Solvatochromic Fluorescence of Piperazine-Modified Bipyridazines for

Jul 14, 2009 - ... a quadrupolar charge distribution in SPBP cannot induce a uniform reaction field at ..... Such a local fluorescence change offers a...
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Solvatochromic Fluorescence of Piperazine-Modified Bipyridazines for an Organic Solvent-Sensitive Film Jaekwon Do, June Huh, and Eunkyoung Kim* Department of Chemical and Biomolecular Engineering, Yonsei University, 262 Seongsanno, Seodaemun-gu, Seoul 120-749, Korea Received January 14, 2009. Revised Manuscript Received June 22, 2009 Bipyridazines were modified with heterocyclic amines such as piperazine to give symmetric quadrupolar (SPBP) and asymmetric dipolar (APBP) bipyridazine. The fluorescence of SPBP and APBP was highly sensitive to solvent polarity, giving a synthetic rainbow of emission in different organic solvents. The solvent-induced changes in the Stokes shift of the bipyridazines resulted in positive solvatochromism. The symmetric bipyridazine showed higher solvatochromic sensitivity than that of the asymmetric bipyridazine and diazines. The positive solvatochromic fluorescence properties were reproduced in a binary system of toluene/dimethyl sulfoxide (DMSO) mixture, which showed a synthetic rainbow of emission by varying the DMSO content in toluene. An organic sensitive poly(methyl methacrylate) film containing SPBP exhibited a visible sensitivity for the detection of solvents by their polarity upon exposure to organic solvent molecules.

Introduction Sensing molecular level and macroscopic environmental changes have become increasingly important in various fields,1-3 including biotechnology and information technology. Optical sensing is the most actively researched technique because of its high sensitivity toward small environmental changes and because it requires relatively simple instrumentation.2,4,5 A wide variety of organic solvent-sensitive probes displaying either negative or positive solvatochromism have been reported.6-9 As solvatochromism7-9 is based on the interaction between the solvent molecules and a fluorophore, solvent-dependent changes in the spectra of these solvatochromic dyes8-10 provide a sensitive response to *To whom correspondence should be addressed. E-mail: eunkim@yonsei. ac.kr. (1) (a) Wolfbeis, O. S., Ed. Fiber Optic Chemical Sensors and Biosensors; CRC Press: Boca Raton, FL, 1991. (b) Borisov, S. M.; Wolfbeis, O. S. Chem. Rev. 2008, 108, 423–461. (c) Grate, J. W. Chem. Rev. 2008, 108, 726–745. (d) Heller, A.; Feldman, B. Chem. Rev. 2008, 108, 2482–2505. (e) Janata, J. Principles of Chemical Sensors; Plenum Press: New York, 1989.(f ) Janata, J.; Josowicz, M.; Vansek, P.; DeVaney, D. M. Anal. Chem. 1998, 70, 179–208. (2) Wolfbeis, O. S. Anal. Chem. 2006, 78, 3859–3874. (3) (a) Franke, M. E.; Koplin, T. J.; Simon, U. Small 2006, 2, 36–50. (b) Potyrailo, R. A. Angew. Chem., Int. Ed. 2006, 45, 702–723. (4) (a) Orellana, G.; Moreno-Bondi, M. C. Frontiers in Chemical Sensors: Novel Principles and Techniques; Springer: New York, 2005. (b) Baldini, F.; Chester, A. N.; Homola, J.; Martellucci, S. Optical Chemical Sensors; NATO Science Series 224; Springer: New York, 2006. (5) (a) Bryan, A. J.; de Silva, A. P.; de Silva, S. A.; Rupasinghe, R. A. D. D.; Sandanayake, K. R. A. S. Biosensors 1989, 4, 169–179. (b) Daniel, S. T.; Ashley, D. C.; Faysal, I.; Javier, S.-P.; Michael, A. M. Chem. Mater. 2008, 20, 6595–6596. (c) Mets, U.; Rigler, R. J. Fluoresc. 1994, 4, 259–264. (d) Tang, B.; Cao, L.; Xu, K.; Zhuo, L.; Ge, J.; Li, Q.; Yu, L. Chem.;Eur. J. 2008, 14, 3637–3644. (e) Scrafton, D. K.; Taylor, J. E.; Mahon, M. F.; Fossey, J. S.; James, T. D. J. Org. Chem. 2008, 73, 2871–2874. (6) (a) Claisen, L. Liebigs Ann. Chem. 1896, 291, 25–137. (b) Knorr, L. Liebigs Ann. Chem. 1896, 293, 70–72. (c) Spange, S.; Keutel, D. Liebigs Ann. Chem. 1992, 1992, 423–428. (d) Kolling, O. W.; Goodnight, J. L. Anal. Chem. 1973, 45, 160–164. (e) Reichardt, C.; Harbusch-Wrnert, E.; Schafer, G. Liebigs Ann.Chem. 1988, 1988, 839– 844. (f ) Janowski, A.; Turowska-Tyrk, I.; Wrona, P. K. J. Chem. Soc., Perkin Trans. 2 1985, 2, 821–825. (g) Kanski, R.; Murray, C. J. Tetrahedron Lett. 1993, 34, 2263–2266. (7) Terenziani, F.; Painelli, A.; Katan, C.; Charlot, M.; Blanchard-Desce, M. J. Am. Chem. Soc. 2006, 128, 15742–15755. (8) Effenberger, F.; Wiirthner, F. Angew. Chem. 1993, 32, 742–744. (9) (a) Brooker, L. G. S.; Craig, A. C.; Heseltine, D. W.; Jenkins, P. W.; Lincoln, L. L. J. Am. Chem. Soc. 1965, 87, 2443–2450. (b) Shin, D.-M.; Schanze, K. S.; Whitten, D. G. J . Am. Chem. Soc. 1989, 54, 8494–8501. (10) (a) Arai, S.; Yamazaki, M.; Nagakura, K.; Ishikawa, M.; Hida, M. J. Chem. Soc., Chem. Commun. 1983, 9, 1037–1038. (b) Freed, B. K.; Biesecker, J.; Middleton, W. J. J. Fluorine Chem. 1990, 48, 63–75.

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changes in the surrounding environment.6,11 To induce the high degree of polarization required for maximum sensitivity, these solvatochromic fluorophores are often modified with polar functionality such as amino, cyano, halogen, carbonyl, nitro, or heteroaryl groups. Although the Reichardt dye has been studied most as a solvatochromic probe,12 pyridazines and bipyridazines have recently been reported as a new family of highly polarizable pushpull molecules. In particular, structures containing a bipyridazine and diazine core have been studied as fluorescent probes13,14 because their asymmetry tends to induce large dipoles. Bipyridazines contain four nitrogens, resulting in donor/acceptor (D/A) pairing or a push-pull structure, when substituted with electronrich D unit, which is potentially solvatochromic, solvatofluorescent, and pH sensitive. Such a solvatochromic fluorophore is particularly useful not only as a molecular sensor in solution but also as a fluorescent dopant in a film sensor and a smart coating material detecting analytes on the coated surface. Here, we report highly polarizable bipyridazines modified with an electron-rich π-conjugated piperazine, namely, symmetric bipyridazine (SPBP) and asymmetric bipyridazine (APBP) (Scheme 1), and characterize their solvatochromic properties. In addition, we fabricated a solvent-sensor film made of SPBP dispersed in a polymer thin film and demonstrated its ability as a solvent-sensitive surface.

Results and Discussion Synthesis and Optical Properties of the Piperazinyl Bipyridazines. The synthesis of bipyridazines modified with a heterocyclic amine was carried out under acidic conditions via a two-step Knoevenagel condensation. The symmetric 6,60 -bis[4(4-methylpiperazin-1-yl)styryl]-3,30 -bipyridazine (SPBP), as a (11) (a) Kaim, W.; Olbrich-Deussner, B.; Roth, T. Organometallics 1991, 10, 410–415. (b) Chen, C. T.; Liao, S. Y.; Lin, K. J.; Lai, L. L. Adv. Mater. 1998, 10, 334– 338. (12) (a) Spange, S.; Reuter, A.; Lubda, D. Langmuir 1999, 15, 2103–2111. (b) Onida, B.; Borello, L.; Fiorilli, S.; Bonelli, B.; Arean, C. O.; Garrone, E. Chem. Commun. 2004, 21, 2496–4697. (c) Fiorilli, S.; Onida, B.; Barolo, C.; Viscardi, G.; Brunel, D.; Garrone, E. Langmuir 2007, 23, 2261–2268. (13) (a) D€urr, H.; Schwarz, R.; Willner, I.; Joselevich, E.; Eichen, Y. J. Chem. Soc., Chem. Commun. 1992, 18, 1338–1339. (b) Gardner, J. S.; Strommen, D. P.; Szulbinski, W. S.; Su, H.; Kincaid, J. R. J. Phys. Chem. A 2003, 107, 351–357. (14) Bosch, P.; Peinado, C.; Martı´ n, V.; Catalina, F.; Corrales, T. J. Photochem. Photobiol. A: Chem. 2006, 180, 118–129.

Published on Web 07/14/2009

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Do et al. Scheme 1. Synthesis of Bipyridazine Derivatives

quadrupolar (D-π-A-π-D) structure, was prepared from 6,60 dimethyl-3,30 -bipyridazine (1)15 and 4-(4-methylpiperazin-1yl)benzaldehyde in the presence of p-toluenesulfonic acid as summarized in Scheme 1. Stoichiometric reaction of an equimolar ratio of 1 and 4-(4-methylpiperazin-1-yl)benzaldehyde afforded a dipolar structure (D-π-A), APBP. The resulting bipyridazines were yellowish solids that were readily soluble in common organic solvents to give transparent yellow solutions. The electronic absorption spectra of SPBP in various solvents showed a strong broadband with a maximum in the 397-422 nm region. Such absorption bands are characteristic of highly π-conjugated molecules14,16 and thus could be attributed to a π-π* transition. Even a significant increase in solvent polarity resulted in only a small shift of the absorption bands. The absorption spectra of the solutions of APBP (Figure 1b, left) showed a maximum in the 379-396 nm region, which is a shift to a shorter wavelength of approximately 20 nm as compared to the SPBP spectra in the same series of solvents. SPBP contains two piperazine groups; thus, the donor-acceptor interaction may be more pronounced than that in APBP, resulting in a red shift of the absorbance. As the solvent polarity increased from methyl ethyl ketone to dimethyl sulfoxide (DMSO), the absorption maximum is shifted by 25 nm toward a longer wavelength region. It was noteworthy that the molar absorption coefficient of SPBP was eight times higher in the chlorinated solvent such as in dichloromethane than in methyl ethyl ketone (ε=34930 and 4140 L mol-1 cm-1, respectively), possibly due to the increased interaction of the chlorinated solvent molecules to the ground state as well as the excited state of the SPBP molecule. However, the solvent-induced spectral shift in absorption was not sufficient to discern a color change between the different solvents. The spectroscopic data are summarized in Tables S1 and S2 of the Supporting Information. Solvatochromic Fluorescence and Solvent Sensitivity. Unlike absorption, fluorescence emission was significantly affected by the polarity of the medium, as typically observed from a quadrupolar chromophores.7 The SPBP in dichloromethane was strongly fluorescent, and the emission was highly solvatochromic. As compared with a small bathochromic shift of the absorption bands upon increasing solvent polarity, the emission maximum (λemi) of SPBP in the same series of solvents varied markedly from 535 nm in chloroform to 618 nm in DMSO, showing a 83 nm (2510 cm-1) shift (Figure 1a, right). The solvatochromic emission band was conveniently located within the visible region of the spectrum, allowing a (15) (a) Attias, A.-J.; Cavalli, C.; Bloch, B.; Guillou, N.; Noel, C. Chem. Mater. 1999, 11, 2057–2068. (b) Do, J.; Kim, Y.; Yoo, J.; Kim, E.; Attias, A.-J. Proc. SPIE 2007, 6659, 665905. (16) Cherioux, F.; Attias, A.-J.; Maillotte, H. Adv. Funct. Mater. 2002, 12, 203– 208.

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Figure 1. Absorption and fluorescence emission spectra excited at absorption maxima of (a) SPBP (6.04  10-6 M) and (b) APBP (6.04  10-6 M) in different solvents: ethanol (0), ethylacetate (2), acetonitrile (1), toluene (3), DMSO ((), methylethylketone ()), n-butanol (4), dichlromethane (9), trichloromethane (b), and methanol (O).

visual estimate of solvent polarity. Thus, the solvent-dependent emission color change in the solution of both SPBP and APBP is readily observable. Figure 2 shows photographs of the solutions ranked by Lippert-Mataga solvent polarity parameter (Δf). Several independent modes of fluorophore-solvent interaction have been identified for solvatochromism.17 The observable parameters of a solute-solvent system are commonly correlated using the linear solvation energy relationship (LSER).18,19 (17) (a) Kosower, E. M. J. Am. Chem. Soc. 1958, 80, 3253-3260; 3261-3267; 3267-3270. (b) Kosower, E. M.; Skorcz, J. A.; Schwarz, W. M.; Patton, J. W. J. Am. Chem. Soc. 1960, 82, 2188–2191. (c) Armand, F.; Sakuragi, H.; Tokumaru, K. J. Chem. Soc. Faraday Trans. 1993, 89, 1021–1024. (d) Buncel, E.; Rajagopal, S. J. Org. Chem. 1989, 54, 798–809. (18) (a) Martins, C. T.; Lima, M. S.; Elseoud, O. A. J. Org. Chem. 2006, 71, 9068–9079. (b) Arey, J. S.; Green, W. H. Jr.; Gschwend, P. M. J. Phys. Chem. B 2005, 109, 7564–7573. (19) (a) Spange, S.; Prause, S.; Vilsmeier, E.; Thiel, W. R. J. Phys. Chem. B 2005, 109, 7280–7289. (b) Lagalante, A. F.; Clarke, A. M.; Bruno, T. J. J. Phys. Chem. B 1999, 103, 7319–7323.

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Figure 3. Plot of fluorescence emission (Eem) vs solvent polarity

Figure 2. Photographs of the solution of (a) SPBP and (b) APBP in different organic solvents. The successive solvents (left to right) correspond to toluene, chloroform, ethylacetate, dichloromethane, DMSO, n-butanol, methylethylketone, ethanol, acetonitrile, and methanol.

Therefore, we correlated the emission maxima with the solvent polarity parameter ET(30) and Lippert-Mataga polarity parameter Δf, as shown in Figure 3. The Lippert-Mataga parameter is given by eq 1, where ε is the dielectric constant and n is the refractive index of the medium.20 Δf ¼ ½ðε -1Þ=ð2ε þ 1Þ -ðn2 -1Þ=ð2n2 þ 1Þ

ð1Þ

The emission energies (Eem) of bipyridazine solutions were linearly correlated to ET(30) for seven solvents having ET(30) values lower than 47 kcal/mol. The slope of the linear plot of the SPBP emission energies was steeper than that of APBP, indicating that SPBP is more sensitive to the polarity changes of the medium. When the solvent polarity ET(30) was larger than 47 kcal/mol, emission energies were saturated, as shown in Figure 3a. Deviation of individual data points from the linear correlation may be attributed to specific solvent effects or solvent-solute interactions such as hydrogen bonding.21 A poor correlation was observed between the emission energies of SPBP and the Kamlet and Taft parameters. Such a poor correlation to simple solvent parameters could be ascribed to the fact that a quadrupolar charge distribution in SPBP cannot induce a uniform reaction field at the location of the solute, as quadrupolar chromophores (20) Mataga, N.; Kaifu, Y.; Koizumi, M. Bull. Chem. Soc. Jpn. 1956, 29, 465– 470. (21) (a) Perochon, E.; Lopez, A.; Tocanne, J. F. Chem. Phys. Lipids. 1991, 17– 28. (b) Werner, T. C.; Hoffman, R. M. J. Phys. Chem. 1973, 1611–1615.

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[ET(30)] of (a) SPBP (filled circle) and APBP (unfilled circle). Inset: correlation of (Eem) vs Δf. (b) Lippert-Mataga plots relating the Stokes shift (in eV) to the solvent polarity parameter (Δf ): DMA-2,3 (9), DMA-2,4 (b), DMA-2,5 (2), APBP (0), and SPBP (O).

experience symmetry breaking in the relaxed excited state because of polar solvation.7 However, it is noteworthy that the correlation to the Lippert-Mataga solvent polarity parameter Δf is linear. This allowed us to estimate the excited state dipole moment change of SPBP and relative sensitivity to organic polarity, as described below. The emission energies in all 10 solvents correlated linearly to the Lippert-Mataga solvent polarity parameter Δf,20 as shown in the inset in Figure 3a. This correlation provides information on the dipole-dipole interactions between the bipyridazines and the solvents.22 The linear relationship between the emission energies of bipyridazines and the Lippert-Mataga Δf may be represented as eqs 2 and 3, respectively. Eem ðeV, SPBPÞ ¼ -1:6  Δf þ 2:60

ð2Þ

Eem ðeV, APBPÞ ¼ -1:2  Δf þ 2:58

ð3Þ

As observed in the ET(30) correlation, the slope of the linear plot for SPBP emission energies against Δf was steeper than that for APBP, indicating that SPBP is more sensitive to the polarity of the organic (solvent) molecules. Because the slopes of the linear relationships between emission energy and Lippert-Mataga parameters are indicative of a molecule’s sensitivity toward the polarity of organic molecules, we extended the correlation to known solvatochromic dimethylaminostyryldiazines (DMAs).14 Figure 3b compares the emission (22) Lewis, F. D.; Weigel, W. J. Phys. Chem. A 2000, 104, 8146–8153.

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energy for SPBP with those of the well-known asymmetric fluorophore, DMAs14 (DMA-2,3, DMA-2,4, and DMA-2,5):

The solvent-dependent emission color change of SPBP is more drastic than the DMA series (Figure 3b). The SPBP emission occurs at a much longer wavelength than that of DMA-2,3, with a steeper slope (r) of the emission energy plot (rSPBP = 1.60 and rDMA-2,3=0.93). The slope of the SPBP plot was also steeper than that of DMA-2,5 (rDMA-2,5 = 1.32), which is reported to be the most sensitive DMA to solvent polarity. Furthermore, the slope of the emission energy plot for APBP (rAPBP =1.25) was larger than that of DMA-2,3 but smaller than that of DMA-2,5. Overall, the bipyridiazines displayed much greater solvatochromic sensitivity by fluorescence emission than by absorption. This could arise from a large change in electronic distribution23 or symmetry breaking at the relaxed excited state7 when the neutral symmetrical chromophore is excited. Thus, we examined the electronic density at the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels, as described below. Structure and Dipole Moment Change of PiperazineSubstituted Bipyridazines. According to the calculations, the optimized geometry of SPBP and APBP in the ground state was almost a planar structure, with the dihedral angle between the two pyridazine rings and that between pyridazine and phenyl being smaller than 2°. The electron in the HOMO is mainly localized in the vicinity of the terminal piperazinyl benzene rings, whereas the electron in the LUMO is found near the bipyridazine rings (Figure 4). This indicates that the absorption and emission bands of the bipyridazines may be ascribed to the electronic states associated with the piperazinyl benzene rings and bipyridazine rings, respectively. Interestingly, according to this difference in electron localization in HOMO and LUMO levels, the onephoton transition from the HOMO level (S0) to the lowest excited state (S1) of the bipyridazines could change the property of the S1 state. The change may eventually lead to symmetry breaking in the relaxed excited state because of polar solvation7 or the conformational change of bipyridazine, as its locally excited state may relax to a minimum of potential energy surface (PES) of excited state by some possible geometry alterations. A typical example of conformation change upon photoexcitation can be provided by the excited state of DMA where the dimethylamino groups are twisted out of the plane of the aromatic group to give anomalous emission in different solvents.14,24 In this case, a twisted intramolecular charge transfer (TICT) structure is the most commonly accepted. However, in our sample, we could not observe such anomalous emission from the steady state fluorescence studies. As a simplified attempt to investigate the nature of emission state, we computed the excited energy as a (23) (a) Grabowski, Z. R.; Rotkiewicz, K.; Rettig, W. Chem. Rev. 2003, 3899– 4032. (b) Kim, E.; Choi, K.; Rhee, S. B. Macromolecules 1998, 31, 5726–5733. (24) (a) Collette, J. C.; Harper, A. W. Proc. SPIE 2007, 4809, 4809122. (b) Dobkowski, J.; Wojcik, J.; Kozminski, W.; Koleos, R.; Waluk, J.; Michl, J. J. Am. Chem. Soc. 2002, 124, 2406–2407.

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Figure 4. HOMO (upper side) and LUMO (lower side) orbitals for SPBP (a) and APBP (b) at optimized geometries.

function of dihedral angle (ψ) about the bond between the two central pyridazine rings, which are responsible for fluorescence according to the calculation above. The resultant excitation energy profile along a single reaction coordinate ψ is a slice of hyperdimensional PES. The excited state energy calculation was performed using time-dependent density functional theory/density functional theory (TDDFT//DFT) at the levels of B3LYP/ 3-21G*//B3LYP/3-21G* with a polarizable continuum model32 (PCM) solvent model, while the dihedral angle ψ is varied in steps of 15° between 0° and 180° (see the Computational Methods). The calculated low-lying excited states for SPBP reveal that the S1 excited energy profile is inverse-parabolic, exhibiting a global minimum at ψ=0° and a local minimum at ψ=180°, as shown in Figure S1 of the Supporting Information. The maximum energy barriers between ψ=0° and ψ=180° appeared at ψ=90° and was estimated as ∼1.0 eV higher than that at ψ = 90° in the forward direction (ψ = 0° f ψ = 180°) and 0.6-0.7 eV higher in the backward direction (ψ=180° f ψ=0°). Considering that thermal energy at room temperature is roughly 0.03 eV, these energy barriers for twisting of the central pyridazine rings seem to be too high for SPBP to undergo flipping of two pyridazine rings over to each other. A similar energy profile is found for APBP. It is also found that the reaction coordinate associated with rotating ψ is not related to a conical intersection with an energetically higher lying state (S2, S3). We therefore infer that the twisting of the bipyridazine ring at the excited state is not likely to occur albeit our choice of ψ as a reaction coordinate for exploring excited energy surface cannot completely rule out other possibilities of geometry change. This suggests that the initially excited dye molecules with a conformation of two pyridazine rings being in the opposite side from each other (ψ = 0°) are relaxed to lower excited states upon solvent interaction without conformational twisting. It is noteworthy that the symmetric SPBP is highly sensitive to solvent polarity, despite the general notion that asymmetric fluorophores display a higher sensitivity to solvent polarity than symmetric fluorophores. The large fluorescence solvatochromism in SPBP suggests the existence of polar excited states in SPBP. This could be rationalized that the nitrogens in the piperazine group facilitate π-electron delocalization into the neighboring benzene to π-bridge and then ultimately to the central pyridazine acceptor to induce internal charge transfer (ICT). Such an extended π-electron delocalized structure could allow interactions of the excited structure with solvents molecules, to lead a relaxed (symmetry broken) excited state, which has been proposed for solvatochomic quadrupolar chromophores (DAD), where D and Langmuir 2009, 25(16), 9405–9412

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A are donor and acceptor, respectively.7 In this model, the charge resonance structures of DAD, D+A-D, and DA-D+ are the basis of a set of three orthogonal states responsible for symmetry breaking issues. As SPBP shows a strong solvatochromism in fluorescence but nonsolvatochromic absorption spectra, it can be classified as a low-quadrupolar molecule (class I chromophores in ref 7). Then, the system relaxes to a polar relaxed excited state after vertical excitation, and symmetry breaking could appear in polar solvents as a result of the combined action of vibrational and solvation coupling, according to the literature. The absorption and emission spectra of fluorophores dissolved in alcohols gave information of the specific interaction between the dye and the surrounding alcohol molecules due to the hydrogen-bonding ability of alcohols.21 It was reported that the relaxed excited state of the class I chromophores in ref 7 is nonpolar in toluene (a case of false symmetry breaking) and acquires an increasing dipolar character (real symmetry breaking) with increasing solvent polarity, reaching an almost “saturated” value in hexanol. Bipyridazines have multiple nitrogen atoms. Thus, we may expect a swerve if there is hydrogen bonding between SPBP (and APBP) with alcohol in the plot of the Lippert-Mataga parameter versus fluorescence emission energy (Figure 3). However, we did not observe any plateau or a swerve. This indicates that the hydrogen-bonding interaction with alcohol may not occur in our bipyridazines. The two piperazine groups in SPBP caused a red shift in emission spectra as compared to those of APBP, which comprises only one piperazine group. This finding explains the higher sensitivity of SPBP toward organic solvent molecules as compared with that of APBP. The difference in energy Δν (in cm-1) between the absorbed and the emitted radiation, known as the Stokes shift,25 informs primarily the solvent relaxation adjacent to the chromophore. The Lippert-Mataga equation is a simple and widely used formula that relates the Stokes shift to the dipole moment of the excited state,23b as shown in eq 4. ems Δν ¼ νabs max -νmax ¼

2ðμe -μg Þ2 Δf þconst hca3

ð4Þ

In the above equation μe and μg are the dipole moments of the excited state and ground state, respectively. The terms h, c, and a are Planck’s constant, the velocity of light, and the radius of the cavity in which the fluorophore resides, respectively. The linear dependence of the bipyridazine Stokes shift (EST) on the solvent parameter Δf is shown in Figure 3b and is represented in eqs 5 and 6 for SPBP and APBP, respectively. In addition, the linear dependence of the Stokes shifts of the asymmetric pyridazines on Δf is represented in eqs 7-9. EST ðeV, SPBPÞ ¼ -1:76  Δf þ0:44 or Δν ðcm -1 Þ ¼ -14427  Δf þ3560

ð5Þ

EST ðeV, APBPÞ ¼ -1:21  Δf þ0:63 or Δν ðcm -1 Þ ¼ -9731  Δf þ5104

ð6Þ

EST ðeV, DMA-2; 3Þ ¼ -0:74  Δf þ0:48

ð7Þ

EST ðeV, DMA-2; 4Þ ¼ -1:10  Δf þ0:38

ð8Þ

EST ðeV, DMA-2; 5Þ ¼ -1:17  Δf þ0:41

ð9Þ

Among the slopes of the plots represented by eqs 5-9, the slope for SPBP was the largest, with r = 1.76. This observation provides (25) Narang, U.; Zhao, C. F.; Bhawalkar, J. D.; Bright, F. V.; Prasad, P. N. J. Phys. Chem. 1996, 100, 4521–4525.

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another indication of the high sensitivity of SPBP to the polarity changes of the medium and suggests that this cyclic aminesubstituted SPBP is a promising candidate for a solvent or organic molecule sensitive molecule. From the slope and Onsager cavity radius, dipole moment changes, between the ground and the excited states of bipyridazines, were determined according to the eq 4 and the optimized structure in Figure 4. Assuming that the Onsager cavity radius a is given as a = (3Vvdw/4π)1/3, where Vvdw is the van der Waals volume of the solute,26 the cavity radius was determined as 5.02 A˚ for SPBP and 4.37 A˚ for APBP. Thus, the dipole moment changes were calculated as 13.2 D for SPBP and 8.9 D for APBP. The dipole moments of SPBP in S0 and S1 states were calculated using the TDDFT//DFT method, which allows us to compare TDDFT results for the dipole moment change (Δμ=μE - μG) with the value determined from the Lippert-Mataga plot (Δμ= 13.2 D). The computed ground state dipole moment of SPBP was nearly zero (μG = 0.36 D), whereas the dipole moment of the excited state increases to μE of 14.7 D. Thus, the dipole moment enhancement (Δμ) corresponds to 14.4 D, which is in reasonably good agreement with the value (Δμ=13.2 D) from the LippertMataga plot. Such a high dipole moment of SPBP at the excited state must be important for solvatochromic fluorescence. Solvatochromism in a Binary Solvent Mixture. The large solvatochromic shift of the fluorescence in the bipyridazines may be utilized to detect organic mixtures. Optical properties of the bipyridazines were examined in toluene/DMSO mixtures, as DMSO induces a large shift in the fluorescence spectra. A fixed amount of bipyridazine was added to binary solvent comprising varying volume ratios of toluene and DMSO. All of the solutions showed a similar absorption maximum, whereas the fluorescence maximum λemi was red-shifted with increasing proportion of DMSO in the binary solvent. The fluorescence intensity increased up to a DMSO/toluene volume ratio of 30/70 and decreased as the DMSO content was increased further, as shown in Figure 5a for SPBP. This indicates that the excited state of the bipyridazines is stabilized by the surrounding polar solvent molecules in the binary system. Polar solvent molecules are attracted more strongly to the highly polarized SPBP molecules in the excited state than to those in the ground state. The magnitude of the fluorescence shift is determined by the number of polar solvent molecules in close proximity to the fluorophore. The plot of emission energy as a function of the mole fraction of DMSO in the SPBP solution shown in Figure 5b clearly indicates saturation at a mole fraction of approximately 0.5. This saturation occurs when the first solvation shell of the excited dipolar molecule is unable to accommodate additional polar solvent molecules.27 The sensitivity of the fluorescence shift in response to DMSO mole fraction changes was higher for SPBP than for APBP, consistent with the larger dipole moment enhancement in the excited state for SPBP than for APBP, as described above. The shift in emission energy was pronounced and could be observed visually, as shown in the photographs of SPBP and APBP solutions in Figure 5c. The synthetic rainbow of emission generated by varying DMSO content in toluene was similar to that observed in Figure 2, suggesting that the polarity range in the binary system is similar to that produced by the different solvents in Figure 2. Organic Molecule Sensing by the Fluorescent Bipyridazine Film Surfaces. The solvatochromic fluorescence and high (26) Rummens, F. Can. J. Chem. 1976, 54, 254–269. (27) (a) Jozefowicz, M.; Heldt, J. R. Chem. Phys. 2003, 294, 105–116. (b) Banerjee, D.; Laha, A. K.; Bagchi, S. J. Chem. Soc. Faraday Trans. 1995, 91, 631–636.

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Figure 5. (a) Fluorescence spectra of SPBP (6.04  10-6 M) in a binary solvent system. (b) Plots of Stokes shift and fluorescence intensity as a function of the mol fraction of DMSO in the binary system. (c) Photographs of the binary solution of SPBP (top) and APBP (bottom) in different DMSO contents. The successive solutions (from left to right) correspond to the volume ratio of toluene/DMSO as 100/0, 95/5, 90/10, 80/20, 70/30, 60/40, 50/50, 40/60, 30/70, 20/80, 10/90, 5/95, and 0/100.

Figure 6. (a) Fluorescence spectra and (b) photograph of the emission from the SPBP-doped PMMA film sensor dipped in different solvents (from left to right: toluene, ethyl acetate, and acetonitrile).

fluorescence quantum yield of SPBP suggest the potential application of this molecule in fluorescent sensors. SPBP and APBP were soluble in chloroform to give transparent solution. SPBP solutions can be easily processed into a film sensor in the presence of a transparent binder by spin coating. Polymethylmethacrylate (PMMA) was chosen as a binder for its high solubility and compatibility with SPBP. The spin-coated film remained transparent after the solvent was evaporated, which indicates that the dyes are homogenously dispersed in the polymer matrix without the formation of dye aggregates. A barrier layer of polyvinyl alcohol (PVA) film was coated on top of the SPBP-doped PMMA layer to prevent SPBP from dissolving in the organic solvent. The dried film sensor showed a blue green emission color. Figure 6a shows fluorescence spectra of the film sensor dipped in three different solvents. The λemi values were 505, 543, and 615 nm for toluene, ethyl acetate, and acetonitrile, respectively, which matched well with those observed in solution (Table S1 of the Supporting Information). The emission color of the film sensor was immediately changed to green when the film sensor was dipped into ethyl acetate and to dark red when dipped into acetonitrile. In toluene, the emission color was slightly changed from that of the dried film to greenblue. Photographs of these color changes are shown in Figure 6b. The emission color was changed in the solvent-wetted film but not in the solvent itself, which remained dark. This indicated that the solvent molecules penetrate into the PMMA film over the PVA layer, while SPBP remains in the PMMA film without dissolution into the solvent. The emission color of the portion of the film 9410 DOI: 10.1021/la901476q

Figure 7. Fluorescence image of SPBP-doped PMMA film sensor (a) before vapor exposure, (b) after exposure to acetonitrile vapor, and (c) after exposure to ethyl acetate vapor at the circular region.

above the liquid line was slightly changed due to the solvent vapor. These data indicate that bipyridazines substituted with piperazine are promising materials for organic molecule sensing because their emission is highly sensitive to the polarity of the molecules. The film sensor from the highly solvent-sensitive SPBP also allows detection of not only fluidic solvent but also solvent vapor. Figure 7a,b shows the film sensor in air (Figure 7a) and under exposure to acetonitrile vapor (Figure 7b). The initial greenish color of fluorescence from the film in air drastically changed to a dark red over the entire area of the film upon exposure to acetonitrile vapor. As shown in Figure 7c, of interest is that when Langmuir 2009, 25(16), 9405–9412

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Article

the ethyl acetate vapor is applied only to the circular region of the film; the film shows a yellow fluorescent circle. Such a local fluorescence change offers a new method not only for a filmpatterning process such as solvent stamping or selective vapor exposure on the film but also for understanding dynamic mechanisms of various solvent-based film processing such as widely used solvent annealing28 for mesophase orientation in block copolymer thin films. Further studies on the lifetime of the fluorescence emission and structural modification of the bipyridazines are in progress to explore the photophysics and application of these molecules in flexible organic film sensors for biological and environmental applications.

Conclusions Highly fluorescent bipyridazines comprising a heterocyclic amine (piperazine) group were synthesized to give SPBP and APBP. The quantum yield of SPBP was extremely high in dichloromethane (φF =0.63) and was higher than that of APBP and other asymmetric diazines. Both of the bipyridazines showed positive solvatochromic fluorescence properties and generated a synthetic rainbow of emission in 10 different organic solvents. The Stokes shift of the bipyridazines was linearly correlated to Lippert-Mataga solvent polarity parameter Δf. The slope of this linear correlation was highest for SPBP, indicating a higher sensitivity to organic solvents as compared with APBP and other asymmetric diazines. From the slope and Onsager cavity radius, dipole moment changes, between the ground and the excited states of bipyridazines, were determined as 13.2 D for SPBP and 8.9 D for APBP. The higher dipole moment enhancement in SPBP correlates well with the greater sensitivity of its fluorescence to solvatochromic effects as compared with that of APBP. A solvatochromic film sensor was fabricated by a simple solutioncoating method using PMMA and PVA as the host matrix and barrier layer, respectively. The fabricated film sensor shows a high sensitivity for the detection of solvent species ranging from toluene to DMSO upon exposure to solvent molecules, which fits into many applications as a polarity-sensitive fluorescent surface. Experimental Section Materials. 3-Chloro-6-methylpyridazine, nickel(II) chloride hexahydrate, triphenylphosphine, zinc powder, N,N-dimethylformamide, and p-toluenesulfonic acid were purchased from Aldrich Chemical. 4-(4-Methylpiperazin-1-yl)benzaldehyde was purchased from TCI chemical. PMMA (MW=35000) was purchased from Acros Organics. Spectroscopic grade toluene, ethylacetate, dichloromethane, trichloromethane, acetonitrile, methylethylketone, ethanol, methanol, n-butanol, and DMSO were purchased from Aldrich Chemical. The abbreviations of solvents are chloroform (CHCl3), ethylacetate (EA), dichloromethane (MC), dimethylsulfoxide (DMSO), n-butanol (BuOH), methylethylketone (MEK), ethanol (EtOH), acetonitrile (CH3CN), and methanol (MeOH). 6,60 -Dimethyl-3,30 -bipyridazine (1) was synthesized from 3-chloro-6-methylpyridazine, according to the method reported in the literature15 and was confirmed with the purchased one from SYNCHEM OHG (HS2-E002; mp 239.4-239.8 °C). Thus, nickel(II) chloride hexahydrate (14.16 g, 60 mmol), 62.4 g of triphenylphosphine (240 mmol), and 300 mL of N,N-dimethylformamide were heated at 65 °C for 2 h under an argon atmosphere. To the cyan-colored reaction mixture, 3.84 g of zinc powder (60 mmol) was added. After vigorous stirring, the reaction temperature cooled to 50 °C, and 7.71 g of 3-chloro-6-methylpyridazine (60 mmol) was added. After 28 h, the reaction mixture was (28) Kim, T. H.; Hwang, J; Hwang, W. S.; Huh, J.; Kim, H. C.; Kim, S. H.; Thomas, E. L.; Park, C. Adv. Mater. 2008, 20, 522–527.

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treated with ammonia solution. The resulting solution was washed with aqueous NaOH (2 M) solution and extracted with dichloromethane. The organic layer was dried over magnesium sulfate and evaporated under vacuum. 6,60 -Dimethyl-3,30 -bipyridazine (3.51 g) was isolated as pale brownish solids by column chromatography (yield, 63%); mp 240.0-240.2 °C. 1H NMR (CDCl3, ppm): 2.80 (s, CH3, 6H), 7.50 (d, Ar-H, 2H), 8.69 (d, Ar-H, 2H).

Synthesis of the Piperazinyl Bipyridazines (SPBP and APBP). Compound 1 (2.1 g, 11.3 mmol), 4-(4-methylpiperazin-1yl)benzaldehyde (6.92 g, 33.9 mmol), and p-toluenesulfonic acid were mixed and stirred at 160 °C. After 63 h, most of 1 was consumed as monitored by thin-layer chromatography (TLC). The mixture was washed with aqueous NaOH (2 M) solution. The crude product was purified by column chromatography using a mixture of dichloromethane and ethanol (10:1, v/v). After the solvents were removed, solid products were dried under reduced pressure at 80 °C for 2 days to give SPBP as yellow lustrous flakes (yield, 62%). The melting peak of SPBP was observed at 332 °C by DSC. 1H NMR (CDCl3, ppm, Figure S2 of the Supporting Information): 2.37 (s, CH3, 6H), 2.60 (t, CH2, 8H), 3.32 (t, CH2, 8H), 6.95 (d, Ar-H, 4H), 7.28 (d, vinyl, 2H), 7.56 (d, Ar-H, 4H), 7.74 (d, Ar-H, 2H), 7.76 (d, vinyl, 2H), 8.75 (d, Ar-H, 2H). Anal. calcd for C34H38N8: C, 73.09; H, 6.86; N, 20.06. Found: C, 72.54; H, 6.94; N, 20.02. The APBP, 6-methyl-60 -(4-(4-methylpiperazin-1-yl)styryl)3,30 -bipyridazine, was synthesized similarly using compound 1 (2.1 g, 11.3 mmol), 4-(4-methylpiperazin-1-yl)benzaldehyde (2.77 g, 13.6 mmol), and p-toluenesulfonic acid. After 41 h, the reaction mixture was worked up and purified by column chromatography to give APBP as yellow lustrous flakes (yield, 74%); mp 250252 °C. 1H NMR (CDCl3, ppm, Figure S3 of the Supporting Information): 2.37 (s, CH3, 3H), 2.60 (t, CH2, 4H), 2.80 (s, CH3, 3H), 3.32 (t, CH2, 4H), 6.95 (d, Ar-H, 2H), 7.28 (d, vinyl, 1H), 7.51 (d, Ar-H, 1H), 7.56 (d, Ar-H, 2H), 7.73 (d, Ar-H, 1H), 7.75 (d, vinyl, 1H), 8.72 (d, Ar-H, 2H). Anal. calcd for C22H24N6: C, 70.94; H, 6.49; N, 22.56. Found: C, 71.02; H, 6.71; N, 22.08. Instruments. 1H NMR spectra were recorded on a Varian Unity 400 spectrometer operating at 400.04 MHz, respectively. Chloroform-d (CDCl3) was used as a solvent, and tetramethylsilane (TMS) was used as an internal standard. Elemental analysis was carried out using a Thermo scientific flash EA 1102 model. DSC was carried out using NETZSCH (DSC 200 F3), and melting points of APBP and 6,60 -dimethyl-3,30 -bipyridazine were analyzed from B€ uchi Labortechnik AG (B-535). The UV-visible absorption spectra were obtained in different solvents and solvent mixtures with Avaspec spectroscopy (S:Avaspec-2048, D:Avalight-DHS). Appropriate concentrations (absorbance below 0.2 au) were chosen to avoid inner filter effects. The fluorescence emission spectra were obtained from the same sample used for absorption measurements, using a Luminescence spectrometer “LS55” (PerkinElmer). To minimize the atmospheric H2O and O2 contamination in the sample, argon gas was purged through the solution before measurements. The scan speed was fixed at 500 nm/min. Molar absorption coefficients (εmax) were determined using the Beer-Lambert law. Fluorescence quantum yields were determined in different solvents by using 2,4,6-triphenylpyrylium tetrafluoroborate (in dichloromethane under argon, φref = 0.5829) as a reference. The quantum yield was  of the 2 sample  n A determined using the equation: j ¼ jref Iref where φ nref Aref I is the relative quantum yield of sample, I is the integrated intensity of fluorescence spectrum of sample, A is the intensity of absorbance spectrum of sample, and n is the refractive index. The subscript “ref” refers to the value of the reference fluorescence material of known quantum yield. Quantum yields were not corrected for the change in refractive index between the chosen solvent and the dichloromethane solution. (29) (a) Akaba, R.; Kamata, M.; Koike, A.; Mogi, K.-I.; Kuriyama, Y.; Sakuragi, H. J. Phys. Org. Chem. 1997, 10, 861–869. (b) Akaba, R.; Sakuragi, H.; Tokumaru, K. J. Chem. Soc., Perkin Trans. 2. 1991, 2, 291–297.

DOI: 10.1021/la901476q

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Preparation of Solvatochromic Film Sensors. A chloroform (13 g) solution of SPBP (1.1 mg) and PMMA (1 g) was coated on a slide glass by spin coating for 1 min with a spin rate of 1800 rpm. Then, the PMMA film was dried under reduced pressure. A barrier layer composition was prepared by using aqueous solution of PVA (3 wt %). The barrier layer solution was coated on top of the SPBP-doped PMMA film by spin coating for 1 min with a spin rate of 1800 rpm. Then, the film was dried under reduced pressure. Computational Methods. The quantum chemical computations of the ground and excited state properties of SPBP and APBP were performed using TDDFT as implemented in Gaussian 03 program30 with the B3LYP hybrid functional31 composed of the exchange functional of Becke and the correlational functional of Lee, Yang, and Parr and 3-21G* Gaussian basis set. To calculate the (30) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr. ; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth,G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision B.04; Gaussian: Wallingford, CT, 2004. (31) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652.

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Do et al. potential energy as a function of dihedral angle (ψ) about the bond between two pyridazine rings (see Scheme 1), the dihedral angle ψ was varied in steps of 15° between 0° and 180°. All other geometrical parameters were fully optimized by density functional theory (DFT) at the level of B3LYP functional with the 3-21G* basis set with a solvent effect included via the PCM.32 The TDDFT-PCM calculations for ground and excited state energy were then carried out using the produced geometries. Regarding the calculation of the dipole moments by Gaussian 03, the x-axis corresponded to the direction of long axis of the molecule and the z-axis corresponded to the perpendicular direction of the cored bipyridazine plane. The dipole moment in ground state and excited state for x, y, z axis were calculated as 0.36 D (x; -0.16 D, y: -0.29 D, z: 0.14 D) and 14.7 D (x: 14.76 D, y: 0.014 D, z: 0.10 D), respectively.

Acknowledgment. This work was supported by a Korea Science and Engineering Foundation (KOSEF) grant funded by the Korean Government (MEST) (R11-2007-050-01001-0) and the Seoul R&BD Program (10816). We thank Prof A. Attias [Universite Pierre et Marie Curie, Paris 6 (UPMC)] for his valuable discussions. Supporting Information Available: Tables of spectral data and NMR spectra of SPBP and APBP. The potential energy of ground (S0) and excited (S1) states of SPBP as a function of the dihedral angle (ψ). This material is available free of charge via the Internet at http://pubs.acs.org. (32) Tomasi, J.; Mennucci, B.; Cammi, R. Chem. Rev. 2005, 105, 2999–3094.

Langmuir 2009, 25(16), 9405–9412