Mesogenic, Luminescence, and Nonlinear Optical Properties of New

Article pubs.acs.org/JPCC

Mesogenic, Luminescence, and Nonlinear Optical Properties of New Bipyrimidine-Based Multifunctional Octupoles Huriye Akdas-Kilig,*,† Maxime Godfroy,† Jean-Luc Fillaut,† Bertrand Donnio,‡,§ Benoît Heinrich,‡ Przemysław Kędziora,∥ Jean-Pierre Malval,⊥ Arnaud Spangenberg,⊥ Stijn van Cleuvenbergen,# Koen Clays,# and Franck Camerel*,† †

Institut des Sciences Chimiques de Rennes, UMR 6226 (CNRS-Université de Rennes 1), Campus de Beaulieu, 35042 Rennes, France ‡ Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), UMR 7504 (CNRS-Université de Strasbourg), 23 rue du Loess, BP 43, Strasbourg 67034 Cedex 2, France § Complex Assemblies of Soft Matter Laboratory (COMPASS), UMI 3254 (CNRS-SOLVAY-University of Pennsylvania), CRTB, 350 George Patterson Boulevard, Bristol, Pennsylvania 19007, United States ∥ Polish Academy of Sciences, Institute of Molecular Physics, Smoluchowskiego 17, 60-179 Poznań, Poland ⊥ Institut de Science des Matériaux de Mulhouse, UMR 7361 (CNRS-Université de Haute-Alsace), 15 rue Jean Starcky, 68057 Mulhouse, France # Department of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200D and F, B-3001 Leuven, Belgium S Supporting Information *

ABSTRACT: A series of new bipyrimidine-based chromophores have been prepared presenting alkoxystyryl donor groups carrying aliphatic chains in the 3,4, 3,5 or 3,4,5 positions, connected to electron-accepting 2,2′-bipyrimidine cores. Their linear and nonlinear optical properties were investigated as well as their mesomorphic properties by various techniques (light-transmission measurements, polarized-light optical microscopy, and differential scanning calorimetry measurements). Only two derivatives, BPM-3,4-C12 and BPM-3,4-C16, were found to exhibit liquid-crystalline behavior with the formation of lamella-columnar phases and/or hexagonal columnar phases over large temperature ranges. Small-angle X-ray scattering analysis allowed proposing a stacking model inside the mesophase in which the molecules are interdigitated alternatively along their long axis and their short axis to form columns. Dielectric measurements were performed as a function of the temperature, showing the centrosymmetric nature of the mesophases. Large quadratic hyperpolarizabilities have been measured for the individual mesogens in solution by using hyper-Rayleigh scattering. These chromophores exhibit also cubic nonlinear optical properties, revealing relatively large two-photon absorption cross sections. The nonlinear optical properties in the liquid crystalline state of compounds BPM-3,4-C12 and BPM-3,4-C16 have been studied by wide-field second-harmonic generation and two-photon fluorescence microscopy, confirming centrosymmetry for these achiral mesogens and the excellent third-order nonlinearity for multiphoton imaging.

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INTRODUCTION

conjugated branches carrying electron-donating fragments. The octupolar properties are significantly influenced by the donor− acceptor strength, the conjugation length, and the nature of the frontier electronic states.7 In contrast, a more limited number of octupoles with three-dimensional tetrahedral (Td) or pseudotetrahedral (D2d) symmetries have been reported so far as efficient NLO-phores.8,9 Recently, a new class of three-dimensional NLO-phores of D2d symmetry based on donor-substituted styryl bipyrimidine cores have been investigated.10 The lone pair’s interactions

Over the last decades, organic materials for nonlinear optics (NLO) and photonics have gained a lot of interest in materials1 and biological science.2 Molecular design of optically active organic or organometallic molecules with nondipolar symmetry has shown outstanding results in nonlinear optics, and numerous synthetic routes have been explored to tune their hyperpolarizabilities and to optimize their NLO responses in solutions and in the solid state.3,4 Octupolar π-conjugated molecules possessing electron donor−acceptor (D−A) characteristics are a promising class of materials for nonlinear optics.5,6 Many of these systems are two-dimensional octupoles with a 3-fold symmetry architecture (D3 or D3h) in which the central aromatic acceptor core is trigonally substituted with © XXXX American Chemical Society

Received: November 17, 2014 Revised: January 20, 2015

A

DOI: 10.1021/jp511486y J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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EXPERIMENTAL SECTION Materials and General Methods. NMR Spectra. NMR spectra (1H, 13C) were recorded at room temperature on BRUKER ASCEND400 and DMX500 spectrometers operating at 400 and 500 MHz. High-resolution mass spectra were obtained on a Bruker Micro-TOF-Q II instrument or a ZabSpec TOF Micromass instrument at CRMPO, University of Rennes 1. Steady-State Absorption and Fluorescence Spectra. The absorption measurements were carried out with a PerkinElmer Lambda 2 spectrometer. Steady-state fluorescence spectra were collected from a FluoroMax-4 spectrofluorometer. Emission spectra are spectrally corrected, and fluorescence quantum yields include the correction due to solvent refractive index and were determined relative to quinine bisulfate in 0.05 molar sulfuric acid (Φref = 0.52).22 Actinometry. Quantum yields for trans → cis photoisomerization were measured under irradiation at 365 nm using a 100 W Mercury−Xenon lamp (Hamamatsu, L2422-02) equipped with a band-pass filter. All irradiated solutions were previously N2-degassed. The progress of the photoreaction was monitored via UV−vis absorption spectra. The absorbance at excitation wavelength was greater than 2 to assume a total absorption of the incident photons. The incident light intensity was measured by ferrioxalate actinometry.23 Two-Photon Excited Fluorescence. The two-photon absorption measurements were performed with femtosecond mode-locked laser pulse using a Ti: Sapphire laser (Coherent, Chameleon Ultra II: pulse duration: ∼140 fs; repetition rate: 80 MHz; wavelength range: 680−1080 nm). A relative two-photon excited fluorescence (2PEF) method24 was employed to measure the two-photon absorption cross sections, δ. A 10−4 M solution of fluorescein25,26 in water at pH = 11 was used as the reference (r). The value of δ for a sample (s) is given by

between the nitrogen atoms of the two pyrimidine fragments allow the molecule to present a pseudotetrahedral octupolar symmetry. Such derivatives exhibit interesting fluorescence and fluoro-solvatochromic properties, as well as large cubic and quadratic NLO responses, which can be controlled by the nature of the substituents within the styryl fragments. Therefore, bipyrimidine derivatives appear particularly interesting in the design of new three-dimensional NLO-phores since they can easily be functionalized by various types of styryl fragments in order to control the hyperpolarizabilities and tune their supramolecular self-assembly abilities in solution and in the solid state such as liquid crystalline materials. Actually, incorporation of photoactive molecules into liquid crystalline materials can allow the modulation of their luminescence properties at the macroscopic scale in response to various external stimuli such as temperature, electrical, or magnetic fields or mechanical constraints.11−13 Mesomorphic properties also allow switching between several states, which would be of great interest due to their potential application for memory devices,14−16 sensors,17 and information displays.18 However, a limited number of attempts have been done up to now to directly incorporate nonlinear optical properties onto mesogens. The adequate functionalization of D3h molecules such as hexaazatriphenylene or triindole has allowed the preparation of octupolar mesogenic molecules.19,20 However, the nonlinear optical properties of these mesogens have been only studied in solution, and there is no report concerning their nonlinear optical properties in the solid state. Only recently, second harmonic generation has been observed on liquid crystal (LC) thin films with an alkynylbenzene derivative carrying chiral hydrocarbon chains.21 Thus, the appropriate functionalization of the nonlinearly optically active bipyrimidine core by long alkyl segments should lead to the emergence of mesomorphic materials able to form soft structured NLO-active thin films. The present contribution describes the synthesis and complete physicochemical characterization (including linear and nonlinear optical, mesomorphic, thermal, structural, dielectric) of a series of octupolar four-branched compounds incorporating a bipyrimidine (BPM) chelating group as an electron-deficient core (Scheme 1) and four divergent lipophilic

δS =

SS Φr ηr cr Sr ΦSηScS

δr

where S is the detected two-photon excited fluorescence integral area, c the concentration of the chromophores, and Φ the fluorescence quantum yield of the chromophores. η is the collection efficiency of the experimental setup and accounts for the wavelength dependence of the detectors and optics as well as the difference in refractive indices between the solvents in which the reference and sample compounds are dissolved. The measurements were conducted in a regime where the fluorescence signal showed a quadratic dependence on the intensity of the excitation beam, as expected for two-photon induced emission. For the calibration of the two-photon absorption spectra, the two-photon excited fluorescence signal of each compound was recorded at the same excitation wavelength (λexc: 782 nm) as that used for fluorescein. The laser intensity was in the range of 0.2−2 × 109 W/cm2. The experimental error on the reported cross section is 15%. Thermal Analyses. DSC was carried out by using the NETZSCH DSC 200 F3 instrument equipped with an intracooler. DSC traces were measured at 10 °C/min down to −30 °C. POM . Optical microscopy investigations were performed on a Nikon H600L polarizing microscope equipped with a Linkam “liquid crystal pro system” hotstage. The microscope is also equipped with a UV irradiation source (Hg Lamp, λ = 350−400 nm) and an ocean optic USB 2000+ UV−vis−NIR

Scheme 1. Molecular Structures of the Lipophilic BPM Chromophores

styryl fragments. Depending on the alkoxy chain-substitution pattern on these fragments, the bipyrimidine derivatives exhibit distinctive mesogenic properties and optical properties such as absorption, emission, or NLO behaviors (two-photon absorption). Such divergent properties will be connected with the structural arrangement of the molecule and its impacts on the three-dimensional organization of the global architecture. B


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horizontal and vertical directions correspond to X and Y, respectively. Solvents and Reactants. All reagents for synthesis were commercially obtained from Aldrich. The solvents employed for absorption and emission analysis were spectroscopic grade. Synthetic Procedures. General Procedure for the Condensation of the Aldehydes with 4,4′,6,6′-Tetramethyl[2,2′]-bipyrimidine. A stirred mixture of 4,4′,6,6′-tetramethyl[2,2′]-bipyrimidine (1.0 mmol) and the corresponding aldehyde (5.0 mmol) in aqueous sodium hydroxide (5 M, 15 mL) containing Aliquat 336 (0.1 mmol) was heated under reflux for 72 h. The mixture was allowed to cool, and the precipitate was filtered off, washed with water, and purified by recrystallization from a dichloromethane/methanol mixture. BPM-3,4-C1: orange solid, 71% yield. 1H NMR (CHCl3, 400.16 MHz): δ ppm 7.92 (d, J = 15.9 Hz, 4H), 7.64 (s, 2H), 7.18 (s, 4H), 7.2 (m, under solvent peak), 6.94 (d, J = 8.0 Hz, 8H), 3.98 (s, 3H), 3.96 (s, 3H). 13C NMR (CD2Cl2, 125.8 MHz): δ ppm 164.2, 151.2, 150.0, 137.4, 129.3, 124.8, 122.4, 111.9, 110.2, 56.3. Anal. found: C, 67.52; H, 5.77; N, 6.39. C48H46N4O8·2/3CH2Cl2 Calcd: C, 67.69; H, 5.53; N, 6.49. m/z (Maldi-TOF) 806.413 ([M]+, C48H46N4O8 requires 806.332). BPM-3,4-C12: yellow solid, 51% yield. 1H NMR (CD2Cl2, 400.16 MHz): δ ppm 7.82 (d, J = 15.9 Hz, 4H), 7.41 (s, 2H), 7.18 (s, 4H), 7.13 (d, J = 6.1 Hz, 4H), 7.06 (d, J = 16.1 Hz, 4H), 6.83 (d, J = 8.0 Hz, 8H), 4.03 (q, J = 8.0 Hz, 16H), 1.73 (m, 16H), 1.50−1.30 (m, 144H), 0.90 (m, 24H). 13C NMR (CD2Cl2, 125.8 MHz): δ ppm 165.0, 164.7, 151.7, 150.3, 137.9, 129.8, 125.1, 122.8, 115.0, 114.2, 112.9, 70.3, 70.15, 32.9, 30.7, 30.68, 30.65, 30.48, 30.44, 30.38, 30.29, 27.1, 27.0, 23.7, 14.8. Anal. found: C, 78.73; H, 10.76; N, 2.70. C136H222N4O8·1/ 2CH2Cl2 Calcd: C, 78.68; H, 10.79; N, 2.69. m/z (Maldi-TOF) 2041.1201 ([M]+, C136H222N4O8 requires 2041.240). BPM-3,4-C16: yellow powder, 73% yield. 1H NMR (CD2Cl2, 400.16 MHz): δ ppm 7.82 (d, J = 15.9 Hz, 4H), 7.41 (s, 2H), 7.18 (s, 4H), 7.13 (d, J = 6.1 Hz, 4H), 7.06 (d, J = 16.1 Hz, 4H), 6.83 (d, J = 8.0 Hz, 8H), 3.95 (q, J = 8.0 Hz, 16H), 1.73 (m, 16H), 1.50−1.30 (m, 208H), 0.90 (m, 24H). 13 C NMR (CDCl3, 100.62 MHz): δ ppm 164.1, 150.6, 149.3, 137.1, 128.9, 124.6, 121.9, 113.2, 111.8, 69.3, 31.9, 29.7, 29.69, 29.66, 29.49, 29.45, 29.39, 29.32, 29.24, 26.1, 22.7, 14.1. Anal. found: C, 78.62; H, 11.22; N, 2.09. C168H286N4O8·CH2Cl2 Calcd: C, 78.83; H, 11.27; N, 2.18. m/z (Maldi-TOF) 2490.092 ([M]+, C168H286N4O8 requires 2490.090). BPM-3,5-C12: sticky solid, 42% yield. 1H NMR (CD2Cl2, 300 MHz): δ ppm 7.93 (d, J = 15.9 Hz, 4H), 7.58 (s, 2H), 7.31 (d, J = 16.1 Hz, 4H), 6.86 (ls, 8H), 6.52 (ls, 4H), 4.03 (t, J = 6.6 Hz, 16H), 1.82 (qt, J = 6.0 Hz, 16H), 1.50−1.30 (m, 144H), 0.90 (t, J = 6.0 Hz, 24H). 13C NMR (CD2Cl2, 500 MHz): δ ppm 163.9, 163.8, 160.5, 137.6, 127.0, 114.0, 106.0, 102.9, 68.2, 31.9, 29.69, 29.66, 29.65, 29.62, 29.4, 29.3, 29.2, 26.0, 22.7, 14.1. Anal. found: C, 78.17; H, 10.65; N, 2.75. C136H222N4O8·2/3CH2Cl2 Calcd: C, 78.24; H, 10.73; N, 2.67. m/z (Maldi-TOF) 2039.963 ([M]+, C136H222N4O8 requires 2039.709). BPM-3,5-C16: creamy solid, 85% yield. 1H NMR (CDCl3, 400.16 MHz): δ ppm 7.88 (d, J = 15.9 Hz, 4H), 7.63 (s, 2H), 7.32 (d, J = 16.1 Hz, 4H), 6.81 (ls, 8H), 6.50 (ls, 4H), 4.03 (t, J = 6.6 Hz, 16H), 1.82 (qt, J = 6.0 Hz, 16H), 1.50−1.30 (m, 208H), 0.90 (t, J = 6.0 Hz, 24H). 13C NMR (CD2Cl2, 100.63 MHz): δ ppm 164.4, 161.5, 138.4, 138.4, 127.5, 115.7, 106.8, 103.7, 69.1, 32.7, 30.59, 30.56, 30.51, 30.49, 30.46, 30.27, 30.22, 30.12, 26.8, 23.5, 14.7. Anal. found: C, 77.74; H, 11.13; N, 2.12.

spectrophotometer based on CCD detection technology. This setup allows the recording of luminescence spectra on solids, liquids, liquid crystalline materials, and gels from −196 °C up to 420 °C between 350 and 1100 nm. SAXS . The SAXS patterns were obtained by transmission focalized geometry. A linear monochromatic Cu−Kα1 beam (λ = 1.5405 Å) was obtained using a sealed-tube generator (600 W) equipped with a bent quartz monochromator. In all cases, the crude powder was filled in Lindemann capillaries of 1 mm diameter and 10 μm wall thickness. The diffraction patterns were recorded with a curved Inel CPS120 counter gas-filled detector linked to a data acquisition computer (periodicities up to 70 Å) and on image plates scanned by STORM 820 from Molecular Dynamics with 50 μm resolution (periodicities up to 120 Å). The sample temperature was controlled within ±0.01 °C, and exposure times were varied from 1 to 24 h. Dielectric and Electrooptic Experiments. Dielectric and electrooptic experiments were performed using the HewlettPackard HP4192A impedance analyzer and a phase-sensitive voltmeter Lock-In SR 850 from Stanford (analyzing the electrooptic effect signal recorded by the photodiode with preamplifier from FLC Electronics). The investigated samples were placed in commercial measuring cells from Linkam (UK) with thicknesses of 7 μm with the semitransparent electrodes of indium−tin oxide (ITO). The cells were placed in a modified Mettler FP 82 HT hot-stage. Their temperature was stabilized using a Unipan model 650H temperature controller with accuracy better than 0.05 K. For measurements of electrooptic response the sample was placed between crossed polarizers on the stage of the polarizing microscope MPI-5 from PZO, Warsaw. The setup is described in full detail elsewhere.27 All measurements were performed with the voltage of 0.5 V rms. The dielectric measurements of ε in frequency domain were performed with a WAYNE KERR 6440B analyzer. CD Spectroscopy Measurements. Measurements were performed on a JASCO J810 CD spectrophotometer. The incident beam is always normal to the sample plane, i.e., the glass surface. NLO Imaging. For NLO imaging, the sample was illuminated wide field under normal incidence with femtosecond pulsed infrared (IR) laser light at 800 nm (Spectra Physics, Tsunami).28 The polarization of the incident IR light is varied by a zero-order half-wave plate for 800 nm mounted in a computer-controlled rotation stage (Thorlabs, PRM-Z8). A set of prisms (Newport, SF10) compensates for possible group velocity dispersion caused by the optical elements. The sample is irradiated by a long focal length lens ( f = 7.5 cm) so that the incident fundamental light can be considered to a good approximation as a collimated beam, and electric field components along the propagation direction (Z) can be neglected. Behind the sample, a high-power objective (Thorlabs, LMU-15X-NUV) combined with the 1.6× objective of the Olympus IX71 microscope frame collimates the transmitted light. While effects due to spherical aberration can be substantial,29 these effects can be neglected in this study due to the use of a low numerical aperture (0.32) objective.26 The spatial resolution under these conditions is 1 μm. Depending on the probed process and the corresponding wavelength range of interest, different filter sets are used (400 and 420−650 nm for SHG and TPF, respectively), and a sheet analyzer can be inserted as well. Finally, an EM-CCD (electron multiplying charge-coupled device; Hamamatsu) camera collects the transmitted light. In the recorded images, the C


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The Journal of Physical Chemistry C C168H286N4O8·1.5CH2Cl2 Calcd: C, 77.78; H, 11.13; N, 2.14. m/z (Maldi-TOF) 2490.088 ([M]+, C168H286N4O8 requires 2490.090). BPM-3,4,5-C12: yellow solid, 51% yield. 1H NMR (CDCl3, 300 MHz): δ ppm 7.81 (s, 2H), 7.76 (s, 2H), 7.63 (s, 2H), 7.24 (s, 2H), 7.19 (s, 2H), 6.85 (s, 8H), 4.03 (t, J = 6.2 Hz, 24H), 1.78 (m, 24H), 1.6−0.95 (m, 216H), 0.88 (m, 36H). 13C NMR (CDCl3, 100.63 MHz): δ ppm 164.7, 164.2, 154.0, 140.2, 137.9, 131.4, 126.6, 125.9, 106.6, 74.1, 69.7, 32.6, 32.57, 30.39, 30.38, 30.34, 30.36, 30.31, 30.24, 30.09, 30.29, 26.8, 26.76, 23.3, 14.5. Anal. found: C, 78.43; H, 11.33; N, 1.61. C232H414N4O12·1/ 2CH2Cl2 Calcd: C, 78.55; H, 11.40; N, 1.99. m/z (Maldi-TOF) 2776.34 ([M]+, C184H318N4O12 requires 2776.44).

NaOH solution using Aliquat 336 as a catalyst, according to a previously reported procedure10 and afforded bipyrimidinebased compounds in good yields after crystallization (average yield 80%). All intermediates and final compounds were fully characterized using a variety of analytical techniques. NMR analyses confirmed that all the styryl fragments adopted the trans configuration of the double bond (δCH = 16 Hz) in solution and present a set of aromatic peaks integrating for even numbers of aromatic protons which confirms the formation of the expected molecular structure and the symmetrical substitution pattern on bipyrimidine scaffold. Linear Optical Properties. Figure 1 displays the normalized absorption and fluorescence spectra of chromophores in 2-methylcyclohexane (apolar medium) and in dichloromethane (polar medium). Table 1 gathers the corresponding spectroscopic data. The low-energy side of the spectra is dominated by a single absorption band whose intensity seems to increase with the length of the alkyl chains. Note that this hyperchromic effect is more significant when considering the 3,4-substituted derivatives for which the crowded alkoxy groups are adjacent. It is also noteworthy that the para-to-meta positioning of the alkoxy substituents (BPM-3,4-Cn → BPM-3,5-Cn) does not lead to a correlated effect with the evolution of the band intensity. However, such a structural change clearly induces a decrease of the maximum absorption wavelength. In 2-methylcyclohexane, for instance, the absorption band of the “pure” meta−meta chromophores (i.e., BPM-3,5-Cn) is blue-shifted by about ∼1900 cm−1 with respect to that of their meta−para regioisomeric homologues (i.e., BPM-3,4-Cn). As previously observed for similar multibranched bipyrimidine-based derivatives,34 the longestwavelength absorption band encompasses multiple π−π* electronic transitions which mainly imply an electronic delocalization along the stilbenyl π-linkers with a charge

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RESULTS AND DISCUSSION Synthesis and Characterization. 4,4′,6,6′-Tetramethyl2,2′-bipyrimidine was synthesized according to previously reported procedures.30,31 The target bipyrimidine compounds were readily prepared by means of a tetra-aldol condensation between 4,4′,6,6′-tetramethyl-2,2′-bipyrimidine and the corresponding aromatic aldehyde (Scheme 2). 3,4- and 3,5Scheme 2. Condensation of 4,4′,6,6′-Tetramethyl-2,2′bipyrimidine with Lipophilic Aromatic Aldehydes

dialkoxybenzaldehydes and 3,4,5-tridodecyloxybenzaldehyde were synthesized following a reported procedure.32,33 The coupling reactions were carried out in boiling aqueous 5 M

Figure 1. Normalized absorption and fluorescence spectra of chromophores in 2-methylcyclohexane, MCH (squares), and dichloromethane, DCM (circles). D


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The Journal of Physical Chemistry C Table 1. Spectroscopic Data of Compounds in 2-Methylcyclohexane (MCH) and Dichloromethane (DCM) εMAX/M−1 cm−1 BPM-3,4-C1 MCH DCM BPM-3,4-C12 MCH DCM BPM-3,4-C16 MCH DCM BPM-3,5-C12 MCH DCM BPM-3,5-C16 MCH DCM a

λabs/nm

λfluo/nm

Φfluo

Φt→c

λ2PA/nm

δ2PA/GM

-a 59200

-a 369

-a 475

-a 0.25

-a 0.05

705

325 ± 40

58300

360b, 370c 373

405, 429c 481

0.16 0.35

0.05 0.03

720

380 ± 41

359, 371c 379

403c, 426 483

0.13 0.37

0.05 0.04

705

372 ± 15

337c, 349 342

380b, 412c 440

0.01 0.06

0.19 0.09

690

118 ± 15

338, 350c 348

381, 401c 439

0.004 0.06

0.20 0.07

680

113 ± 14

87100

57400

70000

Insoluble. bShoulder. cWavelength of maximal absorption is underlined.

all compounds. However, if we consider the solvent effect on Φt→c, two distinctive behaviors can be clearly observed. For the BPM-3,4-Cn derivatives, Φt→c is insensitive to solvent polarity and remains relatively low (i.e., < 0.1). In this case, the solventinduced stabilization of S1 and 1P states should be equivalent which maintains a large value for Ea. Therefore, one can attribute the significant increase of Φfluo to an S1 state relaxation process toward a highly emissive intramolecular charge transfer (1ICT) state. Such a relaxation process has been previously proposed and rationalized for comparable para aminosubstituted chromophores.33 If we consider the BPM-3,5-Cn derivatives, the trans-to-cis photoisomerization quantum yield decreases in concomitance with the increase of Φfluo on going from apolar to polar solvent. Here, the torsional barrier toward the 1P state should be increased in polar medium presumably due to a strong difference in relative polarity of the S1 versus 1P state. It is interesting to note that our multipolar positional isomers exhibit a completely reverse behavior as that observed for the linear 4- and 3-aminostilbenes.38 For these latter chromophores, the meta-to-para substitution effect induces a strong decrease of the CC torsional barrier leading to a fluorescence “switch off” and activation of the trans-to-cis photoisomerization process. Nonlinear Optical Properties in Solution. Since the investigated compounds are all octupolar (and thus lack an overall ground-state dipole moment), hyper-Rayleigh scattering (HRS) is the only available technique to determine the quadratic hyperpolarizability.42,43 Measurements were performed at a wavelength of 800 nm in dichloromethane. The setup is described in full detail elsewhere.44 Samples are analyzed toward crystal violet in methanol (βHRS,800 nm = 208.6 × 10−30 esu), a standard reference compound at this wavelength. The differences in solvent and molecular symmetry between the reference and samples are accounted for by the standard local-field correction factors at optical frequencies and the appropriate factors for the contributing tensor components, respectively. To correct for the contribution of multiphoton fluorescence at the second-harmonic wavelength, the highfrequency demodulation technique has been applied to obtain fluorescence-free first hyperpolarizabilities.44,45 The values of the SHG-active BPM-3,4-C12 and BPM-3,4C16 compounds are given in Table 2. These are within error identical for both compounds, which is a priori logical since the

transfer (CT) from the alkoxy electron-donating groups to the bipyrimidine electron-deficient core. The bathochromic shift observed on going from the meta to para derivatives should be ascribed to a stronger coupling between the donor and the conjugated system.35,36 However, the meta-to-para substitution37,38 effect also induces a reduction of the symmetry within the multipolar architecture.39 As a consequence, an extensive configuration interaction between pure transitions should result in a splitting of the state energy levels as previously observed by Lewis et al.24 for D−A substituted trans-stilbenes. Even though such a symmetry-induced coupling should be relatively weak, it can reasonably explain the absence of a clear correlation between εMAX with the position of the alkoxy substituent (see Table 1). As shown in Figure 1, the solvent-induced spectral shift is more significant for the BPM-3,4-Cn chromophores. This holds especially true when considering the Stokes Shift in the fluorescence spectra. For instance, the emission bands of BPM3,4-C16 and BPM-3,5-C16 redshift, respectively, by about 4110 and 2160 cm−1 on going from apolar to polar solvents. This indicates that the relaxation of the excited state is more significant for the BPM-3,4-Cn isomers as compared to the BPM-3,5-Cn ones when the polarity increases. Interestingly, the radiative and nonradiative deactivation processes at the S1 state are also affected by the position of the alkoxy groups. The BPM-3,4-Cn chromophores are much more emissive than the BPM-3,5-Cn ones whose fluorescence quantum yields (Φfluo) are lower than 10−2 in apolar medium (see Table 1). This very low emissivity is a clear indication of the occurrence of competing nonradiative deactivation processes at the S1 state. The trans-to-cis photoisomerization of the stilbenyl moieties should be considered as one possible relaxation pathway. Indeed, the photoisomerization quantum yield (Φt→c) is about ∼0.2 in MCH for the BPM-3,5-Cn isomers, whereas it is divided by a factor 4 for the BPM-3,4-Cn ones. These noticeable photoisomerization properties observed for the BPM-3,5-Cn chromophores should be ascribed to a weak activation barrier (Ea) between the S1 state and the so-called “phantom” state40,41 (1P). This intermediated species which exhibits a double-bond-twisted conformation acts as a photochemical funnel toward the ground-state surface from which isomerization to cis or trans occurs. It should be noted that the fluorescence quantum yield increases with solvent polarity for E


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localized in the same spectral range as the longest-wavelength 1PA band. However, it should be noted that the maximum 2PA band does not match the 1PA one. This spectral effect which is similar to that observed for comparable multibranched bipyrimidine-based systems33 should be ascribed to the noncentrosymmetric geometry of the dyes (i.e., D2d) which leads theoretically to a two-photon forbidden S0 → S1 transition. The length of alkyl chains hardly influences the 2PA ability of the chromophores, whereas the position of the alkoxy substituents strongly affects the value of the maximum 2PA cross-section which undergoes an ∼3-fold increase on going from BPM-3,5-Cn to BPM-3,4-Cn derivatives (see Table 1). The enhancement of the CT character within the S0 → Sn electronic transitions of these octupolar systems should be reasonably considered as the major factor promoting the increase of δMAX. This assumption is also consistent with the nature of the electron-donor properties of the substituent. For instance, similar chromophores with stronger donating group (i.e., diethylamino) in para position lead to a larger δMAX with value of about 530 GM.33 Thermal Behavior and Self-Organization Properties. The thermal properties of the various BPM compounds were studied by combining polarized-light optical microscopy (POM) observations and differential scanning calorimetry (DSC) measurements, whereas the molecular organizations in the various phases were analyzed by small-angle X-ray scattering (SAXS) experiments. The thermal behaviors and the mesophase parameters are summarized in Tables 3 and 4. None of the two compounds with the aliphatic chains in the positions 3 and 5 (BPM-3,5-Cn) are mesomorphic. DSC and POM analyses revealed that BPM-3,5-C16 (Figure S1, Supporting Information) exhibits a single melting point at 19.7 °C and that BPM-3,5-C12 displays only a glass transition at 8.8 °C (Figure S2, Supporting Information). In contrast, their regio-isomeric homologues, BPM-3,4-C16 and BPM-3,4-C12, exhibit liquid-crystalline behavior. Several first-order reversible thermal transitions were detected on the DSC traces of BPM-3,4-C16 (Table 3, Figure 3). Above 127.5 °C, the compound is in the isotropic fluid. On cooling, the compound becomes birefringent below 123 °C, and a fluid mosaic-like texture develops, with the formation of pseudofan shapes, along the presence of large homeotropic domains, typical of a hexagonal columnar mesophase (Figure 4). The broad transition near 30 °C is likely attributed to the crystallization of the alkyl chains.

Table 2. Hyperpolarizability Values in Solution BPM-3,4-C12 BPM-3,4-C16

β800nma/10−30 esu

β0b/10−30 esu

504 ± 80 485 ± 25

38 ± 8 42 ± 7

Fluorescence-free values, β (800 nm), measured by hyper-Rayleigh scattering in CH2Cl2 solution (10−3 mol L−1). bβ(0) from the threelevel model for octupoles. a

conjugated system, governing the second-order NLO response, is not influenced by the difference in side chain length (as can also be witnessed from the linear absorption). We also note that previous work10,33 on similar compounds revealed that replacing the diethylamino donor group (ICT band λmax = 437 nm) by the weaker alkoxy donor groups results in ca. 60% decrease of the β0 value. These data clearly demonstrate the quadratic NLO activity of these novel octupolar chromophores. Figure 2 shows the one- (1PA) and two-photon (2PA) absorption spectra of the compounds in dichloromethane. The

Figure 2. Linear and two-photon absorption spectra of compounds in DCM.

2PA bands are plotted against half the excitation wavelength in order to have a direct comparison with the one-photon absorption (1PA) spectra. The two-photon absorption cross sections (δ) have been measured in the 680−900 nm range by means of two-photon excited fluorescence.46 According to our spectral resolution, all spectra present two distinctive bands Table 3. Thermal Behavior of the BPM Compounds

thermal behaviora

compounds

G 13.7 (0.24) I I 3.9 (−0.35) G Cr 24.7 (30.8) I I 14.7 (−31.8) Cr Cr 69.7 (−) LamColrec 102.0 (0.7) Colhex 126.8 (1.0) I I 120.5 (−0.8) Colhex 87.6 (−0.9) LamColrec 70.0 (−) Cr Cr 43.0 (38.5) Colhex 127.5 (1.8) I I 123.2 (−1.5) Colhex 29.6 (−36.9) Cr Cr −18.6 (10.95) I I −20.3 (−9.0) Cr

BPM-3,5-C12 BPM-3,5-C16 BPM-3,4-C12 BPM-3,4-C16 BPM-3,4,5-C12

a Abbreviations: Colhex = hexagonal columnar phase, LamColrec = lamello-columnar phase with rectangular symmetry; G = glass, Cr = crystalline phases; I = isotropic liquid. T in °C, ΔH in J·g−1, and ΔCp* in J·g−1·K−1 are given in parentheses.

F


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The Journal of Physical Chemistry C Table 4. X-ray Data and Mesophase Parameters of BPM Compoundsa indexation compound T/°C

dmeas/Åb

hkc

Id

dcalcd/Åb

parameterse

BPM-3,4-C12 (T = 80 °C, LamColrec)

32.69 16.27 14.51 7.37 4.58 3.75

10 20 01 h0 hch h1

VS (sh) M (sh) VW (sh) S (br) VS (br) W (br)

32.55 16.28 14.56 -

BPM-3,4-C12 (T = 110 °C, Colhex)

32.39 16.24 7.8 4.61 36.32 20.92 13.72 12.35 12.07 7.7 4.65 3.84

10 20 h0 hch 10 11 21 h0 30 h1 hch h2

VS (sh) M (sh) S (br) VS (br) VS (sh) M (sh) VW (sh) W (br) VW (sh) VW (br) VS (br) VW (br)

32.43 16.22 36.27 20.94 13.71 12.09 -

LamColrec a = 32.55 Å b = 14.56 Å S = 474 Å2 ρ = 0.95 g/cm3 Vmol = 3580 Å3 N=1 hmol = 7.55 Å a = 37.45 Å S = 1214 Å2 ρ = 0.90 g/cm3 Vmol = 3770 Å3 a = 41.88 Å S = 1519 Å2 ρ = 0.92 g/cm3 Vmol = 4480 Å3 N = 4.18 hmol = 2.95 Å hcol = 12.3 Å

BPM-3,4-C16 (T = 80 °C, Colhex)

a

Abbreviations: Colhex = hexagonal columnar phase; LamColrec = lamellar-columnar with rectangular lattice phase. bdmeas and dcalc are the measured and calculated diffraction spacings, respectively (dcalc is deduced from the following mathematical expression: a = 2 × ∑[dhk(h2 + k2 + hk)1/2(Nhk√3)−1], where Nhk is the number of hk reflections for the Colhex phase and from dhk = 1/[(h2/a2 + k2/b2)1/2] for the rectangular phase). c hk are the Miller indices of the reflections. hch corresponds to the maximum of the diffuse scattering due to lateral distances between molten aliphatic tails (ch); h0, h1, and h2 correspond to some specific stacking periodicities between aromatic cores; see text. dI is the relative intensity of the reflections (VS, very strong; S, strong; M, medium; W, weak; VW, very weak; br and sh stand for broad and sharp signal). ea is the lattice parameter of the Colhex phase and a and b for the LamColrec phase. S is the columnar cross-sectional area: S = a2√3/2 for Colhex and a × b for the LamColrec. Vmol is the molecular volume and ρ the density, and hmol = Vmol/S is the molecular thickness and hcol = N × hmol, where N is the number of molecules in the considered segment; see text.

transmitted light is observed at the isotropic-to-Colhex phase transition around 120 °C, in line with the formation of a birefringent texture. In the Colhex phase, the intensity of the transmitted light slightly and monotonously decreases upon cooling. In marked contrast, below 90 °C, the intensity of the transmitted light strongly decreases to reach a minimum around 70 °C, which weakly evolves in the glassy state upon further cooling. The intensity changes observed as a function of the temperature clearly highlight the presence of two mesophases between the isotropic phase and the glassy state, as suspected on the DSC curves and by POM observations. The formation of a Colhex phase for BPM-3,4-C16 between 30 and 120 °C was unequivocally confirmed by SAXS. The diffractogram recorded at 80 °C on cooling (Figure 6a) reveals the presence of four sharp, small angles, reflections of decreasing intensity, in the ratio 1:√3:√7:3, corresponding to the hk reflections (10), (11), (21), and (30) of a p6mm hexagonal lattice (with the lattice parameter a = 41.9 Å, Table 4). In the wide-angle part, the very intense halo centered at 4.6 Å, corresponding to the mean distance between the hydrocarbon chains in a molten state (hch), is associated with three additional diffusion maxima, two rather weak at 7.7 and 3.85 Å (h1 and h2, respectively, Figure 6a and Table 4) and a slightly more intense one at ca. 12.35 Å (h0) corresponding to some local molecular interactions (vide infra). Upon further cooling, the broad transition near 30 °C corresponds to the crystallization of the compound, as confirmed by SAXS, with the emergence of numerous sharp peaks in the wide-angle region associated with the crystallization of the alkyl chains.

The thermal behavior of BPM-3,4-C12 displays three reversible thermal transitions at ca. 70, 95, and 124 °C (Figure 3b). The high-temperature transition was unambiguously attributed to the isotropization of the material (POM). Below 120 °C, a fluid mosaic-like texture with pseudofan shapes typical of a hexagonal columnar mesophase, similar to the one obtained with BPM-3,4-C16, readily develops (Figure 4). Below 85 °C, the pseudofan-shaped texture is preserved, but the decrease of the extent of the homeotropic regions suggests the preservation of a columnar-like mesophase but of different nature than the one observed at high temperature (Figure S3, Supporting Information). Below 70 °C, no textural change is observed down to room temperature, suggesting that this lowtemperature transition does not alter the previous supramolecular arrangement and that the phase symmetry is kept (glassy state G). Finally, the BPM-3,4,5-C12 compound is also deprived of mesomorphic properties. A single broad melting point is observed around −20 °C (Figure S4, Supporting Information). Light-Transmission Measurements. Light-transmission measurements performed on a polarized microscope equipped with a light detector and voltmeter confirm the phase domains and the dynamic of the phase transitions observed by DSC (Figure 5). For BPM-3,4-C16, the transmitted light rapidly increases at the isotropic to Colhex transition to reach a maximum which remains constant within the mesomorphic temperature range down to the crystallization process starting at 40−50 °C and for which a decrease of the transmitted light is observed. For BPM-3,4-C12, an abrupt increase of the G


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Figure 3. DSC traces of BPM-3,4-C16 (a) and BPM-3,4-C12 (b) compounds (red: 2nd heating curve; black: 1st cooling curve; scan rate = 10 °C/min) (LC and LC′: liquid crystalline mesophase).

Figure 5. Intensity of the transmitted light in the polarizing microscope as a function of the temperature for BPM-3,4-C16 (a) and BPM-3,4-C12 (b) compounds. Dashed and dotted lines indicate the mesophase domains upon heating and cooling, respectively (scan rate = 1 °C/min).

stretching of the aliphatic chains, implying that only small molecular rearrangements must take place within the columns. The X-ray patterns of BPM-3,4-C12 taken at 110 °C on the sample cooled from the isotropic liquid display two sharp reflections (10) and (20) in the small-angle region, a very intense halo centered at 4.6 Å (hch), confirming the liquid crystal nature of the material, and a weak diffusion maxima at ∼7.8 Å (h0) corresponding to some stacking interactions. In agreement with POM observations (carried out between 95 and 124 °C), the high-temperature mesophase was assigned as a Colhex phase (Figure S5, Supporting Information), as for its longer-chain-length homologue, BPM-3,4-C16. The X-ray pattern recorded at 80 °C for BPM-3,4-C12 upon further cooling (Figure 6b) contains three sharp reflections in the small-angle region, fitting the indexation of a simple rectangular lattice. The X-ray patterns also displayed in the wide-angle region three diffuse halos, the broad scattering hch, centered at 4.6 Å, indicative of liquid-like order of the aliphatic chains, and two smaller ones, h0 and h1 (Figure 6b and Table 4). The band at ca. 12.35 Å seen for the homologue compound is no more visible here, but the intensity of both remaining diffusion maxima has been slightly enhanced in comparison. In this lowtemperature mesophase, the columns of BPM-3,4-C12 have merged in the plane perpendicular to the a-axis, and the

Figure 4. Edge of a thin film of BPM-3,4-C16 examined by optical microscopy between crossed polarizers upon cooling (symbolized by the cross in the corner of the picture) at 80 °C revealing the coexistence of black homeotropic regions and fan-shaped textures (Colhex phase).

Identical X-ray patterns were obtained between 120 and 30 °C, confirming that no other mesophase is formed in this temperature range other than a single Colhex phase: the lattice dimension of the mesophase is almost invariant with temperature (a = 38−42 Å) and is essentially due to the swelling and H


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thickness modulations that foreshadow the interruption of the lamellae by a mechanism of mesogen inclination.47 With the assumption that the additional fine reflection corresponds to the periodicity along the rows, d0k, the A/d0k ratio provides the periodicity in the direction along the stacks. The value found (A/d0k = 7.58 Å = hmol) actually coincides with the position of the diffuse band in the diffractogram (Figure 6b, h0), which confirms a structure made of a two-dimensional array of registered rows.48 As the length of the chains (BPM-3,4-C16) or the temperature increases (BPM-3,4-C12), the carbon chains likely fold back and penetrate between the rows to be able to merge and to generate the aliphatic continuum. The twodimensional network of the short homologue is therefore totally erased by the dissociation of the parallel rows, but a new 2D array is formed in the orthogonal direction, made from columns of rigid parts segregated from the aliphatic continuum. The richness of the diffraction patterns of BPM-3,4-C16 allows some geometrical analysis to get more detailed insights about the molecular packing inside the columnar phases.50−52 The combination of the lattice molecular area (S) and partial molecular volumes (Vc and Vmol) permits us to deduce successively hmol, the molecular height in the column (from the ratio hmol = Vmol/S = 2.95 Å), the core columnar crosssection, Ac (Ac = Vc × S/Vmol = 296 Å2), the diameter of the equivalent cylinder, Dcyl (Dcyl = 2 × (Ac/π)1/2 = 19.4 Å), and the surface of the interface per molecule Σcyl (Σcyl = π × Dcyl × hmol = 180 Å2) and per chain Σch (Σch = Σcyl/8 = 22.5 Å2 ≈ σch). It can be verified that the interruption of the rows is achieved in order to optimize the amphipathic segregation by reducing the interface to a value close to the minimum required by the surface of the chains (from 27.5 Å2 in the LamColrec to 22.5 Å2 in the Colhex). The same data indicate that a column segment of equivalent height to the first maximum diffusion (h0 ≈ 12.35 Å = hcol) contains N = h0/hmol = 4.18 ≈ 4 molecules. Given the pseudotetrahedral shape of the molecules, it is difficult to go into further detail of the packing with only geometric data. However, the analysis of the crystal structure of a related compound, with diethylamino end groups instead of the chains,10 permitted us to go further in the description (CCDC 707590, P2/n, a = 17.5345(15) Å, b = 15.2814(14) Å, c = 20.3953(18) Å, α = 90°, β = 96.350(5)°, γ = 90°, V = 5431.43 Å3, Z = 4). Briefly, in the crystalline phase, the molecules associate in layers parallel with the (ab) plane, the axis of the bipyrimidines being almost contained in the same plane. Periodicities are one molecule along c and two in the plane of the layers. The bipyrimidine aromatic cycles make a dihedral angle of 73°, giving the molecules a twisted cross conformation that approximately fits into a parallelepiped with a rectangular base of sides of 15 and 7 Å. However, much of this volume remains unoccupied and is partly filled by the intercalation of vicinal molecules. Specifically, half of the molecules are aligned in rows with the longer side of the rectangle parallel to b, while the other half are oriented orthogonally, with the styril branches intermingled with the molecules of the rows (Figure S6, Supporting Information). The presence of the eight long alkoxy chains should not alter this molecular conformation excessively and should help the confinement of the rigid parts in sublayers alternating with sublayers of molten chains. The difference between the molecular area imposed by the chains and the section of rigid parts is compensated by the folding of the chains toward the rigid parts, which then pile into columns aligned in rows. The

Figure 6. Representative SAXS patterns of (a) BPM-3,4-C16 at 80 °C and (b) BPM-3,4-C12 recorded at 80 °C upon cooling from the isotropic melt.

morphology of the mesophase has changed from columnar to lamellar, with a thickness modulation compensating the excess of the interface generated by the expulsion of the chains from between columns, in ways quite similar to the SmC to Colhex transition observed in polycatenar systems.47 In contrast to these models for calamitic-like systems, the persistence of diffuse bands (h0 and h1) and the appearance of a new reflection (01) indicate that the stacks are preserved within the lamello-columnar structure, with a 2D or 3D long-range network. Such a supramolecular liquid crystalline arrangement has already been previously described for some terpyridine ligands and bipyridine complexes,48,49 in which the complexes are stacked into short-range ordered columns that laterally merge into rows, while still preserving their position at the node of a 2D long-range network. In order to get some more insights about the packing mode of BPM-3,4-Cn (n = 12, 16) inside the columns, it is necessary to access to the molecular volumes: these could be estimated (within 5% systematic error) to be around 3580 Å3 at 80 °C and 3770 Å3 at 110 °C for BPM-3,4-C12 and 4480 Å3 at 80 °C for BPM-3,4-C16 (the volume of the rigid part, Vc = 870 Å3, being estimated from a crystalline structure of a close derivative, vide supra). For the lamellae of BPM-3,4-C12, we calculated a molecular area A = 110 Å2 (defined as A = Vmol/d, d = a being the lamellar periodicity) and thus a section of 27.5 Å2 per terminal chain (A/4). The deviation from the minimum cross-section of the chains (22.2 Å2 at 80 °C) is quite large but may be explained by I


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The Journal of Physical Chemistry C spacing in the piles and along the rows, deduced from the position of the diffuse band assigned to the stack (h0 = 7.37 Å) and the lateral periodicity of the two-dimensional array (d01 = 14.6 Å), is in good agreement with the dimensions deduced from the crystal structure. In the case of BPM-3,4-C16, the folding of the longer chains leads to the interruption of the stacks and to the isolation of the rows within the columns. This process does preserve the radial stretching of the chains near the interface but not the orientation of the rigid parts in the columns. Thus, the observation of several diffuse bands at intermediate positions between the dimensions of the rigid parts (h0 = 12.35 Å and h1 = 7.7 Å) suggests nonhomogeneous directions in the piles. These results demonstrate that the functionalization of the bipyrimidine core by long alkyl peripheral chains allows under some conditions the emergence of liquid crystalline phases in which the molecules can efficiently pack to form columns. The self-organization of these materials into mesophase is indeed highly sensitive to the chain substitution pattern: the absence of a chain in the para-position of the alkoxystyryl fragments (3,5disubstitution pattern) or the crowding of the periphery (3,4,5trisubstitution pattern) is detrimental and prevents the segregation of the different molecular parts in regular structures and thus the emergence of a liquid crystalline mesophase. Substitution in the 4 and 5 positions prevents the interdigitation of the molecules to form columns of orthogonally stacked molecules, as depicted in the proposed molecular packing model. Moreover, the increase of the chain length from 12 to 16 methylene units favors the complete isolation of the columns by refolding the termini between adjacent cores and stabilizes the Colhex phase over a large temperature range. Solid-State Luminescence. The luminescence properties of both compounds, BPM-3,4-C12 and BPM-3,4-C16, in the mesophase were also evaluated with the help of a fluorescence microscope equipped with a heating stage and a UV−vis−NIR spectrophotometer based on CCD detection technology (Figures S7 and S8, Supporting Information). Upon UV irradiation, the Colhex mesophases appeared highly luminescent, and a broad structureless emission band centered at 495 nm was detected for BPM-3,4-C12 and BPM-3,4-C16 (Figure 7). In the bulk, the emission spectra are, as expected, broader and much less resolved than in solution, and the maxima are redshifted by 90−92 nm. The difference should come from peculiar intermolecular interactions inside the column in the LC phase, while the interactions with the solvent dominate in solution. Dielectric Measurements. Temperature-dependent dielectric measurements have been performed on both homologues, BPM-3,4-C12 and BPM-3,4-C16 (Figure 8). The temperature dependence of the capacitance was measured in the temperature range of 30−150 °C. For BPM-3,4-C16, a slight decrease of the capacitance is visible during melting around 50 °C, whereas a major decrease of the capacity at the Colhex phase to isotropic phase transition around 125 °C is clearly observed. The global decrease of the capacitance is related to the decrease viscosity with the temperature, but the abrupt changes of the capacitance observed around 50 and 125 °C are related to drastic changes in the local molecular organizations. These dielectric measurements confirm the extent of the phase domains and the transition temperatures for BPM-3,4-C16. In the case of BPM-3,4-C12, the

Figure 7. Solid-state emission spectra of BPM-3,4-C12 and BPM-3,4C16 in the Colhex phase at 110 °C and at 100 °C, respectively, under excitation at 350−400 nm (mercury lamp). (The small peak at 372 nm is a residue of the excitation beam.)

Figure 8. Evolution of the capacitance as a function of the temperature in the range 30−150 °C (frequency = 440 Hz). Top: BPM-3,4-C16; bottom: BPM-3,4-C12 (red: 2nd heating curve; black: 1st cooling curve; scan rate = 1 °C/min).

capacitance variations are smoother, and thus the phase transitions are much less marked. The variation of the dielectric permittivity (ε), extracted from these measurements, shows little change (about 0.2) over the whole temperature range explored. In addition, the measurements of ε in the frequency domain show that there are no J


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Figure 9. Optical, two-photon fluorescence, SHG images of BPM-3,4-C12 and BPM-3,4-C16 in the isotropic phase (150 °C) and in the liquid crystalline phase (105 °C for BPM-3,4-C12 and 80 °C for BPM-3,4-C16).

formation of a centrosymmetric phase, which is in line with the formation of Colhex and LamColrec mesophases. Nonlinear Optical Properties in the LC State. The nonlinear optical properties in the liquid crystalline state of BPM-3,4-C12 and BPM-3,4-C16 have been imaged by widefield second-harmonic generation (SHG) and two-photon fluorescence (2PF) microscopy performed at a wavelength of 800 nm. The setup has been described in full detail elsewhere.53 Thin films of the mesogens were prepared in standard LC cells of 2.2 μm thickness. After heating the samples to the isotropic liquid phase at 150 °C, the films were subsequently cooled

relaxation processes up to 100 kHz for BPM-3,4-C12 (Figure S9, , Supporting Information). The relaxation band about 1 MHz is connected with the RC circuit of a measuring cell (R ∼ 500 Ω and c ∼ 200 pF). The relaxation of such a circuit is of the order of MHz. Finally, the dielectric constant ε measured at 90, 115, and 145 °C for various applied electric fields up to 1.5 kV/ cm did not show noticeable field effect for BPM-3,4-C12. All these measurements indicate either that the BPM molecules are weakly polar or that a centrosymmetric phase is formed. The 2PA activities measured in mesophases (vide infra) with these molecules exclude the first hypothesis and point to the K


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Cn compounds exhibit strong 2PF upon excitation at 800 nm in the isotropic phase and the liquid crystalline phase, confirming the excellent third-order NLO activity and efficiency of these bipyrimidine derivatives in condensed matter. Both compounds were found to be SHG silent in their liquid crystalline phases. The formation of centrosymmetric columnar mesophases of hexagonal symmetry did not allow the emergence of a SHG response. One possibility to generate a noncentrosymmetric mesophase and to obtain a SHG response will be to graft, on the styryl bipyrimidine core, enantiomerically pure chiral carbon chains in the 3,4-position to obtain right- or left-handed helical columns.21,54 The combined second- and third-order nonlinear optical microscopy will provide a powerful and sensitive tool to detect and study possibly chiral mesophases. Chiral phases will show up in SHG microscopy, while achiral phases, as is the case in this study, will not light up. It is also envisaged to protonate these derivatives toward bipyrimidinium to enhance the NLO properties and also to coordinate the bipyrimidine fragment with metal ions to modulate the electronic properties and the geometry of the molecules.55,56 Works along these lines are currently under progress in our laboratories.

slowly (5°/min) to the liquid crystalline phase, where images were taken at 80 °C for BPM-3,4-C16 and at 105 °C for BPM3,4-C12. At each temperature, optical, 2PF, and SHG images were recorded, as depicted in Figure 9. The 2PF images are affected by interference, partly because of variations in film thickness and local density. However, it is clear that both samples exhibit strong 2PF upon excitation at 800 nm. These data clearly show the excellent third-order NLO activity and efficiency of these bipyrimidine derivatives in the liquid crystalline phase. In contrast, the films for both compounds are SHG silent, in the isotropic as well as in the liquid crystalline phase. This is not surprising since SHG, unlike 2PF, is restricted to structures of noncentrosymmetric symmetry. Indeed, the isotropic phase is centrosymmetric (by definition), while the hexagonal columnar mesophase is most likely centrosymmetric (but not by definition). This also means that no additional information about the configuration of the compounds in these phases could be deducted by SHG microscopy, apart from the fact that they organize in a centrosymmetric manner.

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CONCLUSION An original series of alkoxy-substituted styryl bipyrimidine cores have been synthesized. The substitution pattern around the styryl bipyrimidine core was found to strongly influence the linear and nonlinear optical properties in solution and in solid as well as the self-assembly properties. Peripheral substitution in the 3,4-position of the styryl fragments leads to molecular entities strongly emitting around 400 nm with quantum yields around 10% and displaying interesting cubic and quadratic NLO activities in solution with β0 values around 40 × 10−30 esu and δ2PA best values around 350 GM. The BPM-3,4-Cn compounds with n = 12, 16 were found to be mesomorphic. BPM-3,4-C16 forms a columnar mesophase of hexagonal symmetry over a wide temperature from 35 °C up to 125 °C. The mesomorphism of BPM-3,4-C12 was found to be richer with the formation of the Colhex phase at high temperatures between 95 and 123 °C and of a LamColrec phase at lower temperatures between the Colhex phase and the crystalline phase at 70 °C. On the basis of the crystal structure of a related compound, a packing model was proposed in which the molecules are interdigitated alternatively along their long axis and their short axis to form columns. The BPM-3,4-Cn compounds exhibit clearly excellent cubic and quadratic NLO activities for organic octupolar chromophores in solution. In contrast, the alkoxy substitution in the 3 and 5 positions was found to be detrimental for the photophysical properties and for the emergence of mesomorphic properties. The regioisomers of BPM-3,5-Cn compounds with n = 12, 16 were found to be amorphous at room temperature, and a single glass transition and a melting point have been detected at low temperatures. Substitution in the 4 and 5 positions prevents the interdigitation of the molecules to form columns of orthogonally stacked molecules, as depicted in the proposed molecular packing model of BPM-3,4-Cn compounds. These compounds display weak emission in solution with quantum yields below 1%. This very low emissivity is a clear indication of the occurrence of competing nonradiative deactivation processes, and the trans-to-cis photoisomerization of the stilbenyl moieties should be considered as one possible relaxation pathway. Solid-state nonlinear optical properties of the mesomorphic compounds were also investigated in thin films. Both BPM-3,4-

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ASSOCIATED CONTENT

S Supporting Information *

DSC curves of BPM-3,5-C16, BPM-3,5-C12, and BPM-3,4,5C12. Textures observed with BPM-3,4-C12 by optical microscopy between crossed polarizers upon cooling at 95 °C (a) and 50 °C (b). SAXS patterns of BPM-3,4-C12 at 110 °C after heating into the isotropic phase Proposed model for the stacking of the bipyrimidine cores inside the columns directly deduced from the crystalline structure of a related compound (CCDC 707590, Cambridge database). Pristine powder of the BPM-3,4-C12 compound observed by optical microscopy under polarized light (a) and under UV irradiation at 350 < λex < 400 nm ((b), same area) at room temperature. BPM-3,4-C16 Colhex phase at 80 °C observed upon irradiation at 350 < λex < 400 nm. Measurements of the capacitance as a function of the frequency at 115 °C with the BPM-3,4-C12 compound. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the CNRS, Rennes Metropole, and the University of Rennes 1 for their financial support. S.V.C. is grateful to FWO Flanders and the Hercules foundation for financial support. P.K. is grateful for financial support from the Fonds voor Wetenschappelijk Onderzoek, in the framework of the agreement for scientific cooperation with Polish Academy of Sciences.

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REFERENCES

(1) Marder, S. R. (Ed., themed issue) Nonlinear optics: The Evolving Field of Nonlinear Optics - a Personal Perspective. J. Mater. Chem. 2009, 19, 7392−7393.

L


Article

The Journal of Physical Chemistry C (2) Denk, W.; Strickler, J. H.; Webb, W. W. Two-Photon Laser Scanning Fluorescence Microscopy. Science 1990, 248, 73−76. (3) Wu, W.; Qin, J.; Li, Z. New Design Strategies for Second-Order Nonlinear Optical Polymers and Dendrimers. Polymer 2013, 54, 4351−4382. (4) Clays, K.; Docherty, R. J.; Fielden, J.; Helliwell, M.; Raftery, J.; Shaw, R.; Swanson, C. A.; Coe, B. J. Proc. SPIE 7817, Nanophotonics and Macrophotonics for Space Environments IV, 78170E, 2010. (5) Zyss, J.; Ledoux, I. Nonlinear Optics in Multipolar Media: Theory and Experiments. Chem. Rev. 1994, 94, 77−105. (6) Ishizuka, T.; Sinks, L. E.; Song, K.; Hung, S. T.; Clays, K.; Therien, M. J. The Roles of Molecular Structure and Effective Optical Symmetry in Evolving Dipolar Chromophoric Building Blocks to Potent Octopolar Nonlinear Optical Chromophores. J. Am. Chem. Soc. 2011, 133, 2884−2896. (7) Le Droumaguet, C.; Sourdon, A.; Genin, E.; Mongin, O.; Blanchard-Desce, M. Two-Photon Polarity Probes Built from Octupolar Fluorophores: Synthesis, Structure-properties Relationships and Use in Cellular Imaging. Chem.−Asian J. 2013, 8, 2984−3001. (8) Duncan, T. V.; Song, K.; Hung, S.-T.; Miloradovic, I.; Nayak, A.; Persoons, A.; Verbiest, T.; Therien, M. J.; Clays, K. Molecular Symmetry and Solution-Phase Structure Interrogated by HyperRayleigh Depolarization Measurements: Elaborating Highly Hyperpolarizable D2-Symmetric Chromophores. Angew. Chem., Int. Ed. 2008, 47, 2978−2981. (9) Quintiliani, M.; Pérez-Moreno, J.; Asselberghs, I.; Vazquez, P.; Clays, K.; Torres, T. Synthesis and Nonlinear Optical Properties of Tetrahedral Octupolar Phthalocyanine-Based Systems. J. Phys. Chem. B 2010, 114, 6309−6315. (10) Akdas-Kilig, H.; Roisnel, T.; Ledoux, I.; Le Bozec, H. A New Class of Bipyrimidine-Based Octupolar Chromophores: Synthesis, Fluorescent and Quadratic Nonlinear Optical Properties. New. J. Chem. 2009, 33, 1470−1473. (11) Kato, T.; Mizoshita, N.; Kishimoto, K. Functional LiquidCrystalline Assemblies: Self-Organized Soft Materials. Angew. Chem., Int. Ed. 2006, 45, 38−68. (12) Yamane, S.; Tanabe, K.; Sagara, Y.; Kato, T. Stimuli-Responsive Photoluminescent Liquid Crystals. Top. Curr. Chem. 2012, 318, 395− 406. (13) Hennrich, G.; Omenat, A.; Asselberghs, I.; Foerier, S.; Clays, K.; Verbiest, T.; Serrano, J. L. Liquid Crystals from C3-Symmetric Mesogens for Second-Order Nonlinear Optics. Angew. Chem., Int. Ed. 2006, 45, 4203−4206. (14) Irie, M.; Fukaminato, T.; Sasaki, T.; Tamai, N.; Kawai, T. A. Digital Fluorescent Molecular Photoswitch. Nature 2002, 420, 759− 760. (15) Sagara, Y.; Kato, T. Mechanically Induced Luminescence Changes in Molecular Assemblies. Nat. Chem. 2009, 1, 605−610. (16) Mutai, T.; Satou, H.; Araki, K. Reproducible On−Off Switching of Solid-state Luminescence by Controlling Molecular Packing Through Heat-Mode Interconversion. Nat. Mater. 2005, 4, 685−687. (17) Zhang, X.; Rehm, S.; Safont-Sempere, M. M.; Würthner, F. Vesicular Perylene Dye Nanocapsules as Supramolecular Fluorescent pH Sensor Systems. Nat. Chem. 2009, 1, 623−629. (18) Weder, C.; Sarwa, C.; Montali, A.; Bastiaansen, C.; Smith, P. Incorporation of Photoluminescent Polarizers into Liquid Crystal Displays. Science 1998, 279, 835−837. (19) Oliva, M. M.; Juarez, R.; Ramos, M.; Segura, J. L.; Van Cleuvenbergen, S.; Clays, K.; Goodson, T.; Navarrete, J. T. L.; Casado, J. Linear and Nonlinear Optical Properties of Ramified Hexaazatriphenylenes: Charge Transfer Contributions to the Octupolar Response. J. Phys. Chem. C 2013, 117, 626−632. (20) Van Cleuvenbergen, S.; Asselberghs, I.; Garcia-Frutos, E. M.; Gomez-Lor, B.; Clays, K.; Pérez-Moreno, J. Dispersion Overwhelms Charge Transfer in Determining the Magnitude of the First Hyperpolarizability in Triindole Octupoles. J. Phys. Chem. C 2012, 116, 12312−12321. (21) De Vega, L.; Van Cleuvenbergen, S.; Depotter, G.; GarciaFrutos, E. M.; Gomez-lor, B.; Omenat, A.; Tejedor, R. M.; Serrano, J.

L.; Hennrich, G.; Clays, K. Nonlinear Optical Thin Film Device from a Chiral Octopolar Phenylacetylene Liquid Crystal. J. Org. Chem. 2012, 77, 10891−10896. (22) Meech, R.; Phillips, D. J. Photophysics of some Common Fluorescence Standards. J. Photochem. 1983, 23, 193−217. (23) Montalti, M.; Credi, A.; Prodi, L.; Gandolfi, M. T. Handbook of Photochemistry, Third ed.; CRC Press: Boca Raton, 2006. (24) Lewis, F. D.; Kalgutkar, R. S.; Yang, J.-S. The Photochemistry of Trans-Ortho-, -Meta-, and -Para-Aminostilbenes. J. Am. Chem. Soc. 1999, 121, 12045−12053. (25) Albota, M. A.; Xu, C.; Webb, W. W. Two-Photon Fluorescence Excitation Cross Sections of Biomolecular Probes from 690 to 960 nm. Appl. Opt. 1998, 37, 7352−7356. (26) Tung, C. K.; Sun, Y.; Lo, W.; Lin, S. J.; Jee, S. H.; Dong, C. Y. Effects of Objective Numerical Apertures on Achievable Imaging Depths in Multiphoton Microscopy. Microsc. Res. Technol. 2004, 65, 308−314. (27) Kuczyński, W.; Hoffmann, J. Determination of Piezoelectric and Flexoelectric Polarization in Ferroelectric Liquid Crystals. J. Phys. Rev. E 2005, 72, 041701−1−6. (28) Van der Veen, M. A.; De Vos, D. E.; Verbiest, T. Unraveling Molecular Architecture Inside Zeolites with Second-Harmonic Generation Microscopy. Proc. SPIE 2010, 7758, 77580D. (29) Young, P. A.; Clendenon, S. G.; Byars, J. M.; Decca, R. S.; Dunn, K. W. The Effects of Refractive Index Heterogeneity within Kidney Tissue on Multiphoton Fluorescence Excitation Microscopy. J. Microsc. 2011, 242, 147−165. (30) Nasielski, J.; Standaert, A.; Nasielski-Hinkens, R. Efficient Coupling of 2 Halopyrimidines to 2,2′-Bipyrimidines. Synth. Commun. 1991, 21, 901−906. (31) Vlád, G.; Horváth, I. T. Improved Synthesis of 2,2′Bipyrimidine. J. Org. Chem. 2002, 67, 6550−6552. (32) Cardinaels, T.; Ramaekers, J.; Nockemann, P.; Driesen, K.; Van Hecke, K.; Van Meervelt, L.; Lei, S.; De Feyter, S.; Guillon, D.; Donnio, B.; Binnemans, K. Imidazo[4,5-f]-1,10-phenanthrolines: Versatile Ligands for the Design of Metallomesogens. Chem. Mater. 2008, 20, 1278−1291. (33) Donnio, B.; Bruce, D. W. Liquid-Crystalline, Polycatenar Complexes of Silver(I): Dependence of the Mesomorphism on the Ligand and the Anion. New J. Chem. 1999, 275−286. (34) Savel, P.; Akdas-Kilig, H.; Malval, J.-P.; Spangenberg, A.; Roisnel, T.; Fillaut, J.-L. Metal-Induced Dimensionality Tuning in a Series of Bipyrimidine-based Ligands: A Tool to Enhance Two-Photon Absorption. J. Mater. Chem. C 2014, 2, 295−305. (35) Jin, M.; Xu, H.; Hong, H.; Malval, J.-P.; Zhang, Y.; Ren, A.; Wan, D.; Pu, H. Design of D−π−A Type Photoacid Generators for High Efficiency Excitation at 405 and 800 nm. Chem. Commun. 2013, 49, 8480−8482. (36) Jin, M.; Hong, H.; Xie, J.; Malval, J.-P.; Spangenberg, A.; Soppera, O.; Wan, D.; Pu, H.; Versace, D.-L.; et al. S. π-Conjugated Sulfonium-Based Photoacid Generators: an Integrated Molecular Approach for Efficient One and Two-Photon Polymerization. Polym. Chem. 2014, 5, 4747−4755. (37) Lewis, F. D.; Weigel, W. J. Excited State Properties of Donor− Acceptor Substituted Trans-Stilbenes: The Meta-Amino Effect. Phys. Chem. A 2000, 104, 8146−8153. (38) Lewis, F. D.; Weigel, W.; Zuo, X. Relaxation Pathways of Photoexcited Diamino-Stilbenes: The Meta-Amino Effect. J. Phys. Chem. A 2001, 105, 4691−4696. (39) Xia, R.; Malval, J.-P.; Jin, M.; Spangenberg, A.; Wan, D.; Pu, H.; Vergote, T.; Morlet-Savary, F.; Chaumeil, H.; Baldeck, P.; et al. O. Enhancement of Acid Photogeneration Through a Para-to-Meta Substitution Strategy in a Sulfonium-Based Alkoxystilbene Designed for Two-Photon Polymerization. Chem. Mater. 2012, 24, 237−244. (40) Saltiel, J.; Waller, A. S.; Sears, D. F.; Hoburg, E. A.; Zeglinski, D. M.; Waldeck, D. H. Fluorescence Quantum Yields and Lifetimes of Substituted Stilbenes in n-Alkanes. A Reexamination of the Relationship between Solute Size and Medium Effect on Torsional Relaxation. J. Phys. Chem. 1994, 98, 10689−10698. M


Article

The Journal of Physical Chemistry C (41) Waldeck, D. H. Photoisomerization Dynamics of Stilbenes. Chem. Rev. 1991, 91, 415−436. (42) Verbiest, T.; Clays, K.; Persoons, A.; Meyers, F.; Brédas, J. L. Determination of the Hyperpolarizability of an Octopolar Molecular Ion by Hyper-Rayleigh Scattering. Opt. Lett. 1993, 18, 525−527. (43) Clays, K.; Persoons, A. Hyper-Rayleigh Scattering in Solution. Phys. Rev. Lett. 1991, 66, 2980−2983. (44) Olbrechts, G.; Strobbe, R.; Clays, K.; Persoons, A.; Tuning Octopolar, N. L. O. Chromophores: Synthesis and Spectroscopic Characterization of Persubstituted 1,3,5-tris(ethynylphenyl)benzenes. Rev. Sci. Instrum. 1998, 69, 2233−2241. (45) Olbrechts, G.; Wostyn, K.; Clays, K.; Persoons, A. HighFrequency Demodulation of Multiphoton Fluorescence in LongWavelength Hyper-Rayleigh Scattering. Opt. Lett. 1999, 24, 403−405. (46) Xu, C.; Webb, W. W. Measurement of Two-photon Excitation Cross Sections of Molecular Fluorophores with Data from 690 to 1050 nm. J. Opt. Soc. Am. B 1996, 13, 481−491. (47) Guillon, D.; Heinrich, B.; Ribeiro, A. C.; Cruz, C.; Nguyen, H. T. Thermotropic Lamellar-to-Columnar Phase Transition Exhibited by a Biforked Compound. Mol. Cryst. Liq. Cryst. 1998, 317, 51−64. (48) El-Ghayoury, A.; Douce, L.; Skoulios, A.; Ziessel, R. π-Stacked Ortho-Palladated Bipyridine Complexes Exhibiting Unusual Liquid Crystalline Behavior. Angew. Chem., Int. Ed. 1998, 37, 1255−1258. (49) Camerel, F.; Donnio, B.; Bourgogne, C.; Schmutz, M.; Guillon, D.; Davidson, D.; Ziessel, R. Tuning the Thermotropic and Lyotropic Properties of Liquid-Crystalline Terpyridine Ligands. Chem.Eur. J. 2006, 12, 4261−4274. (50) Morale, F.; Date, R. W.; Guillon, D.; Bruce, D. W.; Finn, R. L.; Wilson, C.; Blake, A. J.; Schröder, M.; Donnio, B. Columnar Mesomorphism from Hemi-Disklike Metallomesogens Derived from 2,6-Bis[3′,4′,5′-tri(alkoxy)phenyliminomethyl]pyridines (L): Crystal and Molecular Structures of [M(L)Cl2] (M=Mn, Ni, Zn). Chem. Eur. J. 2003, 9, 2484−2501. (51) Donnio, B.; Heinrich, B.; Allouchi, H.; Kain, J.; Diele, S.; Guillon, D.; Bruce, D. W. A Generalized Model for the Molecular Arrangement in the Columnar Mesophases of Polycatenar Mesogens. Crystal and Molecular Structure of Two Hexacatenar Mesogens. J. Am. Chem. Soc. 2004, 126, 15258−15268. (52) Myśliwiec, D.; Donnio, B.; Chmielewski, P. J.; Heinrich, B.; Stępień, M. Peripherally Fused Porphyrins via the Scholl Reaction: Synthesis, Self-Assembly, and Mesomorphism. J. Am. Chem. Soc. 2012, 134, 4822−4833. (53) Van Cleuvenbergen, S.; Hennrich, G.; Willot, P.; Koeckelberghs, G.; Clays, K.; Verbiest, T.; Van der Veen, M. A. All Optical Determination of Microscopic and Macroscopic Structure of Chiral, Polar Microcrystals from Achiral, Nonpolar Molecules. J. Phys. Chem. C 2012, 116, 12219−12225. (54) Ziessel, R.; Camerel, F.; Donnio, B. Rational Shaping of Liquid Crystalline Diacylaminophenyl Platforms Equipped with Chelating Fragments, Fluorescent Dyes, and Square Planar Platinum Complexes. Chem. Rec. 2009, 9, 1−23. (55) Asselberghs, I.; Hennrich, G.; Clays, K. Proton-Triggered Octopolar NLO Chromophores. J. Phys. Chem. A 2006, 110, 6271− 6275. (56) Coe, B. J.; Harris, J. A.; Brunschwig, B. S.; Asselberghs, I.; Clays, K.; Garín, J.; Orduna, J. Three-Dimensional Nonlinear Optical Chromophores Based on Metal-to-Ligand Charge-Transfer from Ruthenium(II) or Iron(II) Centers. J. Am. Chem. Soc. 2005, 127, 13399−13410.

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