Second-Order Nonlinear Optical (NLO) Properties of a

J. Phys. Chem. C 2009, 113, 2745–2760

2745

Second-Order Nonlinear Optical (NLO) Properties of a Multichromophoric System Based on an Ensemble of Four Organic NLO Chromophores Nanoorganized on a Cyclotetrasiloxane Architecture Marco Ronchi, Maddalena Pizzotti,* Alessio Orbelli Biroli, Stefania Righetto, and Renato Ugo Dipartimento di Chimica Inorganica Metallorganica e Analitica dell’UniVersita` di Milano “Lamberto Malatesta”, Unita` di Ricerca dell’INSTM e Istituto di Scienze e Tecnologie Molecolari del CNR (ISTM), Via Venezian 21, 20133 Milano, Italy

Patrizia Mussini Dipartimento di Chimica Fisica ed Elettrochimica dell’UniVersita` di Milano, Via Venezian 21, 20133 Milano, Italy

Marco Cavazzini, Elena Lucenti, and Matteo Salsa Istituto di Scienze e Tecnologie Molecolari del CNR (ISTM), Via Golgi 19, 20133 Milano, Italy

Piercarlo Fantucci Dipartimento di Biotecnologie e Bioscienze dell’UniVersita` di Milano Bicocca, Piazza della Scienza 1, 20126 Milano, Italy ReceiVed: October 28, 2008; ReVised Manuscript ReceiVed: December 5, 2008

In this paper we report the synthesis, the photophysical, conductometric, and second-order nonlinear optical (NLO) characterization of an ensamble of four NLO active organic tails nanoorganized on a cyclotetrasiloxane ring, to produce various macrocyclic NLO chromophores. The second-order NLO response of the macrocyclic NLO chromophores measured by the EFISH technique as µβ1.91, where µ is the dipole moment and β1.91 the projection along the dipole moment axis of the vectorial component of the quadratic hyperpolarizability working with an incident wavelength of 1.907 µm, increases, compared to the reference monomeric NLO chromophores, from 2.8 up to 3.5 times for nonionic macrocyclic NLO chromophores. The increase is mainly due to an increase of the dipole moment when compared to reference monomeric NLO chromophores while the β1.91 values remain almost unchanged. These macrocyclic NLO chromophores can be also considered as a simple model of a monolayer of organic NLO chromophores on a chemically engineered silica surface. Since the factor controlling their second-order NLO response is the orientation toward the dipole moment axis of the single organic NLO active tails, this kind of model confirms that the second-order NLO response of a monolayer of organic NLO chromophores on a chemically engineered silica surface is controlled by the topology of the binding sites on the surface, as suggested by previous investigations on multilayers on chemically engineered silica surface. The macrocyclic NLO chromophore with a ionic organic NLO active tail shows a strong concentration dependence of its second-order NLO response to be ascribed to a larger increase of its ionic dissociation by dilution, when compared to that of the reference ionic monomeric NLO chromophore. This behavior is evidence of a cooperative effect due to the nanoorganization, to be ascribed to a more facile ionic dissociation of the single ionic organic tail originated by a local increase of the solvent polarity favored by the closeness of the positive charges of the four organic tails. Finally it was shown that the Si(OSiMe3)3 group behaves in a classical organic push-pull NLO chromophore as a pull group as strong as the nitro group. 1. Introduction In the field of new organic or organometallic materials with nonlinear optical (NLO) properties, a substantial amount of work has been directed, in the last two decades, toward the translation of the properties of molecular NLO chromophores to the macroscopic level of bulk nanostructured materials.1 The noncentrosymmetric arrangement of molecular NLO chromophores, required to produce nanostructured materials with * Corresponding author, [email protected].

second-order NLO activity, can be reached by a series of different approaches such as their incorporation into electrically poled polymeric films2 or their organization in Langmuir-Blodgett films,3 their self-assembly and self-organization into layers guided by hydrogen bonding4 or by ionic interactions in crystal structures,5 or finally by building up, by chemical methods, layers or multilayers on molecularly engineered silica surfaces.6 Investigations have been experimentally carried out on the effects of photophysical properties of molecular NLO chromophores closely packed in Langmuir-Blodgett films.7 Some

10.1021/jp8095242 CCC: $40.75  2009 American Chemical Society Published on Web 01/26/2009

2746 J. Phys. Chem. C, Vol. 113, No. 7, 2009

Ronchi et al.

Figure 1. NLO chromophores investigated.

theoretical studies have been also carried out on the factors governing the bulk second-order NLO properties of a nanostructured assembly of molecular NLO chromophores such as orientation effects, cooperative phenomena, and intermolecular nonbonding interactions.8 However a question not yet completely resolved is the experimental evidence of the role, on the photophysical and in particular second-order NLO properties, of the nonbonding electronic interactions between molecular NLO chromophores and of their orientation when organized as a monolayer on a molecularly engineered silica surface.4b A simplified approach in order to experimentally evidence such a role can be the study of the photophysical and secondorder NLO properties of a simple model of a fragment of chemically engineered silica surface covered by covalently bound molecular NLO chromophores. In order to realize one of these models, some of us recently reported the synthesis and structural characterization of a series of functionalized cyclotetrasiloxane rings [4-RC6H4Si(O)OR′]4 (R ) Cl, Br, CHdCH2, CH2Cl; R′ ) Na, SiMe3) to be used as the basic architecture of a very simple model of a monolayer of second-order organic NLO chromophores on a molecularly engineered silica surface. In fact these cyclotetrasiloxane rings can be considered as fragments of silica surface with attached various properly functionalized aromatic rings, in an all-cis arrangement.9 Various organic polar tails can be thus covalently linked to the functionalized aromatic rings in order to reproduce a fragment of a monolayer of push-pull organic NLO chromophores, whose second-order NLO and photophysical properties can be thus compared to those of structurally related reference monomeric NLO chromophores. In this paper we report thus the synthesis and the theoretical, spectroscopic, photophysical, conductometric, and second-order NLO characterization of a series of cyclotetrasiloxanes functionalized with four push-pull organic tails and of their reference monomeric NLO chromophores in order to investigate the effect of this kind of nanoorganization on the photophysical and second-order NLO properties. Some push-pull NLO chromophores, either tetrameric or monomeric, investigated in this work, are based on the push -NMe2 group while the system [sSi(-O-)3] is acting as the pull group, the spacer being a single, double, or triple bond (1-3, Figure 1). In these latter NLO chromophores the role of the cyclotetrasiloxane ring is not that of a simple scaffold, since it participates to the electronic properties of the push-pull system. We have also investigated a tetrameric NLO chromophore based on a push-pull pyridinium salt covalently linked to the cyclotetrasiloxane ring, which in this case is acting only as a scaffold and not as the

pull system (4, Figure 1). Clearly the NLO chromophore 4 better reproduces a fragment of a monolayer on a molecularly engineered silica surface. The reference monomeric push-pull NLO chromophores 1a-4a (Figure 1) were also synthesized and investigated in order to compare their photophysical, conductometric (for 4 and 4a), and second-order NLO properties with those of their corresponding macrocyclic NLO chromophores 1-4. 2. Experimental Section General Comments. All reagents, and anhydrous solvents were obtained from commercial sources (Sigma Aldrich) and used without additional purification except for triethylamine that was freshly distilled over KOH. Tris(trimethylsilyloxy)chlorosilane was purchased from Alfa Aesar, 4-(chloromethylphenyl)trichlorosilane was purchased from Gelest, and 1,3,5,7tetra(4-bromophenyl)-1,3,5,7-tetra(trimethylsilyloxy)cyclotetrasiloxane (5),9 1,3,5,7-tetra[4-vinylphenyl)]-1,3,5,7-tetra(trimethylsilyloxy)cyclotetrasiloxane (6),9 1,3,5,7-tetra[(4-(chloromethylphenyl)]-1,3,5,7-tetra(trimethylsilyloxy)cyclotetrasiloxane (8),9 and (4-vinylphenyl)trichlorosilane10 were synthesized using the methodology reported in literature. 1 H and 29Si NMR were recorded on a Bruker Avance DRX400 spectrometer in CDCl3 or CD2Cl2 as solvents. Mass spectra were obtained with a Thermo-Finnigan apparatus with a Ion Trap analyzer (positive mode) and an electrospray ionization source (ESI) using a LCQ-Advantage instrument. UV spectra were recorded at room temperature with a Jasco V-570 spectrometer. Emission spectra were recorded with a Jobin-Yvon Fluorolog-3 spectrometer equipped with double monochromators and Hamamatsu-928 photomultiplier tube (PMT) as detector. The relative quantum yields, Φ, of fluorescence were obtained by applying the optically diluted method,11 using anthracene (Φ ) 0.27 in EtOH) as a reference12 for compounds 1-3 and 1a-3a, and rhodamine B (Φ ) 0.56 in MeOH)13 for compounds 4 and 4a. Solvatochromic Measurements. The quadratic hyperpolarizability along the charge transfer direction, βCT, was determined by the solvatochromic method, taking into account the solvatochromic shift of the ICT absorption band in various solvents (toluene, n-hexane, cyclohexane, ethyl acetate, THF, dichloromethane, CCl4, acetone, CH3CN) and the solvatochromic shifts of the emission band in the same solvents. The cavity radius (a) was thus directly evaluated by a parallel use of both absorption and emission solvatochromism.14 The quadratic hyperpolarizability tensor βCT along the charge transfer axis of

Properties of Macrocyclic NLO Chromophores

J. Phys. Chem. C, Vol. 113, No. 7, 2009 2747

the ICT transition controlling the NLO response was calculated according to the Oudar two-level equation15

νa2reg2∆µeg 3 βCT ) 2 2 2 2h c (νa - νL2)(νa2 - 4νL2)

(1)

where reg is the transition dipole moment related to the oscillator strenght f of the absorption band by the equation reg2 ) 2.13 × 10-30 f/νeg, νa is the frequency of the ICT absorption band, νL is the frequency of the fundamental incident radiation, and ∆µeg is the variation of the dipole moment between excited and ground state. EFISH Measurements. The molecular quadratic hyperpolarizabilities were measured in CH2Cl2 and CHCl3 solutions by the electric field induced second harmonic (EFISH) generation technique,16 which provides direct information on the intrinsic molecular NLO properties through eq 2

γEFISH ) (µβλ ⁄ 5kT) + γ(-2ω; ω,ω,0)

(2)

where µβλ/5kT represents the dipolar orientational contribution and γ (-2ω; ω,ω,0), a third-order term at frequency ω of the incident wavelength, is the electronic contribution to γEFISH which is negligible for the kind of molecules here investigated.17,18 βλ is the projection, working with an incident wavelength λ, along the dipole moment axis of the vectorial component βVEC of the tensor of the quadratic hyperpolarizability. All EFISH measurements were carried out working with a nonresonant incident wavelength of 1.907 µm, using a Q-switched, modelocked Nd3+:YAG laser manufactured by Atalaser equipped with a Raman shifter, while the apparatus for the EFISH measurements was made by SOPRA (France). Conductometric Measurements. Specific conductivity κ measurements have been performed by an AMEL 160 conductometer and an AMEL 192 glass-shielded, platinized-Pt conductivity cell of cell constant about 1 cm-1, carefully thermostated at 298 K. Prior to measurement the cell was cleaned with diluted HNO3, rinsed with water, and repeatedly washed with the operating solvent following the conductivity decrease until stable readings where reached (such values, accounting for blank solvent conductivity, were afterward subtracted from the conductivity values measured in solutions). Then conductivity measurements were carried out in solutions of increasingly higher concentrations in the 5 × 10-6 to 10-2 M range. The cell constant was determined by standardization with a 0.01 m KCl solution. The temperature coefficient was also evaluated on a 5 × 10-4 M solution of 4a, yielding 0.0014 µS K-1 at 298 K. Synthesis of 1,3,5,7-Tetra(4-iodophenyl)-1,3,5,7-tetra(trimethylsilyloxy)cyclotetrasiloxane (7). To a stirred solution of 1,3,5,7-tetra(4-bromophenyl)-1,3,5,7-tetra(trimethylsilyloxy)cyclotetrasiloxane (5) (1.20 g, 1.04 mmol), in Et2O (90 mL) at -78 °C, a 1.7 M solution of tert-butyllithium in n-pentane (6.13 mL, 10.4 mmol) was added dropwise, working under rigorous water-free conditions and under N2 atmosphere. After addition, the reaction mixture was stirred for 2 h at -78 °C, then it was added via cannula to a stirred solution of I2 (4.00 g, 15.8 mmol) in Et2O (90 mL) at -78 °C. The reaction was allowed to warm to room temperature and was left overnight under stirring. The solvent was removed in vacuo, and the product extracted with n-hexane and filtered off on celite to eliminate the lithium salts. The solution was evaporated to dryness, and the residue was purified by column chromatography (silica gel, n-hexane/CHCl3 ) 9/1) affording 0.64 g (45.7% yield) of pure product as a white solid.

1

H NMR (400.1 MHz, CDCl3), δ, ppm: 7.52 (AA′XX′, 8H, o-C6H4Si, JH-H ) 8.11 Hz), 6.98 (AA′XX′, 8H, m-C6H4Si, JH-H ) 8.11 Hz), 0.18 (s, 36H, SiMe3). 29Si NMR (79.5 MHz, CDCl3), δ, ppm: 11.54 (s, OSiMe3), -79.75 (s, O3SiC6H4I). MS-ESI+: m/z: 1366.79 [M + Na]+. Anal. Calcd for C36H52I4O8Si8: C, 32,15; H, 3.90. Found: C, 32.34; H, 3.99. Synthesis of (4-Bromophenyl)tris(trimethylsiloxy)silane (5a). To a stirred solution of 1,4-dibromobenzene (7.05 g, 29.9 mmol), in Et2O (110 mL) at 0 °C, a 1.6 M solution of n-BuLi in n-hexane (20.5 mL, 32.8 mmol) was added dropwise, working under rigorous water-free conditions and under N2 atmosphere. After 2 h the solution was added via cannula to a solution of tris(trimethylsiloxy)chlorosilane ClSi(OSiMe3)3 (10.8 g, 44.8 mmol) in Et2O (140 mL) at 0 °C; LiCl immediately precipitated as a white powder. The reaction was allowed to warm to room temperature and was left overnight under stirring, then the solvent was removed in vacuo and the product was extracted with n-hexane and filtered off on celite to eliminate the lithium salts. The solution was evaporated to dryness, and the residue was distilled in vacuo, affording 9.01 g of the product (66.7% yield) as a colorless liquid. 1H NMR (400.1 MHz, CDCl3), δ, ppm: 7.52 (AA′BB′, 2H, o-C6H4Si, JH-H ) 8.34 Hz), 7.46 (AA′BB′, 2H, m-C6H4Si, JH-H ) 8.34 Hz), 0.15 (s, 27H, SiMe3). 29Si NMR (79.5 MHz, CDCl3), δ, ppm: 9.3 (s, OSiMe3), -78.5 (s, O3SiC6H4Br). Synthesis of (4-Iodophenyl)tris(trimethylsilyloxy)silane (7a). A solution of (4-bromophenyl)tris(trimethylsiloxy)silane (5a) (7.00 g, 15.5 mmol) in THF (2 mL) was added dropwise, working under rigorous water-free conditions and under N2 atmosphere, to a 100 mL three-neck round-bottom flask containing magnesium turnings (0.40 g, 16.5 mmol) in THF (20 mL). After the addition, the reaction mixture was stirred for 1 h at room temperature. The Grignard reagent was added via cannula in a dropping funnel and was added dropwise to a solution of I2 (6.70 g, 26.4 mmol) in THF (50 mL) in about 0.5 h. The resulting mixture was stirred overnight, the solvent was removed in vacuo, and the product was extracted with n-hexane and filtered off on celite. The solvent was removed, and the residue was purified by column chromatography (silica gel, n-hexane/ CH2Cl2 ) 9/1) affording 1.30 g of the product as a colorless liquid (16.8% yield). 1H NMR (400.1 MHz, CDCl3), δ, ppm: 7.71 (AA′XX′, 2H, o-C6H4Si, JH-H ) 8.10 Hz), 7.29 (AA′XX′, 2H, m-C6H4Si, JH-H ) 8.10 Hz), 0.13 (s, 27H, SiMe3). 29Si NMR (79.5 MHz, CDCl3), δ, ppm: 9.3 (s, OSiMe3), -78.4 (s, O3SiC6H4I). MS-ESI+: m/z: 521.0 [M + Na]+. Synthesis of (4-Vinylphenyl)tris(trimethylsiylloxy)silane (6a). A 1.0 M solution of Me3SiONa in CH2Cl2 (74 mL, 74 mmol) was added dropwise to a stirred solution of (4vinylphenyl)trichlorosilane (4.40 g, 18.5 mmol) in n-hexane (60 mL), working under rigorous water-free conditions and under N2 atmosphere. The reaction mixture was stirred at reflux temperature for 3 h. Unreacted Me3SiONa was then neutralized with Me3SiCl (4.5 mL). The resulting mixture was washed several times with water and dried over MgSO4, then solvent was removed in vacuo and the residue was purified by column chromatography (silica gel, n-hexane/CHCl3 ) 9/1) affording 0.90 g (12.2% yield) of pure product as a colorless liquid. 1H NMR (400.1 MHz, CDCl3), δ, ppm: 7.58 (AA′BB′, 2H, o-C6H4Si, JH-H ) 8.03 Hz), 7.44 (AA′BB′, 2H, m-C6H4Si, JH-H ) 8.03 Hz), 6.77 (m, 1H, CHd, Jcis ) 10.9 Hz, Jtrans ) 17.6 Hz), 5.84 (d, 1H, CH2d, Jtrans ) 17.6 Hz), 5.31 (d, 1H, CH2d, Jcis ) 10.9 Hz), 0.18 (s, 27H, SiMe3). 29Si NMR (79.5 MHz,

2748 J. Phys. Chem. C, Vol. 113, No. 7, 2009 CDCl3), δ, ppm: 8.8 (s, OSiMe3), -77.9 (s, O3SiC6H4CHdCH2). MS-ESI+: m/z: 399.5 [M + H]+. Synthesis of (4-Chloromethylphenyl)tris(trimethylsiyloxy)silane (8a). A 1.0 M solution of Me3SiONa in CH2Cl2 (40 mL, 40 mmol) was added dropwise to a stirred solution of (4chloromethylphenyl)trichlorosilane (2.80 g, 11.5 mmol) in n-hexane (50 mL), working under rigorous water-free conditions and under N2 atmosphere. The reaction mixture was stirred at reflux temperature for 3 h. Unreacted Me3SiONa was then neutralized with Me3SiCl (1 mL). The resulting mixture was washed several times with water and dried over MgSO4, the solvent was removed in vacuo, and the residue was distilled under vacuum affording 1.20 g (24.8% yield) of the pure product as a colorless liquid. 1H NMR (400.1 MHz, CDCl3), δ, ppm: 7.61 (AA′XX′, 2H, o-C6H4Si, JH-H ) 7.99 Hz), 7.41 (AA′XX′, 2H, m-C6H4Si, JH-H ) 7.99 Hz), 4.62 (s, 2H, CH2Cl), 0.17 (s, 27H, SiMe3). 29Si NMR (79.5 MHz, CDCl3), δ, ppm: 9.05 (s, OSiMe3), -78.4 (s, O3SiC6H4CH2Cl). Synthesis of 1a. In a dry Schlenk tube, 4-(dimethylamino)phenylboronic acid (93 mg, 0.56 mmol) and tetrakis(triphenylphosphine)palladium(0) (16 mg, 0.014 mmol) were added as solid to a solution of (4-bromophenyl)tris(trimethylsiloxy)silane (5a) (130 mg, 0.29 mmol) in 15 mL of THF. The mixture was degassed by a nitrogen stream, and then 1 mL of Na2CO3 1 M acqueous solution was added. The reaction mixture was heated under vigorous stirring at 80-85 °C for 24 h, until complete consumption of starting product, controlled by TLC (silica gel, petroleum ether/AcOEt ) 9.5/0.5). The organic phase was separated and evaporated in vacuo. The residue was purified by column chromatography (silica gel, petroleum ether/AcOEt ) 10/0.1) to afford 103 mg (72% yield) of pure product as a white solid. 1H NMR (400.1 MHz, CD2Cl2), δ, ppm: 7.58 (m, 6H), 6.83 (AA′XX′, 2H, o-C6H4NMe2, JH-H ) 8.80 Hz), 3.02 (s, 6H, CH3), 0.18 (s, 27H, SiMe3). 29Si NMR (79.5 MHz, CD2Cl2), δ, ppm: 8.9 (s, OSiMe3), -77.3 (s, O3SiC6H4). MSESI+: m/z: 492.3 [M + H]+. Anal. Calcd for C23H41NO3Si4: C, 56.16; H, 8.40; N, 2.85. Found. C, 55.98; H, 8.50; N, 2.82. Synthesis of 1. In a dry Schlenk tube, solid 4-(dimethylamino)phenylboronic acid (342 mg, 2.07 mmol) and 1,3,5,7-tetra(4bromophenyl)-1,3,5,7-tetra(trimethylsilyloxy)ciclotetrasiloxane (5) (100 mg, 0.086 mmol) were suspended in 10 mL of toluene. The mixture was degassed by three freeze-pump-thaw cycles, then tetrakis(triphenylphospine)palladium(0) (20 mg, 0.017 mmol) was added, followed by 2 mL of K2CO3 saturated aqueous solution and few drops of Aliquat 336 (tricaprylmethylammonium chloride). The reaction mixture was heated at 80-85 °C for 24 h until complete consumption of starting product, controlled by TLC (silica gel, petroleum ether/CH2Cl2 ) 8/2). The organic phase was separated and evaporated under reduced pressure, and the residue was purified by column chromatography (silica gel, petroleum ether/AcOEt ) 8/2) affording 31 mg (27% yield) of pure product as a white waxy solid. 1H NMR (400.1 MHz, CD2Cl2), δ, ppm: 7.46 (m, 16H), 7.39 (AA′BB′, 8H, m-C6H4Si, JH-H ) 8.27 Hz), 6.77 (AA′XX′, 8H, o-C6H4NMe2, JH-H ) 7.05 Hz), 3.02 (s, 24H, CH3), 0.33 (s, 36H, SiMe3). 29Si NMR (79.5 MHz, CD2Cl2), δ, ppm: 10.6 (s, OSiMe3), -78.7 (s, O3SiC6H4). MS-ESI+: m/z: 1319.1 [M + H]+. Anal. Calcd for C68H92N4O8Si8: C, 61.96; H, 7.03; N, 4.25. Found: C, 61.82; H, 7.08; N, 4.26. Synthesis of 2a. In a dry Schlenk tube, (4-vinylphenyl)tris(trimethylsiyloxy)silane (6a) (200 mg, 0.50 mmol) and 4-bromoN,N-dimethylaniline (91 mg, 0.45 mmol) were dissolved in triethylamine (6 mL). The solution was degassed under nitrogen

Ronchi et al. stream before tris-o-tolylphosphine (27 mg, 0.091 mmol) and palladium(II) acetate (10 mg, 0.045 mmol) were added. The reaction mixture was heated at 90 °C for 20 h until complete consumption of starting product, controlled by TLC (silica gel, petroleum ether/AcOEt ) 9.5/0.5). The mixture was evaporated to dryness, and the residue was first purified by column chromatography (silica gel, petroleum ether/AcOEt ) 9.5/0.5) and then crystallized from methanol to afford 217 mg of pure product (84% yield) as fine yellow needles. 1H NMR (400.1 MHz, CD2Cl2), δ, ppm: 7.56 (AA′BB′, 2H, o-C6H4Si, JH-H ) 7.56 Hz), 7.48 (m, 4H), 7.14 (d, 1H, CHd, Jtrans ) 16.1 Hz), 6.97 (d, 1H, CHd, Jtrans ) 16.1 Hz), 6.82 (AA′XX′, 2H, o-C6H4NMe2, JH-H ) 8.03 Hz), 3.03 (s, 6H, CH3), 0.17 (s, 27H, SiMe3). 29Si NMR (79.5 MHz, CD2Cl2), δ, ppm: 8.9 (s, OSiMe3), -77.6 (s, O3SiC6H4). MS-ESI+: m/z: 518.4 [M + H]+. Anal. Calcd for C25H43NO3Si4: C, 57.97; H, 8.37; N, 2.70. Found: C, 57.92; H, 8.42; N, 2.67. Synthesis of 2. In a dry Schlenk tube, 1,3,5,7-tetra(4vinylphenyl)-1,3,5,7-tetra(trimethylsilyloxy)ciclotetrasiloxane (6) (350 mg, 0.37 mmol), 4-iodo-N,N-dimethylaniline (502 mg, 2.03 mmol), palladium(II) acetate (330 mg, 1.48 mmol), and tris-otolylphosphine (450 mg, 1.48 mmol) were dissolved in triethylamine (40 mL) under nitrogen stream. The mixture was degassed by five freeze-pump-thaw cycles and then heated at 90 °C for 24 h until complete consumption of starting product, controlled by TLC (silica gel, petroleum ether/AcOEt ) 7/3). The mixture was evaporated to dryness. The residue was treated with a mixture of petroleum ether/AcOEt (7/3) and filtered off, and the solution was evaporated to dryness. The residue was purified by column chromatography (silica gel, petroleum ether/ AcOEt ) 7/3) affording 163 mg of pure product as a pale yellow solid (31% yield). 1H NMR (400.1 MHz, CD2Cl2), δ, ppm: 7.34 (AA′BB′, 8H, o-C6H4Si, JH-H ) 8.64 Hz), 7.29 (m, 16H), 7.06 (d, 4H, CHd, Jtrans ) 16.3 Hz), 6.89 (d, 4H, CHd, Jtrans ) 16.1 Hz), 6.80 (AA′XX′, 8H, o-C6H4NMe2, JH-H ) 8.15 Hz), 3.00 (s, 24H, CH3), 0.27 (s, 36H, SiMe3). 29Si NMR (79.5 MHz, CD2Cl2), δ, ppm: 10.9 (s, OSiMe3), -79.3 (s, O3SiC6H4). MSESI+: m/z: 1444.6 [M + Na]+. Anal. Calcd for C76H100N4O8Si8: C, 64.18; H, 7.09; N, 3.94. Found. C, 63.95; H, 7.15; N, 3.86. Synthesis of 3a. In a Schlenk tube under rigorous waterfree conditions, (4-iodophenyl)tris(trimethylsilyloxy)silane (7a) (210 mg, 0.42 mmol), bis(triphenylphosphine)palladium(II) dichloride (5.9 mg, 0.0084 mmol), copper(I) iodide (5.2 mg, 0.027 mmol), and triethylamine (0.5 mL) were added all at once. The mixture was degassed by five freeze-pump-thaw cycles, then the resulting mixture was heated at 80 °C under H2 atmosphere. A solution of 4-ethynyl-N,N-dimethylaniline (61 mg, 0.42 mmol) in 1 mL of triethylamine was degassed by five freeze-pump-thaw cycles, and then it was added under hydrogen stream. The final solution was stirred at 80 °C for 24 h. The solvent was removed in vacuo, and the residue was purified by column chromatography (silica gel, n-hexane/CH2Cl2 ) 7/3) affording 145 mg (67% yield) of pure product as a white solid. 1H NMR (400.1 MHz, CD2Cl2), δ, ppm: 7.60 (AA′BB′, 2H, m-C6H4Si, JH-H ) 8.24 Hz), 7.51 (AA′BB′, 2H, o-C6H4Si, JH-H ) 8.24 Hz), 7.45 (AA′XX′, 2H, m-C6H4NMe2, JH-H ) 8.97 Hz), 6.73 (AA′CC′, 2H, o-C6H4NMe2, JH-H ) 8.97 Hz), 3.03 (s, 6H, CH3), 0.19 (s, 27H, SiMe3). 29Si NMR (79.5 MHz, CD2Cl2), δ, ppm: 9.25 (s, OSiMe3), -78.2 (s, O3SiC6H4). MSESI+: m/z: 516.7 [M + H]+. Anal. Calcd for C25H41NO3Si4: C, 58.20; H, 8.01; N, 2.71. Found: C, 57.52; H, 8.10; N, 2.70. Synthesis of 3. In a Schlenk tube under rigorous water-free conditions, 1,3,5,7-tetra(4-iodophenyl)-1,3,5,7-tetra(trimethylsilyloxy)cyclotetrasiloxane (7) (229 mg, 0.170 mmol), bis(triph-



SCHEME 1

enylphosphine)palladium(II) dichloride (24 mg, 0.034mmol), copper(I) iodide (13 mg, 0.068 mmol), triethylamine (1.5 mL), and anhydrous THF (15 mL) were added all at once. The solution was degassed by five freeze-pump-thaw cycles, and the resulting mixture was stirred at reflux temperature under H2 atmosphere. A solution of 4-ethynyl-N,N-dimethylaniline (197 mg, 1.36 mmol) in 5 mL of THF was degassed by five freeze-pump-thaw cycles, and then it was added under hydrogen stream. The resulting solution was stirred at 80 °C for 24 h, and the solvent was removed in vacuo; the residue was purified by column chromatography (silica gel, n-hexane/ AcOEt ) 6.5/3.5) affording 0.04 g (48% yield) of the product as a pale green solid. 1H NMR (400.1 MHz, CD2Cl2), δ, ppm: 7.40 (AA′XX′, 8H, m-C6H4NMe2, JH-H ) 8.93 Hz), 7.34 (m, 16H), 6.65 (AA′XX′, 8H, o-C6H4NMe2, JH-H ) 8.93 Hz), 3.0 (s, 24H, CH3), 0.27 (s, 36H, SiMe3). 29Si NMR (79.5 MHz, CD2Cl2), δ, ppm: 11.2 (s, OSiMe3), -79.7 (s, O3SiC6H4). MSESI+: m/z: 1436.5 [M + Na]+. Anal. Calcd for C76H92N4O8Si8: C, 64.54; H, 6.56; N, 3.96. Found: C, 64.68; H, 6.61; N, 3.91. Synthesis of 4a. To a solution of (4-chloromethylphenyl)tris(trimethylsilyloxy)silane (8a) (232 mg, 0.551 mmol) in CH3CN (20 mL), 4-[4-(dimethylamino)styryl]pyridine (9) (150 mg, 0.669 mmol) was added, and the resulting mixture was stirred at reflux for 18 h. The solvent was removed in vacuo, and the residue was washed with hot AcOEt (5 × 20 mL), to afford 189 mg (52% yield) of pure 4a. 1 H NMR (400.1 MHz, CDCl3), δ, ppm: 9.16 (AA′XX′, 2H, o-C5H4N, JH-H ) 6.7 Hz), 7.78 (AA′XX′, 2H, m-C5H4N, JH-H ) 6.7 Hz), 7.62 (AA′BB′, 2H, o-C6H4Si, JH-H ) 8.07 Hz), 7.54 (m, 5H), 6.86 (d, 1H, Me2NC6H4CHd, JH-H ) 15.9 Hz), 6.76 (AA′XX′, 2H, o-C6H4NMe2, JH-H ) 8.8 Hz), 6.05 (s, 2H, CH2), 3.1 (s, 6H, CH3), 0.12 (s, 27H, SiMe3). 29Si NMR (79.5 MHz, CDCl3), δ, ppm: 9.3 (s, OSiMe3), -79.0 (s, O3SiC6H4CH2). MSESI+: m/z: 609.3 [M - Cl]+. Anal. Calcd for C31H49ClN2O3Si4: C, 57.68; H, 7.65; N, 4.34. Found: C, 57.26; H, 7.80; N, 4.25. Synthesis of 4. To a solution of 1,3,5,7-tetra(4-(chloromethylphenyl)-1,3,5,7-tetra(trimethylsilyloxy)cyclotetrasiloxane (8) (197 mg, 0.19 mmol) in CH3CN (20 mL), 4-[4-(dimethylamino)styryl]pyridine (9) (173 mg, 0.77 mmol) was added and the resulting mixture was stirred at reflux for 28 h. During this time the formation of a red solid precipitate was observed. The

reaction mixture was cooled at room temperature, and the solid was filtered off and washed with hot ethyl acetate (3 × 20 mL) to afford 169 mg (46% yield) of pure 4 as a red powder. 1 H NMR (400.1 MHz, CDCl3), δ, ppm: 9.27 (AA′XX′, 8H, o-C5H4N, JH-H ) 6.7 Hz), 7.91 (AA′XX′, 8H, m-C5H4N, JH-H ) 6.7 Hz), 7.68 (d, 4H, C5H4NCHd, JH-H ) 15.9 Hz), 7.50 (AA′XX′, 8H, m-C6H4NMe2, JH-H ) 8.9 Hz), 7.26 (AA′BB′, 8H, o-C6H4Si, JH-H ) 8.0 Hz), 7.07 (AA′BB′, 8H, m-C6H4Si, JH-H ) 8.0 Hz), 6.86 (d, 4H, Me2NC6H4CHd, JH-H ) 15.9 Hz), 6.61 (AA′XX′, 8H, o-C6H4NMe2, JH-H ) 8.9 Hz), 6.02 (s, 8H, CH2), 2.99 (s, 24H, CH3), 0.16 (s, 36H, SiMe3). 29Si NMR (79.5 MHz, CDCl3), δ, ppm: 10.70 (s, OSiMe3), -79.08 (s, O3SiC6H4CH2-). MS-ESI+: m/z: 1896.8 [M - Cl]+, 930.1 [M - 2Cl-]2+, 608.9 [M - 3Cl-]3+, 448.1 [M - 4Cl-]4+. Anal. Calcd for C100H124Cl4N8O8Si8: C, 62.15; H, 6.47; N, 5.80. Found: C, 61.95; H, 6.51; N, 5.75. 3. Results and Discussion 3.1. Synthesis. Cyclotetrasiloxanes 1-4 (Figure 1) are characterized by four dipolar second-order NLO active tails on the same side of the macrocycle,9 with their dipole moments oriented in the same direction. They can be thus regarded as an ensamble of dipolar NLO chomophores held together to generate a large macrocyclic NLO chromophore, which should show an enhanced second-order NLO response. Compound 1a was prepared (Scheme 1) in 72% yield by palladium-catalyzed coupling of (4-bromophenyl)tris(trimethylsiloxy)silane (5a) with 4-(dimethylamino)phenylboronic acid according to the Suzuki reaction conditions, working in THF as solvent and with aqueous Na2CO3 as base. Again the Suzuki coupling of 1,3,5,7-tetra(4-bromophenyl)-1,3,5,7-tetra(trimethylsiloxy)cyclotetrasiloxane (5) with 4-(dimethylamino)phenylboronic acid, as described by Laine and co-workers,19 afforded the corresponding cyclotetrasiloxane derivative 1 in 27% yield, after purification by column chromatography (Scheme 1). Palladium-catalyzed cross coupling of (4-vinylphenyl)tris(trimethylsiloxy)silane (6a) with 4-bromo-N,N-dimethylaniline, carried out by the Heck reaction working at 90 °C with triethylamine as solvent, in the presence of catalytic amounts of Pd(II) acetate and tris-o-tolylphosphine,20a afforded compound


Ronchi et al.

SCHEME 2

SCHEME 3

2a in 84% yield, after purification by column chromatography and crystallization from methanol (Scheme 2). The same reaction conditions were used for the preparation of the corresponding cyclotetrasiloxane derivative 2 starting from 1,3,5,7-tetra(4-vinylphenyl)-1,3,5,7-tetra(trimethylsiloxy)cyclotetrasiloxane (6) but with poor and hardly reproducible yields. Only starting from 4-iodo-N,N-dimethylaniline, under the same reaction conditions used for the synthesis of 2a, compound 2

was successfully obtained in 31% yield after column chromatography (Scheme 2). Compounds 3a and 3 were obtained in 67% and 48% yields, respectively, after column chromatography (Scheme 3) by Sonogashira cross coupling of 4-ethynyl-N,N-dimethylaniline with (4-iodophenyl)tris(trimethylsiloxy)silane (7a) and 1,3,5,7tetra(4-iodophenyl)-1,3,5,7-tetra(trimethylsiloxy)cyclotetrasiloxane (7), respectively. Both reactions were carried out



SCHEME 4

under an atmosphere of hydrogen, in order to limit the formation of the homocoupling byproduct of 4-ethynyl-N,N′-dimethylaniline,20b working in the presence of catalytic amounts of bis(triphenylphosphine)palladium(II) chloride and copper(I) iodide at 80 °C and with triethylamine as solvent. Compound 4 was prepared by reaction, in refluxing acetonitrile, of 4-[4-(dimethylamino)styryl]pyridine (9) with the benzyl chloride groups of the cyclotetrasiloxane derivative 1,3,5,7-tetra(4-chloromethylphenyl)-1,3,5,7-tetra(trimethylsiloxy)cyclotetrasiloxane (8). Compound 4 separated directly from the hot reaction solution and was isolated in 46% yield as a pure red powder, after washing with hot ethyl acetate. The structurally related monomer 4a was synthesized in 52% yield following the same experimental procedure (Scheme 4). All compounds were characterized by 1H and 29Si NMR spectra and by mass spectroscopy using the electronspray ionization source (ESI). 3.2. NMR and Mass Spectra. 1H NMR spectra in CD2Cl2 of the macrocyclic NLO chromophores 1-3 are those expected for an average C4V symmetry and, as the reference monomeric NLO chromophores 1a-3a, show two aromatic systems: the first one (C6H4NMe2) at around 7 ppm and the second one (C6H4Si) at around 7.3 ppm. The signals of the aromatic rings of the macrocyclic NLO chromophores 1-3 are always slightly upfield shifted when compared to those of reference monomeric NLO chromophores 1a-3a suggesting some weak shielding due to long-range ring-current effects between the aromatic rings. This effect, as already observed by some of us for the basic cyclotetrasiloxane scaffold,9 is more significant for the more closely packed aromatic rings (C6H4Si) directly linked to the tetracyclosiloxane ring (∆δ ) 0.18-0.23 ppm, average distance between the aromatic rings of about 7 Å) than for the other aromatic rings (C6H4NMe2) (∆δ ) 0.06-0.13 ppm, average distance between the aromatic rings of about 14 Å). These average distances were calculated from the optimized geometrical structures obtained by the DFT theoretical approach (see later section 3.3).

The increased rigidity of the organic tail of 3 produces an increased separation of the C6H4NMe2 aromatic rings, so that the upfield shift of their 1H NMR signals is quite similar for 2 (∆δ ) 0.13 ppm) and 1 (∆δ ) 0.12 ppm) but smaller for 3 (∆δ ) 0.06 ppm). 1 H NMR spectra in CDCl3 of the macrocycle 4 and of the corresponding monomer 4a are more complex due to the presence of the pyridinic ring. Also in this case we have a very small upfield shift of the 1H NMR signals of the aromatic ring C6H4NMe2 (∆δ ) 0.06 ppm), which becomes more significant for the aromatic ring C6H4Si (∆δ ) 0.38 ppm) when compared to the corresponding signals of the reference monomer 4a. 1 H NMR signals of the CH3 groups of the OSiMe3 moieties of 1-4 are downfield shifted (∆δ ) 0.04 - 0.16 ppm) when compared to those of 1a-4a, while the signals of the methyl groups of the NMe2 moiety do not show a significant shift. 29 Si NMR spectra of 1-4 show, in agreement with an average C4V symmetry, only one signal around s79 ppm corresponding to the silicon atoms of the cyclotetrasiloxane ring,9 only slightly upfield shifted when compared to the signal at around s78 ppm of the reference monomers 1a-4a, and one signal at around 11 ppm, assigned to the silicon atoms of the OSiMe3 moieties, slightly downfield shifted when compared to the corresponding signals at around 9 ppm of the reference monomers 1a-4a. The 1H NMR spectra of 1-3, even at -80 °C, are typical for a C4V symmetry; with the four NLO active organic tails all equivalent, suggesting both a fast rotation around the Si-C bond, as discussed later in section 3.3, and a flexible behavior of the cyclotetrasiloxane ring as expected for a fast exchange of the various conformations of the four NLO active organic tails. Mass spectra, performed by using an electronspray ionization source (ESI), show the presence of the molecular peak [M + Na]+ or [M + H]+ with the expected isotopic distribution for compounds 1-3 and 1a-3a. For 4a the quite strong molecular peak is [M - Cl-]+ due to the loss of the counterion Cl- while the ESI-MS spectrum of the macrocyclic compound 4 shows a

2752 J. Phys. Chem. C, Vol. 113, No. 7, 2009 very weak peak due to the [M - Cl-]+ ion, but on the other hand the signals due to the bi-, tri-, and tetrapositive ions [M - 2Cl-]2+, [M - 3Cl-]3+, and [M - 4Cl-]4+, respectively are rather intense. In particular the pattern of the signals of the multicharged ions shows, in high-resolution spectra, the typical peak separation (about 0.50, 0.33, and 0.25, respectively). 3.3. Theoretical Calculations of the Optimized Geometries and of Dipole Moments. The best geometries of 1-3 and 1a-3a were first defined by semiempirical theoretical calculations, using the AM1 Hamiltonian21 and then refined by an ab initio approach based on density functional theory (DFT) calculations carried out using the BP86 functional22 and the splitvalence plus polarization basis set (SVP).23 The DFT calculations were carried out using the TURBOMOLE package of programs;24 the AM1 calculations were done using the MOPAC7 program.25 The best AM1 geometries were thus the basis for the geometry optimizations (Figure 2) carried out by the DFT method. Compound 2a shows a Ph-CdC-Ph system strictly planar, because of the π conjugation of the two rings through the ethylene bridge. However, due to the nonlinearity, 2a does not follow strictly the axial symmetry; on the contrary, 3a shows an axial symmetry due to the linearity of the of the Ph-CtC-Ph system. The π conjugation is broken in the structure of 1a because the absence of the π bridge introduces a steric repulsion of hydrogens belonging to the two directly linked aromatic rings, rotated by about 45°, as found for example for biphenyl in vacuo.26 The extent of the axial symmetry of 1a-3a can be evaluated by the ratio r ) µz/|µ|, where µz and |µ| are the z component along the molecular axis and the modulus of the dipole moment vector, respectively. The computed r values are 0.994, 0.996, and 0.998 for 1a, 2a, and 3a, respectively. Calculated dipole moments of 1a-3a are similar, but with a slight increase in the series 1a < 2a < 3a. Calculated dipole moments of 1-3 increase, when compared to those of reference monomers 1a-3a, up to three times (Table 1), as expected, according to the approximation described in ref 27 for the vectorial sum of the projection along the molecular dipole moment axis of the dipole moments of the single NLO active dipolar organic tails nanoorganized on the cyclotetrasiloxane ring with a not exactly parallel orientation27 (Figure 2). Since in the macrocyclic NLO chromophores 1-3 the planarity and π conjugation of the NLO active dipolar organic tails are very similar to that of the reference monomers 1a-3a, the dipole moments should also be similar27 (Figure 2). We have thus investigated to which extent the DFT calculated dipole moments of 1-3 are far from the vectorial sum of the projection along the molecular dipole moment axis of the dipole moments of the single NLO active dipolar organic tails. A detailed analysis of the optimized geometrical structure of 1-3 (Figure 2) shows that the mutual orientation of the four NLO active organic tails does not seem to obey to any particular symmetry rule. The mutual orientation has been evaluated in terms of the dihedral angle of the planes between the phenyl rings directly attached to the silicon atom of the macrocycle and belonging to two adiacent dipolar organic tails. The values obtained by DFT calculations for 1 are 58.3, 67.6, 75.8, and 103.1° (θ average value 76.2°), for 2 are 43.2, 86.1, 77.0, and 80.2° (θ average value 71.6°), and for 3 are 88.7, 53.5, 100.0, and 116.4° (θ average value 89.2°). In all the optimized geometries of 1-3 we could not observe the typical distances indicative for strong nonbonding electronic interaction such as π stacking.8 We can thus conclude that the mutual spatial orientation of the four

Ronchi et al. organic tails is basically disordered and that the best molecular geometry calculated by DFT optimization is one of the several, nearly degenerate local minima. This latter conclusion is in agreement with the 1H NMR spectra which show that even at -80 °C the four dipolar organic tails of 1, 2, and 3 are equal, corresponding to an average C4V symmetry. If we associate to each dipolar organic tail of 1-3 a dipole vector identical to that computed for the reference monomer 1a-3a, the facile rotation around the Si-C(Ph) bonds produces an effective annihilation of the dipole components perpendicular to the axis Si-N(Me)2 of the dipolar organic tails while the dipole components parallel to this axis add each other. Therefore, the sum of the local dipoles of the organic tails, oriented along the molecular dipole axis perpendicular to the plane of the cyclotetrasiloxane ring, is obtained from the dipole moments of the single dipolar organic tails, according to the equation 4

µT )

∑ µziki · kT

(3)

i)1

where µT and µzi are respectively the molecular dipole moment of the macrocycle (assumed to be fully coincident with the axis perpendicular to the cyclotetrasiloxane ring) and the local dipole moments of the organic tail ith (assumed to be fully coincident with the axis of the organic tail); ki and kT are the versors parallel to the local axis of the organic tails and the molecular dipole axis, respectively. Interestingly eq 3 is the equivalent of the following equation, reported in ref 27

µcyclotetrasiloxane)4(cos θ)µreference where θ is the average angle between the local dipole axis of the dipolar organic tails and the molecular dipole axis of the macrocycle, if we assume that all four organic tails are, as average, symmetrically distributed with a C4V symmetry around the molecular dipole axis perpendicular to the cyclotetrasiloxane ring. On the basis of the optimized DFT geometries of 1-3 (see Figure 2), the four angles between the local dipole axis of the four dipolar organic tails and the molecular dipole axis of the macrocycle are 35.5, 70.7, 24.1, and 54.0° (average value 46.3°), 40.9, 60.0, 43.4, and 51.6° (average value 48.9°), and 42.1, 54.0, 42.4, and 59.4° (average value 49.5°), for 1, 2, and 3, respectively, with a sort of regular alternance of the angle values since, alternatively, one organic tail is more “close” to the molecular dipole axis than the adiacent one. This structural feature is typical for instance of the pinched structure of conical tetraalkynyl calyx[4]arenes.28 Although the single values are scattered, the resulting average values are quite similar for 1-3. Interestingly the vectorial sum of the projection along the molecular dipole moment axis of the macrocycles 1-3, of the calculated DFT dipole moments of the dipolar organic tails (according to eq 3), gives the following values: 8.47, 10.83, and 11.17 D for 1, 2, and 3, respectively, almost coincident with those calculated according to the simplified approach of ref 27 (8.85, 10.88, and 11.20 D, respectively), starting from the calculated DFT dipole moments of 1a, 2a, and 3a, respectively, and of the values of the average angle θ of 1, 2, and 3. In conclusion the approximate assumption formulated in ref 27 is qualitatively confirmed by our theoretical approach, although the dipole moments of 1, 2, and 3 obtained as the vectorial sum of the DFT calcutated values of 1a, 2a, and 3a are smaller than those obtained by full DFT calculations, the difference being about 1 D for 1 and 3 and about 2 D for 2. 3.4. Electronic Absorption and Emission Spectra and Nonlinear Optical Properties. Electronic absorption spectra of 1-4 are characterized by a major internal π f π* charge



Figure 2. Optimized geometries of 1-3 (both side and front view) obtained by DFT calculations carried out with BP8622 functional and SVP23 basis set (see section 3.3 for details).


Ronchi et al.

TABLE 1: Calculated Components and Modulus of the Dipole Moment (Debye) of 1a-3a and 1-3a compound 1a 2a 3a 1 2 3

µx

µy

µz

|µ|

-0.121 0.147 0.130 -0.293 3.904 -0.475

0.300 -0.324 0.246 9.681 11.981 12.387

3.207 4.144 4.320 1.558 -1.994 -1.310

3.223 4.160 4.329 9.810 12.758 12.465

a Obtained by DFT calculations based on BP86 functional and SVP basis set (see text for details).

transfer band (ICT). Macrocyclic NLO chromophores 1-3 show a very small blue shift of this band when compared to the same ICT band of the reference monomeric NLO chromophores 1a-3a (Table 2). Such a blue shift is probably due to very weak π-π interactions, already evidenced by 1H NMR spectroscopy (see section 3.2), between the aromatic rings (C6H4Si) directly linked to the cyclotetrasiloxane ring. For a different ensemble of four dipolar organic NLO active tails such as that of a push-pull tetraalkenyl calyx[4]arene with a nitro pull group and a propoxy push group, a similar very small blue shift was reported.27,29 Significant blue shifts are expected only for strong π-π interactions such as those typical of Langmuir-Blodgett films of NLO active aromatic molecules,7a more closely packed when compared to the much less packed arrangement of the four NLO active organic tails in the optimized structure of the macrocycles 1-3 (see Figure 2). The intensity of the absorption of the ICT bands of 1-3 is slightly less than four times that of the reference monomeric NLO chromophores 1a-3a (Table 2). A similar lower intensity of the ICT bands was reported for tetraalkenyl and tetraalkynyl calyx[4]arenes.27,29 In the case of 4 and 4a, as expected for positively charged NLO chromophores30 the ICT band is shifted at lower energies (Table 2). This shift is basically originated by the more electron withdrawing effect produced by the pyridinium group. The emission spectra of NLO chromophores 1-3 show a small red shift when compared to those of the reference NLO chromophores 1a-3a (Table 2), while 4 and 4a evidence a slight blue shift. The quantum yields (φ) of 1-3 are rather low and slightly lower than those of reference monomeric NLO chromophores 1a-3a, being particularly low for 2 and 2a (Table 2). The dipolar contribution to the quadratic hyperpolarizabilities (β1.91, where 1.91 means for sake of simplicity 1.907) of macrocyclic NLO chromophores and of their reference monomeric NLO chromophores was measured (in CH2Cl2 solution for 1-3 and 1a-3a and in CHCl3 solution for 4 and 4a) by the electric field induced second harmonic generation (EFISH) technique working with an incident wavelength of 1.907 µm (see Experimental Section), in order to avoid as much as possible resonance enhancement for the measured hyperpolarizabilities β1.91 which were corrected in any case for dispersion according to the two level model15 which allows determination of the static quadratic hyperpolarizabilities β0 (Table 3). Of course the EFISH measurement of the quadratic hyperpolarizability takes into account mainly the dipolar contribution of the β tensor of a multichromophoric system such as the macrocyclic NLO chromophores investigated in this work. However in these latter the structure of the basic cyclotetrasiloxane ring, with all the dipolar NLO-active tails in the same orientation of the dipole vector, induces a strong dipolar component of the quadratic hyperpolarizability as reported for

push-pull calix[4]arenes with a cone configuration. It follows that the octupolar component of the β tensor does not play a relevant role in our multichromophoric macrocyclic NLO chromophores and therefore that the EFISH measurements may safely represent their second-order NLO properties.31 Since the values of the dipole moments of the macrocycles 1-3 can be assumed to be the vectorial sum of the projection of the dipole moments, along the molecular dipole axis of the macrocycle, of the individual dipolar organic tails, according to the approximate approach of ref 27 as discussed in section 3.3, we have followed a similar approximation for the values of the EFISH quadratic hyperpolarizability, corresponding to the projection of the vectorial component of the quadratic hyperpolarizability along the molecular dipole moment axis. This approximation was safely applied for instance in the case of other macrocyclic NLO chromophores such as some tetraalkenyl or alkynyl calix[4]arenes.27,29 We have thus assumed, for the macrocyclic NLO chromophores 1-3, that the EFISH quadratic hyperpolarizability can be the sum of the vectorial contribution of the projection along the molecular dipole moment axis of the EFISH quadratic hyperpolarizabilities of each NLO active tail, involving an orientational factor (cos θ)3, where θ is again the average angle, which takes into account a C4V symmetry, as discussed in section 3.3. If the EFISH quadratic hyperpolarizability of the individual NLO active organic tail is quite similar to that of the reference monomeric NLO chromophores,27,29 the µβ1.91 value, obtained by the EFISH measurements, of the macrocyclic NLO chromophores 1-3 can be described, according to ref 27, by eq 427

(µβ1.91)cyclotetrasiloxane)16(cos θ)4(µβ1.91)reference

(4)

where (β1.91)cyclotetrasiloxane ) 4(cos θ) (β1.91)reference and µcyclotetrasiloxane ) 4(cos θ)µreference. It follows that both the values of µβ1.91 and β1.91 of 1-3 are strictly dependent not only on the values of the dipole moment and of the EFISH quadratic hyperpolarizability of the single NLO active organic tails but also on their average opening angle θ. This simplified approach may suffer of the limitations of treating the macrocyclic NLO chromophores 1-3 as an ensemble of isolated NLO chromophores not affected by significant nonbonding electronic interactions between them,8 but since both the theoretically optimized structures of 1-3 and the evidence produced by 1H NMR and electronic absorption spectra do not support significant π-π electronic interactions between the single organic tails, we can safely assume that the second-order NLO properties of the macrocyclic NLO chromophores 1-3 can be in first approximation described by eq 4. Therefore the effect of the nanoorganization of four NLO-active organic tails on the tetracyclosiloxane ring results in an increase of the µβ1.91 value of 1-3 compared to that of the reference monomeric NLO chromophores 1a-3a of 2.8 times for 1, 3.1 for 2, and 3.5 for 3 (Table 3). Such a relatively low increase reflects a rather large average angle θ of the alignement of the single NLO active organic tails which is not only much higher than zero (all the tails parallel to the molecular dipole moment axis) but also higher than 45° (which should correspond to an increase of four times), as confirmed by our theoretical DFT calculations (see section 3.3) of the optimized geometries of 1-3 which have produced an average angle θ always higher than 45°. The increase of µβ1.91 values of 1-3, when compared to those of 1a-3a is mainly due to the increase of the dipole moment, since the value 3



TABLE 2: Absorption and Emission Bands of 1-4 and 1a-4a (in CH2Cl2 Solution for 1-3 and 1a-3a and in CHCl3 Solution for 4 and 4a)

a

λexc: 320 nm (for 1 and 1a), 359 nm (for 2), 358 nm (for 2a), 343 nm (for 3), 345 nm (for 3a), 525 nm (for 4), and 526 nm (for 4a). b Φ means the quantum yields of fluorescence.

TABLE 3: µβ1.91 Measured in CH2Cl2 Working with an Incident Wavelength of 1.907 µm, Theoretical Dipole Moments (µ), and EFISH Quadratic Hyperpolarizability (β1.91 and β0) of 1-3 and 1a-3a

a The µβ values reported are the average of 16 successive measurements performed on each sample at about 10-3 M concentration. The standard deviation was about 10-15%. b Calculated by DFT. c Calculated with the two level model where β0 ) βλ[1 - (2λmax/λ)2][1 - (λmax/ λ)2] (ref 15).

of the EFISH quadratic hyperpolarizability β1.91 of the macrocyclic NLO chromophores 1-3 remains similar to that of the reference monomeric NLO chromophores 1a-3a (Table 3). Some push-pull tetracalix[4]arenes with a cone conformation display a higher enhancement of the EFISH µβ0 value when compared to their reference monomeric NLO chromophores. For instance for tetranitro-tetrapropoxycalix[4]arene and tetra[(E)-1-(4-nitrophenyl)ethenyl]-tetrapropoxycalix[4]arene the enhancement of the µβ0 value is 6.9 and 10.1 times higher than the µβ0 value of their reference monomeric NLO chromophores. In accordance these calix[4]arenes show an increase of the

EFISH β1.91 or β0 values of about two to three times when compared to those of their reference monomeric NLO chromophores.28,29 Such striking difference between these two kinds of push-pull macrocyclic NLO chromophores is due to the more parallel organization of the four dipolar NLO active tails and, therefore, to a lower θ opening angle, in the typical pinched conical structure of calix[4]arenes when compared to the optimized structure of NLO chromophores 1-3 based on a cyclotetrasiloxane ring, as confirmed by the known X-ray structures of some push-pull tetracalix[4]arenes.27-29 In conclusion our macrocy-


Ronchi et al.

TABLE 4: λmax of the ICT Absorption Bands, Dipole Moments, EFISH µβ1.91 Values Measured in CH2Cl2 Solution Working with an Incident Wavelength of 1.907 µm, and β0 Values of 1a-3a vs the NLO Chromophores Carrying a Nitro Group as Electron-Withdrawing Group, Measured in CHCl3 Solution

a The µβ values reported are the average of 16 successive measurements performed on each sample at about 10-3 M concentration. The standard deviation was about 10-15%. b Calculated by DFT. c Reference 32.

clic NLO chromophores 1-3 are characterized by a significantly high dipole moment, due to a framework of four dipolar organic tails all pointing in the same direction. This feature should allow a significant and more stable orientation of these macrocyclic NLO chromophores under the action of a strong electric field, when blended with a poly(methyl methacrylate) matrix, if compared to their reference monomeric NLO chromophores 1a-3a, as was shown to occur for some push-pull tetracalix[4]arenes with a cone conformation.29 In conclusion, if our very simple model can be considered an approximate representation of a fragment of a monolayer of quite independent organic push-pull NLO chromophores bound to a molecularly engineered silica surface, our EFISH investigation has experimentally confirmed that, being the NLO chromophores not enough parallel and perpendicular to the surface due to topology of the bonding sites on the silica surface, the second-order NLO properties of the monolayer are not the sum of the NLO properties of the single NLO chromophores but only the sum of their vectorial components along the direction perpendicular to the surface, due to a partial cancelation of the other components along the plane of the surface. 3.5. Withdrawing Effect of the Si(OSiMe3)3 Group. It is of some interest to discuss the EFISH quadratic hyperpolarizability of the new NLO chromophores 1a-3a. To our knowledge, the electron-withdrawing effect of the Si(OSiMe3)3 group in a classical push-pull organic NLO chromophores was never investigated. The values of the EFISH static quadratic hyperpolarizabilities β0 of the NLO chromophores 1a- 3a, obtained by the two level model,15 are quite similar to those of organic NLO chromophores with the same push-pull structure but with a nitro electron-withdrawing group (Table 4).32 This parallelism suggests that the Si(OSiMe3)3 group can be considered a pull group as strong as the nitro group. This suggestion is at first view unexpected because the ICT bands of 1a-3a are at higher energy than those of structurally similar push-pull organic NLO chromophores with the pull nitro group. Moreover the Si(OSiMe3)3 group produces a lower dipole moment of 1a-3a when

compared to that of structurally similar push-pull organic NLO chromophores with the nitro group, suggesting a limited electron-withdrawing property in electronic structure of the ground state (Table 4). The unexpected relevant pull properties of the Si(OSiMe3)3 group in 1a-3a could be thus originated by a high degree of charge redistribution on population of the excited-state involved in the ICT process possibly favored by the empty d orbitals of the silicon atom or by the σ* orbitals on the Si(OSiMe3)3 group.33 A careful solvatochromic investigation, carried out using both the absorption and emission spectra14 on the NLO chromophore 3a, has given a solvatochromic βCT value (βCT ) 49.3 × 10-30 esu) measured at 1.91 µm, comparable to the β1.91 value obtained by EFISH technique (Table 4), since the ICT charge transfer is located along the dipole moment axis. In particular the solvatochromic investigation produces clear evidence that 3a, even though it has a ICT band at higher energy than that of the structurally similar push-pull organic NLO chromophore with the pull nitro group, may have a comparable value of the quadratic hyperpolarizability, due to higher ∆µeg (7.19 D vs 5.7 D) and f (0.79 vs 0.37) values. This is more evidence that in push-pull NLO chromophores based on a diphenylacetilene structure the ∆µeg value plays a significant role in controlling the value of the quadratic hyperpolarizability.34 The higher value of the second-order NLO response (µβ1.91) of the ionic NLO chromophore 4 (Table 5) compared with those of neutral NLO chromophores 1-3 (Table 3) is basically originated by the more efficient push-pull system produced by the positive charge on the pyridine nitrogen atom.35 It must be pointed out that the macrocyclic NLO chromophore 4 better reproduces a monolayer on a molecularly engineered silica surface, because in its structure the cyclotetrasiloxane ring acts as a real inert scaffold and does not participate as a pull system to the second-order NLO properties as in macrocyclic NLO chromophores 1-3. Moreover the µβ1.91 values of the ionic NLO chromophore 4 and of its reference 4a appear to be strongly concentration



TABLE 5: Molar Conductivities Λ and EFISH µβ1.91 Values, Obtained Working with an Incident Wavelength of 1.907 µm, at Different Concentrations of 4a and 4 in CHCl3 Solution and at 298 K, Taking into Account That 4 Behaves as a Uni-Univalent Species with Four Independent Ionic Tailsa 4a concentration c 5.00 1.00 2.00 3.60 5.00 1.00 2.00 3.60 5.00 1.00 2.00 3.60 5.00 1.00 2.00 3.60 a

× × × × × × × × × × × × × × × ×

10-6 10-5 10-5 10-5 10-5 10-4 10-4 10-4 10-4 10-3 10-3 10-3 10-3 10-2 10-2 10-2

-1

Λ [S mol

1.90 (2) 1.30 (2) 1.01 (2) 0.78 (2) 0.68 (2) 0.52 (2) 0.38 (2) 0.30 (2) 0.26 (2) 0.21 (2) 0.17 (2) 0.16 (2) 0.16 (2) 0.18 (1) 0.20 (1) 0.30 (1)

2

cm ]

4 -48

µβ1.91 × 10

esu

-1

Λ [S mol

2

cm ]

µβ1.91 × 10-48 esu

n.d.[b]

4.39 (4) 3.85 (4)

11220

3490 3400

2.16 (4) 1.49 (4)

10000 8100

2120 1260

0.80 (6) 0.64 (8)

4790 2580

880 750

0.46 (1) 0.45 (1)

1710 1100

Reported are average Λ values (over the measurement replications indicated in parentheses). b Too diluted solution.

(b) whether 4 is more correctly represented as a one to four or as a four times one to one ionic species (assuming that the four ionic NLO-active tails are independent). The value of Λ is controlled by the dissociation degree R and by the ionic mobilities ui, both affected by concentration. For very diluted solutions of a strong electrolyte, characterized by R about 1 and mobilities ui significantly decreasing, a linear Λ vs c0.5 relationship (eq 5) should be obtained (Onsager test)37

Λ ) Λ0 - Q√c

(5)

0

where the intercept Λ is the “limiting” or infinite dilution molar conductivity. For very diluted solutions of a weak electrolyte, R should rapidly decrease but ion mobilities ui can be considered unaffected. In this latter case a linear 1/Λ vs cΛ relationship (eq 6) should be obtained for a uni-univalent weak electrolyte (Kraus and Bray test)37

Figure 3. Evidence of uni-univalent weak electrolyte behavior (Kraus and Bray test), in the case of the monomeric NLO chromophore 4a (circles) and of the macrocyclic tetrameric NLO chromophore 4 (squares), this latter being considered as a monomer with four times the basic concentration (4c).

dependent when measured in CHCl3 solution with a strong increase by decreasing concentration (Table 5). Such a behavior was reported by some of us for some Zn(II) complexes with weakly bound sulfonated ligands, which dissociate in CHCl3 solution into ions by decreasing concentration.36 We have thus carried out an extended conductometric investigation in CHCl3 solution at decreasing concentration of the NLO chromophores 4 and 4a. 3.6. The Effect of Concentration on the Second-Order NLO Response in CHCl3 of Ionic NLO Chromophores 4 and 4a. In order to correlate the increase of the µβ1.91 values with the ionization degree of 4 and 4a, molar conductivities Λ were measured in CHCl3 working at 298 K in the 5 × 10-6-10-2 M concentration range (Table 5). By applying the Onsager and Kraus/Bray tests,37 we tried to answer the following points: (a) whether ion pairs of 4 and 4a have in CHCl3 solution strong or weak electrolyte behavior;

1 cΛ 1 + ) Λ Λ0 K(Λ0)2

(6)

where K is the dissociation constant of the weak electrolyte. For both 4 and 4a in the 5 × 10-6 to 5 × 10-4 M concentration range, only a very good Kraus and Bray linear relationship (Figure 3) was obtained, supporting the view that both 4 and 4a behave in CHCl3 solution as weak uni-univalent electrolytes. We obtained also a satisfactory convergency of the two linear Kraus/Bray plots at infinite dilution (corresponding to a complete ionic dissociation), suggesting that the ionic NLO-active tails linked to the cyclotetrasiloxane scaffold are independent. In fact from the intercepts of the Kraus and Bray plots we obtained equal values of Λ0 for 4a (1.77 Ω-1 cm2 mol-1) and for 4 (1.80 Ω-1 cm2 mol-1), while the calculated electrolitic dissociation constants K are quite different (1.3 × 10-5 for 4a and 3.4 × 10-5 for 4). In summary, the conductivity of the macrocycle 4 is best treated as that of an uni-univalent weak electrolyte but with four times the basic concentration due to the presence of four indipendent ionic tails, while the structural vinculum of the cyclotetrasiloxane scaffold appears to be effective on the dissociation degree R, which is significantly higher for the single ionic tail of 4 than for the reference monomeric ionic NLO


Figure 4. Kraus and Fuoss test for evidence of aggregate formation, in the case of monomeric NLO chromophore 4a (circles) and macrocyclic tetrameric NLO chromophore 4 (squares), this latter being considered as a monomer with four times the basic concentration (4c).

Ronchi et al. and begins at lower concentrations, 10-3 M vs 4 × 10-3 M for 4a and 4 respectively). In conclusion as shown in Figure 5, the µβ1.91 values are in an excellent linear correlation with 1/R for both the NLO chromophores 4a and 4, confirming that the regular increase of the second-order NLO response µβ1.91 is due to an increasing ionic dissociation degree R. The linear plot for the single ionic tails of 4 is significantly higher than that of the reference ionic NLO chromophore 4a, in agreement with the evidence that the binding of the ionic tail to the cyclotetrasiloxane scaffold facilitates its ionic dissociation with parallel higher increase of the NLO response by decreasing concentration. This statement is supported by comparing, in Table 5, the molar conductivity Λ of 4a at 2 × 10-4 M concentration (0.38 S mol-1 cm2) vs the molar conductivity Λ of 4, taking into account four times the concentration of 4, 5 × 10-5 M (2.16 S mol-1 cm2), with a ratio about 1:6. At increasing concentrations the plots of both 4a and 4 are not linear anymore and the effect of ionic dissociation on the µβ1.91 value is not relevant (Figure 5). This is to be ascribed to progressive association into aggregates which, as just shown, is more facile for the monomeric species 4a than for the macrocyclic species 4. The much more facile ionic dissociation of the organic ionic tails nanoorganized on the cyclotetrasiloxane scaffold suggests that by their progressive ionic dissociation an increase of the solvent polarity is locally produced, due to their proximity, which facilitates ionic dissociation. This is interesting evidence of a significative cooperative effect, produced by the nanoorganization of the ionic tails on the cyclotetrasiloxane scaffold. 4. Conclusions

Figure 5. Correlation between the µβ1.91 value and the dissociation degree R for the monomeric NLO chromophore 4a (circles) and the macrocyclic tetrameric NLO chromophore 4 (squares), this latter being considered as a monomer at four times the basic concentration (4c).

chromophore 4a, when the same concentration of the tails of 4 and of 4a is taken into account. We have investigated also the possible formation of higher ionic aggregates at relatively high concentrations, by the Kraus and Fuoss equation,38 which, for a weak 1:1 electrolyte, as we have shown to be both 4 and 4a, implies a linear log Λ vs log c plot with -1/2 slope. Deviations from linearity with increasing concentration (in particular humps) indicate formation of higher aggregates. By assuming that the macrocyclic NLO chromophore 4 behaves as a 1:1 electrolyte with four times its basic concentration, we obtained the plots reported in Figure 4. Both plots (referring to 4a and to the single ionic tail of 4) nearly coincide in the linear region, but as expected for a significantly higher dissociation degree, 4 shows a minor tendency to form aggregates compared to the reference monomeric NLO chromophore 4a (the hump is much more evident

If our macrocyclic multichromophoric systems can be considered a very simple model of a monolayer of push-pull NLO chromophores on a chemically engineered silica surface, the results of our investigation suggest that the orientations of these NLO chromophores relative to the plane of the silica surface play a relevant role on defining its second-order NLO response while the nonbonding electronic interactions within the monolayer are not relevant. Such experimental evidence at the molecular level of the significant role of the orientation of the NLO active subunits of a monolayer, relative to silica surface, is in very good accordance with what proposed for layer-by-layer systems organized by chemical methods as a multilayer structure.39 In fact the average opening up angle θ calculated for our models such as the macrocyclic NLO chromophores 1-3 (around 46-50°) is quite close to the “tilt angle” (46-50°) calculated from experimental data for multilayers of similar organic NLO chromophores on a chemically engineered silica surface.6f,39 Our multichromophoric systems behave also as macrocyclic NLO chromophores which, by the organization of four dipolar NLO-active organic tails as an ensamble of virtually independent dipolar units held together in the same alignement, produce a large dipole moment, which should induce a better dipolar alignement of the macrocyclic NLO chromophores by electrical poling when blended in a polymeric matrix, together with a low tendency to thermal relaxation due to their size, when compared to the reference monomeric NLO chromophores. At the same time the macrocyclic NLO chromophores 1-3 maintain the absorption properties of the reference monomeric NLO chromophores 1a-3a and therefore their transparency window, but with an increased µβ0 second-order NLO response.

Properties of Macrocyclic NLO Chromophores Our work has shown also that, while by the nanoorganization of four neutral dipolar push-pull NLO-active organic tails on a cyclotetrasiloxane architecture, the single NLO active tail behaves as independent NLO chromophores, when the dipolar push-pull NLO-active organic tails are ionic, their nanoorganization on a cyclotetrasiloxane architecture produces a significant cooperative effect between the four tails with increase of the second-order NLO response by decreasing concentration. The origin of this cooperative effect is the more facile dissociation into ions, characterized by a higher second-order NLO response, of the single ionic tail of the macrocyclic system. In fact the nonbonding interactions between the various tails of the macrocyclic structure, not relevant from the electronic point of view when the tails are neutral, become electrostatically significant when the tails dissociate into ions. Probably their progressive dissociation into ions produces, due to their proximity, an increase of the local polarity of the solvent with parallel more facile ionic dissociation of the single tails and strong increase of the second-order NLO response of the macrocyclic NLO chromophore. Finally we have shown that the -Si(OSiMe3)3 group, never investigated up to now, seems to behave as a pull group of a classical organic push-pull NLO chromophore as strong as the nitro group. This result is quite unexpected considering the large blue shifts of the ICT band when compared with that of the analogous nitro-containing NLO chromophore. Although this comparison is actually limited to few examples and it is dependent on the accuracy of DFT-calculated dipole moments, it is quite probable that these pull properties have to be associated with an increase of ∆µeg and transition dipole moment reg values originated by a larger and more extended electronic transfer in the ICT excitation process. Acknowledgment. This work was supported by the MUR (FIRB 2003sResearch Title: Molecular compounds and hybrid nanostructured materials with resonant and non resonant optical properties for photonic devices; PRIN 2005 Research title: Progettazione ed autoorganizzazione di architetture molecolari per nanomagneti e sistemi optoelettronici), Fondazione CARIPLO 2005 (Research title: Nuovi materiali con nanoorganizzazione di cromofori in sistemi Host-Guest o su scaffold inorganico per dispositivi fotoluminescenti o optoelettronici), and by CNR (Project PROMO 2006). We thank Dr. S. Quici for useful discussions. References and Notes (1) (a) Barlow, S.; Marder, S. R. In Functional Organic Materials; Mu¨ller, T. J. J., Bunz, U. H. F., Eds.; Wiley-VCH Verlag GmbH & Co. KgaA: Weinheim, Germany, 2007; pp 393-437, and references therein. (b) Sanchez, C.; Lebeau, B.; Chaput, F.; Boilot, J.-P. In Functional Hybrid Materials; Gomez-Romero, P., Sanchez, C., Eds.; Wiley-VCH Verlag GmbH & Co. KgaA: Weinheim, Germany, 2004; pp 122-171, and references therein. (c) Kauranen, M.; Verbiest, T.; Boutton, C.; Teerenstra, M. N.; Clays, A.; Schouten, A. J.; Nolte, R. J. M.; Persoons, A. Science 1995, 270, 966–969. (d) Verbiest, T.; Samyn, C.; Boutton, C.; Houbrechts, S.; Kauranen, M.; Persoons, A. AdV. Mater. 1996, 8, 756–759. (2) (a) Proutie`re, S.; Ferruti, P.; Ugo, R.; Abbotto, A.; Bozio, R.; Cozzuol, M.; Dragonetti, C.; Emilitri, E.; Locatelli, D.; Marinotto, D.; Pagani, G.; Pedron, D.; Roberto, D. Mater. Sci. Eng., B 2008, 147, 293– 297, and references therein. (b) Vocanson, F.; Seigle-Ferrand, P.; Lamartine, R.; Fort, A.; Coleman, A. W.; Shahgaldian, P.; Mugnier, J.; Zerroukhi, A. J. Mater. Chem. 2003, 13, 1596–1602. (c) van der Boom, M. E. Angew. Chem., Int. Ed. 2002, 41, 3363–3366. (d) Ma, H.; Chen, B.; Sassa, T.; Dalton, L. R.; Jen, A. K.-Y. J. Am. Chem. Soc. 2001, 123, 986–987. (e) Shi, Y.; Zhang, C.; Zhang, H.; Bechtel, J. H.; Dalton, L. R.; Robinson, B. H.; Steier, W. H. Science 2000, 288, 199–122. (f) Kenis, P. J. A.; Noordman, O. F. J.; Scho¨nherr, H.; Kerver, E. G.; Snellink-Rue¨l, B. H. M.; van Hummel, G. J.; Harkema, S.; van der Vost, C.P.J.M.; Hare, J.; Picken,

J. Phys. Chem. C, Vol. 113, No. 7, 2009 2759 S. J.; Engbersen, J. F. J.; van Hulst, N. F.; Vancso, G. J.; Reinhoudt, D. N. Chem. Eur. J. 1998, 4, 1225–1234. (g) Yitzchaik, S.; Di Bella, S.; Lundquist, P. M.; Wong, G. K.; Marks, T. J. J. Am. Chem. Soc. 1997, 119, 2995– 3002. (h) Burland, D. M.; Miller, R. D.; Walsh, C. A. Chem. ReV. 1994, 94, 31–75. (3) (a) Acharya, S.; Krief, P.; Khodorkovsky, V.; Kotler, Z.; Berkovic, G.; Klug, J. T.; Efrima, S. New J. Chem. 2005, 29, 1049–1057. (b) Ashwell, G. J.; Zhou, D.; Skjonnemand, K.; Ranjan, R. Aust. J. Chem. 2001, 54, 19–25. (c) Li, F. Y.; Jin, L. P.; Huang, C. H.; Zheng, J.; Guo, J. Q.; Zhao, X. S.; Liu, T. T. Chem. Mater. 2001, 13, 192–196. (d) Schwartz, H.; Mazor, R.; Khodorkovsky, V.; Shapiro, L.; Klug, J. T.; Kovalev, E.; Meshulam, G.; Berkovic, G.; Kotler, Z.; Efrima, S. J. Phys. Chem. B 2001, 105, 5914– 5921. (e) Ricceri, R.; Neto, C.; Abbotto, A.; Facchetti, A.; Pagani, G. Langmuir 1999, 15, 2149–2151. (f) Ashwell, G. J. J. Mater. Chem. 1999, 9, 1991–2003. (4) (a) Facchetti, A.; Annoni, E.; Beverina, L.; Morone, M.; Zhu, P.; Marks, T. J.; Pagani, G. A. Nat. Mater. 2004, 3, 910–917. (b) Keinan, S.; Ratner, M. A.; Marks, T. J. Chem. Mater. 2004, 16, 1848–1854. (c) Keinan, S.; Ratner, M. A.; Marks, T. J. Chem. Phys. Lett. 2004, 392, 291–296. (5) (a) Cariati, E.; Ugo, R.; Cariati, F.; Roberto, D.; Masciocchi, N.; Galli, S.; Sironi, A. AdV. Mater. 2001, 13, 1665–1668, and references therein. (b) Be´nard, S.; Yu, P.; Audie`re, J. P.; Rivie`re, E.; Cle´ment, R.; Guilhem, J.; Tchertanov, L.; Nakatani, K. J. Am. Chem. Soc. 2000, 122, 9444–9454. (c) Marder, S. R.; Perry, J. W.; Yakymyshyn, C. P. Chem. Mater. 1994, 6, 1137–1147. (6) (a) Gulino, A.; Condorelli, G. G.; Mineo, P.; Fragala`, I. Nanotechnology 2005, 16, 2170–2175. (b) Gulino, A.; Bazzano, S.; Mineo, P.; Scamporrino, E.; Vitalizi, D.; Fragala`, I. Chem. Mater. 2005, 17, 521–526. (c) Facchetti, A.; Abbotto, A.; Beverina, L.; van der Boom, M. E.; Dutta, P.; Evmenenko, G.; Pagani, G.; Marks, T. J. Chem. Mater. 2003, 15, 1064– 1072. (d) van der Boom, M. E.; Evmenkenko, G.; Yu, C.; Dutta, P.; Marks, T. J. Langmuir 2003, 19, 10531–10537. (e) Zhu, P.; van der Boom, M. E.; Kang, H.; Evmenkenko, G.; Dutta, P.; Marks, T. J. Chem. Mater. 2002, 14, 4982–4989, and references therein. (f) Katz, H. E.; Wilson, W. L.; Scheller, G. J. Am. Chem. Soc. 1994, 116, 6636–6640. (g) Katz, H. E. Chem. Mater. 1994, 9, 2227–2232. (7) (a) Locatelli, D.; Quici, S.; Righetto, S.; Roberto, D.; Tessore, F.; Ashwell, G. J.; Amiri, M. Prog. Solid State Chem. 2005, 33, 223–232, and references therein. (b) Li, F. Y.; Huang, C. H.; Jin, L. P.; Wu, D. G.; Zhao, X. S. J. Mater. Chem. 2001, 11, 3002–3007. (8) (a) Di Bella, S.; Ratner, M. A.; Marks, T. J. J. Am. Chem. Soc. 1992, 114, 5842–5849. (b) Datta, A.; Pati, S. K. Chem. Eur. J. 2005, 11, 4961–4969. (c) Datta, A.; Pati, S. K. Chem. Soc. ReV 2006, 35, 1305– 1323, and references therein. (9) Ronchi, M.; Pizzotti, M.; Orbelli Biroli, A.; Macchi, P.; Lucenti, E.; Zucchi, C. J. Organomet. Chem. 2007, 692, 1788–1798. (10) Haddad, T. S.; Viers, B. D.; Phillips, S. H. J. Inorg. Organomet. Polym. 2002, 11, 155–164. (11) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991–1024. (12) Murov, S. L.; Charmichael, I.; Hug, G. L. In Handbook of Photochemistry, 2nd Ed.; Marcel Dekker: New York, 1993. (13) Bindhu, C. V.; Harilal, S. S.; Varier, G. K.; Issac, R. C.; Nampoori, V. P. N.; Vallabhan, C. P. G. J. Phys. D: Appl. Phys. 1996, 29, 1074– 1079. (14) Bruni, S.; Cariati, F.; Cariati, E.; Porta, F. A.; Quici, S.; Roberto, D. Spectrochim. Acta, Part A 2001, 57, 1417–1426, and references therein. (15) (a) Oudar, J. L. J. Chem. Phys. 1977, 67, 446–457. (b) Oudar, J. L.; Chemla, D. S. J. Chem. Phys. 1977, 66, 2664–2668. (16) (a) Levine, B. F.; Bethea, C. G. Appl. Phys. Lett. 1974, 24, 445– 447. (b) Singer, K. D.; Garito, A. F. J. Chem. Phys. 1981, 75, 3572–3580. (c) Ledoux, I.; Zyss, J. Chem. Phys. 1982, 73, 203–213. (17) Roberto, D.; Ugo, R.; Bruni, S.; Cariati, E.; Cariati, F.; Fantucci, P. C.; Invernizzi, I.; Quici, S.; Ledoux, I.; Zyss, J. Organometallics 2000, 19, 1775–1788. (18) (a) Kanis, D. R.; Lacroix, P. G.; Ratner, M. A.; Marks, T. J. J. Am. Chem. Soc. 1994, 116, 10089–10102. (b) Lesley, M. J. G.; Woodward, A.; Taylor, N. J.; Marder, T. B.; Cazenobe, I.; Ledoux, I.; Zyss, J.; Thornton, A.; Bruce, D. W.; Kakkar, A. K. Chem. Mater. 1998, 10, 1355–1365. (19) Brick, C. M.; Ouchi, Y.; Chujo, Y.; Laine, R. M. Macromolecules 2005, 38, 4461–4465. (20) (a) Cerveau, G.; Chappellet, S.; Corriu, R. J. P.; Dabiens, B. J. Organomet. Chem. 2001, 626, 92–99. (b) Elangovan, A.; Wang, Y. H.; Ho, T. I. Org. Lett. 2003, 5, 1841–1844. (21) Dewar, M.; Thiel, W. J. J. Am. Chem. Soc. 1977, 99, 2338–2339. (22) (a) Becke, A. D. Phys. ReV. A 1988, 38, 3098–3100. (b) Perdew, J. P. Phys. ReV. B 1986, 33, 8822–8824. (23) Schaefer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994, 100, 5829–5835. (24) (a) Alrichs, R.; Bar, M.; Haser, M.; Horn, H.; Kolmel, C. Chem. Phys. Lett. 1989, 162, 165–169. (b) Eichkorn, K.; Weigend, F.; Treutler, O.; Ahlrichs, R. Theor. Chem. Acc. 1997, 97, 119–124. (25) Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. P. J. Am. Chem. Soc. 1985, 107, 3902–3909.

2760 J. Phys. Chem. C, Vol. 113, No. 7, 2009 (26) Almenningen, A.; Bastiansen, O.; Fernholt, L.; Cyvin, B. N.; Cyvin, S.; Samdal, S. J. Mol. Struct. 1985, 128, 59–76. (27) Kenis, P. J. A.; Kerver, E. G.; Snellink-Ruel, B. H. M.; van Kummel, G. J.; Harkema, S.; Filipse, M. C.; Woudenberg, R. H.; Engbersen, J. F. J.; Reinhoudt, D. N. Eur. J. Org. Chem. 1998, 1089–1098. (28) Kelderman, E.; Derhaeg, L.; Verboon, W.; Erigbersen, J. F.; Harkema, S.; Persoons, A.; Reinhoudt, D. N. Supramol. Chem. 1993, 2, 183. (29) (a) Hennrich, G.; Murillo, M. T.; Prados, P.; Song, K.; Asselberghs, J.; Clays, K.; Persoons, A; Benet-Bucholz, J.; de Mendoza, J. Chem. Commun. 2005, 2747–2749. (b) Hennrich, G.; Murillo, M. T.; Prados, P.; Al-Saraierh, M.; El-Dali, A.; Thompson, D. W.; Collins, J.; Georghiou, P. E.; Teshome, A.; Asselberghs, J.; Clays, K. Chem. Eur. J., 2007, 13, 7753–7761. (30) Wang, S. L.; Gao, G. Y.; Ho, T. I.; Yang, L. Y. Chem. Phys. Lett. 2005, 415, 217–222. (31) Kenis, P. J. A.; Noordman, O. F. J.; Houbrechts, S.; Van Hummel, G. J.; Harkema, S.; Van Veggel, F.C. J. M.; Clays, K.; Engbersen, J. F. J.; Persoons, A.; Van Hulst, N. F.; Reinhoudt, D. N. J. Am. Chem. Soc. 1998, 120, 7875–7883. (32) (a) Cheng, L. T.; Tam, W.; Stevenson, S. H.; Meredith, G.; Rikken, G.; Marder, S. R. J. Phys. Chem. 1991, 95, 10631–43. (b) Cheng, L. T.;

Ronchi et al. Tam, W.; Marder, S. R.; Stiegman, A. E.; Rikken, G.; Spangler, C. W. J. Phys. Chem. 1991, 95, 10643–10652. (33) Bassindale, A. R.; Taylor P.G. In The Chemistry of Organic Silicon Compounds; Patai, S., Rappoport, Z., Eds.; John Wiley & Sons Ltd: Chichester, U.K., 1989; Part. 2, Chapter 14, pp 893-963. (34) (a) Stiegman, A. E.; Graham, E.; Perry, K. J.; Khundkar, L. R.; Cheng, L. T.; Perry, J. W. J. Am. Chem. Soc. 1991, 113, 7658–7666. (b) Dehu, C.; Meyers, F.; Bredas, J. L. J. Am. Chem. Soc. 1993, 115, 6198– 6206. (35) Alain, V.; Blanchard-Desce, M.; Ledoux-Rak, I.; Zyss, J. Chem. Commun. 2000, 353–354. (36) Tessore, F.; Roberto, D.; Ugo, R.; Mussini, P.; Quici, S.; LedouxRak, I.; Zyss, J. Angew. Chem., Int. Ed. 2003, 42, 456–459. (37) Harned, H. S.; Owen, B. B. In The Physical Chemistry of Electrolytic Solutions, 3rd ed.; Reinhold: New York, 1958; pp 286-291. (38) McInnes, D. A. The Principles of Electrochemistry; Reinhold: New York, 1947; pp 367-380. (39) Morotti, T.; Calabrese, V.; Cavazzini, M.; Pedron, D.; Cozzuol, M.; Licciardello, A.; Tuccitto, N.; Quici, S. Dalton Trans. 2008, 2974– 2982, and references therein.

JP8095242

Second-Order Nonlinear Optical (NLO) Properties of a

Recommend Documents