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J. Phys. Chem. C 2008, 112, 7900–7907
Theoretical and Experimental Investigation of Electric Field Induced Second Harmonic Generation in Tetrathia[7]helicenes† Alberto Bossi,*,‡ Emanuela Licandro,‡ Stefano Maiorana,‡ Clara Rigamonti,‡ Stefania Righetto,§ G. Richard Stephenson,⊥ Milena Spassova,| Edith Botek,# and Benoît Champagne*,# Dipartimento di Chimica Organica e Industriale, UniVersità degli Studi di Milano and UdR dell’INSTM di Milano, Via Golgi 19, I- 20133 Milan, Italy, Dipartimento di Chimica Inorganica, Metallorganica e Analitica, Centro di Eccellenza CIMAINA dell’UniVersità di Milano and UdR dell’INSTM di Milano, Via Venezian 21, I-20133 Milan, Italy, Wolfson Materials and Catalysis Centre, School of Chemical Sciences and Pharmacy, UniVersity of East Anglia, Norwich, Norfolk, NR4 7TJ United Kingdom, Institute of Organic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria, and Laboratoire de Chimie Théorique Appliquée, Facultés UniVersitaires Notre-Dame de la Paix, rue de Bruxelles, 61, B-5000 Namur, Belgium ReceiVed: December 14, 2007
In this paper, we report the first systematic experimental and theoretical investigation of the electric field induced second harmonic response of some tetrathia[7]helicene-based NLOphores. We studied six model compounds carrying the NO2, CHdCHCN, and COCF3 units as accepting groups on both the terminal thiophene positions as well as on the central benzene ring of the helicene backbone. These groups, known to be of medium and medium-strong accepting strength, allow tuning of both the electronic and structural properties of the helicenes studied. This experimental/theoretical study should set a milestone in addressing new structureproperties relationship of this class of nonconventional chiral chromophores able to show second order as well as third order NLO phenomena. 1. Introduction The rapid development of optical telecommunications and information technologies, where the light is the carrier of the information, is pushing research more and more toward the development of efficient systems that manipulate and modulate an incoming optical signal.1 Materials which show these properties must interact strongly with the electric field of an incident laser radiation and generate “nonlinear optical” (NLO) responses. NLO phenomena include a large number of effects, e.g., second and third harmonic generation and electrooptic (EO) changing of refractive index, to name just a few.2 The development of high performance, organic-based, electrooptic materials moves toward both (i) an optimization of the NLO properties of the molecular chromophore of which the material is made and/or (ii) an optimization of the material performances by engineering. Efficient molecular systems and materials have been developed in recent years, providing high second-order NLO effects at both the molecular3 and macroscopic levels.4 Usually, the so-called “first-generation” organic chromophores for secondorder NLO have a general structure in which a π-conjugated bridge links together both a strong electron-accepting (A) unit and a strong electron-donating (D) one, producing A-π-D systems. Later, other structures have been proposed, including octupolar5 and Λ-shape6 molecules. † Part of the “Larry Dalton Festschrift”. * To whom correspondence should be addressed. E-mail: alberto.bossi@ unimi.it;
[email protected]. ‡ Università degli Studi di Milano and UdR dell’INSTM di Milano. § Centro di Eccellenza CIMAINA dell’Università di Milano and UdR dell’INSTM di Milano. ⊥ University of East Anglia. | Bulgarian Academy of Sciences. # Facultés Universitaires Notre-Dame de la Paix.
Figure 1. Three-dimensional view of tetrathia[7]helicene 1.
Within the class of A-π-D NLOphores, the absence of centrosymmetry at the supramolecular level is a requirement to second order phenomena, and therefore, chirality is increasingly important in the design of π-conjugated materials, not only as an additional optical property, but because it can allow optimization of achiral properties.7 In particular, supramolecular structures, fundamental in optoelectronic applications, can be controlled and optimized through the inherent three-dimensional character of chirality and the control of intermolecular interactions. To advance the more rapid and targeted identification of the most suitable candidates for NLO applications, the availability of flexible and efficient synthetic methodologies for preparing the NLOphores as well as efficient theoretical modeling schemes would simplify synthetic efforts toward the most valuable targets. Nevertheless, in the case of complex molecular architectures and unconventional chromophores, like helicenes, both should be applied to address new structure–property relationships. Recently, tetrathia[7]helicenes (Figure 1 and parent compound 1 in Chart 1) have been recognized as an extremely attractive class of polyconjugated chiral molecules, currently investigated for optoelectronic applications and second-order NLO phenom-
10.1021/jp7117554 CCC: $40.75 2008 American Chemical Society Published on Web 04/15/2008
Second Harmonic Generation in Tetrathia[7]helicenes
J. Phys. Chem. C, Vol. 112, No. 21, 2008 7901
CHART 1: Thiahelicene Series Investigated in This Worka
a 1: Tetrathia[7]helicene (TH-[7]); 2: 7,8-di-(n-propyl)-2-trifluoroacetyl-tetrathia[7]helicene; 3: (E)-2-(7,8-di-(n-propyl)-2-tetrathia[7]helicen-2yl)-nitro-ethene; 4: 7-nitro-8-trimethylsilyl-tetrathia[7]helicene; 5: 7,8-di-(n-propyl)-2,13-di-trifluoroacetyl-tetrathia[7]helicene; 6: (E, E)-3,3′-(7,8di-n-propyl-tetrathia[7]helicen-2,13-di-yl)di-acrylonitrile.
SCHEME 1: Synthesis of Compound 3 ena.8 They combine the electronic properties afforded by their extensive π-conjugated system with the chiroptical properties9 associated to their helical structure, which results from the orthocondensation of aromatic and heteroaromatic rings. In addition, the regioselective functionalization of tetrathia[7]helicenes could allow the tuning of both their electronic and structural (i.e., helical pitch) properties and therefore the identification of the most promising derivatives. Our groups have recently been involved in this design of functionalized tetrathia[7]helicenes with specific properties for targeted applications, finding an improved synthetic route10 and characterizing their NLO properties by theoretical11 and experimental12 means. In particular, ref 11c highlighted how strategic are both the terminal thiophene positions (2 and 13 in 1, Chart 1) and the central arene positions (7 and 8 in 1, Chart 1) for enhancing the NLO response. Moreover, a first extensive investigation, focusing on electron transfer (ET) properties of thiahelicenes, has been reported by some of us providing a deep insight on the structure/ electrochemical activity relationship within this attractive class of compound.13 However, despite the potential of thiahelicenes for optoelectronic science, combined theoretical/experimental NLO characterization are still needed for a better understanding of the structure–property relationships and constitute the topic of this paper. The design of materials for second-order NLO applications requires not only an understanding of the contribution of the molecules constituting the medium to its nonlinear polarization but also of the way in which their geometrical arrangement in the medium determines the propagation characteristics of the resulting field components.14 The technique of electric field induced second harmonic generation (EFISH)15 was developed to determine the value of quadratic molecular hyperpolarizability β from measurements on liquids or solutions. In this technique, a strong DC electric field is applied through a liquid or solution causing an orientation of the molecules due to the interaction of the field with the molecule permanent dipoles. The EFISH technique found wide application for determination of the quadratic hyperpolarizability of various dipolar organic16 and organometallic compounds17 providing the projection of the vector part of β on the dipole moment vector. This work reports on the experimental and theoretical investigations of the linear and nonlinear, using the EFISH technique, optical properties of tetrathia[7]helicenes functionalized with electron-accepting groups (2-6), together with the unfunctionalized parent 1 (Chart 1). The presence of the two n-propyl chains on the 7 and 8 positions of the central arene ring in helicenes 2, 3, 5 and 6 is important to ensure their solubility in organic solvents, thus allowing the study of their optical properties. It is important to point out that all of the helicenes studied in this work could represent useful model
systems (having functional groups both in position 2 and 13 and 7 and 8) to compare experimental and theoretical data, and in addition, they can be easily synthesized following well established synthetic protocols, developed by us and other authors.8b,d,e,10 From this study, we expect to establish, and set, useful guidelines for the development of this amazing class of nonconventional compounds. In particular, we have considered thiahelicenes carrying the NO2, CHdCHCN, and COCF3 as accepting groups; these groups could be considered of medium and medium-large accepting strength. It is known how acceptor groups are usually much more effective than donor ones in the modulation of NLO response. This behavior has been ascribed by Cheng et al.18 to the intrinsic nature of sp or sp2 orbital of the main atom of the acceptor. The same authors also found that, in monofunctionalized benzenes bearing only electron-withdrawing groups, the efficacy of the acceptor increases in the series CN < COCF3 < NO2; nevertheless, extended conjugated systems containing CN moieties, like CHdC(CN)2, become stronger acceptors.18 Thus, it appears also of interest to establish whether the acrylonitrile group CHdCHCN, analogously to the cited CHdC(CN)2, could lead to an improved performance exceeding that of COCF3 one as a result of the more extended conjugation. In summary, the six thiahelicenes (Chart 1) can be classified into (a) a parent tetrathiahelicene having no substituents (molecule 1), (b) tetrathiahelicenes having only one acceptor group on the R thiophene position (molecules 2 and 3) as well as on the central benzene ring (molecule 4), i.e., on the opposite side of the molecule, (c) tetrathiahelicenes having the acceptor groups at both the two terminal R thiophene positions (molecules 5 and 6), (d) tetrathiahelicenes having, respectively, only one or two identical accepting groups on the thiophene position (molecules 2 and 5). 2. Experimental Section 2.1. Methodologies and Experimental Procedures. The syntheses of helicenes 1,8b,d,e,10a,d 2 and 510c (Chart 1) have been previously reported. The synthesis of helicene 3 is described in this work and is shown in Scheme 1. On the contrary, the syntheses of helicenes 419 and 6,20 which required an in depth synthetic study and several new reaction steps, are more complex and will be reported in a future specific synthetic work following further optimization. Helicene 3 was obtained by a Henry-type,
7902 J. Phys. Chem. C, Vol. 112, No. 21, 2008 nitroaldolic condensation of the aldehyde 710c with nitromethane (Scheme 1). The reaction was carried out both following a thermal heating approach and by microwave irradiation. Although well-known, this synthetic methodology has never been utilized to insert a nitrovinyl moiety onto helicenes and resulted very useful for the extension of helicene π system and insertion of the strong electron-withdrawing nitro group conjugated to the helix. 2.1.1. General. Reagents obtained from commercial sources were used without further purification. In order to monitor the progress of the reactions, thin layer chromatography (TLC) was performed using Merck silica gel 60 F254 precoated plates. Flash chromatography was performed using Merck silica gel 60, 230–400 mesh. Melting points were determined by means of a Büchi 510 apparatus and are uncorrected. High-resolution mass spectra were recorded on a Vg Analytical 7070 EQ. 1H and 13C NMR were recorded on Bruker AC200, Bruker AC300, and Bruker AMX-300. Microwave assisted reactions were performed with a CEM Discover S-Class apparatus. UV-absorption spectra were measured with Perkin-Elmer Lambda EZ210 spectrophotometer; data obtained from 10-5 M concentration of each helicene were normalized for a better clarity. 2.1.2. Synthesis of 2-(2-NitroWinyl)-tetrathia[7]helicene (3). 2.1.2.1. Method A. Under argon atmosphere, 77 mg of 7 (0.15 mol) and 34.7 mg of AcONH4 (0.45 mmol; 3 equiv) were introduced into a 10 mL microwave reactor; 0.43 mL of AcOH (75 mmol, 50 equiv) was added together with 0.50 mL of dry THF, in order to obtain a clear solution, and 0.016 mL of CH3NO2 (0.30 mmol, 2 equiv). The reaction mixture was irradiated at 110 °C for 10 min in the following conditions: Pmax) 50 W; Pmax) ON; Pmax) 10 bar; ramp time, 40 s; stirring, on. After 10 min, 0.016 mL of CH3NO2 (2 equiv) was added together with 100 mg of activated molecular sieves. The reaction was heated for another 40 min under the same conditions. The reaction mixture was diluted with Et2O and quenched with saturated Na2CO3. After phase separation, the aqueous phase was extracted with 3 × 10 mL of Et2O. The collected organic phases were washed with water and dried over Na2SO4, and the solvent was removed under reduced pressure affording 200 mg of a crude mixture which was purified by means of flash column chromatography, eluted with light petroleum/CH2Cl2/ Et2O 80:20:3. Compound 3 was obtained as a brown-red solid in 26% yield (22 mg). 2.1.2.2. Method B. A total of 70 mg of 7 (0.136 mol; 1 equiv) and 31.4 mg of AcONH4 (0.408 mmol; 3 equiv) were introduced, under argon atmosphere, in a 10 mL round-bottom flask; 0.39 mL of AcOH (6.80 mmol, 50 equiv) was added together with 0.50 mL of dry THF and 0.025 mL of CH3NO2 (0.544 mmol, 4 equiv). The reaction was heated at 100 °C for 2 h. By the method described above, the crude mixture afforded compound 3 in 76% yield (57.1 mg). HRMS Calcd for C30H23NO2S4: 557.06445. Found: 557.061166. 1H NMR (300 MHz, CDCl , ppm): δ 1.60 (t, J ) 7.4 Hz, 3H, 3 CH3), 1.63 (t, J ) 7.1 Hz, 3H, CH3), 1.88 (m, 4H, 2xCH2), 3.10 (m, 4H, 2xCH2), 6.66 (d, J ) 5.6 Hz, 1H), 6.95 (d, J ) 5.6 Hz, 1H), 7.03 (s, 1H), 7.11 (d, J trans ) 13.3 Hz, 1H), 7.47 (d, J trans ) 13.3 Hz, 1H), 7.90 (d, J ) 8.5 Hz, 1H), 8.01 (d, J ) 8.5 Hz, 1H), 8.06 (d, J ) 8.5 Hz, 1H), 8.08 (d, J ) 8.5 Hz, 1H). 13C NMR (75 MHz, CDCl3, ppm): 127.6, 127, 130.7, 121.1, 131.7, 132.4, 133.2, 135.34, 135.35, 136.4, 136.7, 136.8, 138.5, 140.1, 140.3 (Cq); 118.9, 119.6, 120.2, 121.3, 121.9, 124.6, 125.0, 132.1, 133.3, 136.2 (CH); 34.4, 23.3 (CH2); 14.47 (CH3). MS (EI): m/z (%) ) 557 ([M]+,100); 511 ([M-NO2]+, 35); 478 ([M-79]+, 70).
Bossi et al. 2.2. Experimental EFISH Methodology and Dipole Moment. In order to investigate their NLO properties, the helicenes were submitted to EFISH measurements. It is known that the EFISH technique can provide direct information on the intrinsic molecular NLO properties through (1)
γEFISH ) 〈γ(-2ω; ω, ω, 0)〉 µβ|(-2ω; ω, ω) 5kT
) γ|(-2ω; ω, ω, 0) +
(1)
ω is the incident light frequency, γ|(-2ω;ω,ω,0) is the thirdorder term, µβ|(-2ω;ω,ω)/5kT is the dipolar orientational contribution, with β|(-2ω;ω,ω) the projection along the dipole moment axis of the vectorial component of the first hyperpolarizability tensor
β|(-2ω;ω, ω) ) β| ) )
3 5
∑ i
3 5
µ
∑ µi ∑ (βijj + βjij + βjji)
µiβi µ
i
j
(2)
µ is the norm of the dipole moment and µi and βi are the components of the µ and β vectors. When considering the quadratic NLO response of dipolar compounds,17a,21 the γ|(-2ω;ω,ω,0) cubic contribution is often neglected in the analysis of the EFISH results (i.e., though as discussed later, it might raise some complications). In the case of neglecting γ|(-2ω;ω,ω,0), the dipole moment is the unique molecular property required to extract the β| value. To avoid overestimation of the quadratic hyperpolarizabilty value due to dispersion effects producing resonance enhancements, it is necessary to work with an incident wavelength λ whose second harmonic λ/2 is far enough from the λmax of any absorption of the investigated molecules. All EFISH measurements were carried out at 298 K in CHCl3 solutions at the same concentration (10-3 M) working at a nonresonant incident wavelength of 1907 nm, using a Q-switched, mode-locked Nd3+:YAG laser with pulse durations of 15 ns (90 ns) at a 10 Hz repetition rate equipped with a Raman shifter. The EFISH values reported are the average of 16 successive measurements performed on the same sample. Usually the error on the µβ values is roughly 10–15%. All experimental EFISH β| values were defined according to the “phenomenological or X” convention.22 Dipole moments were measured in chloroform solution using the capacitance Guggenheim method23 on a Dipolmetre WTW mod. DM01. The standard deviation on µ amounts to 1 D. 2.3. Quantum Chemical and Computational Procedure. Geometry optimizations of the ground-state equilibrium structures were carried out at the density functional theory (DFT) level of approximation using the B3LYP exchange-correlation functional and the 6-31G* basis set. The EFISH responses (1) were calculated at different levels of approximation (i) using the time-dependent Hartree–Fock (TDHF) approach24 which determines the successive energy derivatives with respect to perturbating static and/or dynamic electric fields, (ii) employing its static analog, the coupled-perturbed Hartree–Fock (CPHF) approach, and (iii) adopting MP2/finite field approach25 in order to estimate the importance of correlation effects on these properties. In the latter case, a finite differentiation procedure is employed and combined with the Romberg procedure to improve the accuracy on the numerical derivatives. Standard wavelength of 1907 nm was considered in the TDHF calculations. To include both electron correlation and dispersion effects on the EFISH responses, the multiplicative correction scheme
Second Harmonic Generation in Tetrathia[7]helicenes
J. Phys. Chem. C, Vol. 112, No. 21, 2008 7903
Figure 2. Normalized optical absorption of compounds 1-6 in CH2Cl2 solution.
was applied. It consists of multiplying the static MP2 value by the TDHF/CPHF ratio
PMP2(ω) ≈ PMP2(0)
PTDHF(ω) PCPHF(0)
TABLE 1: Optical Bandgaps of Helicenes 1-6 compounds
λmax (nm)
λonset (nm)a
Eg (eV)
Eg,onset (eV)
1 2 3 4 5 6
387.0 426.0 429.0 440.5 442.5 430.0
407.0 500.0 521.0 542.5 508.0 471.0
3.21 2.92 2.90 2.82 2.81 2.89
3.05 2.49 2.39 2.29 2.45 2.64
(3)
where P is γ|(-2ω;ω,ω,0) or β|(-2ω;ω,ω). These calculations were performed using the 6-311G* as well as the 6-311+G* basis sets to assess the role of basis set diffuse functions. When considering the solvent, PTDHF(ω) is determined within the IEFPCM scheme whereas the static quantities (CPHF and MP2) are both obtained in vacuo. The excitation energies (∆E0n) and oscillator strengths (f0n) were determined using the time-dependent DFT (TDDFT) scheme26 by restricting the calculation to the 30–40 lowestenergy states. These calculations employed the B3LYP exchange correlation functional and the 6-311G* and 6-311+G* atomic basis sets, which have been shown at several occasion to provide at least a qualitative agreement with experiment.27 Spectra were simulated by associating to each transition a 50/50 Gaussian/ Lorentzian line shape having a height proportional to f0n and a full width at half-maximum (fwhm) ranging from 0.1 to 0.5 eV to accommodate for the differences in the experimental spectra. Thus, in simulating the UV/visible spectra, the vibrational broadening of the transitions is not explicitly considered as could be done through determining Franck–Condon factors. Moreover, to take into account the solvent effect, the integral equation formalism (IEF) of the polarizable continuum model (PCM)28 was employed. Chloroform was selected as solvent in the calculations like in the experimental EFISH determination. The calculations were carried out using the GAMESS29 (TDHF γ calculations) and the GAUSSIAN program30 (TDHF and MP2/ FF β evaluations as well as geometry optimizations and TDDFT calculations). 3. Results and Discussion 3.1. Experimental Linear and Nonlinear Optical Properties. The linear optical data for the six compounds are provided in Figure 2 and Table 1. In all cases, functionalization of 1 leads
a The criterion for reading “onset” wavelengths was that the peak absorbance, after background (subtraction) correction, should attain 5% of the absorbance of the lowest-energy wavelength absorption peak.
TABLE 2: Experimental EFISH Responses for Compounds 1-6 compound
γEFISH (10-34 esu)
1 2 3 4 5 6
-2.2 6.0 9.8 6.5 9.3 11.8
µβ| (10-48 esu) a
120 197 130 180 230
a
Cannot be calculated because the γ| cannot be neglected (see section 2.2).
to a red shift of the onset of absorption by up to 0.75 eV. The same is true for the maximum of absorption of the lowest-energy transition. Although the bandgap values estimated at both the maximum and the onset show small variations for most systems, the shape of the absorption spectrum is dominated by the nature of the substituting groups. A broad absorption is recognized for the nitro-substituted helicenes 3 and 4, whose substitution is present on the thiophene (3) or on the benzene ring (4). A broader band, resulting probably from vibronic coupling, is found for the helicenes 2 and 5 containing the COCF3 groups, with two distinct transitions for compound 5. The EFISH responses are summarized in Table 2, both the full γEFISH and the extracted µβ| (as can be deduced from eq 1 neglecting γ| contribution). The analysis of the experimental
7904 J. Phys. Chem. C, Vol. 112, No. 21, 2008
Bossi et al.
TABLE 3: Vertical Excitation Energies (and Corresponding Wavelengths) for the Dominant Low-Energy Absorption Band of Helicenes 1-6 Determined at the TDDFT/B3LYP/ 6-311G* Level of Approximation, Using the IEFPCM Scheme to Describe the Solvent Effects (CHCl3) compounds
λ0n (nm)
∆E0n (eV)
f0n
1 2 3 4 5 6
389, 385 508, 477 543, 514 534, 495 505, 476 469, 463
3.19, 3.22 2.44, 2.60 2.29, 2.41 2.32, 2.50 2.46, 2.61 2.64, 2.68
0.294, 0.143 0.063, 0.128 0.037, 0.205 0.106, 0.180 0.115, 0.145 0.145, 0.253
NLO responses of helicenes 2-6 show an overall increase in the performance. Passing from the mono functionalized helicene 2, bearing in position 2 a COCF3 group, to helicene 3 in which a stronger acceptor (NO2) is present together with an extension of the π system, the overall response increases. Comparing 4 (NO2 is directly bonded to the helicene in position 7) and 2, however, the response is not so much higher despite the presence of the NO2 group. This consideration might point to a lower efficiency of the benzene position with respect to the thiophene case.13 An analogous conclusion might be drawn comparing helicene 4 with 3 (in which the central NO2 group substantially underperforms the example located at the terminus). An alternative possible explanation might be that in 4 the bulky SiMe3 group next to the NO2 prevents good π overlap between the acceptor and the helicene, by blocking coplanar alignment. However, an examination of the conformation identified in the DFT calculations indicates that the NO2 group in 4 is almost planar to the arene, thus rendering the first explanation more plausible. A second important trend can be identified when helicene 2 is compared with its symmetrically disubstituted analogue 5 which shows a substantially higher response. Structure 2 can be regarded as a single long conjugated structure (comprising seven fused unsaturated rings) polarized by a single terminal COCF3 group. With respect to this seven-ring system, the two substituents in 5 oppose one-another with regard to their polarizing effect on the molecule as a whole. To account for the greater γEFISH and µβ values observed for 5, it is necessary to consider the structure as one containing the combined effects of two shorter fused polycyclic substructures (each with four fused rings and a terminal COCF3 group). The combined effects of these two shorter substructures appears to significantly outperform the monosubstituted segment in 2, as in Λ-shape compounds.6d Among these model systems, we found that diacrylonitrile helicene 6 shows the highest value. Again this can be explained as resulting from the combined effect of the two four-ring substructures, this time polarized by the (E) CHdCHCN group. This combined effect must be substantial, since comparison of 6 and 3 shows a greater NLO response in the former, despite the fact that its electron-withdrawing substituent is less powerful (see above) than the NO2 group present in the corresponding position in 3. The effect of the more extended delocalized structure and the improved performance of CHdCHCN with respect to the COCF3 group, can be noticed comparing 5 and 6. All these data point out the better NLO efficiency of compounds 3 and 6 with respect to the other thiahelicenes. The observed NLO response of the unsubstituted structure 1 is more difficult to measure because in symmetric π-conjugated chromophores (usually derived from linear systems, e.g., diarylethenes, polyphenylenevinylenes, and polythiophenes), without acceptor and donor groups, i.e., with no significant charge transfer over the conjugated system, the EFISH response
Figure 3. TDDFT/B3LYP/6–311G* simulated UV/visible absorption spectrum of compound 4 for different values of fwhm.
Figure 4. TDDFT/B3LYP/6–311G* simulated UV/visible absorption spectrum of compound 1–6 with fwhm ) 0.2 eV.
is small, which explains that the standard deviation is relatively large, and probably coming mostly from the third-order contribution γ|.11c,17e 3.2. Theoretical Linear and Nonlinear Optical Properties. Table 3 lists the major UV/visible absorption characteristics of the 1-6 helicenes as determined at the TDDFT/B3LYP/6–311G* level of approximation, whereas Figure 3 shows, for thiahelicene 4, how the shape can change as a function of the fwhm. With the impact of the n-Pr groups in positions 7 and 8 being assumed to be negligible, they have been replaced by H atoms in modeling the properties of compounds 2, 3, 5, and 6. The low-energy region of the UV/visible absorption spectra is characterized by two transitions, almost degenerate in 1 and 6 while distant by up to 0.18 eV in 4(Table 3). These spectroscopic data as well as the simulations presented in Figure 4 reproduce the observed experimental bathochromatic shift found for compounds 2-6 with respect to the reference compound 1. The differences between simulations and experiment might have different origins, including limitations of the TDDFT scheme and the absence of vibronic structures. In general, the theoretical ∆E0n values are smaller than the experimental results, as often found in TDDFT calculations.26,27
Second Harmonic Generation in Tetrathia[7]helicenes TABLE 4: TDHF/6-311+G* EFISH Molecular Properties (λ ) 1907 nm) of Compounds 1-6, as Calculated by Taking into Account the Effect of the Solvent (CHCl3) using the IEFPCM Schemea µ
β|
compound (10-18 esu) (10-30 1 2 3 4 5 6
0.20 6.18 8.75 7.09 9.42 10.13
µβ| γ| γEFISH esu) (10-48 esu) (10-36 esu) (10-36 esu)
0.04 2.51 8.16 4.77 4.20 5.56
0.01 15.5 71.4 33.8 39.6 56.3
5.6 9.1 16.4 13.1 11.3 21.6
5.7 (99) 84.5 (11) 363.3 (5) 175.4 (7) 203.5 (6) 295.2 (7)
a The values in parentheses in the last column define the contribution (in %) of the third order term to the full EFISH response.
TABLE 5: MP2/6-311+G* EFISH Molecular Properties (λ ) 1907 nm) of compounds 1-6 using eq 3, as Calculated by Taking into Account the Effect of the Solvent (CHCl3) using the IEFPCM Schemea β| µβ| γ| γEFISH µ compound (10-18 esu) (10-30 esu) (10-48 esu) (10-36 esu) (10-36 esu) 1 2 3 4 5 6
0.33 5.09 6.60 5.70 7.55 8.40 b
0.32 4.17 15.60 9.17 7.48 11.15
0.1 21.2 103.0 52.3 56.5 93.6
12.2 15.5 31.9 22.9 19.4 39.4
12.7 (96) 118.6 (13) 532.2 (6) 276.8 (8) 293.7 (7) 494.3 (8)
a The values in parentheses in the last column define the contribution (in %) of the third-order term to the full EFISH response. b Experimentally determined µ value: 8.3 D.
Table 4 lists the calculated values for the molecular properties appearing in 1, as determined at the TDHF/6–311+G* level of approximation with T ) 298.15 K. Although the details are not provided here, removing diffuse functions in the basis set, i.e., going from the 6-311+G* to the 6-311G* basis set, leads to a decrease of β| by 5–12%, except for 1 where the second order NLO response is very small. Table 5 presents the corresponding quantities after accounting for electron correlation through 3. TDHF and CPHF calculations carried out on 1 and 5 showed that the frequency dispersion effects on γ| attain 25% and therefore, a scaling factor of 1.25 has been applied to all the γ| values. The effects of the solvent were only taken into account on the first hyperpolarizability so that the second hyperpolarizabilities are underestimated, and consequently, the third order NLO contributions to γEFISH should be considered as a lower limit. This prediction is substantiated (i) by the impact of the solvent on the β| response, which, as determined using the IEFPCM scheme, corresponds typically to an increase by 120–150% for compounds 2-5 and (ii) by the corresponding calculated gas phase γ|/γEFISH ratios amounting to 97%, 27%, 15%, 21%, 16%, and 20% for compounds 1-6, respectively. In both Tables 4 and 5, the two systems with the largest µβ| responses are compounds 3 and 6 (though the order is reversed with respect to experiment, Table 2). The next compounds in decreasing order of µβ| are 5, 4, 2, and 1. Moreover, calculations attribute to compound 5 a µβ| response, which is larger than twice the response in compound 2, its parent monosubstituted compound. These µβ|-based relationships are confirmed after considering the third-order term to γEFISH since for compounds 2-6, their amplitude is of the order of 5–15%. The largest γ| response is associated with compound 6, demonstrating the dominant role of the extended π-conjugated network. In addition, compound 4 presents a larger γ| value than compound
J. Phys. Chem. C, Vol. 112, No. 21, 2008 7905 TABLE 6: Experimental and Theoretical Ratios Between the Experimental and Theoretical Values for the µβ| and γEFISH Quantities of Compounds 1-6a compound
µβ|
γEFISH
1 2 3 4 5 6
5.7 1.9 2.5 3.1 2.4
-22 5.0 1.8 2.3 3.2 2.4
a The theoretical values were taken from Table 5, whereas the experimental ones are from Table 2.
5, whereas the opposite holds for the µβ| response. This further illustrates that the second and third order responses have different behaviors with respect to the nature and position of the substituents. Finally, as it was indicated as a target in the Introduction, it is possible to address the accepting properties of the substituents studied. From ab initio calculations, the -CHdCHCN group is a better acceptor group than the -COCF3 group but the strongest attractor substituent is the sCHdCHsNO2 group. 3.3. Discussion. At the outset, it is important to emphasize that the discussion is focused on µβ| rather than on β| because the standard deviation on the dipole moment is quite large. Considering the NLO responses of the functionalized helicenes 2-6, from the theoretical (Tables 4 and 5) as well as from the experimental (Table 2) points of view, it is evident that our model compounds well fit the scheme of discussion proposed in the Introduction. In particular, (i) the three helicenes bearing one accepting group (2, 3, and 4) show an overall increase of µβ| as a function of the increasing acceptor strength: 2 (COCF3) < 4 (NO2 in position 7) < 3 (-CHdCHNO2); these responses, inter alia, also substantiate that the terminal thiophene positions are more efficient for achieving large nonlinearities than the benzene positions; (ii) from comparing helicene 2 with its symmetrical disubstituted 5, the larger response of 5 accounts for its peculiar Λ-shape structural feature; considering the 10–15% uncertainty of the experimental EFISH determinations and the inherent limitations of the calculations, both theory and experiment predict that the NLO response of 5 (theory: 56.5 × 10-48 esu, experiment: 172 × 10-48 esu) is about twice larger than for 2 (theory: 21.2 × 10-48 esu, experiment: 120 × 10-48 esu); (iii) in the two Λ-shape helical structures 5 and 6, as expected, the two acrylonitrile groups prompt the corresponding helicene (6) to show the largest NLO response; (iv) finally, for helicene 1, the theoretical results demonstrate that the second hyperpolarizability γ| is the main contributor to the γEFISH response and that the µβ| term is at least 2 orders of magnitude smaller. Table 6 provides a direct comparison between the experimental data and the most evolved theoretical results, i.e. those obtained by considering electron correlation, frequency dispersion, and solvent effects. The experimental values are systematically larger than the theoretical ones. For compounds 3-6, the ratio amounts to 2.5 ( 0.6 for µβ| and 2.5 ( 0.7 for γEFISH. The origin of most of this ratio has probably to be found in the usually simplified local field factors, that are used to extract the molecular properties from the bulk responses. In the case of compound 2, the ratio is however twice larger. 4. Conclusion For the first time, we have synthesized and studied, in a joint experimental and theoretical effort, the linear and nonlinear (by
7906 J. Phys. Chem. C, Vol. 112, No. 21, 2008 means of the EFISH technique) optical properties of six functionalized tetrathiahelicenes, model candidates to unravel the structure–functionalization-property relationships of these nonconventional chiral polyconjugated systems. We have investigated compounds carrying the NO2, CHdCHCN, and COCF3 units as accepting groups on both the terminal thiophene positions as well as on the central benzene ring of the helicene backbone and have shown how these functional groups could tune their first and second hyperpolarizabilities. In particular, in acceptor-monosubstituted helicenes, an increase of the second order NLO response µβ| is found as a function of increasing acceptor strength while the Λ-shape structural feature yields a larger µβ| response than its analogue monosubstituted at one terminal thiophene position. Work on these systems is further in progress in our groups, pointing toward modeling and designing new and efficient helicene-based NLOphores for both second and third order NLO processes. Acknowledgment. The authors are grateful to Prof. R. Ugo, Prof. D. Roberto, and Prof. E. Cariati (Dipartimento di Chimica Inorganica, Metallorganica e Analitica, Università di Milano). The authors gratefully acknowledge joint financial support from the Ministero dell’Istruzione, dell’Università e della Ricerca Scientifica (MUR), Rome, and the University of Milan (FIRB Project, Bando 2003, Progetto RBNE033KMA, title of the project: “Molecular compounds and hybrid nanostructured materials with resonant and non resonant optical properties for photonic devices.” (fellowship to A.B.); Fondazione CARIPLO (2003: Materiali ibridi polimerici, supramolecolari e nanostrutturati, con superiori proprietà di stabilità e di trasmissione di informazioni fotoniche); PRIN 2005 Project: “Nuovi sistemi catalitici stereoselettivi e sintesi stereoselettiva di molecole funzionali”; FIRST). They also acknowledge the Centro di Eccellenza CIMAINA and COST Action D26 and EPSRC. This study results also from scientific cooperations established and supported by the Belgian National Fund for Scientific Research (FNRS), the Commissariat Général aux Relations Internationales (CGRI) of the Communauté française Wallonie-Bruxelles, and the Bulgarian Academy of Sciences. E.B. thanks the IUAP program No. P6-27 for her postdoctoral grant. B.C. thanks the Belgium National Fund for Scientific Research (FNRS) for his research director position. The calculation have been performed on the Interuniversity Scientific Calculation Facility (ISCF) installed at the Facultés 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. References and Notes (1) (a) Verbiest, T.; Van Elshocht, S.; Kauranen, M.; Hellemans, L.; Snauwaert, J.; Nuckolls, C.; Katz, T. J.; Persoons, A. Science 1998, 282, 913. (b) Marder, S. R. Chem. Commun. 2006, 131. (2) Barlow, S.; Marder, S. R. Nonlinear Optical Properties of Organic Materials. In Functional Organic Materials. Syntheses, Strategies, and Applications; Miller, T. J. J., Bunz, U. H. F., Eds.; Wiley-VCH: Weinheim, Germany, 2007; pp 393. (3) (a) Meyers, F.; Marder, S.R.; Pierce, B. M.; Brédas, J. L. J. Am. Chem. Soc. 1994, 116, 10703. (b) Albert, I. D. L.; Morley, J. O.; Pugh, D. J. Phys. Chem. A 1997, 101, 1763. (c) Verbiest, T.; Houbrechts, S.; Kauranen, M.; Clays, K.; Persoons, A. J. Mater. Chem. 1997, 7, 2175. (d) Cho, M.; Am, S. Y.; Lee, H.; Ledoux, I.; Zyss, J. J. Chem. Phys. 2002, 116, 9165. (e) Frediani, L.; Ågren, H.; Ferrighi, L.; Ruud, K. J. Chem. Phys. 2005, 123, 144117. (f) Maury, O.; Le Bozec, H. Acc. Chem. Res. 2005, 38, 691. (g) Sanguinet, L.; Pozzo, J. L.; Rodriguez, V.; Adamietz, F.; Castet, F.; Ducasse, L.; Champagne, B. J. Phys. Chem. B 2005, 109, 11139. (h) Sanguinet, L.; Pozzo, J. L.; Guillaume, M.; Champagne, B.; Castet, F.; Ducasse, L.; Maury, E.; Soulié, J.; Mançois, F.; Adamietz, F.; Rodriguez, V. J. Phys. Chem. B 2006, 110, 10672. (i) Coe, B. J. Acc. Chem. Res. 2006, 39, 383. (j) Xu, H. L.; Li, Z. R.; Wu, D.; Wang, B. Q.; Li, Y.;
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