Photoswitching of the Second Harmonic Generation from Poled

ARTICLE pubs.acs.org/JPCC

Photoswitching of the Second Harmonic Generation from Poled Phenyl-Substituted Dithienylethene Thin Films and EFISH Measurements Daniele Marinotto,*,† Rossella Castagna,‡,§ Stefania Righetto,† Claudia Dragonetti,† Alessia Colombo,† Chiara Bertarelli,‡,§ Michele Garbugli,|| and Guglielmo Lanzani‡,|| †

)

Dipartimento di Chimica Inorganica Metallorganica e Analitica “Lamberto Malatesta” and Centro di Eccellenza CIMAINA dell'Universita degli Studi di Milano and UdR dell'INSTM, V. Venezian 21, 20133 Milano, Italy ‡ Center for Nano Science and Technology @Polimi, Istituto Italiano di Tecnologia, Via Pascoli 70/3, 20133 Milano, Italy § Dipartimento di Chimica, Materiali e Ingegneria Chimica “G. Natta”, Politecnico di Milano, piazza Leonardo da Vinci 32, 20133 Milan, Italy Dipartimento di Fisica, Politecnico di Milano, piazza Leonardo da Vinci 32, 20133 Milan, Italy ABSTRACT: The photoswitching of the second harmonic generation signal of poled phenyl-substituted dithienylethene/polystyrene thin films evidence a different behavior in function by their molecular structure. The three nonzero χXXZ(2), χZXX(2), and χZZZ(2) coefficients of second order susceptibility at 1064 nm have been measured for both isomeric forms of substituted diarylethene in polymer films. The relationship between χXXZ(2), χZXX(2), and χZZZ(2) for symmetrical diarylethene (OMe
OMe and CN
CN) differs from that found for the asymmetrical OMe
CN chromophore, suggesting for the two symmetrically substituted molecules a two-dimensional first hyperpolarizability. The two-dimensional first hyperpolarizability model has been checked by a statistical analysis from the correlation between susceptibility and first hyperpolarizability. Analysis of βxxz, βzxx, and βzzz values confirm for OMe
OMe and CN
CN a twodimensional charge flow pattern, which is different from the monodimensional OMe
CN. Moreover, the goodness of the our statistical model was confirmed by comparing the βz value measured by the EFISH technique with respect to that calculated from the three components βxxz, βzxx, and βzzz.

1. INTRODUCTION Photochromism is the reversible transformation by a suitable light irradiation of a chemical species between two stable forms, whose absorption spectra are significantly different:1 this property has been exploited in the development of molecular and supramolecular photoswitching and optical and optoelectronic devices. In the last two decades 1,2-diarylethenes (DTEs)2 have been used as optical memories3 and astronomical devices4 and have gained much attention as switching units where irradiation can interconvert them in two isomers through a 4n + 2 electrocyclization. In particular, upon UV light irradiation, the colored closed form is obtained, whereas upon visible light irradiation the nonconjugated colorless open form is restored. DTEs offer excellent thermal stability of both isomers, fatigue resistant character, rapid response, and high reactivity at the solid state.5 By a proper molecular design, the different chemical
physical properties can be tailored and optimized. DTEs bearing phenyl groups have been drawing the attention of researchers for a long time: the aromatic substituent increases the ring closure quantum yield and leads to very stable structures with extended delocalized π-systems.6 In this work we present the study of the second harmonic generation (SHG) signal of poled host
guest systems, their r 2011 American Chemical Society

SHG photoswitching capability,7 and the second-order nonlinear optical properties, measured in solution with the EFISH technique8 in both symmetrical and asymmetrical phenyl-substituted diarylethenes (Figure 1). For the first time we report the study of SHG photoswitching of DTE poled films in a polystyrene matrix. The effect of the molecular structure of a series of DTEs on the photoswitching of poled molecules and on their nonlinear optical properties is analyzed.

2. EXPERIMENTAL SECTION 2.1. Synthesis. Photochromic compounds CN
CN, OMe
OMe, and OMe
CN were synthesized according to a previously published procedure.9 Operations were carried out under a dry, oxygen-free argon atmosphere. Reagent grade solvents were distilled from potassium benzophenone ketyl, under argon. All reagents and catalysts were commercial (Aldrich). Calculated dipole moments are reported in Table 1. Received: June 27, 2011 Revised: September 6, 2011 Published: September 14, 2011 20425

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Figure 1. Chemical structure and molecular coordinate system: (a) OMe
OMe and CN
CN phenyl-substituted dithienylethenes and (b) OMe
CN phenyl-substituted dithienylethenes. (xyz is the molecular coordinate system, the y axis is perpendicular to the plane xz).

Looking at the components of the dipole moments and at the molecular structures, it is apparent that the direction of the dipole moment is roughly along the z axis. Molecules can be considered with C2 symmetry for the OMe
OMe and CN
CN molecules, with the dipole moment parallel to the 2-fold rotation axis and C∞v symmetry for the OMe
CN one, again with the dipole moment parallel to the rotation axis. 2.2. Polymer Films: Preparation and Characterization. Composite films were produced by casting assisted by control coater on glass substrates from a chloroform solution of a polystyrene (PS) matrix (40 mg/mL) and the chromophore (5% w/w relative to the polymer) in closed form. Film thickness was measured by spectral reflectance technique (Filmetrics F20-EXR) to be between 1 and 2 μm (see Table 2). UV
vis absorption spectra of photochromic films were carried out using a JASCO V570 spectrometer. Concentrations of the DTEs moieties in the films were calculated from their optical spectra to be 3.40  1025 m
3 (OMe
OMe film), 4.46  1025 m
3 (CN
CN film), and 6.29  1025 m
3 (OMe
CN film).10 2.3. Corona Poling Setup. The fundamental incident light was generated by a 1064 nm Q-switched Nd:YAG (Quanta System Giant G790-20) laser with a pulse of 7 ns and 20 Hz repetition rate. The output pulse was attenuated to 0.57 mJ and was focused with a lens (f = 600 mm) on the sample, placed over the hot stage. Corona poling process was performed inside a specially built drybox, in a N2 atmosphere. The fundamental beam was polarized in the incidence plane (so-called p-polarized) with an angle of about 55
with respect to the sample in order to optimize

Table 1. Predicteda Dipole Moments of the Investigated Molecules in Both Isomeric Forms dipole moments (D) x

chromophore

a

y

z

module

OMe
OMe open

0.01


0.2


2.05

2.06

OMe
OMe closed

0.02


0.24


3.05

3.06

CN
CN open CN
CN closed

0.04 0.04


0.05 0.03


0.81
0.17

0.81 0.18

OMe
CN open


1.69


1.57

7.17

7.53

OMe
CN closed


1.80


0.05

9.05

9.23

Based on calculations: DFT B3LYP, 6-31G(p,d).

the SHG signal. The hot stage temperature was controlled by a GEFRAN 800 controller, while the corona-wire voltage (up to 9.5 kV across a 10 mm gap) was applied by a TREK 610E high voltage supply. After rejection of the fundamental beam by an interference filter and a glass cutoff filter, the p-polarized SHG signal at 532 nm was detected with a UV
vis photomultiplier (PT) Hamamatsu C3830. The output signal from the PT was set to a digital store oscilloscope and then processed by a computer with dedicated software. The films were stored in the dark for at least 24 h after poling, in order to ensure that the surface charges resulting from the deposition of ions on the film surface during the corona wire poling process11 were neutralized. 2.4. Maker Fringe and Second Harmonic Photoswitch. In the Maker fringe experiment, the second harmonic (SH) intensity 20426

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Table 2. Susceptibility Components of Poled Films with Dithienylethenes (a) in the Open Form and (b) in the Closed Form (1esu = 4.19  108 pm/V) thickness chromophore

(μm)

OMe
OMe CN
CN

2.0 1.3

OMe
CN

2.0

(a) Open Form XZZZ(2)
10

XZXX(2) (10
10 esu)

u

6.82 16.22

1.86 4.86

0.95 0.85

10.74

3.10

1.23

Figure 2. Setup photoswitch experiment and Maker Fringe (XYZ is the macroscopic coordinate system).

u

(605 and 594 nm for CN
CN and OMe
OMe, respectively) is likely due to the more extended conjugation, the cyano group providing the triple bond which takes part in overall delocalization of the π-electrons. In this sense the different electronic effect of the ending functional groups is almost negligible. Accordingly, a λclosedOMe
CN intermediate between OMe
OMe and CN
CN would be expected, as the asymmetric molecules only one CN group over which the delocalization can be extended. Differently from this prediction, OMe
CN is characterized by a λclosedOMe
CN of 611 nm, red-shifted if compared to the previous two compounds. This can be ascribed to the push
pull structure when the molecule is in its closed conjugated form. After poling the closed form of the OMe
CN and OMe
OMe showed an absorption band with intensity which was about 40% of their maximum value (see Figure 3a for OMe
CN). The decreasing of absorption peaks in comparison with that observed before poling (both in the open and in the closed forms, see Figure 3a) is the characteristic so-called dichroic effect due to the partial orientation of molecules along the direction of the electric poling field (Z axis).13 No appreciable Stark shift13 of the absorption peaks was observed after poling. To the extent that the polymer film during the poling can be considered as an ideal solid solution system,14 i.e., noninteracting chromophores in thermal equilibrium under the electric field poling, the relative change of Δα/α in the absorption coefficient (i.e., the order parameter j) can be expressed by

(10

esu)

(b) Closed Form chromophore thickness (μm)

XZZZ(2)

XZXX(2)

XXXZ(2)


10


10


10

(10

esu) (10

esu) (10

esu)

OMe
OMe

2.0

22.40

4.77

6.68

1.25

CN
CN

1.0

13.36

2.95

3.29

1.19

OMe
CN

2.0

170.88

20.19

24.00

4.41

was detected as a function of the incidence angle θ of the fundamental beam and normalized with respect to that of a calibrated quartz crystal wafer (X-cut) 1 mm thick whose d11 is 0.46 pm/V. The incidence angle was changed by rotating the poled film along the Y axis (see Figure 2) while the polarization of the fundamental and SH beam could be changed by a half-wave plate and a cube beam splitter, respectively. In order to determine the nonzero independent components of the susceptibility tensor for poled films (C∞v symmetry) Maker fringe measurements were conducted with different polarizations: p f p, s f p, and 45 f s for chromophores in the closed form, p f p and s f p for chromophores in the open form. Kleiman’s symmetries12 were considered valid in films with chromophores in the open form (see section 3.3). In the SHG photoswitch experiment the poled film was rotated at incidence angle of 50
and the fundamental and SH beams were p-polarized. The poled film was alternately irradiated with UV light at 375 nm (continuous wave (cw) mode power, 15mW) and visible light with a cutoff filter at 550 nm (cw mode power, 157 mW) in order to change the state of the photochromic molecules. The two beams crossed the poled film in noncollinear configuration with respect to the fundamental beam (see Figure 2), and the beam widths were about 1 cm. 2.5. EFISH. All EFISH measurements8 of photochromic molecules were carried out in 10
3 M CHCl3 solution working with a nonresonant incident wavelength of 1907 nm, obtained by Raman shifting the fundamental 1064 nm wavelength produced by a Q-switched, mode-locked Nd:YAG laser manufactured by Atalaser. The apparatus for the EFISH measurements was made by SOPRA (France). The EFISH values reported are the average of 16 successive measurements performed on the same sample. Usually the error on the μβλ values is roughly 10
15%.

3. RESULTS AND DISCUSSION 3.1. Linear Absortion Spectra. Upon irradiation with UV light the three photochromic molecules were converted in the closed, blue-colored form, as shown by the typical increasing of the actinic band (see for example Figure 3a). The very slight differences in the absorption maxima of the closed forms can be rationalized as follows: the bathochromic shift of the λclosedCN
CN if compared to the λclosedOMe
OMe

j¼

Δα 3 3 ¼ 1 þ 2
cothðuÞ α u u

ð1Þ

where u = (μ 3 Ep)/kT, in which μ is the molecular ground state dipole moment expressed in debyes, Ep the local electric field expressed in MV cm
1, k the Boltzmann constant, and T the poling temperature in kelvin. In our poling experiments a voltage of 9.5 kV was applied and, consequently, u lies between 0.85 and 4.41 (Tables 2 and 3), depending on the nature of the chromophores and their isomerization, leading to Ep = 2
120 MV/cm. This simple method to evaluate u could have some disadvantages such as fast relaxation processes after poling15 and possible strong molecular interactions (e.g., formation of aggregates). Other different methods have also been proposed,16 but they are based on the same physical principles and so should have the same limitations. The u parameter will be used to correlate the nonzero independent components of the susceptibility tensor χ(2) to the first hyperpolarizability components of the tensor β. 3.2. Poling Measurements. Before poling, films were placed over the stage inside a drybox where at least three cycles of vacuum
nitrogen were applied in order to keep the oxygen presence as low as possible. It was necessary to work under a nitrogen atmosphere because the ozone formed by the high 20427

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Table 3. Susceptibility Components of Poled Films with Dithienylethenes in Open Form after UV Photoisomerization thickness

XZZZ(2)
10

(10

XZXX(2)
10

esu) (10

XXXZ(2)

esu) (10
10 esu)

u

chromophore

(μm)

OMe
OMe CN
CN

2.0 1.3

16.48 12.45

3.81 1.81

4.70 1.43

1.09 1.52

OMe
CN

2.0

94.03

14.65

21.47

2.82

Figure 4. Poling dynamics of three representative films: OMe
OMe in both isomeric forms and OMe
CN in the open form.

Figure 3. (a) UV
vis absorption spectra of film of OMe
CN before and after poling. (b) UV
vis absorption spectra of films of OMe
OMe (red line), CN
CN (blue line), and OMe
CN (black line) in the closed form after poling.

electric field in the corona poling quickly produces degradation, probably by decomposition of the chromophores. First, poling was done on poly(methyl methacrylate) (PMMA) films with chromophores either in the open or closed form. However the SHG signal of diarylethene chromophores in PMMA matrix progressively faded due to a loss of orientation of the dyes. This feature, which has been already reported in many other works,7,17,18 is not surprising if the β transition of PMMA, which has been attributed to rotation of the ester side group,19 is considered. To avoid this problem, polystyrene was used as matrix, showing a better behavior. We observed that the poling dynamics is quite similar for all NLO chromophores investigated independently on their isomerization (see Figure 4). The SHG was negligible before applying the corona voltage, and it quickly increased within 1
2 s after application of the electric field (directed along Z axis). When the temperature was increased up to 80
C, a large increase of the SHG occurred, due

to the decrease of the viscosity of the polymeric matrix near to Tg which allowed a facile orientation of the NLO chromophores. When a stable temperature of 80
C was reached, the sample was then cooled to 24
C and the drybox was open. The final switch off of the electric field caused a typical drop of the SHG. All the phenyl-substituted dithienylethenes were effectively poled due to the relatively high electric field applied, which made negligible both the differences in dipole moments of NLO chromophores and their different point group symmetry. Moreover, alignment of the dipole moment along the direction of the electric field occurred differently for the three chromophores, namely, OMe
OMe and CN
CN along the 2-fold z axis, while OMe
CN along the symmetry z axis that at the two ends presents the OMe and CN groups. 3.3. Determination of the Second-Order Susceptibility Tensor. The standard expression20 used to fit the SHG intensity in a Maker fringe measurement includes the absorption coefficient of the film at the harmonic frequency. In this expression the SHG intensity is proportional to the square of the effective nonlinear optical coefficient (deff) which depends on polarizations of the fundamental and SH beam. Considering the C∞v symmetry expected for poled films and the polarizations of the fundamental and SH beam, the coefficient deff assumes the expression deff ¼ dZXX sin ϑ2

ð2aÞ

for s f p configuration deff ¼ dXXZ sin ϑ1 20428

ð2bÞ

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for 45 f s configuration deff ¼ 2dXXZ sin ϑ1 cos ϑ1 cos ϑ2 þ sin ϑ2 ðdZXX cos2 ϑ1 þ dZZZ sin2 ϑ1 Þ

ð2cÞ

for p f p configuration, where θ1 and θ2 are respectively the angles of refraction inside the poled film for the fundamental and SH beam with refractive indices nω and n2ω (sin θm = sin θ/nmω, m = 1, 2). It is worthwhile remembering that the components of the nonlinear optics coefficient and the susceptibility tensor are related by the formula 2dijk = χijk(2).12 By fitting the Maker fringe measurements using the appropriate expressions (2) the values of χZZZ(2), χZXX(2), and χXXZ(2) have been evaluated (Table 2); the error in these data can be estimated as e20%. Table 2 shows the values χZZZ(2), χZXX(2), and χXXZ(2) calculated one day after poling, for films poled with chromophores in the open and the closed forms, respectively. We considered valid the Kleinman’s symmetry χXXZ(2) = χZXX(2) as no absorption at 532 nm is observed in films with chromophores in the open form (see for example Figure 3a). For films with chromophores isomerized in closed form, we have checked if the Kleinman’s symmetry could be applied. In Table 2b we see that χXXZ(2) ≈ χZXX(2) is valid only for the CN
CN chromophore where the absorption at 532 nm of the CN
CN film can be considered negligible (Figure 3b). In the limit in which u < 1 or u ≈ 1 and the first hyperpolarizability tensor of the chromophore can be considered one-dimensional, the components χXXZ(2) and χZXX(2) are about equal to χZZZ(2)/3.21 From Table 2, we can see that these relations are not verified for OMe
OMe and CN
CN chromophores but only for the OMe
CN chromophore isomerized in open form, whereas for the film of OMe
CN isomerized in closed form the condition of low field is no longer satisfied (see u . 1). This behavior of the OMe
OMe and CN
CN chromophores suggests a dimensionality of the first hyperpolarizability greater than 1 (as it will be shown in section 3.6) to come from a C2 symmetry and reveals the key role of the molecular geometry on the NLO response of the film. It should also be noted that the highest value of χZZZ(2) was obtained for the OMe
CN chromophore in closed form (see Table 2), in which the presence of the methoxy donor group and the cyano withdrawing group enables a push
pull like structure across the π-conjugated bridge. Although the values found for the OMe
OMe and CN
CN chromophores were about an order of magnitude lower than that of OMe
CN in the closed form, the two symmetrically substituted dithienylethenes show interesting behaviors: changing the OMe
OMe with CN
CN groups the χZZZ(2) values of the closed form decrease while those of the open form increase. Moreover, the value for the open form of CN
CN is higher than that of its closed form, whereas the opposite happens for the OMe
OMe chromophore. These unexpected results are emphasized in SHG photoswitching experiments. The χZZZ(2), χZXX(2), and χXXZ(2) components in Table 3 were calculated by Maker fringe measurements on films of Table 2a after UV irradiation. These values confirm the trends observed in Table 2b although they are lower; as we will report in the next section, this is due to the partial loss of chromophores orientation during the photochromic switching experiments. It is important to underline that all films with chromophores isomerized in the closed form have the susceptibility tensor components affected

Figure 5. (a) OMe
OMe Photoswitch. (b) OMe
CN Photoswitch. (c) CN
CN photoswitch.

by resonance enhancement due to the optical absorption of films at 532 nm (see Figure 3b). 3.4. SHG Photoswitching. The in situ SHG photoswitching measurements on films cast from polymer solution of the NLO chromophores in the open form 1 day after poling are reported in Figure 5. 20429

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For films with OMe
CN and OMe
OMe, the SH signal increases after the first UV irradiation (as the closed form was generated from the open one), while the opposite behavior occurs for the film with the CN
CN chromophore. Once a stable plateau is reached, the UV irradiation was stopped and visible light was turned on. The SHG intensity decreased as the closed form converted back to the open form for OMe
CN and OMe
OMe chromophores, while increased for CN
CN chromophores. Interestingly, the SHG drop is the highest for OMe
CN chromophores (85
86%) decreases at about 70% for OMe
OMe, and is only 28
31% for CN
CN. Through two UV
visible irradiation cycles we have monitored the SHG stability. From Figure 5a, we note that the process of disorientation is very slow for the OMe
OMe chromophore and quickly increases going to OMe
CN and CN
CN chromophores (see panels b and c of Figure 5). This loss of the SHG signal has already been observed in spiropyran/photomerocyanine in PMMA,17b,18,22 and it is due to the irreversible loss of orientation during the photoisomerization processes. 3.5. EFISH. In order to investigate the NLO solution properties, EFISH measurements were carried out. It is known that the EFISH technique can provide direct information on the intrinsic molecular NLO properties through eq 3 γEFISH ¼ Æγð
2ω, ω, ω, 0Þæ ¼ γ|| ð
2ω, ω, ω, 0Þ þ

μβλ ð
2ω, ω, ωÞ 5kT

ð3Þ

)

where ω is the incident light frequency, γ (
2ω,ω,ω,0) is the third-order term and μβλ(
2ω,ω,ω)/5kT is the dipolar orientational contribution, with βλ(
2ω,ω,ω) the vector component of the βijk tensor along the dipole moment direction23,24 βλ ¼

∑i

μi βi μ

βi ¼ βiii þ

ð4Þ

∑

1 ðβ þ βjij þβjji Þ 3 j6¼ i ijj

ð5Þ

)

)

μ is the norm of the dipole moment vector with components μi. When considering the quadratic NLO response of dipolar compounds, the γ (
2ω,ω,ω,0) cubic contribution is often neglected in the analysis of the EFISH results. In the case of neglecting γ (
2ω,ω,ω,0), the dipole moment is the unique molecular property required to extract the βλ value. We recall that our chromophores present an almost one-dimensional dipole moment (see Table 1) along the molecular z axis, so in eqs 3 and 4, μx and μy are in first approximation neglected and βλ ≈ βz. To avoid overestimation of the quadratic hyperpolarizability value due to dispersion effects producing resonance enhancements, we have worked with an incident wavelength λ whose second harmonic λ/2 is far enough from the λmax of any absorption band of the investigated molecules. All experimental EFISH βz values were defined according to the “phenomenological or X” convention.25 Molecular first hyperpolarizabilities of the two isomers, closed and open forms, are different, depending on the molecular structures. For instance, the symmetrical substituted diarylethene reported by Atassi et al.18 shows only a small increment of μβz value, measured with the EFISH technique at 1907 nm. Conversely, the asymmetrical substituted diarylethene reported in Gilat et al.26 shows a large increment of μβz value, measured with the EFISH technique at 1064 nm; in this particular case, the μβz

Table 4. EFISH Measurements at λ = 1907 nm μβz (10
30 D cm5 esu
1) chromophore

open form

closed form

OMe
OMe

138

300

OMe
CN

230

800

value could be slightly overestimated due to a resonance enhancement (the absorption maximum of the closed form is located at 828 nm in benzene). We have compared the μβz values, measured by the EFISH technique, for the asymmetrically substituted dithienylethenes (OMe
CN) and for a symmetrically substituted chromophore (OMe
OMe) that have theoretical values of the dipole moments (open and closed form) bigger than 1 D. We have worked with an incident nonresonant 1907 nm wavelength in a 10
3 M CHCl3 solution and the measured values of μβz for the OMe
OMe and OMe
CN chromophore are shown in Table 4. As expected, in the two molecules examinated, the μβz values of the closed forms are higher than those of the analogous open forms. The symmetrically substituted OMe
OMe diarylethene shows lower values of both the open and closed form with respect to OMe
CN compound, and also the modulation between the open and closed form is smaller (a factor of about 2 for OMe
OMe), than for asymmetrical substituted diarylethene (a factor of about 3 for OMe
CN). 3.6. Microscopic Analysis: Determination of the β Tensor. In section 3.3, we have observed that the first hyperpolarizability of the OMe
OMe and CN
CN chromophores does not have a one-dimensional nature. Analyzing their chemical structures (Figure 1) it is possible, with a good approximation, to assign a point group symmetry C2, where the 2-fold axis is parallel to the z axis of the dipole moment. For dipole molecules of C2 symmetry, the first hyperpolarizability tensor has 13 nonzero components, eight of which are independent of application of the intrinsic permutation symmetry (βijk = βikj). In the case of the OMe
CN chromophore a charge transfer occurs between the donor OMe and the acceptor CN substituents, and the corresponding first hyperpolarizability is basically one-dimensional and directed along the charge transfer z axis. To verify if these models are correct, an evaluation of the molecular SHG response, i.e., the nonzero components of the first hyperpolarizability tensor β, from the previously determined χijk(2) components was performed.27 The correlation between the χijk(2) and the βijk tensor components has been established by Kielich28 assuming noninteracting molecules in thermal equilibrium under an applied field. We have assumed, as in section 3.1, that these conditions could be applied for our chromophores embedded in PS films. In the most general case, i.e., without assuming Kleinman’s symmetry and taking into account the C∞v symmetry expected for poled films and C2 molecular symmetry for the OMe
OMe and CN
CN chromophores, the following formulas are obtained
β ð2Þ χZZZ ¼ Nf2ω fω2 βxxz ½L1 ðuÞ
L3 ðuÞ þ zxx ½L1 ðuÞ
L3 ðuÞ 2 þ βzzz L3 ðuÞ þ

βzyy ½L1 ðuÞ
L3 ðuÞ 2 o

þ βyzy ½L1 ðuÞ
L3 ðuÞ 20430

ð6aÞ

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Table 5. First Hyperpolarizability βijk Calculated by Expressions 6 and 7 with Susceptibility Components of Table 2a

Table 6. First Hyperpolarizability βijk Calculated by Expressions 6 and 7 with Susceptibility Components of Table 2b

N

βzzz

βzxx

βxxz

N

βzzz

βzxx

βxxz

chromophore

(1025 m
3)

(10
30 esu)

(10
30 esu)

(10
30 esu)

chromophore

(1025 m
3)

(10
30 esu)

(10
30 esu)

(10
30 esu)

OMe
OMe CN
CN

3.40 4.46

91.13 138.09


54.65
62.83


53.95
61.87

OMe
OMe CN
CN

3.40 4.05

154.17 11.21


74.47
73.12


64.95
71.65

OMe
CN

6.29

26.07

OMe
CN

6.29

156.09

ð2Þ

χZXX ¼

Nf2ω fω2 2


βxxz ½L3 ðuÞ
L1 ðuÞ þ

βzxx ½L3 ðuÞ þ L1 ðuÞ 2

βzyy ½L3 ðuÞ þ L1 ðuÞ þ βzzz ½L1 ðuÞ
L3 ðuÞ þ  2 þ βyzy ½L3 ðuÞ
L1 ðuÞ ð6bÞ ð2Þ

χXXZ ¼


2

Nf2ω fω 2

βxxz L3 ðuÞ þ

βzxx ½L3 ðuÞ
L1 ðuÞ 2

βzyy ½L3 ðuÞ
L1 ðuÞ þ βzzz ½L1 ðuÞ
L3 ðuÞ þ 2 o þ βyzy L3 ðuÞ

ð6cÞ

For chromophores with one-dimensional charge transfer, the first hyperpolarizability βzzz is related to the component of the susceptibility tensor, through the relation21 ð2Þ

χZZZ ¼ Nf2ω fω2 βzzz L3 ðuÞ

ð7Þ

where N is the concentration of chromophores in the film, fω and f2ω are the Lorentz local field factors [ fnω = (nnω2 + 2)/3, n = 1, 2 ], and Ln stands for the n-order Lengevin function.28 According to expressions 6, only five of the eight independent components are present βxxz, βzxx, βzzz, βzyy, and βyzy; therefore the other independent components βxzy, βzyx, and βyxz cannot be determined by our measurements χXXZ(2), χZXX(2), and χZZZ(2). However, it should be noted that, as the transition dipole moments perpendicular to the zx molecular plane are generally small, the components βzyy, βyzy, βxzy, βzyx, and βyxz can be usually negligible.24,29 In Tables 5 and 6 the βijk values calculated from eqs 6 and 7 are listed, using as components of the susceptibility of the open form (Table 2a) and of the closed form (Table 2b), respectively. The key point in these evaluations is the u parameter estimated through eq 1. It has been checked that the βijk values obtained were not too sensitive to uncertainty associated with its calculation (changes of β < 38% for changes of u > 50%). Inspection of the in-plane components of the OMe
OMe and CN
CN chromophore shows high βzzz, βzxx, and βxxz values; this means that the electric field of the fundamental beam induces an appreciable polarization along the molecular plane confirming a two-dimensional charge transfer pattern. Moreover, βzzz values are about 50% greater than those of βzxx and βxxz, indicating a preferential charge flow along the molecular dipole moment and a weaker charge flow along the substituent groups OMe
OMe and CN
CN. These behaviors do not depend on the isomerization of the chromophores. From Tables 5 and 6, we note differences between OMe
OMe and CN
CN chromophores that are similar to that observed for the susceptibility in section 3.3. Such a feature is

Table 7. Comparison between the βz Values Measured by the EFISH Technique with Respect to That Obtainable Using the βijk Components of Tables 5 and 6 through Equation 8 chromophore

βz (10
30 esu)

βz EFISH (10
30 esu) 67.31

OMe
OMe open

36.94

OMe
CN open

26.07

30.54

OMe
OMe closed

86.04

98.36

OMe
CN closed

156.09

88.39

deeply related to the different charge flow pattern inside of the molecules during the SHG. The OMe
OMe chromophore shows βzzz, βzxx, and βxxz values higher in the closed form than in the open form, as a result of an increased conjugation. On the other hand, for the CN
CN chromophore the charge flow pattern during the light excitation leads to an increase of βzzz going from the closed to the open form while the βzxx and βxxz values decrease. It should again be noted that for the OMe
OMe chromophore in the closed form the βzxx 6¼ βxxz so that Kleinman’s symmetry is not satisfied even at the molecular level. The βzzz value for the OMe
CN chromophore (Tables 5 and 6) increases going from the open to the closed form, and its value in the open form is lower than those for OMe
OMe and CN
CN chromophore in the open form as well. These behaviors are consistent with the hypothesis of one-dimensional charge transfer along the z axis. In fact, during the excitation of the fundamental beam, the induced polarization is larger as the conjugation is increased by the closing processes. As a check of our statistical models, we have compared the βz value measured by the EFISH technique with respect to that obtainable from the βijk components of Tables 5 and 6 for one symmetrically substituted chromophore (OMe
OMe) and the asymmetrically (OMe
CN) substituted dithienylethene. From eq 5, in the case of a molecule with C2 symmetry in which the orthogonal components to the plane zx of the first hyperpolarizability are neglected and the intrinsic permutation symmetry is considered, βz is given 1 βz ¼ βzzz þ ðβzxx þ 2βxxz Þ 3

ð8Þ

while for a one-dimensional hyperpolarizability along z, βz = βzzz. In Table 7 the βz values calculated for both OMe
OMe and OMe
CN chromophores are listed, using eq 8 for the OMe
OMe chromophore. For chromophores in the open form, the difference between the βz value obtained by corona poling and that measured by EFISH is within errors associated to the SHG and EFISH techniques. Instead, for chromophores in the closed form the difference is higher. However, it is worth noting that, as already observed in section 3.3 for susceptibility, the components of the first hyperpolarizability are affected by resonance enhancement, 20431

dx.doi.org/10.1021/jp2060409 |J. Phys. Chem. C 2011, 115, 20425–20432

The Journal of Physical Chemistry C higher for OMe
CN than for OMe
OMe. The resonance enhancement is not present in the EFISH measurements because the samples do not absorb at 953 nm. Therefore, we can conclude that the one- and two-dimensional models for OMe
CN and OMe
OMe chromophores, respectively, are in reasonable agreement with the EFISH experimental data.

4. CONCLUSIONS Photoswitching of the SHG signal of poled phenyl-substituted dithienylethene/PS thin films was shown, which evidence a different behavior in function of their molecular structure. The three nonzero, χXXZ(2), χZXX(2), and χZZZ(2) coefficients of secondorder susceptibility at 1064 nm have been measured for both isomeric forms of the diarylethene in polymer films. The relationship between χXXZ(2), χZXX(2), and χZZZ(2) for OMe
OMe and CN
CN differs from that found for the OMe
CN chromophore, suggesting for the two symmetrically substituted molecules a two-dimensional first hyperpolarizability. We have checked the two-dimensional first hyperpolarizability model, by a statistical analysis from the correlation between susceptibility and first hyperpolarizability. Analysis of βxxz, βzxx, and βzzz values has confirmed for OMe
OMe and CN
CN a two-dimensional charge flow pattern, which is different from the OMe
CN. Moreover, the goodness of our statistical model was confirmed by comparing the βz value measured by the EFISH technique respect to that calculated by the three components βxxz, βzxx, and βzzz. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

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