Acidoinduced Second-Order Nonlinear

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Huge Electro-/photo-/acidoinduced Second-order Nonlinear Contrasts from Multiaddressable Indolinooxazolodine Flavie Bondu, Rachid Hadji, Gyorgy Szaloki, Olivier Aleveque, Lionel Sanguinet, Jean-Luc Pozzo, Dominique Cavagnat, Thierry Buffeteau, and Vincent Rodriguez J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.5b03070 • Publication Date (Web): 07 May 2015 Downloaded from http://pubs.acs.org on May 11, 2015

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The Journal of Physical Chemistry

Huge Electro-/Photo-/Acidoinduced Second-Order Nonlinear Contrasts From Multiaddressable Indolinooxazolodine Flavie Bondu,1 Rachid Hadji,2 György Szalóki,2 Olivier Alévêque,2 Lionel Sanguinet,2* Jean-Luc Pozzo,1 Dominique Cavagnat, 1 Thierry Buffeteau,1 Vincent Rodriguez1# 1

Université de Bordeaux, Institut des Sciences Moléculaires, UMR 5255 CNRS, 351 Cours de la Libération, F-33405 Talence Cedex, France

2

Laboratoire Moltech-Anjou, Université d’Angers, UMR 6200 CNRS, 2 Boulevard Lavoisier, F-49045 Angers Cedex, France

Abstract In this work, linear and nonlinear optical properties of electro-/acido-/photo-switchable indolino[2,1-b]oxazolidine derivatives were investigated. The linear optical properties of the closed and open forms have been characterized by UV-Visible and IR spectroscopies associated with DFT calculations. Nonlinear optical properties of the compounds have been obtained by ex-situ and in-situ hyper-Rayleigh experiments in solution. We show that protonated, oxidized and irradiated open forms exhibit the same visible absorption and NLO features. In particular, the closed and open forms exhibit a huge contrast of the first hyperpolarizability with an enhancement factor of 40-45. Additionally, we have designed an original electrochemical cell that allows to monitor in-situ the hyper-Rayleigh response upon electrical stimulus. We report notably a partial but good and reversible NLO contrast in-situ during oxidation/reduction cycles. Thereby, indolinooxazolidine moieties are versatile trimodal switchable units which are very promising for applications in devices.

Corresponding authors:

# [email protected] *[email protected]

Keywords: NLO molecular switches, indolinooxazolidine, electrochrome, photochrome, acidochrome, hyper-Rayleigh scattering

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1-Introduction The elaboration of molecular switches continues to arise a lot of interest in a large scientific community. These systems can be resumed as a molecular system where one property could be modulated by application of an external stimulus such as light, chemicals species, proton or electron.1,2 In this context, numerous molecules were designed in order to modulate their optical,3 photophysical,4 redox,5 or complexation behaviors.6 Among them, the elaboration of molecular systems possessing the ability to modulate their nonlinear optical (NLO) properties have received a rising interest since the last decades. In fact, NLO-active chromophore could be used in various applications such as biomedical applications (imaging, sensing, and treatment), optical communications, optical data processing and storage, microfabrication.7 In this context, the design of molecular systems where their NLO properties can be modulate by external stimulation are still of current interest. 8 , 9 , 10 , 11 To modulate a property, light is certainly one of the most attractive solution due to the ease of addressability, fast response times and the compatibility of light with a wide range of condensed phases. 12 Thus, the development of new photo-switches presenting a large modulation of their NLO properties between initial and photo-induced state continue to arouse a great interest. Disperse red 1 (DR1) is certainly one of the most wide spread example of photoswichable NLO-phore, where the trans/cis isomerization associated with the photochromism of azobenzene reduces its first hyperpolarizability by a factor 5. Semi-empirical calculations indicate a shift from β°trans= 44.6 x10-30 to β°cis= 8.4 x10-30 esu.13 The photomodulation of NLO properties via a modification of the communication between donor and acceptor parts is not restricted to a trans/cis isomerization of stillbene or azobenzene. As recently reviewed, 14 numerous photoswitchable NLO-phores are diarylethene-based photochromic compounds. The large change of conjugated pathways along photocyclisation affords high contrast between initial and photoinduced states. For example, Le Bozec and co-workers reported recently some dithienylethene-based platinum(II) complexes exhibiting an enhancement of NLO properties by a factor 12 after UV irradiation.15 In order to photo-modulate the NLO properties, another attractive strategy consists in modifying by stimulation the donor/acceptor capacity of corresponding groups within push-pull system.16 In this context, some of us have designed systems incorporating a benzazolooxazolidine (BoX) moiety. The ring-opening of the oxazolidine can be reversibly and selectively achieved either under UV irradiation or acidity changes conducting to the corresponding benzazolium which acts as strong electron withdrawing group. Associated with various styrilic residues, several multi-addressable 2 ACS Paragon Plus Environment

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switches in nonlinear optics displaying both photo- and acidochromic performances were prepared.

17 , 18

More recently, we have demonstrated that the association of an

indolinooxazolidine unit with a bithiophene moiety acting as a redox center leads to an interesting trimodal switch where the interconvertion between the colorless closed form and a colored open form can be indifferently performed by light, proton or electrical stimulation.19 The additional possibility to stimulate our system by electrical way would represent a strong benefits as the influence of the stimulation on the first hyperpolarizability could be more precisely studied: the conversion is quick and quantitative in both ways and by avoiding the addition of chemicals aliquots, we can study at constant concentration several cycles of switching on/off process. Although in-situ electrochemical switching of some NLO-phores was already reported,20 we described here a new compact experimental set up that allows us to monitor in-situ the evolution of the first hyperpolarizability of any chromophore as function of the applied potential. This new equipment was used to fully characterize the NLO properties of the trimodal switch and determine NLO contrast between initial, photoinduced and electrogenerated states. In addition, comparative study in IR spectroscopy was carried out on all states and described below. Finally, the last section provides experimental and computational details.

2-Results and discussion 2.1- Linear optical properties The two BoX derivatives are presented in scheme 1. Both of them present some photochromic, acidochromic and electrochromic properties.19 The heterocyclic ring can be either opened by irradiation at 254 nm, by adding an acid or by oxidation at 0.60 V vs Fc/Fc+ and closed back by acidity change or by reduction at -0.80 V vs Fc/Fc+.



Scheme 1. Closed and open forms of the BoX derivatives 1 and 2.

The UV-Vis absorption spectra of molecule 1 (Figure 1) and 2 (not showed), in acetonitrile/chlorobenzene (90/10), match exactly, highlighting the poor influence of the methyl group. The closed form (1c) is almost colorless characterized by a λmax centered at 360 nm. As displayed in figure 1, the addition of acid, UV irradiation (254 nm) or oxidation at 0.6 V (vs Fc/Fc+) conduct in all cases to a drastic change of the UV-Vis absorption spectrum with the appearance of a new band at 521 nm. Absorption spectra of the open forms (1o) obtained from protonation, irradiation and oxidation are superimposable indicating that protonated, photo and electro-generated forms adopt similar structures. This open form displays an intense and broad band (ε ~ 40000 L.Mol-1.cm-1) which is rather expected for BoX moieties with good NLO responses.21 Moreover, the fluorescence emission of protonated compound 1 exhibits a maximum at 702 nm, giving thus a Stokes shift of ~5000 cm-1 which is rather typical of bithiophene π-conjugated linker.22 The Stokes shift decreases to 3900 cm-1 when the experiences are carried out in dioxane which is a low polarity solvent. This indicates a typical positive solvatochromism and reveals a large nuclear reorganization in the emissive state where an intramolecular charge transfer (ICT) transition occurs with probably a strong dipole moment change (∆µ) upon excitation.

4

1c 1o/irradiation 1o/protonated 1o/oxydation

4

3 10

360 nm

-1

ε / L.mol .cm

-1

4 10

Fluorescence emission

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4

521 nm

2 10

702 nm

4

1 10

1o/protonated

0 300

400

500

600

700

800

Wavelength / nm Figure 1. UV-Vis absorption spectra of closed form 1c and open forms 1o (in acetonitrile/chlorobenzene (90/10)) obtained from irradiation at 254 nm, from addition of PFA (protonated) and from oxidation at +0.60 V vs Fc/Fc+. Emission fluorescence spectrum of 4 ACS Paragon Plus Environment

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the protonated form 1o (in acetonitrile/chlorobenzene (90/10)) is also reported (λexc = 520 nm). Hence, the photophysical properties of 1 and 2 are similar and quite strong NLO 2nd order responses are expected. 23 Note however that the introduction of a methyl in position 5 warrants reversible redox cycles, i.e. molecule 1 is expected to be the best stable candidate for electro commutation.19 The experimental IR spectra of closed and open (using irradiation and oxidation processes) forms of compound 1 are reported in Figure 2. These spectra were obtained using an ATR accessory (diamond crystal) after evaporation of an acetonitrile solution of 1 (see experimental section). The frequencies observed on the ATR spectra for the most important bands as well as their assignments are reported in Table 1. This assignment has been performed from the literature24 and on the basis of the visual observation with GaussView software of the fundamentals calculated at the density functional theory (DFT) level using the B3PW91 functional and 6-31G* basis set.

Table 1: Assignment of the major bands of compound 1 in the closed and open forms. Wavenumbers (cm-1) 1c

Assignment 1o

irradiation

oxidation ν(OH) free

3590

1636

3210 (broad) -

ν(OH) bounded

1579

ν(C=C) vinyl

1579

1615, 1595 1620, 1605

1620, 1605 ν8a,b(C=C) phenyl

1547

1540

1540

νa(C=C) thiophene

1450

1450

1450

νs(C=C) thiophene

1430-1400

1430-1400

ν(C=N), ν(C=C) conjugated system

1300

1300

δ(C=C–H) in plane

1278

A satisfactory agreement between the calculated and the ATR experimental spectra was obtained for 1c and 1o (see experimental section). The band associated with the stretching vibration of the vinyl moiety, ν(C=C), is observed at 1636 cm-1 on the ATR spectrum of 1c. This band is shifted to 1579 cm-1 for 1o with an enhancement of its intensity. This feature is 5 ACS Paragon Plus Environment


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due to the higher conjugation along the alternating single and double bonds induced by the opening of the oxazolidine ring. The bands due to the ν8aC=C and ν8bC=C stretching vibration of the phenyl ring of the BoX occur around 1615 and 1595 cm-1 for 1c and 1620 and 1605 cm-1 for 1o, respectively.

1c

1o/irradiation

1o/oxidation

3600

3200

2800

*

2400 1600

1200

*

800

-1

Wavenumber / cm

Figure 2. IR absorption spectra (ATR) of closed form 1c (top) and open form 1o obtained from irradiation at 254 nm (middle) and from oxidation at 0.60 V vs Fc/Fc+ (bottom; *: PF6 bands). 6 ACS Paragon Plus Environment

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For 1c, the antisymmetric and symmetric stretching vibration of the C=C bonds of the thiophene rings give rise to the bands observed at 1547 and 1450 cm-1, respectively. The intensity of these bands slightly increases in the open form, due to the higher conjugation of the system. The two intense bands around 1420 cm-1 observed for 1o come from the conjugation of the open system; they are related to the coupling of the νC=N+, νC=C (vinyl) and νC=C (thiophene) modes. The C=C-H in plane deformation (δC=C-H) of the vinyl group is observed at 1300 and 1278 cm-1 for 1o and 1c, respectively. This frequency difference is due to the different interaction of the C-H bond of the vinyl group with the open or closed oxazolidine ring. Assignment of the vibrational modes in the spectral range lower than 1300 cm-1 is more complex because the observed bands correspond to coupled modes. All these observations clearly indicate that the closed form of 1 can be easily distinguished from its open form using infrared spectroscopy. In the other hand, very small spectral differences were observed for the two open forms of 1 obtained by oxidation or irradiation. Nevertheless, examination of the ATR spectra in the 3800-2800 cm-1 spectral range reveals that the bands corresponding to the stretching vibration of the OH groups (νOH) are different for the two open forms of 1. In the case of the oxidized/protonated form, the weak sharp band at 3590 cm-1 is characteristic to νOH vibration of free OH group which is a stabilizing feature.17, 18, 21 In contrast, for the irradiated form, a broad band is observed around 3210 cm-1, corresponding to νO---H+ vibrations which makes probably easier the backward reaction to the closed form. Besides, the ATR spectrum of the oxidized form 1o is marked by the presence of hexafluorophosphate anions as revealed by the very strong bands at 836 and 557 cm-1, associated with the ν3 and ν4 vibrations of octahedral (XY6) molecules.25

2.2- Nonlinear Optical Properties In order to determine the first hyperpolarizability of the open and the closed form of 1 and 2, typical hyper-Rayleigh scattering (HRS) measurements were carried out at 1064 nm by varying concentration and incident power (Figure 3). For both compounds, a typical 3D representation was obtained, reporting nicely the linear relationship of the HRS response with the concentration of dye as well as the quadratic dependence of the collected signal following equation 4 reported in the experimental section. These experiments are completed by plotting the polarization curves of solvated dyes in the open and closed forms (Figure 3). In order to 7 ACS Paragon Plus Environment


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allow a comparison with other reported photoswichable NLO-phore, similar experiments were conducted on DR1 a well-known dye which is often used as reference. The extracted NLO data for the two dyes, as well as Disperse Red one are gathered in Table 2.

Table 2. Data deduced from HRS measurements at 1064 nm in acetonitrile/chlorobenzene (90/10) and tetrabutylammonium hexafluorophosphate (TBAP) 0.1 M. Hyperpolarizabilities (with relative accuracy ca 5%) are expressed in atomic units a) using convention T (Taylor series). 26 Data from Disperse red one (DR1) in acetonitrile are reported for comparison purpose. β HRS 1c

3

2.8×10

ρ

β J =1

3.5

1.28 ± 0.06

4.6×10

3

/irradiation

106×10

/oxydation

112×10

3

4.7

0.88 ± 0.02

206×10

2c

2.2×10

3

3.9

1.13 ± 0.10

3.8×10

/irradiation

100×10

/oxydation

98×10

2o

3

3

117×10

3

4.7

0.89 ± 0.01

o βc β HRS HRS

β J =3

3

1o

DR1

DR

3

38

5.9×10

194×10

3

172×10

3

3

182×10

3

40

3

4.3×10

3

45

4.8

0.87 ± 0.01

184×10

3

160×10

3

4.8

0.86 ± 0.01

181×10

3

155×10

3

45

4.3

0.99 ± 0.01

208×10

3

206×10

3

-

a) 1 atomic unit of β = 3.62 × 10-42 m4V-1 = 8.641 × 10-33 esu.

As suggested by IR and UV-Vis spectroscopic data analyses, the HRS measurements confirm that the structure and therefore the NLO properties of the open form are not related to the nature of the stimulation. In fact, 1o and 2o present identical (within the experimental error) amplitude (βHRS) and depolarization ratio (DR) whatever the stimulus (irradiation or oxidation). Moreover, the amplitudes βHRS of the open forms are really impressive in comparison with other dyes. As example, their amplitude are about twice time greater than the prototypical octupolar crystal violet, 27 and scale as DR1 which is known to exhibit an extended efficient charge transfer between the HOMO and LUMO states. In fact the HRS amplitude of 1o (2o) scales with other efficient switchable BoX previously reported in the literature, in particular the protonated dimethylaminophenyl substituted BoX (compound 2c in reference28). Actually, a strong NLO response of 1o (and 2o) is expected since from one side bithiophene acts as a highly π-conjugated moiety whereas on the other side thiomethyl is a strong donating group so that the open form achieve a nice asymmetric charge transfer which is optimum for second order nonlinear optical response. 29 The anisotropy parameter 8 ACS Paragon Plus Environment

ρ = 0.9 ≈ 2 3 , or the depolarization ratio DR = 4.7 ≈ 5, are typical of chromophores at near resonance (λmax= 521 nm) with the second harmonic emission at 532 nm. This situation corresponds to the limiting case of the two-state approximation (HOMO/LUMO) with a 1-D component βzzz.30 Note that for all dyes at near resonance with the harmonic response at 532 nm, i.e. all open forms 1o, 2o and even DR1, we had to minimize the input laser energy (operating at 2 kHz with 65 ps pulses) to low range (≤ 10 µJ) to avoid drawbacks from twophoton absorption and fluorescence (possible optical biphotonic pumping) as described previously for octupolar merocyanine dyes.27 Regarding the closed forms, they exhibit similar low amplitude down to a few thousand of atomic units as well as similar anisotropic factors ρ indicating a dominating octupolar character, both in accordance with a loss of planarity of the o βc molecule toward a 3-D bulky molecular structure. The contrast β HRS HRS between the open

and closed forms 1o (2o) is very efficient, 40 (45), and it scales as the best one in comparison with other photochromes and particularly other BoX moieties.15,

28

Thus this NLO study

clearly points out that both 1o and 2o molecules are among the best candidates up to now for NLO switching with trimodal capabilities. Hyper Rayleigh intensity (arb. unit)

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7 6 5 4

I

I

HV

HV

3

I

2

VV

1 0 -100 -50

0

50

100 150 200 250

Ψ (Deg.)

Hyper Rayleigh intensity (arb. unit)

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300 250 200

I

I

HV

HV

150 100

I

VV

50 0 -100 -50

0

50

100 150 200 250

Ψ (Deg.)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 3. HRS responses of 1c (top) and 1o/oxidation (bottom) in solution: 3D representation as a function of the incident power and concentration (left) and extracted plot of the polarization curve of the solvated dye (right). Circles are experimental data and solid lines are best fitted curves according to Equations 4 (left) and 5 (right). 2.3- In-situ nonlinear electro-switching Taking advantage of the good NLO contrast obtained previously from ex-situ experiments, a spectroelectrochemical cell has been developed to monitor in-situ NLO responses. This cell, presented in the experimental section (Figure 7), has been especially designed for in-situ HRS scattering measurements at 90°. Thus, we have fully characterized the HRS response of 1c and 1o in the condition of electrochemical cyclic voltammetry at 10 mV.s-1 between -1 V and 0.8 V vs Fc/Fc+ at the working electrode in order to electroswitch reversibly the molecule. The obtained results in-situ are reported in Table 3 and compared with those obtained from ex-situ oxidation and photo-irradiation. Table 3. HRS data of 1o obtained from irradiation and oxidation ex-situ and in-situ.

βHRS a)

DR

1o/irradiation

(106 ± 8) ×103

4.7 ± 0.1

1o/ oxidation ex-situ

(112 ± 8) ×103

4.7 ± 0.1

1o oxidation in-situ

(100 ± 8) ×103

5.0 ± 0.1

a) in atomic units using convention T (Taylor series).26 The HRS amplitude βHRS measured in-situ is slightly weaker than that determined out of polarization (ex-situ) but it remains at the same order of magnitude. This slight difference could be due to a partial electro-commutation under these conditions. However, more significant is the change of the depolarization ratio DR which increases by about 0.3 to reach the value of 5. This difference could be explained by the electric field which produces an additional EFISH (Electric Field-Induced Second Harmonic) contribution (electro-optic Kerr 10 ACS Paragon Plus Environment

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effect) to the HRS response given by (β + γ EDC). Thus, the variation of the amplitudes between a measure in-situ and ex-situ, could be explained by the contribution of the optical second hyperpolarizability γ which exhibits a frequency dispersion (additive or subtractive contribution at 1064 nm). Independently of its amplitude, the EFISH contribution, γ EDC, to the HRS signal is purely dipolar with DR= 9 and then contributes to an increase of the effective total depolarization ratio. This expectation is in good agreement with our observation since we observe a clear increase of ca 6% of DR for in-situ experiments. Both reversibility and stability are important parameters for a switching molecular unit. Concerning BoX derivative 1, the perfect stability of the signal over cycle translates the perfect reversibility of the electrocommutation between open and closed forms. The oxidation peak at +0.60 V vs Fc/Fc+ corresponds to the oxidation of the closed form towards the open form and the reduction peak at -0.80 V vs Fc/Fc+ corresponds to the reduction of the open form towards the closed form (Figure 4).

15

10

I (µ µA)

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5

0

-5

-10

-0.8

-0.4

0

+

0.4

E (V vs Fc/Fc ) Figure 4. Cyclic voltammetry of the molecule 1 in acetonitrile/chlorobenzene (90 / 10) and (TBA, PF6) 0.1 M at 10 mV.s-1.



200

HRS intensity (arb. unit)

10 0

150

I (µA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-10 100

-20 -30

50 -40 0

0

400

800

1200

-50

1600

Time (s) Figure 5. Current and HRS intensity response (VV polarized) for several oxidation/reduction cycles (10 mV.s-1). During the cyclic voltammetry (CV) experiments (10 mV.s-1), the electro-commutation between 1c and 1o is monitored by following the HRS signal as function of time (Fig. 5). As expected, the scattered NLO intensity increases slowly with a time delay of typically more than a hundred of seconds after the oxidation of the closed form at +0.60 V vs Fc/Fc+ until it reaches a (relative) maximum value of ~105.. Hereafter, the signal intensity decreases also with a time delay after the reduction of the open form at -0.80 V vs Fc/Fc+ to reach a minimum value of 15 after more than a hundred of seconds. Thus the HRS signals and the CV experiments are correlated with a time lag. This is explained by the time required to allow the diffusion of electro-generated species from the working electrode surface to the focus point of the laser situated at ca 500 µm (Figure 7). At this distance, all molecules are not electrocommutated in the volume of the focus point during the electrochemical cycles, a perfect correlation could be obtained only at the electrode surface. Taking into account the diffusion laws (thickness of the diffusion layer = 2()/ ), we thus assume that only a small amount of compound 1 is electro-commuted at this distance with the consequence of a weaker

(

o c NLO intensity contrast than expected ( < β HRS β HRS

)

2

= ( 40 )

2

) for quantitative electro-

commutation. By consequence, the analysis of the contrast value measured at ~ 500 µm permit to determine the molar fraction x of electro-commutation at this distance. The contrast between the maxima and the minima (see Fig. 5) can be estimated as


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I Max I Min

105 I Solvent + (1 − x ) I1c + xI1o = 15 I Solvent + I1c

(1)

where I Solvent , I1c , I1o are respectively the HRS intensity contribution of the solvent, the closed form and the open form. Assuming that the absorption at 532 nm is negligible, we estimate the molar fraction x of 1o

(

VV I  I Solvent + I1c x =  Max − 1 VV VV  I Min  I1o − I1c

where I xVV ∝

(

)

)

(2)

9 2 6 2 β J =1 ( x ) + β J =3 ( x ) .30 Considering that I1VVc 45 105

I Solvent and I1VV o

I1VV c (see

Table 2), Equation 2 becomes

( ) ( )

VV  I Max  I1c x  − 1 VV ≈ 3.6%  I Min  I1o

(3)

Clearly even a small amount of electro-commutated dye gives a very nice HRS contrast which actually makes chromophore 1 a very efficient multiresponsive system.

2.4-Conclusion In summary, we have investigated the UV-Vis absorption and emission spectra of indolinooxazolidine units coupled with a bithiophene moiety as redox center. The structure of closed and open forms have been characterized by the IR ATR technique and compared with DFT calculations. The HRS responses of the two multi-switchable BoX derivatives exhibit the best contrast factor ever reported as high as 40-45. A good stability and reversibility of BoX 1 has been demonstrated during redox cycles and make it very attractive for further applications. Among them, thin films seems to be quite attractive.11 As reported recently by two of us, a direct comparison between thin layer spectroelectrochemistry in solution and spectroelectrochemistry on SAMs has shown that a SAM could be viewed as a highly concentrated solution. 31 For that purpose, the thiomethyl group could be advantageously replaced by a long thioalkyl chain bearing an appropriate anchoring group.

3-Experimental and computational details 3.1- UV-Vis and infrared



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UV-Vis absorption spectra were recorded on a Perkin Elmer Lambda 650 spectrophotometer in acetonitrile/chlorobenzene (90/10). Fluorescence spectra were recorded on a SPEX spectrofluorometer in acetonitrile and dioxane solutions with chromophore concentration of 3.10-6 M.. The instrument response was previously calibrated with a certified lamp. The ATR spectra were recorded on a Thermo-Scientific Nicolet iS50 FT-IR spectrometer equipped with a DLaTGS detector (KBr window) using the diamond iS50 ATR accessory. Each spectrum was obtained in the 4000-400 cm-1 spectral range, at a resolution of 4 cm-1, by coadding 200 scans. The ATR spectrum of 1c was obtained after evaporation of an acetonitrile solution of 1c. The ATR spectra of 1o were obtained after evaporation of an acetonitrile/chlorobenzene (90/10) solution of 1o (irradiation process) or after evaporation of an acetonitrile solution (with presence of hexafluorophosphate anions) of 1o (oxidation process).

3.2- DFT calculations The geometry optimizations, vibrational frequencies, and absorption intensities were calculated by Gaussian 09 program32 on the DELL cluster of the MCIA computing center of the University of Bordeaux (Sciences and Technologies). Calculations of the optimized geometry of the closed and open forms of compound 1 were performed at the density functional theory level using B3PW91 functional and 6-31G* basis set. Vibrational frequencies and IR intensities were calculated at the same level of theory. For comparison to experiment, the calculated frequencies were scaled by 0.968 and the calculated intensities were converted to Lorentzian bands with a half-width of 7 cm-1 (see Figure 6).


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Molar absorptivity / a.u.

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Molar absorptivity / a.u.

Page 15 of 22

1c

1o/protonated

3600

3200

2800

2400 1600

1200

800

-1

Wavenumber / cm

Figure 6. Calculated IR spectra of the closed and open forms of 1.

3.3- Hyper-Rayleigh Scattering HRS measurements were performed with diluted solutions in acetonitrile/chlorobenzene (90/10) and 0.1 M (TBA, PF6) with chromophore concentrations ranging from 10-4 to 10-5 M for the closed forms and from 5.10-5 to 2.10-6 M for the open forms. All solutions were centrifuged during 20 minutes to prevent spurious scattering from particles. The solvent mixture was previously calibrated versus pure acetonitrile and used subsequently as an internal reference.30 DR1 measurements were performed with diluted solutions in acetonitrile ranging from 10-5 to 10-6 M. Full details about the HRS measurements and the procedure are described elsewhere.27 The incident radiation at 1064 nm was obtained from a passively mode-locked Nd:YVO4 laser (EKSPLA) producing trains of 65 ps, ≤ 50 µJ pulses at a repetition rate of 2 kHz. The experimental setup is configured in a classical 90° scattering 15 ACS Paragon Plus Environment


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geometry that allows hyper-Rayleigh but also hyper-Raman detection. 33 , 34 The elliptical polarization of the incident light denoted by the polarization angle Ψ was obtained by using a combination of a rotating half-wave plate (λ/2) and a fixed quarter-wave plate (λ/4). All elliptical states of polarization of the incident light could be obtained, as well as linear polarization H (Ψ = 0°) and V (Ψ = 90°) and left/right circular polarization (Ψ = ±45°). The polarized incident laser beam was focused into the sample cell with a 5X Mitutoyo Plan APO NIR objective (infinity-corrected with NA=0.14) and the scattered light was collected at 90° with f /1.2 optics and focused into a modified Horiba spectrograph, with vertical (V) polarization selection. HRS measurements were analyzed in terms of multipolar description, where β is decomposed into the sum of dipolar (βJ=1) and octupolar (βJ=3) tensorial βJcomponents.35 The total harmonic scattered light for a binary solution (solvent/chromophore) in the diluted regime is given by:

{

}

2  Iψ2Vω = G  CS  β J =1 CΨV  + C X   S  

 β 2 C   .10−α 2ω CX .  I ω  2    J =1 ΨV  X 

(4)

where the subscripts S and X stand for the solvent and chromophore respectively; G is a constant containing geometrical, optical and electrical factors of the experimental setup; C is the concentration (molarity) of the solvent or solute;

accounts for the one-photon absorption of the total second harmonic response from the chromophore. Ψ is the orientational average of the molecular spherical components of the hyperpolarizability of the solute or solvent, given by: 9  12 12 2  4 6 2   20 10 2  2 CΨV i =  + ρ i  + − + ρ i  cos Ψ +  − ρ i  cos Ψ  45 105   45 105   45 105 

()

()

()

( )

()

( )

(5)

The nonlinear anisotropy ρ = |βJ=3|/|βJ=1| compares the relative contributions of the octupolar and dipolar components of the hyperpolarizability tensor β. The commonly used depolarization ratio, defined as:

I DR = VV I HV

 2ρ 2  1+ 7 =9 2 1 + 12 ρ 7 

    

(6)


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ranges from 3/2 in the case of pure octupolar response to 9 for a pure dipolar response. The total HRS intensity (obtained without polarization analysis of the scattered beam) which is a common reference value for the literature is given by β HRS = β J =1

2  1 ρ2   +  33 7   

(7)

3.4-Electrochemistry

Electrochemical experiments were carried out with Radiometer PGP201 potentiostat driven by the VoltaMaster software. For ex-situ experiments, the full electrolysis of a solution of 1 in acetonitrile/chlorobenzene solution with TBAP as electrolyte (0.1M) was carried out in a divided cell. Working and counter electrodes were platinum grids and a silver wire was used as pseudo reference electrode. The initial uncolored solution was electrolyzed at a controlled potential of 0.6 V and monitored by CV.

Figure 7. Scheme of the spectro-electrochemical cell used for in situ HRS measurements. In-situ experiments were performed in a self-made three electrode cell (Figure 7). Working and counter electrodes were Au substrates prepared by deposition of ca. 5 nm of chromium followed by ca. 100 nm of gold onto a glass substrate through a shadow mask (MECACHIMIQUE/France) using a physical vapor deposition system PVD ME300 17 ACS Paragon Plus Environment


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PLASSYS/France. This protocol provides reproducible Au(111) surfaces with high crystallographic quality, low roughness (Ra less than 2 nm) and with a defined geometry. A silver wire was used as pseudo reference electrode. Ex-situ and in-situ experiments were recorded

in

HPLC-grade

acetonitrile

(Sigma-Aldrich)

with

tetrabutylammonium

hexafluorophosphate (Bu4NPF6, electrochemical grade, Fluka) as supporting electrolyte. Potentials were calibrated versus ferrocenium/ferrocene couple (Fc+/Fc).36

Acknowledgments This work was supported by grants (FB, RH, GS) from the ANR (PHOEBUS) and has been done in the framework of the center of Excellence LAPHIA. V.R. and F.B. are grateful to C. Aupetit for fluorescence experimental support and to F. Adamietz for HRS experimental support and technical developments. V. R. thanks CNRS and Région Aquitaine for funding supports.


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Current and hyper-Rayleigh intensity response for several oxidation/reduction cycles of an indolinooxazolidine-based trimodal molecular switch.

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Acidoinduced Second-Order Nonlinear

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