Fluorescence Enhancement of a Cationic FluoreneâPhenylene

Article Cite This: Langmuir XXXX, XXX, XXX-XXX

pubs.acs.org/Langmuir

Fluorescence Enhancement of a Cationic Fluorene−Phenylene Conjugated Polyelectrolyte Induced by Nonionic n‑Alkyl Polyoxyethylene Surfactants María Monteserín,† Hugh D. Burrows,*,‡ Artur J. M. Valente,‡ A. A. C. C. Pais,‡ Roberto E. Di Paolo,§ Antonio L. Maçanita,§ and María J. Tapia*,† †

Departamento de Química, Universidad de Burgos, Plaza Misael Bañuelos, Burgos 09001, Spain Centro de Química de Coimbra (CQC), Department of Chemistry, University of Coimbra, 3004-535 Coimbra, Portugal § Departamento de Engenharia Química e Biologica, Instituto Superior Técnico (IST), Avenida Rovisco Pais, P1049-001 Lisboa, Portugal ‡

S Supporting Information *

ABSTRACT: The modulation of conjugated polyelectrolyte fluorescence response by nonionic surfactants is dependent on the structures of the surfactant and polymer, polymer average molecular weight, and polyelectrolyte−surfactant interactions. In this paper, we study the effect of nonionic n-alkyl polyoxyethylene surfactants (CiEj) with different alkyl chain lengths (CiE5 with i = 6, 8, 10, and 12) and number of oxyethylene groups (C12Ej with j = 5, 7, and 9) on the photophysics and ionic conductivity of poly{[9,9-bis(6′-N,N,Ntrimethylammonium)-hexyl]-2,7-fluorene-alt-1,4-phenylene}bromide (HTMA-PFP) in dimethyl sulfoxide−water 4% (v/v). Molecular dynamics simulations show that HTMA-PFP chains tend to approach as the simulation evolves. However, the minimum distance between the polymer centers of mass increases upon addition of the surfactant and grows with both the surfactant alkyl chain length and the number of oxyethylene groups, although there are no specific polymer−surfactant interactions. A significant increase in the polymer emission intensity has been observed at surfactant concentrations around their critical micelle concentrations (cmcs), which is attributed to polymer aggregate disruption. However, an increase in the solution conductivity for concentrations above the C12E5 cmc has only been observed for the HTMA-PFP/C12E5 system. The enhancement of fluorescence emission intensity and conductivity upon surfactant addition increases with polymer average molecular weights and seems to be controlled by the polymer−surfactant proximity, which is maximum for C10E5 and C12E5.

1. INTRODUCTION

polarities allows increasing polymer emission quantum yields in solution.22−24 A third strategy which enhances CPE emission quantum yields in solution involves the interaction with surfactants, which is usually accompanied by a blue shift of the emission and changes in the vibrational structure of the emission spectra.24−26 These spectroscopic changes are attributed either to conformational changes27 or to the breakup of polymer aggregates;19,21 although there is no complete consensus on this topic, small-angle neutron scattering (SANS)28 and other studies show that deaggregation of CPEs does occur. However, some surfactants quench the emission quantum yield of CPEs instead of enhancing it. This has been observed upon addition of anionic surfactants of the family of n-alkyl sulfonates and sulfates at concentrations below the critical micelle concentration (cmc) to the cationic poly{[9,9-bis(6′-N,N,N-trimethylammonium)-hexyl]-2,7-fluorene-alt-1,4-phenylene}bromide

Conjugated polyelectrolytes (CPEs) are a group of conjugated organic polymers1 with charged groups in their structure to enhance their solubility in water and polar solvents. Considerable attention has focused on CPEs in the last decade because of their large number of applications,2 including biological and chemical sensors,3−6 light-emitting diodes,7−9 photovoltaic cells,2,10−13 and other opto-electronic devices.14,15 Polyfluorenes and related copolymers are present in a particularly valuable group of CPEs for these applications because of their blue emission and high emission quantum yield.16 However, CPEs have a strong tendency to aggregate17 in aqueous solution, which quenches their emission.18−21 Several strategies have been developed to reduce this aggregation. One of them resorts to the synthetic route, which is mainly based on the introduction of bulky groups in their structure, which minimizes the interactions between polymer chains, consequently improving CPE dissolution. The addition of organic cosolvents has also been used to improve CPE solubility. The combination of solvents with different © XXXX American Chemical Society

Received: September 8, 2017 Revised: October 18, 2017

A

DOI: 10.1021/acs.langmuir.7b02818 Langmuir XXXX, XXX, XXX−XXX

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Figure 1. Chemical structures of fluorene-based CPEs (A) and n-alkyl polyoxyethylene surfactants, CiEj (B).

(HTMA-PFP). At this surfactant concentration range, electrostatic interactions between the two species seemed to play a major role, leading to complex aggregate structures, whereas at surfactant concentrations above the cmc (when surfactant micelles are formed), emission enhancement was observed and may be associated with dissolution of the CPEs in surfactant micelles.29 A more recent work, which compares the spectroscopic behavior of different polyfluorene−surfactant systems, shows that most of the nonionic surfactants studied (C12E5, Triton X100, and Tween 20) improve the emission intensity of different polyfluorene CPEs, regardless of their charge, highlighting the importance of mixed polymer−surfactant micelle formation.30 The only system in which the polymer emission was not improved by addition of a nonionic surfactant is poly{9,9′bis[(6″-N-acetyltyrosine)hexyl]-2,7-fluorene-alt-1,4-phenylene}, (Tyr-PFP) with C12E5 addition, and this behavior has been attributed to steric hindrance of the high voluminous substituent at carbon 9 of Tyr-PFP.

The interaction between the anionic poly{9,9-bis[(4butylsulfonate)phenoxy]-2,7-fluorene-alt-1,4-phenylene} (PBSPFP) and C12E5 has been widely studied.19,21,31−34 The increase in the emission intensity of the polymer has been attributed to the disruption of polymer aggregates through the incorporation of polymer chains into the surfactant micelles.33 A similar behavior was observed with HTMA-PFP,30,35 leading to an improvement of its DNA sensing properties.35−37 Also, with poly[9,9-bis(4-sulfonylbutoxyphenyl)fluorene-2,4-diyl2,2′-bithiophene] (PBS-PF2T), a breakup of polymer aggregates upon interaction with liquid crystalline aqueous C12E438 or C12E539 systems has been confirmed. In fact, surfactants have been used to modify the fluorescence response of CPEs both as films40 and as nanoparticles for fluorescence imaging of latent fingerprints.41 However, the lack of a detailed understanding of the molecular parameters and interactions leading to the disruption of the polymer aggregates means that the choice of the appropriate surfactant to improve the optical properties of B


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Polarized fluorescence spectroscopy experiments were carried out, and the fluorescence anisotropy of the sample was calculated according to eq 1:49

polyfluorenes still remains a trial and error matter.30 We have carried out a systematic study of the parameters that govern the polymer−surfactant interaction with one particular CPE/ surfactant system, to obtain insights into them, and their consequences in the improvement of the optical properties of CPEs. This has been carried out by varying both the surfactant structure and the polymer molecular weight. We report a combined experimental (absorption, steady-state and timeresolved emission, and conductivity) and theoretical study [using molecular dynamics (MD) simulations] of the interaction between the cationic HTMA-PFP and nonionic surfactants of the alkyl polyoxyethylene family, CiEj, on varying the length of the alkyl chain of the surfactant (i, CiE5) and the number of oxyethylene groups (j, C12Ej).

⟨r ⟩ =

IVV − GIVH IVV + 2GIVH

(1)

where Iex,em is the intensity of emission, V and H are the vertical and horizontal alignment of the excitation and emission polarizers, and G = IHV/IHH is the instrumental correction factor. These experiments were conducted with a polarization setup consisting of two Glan-Thompson rotatable prism polarizers. The single channel (L-format) method for fluorescence anisotropy determination was used. To calculate each anisotropy value via eq 1, four complete fluorescence spectra, one for each configuration of polarizers, were required. The fluorescence spectra for anisotropy experiments were recorded on a Jobin Yvon Fluoromax-3 spectrometer with right-angle geometry, with excitation at 381 nm and excitation and emission slits of 5 nm. Electrical resistances (R) were measured using a Wayne Kerr model 4265 Automatic LCR meter at 1 kHz. A Shedlovsky-type conductivity cell was used. The cell constant [(l/A) = 0.105 cm−1] was determined to ±0.02% from measurements with aqueous KCl solutions (reagent grade, recrystallized and dried using the procedure and data from Barthel et al.).50 MD simulations were carried out using the GROMACS software package with the standard GROMACS force field,51,52 which is a modified version of the GROMOS87 force field.53 The PRODRG server54 was used to generate the topology files from the initial structures in Cartesian coordinates. Solute molecules were added to a cubic box cell and solvated with SPC (single point-charge) water model,55 with the structure constrained by the SETTLE algorithm.56 The SPC water model considers three interaction sites centered on the atomic nuclei; the intramolecular degrees of freedom are frozen, whereas the intermolecular interactions are described by a conjunction of Lennard-Jones 12-6 potential and Coulombic potentials between sites with fixed point-charges. This simple model of water was considered to accurately reproduce many properties of bulk water, especially under normal conditions:57 300 K and 1 atm. It has also been shown to be superior to more sophisticated water models58,59 and has been recommended for modeling aqueous solutions of biomolecules;60 it has also been previously employed in systems comprising interactions similar to those of the present work.61 To keep the whole system neutral, the bromide anions were added by randomly replacing solvent molecules. First of all, an energy minimization was performed on the system. This was followed by an MD equilibration run under position restraints for 5 ns. Then, the simulation was equilibrated with an unrestrained MD run for 1 ns. Finally, an MD run of 10 ns was carried out. This generates a trajectory of 5 ns with a time step of 2 fs. All simulations were performed with periodic boundary conditions, using the Berendsen coupling algorithm.62 This algorithm needs to specify pressure (P), temperature (T), and the respective coupling times τP and τT. Thus, constant pressure, temperature, and the number of particles (N), that is, NPT conditions are maintained. In this case, we used P = 1 bar, τP = 0.5 ps, T = 300 K, and τT = 0.1 ps. The particle mesh Ewald method63 was used for computation of long-range electrostatic forces. These simulations were carried out with a model based on three trimer chains of HTMA-PFP, with DMSO−water (4% v/v) as the solvent. The number of DMSO and water molecules varies on each simulated system to maintain the 4% ratio between them. Five different systems were studied; four with different CiEj surfactants and the fifth one, taken as the reference, only with the polymer in solution, as summarized in Table 1.

2. EXPERIMENTAL SECTION 2.1. Reagents and Solution Preparation. Three different batches of HTMA-PFP were used and kindly supplied by Professor Ricardo Mallavia (Universidad Miquel Hernandez de Elche, Spain). They have been synthesized via the Suzuki coupling reaction, using 1,4-phenyldiboronic acid and 2,7-dibromo-9,9-bis(6′-bromohexyl)fluorene as precursors.42,43 HTMA-PFP (Figure 1A) was obtained by treating the neutral poly[9,9-bis(6′-hexyl bromide)-fluorene phenylene] products with trimethylamine (Menschutkin reaction).44 Size exclusion chromatography in tetrahydrofuran was used to determine the molecular weight of the three neutral polymer batches. In agreement with previous studies,45 it is assumed that the neutral copolymer is almost quantitatively converted into the cationic one and that the conversion does not affect the degree of polymerization of the polymer. According to this, M̅ n of the three cationic polymer batches synthesized are 14.5, 30.1, and 61.3 kg/mol, and hereafter they will be labeled as P1, P2, and P3, respectively. The highest molecular weight polymer shows a bimodal distribution with 90.7% of the fraction with M̅ n = 71.1 kg/mol and 9.3% of the fraction with M̅ n = 16.6 kg/mol.43 Details of the synthesis and characterization of both polymers have been presented elsewhere.42,44 For the highest molecular weight HTMA-PFP, slight differences in the bimodal distribution may be observed with respect to the precursor neutral polymer as a consequence of the synthesis and purification processes. HTMA-PFP shows low solubility in water, but it dissolves easily in dimethyl sulfoxide (DMSO)−water mixtures. Stock polymer solutions with concentrations around 6.9 × 10−2 g/L (1 × 10−4 mol/L in terms of monomer repeat units) were prepared in DMSO (Aldrich, spectrophotometric grade) by continuous stirring overnight. Aliquots were diluted with Milli-Q water to prepare solutions for the spectroscopic and conductivity measurements with polymer concentrations around 4 × 10−6 M (repeat units) in DMSO−water 4% (v/v). Commercial nonionic surfactants of the family of n-alkyl polyoxyethylenes (CiEj, Figure 1B) were purchased and used without further treatment. Surfactants with different alkyl chain lengths (i) and number of oxyethylene groups (j) have been used: C6E5 (Sigma), C8E5 (Sigma), C10E5 (Fluka), C12E5 (Sigma), C12E7 (Sigma), and C12E9 (Fluka). Quinine sulfate from Fluka in 0.1 M sulfuric acid was used as the standard for quantum yield measurements.46 2.2. Apparatus and Methods. Absorption spectra were recorded on a Shimadzu 2501 PC UV−visible spectrophotometer. For steadystate luminescence spectral measurements, a Shimadzu RF-5301 PC instrument was used in a right-angle configuration. The excitation wavelength was 381 nm, and excitation and emission slits were 3 and 1.5 nm. Both absorption and emission spectra were recorded at 25.0 ± 0.1 °C, below the cloud point of CiEj used.12 Time-resolved fluorescence measurements were carried out using the single-photon counting technique with ps time resolution, as previously described.29,47,48

3. RESULTS AND DISCUSSION The interaction of the cationic polyfluorene HTMA-PFP with different nonionic surfactants of the family of n-alkyl polyoxyethylenes, which will be named as CiEj (where i is the alkyl C


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PFP29 in water and in water−dioxane (70:30%, v/v).21 These ill-defined species are frequently described as clusters.21 The radial distribution functions, g(rdf), between polymer chains also show this tendency of the polymer chains to aggregate. The distance between the polymer trimers decreases after simulation (Figure 2C). The tendency of HTMA-PFP to form clusters in DMSO− water 4% (v/v), indicated by the MD simulations, is compatible with the spectroscopic results shown in Figure 3. By increasing the average molecular weight of the polymer, the background of the absorption spectra for wavelengths shorter than 300 nm increases (Figure 3A), which is indicative of the increase in light scattering compatible with the formation of larger polymer clusters. Moreover, the maxima of the absorption and emission spectra are shifted to red (from 378 to 382 nm and from 412 to 417 nm, respectively, Figure 3A,B). This behavior is characteristic of more extended π systems49 with the increase in the polymer molecular weight, and it is also compatible with the increase in polymer aggregation.19 Small changes in the vibrational structure of the emission spectra are also observed (Figure 3B). Similar spectroscopic changes have previously been observed in poly[9,9-di(ethylhexyl)fluorene] (PF2/6) in methylcyclohexane upon changing the number of repeat units of three oligomers from 3 to 20.64 Additional experimental evidence for the increase of polymer aggregation with molecular weight comes from time-resolved fluorescence experiments. Fluorescence decays of solutions of the three HTMA-PFP batches (P1, P2, and P3) were recorded at 416 and 450 nm (onset and tail of the emission band) with excitation at 380 nm. Good global fits of the fluorescence decays were obtained with the sum of three exponential terms for the three batches. A typical decay for P1 is shown in Figure S2. Multiexponential decays were also observed with poly(9,9dialkylfluorene)s (alkyl = ethylhexyl or octyl, PF2/665 or PFO,66 respectively), which in addition to the polymer lifetime show fast decay times at the band onset, appearing as rise-times at the emission tail. These fast decays were assigned to torsional relaxation of the polymer backbone in the excited state, possibly accompanied by some singlet energy migration along the conjugated backbone.67 However, the decays of the HTMAPFP polymers do not show the characteristic rise-time (negative pre-exponential coefficient) in the fast components at the band tail (450 nm), displaying, instead, similar preexponential coefficients at 450 and 416 nm. Actually, the two decays are visually superimposable. This means that torsional relaxation is not responsible for the two faster decays, and we attribute the two shortest times to the emission of the distribution of HTMA-PFP aggregates (clusters). From the decay times τi and the respective pre-exponential factors a i (Table 2), the percentage of fluorescence contributions of the three decay components to the total time-integrated fluorescence (% Fi) can be calculated as follows:49 aiτi % Fi = ∑i aiτi (2)

Table 1. Composition of the Simulation Box surfactant

HTMA-PFP trimers

Br−

H2O molecules

DMSO molecules

CiEj molecules

C6E5 C12E5 C16E5 C12E9

3 3 3 3 3

18 18 18 18 18

4131 3917 3881 3853 3822

45 47 48 42 41

9 9 9 9

chain length and j represents the number of oxyethylene groups) and the effect of the molecular weight of the polymer on the interaction with C12E5 have been studied. In the first section, the simulation studies and optical properties are discussed. The conductivity results are shown in a separate section. 3.1. Simulation Studies and Electronic Spectroscopy Results. The simulation results obtained for the polymer in DMSO−water mixtures 4% (v/v) and those for the polymer with n-alkyl polyoxyethylenes will be discussed separately. 3.1.1. Polymer. MD simulations carried out in systems with three trimers of HTMA-PFP with DMSO−water mixtures 4% (v/v) as the solvent show a clear tendency of HTMA-PFP to form aggregates in aqueous solution. The composition of the simulation box is shown in Table 1. The snapshot of the simulation box before and after simulation shows that, although the polymer chains are initially placed at significant separation in the simulation box (Figure 2A), they approach during

Figure 2. (A) HTMA-PFP trimers in the initial simulation box. (B) HTMA-PFP trimers after 10 ns of MD simulation. Bromide counterions and the solvent [DMSO−water 4% (v/v)] have not been included for clarity of the representation. (C) g(rdf) between polymer chains at the beginning of the simulation (solid line) and after 10 ns of MD (dashed line).

The percentage of fluorescence contribution of the longest decay time, % F1, and the sum of the fluorescence contribution of the two shortest lived species behave in the opposite fashion (Figure 3C). It seems that the long-lived component is replaced by the short-decay time species (% F2 + % F3) with the increase in the polymer molecular weight. This is similar to what was

equilibration, and after 10 ns of simulation, they are close together (Figure 2B). Although not explicitly included, a certain degree of π−π interaction between the conjugated backbones is observed, whereas the alkyl chains with the charged trimethylammonium groups are oriented toward the solvent. A similar behavior was observed in the case of anionic PBSD


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Figure 3. (A) Normalized absorption and (B) normalized emission spectra of HTMA-PFP in DMSO−water 4% (v/v) mixtures of P1 (black), P2 (dark gray), and P3 (gray) lines. The polymer concentrations (in terms of repeat units) are 3.0, 3.8, and 3.9 μM for P1, P2, and P3, respectively. Excitation wavelength = 381 nm. (C) Percentages of the long (τ ≈ 535 ps, full square) and short fluorescence lifetime species [as a sum-up of the percentages of the contributions of the intermediate (τ ≈ 275 ps) and shortest component (τ ≈ 45 ps), empty square] and (D) anisotropy of HTMA-PFP at 500 nm vs M̅ n of HTMA-PFP in kg/mol. Polymer repeat unit concentrations are 3.6 μM (P1), 4.2 μM (P2), and 7.2 μM (P3). Values in (D) are mean values of Figure S1.

Table 2. Molar Absorption Coefficient at 381 nm in Terms of Repeat Unit Concentration (ε), Emission Quantum Yield (ϕ), and lifetimes (τi) of HTMA-PFP in 4% (v/v) DMSO−Water Observed at 416 nm (Excitation Wavelength 381 nm)a M̅ n HTMA‑PFP (kg/mol)

ε (cm−1 mol−1 L)

ϕ

τi (ps)

ai

χ2

% Fi

14.5

35 400 (±334)

0.52 (±0.03)

35 100 (±410)

0.54 (±0.03)

61.3

24 400 (±217)

0.56 (±0.08)

0.22 0.31 0.47 0.47 0.25 0.28 0.32 0.37 0.31

1.01

30.1

45 321 534 42 218 544 54 297 534

2.74 27.61 69.64 8.71 24.06 67.23 5.90 37.54 56.55

1.43

1.2

a Normalized pre-exponential factors (ai), goodness of the fit of the emission decays (χ2), and percentage of the fluorescence contribution (% Fi). Polymer concentrations (in terms of repeat units) for lifetime experiments: 5.3, 4.7, and 5.5 μM for M̅ n = 14.5, 30.1, and 61.3 kg/mol, respectively.

The loss in fluorescence anisotropy in HTMA-PFP can be attributed to three different mechanisms: rotational diffusion,49 exciton migration, and conformational relaxation (twisting of part of the aromatic backbone).64 From our experiments, rotational diffusion cannot be an explanation because in this case, the smaller polymers should also show lower anisotropy. Similar conclusions have been reached with PF2/6 and with HTMA-PFP/DNA systems.36 The small changes in the shape of the absorption and emission spectra of the three polymer batches (Figure 3A,B) indicate that there are no significant differences between them in conformational terms. As a consequence, the most likely explanation to justify the differences in the anisotropy is interchain excitation migration favored by the polymer aggregation shown by the MD simulations (Figure 2B). A similar conclusion was reached in the case of HTMA-PFP/DNA systems.36 No significant differences were observed in the emission quantum yields (ϕ) of HTMA-PFP in DMSO−water 4% (v/v)

previously observed upon interaction of HMTA-PFP with ndecyl sulfonates29 or DNA.36 As was discussed in those cases,29,36 the two short-lived components are likely to be due to the interchain energy migration in polymer aggregates formed upon neutralization of the polymer charge by anionic surfactants or DNA. In the present case, the rise of polymer aggregation is not induced by charge neutralization upon interaction with oppositely charged species, as in the case of the HTMA-PFP/DNA system, but by the increase in the average polymer molecular weight. Interchain energy migration in aggregated polymer chains can also explain the changes in anisotropy with the average polymer molecular weight. The fluorescence anisotropy (⟨r⟩) of the three polymer samples (P1, P2, and P3), measured in the 470−530 wavelength range (Figure S1 in the Supporting Information) decreases quasi-linearly with the increase of M̅ n, as shown in Figure 3D. The decrease of anisotropy with the increase in the number of repeat units has also been observed for PF2/6.64 E


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Langmuir mixtures, which show statistically insignificant variations from 0.52 (P1) to 0.56 (P3) (Table 2). In spite of the tendency of the polymer to form clusters, the absorbance and the emission intensity in the maxima at about 381 and 412 nm, respectively, grow linearly with the polymer concentration (expressed hereafter in terms of the repeat units) for the three polymer batches (P1, P2, and P3), at least for optical density < 0.1 (concentration range between 0.50 and 3.0 μM, Figure S3 in the Supporting Information). This indicates that in this concentration range, no significant changes in polymer conformation take place, and the polymer suspension behaves as an ideal solution. The molar absorption coefficients are calculated with Beer−Lambert Law by dividing the experimental absorbance at 381 nm by the polymer experimental concentration expressed in terms of repetitive units and are shown in Table 2. This value decreases with increasing polymer molecular weight, as can clearly be seen for P3; values for the other two polymers are similar, within the experimental error. This decrease could be partially due to a stronger tendency to form aggregates of the polymer with the highest average molecular weight. 3.1.2. Polymer with n-Alkyl Polyoxyethylene Surfactants. Addition of CiEj surfactant molecules in the simulation box with three polymer trimers (previously equilibrated) at a surfactant to polymer concentration ratio of 1:1 separates the polymer chains (Figure 4). A detailed description of the composition of the simulation box of each system is given in Table 1.

Figure 5. (A) Minimum distance between the centers of masses of the closest HTMA-PFP trimers vs the number of alkyl chain carbon atoms (i, open squares, solid line) and vs the number of oxyethylene groups (j, black circles, dashed line) of the surfactant C12Ej after 10 ns of simulation. Distance = 0 indicates the separation between polymer chains in the system without the surfactant. (B) HTMA-PFP/CiEj distance vs simulation time. Systems with C6E5, C12E5, C16E5, and C12E9 are shown in black, red, blue, and green, respectively.

(N,N,N-trimethylammonium)alkyl]fluorene-co-1,4-phenylene} iodides in aqueous solution in the presence of C12E5.28 One of the polymers used in that work was the iodide oligomer derivative of HTMA-PFP (M̅ n = 3.5 kg/mol). All results show that the polymer is solubilized by the surfactant through the formation of a mixed polymer−surfactant species. SANS data suggest that upon dissolution, C12E5/polymer systems form long wormlike particles with rigid segments with a diameter of 2.5 nm. For the above-mentioned oligomer, the length of the rigid segment is 45 nm and grows to 60 nm when the molecular weight of the polymer increases from M̅ n = 3.5 to 10 kg/mol.28 Wormlike cylinders are found for semiflexible π-conjugated polymers28 and for micelles of polyoxyethylene alkyl ethers CiEj, with average values of the cross-sectional diameter of about 2.5 nm, regardless of the alkyl chain length (i) or the number of oxyethylene groups (j).12 This is in very good agreement with the diameters obtained from the SANS data for the C12E5/polymer28 and indicates that the diameter of the C12E5 micelle is hardly affected by the incorporation of the polymer. For pure surfactants, the micellar length depends on the surfactant structure (i.e., on the i and j values), surfactant concentration, and system temperature.12 For CiE5 surfactants, the micelle length grows with increasing alkyl chain length (i) because of the increase in the hydrophobic interactions that facilitate micellar growth. However, the length of the micelles becomes shorter with the increase in the number of oxyethylene groups (j). This is assumed to be due to the affinity between the oxyethylene groups and water molecules, which induces repulsive forces between adjacent oxyethylene chains on the surface of the micelles. The repulsive force is stronger for the longer oxyethylene group, increasing the micelle surface area and making them less stable. For this reason, C14Ej molecules form shorter micelles upon increasing the number of oxyethylene groups (j).12 In addition, the length of the CiEj micelles grows with the surfactant concentration and the temperature.12 Simulation results indicate that the minimum distance between the centers of mass of the polymers grows linearly with the surfactant alkyl chain length and with the number of

Figure 4. (A) g(rdf) of the distances between HTMA-PFP chains in the systems without the surfactant and with C6E5, C12E5, and C16E5 in black, blue, red, and green, respectively. (B) g(rdf) of the distances between HTMA-PFP chains in the systems without the surfactant and with C12E5 and C12E9 in black, red, and blue, respectively.

The radial distribution functions between the polymer trimer chains in the absence of the surfactant and in the presence of C6E5, C12E5, C16E5, and C12E9 are shown in Figure 4. The first peaks of g(rdf) represent the separation between the two closest HTMA-PFP chains, which shift to longer distances when CiEj is added to the simulation box compared with the distance of the first peak of the g(rdf) in the absence of the surfactant. A similar conclusion is reached from the increase in the minimum distance between the polymer centers of mass upon addition of the surfactant to the simulation box (Figure 5A), as was previously observed in simulations carried out with PBS-PFP/C12E5 systems,33 supporting the idea that the surfactants break up the polyelectrolyte clusters. These simulation results are in good agreement with the conclusions of structural studies carried out on two cationic poly{9,9-bis[6F


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Figure 6. MD simulation with three HTMA-PFP trimers (pale blue) and nine surfactant molecules (red) after 10 ns of simulation: (A) C6E5, (B) C12E5, and (C) C12E9. Solvent molecules [DMSO−water 4% (v/v)] and bromide counterions have not been shown for clarity. N(CH3)3+ groups of the polymer are shown in dark blue.

Table 3. CiEj cmc from the Literature; Surfactant Concentration at Half the Height of the Sigmoid-Shaped Curves Obtained by Plotting the Emission Intensity Ratios: I412/I434 (Figure S5) and I412(Polymer−Surfactant)/I412(Polymer), Figure 7, of HTMAPFP (P1) vs the Surfactant Molar Concentrations. Inflection point in the increase of conductivity, Δκ′ ′ ′ (Δκ′ = κHTMA ‐PFP/C12E5 − κHTMA ‐PFP) with surfactant additions through conductivity experiments (see section below). All experiments were carried out in DMSO−water 4% (v/v) surfactant C6E5 C8E5 C10E5 C12E5 C12E7 C12E9 a

cmc bibliography (mol L−1)68 7.5 7.1 8.6 7.2 7.3 1.2

(±2.2) (±2.2) (±2.0) (±2.2) (±1.4) (±0.7)

−2 69

× 10 × 10−3 70 × 10−4 71,72 × 10−5 73 ×10−5 73,74 ×10−4 75−77

I412/I434 5.4 5.5 8.3 8.3 6.5 1.2

(±0.3) (±0.2) (±0.1) (±3.7) (±0.4) (±0.2)

I412(polymer−surfactant)/I412(polymer) −2

× 10 × 10−3 × 10−4 × 10−5 a ×10−5 × 10−4

6.3 7.9 9.8 9.3 8.2 1.3

(±0.1) (±0.3) (±0.4) (±3.7) (±0.2) (±0.2)

× × × × × ×

conductivity, Δκ′

−2

10 10−3 10−4 10−5 a 10−5 10−4

7.5 (±1.0) × 10−5

Average values for the three molecular weights (M̅ n = 14.5, 30.1, and 61.3 kg/mol, Table S1).

HTMA-PFP absorption and emission spectra in DMSO− water (4% v/v) with different concentrations of CiEj surfactants were recorded to study the effect of the surfactant concentration and structure on the optical properties of the polymer in aqueous solution. The spectroscopic results will be linked to those of MD simulations. Increasing the surfactant concentration does not seem to significantly affect the shape of the absorption spectra, except for a slight increase in the absorption background at a very high surfactant concentration because of the greater scattering of the solutions, as shown in Figure S4A in the Supporting Information. Moreover, no significant shifts of the normalized absorption and emission spectra of HTMA-PFP are observed for various C12E5 concentrations below and above the cmc (Figure S4A). However, significant changes in the vibrational structure of HTMA-PFP-normalized emission spectra (Figure S4B) are observed when increasing the surfactant concentrations. The increase in the vibrational structure of the polymer emission spectra in the presence of C12E5 can be explained by the increased rigidity due to the surfactant winding up the HTMAPFP backbone, as shown in Figure 6. Sigmoid-shaped curves are obtained by plotting the ratio between the emission intensity of the maximum of the spectra (at about 412 nm) and that of the first shoulder (at around 434 nm), I412/I434 (Figure S5 in the Supporting Information). I412/ I434 ratio is calculated by dividing the polymer emission intensity at 412 nm by that at 434 nm. The surfactant concentrations needed for the polymer I412/ I434 intensity ratio to reach half of the height of the sigmoid shape increase shown in Figure S5 (Table 3) are within the surfactant concentration cmc range provided in the literature for each surfactant (Table 3). This behavior has been observed in HTMA-PFP/C12E5, regardless of the polymer molecular

oxyethylene groups of the surfactant, as shown in Figure 5A. The slopes of the linear fittings of the minimum distance between the polymer centers of mass versus the surfactant alkyl chain length and as a function of the number of oxyethylene groups are 1.04 (±0.05) × 10−2 and 5.2 (±1.7) × 10−2, respectively. From the comparison of the slopes, it can be concluded that the effect of the number of oxyethylene groups of the surfactant seems to be higher than the effect of the alkyl chain length in inducing the separation of the polymer chains. This means that the effect of the hydrophilic length (j) is stronger than the effect of the hydrophobic length (i) of the surfactant to induce the disruption of polymer aggregates. Another parameter of interest to understand the experimental behavior, as will be discussed below, is the polymer to surfactant distance which is plotted versus the time of simulation, as shown in Figure 5B. During the simulation, surfactant molecules get close to HTMA-PFP chains, regardless of the surfactant considered. After 10 ns of MD simulations, polymer backbones become wrapped around by CiEj molecules, which are located mainly around the polymer backbone with the N(CH3)3+ groups of the polymer oriented toward the solvent (Figure 6). Similar nonspecific interactions between C12E5 and the polyfluorene backbone with the sulfonate groups exposed to the solvent were observed in simulations carried out with PBS-PFP/C12E5 systems.33 Moreover, the formation of polymer−surfactant aggregates has been experimentally tested through the study of the cloud point of the PBS-PFP/C12E5 system32 and other fluorene copolymers and nonionic surfactants.32 In those studies, it was observed that the cloud point of the solution at postmicellar surfactant concentrations decreases if there is polymer in the solution. This behavior is due to the formation of polymer− surfactant aggregates through a process similar to phase separation. G


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Langmuir weight considered, as shown in Table S1 in the Supporting Information. Another spectroscopic parameter that dramatically changes with CiEj concentrations is what we term normalized emission intensity, which is calculated by dividing the polymer emission intensity at 412 nm (I412) upon addition of the surfactant by that of the polymer alone (Figure 7). Sigmoid-shaped curves,

Figure 8. Ratio of the number of surfactant molecules per number of polymer repetitive units at half the height of (A) I412/I434 and (B) I412(polymer−surfactant)/I412(polymer) sigmoid curves as a function of the surfactant concentration (Figures S5 and 7, respectively) vs the number of carbon atoms in the surfactant alkyl chain (i). Figure 7. HTMA-PFP (P1, 5.0 μM)-normalized emission intensity [I412(polymer−surfactant)/I412(polymer)] vs (A) CiE5 molar concentration in open black squares, red circles, green up triangles, and blue down triangles are for C6E5, C8E5, C10E5, and C12E5, respectively, and (B) C12Ej molar concentration in open blue squares, red circles, and black triangles are for C12E5, C12E7, and C12E9, respectively. Surfactant molar concentrations are shown in logarithmic scales. Solid lines represent cmc.

that the presence of the polymer does not affect significantly the surfactant micelle formation. As a consequence, the polymer is a good spectroscopic probe to test the CiEj cmc (for 6 ≤ i ≤ 12 and 5 ≤ j ≤ 9). The enhancement of polyfluorene emission intensity upon addition of C12E5 concentrations above its cmc has been previously described for the anionic PBS-PFP aqueous solutions, anionic fluorene−phenylene co-polymer,19,33 and cationic poly{9,9-bis[6-(N,N,N-trimethylammonium)alkyl]fluorene-co-1,4-phenylene}iodides in aqueous solution.28 In those systems, the spectroscopic changes are attributed to the breakup of polymer aggregates as a consequence of its interaction with the surfactant.19,21,28,31−33 The same process is believed to take place in the HTMA-PFP/CiEj systems studied, as suggested by the MD simulation results discussed above. HTMA-PFP tends to form aggregates in aqueous solution (Figure 2B),36 and the polymer chains split up upon addition of CiEj, as shown by the g(rdf) (Figure 4) and the minimum distance between the centers of mass of the polymer chains (Figure 5A), and this is responsible for the increase in the polymer emission intensity. The enhancement of the normalized emission intensity seems to depend on the surfactant structure, and generally speaking, it seems to increase with the surfactant alkyl chain length (for i between 6 and 10) and decrease with the increase in the number of oxyethylene groups (5 ≤ j ≤ 9), as shown in Figure 7. Experimental (maximum normalized emission intensity upon addition of surfactant concentrations above their cmc) and MD simulation (average distance between the center of masses of the polymer and surfactant) results, presented in Figure 9, show a good correlation. The polymer−surfactant proximity seems to be a key factor, which favors the increase in the emission intensity and defines the ability of the surfactant to solubilize the polyelectrolyte. The interaction between HTMA-PFP and CiEj surfactants is probably maximum when an optimal balance is reached between the hydrophilic and hydrophobic parts of the surfactant structure, and this seems to occur for C10E5 and C12E5.

similar to those obtained by plotting I412/I434 as a function of the surfactant concentration (Figure S5), are also obtained and significant rises in the normalized emission intensities are observed for surfactant concentrations around the cmc, in most of the cases. Surfactant concentrations, at half the height of the jump of the normalized emission intensities area, are also shown in Table 3. To know whether specific interactions between the polymer and surfactants are taking place, the ratio between the number of surfactant molecules and the number of polymer repeat units at half the height of the sigmoid curves I412/I434 (Figure S5) and I412(polymer−surfactant)/I412(polymer) (Figure 7) were plotted as a function of the number of surfactant carbon atoms (i) and the number of surfactant oxyethylene groups (j). No correlation was found in the latter case. However, exponential curves were obtained for the plots versus the number of surfactant carbon atoms (i) (Figure 8). These curves are similar to those obtained by plotting the data of the surfactant cmc (Table 3) versus the number of carbon atoms (i) for CiEj surfactants (Figure S6 in the Supporting Information). From these results, it can be suggested that no specific interactions are present between HTMA-PFP and CiEj surfactants; in contrast, when HTMA-PFP interacts electrostatically with anionic polyelectrolytes such as DNA, maximum emission quenching is observed for the concentration of species, which corresponds to the polymer charge neutralization.36 Moreover, it can be deduced that the main factor controlling the increase in the polymer emission intensity and the changes in the vibrational structure of the HTMA-PFP emission spectra is the surfactant aggregation and the formation of micelles at surfactant concentrations around the cmc, which indicates H


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Figure 9. Plots of the average distance between the mass center of HTMA-PFP and CiE5 (solid line) and maximum normalized emission intensity (dotted line) vs the number of surfactant alkyl carbon atoms.

The effect of the polymer average molecular weight has also been studied. For that, increasing amounts of C12E5 were added to HTMA-PFP solutions in DMSO−water (4%) of the three batches (P1, P2, and P3) to give surfactant concentrations below and above the surfactant cmc. The spectroscopic results are summarized in Figure S7 of the Supporting Information and are similar to those shown for the lowest molecular weight (P1) in Figures 7 and S5. It can be observed that a dramatic increase in the normalized emission intensity (I412/I434) is found around the surfactant cmc, and that the vibronic structure of the emission spectra changes (Figure S7A). The normalized emission intensity in the maximum (Figure S7B) as well as the emission quantum yields (ϕ, Figure S7C) also dramatically increase for C12E5 around its cmc. Whereas the emission quantum yields of the three batches of polymer are around 0.55 in the premicellar region, the maximum values reached in the postmicellar region are 0.67, 0.77, and 0.72 for P1, P2, and P3, respectively. The stronger increase in the emission quantum yield for the batches with higher molecular weight is compatible with the fact that those batches show the most aggregation. As a consequence, the improvement of polymer fluorescence emission upon surfactant addition is more significant for the higher molecular weight polymer batches. Also, time-resolved fluorescence experiments have been carried out with the three polymer batches. Fluorescence decays of HTMA-PFP (P1, P2, and P3) were recorded at 416 nm (excitation at 380 nm) upon various C12E5 additions in the premicellar and postmicellar region in such a way that the effect of both the polymer molecular weight and surfactant concentration can be analyzed. Good global fits of the HTMA-PFP/C12E5 fluorescence decays were obtained with the sum of three exponential terms. A long-lived component at about 450 ps related to the polymer backbone with the highest contribution to the decay (>60% in the less favorable case) and two short-lived components related at around 170 and 30 ps. These lifetimes are rough average values for all decays at different C12E5 concentrations and for the three polymer batches. Data for the fittings are shown in Table S2 in the Supporting Information, and the changes in the species lifetimes and the percentage of contribution of each species to the total emission (% Fi, eq 2) as a function of the surfactant concentrations are shown in Figure 10. Considering the effect of the average polymer molecular weight, the most notable fact is probably related to the differences in the contributions of the longest and short-lived species to the total emission. The contribution of the longest

Figure 10. Lifetimes of the (A) longest, (B) intermediate, and (C) shortest component of HTMA-PFP decays at 416 nm in DMSO− water 4% (v/v) with several C12E5 additions, on the logarithmic scale. Percentage of contribution of the (D) longest, (E) intermediate, and (F) shortest component to the total HTMA-PFP emission vs C12E5 concentration on the logarithmic scale. HTMA-PFP molar concentrations are 5.3, 4.7, and 5.5 μM in the experiments carried out with the three polymer batches P1 (in black full squares), P2 (in blue full triangles), and P3 (in red open squares), respectively. Excitation wavelength = 380 nm. The solid line represents C12E5 cmc.

component (∼450 ps), regardless of the surfactant concentration considered, is higher for the lowest molecular weight polymer and decreases with the increase in the polymer molecular weight (i.e., P1 ≫ P2 > P3 kg/mol, Table S2 and Figure 10D). The contribution of the short-lived species (∼170 and 30 ps) grows with the polymer molecular weight P3 > P2 ≫ P1 (Table S2 and Figure 10E,F). In addition, the differences in % Fi for the smallest and intermediate average molecular weight polymers are higher than the differences between the values of % Fi for the intermediate and highest molecular weight one. If we accept that polymer aggregation increases upon increasing the average polymer molecular weight,78 we can conclude that the percentage contributions of the three lifetime components (particularly the longest and intermediatelived components) are very sensitive parameters to polymer aggregation. This is in good agreement with the previous results obtained with HTMA-PFP/DNA36 and HTMA/sodium alkkysulfonate surfactant29 systems, which associate the shortlived components with polymer aggregation upon charge neutralization between species with an opposite charge. Moreover, the contribution of the longest-lived component reaches maximum values for C12E5 concentrations slightly above the surfactant cmc (Figure 10D and Table S2), whereas at this surfactant concentration, the contribution of the shortlived components reached minimum values (Table S2 and Figure 10E,F). These results are in very good agreement with the molecular simulation results, which show that the cluster of polymer chains splits up upon addition of CiEj (Figure 4). However, for surfactant concentration above the cmc, the contribution of the 450 ps component emission decreases (Figure 10D), whereas those of the 170 and 30 ps species are enhanced (Figure 10E,F). This could be attributed to variations in the polymer chain conformations, which are in good agreement with the changes observed in the vibrational I


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subtracting the specific conductivity of the solvent (and/or CiE5 aqueous solutions) for each system (κexp − κsolvent = κ′). For HTMA-PFP/CiE5 systems with the shortest alkyl chains (C6E5, C8E5, and C10E5), no significant changes in the conductivity of solutions were noted, suggesting that the increase in the charge density of the polymer is dependent on the hydrophobic interactions.33 However, upon addition of C12E5 to the polymer, κ′ starts to increase upon addition of the surfactant, and a change in the slope of κ′ versus C12E5 molar concentrations is observed around the surfactant cmc. This phenomenon was not observed on adding the shortest alkyl chains (C6E5, C8E5, and C10E5). However, results similar to those of the HTMA-PFP/C12E5 system were obtained for the anionic PBS-PFP polyfluorene upon addition of C12E519 and were interpreted in terms of the increase in counterion mobility upon incorporation of the polymer into C12E5 micelles, as confirmed using a sodium selective electrode for the PBS-PFP/C12E5 system.19 An additional factor that contributes to the increase in specific conductivity is the enhancement in the charge density of the polymer, which can be explained by the presence of a higher fraction of free polycation chains due to the polymer aggregate breakup.79 This is in very good agreement with the spectroscopic and MD simulation results. The growth in conductivity is not observed if the surfactant is not able to solubilize the polymer aggregates, as in the case of C8E5. From the MD simulations, it has been concluded that only surfactants with the appropriate alkyl chain length can approximate close enough to the polymer to induce enhancement in the emission intensity (Figure 9) and electrical conductivity (Figure 11).

structure of the emission spectra for C12E5 concentrations around and above the cmc (Figure S4). For this concentration regime, SANS data show the formations of wormlike C12E5/ polymer complexes, which contain rigid segments of 2.4 nm diameter, with the length depending on the polymer molecular weight.28 The formation of these surfactant/polymer complexes can be expected to induce changes in the conformation of the polymer chain, as suggested by the vibrational structure of the polymer emission spectra (Figure S4). It can be concluded that the percentage contribution of short-lived components to the total emission is sensitive to polymer aggregate disruption below the C12E5 cmc and to the changes in the polymer conformation above the cmc because of the formation of wormlike polymer−surfactant complexes. It is also worth noting that there is a decrease in the three lifetime components for C12E5 below the cmc and an increase after the cmc (Figure 10A−C). The decrease in the polymer lifetime below the cmc cannot be explained in terms of polymer aggregate disruption for surfactant concentrations around the cmc because previous studies carried out with two HTMA-PFP molecular weight polymers in acetonitrile/water mixtures show that the lifetime of the three components increases with the improvement of polymer solubility when the percentage of acetonitrile is increased between 10 and 30%.78 It is still unclear why polymer aggregate disruption leads to an increase in lifetimes in acetonitrile/water solution and a decrease in lifetimes when HTMA-PFP interacts with C12E5. This can tentatively be ascribed to the increase in nonradiative deactivation processes (probably due to reorganization of lateral chain polymer substituents upon the initial surfactant additions) or to a hypothetical increase of intrachain energy migration that could be favored by initial polymer conformation changes induced by a low surfactant concentration ( P3), and above the cmc and below 2 × 10−4 M, the increment in conductivity is significantly higher for the highest molecular weight polymer (P3 ≫> P2 > P1). The fact that the effect of the addition of C12E5 on the conductivity of the solution increases with polymer molecular weight for concentrations above the surfactant cmc and J


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aggregated in the absence of a surfactant, show a stronger response to the CiEj surfactant additions. A good correlation has been found between the emission intensity and the polymer−surfactant distances. HTMA-PFP emission intensity is maximum for the systems that show minor polymer−surfactant distances (C10E5 and C12E5), proving that the polymer to surfactant distance is a key factor to be considered to enhance the emission of CPEs through the interaction with surfactants. Time-resolved fluorescence experiments show triexponential decays with a long-lived component at about 450 ps related to the polymer backbone and two short-lived components around 170 and 30 ps. The percentage contribution of short-lived components to the total emission seems to be determined by polymer aggregate disruption for C12E5 concentration below the cmc and for polymer changes in conformations above the cmc. The latter seems to be the main factor affecting the HTMA-PFP lifetimes. Specific conductivity of the polymer in HTMA-PFP/C12E5 systems grows significantly at surfactant concentrations close to the cmc and is explained in terms of the breakup of polymer aggregates. In addition, with polymer molecular weight, variations in the specific conductivity increments, Δκ′, ′ ′ (Δκ′ = κHTMA ‐PFP/C12E5 − κHTMA ‐PFP) of the polymer have been observed. Below the cmc, the lowest molecular weight polymer shows the highest increases, whereas above the cmc, the largest increases are observed for the highest molecular weight polymer. The counterion mobility is thought to be responsible for the changes observe in Δκ′ below the cmc and the increase in the charge density of the nonaggregated polymer chain is thought to determine Δκ′ variations above the surfactant cmc. Because conductivity is less sensitive than fluorescence, only for the most favorable system (HTMA-PFP/C12E5), HTMAPFP can be used as a probe to determine the cmc. We believe that these results may have implications in the design of CPE/surfactant systems for applications in sensing or other areas of colloidal and materials science.

decreases below the cmc (Figure 11) could be an indication that two different phenomena are leading to changes in conductivity in the two surfactant concentration ranges. Tentatively, an increase in the counterion mobility may be the main factor responsible for the increase Δκ′ below the surfactant cmc, provided that dissociation can be expected to be higher for the lower charge (P1). However, the increase in the charge density of the polymer after the polymer aggregate breakup can be thought to govern the increase of Δκ′ above the surfactant cmc. This is easily justified by an increase in the number of repeat units available for ionic transport with the polymer molecular weight; in fact, taking the molecular weight of the repeat units as 684.62 g/mol and the M̅ n values, the number of repeat units for P1, P2, and P3 are, respectively, 21, 44, and 89. It is worth noting that only the interaction between HTMAPFP and C12E5 is favorable enough for the polymer to be used as a conductivity probe for surfactant micelle formation (Table 3), confirming the spectroscopic and simulation results which points out to C12E5 as one of the surfactants of the family of CiEj, which solubilize better HTMA-PFP aggregates and approximate closer to the polymer backbone.

4. SUMMARY AND CONCLUSIONS The effect of nonionic alkyl polyoxyethilene surfactants (CiEj) on the spectroscopy and conductivity of three molecular weight HTMA-PFP polyelectrolyte batches has been studied. In addition, MD simulations have been carried out to rationalize the experimental results. Snapshots of the simulation box show that HTMA-PFP trimers approximate each other during the equilibration time, and to some extent, π−π interaction occurs between the conjugated polymer backbones. However, the minimum distance between the center of masses of the closest HTMAPFP trimers increases upon addition of the surfactant to the simulation box and grows linearly with the surfactant alkyl chain length and with the number of oxyethylene groups of the surfactant. The effect of the hydrophilic length (j) seems to be stronger than the effect of the hydrophobic length (i) to separate HTMA-PFP trimers, according to the slope of the fitting lines. From the simulations, nonspecific interactions between CiEj and HTMA-PFP backbone are observed, and polymer−surfactant complexes are formed with surfactant molecules wrapped around the polymer backbone. The photophysical study shows a dramatic increase in the I412(polymer−surfactant)/I412(polymer) emission intensity ratio with the addition of the surfactant and relevant changes in the vibrational structure of the emission spectra followed by the ratio of emission intensities, I 412 /I 434 . Plots of I412(polymer−surfactant)/I412(polymer) and I412/I434 emission intensity ratios versus surfactant molar concentration lead to sigmoid-shaped curves. The surfactant concentrations at half the height of the increase of both ratios of intensities are close to the surfactant critical micelle concentrations, indicating that HTMA-PFP is a good fluorescent probe to determine the CiEj cmc (for 6 ≤ i ≤ 12 and 5 ≤ j ≤ 9). This indicates that the presence of the polymer does not affect significantly the surfactant micelle formation. Moreover, from the photophysical results, it can also be concluded that no specific interactions take place between HTMA-PFP and CiEj surfactants, which is in good agreement with MD simulations. The polymer molecular weight plays an important role, and batches with higher molecular weights, which are more

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Informationis available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b02818. HTMA-PFP anisotropy versus wavelength for P1, P2, and P3; triple-exponential global analysis of fluorescence decays of HTMA-PFP (P1) in DMSO−water 4% (v/v), excited at 380 nm and measured at 416 and 450 nm; variation of absorbance at 381 nm and emission intensity at 412 nm of HTMA-PFP in DMSO−water 4% (v/v) mixtures versus polymer concentration for P1, P2, and P3; normalized absorption and emission spectra of HTMA-PFP in DMSO−water 4% (v/v) with C12E5 concentrations below and above the surfactant cmc; ratio of emission intensities of HTMA-PFP (P1) in DMSO−water 4% (v/v) in the maximum (412 nm) and in the first shoulder (434 nm) as a function of the surfactant molar concentration on the logarithmic scale; surfactant concentration at half the height of the increase in the ratio (I412/I434) and the increase in the normalized emission intensity of HTMA-PFP versus C12E5 concentration; exponential fitting of cmc values for CiEj K


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C14E8, Hexadecyl C16E8, and Octadecyl C18E8 Ethers. J. Phys. Chem. B 2005, 109, 6990−6998. (10) Günes, S.; Neugebauer, H.; Sariciftci, N. S. Conjugated Polymer-Based Organic Solar Cells. Chem. Rev. 2007, 107, 1324− 1338. (11) Imanishi, K.; Einaga, Y. Wormlike Micelles of Polyoxyethylene Alkyl Ether Mixtures C10E5 + C14E5 and C14E5 + C14E7: Hydrophobic and Hydrophilic Chain Length Dependence of the Micellar Characteristics. J. Phys. Chem. B 2007, 111, 62−73. (12) Einaga, Y. Wormlike Micelles of Polyoxyethylene Alkyl Ethers CiEj. Polym. J. 2009, 41, 157−173. (13) Einaga, Y.; Kusumoto, A.; Noda, A. Effects of Hydrophobic Chain Length on the Micelles of Heptaoxyethylene Hexadecyl C16E7 and Octadecyl C18E7 Ethers. Polym. J. 2005, 37, 368−375. (14) Sirringhaus, H.; Kawase, T.; Friend, R. H.; Shimoda, T.; Inbasekaran, M.; Wu, W.; Woo, E. P. High-Resolution Inkjet Printing of All-Polymer Transistor Circuits. Science 2000, 290, 2123−2126. (15) de Gans, B.-J.; Duineveld, P. C.; Schubert, U. S. Inkjet Printing of Polymers: State of the Art and Future Developments. Adv. Mater. 2004, 16, 203−213. (16) Scherf, U.; List, E. J. W. Semiconducting Polyfluorenes Towards Reliable Structure-Property Relationships. Adv. Mater. 2002, 14, 477−487. (17) Burrows, H. D.; Knaapila, M.; Fonseca, S. M.; Costa, T. Aggregation Properties of Conjugated Polyelectrolytes. In Conjugated Polyelectrolytes. Fundamentals and Applications in Emerging Technologies; Liu, B., Bazan, G. C., Eds.; Wiley-VCH: Weinheim, 2013; Chapter 4. (18) Dwight, S. J.; Gaylord, B. S.; Hong, J. W.; Bazan, G. C. Perturbation of Fluorescence by Nonspecific Interactions Between Anionic Poly(phenylenevinylene)s and Proteins: Implications for Biosensors. J. Am. Chem. Soc. 2004, 126, 16850−16859. (19) Burrows, H. D.; Lobo, V. M. M.; Pina, J.; Ramos, M. L.; de Melo, J. S.; Valente, A. J. M.; Tapia, M. J.; Pradhan, S.; Scherf, U. Fluorescence Enhancement of the Water-Soluble Poly{1,4-phenylene[9,9-bis(4-phenoxybutylsulfonate)]fluorene-2,7-diyl} copolymer in ndodecylpentaoxyethylene Glycol Ether Micelles. Macromolecules 2004, 37, 7425−7427. (20) Decher, G. Fuzzy Nanoassemblies: Toward layered Polymeric Multicomposites. Science 1997, 277, 1232−1237. (21) Burrows, H. D.; Fonseca, S. M.; Silva, C. L.; Pais, A. A. C. C.; Tapia, M. J.; Pradhan, S.; Scherf, U. Aggregation of the Hairy Rod Conjugated Polyelectrolyte Poly{1,4-phenylene-[9,9-bis(4phenoxybutylsulfonate)]fluorene-2,7-diyl} in Aqueous Solution: an Experimental and Molecular Modelling Study. Phys. Chem. Chem. Phys. 2008, 10, 4420−4428. (22) Ma, W.; Iyer, P. K.; Gong, X.; Liu, B.; Moses, D.; Bazan, G. C.; Heeger, A. J. Water/Methanol-Soluble Conjugated Copolymer as an Electron-Transport Layer in Polymer Light-Emitting Diodes. Adv. Mater. 2005, 17, 274−277. (23) Wang, S.; Bazan, G. C. Solvent-Dependent Aggregation of a Water-Soluble Poly(fluorene) Controls Energy Transfer to Chromophore-Labeled DNA. Chem. Commun. 2004, 0, 2508−2509. (24) Chen, L.; Xu, S.; McBranch, D.; Whitten, D. Tuning the Properties of Conjugated Polyelectrolytes Through Surfactant Complexation. J. Am. Chem. Soc. 2000, 122, 9302−9303. (25) Lavigne, J. J.; Broughton, D. L.; Wilson, J. N.; Erdogan, B.; Bunz, U. H. F. “Surfactochromic” Conjugated Polymers: Surfactant Effects on Sugar-Substituted PPEs. Macromolecules 2003, 36, 7409− 7412. (26) Burrows, H. D.; Valente, A. J. M.; Costa, T.; Stewart, B.; Tapia, M. J.; Scherf, U. What Conjugated Polyelectrolytes Tell us About Aggregation in Polyelectrolyte/Surfactant Systems. J. Mol. Liq. 2015, 210, 82−99. (27) Abe, S.; Chen, L. Tuning the Photophysical Properties of an Ionic Conjugated Polymer Through Interactions with Conventional Polyelectrolytes. J. Polym. Sci., Part B: Polym. Phys. 2003, 41, 1676− 1679. (28) Burrows, H. D.; Knaapila, M.; Monkman, A. P.; Tapia, M. J.; Fonseca, S. M.; Ramos, M. L.; Pyckhout-Hintzen, W.; Pradhan, S.;

surfactants as a function of the number of carbon atoms (i); ratio of the emission intensities at the maximum I(412) and first shoulder I(434); normalized emission intensity in the maximum and emission quantum yield of the three HTMA-PFP batches versus C12E5 molar concentration on the logarithm scale; and HTMA-PFP lifetime decays at 416 nm in DMSO−water 4% (v/v) mixtures with several C12E5 additions (below and above its cmc) for the three polymer batches (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +351 239 854482. Fax: (+351) 239 827703 (H.D.B.). *E-mail: [email protected]. Phone: +34 947 258820. Fax: (+34) 947 258831 (M.J.T.). ORCID

Artur J. M. Valente: 0000-0002-4612-7686 A. A. C. C. Pais: 0000-0002-6725-6460 María J. Tapia: 0000-0001-9943-245X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Dr. Ricardo Mallavia is thanked for kindly providing us with HTMA-PFP. MEC and FEDER are thanked for the financial support through MAT2004-03827 and MAT MAT2008-06079. The group in Coimbra are grateful for funding from “The Coimbra Chemistry Centre”, which is supported by the Fundaçaõ para a Ciência e a Tecnologia (FCT), Portuguese Agency for Scientific Research, through the programs UID/ QUI/UI0313/2013 and COMPETE.

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