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Synthesis and third-order nonlinear optical studies of a novel four-coordinatedorganoboron derivative and a bidentate ligandThe effect of the N → B coordinative bond
Mario Rodrígueza, Rigoberto Castro-Beltrána, Gabriel Ramos-Ortiza,∗, José Luis Maldonadoa,Norberto Farfánb, Oscar Domínguezc, Jesús Rodríguezc, Rosa Santillanc,Marco Antonio Meneses-Navaa, Oracio Barbosa-Garcíaa, Jorge Peond
a Centro de Investigaciones en Óptica, Apartado Postal 1-948, 37000 León, Gto., Mexicob Facultad de Química, Departamento de Química Orgánica, Universidad Nacional Autónoma de México, México, D.F., 04510, Mexicoc Departamento de Química, Centro de Investigación y de Estudios Avanzados del IPN, 07000, Apartado Postal 14-740, México, D.F., Mexicod Instituto de Química, Universidad Nacional Autónoma de México, Circuito Exterior Cd. Universitaria, México, D.F., 04510, Mexico
a r t i c l e i n f o
Article history:Received 11 October 2008Received in revised form 18 February 2009Accepted 19 February 2009Available online 24 March 2009
Keywords:Boronates
a b s t r a c t
The third-order nonlinear optical characterization of a new boronate (2) derived from 4-dimethylaminocinnamaldehyde was performed by third-harmonic generation (THG) at the infraredwavelength of 1550 nm. Compound 2 was prepared from the reaction of diphenylboronic acid and thebidentate ligand (1) and characterization was made through UV, IR, 1H, 11B, and 13C NMR and X-ray diffrac-tion. The THG experiments showed that the N → B coordinative bond in 2 enhanced the second molecularhyperpolarizability of the type � (3)(−3�, �, �, �) by a factor of three with respect to the value exhib-ited by the ligand 1. On the other hand, Z-scan studies at 800 nm (femtosecond (fs) pulses) also showedthat such coordinative bond increased the nonlinear absorption (two-photon absorption (TPA)) in 2 with
NLO
Z-scan11B NMRX-ray
respect to 1. These studies demonstrate that the N → B coordinative bond facilitates the polarization ofthe electronic �-system, a situation that optimizes the third-order NLO response. Results on the excitedstate absorptions in these compounds are also presented.
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. Introduction
The need of new materials for applications in photonics suchs telecommunications, all-optical switching, data processing andptical limiting has inspired an intense research in organic com-ounds. These materials comprise highly polarizable �-electronragments, which make possible the observation of efficient non-inear optical (NLO) phenomena [1]. Furthermore, the organic
aterials are of major interest because of their relative low cost,asy integration, enormous design flexibility and their large andast nonlinear optical response [2–3].
In the search for novel organic materials, recently, molecular andolymeric boron-containing compounds have been investigated
ecause of their large and fast nonlinear optical response and poten-ial applications in optical communication, data processing, opticalwitching, optical limiting devices, etc. [4–12]. They are also ofnterest for sensors [13–14] and electroluminescence applications,
∗ Corresponding author.E-mail address: [email protected] (G. Ramos-Ortiz).
i.e., for the development of charge transport and luminescent mate-rials for OLEDs [15–20]. For these applications, three-coordinatedboron species have been widely studied due to the fact that theirvacant p-orbital is a strong �-electron acceptor which can lead tosignificant delocalization with an adjacent organic conjugated sys-tem. In contrast to the nitro group (a well known strong �-acceptor),the three-coordinated boron atom can also function as a �-donorbecause of its low electronegativity [6]. Additionally, boron hasbeen also utilized in 12-vertex clusters to form donor groups in�-conjugated systems [21].
On the other hand, four-coordinated boron has also been incor-porated into NLO organic systems. For instance, from experimental[22] and theoretical [23] studies it was demonstrated that thesecond-order NLO properties of boron complexes of stilbazolesand pyridines are enhanced with respect to their correspond-ing free-boron compounds; similarly, Zwitterionic borates showed
much larger second-order NLO properties and higher transparencycompared with their analogous uncharged push–pull species [24].Our group has studied second- and third-order nonlinearities infour-coordinated boron systems prepared from tridentate ligands[25–26]. These studies showed that boron derivatives of salicyli-
Fig. 1. Molecular structures of ligand 1 and boronate 2.
eniminophenols exhibit quadratic nonlinearities with the product× ˇ (� being the dipole moment and ˇ the first molecular hyper-
olarizability) ranging from 630 to 845 × 10−30 cm5 esu−1 D. Thesetudies also showed that the appropriate combination of groupsdonor–acceptor) and the formation of the N → B coordinative bondptimize quadratic nonlinear effects such as the second-harmoniceneration [26]. Additionally, the cubic nonlinearities in the boronomplexes reported by our group have been investigated exhibit-ng third-order nonlinear susceptibility (�(3)) values in the range of0−12 esu [25].
In continuation of our research on the structural-opticalesponse in boron-containing systems, and in order to get furthernsight about the effect of the �-electronic polarization and third-rder nonlinearities due to the formation of the N → B coordinativeond, in this work we present studies of nonlinear absorption andarmonic generation in a hetero-aromatic �-conjugate system con-aining a four-coordinated boron and a NMe2 donating group (seeig. 1); the nonlinear response of this system is compared with therecursor bidentate ligand.
. Experimental procedure
.1. Starting materials and equipment
All starting materials were purchased from Aldrich Chemical Co.olvents were used without further purification. Melting pointsere obtained with an Electrothermal 9200 apparatus and arencorrected. Infrared spectra were measured on a FT-IR spectrom-ter Spectrum RX1 PerkinElmer using KBr pellets. 1H, 11B and 13CMR spectra were recorded on Jeol FX 270 spectrometer. Chemical
hifts (ppm) are relative to (CH3)4Si for 1H and 13C and BF3·Et2Oor 11B. Mass spectra were recorded on a Hewlett Packard 5989 Apectrometer using the electron ionization technique at 20 eV. Theigh resolution mass spectrometry (HRMS) was performed with angilent G1969A electrospray ionization time-of-flight spectropho-
ometer using the APCI technique. UV–vis spectra were recordedn a PerkinElmer Lambda 900 spectrophotometer. X-ray diffractiontudies of single crystal were performed on a Kappa CCD diffrac-ometer. Solution and refinement: direct method SHELXS-86 fortructure solution and the SHELXL-97 [27] software package forefinement and data output.
.2. Synthesis and chemical characterization
2-[3-(4-Dimethylamino)allylideneamino]phenol (1). Theitle compound was prepared from 4-dimethylamino-trans-innamaldehyde (1.75 g, 10 mmol) and 2-aminophenol (1.09 g,0 mmol) under reflux of methanol for 1.5 h, to give 2.35 g8.8 mmol, 88% yield) of 1. M.P.: 145–146 ◦C. IR (KBr) �max: 3323,
Crystal data for 1: C17H18N2O, F.W = 266.34, monoclinic,space group, P21/c (No. 14), a = 6.3352(2), b = 12.8076(5),c = 18.0578(10) Å, ˛ = � = 90◦, ˇ = 97.054(2)◦, V = 1451.09(10) Å3,Z = 4, Dcalcd = 1.219 g cm−3, T = 293 K, 3257 unique reflections,(Rint = 0.0621). The structure was refined to R1 = 0.0547, wR2 = 0.1241for 2068 reflections with I > 2�(I) and 187 parameters, goodness-of-fit = 1.019. Crystal data for 2: C29H27B1N2O1, F.W = 430.33,Triclinic, space group Pı (No. 2), a = 11.2058(3), b = 12.1600(4),c = 18.9518(6) Å, ˛ = 97.218(2)◦, ˇ = 92.385(2)◦, � = 111.7470(10)◦,V = 2368.70(13) Å3, Z = 4, Dcalcd = 1.207g cm−3, T = 293 K, 9445unique reflections, (Rint = 0.0516). The structure was refinedto R1 = 0.0748, wR2 = 0.0509 for 7219 reflections with I > 2�(I)and 516 parameters, goodness-of-fit = 1.047. CCDC-705065 andCCDC-716722 contain the supplementary crystallographic datafor 1 and 2, respectively. These data can be obtained free ofcharge from the Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/request/cif.
2.3. Sample preparation
Compounds 1 and 2 were studied in solid polymer films forthird-harmonic generation (THG) experiments. In this case, ratiosof 70:30 wt.% of polystyrene and the molecule under test were dis-solved in chloroform. The solid films were then deposited on fusedsilica substrates (1 mm-thick) by using the spin coating technique.With this procedure the prepared films had typical thickness ofabout 120 nm and had good optical quality. The film thicknesseswere measured with a surface profiler (Dektak 6 M, Veeco). On theother hand, compounds 1 and 2 were also studied in chloroformsolutions for nonlinear absorption experiments (Z-scan).
2.4. Third-harmonic generation experiments
The second molecular hyperpolarizability � of 1 and 2 wasobtained from the third-order nonlinear susceptibility �(3)(−3�, �,
M. Rodríguez et al. / Synthetic Metals 159 (2009) 1281–1287 1283
F ne of tr ntactn e mot
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ig. 2. Crystal packing of ligand 1 and boronate 2. (a) Packing of 1 viewed from ac plaunning along a axis through intramolecular O–H· · ·N and intermolecular C–H· · ·O coeighboring molecules with the aid of C–H· · ·� contacts. H atoms not involved in th
, �), which was measured by using the THG Maker-fringes tech-ique at the IR wavelength of 1550 nm. Details of this experimentan be found in ref. [25]. Briefly, it consisted of a Nd-YAG laser-umped optical parametric oscillator (OPO) that delivered pulsesf 7 ns at a repetition rate of 10 Hz. The idler beam of the OPO sys-em tuned at 1550 nm was focused into the polystyrene films dopedith the compounds under test. Typical pump irradiance at sampleosition was about 0.5 GW/cm2. The third-harmonic beam emerg-
ng from the films was separated from the pump beam by usingcolor filter and detected with a PMT and a lock-in amplifier. TheHG measurements were performed for incident angles in the rangerom −40◦ to 40◦ with steps of 0.27◦. Whole of the experiment wasomputer controlled.
.5. Z-scan experiments
To study the nonlinear absorption properties of 1 and 2 in solu-ions, Z-scan measurements were performed with femtosecond (fs)nd nanosecond (ns) laser pulses. In the first case a femtosecondi:sapphire regenerate amplifier delivering pulses of 280 fs (1 kHzepetition rate) at 800 nm was used. In the second case, 7 ns pulsest 532 nm (10 Hz) were obtained from a frequency doubled Q-witched Nd:YAG laser. The laser pulses were focused into a beamaist radius of 29 and 20 �m for fs and ns pulses, respectively. The Z-
can curves were taken in a 1 mm quartz cell, which is less than theayleigh range for the focused beams. To avoid sample degradation,he peak irradiance was kept below 60 GW/cm2 and 90 MW/cm2 fors and ns excitation, respectively.
. Results and discussion
.1. NMR spectroscopy
The compounds 1 and 2 (see Fig. 1) were characterized throughH NMR experiments that showed a signal in 8.41 ppm correspond-ng to an imine proton (–N CH–) in 1, which is shifted to 8.30 ppmn 2 after coordination to boron. The vinyl fragment gives an ABM
ystem where the signal for H-8 appears in 6.93 and 6.88 ppm forand 2, respectively, and shows characteristic couplings with H-(J = 8.9 Hz in 1 and J = 10.6 Hz in 2) and H-9 (J = 15.5 Hz in 1 and
= 15.8 Hz in 2). The comparison of the JH8-H9 reported values [28]s in agreement with trans configuration in 1 and 2.
he unit cell, showing the formation of five-membered-rings and chains of moleculess. (b) Supramolecular structure of 2, showing the formation of interdimers betweenifs have been omitted for clarity.
The 13C NMR spectra of 1 exhibited a signal in 159.3 ppm corre-sponding to an imine carbon, which is shifted to lower frequencies(152.3 ppm) in 2 due to the polarization of the electronic �-systempromoted by the N → B coordinative bond. Another important sig-nal which shows a shift is C-1, which appears in 151.9 ppm in thefree ligand and is shifted around 9 ppm to higher frequency in theboronate (160.4 ppm), due to the partial � character exhibited bythe O–Cph bond; this is in agreement with the observations of theNMR analysis and with the X-ray diffraction data for the boronatederivatives of salicylideniminophenols [26]. The 11B NMR spectra of2 exhibited a signal with a chemical shift of 9.2 ppm correspondingto a tetracoordinate boron atom (from 6 to 12 ppm) [25,26,29].
3.2. X-ray characterization
The structures of 1 and 2 were corroborated by X-ray diffrac-tion. Fig. 2a depicts the crystal structure of 1, showing the twomono-substituted aromatic rings connected by an aza-butadienesystem with a trans arrangement, and also showing hydrogen bond-ing between the nitrogen (from iminic moiety) and oxygen (fromhydroxyl group) atoms which assure the boron complexation in 2and formation of a five-membered heterocycle containing N → Bbond [29,30].
The crystal packing of 1 showed an intra-molecular hydro-gen bond (D-H · · · A) between O(1)–H(1)· · ·N(1) with an A· · ·Hand D· · ·A distances of 2.05(3) and 2.6311(18) Å, respectively. Thisbond gives origin to the formation of a five-membered hetero-cycle which is nearly planar, with a torsion angle value 1.4(2)◦
for N(1)–C(6)–C(1)–O(1) fragment, and bond length values of1.4133(19), 1.401(2) and 1.366(2) Å, respectively. These values evi-dence certain degree of electronic delocalization over the fragment;this is deduced by comparing them with standard distancesreported in the literature [31]. Additionally, the inter-molecularhydrogen bond 3.428(2) Å between the imine proton and thehydroxyl O atom of a neighboring molecule related by a symmetrycode −1 + x, y, z, produces chains along the a axis.
On the other hand, the four-coordinated boron atom in 2observed by NMR spectroscopy was also confirmed by X-ray stud-
ies. The geometry around the boron atom resulted in a distortedtetrahedral with bond angles between 99.0(3)◦ and 116.1(3)◦. So,the unit cell of 2, depicted in Fig. 2b, is constituted by two moleculesfor each asymmetric unit. Furthermore, the boron coordinationgeometry forms a five-membered chelate ring, as it should be
1284 M. Rodríguez et al. / Synthetic Metals 159 (2009) 1281–1287
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3
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Fig. 4. Normalized transmittance T in open-aperture Z-scan experiments (fem-tosecond excitation) for ligand 1 (filled circles) and boronate 2 (open squares) at
2 2
using femtosecond excitation with peak irradiance of 53 GW/cm at
ig. 3. Linear absorption spectra of compounds 1 and 2 in chloroform (concentrationf 100 �M).
xpected with the aid of the newly formed N(sp2) → B coordinativend O − B covalent bonds, whose values are 1.625(6) and 1.505(5) Ån one molecule and 1.614(5) and 1.524(5) Å in the other, respec-ively. The consequence in the structural conformation derived fromhe boron complexation is that the N C(7) bond length in eachndependent molecule of the boronate is 1.315(5) and 1.307(4) Å,espectively, which is longer than the value of 1.283(2) Å shown byhe ligand. It is worth to notice that compound 2 exhibits a morelanar structure than 1, this is deduced from the dihedral angleetween the two aromatic rings present in the molecules, which are2.9(2) for 1 and 10.8(4) and −11.2(4)◦ for each one of the indepen-ent molecules obtained in 2. The analysis of the X–H· · ·Cg (�-ring)ontacts in the crystal structure of compound 2 shows C–H· · ·� con-acts (symmetry code x, y, z,) between the two crystallographicndependent molecules present in the asymmetric unit, formingupramolecular dimers in the crystal state.
.3. Linear and nonlinear optical characterization
Fig. 3 presents the linear absorption spectra for compoundsand 2 in solution (at the concentration of 100 �M). These
pectra are characterized by absorption bands attributed to(NMe2) → �*(aryliminic) transitions whose absorption maximare located at 400 and 510 nm for 1 and 2, respectively. More than00 nm of a red shift is observed from the ligand to the boronatendicating that the N → B coordinative bond polarizes the �-system.dditionally, the maximum of absorption showed a small decrease
n boronate 2 compared to the ligand 1.The NLO properties were first evaluated at the telecommunica-
ion wavelength of 1550 nm with the THG signal (517 nm) obtaineds a function of the angle of incidence for the pump beam inolystyrene solid films doped with 30 wt.% of 1 (120-nm-thick film)nd 2 (125-nm-thick film), respectively. In the Maker-fringes tech-ique, the THG peak intensity I3ω from the substrate-film structure
s compared to that produced from the substrate alone. Then, fol-owing the same procedure as in Ref. [25] the third-order nonlinearusceptibilities �(3)(−3�, �, �, �) resulted to be 4.4 × 10−12 and.5 × 10−12 esu for 1 and 2, respectively. Here, the nonlinear sus-eptibilities values measured for 2 are larger than those previouslyeported for other four-coordinated boronates synthesized in our
roup [25].
The macroscopic nonlinear susceptibility �(3) was used to cal-ulate the second hyperpolarizability � , which is the moleculararameter of interest, through 〈�〉 = �(3)/L4Ns where Ns is the den-
irradiance of 53 GW/cm ; and 2 at irradiance of 30 GW/cm (open circles). Inset:closed-aperture (S = 0.4) Z-scan curves for 1 (filled circles) and 2 (open squares) atirradiance of 53 GW/cm2. Solutions of 10 mM were employed. Continuous lines aretheoretical fit to experimental data.
sity of molecules in the polymer films, L = (n2 + 2)/3 is the correctionfactor due to local field effects and n is the refractive index. Assum-ing for the films the refractive index and density of polystyrene, andusing a molecular doping level of 30%, the � values for the boronateand its ligand resulted to be 4060 × 10−36 and 1440 × 10−36 esu,respectively. Then we have that the boronate exhibits an enhancednonlinear response of about a factor of three with respect to itsfree ligand. It must be observed, however, that although these cal-culations give the correct order of magnitude (10−33 esu) for thehyperpolarizabilities, the actual � values for both molecules are alittle smaller. This is because we assumed for the films the refrac-tive index of polystyrene but for doped polystyrene the refractiveindex is increased; this increment in the refractive index is partic-ularly notorious near the absorption band of the dopant, but at thewavelength of our interest (1550 nm) is just a residual effect thatslightly increases the factor L.
Note that the study of third-order nonlinearities in boron com-pounds through THG spectroscopy at infrared wavelengths hasnot been extensively reported. For example, in a study on thestructure-property in �-conjugated asymmetric and symmetricdimesitylboranes, performed by using THG at 1907 nm, maxi-mum second hyperpolarizability resulted about 229 × 10−36 esu[4–5,32,33]. The order of magnitude here reported for � in theboron compound 2 is certainly much larger than those reportedfor dimesitylboranes, but it must be considered that in the for-mer case resonant nonlinear response plays an important role asthe measurements were performed at 1550 nm, i.e., the � value isenhanced by a three-photon resonance since the THG wavelength(517 nm) almost coincides with the peak of linear absorption bandof the molecule. In the case of dimesitylboranes, however, the �values at 1907 nm are out of resonance. Another report on the THGresponse describes that boron subphthalocyanines exhibit resonantnonlinearities at infrared wavelengths comparable to the resonantvalue here reported for 2 [34,35].
The enhancement of NLO properties due to the coordinativeN → B bond was also observed for nonlinear absorption experi-ments. For instance, Fig. 4 shows the open-aperture Z-scan curvesfor 1 and 2 in chloroform at the concentration of 10 mM, in this case
2
the focal region. At this level of excitation it is clearly observed that2 exhibits nonlinear absorption while for 1 such kind of absorptionis practically zero or below the level of sensitivity of the experi-ment. The figure also presents the Z-scan (open-aperture) for 2 at
M. Rodríguez et al. / Synthetic Metals 159 (2009) 1281–1287 1285
Table 1Optical properties of ligand 1 and boronate 2 obtained with THG and Z-scan techniques.
Compound max (nm) � (3) (esu) (ns) at 1550 nm ˇ (cm/GW) (fs) at 800 nm ˇ (cm/GW) �0 (cm2) (ns) at 532 nm �exc (cm2)
1 400 1440 × 10−36 Too weak to be detecteda 9.2b × 10−8 6.9b × 10−18 13.0b × 10−18
ower excitation (30 GW/cm2). The nonlinear absorption coefficientfor this compound obtained from open-aperture Z-scan data was
btained using the relations [36]:
(z, s = 1)=∞∑
m=0
[−q0(z)]m
(m + 1)3/2whereq0(z)=
[ˇI0Leff
(1 + z2/z20)
]< 1 (1)
ere I0 is the peak irradiance, Leff = (1-exp(-˛0L))/˛0 the effectivehickness with L the sample thickness, while z is the sample positionnd z0 the Rayleigh range. By fitting the experimental data to Eq.1) the ˇ value for 2 resulted to be in average 3.7 × 10−11 cm/W foroth levels of irradiance shown in Fig. 4. To verify the validity of theeasurements, the Z-scan set-up was previously calibrated with
he standard CS2. Thus, owing to the type of excitation (femtosec-nd pulses far from the linear absorption band) it is then recognizedhat the origin of the nonlinear absorption exhibited by 2 is due tohe two-photon absorption (TPA) process. We remark that the exci-ation wavelength (800 nm) should produce resonant two-photonrocess in the case of compound 1 as its linear absorption spec-rum has a maximum peak at 400 nm; this fact however did notead to an observable TPA effect. In contrast, compound 2 is out ofesonance and exhibited appreciable TPA. We ascribe these resultso the formation of the N → B coordinative bond that increases the
onlinearities in the boronate with respect to its ligand. On thether hand, the inset of Fig. 4 presents the ratio of closed-aperture-scan curves for both 1 and 2 where the effect of nonlinear absorp-ion was eliminated by dividing these curves by their respectivepen-aperture curves. The closed-aperture Z-scan was obtained
Fig. 5. Main resonance forms fo
with aperture transmittance of 40% (S = 0.4). Here the nonlinearrefractive response for solutions of both molecules were positiveand did not differ appreciably from that exhibited by the solvent.
In the following, we compared the two-photon absorption cross-section �2PA for compound 2 with other values reported in theliterature for boron-containing molecules. �2PA can be evaluatedfrom �2PA = hˇ/N where h is the photon energy and N is thedensity of molecules in the solution, giving a value of 152 GM(1 GM = 10−50 cm4 s photon−1) for 2. Although this value is some-what moderate, it demonstrates the effectiveness of the N → Bcoordinative bond to enhance nonlinear absorption. In regard toother boron-containing compounds, the two-photon absorptioncross section value measured for 2 is in the range of those exhibitedby a series of push–pull type compounds comprising the trivalentboron B(Mes)2 group as electron acceptor. In those compounds themeasured �2PA values ranged from 36 to 300 GM when pumpedwith 200 fs pulses in the 700–900 nm wavelength range [37]. Thevalue of �2PA was further enhanced up to 377 [38] and 1340 GM[39] by using dimesitylboryl as end-groups in acceptor–�–acceptorquadrupoles, and up to approximately 1000 GM by using dime-sitylboryl as acceptor group in octupolar compound [40]; similarvalues (1350 GM) were observed in a �–�–� systems comprisinga cyclodiborazane core [41]. The two-photon absorption process
has also been reported in closo-dodecaborate clusters with �2PAof 35 GM [21]. On the other hand, very recent theoretical studiessuggest that values near to 104 GM are possible in three-branchedcompounds with a boron center [8,9]. Note that in all these citedworks the �2PA values were measured through two-photon excited
r ligand 1 and boronate 2.
1 tic Me
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286 M. Rodríguez et al. / Synthe
uorescent experiments about 800 nm with fs excitation; in ourase, however, the two-photon transition was directly measuredhrough nonlinear transmission experiments since 1 and 2 are notuorescent molecules. The use of the Z-scan technique with fem-osecond pulses allowed us to observe instantaneous two-photonransitions, i.e., other types of effects such as nonlinear absorptionue to excited state absorption are precluded with this very ultra-hort optical excitation. To evaluate nonlinear absorption effectsriginated by non-instantaneous two-photon transitions, in the lastection of the paper we present nonlinear absorption experimentsith nanosecond excitation.
Table 1 summarizes the nonlinearity enhancement observedrom 1 to 2 by using THG and TPA (Z-scan fs excitation) experi-
ents. This enhancement is explained as follows: the ligand andoronate have the neutral aromatic-like resonance forms 1a and 2a,espectively, but upon photoexcitation there is a redistribution ofhe �-electrons over the backbone to produce the resonance formsb and 2b as shown in Fig. 5 Thus, the presence of the N → B bondavors the increase of the nonlinear response due to a larger con-ribution to the charge transfer process in the resonance structureb and probably due to the fact that the boron atom (in 2b) sta-ilizes the negative charge more efficiently than the nitrogen inb (even though this nitrogen atom forms part of an acceptor inn intramolecular hydrogen bond with the OH group), at the timehat the nitrogen–boron bond changes from coordinative in 2a to aigma bond in 2b.
We also measured the nonlinear absorption of molecules 1 andwith nanosecond laser excitation at the visible wavelength of
32 nm, where the compounds show linear absorption. To limit theevel of linear absorption the solutions were diluted to 100 and5 �M for 1 and 2, respectively. Fig. 6 shows the obtained open-perture Z-scan curves using irradiance of 88 MW/cm2. Followingimilar data fitting as in the fs excitation case, the ligand 1 exhibitsˇ value of 9.2 × 10−8 cm/W while the boronate 2 now exhibits aegative ˇ value of −7.8 × 10−8 cm/W. In contrasts to the simulta-eous two-photon absorption that occurred in 2 under fs excitationFig. 4), in the nanosecond regime it is reasonable to assume sequen-ial single-photon absorption to explain the nonlinear effect [36].he observed nonlinear transmission in Fig. 6 is then explained by ave-level model involving singlet Sn (n = 0, 1, 2) and triplet Tn (n = 1,
) manifolds. Initially, the linear absorption at 532 nm is responsibleor the transition to the singlet excite state S1 from the ground state0; then, the relative long duration of the pulse allows further pro-otion to the higher excited singlet state S2 or to the excited triplet
ig. 6. Normalized transmittance T in open-aperture Z-scan experiments (nanosec-nd excitation) for ligand 1 (filled circles) and boronate 2 (open squares) at irradiancef 88 MW/cm2. Solutions of 100 and 25 �M were employed for 1 and 2, respectively.ontinuous lines are theoretical fit to experimental data.
[
[[[
tals 159 (2009) 1281–1287
state T2 (provided that intersystem crossing takes place from thefirst excited state S1 to the first triplet state T1). Note that this kindof multi-photon transitions were not present for the femtosecondexcitation because at 800 nm the linear absorption was too weakand because such ultra-fast excitation is much shorter than the typ-ical lifetime of excited states in organic compounds. It follows that1 exhibits reverse saturable absorption (RSA) since the excited stateabsorption dominates over the initial absorption, as proved by com-puting the ratio between the excited (�exc) and ground (�0) stateabsorption cross sections, which is �exc/�0 ∼ 1.9 (see Table 1). Toperform this calculations, and owing to the fact that the populationof excited states depends on the fluence F0 rather than the irra-diance, it was necessary to replace ˇI0 by ˛0�excF0/2h in Eq (1).Note that for 2 the ratio becomes �exc/�0 ∼ 0.0013 such that nowthe absorption saturates, which means that during the interval oftime when excitation is present (nanosecond interval), the groundstate population is significantly depleted.
4. Conclusions
In summary, the formation of the N → B coordinative bondincreases the nonlinearities in hetero-aromatic �-conjugate sys-tems. In particular, under ultra-fast (fs) excitation at 800 nm thenonlinear process of TPA was increased in the boronate 2 withrespect to its ligand 1, i.e., the boronate exhibited two-photonabsorption characterized by a cross section of 152 GM, while sucheffect was practically absent for the ligand. These results suggestthat the N → B bond allows more effective delocalization of the�-system and that the boron atom stabilizes the negative chargemore efficiently than the nitrogen in the ligand. On the other hand,through THG spectroscopy the resonant second hyperpolarizabil-ity � for the boronate resulted in the order of 10−33 esu, this undernanosecond excitation at the telecommunications wavelength of1550 nm. The compounds here studied also exhibited excited stateabsorptions at the visible wavelength of 532 nm with ns excita-tion: saturable absorption for the boronate and reversible saturableabsorption for the ligand. Further work is in progress in order toincorporate the N → B coordinative bond to more efficient organicsystems in order to improve their nonlinear response.
Acknowledgements
This work was supported by CONCyTEG (Project 07-04K662-080A3), CONACyT (Projects J49512F and 58783) and UNAM (PAPIIT IN-203207). The authors thank Rafael Espinosa for the film thicknessmeasurements.
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