Supramolecular Liquid Crystalline π-Conjugates: The Role of

Aug 20, 2010 - OPVs with an even number of methylene units in the side chains showed higher enthalpies with respect to their highly packed solid struc...
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J. Phys. Chem. B 2010, 114, 12508–12519

Supramolecular Liquid Crystalline π-Conjugates: The Role of Aromatic π-Stacking and van der Waals Forces on the Molecular Self-Assembly of Oligophenylenevinylenes Mahima Goel and M. Jayakannan* Department of Chemistry, Indian Institute of Science Education and Research (IISER)-Pune, 900, NCL InnoVation Park, Dr. Homi Bhabha Road, Pune -411008, Maharashtra, India ReceiVed: June 24, 2010; ReVised Manuscript ReceiVed: July 21, 2010

Here, we report a unique design strategy to trace the role of aromatic π-stacking and van der Waals interactions on the molecular self-organization of π-conjugated building blocks in a single system. A new series of bulky oligophenylenevinylenes (OPVs) bearing a tricyclodecanemethylene (TCD) unit in the aromatic π-core with flexible long methylene chains (n ) 0-12 and 16) in the longitudinal position were designed and synthesized. The OPVs were found to be liquid crystalline, and their enthalpies of phase transitions (also entropies) showed odd-even oscillation with respect to the number of carbon atoms in alkyl chains. OPVs with an even number of methylene units in the side chains showed higher enthalpies with respect to their highly packed solid structures compared to odd-numbered ones. Polarized light microscopic analysis confirmed the formation of cholesteric liquid crystalline (LC) phases of fan shaped textures with focal conics in OPVs with 5 e n e 9. OPVs with longer alkyl chains (OPV-10 to OPV-12) produced a birefringence pattern consisting of dark and bright ring-banded suprastructures. The melting temperature followed a sigmoidal trend, indicating the transformation of molecular self-organization in OPVs from solid to ring-banded suprastructures via cholesteric LC intermediates. At longer alkyl chain lengths, the van der Waals interactions among the alkyl chains became predominant and translated the mesogenic effect across the lamellae; as a consequence, the lamellae underwent twisted self-organization along the radial growth direction of the spherulites to produce bright and dark bands. Scanning electron microscope (SEM) analysis of cholesteric LC and ring-banded textures strongly supported the existence of twisted lamellae in the OPVs with ring-banded textures. Variable temperature X-ray diffraction analysis confirmed the reversibility of the molecular self-organization in the solid state and also showed the existence of the higher ordered lamellar structure in ring-banded OPVs. Photophysical characterizations such as excitation, emission, and time resolved fluorescence decay measurements were employed to trace molecular self-organization in their liquid crystalline phases. The emission spectra of the OPV samples showed odd-even oscillation in their emission wavelengths with respect to the length of alkyl chains. Highly packed evenOPVs showed more blue shift compared to that of less crystalline odd-OPVs. Time dependent fluorescence decay of OPVs followed a biexponential fit, and their lifetimes (τ1 and τ2 values) revealed that the decay is faster for odd-OPVs compared to even-OPVs. Among all the OPVs, the τ2 values for OPV-8 and OPV-12 were found to be much higher, indicating their high luminescent characteristics. In a nut shell, bulky liquid crystalline OPV chromophores were cleverly utilized, for the first time, to probe the aromatic π-stacking versus van der Waals interactions on the molecular self-organization of π-conjugated system. Introduction Supramolecular π-conjugated materials based on organic molecules and polymers have been widely studied due to their potential applications in electronic devices like light emitting diodes, photovoltaics, and chemical and biosensors.1-4 Molecular self-assembly via noncovalent forces such as hydrogen bonding, aromatic π-π stacking, and hydrophilic/hydrophobic and van der Waals interactions were employed as tools for obtaining supramolecular architectures.5-7 The macroscopic outcome of the self-assembly at the molecular level is routinely visualized as semisolid organo-gels, viscous liquids, or three dimensional ordered liquid crystals (LC) in the solid state.8-13 The liquid crystalline materials are particularly important since self-organized structures could be obtained via solvent free crystallization process. Optimization of precise structure and geometry in π-Conjugated molecules to tune their LC phases * Corresponding author. E-mail: [email protected]. Fax: +9120-25899790.

such as nematic, smectic, cholesteric or columnar is a very challenging task.14,15 Oligophenylenevinylenes (OPV),16 oligophenylenes,17 oligothiophenes,18 oligofluorenes,19 and oligophenyleneethylene20 chromophores were reported for selforganization in π-conjugated materials. Among these, multidimensional nanoarchitectures of oligothiophenes18d and cooperative conformation transition in oligopheylenethylenes20d were found to be more attractive. High luminescence characteristics, thermal and optical stability, solubility in organic solvents, film forming tendency, easy synthetic approaches, and so forth make OPV-chromophores very unique compared to other π-conjugates.16 Efforts had been put to self-organize OPVs building blocks via hydrogen bonding,8-11 π-π-stacking,12,13 and metal-ion interactions,21 and the resultant structures were tested successfully as active layers in electronic devices.22 More often, other interactions like hydrogen bonding and metal-ion interactions were introduced as additional secondary forces to strengthen the aromatic π-π-stacking self-assembly.23 However, the nonconjugated chemical linkages such as hydrogen bonding units

10.1021/jp105839f  2010 American Chemical Society Published on Web 08/20/2010

Supramolecular Liquid Crystalline π-Conjugates

Figure 1. Supramolecular self-organization of OPVs via aromatic π-stacking and van der Waals interactions.

(or metal-ion units) in the π-conjugated molecules behaved as insulting matrix which further influenced on the performance electronic devices.24 Further, in most cases, it was very difficult to rationalize the extent of π-π stacking and hydrogen bonding interaction toward the molecular self-assembly in supramolecular structures. It has been now widely realized that retaining the aromatic π-π stacking among the building blocks without compromising on the molecular self-assembly is very much essential for the success of these materials in electronic devices.25-27 Alkyl chains assisted self-organization based on weak van der Waals forces is another important secondary interaction; however, it was not explored to its full potential for self-assembly in π-conjugated materials. Thus, developing new molecular designs in the π-conjugated molecules for molecular self-organization via aromatic π-stacking and van der Waals forces are important issues to be addressed both for fundamental research and for developing new classes of materials for electronic devices. Recently, we reported a new structural design strategy for tuning the liquid crystallinity in OPVs purely based on aromatic π-stacking with the help of bulky cycloaliphatic ring substitutions.28-32 The preliminary investigation had revealed that among a dozen examined OPV structures, one molecule with symmetric substitution of two TCD units in the OPV backbone produced ring-banded LC textures.28 It provided us a new opportunity to design OPV building blocks having identical aromatic cores with variable alkyl chains in the longitudinal positions (Figure 1). This facilitates the investigation of the role of the aromatic π-stacking (via OPV core) and van der Waals interactions (via alkyl chains) on the molecular self-organization phenomenon in a single molecular building block system. OPVs are optically active chromophores, and therefore, the macroscopic outcome of selforganization could be traced by studying their liquid-crystalline and photophysical properties, which would be very useful to trace the origin of the molecular self-assembly. The present investigation emphasizes designing new series of bulky OPVs bearing a tricyclodecanemethylene (TCD) anchoring unit and self-organizing them via aromatic π-stacking

J. Phys. Chem. B, Vol. 114, No. 39, 2010 12509 and van der Waals interactions in the solid state. The design strategy of oligophenylenevinylene (OPV) molecular skeletons was adapted in such a way that rigid 1,8-tricyclodecanemethylene (TCD) is carried in the π-conjugated backbone and flexible long methylene chains in the longitudinal positions along the molecular axis (Figure 1). On the basis of our previous experience,28 the π-conjugation was fixed with a bis-distyryl unit and methylene chains were varied with -(CH2)n units where n ) 0-12 and 16 at both sides of the bis-distyryl conjugated block (Figure 1). The thermal analysis of the OPVs revealed that the OPV chromophores underwent sigmodial transformation in self-assembly from crystalline (n e 4) to three-dimensional cholesteric LC (5 e n e 9) and subsequently (n ) 10 and above) to higher order ring-banded suprastructures (Figure 1). This is the first time systematic transformation from crystalline to cholesteric to ring-banded textures has been observed in any type of molecular self-organization including π-conjugated materials and polymers. Further, the OPVs also showed an odd-even effect in the enthalpies and entropies of the LC-toisotropic and isotopic-to-crystalline transitions. OPVs with even numbers of methylene units showed higher enthalpies with respect to their highly packed structures compared to that of their odd-OPV counterparts. The molecular self-organization in the OPVs was studied in detail using a polarized light microscope, an electron microscope, and wide angle X-ray diffraction techniques. Since the building blocks possessed optically active OPV chromophores, the LC-frozen thin films were subjected to excitation, emission, and time resolved fluorescence decay lifetime techniques to understand the role of aromatic π-stacking. With the increase in the methylene chain length, the van der Waals forces became the predominant factor in determining the self-assembly of OPV units. In a nut shell, the unique LC forming tendency of the TCD-anchored bulkyOPV chromophores was cleverly utilized in the present investigation to probe the factors behind the molecular selforganization more exclusively based on aromatic π-stacking versus van der Waals interactions. Experimental Methods Materials. 1,8-Tricyclodecanemethanol was donated by Celanese Chemicals and Co. and was used without further purification. p-Toulenesufonyl chloride, triethylamine, hydroquinone, triethlyphosphite, potassium tert-butoxide (1 M in THF), 4-hydroxybenzaldehyde, 1-bromobutane, 1-bromopentane, 1-bromohexane, 1-bromoheptane, 1-bromooctane, 1-bromononane, 1-bromodecane, 1-bromoundecane, 1-bromododecane, 1-bromohexadecane, 4-methoxybenzaldehyde, 4-ethoxybenzaldehyde, and 4-(propyloxy)benzaldehyde were purchased from Aldrich Chemicals. HBr in glacial acetic acid, paraformaldehyde, KI, K2CO3, and NaOH were purchased locally. Solvents were purchased locally and purified by standard procedures. 1,4Bis(bromomethyl)-2,5-bis[(1,8-tricyclodecylmethylene)oxy]benzene and its corresponding ylides were synthesized according to our earlier procedures.29,32 General Procedures. 1H and 13C NMR were recorded using a 400 MHz JEOL NMR spectrometer. All NMR spectra were recorded in CDCl3 containing TMS as internal standard. Infrared spectra were recorded using a Thermo-Scientific Nicolet 6700 FT-IR spectrometer with the solid state in KBr. The mass of all the OPVs was determined by using the Applied Biosystems 4800 PLUS MALDI TOF/TOF analyzer. The samples were dissolved in dichloromethane and R-cyano-4-hydroxycinnamic acid was used as the matrix. The purity of all OPVs was further checked by gel permeation chromatographic (GPC) analysis,

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which was performed using a Viscotek VE 1122 pump, Viscotek VE 3580 RI detector, and Viscotek VE 3210 UV/vis detector in tetrahydrofuran (THF) using polystyrene as standards. Thermal analyses of all the OPVs were performed using a TA Q20 differential scanning calorimeter (DSC). The instrument was calibrated with indium standards. All the OPVs were heated to melt before recording their thermograms to remove their previous thermal history. OPVs were heated and cooled at 10 °C/min under a nitrogen atmosphere and their thermograms were recorded. To study the different LC phases as shown by different OPVs, we used a LIECA DM2500 P polarized light microscope equipped with Linkam TMS 94 heating and freezing stage connected to a Linkam TMS 600 temperature programmer. SEM images were recorded using the FEI QUANTA 200 3D scanning electron microscope. For SEM analysis, the samples were prepared in a hot stage (at 10 °C/min) under a polarized light microscope on glass coverslips (to confirm the formation of textures), subjected to gold coating. Variable temperature X-ray diffraction patterns were recorded using Philip X’pert Pro powder X-ray diffractometer with a copper target. The system consisted of a rotating anode generator with a copper target and a wide angle powder goniometer fitted with a high temperature attachment. Spectra were recorded using Cu KR radiation in the range of 2θ ) 3-50° at a heating and cooling rate of 10 °C/min. Absorption spectra were recorded using a Perkin Elmer Lambda 45 UV spectrophotometer. Steady state fluorescence emission, excitation spectra, and time resolved fluorescence lifetime measurements were performed using a Fluorolog HORIBA JOBIN VYON fluorescence spectrophotometer. For these photophysical studies, samples were heated to melt and then cooled to freeze them in their respective LC states between two glass coverslips. These thin transparent films were put under a polarized light microscope to confirm the formation of LC textures and then used for photophysical studies including fluorescence lifetime measurements. A HORIBA JOBIN YVON Nano-LED source with wavelength 371 nm was used to excite all the samples, and emission was collected at 500 nm. Fluorescence lifetime values were determined by deconvoluting the data with exponential decay using DAS6 decay analysis software. The quality of fit was judged by fitting parameters such as χ2 ≈ 1, as well as the visual inspection of the residuals. Synthesis of 4-Alkoxybenzaldehyde. The detailed synthesis is described for 4-(decyloxy)benzaldehyde, and other derivatives were synthesized using similar procedures. 4-Hydroxybenzaldehyde (3.66 g, 0.030 mol), anhydrous KI (4.98 g, 0.030 mol), and powdered anhydrous K2CO3 (8.28 g, 0.060 mol) were taken in dry acetone (50.0 mL) and refluxed for 2 h under a nitrogen atmosphere. 1-Bromodecane (6.8 mL, 0.033 mol) was added dropwise to the above hot reaction mixture. The reaction mixture was further refluxed for 24 h under a nitrogen atmosphere. It was cooled, acetone was removed under vacuum, and the residue was poured into water (100 mL). It was extracted into dichloromethane and washed with NaOH (150 mL, 2% solution) and with brine. The organic layer was dried over anhydrous Na2SO4 and condensed to get a pale yellow liquid as product. It was further purified by passing through a silica gel column using ethyl acetate (2% v/v) in hexane as eluent. Yield ) 6.6 g (84 %). 1H NMR (CDCl3, 400 MHz), δ: 9.85 (s, 1H, Ar-CHO), 7.80 (d, 2H, Ar-H), 6.97 (d, 2H, Ar-H), 4.01 (t, 2H, Ar-OCH2), 1.79 (m, 2H, Ar-OCH2CH2), 1.44 (m, 2H, Ar-OCH2CH2CH2), 1.31-1.25 (12H, aliphatic), 0.86 (t, 3H, -CH3). 13C NMR (CDCl3, 100 MHz), δ: 190.8 (-CHO), 164.21, 131.9, 129.63, 114.7 (Ar-C), 68.4 (-OCH2), 31.84, 29.5, 29.3, 28.9, 25.9, 22.6, 14.1 (aliphatic). FT-IR (KBr, cm-1):

Goel and Jayakannan 2924.5, 2854.1, 2731.9, 1694.1, 1601.3, 1577.9, 1509.3, 1467.9, 1428.6, 1393.3, 1311.7, 1258.8, 1215.3, 1159.4, 1109.3, 1016.0, 856.9, 832.3, 721.9, 651.4. Synthesis of Compounds 2-4. Tosylated TCD was coupled with hydroquinone to produce 2, which was then reacted with p-HCHO and HBr in acetic acid to get the bisbromomethylated compound (3). 3 was heated with triethyl phosphite to obtain the corresponding ylide (4). The detailed synthetic procedures for these compounds, yield, and NMR and FT-IR data are given in the Supporting Information. Synthesis of Oligophenylenevinylenes (OPV-n)s. All OPVs were synthesized using the general method described for OPV10. Tetraethyl 1,4-bis[(tricyclodecylmethylene)oxy]-2,5-xylenediphosphonate (0.47 g, 66.0 mmol) and 4-(decyloxy)benzaldehyde (0.35 g, 132 mmol) were taken in dry THF (30 mL) and kept under ice cold condition. Potassium tert-butoxide (3.96 mL, 1M THF solution) was added dropwise to the reaction mixture under a nitrogen atmosphere. It was stirred at 30 °C for 24 h. The resultant yellow solution was concentrated and poured into a large amount of methanol. The yellow green precipitate was filtered and washed with a large amount of methanol until the filtrate become colorless. It was purified by passing though a silica gel column using 1% ethyl acetate in hexane as eluent. Yield ) 0.22 g (36.0 %). 1H NMR (CDCl3, 400 MHz), δ: 7.45 (d, 4H, ArsH), 7.3 (d, 2H, CHdCH), 7.11 (d, 2H, CHdCH), 7.05 (s, 2H, ArsH), 6.89 (d, 4H, ArsH), 3.99 (t, 4H, ArsOCH2salkyl), 3.82-3.75 (m, 4H, ArsOCH2sTCD), 2.5-0.88 (m, 68H, cyclic-H and aliphatic-H).13C NMR (CDCl3, 100 MHz), δ: 158.8, 151.2, 130.8, 128.5, 127.7, 126.8, 121.6, 114.8, 111.1 (Ar-C), 73.7 (Ar-OCH2-D), 68.2 (ArsOCH2s TCD) 45.8, 45.4,44.1, 34.8,32, 29.7,29.5, 29.4, 29.17, 28.1, 26.6, 26.1, 22.8, 14.2. FT-IR (KBr, cm-1): 2924.4, 2855.6, 1605.0, 1510.8, 1469.7, 1245.6, 1178.6, 1018.4, 971.8, 849.7, 816.9. MALDI-TOF-TOF-MS (MW ) 923.40): m/z ) 922.677. Anal. Calcd for C64H90O4: C, 83.25; H, 9.82. Found: C, 84.03; H, 10.45. 1 H NMR, 13C NMR, and MALDI TOF/TOF data for all other OPVs are provided in the Supporting Information. Results and Discussion We synthesized a homologous series of tricyclodecanemethanol(TCD) based oligophenylenevinylenes by Wittig Horner reaction of 4-alkoxybenzaldehydes (1a-m) with tetraethyl 1,4bis[(tricyclodecylmethylene)oxy]-2,5-xylenediphosphonate (4) as shown in Scheme 1. The structures of OPVs were confirmed by 1H NMR, 13C NMR, and MALDI-TOF-TOF mass spectroscopy, and the purity was further confirmed by gel permeation chromatography (Supporting Information). The number of methylene units in alkoxy side chains was varied from 0 to 12 and 16. 4-Alkoxybenzaldehyde derivatives (1a to 1m) were synthesized by reacting their corresponding 1-alkyl bromides with 4-hydroxybenzaldehyde in the presence of base. The OPVs were named as OPV-n where “n” represents the number of methylene units in the alkoxy side chains. To trace the liquid crystallinity in the OPV-n, powdered samples were subjected to differential scanning calorimetry (DSC). All the samples were subjected to 10 °C/min heating and cooling and first heating cycle data were discarded since they possessed prehistory of the sample. DSC thermograms of OPV-n with n ) 1, 8, 9, 11, 12, and 16 are shown in Figure 2 (DSC thermograms of all other OPVs are shown in the Supporting Information). OPV-1, OPV-8, OPV-9, OPV-11, and OPV-12 showed two crystallization peaks when cooled from the melt. This is the typical nature of thermal transitions for thermotropic liquid crystalline materi-

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SCHEME 1: Synthesis of Liquid Crystalline Bulky OPVs with Various Alkyl Chains

als. The first peak was assigned to the isotropic-to LC and the second one to the LC-to-crystalline transition ( Figure 2a); for example, in OPV-12, the first peak at 115 °C and the second peak at 43 °C were assigned to isotropic-to-LC and LC-tocrystalline transitions, respectively. OPV-16 showed three peaks corresponding to isotropic-to-LC (at 88 °C), LC-crystalline (at 52 °C), and crystalline-to-crystalline (at 2 °C). In the subsequent heating cycle (second heating), OPV-9, OPV-11, and OPV-12

Figure 2. DSC thermograms of OPVs in the cooling (a) and heating cycle (b) at 10 °C/min.

showed two melting transitions and OPV-16 showed three melting transitions (Figure 2b). These melting transitions were assigned to their reverse phase changes, for example, in the OPV-11, crystalline-to-LC (at 15 °C) and LC-to-isotropic (at 162 °C). The samples OPV-0, OPV-1, and OPV-8 showed two thermal transitions in the cooling cycle, but only one melting transition in the heating cycle, indicating their monotropic liquid crystalline nature.33-35 Two important trends could be observed from the DSC plots: (i) the liquid crystalline active window (temperature range of isotopic-LC to LC-crystalline in the cooling cycle) increased from 5-15 °C (in OPV-1 to OPV-8) to 80-125 °C (in OPV-9 to OPV-12) and (ii) with the increase in the number of methylene units in alkoxy side chains, the tendency for OPVs to become liquid crystalline also increased. Another important observation was that the LC active window for OPV-16 was significantly smaller as compared to OPV-12, which suggested that beyond an optimum chain length the tendency for liquid crystallinity in OPVs decreased. DSC analysis clearly revealed that TCD substitution is very unique in producing a large number of OPV liquid crystalline chromophores that were only different in the number of carbon atoms in the methylene chains in the longitudinal position. The enthalpies of the melting and crystallization transitions were determined and summarized in Table 1 in the Supporting Information. The entropies of the transitions were calculated using the thermodynamic expression for the phase transitions,36 i.e., ∆S ) ∆H/T, where ∆H is the enthalpy of the transition at the temperature T (in Kelvin scale) and reported in Table 1. The enthalpies and entropies of the melting and isotropic-toLC transitions were plotted against the number of methylene units in the spacer and are shown in Figure 3. It is interesting to note that the OPVs followed odd-even oscillation in their enthalpy and entropy plots. Both enthalpy and entropy of the melting transition were found to be very high for OPVs with even numbers of carbon atoms in the spacer compared to those of their odd-numbered counterparts. The highly packed structure requires more energy for melting (endothermic) and similarly releases more energy (exothermic) while crystallizing compared to the case of weakly packed molecules. This suggests that the

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Figure 3. Odd-even oscillation of OPVs in the enthalpies and entropies of melting transition and crystallization. Schematic representations of OPV-6 and OPV-5 for molecular packing.

packing of the OPV chromophores is enhanced if even numbers of carbon atoms are present in the alkyl chains as compared to odd numbers of carbon atoms. Further, OPVs with spacers n ) 4, 8, 12, and 16 showed significantly higher energy terms as compared to other even-OPVs (n ) 2, 6, and 10) and oddOPVs. It suggests that OPVs with these carbon atoms 4, 8, 12, and 16 possessed higher packing in solid state and these numbers turned to be the magic numbers for solid state packing of OPV chromophores. Odd-even oscillation is typically observed in liquid crystalline materials having molecular twins in which azobenzene,37,38 biphenyl chromophores,39 dimeric salicylaldiamine,40 and LC polyesters41 were usually connected via methylene spacers. However, there are no reports for odd-even behavior in π-conjugated oligomers and polymers. Here, for first time, such an odd-even oscillation is observed in the LC properties of π-conjugated materials. This provides opportunity to manipulate the cyrstallinity and solid state ordering in the optical active OPV chromophores, which could be exploited in their electronic devices. To study the temperature dependent LC textures, all OPVs were subjected to polarized light microscopic analysis with a programmable hot stage. A pinch of sample was placed on the glass substrate, heated to melt at 10 °C/min, and kept isother-

mally at 20 °C above their melting temperature for 2-3 minutes. The melt was subsequently cooled at 10 °C/min to capture the image using camera. LC textures of OPVs are shown in Figures 4 and 5. OPV molecules with shorter alkoxy chains (n ) 0-4) did not produce prominent LC textures. The LC textures of OPV-5 to OPV-9 were found to be fan shaped, exhibiting focal conics and OPV-10 to OPV-12 produced beautiful birefringence patterns consisting of concentric dark and bright rings. Closer observation revealed that though OPV-0 and OPV-1 started to develop some type of LC texture, they immediately crystallized out to produce fanlike textures.33 Though these two samples clearly showed two crystallization peaks in the cooling cycle in DSC, the narrow LC-active window resulted in the formation of the mixture of the (LC + crystalline) phases. The fanlike textures with focal conics observed for OPV-5 to OPV-9 resembled the typical textures observed for cholesteric LC samples.33 The ring-banded textures found in OPV10, OPV-11 and OPV-12 were seen uniformly throughout the samples. The images captured for all the three samples showed both fully grown spherulites and the new born spherical nuclei (see site A and A′ in OPV-11 and B and B′ in the case of OPV-12 in Figure 5). Both fully grown spherulites and the new born spherical nuclei followed a similar trend of producing ring-

Supramolecular Liquid Crystalline π-Conjugates

Figure 4. Cholesteric liquid crystalline textures of OPVs under polarizing light microscope.

banded textures. Closer observation of these textures revealed that rings are not concentric but originate from a common center and continue to grow vertically outward in a left-handed helical fashion (see expanded image in the Supporting Information). The width of the dark and bright bands was found to be 9-18 and 9 µm, respectively. Surprisingly, OPV-16 did not produce ring-banded textures and the textures appeared to be typical textures of liquid crystals. From the above PLM studies, we understood that OPVs exhibited systematic transition in the mesophases from crystalline to cholesteric to ring-banded textures with an increase in the methylene units in the alkyl chains. This is for the first time; such a beautiful transformation of LC textures has been observed in a single chromophoric

J. Phys. Chem. B, Vol. 114, No. 39, 2010 12513 system. The transformation in the LC morphology was obtained in OPVs having identical aromatic π-cores, and therefore, the role of van der waals interactions among the alkyl chain in the OPV chromophores are expected to play a crucial role in determining the molecular self-organization during the crystallization process. To understand the nature of the molecular self-organization, the melting transition temperatures of the OPVs were plotted against the number of carbon atoms in alkyl chains and shown in Figure 6. The melting transition showed a sigmoidal relationship with the spacer length. There are two flat regions in the plot: (i) crystalline OPVs of shorter alkyl chains (OPV-0 to OPV-4) and (ii) ring-banded OPVs with alkyl chains 10 e n. The melting temperatures showed a sharp decrease from OPV-5 to OPV-9 where the molecules exist in the form of cholesteric LC phases. The combination of PLM textures and the sigmoidal curve indicated that van der Waals interactions among the methylene chains attached in the longitudinal axis played a key role in self-organizing OPVs, which transformed from crystalline solid to ring-banded supramolecules via cholesteric intermediates. To further understand the sigmodial transitions for the OPVs in the melt crystallization process, the LC temperatures were also plotted against the spacer length. The crystallization temperature versus spacer length was also fitted into a sigmoidal function; however, the data were found to be more scattered. Sigmoidal transformation is usually observed in the biological systems, for example, in the denaturation of proteins in the presence of appropriate denaturant moiety.42 However, sigmoidal (highly cooperative) selforganization is very rare in synthetic macromolecules due to the lack of cooperativity between secondary interactions unlike biological systems. Interestingly, in the present system, OPVs have followed typical sigmoidal self-organization indicating very good cooperativity between aromatic π-stacking and van der Waals interactions present in the chromophores. To trace the molecular packing of the OPV-mesogens, energy minimized structures of the OPVs were obtained by the Chem Draw 8.0 programme. Energy minimized structures of OPV-8

Figure 5. Ring-banded structures of OPV-10 to OPV-12 and the LC textures of OPV-16 under a polarizing light microscope.

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Figure 6. Sigmoidal plot of OPVs and the proposed mechanism for the self-organization of OPV chromophores in cholesteric LC phase and ring-banded structures.

and OPV-12 (Supporting Information) indicated that the two TCD-bulky groups attached in the aromatic rings projected away in opposite directions from the plane of the aromatic backbone. The long alkyl chains are extended along the axis of the molecule and stay in the plane of the aromatic backbone (longitudinally). As a result, the OPV chromophores were found to be nonsuperimposable on their mirror images. The molecular packing of nonsuperimposable liquid crystalline mesogens are well known to induce helical packing along the molecular axis and produce cholesteric-like LC phases.33 Therefore, in the present case also it is reasonable to think that the nonsuperimposable OPV-mesogens self-organize in the nonlinear fashion to produce cholesteric LC phases (in OPV-5 to OPV-9, Figure 4). In the present system, depending upon their alkyl chain lengths in the longitudinal position, OPV chromophores produced textures resembling either cholesteric fanlike or higher order ring-banded textures. As the alkyl chain length increases further (in OPV-10 to OPV-12), the packing of the nonsuperimposible OPV chromophores differed from their medium alkyl chain length counterparts (n ) 5-9). As a result, the birefringence pattern consisting of concentric dark and bright rings was produced in OPV-10 to OPV-12. Therefore, the origin of the ring-banded textures may not be merely the extension of helical arrangements of the LC mesogens as observed in the cholesteric LC phases. The crystallization phenomenon is one of the most important fundamental properties of solids and it typically consists of two consecutive steps: nucleation and crystal growth.34,35 The crystal growth predominantly determines the structure and orientation of the three-dimensional growth of the spherulites. Ring-banded spherulitic structures are not common in crystalline solids; however, recently it was observed in a few commercial polymers like polyethylene, polylactides, and poly(aryl ether ketones), etc.43-45 Keith et al. and few others had studied the fundamental understanding of these ring-banded structures using theoretical, microscopic studies, and computational simulations.46-51 The origin for the ring-banded textures in the melt crystallization was universally attributed to the periodic twisting of lamellar crystals along the radial growth direction of the spherulites.52-54 The formation of dark and bright

bands in the PLM images is the result of birefringence of plane polarized light when it passes through a twisted lamella.45a-d Here, the ring-banded textures are obtained from small liquid crystalline OPV molecules unlike earlier observed examples of semicrystalline commercial polymers. Therefore, the origin of the ring-banded textures in OPVs is the combination of the crystallization processes observed in typical cholesteric liquid crystalline molecules and the ring-banded structures of high molecular weight polymers. On the basis of above discussion, the following mechanism is proposed for OPV self-organization (Figure 6): (i) lamella consisting of helically oriented LC mesogens are produced by nonsuperimposable OPV chromophores, (ii) for the medium alkyl chains in the longitudinal positions, these lamellae undergo face-to-face packing and produce three directional spherulites in cholesteric phases, (iii) at longer alkyl chains, the strong van der Waals interactions among the alkyl chains translate the mesogenic effect across the lamella, (iv) as a consequence of this effect, the lamellae undergo twisted self-organization along the radial growth direction of the spherulites, and (v) the lamellae oriented parallel to the plane of polarized light produced bright bands whereas dark bands were observed from the lamellae that were perpendicular to the plane of the light. Further, the ring bands were observed as left-handed helices in both the growing spherulites and the new born ones. Thus, the formation of the ring-banded textures in the OPV-10 to OPV-12 is the result of higher order twisted molecular self-assembly in the OPV chromophores during crystallization form isotropic state. In the present design, aromatic π-stacking (via bis-distyrl unit) and van der Waals interaction (through alkyl chains) are the only two main noncovalent interactions available for molecular self-assembly. Since all the OPV mesogens possessed a fixed aromatic counterpart, ring-banded texture formations are directly correlated to the difference in the van der Waals interactions at higher alkyl chain lengths. To confirm the above mechanism, the LC frozen samples were subjected to scanning electron microscope (SEM) analysis. SEM images of the OPV-8 (cholesteric LC textures) and OPV12 (ring-banded textures) are given in Figure 7. The crystalline

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Figure 7. Scanning electron microscope images of OPV-8 and OPV-12 samples frozen in their LC phases.

vectors corresponding to the cholesteric phases in OPV-8 showed uniform long rectangular planks with thickness (600 nm. The rectangular planks were straight (length more than 10 µM) and loosely packed; however, all of them projected upward in one direction. The SEM image of OPV-12 was completely different from that of OPV-8. It showed the formation of thin twisted sheets. The sheets were twisted within, tightly packed and also twisted together along the same direction. The crystalline vectors are typically composed of a large number of lamellae, which could be in thousands, and these subnanometer size species pack in the solid state to produce the images shown in Figure 7. The straight planks in OPV-8 and twisted sheets in OPV-12 revealed that the orientation of the lamellae were obviously different in both the cases. SEM analysis strongly supported the existence of twisted lamellae in OPV-12 and directly supports the mechanism proposed in Figure 6. The twisted lamellae produced ring-banded textures whereas the normal packing produced cholesteric textures. To identify the lamellar ordering, OPV-8 and OPV-12 were further subjected to variable temperate X-ray diffraction analysis and their respective diffraction patterns are shown in Figure 8. The powder sample showed multiple sharp crystalline peaks corresponding to a highly crystalline state. The samples were heated to melt (180 °C for OPV-12 and 200 °C for OPV-8) and subsequently cooled to the desired LC active temperature. Upon heating, the crystalline peaks completely disappeared (see H-180 and H-200 °C in Figure 8) in the isotropic region, and in the subsequent cooling cycle, the peaks re-appeared. The temperature dependent X-ray analysis revealed that the molecular selforganization is completely reversible in the repetitive heating/ cooling cycles (as already seen in PLM and DSC analysis). The bunch of peaks that appeared in the higher angle region from

Figure 8. Variable temperature wide angle X-ray diffraction patterns of OPV-8 and OPV-12.

2θ ) 15-25° (d ) 4-6 Å) were almost identical in both OPV-8 and OPV-12 and corresponded to aromatic core packing.55,56 In the low angle region (2θ < 15), there are three peaks

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appearing at 2θ ) 6.5, 10.0, 12.8° in OPV-12 whereas only two peaks are seen in OPV-8 at 2θ ) 10.4 and 11.1°. The d-spacing values of these reflections corresponded to 13.6, 8.8, and 6.9 Å with regular periodicity. The comparison of the X-ray diffraction patterns revealed that the two sharp low angles peaks at 13.6 and 6.92 Å in OPV-12 are not present in the OPV-8. These peaks were assigned to higher order reflections 002 and 003, respectively, in the lamellae.28,55,56 The presence of the fundamental peak at the low angle region proved the existence of the higher ordered lamellar structure followed by the close packing of mesogens in OPV-12 (as seen in the SEM image). Single crystal analysis of these OPV molecules would be very useful to confirm the above hypothesis; however, our attempts to grow single crystals are not successful so far. Nevertheless, the detailed analysis of OPVs by DSC, sigmoidial trend, PLM, SEM, and variable temperature WXRD provided ample evidence to support the postulated mechanism for the ring-banded textures in OPVs (as shown in Figure 6). Therefore, it can be concluded that the formation of ring-banded textures in OPVs is the result of long range solid state ordering induced by the alkyl chain packing. The ring-banded textures in OPV-10 to OPV-12 basically originated from the twisted molecular self-assembly of mesogens across the lamella, which was facilitated by van der Waals interactions at the molecular level. To the best of our knowledge, this is the first time ring-banded supramolecular textures were observed in the any type of π-conjugated oligomers or conducting materials. The OPV building block is a highly luminescent π-conjugated unit, and therefore, photophysical characterization such as excitation, emission, and time resolved fluorescence decay measurements were employed to trace molecular self-organization in the liquid crystalline phase.57 Thin films of OPVs were prepared on a glass substrate, as described earlier in PLM studies. These samples were excited with 380 nm monochromatic light and their emission spectra are given in Figure 9a. OPV-0 and OPV-1 were omitted for the photophysical studies due to the following reasons: (i) OPV-0 did not have alkoxy units in the peripheral phenyl ring and (ii) the Ar-OCH3 inductive effect causes OPV-1 π-electronic structure different from that of other OPVs. The emission spectra of OPVs were very broad (Figure 9a), and therefore, to quantify the extent of the red or blue shift among the OPV samples, emission wavelengths (in nm) at PL intensities of 0.8 and 1.0 were compared in their normalized spectra. The plots of the emission wavelengths at PL intensities 0.8 and 1.0 are in Figure 9b. Two important observations could be made from the emission spectra of OPV-n films: (a) both the plots in Figure 9b showed odd-even oscillation in the emission wavelength with respect to the number of carbon atoms in the alkyl chains and (b) OPV-8 and OPV-12 were distinctly blue-shifted compared to all other OPVs. Interestingly, the odd-even oscillation in the emission spectra was not restricted to any particular type of LC texture. It was observed across the entire series in crystalline solids (OPV-2 to OPV-4), cholesteric LCs (OPV-5 to OPV-9), and ring-banded LCs (OPV-10 to OPV-12). The comparison of the odd-even oscillation behavior in the enthalpy plots (in Figure 3) and emission spectra (in Figure 9b) revealed that the highly crystalline and packed samples showed more blue shift compared to that of less crystalline counterparts. The extent of the blue shift was observed to be very large in OPV-8 and OPV12, which possessed highly packed structures with higher energy terms. The highly packed structures were expected to possess strong π-overlap among the OPV-chromophores, which led to a blue shift in their emission spectra (OPV-8 and OPV-12)

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Figure 9. (a) Photoluminescence spectra of OPVs in the LC frozen films (excitation ) 380 nm) and (b) plot of the PL emission wavelengths versus the number of carbon atoms in the alkyl chain.

compared to the spectra of the weakly packed samples (OPV-9 and OPV-11). A large number of PL experiments were carried out to verify the reproducibility of the odd-even oscillation behavior in the emission spectra (Supporting Information). More than four sets of newly prepared LC films for each OPV samples were subjected to photoluminescence studies. Since the excitation spectra of the OPVs are very broad from 320 to 420 nm (Supporting Information), the emission spectra were also collected by exciting the samples at 320, 340, 360, 380, 400, 420, and 440 nm. Though there is a slight deviation in the wavelength of the emission spectra, all these control experiments showed perfect reproducibility in the odd-even oscillation in their emission spectra (Supporting Information). Therefore, the odd-even oscillation in the PL spectra is not an artifact; it is a molecular property that arose via the differences in the molecular packing among the OPV chromophores. The highly packed even numbered OPV-n molecules showed a blue shift compared to that of less or weakly packed odd number counterparts. The extent of the packing and blue shift was found to be predominant for alkyl chain lengths n ) 8 and n ) 12. Time dependent fluorescence decay measurements were carried out by the TCSPC technique using 370 nm nano-LED light sources. The thin films of samples employed for the PL studies were used for the fluorescence decay measurements. At least two sets of samples were subjected for each OPV to confirm the reproducibility of the decay profiles. The decay profiles for few representative OPV molecules are shown in Figure 10a,b (for all other samples, see Supporting Information). All the decay data followed a biexponential fit and their lifetime values are summarized in the Supporting Information, Table 2. All the OPV samples typically showed a fast decay with a lifetime of τ1 ) 0.28-0.40 ns and slow second decay with τ2 ) 1.2-2.4 ns. The τ1 and τ2 values match that of the earlier reports for OPV systems.28,58 The τ1 and τ2 values were plotted

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Figure 10. Fluorescence decay profiles of OPVs and plot of lifetimes versus the number of carbon atoms in the alkyl chains.

against the number of carbon atoms in the alkyl chains and shown in Figure 10c. The overall trend of the lifetime values showed that the decay is faster for odd-OPVs when compared to their even-OPV counterparts. Among all the OPVs, the τ2 values for OPV-8 (1.95 ns) and OPV-12 (1.77 ns) were found to be much higher, indicating their high luminescent characteristics. The origin for the large blue shift in OPV-8 and OPV12 is not very clear at present; however, all the above analysis confirmed their strong self-organization for the existence of H-type aggregates in these two OPVs. We are currently engaged in studying these aggregates in detail, and the results will be communicated at a later stage. Nevertheless, it has been proven in the present system that the difference in the alkyl chain packing in the π-conjugated OPV backbone results in the difference in LC textures, molecular packing, and subsequently, their photophysical properties. In a nut shell, the liquid crystalline OPV core has been cleverly chosen to study the role of aromatic π-stacking versus the van der Waals interactions in a single system. This was achieved in the present system without any change in the structure of the rigid optical chromophore (OPV part), but by just varying the length of the alkyl chains from 0 to 12 along the axis of the molecule. We are currently engaged in designing off novel liquid crystalline OPVs with optical active units based on the above molecular skeleton to trace the self-organization in the cholesteric and ringbanded suprastructures via induced molecular chirality. The OPVs demonstrated here have precise self-organization in the solid state, which would be very useful for opto-electronic applications. Conclusion In conclusion, a new series of bulky π-conjugated oligophenylenevinylenes based on tricylodecanemethylene units with variable alkyl chain lengths in the longitudinal position were synthesized to study the role of aromatic π-stacking and van der Waals interactions on the molecular self-assembly. The important novelties of the present investigation are as follows:

(i) A new molecular skeleton for liquid crystalline OPV chromophore was developed on the basis of a rigid aromatic π-core with flexible chains; as a result, more than ten new liquid crystalline materials were synthesized. (ii) The LC-active window increased from 5-15° to 80-125° by increasing the number of methylene units in alkoxy side chains. (iii) The enthalpies and entropies of the melting and isotropic-to-LC transitions interestingly followed odd-even oscillation, which provided an opportunity to manipulate the crystallinity of the OPV chromophores with respect to the alkyl chain length. (iv) OPV molecules with shorter alkoxy chains (n ) 0-4) were found to be crystalline solids. (v) OPV-5 to OPV-9 were found to be cholesteric LCs and produced fan shaped textures with focal conics in their PLM images. (vi) A birefringence pattern consisting of concentric dark and bright rings was observed in OPV-10 to OPV-12; as a result, for the first time, such a beautiful transformation of LC textures has been observed in a single chromophoric system. (vii) The sizes of dark and bright bands were obtained as 9-18 and 9 µm, respectively. (viii) The plot of melting temperatures against the number of carbon atoms in the alkyl chains showed a sigmoidal trend, indicating very good cooperativity between the aromatic π-stacking and van der Waals interactions present in the OPVs. (ix) Energy minimized structures helped in understanding the molecular packing of nonsuperimposable liquid crystalline OPVs and the nonlinear OPV-mesogenic self-organization in the lamellae producing cholesteric LC phases. (x) The formation of the ringbanded textures in OPV-10 to OPV-12 was correlated to the twisted self-assembly of lamellae during the crystallization form isotropic state. (xi) In the present design, all the OPV mesogens possessed a fixed aromatic π-core, and therefore, the van der Waals interactions at higher alkyl chain lengths account for the formation of ring-banded textures. (xii) SEM and variable temperate X-ray diffraction analysis confirmed the existence of twisted lamellar self-organization in the ring-banded textures. (xiii) The photophysical studies of OPVs revealed odd-even oscillation in the emission wavelength with the variation in the

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alkyl chain lengths. (xiv) Time dependent fluorescence decay studies showed biexponential decay, and their lifetimes (τ1 and τ2 values) were found to be much higher for OPV-8 and OPV12, indicating their highly luminescent characteristics. The approach demonstrated here allows one to synthesize and manipulate the liquid crystallinity, solid state ordering, molecular self-assembly, and photophysical characteristics in π-conjugated chromophores, which have continuous demand in the electronic industry. Further, the liquid crystalline materials are produced in an eco-friendly solvent free self-organization via a melt crystallization process and bear potential for large scale processing applications. Acknowledgment. We thank Department of Science and Technology, New Delhi, India, under Scheme: NSTI Programme-SR/S5/NM-06/2007 and SR/NM/NS-42/2009 for financial support. We thank Dr. P. A. Joy, NCL-Pune for WXRD and Dr. B. L. V Prasad for SEM analysis. M.G. thanks CSIRNew Delhi, India, for a Junior Research Fellowship. Supporting Information Available: 1H and 13C NMR and MALDI TOF/TOF mass spectra for all the OPVs; synthetic details for precursors 2-4 and OPV-0-12 and -16; table for enthalpies of melting and crystalline transitions; table of fluorescence decay lifetimes of OPVs; DSC thermograms of OPV-0, -2, -3, -4, -5, -6, -7, and -10; expanded PLM images for OPV-10-12; energy minimized structures; excitation and absorption spectra; even-odd oscillation plots at variable excitation wavelengths; even-odd oscillation plot for four sets at 380 nm excitation wavelength; emission maximum vs excitation wavelength plot; decay profiles of OPVs and GPC chromatograms of OPVs are available. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem. Int. Ed. 1998, 37, 402. (2) Zgierski, M. Z.; Fujiwara, T.; Lim, E. C. Acc. Chem. Res. 2010, 43, 506. (3) Bowden, N. B.; Weck, M.; Choi, I. S.; Whitesides, G. M. Acc. Chem. Res. 2001, 34, 231. (4) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning, A. P. H. J. Chem. ReV. 2005, 105, 1491. (5) (a) Schenning, A. P. H. J.; Jonkheijm, P.; Peeters, E.; Meijer, E. W. J. Am. Chem. Soc. 2001, 123, 409. (b) Jonkheijm, P.; Miura, A.; Zdanowsk, M.; Hoeben, F. J. N.; Feyter, S. D.; Schenning, A. P. H. J.; Schryver, F. C. D.; Meijer, E. W. J. Angew. Chem. Int. Ed. 2004, 43, 74. (c) Pisula, W.; Tomovic, Z.; Wegner, M.; Graf, R.; Pouderoijen, M. J.; Meijer, E. W.; Schenning, A. P. H. J. J. Mater. Chem. 2008, 18, 2968. (d) Herrikhuyzen, J. V.; Shyamakumari, A.; Schenning, A. P. H. J.; Meijer, E. W. J. Am. Chem. Soc. 2004, 126, 10021. (e) Hoeben, F. J. M.; Wolffs, M.; Zhang, J.; Feyter, S. D.; Leclere, P.; Meijer, E. W. J. Am. Chem. Soc. 2007, 129, 9819. (6) (a) Srinivasan, S.; Babu, S. S.; Praveen, V. K.; Ajayaghosh, A. Angew. Chem. Int. Ed. 2008, 47, 5746. (b) Srinivasan, S.; Babu, S. S.; Mahesh, S.; Ajayaghosh, A. J. Am. Chem. Soc. 2009, 131, 15122. (c) Srinivasan, S.; Praveen, V. K.; Philip, R.; Ajayaghosh, A. Angew. Chem. Int. Ed. 2008, 47, 5750. (7) Yagai, S.; Kubota, S.; Saito, H.; Unoike, K.; Karatsu, T.; Kitamura, A.; Ajayaghosh, A.; Kanesato, M.; Kikkawa, Y. J. Am. Chem. Soc. 2009, 131, 5408. (8) Reddy, R. A.; Dantlgraber, G.; Baumeister, U.; Tschierske, C. Angew. Chem. Int. Ed. 2006, 45, 1928. (9) Wicklein, A.; Lang, A.; Muth, M.; Thelakkat, M. J. Am. Chem. Soc. 2009, 131, 14442. (10) Kuo, L. C.; Huang, W. T.; Yang, K. H.; Hsu, H. F.; Jin, B. Y.; Leung, M. K. J. Phys. Chem. B 2010, 114, 2607. (11) Kimura, M.; Miki, N.; Adachi, N.; Tatewaki, Y.; Ohta, K.; Shirai, H. J. Mater. Chem. 2009, 19, 1086. (12) Hirai, Y.; Babu, S. S.; Praveen, V. K.; Yasuda, T.; Ajayaghosh, A.; Kato, T. AdV. Mater. 2009, 21, 4029. (13) Ajayaghosh, A.; Praveen, V. K. Acc. Chem. Res. 2007, 40, 644. (14) Hoag, B. P.; Gin, D. L. Liq. Cryst. 2004, 31, 185.

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