whereas, the ruthenium complex part is hydrophilic. This imparts an amphiphilic character to the RuFDN. Amphiphilic systems display a tendency to self-assemble into diverse supramolecular architectures depending on different conditions such as concentration, molecular architecture and solvent environment.55-57 We observed that RuFDN exhibited different morphologies in different solvents. In water, they existed as particles while in dichloromethane, they formed fiber-networks. RuFDNs selfassembled into nanocapsules in acetone and acetonitrile. The morphological transformations of RuFDNs were studied using TEM and AFM analysis. In these studies, the concentration of RuFDN was calculated assuming that two units of ruthenium complex are attached per FDN. Figure 3 shows the TEM images of RuFDNs in water at two different concentrations. At 5 × 10-6 M concentration, TEM analysis showed particles in the size range of 4 to 28 nm (Figure 3a). Zoomed in images of these particles (Figure 3b) revealed that these are formed by the aggregation of smaller particles. On increasing the concentration of RuFDNs to 5 × 10-5 M we observed particles in the size range of 30-60 nm (Figure 3c). On further increasing the concentration to 5 × 10-4 M, a significant change was not observed in the particle size distribution (45-80 nm; Figure S8 a, ESI), indicating that the particles do not show a tendency to undergo further aggregation. The corresponding AFM image is given in Figure S8b (ESI), which shows a particle size distribution of 65-82 nm. Heights of the particles were observed to be in the


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range of 1-4 nm. In water, the hydrophobic FDN part of RuFDN will be directed away from the solvent and the hydrophilic ruthenium complex will be present at the periphery. The repulsion between the positively charged ruthenium complexes at the surface may be the factor which prevents further aggregation of the particles in water.

Figure 3.TEM images of RuFDN (in water) at different concentrations: 5 × 10-6 M (a) and 5 × 10-5 M (c). b. Zoomed in image of particles at 5 × 10-6 M concentration. RuFDNs formed fiber-networks in dichloromethane, as shown by TEM analysis (Figure 4). At a lower concentration (5 × 10-6 M), we could observe particles in the size range of 27-83 nm joining to form fibers (Figure 4a,b). The fibers have thickness in the range 29-100 nm. On increasing the concentration to 5 × 10-5 M, the thickness increased to 33-265 nm (Figure 4c). At a higher concentration of 5 × 10-4 M, the dense network of fibers with thickness in the range 138745 nm were observed (Figure 4d). The fibers exhibited more contrast at the core than at the periphery (inset of Figure 4d). Dichloromethane is a good solvent for FDNs, but a poor solvent for the ruthenium complex. So RuFDNs may be arranging themselves in such a way that FDN part is exposed to the solvent while the ruthenium complex part is directed toward the core. The presence of electron dense Ru at the core imparts more contrast to the core compared to the periphery.43,47


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Figure 4.TEM images of RuFDN (in dichloromethane) at different concentrations: 5 × 10-6 M (a), 5 × 10-5 M (c) and 5 × 10-4 M (d). (b) is zoomed in image of particles joining to form fibers at 5 × 10-6 M concentration. Inset of (d) shows zoomed in image of fiber showing contrast difference at core and periphery. AFM measurements were found to be in close agreement with the TEM results (Figure S9, ESI). At 5 × 10-6 M concentration of RuFDN in dichloromethane, fiber-networks with thickness in the range of 37- 123 nm were observed. Heights of these fibers were in the range of 3-4 nm. On increasing the concentration to 5 × 10-5 M and 5 × 10-4 M, the thickness increased to 44-272 nm and 157-786 nm, respectively. The heights of the fibers were observed to be in the range of 3-7 nm and 8-12 nm at 5 × 10-5 M and 5 × 10-4 M concentration, respectively. Figure 5 shows the TEM images of RuFDN (in acetone) at three different concentrations, viz. 5 × 10-6 M , 5 × 10-5 M and 5 × 10-4 M. At all these concentrations, we could observe interlinked nano-aggregates of varying diameters. Diameters of the aggregates were observed to be in the range of 40-136 nm at 5 × 10-6 M concentration. At 5 × 10-5 M concentration, TEM image displayed spherical aggregates with diameters in the range of 80 to 350 nm. Large


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spherical aggregates with outer diameters in the range 250-580 nm, were observed at 5 × 10-4 M concentration. A shell was clearly visible in the case of these nanospheres. The shell thickness varied from 10-115 nm. The thickness of the shell was not uniform around the same nanosphere.

Figure 5.TEM images of RuFDN (in acetone) at different concentrations: (a) 5 × 10-6 M,(b) 5 × 10-5 M and (c) 5 × 10-4 M. TEM images of these nanoaggregates were carefully examined to get further insights into their structures. Small holes could be observed on the surface of the nanoparticles, which indicates their hollow nature. In order to validate our assumption, we allowed immediate evaporation of the solution after drop-casting the sample by applying vacuum. On rapid evaporation, solvent bubbles present inside the hollow sphere will try to escape by breaking through the thinnest part of the aggregates and this would lead to opening of more holes on the surface.58,59 Figure 6 shows the TEM images obtained under the above mentioned conditions. At all concentrations, openings could be observed on the nanoparticle surface confirming their hollow nature. The nanocapsules obtained exhibited three different regions of contrast – a light central core and periphery and a dark middle region (Figure 6d). The darker area indicates regions of higher ruthenium density. This suggests that the ruthenium complex moieties of RuFDNs are sheltered in the interior of the hollow capsules while the FDN units are exposed to the solvent.43,47 RuFDN exhibited similar morphology in acetonitrile also (Figure S10, ESI).


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Figure 6.TEM images of RuFDN (in acetone) obtained by fast evaporation of solvent at different concentrations: 5 × 10-6 M (a), 5 × 10-5 M (b) and 5 × 10-4 M (c).(d) is zoomed in image of a nanocapsule. The formation of nanocapsule structures was also supported via AFM analysis (Figure S11, ESI). Opening on the surface of the particles can be clearly observed in the AFM images. The diameter to height ratios of the nanocapsules formed at 5 × 10-5 M and 5 × 10-4 M were estimated to be in the range of 5-8 and 4-6, respectively. This result can be attributed to the shell collapse caused by the removal of solvent molecules from the nanocapsules as well as the high local force exerted by the AFM tip. At higher concentrations (1.3 × 10-3 M, in acetone) RuFDN self-assembled into micrometer-sized hollow capsules as revealed by SEM and confocal fluorescence images (Figure S12, ESI). Zeta Potential analysis enabled us to determine the surface charge of nanoparticles in solution. Zeta potential measurements of RuFDN in different solvents are given in Figure S13 (ESI). RuFDN exhibited neutral zeta potential (in the range of ~ ± 6 mV) in solvents like dichloromethane, acetone and acetonitrile. However, a positive zeta potential of ~ +38 mV was observed for RuFDN in water. These results provide evidence for our assumption that in water


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RuFDNs self-assemble in such a way that the ruthenium(II) bipyridine complex units are present at the periphery of the nanoparticles and the FDN units are present at core, whereas in other solvents FDN units are exposed to the solvent while ruthenium complexes are present in the interior of the nanostructures. To get further insight into the RuFDN nanostructures, their photophysical behavior in different solvents were studied. Absorption spectra of RuFDN nanostructures exhibited some solvent dependence and in order to highlight the differences the spectra are normalized at 457 nm, which corresponds to the maximum of Ru complex absorption. The normalized absorption spectra in various solvents are given in Figure 7a. In all the solvents, the absorption spectra displayed the characteristic features of both the FDN units and ruthenium(II) bipyridine moiety. The absorption spectra of RuFDN were observed to be more broadened in solvents like dichloromethane, acetone and acetonitrile most probably due to the predominance of the FDN feature in these solvents. However, in water the characteristic MLCT band of the ruthenium(II) bipyridine was observed to be more predominant. The normalized emission spectra of RuFDN in different solvents are given in Figure 7b. RuFDNs exhibited emission maxima in the range of 622–636 nm in different solvents. The small bathochromic shift of the emission bands on going from dichloromethane to water is due to the more stabilization of excited states in more polar media. The emission spectra of RuFDN exhibited a broad shoulder around 527 nm in dichloromethane, acetone and acetonitrile (Figure 7a and b), which is due to the emission of the FDN unit (Figure S2 d). This feature was not observed in the emission spectrum of RuFDN in water.


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Figure 7.Absorption (a) and normalized emission (b) spectra of RuFDN in different solvents. For (b) λexc was 440 nm. Our proposal that in organic solvents the ruthenium(II) bipyridine moieties of RuFDN are buried inside and in water they are exposed to the solvent gained support from luminescence quenching studies. In organic solvents we used 2,6-dimethyl-9,10-anthraquinone as the quencher whereas in water anthraquinone-2-sulfonic acid sodium salt was employed. We observed substantial decrease of the luminescence intensity with increasing quencher concentration in water (Figure S14 a). However, negligible quenching of RuFDN emission was observed in organic solvents (Figure S14 b-d). This confirms that, in these organic solvents, the ruthenium complex units are present in the interior of the nano-assemblies and thus are protected from the quencher molecules. In water, ruthenium complex units are present at the periphery of the nanoparticle and are accessible to the quencher molecules. In the absorption and emission data obtained for RuFDN, the features due to FDN unit are observed to be more prominent compared to ruthenium(II) bipyridine complex in the organic solvents. On the contrary, in water, features due to ruthenium(II) bipyridine complex was more predominant. This strengthens our notion that in water, the hydrophobic FDN units are sheltered by the hydrophilic ruthenium complex units, whereas in organic solvents ruthenium complex


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units are present in the inner part of the nano-assemblies, protected by the FDN units. Quenching experiments provide further proof to the proposed arrangement of RuFDNs within the nanoaggregates. Schematic illustration of the arrangement of RuFDNs in different solvents is given in Scheme 3. Scheme 3. Graphical illustration of the arrangement of RuFDNs in different solvents.

The FDNs exhibit very high tendency for aggregation even at very low concentrations. This aspect is discussed in detail in our previous paper.46 The exact reason for the aggregation is not known, but we believe that π−π interactions involving the phenyl rings present on the periphery of the FDN are responsible for this aggregation. Even when FDN is functionalized with ruthenium(II) bipyridine complex, these materials are soluble in organic solvents if the counter ion is PF6-. Since the organic solvents do not stabilize charges, Ru(bpy)2(bpy-ph)2+2PF6- will be existing as a tight ion pair. When the counter ion is Br-, RuFDN is water-solule. Since water stabilizes both positive and negative charges, the ions may be present as free solvated ions in water.


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The aggregation of RuFDN into different supramolecular structures in different solvents can be explained based on the solubility of FDN and the ruthenium complex moieties and zeta potentials of the aggregates. In water the aggregates are formed in such a way that the Ru(bpy)2(bpy-ph)2+ moiety is fully exposed to water and the counter bromide ions are fully solvated. When several RuFDNs undergo aggregation the particles formed will have large numbers of Ru(bpy)2(bpy-ph)2+ moieties on the outside. The particle then becomes positively charged as revealed by zeta potential measurements. Further aggregation will be prevented by charge repulsion. In organic solvents, the RuFDNs aggregate in such a way that the charged Ru(bpy)2(bpy-ph)2+ and the hexafluorophosphate counter ion form a tight ion pair and would exist as buried inside without getting exposed to the solvent. Since the charged locations are scattered within the aggregates, very large particles or different networks may be formed. 2 Conclusions Covalent functionalization of FDN with a ruthenium(II) bipyridine complex was realized by chemical reduction of the diazo derivative of the ruthenium(II) bipyridine complex in the presence of FDN. The RuFDN formed exhibited solvent dependent morphological transformations. The different morphologies displayed by RuFDN were studied using various techniques such as UV-Vis, IR, NMR, AFM, TEM, zeta potential measurements and luminescence quenching studies. ASSOCIATED CONTENT Supporting Information. Figure S1: Chemical structure of RuN2+. Characterization of RuFDN. Figure S2: Absorption and emission spectra of RuFDN. Figure S3: IR spectra of RuN2+ and RuFDN, FDN, Rubpy. Figure S4: EDX spectrum. Table ST1: Elemental composition of RuFDN. Figure S5: Thermogram of RuFDN and FDN. Figure S6: Possible structures formed on multiple layering. Figure S7: Emission decay profiles of RuFDN and FDN. Table ST2: Fluorescence decay parameters for RuFDN and FDN. Self-assembly of RuFDNs. Figure S8: TEM and AFM


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images of RuFDN in water. Figure S9: AFM images of RuFDN in dichloromethane. Figure S10: TEM image of RuFDN in acetonitrile. Figure S11: AFM images of RuFDN in acetone. Figure S12: SEM and confocal fluorescence images of RuFDN. Figure S13: Zeta potential measurements. Figure S14: Quenching studies. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest ACKNOWLEDGMENT We gratefully acknowledge CSIR and the Network projectCSC0125 for financial support. We are thankful to RGCB, Trivandrum for confocal imaging. We would also like to thank Mr. Kiran Mohan and Mr. Robert Philip for TEM images. S.K. thanks CSIR for a research fellowship. REFERENCES (1)

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