Optical Nonlinearity in (EDT-DSDTFVO)2·FeBr4, an Intriguing

Optical Nonlinearity in (EDT-DSDTFVO)2·FeBr4, an Intriguing Molecular Material with Metallic Conductivity, Magnetoresistance Effects, and Quadratic N...
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J. Phys. Chem. C 2010, 114, 21762–21769

Optical Nonlinearity in (EDT-DSDTFVO)2 · FeBr4, an Intriguing Molecular Material with Metallic Conductivity, Magnetoresistance Effects, and Quadratic Nonlinear Optical Properties Pascal G. Lacroix,*,† Toshiki Hayashi,‡ Toyonari Sugimoto,‡ Keitaro Nakatani,§ and Vincent Rodriguez# Laboratoire de Chimie de Coordination du CNRS, 205 route de Narbonne, 31077 Toulouse, France, Department of Chemistry Graduate School of Science, Osaka Prefecture UniVersity, Osaka 599-8570, Japan, PPSM, Ecole Normale Supe´rieure de Cachan, UMR 8531, 61 AVenue du Pdt Wilson, 94235 Cachan, France, and Institut des Sciences Mole´culaires, UMR 5255 CNRS, UniVersite´ de Bordeaux, 351 Cours de la Libe´ration, 33405 Talence, France ReceiVed: October 8, 2010

The quadratic (∝ E2) nonlinear optical (NLO) properties of (EDT-DSDTFVO)2 · FeBr4 have been investigated: an efficiency equal to 3-6 times that of urea in second-harmonic generation at 1.907 µm has been observed by a powder test, and an effective (deff) nonlinear optical susceptibility (χ(2)) tensor component equal to 110 pm/V at 1.064 µm has been determined by confocal µ-SHG in the backscattering geometry. The origin of this effect is ascribable to the presence of strongly interacting EDT-DSDTFVO species organized in noncentrosymmetric stacks in solid state. A computational (ZINDO) investigation carried out on a [(EDTDSDTFVO)4]2+ tetrameric entity at the fractional (F ) 0.5) oxidation state leads to a static molecular hyperpolarizability (β0) equal to 854 × 10-30 cm5 esu-1 per tetrameric unit. (EDT-DSDTFVO)2 · FeBr4 is the first multifunctional molecular material exhibiting simultaneously conducting, magnetic, and NLO effects. The possibilities of using such absorbent species as micrometer size materials with efficient NLO properties are thoroughly evaluated. Introduction The last few decades have witnessed an increasing interest for molecular materials and have led to the emergence of new magnets,1,2 molecular devices for data storage,3,4 conductors and superconductors,5-7 and second-order nonlinear optical (NLO) materials with ultrafast response time and enhanced NLO efficiencies.8,9 These materials offer benchmark units, which can be used to test theoretical models describing the physical properties of solids, by means of molecular parameters: e.g., transfer integral (τ) between overlapping species in one-dimensional conductors, coupling constant (J) between metal centers in paramagnetic chains, and quadratic molecular hyperpolarizability (β) in molecular nonlinear optics. Furthermore, molecular materials offer a unique opportunity to meet additional challenges, such as the design of multifunctional materials, in which molecular properties could be linked at the microscopic scale, with the challenging target of their possible interplay. To date, few reports have been devoted to hybrid materials exhibiting two electronic properties: (i) magnetic-conductors, (ii) magneticNLO materials, and (iii) intriguing structures combining NLO response and intermolecular electron delocalization. (i) Linking conductivity and magnetism has been stimulated by the discovery of superconductivity in paramagnetic materials in 1995,10,11 although both properties had been considered to be somewhat inimical for quite a long time. The restoration of * Author to whom correspondence should be addressed. E-mail: [email protected]. † Laboratoire de Chimie de Coordination du CNRS. ‡ Osaka Prefecture University. § PPSM. # Institut des Sciences Mole´culaires.

a metallic state has been observed in a paramagnetic conductor, as a step toward an efficient interplay between both properties.12 A molecular ferromagnet with metallic behavior has also been reported.13 (ii) Few investigations devoted to NLO properties in magnetized molecular materials have appeared in the literature.14 They were occasionally discussed in relation to the concept of Faraday rotation,15 a property which has the potential to modify the propagation of the light and hence to modulate the NLO response, the β value being not affected. A possible interplay between nonlinear optics and magnetism has also been envisioned in polynuclear paramagnetic species16 and in spin crossover complexes.17,18 (iii) Finally, the issue of designing molecular material with through-space charge transfer and NLO response has also been addressed both theoretically19,20 and experimentally.20a,21 Although long-range electron delocalization usually results in very weak transparencies over a large frequency range, both conducting and NLO behaviors were observed in some cases.22 In a research effort aiming at connecting three properties (instead of two) in a single crystal cell, and to expand the range and complexity of molecular materials, we now wish to investigate the possibility to combine simultaneously magnetic effects, metallic behavior, and solid-state NLO efficiency in a molecular material. The main bottleneck in this approach is related to the fact that molecular metals quite invariably crystallize in centrosymmetric space groups, with the deleterious effect of cancelation of any quadratic (∝ E2) NLO effect. For instance, a statistical search conducted from the Cambridge Crystallographic Data Base (CCDB) on every reported structure based on the conducting precursor tetracyanoquinidimethane

10.1021/jp109646u  2010 American Chemical Society Published on Web 11/19/2010

Optical Nonlinearity in (EDT-DSDTFVO)2 · FeBr4 SCHEME 1

(TCNQ, Scheme 1) has revealed that only four structures among six hundreds entries actually show non-centrosymmetric TCNQ stacks.20a A previous report, based on nickel bis(dithiolene) metal complexes ([Ni(dmit)2], Scheme 1), has revealed only two noncentrosymmetric structures in about one hundred.23 A possible strategy toward non-centrosymmetric molecular metals could be based on the use of non-centrosymmetric organic precursors, such as ethylenedithiodiselenadithiafulvalenoquinone-1,3-dithiolemethide and ethylenedithiotetrathiafulvalenoquinone-1,3dithiolemethide (EDT-TTFVO and EDT-DSDTFVO, in Scheme 1). Recently, ferromagnetic interactions and metallic behaviors were reported in (EDT-TTFVO)2 · FeBr4, while metallic behavior and magnetoresistance effects were observed in (EDTDSDTFVO)2 · FeBr4.24 Furthermore, this latter material crystallizes in the non-centrosymmetric Cmc21 orthorhombic space group, which leads to the possibility of solid-state NLO efficiency. In the present contribution, the NLO capabilities of this compound are investigated and discussed on the basis of a computational approach conducted on a simplified oligomeric [(EDT-DSDTFVO)4]2+ fragment, used to provide a qualitative picture of the charge transfer behavior, as well as a rationale for the origin of the NLO response in this hybrid material. Experimental Section Starting Materials and Equipment. (EDT-DSDTFVO)2 · FeBr4 was obtained as microcrystals by electrochemical oxidation of EDT-DSDTFVO at constant current, in the presence of NBu4FeBr4 supporting electrolyte, following the procedure previously reported.24a Optical spectra were recorded in KBr matrix (2 wt.-% NLO material) on a Perkin-Elmer lambda 35 UV/vis spectrometer and on a Perkin-Elmer Spectrum GX FTIR system spectrometer. A spectrum of urea (2 wt.-% in KBr matrix) was used as a “blank” for the measurement to account for the screening of the light within the KBr disks. NLO Measurements. The NLO properties of (EDTDSDTFVO)2 · FeBr4 have been tentatively investigated as the solid-state efficiency in second harmonic generation (SHG). SHG measurements were carried out by the Kurtz-Perry powder technique,25 using a picosecond Nd:YAG pulsed (10 Hz) laser operating at λ ) 1.064 µm. The outcoming Stokesshifted radiation at 1.907 µm generated by Raman effect in a hydrogen cell was used as the fundamental beam for second

J. Phys. Chem. C, Vol. 114, No. 49, 2010 21763 harmonic generation. The SHG signal was detected by a photomultiplier and recorded on a Tektronic 7834 oscilloscope. To overcome problems related to the strong absorbance of the conducting material, signals were recorded on pressed pellets, following a method described in the literature26 and previously used in the case of conducting materials.22 KBr disks were prepared (10 mm diameter, 70 mg) with e1 wt.-% NLO material. A sample of urea (15% in KBr) was measured under the same conditions and its efficiency used as a reference. Under the present conditions, the optical losses due to absorption in the (EDT-DSDTFVO)2 · FeBr4 sample were found to fall approximately in the range of magnitude of those due to scattering in the urea/KBr pellet, with an absorbance ≈2 measured in all instances, and the SHG efficiencies were therefore assumed to be proportional to the λ/2 signal divided by the concentration of the materials in the KBr disks. No other corrections were made to account for the absorbance of the conducting material. (EDT-DSDTFVO)2 · FeBr4 and urea used to prepare the pellets were microcrystalline powders obtained by grinding and were calibrated in the 0-50 µm range. They were carefully mixed with grinded KBr, but no additional grinding was performed before packing the samples in a press (7 tons, 2 mn). In another approach aiming at reducing the deleterious effect of light absorption by the conducting material, the SHG signal was measured by the technique of confocal µ-SHG using a modified µ-Raman spectrometer (Horiba HR800) so that the backward waves are analyzed.27 This technique of high depth selectivity allows the investigation of the NLO response within a volume restricted to the vicinity of the surface (depth of focus), thus avoiding coherent effects and most of the absorption effect occurring in an experiment conducted by transmission.28 In the present case, the thickness of the (EDT-DSDTFVO)2 · FeBr4 crystals were equal to ca. 2 µm that minimizes absorption losses at the fundamental and harmonic waves. Hence, no correction for absorption loss has been considered. The source was a 1064 nm diode pumped picosecond laser (EKSPLA PL2200: pulse duration 65 ps, repetition rate 2 kHz) focused at the surface of the sample with a 50× NIR objective (NA ) 0.42) and using an optimal pinhole of 100 µm. The depth of focus was estimated ca. 3 µm. The energy per pulse was always less than 100 nJ and adjusted by a power unit composed of a rotating half-wave plate followed by a GLAN Taylor. The incident power pulse was monitored by a fast InGaAs photodiode for quantitative power calibration purpose. A transparent polarized glass (formula: (1 - x)NaPO3 + xNb2O5, x ≈ 0.47) where a NLO layer of ca. 3 µm has been implemented at the anode during a poling process,29 and in which the d tensor components were previously determined (d33 ) 2.1 pm/V, d31 ) d33/3), was used as a reference for the calibration of the d value. Owing to the size of the crystal (50 × 30 × 2 µm3), no preferred orientation was possible except that, using a rotating stage, the crystal plate was oriented in-plane so that the reflected µ-SHG signal was maximum. In addition, it was necessary to use a calibrated density filter to attenuate the excitation line (OD ) 3 at 1064 nm) for the [(EDT-DSDTFVO)2] · (FeBr4) crystal because of the very high efficiency of its response with regards to the reference sample. Finally, for both the reference and the crystal, polarized SHG intensities have been recorded as a function of the incident power so as to obtain a quadratic dependence of the harmonic NLO response (Figure 1). Assuming that the probed thickness is defined by the depth of focus (DOF) of the focused beam, the SHG intensity (I2ω) is related to the NLO susceptibility deff as follows:

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Figure 1. Quadratic dependence of the SHG signal of the reference poled glass sample (squares) and of the crystal of (EDTDSDTFVO)2 · FeBr4 (circles) represented in the inset. Note the difference of intensity scales between the two materials.

(I2ω) ∝ DOF2 × (deff)2 × (Iω)2

(1)

Note first that the NLO interaction reduces the volume of the sample that is probed and defined by the DOF here, and second that because of the weak numerical aperture (NA ) 0.42) used in the experiment, the shape of the volume of interaction is rather cylinder-like so that the propagation of light inside the material through the microscope setup is somehow quite similar to classical SHG macroscopic experiments.28

Figure 2. Optical spectrum of (EDT-DSDTFVO)2 · FeBr4 recorded in KBr pellets in the UV-visible (a) and near-infrared (b) domains.

SCHEME 2

Computational Methods The all-valence INDO (intermediate neglect of differential overlap) method,30,31 in connection with the sum over state (SOS) formalism was employed for the calculation of the electronic spectra and the molecular hyperpolarizability of (EDT-DSDTFVO)2 · FeBr4.32 Details for the computationally efficient INDO-SOS-based method for describing molecular optical nonlinearities have been reported elsewhere.33 In the present approach, the close-shell restricted Hartree-Fock (RHF) formalism was adopted and the calculation performed on a fragment built up from four EDT-DSDTFVO entities sharing a 2+ charge, which results in a singlet (S ) 0) ground state. The monoexcited configuration interaction (MECI) approximation was employed to describe the excited states. The lowest 200 energy transitions obtained from the 20 highest occupied orbitals and the 20 lowest ones (CI level ) 20) were chosen to undergo CI mixing and used for the β calculation. All calculations were performed using the INDO/1 Hamiltonian incorporated in the commercially available software package ZINDO.34 Metrical parameters used for the calculations were those obtained from the crystal structure available.25a Additional calculations used as references were made in the same conditions for 3-methyl-4-methoxy-4′-nitrostilbene (MMONS),35 3-nitroaniline (m-NA),36 2-cyclooctylamino-5nitropyridine (COANP),37 and [N-(4-nitrophenyl)-N-methylamino]acetonitrile (NPAN).38 They were based on the previously reported crystal structures, which all belong to the same mm2 point group, for the consistency of the investigation of the structure-property relationships. Results and Discussion Optical and Nonlinear Optical Properties of (EDTDSDTFVO)2 · FeBr4. Optical Spectra. The previously reported conducting behavior of (EDT-DSDTFVO)2 · FeBr4 leads to the

expectation that the material is strongly absorbent over a large frequency range. The optical spectra recorded in KBr pellets are shown in Figure 2. As anticipated, the spectra reveal two broad domains of intense absorptions centered around 500 and 1000 nm. These features prohibit the use of the fundamental radiation of the Nd:YAG laser, as both incident (λ ) 1064 nm) and second harmonic (λ ) 532 nm) beams are strongly absorbed by the samples and justify the use of the 1907 nm wavelength as the incident radiation in the measurement of samples calibrated in the 0-50 µm range. Nevertheless, the presence of a group of transitions in the 800-1600 nm range suggests that the NLO response may be strongly enhanced by resonance at the 2ω second harmonic, but also possibly reduced by a reabsorption process (vide infra). To provide computational support for the optical properties of (EDT-DSDTFVO)2 · FeBr4, a ZINDO calculation has been carried out on a stack of four interacting EDT-DSDTFVO species (Scheme 2), built up from the crystal structure available. This approach assumes that a tetrameric stack should provide qualitative insights for the possibility of electronic delocalization along the chains and hence potential NLO enhancement by through-space charge transfers. The calculated data gathered in

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TABLE 1: ZINDO-Calculated Low Lying (f > 0.1) Transitions for [(EDT-DSDTFVO)2]4+ with Absorption Maxima (λmax in nm), Oscillator Strength (f), and Related Dipole Moment Change (∆µ in D) transition

λmax

f

∆µ

1f4 1f5 1f6 1f8 1f9

1451 1421 1321 1106 922

0.52 0.23 2.36 0.29 0.36

21.4 21.7 3.0 1.2 3.7

Table 1 reveal that a group of low lying and intense transitions is present in the 900-1500 nm range, with an absorption maximum (λmax) at 1321 nm (f ) 2.36). The energy shift observed between experimental (1130 nm, Figure 2b) and calculated (1321 nm) λmax falls in the range of uncertainty encountered within TTF-containing species39 or large chromophores with inorganic counterparts.40 Few additional transitions are present at higher wavelengths; however, they all exhibit very weak intensities (f < 0.1). As indicated in Table 1, the intense transitions are associated with large dipole moment changes (∆µ) which strongly support their contribution to the NLO properties of the material. NLO Properties. The efficiencies in second-harmonic generation measured on KBr pellets are shown in Table 2 and compared with that of urea. Note that the low transparency of the (EDT-DSDTFVO)2 · FeBr4 material hampers reliable measurements on samples containing more than 1 wt.-% NLO material, which are too absorbent to allow the observation of reliable NLO signals. Therefore, and under the present experimental conditions, the uncertainties remain important. Nevertheless, a nonzero SHG signal is evidenced, which confirms the potential NLO capabilities of these systems. The weak SHG intensities measured in the diluted (EDT-DSDTFVO)2 · FeBr4 samples lead to an estimated SHG efficiency of 3 to 6 times that of urea, within the crude assumption introduced in the Experimental Section. Additionally, the measurements performed by µ-SHG were carried out on thin microcrystals (thickness around 2 µm) in which, to a large extent, the major deleterious effect of light absorption may be passed over. The measurements confirm some SHG ability in the material and lead to deff ≈ 110 pm/V, which is presumably slightly underestimated because of the absorption at both the fundamental and harmonic waves. However, this result is sizable, once compared with those recorded at 1.064 µm on typical molecular materials, N-(4-nitrophenyl)-L-prolinol (NPP)41 and N,N-dimethylamino-N′-methylstilbazolium p-toluenesulfonate (DAST),42 in Table 3. DAST shows exceptionally large SHG efficiency at 1.907 µm (1000 times that of urea),42 a value which is here reduced to 15 at 1.064 µm (Table 3), due to second harmonic absorption. This limitation, present in (EDTDSDTFVO)2 · FeBr4 at both incident and second harmonic frequencies, will necessarily restrict the use of such absorbent materials to applications in the micrometer scale (vide infra).

Beside discussing the efficiency/transparency trade-off in these materials, one must address the intriguing issue of the origin of the NLO behavior in the grossly symmetric (EDTDSDTFVO)2 · FeBr4 crystals. As is well-known, the solid-state SHG efficiency of molecular materials is ultimately related to the underlying molecular hyperpolarizability (β) arising from intense transitions having non-centrosymmetric (∆µ * 0) charge transfer character.43 In the case of (EDT-DSDTFVO)2 · FeBr4, the origin of the NLO effect is somewhat unclear, as no strong “push-pull” character appears at first glance from the examination of both the molecular formula of EDT-DSDTFVO and the overall crystal packing. Nevertheless, and following the few previous suggestions that non-centrosymmetric through-space charge transfers could lead to extremely large NLO responses,19,20b the possibilities for large β values arising from overlapping EDT-DSDTFVO species will be discussed in the next section with the help of the computational quantum procedures. A Computational Approach to β in Chains of Overlapping EDT-DSDTFVO Species. During the last 20 years, ZINDObased quantum mechanical computations have profoundly changed the science of chromophore design.33 However, these investigations are restricted to molecules of limited size (around 100 heavy atoms), in relation to the increasingly large (several hundreds) number of transitions required in the configuration interaction (CI) process to ensure the reliability of the β calculation, as the size of the molecule is getting larger. In a few instances, β calculations of small molecules were envisioned on the entire extent of the crystal cell, the overall size limitation remaining around 100 heavy atoms.44 Computational approaches of infinite chains therefore imply drastic simplifications, with the assumption that, to a large extent, the behavior of the chain can be rationalized from that of a limited number of interacting species. In the present material, an additional limitation arises from the fact that our ZINDO release does not allow β computation for open-shell systems. Therefore, the only computed β value available is that of [(EDT-DSDTFVO)4]2+. Indeed, [(EDTDSDTFVO)2]+ and [(EDT-DSDTFVO)6]3+ are paramagnetic, while [(EDT-DSDTFVO)8]4+ is too large (160 heavy atoms). The computation performed on [(EDT-DSDTFVO)4]2+ at the experimental (λ ) 1.907 µm) wavelength leads to a giant β value of 9715 × 10-30 cm5 esu-1 (2429 per molecule). In order to find a rationale for this unexpectedly large hyperpolarizability, it is important to note that, within the SOS procedure, β can be strongly enhanced by the resonance effect, when the incident or second harmonic laser energies (pω and 2pω, respectively) are close to the chromophore transition energies Ei, as illustrated by the following simplified, but widely used, “two-level” quantum description of β:45

TABLE 2: SHG Efficiencies Recorded in KBr Pellets for (EDT-DSDTFVO)2 · FeBr4 and Urea at the Incident 1.907 µm Wavelength sample

concentration (% in KBr pellet)

log (I0/I)a (absorption + scattering)

measured I2ω signal

SHG efficiency

(EDT-DSDTFVO)2 · FeBr4

0.5 1.0 15.0

1.8 2.1 1.9

10 ( 10 30 ( 10 90 ( 10

3 ( 3b 5 ( 2b 1c

urea a

At 953 nm. b Calculated as (I2ω(X)/[X])/(I2ω(urea)/[urea]). c Reference value.

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TABLE 3: Macroscopic SHG Properties of (EDT-DSDTFVO)2 · FeBr4 (experimental d tensor component, and SHG efficiencies) Recorded at 1.064 µm, with the Indication of Transparencies at ω and 2ω Frequencies, Compared to Those of Relevant NLO Molecular Materials

a

Data for DAST and NPP from ref 9b, chapter 4.

βpω )

∑ i

3e2fi∆µi 2mEi3

×

Ei4 (Ei2 - (2pω)2)(Ei2 - (pω)2)

(2) In this relation, fi and ∆µi are respectively the oscillator strength and dipole moment change associated with the ith transition, while the second term of the right-hand equation quantifies β enhancement by resonance effects. For instance, with a laser operating at 1.907 µm (λ2ω ) 953 nm), the resonance term of eq 2 is equal to 21 for the 1 f 9 transition (Table 1), which strongly suggests that the contribution of this transition to the intrinsic NLO effect is overestimated. It is therefore more relevant to provide the static β value (β0), calculated at zero frequency (λ ) ∞), free of any resonance effect. β0 is equal to 854 × 10-30 cm5 esu-1 (213.5 × 10-30 cm5 esu-1 per EDTDSDTFVO entity), which confirms that the material undoubtedly possesses sizable intrinsic NLO capabilities. From Molecular (β) to Bulk NLO Efficiency in (EDTDSDTFVO)2 · FeBr4. As is well-known, nonzero SHG efficiencies require that the chromophores be engineered in a noncentrosymmetric solid-state environment, if the molecular (β) NLO response is to contribute to an observable macroscopic effect. (EDT-DSDTFVO)2 · FeBr4 crystallizes in the non-centrosymmetric Cmc21 orthorhombic space group, which belongs to the mm2 point group. Several investigations have previously pointed out large NLO efficiencies within this promising point group.46-49 However, the fact that a chromophore crystallizes in a non-centrosymmetric space group does not guarantee that the molecular packing is optimized for NLO purposes. Indeed, the charge transfer directions may be engineered in a pseudocentrosymmetric solid-state environment in the crystal, thus canceling most of the macroscopic effect. Therefore, the β orientation within the crystal symmetry has to be carefully examined in order to evaluate the potential NLO efficiency of (EDT-DSDTFVO)2 · FeBr4 in the solid state. At the macroscopic level, the hyperpolarizability tensor (component βijk in the molecular framework) is related to the corresponding crystalline first-order nonlinearity tensor χ(2) (component dijk in the crystalline framework).50 In a simplified approach, we assume a strong vectorial character for β along a

dominant charge transfer (CT) direction, so β ≡ βCT. In the orthorhombic mm2 space group, this leads to three independent tensor components expressed as follows:46a,50

dzxx ) Nf z2ω(f ωx )2 cos2 φ cos θ sin2 θ × βCT dzyy ) Nf z2ω(f ωy )2 sin2 φ cos θ sin2 θ × βCT dzzz ) Nf

2ω ω 2 z (f z )

(3)

cos θ × βCT 3

where N is the number of molecules per unit volume, fiω,2ω are local field factors, θ is the angle between βCT, and the crystal axis c, and φ the angle between the crystal axis a and the projection of βCT in the ab plane. The optimization of dzzz can be achieved with θ ) 0°, a situation which corresponds to a strict one-dimensional NLO solid, and which is of no practical use, for reasons related to the absence of phase-matching capability.41 More importantly, the present crystal structure leads to θ and φ equal to 68.27° and 37.67°, respectively. This provides a large angular factor value (cos2 φ cos θ sin2 θ) equal to 0.20 in the expression of dzxx. Zyss et al.50 have found that the optimized angular factors among any space groups cannot be larger than 0.38, thus suggesting that the present 0.20 factor is still capable to lead to a large macroscopic effect. To further estimate the intrinsic NLO capability of (EDT-DSDTFVO)2 · FeBr4, we have found interesting to compare its solid-state tensor components (dzxx and dzyy) with those of various well-known organic NLO materials, previously investigated. These materials were chosen in relation to their crystal structures, which are all derived from the same mm2 point group, as does (EDT-DSDTFVO)2 · FeBr4. Therefore, they provide reliable comparisons for the evaluation of the solidstate NLO responses. These reference molecules are presented in Scheme 3. It is important to point out that some of them (e.g., MMONS51 and NPAN52) have provided NLO crystals with excellent figures of merit. A comparison of the structure-property relationships is provided in Table 4, for the five different materials. Within the assumption that the overall solid-state NLO efficiency arises from the value of the d tensor components, the data gathered in Table 4 lead to the somewhat surprising

Optical Nonlinearity in (EDT-DSDTFVO)2 · FeBr4

J. Phys. Chem. C, Vol. 114, No. 49, 2010 21767 DSDTFVO)2 · FeBr4 for this purpose remains highly speculative, for reasons related to the transparency/conductivity trade-off, which is the fundamental issue to be addressed for these hybrid systems. It has long been known that the molecular conductors (e.g., TCNQ55 or TTF-based56 charge transfer materials) possess solidstate absorption spectra with a large band in the near-infrared domain, with extinction coefficients (ε) between 103 and 104 L · mol-1 · cm-1, ascribable to intermolecular charge transfer processes, as follows:

SCHEME 3

TCNQ- + TCNQ0 f TCNQo + TCNQTTF0 + TTF+ f TTF+ + TTF0

(4)

The loss of most of the transmitted signal in thick absorbent samples prohibits their application in optical devices. However, future photonics will require ultrashort (subpico) pulses of light,57 where the radiation cannot be regarded as strictly monochromatic. Therefore, photonics should benefit from the use of micrometer-thick materials to avoid velocity dispersion during the transversal of the devices, the overall NLO efficiency being ensured by the possibility of giant β values. In an attempt to relate the rather weak SHG efficiency of (EDT-DSDTFVO)2 · FeBr4 (3-6 times that of urea at 1.907 µm) estimated by transmission to the modest transparency of the material, carefully ground KBr pellets (ca. 1 wt.-%) were observed to exhibit typical absorbance equal to 1, at 952 nm, for a pellet thickness of 0.25 mm. This corresponds to an absorbance around 0.40 per micrometer of pure (EDTDSDTFVO)2 · FeBr4, while the related resulting absorbance at the incident (1.907 µm) wavelength is reduced to 0.15. The evaluation of the balance between absorption and optical nonlinearity in oriented samples (e.g., crystals or poled polymers) has recently been approached computationally.58 Along this line, and in the idealized case of perfect phase-matchable material (no velocity dispersion between ω and 2ω waves), the SHG intensity (I2ω) is related to the film thickness (l) through the following relation:

result that (EDT-DSDTFVO)2 · FeBr4 possesses a potential efficiency (calculated d value) around 40 times that of the best organic candidates. However, several limitations must be pointed out, which all lead to a lowering of the intrinsic capabilities of this material: (i) a large part of the NLO response is invariably lost by absorption in conducting materials, and it is therefore not surprising that the experimental d values available (Table 4) indicate a value for (EDT-DSDTFVO)2 · FeBr4 only twice as large as those of the organic candidates. (ii) Furthermore, most traditional applications of NLO materials (e.g., for frequency doubling) require phase-matchable26,53 crystals with a thickness of a few millimeters, if a large part of the incident (ω) energy has to be converted to the second-harmonic (2ω) frequency. This prohibits the use of highly conducting materials as large single crystals currently identified as potential candidates in electro-optical devices. Nevertheless, the possibility of using these absorbent species as thin-layered materials for nontraditional applications will be evaluated in the last section. Critical Evaluation of Conducting Molecular Materials for NLO Applications. Linking conductivity and nonlinear optics within a single crystal cell could become a challenging target in technologies aiming at combining both properties, for instance, the technology of optoelectronics, which integrates photonic devices with standard semiconductor electronics.54 However, the possibility of using materials such as (EDT-

I2ω ∝ l2 × D(l)

(5)

In this expression, D(l) is a diminution factor accounting for the absorption, and expressed by:58

TABLE 4: Relevant Macroscopic Tensor Component for (EDT-DSDTFVO)2 · FeBr4, Compared with Those of Various Molecular NLO Materials, with the Same mm2 Point Group Symmetry in the Solid State orientation anglea

d tensor component βCT(in 10-30 esu)

compd

space group

N

θ (deg)

φ (deg)

(EDT-DSDTFVO)2 · FeBr4

Cmc21

2/(3686 × 10-24)

68.27

37.67

MMONS

Aba2

8/(2833 × 10-24)

34.94

59.12

m-NA

Pca21

4/(624 × 10-24)

37.42

89.60

COANP

Pca21

4/(1310 × 10-24)

59.91

61.21

15.6

NPAN

Fdd2

16/(3683 × 10-24)

60.60

18.42

18.5

a

9715 30.9 5.41

calculatedb (in 10-9 esu)

experimentalc (in pm/V)

dzxx ) 1054 dzyy ) 628 dzxx ) 6.20 dzyy ) 17.3 dzxx ) 0.00 dzyy ) 13.3 dzxx ) 4.14 dzyy ) 13.7 dzxx ) 26.9 dzyy ) 2.97

deff ) 110 41 1.6 20 15 26 deff ) 59

Equation 3. b Calculated from eq 3 with the assumption that all fiω,2ω local field factors are equal to 1. c Values from ref 9b, chapter 4, for MMONS, m-NA, COANP, and NPAN.

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1 D(l) ) exp - (R2ω + 2Rω) × l × 2 1 sinh (R2ω - 2Rω) × l 4 1 (R - 2Rω) × l 4 2ω

[

[

] [

]

]

2

(6)

where aω and R2ω are the absorption coefficients at the incident and second harmonic frequencies, respectively. The resulting computed efficiencies are shown in Figure 3a, for samples having the same d solid-state tensor components, perfect transparencies at ω (Rω ) 0), and absorption coefficients ranging from 0.2 to 0.5 µm-1, at the 2ω frequency. In any cases, nonzero SHG efficiencies are observed whatever the sample thickness. By contrast, the situation with R2ω ) 2.5Rω, which corresponds to the present conducting species, is investigated in Figure 3b. The figure indicates that the SHG intensities increase with l for very thin samples, reach a maximum, and finally decrease to zero when the sample thickness becomes such that the incident ω signal is fully absorbed by the material. The situation for the present (EDT-DSDTFVO)2 · FeBr4 is that of curve 3 in Figure 3b. Although modest, the present absorption at ω turns out to be the dominant factor, which leads to vanishing NLO response at the macroscopic level for thicknesses larger than 20 µm, while the best SHG efficiency is expected at a crystal thickness around 5 µm. This may account for a rather disappointing SHG efficiency recorded on particles sizes in the 0-50 µm range. To further compare the NLO capabilities of (EDTDSDTFVO)2 · FeBr4 with those of traditional NLO materials (e.g., Scheme 3), The SHG intensities (calculated as l2 × D(l) × d2) are drawn in Figure 4 against the samples thickness. In Figure 4, the d values are equal to 1054 × 10-9 for (EDTDSDTFVO)2 · FeBr4, and 26.9 × 10-9 for the efficient NPAN molecule, used as a transparent reference (Table 4). NPANlike materials are unambiguously more efficient at the standard (>100 µm) crystal size. Nevertheless, and within a thickness range of 1 to 10 µm, absorbent materials can exhibit extremely large efficiencies, which could attract some interest in application where very thin devices would be required. To date, hundreds of new molecules combining good transparencies and high SHG efficiencies have been reported as potential candidates for applications in nonlinear optics. In this respect, the present hybrid conducting and magnetic (EDTDSDTFVO)2 · FeBr4 material exhibits rather modest capabilities, for reasons related to its strong intrinsic absorbance. However, this compound perfectly illustrates that the field of molecular material now encompasses chemical structures of much greater electronic complexity than those of the first generations of molecules in which the electronic structures were specifically designed for a single (e.g., conducting, magnetic, or optical) purpose. Multifunctional materials, in which molecular properties are expected to be linked and furthermore to interplay at the microscopic scale, should become an important class of materials and greatly stimulate the development of many research areas at the interface between chemistry and material science. Conclusion (EDT-DSDTFVO)2 · FeBr4 is a hybrid material, which exhibits a conducting behavior, a magnetoresistance effect, and an observable quadratic optical nonlinearity by virtue of its noncentrosymmetric crystal structure. Owing to these nontraditional

Figure 3. Efficiencies expressed as a function of the sample thickness (l) for materials having the same macroscopic d tensor component, and various absorption coefficients (Rω and R2ω). In all instances, R2ω is equal to 0.2 (curve 1), 0.3 (curve 2), 0.4 (curve 3), and 0.5 µm-1 (curve 4)). In part a, Rω ) 0, and in part b, R2ω) 2.5Rω.

Figure 4. Efficiencies plotted against sample thickness for (EDTDSDTFVO)2 · FeBr4 (curve 1), compared with those of transparent NPAN crystals, in which the assumption is Rω ) R2ω ) 0 (curve 2), or Rω ) 0; R2ω ) 0.005 µm-1 (curve 3).

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