J. MATER. CHEM., 1991, 1(6), 997-999 Trivalent Boron as Acceptor Chromophore in Asymmetrically Substituted 4,4'-Biphenyl and Azobenzene for Non-linear Optics Minh Lequan,* Rose Marie Lequan and Kathleen Chane Ching Laboratoire de Chimie et d'Electrochimie des Materiaux Moleculaires, Ecole Superieure de Physique et de Chimie lndustrielle de Paris, CNRS UA 429, 10 Rue Vauquelin, 75231 Paris, Cedex 05, France [4'-( Di methyl am ino) bi phenyl-4-ylldi mesi tyl borane (BN B) and [4-(d imethyl am ino) phenylazophenyl-4-yl]d imesityl-borane (BNA) have been prepared and studied by a solvatochromic technique. The high second-order hyperpolarisability coefficients PCTdetermined by this technique, 37 and 240 x esu,t respectively, show that this new family of materials is of interest for non-linear optics.Keywords: Boron; Solvatochromic technique; Non-linear optical material The search for materials possessing a large hyperpolarisability coefficient, /j, for potential applications in non-linear optics has been developed extensively in recent years. So far, exper- imental efforts have been focused on benzene, stilbene or azo dyes substituted by organic donor and acceptor moieties, the latter being generally limited to nitro, nitrile, pyridinium or sulphone groups. The use of Lewis acids as electron-acceptor chromophores has been suggested recently by Kanis et al.' These novel groups should be interesting because, according to the theor- etical calculations of the authors, they could enhance the hyperpolarisability coefficient.However, Zheng Yuan et al. * have published the results of their investigation on a series of boron compounds prepared principally by addition of borane to acetylenic derivatives. In a previous paper3, the use of the phosphonium cation Ph2MeP+ or phosphine oxide Ph,P-+O as electron-acceptor groups combined with amine or borate anions as electron donors, was mentioned. We now propose a trivalent boron species as the acceptor chromophore and a dimethylamino group as the donor, substituted in positions 4 and 4 of a biphenyl (BNB) or azobenzene (BNA) backbone. The results will be compared with those obtained for phosphorous derivatives. BNB BNA The present paper reports the synthesis of these compounds, the results of solvatochromic studies in both absorption and emission spectroscopy and the determination of hyperpolaris- ability properties in solution.Experimental Synthesis Fluorodimesitylborane was synthesised by Brown's meth~d.~ The 4'-bromodiphenyl-4-ylamine was obtained by reduction of the corresponding nitro derivative5 by NaBH, in ethanol according to the procedure described in the literature.6 The amino group was methylated into a dimethylamino group by trimethyl pho~phate.~ The [4'-(dimethylamino)biphenyl-4-yl]dimesitylborane (BNB) was prepared according to the method used for aryl- dimesitylboranes.8 6.5 cm3 of BuLi (1.5 mol dm-3 in hexane) was added to a solution of 2.76 g mol) of 4'-bromobiphe- nyl-4-yldimethylamine in 50 cm3 of distilled and deaerated tetrahydrofuran (THF) at -60 "C under argon.The tempera- ture was allowed to reach -40 "C then 2.6 g of fluorodimes- itylborane in 10cm3 of THF were introduced. The mixture was heated under agitation at 50 "C for 3 h. The reaction was hydrolysed and the product extracted with ether. The organic solution was dried over magnesium sulphate and the solvent evaporated to yield the crude product which was recrystallised in acetonitrile to give 2.9 g, m.p. 156 "C. Elemental analysis confirmed the expected formula. dH in CD,COCD,: 2.9 (NCH,); 2.3 (p-CH3); 2.03 (0-CH3); 6.81 HA; 7.63 HB; JHAHB =9 Hz; 7.66 HA,; 7.5 HB'; JHA,HB, =9 Hz. The (4'-bromophenylazophenyl-4-yl)dimethylaminewas prepared by electrophilic substitution of N-dimethylaniline by the diazonium salt derived from p-bromoaniline.The [4'- (dimethylamino)phenylazophenyl-4-yl]dimesitylborane(BNA) was synthesised by a similar procedure to that used for BNB. Recrystallisation in acetone gave red crystals, m.p. 180 "C. Satisfactory elemental analysis was obtained. 6, in CD3COCD3: 3.1 (NCH3); 2.26 (p-CH,); 2.00 (0-CH,); 6.83 HA; 7.86 HB; JHAHa 9.3 Hz; 7.8 1 HA-; 7.58 HB,; JHA,Hs, =8.4 Hz. Dipole Moment Determination The permittivities E for different concentrations of solutes in toluene (0.1-0.02 mol dmP3) were derived from the measured capacitance C, of solutions by the use of a homemade dipole meter equipped with a cylindrical nickel-plated condenser cell. The cell's constants were determined for four weakly polar reference solvents, these being carbon tetrachloride, cyclohex- ane, dioxane and toluene (HPLC grade).Measurement of the capacitance of the cell filled with reference solvents and knowledge of the corresponding relative permittivity of sol- vents, gives the cell's constants via linear regression C, =AE+ B which allows calculation of the permittivity E of each solution. The refractive indices were measured with an Abbe refrac- tometer. The temperature control for both dipole meter and refractometer was achieved by circulating water from a ther- moregulator adjusted to 20 +0.5 "C. The dipole moments pg were calculated using the Guggen- heim-Debye eq~ation.~ UV-VIS Absorption and Fluorescence Absorption and fluorescence spectra were recorded with an UVIKON 860 spectrometer and a FICA 55 fluorimeter.All solvents used were HPLC grade purchased from Aldrich and kept dried over molecular sieves. Results and Discussion The contribution of a trivalent boron chromophore as an acceptor group in the delocalisation of the lone pair from the amino group, in the ground state, can be estimated from the corresponding lLma,measured in UV- VIS absorption and compared with the withdrawing power of phosphine oxide P+O, the phosphonium cation P+ and the nitro group. In this case, the electron donor is the N,N-dimethylamino group for all the compounds. The dimesityl boron chromophore is seen to be as good an electron attractor as a nitro group.When the aromatic rings of biphenyl are separated by a diazo N=N bridge the delocalisation is greater and the absorption reaches 440 nm. In contrast, the phosphine -P< is a very poor donor group (Table 1). Moreover, a red shift appeared clearly for each compound when the polarity of solvents was increased. In fluorescence spectra, the red shift was also observed for BNB (Table2; Fig. 1). By contrast, no fluorescence was detected for BNA, the result of either a non-radiative deactivation or an emission in the IR region. This sensitivity to the polarity of the solvent allowed investigation of these new materials by a solvato-chromic technique. The variation of their dipole moment between the excited and the ground states was determined according to Varma and Groenen's method." Two parameters Xij and Yij were defined for various couples of solvents i and j, Xij depending on the relative permittivities and refractive indices and Yij the wavenumber corresponding to the maxima of the absorption or fluorescence bands of the solute and the same refractive indices (Fig.2). Xij= Si- Sj/Qi-Qj with Si=(E~-1)/(2~,+1) Qi= (n? -1)/(2n?+1) y..IJ =v.J -fi/Q.1 J-Q . vi = 107/~,,,~, Table 1 UV-VIS absorption of 4X (C,H,),N(Me), in ethanol Br 308 Ph,P+O 340 Ph, MeP 360+ Mes,B (BNB) 373 NO2 373.911 Mes, BC,H,N= NC,H,N( Me), 440 Ph,MeP'(C,H,),PPh, 2753 J. MATER. CHEM., 1991, VOL. I Table 2 Absorption (abs.) and emission (emiss.) of BNB and BNA in solvents with increasing polarity 'TI,, solvent abs.(BNB) emiss. abs. (BNA) cyclohexane 367.5 -426.5 dioxane 375 -438.7 carbon tetrachloride 369.5 -432.2 toluene 378 -440.5 ethyl acetate 373.7 489 440 dichloromethane 376 48 8 446.2 acetone -505 448 ethanol 373.2 495 440 acetonit rile 373.7 51 1 448 dimethylformamide 383 513 457.5 dimethyl sulphoxide 386.2 515 468 Plotting Yij us. Xij gave a straight he, the slope, b, of which is from absorption data and be, =2peAp/hca; (2) from emission data, where pg is the ground-state dipole moment, pe the excited-state dipole moment, Ap=pe -pg, h= Planck's constant, c =the velocity of light, a. =the radius of the spherical cavity occupied by the solute in the solvent (Onsager's cavity) which remains unchanged for the ground and excited states.pg was determined from dielectric measurements by the use of the Guggenheim-Debye eq~ation:~ pi =(9k,T/4dV*)[3/(~0 +2)(ni +2)]A where E =relative permittivity of solutions determined from 1 350 400 500 600 A/nm Fig. 1 (a) UV absorption spectrum for BNB in acetonitrile, (b) UV absorption spectrum for BNA in acetonitrile. Molar absorption coefficients: (a) 28 000dm3 cm-' mol-'; (b)37 500 dm3 cm-' mol-'. (c) fluorescence spectrum of BNB, in (------) ethyl acetate, (----) ethanol, (-) acetonitrile, (----) dimethyl sulphoxide J. MATER. CHEM., 1991, VOL. 1 the cell's constant equation, E, =relative permittivity of pure nsolvent (t~luene),'~ =refractive index of solutions, no= refractive index of pure solvent (t~luene),'~ NA=Avogadro's number, kB=Boltzmann's constant, A =slope of the curve (E -n')=/lC) with C in mol cmP3 and pg in D.? For BNB, pe can be estimated from the experimental value of pg (2.0 D) and eqn.(3) deduced from eqn. (1) and (2) pe =(bern/babs)Pg (3) Hence pe= 11 D and Ap=9 D. This calculation does not take account of Onsager's cavity for BNB; nevertheless, it can be calculated from eqn. (1) (a, = 4.0A) and compared to the value determined by immersion of a molecular model in water (a, =4.1 A).'. This result shows that the immersion technique may be a valuable method for estimation of the solvent cavity. In the case of BNA for which fluorescence data cannot be obtained, the parameter a, was determined by this method (ao=4.2 A).Using eqn. (1) we obtained Ap= 15 D for an experimentally measured value of pg=3.2 D. The hyperpolarisability coefficients can be now calculated from the dipole-moment variation of BNB and BNA in making use of Blombergen's equation, PCT =(3fi2e2/2m)F(@/fAlL (12) where e=the charge of an electron, m=the mass of an 70 000' 30 000 -1 0000 's-50000 . . * . . '. . . . . . ' ' * . .' -20 -15 -10 -5 0 5 10 15 20 150 50 -50 -1 50 -1 0 -5 0 5 10 ,-= -20 -1 0 0 10 20 Xij Fig. 2 Yij us. Xijobtained for BNB (a)and (b),and BNA (c).Linear-regression curves: (a)Yij= 17810+2818Xij,R=0.91 (absorption data); (b)Yij=5844 +15570Xij, R =0.99 (fluorescence data) (c)Yij= 20770 +5531Xi,, K =0.96 (absorption data) t 1 Dz3.335 64 x lop3' C In.electron,f= the oscillator strength derived from the area under the absorption band and F(o)=the frequency factor depen- dent on the transition energy of the solute and the laser energy, in the present case 1.17 eV. For BNB, f=0.63 (CH,Cl,), PCT=37 x lop3' esu CGS and Po= 16.5 x esu (at zero frequency). For BNA, f=0.90 (CH,CI,) PCT= 2lO~lO-~~esu CGS and ~,=50x10-30esu. These results show clearly that the trivalent boron chromo- phore behaves as a very good electron acceptor, as good as the nitro group. It is of interest to compare the experimental hyperpolarisability coefficient of BNB (37 x lop3' esu) to the theoretical value of the corresponding nitro compound calcu- lated by Morley et al." (31 x lop3' esu), recalling that PCT obtained by the solvatochromic method represents ca.75% of the overall effect. Moreover, a considerable enhancement of the molecular effect occurs when the two aromatic rings of the biphenyl are separated by an unsaturated bond, here, a diazo chain. The mesityl boron derivatives are stable to air, insensitive to light and can be recrystallised easily in organic solvents. X-Ray structure is under investigation, nevertheless one might expect, as is frequently the case for organic crystals, a centro- symmetric space group. However, the utility of such molecules in non-linear optics results in the possibility of incorporating organic functional groups, e.g. hydroxy or halogen groups, either in N-substituted alkyl chains or in aromatic mesityl groups.These functional groups allow attachment of the new chromophores to a polymeric matrix and polarisation under electric field to obtain a non-centrosymmetric material. Conclusions We have shown the preparation of a new family of organic materials possessing an important dipole-moment variation when excited by electromagnetic radiation. The use of a trivalent boron chromophore as an electron acceptor gives very promising results for obtaining high hyperpolarisability coefficients. The biphenyl derivatives and their homologues may be a good choice for potential applications in the field of non-linear optics. 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