Porphyrins Sheathed in Quadrupolar Solvation Spheres of

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Porphyrins Sheathed in Quadrupolar Solvation Spheres of Hexafluorobenzene: Solvation-Induced Fluorescence Enhancement and Conformational Confinement Mitsuhiko Morisue, Ikuya Ueno, Masaki Shimizu, Takashi Yumura, and Noriaki Ikeda J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04083 • Publication Date (Web): 09 Aug 2017 Downloaded from http://pubs.acs.org on August 11, 2017

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The Journal of Physical Chemistry

Porphyrins Sheathed in Quadrupolar Solvation Spheres of Hexafluorobenzene: SolvationInduced Fluorescence Enhancement and Conformational Confinement Mitsuhiko Morisue,*,† Ikuya Ueno,† Masaki Shimizu,† Takashi Yumura,‡ and Noriaki Ikeda‡ †

Faculty of Molecular Chemistry and Engineering, and ‡Faculty of Materials Science and

Engineering, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

ABSTRACT. Hexafluorobenzene (C6F6) strongly solvated porphyrin ring via a quadrupolar interaction. The solvation sphere of C6F6 hindered the thermal fluctuations near the porphyrin ring and evoked remarkable photoelectronic properties of the porphyrins, such as fluorescence enhancement and spectral sharpening due to confined torsional planarity.

INTRODUCTION A π-stacked interaction exemplifies an omnipresent aromatic interaction.1–3 A quadrupolar interaction exists between a positively polarized π-surface of an electron-deficient aromatic molecule and a negatively polarized counterpart of an electron-rich aromatic molecule, and therefore, this electrostatic interaction can drive the self-assembly of an equimolar mixture of these two molecules into alternatingly π-stacked columnar assemblies.4–7 Hexafluorobenzene (C6F6) is a preeminent electron-deficient molecule because of the negative inductive effect of its

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fluorine atoms.4–9 Such cohesive interactions between arenes and perfluorinated arenes have hitherto been employed to design supramolecular systems in relatively condensed conditions, including crystals and liquid crystals,8–13 where relatively small aromatic hydrocarbons have been the principal focus. Meanwhile, the insufficient binding strengths of the quadrupolar interactions have limited their studies in dilute solutions.14–16 For large arenes under dilute conditions, on the other hand, multiple C6F6 molecules could produce a strong solvating sphere, which may induce unusual solvent effects on the solute molecules. Porphyrins are one of the largest chromophore frameworks, and quadrupolar interactions between porphyrins and C6F6 have never been found, even in crystals.17–19 The meso–ethynylene conjugated porphyrins exhibit the excellent near-infrared (NIR) photoelectronic properties, as independently established by Therien and coworkers20 and Anderson and coworkers.21 In the last decades, the meso–ethynylene conjugated porphyrins and their oligomers have played pivotal roles in the materials science featuring porphyrins, such as dye-sensitizing solar cells22 and nonlinear optical materials,23 instead of conventional meso–tetraarylporphyrins. To enhance NIR photophysical properties, insulation of the porphyrin rings from thermal fluctuations should be effective. Based on the above regards, our survey found unexpected solvatochromic effects of C6F6 on meso–ethynylporphyrin in comparison to C6F6 with cyclohexane (C6H12). The solvatochromic effect is empirically parameterized by a standard solvent-polarity index, ET(30) (0.13 kJ mol–1 for C6H12 and 0.14 kJ mol–1 for C6F6).24 The ET(30) magnitude excludes usual solvent polarity effects, and instead suggests the existence of non-conventional solvent effects. These unusual observations prompted us to explore the quadrupolar interactions between meso– ethynylporphyrins and C6F6. Here, we disclose two most representative solvation effects arising from the quadrupolar interactions between meso–ethynylporphyrins and C6F6. This study


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employed highly soluble porphyrin 1 bearing two 3,4,5-tri((S)-3,7-dimethyloctyloxy)phenyl groups at the meso-positions, that adopted the form of sticky liquid at ambient temperature under the solvent-free conditions,25,26 and the dithienosilole-linked dimer 2, whose spectroscopic signature was susceptible to the torsional conformations due to the freely rotatable ethynylene linkages (Chart 1).27

Chart 1. Chemical structures of meso–ethynylene conjugated porphyrin 1 and 2. The arrows indicate the direction of transition dipole moments.

RESULTS AND DISCUSSION Initially, a titration experiment of 1 with C6F6 in C6H12 was conducted to quantitatively evaluate the porphyrin–C6F6 quadrupolar interaction (Figure 1). The spectral changes through several isosbestic points indicates an all-or-nothing two-state equilibrium. A global analysis of the titration isotherms revealed the formation of the 1·(C6F6)2 complex with the binding constants, K1 = 17.2 ± 1.7 M–1 and K2 = 8.3 ± 0.9 M–1 in C6H12 (Scheme 1, Figure 1B). The binding strength was on the order comparable to moderate hydrogen bonding interactions,


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denoting that the quadrupolar interaction is insufficient to form a discrete cofacial 1·C6F6 complex, but is in the range of solute–solvent interactions. Notably, the isosbestic points did not overlap with the electronic absorption of 1 in neat C6F6, indicating that multiple C6F6 molecules clearly exhibit electronic coupling with 1; however, the electronic structure of 1 is particularly diagnostic of the first binding event of C6F6.

Scheme 1. Binding equilibrium of 1 with C6F6 to form a 1·(C6F6)2 complex.

Figure 1. (A) Spectrometric titrations of 1 ([1]0 = 3.7 × 10–6 M, red line) with C6F6 at 25 °C in C6H12 (gray lines), together with spectrum observed in neat C6F6 (green line). The inset magnifies the Q band. (B) Titration isotherms at 435, 453, 616, and 641 nm with least-squares regression analysis curves assuming the 1·(C6F6)2 complex formation (K1 = 17.2 ± 1.7 M–1 and K2 = 8.3 ± 0.9 M–1).


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The nearly degenerated Soret band at 435 nm in C6H12 was split in C6F6 (Figure 1A). C6F6 stabilized the electronic states along the long axis (x) of 1, which was differentiated from the interaction along the short axis (y). At the same time, the Q band showed a large bathochromic shift in C6F6, presumably due to the slightly enhanced charge-transfer (CT) nature. The entire spectral characteristics of 1 in C6F6 is close reminiscent of the spectral changes observed for 1 during the addition of an axially coordinating ligand, such as pyridine.25 The interactions between 1 and C6F6 were rationalized by a density functional theory (DFT) study using the M06-2X functional (details described in the Experimental Section). The longitudinal displacement of the cohesively bound C6F6 on the porphyrin plane accounts for the dissymmetric electronic structure (Figure 2). Intriguingly, the 1·(C6F6)2 complex formation includes not only cofacial π-stacking via the quadrupolar interaction but also η2-type coordination bond formation of C6F6 to the central zinc atom of the porphyrin ring; the optimized separations between the zinc atom and the two proximal carbon atoms of C6F6 are 2.92 and 2.95 Å or 2.93 and 2.96 Å, which can be seen in Figure 2A. This η2 coordination bond formation of C6F6 is known for rhodium, iridium, and nickel complexes.28–31 Due to the interactions, the 1·(C6F6)2 complex provides a 7.1 and 6.6 kcal mol–1 relative stability compared to the dissociated state of 1 and C6F6 and the dissociated state of 1 and 1·C6F6 complex, respectively (Equation 4 and 5 in Experimental Section). This theoretical result corresponds to only the first and second binding event of C6F6 to 1 in the titration experiment. The meso-ethynylene conjugated porphyrin ring possesses a planar surface without any steric hindrance of the meso-ethynylene group (Figures 2B and 2C), which can tolerate a relatively large displacement of the cofacially stacked C6F6, unlike the π-surface of meso-tetraarylporphyrin.17,18 The η2-type coordination bond formation is able to assist the quadrupolar interaction between 1 and C6F6. The comprehensive


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experimental and theoretical study firmly established the porphyrin–C6F6 interaction; two C6F6 molecules bind to the porphyrin via a quadrupolar interaction as well as two η2-type coordination bonds to zinc to form a sandwich-type 1·(C6F6)2 complex. Eventually, C6F6 produced a strong solvated sphere surrounding the porphyrin ring.

Figure 2. Optimized geometry of the 1·(C6F6)2 complex obtained from the M06-2X DFT calculations. (A) and (B) Side of view and (C) top of view of the optimized geometry. The key C···Zn bond lengths are displayed in Å. The alkyl chains are replaced by methyl groups for visual clarity.

Marvelously, the absolute quantum yield (Φ) of 1 was markedly enhanced from 5.4% in C6H12 up to 32% upon addition of C6F6 (Figure 3A). This result is in sharp contrast to a report claiming that C6F6 quenches the fluorescence of pyrene in C6H12.15 Upon addition of C6F6, the fluorescence of 1 increased in a biphasic manner, indicative of a stepwise formation of a 1·(C6F6)2 complex with K1 = 13.7 ± 1.5 M–1 and K2 = 1.5 ± 0.6 M–1 (Figure 3B). The fluorescence enhancement occurred upon the 1·(C6F6)2 complex formation, although the 1·C6F6 complex formation was insignificant. A further increase of the content of C6F6 was, however, ineffective in further enhancing the fluorescence. The radiative rate constant, kem = Φ/τ, in 30 vol% of C6F6 in C6H12 was 2.3 × 108 s–1, and much greater than those in C6H12 or in neat C6F6


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(Table 1, Figure S1). The fluorescence of 1 was enhanced in the binary C6F6–C6H12 mixture but not in neat C6F6, indicating the occurrence of preferential solvation.32

Figure 3. (A) Fluorescence spectra of 1 ([1]0 = 3.7 × 10–6 M, red) upon addition C6F6 up to 7.7 × 105 equivalent at 25 °C in C6H12 (gray lines), together with emission spectrum of 1 in neat C6F6 (green line), obtained by excitation at 435 nm. (B) Titration isotherm for the fluorescence of 1 at 622, 679 nm (fluorescence maxima in C6H12), and 645 and 708 nm (fluorescence maxima in C6F6), together with global titration analysis (solid lines) assuming formation of a 1·(C6F6)2 complex with K1 = 13.7 ± 1.5 M–1 and K2 = 1.5 ± 0.6 M–1.

Table 1. Solvent dependence of photophysical properties of 1.

Φa)

τ/nsb)

kem/s–1 d)

0.054

1.74

3.1 × 107

30% C6F6 in C6H12 0.32

1.4 c)

2.3 × 108

C6F6

1.54

3.9 × 107

C6H12

0.060

a) Absolute quantum yield, Φ, was determined using an integration sphere. b) Fluorescence lifetime, τ, was determined from streak scope measurement. c) Average lifetime including 0.39 ns (13%) and 1.59 ns (87%). d) Radiative rate constant; kem = Φ/τ.

The Φ value is defined by the following equation;


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Φ = kem/(kem + kisc + knr)

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(1)

wherein kem, kisc, and knr refer to the rate constants for possible three relaxation processes from the lowest singlet excited state (S1); such as fluorescent radiative decay, intersystem crossing (ISC) to produce a triplet excited state (T1), and S1 → S0 nonradiative decay, respectively. The Strickler–Berg equation is widely applicable to a relationship between intrinsic fluorescence lifetime, τ0, and absorption of organic fluorophores;33 radiative rate constant is proportional to the integrated oscillator strength of the lowest absorption band, i.e., kem = 1/τ0 ∝ ∫ε d ln߭෤, wherein ε is molar extinction coefficient in wavenumbers. The ε magnitude of the Q band of 1 remained a similar level throughout the titration experiment with C6F6 (the inset in Figure 1A). This law predicts that the kem magnitude of 1 remained a similar level regardless of the solvent fraction of C6H12 and C6F6 (Figure 1A). Large deviation of the kem magnitude in 30 vol% of C6F6 in C6H12 ruled out the possibility that the increased transition probability enhanced the fluorescence intensity. A heavy atom effect of the central zinc atom of porphyrins gives rise to a spin–orbit coupling, and therefore kisc is typically greater than kem.34 Pump–probe transient absorption spectroscopy is effective to observe the ISC processes. Based on the reported absorption peaks of analogous porphyrins,35,36 the S1 → Sn absorption emerges as a rise component at approximately 470 nm instead of bleaching of the Soret (S0 → S2 absorption) and Q band (S0 → S1 absorption), and the subsequent T1 → Tn absorption gives an intense absorption at 460 nm and the concomitant broad absorption at approximately 700 nm. Photoexcitation of 1 at 532 nm in C6H12, the absorption at 470 nm immediately appeared due to a prompt generation of the S1 sate, and a gradual rise at 460 nm and 700 nm persistent over the several nanoseconds was observed due to


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a subsequent evolution of the T1 sate (Figure 4A and 4B). The recovery of the bleached Q band was compatible with the fluorescence lifetime. Even in the presence of C6F6, a modest rise of the T1 → Tn absorption was also observed, although the absorption bands were slightly shifted (Figure 4C and 4D). Then, the kISC magnitude remained nearly constant regardless of the presence of C6F6. In this consequence, according to equation 1, it was deduced that C6F6 significantly decelerated the S1 → S0 nonradiative decay of 1.

Figure 4. Transient absorption spectra and time profiles of 1 in C6H12 (A, B) and in 30 vol% of C6F6 in C6H12 (C, D). Excitation = 532 nm.

Based on the entire experimental results, the rate constant of nonradiative decay, knr, in the presence of C6F6 was at least ten times smaller than that in C6H12 to satisfy equation 1. The knr magnitude is described in Equation 2;37 knr ≈ fv kic,max

(2)


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wherein fv is the Frank–Condon factor and kic,max is the maximum rate of internal conversion. The Frank–Condon factor is significantly depending on energy gap, ∆E, and a proportionality constant, α, as described in Equation 3;37 fv = exp(–α∆E)

(3)

Accordingly, “energy gap law” predicts that the emission efficiency becomes lower as the wavelength becomes closer to the NIR region.38,39 C6H12 as a solvent typically does not deactivate the fluorescence in the UV and visible wavelength regions. However, nonpolar C6H12 is typically studied only with small chromophore, unlike porphyrins. To circumvent the thermal deactivation pathways, the replacement of high-energy oscillators, such as the protons, by heavier atoms, such as fluorine, is effective, because of the lower vibrational frequencies of C–F bonds (νC–F ≈ 1000–1400 cm–1) compared to C–H bonds (νC–H ≈ 3000 cm–1), as exemplified by NIR luminescent lanthanoid complexes.40,41 Therefore, low vibrational frequencies reduces the Frank–Condon factor to mitigate intrinsic thermal dissipation of photoexcited energy. The deductive interpretation was pertinent to the expected extent of reduced knr of 1 in the presence of C6F6.41 Moreover, the porphyrin ring is ruffled in the photoexcited state,42,43 and the photoexcited porphyrin plane can mirror environmental perturbations.44 A conceivable rationalization for the fluorescence enhancement is that the strongly solvating C6F6 insulated the porphyrin ring from thermal fluctuations to suppress the non-radiative dissipation of excitons. Presumably due to preferential solvation,32 C6H12 partially solvated the (S)-3,7-dimethyloctyloxy chains to extend them further from the porphyrin segment. This interpretation is compatible with the following experiments.


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A similar effect of the solvation sphere of C6F6 on the porphyrin unit was also observed for 2 in the ground state. The electronic structure of 2 is typically understood by the point-dipole approximation,45,46 wherein the relative orientation of two porphyrin rings produces interchromophore electronic coupling. The porphyrin dimer 2 intervened by two freely rotatable ethynylene linkages adopts staggered torsional conformations in the solution phase. The spectroscopic signatures of 2 were somewhat broadened in C6H12 (Figure 5) as well as in nhexane, toluene, tetrahydrofuran, and dichloromethane as shown in our previous report.27 Upon the addition of C6F6, 2 showed unequivocal spectral sharpening and a bathochromic shift (Figure 5A). The spectral changes through several isosbestic points indicates a two-state equilibrium. Based on a global titration analysis, we obtained a reliable fitting curves assuming that two C6F6 molecules sequentially bound to 2 to afford a 2·(C6F6)2 complex, wherein K1 = 22.6 ± 1.4 M–1 and K2 = 8.6 ± 1.5 M–1, ignoring possible further binding of C6F6 to 2 (Figure 5B). The splitting of the Soret band became much more explicit in C6F6 compared to that in C6H12, suggesting enhanced π-conjugation due to the planarization of the torsional conformations of 2. The spectral sharpening of the electronic absorption band is evaluated by the full-width-at-the-half-maximum (FWHM) of the Q band, since the electronic spectra appear as the sum of the electronic transitions and since the FWHM mirrors the distribution of the torsional conformations. The FWHM narrowed from 1395 cm–1 in C6H12 to 890 cm–1 in C6F6. This spectral sharpening in C6F6 indicated that the broadly distributed torsional conformations of 2 in C6H12 were significantly confined in C6F6. The result inferred that C6F6 produced a solvation sphere near the porphyrin planes and dithienosilole unit as a confined space.


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Figure 5. (A) Spectrometric titration of 2 ([2]0 = 5.0 × 10–7 M, red line) with C6F6 at 25 °C in C6H12; up to 7.3 × 105 equivalent (gray lines), together with spectrum of 2 in neat C6F6 (green line). The inset shows the fluorescence spectra of 2 in C6H12 (red line) and in neat C6F6 (green line), obtained by excitation at 450 nm. (B) Titration isotherms at 435, 476, 658, 702 nm with least-squares regression analysis curves assuming the stepwise formation of a 2·(C6F6)2 complex (K1 = 22.6 ± 1.4 M–1 and K2 = 8.6 ± 1.5 M–1).

A rational explanation for the conformational planarization of 2 in C6F6 may be given by similar reasoning as the fluorescence enhancement of 1. A marginal torsional barrier of only 1.6 kcal mol–1 around the ethynylene linkage allowed a staggered torsional conformation of 2 in C6H12 due to thermal fluctuations; however a planar conformation is preferred because of the quinoidal–cummulenic π-conjugated nature of the dithienosilole–ethynylene linkage.27 Analogous conformational planarizations have been reported for some conjugated porphyrin dimers at cryogenic temperatures in frozen solvents.47–49 Accordingly, we interpreted that C6F6 produced a solvation sphere as a “pseudo-frozen matrix” surrounding 2, and therefore, the torsional conformations were thermodynamically accessible along with the planar one even at ambient temperatures. The solvation sphere of C6F6 likely accounts for the overall observations. The low rotational barrier around the ethynylene linkage of 2 allows repopulation of the twisted conformations to the planar one due to the enhanced quinoidal–cummulenic π-


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conjugated nature in the photoexcited state, as reported in our previous paper.27 In the photoexcited state, the torsional conformations of 2 is confined to the fully planar one even when a locally excited state initially produced. The Einstein–Stokes equation defines the viscosity of a Newtonian fluid as a function of the hydrodynamic radii. Then, the solvation strengths of C6H12 and C6F6 may affect the torsional dynamics of porphyrin dimers in similar mechanisms as those in the reports of detecting the viscosity of lipid membranes or solvents.50–53 To assess the photodynamic process involving solvation dynamics, time-resolved fluorescence spectroscopy is a well-established technique.54,55 The laser irradiation at 452 nm mostly photoexcited the porphyrin-localized Soret band along the y-axis. In C6H12, fast decay of the locally excited state at relatively shorter wavelength and concomitant rise of the fully conjugated state at relatively longer wavelength occurred on the subnanosecond time scales (Table 2, Figure S4 and S5). Although the initial population of the locally excited state in the orthogonal conformation was not dominant in C6F6, light irradiation at 452 nm can also produce the porphyrin-localized excited state despite of that being a minor proportion. The simultaneous rise in time constants of ~100 ps both at shorter and longer wavelength regions in C6F6. It was interpreted that a less fluorescent locally excited state in C6F6 was repopulated to a relatively fluorescent fully conjugated state. The shorter time scales of the torsional relaxations in C6F6 than those in C6H12 indicated that C6F6 somewhat fastened the torsional reorientation of 2 presumably the enhanced quinoidal–cummulenic π-conjugated nature. Thus, the solvation sphere of C6F6 is robust enough to insulate thermal fluctuations, but is too elusive to prevail in the photodynamic relaxation owing to the enhanced quinoidal–cummulenic π-conjugation.


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Table 2. Fluorescence decay components of porphyrin 2 in C6H12 and C6F6. Solvent

τ1/ns (α1), τ2/ns (α2) and τ3/ns (α3) Shorter wavelength region

Longer wavelength region

C6H12

0.17 (0.69), 0.48 (0.29), 1.85 (0.02)

0.16 (–1.18), 0.69 (1.97), 1.16 (0.21)

C6F6

0.09 (–2.66), 0.24 (2.79), 0.83 (0.88)

0.11 (–10.7), 0.77 (10.5), 0.97 (1.2)

Fluorescence decay constant (τ) and the normalized amplitude (α) were determined from the fluorescence decay profiles integrated in the shorter and longer wavelength regions, i.e., 660–670 and 695–705 nm in C6H12, and 680–690 and 715–725 nm in C6F6. Data were obtained by the streak scope by excitation with a 200-fs laser pulse at 452 nm (Figure S4 and S5).

CONCLUSION In conclusion, we performed the first comprehensive experimental and theoretical study to establish the porphyrin–C6F6 quadrupolar interaction, wherein the existence of an η2-type coordination bond of C6F6 to the central zinc atom of the porphyrin ring was proposed. Accordingly, we suggested that C6F6 produces a relatively robust solvation sphere in the vicinity of the porphyrin surface and that the porphyrin ring is likely insulated from thermal fluctuations. This interpretation accounts for unique solvent effects of C6F6 on the porphyrins, including the fluorescence enhancement and thermodynamic conformational confinement. Analogous insulation effects are widely established for other host–guest systems; e.g., the cavity of cyclodextrin insulates the encapsulating guest molecule from thermal fluctuations and reduces the deactivation pathways of excitons to enhance the emission intensity of the guest chromophore.56–58 The results concluded that the solvation sphere of C6F6 serves as a new confined space. The study highlighted a novel approach to advancement of versatile NIR photophysical functionality of the meso–ethynylene conjugated porphyrins. The preferential solvation of the porphyrins in the C6H12–C6F6 mixture is under current investigation.


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Experimental Section Photophysical Measurements: UV/vis absorption spectra were recorded on a spectrophotometer (Shimadzu, UV-1800) equipped with a Peltier thermoelectric temperature controlling unit (Shimadzu, TCC-240A). Fluorescence spectra recorded on a fluorescence spectrophotometer (JASCO, FP-8300ST equipped with WRE-362) are shown with correction of spectral sensitivity, and absolute quantum yields were determined using an integration sphere as an equipment attached to the same spectrometer. Time-resolved fluorescence measurements were carried out by employing a circularly polarized beam of 200-fs laser pulse at 488 nm, second harmonic generation (Spectra Physics, Model 3980) of a continuous wave (CW) from 200-fs Ti:sapphire laser (Spectra Physics, Mai Tai), and monitored by a streak camera (Hamamatsu Photonics, Streak Scope C4334) as a detecting apparatus, wherein the timeresolution was approximately 30 ps. Pump–probe transient absorption spectroscopy was conducted using a mode-locked Nd3+:YAG laser; a 17-ps pulse at 532 nm (10 Hz) as an excitation light and a delayed white pulse as an probe light. The detail experimental setup was described elsewhere.59 Under our experimental conditions, thermal lens effect of neat C6F6 solvent was unavoidable, and large scattering of the probe light disturbed reliable observations in nanosecond time region (Figure S2). Computational investigation: To investigate interactions between 1 and C6F6, density functional theory (DFT) calculations were performed by using the M06-2X functional. To construct a model of 1, (S)-3,7-dimethyloctyloxy groups were replaced with methoxy groups. By using the model (Zn C60H72N4O6Si2·C6F6), the 1·C6F6 structure was fully optimized. During the


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DFT optimization implemented in the Gaussian 09 code, we used the 6-311G* basis set for the Zn atoms, and the 6-31G* basis set for the other atoms. After obtaining the optimized structure for 1·C6F6 complex, the interaction energy (Ebind) was estimated in the following equation. Ebind = Etotal(1·C6F6) − Etotal(1) − Etotal(C6F6)

(4)

Ebind = Etotal(1·(C6F6)2) − Etotal(1·C6F6) − Etotal(C6F6)

(5)

where Etotal(1·(C6F6)2), Etotal(1·C6F6), Etotal(1), and Etotal(C6F6) are the total energies of 1·(C6F6)2, 1·C6F6, 1, and C6F6, respectively. Then, basis set superposition errors (BSSEs) were corrected in estimating the Ebind value in the 1·C6F6 or 1·(C6F6)2 complex. Figures 2 and S3 display the optimized geometry for the 1·(C6F6)2 complex and the 1·C6F6 complex, respectively. We found

η2-type coordination bond formation of C6F6 to the central zinc of porphyrin ring; the optimized separations between the zinc atom and a carbon of C6F6 are 2.93 and 2.94 Å in a 1·C6F6 complex (Figure S3). The optimized 1·C6F6 complex is 7.1 kcal·mol–1 stable relative to the dissociation limit toward 1 and C6F6. The value was comparable to the Gibbs energy change in the 1·C6F6 complex formation (∆G1 = –1.6 kcal mol–1) as the diagnostic step for bathochromic shift of the Q band (1.9 kcal mol–1) from 614 nm in C6H12 to 640 nm in C6F6.

ASSOCIATED CONTENT Supporting Information. Fluorescence decay profiles together with streak scope images are described.

AUTHOR INFORMATION


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Notes The authors declare no competing financial interests. ACKNOWLEDGMENT We are grateful to Prof. Joji Ohshita (Hiroshima University) for his encouragement at the initial stage of this work, Prof. Shinjiro Machida (Kyoto Institute of Technology) for time-resolved fluorescence measurements, and Prof. Yasuhisa Kuroda (Kyoto Institute of Technology) for global titration analysis. This work was partly supported by a Grant-in-Aid for Scientific Research on Innovative Areas “New Polymeric Materials Based on Element-Blocks (No.2401)” (M.M., JP15H00741) and “Stimuli-responsive Chemical Species for the Creation of Fundamental Molecules (No. 2408)” (T.Y., JP15H00941), and KAKENHI (M.M., JP16K05749) and Grant-in-Aid for Young Scientists (B) (T.Y., No. 26790001) from JSPS. REFERENCES (1) Hunter, C. A.; Lawson, K. R.; Perkins, J.; Urch, C. J. Aromatic interactions. J. Chem. Soc., Perkin Trans 2, 2001, 651–669. (2) Martinez, C. R.; Iverson, B. L. Rethinking the term “pi-stacking”. Chem. Sci. 2012, 3, 2191–2201. (3) Salonen, L. M.; Ellermann, M.; Diederich, F. Aromatic rings in chemical and biological recognition: energetics and structures. Angew. Chem. Int. Ed. 2011, 50, 4808–4842. (4) Patrick, C. R.; Prosser, G. S. A molecular complex of benzene and hexafluorobenzene. Nature 1960, 187, 1021. (5) Williams, J. H.; Cockcroft, J. K.; Fitch, A. N. Structure of the lowest temperature phase of the solid benzene–hexafluorobenzene adduct. Angew. Chem. Int. Ed. Engl. 1992, 31, 1655–1657.


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Porphyrins Sheathed in Quadrupolar Solvation Spheres of

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