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First hyperpolarizability in a new benzimidazole derivative
Chemical Physics 305 (2004) 115–121 www.elsevier.com/locate/chemphys
First hyperpolarizability in a new benzimidazole derivative Fabiano Severo Rodembusch a, Tiago Buckup b, Maximiliano Segala c, Luciana Tavares b, Ricardo Rego Bordalo Correia b, Valter Stefani a,* a
Instituto de Quı´mica, Universidade Federal do Rio Grande do Sul, Laborato´rio de Novos Materiais Orgaˆnicos, Av. Bento Gonc¸alves, 9500, CP 15003, CEP 91501-970 Porto Alegre-RS, Brazil b ´ ptica & Laser, Av. Bento Gonc¸alves, 9500, CP 15051, Instituto de Fı´sica, Universidade Federal do Rio Grande do Sul, Laborato´rio de O CEP 91501-970 Porto Alegre-RS, Brazil c Instituto de Quı´mica, Universidade Estadual de Campinas, Cidade Universita´ria Zeferino Vaz s/n, CP 6154, CEP 13083-970 Campinas-SP, Brazil Received 15 April 2004; accepted 21 June 2004 Available online 20 July 2004
Abstract A new benzimidazole, 2-(4 0 -amino-2 0 -hydroxyphenyl)-6-nitrobenzimidazole (LEN), with promising applications in nonlinear optics were synthesized, purified and characterized by classical techniques. The UV–Vis and steady-state fluorescence of LEN in solution were applied in order to characterize the photophysical behaviour of this new fluorescent dye. The Hyper-Rayleigh Scattering study indicates a remarkable increase in the b absolute value (1197.3 ± 1.2 · 1030 esu) of the LEN in acetone at 1064 nm, when comparing with a previously described benzoxazole (BO6). The ratio between the experimental b and the maximum theoretical value using a two-level model was also improved by a factor 6. Ab initio and semi-empirical calculations of the dipole moments and the first hyperpolarizability are also presented. The enhancement in the experimental hyperpolarizability value in relation to BO6 is explained in terms of the benzimidazolic ring basicity and a prototropic effect (annular tautomerism) present in the LEN, identified as a resonance-assisted hydrogen bond, which leads to a stronger electronic delocalization. 2004 Elsevier B.V. All rights reserved.
1. Introduction Organic molecules are useful materials for nonlinear optical (NLO) applications in photonics, bio-sensors and two-photon-induced fluorescence probes [1–4]. The NLO properties of these molecules are often enhanced or modified using different types of radicals as acceptor and donor groups, in analogy to the wellknown model v(2) compounds for NLO [5]. Furthermore, in the last years, several studies about the incorporation of these molecules into polymeric chains * Corresponding author. Tel.: +55-51-3316-6285; fax: +55-51-33167304. E-mail address: [email protected] (V. Stefani). URL: http://www.iq.ufrgs.br/lnmo.
0301-0104/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2004.06.046
have been made [6–15]. In this field, studies regarding the NLO properties of some benzazole derivatives have been done due the great thermal and photophysical stability of these dyes [16–18]. This stability characteristic is mainly caused by an intramolecular proton transfer mechanism in the excited state (ESIPT), a photoprototropic effect [19–23]. In the ESIPT, the enol–cis conformer (E) in non-polar and aprotic solvents, with an intramolecular hydrogen bond between the phenolic hydrogen and the nitrogen in the benzazolic ring, is the predominant specie in the ground state (Fig. 1) [24]. The UV light absorption through the enol–cis (E) produce an excited enol–cis (E*) which is quickly [25,26] converted to an excited keto tautomer (K*) by an intramolecular proton transfer [27]. The excited keto tautomer (K*) decays emitting fluorescence to a keto
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2.2. Synthesis of 2-(4 0 -amino-2 0 -hydroxyphenyl)-6-nitrobenzimidazole
Fig. 1. Chemical structure of the tautomeric forms enol–cis (E) and keto (K) involved in the ESIPT mechanism (X = O, S and NH).
tautomer in the ground state (K). Since the enol–cis tautomer (E) is more stable than the keto tautomer in the ground state, the initial enol–cis form is regenerated without any photochemical change [28,29]. The characteristics of these ESIPT compounds [30–36], as the fast proton-transfer cycle, thermal and photo-stabilities and the strong interaction with the neighborhood make these molecules with ESIPT suitable for many NLO applications. In this work, we present the synthesis and characterization of a new compound with such properties and with a large first hyperpolarizability. It was derived from a molecule with similar characteristics of the benzoxazole family studied in a previous work [37]. UV–Vis and fluorescence emission were applied in order to characterize the photophysical behavior of the dye. HyperRayleigh scattering approach was used in order to measure the first-order hyperpolarizability coefficient b [38]. The experimental results are compared with the theoretical calculations of the dipole moments and the hyperpolarizabilities.
2. Experimental 2.1. Materials Reagent grade 4-nitro-1,2-phenylenediamine, 4-aminosalicylic acid and p-nitroaniline (pNA) (Aldrich) were used as received. Polyphosphoric acid (PPA) was purchased from ACROS. Silicagel 60 (Merck) was used for chromatographic column separations. All the solvents were used as received or purified using standard procedures [39]. For all UV–Vis and fluorescence measurements spectroscopic grade solvents were used.
The 2-(4 0 -amino-2 0 -hydroxyphenyl)-6-nitrobenzimidazole (3) was prepared using the methodology presented in Fig. 2. A mixture of 4.0 g (26.12 mmol) of 4-aminosalicylic acid (1) and 4.0 g (26.12 mmol) 4-nitro-1,2-phenylenediamine (2) in 40 ml of polyphosphoric acid was heated at 180 C for 5 h under N2 atmosphere. The reaction was accompanied by TLC using dichloromethane/acetone (4:1) as eluent. The reaction mixture was poured into ice, and the precipitated was filtered, neutralized with NaHCO3 (10%) and dried at room temperature. The purification was made with a flash column chromatography using acetone as eluent. The final yield was of 11% due the steps needed to achieve the optical purity grade required for photophysical characterization. The role synthesis description and spectroscopic characterization will be published elsewhere. 2.3. Methods and instruments UV–Vis absorption spectra were taken on a Shimadzu UV-1601PC spectrophotometer. Fluorescence spectra were measured with a Hitachi spectrophotometer model F-4500. All experiments were performed at room temperature in a concentration range of 106 M. The HRS setup was described elsewhere [37]. The laser source was the fundamental at 1064 nm of a 20-Hz Q-switched Nd:YAG with 10 ns pulse duration. A more suitable photomultiplier (Thorn EMI 9781R) and a thoroughly energy variation scheme were added in this work. This last scheme is a combination of an adjustable home-made half-wave plate made of three birefringent sheets as develop by Darsht et al. [40], with a thin film polarizer (CVI TFP-1064-PW-1025-C), as presented in Fig. 3. This method provided a continuous variation within an energy interval about two orders of magnitude. The signal detected after narrow bandpass and color filters were integrated over 1200 shots and background corrected. The solutions of LEN and pNA were prepared in acetone and methanol, respectively, using Millipore filter membranes. It was used Durapore (0.22 lm) to methanol and Fluoropore (0.5 lm) to acetone. The solutions were filtered into dust-free cells, in order to avoid unwanted light scattering and plasma formation. The cell windows were made of quartz. Optimization of the geometry calculation was carried out using the Austin Model 1 (AM1) [41] semi-empirical method as implemented in MOPAC93 program [42]. Calculations of hyperpolarizabilities were carried out with Time-Dependent Hartree–Fock (TDHF) approach [43], implemented in MOPAC93. The dipole moment calculations in the S0 and S1 states (see Fig. 1) were done with the ab initio method in GAMESS program [44] ver-
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Fig. 2. Synthesis of the 2-(4 0 -amino-2 0 -hydroxyphenyl)-6-nitrobenzimidazole (LEN).
Fig. 3. Experimental setup for the HRS experiment using a new attenuation scheme based on half-wave plates [46]. The light collection system was made of a short focus length lens (L2) and a filter set (F2) (see text for details). The long-pass filter F1 was introduced to eliminate the secondharmonic from our Nd:YAG laser. The remaining laser at 1064 nm was focused with the lens L1 into the sample (f = 100 mm).
sion of the 20 June 2002 (R2). The ab initio calculations were done using a 6-31G(d,p) base with the Restricted Hartree–Fock (RHF) method in S0 state and with the Restricted Open-Shell Hartree–Fock (ROHF) method in S1 state.
this electron acceptor group [45]. All molar extinction coefficients values are in order of 104 l mol1 cm1, as expected to p–p* transitions. A dominant fluorescence emission band with a maximum fluorescence emission ðkem max Þ located between 439 and 454 nm could be observed. In emission, the corresponding energy range of
3. Results and discussion 3.1. Photophysical behavior The UV–Vis absorption and emission spectra were performed in solvents with different polarity in order to characterize the photophysical behavior of this new benzimidazole. The UV–Vis absorption and fluorescence emission spectra are presented in Figs. 4 and 5. Absorption and emission maxima wavelengths as well as the Stokes are listed in Table 1. As can be seen in Fig. 4, the wavelength of the low energy (S0–S1) absorption maximum was located between 373 and 378 nm. A subtle solvent influence was noticed on the displacement of the maximum location within these 355 cm1. The observed UV–Vis bands occur spectrally in agreement with the expected structure since the addition of a nitro group could introduce some red shift on the bands relatively to the molecule without
Fig. 4. Normalized UV–Vis absorbance spectra of LEN.
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using the ground and the excited states as different logical positions of the switch, this new compound LEN could be used due to the variation of dipole moments. Depending on the molecule and its deactivation (via proton and back-transfer rate and enol fluorescence), it can be possible to reach switch rates in the terahertz range. This all optical switch controlled by an external excitation still has to be demonstrated. 3.2. Hyper-Rayleigh scattering
Fig. 5. Normalized fluorescence emission spectra of LEN.
solvent dependent Stokes shift spans a broader region of 750 cm1. The slight change of maxima absorption location and a more pronounced shift on fluorescence emission, relative to the solvent polarity permutation, indicates a change of molecular polarity between ground and first excited states [37]. A significant blue shift in the fluorescence emission maximum is noticed increasing the solvent polarity, as already reported to molecules which decays trough the ESIPT mechanism [46,47]. In polar and protic solvents, an angular conformer can be stabilized through dipole– dipole interaction with the solvent. This conformer is originated from the intramolecular hydrogen bond rupture between the hydrogen of the hydroxyl group and the nitrogen in 3-position followed by a rotation of 180 of the 2-hydroxyphenyl group under the C2–C1 bond. The intramolecular hydrogen bond allows in the benzazoles higher stabilization energy. The solvent interaction with the angular conformer induces the intramolecular hydrogen bond breaking [48]. This explains the absence of the keto tautomer emission band. A good agreement of the photophysical behavior can be found with the dipole moments calculations for the ground and excited states of the LEN using an ab initio approach, where it was found 8.5 and 10.1 D, respectively. If an optical switch is envisaged, for instance,
The HRS signal for pNA in methanol [0.20 M] and for LEN in acetone [7.70 · 103 M] is shown in Fig. 6(a) and (b). The incident pulse energy in the sample was kept low (less than 5 mJ) to avoid undesired nonlinear effects in the liquid, such as self-focusing and plasma formation, but enough to measure the HRS signal. For both solutions, it was checked carefully if any contribution of 3-photon fluorescence was present, since both present absorption bands in wavelength regions below 532 nm. Contributions of multi-photon fluorescence were not observed for the used energy range. In this way, no ESIPT is taking place during this HRS experiment. In order to calculate the first hyperpolarizabilities (b) from the experimental values, it was measured that the contribution of the solvent was smaller when compared to the solute, i.e., all measurable scattered signal at 2x from solution comes from the solute. The evaluation of b for LEN was referenced to the b value of pNA as a standard, after corrected for the solution reabsorption: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi N pNA S LEN bLEN ¼ bpNA ; ð1Þ N LEN S pNA where NpNA and NLEN are the concentrations of the pNA and LEN, and S stands for the HRS signal measured under the same conditions for both molecules. Actually, the ratio (SLEN/SpNA) simplifies to the ratio between the coefficients of the second-order polynomial function adjusted to the corresponding data points of Fig. 6. The ratio between these coefficients (bLEN/bpNA) and the concentrations gives the proportionality between the b values of the reference and the sample. The calculated experimental and theoretical maximum allowed first hyperpolarizabilities are shown in
Table 1 em Maximum wavelength of UV–Vis absorption ðkabs max Þ and fluorescence emission ðkmax Þ, molar extinction coefficient (e) and Stokes shift (Dk) of LEN Solvent
Dielectric constant
kabs max (nm)
e · 104 (l mol1 cm1)
kem max (nm)
Dk (nm)/(cm1)
Dioxane Ethyl acetate Dichloromethane Ethanol Acetonitrile
2.2 6.0 9.1 24.3 37.5
378 377 373 373 373
1.1 3.3 2.1 2.3 2.9
454 449 448 439 439
76/4428 72/4253 75/4488 66/4030 66/4030
The dielectric constant of the organic solvents is also presented.
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is approximately twice the number of double or triple bonds in the molecule, and the absorption maximum of the S0–S1. The first hyperpolarizability is correlated with the number of participating electrons and the wavelength of the maximum absorption. It is just a parameter indicating how large should be the polarization if the electrons of the double and triple-bonds could participate in the best way. It was also supposed the presence of just one main excited state, making a two-level system with the ground state, what is not so far from the situation here studied. The bmax is calculated according to the following equation: p ffiffiffi n2 þ 2 3 eh 3 N 3=2 4 p ffiffiffi ffi ¼ 3 ; ð2Þ b2L max 3=2 3 m E10
Fig. 6. HRS signal of [pNA]methanol = 0.20 M (a) and [LEN]acetone = 7.70 · 103 M (b). The continuous lines are fits with a second-order polynomial (y = y0 + bx2).
Table 2. The b of the pNA was corrected to acetone with the purpose of comparison. In Table 2, the bmax were calculated according to Kuzyk [49], introducing a rich intuitive picture based on the molecular structure. The main parameters of this calculation are the number of participating electrons N, what
where e is the electron charge, h is the Planck constant and m stand for the electron mass. The energy of the first excited state E10 is obtained from the absorption maximum for each molecule. The term in the first parenthesis is just the coefficient of the Lorentz local field model. The ratio between the maximum allowed b and the experimental value gives an indication of the molecule structure optimization relative to the number of double bonds. In our previous work [37], the modification of the basic structure of the HBO with the amino and the nitro groups, increased the number of electrons participating but did not change the main bottleneck to the first hyperpolarizability, i.e., the basic HBO structure. The ratio bmax/bexp is quite similar for the HBO and the BO6, as can be seen in Table 2. The modification achieved on the BO6 structure into LEN, not modifying the donor/acceptor groups as calculated for other HBO derivatives, overcome this bottleneck. It was intended, with the substitution of the oxygen by the nitrogen, a much effective distortion of the electronic cloud to the benzoxazolic ring, i.e., the acceptor side. Indeed, this modification allowed a remarkable increase in the bmax/bexp ratio, what cannot be completely explained in terms of changes in the number of p electrons (9) or of the absorption maximum in acetone, since both remained practically the same for the LEN and the BO6. Since the benzimidazole ring is usually more basic than
Table 2 Comparison between experimental and semi-empirical calculated first-hyperpolarizabilities at 1064 nm Compound
Maximum allowed b in acetone
Experimental b in acetone 30
(10 pNAa LEN BO6a HBOa
1128 6318 7036 2337
esu)
24.5 1197.3 ± 1.2 213.4 ± 25.7 77.7 ± 9.3
bmax/bexp
Theoretical b in gas
46 5.3 33 30.1
10.3 72.6 93.96 5.84
(·bpNA) 1 48.9 ± 0.5 8.7 ± 1.1 3.2 ± 1.1
The column with maximum allowed b was calculated taking into account the conjugated length and a two-level model. Maximum allowed and theorethical b are presented in (1030 esu). a Ref. [44].
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benzoxazole [50], charge transfer interactions contributions are higher in structures which contain the benzimidazole ring, as LEN, showing that even using the same donor/acceptor groups in the molecule structure, different values could be expected in the same solvent. It is important to notice that the b of the LEN was optimized as much in relation to the BO6 as to its maximum allowed theoretical value. These results are a message to those trying to optimize structures, which the well evaluated choice of a correct starting compound is of fundamental importance. Based on the calculated and measured values for BO6 and HBO, an explanation for the remarkable increase in b of the LEN could also be addressed to the interaction between the NLO molecule and the solvent. It is well known that different types of specific and non-specific interactions can take place between the NLO molecule and the solvent, such as H-bonds and dipole–dipole, respectively [37]. In the later case, the first hyperpolarizability has been observed to depend on the dipole moment and molar volume of solvent [50]. In a previous work [37], we showed that the difference between the theoretical and experimental b for the BO6 and HBO could be explained in terms of those non-specific dipole–dipole interactions. Since non-specific interactions depend on the magnitude of the solvent dipole moment and the mean distance between the NLO molecule and the solvent are still the same for LEN, the same explanation used for BO6 does not find arguments. The solvent used in both cases was the same, acetone. BO6 and LEN have similar structures, differing only by the heteroatom X (O or NH for BO6 or LEN, respectively) which does not change significantly the molar volume (van der Wa˚ for N and 1.4 A ˚ for O). Besides, the als radius of 1.5 A calculated dipole moment of both molecules in the ground state is similar, what could not explain such measured difference. This other cause must originate from some site-specific interaction with the new introduced atom. This particular behaviour of the LEN in solution can also be seen in the solubility. Whereas the BO6 and HBO derivatives can be dissolved without further difficulties, the LEN is an exception among many benzimidazole compounds, which are hardly soluble. Semi-empirical and ab initio calculations of the first hyperpolarizability were also performed for the LEN (Table 2). Since the semi-empirical and the used ab initio calculations do not simulate any kind of molecule–solvent interaction, the large experimental b obtained for the LEN could not be reproduced at all. It is well accepted that the dipole moment correlates fairly well with the first hyperpolarizability for similar molecular structures. Observing the calculated dipole moments of the LEN and the BO6, 8.5 D and 7.3 D [37], respectively, one would expect that the LEN would have a larger calculated b than BO6. However, the semi-empirical calculations do not reproduce it, as can be seen in Table 2. It
is well recognized that semi-empirical calculations have severe restraints because these methods use a simpler Hamiltonian than the correct molecular Hamiltonian and use parameters whose values are adjusted to fit experimental data or the results of ab initio calculations. Probably in this case, the issue may reside in an annular tautomerism [51,52], which involves the movement of a proton between two annular nitrogen atoms. For unsubstituted imidazoles, the two tautomers are identical, but this does not apply to substituted derivatives [53], as LEN. This prototropism is not taken in account in the calculations and was not possible before in the BO6 structure. This type of prototropism would modify the conjugation length of the LEN, since now the H of the NH group would be delocalized between the nitrogens of the benzimidazolic ring, enabling simultaneously more than one conjugation path. We identify it as a manifestation of a resonance-assisted hydrogen bond (RAHB), which leads to a greater electronic delocalization in the benzimidazolic ring [54,55]. In the maximum allowed b model of Kuzyk, this stronger electron mobility would generate a bigger hyperpolarizabilty for the LEN. Since the topology of the LEN prototropism potential is not known yet, the electronic delocalization degree can not be evaluated. It seems clear that ab initio calculations involving annular tautomerism are necessary.
4. Conclusions The synthesis and characterization of a new benzimidazole (LEN) derivative with promising applications in NLO were presented. When compared with a previously described benzoxazole derivative (BO6), this new benzimidazole presents an increase of the b value by a factor of 5.6. Since the donor/acceptor groups and its localization were the same in LEN-BO6, the LEN main structure (X = NH) played a fundamental role in the molecule hyperpolarizability. The increase of the first-hyperpolarizabilty is originated by the prototropism between the nitrogen atoms, just present in the LEN. It leads to a stronger electronic delocalization in the benzimidazolic ring and enables two p-conjugation paths in the molecule. It can be a new general molecular mechanism to synthesize molecules with large b. Molecules with similar structures will be investigated in the future.
Acknowledgements We are grateful for financial support and scholarships from Conselho Nacional para o Desenvolvimento Cientı´fico e Tecnolo´gico (CNPq), Fundac¸a˜o de Amparo a` Pesquisa do Estado do Rio Grande do Sul (FAPERGS), and
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Coordenac¸a˜o de Aperfeic¸oamento de Pessoal de Nı´vel Superior (CAPES).
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First hyperpolarizability in a new benzimidazole derivative Fabiano Severo Rodembusch a, Tiago Buckup b, Maximiliano Segala c, Luciana Tavares b, Ricardo Rego Bordalo Correia b, Valter Stefani a,* a
Instituto de Quı´mica, Universidade Federal do Rio Grande do Sul, Laborato´rio de Novos Materiais Orgaˆnicos, Av. Bento Gonc¸alves, 9500, CP 15003, CEP 91501-970 Porto Alegre-RS, Brazil b ´ ptica & Laser, Av. Bento Gonc¸alves, 9500, CP 15051, Instituto de Fı´sica, Universidade Federal do Rio Grande do Sul, Laborato´rio de O CEP 91501-970 Porto Alegre-RS, Brazil c Instituto de Quı´mica, Universidade Estadual de Campinas, Cidade Universita´ria Zeferino Vaz s/n, CP 6154, CEP 13083-970 Campinas-SP, Brazil Received 15 April 2004; accepted 21 June 2004 Available online 20 July 2004
Abstract A new benzimidazole, 2-(4 0 -amino-2 0 -hydroxyphenyl)-6-nitrobenzimidazole (LEN), with promising applications in nonlinear optics were synthesized, purified and characterized by classical techniques. The UV–Vis and steady-state fluorescence of LEN in solution were applied in order to characterize the photophysical behaviour of this new fluorescent dye. The Hyper-Rayleigh Scattering study indicates a remarkable increase in the b absolute value (1197.3 ± 1.2 · 1030 esu) of the LEN in acetone at 1064 nm, when comparing with a previously described benzoxazole (BO6). The ratio between the experimental b and the maximum theoretical value using a two-level model was also improved by a factor 6. Ab initio and semi-empirical calculations of the dipole moments and the first hyperpolarizability are also presented. The enhancement in the experimental hyperpolarizability value in relation to BO6 is explained in terms of the benzimidazolic ring basicity and a prototropic effect (annular tautomerism) present in the LEN, identified as a resonance-assisted hydrogen bond, which leads to a stronger electronic delocalization. 2004 Elsevier B.V. All rights reserved.
1. Introduction Organic molecules are useful materials for nonlinear optical (NLO) applications in photonics, bio-sensors and two-photon-induced fluorescence probes [1–4]. The NLO properties of these molecules are often enhanced or modified using different types of radicals as acceptor and donor groups, in analogy to the wellknown model v(2) compounds for NLO [5]. Furthermore, in the last years, several studies about the incorporation of these molecules into polymeric chains * Corresponding author. Tel.: +55-51-3316-6285; fax: +55-51-33167304. E-mail address: [email protected] (V. Stefani). URL: http://www.iq.ufrgs.br/lnmo.
0301-0104/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2004.06.046
have been made [6–15]. In this field, studies regarding the NLO properties of some benzazole derivatives have been done due the great thermal and photophysical stability of these dyes [16–18]. This stability characteristic is mainly caused by an intramolecular proton transfer mechanism in the excited state (ESIPT), a photoprototropic effect [19–23]. In the ESIPT, the enol–cis conformer (E) in non-polar and aprotic solvents, with an intramolecular hydrogen bond between the phenolic hydrogen and the nitrogen in the benzazolic ring, is the predominant specie in the ground state (Fig. 1) [24]. The UV light absorption through the enol–cis (E) produce an excited enol–cis (E*) which is quickly [25,26] converted to an excited keto tautomer (K*) by an intramolecular proton transfer [27]. The excited keto tautomer (K*) decays emitting fluorescence to a keto
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2.2. Synthesis of 2-(4 0 -amino-2 0 -hydroxyphenyl)-6-nitrobenzimidazole
Fig. 1. Chemical structure of the tautomeric forms enol–cis (E) and keto (K) involved in the ESIPT mechanism (X = O, S and NH).
tautomer in the ground state (K). Since the enol–cis tautomer (E) is more stable than the keto tautomer in the ground state, the initial enol–cis form is regenerated without any photochemical change [28,29]. The characteristics of these ESIPT compounds [30–36], as the fast proton-transfer cycle, thermal and photo-stabilities and the strong interaction with the neighborhood make these molecules with ESIPT suitable for many NLO applications. In this work, we present the synthesis and characterization of a new compound with such properties and with a large first hyperpolarizability. It was derived from a molecule with similar characteristics of the benzoxazole family studied in a previous work [37]. UV–Vis and fluorescence emission were applied in order to characterize the photophysical behavior of the dye. HyperRayleigh scattering approach was used in order to measure the first-order hyperpolarizability coefficient b [38]. The experimental results are compared with the theoretical calculations of the dipole moments and the hyperpolarizabilities.
2. Experimental 2.1. Materials Reagent grade 4-nitro-1,2-phenylenediamine, 4-aminosalicylic acid and p-nitroaniline (pNA) (Aldrich) were used as received. Polyphosphoric acid (PPA) was purchased from ACROS. Silicagel 60 (Merck) was used for chromatographic column separations. All the solvents were used as received or purified using standard procedures [39]. For all UV–Vis and fluorescence measurements spectroscopic grade solvents were used.
The 2-(4 0 -amino-2 0 -hydroxyphenyl)-6-nitrobenzimidazole (3) was prepared using the methodology presented in Fig. 2. A mixture of 4.0 g (26.12 mmol) of 4-aminosalicylic acid (1) and 4.0 g (26.12 mmol) 4-nitro-1,2-phenylenediamine (2) in 40 ml of polyphosphoric acid was heated at 180 C for 5 h under N2 atmosphere. The reaction was accompanied by TLC using dichloromethane/acetone (4:1) as eluent. The reaction mixture was poured into ice, and the precipitated was filtered, neutralized with NaHCO3 (10%) and dried at room temperature. The purification was made with a flash column chromatography using acetone as eluent. The final yield was of 11% due the steps needed to achieve the optical purity grade required for photophysical characterization. The role synthesis description and spectroscopic characterization will be published elsewhere. 2.3. Methods and instruments UV–Vis absorption spectra were taken on a Shimadzu UV-1601PC spectrophotometer. Fluorescence spectra were measured with a Hitachi spectrophotometer model F-4500. All experiments were performed at room temperature in a concentration range of 106 M. The HRS setup was described elsewhere [37]. The laser source was the fundamental at 1064 nm of a 20-Hz Q-switched Nd:YAG with 10 ns pulse duration. A more suitable photomultiplier (Thorn EMI 9781R) and a thoroughly energy variation scheme were added in this work. This last scheme is a combination of an adjustable home-made half-wave plate made of three birefringent sheets as develop by Darsht et al. [40], with a thin film polarizer (CVI TFP-1064-PW-1025-C), as presented in Fig. 3. This method provided a continuous variation within an energy interval about two orders of magnitude. The signal detected after narrow bandpass and color filters were integrated over 1200 shots and background corrected. The solutions of LEN and pNA were prepared in acetone and methanol, respectively, using Millipore filter membranes. It was used Durapore (0.22 lm) to methanol and Fluoropore (0.5 lm) to acetone. The solutions were filtered into dust-free cells, in order to avoid unwanted light scattering and plasma formation. The cell windows were made of quartz. Optimization of the geometry calculation was carried out using the Austin Model 1 (AM1) [41] semi-empirical method as implemented in MOPAC93 program [42]. Calculations of hyperpolarizabilities were carried out with Time-Dependent Hartree–Fock (TDHF) approach [43], implemented in MOPAC93. The dipole moment calculations in the S0 and S1 states (see Fig. 1) were done with the ab initio method in GAMESS program [44] ver-
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Fig. 2. Synthesis of the 2-(4 0 -amino-2 0 -hydroxyphenyl)-6-nitrobenzimidazole (LEN).
Fig. 3. Experimental setup for the HRS experiment using a new attenuation scheme based on half-wave plates [46]. The light collection system was made of a short focus length lens (L2) and a filter set (F2) (see text for details). The long-pass filter F1 was introduced to eliminate the secondharmonic from our Nd:YAG laser. The remaining laser at 1064 nm was focused with the lens L1 into the sample (f = 100 mm).
sion of the 20 June 2002 (R2). The ab initio calculations were done using a 6-31G(d,p) base with the Restricted Hartree–Fock (RHF) method in S0 state and with the Restricted Open-Shell Hartree–Fock (ROHF) method in S1 state.
this electron acceptor group [45]. All molar extinction coefficients values are in order of 104 l mol1 cm1, as expected to p–p* transitions. A dominant fluorescence emission band with a maximum fluorescence emission ðkem max Þ located between 439 and 454 nm could be observed. In emission, the corresponding energy range of
3. Results and discussion 3.1. Photophysical behavior The UV–Vis absorption and emission spectra were performed in solvents with different polarity in order to characterize the photophysical behavior of this new benzimidazole. The UV–Vis absorption and fluorescence emission spectra are presented in Figs. 4 and 5. Absorption and emission maxima wavelengths as well as the Stokes are listed in Table 1. As can be seen in Fig. 4, the wavelength of the low energy (S0–S1) absorption maximum was located between 373 and 378 nm. A subtle solvent influence was noticed on the displacement of the maximum location within these 355 cm1. The observed UV–Vis bands occur spectrally in agreement with the expected structure since the addition of a nitro group could introduce some red shift on the bands relatively to the molecule without
Fig. 4. Normalized UV–Vis absorbance spectra of LEN.
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using the ground and the excited states as different logical positions of the switch, this new compound LEN could be used due to the variation of dipole moments. Depending on the molecule and its deactivation (via proton and back-transfer rate and enol fluorescence), it can be possible to reach switch rates in the terahertz range. This all optical switch controlled by an external excitation still has to be demonstrated. 3.2. Hyper-Rayleigh scattering
Fig. 5. Normalized fluorescence emission spectra of LEN.
solvent dependent Stokes shift spans a broader region of 750 cm1. The slight change of maxima absorption location and a more pronounced shift on fluorescence emission, relative to the solvent polarity permutation, indicates a change of molecular polarity between ground and first excited states [37]. A significant blue shift in the fluorescence emission maximum is noticed increasing the solvent polarity, as already reported to molecules which decays trough the ESIPT mechanism [46,47]. In polar and protic solvents, an angular conformer can be stabilized through dipole– dipole interaction with the solvent. This conformer is originated from the intramolecular hydrogen bond rupture between the hydrogen of the hydroxyl group and the nitrogen in 3-position followed by a rotation of 180 of the 2-hydroxyphenyl group under the C2–C1 bond. The intramolecular hydrogen bond allows in the benzazoles higher stabilization energy. The solvent interaction with the angular conformer induces the intramolecular hydrogen bond breaking [48]. This explains the absence of the keto tautomer emission band. A good agreement of the photophysical behavior can be found with the dipole moments calculations for the ground and excited states of the LEN using an ab initio approach, where it was found 8.5 and 10.1 D, respectively. If an optical switch is envisaged, for instance,
The HRS signal for pNA in methanol [0.20 M] and for LEN in acetone [7.70 · 103 M] is shown in Fig. 6(a) and (b). The incident pulse energy in the sample was kept low (less than 5 mJ) to avoid undesired nonlinear effects in the liquid, such as self-focusing and plasma formation, but enough to measure the HRS signal. For both solutions, it was checked carefully if any contribution of 3-photon fluorescence was present, since both present absorption bands in wavelength regions below 532 nm. Contributions of multi-photon fluorescence were not observed for the used energy range. In this way, no ESIPT is taking place during this HRS experiment. In order to calculate the first hyperpolarizabilities (b) from the experimental values, it was measured that the contribution of the solvent was smaller when compared to the solute, i.e., all measurable scattered signal at 2x from solution comes from the solute. The evaluation of b for LEN was referenced to the b value of pNA as a standard, after corrected for the solution reabsorption: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi N pNA S LEN bLEN ¼ bpNA ; ð1Þ N LEN S pNA where NpNA and NLEN are the concentrations of the pNA and LEN, and S stands for the HRS signal measured under the same conditions for both molecules. Actually, the ratio (SLEN/SpNA) simplifies to the ratio between the coefficients of the second-order polynomial function adjusted to the corresponding data points of Fig. 6. The ratio between these coefficients (bLEN/bpNA) and the concentrations gives the proportionality between the b values of the reference and the sample. The calculated experimental and theoretical maximum allowed first hyperpolarizabilities are shown in
Table 1 em Maximum wavelength of UV–Vis absorption ðkabs max Þ and fluorescence emission ðkmax Þ, molar extinction coefficient (e) and Stokes shift (Dk) of LEN Solvent
Dielectric constant
kabs max (nm)
e · 104 (l mol1 cm1)
kem max (nm)
Dk (nm)/(cm1)
Dioxane Ethyl acetate Dichloromethane Ethanol Acetonitrile
2.2 6.0 9.1 24.3 37.5
378 377 373 373 373
1.1 3.3 2.1 2.3 2.9
454 449 448 439 439
76/4428 72/4253 75/4488 66/4030 66/4030
The dielectric constant of the organic solvents is also presented.
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is approximately twice the number of double or triple bonds in the molecule, and the absorption maximum of the S0–S1. The first hyperpolarizability is correlated with the number of participating electrons and the wavelength of the maximum absorption. It is just a parameter indicating how large should be the polarization if the electrons of the double and triple-bonds could participate in the best way. It was also supposed the presence of just one main excited state, making a two-level system with the ground state, what is not so far from the situation here studied. The bmax is calculated according to the following equation: p ffiffiffi n2 þ 2 3 eh 3 N 3=2 4 p ffiffiffi ffi ¼ 3 ; ð2Þ b2L max 3=2 3 m E10
Fig. 6. HRS signal of [pNA]methanol = 0.20 M (a) and [LEN]acetone = 7.70 · 103 M (b). The continuous lines are fits with a second-order polynomial (y = y0 + bx2).
Table 2. The b of the pNA was corrected to acetone with the purpose of comparison. In Table 2, the bmax were calculated according to Kuzyk [49], introducing a rich intuitive picture based on the molecular structure. The main parameters of this calculation are the number of participating electrons N, what
where e is the electron charge, h is the Planck constant and m stand for the electron mass. The energy of the first excited state E10 is obtained from the absorption maximum for each molecule. The term in the first parenthesis is just the coefficient of the Lorentz local field model. The ratio between the maximum allowed b and the experimental value gives an indication of the molecule structure optimization relative to the number of double bonds. In our previous work [37], the modification of the basic structure of the HBO with the amino and the nitro groups, increased the number of electrons participating but did not change the main bottleneck to the first hyperpolarizability, i.e., the basic HBO structure. The ratio bmax/bexp is quite similar for the HBO and the BO6, as can be seen in Table 2. The modification achieved on the BO6 structure into LEN, not modifying the donor/acceptor groups as calculated for other HBO derivatives, overcome this bottleneck. It was intended, with the substitution of the oxygen by the nitrogen, a much effective distortion of the electronic cloud to the benzoxazolic ring, i.e., the acceptor side. Indeed, this modification allowed a remarkable increase in the bmax/bexp ratio, what cannot be completely explained in terms of changes in the number of p electrons (9) or of the absorption maximum in acetone, since both remained practically the same for the LEN and the BO6. Since the benzimidazole ring is usually more basic than
Table 2 Comparison between experimental and semi-empirical calculated first-hyperpolarizabilities at 1064 nm Compound
Maximum allowed b in acetone
Experimental b in acetone 30
(10 pNAa LEN BO6a HBOa
1128 6318 7036 2337
esu)
24.5 1197.3 ± 1.2 213.4 ± 25.7 77.7 ± 9.3
bmax/bexp
Theoretical b in gas
46 5.3 33 30.1
10.3 72.6 93.96 5.84
(·bpNA) 1 48.9 ± 0.5 8.7 ± 1.1 3.2 ± 1.1
The column with maximum allowed b was calculated taking into account the conjugated length and a two-level model. Maximum allowed and theorethical b are presented in (1030 esu). a Ref. [44].
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benzoxazole [50], charge transfer interactions contributions are higher in structures which contain the benzimidazole ring, as LEN, showing that even using the same donor/acceptor groups in the molecule structure, different values could be expected in the same solvent. It is important to notice that the b of the LEN was optimized as much in relation to the BO6 as to its maximum allowed theoretical value. These results are a message to those trying to optimize structures, which the well evaluated choice of a correct starting compound is of fundamental importance. Based on the calculated and measured values for BO6 and HBO, an explanation for the remarkable increase in b of the LEN could also be addressed to the interaction between the NLO molecule and the solvent. It is well known that different types of specific and non-specific interactions can take place between the NLO molecule and the solvent, such as H-bonds and dipole–dipole, respectively [37]. In the later case, the first hyperpolarizability has been observed to depend on the dipole moment and molar volume of solvent [50]. In a previous work [37], we showed that the difference between the theoretical and experimental b for the BO6 and HBO could be explained in terms of those non-specific dipole–dipole interactions. Since non-specific interactions depend on the magnitude of the solvent dipole moment and the mean distance between the NLO molecule and the solvent are still the same for LEN, the same explanation used for BO6 does not find arguments. The solvent used in both cases was the same, acetone. BO6 and LEN have similar structures, differing only by the heteroatom X (O or NH for BO6 or LEN, respectively) which does not change significantly the molar volume (van der Wa˚ for N and 1.4 A ˚ for O). Besides, the als radius of 1.5 A calculated dipole moment of both molecules in the ground state is similar, what could not explain such measured difference. This other cause must originate from some site-specific interaction with the new introduced atom. This particular behaviour of the LEN in solution can also be seen in the solubility. Whereas the BO6 and HBO derivatives can be dissolved without further difficulties, the LEN is an exception among many benzimidazole compounds, which are hardly soluble. Semi-empirical and ab initio calculations of the first hyperpolarizability were also performed for the LEN (Table 2). Since the semi-empirical and the used ab initio calculations do not simulate any kind of molecule–solvent interaction, the large experimental b obtained for the LEN could not be reproduced at all. It is well accepted that the dipole moment correlates fairly well with the first hyperpolarizability for similar molecular structures. Observing the calculated dipole moments of the LEN and the BO6, 8.5 D and 7.3 D [37], respectively, one would expect that the LEN would have a larger calculated b than BO6. However, the semi-empirical calculations do not reproduce it, as can be seen in Table 2. It
is well recognized that semi-empirical calculations have severe restraints because these methods use a simpler Hamiltonian than the correct molecular Hamiltonian and use parameters whose values are adjusted to fit experimental data or the results of ab initio calculations. Probably in this case, the issue may reside in an annular tautomerism [51,52], which involves the movement of a proton between two annular nitrogen atoms. For unsubstituted imidazoles, the two tautomers are identical, but this does not apply to substituted derivatives [53], as LEN. This prototropism is not taken in account in the calculations and was not possible before in the BO6 structure. This type of prototropism would modify the conjugation length of the LEN, since now the H of the NH group would be delocalized between the nitrogens of the benzimidazolic ring, enabling simultaneously more than one conjugation path. We identify it as a manifestation of a resonance-assisted hydrogen bond (RAHB), which leads to a greater electronic delocalization in the benzimidazolic ring [54,55]. In the maximum allowed b model of Kuzyk, this stronger electron mobility would generate a bigger hyperpolarizabilty for the LEN. Since the topology of the LEN prototropism potential is not known yet, the electronic delocalization degree can not be evaluated. It seems clear that ab initio calculations involving annular tautomerism are necessary.
4. Conclusions The synthesis and characterization of a new benzimidazole (LEN) derivative with promising applications in NLO were presented. When compared with a previously described benzoxazole derivative (BO6), this new benzimidazole presents an increase of the b value by a factor of 5.6. Since the donor/acceptor groups and its localization were the same in LEN-BO6, the LEN main structure (X = NH) played a fundamental role in the molecule hyperpolarizability. The increase of the first-hyperpolarizabilty is originated by the prototropism between the nitrogen atoms, just present in the LEN. It leads to a stronger electronic delocalization in the benzimidazolic ring and enables two p-conjugation paths in the molecule. It can be a new general molecular mechanism to synthesize molecules with large b. Molecules with similar structures will be investigated in the future.
Acknowledgements We are grateful for financial support and scholarships from Conselho Nacional para o Desenvolvimento Cientı´fico e Tecnolo´gico (CNPq), Fundac¸a˜o de Amparo a` Pesquisa do Estado do Rio Grande do Sul (FAPERGS), and
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Coordenac¸a˜o de Aperfeic¸oamento de Pessoal de Nı´vel Superior (CAPES).
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