Acido- and Phototriggered NLO Properties Enhancement - The

May 14, 2005 - Obviously, for this compound, we find a polarization ratio / = 4.94 (see Table 2), very close to the value of the pure rodlike dipole w...
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J. Phys. Chem. B 2005, 109, 11139-11150

11139

Acido- and Phototriggered NLO Properties Enhancement Lionel Sanguinet and Jean-Luc Pozzo* Chimie Supramole´ culaire, Biomime´ tisme et Nanoscience - UMR 5802 CNRS - UniVersite´ Bordeaux I, Cours de la Libe´ ration, 351, F-33405 Talence CEDEX, France

Vincent Rodriguez,* Fre´ de´ ric Adamietz, Fre´ de´ ric Castet, and Laurent Ducasse Laboratoire de Physico-Chimie Mole´ culaire - UMR 5803 CNRS - UniVersite´ Bordeaux I, Cours de la Libe´ ration, 351, F-33405 Talence CEDEX, France

Benoıˆt Champagne Laboratoire de Chimie The´ orique Applique´ e, Faculte´ s UniVersitaires Notre-Dame de la Paix, rue de Bruxelles, 61, B-5000 Namur, Belgium ReceiVed: December 17, 2004; In Final Form: April 4, 2005

In this paper, acido- and phototriggered enhancement of the nonlinear optical (NLO) properties are achieved by combining the 10-(2-arylethenyl)indolino[2,1-b]oxazolidine unit with various styrylic residues. The synthetic pathway allows the preparation of phenyl, naphthyl, anthryl, and phenanthryl derivatives, which have been shown to display photochromic and acidochromic behavior at ambient temperature. The multiaddressable molecular switches are characterized by a large contrast of the NLO response along the reversible transformations. Hyper-Rayleigh scattering (HRS) experiments clearly indicate the strong influence of the electrodonating substituent borne by the aromatic system. Theoretical calculations have been performed for representative molecules showing a good agreement with the experimental hyperpolarizabilities. This enables a better understanding of the NLO responses in terms of structural and electronic parameters for both closed and protonated open forms.

I. Introduction A photochromic compound is characterized by its ability to alternate between two different chemical forms having different absorption spectra in response to irradiation of appropriate wavelengths.1 Photochromism is defined as a light-induced, reversible change of color. Some chemical species undergo reversible photochemical reactions that encompass reversible physical phenomena, leading to devices that act as switches,2 optical memories,3 sources of variable electrical current, enhancers of ion transport through membranes, and modulators of wettability. Organic photochromism also offers the unique possibility to modify the self-assembly process of the individual molecules and the resulting supramolecular network by means of light.2 Moreover, the photochromic molecules can be used as molecule-scale switching units of electronic and magnetic interactions.3 Switching of metal ion capture and energy or electron transfer can also occur as a consequence of a photochromic reaction.4 The current interest in miniaturizing the components of electronics down to the molecular level is a major driving force in the research of molecular systems with switchable properties.5 Among the molecular properties that could be tuned along with this transformation, nonlinear optical (NLO) properties have motivated numerous groups.6 Organic photochromes that exhibit a large change in electronic distribution between the two states are promising candidates.7 For instance, one could expect a pronounced change between respective first hyperpolarizabilities (β) for photochromic * Corresponding authors. V. Rodriguez, [email protected]. J. L. Pozzo, [email protected].

SCHEME 1: Photochromic Equilibrium for 10-(2-Arylethenyl)indolino[2,1-b]oxazolidines

compounds alternating between a neutral colorless form and a charged colored form. Here, we report the synthesis and the NLO properties of a new series of photochromes based on the indolino[2,1-b]oxazolidine moiety. Our approach in the design of multiaddressable chromophores that could exhibit interesting photo- and acidotunable NLO properties is based on cyanine dye derivatives which exhibit a pure zwitterionic character. Because one key aspect toward the progress of molecular switching technology is the integration of several switchable functions into a single molecule, the drastic change in linear and nonlinear optical properties due to the photostimulated ringopening is also reversibly revealed on pH variation. In the photochromic system based on the oxazolinic ring-opening, the basic process is the breaking of a σ-bond, leading to a zwitterionic species (Scheme 1). The protonation of the oxazoline ring leads to the formation of a colored protonated open form (POF) which could be reversibly reverted upon base addition (Scheme 2). To characterize the electronic as well as linear and nonlinear optical properties of the selected indolino-oxazolidines, hyper-

10.1021/jp0442450 CCC: $30.25 © 2005 American Chemical Society Published on Web 05/14/2005

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TABLE 1: UV-vis Spectroscopic Data for Open Forms 5a′-l′ compounds

5a′

5b′

5c′

5d′

5e′

5f ′

5 g′

5h′

5i′

5j′

5k′

5l′

λmax (nm) fwhma (cm-1)

401 4650

426 4500

430 4750

548 2600

573 4100

551 4950

571 4500

550 5100

439 4500

520 4100

523 4200

445 4650

a

Full width at half-maximum.

SCHEME 2: Acidochromic Equilibrium for 10-(2-Arylethenyl)indolino[2,1-b]oxazolidines

SCHEME 4: Formulas of Target Molecules 5a-l

SCHEME 3: Synthesis of Target Moleculea

a Reagents: (a) 2-iodoethanol, (b) various aromatic aldehydes, (c) NaOH.

Rayleigh scattering (HRS) measurements have been combined with quantum chemical simulations. HRS is known to be the method of choice for determining the second-order NLO responses of charged species.8 On the other hand, theoretical simulations help in interpretating structure-property relationships9 so that the mixed theory/experiment analysis is expected to be useful for rational design of new multiaddressable NLOphores. Section II reports the general synthesis and photo- and acidochromic properties of a wide class of compounds. Sections III and IV detail the experimental and theoritical studies of representative NLO chromophores. II. Synthesis and Photo- and Acidochromic Properties Taking advantage of both acido- and photoinduced ringopening of 10-styrylindolino[2,1-b]oxazolidine, we designed target molecules having aromatic residues varying from phenyl, naphthyl, anthryl, or phenanthryl moieties. In that regard, we set up a general synthetic pathway which greatly extends the range of previously reported parent molecules.10 Furthermore, as dipolar push-pull chromophores probably constitute the widest class of compounds investigated for their NLO properties,11 various donor groups such as methoxy or N,N-dimethylamino were introduced on the aromatic part. Tetraalkylindoleninium iodides 2a-d can be prepared in reasonable yield by quaternarisation of suitable 2,3,3-trialkylindolenines 1a-d with 2-iodoethanol (Scheme 3). Resulting 1-(2-hydroxyethyl)-2,3,3-trialkylindoleninium iodides and various aromatic aldehydes were dissolved in absolute ethanol, and then, a stoichiometric amount of N-methylmorpholine was added by portion. Starting from 1a, this experimental setup prevents the formation of 9,9,10-trimethylindolino[2,1-b]oxazolidine 3 as a byproduct. The intermediate Schiff base reacts on the carbonylated compounds, and subsequent water elimination leads to the formation of the protonated open forms 4a-l of the target molecules, denoted POF for convenience. The

indolino-oxazolidines 5a-l (Scheme 4) are directly obtained when the aromatic nucleus does not possess a electrodonating substituent as carbon C-2 is a sufficiently electrophilic center to react with the alcohol in an intramolecular process. When substituents such as methoxy and N,N-dimethylamino groups are present, the conversion of the stable POF to the closed forms is simply achieved upon basic treatment using NaOH. This strategic approach was successfully used for the preparation of extended-conjugated molecules in 54-67% yield. All prepared molecules 5a-l are photochromic at ambient temperature, and the corresponding light-induced open forms, 5a′-l′ (labeled OF in Scheme 1) exhibit an absorption band in the visible. The 2-arylethenyl group connected to the indolino[2,1-b]oxazolidine on the 10-position had only a minor influence on the absorption bands of the heterocyclic moiety, since the absorption spectra were similar (λmax ca. 330 nm). In contrast, the wavelength maxima of the colored open forms (5a′-l′) are strongly correlated to the nature of the aryl group borne by the double bond and vary in a huge range from 401 to 573 nm (see Table 1). This bathochromic shift arises from two distinct effects, (i) the number and position of the rings involved in the aromatic system, as illustrated by the 122 nm shift when going from the tolyl to the 9-methylanthryl derivatives, and (ii) the electrodonating character of the substituent. In that connection, a shift of ca. 120-150 nm is observed with the N,N-dimethylamino group regardless of the aromatic system used. Besides 5d′, which presents a smaller bandwidth, the full width at halfmaximum (fwhm) values range between 4100 and 5100 cm-1. Moreover, the absorption spectra arising from UV irradiation and upon acidic addition are totally superimposable, indicating that photoinduced and acidogenerated color forms adopt similar geometries. Noticeably, the compounds under their open forms do not exhibit a strong dependence toward solvent polarity, which is not so common for zwitterionic species. This character is well-supported by the chemical shifts found for the methylene group borne by the nitrogen atom under the protonated open forms 4d, 4.73 and 51 ppm, respectively, for 1H and 13C NMR. Photogenerated absorption maxima were respectively found at 548, 547, and 546 nm for 5d′ in ethanol, dimethyl sulfoxide, and acetonitrile solutions. This indicates that the negatively charged oxygenated group does not participate in the visible electronic transition. Furthermore, UV irradiation reveals a large absorption band centered at 548 nm (5d′: OF) which is accompanied by a decrease of the absorption at 330 nm (5d: CF) in acetonitrile (Figure 1). Moreover, a vibronic structure of ∼1400 cm-1 is observed.

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TABLE 2: Optical and NLO Properties of Selected Chromophores at 1064 nma λmax (nm) 5d 4b 4c 4d 4e DR1

310 426 430 544 573 488

λF (nm) 633 584 604 652 640

IF/IHRS

β1064 zzz (au)

R ) βzxx/βzzz

2ω I2ω VV/IHV

β0 (au)

0.13 0.81 0.60 1.01 0.26

4 600 ( 1 000 18 800 ( 1 500 21 500 ( 1 000 144 000 ( 6 000 91 500 ( 7 500 107 500 ( 2 000

-0.307 ( 0.052 -0.090 ( 0.040 -0.070 ( 0.090 -0.006 ( 0.013 -0.060 ( 0.050 -0.057 ( 0.012

2.62 ( 0.06 4.16 ( 0.04 4.34 ( 0.04 4.94 ( 0.05 4.44 ( 0.05 4.46 ( 0.05

2 800 4 850 6 300 4 850 11 550 13 400

a The absorption maximum and the two-photon induced fluorescence maximum are given by λ max and λF, respectively. The integrated twophoton induced fluorescence intensity and the integrated HRS intensity are indicated by IF and IHRS, respectively. Absolute values of the longitudinal 1064 -42 m4 V-1 ) hyperpolarizability, β1064 zzz , were obtained by comparison with βxyz ) 19 au of CCl4 and are given in atomic units (1 au ) 3.62 × 10 3.2063 × 10-53 C3 m3 J-2 ) 8.641 × 10-33 esu). The static hyperpolarizability, β0 (β∞zzz), is estimated from the two-states model. DR1 (Disperse Red 1) results are included for comparison purpose, since it is a reference compound.

Figure 1. Absorption spectral change of 5d/5d′ in acetonitrile solutions (10-3 mol L-1) under photoirradiation (330 nm).

Back total photobleaching could be achieved using an appropriate visible irradiation. Thermal irreversibility and interconversion could also be reached through the use of acidbase treatment cycles. A Stoichiometric amount of acetic acid or gaseous hydrochloric acid provokes total conversion toward protonated open forms (POF, 4a-l) which can be reverted to indolino[2,1-b]oxazolidine (CF, 5a-l) by simply adding sodium hydroxide. This cyclic treatment could be repeated several times until dilution diminishes initial absorbance. The open- and closed-ring isomers of the indolino[2,1-b]oxazolidines differ from each other not only in their absorption spectra but also, as we will show, in their NLO responses. The NLO properties of a representative selection among these molecules are now addressed. III. NLO Properties The molecules have been selected to display various donor/ acceptor characteristics, starting from the methoxy-substituted phenyl compound (4b). The effect of increasing the donor strength has been investigated by considering a phenyl ring substituted by three methoxy groups (4c) or by replacing the methoxy group of 4b by a N,N-dimethylamino group (4d). In regard to 4d, the phenyl moiety has been replaced by a naphthyl group to address the impact of the nature of the aromatic ring (4e). Finally, the closed form associated with 4d has been considered (5d). All the details about the NLO measurements and theoretical modeling are reported in the Experimental Section. III.a. Polarized Hyper-Rayleigh Scattering Experiments. It is well-known that many chromophores of interest for applications in nonlinear or electro optics also exhibit a large two-photon induced fluorescence effect. Typically, the twophoton induced fluorescence overlaps with the HRS spectrum to a significant extent. As a result, the values for β obtained

Figure 2. Absorption spectra (right, dotted line) and two-photon induced fluorescence (left, solid line) in the VV scattering geometry of 4d in acetonitrile (10-5 mol/L) with irradiation at 1064 nm. Although the nonlinear fluorescence intensity is very strong, the HRS peak at 2ω ) 18 797 cm-1 is easily detectable here (see inset) because of the high signal/noise ratio and the high spectral resolution (∼2.5 cm-1). A vibronic structure of ∼1500 cm-1 is observed in the two-photon fluorescence spectrum.

from HRS measurements can be overestimated. It has been shown that either time-resolved femtosecond experiments12,13 or spectral analyses14,15 give more accurate determination of the hyperpolarizabilities. This is illustrated in Figure 2 for the 4d compound, which gives the most intense two-photon fluorescence scattering. In any case, a quadratic power dependence of the harmonic light process has been evidenced for all compounds under study. The molecular hyperpolarizabilities, βzzz and βzxx, have been determined for all compounds, and the results are reported in Table 2. The obtained values are listed in atomic units (au) and were measured at 1064 nm in acetonitrile. If compound 4d is used as an illustration, Figure 3 details both the linear dependence with chromophore concentration and the quadratic power dependence of the scattered harmonic light. In Figure 4, which gives the polar plot of the obtained polarization ratios 2ω 2ω IψV /IHV using setting S2 (see Experimental Section), the quadratic dependence of this ratio with the retardation angle ψ is evidenced well. Obviously, for this compound, we find a 2ω polarization ratio I2ω VV/IHV ) 4.94 (see Table 2), very close to 2ω the value of the pure rodlike dipole where I2ω VV/IHV ) 5. III.b. Theoretical Simulations. The molecular structures of the chromophores 5d and 4d obtained by geometry optimization at the B3LYP/6-31G level of approximation are schematized in Figure 5. The POFs present a dihedral angle between the planes of the six-membered rings on both sides of the conjugated bridge ranging between 12° (4d) and 17° (4b). On the contrary, the indolinic and styryl parts of the 5d molecule are almost perpendicular, with a dihedral angle equal to 82°. The transition energies, oscillator strengths, changes in dipole moment, and molecular orbital (MO) characteristics of the most

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Figure 3. Experimental (points) and best fitted curves (lines) of harmonic light, following eq A-4 (see Experimental Section 2): 3-D (left) and 2-D (right) plots as a function of the incident power and concentration (in mol/L) of 4d with irradiation at 1064 nm.

TABLE 3: Transition Energies (eV), Wavelengths (nm), Oscillator Strengths, and Changes of the Dipole Moment (au) for the Dominant Low-Energy Charge Transfer Excited State Calculated at the CIS/INDO/S Levela ∆Ege ∆Ege (exp) λge 5d 3.936 3.989 4b 2.756 4c 2.670 4d 2.495 4e 2.240

Figure 4. Polar plot of the measured (solid squares) and calculated 2ω 2ω (solid line) polarization ratios, IψV /IHV, of 4d with irradiation at 1064 nm, as a function of the retardation angle ψ (in degrees), using setting S2 (see Experimental Section). The curve shows that the HRS polarization data can be well-fitted assuming 4d has a pseudo-planar 1064 C2V symmetry with R ) β1064 zxx /βzzz ) -0.006 ( 0.013.

4.00 2.91 2.88 2.28 2.16

315 311 450 464 497 554

fge

∆µge

main CI coefficients

0.588 0.332 1.230 1.216 1.405 1.286

0.94 0.58 0.97 1.82 1.85 1.36

65% H f L, 23% H f L + 2 25% H f L, 57% H f L + 2 93% H f L 91% H f L 91% H f L 92% H f L

a For comparison, the experimental transition energies [Ege(exp)] are also reported. The last column lists the main CI expansion coefficients. H ) HOMO ) highest occupied molecular orbital; L ) LUMO ) lowest unoccupied molecular orbital.

Figure 5. Molecular structures and Cartesian frame used in the theoretical calculations for 5d and 4d. The H atoms are omitted for clarity.

significant charge transfer excitations, obtained at the INDO/S level, are reported in Table 3. The absorption wavelengths of the POF chromophores range from 450 nm (4b) to 554 nm (4e). The most absorbing charge transfer state is dominated by a π-π* transition between the HOMO and the LUMO, both delocalized over the whole molecule, as illustrated in Figure 6 for the 4d compound. It is associated with a transfer of charge from the N,N-dimethylaminophenyl moiety toward the CHdCH bridge and the oxazolidine group (Figure 7). The phenyl part at the extremity of the oxazolidine group does not display significant variation of charge upon excitation, consistent

Figure 6. HOMO (top) and LUMO (bottom) for the 4d compound.

with the shape of the HOMO and LUMO involved in this excitation (Figure 6). The closed form 5d presents two quasi-degenerate low-energy charge transfer excited states. These two excited states are both characterized by a significant transition moment and a transition energy roughly 1.5 eV larger than the corresponding POF, 4d. The lower-energy excited state involves a dominant contribution from the HOMO-LUMO (π-π/1) transition, both localized on

Acido- and Phototriggered NLO Properties Enhancement

J. Phys. Chem. B, Vol. 109, No. 22, 2005 11143 TABLE 4: Main Static First Hyperpolarizability Tensor Components (au), Averaged Macroscopic Hyper-Rayleigh 0 /I0HV a VV Responses, and Depolarization Ratios, DP ) IVV CPHF/6-31G 5d βzzz βzxx βyzz βxzz 2 1/2 〈βzzz 〉 0 IVV/I0HV 4b βzzz βzxx βyzz

Figure 7. Mulliken charge differences between the ground state and the first excited state of the 4d compound calculated at the INDO/S level.

the oxazolidine moiety, as well as a nonnegligible contribution from the HOMO-LUMO + 2 (π/2, localized on the styryl part). The situation is reversed for the second excited state having a major (minor) contribution from the π-π/2 (π-π/1) transition. To assess the effects of including higher-order excitations in the configuration interaction (CI) space, similar calculations were also performed using a truncated CI scheme in combination with the AM1 Hamiltonian. Both sets of results on the transition energies as well as on the characteristics of the excited states lead to identical conclusions. Table 4 gathers the static electronic first hyperpolarizability components obtained within various semiempirical and ab initio schemes. The molecular axes used in the calculations are those reported in Figure 5. With respect to the coupled perturbed Hartree-Fock (CPHF) approach, the inclusion of electron correlation at the MP2 level leads to an increase of βzzz by 93% to 138%, similar to the effect evidenced on other push-pull chromophores such as p-nitroaniline and 2-methyl-4-nitroaniline.16,17 When changing the basis set from 6-31G to 6-31G*, β decreases slightly. For example, the dominant βzzz tensor component of 4d is reduced by 8% and 13% within the CPHF and MP2 approaches, respectively. For all POF systems, the CPHF/AM1 βzzz values are slightly larger (18-23%) than the corresponding MP2/6-31G values, while they are very similar for the closed form 5d. The SOS/INDO/S results are very similar to the CPHF/AM1 ones for 5d, 4d, and 4e. For 4b and 4c, involving methoxy groups, the difference is a bit larger and amounts to 28-29%. Although the absolute values of βzzz cover a rather wide range, the relative values with respect to 4b (given in parentheses in Table 4) show a qualitative agreement. Nevertheless, the order of the βzzz amplitudes is inverted between 4c and 4e, from CPHF (4b < 4c < 4e < 4d) to MP2 (4b < 4e < 4c < 4d). Moreover, the relative AM1 βzzz values are very similar to the MP2 ones,

βxzz 2 1/2 〈βzzz 〉 0 0 IVV/IHV 4c βzzz βzxx βyzz βxzz 2 1/2 〈βzzz 〉 0 IVV/I0HV 4d βzzz βzxx βyzz βxzz 2 1/2 〈βzzz 〉 0 IVV/I0HV 4e βzzz βzxx βyzz βxzz 2 1/2 〉 〈βzzz 0 IVV/I0HV

MP2/6-31G

CPHF/AM1

SOS/INDO/S

771 (0.22)b 1487 (0.18) 1538 (0.16) 400 (0.20) 32 42 34 66 -241 -372 -422 -416 -100 -148 -150 -178 325 616 637 591 4.40 4.87 4.66 4.59 3440 (1.00) 8122 (1.00) 9751 (1.00) 6886 (1.00) -255 -86 -382 -887 245 421 -528 -663 -252 455 218 -783 1260 3066 3616 2506 4.24 4.85 4.60 3.78 4571 (1.33) 10 883 (1.34) 13 265 (1.36) 9510 (1.38) -224 -49 -356 -793 260 471 -566 -778 -234 475 303 -824 1686 4114 4947 3496 4.48 4.91 4.73 4.18 6099 (1.77) 12 732 (1.57) 15 087 (1.55) 14 506 (2.10) -377 -334 -671 -1387 468 830 -985 -1281 -590 -185 -503 -1399 2260 4760 5592 5314 4.33 4.73 4.55 4.06 5140 (1.49) 10 362 (1.28) 12 786 (1.31) 12 806 (1.86) -457 -337 -724 -1555 668 1341 -1355 -1759 -588 -262 -505 -1480 1916 3942 4774 4734 4.08 4.69 4.47 3.87

a Calculated using various ab initio or semiempirical schemes. b The values given in parentheses are the magnitudes of the βzzz compared to 4b.

which indicates that the AM1 parametrization implicitly includes a part of the electron correlation, necessary for a correct description of the NLO properties. On the contrary, the SOS/ INDO/S approach overestimates (with respect to MP2/6-31G and CPHF/6-31G) the relative βzzz values of 4d and 4e. To compare the theoretical and experimental results, we also report 2 1/2 〉 values, calculated within convenin Table 4 the mean 〈βzzz tions described in ref 18, as well as the theoretical static depolarization ratio DP ) I0VV/I0HV. The dipolar nature of the chromophores are clearly evidenced, since all the DP ratios are found close to 5. Dynamic first hyperpolarizabilitiy values evaluated at the semiempirical SOS/INDO/S and TDHF/AM1 levels are reported in Table 5 along with the DP ratios. With respect to the static values, the order of the dynamic SOS/INDO/S βzzz amplitudes is the same, with the exception of 4d and 4e. This inversion comes from the smaller excitation energy of the 4e compound (Table 3). A similar impact of frequency is found at the TDHF/ AM1 level where the βzzz value of 4e is enhanced relatively more than for the other compounds. DP ratios are found to increase and get closer to 5 with frequency dispersion. IV. Discussion and Concluding Remarks All POF compounds exhibit substantial NLO responses. Moreover, they display large variations with the nature of the donor group, which are now addressed by combining the experimental and theoretical data. The experimental absorption maximum wavelengths of compounds 4b and 4c, containing methoxy-substituted phenyl

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TABLE 5: Longitudinal SHG First Hyperpolarizabilities βzzz 2ω 2ω a (au) and Depolarization Ratios DP ) IVV /IHV SOS/INDO/S 5d 4b 4c 4d 4e

βzzz 2ω I2ω VV/IHV βzzz 2ω I2ω VV/IHV βzzz 2ω I2ω VV/IHV βzzz 2ω I2ω VV/IHV βzzz 2ω I2ω VV/IHV

TDHF/AM1

1500 nm

1064 nm

1500 nm

1802 (0.16)b 4.73 11 514 (1.00) 4.16 17 456 (1.52) 4.52 29 286 (2.54) 4.47 32 672 (2.84) 4.52

2418 (0.09) 4.88 27 064 (1.00) 4.50 51 364 (1.90) 4.81 146 150 (5.40) 4.83

2184 (0.10) 4.80 21 549 (1.00) 4.82 34 585 (1.60) 4.92 44 065 (2.04) 4.84 56 707 (2.63) 4.89

a Calculated using the SOS/INDO/S and TDHF/AM1 schemes with wavelengths of 1500 and 1064 nm. b The values given in parentheses are the magnitudes of the βzzz compared to 4d.

groups, are almost identical (∼430 nm). The same trend is observed for the calculated values. The experimental longitudinal static values, β0, are obtained here from extrapolation, following the two-states model19 of experimental values measured under off-resonance conditions. In agreement with all theoretical results, β0 values of 4b and 4c are found to be within the same order of magnitude of 5000-6000 au. Moreover, the increase of the donor strength from 4b to 4c is illustrated well through their relative NLO static and dynamic responses. This effect is mainly due to a larger variation of the dipole moment ∆µge in 4c than in 4b (1.82 vs 0.97 au, see Table 3). In the case of the N,N-dimethylaminophenyl derivative (4d), the measured first hyperpolarizability is the largest among the compounds under investigation. Thus, the 5d/4d couple displays a β variation of about a factor of 31 when switching from the closed form (CF) to the POF. The SOS/INDO/S scheme predicts that this enhancement is frequency-sensitive and amounts to 10, 16, and 61 for λ ) ∞, 1500 nm, and 1064 nm, respectively. Since the second harmonic generation (SHG) experiments were performed near resonance (λmax ) 544 nm), the extrapolation of β0 using the two-state model (Table 2) is less accurate than for 4b and 4c. Indeed, the measured value, which is similar to the 4b value, is probably underestimated, since its donor strength is larger than for the methoxy group. This idea is confirmed by the theoretical results (Table 4) where the β0 ratios between 4d and 4b range from 1.55 to 2.10. Moreover, in addition to the decrease of the excitation energy, the change in the dipole moment upon excitation and the oscillator strength increase by going from 4b to 4d (Table 3). The N,N-dimethylaminonaphthyl derivative, 4e, presents the lowest transition energy among all the POF compounds, which is associated with the largest frequency dispersion. When going from 4d to 4e, the extent of the conjugated bridge perpendicular to the charge transfer direction should not significantly change the NLO static responses, as indicated by the calculations. The experimental values fail with this point, but this descrepancy may be related to stronger resonant conditions encountered in 4d. The similar β0 value encountered for 4d and 4e results, however, from a compensation between the variations of ∆Ege and ∆µge. The various indolino-oxazolidines do not present significant variations in their DP ratios, with the exception of the small experimental value of 5d. In this paper, we have revisited and greatly enlarged the 10(2-arylethenyl)indolino[2,1-b]oxazolidine series. We have clearly demonstrated that acido- and phototriggered enhancement of the NLO properties could be readily obtained from the

combination of a reliably versatile indolino-oxazolidine unit and various styrylic residues. The synthetic pathway allows the preparation of phenyl, naphthyl, anthryl, and phenanthryl derivatives which have been shown to display photochromic and acidochromic behavior at ambient temperature. As expected, the multiaddressable molecular switches are characterized by a large contrast of the NLO response along the reversible transformation. Furthermore, HRS experiments indicate the strong influence of the electrodonating substituent borne by the aromatic system. The theoretical calculations globally reproduce the experimental trends within the series and enable a rationalization of the NLO responses in terms of structural and electronic parameters. These experimental/theoretical studies provide a promising way to design new multiaddressable NLOphores with improved efficiencies. Within the indolino-oxazolidine series, this might be achieved by modifying the indolinic residue. V. Experimental Section V.a. Synthesis. General. 1H NMR and 13C NMR characterizations were performed on a Bruker AC250 instrument. The residual protons from the solvent are taken as an internal reference, and chemical shifts are reported in parts per million relative to tetramethylsilane. UV-vis measurements were performed using a Hitachi U-3300 spectrophotometer. Melting points were uncorrected. All commercially available materials were used without further purification unless otherwise stated. Solvents used for spectroscopic measurements were all spectrograde. Column chromatography was performed using silica gel Si 60 (40-63 µm) from Merck. General Procedure for the Preparation of Indoleninium Iodides 2a-d. A mixture of appropriate arylhydrazinium hydrochloride (30 mmol) and ketone (30 mmol) is dissolved in glacial acetic acid (50 mL) and then stirred at 60 °C under nitrogen for 5 h. The medium is concentrated in vacuo. The residue obtained is dissolved in CH2Cl2 (100 mL) and washed with aqueous K2CO3 10% (2 × 100 mL), dried over Na2SO4, and evaporated. After 12 h drying in a vacuum, the indolenines 1a-d were used in the next reaction without further purification. A mixture of indolenine (30.00 mmol) and 2-iodoethanol (30.00 mmol) is refluxed under nitrogen for 12 h; then, the medium is allowed to cool and washed with Et2O (3 × 20 mL). The precipitate is filtered and washed with Et2O (20 mL)/EtOH (0.1 mL). 5-Methoxy-2,3,3-trimethylindolenine (1b). Compound 1b was obtained as a yellowish oil (94%). 1H NMR (250 MHz, CDCl3, δ): 7.41 (d, J ) 8.0 Hz, 1H), 6.81 (d, J ) 2.0 Hz, 1H), 6.79 (dd, J ) 8.0, 2.0 Hz, 1H), 3.81 (s, 3H), 2.22 (s, 3H), 1.26 (s, 6H). 13C NMR (63 MHz, CDCl3, δ): 185.8, 157.9, 147.4, 147.3, 120.0, 112.0, 108.1, 55.7, 53.7, 23.2, 15.3. 2′-Methylspiro[cyclohexane-1,3′-indolenine] (1c). Compound 1c was obtained as a yellow-orange solid (94%). mp 43 °C. 1H NMR (250 MHz, CDCl3, δ): 7.70 (d, J ) 7.4 Hz, 1H), 7.53 (d, J ) 7.4 Hz, 1H), 7.31 (dt, J ) 7.4, 1.2 Hz, 1H,), 7.14 (dt, J ) 7.4, 1.2 Hz, 1H), 2.26 (s, 3H), 2.14-1.24 (m, 10H). 13C NMR (63 MHz, CDCl3, δ): 187.7, 154.2, 144.5, 127.4, 124.2, 124.1, 120.1, 57.8, 31.1, 25.2, 21.6, 16.1. 5′-Methoxy-2′-methylspiro[cyclohexane-1,3′-indolenine] (1d). Compound 1d was obtained as a yellowish oil (96%). 1H NMR (200 MHz, CDCl3, δ): 7.43 (d, J ) 8.5 Hz, 1H), 7.27 (d, J ) 2.5 Hz, 1H), 6.83 (dd, J ) 8.5, 2.5 Hz, 1H), 3.82 (s, 3H), 2.22 (s, 3H), 1.95-1.25 (m, 10H). 1-(2-Hydroxyethyl)-2,3,3-trimethylindoleninium iodide (2a). Compound 2a was obtained as colorless needles (70%). mp 169

Acido- and Phototriggered NLO Properties Enhancement °C (acetone). UV λ (EtOH) nm (): 220 (13715), 280 (3768). 1H NMR (250 MHz, DMSO-d , δ): 7.97 (m, 1H), 7.87 (m, 6 1H), 7.63 (m, 2H), 5.00 (bs, 1H), 4.61 (t, J ) 5.0 Hz, 2H), 3.88 (t, J ) 5.0 Hz, 2H), 2.83 (s, 3H), 1.56 (s, 6H). 13C NMR (63 MHz, DMSO-d6, δ):197.7, 141.8, 141.1, 129.3, 128.8, 123.5, 115.6, 57.8, 50.3, 22.0, 14.6. MS (EI) m/z (relative intensity): 203 (M+, 46), 188 (M+ - 15, 33), 172 (23), 158 (50), 146 (100), 128 (81). MS (FAB) m/z: 204 [M + H]+. 1-(2-Hydroxyethyl)-5-methoxy-2,3,3-trimethylindoleninium iodide (2b). Compound 2b was obtained as an amorphous solid (96%). mp 209 °C. UV λ (EtOH) nm (): 202 (24562), 218 (18638), 307 (5363). 1H NMR (250 MHz, DMSO-d6, δ): 7.87 (d, J ) 8.9 Hz, 1H), 7.52 (d, J ) 2.4 Hz, 1H), 7.14 (dd, J ) 8.9, 2.4 Hz, 1H), 4.56 (t, J ) 4.7 Hz, 2H), 4.30 (bs, 1H), 3.86 (t, J ) 4.7 Hz, 2H), 3.86 (s, 3H), 2.77 (s, 3H), 1.54 (s, 6H). 13C NMR (63 MHz, DMSO-d6, δ):194.7, 160.5, 143.8, 134.2, 116.5, 114.2, 109.3, 57.8, 56.1, 54.0, 50.2, 22.0, 14.2. MS (EI) m/z (relative intensity): 233 (M+, 65), 218 (M+ - 15, 35), 188 (51), 176 (100), 128 (85). MS (FAB) m/z: 234 [M + H]+. 1′-(2-Hydroxyethyl)-2′-methylspiro[cyclohexane-1,3′-indoleninium] iodide (2c). Compound 2c was obtained as an amorphous solid (90%). mp 224 °C. UV λ (EtOH) nm (): 203 (46296), 219 (23597), 282 (5551). 1H NMR (250 MHz, DMSOd6, δ): 8.15 (d, J ) 7.6 Hz, 1H), 8.01 (d, J ) 7.6 Hz, 1H), 7.68 (t, J ) 7.6 Hz, 1H), 7.60 (t, J ) 7.6 Hz, 1H), 4.62 (t, J ) 4.9 Hz, 2H), 3.87 (t, J ) 4.9 Hz, 2H), 3.49 (bs, 1H), 2.86 (s, 3H), 2.12-1.39 (m, 10H). 13C NMR (63 MHz, DMSO-d6, δ): 196.7, 141.4, 140.3, 128.7, 128.5, 125.8, 115.7, 57.9, 57.7, 50.2, 29.6, 23.7, 20.0, 15.1. MS (EI) m/z (relative intensity): 243 (M+ - 1, 38), 228 (74), 188 (87), 144 (81), 128 (100). MS (FAB) m/z: 244 [M]+. 1′-(2-Hydroxyethyl)-5′-methoxy-2′-methylspiro[cyclohexane1,3′-indoleninium] iodide (2d). Compound 2d was obtained as an amorphous solid (79%). mp 181 °C. UV λ (EtOH) nm (): 202 (33197), 218 (19295), 299 (3443). 1H NMR (200 MHz, DMSO-d6, δ): 7.93 (d, J ) 8.9 Hz, 1H), 7.54 (d, J ) 2.3 Hz, 1H), 7.23 (dd, J ) 8.9, 2.3 Hz, 1H), 4.57 (t, J ) 4.9 Hz, 2H), 3.89 (s, 3H), 3.85 (t, J ) 4.9 Hz, 2H), 2.79 (s, 3H), 2.06-1.41 (m, 10H). 13C NMR (50 MHz, DMSO-d6, δ): 193.7, 159.6, 142.2, 134.8, 116.6, 113.4, 112.2, 57.8, 57.6, 56.1, 50.2, 29.5, 23.6, 19.8, 14.9. MS (EI) m/z (relative intensity): 273 (M+ 1, 67), 258 (91), 218 (77), 174 (55), 128 (100). MS (FAB) m/z: 274 [M]+. General Procedure for the Preparation of IndolinoOxazolidines 5a-l. A mixture of the corresponding indoleninium iodide (3.12 mmol) and the aldehyde (3.12 mmol) was dissolved, under nitrogen, in absolute EtOH (5 mL). Nmethylmorpholine (3.12 mmol) was added over 60 min onto the medium under reflux; after the end of the addition, the reflux was maintained for 8 h. After cooling, the mixture was concentrated under vacuo, and the residue was purified by column chromatography, (SiO2, CH2Cl2/CH3OH). 9,9-Dimethyl-10-[2-(p-tolyl)ethenyl]indolino[2,1-b]oxazolidine (4a). Compound 4a was obtained as colorless crystals (65%). mp 110 °C (ethyl ether). UV λ (EtOH) nm (): 257 (24234), 286 (6187), 296 (3250). UV λ (EtOH/HCl) nm (): 252 (7724), 401 (34929). UV λ (MCH) nm (): 257 (23273), 286 (6615). 1H NMR (250 MHz, CDCl3, δ): 7.35 (d, J ) 7.9 Hz, 2H), 7.16 (m, 1H), 7.14 (d, J ) 7.9 Hz, 2H), 7.08 (bd, J ) 7.3 Hz, 1H), 6.93 (t, J ) 7.3 Hz, 1H), 6.83 (d, J ) 15.9 Hz, 1H), 6.79 (d, J ) 7.3 Hz, 1H), 6.22 (d, J ) 15.9 Hz, 1H), 3.813.41 (m, 4H), 2.34 (s, 3H), 1.43 (s, 3H), 1.15 (s, 3H). 13C NMR (63 MHz, CDCl3, δ): 150.6, 139.8, 137.8, 133.6, 132.1, 129.3, 127.5, 126.6, 124.7, 122.4, 121.6, 112.0, 109.9, 63.5, 50.1, 47.9,

J. Phys. Chem. B, Vol. 109, No. 22, 2005 11145 28.4, 21.2, 20.3. MS (EI) m/z (relative intensity): 305 (M+, 26), 290 (M+ - 15, 13), 200 (32), 145 (100), 115 (49). MS (FAB) m/z: 306 [M + H]+; Anal. Calcd. for C21H23NO: C, 82.58; H, 7.59; N, 4.59. Found: C, 82.63; H, 7.74; N, 4.49. 1-(2-Hydroxyethyl)-2-[2-(4-methoxyphenyl)ethenyl]-3,3-dimethylindoleninium iodide (4b). Compound 4b was obtained as orange crystals (67%). mp 173 °C. 1H NMR (250 MHz, CDCl3, δ): 8.33 (d, J ) 8.5 Hz, 2H), 8.11 (d, J ) 15.5 Hz, 1H), 8.02 (d, J ) 15.5 Hz, 1H), 7.60-7.52 (m, 4H), 7.08 (d, J ) 8.5 Hz, 2H), 4.93 (t, J ) 5.5 Hz, 2H), 4.52 (t, J ) 7.5 Hz, 1H), 4.22 (dt, J ) 7.5 and 5.5 Hz, 2H), 3.94 (s, 3H), 1.85 (s, 6H). 13C NMR (63 MHz, CDCl3, δ): 166.6, 155.5, 145.2, 143.3, 141.1, 135.0, 129.7, 127.2, 123.0, 117.2, 115.8, 114.8, 111.4, 59.2, 56.3, 51.3, 50.7, 28.0. MS (LSIMS) m/z: 332. Anal. Calcd. for C21H2NO2I: C, 56.13; H, 5.38. Found: C, 56.21; H, 5.54. 1-(2-Hydroxyethyl)-2-[2-(3,4,5-trimethoxyphenyl)ethenyl]3,3-dimethylindoleninium iodide (4c). Compound 4c was obtained as purple crystals (67%). mp 173 °C. 1H NMR (250 MHz, CDCl3, δ): 8.06 (bs, 2H), 7.60-7.52 (m, 4H), 7.35 (s, 2H), 5.03 (t, J ) 5.5 Hz, 2H), 4.54 (t, J ) 7.5 Hz, 1H), 4.24 (dt, J ) 7.5 and 5.5 Hz, 2H), 4.08 (s, 6H), 3.98 (s, 3H), 1.89 (s, 6H). 13C NMR (63 MHz, CDCl , δ):155.9, 154.1, 145.2, 143.3, 141.1, 3 135.0, 130.1, 130.0, 129.3, 123.0, 117.2, 115.0, 113.1, 109.6, 61.6, 59.2, 58.4, 51.1, 48.3, 28.0. MS (LSIMS) m/z: 368. Anal. Calcd. for C23H28NO4I: C, 54.23; H, 5.54. Found: C, 54.21; H, 5.71. 1-(2-Hydroxyethyl)-2-[2-(4-dimethylaminophenyl)ethenyl]3,3-dimethylindoleninium iodide (4d). Compound 4d was obtained as purple crystals (72%). mp 197 °C. UV λ (EtOH) nm (): 279 (6915), 547 (61231). 1H NMR (250 MHz, CDCl3, δ): 8.17 (bs, 2H), 8.01 (d, J ) 15.5 Hz, 1H), 7.63 (d, J ) 15.5 Hz, 1H), 7.40 (m, 4H), 6.75 (d, J ) 9.1 Hz, 2H), 4.73 (t, J ) 5.5 Hz, 2H), 4.42 (t, J ) 7.5 Hz, 1H), 4.12 (dt, J ) 7.5 and 5.5 Hz, 2H), 2.94 (s, 6H), 1.69 (s, 6H). 13C NMR (63 MHz, CDCl3, δ): 180.4, 155.2, 155.0, 142.1, 141.1, 135.7, 129.2, 127.6, 122.6, 122.3, 112.9, 112.5, 106.2, 58.6, 50.9, 49.0, 40.4, 28.0. MS (EI) m/z: 334 (M+, 20), 304 (12), 174 (100), 146 (35). Anal. Calcd. for C22H27N2OI: C, 57.15; H, 5.89 Found: C, 57.26; H, 5.73. 10-[2-(4-Dimethylaminophenyl)ethenyl]-9,9-dimethylindolino[2,1-b]oxazolidine (5d). Compound 5d was obtained as yellowish crystals (61%). mp 142 °C (petroleum ether). UV λ (EtOH) nm (): 235 (16861), 298 (21958), 546 (3024). UV λ (EtOH/HCl) nm (): 278 (5558), 548 (57516). UV λ (MCH) nm (): 205 (54880), 235 (25013), 297 (29373). 1H NMR (250 MHz, CDCl3, δ): 7.35 (bd, J ) 8.7 Hz, 2H), 7.16 (dt, J ) 7.5 and 1.2 Hz, 1H), 7.08 (bd, J ) 7.5 Hz, 1H), 6.93 (dt, J ) 7.5 and 1.2 Hz, 1H), 6.79 (d, J ) 7.5 Hz, 1H), 6.77 (d, J ) 15.9 Hz, 1H), 6.70 (d, J ) 8.7 Hz, 2H), 6.06 (d, J ) 15.9 Hz, 1H), 3.78-3.47 (m, 4H), 2.96 (s, 6H), 1.43 (s, 3H), 1.15 (s, 3H). 13C NMR (63 MHz, CDCl , δ): 150.7, 150.3, 139.9, 132.1, 3 127.7, 127.4, 124.9, 122.4, 121.5, 120.9, 112.4, 112.0, 110.2, 63.4, 50.0, 47.8, 40.5, 28.4, 20.3. MS (EI) m/z (relative intensity): 334 (M+, 23), 304 (M+ - 30, 14), 174 (100), 146 (23). MS (FAB) m/z: 335 [M + H]+; Anal. Calcd. for C22H26N2O: C, 79.00; H, 7.84. Found: C, 78.94; H, 7.88. 10-{2-[1-(4-Dimethylaminonaphthyl)]ethenyl}-9,9-dimethylindolino[2,1-b]oxazolidine (5e). Compound 5e was obtained as colorless crystals (65%). mp 180 °C (ethyl ether-acetone). UV λ (EtOH) nm (): 222 (18886), 333 (6814). UV λ (EtOH/ HCl) nm (): 256 (7078), 299 (12632), 573 (26592). UV λ (MCH) nm (): 204 (47300), 224 (28515), 334 (10715). 1H NMR (250 MHz, CDCl3, δ): 8.29 (m, 1H), 8.16 (m, 1H), 7.57 (m, 3H), 7.56 (d, J ) 15.9 Hz, 1H), 7.20 (t, J ) 7.4 Hz, 1H), 7.13 (d, J ) 7.4 Hz, 1H), 7.09 (d, J ) 7.9 Hz, 1H), 6.97 (t, J

11146 J. Phys. Chem. B, Vol. 109, No. 22, 2005 ) 7.4 Hz, 1H), 6.84 (d, J ) 7.4 Hz, 1H), 6.25 (d, J ) 15.9 Hz, 1H), 3.93-3.47 (m, 4H), 2.90 (s, 6H), 1.59 (s, 3H), 1.19 (s, 3H). 13C NMR (63 MHz, CDCl3) 151.1, 150.7, 139.7, 132.4, 129.7, 129.0, 128.6, 127.6, 127.3, 126.0, 125.1, 124.7, 124.2, 124.0, 122.4, 121.6, 113.8, 112.0, 110.1, 63.6, 50.2, 47.8, 45.2, 28.5, 20.4. MS (EI) m/z (relative intensity): 384 (M+, 75), 354 (M+ - 30, 26), 224 (100), 209 (63), 152 (52). MS (FAB) m/z: 385 [M + H]+; Anal. Calcd. for C26H28N2O: C, 81.21; H, 7.34. Found: C, 81.34; H, 7.31. 10-{2-[1-(4-Dimethylaminonaphthyl)]ethenyl}-7-methoxy9,9-dimethylindolino [2,1-b]oxazolidine (5f). Compound 5f was obtained as colorless crystals (53%). mp 194 °C (ethyl etheracetone). UV λ (EtOH) nm (): 221 (50867), 333 (13733). UV λ (EtOH/HCl) nm (): 281 (13400), 551 (37267). UV λ (MCH) nm (): 225 (41230), 317 (13125), 333 (13710). 1H NMR (250 MHz, CDCl3, δ): 8.26 (m, 1H), 8.14 (m, 1H), 7.59 (d, J ) 15.4 Hz, 1H), 7.59 (d, J ) 7.8 Hz, 1H), 7.50 (m, 2H), 7.07 (d, J ) 7.8 Hz, 1H), 6.70 (m, 3H), 6.22 (d, J ) 15.4 Hz, 1H), 3.86-2.98 (m, 4H), 3.78 (s, 3H), 2.89 (s, 6H), 1.47 (s, 3H), 1.21 (s, CH3, 3H). 13C NMR (63 MHz, CDCl3, δ): 155.4, 151.0, 144.2, 141.1, 132.4, 129.8, 129.0, 128.6, 127.3, 126.0, 125.1, 124.7, 124.2, 124.0, 113.8, 112.4, 112.3, 110.7, 63.6, 55.8, 50.6, 48.1, 45.2, 28.4, 20.3. MS (EI) m/z (relative intensity): 414 (M+, 57), 384 (M+ - 30, 20), 224 (100), 209 (59), 184 (42), 152 (38). MS (FAB) m/z: 415 [M + H]+. Anal. Calcd. for C27H30N2O2: C, 78.23; H, 7.29. Found: C, 78.14; H, 7.43. 10-{[1-(4-Dimethylaminonaphthyl)]-2-ethenyl}-9-spiroyclohexylindolino[2,1-b]oxazolidine (5g). Compound 5g was obtained as a brownish foam (80%). mp 118-119 °C. UV λ (EtOH) nm (): 203 (69424), 219 (43409), 333 (14005). UV λ (EtOH/HCl) nm (): 212 (54081), 299 (9919), 571 (37556). UV λ (MCH) nm (): 204 (52996), 222 (32551), 334 (11146). 1H NMR (250 MHz, CDCl , δ): 8.26 (m, 1H), 8.14 (m, 1H), 3 7.63 (d, J ) 7.9 Hz, 1H), 7.57 (d, J ) 15.6 Hz, 1H), 7.50 (m, 2H), 7.40 (d, J ) 7.6 Hz, 1H), 7.18 (t, J ) 7.6 Hz, 1H), 7.08 (d, J ) 7.9 Hz, 1H), 6.92 (t, J ) 7.6 Hz, 1H), 6.83 (d, J ) 7.6 Hz, 1H), 6.25 (d, J ) 15.6 Hz, 1H), 3.87-3.49 (m, 4H), 2.90 (s, 6H), 2.08-1.24 (m, 10H). 13C NMR (63 MHz, CDCl3, δ): 151.2, 151.0, 139.0, 132.4, 129.0, 128.9, 128.7, 128.6, 127.4, 126.0, 125.1, 125.0, 124.7, 124.2, 123.8, 121.0, 113.8, 112.0, 110.3, 63.2, 51.3, 50.3, 45.2, 35.7, 29.9, 25.8, 22.8, 21.9. MS (EI) m/z (relative intensity): 424 (M+, 43), 240 (38), 224 (100), 209 (48), 184 (59), 152 (32). MS (FAB) m/z: 425 [M + H]+. Anal. Calcd. for C29H32N2O: C, 82.04; H, 7.60. Found: C, 81.89; H, 7.63. 10-{[1-(4-Dimethylaminonaphthyl)]-2-ethenyl}-7-methoxy9-spiroyclohexylindolino[2,1-b]oxazolidine (5h). Compound 5h was obtained as a yellowish foam (51%). mp 140-142 °C. UV λ (EtOH) nm (): 220 (33597), 334 (12122). UV λ (EtOH/ HCl) nm (): 213 (46374), 282 (10945), 550 (31722). UV λ (MCH) nm (): 223 (32471), 334 (12123). 1H NMR (250 MHz, CDCl3, δ): 8.26 (m, 1H), 8.13 (m, 1H), 7.62 (1H, d, J ) 7.8 Hz, H2), 7.56 (1H, d, J ) 15.1 Hz, H), 7.50 (m, 2H), 7.07 (1H, d, J ) 7.8 Hz, H3), 7.01 (1H, t, J ) Hz, H), 6.73 (m, 2H), 6.24 (d, J ) 15.6 Hz, 1H), 3.78 (s, 3H), 3.88-3.42 (m, 4H) 2.89 (s, 6H), 2.14-1.19 (m, 3H). 13C NMR (63 MHz, CDCl3, δ): 154.7, 151.0, 144.8, 140.4, 132.4, 129.0, 128.9, 128.7, 128.6, 126.0, 125.1, 124.7, 124.2, 123.8, 113.8, 112.5, 112.0, 111.6, 110.9, 63.2, 55.8, 51.5, 50.7, 45.2, 35.6, 29.7, 25.7, 22.8, 21.9. MS (EI) m/z (relative intensity): 454 (M+, 34), 270 (38), 224 (100), 209 (48), 184 (50). MS (FAB) m/z: 455 [M + H]+. Anal. Calcd. for C30H34N2O2: C, 79.26; H, 7.54. Found: C, 79.35; H, 7.63. 9,9-Dimethyl-10-[2-(1-naphthyl)ethenyl]indolino[2,1-b]oxazolidine (5i). Compound 5i was obtained as a yellowish oil

Sanguinet et al. (60%). UV λ (EtOH) nm (): 203 (127667), 228 (150560), 294 (34487). UV λ (EtOH/HCl) nm (): 220 (119787), 281 (22847), 439 (68333). UV λ (MCH) nm (): 204 (41905), 230 (51610), 294 (11821). 1H NMR (250 MHz, CDCl3, δ): 8.15 (m, 1H), 7.84 (m, 2H), 7.63 (m, 2H), 7.50 (m, 3H), 7.46 (d, J ) 15.9 Hz, 1H), 7.19 (dt, J ) 7.6 and 1.2 Hz, 1H), 7.11 (d, J ) 7.6 Hz, 1H), 6.96 (dt, J ) 7.6 and 1.2 Hz, 1H), 6.83 (d, J ) 7.6 Hz, 1H), 6.31 (d, J ) 15.9 Hz, 1H), 3.91-3.53 (m, 4H), 1.53 (s, 3H), 1.22 (s, 3H). 13C NMR (63 MHz, CDCl3, δ): 150.6, 139.6, 134.2, 133.5, 131.2, 129.6, 129.1, 128.5, 128.1, 127.5, 126.1, 125.8, 125.5, 123.8, 123.7, 122.3, 121.6, 111.9, 109.9, 63.5, 50.1, 47.8, 28.4, 20.4. MS (EI) m/z (relative intensity): 341 (M+, 33), 326 (M+ - 15, 17), 200 (37), 181 (100), 152 (68). MS (FAB) m/z: 342 [M + H]+. Anal. Calcd. for C24H23NO: C, 84.42; H, 6.79. Found: C, 84.30; H, 6.83. 10-[2-(9-Anthryl)ethenyl]-9,9-dimethylindolino[2,1-b]oxazolidine (5j). Compound 5j was obtained as a colorless powder (65%). mp 156 °C. UV λ (EtOH) nm (): 257 (105075), 351 (4183), 368 (6317), 386 (6067). UV λ (EtOH/HCl) nm (): 254 (112550), 520 (13892). UV λ (MCH) nm (): 257 (82150), 354 (56765), 371 (8350), 388 (7453). 1H NMR (250 MHz, CDCl3, δ): 8.41 (m, 1H), 8.31 (m, 2H), 8.02 (m, 2H), 7.71 (d, J ) 16.0 Hz, 1H), 7.50-7.47 (m, 4H), 7.22 (t, J ) 7.5 Hz, 1H), 7.16 (d, J ) 7.5 Hz, 1H), 7.00 (t, J ) 7.5 Hz, 1H), 6.89 (d, J ) 7.5 Hz, 1H), 6.18 (d, J ) 16.0 Hz, 1H), 4.13-3.80 (m, 4H), 1.55 (s, 3H), 1.26 (s, 3H, s). 13C NMR (63 MHz, CDCl3, δ): 151.3, 139.0, 134.5, 132.2, 131.5, 129.4, 128.9, 128.7, 127.7, 126.5, 125.8, 125.7, 125.6, 122.5, 121.8, 112.2, 109.5, 64.0, 50.7, 47.9, 28.8, 20.6. MS (EI) m/z: 391 (M+, 55), 376 (M+ 15, 22), 231 (92), 202 (100), 144 (27). MS (EI) m/z (relative intensity): 392 [M + H]+. Anal. Calcd. for C28H25NO: C, 85.90; H, 6.44. Found: C, 85.81; H, 6.33. 9,9-Dimethyl-10-{9-[2-(10-methylanthryl)]ethenyl}indolino[2,1-b]oxazolidine (5k). Compound 5k was obtained as yellowish crystals (50%). mp 176 °C (methanol). UV λ (EtOH) nm (): 261 (74960), 360 (6064), 378 (9767), 398 (9243). UV λ (EtOH/HCl) nm (): 257 (104014), 307 (11248), 355 (10905), 372 (9779), 391 (6788), 540 (17793). UV λ (MCH) nm (): 262 (86734), 361 (4751), 380 (7591), 399 (7160). 1H NMR (250 MHz, CDCl3, δ): 8.33 (m, 4H), 7.69 (d, J ) 16.1 Hz, 1H), 7.50 (m, 4H), 7.21 (t, J ) 7.6 Hz, 1H), 7.15 (d, J ) 7.6 Hz, 1H), 6.98 (d, J ) 7.6 Hz, 1H), 6.88 (t, J ) 7.6 Hz, 1H), 6.10 (d, J ) 16.1 Hz, 1H), 4.12-3.77 (m, 4H), 3.12 (s, 3H), 1.60 (s, 3H), 1.30 (s, 3H). 13C NMR (63 MHz, CDCl3, δ): 150.6, 139.7, 134.3, 130.9, 129.9, 129.8, 129.4, 129.1, 127.7, 126.5, 125.0, 122.4, 121.8, 112.1, 110.1, 63.9, 50.6, 47.9, 28.8, 20.6, 14.2. MS (EI) m/z (relative intensity): 405 (M+, 77), 390 (M+ - 15, 28), 245 (100), 215 (66), 202 (82). MS (FAB) m/z: 406 [M + H]+. Anal. Calcd. for C29H27NO: C, 85.89; H, 6.71. Found: C, 86.02; H, 6.79. 9,9-Dimethyl-10-[2-(9-phenanthryl)ethenyl]indolino[2,1-b]oxazolidine (5l). Compound 5l was obtained as an orange powder (68%). mp 170 °C. UV λ (EtOH) nm (): 256 (37948), 298 (10800), 242 (33538). UV λ (EtOH/HCl) nm (): 248 (42503), 296 (10797), 320 (9045), 445 (25986). UV λ (MCH) nm (): 244 (49241), 256 (53843), 273 (24583), 297 (16176). 1H NMR (250 MHz, CDCl3, δ): 8.73 (m, 2H), 8.21 (m, 1H), 7.94 (m, 1H), 7.65 (m, 5H), 7.63 (d, J ) 15.5 Hz, 1H), 7.20 (dt, J ) 7.5 and 1.0 Hz, 1H), 7.13 (d, J ) 7.5 Hz, 1H), 6.98 (dt, J ) 7.5 and 1.0 Hz, 1H), 6.85 (d, J ) 7.5 Hz, 1H), 6.40 (d, J ) 15.5 Hz, 1H), 3.94-3.68 (m, 4H), 1.55 (s, 3H), 1.25 (s, 3H). 13C NMR (63 MHz, CDCl3, δ): 151.7, 139.7, 133.3, 131.7, 130.6, 130.4, 130.2, 129.6, 128.6, 127.6, 126.8, 126.7, 126.6, 124.8, 124.6, 123.1, 122.5, 122.4, 121.7, 112.1, 110.2, 63.7, 50.3, 47.9,

Acido- and Phototriggered NLO Properties Enhancement

J. Phys. Chem. B, Vol. 109, No. 22, 2005 11147

Figure 8. Experimental setup for spectral measurements of harmonic light scattered at 90°. The components are denoted by λ/2, half-wave plate; λ/4, quarter-wave plate; P, Glan-Laser polarizer; BS, beam splitter; L1, 25.4 mm plano-convex lens; L2, 50 mm camera lens; S, sample cell; A, sheet polarizer. The spectrograph is also equipped with an external camera to visualize the image of the scattered beam focalized at the entrance of the slits.

28.6, 20.5. MS (EI) m/z (relative intensity): 392 [M + H]+, 391 (M+, 40), 376 (M+ - 15, 23), 231 (100), 202 (74), 144 (22). MS (FAB) m/z: 392 [M + H]+. Anal. Calcd. for C28H25NO: C, 85.90; H, 6.44. Found: C, 86.09; H, 6.63. V.b. Hyper-Rayleigh Scattering Experiments. All the compounds were prepared as dilute solutions (ranging typically from 10-6 to 10-4 mol/L) in acetonitrile, which has an HRS signal for all polarization geometries. Acetonitrile was obtained from Aldrich with a purity of 99.9% (HPCL grade). Approximately 1 cm3 of solution was filtered through a 0.2 µm micropore filter to remove dust particles that could be a source of spurious scattered light. A schematic diagram of the experimental apparatus is shown in Figure 8. The incident radiation at 1064 nm (1.16 eV) was obtained from an acousto-optically Q-switched Nd:YAG laser (Spectra Physics T40-X30S) operating in a near Gaussian TEM00 mode with M2 < 1.1. The laser produced trains of ∼15 ns, e1 mJ pulses at a repetition rate of 3 kHz. The input power level was selected with a half-wave plate followed by a Glan-Laser polarizer. The input polarization was adjusted by half-wave/ quarter-wave plates set up to obtain either a linear, circular, or elliptical state of light controlled with a photopolarimeter (Thorlabs) operating in the 900-1100 nm range. The incident laser beam was focused into the sample cell (normal 1 cm spectroscopic fused silica cuvettes) with a plano-convex lens (focal length 25.4 mm) and positioned to pass at a distance of 1-2 mm from the inside of the cell wall facing the collecting lens. The beam waist had a diameter of ∼8 µm with a Rayleigh range of ∼200 µm. The scattered light was collected at 90° with f/1.7 optics and focused into a spectrograph (Jobin-Yvon Horiba) with vertical polarization selection by a sheet polaroid and using an 1800 grooves/mm grating. The spectrally dispersed light around the harmonic response at 532 nm (18 797 ( 600 cm-1) was detected by a standard WRIGHT air-cooled charge coupled device (CCD) camera (1024 × 256 pixels) in a continuous acquisition mode. The entrance slits of the spectrograph were closed enough to fit the full beam waist image, providing then a spectral slit width of ∼2.5 cm-1. Each spectrum was obtained by averaging at least two spectra with an integrating time of 2-20 s, depending on the experimental conditions. All polarized spectra were recorded at room temperature, using the 1064 nm fundamental line, in conditions where the quadratic dependence of the integrated HRS intensity is verified (i.e., experiments where thermal lensing effect has been evidenced were excluded). Therefore, the maximum incident mean power for the acquisitions was around 2.3 W,

corresponding to 750 µJ pulses. It is noteworthy to point out that this experimental setup has been proven to be very efficient for the recording of polarized hyper-Raman spectra of simple liquids.18 In the HRS technique, the hyperpolarizability, β, is derived from the intensity of the incoherent scattered light at optical frequency 2ω on incidence of an intense laser pulse at ω. Although second-harmonic light is not observed for molecules in isotropic solutions, HRS light can be detected by orientation fluctuations.20,21 The second-harmonic light intensity for a collection of N noninteracting molecules without orientational correlation (pure incoherent signal) is given by I2ω ∝ N〈µi(2ω) µ/i (2ω)〉, where the brackets indicate averaging over all possible molecular orientations. The ensemble averaging over the molecular motions indicated by the brackets will involve products of components of the first hyperpolarizability tensor β of the form 〈βIJKβLMN〉. Assuming an incident light propagating along the X direction, with a state of polarization fully characterized with two angles (ψ, δ), the intensity of the harmonic light scattered at 90° along the Y direction and vertically polarized, V, (along the Z axis) is given by20 2ω I(ψ,δ)V ∝ 〈µZ2〉 ∝ 〈βZYY2〉cos4 ψ + 〈βZZZ2〉sin4 ψ +

sin2 ψ cos2 ψ〈(βZYZ + βZZY)2 + 2βZZZβZYY cos 2δ〉 (A-1) We have used two kinds of experimental settings with different δ values to control the variation of the state of polarization of the incident light when varying ψ. In the first setting (S1), the polarization of the incident light controlled with a half-wave plate alone (δ ) π) induces a simple rotation of the plane of polarization of the linearly polarized incident light with an angle of ψ. The second setting (S2) modulates elliptically polarized light when putting a quarter-wave plate after a half-wave plate, providing then a phase retardation δ ) π/2. Circularly polarized left and right lights are described by ψ ) ({π/4}. Linearly polarized lights are described by ψ ) 0 (horizontal, H) or ψ ) π/2 (vertical, V) giving then the standard HV and VV scattering geometries, respectively. For all the chromophores under investigation, we have assumed a pseudo C2V molecular symmetry where the molecules lay in a mean (xz) plane, with z the twofold symmetry axis. Finally, assuming Kleinman symmetry (time reversibility),22 partially justified here because of the strong dipolar nature of the photochromes, there are two independent coefficients, which implies here βzyy ) βyyz ) 0, βzxx ) βxxz * 0, and βzzz * 0. The

11148 J. Phys. Chem. B, Vol. 109, No. 22, 2005 macroscopic averages 〈βIJKβLMN〉 that determine the amount of HRS scattering in eq A-1 were calculated following Bersohn et al.20 For setting S1 (linear polarization of the incident light), we obtain 2ω IψV ∝

1 β 2[(15 + 18R + 27R2) - (12 + 20R + 105 zzz 16R2)cos2 ψ] (A-2)

and for setting S2 (elliptical polarization of the incident light), we have 2ω ∝ IψV

1 β 2[(15 + 18R + 27R2) - (24 + 60R + 105 zzz 4R2)cos2 ψ + (12 + 40R - 12R2)cos4 ψ] (A-3)

where R ) βzxx/βzzz. It is evidenced through eq A-2 that a linear polarization of the fundamental light introduces a simple quadratic dependence in cos ψ in contrast to the quartic dependence (eq A-3) due to the elliptical polarization of light. As it is pointed out in section III.a, the simplest setting S1 is sufficient enough to determine precisely, within the experimental error, the two independent coefficients βzzz and βzxx, since we analyze in this work dipolar compounds which give essentially a quadratic dependence feature (R ≈ 0). It is known, for a single molecule, that the HRS light intensity, I2ω, is proportional to the square of the incident light, Iω, and the hyperpolarizability, β. For a two-component chromophoresolvent system, the polarized total harmonic scattered light in the VV geometry, I2ω VV, is given as the sum of each composition ω 2 ω 2 solvent 〈(βZZZsolvent)2〉 + I2ω VV ) G(I ) B ) G(I ) {[N

Nchromophore〈(βZZZchromophore)2〉]10-R Nchromophore} (A-4) 2ω

where G is a constant, containing theoretical, geometrical, and electrical factors, N is the number density of the solution components, and the product term, R2ωNchromophore, accounts for the absorption losses at the second harmonic wavelength (532 nm). The macroscopic averages 〈(βZZZ)2〉 are given through eqs A-2 or A-3, taking ψ ) π/2 for the chromophore, and are different for the solvent, depending on its molecular symmetry. Here, we do not need to detail explicitly the macroscopic averages of the solvent (acetonitrile), which has an HRS molecular response, because it is beyond the realm of this paper. Nevertheless, from the VV scattering geometry, the determination of the absolute β values of the chromophores has been accomplished with respect to the response of acetronitrile taken as an internal reference. We have calibrated the acetonitrile response with respect to the carbon tetrachloride response 4 -40 m4 V-1), following the same (βCCl xyz ) 19 au ) 0.69 × 10 procedure described in detail by Kaatz and Shelton23,24, and that we have successfully checked with our experimental setup and conditions. Finally, at a sufficiently low concentration of the chromophore (solute), where the number density of the solvent, ω 2 Nsolvent, is constant, it is evidenced from eq A-4 that I2ω VV/(I ) is chromophore a linear function of the number density of the solute, N , and that I2ω VV/B is a quadratic function of the incident intensity, Iω. In conclusion, our experimental procedure to determine with a good reliability the hyperpolarizability tensor components of each chromophore can be decomposed in two steps. For the first step, we aim to determine the macroscopic average chromophore 2 〈(βZZZ ) 〉 of the chromophore under study. Then, to

Sanguinet et al. obtain the maximum reliability from the data, we determine the best surface curve, following eq A-4, that fits the whole polarized (VV) HRS responses of binary mixtures with several number densities (including the pure solvent solution) and varying the incident intensity. In the second step, under constant incident irradiation and keeping the experimental conditions, we record, as a function of the incident polarization using either the S1 (eq A-2) or S2 (eq A-3) setup, the HRS intensities of a pure solution of solvent and of a binary mixture with an adequate number density (i.e., where the solvent contribution is sufficiently low or negligeable for all incident polarization), accounting for the absorption losses. From these two sets of experiments, the longitudinal molecular tensor component, βzzz, as well as the minor molecular tensor component, βzxx, of the chromophores are determined. V.c. Theoretical Details. The molecular structures were optimized at the B3LYP/6-31G level using the Gaussian98 package.25 The excitation energies ∆Ege and excited-state properties of the chromophores were determined using the CIS (configuration interaction/singles) scheme based on the Zerner’s INDO (intermediate neglect of diatomic overlap) Hamiltonian26 available in the MOS-F program.27 In addition to its applicability to large molecules, the CIS method associated with semiempirical Hamiltonians has been shown to yield accurate predictions of transition energies, because, on one hand, most of the low-lying excitations are dominated by single excitations and, on the other hand, the parameters defining such Hamiltonians are fitted to spectroscopic data.28 The CIS/INDO/S approach also provides the transition dipole moments between the ground and excited states, µge ) 〈0|µˆ |e〉, and the excited-state dipole moments, µee ) 〈e|µˆ |e〉, as well as the oscillator strengths, fge ) (2/3)∆Egeµge2, which are proportional to the transition probability and absorption intensity. The comparison between the ground- and excited-state charges within the Mulliken population analysis is used to characterize the nature of the transfer of charge accompanying the electronic excitations. In all calculations, the complete set of occupied and unoccupied molecular orbitals has been included in the CIS expansions. Subsequently, the hyperpolarizabilities were evaluated using the summationover-states (SOS) perturbation approach combined with the INDO/S Hamiltonian. Although this scheme is known to overestimate the β values, it has the advantages of expressing the NLO responses in terms of spectroscopic quantities and of providing a qualitative understanding of the NLO properties. This method has been widely popularized by Bre´das,29 Morley,30 and their co-workers for deducing structure-property relationships in a large range of conjugated molecules. It has also been used by three of the authors to characterize the crystal packing effects on the electronic excitations and NLO properties of 2-methyl-4-nitroaniline31 and 3-methyl-4-nitropyridine-1-oxyde32 clusters. Within this approach, the expression of the first hyperpolarizability components reads

βijk(-ωσ; ω1, ω2) ) 1 2

∑ P-σ,1,2 n*0 ∑m*0 ∑ (∆E

k µi0n µ j jnm µm0

0n

- ωσ)(∆E0m - ω2)

(A-5)

where the sums run over all excited states |n〉 and |m〉 of energy En and Em. |0〉 is the ground-state wave function of energy E0. µ j jmn ) 〈m|µˆ j|n〉 - δmn〈0|µˆ j|0〉 and ∆E0n ) pω0n ) ω0n ) En E0. Σ P-σ,1,2 is the sum over the six permutations of the pairs (-ωσ, i), (ω1, j), and (ω2, k). When considering the two-states

Acido- and Phototriggered NLO Properties Enhancement approximation for the diagonal components related to the SHG response, the above relation simplifies into

β(-2ω; ω, ω) ) 3µ0e2µ j ee

∆E2 (∆E2 - 4ω2)(∆E2 - ω2)

(A-6)

All SOS calculations of β were performed using the MOS-F program by including the whole set of excited states in the summations. The NLO properties of the chromophores were also computed using the time-dependent Hartree-Fock (TDHF) method. It consists of expanding the matrices of the TDHF equation in the Taylor series of the external electric field and in solving it analytically order by order.33 The so-obtained successive field derivatives of the density matrix are then inserted into the hyperpolarizability expressions

βijk(-ωσ; ω1, ω2) ) -Tr{miDjk(ω1, ω2)}

(A-7)

where mi is the dipole moment matrix along the i-axis and Djk(ω1, ω2) is the second-order derivative of the density matrix with respect to electric fields of pulsation ω1 and ω2 oriented in the j and k directions, respectively. In the static limit, the TDHF method is also known as the coupled-perturbed HartreeFock (CPHF) method. The TDHF scheme was employed in combination with the semiempirical AM134,35 Hamiltonian. The AM1 approximation makes it possible to study large conjugated compounds within reduced calculation times, and it was shown to reproduce fairly well the quality of ab initio correlated methods.17 Calculations were also performed in the static limit at both semiempirical AM1 and ab initio levels of treatment. In particular, to assess the importance of electron correlation effects, static first hyperpolarizabilities were calculated at the Hartree-Fock and second-order Møller-Plesset (MP2) levels of approximation within the finite field (FF) procedure.36 The field-dependent energies have been determined using Gaussian98 in combination with an iterative Romberg procedure to improve the accuracy of the FF differentiation procedures.37 The numerical accuracy of the FF procedure is on the order of 1.0 au, as determined by comparing the equivalent HF/FF and CPHF results. The ab initio calculations have been carried using the 6-31G and 6-31G* basis sets. All first hyperpolarizability values are consistent with convention B of ref 38. Acknowledgment. Corning-CERF (Fontainebleau, France) is greatly acknowledged for its financial support. V.R. is indebted to the Re´gion Aquitaine for financial support in optical, laser, and computer equipment. B.C. thanks the Belgian National Fund for Scientific Research (FNRS) for his senior research associate position. This work has benefited from a scientific cooperation established and supported by the Centre National de la Recherche Scientifique (CNRS), the FNRS, and the Commissariat Ge´ne´ral aux Relations Internationales (CGRI) de la Communaute´ Wallonie-Bruxelles. The calculations were performed thanks to computing time made available by the SiMoA (Simulation et Mode´lisation en Aquitaine, France), by the intensive calculation pole “M3PEC-MESOCENTRE” of the University Bordeaux I, for which the authors acknowledge the financial support of the Regional Council of Aquitaine, and by the Interuniversity Scientific Computing Facility (ISCF), installed at the Faculte´s Universitaires Notre-Dame de la Paix (Namur, Belgium), for which the authors gratefully acknowledge

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