J. Phys. Chem. B 2007, 111, 9795-9802
9795
Acido-Triggered Nonlinear Optical Switches: Benzazolo-oxazolidines Fabien Manc¸ ois,† Lionel Sanguinet,† Jean-Luc Pozzo,† Maxime Guillaume,‡ Benoıˆt Champagne,‡ Vincent Rodriguez,† Fre´ de´ ric Adamietz,† Laurent Ducasse,† and Fre´ de´ ric Castet*,† Institut des Sciences Mole´ culaires, UMR 5255 CNRS, UniVersite´ Bordeaux I, Cours de la Libe´ ration, 351, F-33405 Talence CEDEX, France, and Laboratoire de Chimie The´ orique Applique´ e, Faculte´ s UniVersitaires Notre-Dame de la Paix (FUNDP), rue de Bruxelles, 61, B-5000 Namur, Belgium ReceiVed: May 3, 2007; In Final Form: June 15, 2007
This work is the continuation of our previous experimental and theoretical studies aiming at designing efficient nonlinear optical (NLO) switches derived from the benzazolo-oxazolidine core. Here, we report the synthesis and the characterization of the linear and nonlinear optical properties of benzothiazolo[2,3-b]oxazolidine acidochromes by means of hyper-Rayleigh scattering as well as quantum chemical calculations. It is shown that these new derivatives incorporating a benzothiazole subunit exhibit very high static first hyperpolarizability values in their acido-generated form. On the basis of previously reported NLO responses of indolino- and benzimidazolo-oxazolidines, structure-properties relationships within the benzazolo-oxazolidine series are proposed.
1. Introduction Organic molecular switches possessing large contrasts in their nonlinear optical (NLO) responses are sought for their potential applications in optical communication or data storage.1,2 Due to the large change of their electronic spectrum, which can be induced upon irradiation, photochromic compounds are attractive candidates for incorporation in materials. Among them, several classes of compounds, such as azobenzene derivatives,3 Schiff bases,4 diarylethenes,5 spiropyrans,6 or nitrobenzylpyridines,7 have been shown to act as efficient switchable frequency doublers. Within this framework, we recently presented a new series of phototunable molecular switches, combining the indolino[2,1-b]oxazolidine core with various styrylic residues8 (Scheme 1, X ) CMe2). As illustrated in Scheme 1, the UV irradiation of the closed form induces a rupture of the σC-O bond on the oxazolidine moiety, leading to a colored zwitterionic open form. A particular interest of these systems lies in the fact that the reversible transformation may be triggered indifferently using light or pH variations. Upon acidic addition, a protonated open form (POF) is generated, whose absorption spectrum is identical to the one of the zwitterionic form, indicating that these two species adopt similar geometries. In the following notation, the c and o letters refer to as the closed and protonated open forms of the compounds, respectively. NLO properties of different systems were characterized by using the hyper-Rayleigh scattering (HRS) technique, which allows investigation of both neutral and charged species. All POF compounds were shown to display large NLO responses that depend on the nature of the aryl group, Ar, as well as on the nature of the conjugated bridge linking the oxazolidine moiety to Ar. However, theoretical calculations revealed a poor electron conjugation on the oxazolidine moiety, reducing the efficiency of the light-induced charge transfer between the * Corresponding author. E-mail:
[email protected]. † Universite ´ Bordeaux I. ‡ FUNDP.
SCHEME 1: Photo-/acidochromic Equilibrium for Indolino[2,1-b]oxazolidines (X ) CMe2) and Benzimidazolo[2,3-b]oxazolidine (X ) NMe)a
a
Ar represents an aryl residue.
acceptor and donor parts. Two complementary strategies were then implemented to increase the efficiency of the NLO responses. The first one consists of grafting acceptor substituents onto the oxazolidine core to improve the strength of the pushpull system;9 the second consists of increasing the number of delocalizable electrons by replacing the indolinic unit with a benzimidazolic unit10 (Scheme 1, X ) NMe). Compared to their indolinic analogs, the benzimidazolo-oxazolidine derivatives were found to significantly increase the static first hyperpolarizability of the POF. In line with the latter study, we address here the synthesis as well as the linear and nonlinear optical properties of benzothiazolo-oxazolidine derivatives. The benzothiazolium unit was shown to act as an effective electron acceptor in a recent investigation on the quadratic NLO properties of a series of benzothiazolium salts.11 Two new chromophores, 3o and 4o (Scheme 2), are investigated in this work and compared to their carbonated and nitrogenated analogs 1o and 2o. In all of the compounds, the Ar residue is a p-N,N-dimethylaminophenyl group, which was shown to possess the most efficient donor character. Going from 1o to 3o allows, therefore, assessment of the influence of the nature of the X group on the structural, electronic, and optical properties of the chromophores. Moreover, the impact of elongating the length of the conjugated bridge between the oxazolidine core and the dimethylaminophenyl is addressed by replacing the ethylenyl bridge with a 2,5-diethenylthiophene unit (compound 4o).
10.1021/jp073386+ CCC: $37.00 © 2007 American Chemical Society Published on Web 08/01/2007
9796 J. Phys. Chem. B, Vol. 111, No. 33, 2007 SCHEME 2: Benzazolo-oxazolidine Derivatives in Their Protonated Open Forms
SCHEME 3: Synthetic Pathway for 3o-4o
In the next section, the experimental details related to the synthesis and the HRS experiments are provided, as well as details on models and computational procedures used in the theoretical analysis. Then, the structural and (non)linear optical properties of compounds 3 and 4 are described in Section 3, accounting for the possible existence of various rotational isomers in solution at room temperature. Finally, the last section presents a brief discussion on structure-property relationships within the benzazolo-oxazolidine series by comparing the NLO responses of the different compounds. 2. Experimental Section 2.1. Synthesis. 2-Methylbenzothiazole was first reacted with 2-iodoethanol to afford 3-(2-hydroxyethyl)-2-methylbenzothiazolium iodide 5. This quaternary salt was then condensed with aromatic aldehyde in ethanolic solution in the presence of N-methylmorpholine as a base. This two-step strategy (Scheme 3) affords directly the protonated open forms of 10-(2-aryl)ethenylbenzothiazolo[2,3-b]oxazolidine 3o and 4o. In our previous synthetic work that was designed to use indoleninium or benzimidazolium as electron withdrawing heterocyclic groups, we showed that a stronger base is needed to obtain the corresponding closed forms,8,10 these reactional conditions being often accompanied by lowered yields. Because the salts are easily purified by column chromatography, we directly characterize their NLO responses. 3-(2-Hydroxyethyl)-2-methylbenzothiazolium Iodide (5). 2-Methylbenzothiazole (5 g, 33.5 mmol) is dissolved in toluene (100 mL), then 2-iodoethanol is added (15 g, 87.2 mmol), and the reactionnal mixture is heated under reflux for 24 h. On cooling, a precipitate is formed, which is collected, washed with diethylether, and dried under vacuo to afford 3-(2-hydroxyethyl)2-methylbenzothiazolium iodide 5 (2.8 g, 8.71 mmol) as a white solid (Y: 26%). mp 178 °C. UV λ (EtOH) nm (): 219 (26900),
Manc¸ ois et al. 278 (5000). 1H NMR (250 MHz, DMSO-d6, δ): 8.47 (d, J ) 7.9 Hz, 1H, H-7), 8.34 (d, J ) 7.9 Hz, 1H, H-4), 7.89 (dt, J ) 7.9 and 1.3 Hz, 1H, H-5), 7.81 (dt, J ) 7.9 and 1.3 Hz, 1H, H-6), 4.87 (t, J ) 4.9 Hz, 2H, CH2N+), 3.90 (d, J ) 4.9 Hz, 2H, CH2OH), 3.24 (s, 3H, CH3). 13C NMR (63 MHz, DMSOd6, δ): 177.9, 141.0, 129.1, 128.9, 128.0, 124.5, 117.1, 58.5, 51.9, 17.3. MS (EI) m/z: 193 (M ( 1, 31), 150 (32), 136 (100), 128 (48). MS (FAB) m/z: 194. General Procedure for the Preparation of Protonated Open Forms of Benzothiazolo[3,2-b]oxazolidine 3o, 4o. A stoechiometric mixture of 3-(2-hydroxyethyl)-2-methylbenzothiazolium iodide and the aldehyde was dissolved under nitrogen in absolute ethanol (10 mL for 5.6 mmol). One equivalent of N-methylmorpholine was added over 60 min onto the reaction mixture under reflux which was maintained for an additional 8 h. After cooling, the mixture was concentrated under vacuo, and the residue was purified by column chromatography (SiO2, CH2Cl2/CH3OH). 1-(2-Hydroxyethyl)-2-[2′-(4′′-N,N-dimethylaminophenyl)ethenyl]benzothiazolinium Iodide (3o). Compound 3o was obtained as a purple solid (Y: 98%, 2.49 g, 5.5 mmol). mp 183 °C. UV λ (EtOH) nm (): 526 (51200). 1H NMR (250 MHz, DMSOd6, δ): 8.27 (d, J ) 7.9 Hz, 1H, H-4), 8.09 (d, J ) 7.9 Hz, 1H, H-7), 8.01 (d, J ) 15.0 Hz, 1H, H-1′), 7.84 (d, J ) 9.1 Hz, 2H, H-2′′), 7.74-7.65 (m, 2H, H-5 and H-6), 7.59 (d, J ) 15.0 Hz, 1H, H-2′), 6.76 (d, J ) 9.1 Hz, 2H, H-3′′), 5.14 (bs, 1H, OH), 4.89 (t, J ) 4.9 Hz, 2H, CH2N+), 3.88 (t, J ) 4.9 Hz, 2H, CH2O), 3.01 (s, 6H, N(CH3)2). 13C NMR (63 MHz, DMSO-d6, δ): 171.1, 153.3, 149.7, 141.5, 132.7, 128.6, 127.3, 126.9, 123.7, 121.3, 116.3, 111.8, 106.6, 59.0, 50.6, 48.6. MS (EI) m/z: 324 (20), 294 (12), 174 (100), 146 (35). Anal. Calcd. For C19H21N2OSI: C, 50.45; H, 4.68. Found: C, 50.71; H, 4.54. 1-(2-Hydroxyethyl)-2-[2′-(4′′-N,N-dimethylaminophenyl)ethenyl]benzothiazolinium iodide (4o). Compound 4o was obtained as a greenish solid (Y: 84%, 249 mg, 0.44 mmol). mp 133 °C. UV λ (EtOH) nm (): 575 (34500). 1H NMR (250 MHz, DMSO-d6, δ): 8.38 (d, J ) 7.9 Hz, 1H, H-4), 8.37 (d, J ) 15.0 Hz, 1H), 8.20 (d, J ) 7.9 Hz, 1H, H-7), 7.83-7.70 (m, 2H, H-5 and H-6), 7.81 (d, J ) 15.0 Hz, 1H), 7.49 (d, J ) 9.0 Hz, 2H, H-7′), 7.47 (d, J ) 4.1 Hz, 1H, H-thienyl), 7.29 (d, J ) 15.9 Hz, 1H), 7.27 (d, J ) 4.1 Hz, 1H, H-thienyl), 7.10 (d, J ) 15.9 Hz, 1H), 6.71 (d, J ) 9 Hz, 2H, H-8′), 5.23 (t, J ) 5.9 Hz, 1H, OH), 4.93 (t, J ) 5.3 Hz, 2H, CH2N+), 3.88 (m, 2H, CH2O), 2.96 (s, 6H, N(CH3)2). 13C NMR (63 MHz, DMSOd6, δ): 172.2, 153.7, 143.7, 141.8, 139.6, 138.2, 132.3, 129.9, 129.6, 128.8, 126.9, 125.2, 124.5, 124.2, 122.8, 121.6, 117.5, 115.8, 108.6, 59.3, 50.1, 48.9. MS (EI) m/z: 433 (25), 282(100), 209 (63). Anal. Calcd. For C25H25N2OS2I: C, 53.57; H, 4.50. Found: C, 53.78, H, 4.33. 2.2. Hyper-Rayleigh Scattering Experiment. We have performed HRS experiments on the compounds in the form of dilute solutions (between 10-4 and 10-6 M) in acetonitrile that correspond to typical concentrations used to check the linear dependency of absorbance and determine the extinction coefficient, . In this NLO scattering technique, the intensity of the incoherent scattered light at the second harmonic frequency of an IR Nd:YAG pulsed laser is used to determine the hyperpolarizability, β. The scattered harmonic light is related to quadratic products between components of the molecular β-tensor that correspond to isotropic averaging over the molecular motions (noninteracting molecules). For all the chromophores under investigation, assuming a pseudo C2V molecular symmetry where the molecule lays in a mean (x-z) plane, with z the 2-fold symmetry axis, and assuming Kleinman symmetry, two inde-
Acido-Triggered NLO Switches
J. Phys. Chem. B, Vol. 111, No. 33, 2007 9797
pendent components βzxx and βzzz remain. More details about the procedure can be found elsewhere.8 2.3. Theoretical Simulations. The B3LYP functional12,13 and the 6-31G(d) basis set were used to characterize the molecular structures in vacuum. Vibrational frequency calculations provided zero-point and thermal corrections, and the effect of solvent was approximated by subsequent single-point calculations using the polarizable continuum model14 with the same basis set. A dielectric constant, 0 ) 36.640 (∞ ) 1.806), was used for acetonitrile. Time-dependent density functional (TD-DFT) calculations were carried out at the B3LYP/6-31G(d) level to determine the vertical excitation energies, ∆Ege, and excited-state properties of the chromophores and at the BHandHLYP/SV(P) level to optimize the excited-state geometry. Maximum absorption wavelengths and oscillator strengths are compared to those obtained using the semiempirical INDO/S scheme,15 whose parameters are specifically optimized to reproduce the absorption spectra of organic molecules. In the latter case, the complete set of occupied and virtual MOs was included in the single CI calculations. The time-dependent Hartree-Fock (TDHF) method16,17 and its static analog, the coupled-perturbed Hartree-Fock (CPHF) method,18 were used to calculate the static and frequencydependent electronic hyperpolarizabilities. Most of these calculations employ the 6-31G(d) basis set. To evaluate electron correlation effects, the static first hyperpolarizabilities were also calculated at the second-order Møller-Plesset (MP2) level within the finite field procedure,19 in which a Romberg procedure is applied to improve the accuracy of the numerical derivatives.20,21 No density functional theory approaches were used for determining the NLO responses, owing to the fact that conventional DFT schemes fail drastically, whereas the optimized effective potential procedure for exact exchange still requires being combined with a suitable correlation functional.22 Frequency dispersion effects on correlated hyperpolarizabilities were estimated by adopting the multiplicative correction scheme, which has been shown to be suitable for different systems, including push-pull polyenes:23-25
βMP2(-2ω; ω, ω) ≈ βMP2(0; 0, 0) ×
βTDHF(-2ω; ω, ω) βCPHF(0; 0, 0)
(1)
A wavelength of 1064 nm was adopted in all TDHF calculations. In addition to the longitudinal component βzzz of the hyperpolarizability tensor, the square root of the hyper-Rayleigh scattering intensity for plane-polarized incident light observed perpendicularly to the propagation plane was also evaluated for direct comparison with experimental data.
βHRS(-2ω; ω, ω) ) x{〈β2zzz〉 + 〈β2zxx〉}
(2)
The associated depolarization ratio (DR) is given by
DR )
I2ω VV I2ω HV
)
〈β2zzz〉 〈β2zxx〉
(3)
Full expressions for 〈βzzz2〉 and 〈βzxx2〉 without assuming Kleinman’s conditions are given in ref 26 and correspond to isotropic orientational averages of the β tensor components. All first hyperpolarizability values are consistent with convention B of ref 27. Ab initio calculations have been performed using Gaussian 03,28 except excited-state geometry optimizations for
SCHEME 4: Atom Labels and Cartesian Frame Used in the Calculationsa
a The arrows indicate the bonds where rotations are allowed and are considered in the study.
TABLE 1: Relative Free Energies (kcal/mol) and Populations (in Parentheses) of the Rotational Conformers of 3o and 4o Calculated at the B3LYP/6-31G(d) Levela structure
in gas phase
in acetonitrile
3o-t 3o-c
0.00 (99.95%) 4.50 (0.05%)
0.00 (99.89%) 4.01 (0.11%)
4o-ttt 4o-ttc 4o-cct 4o-ccc 4o-tcc 4o-tct 4o-ctc 4o-ctt
0.00 (64.42%) 0.76 (17.94%) 4.75 (0.02%) 5.29 (0.01%) 1.46 (5.51%) 1.04 (11.21%) 3.79 (0.11%) 2.62 (0.78%)
0.00 (51.59%) 0.13 (41.13%) 5.46 (0.01%) 5.92 (0.00%) 1.73 (2.78%) 1.47 (4.33%) 4.71 (0.02%) 3.51 (0.14%)
a The different conformers are labeled 3o-ξ or 4o-ξψζ, where ξ, ψ and ζ indicate the cis (c) or trans (t) orientations of the C4-C5, C6-C7 and C10-C11 bonds, respectively.
which the Turbomole package29 has been used. INDO/S calculations have been performed using MOS-F.30 3. Results and Discussion 3.1. Molecular Structures. It was demonstrated recently that isomerization effects may have a significant impact on the nonlinear optical properties. In particular, in the case of pushpull chromophores in which the conjugated backbones connecting the donor and acceptor groups contain single bonds joining rigid thiophene rings and 1,2-ethylenic linkages, rotational isomerism may affect the βHRS values by up to 35%.31 The structures and relative energies of all possible rotational isomers of compounds 3 and 4 have then been calculated at the B3LYP/6-31G(d) level. For compound 3o, the rotation around the C4-C5 bond was considered (Scheme 4). The most probable rotational states, in which the N-C4-C5-C6 torsion angle is near 180° and 0°, are referred to as transoid and cisoid, as they bring the N-C4 and C5-C6 double bonds into trans and cis orientations relative one to another. Similarly, eight different rotamers were obtained for 4o from rotations around the three single bonds shown in Scheme 4. The relative free energies of the various rotamers calculated in vacuum and in acetonitrile are given in Table 1. In the following, the different conformers of compounds 3 and 4 in
Manc¸ ois et al.
9798 J. Phys. Chem. B, Vol. 111, No. 33, 2007
TABLE 2: Torsion Angles (in Degrees) Calculated at the B3LYP/6-31G(d) Level for the Compounds 1-4 in Their Most Stable Closed and Protonated Open Formsa 1c-Ob 1o-tb 2c-Oc 2o-tc 3c-Od 3o-td 4c-Ottd 4o-tttd 4o-ttcd a
X-C4-C5-C6
C4-C5-C6-C7
C5-C6-C7-C8
N-C4-C7-C8
109.2 6.6 123.6 32.6 119.0 12.4 119.2 -9.3 -2.5
-179.0 -177.8 179.0 -179.8 179.8 -177.4 -179.5 177.7 179.1
0.7 2.4 -3.2 4.3 2.1 2.7 -179.2 178.5 179.9
59.6 10.0 -68.0 45.2 66.0 19.8 -114.6 166.4 176.3
See Scheme 4 for atom labeling. b X ) CMe2. c X ) CMe. d X ) S.
TABLE 3: Bond Lengths and Bond Length Alternations Calculated at the B3LYP/6-31G(d) Level for the Compounds 1-4 in Their Most Stable Closed and Protonated Open Formsa S-C4 C4-C5 C5-C6 C6-C7 C7-C8
1c-O
1o
2c-O
2o
3c-O
3o-t
4c-Ott
4o-ttt
4o-ttc
1.504 1.347 (-0.157) 1.466 (0.119)
1.405 1.386 (-0.019) 1.425 (0.039)
1.502 1.343 (-0.159) 1.466 (0.123)
1.428 1.367 (-0.061) 1.433 (0.066)
1.889 1.503 1.343 (-0.160) 1.464 (0.121)
1.754 1.412 1.379 (-0.033) 1.423 (0.044)
1.91 1.501 1.345 (-0.156) 1.445 (0.100) 1.381 (-0.064) 1.415 (0.034) 1.383 (-0.032) 1.442 (0.059) 1.355 (-0.087) 1.456 (0.101)
1.760 1.402 1.390 (-0.012) 1.398 (0.008) 1.408 (0.010) 1.386 (-0.022) 1.409 (0.023) 1.418 (0.009) 1.372 (-0.046) 1.432 (0.060)
1.756 1.404 1.389 (-0.015) 1.389 (0.000) 1.409 (0.020) 1.385 (-0.024) 1.410 (0.025) 1.421 (0.011) 1.373 (-0.048) 1.432 (0.059)
C8-C9 C9-C10 C10-C11 C11-C12 C12-C13
a See Scheme 4 for atom labeling. Values in parentheses are the difference between the length of the Cn-Cn+1 and Cn-1-Cn bonds. All values are in Å.
their protonated open forms are labeled 3o-ξ or 4o-ξψζ, where ξ, ψ, and ζ stand for the cis (c) or trans (t) orientations of the C4-C5, C6-C7, and C10-C11 bonds, respectively. The relative populations were calculated by performing a Maxwell-Boltzmann statistics using the relative free energies and a temperature of 298.15 K. Regarding 3o, the free energy differences obtained in the gas phase and in solvent are very similar and indicate that the cis conformer is too high in energy to exist in solution at room temperature. A similar energy difference between the cis and trans forms of the N-methyl analogue of 3o was recently obtained by Coe and co-workers from B3P86/6-31G(d) calculations.11 In the case of 4o, the four conformers displaying a trans orientation with respect to the C4-C5 bond are the most stable. Among them, the all-trans 4o-ttt is the major compound. Calculations performed accounting for the solvent indicate that the 4o-ttc conformer should also exist in similar proportion. The relative energies of the possible conformers of 3 and 4 in their closed forms (i.e., 3c and 4c) are available in the Electronic Supporting Information. In the case of 3c, the oxygen, the nitrogen, or the sulfur atom of the benzothiazolo-oxazolidine moiety may be located in the ethenyl plane defined by the carbons C4, C5, and C6, giving rise to three different stable structures. The most stable form corresponds to the 3c-O conformer, in which the oxygen atom belongs to the ethenyl plane, and the minor product corresponds to 3c-N. These two conformers are expected to coexist in acetonitrile solution in a ratio of 84:14, the 3c-S conformer being less stable. In the case of 4c, the three possible positions of the heteroatom of the benzothiazolo-oxazolidine part, together with the possible rotations around the C6-C7 and C10-C11 single bonds give rise
to a total number of 12 conformers. In a manner similar to 3c, the major conformer is found to be 4c-Ott, in which the oxygen is located in the ethenyl plane. Three other conformers are associated with relative populations larger than 10%: 4c-Ntt, 4c-Otc, and 4c-Oct. The representative torsion angles and bond lengths of the most stable 1-4 conformers are reported in Tables 2 and 3 following the atom labeling given in Scheme 4. In the closed forms, the benzothioazole and the arylethylenyl parts are close to the tetrahedral angle, but these two moieties are close to planarity in the POF, as indicated by the X-C4-C5-C6 torsion angles (with X ) CMe2, CMe, or S). When going from the closed form to the POF, the gain of electron conjugation due to the ring opening reinforces the π-donor character of the sulfur atom and leads to a large decrease in the S-C4 bond length (∼0.15 Å). In 3o, the dihedral angle between the planes of the sixmembered rings on both sides of the conjugated backbone (last column) is small (19.8°), which is consistent with appreciable donor-acceptor π-couplings. However, it is a bit larger than the dihedral angle between the planes of 4-[dimethylamino]phenyl and benzothiazolium rings measured by X-ray crystallography in 3-methylbenzothiazolium salts.11 In addition, this value of 19.8° lies between the value of 10.0° obtained in the corresponding indolino derivative 1o and the value of 45.2° observed in the benzimidazole derivative 2o, where the NMe group induces larger steric strains.10 In 4o, the central thienyl group and the N,N-dimethylaminophenyl ring are coplanar. Table 3 also reports the bond length alternations (BLA), which provide a complementary insight into the extent of electron delocalization32,33 along the chromophores. As expected,
Acido-Triggered NLO Switches
J. Phys. Chem. B, Vol. 111, No. 33, 2007 9799 TABLE 4: Experimental and Theoretical (Vertical) Excitation Energies (eV) of 1o-4o Corresponding to the Main Low-Energy Charge-Transfer Excited Statea TD:B3LYP/6-31G(d) 1o 2o 3o 4o
exptl
INDO/S
in gas phase
in acetonitrile
∆Erelax ge
2.28 3.09 2.37 2.15
2.50 2.67 2.47 1.80
2.73 2.90 2.72 1.95
2.62 2.90 2.61 1.86
2.53 2.76 2.55 /
∆Erelax accounts for both solvent and excited state relaxation ge effects. a
Figure 1. UV-visible absorption spectra of the compounds 1o-4o in acetonitrile.
the differences between the lengths of the successive C-C bonds indicate that the electron conjugation is more efficient in the open forms than in their closed analogs. As a representative indicator, the difference between the lengths of the C6-C7 single bond and its adjacent C5-C6 double bond, 0.121 Å in 3c, is reduced to 0.044 Å in 3o, which is slightly smaller than the value of 0.058-0.090 measured in 3-methylbenzothiazolium salts.11 As reported by Marder, this value of the BLA (0.044 Å) is optimal for maximizing β in polyene backbones. The corresponding values obtained for the indolino (1o) and benzimidazolo (2o) derivatives amount to 0.039 and 0.066, consistent with the difference in planarity of the three chromophores, as shown by the values of the torsion angle N-C4-C7-C8. The differences between the lengths of the successive C-C bonds in 4o-ttt and 4o-ttc are very similar. Two characteristic values for the BLA, corresponding to the 1,2-vinylic linkages, may be pointed out: (i) The difference in length between the C6-C7 and C5-C6 bonds, very close to zero, is representative of a delocalization much more pronounced than in 3o. (ii) The difference in length between the C12-C13 single bond and the adjacent C11-C12 double bond is 0.060 Å. 3.2. Linear Optical Properties. Figure 1 displays the absorption spectra of the colored forms 1o-4o in acetonitrile, with maximum absorption wavelengths of 544 (18380 cm-1), 402 (24880 cm-1), 524 (19080 cm-1), and 576 nm (17360 cm-1), respectively. The molar extinction coefficient of 3o (∼50 000 L mol-1 cm-1) lies between the molar extinction coefficient of the corresponding indolino (1o) and benzothiazolo (4o) derivatives. The absorption spectrum of 2o is widely different from the other compounds of the series, with a molar extinction coefficient of roughly one-third of that found for 1o. The INDO/S transition energies, oscillator strengths, and changes in dipole moments between the ground and the main low-energy charge transfer state of all the possible conformers of 3o and 4o are reported in Table S2 (ESI), and TD-B3LYP/ 6-31G(d) results obtained in vacuum and in acetonitrile are given in Table S3. To account for the existence of various stable conformers in the case of 4o, the averaged excitation energy, ∆Eav ge, has been calculated from application of the MaxwellBoltzmann distribution scheme using the relative free energies in solution reported in Table 1. The excitation energies of 1o4o are compared in Table 4 to experimental data. The most absorbing charge-transfer excited-state of all POFs is dominated by a π f π* transition between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Considering the 1o-3o series, gas-phase calculations performed either at the INDO/S level or at the TD-DFT level
globally overestimate the transition energies. Moreover, the hierarchy among the various values is not well reproduced, ∆Ege being smaller for 3o than for 1o, in opposition to the experimental findings. Except for 2o, accounting for solvent leads to a decrease in the transition energies by ∼0.1 eV, but does not correct the relative ordering of the excitation energies. These discrepancies might originate, at least in part, from the lack of treatment of vibronic effects. Indeed, experimental excitation energies take implicitly into account the geometry relaxations in the excited state and the vibrational structure, whereas the theoretical values are merely vertical electronic excitation energies. Adopting the same approach as in a recent investigation on the vibronic structure of merocyanine dyes,34 the excited-state geometry of the 1o-3o compounds were then optimized. The stabilization of the excited state amounts to 0.09, 0.14, and 0.06 eV, respectively. The slightly higher stabilization encountered for 2o may be related to a more pronounced variation in the BLA when going from the ground to the excited state. The analysis of the geometries of the two states within the series does not reveal any significant variation in the torsion angles. Then, assuming that geometry relaxation and solvent effects are additive, a new theoretical estimate for 1o-3o can be determined (∆Erelax ge , Table 4), in slightly better agreement with experiment. Finally, according to experiments, all levels of calculation predict an excitation energy for 4o widely smaller than for 3o, although the shift of +52 nm observed experimentally when going from 3o to 4o is significantly overestimated. Although relaxing the geometry of both the ground and excited states gives rise to a better agreement with experimental measurements, INDO/S and TD-DFT calculations performed on rigid geometries nevertheless allow one to gather useful insights into the electronic properties of the most absorbing excited states. In particular, the efficiency of the charge transfer occurring under light irradiation may be qualitatively highlighted by analyzing the differences in the INDO/S Mulliken charge distributions between the ground and the first dominant dipoleallowed excited states, as reported in Scheme 5. Similarly to what was observed in the indolino- and benzimidazolo-oxazolidine series, an alteration of the delocalization path is observed in 3o, giving rise to a weak charge transfer on the terminal phenyl ring (0.05 e). The total Mulliken charge variation on the oxazolidine moiety is equal to -0.26 e which is on the same order of magnitude as that observed in the indoline derivative (-0.21 e), and roughly twice as small as the benzimidazole derivative (-0.41e), which is consistent with the relative magnitude of the dipole moment variation between the ground and the main low-energy charge transfer states (4.70, 9.10, and 5.12 Debye from 1o to 3o at the INDO/S level). As was recently shown, an increase in the efficiency of the push-pull system might be achieved by grafting electron-attracting substituents onto the indolinic residue.9 In 4o, the Mulliken charge variation on the oxazolidine moiety is smaller, indicating that the charge-
Manc¸ ois et al.
9800 J. Phys. Chem. B, Vol. 111, No. 33, 2007 SCHEME 5: Mulliken Charge Differences (in Electron Units) between the Ground and the First Excited States Calculated at the INDO/S Level for the 3o (up) and the 4o (bottom) Compounds
transfer mainly occurs between the N,N-dimethylaminophenyl and the thienyl group. 3.3. Nonlinear Optical Properties. The molecular hyperpolarizabilities as well as dynamic depolarization ratios I2ω VV/ I2ω HV for compounds 1o-4o issued from HRS measurements are reported in Table 5. The β values reported for compound 1o differ slightly from our previous publication.8 Indeed, the hyperpolarizability of 1o has been revisited within the framework of a new series of HRS measurements dealing with indolino-oxazolidine derivatives, implying electron-withdrawing substituents on the oxazolidine moiety.9 All quantities are given in atomic units and were measured at 1064 nm in acetonitrile. Figure 2 illustrates the quadratic power dependence of the scattered harmonic light for several concentrations of 3o. The static β values (β0) listed in Table 5 have been obtained by extrapolating the dynamic quantities using the two-state approximation,35 in which only two electronic states, g and e, are assumed to contribute to the first hyperpolarizability. When considering this approximation for the diagonal components, the static and optical second harmonic responses are related through
ωge4 β(-2ω; ω, ω) ) β(0; 0, 0) (ωge2 - 4ω2)(ωge2 - ω2)
(4)
where ∆Ege ) pωge. However, as revealed by previous HRS studies,36,37 the above expression is only applicable in the offresonance region, where 2ω , ωge, whereas the dynamic first hyperpolarizabilities of the compounds presented here (except 2o) were clearly measured under or near resonant conditions. A more general expression than 4, valid in the resonance regime, implies a damping parameter, γ, associated with the optical transition.38-40
ωge2(ωge - iγ)2 β(-2ω; ω, ω) ) β(0; 0, 0) ([ωge - iγ]2 - 4ω2)([ωge - iγ]2 - ω2) (5) The correction due to damping is important if the two-photon scattering frequency (2ω) lies within the absorption band of the chromophore. However, the question arises from the above
expression of the value of the damping factor. Several authors have taken for γ the half width at half-maximum (hwhm) of the main absorption band.40,41,42 However, for the compounds presented here, the observed hwhm of the absorption toward the main low-energy excited state ranges from ∼800 (3o) to ∼3150 cm-1 (2o), which is much larger than the typical γ value of ∼100 cm-1 expected for solvated molecules.43 To qualitatively evaluate the order of magnitude of the errors arising from the use of expression 4, we report in Table 5 several values of β0 obtained using γ values ranging from 100 to 1000 cm-1. However, the two-state model, even with damping, is likely an oversimplification for the evaluation of the static second-order nonlinear response, in which more than a single excited state may significantly contribute. 2ω 2ω Due to frequency dispersion, the βzzz and βHRS values for 2o are significantly smaller than for the three other compounds. Using eq 4 to evaluate the static hyperpolarizabilities, the replacement of the gem-dimethyl group (1o) by a sulfur atom (3o) induces an increase in the β0 value of 8%. This exaltation of the intrinsic nonlinear response is much smaller than that found by using a NMe group (2o), in which the β0 is enhanced by 159%. On the other hand, the extent of the conjugation pathway from 3o to 4o is at the origin of a remarkable enhancement of β0 by a factor of ∼6. Accounting for the damping of the first excited state does not induce significant changes in the relative β0 values when using a γ of 100 cm-1. However, taking γ ) 500 cm-1 affects the hierarchy of the β0 values, which turns out to be 1o < 2o < 3o < 4o. Increasing again the damping factor to 1000 cm-1 leads to a new ordering of the β0 values (2o < 1o < 4o < 3o), but this latter value of γ is probably overestimated regarding the expected effects on β0 of increasing the number of delocalizable electrons when going from 1o to 2o and of increasing the size of the conjugated bridge between 3o and 4o. Assuming that substituting the CH2CH2OH group with a methyl group on the benzothiazolic nitrogen has no significant impact on the quadratic NLO response of the chromophore, the β0 value obtained for 3o can be compared to the β0 value of ∼39400 a.u. estimated by Coe et al. for the 3-methylbenzothiazolium derivative.11 This value, obtained by applying the two-state approximation (eq 4) from HRS measurements performed in a less resonant regime (using an 800 nm excitation laser), suggests that the damping factor lies between 500 and 1000 cm-1 for this compound. Relying on quantum chemistry, the longitudinal and hyperRayleigh static and dynamic first hyperpolarizabilities, as well as the depolarization ratios of the 1o-4o compounds, are given in Table 6. All calculations were performed within the 6-31G(d) basis set. Additional calculations, using the 6-31+G(d) basis set, demonstrated that the relative second-order NLO responses were not significantly affected by the addition of diffuse functions. In the case of 4o, the reported CPHF values of β and DR result from application of Maxwell-Boltzmann statistic probabilities, including all the possible conformers, calculated using the solvated relative free energies given in Table 1, whereas MP2 calculations combine results obtained for the two major conformers only. The static first hyperpolarizabilities of the various conformers of 4o, calculated at the CPHF level, are given Table S4 (ESI). The variations of βHRS induced by rotational isomerism can reach 22%, with values ranging from 12631 a.u. (4o-ctc) to 15344 a.u. (4o-cct). As usually observed for organic push-pull systems, accounting for electron correlation at the MP2 level leads to an increase in the hyperpolarizability values by roughly a factor of 2 with respect to the CPHF values.22,44 Moreover, gas-phase CPHF
Acido-Triggered NLO Switches
J. Phys. Chem. B, Vol. 111, No. 33, 2007 9801
2ω TABLE 5: Absolute Values of the Dynamic Longitudinal (β2ω zzz ) and HRS (βHRS) hyperpolarizabilities, Given in Atomic Units (1 2ω 2ω a au ) 3.62 10-42 m4 V-1 ) 3.2063 10-53 C3 m3 J-2 ) 8.641 10-33 esu) as well as Dynamic Depolarization Ratios IVV /IHV
1o 2o 3o 4o
β2ω zzz
2ω β2ω zxx/βzzz
β2ω HRS
2ω I2ω VV/IHV
β0b
β0c
β0d
β0e
117 800 ( 4000 27 500 ( 3000 184 000 ( 7000 206 000 ( 8000
- 0.109 ( 0.012 + 0.052 ( 0.014 - 0.019 ( 0.007 + 0.03 ( 0.07
47000 11700 75500 86500
4.00 5.47 4.81 5.28
3900 (1.00) 10100 (2.59) 4200 (1.08) 25000 (6.41)
4200 (1.00) 10200 (2.43) 4700 (1.12) 25100 (5.98)
9500 (1.00) 10600 (1.12) 17200 (1.81) 27500 (2.89)
25700 (1.00) 14400 (0.56) 57300 (2.23) 35200 (1.37)
a The static hyperpolarizabilities, β0 (β∞zzz), are estimated from the two-state model using different values for the γ damping factor. Relative values are given in parentheses. b From eq 4. c From eq 5, γ ) 100 cm-1. d From eq 5, γ ) 500 cm-1. e From eq 5, γ ) 1000 cm-1.
TABLE 6: Static and Dynamic Longitudinal First Hyperpolarizability (βzzz), HRS First Hyperpolarizability (βHRS), and Depolarization Ratio (DR) of 1o-4o Evaluated at Different Levels of Approximation Using the 6-31G(d) Basis Seta 1o
2o
3o
4o (av)
λ ) ∞, Gas Phase
Figure 2. Experimental points and fitted curves of harmonic light intensity as a function of the incident power for different concentrations (mol/L) of 3o under 1064 nm irradiation.
calculations predict a static first hyperpolarizability for 2o slightly smaller than for 1o, and MP2 calculations predict similar β0 values for these two compounds. The relative ordering of the β0 values obtained within MP2 is 1o ≈ 2o < 3o , 4o, in good agreement with the β0 values deduced from dynamic experimental values, provided a damping factor of 500 cm-1 is employed to extract static values from experimental dynamic results. Accounting for solvent interactions at the CPHF/6-31G(d) level leads to a large enhancement of the static βHRS responses by a factor 2.36, 1.59, 2.21, and 2.30 for 1o, 2o, 3o, and 4o, respectively. Thus, the solvent effects are on the same order of magnitude for 1o, 3o, and 4o and significantly smaller for 2o, indicating that the electronic states contributing to the static first hyperpolarizability of 2o are less sensitive to the surroundings. Frequency dispersion effects were also sized up for compounds 1o-3o using the TDHF scheme. In the case of 4o, it was not possible to obtain reliable values at λ ) 1064 nm due to numerical divergence associated with the fact that the corresponding TDHF excitation energy is close to 2.33 eV (λ ) 1064/2 nm). For gas-phase calculations, the factors of enhancement due to frequency dispersion range from 1.85 for 2o to 2.18 for 3o; accounting for solvent, they are all reduced to 1.25 (1o), 1.19 (2o), and 1.29 (3o), accordingly to the strong decrease in the dielectric constant of acetonitrile from (λ ) ∞) to (λ ) 1064 nm). Finally, dynamic first hyperpolarizabilities accounting for electron correlation as well as solvent effects have been evaluated by using eq 1 with βTDHF and βCPHF calculated in acetonitrile. These estimations, corresponding to the highest level of calculation used in this study, predict similar βHRS values for 1o and 2o, whereas βHRS for 3o is 36% higher. The ratio between the βHRS (3o)/βHRS (1o) is thus in good agreement with experimental data, but the very small relative βHRS value of 2o is not reproduced. Although contrary to compounds 1 and 2, we did not characterize experimentally the closed forms 3c and 4c, their first hyperpolarizabilities were computed at the CPHF/6-31G(d) level of approximation (Table S5). As for 1c vs 1o, the closed
CPHF βzzz βHRS DR MP2 βzzz βHRS DR
5384 (1.00) 2192 (1.00) 4.30
4706 (0.87) 1949 (0.89) 4.53
6587 (1.22) 2684 (1.22) 4.48
26664 (4.95) 13924 (6.35) 4.86
10070 (1.00) 4112 (1.00) 4.68
10377 (1.03) 4412 (1.07) 4.85
13367 (1.33) 5479 (1.33) 4.78
66363 (6.59) 34304 (8.34) 4.94
λ = 1064 nm, Gas Phase TDHF βzzz βHRS DR MP2b βzzz βHRS
10926 (1.00) 4437 (1.00) 4.61
8625 (0.79) 3599 (0.81) 4.73
14323 (1.31) 5841 (1.32) 4.76
/ / /
20436 (1.00) 8323 (1.00)
19019 (0.93) 8147 (0.98)
29066 (1.42) 11924 (1.43)
/ /
λ ) ∞, in Acetonitrile CPHF βzzz βHRS DR
12736 (1.00) 5173 (1.00) 4.25
7590 (0.60) 3115 (0.60) 4.34
14559 (1.14) 5931 (1.15) 4.38
62183 (4.88) 32063 (6.20) 4.70
λ ) 1064 nm, in Acetonitrile TDHF βzzz βHRS DR MP2c βzzz βHRS
16027 (1.00) 6507 (1.00) 4.64
8911 (0.56) 3704 (0.57) 4.66
18722 (1.17) 7628 (1.17) 4.75
/ / /
12672 (1.00) 5172 (1.00)
12183 (0.96) 5246 (1.01)
17189 (1.36) 7047 (1.36)
/ /
a
All values are given in a.u. Values relative to 1o are given in parentheses. b Using eq 1 with βTDHF and βCPHF calculated in gas phase. c Using eq 1 with βTDHF and βCPHF calculated in acetonitrile.
form displays β values about 1 order of magnitude smaller than the corresponding open form (βHRS (3c)/βHRS (3o) ) 0.123, βHRS (4c)/βHRS (4o) ) 0.075), confirming the large contrast of NLO response in this family of compounds. 4. Conclusions This study is a part of a broad investigation aimed at designing efficient photo- and acido-tunable NLO switches by integrating organic synthesis, optical characterization, and theoretical interpretations. Following our recent works on indolino- and benzimidazolo-oxazolidine derivatives, the linear and nonlinear optical properties of benzothiazolo-oxazolidine derivatives are characterized by means of HRS and quantum chemical calculations. The static hyperpolarizability measurements and theoretical estimates indicate that the benzothiazolium acceptor is 30% more effective than the indoleninium one, and furthermore, elongating an ethylenic bridging unit to a 2,5-diethenylthiophene further increases the NLO response of the POF by a factor of
9802 J. Phys. Chem. B, Vol. 111, No. 33, 2007 6. These conclusions have been drawn using the experimental data and have been supported by calculations performed at different levels of approximation, including electron correlation and solvent, as well as conformational effects. Future design of NLO-phore will combine these strategies of modifying the nature of the ring and extending the conjugation path with the insertion of donor/acceptor substituents, as discussed recently on the basis of theoretical investigation.9 Acknowledgment. This work has benefited from a scientific cooperation established and supported by the Centre National de la Recherche Scientifique (CNRS), the Belgian National Fund for Scientific Research (FNRS), and the Commissariat Ge´ne´ral aux Relations Internationales (CGRI) of the Communaute´ franc¸ aise Wallonie-Bruxelles. V.R. is indebted to the Re´gion Aquitaine for financial support in optical, laser, and computer equipment. B.C. thanks the Belgium National Fund for Scientific Research (FNRS) for his research director position. The calculations have been performed on the intensive calculation pole “M3PEC-Mesocentre” of the University Bordeaux I as well as on the Interuniversity Scientific Calculation Facility (ISCF) installed at the Faculte´s Universitaires Notre-Dame de la Paix (Namur, Belgium), for which the authors gratefully acknowledge the financial support of the FNRS-FRFC and the Loterie Nationale for the convention No. 2.4578.02, and of the FUNDP. Supporting Information Available: Table S1: Relative free energies and populations of the rotational conformers of 3c and 4c. Tables S2-S3: Linear optical properties of the rotational conformers of 3o and 4o. Tables S4-S5: Nonlinear optical properties of the rotational conformers of 3 and 4 in their closed and protonated open forms. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Delaire, J. A.; Nakatani, K. Chem. ReV. 2000, 100, 1817. (2) Coe, B. Chem.sEur. J. 1999, 5, 2464. (3) Loucif-Saı¨bi, R.; Nakatani, K.; Delaire, J.; Dumont, M.; Sekkat, Z. Chem. Mater. 1993, 5, 229. (4) Sliwa, M.; Le´tard, S.; Malfand, I.; Nierlich, M.; Lacroix, P. G.; Asahi, T.; Masuhara, H.; Yu, P.; Nakatani, K. Chem. Mater. 2005, 17, 4727. (5) Gilat, S. L.; Kawaı¨, S. H.; Lehn, J.-M. Chem.sEur. J. 1995, 1, 275. (6) Berkovic, G.; Krongauz, V.; Weiss, V. Chem. ReV. 2000, 100, 1741. (7) Houbrechts, S.; Clays, K.; Persoons, A.; Pikramenou, Z.; Lehn, J.-M. Chem. Phys. Lett. 1996, 258, 485. (8) Sanguinet, L.; Pozzo, J. L.; Rodriguez, V.; Adamietz, F.; Castet, F.; Ducasse, L.; Champagne, B. J. Phys. Chem. B 2005, 109, 11139. (9) Manc¸ ois, F.; Rodriguez, V.; Pozzo, J.-L.; Champagne, B.; Castet, F. Chem. Phys. Lett. 2006, 427, 153. (10) Sanguinet, L.; Pozzo, J. L.; Guillaume, M.; Champagne, B.; Castet, F.; Ducasse, L.; Maury, E.; Soulie´, J.; Manc¸ ois, F.; Adamietz, F.; Rodriguez, V. J. Phys. Chem. B 2006, 110, 10672. (11) Coe, B. J.; Harris, J. A.; Hall, J. J.; Brunschwig, B. S.; Hung, S.T.; Libaers, W.; Clays, K.; Coles, S. J.; Horton, P. N.; Light, M. E.; Hursthouse, M. B.; Garin, J.; Orduna, J. Chem. Mater. 2006, 18, 5907. (12) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (13) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. 1988, 37, 785. (14) Tomasi, J.; Persico, M. Chem. ReV. 1994, 94, 2027. (15) (a) Zerner, M. C. In ReViews of Computational Chemistry; Lipkowitz, K.B., Boyd, D.B., Eds.; VCH: New York, 1991; Vol. 1, p 313.
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