Formation and Growth of Molecular Nanocrystals Probed by their

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Formation and Growth of Molecular Nanocrystals Probed by their Optical Properties A. Patra,† N. Hebalkar,‡,§ B. Sreedhar,‡ and T. P. Radhakrishnan*,† School of Chemistry, UniVersity of Hyderabad, Hyderabad 500 046, India, and Inorganic and Physical Chemistry DiVision, Indian Institute of Chemical Technology, Hyderabad 500 007, India ReceiVed: June 30, 2007; In Final Form: August 8, 2007

A red fluorescent zwitterionic molecule based on the diaminodicyanoquinodimethane framework is synthesized and structurally characterized. Formation of nanocrystals of this molecule through the reprecipitation protocol is followed by examining the optical absorption and emission. Computational modeling based on molecular and crystal structures provides insight into the assembly of molecules during colloid formation. Molecular nano/microcrystals of increasing size are fabricated through a digestion procedure, and the size-dependent optical properties are investigated by spectroscopy and microscopy. Utility of polymer wrapping to arrest the growth of these crystals is described.

Introduction Nanostructures and nanomaterials actively being explored in various areas of nanoscience and technology are largely based on metals, semiconductors, and polymers. However, in recent years considerable interest has emerged in nanocrystals built from small molecules. This is primarily due to the flexibility and versatility inherent in the assembly of molecular materials in general, unique characteristics and attributes of molecular nanomaterials, and the novel application potential they possess. The unique properties of molecular nanomaterials arise from the relatively weak and varied interactions between the molecular building blocks1 and the highly anisotropic character of these interactions. A case in point is that molecular nanocrystals are associated with Frenkel or charge-transfer excitons,2 rather than the Mott-Wannier type excitons that give rise to the prominent quantum size effects in semiconductor nanocrystals.3 A wide range of applications of molecular nanostructures have been demonstrated. They show enhanced and fast optical and nonlinear optical responses4,5 and are promising materials for sensors and switches,6 light-emitting devices,7 biodiagnostics,8 photocatalysis,9 and photonics and microelectronics.10 Several methodologies have been developed for the fabrication of molecular nanoparticles. Simple reprecipitation is the most-popular technique because of the ease and versatility of its implementation.11,12 Crystallization in confined environments of microemulsions,13 nanoporous glasses,14 and sol-gel matrices15 allow growth control. Approaches such as laser ablation,16 flash evaporation of solutions in supercritical fluid,17 electrochemical synthesis,18 sonochemical treatment,19 self-assembly,20 and physical vapor deposition21 have also been used for the fabrication of molecular nanoparticles. Among the various attributes of molecular nanomaterials, optical properties have attracted considerable attention.12,20-29 Recent studies have revealed interesting size-dependent optical and electronic properties in several molecular nanostructures including particles,24-26 * Corresponding author. E-mail: [email protected]. Fax: 91-402301-2460. Phone: 91-40-2301-1068. † University of Hyderabad. ‡ Indian Institute of Chemical Technology. § Present address: International Advanced Research Centre for Powder Metallurgy and New Materials, Balapur, Hyderabad 500 005, India.

wires,27 tubes,28 and plates.29 The size dependence has been attributed to a variety of mechanisms including aggregation26 and surface24 effects and tuning of the strength of intermolecular interactions by variations in lattice softness.25,29 Recently, we have studied molecular nano/microcrystals of the zwitterionic molecule, 7,7-bis(4-chloroanilino)-8,8-dicyanoquinodimethane (BCADQ), and their size-dependent optical properties.29 A model invoking the hierarchical emergence of intermolecular interactions was developed to explain the sizedependent phenomena. We have now investigated the optical attributes of the other halo derivatives in this family of redemitting molecules. Under similar fabrication conditions, the chloro and bromo derivatives are found to yield more welldefined and relatively larger nano/microcrystals than the fluoro and iodo derivatives;30 the trend is also reflected in the growth of good-quality single crystals in the case of the former two. These observations reflect the impact of a subtle balance between the electronegativity of the halogen atom substituent and its size and polarizability, factors that influence the interactions between molecules and hence their packing. The bromo derivative, 7,7-bis(4-bromoanilino)-8,8-dicyanoquinodimethane (BBADQ),

in particular shows distinct absorption and emission spectroscopic signatures in the solution and colloidal states and sharp

10.1021/jp075103j CCC: $37.00 © 2007 American Chemical Society Published on Web 10/04/2007

Molecular Nanocrystals transition between the two. In view of these factors, this paper is focused on BBADQ, which facilitated a systematic exploration of the formation of molecular nanocrystals. The observations are substantiated through computational modeling of the electronic structure of the molecule and its clusters, aided by crystal structure analysis. Optical properties of the colloidal particles with sizes varying from the nano to the micro domain are reported. We also present a simple approach to arrest the growth of nanocrystals to achieve stable particle size and optical responses in the colloidal dispersions. Experimental Section Synthesis. BBADQ was prepared by the condensation of 7,7,8,8-tetracyanoquinodimethane with 4-bromoaniline in acetonitrile, following a procedure similar to that used for BCADQ earlier29 (CAUTION: HCN is the byproduct in this reaction). The product was recrystallized from DMF and dried under vacuum at 135-140 °C for 4 h. Yield ) 65%; M.P./°C ) 300 (dec); FTIR (KBr): υ/cm-1 ) 3157.8, 2185.5, 2129.6, 1612.6, 817.9; 1H NMR (d6-DMSO): δ/ppm ) 11.25 (s, 2H), 7.47 (d, 6H, J ) 8.4 Hz), 7.11 (m, 4H), 6.83 (d, 2H, J ) 8.4 Hz) [exchange experiment with D2O showed that the signal at 11.25 ppm is due to the NH protons]; 13C NMR (d6-DMSO): δ/ppm ) 159.5, 151.1, 136.7, 132.1, 125.7, 122.7, 119.0, 117.7, 113.6; elemental analysis (calculated for C22H14N4Br2): %C ) 53.58 (53.47), %H ) 2.87 (2.85), %N ) 11.30 (11.34). The fluoro and iodo analogs of BBADQ were prepared using similar protocols and characterized.30 Crystal Structure. X-ray diffraction data was collected on a Bruker Nonius Smart Apex diffractometer (with CCD detector) at 298 K. Mo KR radiation with a graphite crystal monochromator in the incident beam was used. Data was reduced using the program SAINT and all non-hydrogen atoms were found using the direct method analysis in SHELXTL.31 Details of data collection, solution and refinement, as well as the CIF are submitted as Supporting Information. Preparation of Colloids. A 0.01 M solution of BBADQ in DMSO (HPLC grade) was filtered through a 100-nm pore alumina membrane (Whatman, Anodisc 13). One hundred

J. Phys. Chem. C, Vol. 111, No. 44, 2007 16185 microliters of this solution was rapidly injected into 20 mL of high-purity water (Millipore MilliQ, resistivity ) 18 MΩ cm) at 30 °C under ultrasonication, which was continued for 2 min. Colloid formation was immediately noticeable. The colloidal solution was digested in a constant temperature bath (at 30, 45, 60, 75, and 90 °C) for 1 h each; it was then brought to room temperature (∼30 °C). In the preparation of polymer-stabilized BBADQ nanocrystals, prior to the injection of the DMSO solution, 300 µL of aqueous solution (0.2 g L-1) of poly(vinyl alcohol) (PVA, average Mw ) 13-23 kDa, % hydrolysis ) 87-89) was added to 20 mL of water and sonicated for 2-3 min. Spectroscopic experiments and microscopy sample preparations were carried out at ambient temperature, within 15 min of colloid synthesis, unless stated otherwise. Microscopy. Size and morphology of the colloidal crystals were examined using (a) TECNAI G2 FEI F12 transmission electron microscope and (b) Solver Pro M (NT-MDT) atomic force microscope. The samples for TEM were prepared by placing a drop of the colloidal solution on a polymer (polyvinyl Formvar) coated copper grid and drying at room temperature; the accelerating voltage used was 120 kV. Colloidal solution was filtered through a 50-nm pore mixed cellulose esters membrane (Millipore) to prepare samples for the AFM investigation; the microscopy was done in the semicontact mode using a cantilever with a force constant of 10 N/m. Spectroscopy. Electronic absorption spectra were recorded on a Shimadzu model UV-3100 or Cary 100 Bio UV-Visible spectrophotometer. Colloid samples for the studies involving the same “concentration” were prepared by mixing 1 mL of the digested colloid with 2 mL of water; the effective concentration of BBADQ was 1.67 × 10-5 M. Steady-state fluorescence excitation and emission spectra were recorded on a Jobin Yvon Horiba model Fluoromax-3 spectrofluorimeter. A UV filter was used to cut off second-order reflections from the exciting radiation. It was ensured that the optical density of the samples used in all experiments is low enough to avoid artifacts due to inner-filter effects. Computations. All computations were carried out using the

Figure 1. (a) Molecular structure and (b) crystal packing of BBADQ (only one of the disordered positions of the nitrogen and carbon atoms in the DMF molecule is shown) from single-crystal X-ray analysis. The broken (green) lines indicate H-bonds; all H atoms except those in the amino groups are omitted for clarity; 99% probability thermal ellipsoids are indicated. Grey (C), white (H), blue (N), red (Br), magenta (O).

16186 J. Phys. Chem. C, Vol. 111, No. 44, 2007 TABLE 1: Crystallographic Data for BBADQ.2DMF BBADQ.2DMF empirical formula crystal system space group a/Å b/Å c/Å R/deg β/deg γ/deg V/Å3 Z Fcalc/g cm-3 µ/mm-1 temperature/K λ/Å min, max transmission no. of reflections no. of parameters GOF R [for I g 2σI] wR2 largest diff peak, hole/eÅ-3

C28H14Br2N6O2 triclinic P1h 9.2737(6) 12.4464(8) 14.1679(9) 94.5810(10) 107.8370(10) 110.3350(10) 1427.54(16) 2 1.457 2.874 298(2) 0.71073 0.5972, 0.7820 5033 423 1.046 0.0368 0.1003 0.642, -0.492

semiempirical quantum chemical module, VAMP in Materials Studio program,32 employing the AM1 Hamiltonian. Geometry optimization calculations used the molecular structure from the crystal analysis for the initial geometry and the eigen vector following routine; either hydrogen atom positions alone (exclud-

Patra et al. ing those involved in H-bonds) or the full geometry were optimized. Excited-state energies and oscillator strengths were computed using the PECI option (single and pair doubles excitation configuration interaction) involving six MO’s bracketing the HOMO/LUMO. Results and Discussion The structure of molecules and supramolecular clusters in the crystal lattice provide the starting point for investigating computationally the electronic structure and hence spectroscopic features of solutions and colloidal particles. Therefore, we have determined first the crystal structure of BBADQ through X-ray diffraction. Single crystals grown by cooling DMF solution were found to belong to the triclinic space group, P1h with one BBADQ and two DMF molecules in the asymmetric unit. The molecular structure of BBADQ (Figure 1a) indicates that the central ring (C1-C6) is nearly benzenoid owing to the zwitterionic nature of the molecule;33 the diaminomethylene unit is twisted with respect to the benzenoid ring plane with dihedral angles, τN9-C7-C1-C2 ) 27.6° and τN10-C7-C1-C6 ) 30.8°. The significant crystallographic data are collected in Table 1, and the crystal packing is shown in Figure 1b. Intermolecular H-bonds and dipole-dipole interactions observed are very similar to those found in BCADQ.29,30 Strong intermolecular H-bonding between one of the amino and cyano groups (rN9....N13′ ) rN9′....N13 ) 2.899 Å, θN9-H9...N13′ ) θN9′-H9′...N13 ) 172.2°) leads to a dimer structure.

Figure 2. (a, b) Electronic absorption and (c, d) fluorescence emission (excited at the respective excitation maxima) spectra of BBADQ in DMSOwater mixtures of different composition with increasing percentages of water: (i) 0, (ii) 20, (iii) 40, (iv) 45, (v) 50, (vi) 55, (vii) 60, (viii) 80, and (ix) 100. The concentration of BBADQ is kept constant in all the experiments (1 × 10-5 M).

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Figure 3. Enthalpies of formation (∆Hf) of BBADQ (per monomer): AM1/CI computed values for the fully optimized ground (GS) and excited (ES) states of the monomer as well as for the ground and excited states of the H-bonded dimer and monomer with geometries from crystal structure (Figure 1). The computed absorption/emission wavelengths as well as the experimentally observed values (in parentheses) for the solution and colloid are shown. In the case of the monomer with crystal geometry, vibrational relaxation in the excited state is schematically indicated.

Figure 5. AFM image of BBADQ crystals fabricated by digestion of the colloid at 30 °C for 1 h.

Figure 6. TEM images of BBADQ crystals fabricated by digestion of the colloid at (i) 60, (ii) 75, and (iii) 90 °C for 1 h. Scale bar ) 200 nm.

Figure 4. Relative fluorescence intensities of BBADQ solution and colloid peaks in DMSO-water mixtures with varying compositions.

TABLE 2: AM1/CI Computed Lowest Excitation (with Appreciable Oscillator Strength) Peak for the Different BBADQ Structures (see Text for Details of the Computation). structure

geometry

monomer

fully optimized (ground state) fully optimized (first excited state) crystal structure geometry; H atoms alone optimized crystal structure geometry; H atoms alone optimized

dimer

λmax (nm) [oscillator strength] 459.1 [1.350] 579.7 [1.217] 555.2 [1.111] 406.5 [2.047]

Solution to Colloid Transformation. Because BBADQ is soluble in DMSO and insoluble in water, the transformation from the solution to the colloidal state was followed by exploring the photophysical properties of BBADQ in DMSO-water mixtures of varying composition. The evolution of the absorption spectrum with increasing content of water is presented in

Figure 2a and b; the samples were prepared by adding 10 µL solutions of BBADQ in DMSO to 10 mL DMSO-water mixtures having 0-100 volume % water, maintaining the concentration constant (1.0 × 10-5 M). The spectrum of the solution in neat DMSO shows a peak in the visible region with λmax at 458 nm. The lowest excitation energy computed for the fully optimized ground-state geometry of BBADQ (Table 2, Figure 3) is in good agreement with this value; the absorption arises due to the intramolecular charge transfer in the zwitterionic molecule (Figure 1a) in solution.33 With increasing percentage of water, this peak shows a steady blue shift, reaching 430 nm at 40%. The negative solvatochromism is typical of molecules exhibiting lower dipole moments in the excited-state compared to that in the ground state.34 When the content of water is 45%, a new peak appears at 351 nm, in addition to the solution peak at 426 nm. With further increase in the content of water, the solution peak vanishes gradually and a sharp and narrow absorption emerges; at 100% water, the λmax is 359 nm and the fwhm of the peak is ∼18 nm. The peak at ∼360 nm cannot arise due to the change of polarity or viscosity of the medium; recall that BBADQ is insoluble in water. A computational enquiry suggests that this can be attributed to the lowest excitation of the H-bonded dimer (Figure 1b) present in the crystal lattice of BBADQ (Table 2, Figure 3) and hence is indicative of the incipient formation of nanocrystals. It is inferred that the formation of colloid starts at 45% water while the dispersions remain transparent and homogeneous with no precipitation. All BBADQ molecules appear to be aggregated into colloidal particles at g80% water. The large shift (∼100 nm) in the absorption maximum from the solution (0% water) to the colloid (∼100% water) indicates the impact of intermolecular interactions in the crystalline lattice.

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Figure 7. (a) Electronic absorption and (b) fluorescence emission (λexc ) 365 nm) spectra of BBADQ colloids digested at (i) 30, (ii) 45, (iii) 60, (iv) 75, and (v) 90 °C for 1 h. The concentration and volume of BBADQ solution in DMSO and the volume of water used for the colloid preparation are kept constant in these experiments, ensuring that the “concentration” of BBADQ is the same in all cases.

Figure 8. Photographs of BBADQ colloids with identical “concentration”, digested at (i) 30, (ii) 60, and (iii) 90 °C for 1 h, illuminated by (a) visible light and (b) UV light at 365 nm.

Figure 2c and d shows the steady-state fluorescence emission spectra of BBADQ in DMSO-water mixtures, recorded by exciting at the λmax of the respective excitation spectra (the excitation spectra in the different solvent mixtures are found to be very similar to the respective absorption spectra30). Emission of BBADQ in DMSO solution (0% water) peaks at 580 nm; this is modeled adequately by the lowest excitation computed for the fully optimized excited-state geometry of BBADQ (Table 2, Figure 3). With increasing content of water, this peak shows a steady blue shift maintaining a trend similar to that of the absorption spectra; at 40% water, the emission λmax is observed at 570 nm. The solution emission nearly vanishes when the water content is g50%, and a red-shifted peak appears at ∼650 nm. With 100% water, the emission peaks at 656 nm. The systematic variation of the peak intensity of the solution and colloid emissions of BBADQ in the different DMSO-water mixtures (Figure 4) depicts clearly the transformation from the solution to the colloidal state. Even though the chloro derivative BCADQ showed similar absorption and emission peaks in the solution and colloidal states,29 the transition between the two states is not so clear because the associated spectral changes are complex and an intensity plot does not reveal a sharp change as seen in Figure 4. The absorption and emission spectra of BBADQ corresponding to 100% water are very similar to those of colloids digested at 30 °C, discussed below. The unusually large Stokes shift (∼297 nm) observed in these systems cannot be accounted for by emission from the vertically excited state. A possible mechanism involves excitation of the H-bonded dimer as suggested above, followed by energy migration to a lower-lying monomer excited state and emission occurring from the latter

(Figure 3). This scenario is supported by the fact that the colloid emission occurs within the range of typical vibrational relaxation of the computed lowest excited state of the BBADQ monomer having the geometry observed in the crystal structure (Table 2). Growth of Nanocrystals and Size-Dependent Optical Properties. The experiments above clearly show that injection of a solution of BBADQ in DMSO into excess water leads to the formation of a colloid. The colloid is found to be stable for several weeks with no detectable precipitation even without any special stabilizing or capping agent. However, microscopy reveals that aging of the colloids with time or digestion by heating leads to increase in the particle size; parallel changes are observed in the spectroscopic properties as well. Colloids were digested at different temperatures in the 30-90 °C range for 1 h to effect systematic control of the particle growth. An AFM image of the crystals obtained by digestion at 30 °C is shown in Figure 5. Because the crystals obtained by digestion in the higher temperature ranges were more amenable to TEM imaging, which revealed sharply the crystalline features, those images are provided in Figure 6. The hexagonal plate morphology and size variation from ∼0.3 to 1 µm are clearly visible; the plates are ∼30-70 nm thick. Absorption and emission spectra of the colloids digested at 30-90 °C are collected in Figure 7. The size-dependent optical properties of BBADQ are very similar to that of BCADQ. Because the molecular structure as well as intermolecular interactions are also similar in the two cases, the model used to explain the optical properties of BCADQ should be applicable to BBADQ as well.29 The red shift of the absorption and blue shift of the emission with increase in the particle size result from the increasing rigidification of the crystal lattice and the associated decrease of vibrational relaxation as suggested in the case of other molecules as well.25 The evolution of the absorption spectra may be attributed to the hierarchical emergence of the intermolecular interactions with increasing rigidity of the crystal lattice, the strongest interaction (H-bonded dimer) alone appearing in the smaller crystals and the weaker ones30 getting added on in the larger ones. The increase of fluorescence intensity with particle size (Figure 8) is likely to be the result of decrease in nonradiative channels as the crystal lattice becomes harder. Microscopic and spectroscopic investigations on colloids of the fluoro and iodo analogs of BBADQ revealed very similar size-dependent optical responses.30

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Figure 9. Electronic absorption spectra of BBADQ colloids (a) without PVA and (b) with PVA (3 × 10-3 g L-1) fabricated at (i) 30 °C, 15 min, (ii) 75 °C, 1 h, and (iii) 30 °C, 7 days.

Figure 10. AFM images of BBADQ colloids prepared (a) without PVA, (b) with PVA, digested at (i) 30 °C for 15 min, and (ii) 30 °C for 7 days.

Arresting the Growth of Nanocrystals. As is the case with all nanomaterials, control of aggregation and growth of organic nanoparticles prepared through reprecipitation11,12 is a critical issue. Aggregation of the nanocrystals is driven by the tendency

to minimize the surface free energy; aging, often accelerated by temperature, leads to particle growth. We have explored the utility of water-soluble polymers like poly(vinyl alcohol),23 sodium poly(styrene sulfonate), and potassium poly(vinyl

16190 J. Phys. Chem. C, Vol. 111, No. 44, 2007 sulfate) in the fabrication of stable dispersions of BBADQ nanocrystals. PVA is found to be an efficient agent for this purpose. If in the reprecipitation procedure the DMSO solution of BBADQ is injected into water containing appropriate amount of PVA, then the resulting dispersion is found to be stable with no particle growth even after several days as revealed by the following spectroscopic studies and AFM images. Figure 9a shows the distinct variations in the absorption spectra of BBADQ colloids prepared at 30 °C without addition of PVA and subjected to digestion at higher temperature as well as aged for several days. Figure 9b shows that the absorption spectrum remains identical when the colloid prepared in the presence of PVA at 30 °C is subjected to digestion at 75 °C for 1 h or aged for several days. AFM images of the BBADQ colloidal particles before and after aging in the absence and presence of PVA (Figure 10) clearly demonstrate the arresting of the growth of the nanocrystals by PVA wrapping. The well-defined nanocrystals of BBADQ formed at ambient temperature and their higher monodispersity compared to those of the other halo derivatives in the series are critical factors that have enabled the unambiguous illustration of the impact of polymer wrapping. Dispersions of BBADQ nano/microcrystals with different sizes can be stabilized by adding PVA to the colloids aged for appropriate extent of time or digested at the required temperature. It is likely that the hydroxy groups of PVA form strong H-bonds with the cyano and amino groups of the zwitterionic BBADQ molecules on the surface of the nanocrystals, effectively disrupting the growth of the crystals and stabilizing the particle size and optical properties. Conclusions The zwitterionic diaminodicyanoquinodimethane molecule, BBADQ, which shows red emission in the solution and crystalline states, is synthesized and characterized crystallographically. Evolution of the molecule from the solution to the colloidal state is investigated by examining the dramatic changes in its spectroscopic properties; computational modeling studies provide insight into the origin of the observed optical responses. The nano/microcrystals fabricated through the reprecipitation route can be size-tuned from ∼80 nm to micrometers by a simple digestion protocol; systematic changes are observed in the spectral responses. The growth of nanocrystals upon aging or digestion is shown to be inhibited effectively by a polymer wrapping procedure. The fluoro and iodo analogs of BBADQ show very similar colloid formation and optical properties. The stabilization of the molecular nano/microcrystals by addition of polymer suggests the possibility of fabricating composite thin films and exploring their optical properties. Preliminary explorations indicate that the nano/microcrystals of these zwitterionic molecules show size-dependent optical limiting behavior. The materials and processes developed and the phenomena demonstrated in this study contribute to the general understanding of the relatively less-explored area of molecular nanocrystals. Acknowledgment. Financial support from the DST and CSIR, New Delhi and infrastructural support from the National Single-crystal X-Ray Diffractometer Facility at the School of Chemistry are acknowledged. We thank Mr. S. I. Leesment for helpful discussions regarding the microscopy work. A.P. thanks Dr. A. Dey for help with crystal structure analysis and the UGC, New Delhi for a Senior Research Fellowship. Supporting Information Available: Details of synthesis, characterization, crystallography, microscopy, spectroscopy, and

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