Mechanical Control of Molecular Aggregation and Fluorescence

Article pubs.acs.org/Langmuir

Mechanical Control of Molecular Aggregation and Fluorescence Switching/Enhancement in an Ultrathin Film B. Balaswamy, Lasya Maganti, Sonika Sharma, and T. P. Radhakrishnan* School of Chemistry, University of Hyderabad, Hyderabad 500 046, India S Supporting Information *

ABSTRACT: Optical responses of molecular aggregates and assemblies are often different from that of the individual molecules. Self-assembly approaches provide little physical control on the extent of aggregation. Mechanical compression of amphiphilic molecules (with chromophore/fluorophore head groups) at the air−water interface, followed by transfer as Langmuir−Blodgett (LB) films, should prove to be an elegant route to molecular assemblies with systematically tunable aggregation and optical responses. This concept is demonstrated using monolayer LB films of a diaminodicyanoquinodimethane (DADQ)-based amphiphile fabricated at different surface pressures. Films deposited above a threshold pressure exhibit a strong blue-shift in the absorption and fluorescence relative to those deposited below; computational investigations suggest that this is due to the formation of 2-dimensional close-packed assemblies. Significantly, the blue emission of the films deposited above the threshold pressure increases with compaction, demonstrating aggregation-induced fluorescence enhancement in ultrathin films, a phenomenon well-established in crystals and nanocrystals of selected classes of molecules including the DADQs. The sharp contrast with aggregation-induced fluorescence quenching observed with most dye molecules is illustrated by a parallel investigation of LB films of a hemicyanine-based amphiphile. The present study illustrates the efficacy of simple mechanical compression and the LB technique in fabricating ultrathin films with tailored supramolecular assembly and optical responses.

■

structures.3 Selective formation of the polar form of a molecular crystal could be induced by growing it on noncentrosymmetric inorganic crystals.4 Aggregation of the chromophore head groups in Langmuir−Blodgett (LB) films can be effectively suppressed through the complexation with polyelectrolytes introduced from the subphase, leading to enhanced optical and nonlinear optical responses.5−8 Such a technique can also be exploited to grow oriented nanostructures of conjugated polymers at the air−water interface.9 The LB technique is a unique approach for the assembly of molecules into ultrathin films. Organization of molecules in LB films can be tailored through various protocols, including mixing the desired molecules with amphiphiles like fatty acids and lipids10,11 and the formation of multilayer structures.7,12,13 Special methods such as application of shear on the Langmuir film have been used to induce preferential orientation of molecules within the corresponding LB film.14 Compression of the Langmuir film at the air−water interface to different surface pressures is perhaps the most direct approach to control the extent of aggregation of the amphiphilic molecules. An obvious effect of the deposition pressure is on the packing density of the

INTRODUCTION A wide range of molecular crystals, nanostructures, and ultrathin films exhibiting various electrical, magnetic, and optical properties have been developed over the past few decades. Fabrication of molecular materials with specific characteristics and functions is realized through the design and synthesis of the molecules and their assembly. The molecular structure determines not only its properties but also the assembly patterns. The latter aspect is key to the selfassembly approaches that exploit relatively weak noncovalent interactions to achieve the desired assembly motifs. As the properties of the molecule and its assembly are thus interlinked, subtle structural alterations at the molecular level can transform the materials' attributes significantly and in complex ways. While this forms the basis of the versatility of molecular materials, it also highlights the challenges involved in tailoring their responses and functions. External fields and interactions can be used to steer the assembly of molecules. Examples include the fabrication of parallel arrays of oriented crystalline needles of p-hexaphenyl using a focused argon ion laser1 and the use of an electric field in the formation of conducting organic wires and nanotransistors of tetrathiafulvalene-tetracyanoquinodimethane.2 Application of a magnetic field during the deposition of a nickel complex was shown to promote oriented layer-by-layer © 2012 American Chemical Society

Received: September 3, 2012 Revised: October 29, 2012 Published: December 6, 2012 17313

dx.doi.org/10.1021/la303549z | Langmuir 2012, 28, 17313−17321

Langmuir

Article

close-packed monolayer films. The contrasting scenario is illustrated using a parallel exercise on the hemicyanine amphiphile, N-octadecyl-4-[2-(4-(N,N-ethyloctadecylamino)phenyl)ethenyl]pyridinium bromide (OEOEP+Br−)8,27 (Figure 1), which shows the conventional fluorescence quenching with increasing molecular aggregation. This study demonstrates the power of the LB technique to create materials with tunable responses and presents a DADQ-based ultrathin film showing aggregation-induced enhancement of blue emission.

amphiphiles. A steady decrease of the unit cell parameters of arachidic acid LB films with increasing deposition pressure has been demonstrated.15 Closer packing of molecules can influence surface characteristics of the ultrathin film such as the contact angle.16 Increasing deposition pressure led to the formation of H-aggregates and absorption spectral shifts along with a decrease in the optical second harmonic generation capability of LB films of a squarine dye-based amphiphile.17 LB films of dipalmitoylphosphatidylethanolamine labeled with nitrobenzoxadiazole fabricated at increasing surface pressures showed significant morphological changes and fluorescence quenching at high deposition pressures.18,19 Similar fluorescence quenching has been observed in LB films of a styrylpyridinium-based amphiphile, fabricated at increasing pressures.20 As the deposition pressure exerts a strong influence on the molecular assembly and characteristics of LB films, it should prove to be a convenient handle to control molecular aggregation and systematically fine-tune and enhance materials' attributes in ultrathin films. Aggregation generally leads to quenching of fluorescence emission in dye molecules, a fact borne out by the LB film examples above. Selected classes of molecules that exhibit enhanced emission in the aggregated state have attracted considerable attention, in view of their potential optical and photonic applications. 21 Diaminodicyanoquinodimethane (DADQ) derivatives form one such class of molecules, crystals22−24 and nanocrystals,25,26 of which have been investigated in our laboratory. These push−pull molecules with a characteristic dihedral twist angle between the diaminomethylene and benzenoid ring planes are zwitterionic with a large ground state dipole moment. Aggregates of these molecules in the solid state exhibit fluorescence emission typically a couple of orders of magnitude stronger than that in solution, due to the inhibition of excited state geometry relaxation.22 We have now investigated Langmuir films of a novel amphiphilic DADQ derivative, 7,7-bis(4-octadecyloxyethylpiperazino)-8,8-dicyanoquinodimethane (BODEPDQ) (Figure 1), at the air−water interface and the LB films deposited at different surface pressures to explore the impact of aggregation on their optical responses. Surface pressure is shown to be an effective mechanical control parameter to systematically induce and increase the molecular aggregation and obtain fluorescence switching and enhancement in the

■

EXPERIMENTAL AND COMPUTATIONAL

Synthesis. BODEPDQ was synthesized by the reaction of tetracyanoquinodimethane with N-octadecyloxyethyl piperazine.28 OEOEP + B r − was s ynthesized by condensing 4-( N,N ethyloctadecylamino)benzaldehyde with 4-methyl-N-octadecylpyridinium bromide.27 Langmuir and LB Film Fabrication. Surface pressure−area (π− A) isotherms at the air−water interface were recorded on a Nima Model 611 M LB trough equipped with a Wilhelmy plate for pressure sensing. All experiments were carried out at 25 °C in a clean environment. High purity water (Millipore Milli-Q, resistivity = 18 MΩ cm and surface tension = 73.0 ± 0.3 mN/m) was used as the subphase. In a typical experiment, 60−80 μL of a ∼0.6 mM solution of the amphiphile in chloroform (EMerck, Uvasol grade) was spread on the subphase. Thirty minutes was allowed for the solvent to evaporate and the monolayer to equilibrate at the air−water interface. The monolayer was compressed by moving the barrier at a speed of 3 cm/ min (rate of area change = 30 cm2/min). Hydrophilic glass and quartz substrates used for depositing the LB films were prepared by immersing in a piranha solution for 10 h, followed by sonication and rinsing in high purity water. LB films were fabricated by vertical dipping of the substrate at a speed of 3 mm/min. All experiments were repeated on at least 4−5 fresh samples to ascertain the reproducibility of the observations. Brewster Angle Microscopy (BAM). Morphology of the Langmuir films at the air−water interface was examined using BAM. Images were recorded with a Nanofilm model BAM2Plus microscope employing a 20 mW power, 532 nm laser beam. The films were examined at different stages of compression. Images were corrected for the angle of incidence of the beam. Atomic Force Microscopy (AFM). A SEIKO model SPA400 AFM was used in dynamic force mode with a cantilever having a force constant of 12 N m−1. LB films deposited at different surface pressures on glass substrates were imaged; films deposited on freshly cleaved mica were also examined and found to yield similar images. Absorption and Emission Spectroscopy. Electronic absorption spectra of the LB films deposited on quartz plates were recorded on a Varian model Cary 100 UV−visible spectrometer; a clean quartz plate was used as the reference. Steady-state fluorescence excitation and emission spectra were recorded on a Horiba Jobin Yvon model Fluorolog-3 spectrofluorimeter in a front-face detection geometry. Fluorescence Lifetime Imaging Microscopy (FLIM). FLIM was carried out using a time-resolved confocal fluorescence microscope (MicroTime 200, PicoQuant) coupled to an Olympus IX71 microscope (PicoQuant). Excitation was achieved using a 405 nm pulsed-laser diode and the fluorescence observed through a 430 nm long-pass filter. The fwhm of pulse response function was 176 ps. Data acquisition was performed with a PicoHarp 300 TCSPC module using PicoHarp300 version 2.3 in a time-tagged time-resolved mode. LB films deposited on glass substrates were imaged. Computational. Quantum chemical computations were carried out at the semiempirical and ab initio levels. Molecular geometry was optimized using the AM1 method within the VAMP routine in Materials Studio.29 Effect of the molecular environment in the condensed phase was mimicked using the COSMO option.30 Models for the molecular packing in the Langmuir/LB films were constructed using the “Build/Surface” option in Materials Studio. Excitation energies and oscillator strengths of molecular trimers (aggregated and

Figure 1. Structure of the amphiphilic molecules used in this study. 17314


Langmuir

Article

pressure of ∼50 mN/m; the general features resemble those of double-chain lipid molecules.32 An abrupt change in the slope is observed at ∼35 mN/m with some kinks up to a pressure of ∼43 mN/m, indicating a shrinkage of the film accompanied by a small pressure rise. The molecular area obtained by extrapolation of the isotherm above this transition is ∼43 Å2, suggestive of a close packing of the double-chain amphiphilic molecule [a model for the 2-dimensional (2D) molecular assembly is discussed later]. The Langmuir film of BODEPDQ was imaged using BAM at various points along the π−A isotherm (Figure 3). No special features are visible up to a surface pressure of 30 mN/m but well-defined domains appear thereafter. On further compression the domains are compacted into definitive shapes, which merge into extended homogeneous regions at higher pressures. Formation of the domains, their penetration into each other, and eventual merger appear to give rise to the transition region in the isotherm. The occasional bright streaks observed at the domain boundaries could be due to multilayer formation33 or the high refractive index of dense boundary regions. OEOEP+Br− with a broadly similar π−A isotherm (Figure 2) and significant optical properties27 serves as a suitable control system in the present study. The morphological evolution and the origin of the plateau in OEOEP+Br− are, however, quite different, a fact reflected in the plateau characteristics.27 LB films of BODEPDQ were fabricated by transferring the Langmuir film at various pressures in the range 5−45 mN/m after several compression−expansion cycles (isocycles); the transfer ratios are generally ∼0.8−0.9. Those deposited at pressures ≤35 mN/m were subjected to isocycling below the transition region (0−35 mN/m) prior to deposition, whereas those deposited at ≥40 mN/m were subjected to isocycling crossing the transition (0−45 mN/m). AFM images of the films fabricated at lower pressures show evidence of material deposited on the substrate (seen also in the FLIM images discussed later) but no clear monolayer formation (Figure 4);

separated) were calculated using the AM1 method, including pair excitation configuration interaction (PECI) involving 36 molecular orbitals (973 singlet microstates). Excitation profiles were calculated also using the ab initio time-dependent density functional (TD-DFT) method. B3LYP/6-31G* level computations (Gaussian03)31 were carried out on the molecular geometries optimized in the semiempirical calculations; retaining the relevant chromophore moiety, the octadecyl chains were replaced by methyl groups (which alone were reoptimized) in order to save on computation. Influence of the neighboring molecular dipoles was mimicked by placing point charges at positions dictated by the packing models; the details are provided in the relevant discussions.28

■

RESULTS AND DISCUSSION BODEPDQ forms a stable monolayer at the air−water interface. The π−A isotherm (Figure 2) shows a collapse

Figure 2. Pressure−area isotherms of BODEPDQ and OEOEP+Br− at the air−water interface at 25 °C. The molecular area obtained by extrapolation of the high pressure region of the BODEPDQ isotherm is shown.

Figure 3. BAM images of BODEPDQ at the air−water interface at 25 °C at different surface pressures. Scale bar = 25 μm. 17315


Langmuir

Article

Figure 4. AFM images (10 × 10 μm) of BODEPDQ LB films deposited at different surface pressures; the vertical scale is indicated.

average thickness of the patches is less than 1 nm.28 Films deposited at 35 and 45 mN/m show clear evidence of monolayer formation with a thickness of ∼3.3 nm. While the film deposited at 35 mN/m (onset of the transition) shows several gaps, the one deposited at 45 mN/m (above the transition) shows a densely packed and continuous film. AFM images of the LB films are consistent with the BAM observations on the Langmuir films. Electronic absorption spectra of BODEPDQ LB films fabricated at different surface pressures are collected in Figure 5a. The characteristic UV absorption peak of DADQs34 at ∼240 nm is seen in all cases. The lowest energy absorption due to the intramolecular charge transfer in the zwitterionic chromophore occurs at ∼440 nm in the films deposited at ≤25 mN/m; the small blue shift relative to the absorption of BODEPDQ in chloroform solution28 may be attributed to the medium effects on the strongly dipolar chromophore. Interestingly, this peak disappears in the films deposited at ≥30 mN/m; a weak absorption at ∼360 nm becomes visible for films deposited at the higher pressures (Figure 5a). Variations of the absorbance at the two λmax are shown in Figure 5b. It may be noted that the LB film of a related amphiphile, 7,7bis(octadecylamino)-8,8-dicyanoquinodimethane fabricated at 45 mN/m showed no trace of a low energy charge transfer absorption;34 this was explained on the basis of the specific organization of the zwitterionic chromophores in the 2D assembly. The control case of OEOEP+Br− films deposited at similar surface pressures showed a steady increase in the absorbance (Figure 5, panels c and d); the slight broadening of the spectra of films formed at higher pressures may be

attributed to the emergence of an absorption due to aggregates.27 As the BODEPDQ LB films exhibit spectral variations, in order to probe the extent of transfer of molecules during the deposition process, we have dissolved the films in chloroform and measured the optical absorption of the solutions. The spectra showed a steady increase in intensity with deposition pressure, with slight enhancement at higher pressures due to the higher density of molecules.28 Even though a precise quantification is difficult in view of the extremely low concentrations and absorbances involved, this experiment demonstrated that the molecule transfer increases with deposition pressure. The number density of molecules in the LB films estimated from the π−A isotherm with corrections for the transfer ratios is shown in Figure 5b (and for OEOEP+Br− in Figure 5d); these values are the upper bounds in view of possible creep deformation of the Langmuir films during the deposition (the isotherms show some hysterisis28). The concentration of molecules in the LB films increases with the deposition pressure with an enhanced growth at pressures above 30 mN/m. The switching of the absorption maximum of the monolayer LB films of BODEPDQ from 440 to 360 nm close to the formation of domains points to the effect of molecular aggregation; it is quite likely that the aggregate formation is induced just before the domains become distinctly visible in the BAM images. As the films deposited at low pressures show the absorption at ∼440 nm in spite of the isocycling up to 35 mN/m, it is clear that the aggregation which sets in at ∼30 mN/m pressure is reversible. Films deposited at low pressures after isocycling up to 45 mN/m did not exhibit 17316


Langmuir

Article

Figure 5. (a) Absorption spectra and (b) plot of the absorbance at λmax and number density of molecules of BODEPDQ LB films deposited at different pressures. (c) Absorption spectra and (d) plot of the absorbance at λmax and number density of molecules of OEOEP+Br− LB films deposited at different surface pressures.

simplest space group P2 that allows packing of the head groups with antiparallel orientation of the dipoles. The packing was tuned manually to achieve maximum density; the resulting unit cell parameters are u = 16.0 Å, v = 6.0 Å, and α = 90°. This model leads to an area/molecule of 48 Å2, in fair agreement with that observed for the close-packed Langmuir film with merged domains (Figures 2 and 3); the slightly lower experimental value of the area may be due to the multilayer defects. Height of the molecular assembly in the model is ∼32.0 Å, consistent with the monolayer thickness observed in the AFM image (Figure 4). As the hydrocarbon chains do not influence the optical properties, the octadecyl chains in the optimized structure of BODEPDQ were replaced by methyl groups, and the resulting molecule BMEPDQ was used to estimate the excitation energies and oscillator strengths using two computational approaches. AM1/PECI calculations were carried out on molecular trimers extracted from the assembly shown in Figure 6b, involving a headgroup chromophore and the adjacent neighbors with antiparallel orientation of the dipoles (Figure 7a). Average interplanar distance between adjacent benzenoid ring planes is ∼3.4 Å; the trimer represents a model system for BODEPDQ in the aggregated state in LB films deposited at high pressures. A model for the separated molecules in the LB films deposited at low pressures was constructed by increasing the intermolecular distance in the trimer to ∼15 Å (Figure 7b),

this absorption, indicating that the molecular aggregation is irreversible if the transition region is crossed. Spectra of the solutions of the LB films28 show that the molecular aggregation in the films can of course be reversed by dissolution. In order to explore the origin of the distinct blue shift of the absorption in the LB films deposited at pressures ≥30 mN/m, we have carried out a detailed computational study of BODEPDQ molecules in aggregated and isolated states. Molecular geometry of BODEPDQ was determined using semiempirical AM1 optimization; a dielectric constant of 2 was used to mimic the environmental effect and reproduce the typical dihedral twist angle observed in the 7,7-bis(piperazino)8,8-dicyanoquinodimethane headgroup.30 The initial geometry for optimization was constructed by orienting both the alkyl chains in the same direction, with the piperazine rings in chair conformations and the major dipole axis of the headgroup (connecting the diaminomethylene and dicyanomethylene carbons) approximately orthogonal to the chain direction. Such a structure is appropriate for the asymmetric environment at the air−water interface and facilitates dipole−dipole interactions between the strongly zwitterionic head groups, most likely a dominant stabilizing effect in the 2D assembly. The geometry obtained by full optimization is shown in Figure 6a; the computed dipole moment is 20.8 D, with the major component along the dipole axis noted above. A plausible 2D lattice of the amphiphiles (Figure 6b) was constructed using the 17317


Langmuir

Article

Figure 6. (a) AM1 optimized structure of BODEPDQ; the ball and stick model (top) and the Connolly surface rendering (bottom) are shown. (b) Top (top) and side (bottom) view of the 2D packing models (see text for details); the axes are indicated, and in the side view the height of the layer is shown.

Figure 7. BMEPDQ structures used in the computational studies. (a and b) Trimers packed as in the 2D assemblies (Figure 6b) with the intermolecular distance enhanced and retained. (c and d) Single BMEPDQ molecule and the molecule with adjacent dipoles [as in (b)] simulated by point charges.

a distance at which no exciton coupling is likely to occur. The strongest low-energy excitations computed for the separated and aggregated trimers are at 469.0 and 383.9 nm, respectively.28 TD-DFT computations were carried out on the BMEPDQ molecule (Figure 7c) and the molecule with point charges placed on either side to simulate the presence of the neighboring antiparallel dipoles (Figure 7d); the positive and negative charges are placed at the positions of the diaminomethylene and dicyanomethylene carbons, respectively.

Such a model that takes into account the most significant effect of the local electric field due to the neighboring, strongly dipolar zwitterionic molecules has been used to explain the vanishing electronic absorption in LB films of a related molecule.34 The lowest energy excitations with significant oscillator strengths computed in the two cases (Figure 7c and 7d) are 443.0 and 373.4 nm, respectively.28 The excitation energies computed at both levels are clearly consistent with the lowest-energy electronic absorptions manifested by the LB 17318


Langmuir

Article

Figure 8. (a) Fluorescence emission spectra of BODEPDQ LB films deposited at different pressures; λexc corresponds to the peak of the respective excitation spectra, and the intensities are relative to the highest in each set. (b) Plot of the fluorescence intensity normalized with respect to the respective absorption and the average value in each set. (c) Fluorescence emission spectra of OEOEP+Br− LB films deposited at different surface pressures; λexc corresponds to the peak of the excitation spectra. (d) Plot of the fluorescence intensity normalized with respect to the respective absorption and the average value of the set.

films deposited at low and high pressures. This strongly supports the model of BODEPDQ molecules in separated and aggregated states in the two cases. The final significant observation in this study is the evolution of the fluorescence emission from the LB films as a function of their deposition pressure. Emission spectra of the BODEPDQ films deposited at various pressures are collected in Figure 8a; the spectra were recorded by exciting at the peak of the relevant excitation spectrum, i.e., at 440 and 360 nm for films deposited at 5−25 mN/m and 30−45 mN/m, respectively (if all the films are excited at 360 nm, spectral intensity increases nearly continuously with deposition pressure28). The emission peak occurs at ∼530 nm for the films deposited at the lower pressures. This green emission is close to the weak fluorescence observed in chloroform solutions of BODEPDQ.28 Films deposited at 30 mN/m and above show not only a blue emission with λmax equal to ∼409 nm but also a fairly structured spectrum, typical of ordered molecular assemblies. The spectra in each set grow with the deposition pressure, consistent with the increasing concentration of molecules in the LB films. In order to assess the variation of the fluorescence efficiency, the integrated intensity of the emission spectrum of each film was divided by the integrated intensity of its absorption spectrum. The relative trends in the normalized fluorescence intensities for the two sets of films, showing green and blue fluorescence, respectively, are shown in Figure 8b. While the films deposited

at ≤25 mN/m show a gently decreasing intensity, the blueemitting LB films deposited at ≥30 mN/m show a steady increase of emission with deposition pressure. On the basis of the absorption spectra and computational investigations discussed above, it is clear that the fluorescence switching and the enhancement thereafter are aggregation-induced. The stark contrast with the case of OEOEP+Br− is shown in Figures 8c and 8d; strong fluorescence quenching commonly associated with aggregation of dye molecules is clearly manifested. Fluorescence lifetime imaging provides a visualization of the BODEPDQ films based on the average excited state lifetimes of their emission. Due to the instrument limitation, the lowest excitation wavelength that could be used was 405 nm; as noted in the Experimental section, emission above 430 nm was collected. Hence, the complete emission of the films deposited at higher pressures does not contribute to these images. Images of the LB films in Figure 9 clearly reveal morphologies consistent with those observed in the BAM and AFM images.

■

CONCLUSIONS Facile tuning of molecular aggregation by mechanical control, possible through the Langmuir−Blodgett technique, is demonstrated in this study. The specific case of BODEPDQ shows that the surface pressure for deposition of the LB film is a convenient parameter that can be exploited to induce and control molecular aggregation, leading to a significant shift in 17319


Langmuir

Article

Figure 9. FLIM images of BODEPDQ LB films deposited at different surface pressures. An image recorded on plane cover glass (control) is also shown. The scale bar = 5 μm.

■

optical absorption and emission; computational modeling provides support for the experimental observations. The interesting phenomenon of aggregation-induced switching and enhancement of emission, studied extensively in molecular crystals and nanocrystals, is demonstrated in ultrathin films of BODEPDQ. The sharp contrast with the commonly observed aggregation-induced emission quenching is illustrated using LB films of a hemicyanine amphiphile. The present study thus reveals the fundamental concept of mechanical control of molecular aggregation and optical responses and presents a novel blue-emitting molecular ultrathin film.

■

ASSOCIATED CONTENT

S Supporting Information *

Details of synthesis and characterization, microscopy, spectroscopy, and computations. This material is available free of charge via the Internet at http://pubs.acs.org.

■

REFERENCES

(1) Balzer, F.; Rubahn, H. Laser-controlled growth of needle-shaped organic nanoaggregates. Nano Lett. 2002, 2, 747−750. (2) Sakai, M.; Iizuka, M.; Nakamura, M.; Kudo, K. Organic nanotransister fabricated by co-evaporation method under alternating electric field. Synth. Met. 2005, 153, 293−296. (3) Dey, S.; Pal, A. J. Layer-by-layer electrostatic assembly with a control over orientation of molecules: Anisotropy of electrical conductivity and dielectric properties. Langmuir 2011, 27, 8687−8693. (4) Prakash, M. J.; Raghavaiah, P.; Krishna, Y. S. R.; Radhakrishnan, T. P. Growing molecular crystals on inorganic crystals: Polar structure induced by noncentrosymmetric templates. Angew. Chem., Int. Ed. 2008, 47, 3969−3972. (5) Chandra, M. S.; Ogata, Y.; Kawamata, J.; Radhakrishnan, T. P. Polyelectrolyte-assisted deaggregation and second harmonic generation enhancement in hemicyanine Langmuir−Blodgett film. Langmuir 2003, 19, 10124−10127. (6) Chandra, M. S.; Krishna, M. G.; Mimata, H.; Kawamata, J.; Nakamura, T.; Radhakrishnan, T. P. Laser-induced second harmonic generation decay in a Langmuir−Blodgett film: Arresting by poly elecrtolyte templating. Adv. Mater. 2005, 17, 1937−1941. (7) Rajesh, K.; Chandra, M. S.; Hirakawa, S.; Kawamata, J.; Radhakrishnan, T. P. Polyelectrolyte templating strategy for the fabrication of multilayer hemicyanine Langmuir−Blodgett films showing enhanced and stable second harmonic generation. Langmuir 2007, 23, 8560−8568. (8) Rajesh, K.; Balaswamy, B.; Yamamoto, K.; Yamaki, H.; Kawamata, J.; Radhakrishnan, T. P. Enhanced optical and nonlinear optical responses in a polyelectrolyte templated Langmuir−Blodgett film. Langmuir 2011, 27, 1064−1069. (9) Chandra, M. S.; Radhakrishnan, T. P. Polyelectrolyte templated polymerization in Langmuir films: Nanoscopic control of polymer chain organization. Chem.Eur. J. 2006, 12, 2982−2986. (10) Carpenter, M. A.; Willand, C. S.; Penner, T. L.; Williams, D. J.; Mukamel, S. Aggregation in hemicyanine dye Langmuir−Blodgett films: Ultraviolet-visible absorption and second harmonic generation studies. J. Phys. Chem. 1992, 96, 2801−2804.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 91-40-2313-4827. Fax: 9140-2301-2460. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Financial support from the Department of Science and Technology, New Delhi, is gratefully acknowledged. B.B. thanks the CSIR, New Delhi, for a Senior Research Fellowship, and L.M. thanks the UGC, New Delhi, for a Junior Research Fellowship. We thank Mr. K. Santhosh for help with the fluorescence lifetime-based imaging. 17320


Langmuir

Article

(11) González-Delgado, A. M.; Rubia-Payá, C.; Roldán-Carmona, C.; Giner-Casares, J. J.; Pérez-Morales, M.; Muñoz, E.; Martín-Romero, M. T.; Camacho, L.; Brezesinski, G. Control of the lateral organization in Langmuir monolayers via molecular aggregation of dyes. J. Phys. Chem. C 2010, 114, 16685−16695. (12) Ashwell, G. J.; Jackson, P. D.; Jefferies, G.; Gentle, I. R.; Kennard, C. H. L. Controlling the structure of transparent Langmuir− Blodgett films for nonlinear optical applications. J. Mater. Chem. 1996, 6, 137−141. (13) Correia, F. C.; Wang, S. H.; Péres, L. O.; Caseli, L. Langmuir and Langmuir−Blodgett films of a quinoline-fluorene based copolymer. Colloids Surf., A 2012, 394, 67−73. (14) Mingotaud, C.; Jego, C. Control of the molecular in-plane orientation in Langmuir−Blodgett films by shearing. In Modular Chemistry; NATO ASI Series. Series C, Mathematical and Physical Sciences, No. 499; Kluwar Academic Publishers: Dordrecht, The Netherlands, 1997; 543−550. (15) Evenson, S. A.; Badyal, J. P. S.; Pearson, C.; Petty, M. C. Variation in intermolecular spacing with dipping pressure for arachidic Acid LB films. J. Phys. Chem. 1996, 100, 11672−11674. (16) Yue, X. L.; Ma, F.; Dai, Z. F.; Liu, M.; Xing, L.; Fan, H. Y.; Xing, Z. W.; Liu, S. Q. New insight into surface properties of LB film of an amphiphilic terpolymer. Chin. Chem. Lett. 2011, 22, 241−244. (17) Ashwell, G. J.; Wong, G. M. S.; Bucknall, D. G.; Bahra, G. S.; Brown, C. R. Neutron reflection from 2,4-bis(4-(N-methyl-Noctylamino)phenyl)squaraine at the air−water interface and the linear and nonlinear optical properties of its Langmuir−Blodgett and spuncoated films. Langmuir 1997, 13, 1629−1633. (18) Ivanov, G. R.; Petkova, J. I.; Okabe, Y.; Aoki, D.; Takano, H.; Kawate, H.; Fujihira, M. Scanning probe microscopy studies of aggregation in Langmuir−Blodgett films. Supramol. Sci. 1997, 4, 549− 557. (19) Ivanov, G. R.; Fujihira, M. Single component LB film investigated by five scanning probe microscopy methods. Colloids Surf., A 2002, 198−200, 305−312. (20) Bossi, M. L.; Turshatov, A. A.; Zaitsev, S. Y.; Alfimov, M. V.; Möbius, D.; Hell, S. W. Influence of monolayer state on spectroscopy and photoisomerization of an amphiphilic styryl-pyridinium dye on a solid substrate. Langmuir 2007, 23, 3699−3705. (21) Hong, Y.; Lam, J. W. Y.; Tang, B. Z. Aggregation-induced emission: Phenomenon, mechanism and applications. Chem. Commun. 2009, 4332−4353. (22) Jayanty, S.; Radhakrishnan, T. P. Spontaneous resolution through helical assembly of a conformationally chiral molecule with an unusual zwitterionic structure. Chem.Eur. J. 2004, 10, 2661−2667. (23) Patra, A.; Radhakrishnan, T. P. Molecular materials with contrasting optical responses from a single-pot reaction and fluorescence switching in a carbon acid. Chem.Eur. J. 2009, 15, 2792−2800. (24) Chandaluri, C. G.; Radhakrishnan, T. P. Zwitterionic diaminodicyanoquino-dimethanes with enhanced blue-green emission in the solid state. Opt. Mater. (Amsterdam, Neth.) 2011, 34, 119−125. (25) Patra, A.; Hebalkar, N.; Sreedhar, B.; Radhakrishnan, T. P. Formation and growth of molecular nanocrystals probed by their optical properties. J. Phys. Chem. C 2007, 111, 16184−16191. (26) Chandaluri, C. G.; Patra, A.; Radhakrishnan, T. P. Polyelectrolyte-assisted formation of molecular nanoparticles exhibiting strongly enhanced fluorescence. Chem.Eur. J. 2010, 16, 8699− 8706. (27) Rajesh, K.; Rajendra, K.; Radhakrishnan, T. P. Fluorescence enhancement in Langmuir−Blodgett films: Role of amphiphile structure, orientation, and assembly. J. Phys Chem. B 2010, 114, 849−856. (28) See Supporting Information. (29) Materials Studio, version 5.5, Accelrys Software Inc.: San Diego, CA, 2010. (30) Jayanty, S.; Radhakrishnan, T. P. Modeling molecule-in-acrystal: The case of push-pull quinonoids. Chem. Mater. 2001, 13, 2460−2462.

(31) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery , J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision E.01; Gaussian, Inc.: Wallingford, CT, 2004. (32) Kewalramani, S.; Hlaing, H.; Ocko, B. M.; Kuzmenko, I.; Fukuto, M. Effects of divalent cations on phase behavior and structure of a zwitterionic phospholipid (DMPC) monolayer at the air-water interface. J. Phys. Chem. Lett. 2010, 1, 489−495. (33) Gehlert, U.; Vollhardt, D. Molecular packing and textures of 1stearylamine-rac-glycerol monolayers. Langmuir 2002, 18, 688−693. (34) Rajesh, K.; Radhakrishnan, T. P. Optical response sensitive to the assembly in a molecular material: Ultrathin film with a vanishing electronic absorption. Chem.Eur. J. 2009, 15, 2801−2809.

17321


Mechanical Control of Molecular Aggregation and Fluorescence

Recommend Documents