Preferential Solvation Unveiled by Anomalous Conformational

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Preferential Solvation Unveiled by Anomalous Conformational Equilibration of Porphyrin Dimers: Nucleation-Growth of Solvent–Solvent Segregation Mitsuhiko Morisue, and Ikuya Ueno J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b02558 • Publication Date (Web): 01 May 2018 Downloaded from http://pubs.acs.org on May 2, 2018

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The Journal of Physical Chemistry

Preferential Solvation Unveiled by Anomalous Conformational Equilibration of Porphyrin Dimers: Nucleation-Growth of Solvent–Solvent Segregation Mitsuhiko Morisue*,†and Ikuya Ueno† †

Faculty of Molecular Chemistry and Engineering, Kyoto Institute of Technology, Matsugasaki,

Sakyo-ku, Kyoto 606-8585, Japan.

ABSTRACT. Preferential solvation was explored using ethynylene- or butadiynylene-linked porphyrin dimers bearing 3,4,5-tri((S)-3,7-dimethyloctyloxy)phenyl groups at the meso-positions in binary hexafluorobenzene (C6F6) and cyclohexane (C6H12) mixture, expecting contrasting solvent affinity of the porphyrin core and the alkyl side-chains toward the individual solvent component. Although the solvent polarity remained nearly constant along with the continuous variation of the solvent composition, the porphyrin dimer showed dramatic change in spectroscopic signatures, indicating the occurrence of preferential solvation. Due to small rotational barrier around the ethynylene- and butadiynylene-linkage, the torsional conformations of the porphyrin dimers varied from orthogonal to planar due to continuous variation of molar fraction of C6H12–C6F6 mixture. Thorough thermodynamic analyses inferred that nucleation as the enthalpic component and phase segregation as the entropic component operated preferential

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solvation. The porphyrin dimer nucleated the C6H12–C6F6 segregation, and the torsional conformation was diagnostic of the inversion of the interfacial curvature of the solvent segregation along with the continuous variation.

INTRODUCTION Preferential solvation is a phenomenon, whereby solvent proportion of binary mixed solvent in the vicinity of a solute molecule differentiates from the statistic proportion in bulk.1–9 Polymers can kinetically confine the solvation spheres as the local energy minima apart from the ideal thermodynamic systems, as exemplified by block copolymers; orthogonal miscibility of the two solvents toward the covalently linked two segments of block copolymers is effective in sequestering the preferable segment from the immiscible segment to control the sophisticated mesophases.10–12 Polymer segments nucleates solvation spheres depending on the solute–solvent preference, resulting in a segment–segment segregation concomitant with a solvent–solvent segregation (Figure 1A). Theoretical study also predicted a principle that a cohesive solvent– solvent interaction is disrupted in the vicinity of solute molecule other than polymers.8 In practice, microscopic phase separations certainly exists at least in photodynamic time scales.5,7,9 The solvation spheres surrounding small molecules are highly fluctuating due to translational motions of the solvent molecules,5,9 and thus microscopic phase separations may be inconspicuous in the stationary state. As another facet, equally cohesive interaction among the isotropic bulk solvent molecules should produce a tension at anisotropic environments near the solute molecule, where solvent molecules orient for the solute molecule to produce a solvation


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sphere. In this regard, disruption of cohesive solvent–solvent interaction results in a possible existence of bicontinuous interface among the primary solvation sphere and a bulk solution.

FIGURE 1. (A) Schematic ternary phase diagram of preferential solvation. Microscopic phase segregation of binary “A”–“B” solvent mixture induced in the vicinity of a solute molecule. (B) Shape of amphiphilic block that governs the interfacial curvature of their assemblies as a function of packing parameter.

Insufficient entropy cost does not mix two or more compartments, and drives phase segregation. The resultant interfacial morphologies of the bicontinuous structures are defined by the minimal surface. For example, the assembling morphologies of amphiphilic surfactants are typically predictable from “packing parameter”, P = V/(a0lc), wherein V, a0, and lc refer the volume of the lipophilic domain, the cross-sectional area of the hydrophilic segment, and the critical length of the lipophilic chain, respectively; the interfacial curvature of the bicontinuous structure is known to inverse across the point of P = 1 (Figure 1B).13 The packing parameter also governs the interfacial curvature of phase segregated block-copolymers.14 The interfacial


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curvature defines the sophisticated morphologies of block-copolymer assemblies or polymer blends, as mathematically described using Gaussian curvatures.15,16 The interfacial curvature inverses at the point corresponding to P = 1 along with a continuous variation of solvent composition. In this context, the inversion of the interfacial curvature could be an indirect evidence of the existence of the bicontinuous structures near the small molecule. Solvophobic interaction may be another description on less cohesive relationship between solvents and solutes, ultimately leading to mutual immiscibility. Torsional equilibration of freely rotatable molecules highly susceptible to the microscopic environments provides an effective implement to quantify such elusive noncovalent interactions, 17–20 including solvophobic interactions21,22 and London dispersion force.23–25 Very low torsional barriers around a key rotatable linkage of ethynylene- and butadiynylene-conjugated porphyrin dimers also allow the re-distribution of the torsional angles, and, therefore, they are diagnostic of environmental media.26–33 We envisioned that orthogonal solubility of the porphyrin rings and pendant allyl groups could dominate the torsional equilibration of the porphyrin dimers. Then, the inversion of the possible interface produced by preferential solvation could twist torsional conformer of porphyrin dimers. Recently, we found that hexafluorobenzene (C6F6) binds to porphyrin 1 via quadrupolar interactions in cyclohexane (C6H12), where two C6F6 rings are stacked on one of the pyrrole rings constituting the porphyrin ring as well as η2-type coordination bonding interaction.34 The fluorescence intensity of 1 was maximized in a binary mixture of C6F6 and C6H12 up to approximately 6-fold greater than in neat C6H12 or C6F6. Nonlinear response of the fluorescence efficiency against the solvent composition is a striking feature of preferential solvation. The binary C6H12–C6F6 mixed solvent system is ideal to rule out the profound impacts of solvent


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polarity on electronic structures of chromophores in solutions, since an empirical solvent polarity index, ET(30), remains nearly constant for C6H12 (129 kJ mol–1) and for C6F6 (143 kJ mol–1).35–38 Additionally, solvophobic effect is expected to exert among C6H12 and C6F6,22 whereas C6H12 is miscible with C6F6 in the absence of solute molecules. We here employed ethynylene- and butadiynylene-conjugated porphyrin dimers, 2 and 3, respectively (Chart 1). These porphyrin dimers adopted an glass form under solvent-free conditions by the virtue of the elastic 3,4,5tri((S)-3,7-dimethyloctyloxy)phenyl groups,39 and showed moderate solubility even in less polar solvent, such as C6H12. The porphyrin dimer 2 and 3 could serve a possible molecular torsion balance to elucidate solvent–solvent segregation in the C6H12–C6F6 binary mixture, expecting the orthogonal solvent preference that could sequester the C6F6-solvated porphyrin segment from the C6H12-solvated aliphatic chains. Continuous variation of the solvent composition can control the C6H12–C6F6 interfacial morphology relevant to the boundary of the aliphatic and porphyrin segments. Based on the torsional conformations of the porphyrin dimer 2 or 3, we discuss the change of the interfacial curvature of the stationary microscopic C6H12–C6F6 segregation.

Chart 1. Chemical structures of porphyrin 1–3.

EXPERIMENTAL SECTION


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The porphyrin 1–3 was synthesized elsewhere.39,40 Electronic absorption spectra were recorded on a spectrophotometer (Shimadzu, UV-1800) equipped with a Peltier thermoelectric temperature controlling unit (Shimadzu, TCC-240A). Fluorescence spectra were recorded on a fluorescence spectrophotometer (JASCO, FP-8300ST equipped with WRE-362) are shown with correction of spectral sensitivity. Time-resolved fluorescence spectroscopy was performed by using second harmonic generation (Spectra-Physics, Model 3980) of a continuous wave (CW) from 200 fs Ti:sapphire laser (Spectra-Physics, Mai Tai) as the excitation laser pulse at 452 nm and a streak camera (Hamamatsu Photonics, Streak Scope C4334) as a detecting apparatus with approximately 30 ps of the time resolution. B3LYP DFT calculations were conducted by using the Gaussian 09 suite of programs.41 The B3LYP calculations42–44 have been generally used in chemistry because the calculations can effectively reproduce the corresponding experimental data. All titration experiments were carried out in an optical cuvette using capillary micropipette (Acura® 846, Socorex isba SA, Swiss). The titration isotherms were analyzed assuming that partial molar volumes remained unchanged throughout the entire titration experiments. However, the definition of preferential solvation assumes coexistence of two extreme situations in the system; one is a bulk state free from the solute molecule and another is a solvated solute molecule. In the most accurate sense, ignorance of partial molar volumes in thermodynamic analyses in this study is contradictory to the principal definition of preferential solvation because of differences in isothermal compressibility and partial molar volumes. To circumvent this confliction, we employed highly diluted experimental conditions below the order of 10–6 M of porphyrins. Additionally, due to the fact that the real molecules practically occupy certain volumes, higher concentration of porphyrin as the solute should give rise to large deviation of the experimental conditions from the ideal state. Under the circumstance, only


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photodynamic studies have assessed miscroscopic phase separation on preferential solvation. Exceptional molar absorptivity of porphyrin presented an effective approach to make preferential solvation observable.

RESULTS AND DISCUSSION The aromatic and aliphatic segments of 1 could show a contrasting solvent affinity to solvent components. Density functional theory (DFT) B3LYP calculations revealed that the electronic potential surfaces of 1, C6F6, and C6H12 (Figure 2). The electronegative surface of the porphyrin ring could be much more electrostatically attractive to the electropositive surface of C6F6 than the surfaces of 3,4,5-tri((S)-3,7-dimethyloctyloxy)phenyl groups of 1 and C6H12. Simultaneously, the electropositive surface of C6F6 could be non-preferable to the 3,4,5-tri((S)3,7-dimethyloctyloxy)phenyl groups of 1 and C6H12, in line with the expected solvophobic effect.

FIGURE 2. Electrostatic potential surfaces of porphyrin 1 (left), C6F6 (right top) and C6H12 (right bottom) generated using DFT B3LYP/6-31G calculations. The red and blue surfaces correspond to a region of negative and positive electrostatic potentials, respectively, and the


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potential values for all the molecules are displayed in the same range. The electrostatic potential surfaces were mapped onto 0.0004 atomic units isovalue.

To appreciate the porphyrin dimer as molecular torsion balance, we discuss the relative orientation of two porphyrin rings that governs the electronic structures of the porphyrin dimers,26–33 amenable to the Kasha’s point-dipole approximation.45,46 A planar conformation enhances π-electronic coupling between two porphyrin rings and orthogonal conformation mitigates π-conjugation (Figure 3). The planar conformer intensifies the longer Soret band (Bx) and weakens the shorter Soret band (By). In contrast, the orthogonal conformation bears an opposing trend in spectral properties.

FIGURE 3. Energy diagrams based on exciton coupling model for the Soret band of planar and orthogonal conformers of porphyrin dimers. Geometry-optimized model of 3 produced using DFT B3LYP (upper). θ = 0; planar conformer, 0

Preferential Solvation Unveiled by Anomalous Conformational

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