Synthesis, Self-Assembly, and Photophysical Properties of Cationic

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Synthesis, Self-Assembly, and Photophysical Properties of Cationic Oligo(p-phenyleneethynylene)s Yanli Tang,† Eric H. Hill,† Zhijun Zhou,†,‡ Deborah G. Evans,*,‡ Kirk S. Schanze,*,§ and David G. Whitten*,† †

Department of Chemical and Nuclear Engineering, Center for Biomedical Engineering, and ‡Department of Chemistry and Chemical Biology, University of New Mexico, Albuquerque, New Mexico 87131-1341, United States § Department of Chemistry, University of Florida, Gainesville, Florida 32611-7200, United States

bS Supporting Information ABSTRACT: Three series of cationic oligo p-phenyleneethynylenes (OPEs) have been synthesized to study their structure-property relationships and gain insights into the transition from molecular to macromolecular properties. The absorbance maxima and molar extinction coefficients in all three sets increase with increasing number of repeat units; however, the increase in λmax between the oligomers having 2 and 3 repeat units is very small, and the oligomer having 3 repeat units shows virtually the same spectra as a p-phenyleneethynylene polymer having 49 repeat units. A computational study of the oligomers using density functional theory calculations indicates that while the simplest oligomers (OPE-1) are fully conjugated, the larger oligomers are nonplanar and the limiting “segment chromophore” may be confined to a near-planar segment extending over three or four phenyl rings. Several of the OPEs selfassemble on anionic “scaffolds”, with pronounced changes in absorption and fluorescence. Both experimental and computational results suggest that the planarization of discrete conjugated segments along the phenylene-ethynylene backbone is predominantly responsible for the photophysical characteristics of the assemblies formed from the larger oligomers. The striking differences in fluorescence between methanol and water are attributed to reversible nucleophilic attack of structured interfacial water on the excited singlet state.

’ INTRODUCTION There has been growing interest in the synthesis of welldefined conjugated oligomers.1-6 While conjugated polymers and polyelectrolytes have been the subject of much investigation,7-14 the polymers consist of a mixture of molecules with a broad range of molecular weights due to the statistical nature of the polymerization processes.1,15-18 The precisely characterized oligomers often have well-defined chemical and physical properties, and they also serve as important model systems to provide a basis for determining the structure-property relationships of the larger polymers.1,3 The newer synthetic strategies based on the Sonogashira cross-coupling reaction and its modifications have provided possibilities for the design and synthesis of rigid molecular systems based on the p-phenyleneethynylene spacer group.4,19 In previous research, poly(p-phenyleneethynylene)s (PPE)s were explored for their use in fluorescence-based sensing and as light-activated antimicrobial agents.13,14,20-32 To investigate structure-reactivity relationships through photophysical and antimicrobial properties, we designed and synthesized three series of well-defined cationic oligomers with different chain lengths and different end groups on the main chain. The structure of these oligomeric p-phenyleneethynylenes1,2,4,10,19,33 (OPEs) are shown in Scheme 1. The OPE-n (n = 1, 2, 3) oligomers were synthesized first and studied both in r 2011 American Chemical Society

solution and attached onto solid surfaces by covalent linkages. To investigate the effect of carboxyester end groups on the photophysical, self-assembly, and antimicrobial properties, the other two sets of oligomers S-OPE-n (H) (with a hydrogen on both ends) and S-OPE-n (COOEt) (with a carboxyester group on both ends) were synthesized. We have published preliminary reports of photophysical properties and complexation with anionic scaffolds of smaller oligomers.34,35 In these studies two major questions were raised that could not be completely resolved. It was noted that fluorescence quantum yields of certain OPEs are much higher in methanol than in water. Specifically, the presence of carboxyester end groups greatly decreased the fluorescence of OPE’s in water, but complexation with anionic scaffolds results in shifting of the absorption and fluorescence and increase of fluorescence. A second unresolved issue was whether the shifts in fluorescence upon complex formation could be attributed to J-aggregate formation or to planarization. A resolution to both of these questions is proposed in this paper. Herein we report the photophysical properties, complex formation with carboxymethylcellulose (CMC)36 and carboxymethylamylose Received: December 18, 2010 Revised: February 15, 2011 Published: March 15, 2011 4945

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Langmuir Scheme 1. Structures of OPEs

(CMA),37,38 and fluorescence quenching by the anionic electron acceptor 9,10-anthraquinone-2,6-disulfonic acid(AQS).39-41 Self-assembly on these anionic “scaffolds” results in a red shift of the absorption, which we attribute to segment planarization of larger OPEs. Computational studies using density functional theory (DFT) calculations have previously shown reliable results with similar oligo(phenylenethynylene)s.5,42 To further understand the relationship between structure and photophysical properties, DFT calculations were performed using the Gaussian 03 software package.43 The results of the computational study, taken together with the experimental results, provide a consistent explanation for the structure, selfassembly and photophysics of this interesting series of cationic conjugated oligomers.

’ EXPERIMENTAL METHODS Synthesis of OPEs. The synthesis of S-OPE-1 (H), S-OPE1(COOEt), OPE-1, and OPE-2 was reported in the previous communications.34,35 The synthesis of OPE-3 is provided as Supporting Information. The summary (Scheme S1, Supporting Information) of synthesis of OPEs indicates that it is more difficult to synthesize unsymmetrical OPEs than symmetrical OPEs. Compounds 8 and 11 were synthesized as intermediates to OPE-2 and OPE-3. The yield of compound 11 is relatively low since the double-substituted byproduct was formed more readily. Scheme S2, in the Supporting Information, shows the summary of the synthesis of S-OPE-n (H) and S-OPE-n (COOEt) (n = 1, 2, 3). The intermediates and final products were characterized by 1H and 13C NMR and mass spectrometry. Materials and Instruments. Carboxymethylcellulose (CMC, Mw 90 000, DS 0.70), carboxymethylamylose (CMA), and 9,10-anthraquinone-2,6-disulfonic acid (AQS) were purchased from Sigma-Aldrich (St. Louis, MO). The precursors used to synthesize the OPEs were purchased from Aldrich or Alfa-Aesar (see Schemes S1 and S2 in the Supporting Information). The UV-vis absorbance and fluorescence measurements were carried out in a 3 mL quartz cuvette at room temperature using a Spectramax M5 spectrometer. Circular dichroism spectra were recorded on an Aviv CD spectrometer in a 3 mL quartz cuvette. Water was purified using a Millipore filtration system with a resistivity of 18.2 MΩ 3 cm-1. Self-assembly Experiments. The self-assembly experiments for all compounds were performed as follows: successive addition of aliquots of (aliquot volume was increased from 0.9 to 1.5 μL gradually) 1.0
10-2 M CMC or CMA to a 3 mL solution of the OPE (1.5
10-5 M) in water at room temperature, and the UV-vis absorption and fluorescence spectra were measured. Fluorescence Quenching Experiments. The fluorescence quenching experiments for all compounds were performed as follows: successive addition of aliquots of (aliquot volume was increased from 1.5

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to 6.0 μL gradually) 1.0
10-3 M AQS to a 3 mL solution of the OPE (1.5
10-5 M) in water at room temperature, and the UV-vis absorption and fluorescence spectra were measured. Measurements of Circular Dichroism. Aliquots of (aliquot volume was increased from 0.9 to 1.5 μL gradually) 1.0
10-2 M CMC or CMA was added successively to a 3 mL solution of the OPE (1.5
10-5 M), and the circular dichroism (CD) spectra were measured at room temperature. Computational Methods. The oligomers were modeled theoretically using electronic structure methods at the density functional theory (DFT) level. Starting with several conformations, DFT calculations were used to generate optimized ground state geometries and molecular orbital information. The initial geometries were first optimized at the semiempirical AM1 level. Further optimizations of the ground state were done at the DFT level of theory. All density functional calculations were performed using the hybrid B3LYP (Becke, threeparameter, Lee-Yang-Parr) exchange-correlation energy functional. The exchange term of B3LYP consists of hybrid Hartree-Fock and local spin density (LSD) exchange functions with Becke’s gradient correlation to LSD exchange. This level of theory provides an efficient method of accounting for electron correlation in larger molecules and oligomers with reasonable computational cost and resources. Successive geometry optimizations with DFT were completed using the 3-21g and the 6-31g basis sets, ultimately arriving at the 6-31 g** basis set. To ensure that a global minimum was reached for the optimized structures, the frequencies of the molecular vibrations were also calculated. The frontier molecular orbitals (the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO)) were also analyzed at the B3LYP/6-31g** level. In addition, the HOMO and LUMO of a forced-planarized S-OPE-2(H) were calculated to compare conjugation with the unconstrained ground state. For all calculations, the Gaussian 03 and Gaussview 4 software packages by Gaussian Inc. were used.43

’ EXPERIMENTAL RESULTS Photophysical Properties of OPEs. OPE-n, S-OPE-n (H), and S-OPE-n (COOEt) (n = 1, 2, 3) were dissolved readily in H2O and in organic solvents such as methanol. The absorption and emission spectra of S-OPE-n (COOEt) in H2O and methanol are shown in Figure 1. The absorption and emission spectra of OPE-n and S-OPE-n (H) are provided as Supporting Information (Figures S1 and S2). Table 1 provides a summary of absorption and emission maxima for all OPEs that are characterized by similar two-banded absorption spectra in both water and methanol. The absorbance maxima of S-OPE-n (COOEt) in methanol are red-shifted 2-6 nm compared to the values in H2O. All compounds in methanol show a red shift compared to the values in H2O, as summarized in Table 1. The fluorescence quantum yields of all compounds are listed in Table 2 (quinine sulfate was used as a standard). For the S-OPE-1 (H) similar quantum efficiencies for fluorescence were obtained in both water and methanol; for all other OPE the quantum yields in methanol are moderate to high (0.68-0.87) but much lower in water. A summary of the data from the self-assembly of OPEs with CMC and CMA is shown in Table 3. Self-Assembly of OPEs with CMC. Since OPEs are monomeric in water and in methanol, the possibility that they could self-assemble on anionic scaffolds to form complexes was investigated in our initial research.34,35 Changes occurred in the absorption and emission spectra for the OPE-2 series when CMC was added, as shown in Figure 2 for S-OPE-2 (H) and 4946

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Figure 1. Absorbance and fluorescence spectra of S-OPE-n (COOEt) in H2O (A and B) and methanol (C and D). Excitation wavelengths for S-OPE-1 (COOEt), S-OPE-2 (COOEt), and S-OPE-3 (COOEt) are 362, 378, and 384 nm in H2O, and 366, 380, and 388 nm in methanol, respectively.

Table 1. Absorption and Emission Maxima of OPEs abs in H2O abs in MeOH em in H2O em in MeOH (nm)

(nm)

(nm)

(nm)

OPE-1

303, 355

304, 356

416

438

OPE-2

318, 375

320, 378

442

430

OPE-3

320, 384

322, 388

438

434

S-OPE-1 (H)

303, 348

302, 348

398

392

S-OPE-2 (H)

318, 370

318, 374

454

420

S-OPE-3 (H)

320, 380

320, 384

438

432

S-OPE-1 (COOEt) 314, 362 S-OPE-2 (COOEt) 320, 378

314, 366 320, 380

454 448

436 432

S-OPE-3 (COOEt) 320, 384

320, 388

440

434

S-OPE-2 (COOEt). An induced CD signal was also observed upon complex formation, as shown below in Figure 3. Sharp, redshifted absorbance spectra are observed and the fluorescence increases more than 18-fold for S-OPE-1 (COOEt) upon addition of CMC. A weaker red-shifted absorbance and fluorescence enhancement were detected for OPE-1 with CMC, and a slight absorbance red shift was seen with S-OPE-1 (H). For S-OPE1(H), the fluorescence intensity decreased when aliquots of CMC were added. Figure 2 shows the assembly of S-OPE-2 (H) and S-OPE-2 (COOEt) upon addition of the scaffold CMC. The absorbance maximum red-shifted 54 nm for S-OPE-2 (COOEt) upon addition of CMC and complete conversion was observed at 65 μM. The complex absorbance peaks are

sharper for S-OPE-2 (COOEt) than those observed for S-OPE-2 (H) and CMC, for which the absorbance maximum red-shifted 46 nm. The fluorescence intensity increased over 18-fold for S-OPE-2 (COOEt) after titration by CMC, whereas the enhancement in fluorescence for OPE-2 is about 14-fold and for S-OPE-2 (H) it is less than 3-fold. Similar results were obtained for all oligomers with 3 repeat units. Figure S3 (Supporting Information) shows the increase in fluorescence is 4.2-fold for S-OPE-3 (COOEt), which is larger than that for OPE-3 and S-OPE-3 (H). Self-Assembly of OPEs with CMA. CMA is an anionic chiral polyelectrolyte, where the conformation is a heavily disrupted helix or random coil in water due to electrostatic repulsions.38 In this earlier study it was found that nonfluorescent (in water) cyanine dyes self-assemble on CMA and CMC to form highly fluorescent “J” aggregates and the aggregates exhibit a strong “excitonic” induced CD. We thought that similar aggregation/ self-assembly would occur for the oligomers and CMC and CMA and in fact for the OPE-1’s the spectral changes are similar; however, as will be discussed later, it seems that there are other explanations to consider. Changes of absorption, fluorescence, and induced CD signal similar to those observed upon addition of CMC to the OPE-2 series were observed when CMA was added to the OPE-3 series, as shown in Figures 4 and 5. For the compounds with 1 repeat unit, only S-OPE-1 (COOEt) with CMA causes a 56 nm red-shift of λmax and a 16-fold increase in fluorescence intensity (Figure S5, Supporting Information). Neither S-OPE-1 (H) nor OPE-1 formed a complex and their fluorescence intensity decreased upon titrating 4947

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Table 2. Molar Extinction Coefficients and Fluorescence Quantum Yields and Lifetimes of OPEs ε in H2O (L 3 mol-1 3 cm-1)

j in H2O

j in MeOH

τ in MeOH (ns)

OPE-1

(3.14 ( 0.016)
104

0.15 ( 0.02

0.68 ( 0.03

1.66

OPE-2

(7.39 ( 0.06)
104

0.012 ( 0.008

0.68 ( 0.02

0.89

OPE-3

(1.15 ( 0.008)
105

0.025 ( 0.001

0.74 ( 0.02

0.71

S-OPE-1 (H)

(2.94 ( 0.023)
104

0.64 ( 0.02

0.64 ( 0.02

S-OPE-2 (H)

(6.83 ( 0.031)
104

0.13 ( 0.02

0.73 ( 0.02

S-OPE-3 (H)

(1.16 ( 0.021)
105

0.04 ( 0.02

0.72 ( 0.02

S-OPE-1 (COOEt)

(3.92 ( 0.013)
104

0.023 ( 0.001

0.75 ( 0.02

1.53

S-OPE-2 (COOEt) S-OPE-3 (COOEt)

(8.29 ( 0.033)
104 (1.10 ( 0.004)
105

0.039 ( 0.001 0.069 ( 0.001

0.71 ( 0.01 0.70 ( 0.02

0.89 0.71

Table 3. Absorption and Emission Maxima of OPEs/CMC and OPEs/CMA abs (nm)

em (nm)

conc of oligomer (μM)

conc of scaffold (μM)

OPE-3/CMC

336, 435

S-OPE-2 (H)/CMC S-OPE-3 (H)/CMC

330, 416 332, 429

500

5.0

24

456 485

15 5.0

75 25

S-OPE-2 (COOEt)/CMC S-OPE-3 (COOEt)/CMC

336, 432

460

15

65

334, 438

462

5.0

30

OPE-3/CMA

332, 434

493

5.0

20

S-OPE-2 (H)/CMA

326, 404

466

S-OPE-3 (H)/CMA

330, 408

487

S-OPE-1 (COOEt)/CMA

330, 416

450

15

35

S-OPE-2 (COOEt)/CMA S-OPE-3 (COOEt)/CMA

336, 440 334, 438

466 468

15 5.0

43 26

with CMA. In the series of OPEs with 2 repeat units (Figure S5, Supporting Information) the λmax red-shifted 50 nm (OPE-2) and 62 nm (S-OPE-2 (COOEt)) and the fluorescence intensity increased over 7-fold. However, weak complexes were formed between S-OPE-2 (H) and CMA and the fluorescence intensity increased less than 2-fold. As shown in Figure 4, the strength of complexation between CMA and OPEs with 3 repeat units increased with the number of carboxyester end groups. Upon forming complexes with CMA, the λmax red-shifted 54, 50, and 28 nm for S-OPE-3 (COOEt), OPE-3, and S-OPE-3 (H), respectively. Fluorescence Quenching of OPEs by AQS. AQS as an anionic electron acceptor can quench cationic PPE polymers as was previously reported.35,41,44,45 Furthermore, the quenching efficiency is related to the Stern-Volmer constant, KSV, that is determined by monitoring initial changes in the fluorescence via the Stern-Volmer equation,46 shown below in eq 1, I 0 =I ¼ 1 þ K SV ½Q 

ð1Þ

where I0 is the unquenched fluorescence intensity of OPEs, I is the fluorescence intensity of OPEs in the presence of quencher, and [Q] is the concentration of quencher. The value of KSV for other OPEs can be obtained using the same procedure, and all the values are summarized in Table 4. In each series of OPEs, the KSV values increase with the number of repeat units from 1 to 3, as shown in Table 4. The nonlinearity at higher [AQS] is most likely due to competitive absorption of the irradiating light by the OPE/AQS complex (Figure 6). Computational Results. The structures of the S-OPE-n (H) series optimized by the DFT:B3LYP/6-31g** basis set are shown in Figure 7.

15 5.0

50 25

The frequencies of the vibrational movements of optimized structures were calculated to ensure that a global minimum had been reached, and the resulting calculated frequencies are given in the Supporting Information. The optimized structures in Figure 7 show decreased planarity in the ground state as the number of subunits increases. S-OPE-1(H), the smallest molecule, is nearly planar. The larger oligomers have much larger rotations about the ethynyl group which break the planarity of the backbone. The Cartesian coordinates of the structures are included in the Supporting Information. The frontier orbitals of the S-OPE-n (H) series optimized by the DFT:B3LYP/6-31g** basis set are shown in Figure 8. It is shown in the orbitals in Figure 8 that as the length of the oligomer increases, the effective length of the π-conjugated chromophore is limited. In the smallest oligomer there is complete conjugation, but in the larger oligomers breaks in the π-conjugation are seen. It is shown in Figure 9 for the molecular orbitals of S-OPE-2(H), when optimized with constrained rotation around the triple bond, that the conjugation extends much further than seen with the optimized ground state structure for S-OPE-2(H) seen in Figure 8. It has been shown by Li and co-workers42 that fully planar OPEs have more extended π-conjugation than OPEs with orthogonal segments that break conjugation. This is seen in Figure 8, where the breaks in π-conjugation in the larger oligomers result from the twisted phenylene backbone.

’ DISCUSSION Some of the most interesting points of this study include the change in absorbance and fluorescence of OPEs as a function of 4948

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Figure 2. (A and B) Absorbance and fluorescence spectra of S-OPE-2 (H) upon addition of CMC. [S-OPE-2 (H)] = 1.5
10-5 M, [CMC] = (0-9.5)
10-5 M. (C and D) Absorbance and fluorescence spectra of S-OPE-2 (COOEt) upon addition of CMC. [S-OPE-2 (COOEt)] = 1.5
10-5 M, [CMC] = (0-6.5)
10-5 M. The excitation wavelength for S-OPE-2 (H) is 370 nm and for S-OPE-2 (COOEt) is 378 nm.

Figure 3. (A) CD spectrum of S-OPE-2 (H) upon addition of CMC. [S-OPE-2 (H)] = 1.5
10-5 M, [CMA] = (0-6.5)
10-5 M. (B) CD spectrum of S-OPE-2 (COOEt) upon addition of CMC. [S-OPE-2 (COOEt)] = 1.5
10-5 M, [CMC] = (0-5.5)
10-5 M. (C) CD spectrum of S-OPE-1 (COOEt) upon addition of CMC. [S-OPE-1 (COOEt)] = 1.5
10-5 M, [CMC] = (0-3.5)
10-5 M.

the number of subunits, the differences in OPE fluorescence between methanol and water, the self-assembly of the OPEs on CMC, CMA, and AQS, and the concurrent changes in absorbance, fluorescence, and induced CD spectra. We discuss these results and their significance in the following sections. The absorption spectra of the various oligomers show no concentration dependence in the range of concentrations used in this study, and there are negligible differences in the absorption spectra between the two solvents. In contrast, the fluorescence quantum yields are solvent-dependent, and the yields for all compounds in methanol are much higher than those in H2O, except for S-OPE-1 (H), which has similar fluorescence quantum yields in both H2O and methanol. The λmax of OPEs red shift with increasing number of

repeat units from 1 to 3 in both solvents; however, the change of λmax between OPE-1 and OPE-2 is much larger than that between OPE-2 and OPE-3 for all three series. The shape of the absorption spectra and location of the absorbance maximum show negligible changes on going from OPE-3 to a similarly structured polymer with 49 repeat units.47,48 Therefore, we conclude that there is a limiting “segment chromophore” reached perhaps already with the OPE-2 compounds and certainly with the OPE-3 compounds. The present results may be compared with those of Pearson and Tour for oligo(2,5thiopheneethynylenes) where a saturation of the optical absorption was noted between the oligomers having 8 and 16 thiophene rings.54 For the OPEs a similar saturation is already evident between the oligomers (OPE-2 and OPE-3) having 5 and 7 phenyl rings. 4949

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Figure 4. (A and B) Absorbance and fluorescence spectra of S-OPE-3 (H) upon addition of CMA [S-OPE-3 (H)] = 5.0
10-6 M, [CMA] = (0-2.5)
10-5 M. (C and D) Absorbance and fluorescence spectra of OPE-3 upon addition of CMA. [OPE-3] = 5.0
10-6 M, [CMA] = (0-2.0)
10-5 M. (E and F) Absorbance and fluorescence spectra of S-OPE-3 (COOEt) upon addition of CMA. [S-OPE-3 (COOEt)] = 5.0
10-6 M, [CMA] = (0-2.6)
10-5 M. Excitation wavelengths for S-OPE-3 (H), OPE-3, and S-OPE-3 (COOEt) are 380, 384, and 384 nm, respectively.

Figure 5. (A) CD spectrum of S-OPE-3 (H) upon addition of CMA. [S-OPE-3 (H)] = 5.0
10-6 M, [CMA] = (0-3.6)
10-5 M. (B) CD spectrum of OPE-3 upon addition of CMA. [OPE-3] = 5.0
10-6 M, [CMA] = (0-2.8)
10-5 M. (C) CD spectrum of S-OPE-3 (COOEt) upon addition of CMA. [S-OPE-3 (COOEt)] = 5.0
10-6 M, [CMA] = (0-3.0)
10-5 M.

The fluorescence quantum efficiencies in water are much lower for OPE-1 and S-OPE-1 (COOEt) than for S-OPE-1 (H) and yet somewhat similar and much higher in methanol. For the larger oligomers all of the fluorescence quantum yields are

much lower in water than in methanol. Triplet transient absorption studies49 indicate that triplet excited state production is much lower for these compounds as well. For the OPE-1 series, conversion of the carboxyester group to a free carboxylate results 4950

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in a large increase in fluorescence in water and there is little difference between the fluorescence efficiency for this compound in water and in methanol. It has been found that diphenylacetylenes and phenyleneethynlene monomers add water across the triple bond via a mechanism that has been proposed to involve either an initial photoprotonation or, for electron deficient derivatives, nucleophilic attack of structured interfacial water.50,51 We have observed that several OPEs undergo a reasonably rapid photobleaching reaction which is consistent with net addition of water to the triple bond.55 It is reasonable to infer that the low fluorescence yields may be connected with a reversible attack of water on the excited singlet state. For OPE-1 and S-OPE-1 (COOEt) the presence of the electrophilic end groups may enhance the likelihood of nucleophilic attack of water on the excited singlet. The lower fluorescence yields for the OPE-2 and OPE-3, regardless of end group, may be consistent with either protonation or nucleophilic attack of interfacial water on the excited singlet states of the larger oligomers. The OPEs self-assemble upon anionic scaffolds such as CMC and CMA with striking changes in absorbance and fluorescence.

Since the complex between the anionic carbohydrate scaffolds and various OPE are likely controlled by Coulombic and electrophile/nucleophile interactions as well as favorable hydrophobic interactions (release of interfacial water), it is reasonable to expect that stronger complexation will occur with the OPE having electrophilic end groups. The increase in fluorescence for the complexed OPE is also reasonable if the quenching of the hydrated monomers occurs in part because of associated interfacial water. The red shift in absorption and fluorescence upon self-assembly of OPE-1 and OPE-2 was previously attributed to either formation of a J-dimer or to planarization during complex formation. As discussed above, for OPE-1 (actual calculation on S-OPE-1 (H)) the calculated optimized structure is very nearly planar and fully conjugated. Thus for this oligomer it is reasonable that formation of a J-dimer could account for the observed spectral changes.39,40 However, for the OPE-2 and OPE-3, Jdimerization can be excluded as a potential explanation for the spectral changes occurring. A closer inspection of the variation of absorption spectral changes between complexes of the oligomers with CMC and

Table 4. KSV of Oligomers Quenched by AQS OPEs

KSV (M-1)

OPEs

KSV (M-1)

OPEs

KSV (M-1)

S-OPE-1 (H)

3.40
104

OPE-1

2.84
104

S-OPE-1 (COOEt)

7.95
104

S-OPE-2 (H)

1.97
10

OPE-2

1.28
10

5

S-OPE-2 (COOEt)

2.05
105

S-OPE-3 (H)

1.82
10

OPE-3

2.30
10

5

S-OPE-3 (COOEt)

2.51
105

5 5

Figure 6. (A and B) Absorbance and fluorescence spectra of S-OPE-1 (COOEt) with successive addition of AQS ([S-OPE-1 (COOEt)] = 1.5
10-5 M and [AQS] = (0-1.5)
10-5 M). (C) Ksv plot of S-OPE-1 (COOEt) in the presence of AQS. (D) Ksv plot in low AQS concentration (0-4)
10-6) profiles. The excitation wavelength is 360 nm. 4951

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Figure 7. DFT:B3LYP/6-31g** optimized structures for the S-OPE-n (H) series.

Figure 8. Molecular orbitals of the S-OPE-n (H) series calculated using the DFT:B3LYP/6-31g** method and basis set.

CMA and across the series is instructive. For example, when the absorption spectra of complexes for S-OPE-2 (H) and CMC (Figure 2) and CMA (Figure S5, Supporting Information) are

compared, it is clear that the absorption spectrum for S-OPE-2 (H)/CMC is sharper, more red-shifted, and more intense than that for S-OPE-2 (H)/CMA. In contrast, when S-OPE-2 4952

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Figure 9. Molecular orbitals of S-OPE-2 (H) optimized with planarity constraints for the triple bonds.

(COOEt) assembles with the same scaffolds (Figure 2 and Figure S5, Supporting Information), both spectra are sharp, strongly red-shifted and intense. Spectra obtained for the OPE2 with CMC and CMA are intermediate between S-OPE-2(H) and S-OPE-2(COOEt). For the OPE-3 oligomers a similar trend can be noted; the complexes are generally sharper and more redshifted for CMC (Figure S3, Supporting Information) than for complexes with CMA (Figure 4) and there is a trend to sharper, more red-shifted absorption spectra from S-OPE-3 (H) to S-OPE-3 (COOEt). We believe this trend is consistent with different extents of “segment planarization” among the complexes that can be attributed to the environment provided by the different scaffolds. In general, the strength of complex formation and apparent degree of “segment planarization” increases from CMA to CMC; this can be attributed to the fact that CMA consists of interrupted helices and random coils in water while CMC exists as a more sheet-like structure.36-38 The greater strength of complex formation and segment planarization also increases as a function of the number of electrophilic (COOEt) end groups that could be attributed in part to favorable iondipole interactions between the end groups and the scaffold. The fluorescence quenching of the cationic OPEs by the anionic electron acceptor AQS was investigated and an absorbance red shift somewhat similar to that observed for complexation with CMC and CMA was observed upon addition of AQS, which may likewise be attributed to segment planarization. The plot of S-OPE-1 (COOEt) quenching by AQS (Figure 6) demonstrates two identical features: (1) a linear region at relatively low quencher concentration and (2) a sharp upward curved fit at higher concentration. At the low concentrations (04 μM) of AQS, the linear Stern-Volmer plot was obtained and the value of KSV calculated for S-OPE-1 (COOEt) using eq 1 was 7.95
104 M-1. The fluorescence lifetime is around one nanosecond for OPEs.35 A dynamic quenching constant (∼1013 M-1s-1) is estimated using this value; this is considerably larger than the maximum possible for diffusion controlled quenching.46 Therefore, the dominant quenching mechanism is static quenching. The KSV values obtained in this study increased with increasing number of repeat units in the OPEs. It appears that much of this increase may result from the increased attraction between the highly charged oligomer and the quencher.39 However, the KSV values obtained in this study are lower than those obtained for other conjugated polyelectrolyte polymers, which is consistent with the idea that excited states for other polymers (such as PPV) are much more delocalized than these PPE derivatives.40,53 An induced circular dichroism signal was observed when achiral oligomers self-assemble onto chiral CMC and CMA, as

was shown for OPE-1 and OPE-2 in our earlier report.35 The induced optical activity of S-OPE-1(COOEt) changes from a positive to negative Cotton effect at the absorbance maximum indicating perhaps incipient left-handed helix formation,52 opposite to that for the S-OPE-20 s. This is similar to what was detected in a complex of cyanine dye and CMA that was reported earlier.38 Figure 5 shows the CD of OPEs of n = 3 with CMA. A right-handed helix was induced between S-OPE-3 (H) and CMA, as the two opposite peaks changed from a positive to negative Cotton effect with decrease in wavelength. Interestingly, the absorption spectrum of the complex does not vary (other than continuing to grow in while retaining the isosbestic point) as the striking changes in the induced CD spectrum occur. This change occurs in the range where the number of glucose units/ OPE repeat unit approaches unity and may result from an effective “dilution” of the OPE molecules on the scaffold that affords closer interaction. The frontier molecular orbitals for the optimized S-OPE-n structures are shown in Figure 8. From these molecular orbitals, it is evident that as the length of the oligomer increases, the effective length of the π-conjugated chromophore becomes finite. The DFT calculations demonstrate that S-OPE-1(H) is not only planar but the HOMO and LUMO orbitals are also fully conjugated. A combined experimental and computational study by Suresh et al.5 of a structurally related but noncharged OPE-1 suggests that this compound is planar in the ground state, but that there is little barrier to rotation along the long axis. In contrast, the optimized structures for S-OPE-2 (H) and S-OPE-3 (H), shown in Figure 7, are decidedly nonplanar and the frontier orbitals for both compounds are largely confined to a partially planar region that extends over little more than three phenyl rings. We suggest that this unit is likely the “segment chromophore” that is responsible for the absorption maximum and that larger polymers may likely consist of several of these “segment chromophores” where π-conjugation is effectively broken between segments. It has been previously shown in the aforementioned computational study by Suresh et al.5 that the energy difference between a fully planar and perpendicular torsional angle between phenyl rings is 0.5 kcal/mol, while kT at room temp (298 K) is 0.59 kcal/mol. This suggests that the oligomers have a nonplanar geometry in the ground state, as is clearly shown in the optimized structures of the OPEs where S-OPE-2 and 3 (H) deviate from planarity. It would seem that these deviations from planarity would cause discrete “segment chromophores”. The results shown in Figure 8 suggest that such a chromophore may be 3-4 phenyl rings in length, including the ethynyl groups in between. It is also observed when Figures 8 and 9 are compared that when S-OPE-2(H) is forced into a planar state, the extent of 4953

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Langmuir conjugation in along the backbone increases by at least one phenyleneethynylene unit. Semiempirical calculations by Miteva and co-workers have also shown that there is a decrease in the HOMO/LUMO gap as the planarity of the backbone is increased,56 which correlates to the red-shifting of absorption upon complexation. We suggest that complexation may likely result in planarization of the “segment chromophores” described above or some other mode of extension of the “segment chromophore” during the complexation process. As noted above, Pearson and Tour found for oligo(2,5thiopheneethynlene)s that saturation of optical absorption spectra occurs between 8 and 16 thiophene rings and their simulations suggest an extended planar zigzag conformation for the oligomer having 16 thiophene rings. Experimental work by Godt and co-workers has also shown that little redshift of λmax occurs after 4-5 phenyl rings is reached,57 which we would attribute to reaching the limit of chromophore length. In a computational study by Na Li and co-workers, it was shown in calculated molecular orbitals and electronic spectra that the extent of the chromophore in dimethoxy-p-phenyleneethynylene oligomers in a fully planar configuration was reached at around 8-10 repeat units.42 In addition, the length of a conjugated unit seen in this study was found to be highly dependent on the planarity of the phenylene-ethynylene backbone. In a DFT-level study by Magyar and co-workers the electronic excitation energy of phenylene-acetylene oligomers were calculated with various conformational arrangements.58 This research showed that most conformations resulted in reaching a limiting chromophore length, around 4-6 units for the planar configurations and 3 units for the twisted conformations. These results lend support to the hypothesis that the predominant cause of photophysical changes from complex formation are a result of segment planarization, especially in the larger oligomers.

’ SUMMARY In summary, a series of cationic p-OPEs with different end groups has been synthesized and their photophysical and self-assembly properties investigated. The OPEs generally exhibit higher fluorescence quantum yields in methanol than in H2O that is consistent with a reversible attack of water on the excited singlet state. The molar extinction coefficients of the OPE in water increase as the number of repeat units increase; however, the change in absorption is sharply attenuated from OPE-2 to OPE-3 and there is little additional change when the spectrum of OPE-3 is compared to that for a cationic PPE with similar structure having 49 PRU. From these observations and a computational investigation we conclude that a limiting “segment chromophore” is reached by OPE-3 that extends over 3 or 4 phenyleneethynylene units. Self-assembly of the OPE on anionic CMC or CMA scaffolds results in sharp spectral changes that we infer may involve planarization of the limiting “segment chromophores” of OPE-2 and OPE-3 derivatives. ’ ASSOCIATED CONTENT

bS

Supporting Information. Detailed synthetic procedures and characterization data for compounds, absorption and emission spectra of OPEs in water and methanol and in the presence of CMA or CMC, CD spectra of OPEs with CMA or CMC, and Cartesian coordinates. This material is available free of charge via the Internet at http://pubs.acs.org.

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: D.G.E., [email protected]; K.S., [email protected]fl.edu; D.G.W., [email protected].

’ ACKNOWLEDGMENT We thank the Defense Threat Reduction Agency for support of this research through grant numbers W911NF07-1-0079 and HDTRA1-08-1-0053. We thank Mr. Ken Sherrell and the University of New Mexico Mass Spectrometry Facility for assistance in characterization of OPEs. ’ REFERENCES (1) Tour, J. M. Chem. Rev. 1996, 96, 537–553. (2) Moore, J. S. Acc. Chem. Res. 1997, 30, 402–413. (3) M€ullen, K.; Wegner, G. Electronic Materials: The Oligomer Approach; Wiley-VCH: Weinheim, 1998. (4) Ziener, U.; Godt, A. J. Org. Chem. 1997, 62, 6137–6143. (5) James, P. V.; Sudeep, P. K.; Suresh, C. H.; Thomas, K. G. J. Phys. Chem. A 2006, 110, 4329–37. (6) Stone, M. T.; Heemstra, J. M.; Moore, J. S. Acc. Chem. Res. 2006, 39, 11–20. (7) Chen, L. H.; McBranch, D. W.; Wang, H. L.; Helgeson, R.; Wudl, F.; Whitten, D. G. Proc. Natl. Acad. Sci. U. S. A. 1999, 96, 12287–12292. (8) Peyratout, C. S.; Dahne, L. Angew. Chem., Int. Ed. 2004, 43, 3762–3783. (9) Haskins-Glusac, K.; Pinto, M. R.; Tan, C. Y.; Schanze, K. S. J. Am. Chem. Soc. 2004, 126, 14964–14971. (10) Sluch, M. I.; Godt, A.; Bunz, U. H. F.; Berg, M. A. J. Am. Chem. Soc. 2001, 123, 6447–6448. (11) Jiang, H.; Taranekar, P.; Reynolds, J. R.; Schanze, K. S. Angew. Chem.-Int. Ed. 2009, 48, 4300–4316. (12) Liu, B.; Bazan, G. C. Chem. Mater. 2004, 16, 4467–4476. (13) Zhu, S. S.; Carroll, P. J.; Swager, T. M. J. Am. Chem. Soc. 1996, 118, 8713–8714. (14) Thomas, S. W.; Joly, G. D.; Swager, T. M. Chem. Rev. 2007, 107, 1339–1386. (15) Ho, H. A.; Brisset, H.; Elandaloussi, E. H.; Frere, P.; Roncali, J. Adv. Mater. 1996, 8, 990–994. (16) Roncali, J. Chem. Rev. 1997, 97, 173–205. (17) Pinto, M. R.; Schanze, K. S. Synthesis-Stuttgart 2002, 1293–1309. (18) Tan, C. Y.; Pinto, M. R.; Schanze, K. S. Chem. Commun. 2002, 446–447. (19) Kukula, H.; Veit, S.; Godt, A. Eur. J. Org. Chem. 1999, 277–286. (20) Bunz, U. H. F. Chem. Rev. 2000, 100, 1605–1644. (21) Kumaraswamy, S.; Bergstedt, T.; Shi, X. B.; Rininsland, F.; Kushon, S.; Xia, W. S.; Ley, K.; Achyuthan, K.; McBranch, D.; Whitten, D. Proc. Natl. Acad. Sci. U. S. A. 2004, 101, 7511–7515. (22) Pinto, M. R.; Schanze, K. S. Proc. Natl. Acad. Sci. U. S. A. 2004, 101, 7505–7510. (23) Achyuthan, K. E.; Bergstedt, T. S.; Chen, L.; Jones, R. M.; Kumaraswamy, S.; Kushon, S. A.; Ley, K. D.; Lu, L.; McBranch, D.; Mukundan, H.; Rininsland, F.; Shi, X.; Xia, W.; Whitten, D. G. J. Mater. Chem. 2005, 15, 2648–2656. (24) Zhao, X. Y.; Liu, Y.; Schanze, K. S. Chem. Commun. 2007, 2914–2916. (25) Liu, Y.; Ogawa, K.; Schanze, K. S. Anal. Chem. 2008, 80, 150–158. (26) Liu, Y.; Schanze, K. S. Anal. Chem. 2008, 80, 8605–8612. (27) Liu, Y.; Ogawa, K.; Schanze, K. S. J. Photochem. Photobiol., C 2009, 10, 173–190. (28) Liu, Y.; Schanze, K. S. Anal. Chem. 2009, 81, 231–239. 4954

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