Letter pubs.acs.org/JPCL
Remarkable Photophysics and Amplified Quenching of Conjugated Polyelectrolyte Oligomers Fude Feng,† Jie Yang,† Dongping Xie,† Tracy D. McCarley,‡ and Kirk S. Schanze*,† †
Department of Chemistry and Center for Macromolecular Science and Engineering, University of Florida, P.O. Box 117200, Gainesville, Florida 32611-7200, United States ‡ Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803-1804, United States S Supporting Information *
ABSTRACT: We report the photophysics and fluorescence quenching of a series of monodisperse, anionic π-conjugated oligomers that are molecularly dissolved in aqueous solution. These structurally welldefined oligomers feature oligo(phenylene ethynylene) backbones with two −CH2COO− units on each repeat unit, with overall lengths of 5, 7, and 9 repeats. The ionic oligomers display a structured fluorescence band with high quantum efficiency in water, in contrast to the low fluorescence quantum efficiency and pronounced aggregation displayed by structurally similar oligomeric and polymeric (phenylene ethynylene) conjugated polyelectrolytes studied previously. Stern−Volmer (SV) fluorescence quenching studies using cationic charge- and energytransfer quenchers reveal that all of the oligomers give rise to SV quenching constants (KSV) in excess of 106 M−1, with values increasing with oligomer length, consistent with the amplified quenching effect. The amplified quenching effect is proposed to occur due to the formation of comparatively small oligomer aggregates. SECTION: Spectroscopy, Photochemistry, and Excited States
I
vinylene) oligomer are quenched rather inefficiently by MV2+. By contrast, in the presence of a cationic surfactant, MV2+ quenches the oligomer fluorescence more efficiently than the structurally analogous anionic conjugated polymer.10 Relatively few studies have explored the photophysics and fluorescence quenching of monodisperse π-conjugated oligomer electrolytes, and consequently, detailed insight concerning the relationship between conjugation length, aggregation, and the amplified quenching effect is lacking.11−19 For this reason, studies of structurally well-defined conjugated oligomer electrolytes with controlled aggregation properties are needed to provide insight into the amplified quenching effect. Here, we describe the photophysics and amplified quenching of a series of anionic monodisperse π-conjugated oligomers (PEnNa, Chart 1) that were specifically designed to reduce their tendency to aggregate in aqueous solution. These oligomers feature a poly(phenylene ethynylene) (PPE) conjugated backbone that is substituted with two anionic carboxylate groups on every repeat unit. Optical absorption and fluorescence spectra combined with fluorescence correlation spectroscopy (FCS) results show that the PEnNa oligomers do not form large aggregate structures in water. Nonetheless, fluorescence quenching studies of oligomers by the cationic
n the past decade, conjugated polyelectrolytes (CPEs) have found broad application as optical materials in chemo- and biodetection, disease diagnosis and therapeutics, and optoelectronics due to their unique light-harvesting properties.1−4 Water-soluble fluorescent CPEs have been of especial interest in sensory applications, where the amplified quenching effect plays an important role.3 Amplified quenching, first reported by Zhou and Swager in describing the molecular wire effect for conjugated polymers, gives rise to sensory amplification via highly efficient fluorescence quenching by a quencher that is associated with the backbone via ion pairing or molecular recognition.5 Whitten and co-workers showed that the fluorescence of an anionic CPE is quenched with remarkable efficiency by an oppositely charged quencher ion (e.g., methyl viologen, MV2+) in part due to ion pairing between the CPE and MV2+.6 Aggregation of CPEs has also been investigated and found to play an important role in the amplified quenching effect.7 Aggregation of CPEs is induced by quencher ions via electrostatic and/or hydrophobic interactions that allow a single quencher ion to quench the fluorescence of many CPE chains that are within the quenching sphere (radius ≈ 400 Å).8,9 However, aggregation of CPEs in the absence of quencher ions frequently occurs in aqueous solution, which gives rise to long-distance exciton transport as well as sites that are inaccessible to quencher ions. For example, Bazan and coworkers reported that aggregates of an anionic (p-phenylene © 2013 American Chemical Society
Received: February 25, 2013 Accepted: April 2, 2013 Published: April 2, 2013 1410
dx.doi.org/10.1021/jz400421g | J. Phys. Chem. Lett. 2013, 4, 1410−1414
The Journal of Physical Chemistry Letters
Letter
Chart 1
Figure 1. Normalized absorption spectra of (a) PEn in CHCl3 and (c) PEnNa in H2O. Normalized fluorescence spectra of (b) PEn in CHCl3 and (d) PEnNa in H2O, λex = 360 nm. PEn’s and PEnNa’s were used at 1.0 μM. PPE and PPECOONa were used at 5.0 μM.
Table 1. Photophysical Properties of Oligomers and the Parent Polymer CHCl3 λmax, nm λem, nm ΦF τ, ns kr, s−1 knr, s−1
H2O
PE5
PE7
PE9
PPE
364 403 0.78a 0.50 1.6 × 109 4.4 × 108
374 416 1.00a 0.46 2.2 × 109 0
382 420 0.92a 0.41 2.2 × 109 2.0 × 108
391 425 0.52a 0.33 1.6 × 109 1.5 × 109
PE5Na 364 402 0.69a 0.69 1.0 × 109 4.5 × 108
PE7Na 374 415 0.73a 0.58 1.3 × 109 4.7 × 108
PE9Na 383 419 0.94a 0.51 1.8 × 109 1.2 × 108
PPECOONa 391 425 0.42a 0.42 1.0 × 109 1.4 × 109
Quinine sulfate in 0.1 M H2SO4 is used as an actinometer, ΦF = 0.54. The estimated error in the quantum yields is ±10%. For the quantum yield measurements, [PEn] and [PEnNa] = 1.0 μM (oligomer concentration), and [PPE] and [PPECOONa] = 5.0 μM (polymer repeat unit concentration).
a
Absorption and fluorescence spectra for the PEn and PEnNa series in several solvents are compared in Figure 1, with important photophysical parameters listed in Table 1. The ester oligomers’ absorption is dominated by a strong π−π* transition in the near-UV which red shifts (longer λ) in the order PE5 < PE7 < PE9 < PPE, and the molar absorptivity increases linearly with oligomer length (see Figure S5, Supporting Information; ε
quencher ions MV2+ and 3,3′-diethyloxacarbocyanine iodide (DOC) give Stern−Volmer (SV) quenching constants (KSV) in excess of 106 M−1, consistent with the amplified quenching effect. The results are interpreted to suggest that the oligomers form comparatively small aggregate structures in aqueous solution. The results highlight the important roles played by ion pairing and aggregation in the amplified quenching effect. 1411
dx.doi.org/10.1021/jz400421g | J. Phys. Chem. Lett. 2013, 4, 1410−1414
The Journal of Physical Chemistry Letters
Letter
Figure 2. (a) Absorption spectra of PE9Na (1.0 μM) in water upon adding CaCl2; (b) fluorescence spectra of PE9Na (1.0 μM) in water upon adding CaCl2, λex = 360 nm; (c) principal component spectra; (d) factor loadings.
≈ 24 000 M−1 cm−1 per phenylene ethynylene repeat). These observations are in accord with previous investigations of PPEtype oligomers and are consistent with a conjugation length of ∼10 PE repeat units.20,21 The fluorescence of each PEn oligomer (and the corresponding polymer, PPE, Chart 1) appears as a structured band with a comparatively small Stokes shift, and small red-shifts are observed with increasing chain length. The fluorescence of the PEn series in CHCl3 has a very high quantum efficiency, with values greater than 0.78 for all of the oligomers.22 Remarkably, the absorption and fluorescence spectra of the oligomer polyelectrolytes (PEnNa, shown in Figure 1c and d) in water (pH 8) are almost identical to those of the organic soluble ester forms in CHCl3. In addition, the fluorescence quantum yields for the PEnNa ionic oligomers in water remain very high (Table 1), signaling that the primary pathway for excited-state decay is via fluorescence. This feature is distinctly different from that observed in previous studies of ionic πconjugated oligomers (and polymers) in water, where the fluorescence is typically red shifted and significantly reduced in quantum yield compared to that of the nonionic forms in organic solvents.15,23 The lack of a distinct difference in the absorption and fluorescence of ester PEn (in CHCl3) and ionic PEnNa (in water pH 8) strongly suggests that the latter are likely molecularly dissolved in the aqueous solution in the absence of additives. This premise is supported by fluorescence anisotropy decay experiments carried out on the PEnNa in water, which give rotational correlation times consistent with the size of the molecules (Table S1, Supporting Information). In previous work, we and others have shown that addition of acid (H+) or mono- and divalent metal ions to carboxylatesubstituted CPEs gives rise to aggregation accompanied by distinct red shifts of the absorption and fluorescence along with reduced fluorescence yield. In order to explore these effects in these structurally well-defined conjugated oligomers, here, we studied the effect of addition of H+, Na+, and Ca2+ on the spectral properties of the PEnNa oligomers and the corresponding polymer PPECOONa. Qualitatively similar
results were obtained in each case, and detailed plots of the absorption and fluorescence spectral shifts are provided in the Supporting Information. Herein, we focus on the effects of addition of divalent Ca2+, which gave the most pronounced changes. Figure 2 summarizes data obtained for titration of a solution of PE9 in water (pH 8) with Ca2+; corresponding data for PE5, PE7, and PPECOONa are shown in Figure S7, Supporting Information. As can be seen, addition of Ca2+ induces a distinct red shift and sharpening of the absorption band, along with concomitant quenching of the structured fluorescence band accompanied by the appearance of a broad and significantly red-shifted fluorescence band. These changes are induced by Ca2+ in the sub-mM range for PE9. For the shorter oligomers, the effects are less pronounced, and they occur at higher [Ca2+], but for the polymer PPECOONa, the effects are very similar to those observed for PE9. Quite interestingly, principal component analysis of the absorption spectral changes seen for the PE9/Ca2+ system reveals two distinct spectral components (eigenspectra, Figure 2c) that can be used to accurately model the data during the entire titration.24 This strongly suggests that addition of Ca2+ induces a change in “state” of the oligomer; we suggest that this may be the result of a change in backbone conformation. The Ca2+-induced changes in the spectra arise from the effect of the divalent metal ion to induce aggregation of the conjugated oligomers (and polymer) via the formation of ionic cross-links between −CO2− groups on adjacent chains.9,23 The absorption and fluorescence shifts are likely associated with planarization of the phenylene ethynylene backbone and interchain contacts that lead to the formation of “excimerlike” interchain excited states. The supramolecular structure of the aggregates is likely analogous to a disordered array of pickup sticks as suggested by Bunz and co-workers to explain the spectral properties of aggregates of PPE-type polymers in spincast films.25 Despite the striking similarity in the spectral changes seen for the PE9/Ca2+ and PPECOONa/Ca2+ systems, suggesting similar aggregate structures, FCS reveals that there is a 1412
dx.doi.org/10.1021/jz400421g | J. Phys. Chem. Lett. 2013, 4, 1410−1414
The Journal of Physical Chemistry Letters
Letter
425 nm) is accompanied by the appearance of DOC emission (λ = 525 nm), confirming that quenching occurs via singlet− singlet (Fö rster) energy transfer. The DOC quenching efficiencies are extraordinary, increasing with oligomer length and ranging from 3.8 × 106 (PE5Na) to 1.1 × 107 M−1 (PE9Na), compared to 1.9 × 107 M−1 for PPECOONa. Importantly, SV plots for PE9Na and PPECOONa are very similar, which clearly shows that the amplified quenching effect is as strong for the 9-mer as it is in the polymer, where exciton transport along the polymer chain could play a role. Another very significant result is that 90% quenching is seen for PE9Na with [DOC] ≈ 275 nM; given that the PE9Na concentration is 1 μM, this result shows that, on average, a single DOC quencher can effectively quench more than three oligomers. From this result, we conclude that, on average, a single DOC molecule resides well within the Förster radius ( 1 μM; similar effects are seen for amplified quenching in CPE/ionic quencher systems, where the effect has been attributed to quencher-induced polymer aggregation.9 Striking quenching results are seen for the PEnNa/DOC quenching system (Figure 3). First, as seen in Figure 3a for PE9Na/DOC, the quenching of the oligomer fluorescence (λ =
shown in this scheme, the quencher ion associates with a PEnNa aggregate, which likely consists of
