Article pubs.acs.org/JPCA
Influence of Cations on the Fluorescence Quenching of an Ionic, Sterically Congested Pyrenyl Moiety by Iodide in Water Michael J. Bertocchi,† Adam Lupicki,† Alankriti Bajpai,‡ Jarugu N. Moorthy,‡ and Richard G. Weiss*,†,§ †
Department of Chemistry and §Institute for Soft Matter Synthesis and Metrology, Georgetown University, Washington, DC 20057-1227, United States ‡ Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208 016, India S Supporting Information *
ABSTRACT: Quenching of the excited singlet states of a watersoluble, sterically congested tetraarylpyrene, 1,3,6,8-tetrakis(2,6dimethyl-4-(α-carboxy)methoxyphenyl)pyrene (Py4C), by a series of iodide salts has been investigated by steady-state and time-resolved fluorescence measurements. Access to the pyrenyl group of Py4C is restricted sterically as a result of the four flanking (2,6dimethylphenoxy)acetic acid groups and the energy costs associated with their rotation. Deprotonation of the carboxylic acid groups of Py4C permits examination of ion−ion electrostatic interactions on the rates of quenching by iodide salts in which different steric and electrostatic factors are introduced by varying the cationic portions. At the same concentrations and with the same cations, chloride anions are ineffective quenchers. The quenching rate constants of Py4C by iodide are found to correlate linearly with the ionic radii of the cations and their enthalpies of hydration. These correlations are discussed in terms of the Hofmeister series. Furthermore, the results indicate that the cations that flank Py4C decrease the quenching efficiency of iodide through polarization and shielding effects (i.e., lowering the effective charge), which isolate to varying degrees the π-system. The effects of the different cations on quenching the fluorescence of a simpler and sterically unencumbered pyrenyl derivative, 1-pyrenylbutyric acid (PyBu), by iodide are much smaller. Overall, the results with Py4C indicate that the fluorescence quenching efficiency by iodide is influenced by direct interactions with the cations associated with the carboxylate groups of Py4C and not the solvation of water molecules. This observation is germane to a topic of current debate: Are the effects of the cations more closely related to bulk water properties or to direct ion−ion interactions? The conclusions obtained from these studies are applicable clearly to a wide variety of other systems in which ion pairing influences cooperative or inhibitory interactions. steric congestion around the pyrenyl π-surface of PyOMe is introduced by the four flanking 2,6-dimethylanisyl rings, which adopt a nearly orthogonal orientation (ca. 88° angles) with respect to the plane defined by the pyrenyl core. By assessing the shape, size, electronic interactions, and conformational lability of the amines, it has been possible to dissect the steric requirements for quenching of PyOMe excited singlet states. In general, they are found to limit the approach of the amines and their abilities to associate closely with the π-system of pyrenyl. Here, quenching of the fluorescence of a water-soluble analogue of PyOMe, 1,3,6,8-tetrakis(2,6-dimethyl-4-(αcarboxy)methoxyphenyl)pyrene (Py4C) (Scheme 1), is examined in aqueous media. The quenchers are a series of iodide salts in which different steric factors are introduced by the cationic portion of the salts, an alkali metal, or a
1. INTRODUCTION The spectroscopic properties of pyrenyl derivatives have been used extensively to examine electrostatic interactions of polyelectrolytes, aggregates, and microheterogenity in aqueous solutions.1−4 Many of these investigations have exploited the fluorescence from pyrenyl groups and its quenching to obtain detailed information about micellar structure, ground-state association constants, and residual charges on aggregates.5−8 However, pyrene itself can be solubilized in significant concentrations only within the heterogeneous, hydrophobic microenvironments within aqueous media (e.g., the interiors of micelles); the use of unsubstituted pyrene as a probe for ionic and electrostatic interactions directly within aqueous environments is very difficult. Thus, the solubility issues are typically overcome by the derivatization of pyrene with polar groups.3 Recently, we examined the steric effects on fluorescence quenching of 1,3,6,8-tetrakis(4-methoxy-2,6-dimethylphenyl)pyrene (PyOMe) (Scheme 1) by a diverse series of tertiary aliphatic and aromatic amines in organic solvents.9 Significant © XXXX American Chemical Society
Received: August 7, 2017 Revised: September 6, 2017
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DOI: 10.1021/acs.jpca.7b07853 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A Scheme 1. Structures of PyOMe and the Anions of Py4C and PyBu
99%), tetramethylammonium chloride (Alfa, >98%), tetraethylammonium iodide (TEAI, Aldrich, 98%), tetraethylammonium chloride (Alfa, >98%), and PyBu (mp 187.1−188.0 °C (Lit:29 187−188 °C); Alfa, 97%) were recrystallized twice from absolute ethanol. 1,3,6,8-Tetrakis(2,6-dimethyl-4-(α-carboxy)methoxyphenyl)pyrene (Py4C) was available from other studies and was >99% pure according to HPLC analyses.30 2.2. Instrumentation. UV/vis absorption spectra were collected using a Varian UV−vis (Cary 300 Bio) spectrophotometer. Steady-state excitation and emission spectra at room temperature were recorded on a Photon Technology International Fluorimeter (SYS 2459) with an Ushio UXL-75 (65 W) xenon lamp and a PTI 814 photomultiplier detection system. The spectra were acquired at a right angle geometry with slit widths of 0.5 mm using an excitation wavelength of 359 nm (for Py4C) or 344 nm (for PyBu). Fluorescence decay histograms were obtained on an Edinburgh Analytical Instruments time-correlated single-photon counter (model FL900) with H2 as the lamp gas. An instrument response function (IRF) was collected periodically using a Ludox solution as the scatterer. Data were collected in 1023 channels and were treated by an exponential deconvolution method, which minimizes χ2 using the FAST software supplied by Edinburgh. Fits were considered acceptable when χ2 ≤ 1.3, and residual plots exhibited no systematic deviations from zero. Data were collected at 23 °C, unless indicated otherwise. 2.3. Titration Procedures. Solutions of Py4C (4 mM) and PyBu (10 mM) were prepared in water. Excess NaOH (100 mM) was added until the pH reached a plateau (ca. 10.5) to ensure complete deprotonation of the carboxylic acid groups. The solutions were then titrated with HCl (4 mM for Py4C or 10 mM for PyBu) using a Veirner Lab Quest pH probe. The pKa values of Py4C and PyBu were then determined by the equivalence point using the Henderson−Hasselbalch equation.22 2.4. Sample Preparation Procedures. Stock solutions of 2.2 × 10−5 M Py4C and PyBu were prepared in water and diluted with aqueous solutions of NaOH equivalent to the number of carboxylic acid groups, either 8.8 × 10−5 M for Py4C or 2.2 × 10−5 M for PyBu. Solutions with the iodide quenchers were prepared by dilution of an aliquot of the stock solution with solutions of various iodide salts. In each case, the ionic strength was kept constant (100 mM total salt concentration) by adding chloride salts (nonquenchers) with the same cation. For the samples containing NaCl only, the total salt concentration was not kept constant (Figure S1). The solutions with Na+ or K+ as the cation, where the total salt concentration
tetraalkylammonium group. Deprotonation of the carboxylic acid groups of Py4C make them more soluble in water and, thus, opens the possibility to probe cation−carboxylate interactions as they influence the quenching rates by iodide. Although electrostatic interactions between a cation and sites of negative charge are well-known, they remain poorly understood;10,11 they have been attributed primarily to lowering the effective charge of the anion (iodide) by either polarization or shielding effects.12−16 In general, the effects of cations have been difficult to explore because they influence the surrounding environments to a lesser degree than do anions.10,17,18 The presence of the four flanking carboxylate groups on Py4C creates several sites of positive charge near the π-system, and the quenching rates of iodide are affected to the extent that the cations interact with the carboxylate groups and with the iodide anions. The results with Py4C are compared with those employing the structurally much simpler and sterically unencumbered pyrenyl derivative, 1-pyrenylbutyric acid (PyBu), as a means to differentiate the cation−iodide and carboxylate−cation interactions. The quenching rate constants are correlated with a variety of properties associated with each cation, such as its activity coefficient, dissociation constant, ionic radius, place in the Hofmeister series, and enthalpy of hydration. It is found that the rate constants for quenching the excited singlet state of Py4C by iodide increase linearly with increases of the ionic radii of the cations, but no obvious correlations are found with the activities of the ions.19−22 Experiments with the corresponding chloride salts (that do not lead to appreciable quenching) are reported as well. The ionic radii of the cations are discussed in terms of the Hofmeister series and their enthalpies of hydration.10,18,23,24 Notably, still under debate is whether the influence of the cations on solvating water molecules or their direct interactions with their counterions is the more important factor in determining bulk structure.11 The current opinion favors ionic interactions,13,25−28 and our results with Py4C support this view.
2. EXPERIMENTAL SECTION 2.1. Materials. Water, (Fisher, HPLC grade, submicron filtered, pH 7), lithium iodide (LiI, Aldrich, 99.8%), lithium chloride (Aldrich, >99.0%), sodium hydroxide (Aldrich, pellets, >99%) sodium iodide (NaI, Aldrich, >99.5%), sodium chloride (Aldrich, 99%), potassium iodide (KI, Aldrich, 99%), potassium chloride (Aldrich, >99.0%), cesium iodide (CsI, Aldrich, 99.9%), and cesium chloride (Aldrich, 99%) were used as received. Tetramethylammonium iodide (TMAI, Aldrich, B
DOI: 10.1021/acs.jpca.7b07853 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A was 100 mM, or when no quencher was added, were flamesealed in flattened 4 cm (width) × 7 cm (length) × 0.3 (i.d.) glass capillaries (Vitro Dynamics) after being degassed until the pressure remained constant by freeze−pump−thaw cycles (4−5 cycles) at 90 mTorr on a mercury-free vacuum line. The remainder of the samples were purged under a stream of N2 for 20 min immediately before taking measurements. The ionic strength of the aqueous solutions was 100 mM in all cases. Data were collected at 23 °C, unless otherwise indicated.
one equivalence point, a pKa ≈ 4.5, indicating that the carboxylic groups of Py4C behave nearly independently (rather than synergistically); they do not interact significantly with each other. This pKa is slightly lower than that of PyBu (4.6)6,32 and of other aliphatic carboxylic acids (∼4.6),22 perhaps as a result of an inductive effect by the nearby oxygen atom of the linker groups. The independence of the four carboxylic acid groups of Py4C was anticipated also based on its rigidity and the long distance between the carboxylic groups. The distance between carboxylate groups, as measured by the nearest oxygen atoms in the crystal structure of Py4C,33 ranges between 13.3 and 16.0 Å. This distance also suggests that a uniform distribution of ions will be present around the periphery of Py4C.
3. RESULTS AND DISCUSSION 3.1. Communication among the Carboxylate Groups of Py4C. An important feature of Py4C is the presence of its four flanking carboxylate groups. The behavior and communication among them depends on their local environments, their ability to make hydrogen bonds, and the distance to other nearby functional groups, such as aromatic rings or vicinal carboxylates.22 Polycarboxylic acids in which the groups are in proximity usually have a unique pKa for each acid group because the protonated−deprotonated nature of one influences strongly the electrostatic field and solvation experienced by the others.31 However, the rigidity of Py4C and the distance between its adjacent carboxylate groups suggest that their ability to form intramolecular hydrogen bonds or other cooperative interactions may be attenuated significantly. In that regard, the titration curve of Py4C (Figure 1) shows only
4. QUENCHING OF PY4C EXCITED SINGLET STATES Addition of an iodide salt within the concentration ranges employed did not alter the aromatic 0S → 1S absorption spectra of either Py4C or PyBu, and no additional absorption features indicative of ground-state complexes could be detected. These and other data indicate that quenching of the excited singlet states of Py4C and PyBu by iodide are dynamic processes (Figure 2). Fluorescence quenching of pyrenyl molecules by iodide in aqueous media may be a combination of a heavy atom effect, which involves an increase in the rate of intersystem crossing (1Py4C → 3Py4C) of pyrenyl and electron-transfer from iodide (Scheme 2).34,35 The spin−orbit coupling constant (SO) of Scheme 2. Simplified Mechanism for Quenching Pyrenyl (Py) Excited Singlet States by Iodide
iodide is 5069 ζl/cm−1, and its oxidation potential in water is 0.45 eV versus SCE.36,37 Electron transfer from iodide to the excited singlet states of Py4C or PyBu in water is calculated to be favored thermodynamically by a large driving force, ca. −1 eV, as shown in Table S1 and eqs S1 and S2. Chloride (SO = 587 ζl/cm−1) does not quench the fluorescence of PyBu,8 and the addition of NaCl (up to 100 mM) to solutions of Py4C did not result in appreciable quenching either (Figure S1). Thus,
Figure 1. Back-titration curve of Py4C (4 mM) with HCl (4 mM). The inset is a magnified portion of the equivalence point region. The dashed red line shows the pH at the equivalence point.
Figure 2. Steady-state fluorescence spectra of 2.2 × 10−6 M Py4C (left) or PyBu (right) at 23 °C in water upon addition of iodide and chloride salts with the same cation. In each sample, the total ion concentration is 100 mM, and the concentrations of the iodide salts are shown in the insets. C
DOI: 10.1021/acs.jpca.7b07853 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A chloride salts with the same cation were added to maintain a constant strength (100 mM of total salt concentration) in each case. Fortunately, chloride and iodide are known to have almost identical activity coefficients and mobilities in aqueous solutions.18,21 As a result, they have the same influence on solvation by nearby water molecules. Furthermore, addition of 100 mM of the chloride salts of the cations with the largest SOs, K+ and Cs+ (SO = 38 and 370 ζl/cm−1, respectively), resulted in
