Cross-Conjugated Polymers with Large Two-Photon Absorption Cross

Alex K.-Y. Jen*. Department of Materials Science and Engineering, UniVersity of Washington, Box 352120,. Seattle, Washington 98195-2120. ReceiVed: Mar...
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J. Phys. Chem. C 2007, 111, 10673-10681

10673

Cross-Conjugated Polymers with Large Two-Photon Absorption Cross-Sections for Metal Ion Sensing Fei Huang, Yanqing Tian, Ching-Yi Chen, Yen-Ju Cheng, A. Cody Young, and Alex K.-Y. Jen* Department of Materials Science and Engineering, UniVersity of Washington, Box 352120, Seattle, Washington 98195-2120 ReceiVed: March 7, 2007; In Final Form: May 3, 2007

The synthesis and metal ion sensing properties of two new cross-conjugated polymers, poly[(9,9-di-nhexylfluorene-2,7-diyl)-alt-co-(2-(4-(N,N-(dibutylamino)styryl)benzene)-1,4-diyl)] (P1) and poly[(9,9-di-nhexylfluorene-2,7-diyl)-alt-co-(2,5-bis(4- (N,N-(dibutylamino)styryl)benzene)-1,4-diyl)] (P2) are reported. These two polymers show very different sensing characteristics from those observed for linear conjugated polymers. Upon binding with different metal ions, P1 and P2 show clear intensity changes and large shift of their fluorescence emission peaks. This combined response significantly improves their binding selectivity. Moreover, the donor-π-bridge-donor (D-π-D) conjugated side chain motifs on P2 endow it with efficient two-photon absorbing (2PA) property. This allows it to be used in two-photon laser scanning microscopy and sensing.

Introduction Conjugated polymers (CPs)-based chemo- and biosensors have recently attracted great attention due to their high sensitivity in detecting targeted spieces.1 Due to their conjugated backbone, CPs allow efficient electron delocalization and exciton migration over long distance. As a result, their electrical, optical, electrochemical, and optoelectronic properties could be manipulated easily by minor perturbations of environmental stimuli due to the amplification actions of a collective system response. This characteristic offers them a significant advantage over small chromophores for sensor applications.1 On the basis of this unique property, numerous CP-based sensors have been demonstrated to be able to detect trace analytes in various environments.1-5 For example, several highly sensitive CP-based metal ion sensors have been reported recently that can detect metal ions in environmental and biological systems.4,5 In most of these sensors, the detecting mechanism is based on either the “turn-off” (quenching) or “turn-on” (enhancing) effect of linear CPs in the presence of analytes. Almost all of them are limited by their capability that can only monitor the changes of their single-photon absorption or emission properties.4 Since the fluorescence intensity of CP could be easily affected by many factors such as nonspecific binding of metal ions with the CP’s ligands, it is very challenging to gain good selectivity in these sensors.6 One alternative to improve selectivity is to ensure that the PL of these polymers exhibits both intensity change and peak shift when detecting different analytes. In literature, there are several highly sensitive and selective DNA sensors that have been reported by taking advantage of the fluorescence resonance energy transfer (FRET) to enhance their properties. These systems not only exhibit a large shift in emission but also their fluorescence intensity change could be ratio-metrically measured to avoid the influence of nonspecific binding.3 Several other reports also mentioned the use of emission peak shift of CPs as readout for detecting different metal ions binding.5 In these sensors, pyridine or phenanthroline derivatives are often used * Address correspondence to this author. E-mail: [email protected]. edu.

as the binding groups on the rigid polymers main chain. The rigid polymer backbone strongly limits its conformational change, and as a result, it affects the spatial-matching interactions between the binding groups and the analytes. This severely decreases the sensitivity of the sensors.4d One possible way to alleviate this problem is to develop CP with binding groups that are located at more outreaching side chains but still maintain conjugation with CPs’ backbone. This requires polymer with a cross-conjugated structure. While cross-conjugated CPs have been used in photovoltaic cells and thin film transistors,7 their application in sensors has rarely been explored.1-6 Herein, we report the design and synthesis of two new crossconjugated conjugated polymers: poly[(9,9-di-n-hexylfluorene2,7-diyl)-alt-co-(2-(4-(N,N-(dibutylamino)styryl)benzene)-1,4diyl)] (P1) and poly[(9,9-di-n-hexylfluorene-2,7-diyl)-alt-co(2,5-bis(4-(N,N-(dibutylamino)styryl)benzene)-1,4-diyl)] (P2) comprising a cross-conjugated side chain and main chain (Scheme 2). The dibutylamino sensing groups on P1 and P2 are located at the end of the side chains so they can have better spatially matched interactions with analytes. Since these donor groups are connected with the conjugated polymer main chain, they also endow P1 and P2 with a strong intramolecular charge transfer (ICT) character.8 As a result, P1 and P2 show large fluorescence intensity changes and emission peak shift upon interacting with different metal ions. This potentially can reduce the intensity fluctuation that is often accompanied by unspecific binding. It can also enhance the selectivity and sensitivity of the metal ion detection through the change of fluorescence sites.9 Moreover, since P2 also possesses an efficient two-photon absorbing (TPA) donor-π-bridge-donor (D-π-D) conjugated side chain,8a-c it could be excited with near-infrared (NIR) light in the range of 700 to 1000 nm. This greatly enhances the choice of excitation wavelength for sensing. It also provides numerous advantages over the commonly used one-photon chromophores, such as increased penetration depth for tissue imaging and reduced photobleaching and photoxicity.10 Up to now, there are very few two-photon chemsensors reported and most of them are based on small molecular chromophores.11 Among these, a two-photon metal-ion sensor using aza-crown ether substituted

10.1021/jp0718799 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/21/2007

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SCHEME 1: Synthesis of Monomers

SCHEME 2: Synthesis of P1 and P2

distyrylbenzenes was reported by Pond et al. Their results indicate that higher sensitivity and contrast could be obtained by two-photon excitation.11a Considering the “amplification property” of CPs, it will be advantageous if their two-photon absorption or emission properties could also be explored for sensing. Results and Discussion 1. Synthesis. The synthetic routes for monomers and polymers are shown in Schemes 1 and 2. 4-(N,N-Dibutylamino)-

benzaldehyde (1), 2-diethylphosphonatomethyl-1,4-dibromobenzene (2), and 2,5-bis(diethylphosphonatomethyl)-1,4-dibromobenzene (3) were prepared according to the known procedures.12,13 Treating 1 equiv of aromatic aldehyde 1 with monophosphonate 2 under the Horner-Emmons-Witting coupling conditions, using potassium tert-butoxide as base, afforded 4-dibromo-2-(4-[N,N-(dibutylamino)styryl])benzene (4) in 67% yield. By using bis-phosphonate 3 instead of mono-phosphonate 2 under the similar condition, a longer π-conjugated 4-dibromo2,5-{bis(4-[N,N-(dibutylamino)styryl])}benzene (5) was ob-

Synthesis of Cross-Conjugated Polymers

Figure 1. Absorption and emission spectra of P1 (a) and P2 (b) in different solvents.

tained in 58% yield. The monomers 4 and 5 were purified by silica gel column, and their molecular structures were confirmed by NMR spectroscopy. The trans geometric structures of monomers 4 and 5 were confirmed by verifying the coupling constant of their vinylic protons in the 1H NMR spectra (J ) ∼16 Hz). The cross-conjugated polymers poly[(9,9-di-n-hexylfluorene-2,7-diyl)-alt -co-(2-(4-(N,N-(dibutylamino)styryl)benzene)-1,4-diyl)] (P1) and poly[(9,9-di-n-hexylfluorene-2,7-diyl)alt-co-(2,5-bis(4-(N,N-(dibutylamino)styryl)benzene)-1,4diyl)] (P2) were synthesized in 73% and 78% yield, respectively, via the Suzuki coupling reaction under reflux in a mixture of 2 M K2CO3 and toluene with Pd(PPh3)4 as catalyst. By using different monomers 4 and 5, P1 and P2 comprise different conjugated lengths of side chains, which provide them completely different sensing characteristics. The resulting polymers P1 and P2 have good solubility in common organic solvents such as tetrahydrofuran (THF), dichloromethane (DCM), chloroform, toluene, and xylene. The number average molecular weight (Mn) estimated by gel permeation chromatography (GPC) against the polystyrene standard with THF as eluent was 25 200 with a polydispersity of 2.9 for P1 and 10 500 with a polydispersity of 2.1 for P2, respectively. 2. Linear Absorption and Photoluminescence Properties. Figure 1 shows UV-visible absorption and photoluminescence (PL) spectra of P1 and P2 in several different solvents. P1 shows a single absorption band with λmax at ∼356 nm, which corresponds to its π-π* transition. P2 has a very different UVvisible spectrum compared to that of P1 due to its D-π-D side chain, which has a strong ICT character. As shown in Figure 1b, P2 has two obvious absorption peaks, one is at ∼338 nm, which corresponds to the π-π* transition, and the other is at ∼414 nm, which is due to strong ICT between two dibutylamino donors on the side chain. The absorption spectra of P1 and P2 undergo only small shifts when solvent polarity was changed, which is very similar to that reported for small amino-containing stilbene and distyrylbenzene chromophores. This indicates that the ICT state originates from the relaxation of the initially formed Frank-Condon excited state.14 Unlike the absorption spectra, the PL spectra of P1 and P2 show a considerable red shift when the solvent was changed from

J. Phys. Chem. C, Vol. 111, No. 28, 2007 10675 toluene to more polar DCM. The PL spectra of P1 in toluene, THF, and DCM peak at 457, 486, and 490 nm, respectively, while the PL spectra of P2 peak at 472, 485, and 496 nm. Table 1 also lists the PL quantum efficiency (η) of P1 and P2 in different solvents compared to that of the standard compound, 9,10-diphenylanthracene in cyclohexane. It was found that the PL efficiency of P1 and P2 decreases with the increase of solvent polarity. The large red shifts in the PL spectra of P1 and P2, together with a decreased η, when the solvent polarity was increased, are typical characteristics derived from their strong ICT character. It is well-known that the ICT excited state is polar and sensitive to the polarity of its environment.14,15 3. One-Photon Metal Ion Responsive Properties. The response of P1 and P2 to different metal ions was monitored by titrating the perchlorate salt solution of different ions with P1 or P2 in DCM. Three kinds of physiologically important ions, Ca2+, Mg2+, and Zn2+, were used in this study. Recent work by Bunz et al.8d,e showed that dialkylaniline donor groups (the terminal groups of P1 and P2’s side chain) on aromatic cruciforms could complex with all three metal ions. And this kind of binding originated from the interaction between the free electron pairs of the dialkylaniline groups and the metal ions, which is very similar to the interaction between the acid protons and the basic dialkylaniline groups.8e The binding strength is determined by the size of the charge density (hardness/softness) of the metal ions and it has been shown that Ca2+, Mg2+, and Zn2+ have similar binding properties with (dibutylamino)aniline groups on the chromophore,8e which makes it difficult to differentiate among them if intensity change is the only indicator provided by these chromophores. However, it is interesting to note that both P1 and P2 show large intensity change and shift of their emission peaks when they bind with Ca2+, Mg2+ and Zn2+. 3.1. Metal Ion Sensing Properties of P1. As shown in Figure 2a, upon coordination with a small amount of Zn2+ (8.5 × 10-7M), the emission of P1 (5.2 × 10-6 M) at 490 nm is quenched by almost half (∼59%). When the concentration of Zn2+ was increased to 1.7 × 10-6 M, the emission of P1 was almost completely quenched (∼90%). However, when the concentration of Zn2+ was further increased (more than 3.4 × 10-6 M), the emission of P1 was surprisingly enhanced and blue-shifted to 415 nm. This indicates that the P1-Zn2+ complex possesses a larger band gap than that of P1. Figure 2b shows the UV-vis spectra of P1 upon the addition of different concentrations of Zn2+. The peak absorbance of P1 (at 356 nm) not only decreased but also showed an obvious blue shift of its absorption band edge. This is consistent with the significantly blue-shifted emission of the P1-Zn2+ complex. On the contrary, the addition of Mg2+ and Ca2+ only quenched the emission of P1. As shown in Figure 3a, when Mg2+ (1.7 × 10-6 M) was added to the solution of P1 (5.2 × 10-6 M), the emission of P1 was only slightly quenched (∼15%), which indicates that P1 can form weaker binding with Mg2+. When the concentration of Mg2+ was increased, the emission of P1 was further quenched. Nevertheless, there is no blue-shifted emission at around 415 nm even when the concentration of Mg2+ exceeds 5.0 × 10-5 M, which is almost 10 times higher than the concentration for P1 (by repeating units) (Figure 3a). The titration of P1 with Ca2+ gave a similar emission quenching result (Figure 3b). Scheme 3 elucidates the possible mechanism for the dramatic emission change when P1 binds with different metal ions. There are two possible species that may coexist on the polymer chain of P1 when it binds with metal ions: P1 (free species) and

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TABLE 1: Summary of Absorption and Photoluminescence Properties of P1 and P2 in Different Solvents toluene P1 P2 a

THF

DCM

λabs (nm)

λem (nm) (η)a

λabs (nm)

λem (nm) (η)

λabs (nm)

λem (nm) (η)

356 338, 412

457 (0.57) 472 (0.71)

354 336, 414

486 (0.31) 485 (0.51)

356 340, 418

490 (0.26) 496 (0.39)

Quantum yields were measured relative to 9,10-diphenylanthracene in cyclohexane.

Figure 2. Emission spectra (a) and UV-vis spectra (b) of 5.2 × 10-6 M P1 upon addition of zinc perchlorate in DCM (λex ) 375 nm).

P1:1M (1:1 complex). When there are only a small number of metal ions in the solution, the two species P1 and P1:1M coexist at the same time. Because P1:1M has a larger band gap than the P1 species, there exists an efficient energy transfer between these two species. As a result, it only shows the exclusive emission from P1 but with a decreased PL intensity. However, when the metal ion concentration increases, the P1:1M species started to dominate the whole emission in solution. Therefore, the whole sensing system showed both a blue-shifted and an enhanced emission. It is expected that Zn2+, the softest one among all three metal ions, binds better with the basic dibutylamino groups than Mg2+ and Ca2+.8e This is evident since even under very high Mg2+ and Ca2+ concentrations, there are still some free P1 species that exist on the polymer chain. It is known that if there are some narrow band gap (NBG) units available on the conjugated polymer’s backbone, the NBG units act as the low-energy traps for the whole polymer chain. This allows efficient energy transfer along the polymer chain and the emission from the NBG units will dominate the emission of the whole polymer system.16 Thus, the available P1 species act as an efficient narrow band gap trap on the polymer main chains and the sensing system shows only the exclusive emission from P1. It should be pointed that the added metal ions have the possibility of interacting with different amino groups on either the same polymer chain or different polymer chains. This may cause aggregation or bending of the polymer chains thus quench the emission of P1. The aggregation/bending-induced quenching is a common problem for the conjugated polymerbased sensors and it significantly affects their selectivity.6

Figure 3. Emission spectra of 5.2 × 10-6 M P1 upon addition of magnesium perchlorate (a) and calcium perchlorate (b) in DCM (λex ) 375 nm).

However, when Zn2+ ion was added, the emission of P1 showed both the dramatic intensity change and peak shift. By monitoring the change of blue emission of P1 (415 nm), the selective binding of Zn2+ ions can be differentiated from that of Mg2+ and Ca2+, because those two ions do not cause any observable emission change (Figure 3). 3.2. Metal Ion Sensing Properties of P2. Compared to P1, P2 has a more extended side chain with two conjugated metal ion sensing groups connected by the distyrylbenzene bridge, and it shows more interesting emission changes upon titration with different metal ions. As shown in Figure 4a, the emission spectra of P2 showed more sensitive changes when Zn2+ was added. Upon coordination with a small amount of Zn2+ (8.5 × 10-7M), the emission of P2 (496 nm) is significantly quenched and red-shifted to 570 nm. However, when the concentration of Zn2+ was increased (more than 3.4 × 10-6 M), the emission of P2 was dramatically enhanced and blue-shifted to 440 nm. Figure 4b showed the change of the absorbance spectra of P2 upon the addition of Zn2+. As the titration continued, the ICT peak at around 418 nm disappeared and the absorbance at around 340 nm intensified as the Zn2+ concentration was increased. Similar spectral shifts were also observed in two-photon small molecular metal ion sensing fluorophores, which is due to the deactivation of the terminal donor groups upon interacting with metal ions.11a Figure 5a shows the change of emission of P2 upon adding Mg2+. When a small amount of Mg2+ (8.5 × 10-7M) was added, the emission of P2 was slightly quenched

Synthesis of Cross-Conjugated Polymers

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SCHEME 3: Effects of Coordinated Metal Ions Number on the Emission Color of P1 and P2

and broadened, indicating a weaker binding strength than that of Zn2+. The emission of P2 was further quenched and redshifted when the concentration of Mg2+ was increased. However, there is no blue shift of P2’s emission regardless of the amount of Mg2+ added (even as high as 1.0 × 10-4 M). Ca2+ also causes similar quenching and a red shift of P2’s emission as those derived from binding with Mg2+ (Figure 5b). Scheme 3 also depicted the possible reason for the red- and/ or blue-shifted PL emission of P2 upon binding with different metal ions. Compared to P1, P2 has three possible components that can coexist when interacting with metal ions: P2 (free species), P2:1M (1:1 complex), and P2:2M (1:2 complex). In the beginning of titration, the P2:1M complex was formed and it dominated the emission of the sensing solution. This P2:1M complex has a D-π-DM structure, which is very different from that of P1:1M. Recent studies indicate that the amino group at the end of the side chain will change its nature from electron donating to weak electron accepting when it binds with a metal ion.11e,17 Thereby, upon binding with 1 equiv of metal ion, P2: 1M forms a D-π-A structure that shows a significant red shift in its emission (to ∼570 nm). With further increase of the metal ion concentration, P2:2M becomes the dominating species in the sensing system and the emission peak blue shifts to 440 nm. In the cases of Mg2+ and Ca2+, there are still many P2:1M species available even under high concentration of metal ions, due to their relatively weak binding strength with P2. Hence, the whole sensing system only exhibits a red-shifted emission from the low-energy species, P2:1Mg2+ (or P2:1Ca2+). 3.3. Metal Ion Sensing Properties Study of Monomers. To gain a better understanding of the dramatic change of polymer emissions upon coordinating with different metal ions, a small model compound of P2, 1,4-{bis(4-[N,N-(dibutylamino)styryl])}-

benzene (M2),12 was used as the sensor material. M2 possesses two binding sites for metal ions so it can potentially coordinate with either one or two metal ions in solution. It has been reported earlier that this kind of chromophore possesses very different fluorescent properties when binding with different equivalents of metal ions.11a,14a The formation of a (1:1) chromophore:metal ion (or proton) complex causes a red-shifted and slightly quenched emission, whereas the formation of a (1:2) chromophore:metal ion complex causes a blue-shifted and enhanced chromophore emission. This is because the original amino group has changed its nature from the original electron donating to electron withdrawing upon binding with metal ions.11,17 As shown in Figure 6a, M2 (2.7 × 10-6 M) shows a very broad and quenched emission upon interacting with Zn2+ (1.7 × 10-6 M). This indicates that all three components (M2, M2: 1Zn2+ and M2:2Zn2+) coexist in the solution. When the concentration of Zn2+ was increased, the M2:2Zn2+ complex became the main component among all three species. As a result, the emission not only blue-shifted but showed higher intensity, which is very similar to that observed for P2. However, when Mg2+ was added (Figure 6b), M2 showed a broad emission even under high Mg2+ concentration (2.0 × 10-5 M), indicating that all three species (M2, M2:1Mg2+, and M2:2Mg2+) are present in the solution due to weaker binding between M2 and Mg2+. These results show that the complexation of metal ions with two binding sites of M2 (or every repeating unit of P2) occurs at the same time. However, there is no emission broadening observed in the P2 sensor system. As was shown in Figure 5a, P2 exhibited a completely red-shifted emission peak even under high Mg2+ concentration (1.0 × 10-4 M). There is no emission observed in the blue region as the sign of P2:2Mg2+ formation. This can be explained that the conjugated backbone of P2 allows

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Figure 4. Emission spectra (a) and UV-vis spectra (b) of 2.7 × 10-6 M P2 upon addition of zinc perchlorate in DCM (λex ) 385 nm).

Figure 5. Emission spectra of 2.7 × 10-6 M P2 upon addition of magnesium perchlorate (a) and calcium perchlorate (b) in DCM (λex ) 385 nm).

exciton migration over a long distance; therefore, it provides the opportunity for efficient energy transfer from higher energy segments (such as P2:2Mg2+ or P2) to a lower energy segment (such as P2:1Mg2+). As a result, only the red-shifted emission could be observed. The result in Figure 6 demonstrates the importance of main chain conjugation of P1 and P2 on the overall sensing

Huang et al.

Figure 6. Emission spectra of 2.7 × 10-6 M M2 upon addition of zinc perchlorate (a) and magnesium perchlorate (b) in DCM (λex ) 385 nm).

performance. It allows efficient energy transfer between different species when P1 and P2 interact with different metal ions. Consequently, P1 and P2 show only the emission from the dominating species instead of the broadened emission from all species as observed for M2. This favors better sensing selectivity. It is interesting to note that the sensing performance of P1 and P2 is very different from that of most of the metal ion sensors based on linear conjugated polymers which only show either quenching or emission intensity change upon binding with metal ions. Most of the conjugated polymer sensors only show the “turn-off” (quenching) or “turn-on” (enhancing) effect upon the addition of analytes, which follow a conventional “SternVolmer” relationship.1,2 However, the sensors based on P1 and P2 show both emission intensity change and dramatic red- or/ and blue-shifted PL emission upon binding with metal ions. This improves both the sensitivity and selectivity of metal ion sensing.9 It is worth noting that P1 and P2 show different binding results with Zn2+ compared to those with Mg2+ or Ca2+ which render them good candidates for biological applications. This kind of binding selectivity could be adjusted easily by tuning the metal ion binding groups on the side chains of the cross-conjugated polymers, which is straightforward by standard organic synthesis.7 4. Two-Photon Absorption and Sensing. The strong ICT character of P2 also renders itself with an efficient TPA property. Figure 7 shows the TPA spectra of P2 in different solvents, which were measured by using the two-photon-excited fluorescence (TPEF) method with fluorescein in water (pH 11) as reference.18 It was found that P2 possesses a large TPA cross section (δ value of 1440 GM, 1 GM ) 1 × 10-50 cm4‚s‚photon-1‚molecule-1) in THF, while it shows a decreased δ in toluene and DCM, which is consistent with the results recently reported by Bazan et al. on small distyrylbenezne chromophores.14a The reasons for the solvent effect on TPA chromophores are quite complicated and many factors, such as solute-solvent interactions, changes in the chromophores

Synthesis of Cross-Conjugated Polymers

J. Phys. Chem. C, Vol. 111, No. 28, 2007 10679 the red-shifted emission. When more Zn2+ was added (>5.1 × 10-6 M), most of the residual neutral P2 species coordinated with Zn2+ and the emission of the whole system was completely quenched (Figure 8). This shows that TPA properties of P2: 1Zn2+ and P2:2Zn2+ species were greatly reduced when bind with metal ions. This trend correlates very well with the results obtained from small molecule-based TPA metal ion sensors.11 It also proves that metal ion sensing from P2 could be measured under the excitation of a NIR light. This phenomenon has never been reported before for conjugated polymer-based sensors. 5. Conclusion

Figure 7. TPA spectra of P2 in different solvents.

Figure 8. TPEF spectra of 5.0 × 10-6 M P2 upon adding zinc perchlorate in DCM (λex ) 800 nm).

geometry, and multichromophore aggregation, can all affect the ICT states.14a Nevertheless, the result from Figure 7 indicates that both the UV light and the NIR light can be used to excite P2, which offers better flexibility in choosing a suitable excitation light source when P2 is used as a sensor material under different environments. It avoids the potential damage to the substrates caused by shorter wavelength excitation (350560 nm).10 Figure 8 shows the TPEF spectra of P2 (5.0 × 10-6 M) upon titrating with Zn2+ under NIR light (800 nm). The emission of P2 gradually red-shifted and quenched upon the addition of Zn2+, which agrees fairly well with the results obtained from one-photon metal ion sensing. However, there is no blue emission that can be observed even under very high Zn2+ concentration (1.0 × 10-5 M), which is very different from that obtained in one-photon fluorescence spectra of P2 (Figure 4a). Such a big difference in one-photon and two-photon sensing has also been reported by Liu et al.11d They observed an enhanced blue-shifted emission of an organoboron compound upon complexation with fluoride anion, whereas its original TPA property has completely disappeared. The metal ion complexation induced a significantly decreased TPA cross section in small molecule-based two-photon chromophores. Pond et al.11a reported that the TPA cross section of bis-aza-crown ether substituted distyrylbenzene is reduced by almost an order of magnitude upon binding with Mg2+ ion when measured at the peak wavelength. Kim et al.11b also found that an aza-crown ether functionalized TPA chromophore’s TPEF intensity decreases gradually upon adding metal ions. The TPEF spectra of P2 not only showed a gradual decrease but also a significantly red shift upon the addition of Zn2+. The red-shifted emission from P2:1Zn2+ indicates that the TPEF spectrum of P2 is still dominated by the low-energy species. When excited, the efficient energy transfer from P2 to P2:1Zn2+ contributes to

In summary, two new cross-conjugated polymers P1 and P2 have been designed and synthesized for studying their responses in metal ion sensing. These polymers show totally different characteristics when they bind with metal ions compared to those observed for traditional linear conjugated polymers. Both P1 and P2 show significant changes in their fluorescence intensity and emission peak shift upon coordinating with different metal ions. This enhances their sensitivity and selectivity in detecting trace metal ions. Among three kinds of physiologically important metal ions for binding with P1 and P2, only Zn2+ introduces significantly enhanced blue-shifted emission, whereas Mg2+ and Ca2+ only show quenched or/and red-shifted emission, due to their weaker binding strength than Zn2+. The control experiments based on using a small molecule M2 (which is the sensing component of P2) indicate that the conjugation of P1 and P2 plays an important role in enhancing the selectivity for metal ion sensing. The D-π-D conjugated side chains on P2 endow it with efficient TPA property and enable its use in two-photon laser scanning microscopy. Considering the flexibility of modifying the chemical structures of these cross-conjugated conjugated polymers, this new architecture of conjugated polymers provides an excellent avenue to improve both sensitivity and selectivity of CP sensors. Experimental Section General Details. 9,9-Dihexylfluorene-2,7-bistrimethyleneborate was purchased from Aldrich and recrystallized from hexane before use. All reagents, unless otherwise specified, were obtained from Aldrich and used as received. All the solvents used were further purified before use. 1H and 13C NMR spectra were measured with a Bruker 500 spectrometer operating at 125 MHz for 13C and 500 MHz for 1H in deuterated chloroform solution with TMS internal standard as a reference for chemical shifts. UV-vis spectra were recorded with a Hewlett-Packard 8452A Diode Array UV-vis spectrophotometer. One-photon fluorescence spectra were recorded with a Perkin-Elmer Luminescence Spectrometer LS 50B, using a Xenon lamp as a light source, with the emission and excitation slit of 5 nm. Twophoton excitation spectra were measured by using the twophoton induced fluorescence technique18 with a mode-locked Ti:Sapphire laser excitation source (Coherent, Mira 900). The laser provides a pulse of approximately 120 fs pulse width at a pulse repetition frequency of 76 MHz in the wavelength range of 730 to 1000 nm. The pumping wavelengths were determined by a monochromator-CCD system. Fluorescein in pH 11 water was used as a reference (r). The two-photon absorption cross section of a sample compound (s) can be calculated at each wavelength according to

δs )

SsηrφrCr δ SrηsφsCs r

(1)

10680 J. Phys. Chem. C, Vol. 111, No. 28, 2007 where S is the detected two-photon induced fluorescence signal, η is the fluorescence quantum yield, C is the concentration of the chromophore, and φ is the overall fluorescence collection efficiency of the experimental apparatus. The concentrations of the solutions were in the range of 5 × 10-6 to 6 × 10-6 M. The measurements were conducted in an intensity regime where the fluorescence signal showed a quadratic dependence on the intensity of the excitation beam. The uncertainty in the measured cross sections is about 15%. 1,4-Dibromo-2-(4-[N,N-(dibutylamino)styryl])benzene (4). To a mixture of 4-(N,N-dibutylamino)benzaldehyde (1) (1.0 g, 4.29 mmol) and 2-diethylphosphonatomethyl-1,4-dibromobenzene (2) (1.5 g, 3.89 mmol) in 50 mL of dry THF was added 4.5 mL of 1 M potassium tert-butoxide methanol solution. The reaction mixture was stirred overnight at room temperature and then was diluted with dichloromethane and water. The two phases were separated, and the water phase was extracted twice with dichloromethane. The combined organic extracts were washed three times with water, dried over magnesium sulfate, evaporated, and purified with column chromatography (silica gel, hexane/dichloromethane (4/1) as eluent to yield 1.20 g (67%) of 1,4-dibromo-2-(4-[N,N-(dibutylamino)styryl])benzene (4) as a yellowish oil. 1H NMR (500 MHz, CDCl3) δ (ppm) 7.83 (d, J ) 2.5 Hz, 1H), 7.49 (d, J ) 9.0 Hz, 2H), 7.45 (d, J ) 8.5 Hz, 1 H), 7.22-7.19 (m, 2H), 7.03 (d, J ) 16.0 Hz, 1H), 6.71 (d, J ) 9.0 Hz, 2H), 3.38 (t, J ) 7.5 Hz, 4H), 1.69-1.64 (m, 4H), 1.48-1.43 (m, 4H), 1.08-1.03 (m, 6H). 13C NMR (125 MHz, CDCl3) δ (ppm) 148.28, 139.84, 134.10, 132.79, 130.09, 128.54, 128.34, 123.51, 121.94, 121.31, 120.54, 111.42, 50.67, 29.38, 20.27, 13.98. 1,4-Dibromo-2,5-{bis(4-[N,N-(dibutylamino)styryl])}benzene (5). To a mixture of 4-(N,N-dibutylamino)benzaldehyde (1) (1.2 g, 5.15 mmol) and 2,5-bis(diethylphosphonatomethyl)1,4-dibromobenzene (3) (1.0 g, 1.80 mmol) in 50 mL of dry THF was added 5.0 mL of 1 M potassium tert-butoxide methanol solution. The reaction mixture was stirred overnight at room temperature and then was diluted with dichloromethane and water. The two phases were separated, and the water phase was extracted twice with dichloromethane. The combined organic extracts were washed three times with water, dried over magnesium sulfate, evaporated, and purified with column chromatography (silica gel, hexane/dichloromethane (4/1) as eluent) to yield 0.72 g (58%) of 1,4-dibromo-2,5-{bis(4-[N,N(dibutylamino)styryl])}benzene (5) as an orange yellow powder. 1H NMR (500 MHz, CDCl ) δ (ppm) 7.86 (s, 2H), 7.46 (d, J 3 ) 8.5 Hz, 4H), 7.17 (d, J ) 16.0 Hz, 2H), 7.00 (d, J ) 16.0 Hz, 2H), 6.68 (d, J ) 9.0 Hz, 4H), 3.35 (t, J ) 7.5 Hz, 8H), 1.67-1.61 (m, 8H), 1.46-1.38 (m, 8H), 1.04-1.01 (m, 12H). 13C NMR (125 MHz, CDCl3) δ (ppm) 148.21, 137.02, 131.75, 129.41, 128.28, 123.87, 122.52, 120.59, 111.50, 50.75, 29.43, 20.32, 13.99. Poly[(9,9-di-n-hexylfluorene-2,7-diyl)-alt-co-(2-(4-(N,N(dibutylamino)styryl)benzene)-1,4-diyl)] (P1). 9,9-Dihexylfluorene-2,7-bistrimethyleneborate (251 mg, 0.5 mmol), 1,4dibromo-2-(4-[N,N-(dibutylamino)styryl])benzene (4) (233 mg, 0.5 mmol), and Pd(PPh3)4 (5 mg) were placed in a 25 mL roundbottomed flask. A mixture of 2 M K2CO3 aqueous solution (3 mL) and toluene (5 mL) were added to the flask and the reaction mixture was degassed. The mixture was refluxed with vigorous stirring for 3 days under nitrogen atmosphere. After the mixture was cooled to room temperature, it was poured into 200 mL of methanol. The precipitated material was recovered by filtration through a funnel. The resulting solid material was washed for 24 h with acetone to remove oligomers and catalyst

Huang et al. residues (0.28 g, 88%). 1H NMR (500 MHz, CDCl3) δ (ppm) 8.11 (s, 1H), 7.91-7.09 (m, 12H), 6.62-6.61 (m, 2H), 3.30 (m, 4H), 2.08 (m, 4H), 1.59 (m, 8H), 1.40-1.35 (m, 4H), 1.15 (m, 12H), 1.02-0.91 (m, 6H), 0.85-0.79 (m, 6H). Poly[(9,9-di-n-hexylfluorene-2,7-diyl)-alt-co-(2,5-bis(4-(N,N(dibutylamino)styryl)benzene)-1,4-diyl)] (P2). 9,9-Dihexylfluorene-2,7-bistrimethyleneborate (251 mg, 0.5 mmol), 1,4dibromo-2,5-{bis(4-[N,N-(dibutylamino)styryl])}benzene (5) (347 mg, 0.5 mmol), and Pd(PPh3)4 (5 mg) were placed in a 25 mL round-bottomed flask. A mixture of 2 M K2CO3 aqueous solution (3 mL) and toluene (5 mL) were added to the flask and the reaction mixture was degassed. The mixture was refluxed with vigorous stirring for 3 days under nitrogen atmosphere. After the mixture was cooled to room temperature, it was poured into 200 mL of methanol. The precipitated material was recovered by filtration through a funnel. The resulting solid material was washed for 24 h with acetone to remove oligomers and catalyst residues (0.31 g, 72%). 1H NMR (500 MHz, CDCl3) δ (ppm) 7.94-7.85 (m, 4H), 7.59-7.10 (m, 12H), 6.60-6.58 (m, 4H), 3.28 (m, 8H), 2.08 (m, 4H), 1.59 (m, 12H), 1.41-1.34 (m, 8H), 1.15 (m, 12H), 1.02-0.98 (m, 12H), 0.86-0.77 (m, 6H). Acknowledgment. Financial support from the National Science Foundation (NSF-STC Program under Agreement No. DMR-0120967), the Air Force office of Scientific Research (AFOSR) under the Bio-inspired Concept, the Microscale Life Sciences Center (an NIH Center of Excellence), and the BoeingJohnson Foundation is acknowledged. References and Notes (1) (a) Zhou, Q.; Swager, T. M. J. Am. Chem. Soc. 1995, 117, 12593. (b) Swager, T. M. Acc. Chem. Res. 1998, 31, 201. (c) Leclerc, M. AdV. Mater. 1999, 11, 1491. (d) Chen, L.; Mcbranch, D. W.; Wang, H. L.; Helgeson, R.; Wudl, F.; Whitten, D. G. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 12287. (e) Heeger, P. S.; Heeger, A. J. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 12219. (f) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. ReV. 2000, 100, 2537. (g) Harrison, B. S.; Ramey, M. B.; Reynolds, J. R.; Schanze, K. S. J. Am. Chem. Soc. 2000, 122, 8561. (h) Bunz, U. H. F. Chem. ReV. 2000, 100, 1605. (i) Ho, H. A.; Boissinot, M.; Bergeron, M. G.; Corbeil, G.; Dore´, K.; Boudreau, D.; Leclerc, M. Angew. Chem. 2002, 41, 1548. (j) Pinto, M. R.; Schanze, K. S. Synthesis 2002, 9, 1293. (k) Rose, A.; Zhu, Z. G.; Madigan, C. F.; Swager, T. M.; Bulovic, V. Nature 2005, 434, 876. (l) Liu, B.; Bazan, G. C. J. Am. Chem. Soc. 2006, 128, 1188. (2) (a) Korri, Y. H.; Garnier, F.; Srivastava, P.; Godillot, P.; Yassar, A. J. Am. Chem. Soc. 1997, 119, 7388. (b) Ba¨uerle, P.; Emge, A. AdV. Mater. 1998, 10, 324. (c) DiCesare, N.; Pinto, M. R.; Schanze, K. S.; Lakowicz, J. R. Langmuir 2002, 18, 7785. (d) Kim, T.; Swager, T. M. Angew. Chem., Int. Ed. 2003, 42, 4803. (e) Nilsson, K. P. R.; Ingana¨s, O. Nat. Mater. 2003, 2, 419. (f) Kumaraswamy, S.; Bergstedt, T.; Shi, X.; Rininsland, F.; Kushon, S.; Xia, W. S.; Ley, K.; Achyuthan, K.; McBranch, D.; Whitten, D. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 7511. (g) Pinto, M. R.; Schanze, K. S. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 7505. (h) Disney, M. D.; Zheng, J.; Swager, T. M.; Seeberger, P. H. J. Am. Chem. Soc. 2004, 126, 13343. (i) Ho, H. A.; Bers-Aberem,M.; Leclerc, M. Eur. J. Chem. 2005, 11, 1718. (j) Wosnick, J. H.; Mello, C. M.; Swager, T. M. J. Am. Chem. Soc. 2005, 127, 3400. (k) Le Floch, F.; Ho, H. A.; Harding, L. P.; Bedard, M.; Neagu, P. R.; Leclerc, M. Adv. Mater. 2005, 17, 1251. (l) Nilsson, K. P. R.; Olsson, J. D. M.; Stabo-Eeg, F.; Lindgren, M.; Konradsson, P.; Ingana¨s, O. Macromolecules 2005, 38, 6813. (m) Herland, A.; Nilsson, K. P. R.; Olsson, J. D. M.; Hammarstrom, P.; Konradsson, P.; Ingana¨s, O. J. Am. Chem. Soc. 2005, 127, 2317. (n) Kim, I. B.; Wilson, J. N.; Bunz, U. H. F. Chem. Commun. 2005, 10, 1273. (3) (a) Gaylord, B. S.; Heeger, A. J.; Bazan, G. C. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 10954. (b) Gaylord, B. S.; Heeger, A. J.; Bazan, G. C. J. Am. Chem. Soc. 2003, 125, 896. (c) Liu, B.; Wang, S.; Bazan, G. C.; Mikhailovsky, A. J. Am. Chem. Soc. 2003, 125, 13306. (d) Wang, S.; Bazan, G. C. Adv. Mater. 2003, 15, 1425. (e) Liu, B.; Bazan, G. C. J. Am. Chem. Soc. 2004, 126, 1942. (f) Liu, B.; Bazan, G. C. Chem. Mater. 2004, 16, 4467. (g) Liu, B.; Bazan, G. C. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 589. (h) Xu, H.; Wu, H.; Huang, F.; Song, S.; Li, W.; Cao, Y.; Fan, C. Nucleic Acids Res. 2005, 33, e83.

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