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Dyes and Pigments 112 (2015) 262e273

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Dyes and Pigments

journal homepage: www.elsevier .com/locate/dyepig

Self-association of 5,10,15,20-tetrakis-(4-sulfonatophenyl)-porphyrintuned by poly(decylviologen) and sulfobutylether-b-cyclodextrin

Mario E. Flores a, Naoki Sano b, Rodrigo Araya-Hermosilla a, Toshimichi Shibue c,Andr�es F. Olea d, Hiroyuki Nishide b, Ignacio Moreno-Villoslada a, *

a Instituto de Ciencias Químicas, Facultad de Ciencias, Universidad Austral de Chile, Casilla 567, Valdivia, Chileb Department of Applied Chemistry, School of Science and Engineering, Waseda University, Tokyo 169-8555, Japanc Materials Characterization Central Laboratory, School of Science and Engineering, Waseda University, Tokyo 169-8555, Japand Facultad Ciencias de la Salud, Universidad Aut�onoma de Chile, Santiago, Chile

a r t i c l e i n f o

Article history:Received 11 February 2014Received in revised form17 July 2014Accepted 18 July 2014Available online 28 July 2014

Keywords:5,10,15,20-Tetrakis-(4-sulfonatophenyl)-porphyrinSulfobutylether-b-cyclodextrinPoly(decylviologen)Molecular assemblyKineticsTernary complex

* Corresponding author. Fax: þ56 63 2293520.E-mail address: imorenovilloslada@uach.cl (I. Mor

http://dx.doi.org/10.1016/j.dyepig.2014.07.0150143-7208/© 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

With the aim of achieving supramolecular structures containing dyes with controlled state of aggrega-tion, the formation of ternary complexes between 5,10,15,20-tetrakis-(4-sulfonatophenyl)-porphyrin,poly(decylviologen), and sulfobutylether-b-cyclodextrin at low excess of the cyclodextrin is described. Bycontrolling the stoichiometry of the reactants, both non-fluorescent H-aggregates of the dye and sta-bilized fluorescent monomers may be obtained. The dye undergoes self-aggregation in the presence ofthe flexible, cationic polymer, and by addition of the negatively charged cyclodextrin the aggregates arecleaved. The cyclodextrin primarily induces the inclusion of 5,10,15,20-tetrakis-(4-sulfonatophenyl)-porphyrin in its cavity, and the ensemble is stabilized by the polymer by means of electrostatic, hy-drophobic, and/or aromaticearomatic interactions. The ternary complex thus produced tends to nano-fiber formation, as seen by FE-TEM.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The self-aggregation tendency of aromatic dyes such as xan-thene dyes and porphyrins at increasing concentrations in aqueoussolution is well known [1e15]. Dispersion forces and aroma-ticearomatic interactions stabilize the aggregates. Higher orderaggregates of charged dyes can be found on complementarycharged surfaces. The charge compensation produced on theseenvironments minimizes the charge repulsion, produces a higherlocal concentration of dyes, and the dyes self-aggregate, frequentlyundergoing cooperative binding [16]. Examples of dye self-aggregation in the presence of polyelectrolytes are methyleneblue (MB) [11] and the tetra-anionic form of 5,10,15,20-tetrakis-(4-sulfonatophenyl)-porphyrin (H2TPPS4�) [4,6] interacting with thepolyanion poly(sodium vinylsulfonate) (PVS) and the polycationpoly(allylamine) (PALA), respectively. The interaction of these dyeswith these polyelectrolytes is considered non-site-specific,

eno-Villoslada).

producing territorial binding, thus governed by long-range elec-trostatic interactions. That makes this interaction very sensitive tothe presence of other ions in solution. As a consequence of this, theincorporation of such dye/polyelectrolyte complexes in solid ma-terials by solvent evaporation or deposition techniques may findthe drawback of dye release from the complexes, and a consequentcrystallization, non-homogenous accumulation, polydisperse ag-gregation, or bad control of the dye distribution in the solid.

The formation of materials where the dyes are homogenouslydispersed, together with furnishing a good control of the dye stateof aggregation is then of interest. For many dye applications such aselectron transfer agents in electronic devices [17], or photosensi-tizers in photodynamic therapy [18], aggregation may be an un-wanted feature since it may reduce light harvesting and minimizethe efficiency of energy transfer. However, recent studies highlightthe potential of organic dyes aggregates for improving the globalenergy-conversion efficiencies of organic dye-sensitized solar cellsand photocatalysts under the requirement of achieving a goodcontrol of the dye aggregation in the assembled structures [19,20].It is by means of secondary interactions such as hydrogen bondingor aromaticearomatic interactions, apart from primary long-range

M.E. Flores et al. / Dyes and Pigments 112 (2015) 262e273 263

electrostatic interactions, that a good control of the assembledstructures is achieved by ionic self-assembly (ISA) [21e28]. Thisproperty has been elegantly used to form interesting nano-structures between polyelectrolytes or dendrimers and dyes[25e28].

We have shown that when polyelectrolytes furnish comple-mentary charged aromatic groups, interactions between the dyesand the polymeric aromatic groups are stabilized by short-rangearomaticearomatic interactions, which leads to site-specific bind-ing [2,4,5,11,12,14,29e34]. The extent of binding and the state ofaggregation of different dyes in the presence of such poly-electrolytes strongly depends on the structure of the polymer as awhole, and variables such as functional groups, localization of thecharge, flexibility, hydrophobia, linear charge density, and lineararomatic density determine the behavior of the system [4,5,12,34].Among aromatic polyelectrolytes, polyviologens are interestingpolymers due to their electro- and photoactive properties. Viol-ogens, 1,10-disubstituted-4,40-bipyridinium dications, are wellknown in both electrochemistry and photochemistry [35e38], inparticular for their reversible redox behavior accompanied by acolor change that make them useful for electrochromic displays[35,36]. Viologen derivatives are important molecules used inelectronic devices as electron acceptors. Polyalkylviologens areparticularly attractive due to the possibility of modulating theinterunit spacing through changes of the alkyl chain length. Theyare usually prepared by theMenschutkin reaction of 4,40-bipyridineand dihaloalkanes in aprotic solvent [39]. The resulting linearpolymers are water-soluble. Ionic porphyrins are commonly usedas water-soluble electron-transfer photosensitizers, and are oftenused in conjunction with viologens as electron acceptors [40,41].Porphyrin-viologen pairs have been used for photoinduced elec-tron transfer in a variety of solution-phase and supramolecularassemblies [42,43]. As aromatic cationic polyelectrolytes, poly-alkylviologens produce strong binding to anionic porphyrins,which may be useful for the incorporation of the dye in differentstructures in a controlled manner. Therefore, designing strategiesfor the control of the dye state of aggregation in the presence ofpolyalkylviologens are of interest.

The biocompatible cyclodextrins (CDs) are good hosts for in-clusion complexes [44e48]. Low molecular-weight molecules

Fig. 1. Molecular

containing hydrophobic groups of a defined size may form com-plexes with CDs by interaction with the hydrophobic internal faceof the CD torus. Among these guest molecules, both substitutedporphyrins [49e53] and viologen derivatives [49,54] have beendescribed to be hosted by CDs. They can also form rotaxanes andpseudorotaxanes [55e60] by means of threading on polymericchains, in some cases containing viologen units [58]. The CDs areable to spin around the axes of the polymer chains as well as moveback and forth along the polymer backbones. These molecular ar-chitectures find potential applications in nanostructured functionalmaterials, molecular switches, molecular logic gates, molecularwires, and memory devices, and contribute to advances in molec-ular electronics and the construction of artificial molecular ma-chines. The low solubility inwater of some CDs has limited their usein several applications such as pharmaceutical formulations. In thisrespect, sulfobutylether-b-cyclodextrin (SB-b-CD) overcomes thisproblem by bearing 6e9 negative charges permolecule that furnishhigh solubility in water.

The photosensitizer 5,10,15,20-tetrakis-(4-sulfonatophenyl)-porphyrin (TPPS) has interesting physicochemical properties con-cerning electronic transitions and acidebase equilibriumwhich aresensitive to its close environment. At pH 7, the basic form of TPPS(H2TPPS4�) is found, which bears four negative charges at its pe-riphery, as can be seen in Fig. 1 [61,62]. In this paper, molecularassociation at pH 7 between H2TPPS4� and the viologen derivativesmethylviologen (MV) and poly(decylviologen) (PV10) will beshown. The interaction between PV10 and SB-b-CD, and the inter-action between H2TPPS4� and SB-b-CD will be also analyzed. Then,as an absolute novelty of the present work, the formation of aternary complex produced by adding SB-b-CD to H2TPPS4�/PV10complexes will be described, revealing monomeric H2TPPS4�

imbibed in the H2TPPS4�/SB-b-CD/PV10 ternary complex, so that agood control of the state of aggregation of the dye can be achievedin these complexes. Ternary complexes that include CDs, poorwater-soluble substances, and hydrophilic polymers have beendescribed mainly to increase the solubility of poor water-solubledrugs and decrease the excess of CD needed to generate the drug/CD inclusion complex [63e66], and the hydrophilic polymers usedare mainly uncharged polymers currently approved for pharma-ceutical uses. However, the components of our research are all

structures.

Fig. 2. A) UVevis spectra of 1 � 10�5 M H2TPPS4� solutions in the absence of anypolyelectrolyte (a), and in the presence of 1 � 10�4 M of PV10 (b), 1 � 10�3 M of SB-b-CD (c), and 1 �10�4 M of PV10 and 1 �10�3 M of SB-b-CD (d), 1 �10�2 M of MV (e); B)corresponding fluorescence spectra divided by the absorbance at the excitationwavelength: 425 for (a’ and c’), 394 nm for (b’), 421 for (d’).

M.E. Flores et al. / Dyes and Pigments 112 (2015) 262e273264

charged, readily water-soluble, so that the assemblies are based onISA principles. We will show by performing kinetics studies, thatthe charge repulsion between the negatively charged SB-b-CD andH2TPPS4� can be overcome by electrostatic interaction with thepositively charged PV10, so that the inclusion of the dye in the CDcavity is promoted. Besides, nanofibers of the ternary complex willbe shown. DLS, TEM, 1H NMR and UVevis spectroscopies are usedfor these analyses.

2. Experimental

2.1. Reagents

SB-b-CD (Cargill), MV (Aldrich), and TPPS (TCI) were commer-cially available and used as received. PV10 was synthesized usingmethanol (Kanto), dimethylformamide (Kanto), and chloroform(Kanto) as solvents, and 4,4-bipyridine (TCI) and 1,10-dibromodecane (Aldrich) as reactants. Solutions of these re-actants were prepared in deionized distilled water, or in D2O(Aldrich) for NMR studies. The structures of the molecules used inthis study are shown in Fig. 1. The concentration of PV10 is given inABmolar basis (A and B being the bipyridinium and decamethyleneunits, respectively), considering a molecular mass of 456 g/mol. SB-b-CD presents a certain polydispersity concerning the number ofsulfobutyl ether groups per b-CD unit, which is informed by themanufacturer to be between 7 and 9 and its molecular mass hasbeen averaged to 2399 g/mol.

2.2. Equipment

Distilledwater was deionized in a SimplicityMillipore deionizer.The pHwas controlled on a UltraBasic Denver Instrument pHmeter.A thermoregulated SHEL LAB bath was used for temperature ad-justments. UVevis measurements were performed in a Helios gspectrophotometer. Fluorescence measurements were done in aPerkin Elemer LS50b fluorometer. 1H NMR measurements weremade in a JNM-Lambda 500 (JEOL, 500MHz) spectrometer. Kineticsmeasurements were done in a stopped flow reactor SFM-3000/Sequipped with MOS-200 optical system (Bio-Logic) and governedby the BioKine32 V4.66 software. Apparent size of molecular as-semblies were obtained by dynamic light scattering (DLS) in a NanoZS zetasizer instrument (Malvern) with backscatter detection(173�). Size results were considered valid under the criteria ofDispersion Technology Software (DTS 6.2, Malvern). Microscopymeasurements were done in a Hitachi Field Emission TransmissionElectron Microscope (FE-TEM, model HD-2200) using 100 Meshcarbon-coater copper grids.

2.3. Procedures

Conventional and well-known procedures have been followed.Particular experimental conditions are provided in the captions toFig. 2 e 15. Aqueous samples were prepared from stock solutions ofthe different reactants. The pH was kept at 7 at which the dye isfound in its tetra-anionic form (H2TPPS4�). In order to avoid anincrease on the ionic strength, no buffer was used to keep a con-stant pH value, but instead, it was adjusted with minimumamounts of NaOH and HCl when necessary. The temperature hasbeen kept at 298 K unless other temperature conditions are re-ported. Quartz vessels of 0.2 and 1.0 cm of optical path length havebeen used for UVevis absorption analyses. When necessary forcomparisons, the data has been corrected by multiplying by 0.2 or5.0 depending on the vessel used. Fluorescence measurementshave been done in a quartz vessel of 1 cm of optical path length. 1HNMR experiments have been done in D2O at 298 K, as well as DLS

measurements. At least three repetitions of all the experimentswere done under comparable conditions with good reproducibility.Prior to FE-TEM measurements, the samples were prepared bydrop-casting onto copper grids of a few drops of the samples in ahot plate.

Synthesis of PV10: Poly(decyl viologen) (PV10), was synthesizedby the methodology previously reported in the literature [35,39].4,40-bipyridine (5.21 g, 0.033 mol) and 1,10-dibromodecane (10.0 g,0.033 mol) were reacted in 50 mL of a 1/1 methanol/DMF mixtureat 60� C overnight. The resulting yellow precipitate was collected,washed with chloroform and dried under vacuum to yield PV10 asa yellowish powder (11.5 g, 0.025 mol, 76% yield). The molecularweight of the resulting polymer was determined by 1H NMR to beMn ¼ 6000 g/mol, by comparison of the intensities of the peaksascribed to the protons on the terminal pyridyl group at 8.79, 8.61,8.24 and 7.73 ppm, and those of the pyridyl groups in the mainchain around 8.95 and 8.38 ppm. That resulted on an average of 13bipyridinium groups per macromolecular chain. The elementalanalysis of PV10 gives 51.5% of C, 6.4% of H, and 6.1% of N, whilstthe theoretical values are 50.8% of C, 6.4% of H, and 5.9% of N. 1HNMR characterization (500 MHz; D2O; Me4Si): d (ppm) 8.95 (2H,broad, a-H of pyridinium CH), 8.79 (2H, d, term a-H of pyridiniumCH), 8.61 (2H, d, term a-H of pyridinium CH), 8.38 (2H, broad, b-Hof pyridinium CH), 8.24 (2H, d, term b-H of pyridinium CH), 7.73(2H, d, term b-H of pyridinium CH), 4.60 (4H, broad,2 � NCH2CH2C3H6), 3.47 (2H, d, term CH2), 1.92 (4H, broad,2 � NCH2CH2C3H6), 1.15 (12H, broad, 2 � NCH2CH2C3H6), 1.00 (2H,broad, term CH2).

3. Results and discussion

3.1. Complexation of H2TPPS4� with viologen derivatives

Complexes between porphyrins and viologen derivatives havebeen reported in the literature [67]. Shifting of the Soret band tolower energies in UVevis spectroscopic experiments due to closebinding has been reported as a sign of molecular association. Inorder to verify this, the interaction of the model compound MVwith H2TPPS4� has been studied by both UVevis and 1H NMRspectroscopies. As can be seen in Fig. 2A, upon a large excess (1000-fold) of the aromatic cation MV, the Soret band of a 1 � 10�5 M ofH2TPPS4� solution is shifted from 414 nm to 417 nm, a fact that isascribed to the stacking of MV molecules on the porphyrin [67].

Table 1Chemical shifts (ppm) of 1H of MV and their upfield shifting in the presence ofH2TPPS4�.

Solution d H0o�

(ppm)Dd H0

o�

(ppm)d H0

m�

(ppm)Dd H0

m�

(ppm)d CH3-(ppm)

Dd CH3-(ppm)

MV 2 � 10�3 M 8.9426 8.4195 4.4013MV 2 � 10�3 M/H2TPPS4�

2 � 10�4 M8.7663 0.1763 8.1676 0.2519 4.2754 0.1259

M.E. Flores et al. / Dyes and Pigments 112 (2015) 262e273 265

The stacking between both molecules is due to aroma-ticearomatic interactions. 1H NMR is a good technique to probe thestacking of aromatic molecules, since upfield shifts of 1H reso-nances for the stacked rings can be foundwhen one ring is placed inthe shielding cone of the other. The drawback of this technique isthe high concentration detection limit of the molecules. This isparticularly clear when trying to observe TPPS by 1H NMR. Theconcentration needed is often too high to avoid aggregation of themolecule in form of large aggregates, associated or not with othermolecules. This aggregation produces a decrease on the molecularmobility, so that the 1H NMR signals broaden and tend to disappear.The spectrum of 2 � 10�4 M of H2TPPS4� is shown in Fig. 3, and theassignments are related to the structures shown in Fig. 1. In thepresence of 10-fold MV, the signals of the porphyrin are scarcelynoticeable due to its relative low concentration. The two smallpeaks appearing at 7.8 and 8.1 ppm may be attributable toporphyrin protons, upfield shifted by the influence of the viologenunits close to it, as a consequence of molecular association. Besides,it can be clearly seen that the signals of MV are upfield shifted asmuch as 0.18 and 0.25 ppm for the aromatic protons, H0

o and H0m, as

can be read in Table 1. The methyl groups of MV are also upfieldshifted 0.13 ppm, as can be seen in Table 1, a fact that may be dueboth to the influence of the magnetic fields produced by the aro-matic electronic currents, and by the decrease on the electronwithdrawing effect of the bipyridine group, since the positivecharge is stabilized by the negative charges of the porphyrin. Notethat a lower excess of MV is needed to observe significant changesin NMR signals. This is due to that the higher concentration of thereactants produces an increase on the probability to undergomutual stacking.

As observed for other charged aromatic polymeric compounds[2e6], dispersion of the porphyrinmolecules on PV10was expecteddue to aromaticearomatic interactions between the dye and thepolymeric aromatic charged groups, producing higher shifts tolower energies of the dye Soret band. On the contrary, when thepolycations are aliphatic, such as in the case of PALA, the highcharge density of the polymers produces a higher local concen-tration of H2TPPS4� around them, inducing its self-stacking in theform of sandwich-like aggregates (H-aggregates) [2e6]. However,

(ppm)Fig. 3. 500 MHz 1H NMR spectra of solutions containing: (a) 2 � 10�4 M of H2TPPS4�;(b) 2 � 10�3 M of MV; (c) 2 � 10�4 M of H2TPPS4� and 2 � 10�3 M of MV.

clear H-type self-aggregates of H2TPPS4� are also formed in thepresence of PV10, and a band at 393 nm (H-band) appears inUVevis experiments, as can be seen in Fig. 2A. Consistent to theformation of H-aggregates, the fluorescence of the samples isquenched, as can be seen in Fig. 2B, since the triplet state becomesmore stable than the singlet state of the excited dye for these ag-gregates. This results strongly support the observations made withpolyanions and positively charged dyes, concerning the variablesthat determine the behavior of these systems, among which thenature of the functional groups, localization of the charges in thepolymer structure (mainly on aromatic rings or in aliphatic groups),flexibility, hydrophobia, linear charge density, and linear aromaticdensity in the polyelectrolytes are determinant [12,34]. If normallya competition between binding to the polymeric charged aromaticgroups and the self-binding can be observed for vinyl aromaticpolyelectrolytes [4], in this case, the stabilization of H-aggregatesby PV10 may be related to that the polymeric aromatic groups areplaced along the polymer backbone, and thus the stabilized struc-tures do not present intercalation of aromatic cationic groups be-tween dyes; the low linear charge and aromatic density of thispolymer, and its high flexibility may contribute to stabilize acompact structure.

The molecular assembly thus produced showed submicron size,as revealed by DLS experiments. This can be seen in Fig. 4, wherecorrelograms corresponding to mixtures containing 1 � 10�5 M ofH2TPPS4� and 1 �10�4 M of PV10 are plotted. The apparent size ofthe assembly takes a value 240 nm, showing a zeta potential of32.5mV, (plotted as ordinate at the origin in Fig. 5). Assays to obtain

Fig. 4. Correlograms obtained by DLS of aqueous solutions containing 1 � 10�5 M ofH2TPPS4�, 1 �10�4 M of PV10, and SB-b-CD at concentrations: (a) 0 M; (b) 2 � 10�6 M;(c) 4 � 10�6 M; (d) 1 � 10�5 M; (e) 6 � 10�5 M.

Fig. 5. Zeta potential of aqueous solutions containing 1 � 10�5 M of H2TPPS4�,1 � 10�4 M of PV10, and variable amounts of SB-b-CD as a function of the SB-b-CD/H2TPPS4� relative concentration.

Table 2Fit parameters for complexation kinetics of H2TPPS4� with PV10 and SB-b-CD onPV10.

Complex a b k1 (s�1) c k2 (s�1) pfast pslow

PV10/H2TPPS4- 0.1029 0.0619 0.3113 0.0222 0.0726 0.74 0.26H2TPPS4�/PV10/SB-b-CD 0.0718 0.0077 1.268 0.0037 0.2315 0.68 0.32

M.E. Flores et al. / Dyes and Pigments 112 (2015) 262e273266

a 1H NMR spectra of the PV10/H2TPPS4� complex at concentrationsof the dye higher than 10�4 M produced disappearance of theH2TPPS4� signals, consistent with the nanoprecipitate nature of thecomplex.

Accommodation of the molecules forming the complexes re-quires relative long time, as can be concluded after kinetics studiesperformed with the aid of a stopped-flow reactor. The disappear-ance of the band at 414 nm corresponding to monomeric H2TPPS4�

has been followed as shown in Fig. 6. It can be seen that the com-plex formation process ranges in the order of several seconds. Theexperimental curves of absorbance at 414 nm against time fit tobiexponential kinetics according to the following equation:

AðtÞ ¼ aþ b expð � k1tÞ þ c expð � k2tÞ (1)

where ki denotes the rate constants and k1 » k2. This adjustmentindicates that two different processes are contributing to thedisappearance of the band at 414 nm. The coefficients b and c give ameasure of the contributions of each process to the reaction: thefraction of H2TPPS4� reacted by the fastest process is given bypfast ¼ b/(b þ c), and by the slowest process is given by pslow ¼ c/(b þ c). The parameter a corresponds to the ordinate at the origin

0 10 20 30 40

0,10

0,12

0,14

0,16

0,18

0,20

0,072

0,074

0,076

0,078

0,080

0,082

0,084

abso

rban

ce (4

21 n

m)

abso

rban

ce (4

14 n

m)

time (s)

Fig. 6. Complex formation kinetics profiles obtained by mixing two equal volumes of(a) PV10 2 � 10�4 M, and H2TPPS4� 2 � 10�5 M, and (b) H2TPPS4� 2 � 10�5 M/PV102 � 10�4 M complex and SB-b-CD 2 � 10�4 M.

and its value is affected by the dead time between mixing andstartingmeasuring. The fit parameters given in Table 2 indicate thata fraction of 74% of H2TPPS4� reacted by the fastest process with arate constant k1 ¼0.3113 s�1. A fraction of 26% followed the slowestprocess with a value of k2 of 0.0726 s�1. The initial interaction be-tween H2TPPS4� and PV10 leads to the adsorption of H2TPPS4� onthe polymer backbone. This step should be a diffusion-controlledreaction and consequently is beyond the time resolution of thestopped-flow technique. Therefore, the decrease of absorbance at414 nm occurring in the millisecond range is produced by muchslower reactions leading to formation of H-aggregates. The aggre-gation of H2TPPS4� involves the redistribution of the initiallyadsorbed H2TPPS4� molecules and necessarily requires the move-ment of polymer chains. Considering that the PV10 concentration,given in mole of bipyridinium groups per liter, is one order ofmagnitude higher than that of H2TPPS4�, and that each polymerchain carries an average of 13 bipyridinium groups, in the initialdistribution a number of PV10 molecules may host only oneH2TPPS4� molecule by polymer chain. Thus, the two processescontributing to the disappearance of the band at 414 nm may berelated to the aggregation of interacting dyes bound in a singlepolymeric chain, for the fastest process, and, for the slower process,to aggregation of dyes confined in separate polymer chains, thusbeing necessary the movement from one polymeric chain toanother. Note that this model implies a cooperative binding ofH2TPPS4� in the presence of PV10.

3.2. PV10 e SB-b-CD interaction

Rotaxanes and pseudorotaxanes formation between CDs andviologen derivatives has been reported in the literature [57e59],where complexation between PV10 of 17 AB units (A and B beingthe bipyridinium and the decamethylene units, respectively) andneutral CDs has been reported. The complex consisted of one CDfor each AB unit, whereas the length of every decamethylene unitdoubles the depth of the CD cavity, allowing CD translationmovements. For uncharged CDs, slow kinetics are reported[57,59] and the threading process is completed in the course ofseveral hours. This is reported to be due to that the viologencharges act as stoppers on the threading process. In the case ofthe interaction between PV10 and SB-b-CD, two different mech-anisms are expected. A fast mechanism consists on the electro-static interaction between both species, since they bearcomplementary charges. Electrostatic complexes may thus beformed. Threading of the SB-b-CD on the polymer due to hydro-phobic forces may constitute a second slower mechanism ofinteraction. Stabilization of a pseudorotaxane between bothspecies is in principle possible due to both electrostatic and hy-drophobic interactions. However, close electrostatic interactionbetween external sulfate CD groups and viologen units mayproduce a high energy barrier regarding the conformational andgeometrical requirements of the threading process, which could,in last term, be kinetically minimized.

We studied by 1H NMR the interaction between SB-b-CD andPV10. Solutions containing 5�10�3 M of PV10 and/or 2� 10�3 M ofSB-b-CD in D2O have been prepared and let stabilize for severalhours. The corresponding spectra are shown and compared in Fig. 7,

Fig. 7. 500 MHz 1H NMR spectra of D2O solutions containing: (a) 5 � 10�3 M of PV10;(b) 5 � 10�3 M of PV10 and 2 � 10�3 M of SB-b-CD; (c) 2 � 10�3 M of SB-b-CD.

M.E. Flores et al. / Dyes and Pigments 112 (2015) 262e273 267

where the assignments of the different signals are related to thestructures drawn in Fig. 1. It can be seen that the signals corre-sponding to aromatic PV10 protons are downfield shifted andbroaden in the presence of the macrocycle. Besides, in the presence

Fig. 8. 600 MHz NOESY spectra of a D2O solution containing 5 � 10�3

of the polymer, the signals corresponding to the aliphatic protons ofSB-b-CD are also shifted and distorted with respect to the pristinemolecule. All these spectroscopic changes are related to the newenvironment of the corresponding protons. However, thesechanges could be attributed to the formation of both electrostaticcomplexes and pseudorotaxanes.

More insights on the formation of specific molecular arrange-ments are searched by NOESYexperiments. As can be seen in Fig. 8,clear cross-signals appear between PV10 protons and SB-b-CDprotons. The negative sign of the intensity is consistent with themacromolecular nature of the complexes. The signals correspond-ing to internal SB-b-CD protons H3 and H5 are included in amultiplet comprising thesementioned signals and those of externalprotons H2, H4, H6, and H10. This multiplet is distorted in thepresence of PV10 as if the signals of the SB-b-CD protons H3 and H5,which appeared for the pristine SB-b-CD as the most deshieldedsignals of the multiplet, were upfield shifted and broadened [60].Strong NOE signals appear between protons included in thismultiplet andmethylene groups at the center of the decamethylenesegments, with chemical shifts at 1.8 and 1.1 ppm. Besides,considerable less intense NOE signals are found between these SB-b-CD protons and the aromatic protons of PV10, indicating that thepossible pseudorotaxane formed presents the PV10 decamethylenegroups inside the SB-b-CD cavity, whilst the charged viologen unitsshould remain outside the SB-b-CD cavity, as described in theliterature [57,58]. It is interesting to note that weak NOE signals alsoappear between PV10 protons (both decamethylene and aromaticprotons) and external SB-b-CD protons corresponding to the sul-fobutyl branches H20, H30, and H40. Although this can be interpretedas rapid magnetization transfer through protons in the SB-b-CD

M of PV10 and 2 � 10�3 M of SB-b-CD. See Fig. 7 for assignments.

Fig. 9. H2TPPS4� spectra in aqueous solutions containing 1 � 10�5 M of H2TPPS4� andvariable amounts of SB-b-CD furnished by titration: A) in the absence of any otheradditive, B) in the presence of 1 � 10�4 M of PV10, C) in the presence of 1 � 10�4 M ofPV10 and 0.1 M NaCl; SB-b-CD concentrations are: Left: (a) 0 M; (b) 5 � 10�5 M; (c)1 � 10�4 M; (d) 2 � 10�4 M; (e) 3 � 10�4 M; (f) 4 � 10�4 M; (g) 5 � 10�4 M; (h)1 � 10�3 M; (i) 2 � 10�3 M; (j) 4 � 10�3 M; (k) 5 � 10�3 M; Right: (a) 0 M; (b)1 � 10�5 M; (c) 2 � 10�5 M; (d) 4 � 10�5 M; (e) 6 � 10�5 M; (f) 8 � 10�5 M.

M.E. Flores et al. / Dyes and Pigments 112 (2015) 262e273268

structure in addition to transfer between internal SB-b-CD protonsand PV10 protons, a close approach of these external protons toPV10 protons that allows direct magnetization transfer is justifiedsince short-range electrostatic interactions may be held betweenthe anionic sulfate groups in SB-b-CD and the cationic viologengroups in PV10.

The complex formation kinetics could not be studied by color-imetric methods, since both species do not absorb in the visibleregion, and the representative 1H NMR spectra of the complexshown in Fig. 7 appeared within the first 15 min after mixing thereactants, and presented no modification after 54 h. Thus, whetherSB-b-CD and PV10 are forming pseudorotaxanes or simply elec-trostatic complexes is not completely elucidated, and probably,both arrangements may coexist at the conditions of ourexperiments.

3.3. H2TPPS4� e SB-b-CD interaction

It has been shown in the literature that H2TPPS4� can forminclusion complexes with one or more uncharged CDs, so that thesulfonatophenyl groups insert on the CD cavity [51e53]. However,both H2TPPS4� and SB-b-CD are negatively charged, so that, inorder to form inclusion complexes, their mutual attractive inter-action must overcome the charge repulsion. That makes theinteraction between both species less probable, as shown in theliterature by a lower binding constant, when comparing withneutral CD [68]. Our own titration studies made by UVevis showthat the Soret band of a 1 $ 10�5 M H2TPPS4� solution, whichappears at 414 nm for the pristine dye, is shifted to lower energiesin the presence of increasing amounts of SB-b-CD, as can be seenin Fig. 9A, achieving a value of 419 nm in the presence of 30-foldexcess of the CD as can be seen in Fig. 10. The H2TPPS4� fluores-cence maximum is also shifted 2 nm in the presence of an excessof SB-b-CD, as can be seen in Fig. 2B, achieving its maximum at642 nm, whereas the emission sprectrum of the pristine dye at thesame concentration is centered at 640 nm. The fluorescence in-tensity relative to the absorbance at the excitation wavelengthdecreases. This magnitude should be proportional to the respectivequantum yield; a decrease on the quantum yield in the presence ofthe SB-b-CD may be due to stabilization of the triplet excited state,since the driving force for the formation of the complex is mainlyhydrophobic, thus increasing the probability of intersystemcrossing between the singlet and the triplet excited states of thedye. The formation of the inclusion complex has been alsoobserved by 1H NMR spectroscopy at pH 7. Upon inclusion com-plex formation with the SB-b-CD, the high symmetry of the dyemolecule is broken, so that splitting of signals is observed, as canbe noticed in Fig. 11 [1].

3.4. PV10 e SB-b-CD e H2TPPS4� interaction

So far we have shown clear spectroscopic changes that witnesscomplex formation between PV10 and H2TPPS4�, PV10 and SB-b-CD, and H2TPPS4� and SB-b-CD. Titration of the PV10/H2TPPS4�

complex with SB-b-CD gave evidences about equilibriumdisplacement and ternary complex formation. Upon adding SB-b-CD to an aqueous solution containing 1 � 10�5 M of H2TPPS4� and1 �10�4 M of PV10, a band at 421 nm corresponding to monomericdyes forming inclusion complexes with the SB-b-CD increasinglyarises at the expense of that at 393 nm, as can be seen in Fig. 9B. Themonomeric state of the complexed dyes allows the samples tofluoresce showing a fluorescence maximum at 645 nm (see Fig. 2B)whose intensity relative to the absorbance at the excitation wave-length, proportional to the fluorescence quantumyield, increases asthe SB-b-CD to H2TPPS4� molar ratio increases, as can be seen in

Fig. 12. It can be noticed that up to 10-fold excess of the SB-b-CD,the quantum yield is stabilized at a low value, which increases forhigher SB-b-CD excess. This may be a consequence of that, at lowexcess of the CD, remaining aggregated molecules are alsoabsorbing light at 421 nm, and monomeric H2TPPS4� speciescomplexed by the SB-b-CD undergo intersystem crossing uponexcitation, since triplets may be stabilized in a hydrophobic envi-ronment furnished both by the polymer and the CD. The possibilitythat the H-aggregates turn into low fluorescent twisted H-aggre-gates in the presence of low excess of the SB-b-CD seems to be

Fig. 10. Absorbance maxima for solutions containing 1 � 10�5 M of H2TPPS4� andvariable amounts of SB-b-CD as a function of the SB-b-CD/H2TPPS4� relative concen-tration: ( 2) in the absence of any other additive, ( 3) in the presence of 1 �10�4 M ofPV10, ( ) in the presence of 1 � 10�4 M of PV10 and 0.1 M NaCl.

Fig. 12. A) Fluorescence spectra relative to the absorption at 421 nm of solutionscontaining 1 �10�5 M of H2TPPS4�, 1 �10�4 M of PV10, and variable amounts of SB-b-CD: (a) 1 � 10�5 M; (b) 2 � 10�5 M; (c) 4 � 10�5 M; (d) 6 � 10�5 M; (e) 8 � 10�5 M; (f)1 � 10�4 M; (g) 5 � 10�4 M; (h) 1 � 10�3 M; (i) 5 � 10�3 M; B) fluorescence at 645 nmrelative to the absorption at 421 nm of solutions containing 1 � 10�5 M of H2TPPS4�,1 � 10�4 M of PV10, and variable amounts of SB-b-CD as a function of the SB-b-CD/H2TPPS4� relative concentration.

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improbable since steric hindrance will induce dispersion of the dyeupon inclusion complex formation [69].

The formation of the inclusion complex is achieved at a lowerexcess of SB-b-CD in the presence of PV10, comparing to the case inthe absence of the polymer, as can be deduced from the evolution ofthe values of the absorbance maxima obtained in both cases whenthe excess of the CD is varied, as shown in Fig. 10. Yet from a stoi-chiometric SB-b-CD concentration with respect to that of the dye,the band at 421 appears in the presence of PV10. This is revealingthat the polymer plays a role in the stabilization of the H2TPPS4�/SB-b-CD inclusion complex. The condition of low excess of the SB-b-CD is interesting for practical applications such as in pharma-ceutical technology [64]. The confinement of the inclusion complexin a hydrophobic environment furnished by the polymer due to itsamphiphilia and electrostatic compensation may be corroboratedby DLS. Experiments show that larger and polydisperse particlesare formed by adding the CD, as can be seen in Fig. 4, where cor-relation functions obtained analyzing the three component mix-tures at variable SB-b-CD concentrations are shown. As discussedbefore, in the absence of the SB-b-CDwell defined nanoparticles areobtained for 10�5 M of H2TPPS4� and 10�4 M of PV10, withapparent size of 240 nm. However, in the presence of 4 � 10�6 M,

Fig. 11. 500 MHz 1H NMR spectra of D2O solutions containing: (a) 2 � 10�4 M ofH2TPPS4�; (b) 2 � 10�4 M of H2TPPS4� and 2 � 10�3 M of SB-b-CD.

1 � 10�5 M, and 6 � 10�5 M of SB-b-CD, the correlation functionsfall at higher times, indicating the formation of larger particles, andshow a noisy behavior at high time values indicating the aggrega-tion of particles. An inversion in the zeta potential is detected forthese particles at a SB-b-CD/H2TPPS4� molar ratio of almost 1, ascan be seen in Fig. 5, indicating electroneutrality in the system atthis condition. Thus, the zeta potential of the particles decreasesfrom 32.5 mV in the absence of the SB-b-CD, achieving a plateau ofaround �30 mV at an excess of SB-b-CD of 6-fold.

Kinetics studies help to highlight the dynamics of thecomplexation process. The appearance of the band at 421 nm in thepresence of 10-fold excess of the SB-b-CD at the expense of theband at 393 nm has been followed by the aid of a stopped-flowreactor, as can be seen in Fig. 6. The process is completed inseveral seconds. The experimental curves also follow a biexpo-nential pattern according to the following equation:

AðtÞ ¼ aþ b½1� expð � k1tÞ� þ c½1� expð � k2tÞ� (2)

and related with the co-existence of two processes occurring atdifferent rates. The fit parameters given in Table 2 indicate that afraction of 68% of TPPS reacted by the fastest process with a rateconstantk1¼1.268 s�1. A fractionof 32% followed the slowest processwith avalue of k2 of 0.2315 s�1. Note that the fastest process has a rateconstantoneorderofmagnitudehigher than the fastest rate constantin the formation process of the PV10/H2TPPS4� complex, whose ki-netics may be related to conformational changes on the polymer. Afaster kinetics should be related to processes involving moleculeswith higher mobility as is the case of the SB-b-CD and H2TPPS4�.Thus, the fastest process observed is attributable to the formation ofan inclusion complex between both molecules. The slowest processshows a rate constant similar to that of the fastest process in theformation of the PV10/H2TPPS4� complex. Thus, this process may berelated to the change on the conformation of the polymer, fromfolded aroundH2TPPS4� to expanded andwrapped around SB-b-CD,and allowing the release of residual trapped dyes.

Both UVevis and DLS results indicate that in this low SB-b-CDexcess range, the H2TPPS4�/SB-b-CD complex should be imbibedinto the PV10 domain forming a ternary complex, as depicted in

Fig. 13. Schematic representation of ternary complex formation and structure. Pictures are obtained at the concentrated regime, so that A) is obtained at final concentrations of10�3 M of PV10 and 2 $ 10�3 M of H2TPPS4� and B) is obtained at final concentrations of 10�3 M of PV10, 10�3 M of H2TPPS4�, and 10�3 M of SB-b-CD.

M.E. Flores et al. / Dyes and Pigments 112 (2015) 262e273270

Fig. 13. 1H NMR studies could not be undertaken, since at the highconcentration needed to perform the experiments, the systemsundergo precipitation. As can be seen in Fig. 13, the color of theprecipitates corresponding to the ternary complexes obtained atthe concentrate regime is green, whilst that of the PV10/H2TPPS4�

complex is brown, consistent with the absorbance band shift dif-ference in both systems at dilute conditions. The shift of the Soretband to 421 nm is probably due to a stoichiometry of the SB-b-CD/H2TPPS4� complex of 2:1, provided that two SB-b-CD are able tobind one porphyrin molecule at opposite sides [53]. Besides, pref-erential solvation of the dye by PV10 segments may contribute to ashift on the absorption band to lower energies, comparing with theSB-b-CD/H2TPPS4� complex in the absence of the polymer [70]. Thestabilization of the inclusion complex by the polymer is associatedto hydrophobic and aromaticearomatic interactions besides elec-trostatic interactions. Less likely, threading of SB-b-CD on PV10mayalso allow an extended polymer conformation to which the dyecould be bound as monomers by aromaticearomatic interactions.However, the threading of CDs on polyalkylviologens showedmuchlower kinetics than those found in our experiments [59,71]. Eventhe inclusion of low molecular-weight viologen containing mole-cules showed rate constants in the order of 10�3 s�1 [58].

Fig. 14. H2TPPS4� spectra in aqueous solutions containing 1 � 10�5 M of H2TPPS4�, 1 � 10�4

and D) 80 �C; E) normalized absorbance at 393 nm as a function of the SB-b-CD/H2TPPS4�

The contribution of interactions other than long-range electro-static interactions to the free energy is witnessed by adding NaCl tothe systems. Long-range electrostatic interactions between mole-cules are screened by the addition of simple electrolytes such asNaCl. However, in the presence of 0.1 M of NaCl, the PV10/H2TPPS4�

complex precipitate, as could be noticed at naked eye, revealing aresistance to the cleaving effect of added electrolytes, so that thesalting out effect is produced. This causes a decrease on the ab-sorption intensity of the complex in UVevis experiments, as can benoticed in Fig. 9C. Titration with SB-b-CD at these conditions pro-duced the re-dissolution of the complex, together with the rise ofthe H2TPPS4�/SB-b-CD inclusion complex band at 420 nm. As can beseen in Fig. 10, the complex is also formed at low excess of the CD,revealing the stabilization effect of the polymer, despite the highionic strength of the mixture. Titrations made at different temper-atures showed a higher tendency towards the formation of theternary complex at low SB-b-CD excess as the temperature in-creases, as can be seen in Fig.14. Thismaybe due to a highermobilityof H2TPPS4� molecules in the PV10/H2TPPS4� complex. Related tothis, the H-band appears shifted to lower energies at higher tem-peratures, being centered at 499 nm at 80 �C, indicating the for-mation of lower-order aggregates. At this temperature, the final

M of PV10, and variable amounts of SB-b-CD furnished by titration: A) 10, B) 25, C) 40,relative concentration at 10 ( ), 25 ( ), 40 ( ), and 80 �C ( ).

Fig. 15. FE-TEM images at 3x (1) and 10x (2) of drop-casted suspensions containing 1 � 10�5 M of H2TPPS4�, 1 � 10�4 M of PV10, and SB-b-CD at concentrations: (a) 0 M; (b)2 � 10�6 M; (c) 6 � 10�5 M.

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structure of the ternary complex under an excess of SB-b-CD of 10-fold over the dye is achieved yet under an excess of 2-fold the CD, ascanbe seen in Fig.14E,where the relative intensity of theH-bandhasbeen plotted versus the SB-b-CD/H2TPPS4� ratio. Higher excess isneeded for lower temperatures, as can be also seen in Fig. 14E.

FE-TEM images of suspensions casted on copper grids show de-posits of lowdegree of organizationwhen SB-b-CD is absent, as canbeseen in Fig.15a. However, in the presence SB-b-CD (Fig.15b and c) theformationof nanofibers is observed. Thesenanofibers aremore clearlyformed in the presence of excess of the SB-b-CD, and at 6 � 10�5 Mthey show an average diameter of 80 nm. Synthesis of nanorods ofTPPS by means of ISA has been described [72e75]. Cationic moietiesare necessary for this, which may include the protonated pyrroles ofthe acid formof thedye,metal ions coordinatedwith theporphyrin, orstrongly interacting counterions such as cationic porphyrins. Theformation of nanofibers is justified under the scope of ISA for thissystem, since the flexible cationic polymer may act as an ionic linkerbetween SB-b-CD/H2TPPS4� inclusion complexes, at increasing con-centration of the reactants and upon solvent evaporation.

As final remarks, the possibility shown here of achieving a goodcontrol of the H2TPPS4� state of aggregation in the presence ofPV10, together with regular morphologies of the assembled struc-tures in the form of nanofibers, may allow projecting the applica-bility of this system in fields such as photoinduced electron transferand photodynamic therapy [76]. In addition, the electro- and che-mochromic properties of PV10 may serve as a tool for the appli-cation of these complexes as sensors for a variety of substances andprocesses. The incorporation of these complexes in functional solidmaterials is under research.

4. Conclusions

H2TPPS4� interacts with both PV10 and SB-b-CD. With theformer, the dye undergoes self-aggregation forming H-type

(sandwich-like) aggregates, revealed by the shift of the Soret bandfrom 414 nm to 393 nm, and consequent fluorescence quenching,forming structures of apparent diameter of 240 nm and zeta po-tential of 32.5 mV, as seen by DLS. With the latter, H2TPPS4� formsan inclusion complex revealed by a shift of the Soret band from 414to 419 nm. By the addition of SB-b-CD to a H2TPPS4�/PV10 complex,the dye undergoes inclusion on the SB-b-CD cavity up to 6-fold SB-b-CD excess, revealed by a appearance of a band at 421 nm at theexpense of that at 393 nm. The large shift of the Soret band of thedye to lower energies at relative low concentrations of SB-b-CD,and the appearance of precipitates at higher concentrations of thereactants in the presence of PV10, reveal the formation of ternarycomplexes between the three species. Spectrometric and kineticsstudies allow proposing the formation of a 2:1 inclusion complexbetween SB-b-CD and H2TPPS4�, which is stabilized by the posi-tively charged PV10 that may coil around. SB-b-CD/H2TPPS4�/PV10complexes self-assemble at increasing concentrations of the re-actants, and nanofibers of around 80 nm of diameter are formed, asseen by FE-TEM. Besides, the state of self-aggregation of the dyemay be controlled by the stoichiometry of the reactants. This isimportant for the inclusion of H2TPPS4� to different substrates forwhich achieving a control of the aggregation properties of the dyeis important such as formulations for photodynamic therapy or forelectronic devices. In addition, the electro- and chemochromicproperties of PV10 may serve as a tool for the application of thesecomplexes as sensors for a variety of substances and processes.

Acknowledgment

The authors thank Fondecyt (Grants No 1090341 and 1120514,Chile), FIC-R Los Ríos 2011 (Chile), Grant-in-Aid for ScientificResearch (No. 24225003, MEXT, Japan) for financial support. Be-sides, the authors want to thank Dr. Jorge Nimptsch for fruitfulcollaboration.

M.E. Flores et al. / Dyes and Pigments 112 (2015) 262e273272

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