Conformational preference of a porphyrin rotor in confined environments

1 SUPPORTING INFORMATION Conformational preference of a porphyrin rotor in confined environments Laramie P. Jameson, Milan Balaz, Sergei V. Dzyuba, and Noriho Kamiya* a Department of Applied Chemistry, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 8128581 Japan. Fax: +81 92 802-2810; Tel: +81 92 802-2807; E-mail: nori_kamiya@mail.cstm.kyushu–u.ac.jp b Department of Chemistry, University of Wyoming, Laramie, WY 82071, USA. Fax: +1 307 7662807; Tel: +1 307 766-4330; E-mail: mbalaz@uwyo.edu c Department of Chemistry, Texas Christian University, Fort Worth, TX 76129, USA. Fax: +1 817 257 5851; Tel: +1 817 257 6218; E-mail: s.dzyuba@tcu.edu d Center for Future Chemistry, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 812-8581 Japan.

Ionic liquids (ILs) have been touted as customizable materials because their physical properties can easily be manipulated via modication of cation and anion pairs. 1 Consequently, the physical properties of ILs could impact various processes, which has lead to the idea of ILs as "designer solvents". 26][7][8] The structural and physical properties of ILs were found to be crucial in modulating the aforementioned processes.The paradigm of controlling chemical, physical and structural properties of small molecules simply by placing them in structurally similar (yet distinct in regards to various physical properties) solvents is an interesting and potentially useful phenomenon which could result in intriguing applications of ILs.
0][11] In particular, encapsulation of an IL and a small molecule within a reverse micelle (RM) might give a different distribution of the conformations of the small molecule from that which was present when that molecule was dissolved in the neat IL.Notably, Falcone and co-workers reported that the connement of ILs within IL-RMs could modify the microstructure of IL as compared to that observed in the bulk. 12,13Therefore, we set out to investigate the effect of connement on the conformation of a molecular rotor.
For our studies we selected a conjugated zinc porphyrin dimer, PD (Fig. 1). 14,15PD undergoes a rotation around the diyne moiety resulting in a collection of conformations, with the twisted and planar conformations as two extremes (Fig. 1). 15hese twisted and planar conformations exhibit distinct emission maxima at around 710 and 780 nm, respectively, which allows for the facile determination of the conformation of PD in a variety of different media.In single component molecular solvents and mixtures of molecular solvents the conformational preference of PD depends solely on the viscosity of the media, whereas the twisted conformation dominates in more viscous media such as glycerol or methanol-glycerol mixtures, for example, and the planar conformer exists in less viscous media such as neat methanol or ethanol. 7,15n the case of ILs on the other hand, the structure of IL was found to play an important role in the conformational bias of PD. 7 Specically, in all ILs with a common cation, a similar conformation of PD was present, regardless of viscosity.Furthermore, an order of magnitude larger viscosity was required to promote a similar amount of the twisted conformation as compared to that found in the molecular solvents.This illustrated that the solute-solvent interactions of ILs with PD could control the conformation of PD, rather than the viscosity. 7n order to evaluate the effect of connement on the conformation of PD, we prepared IL-RMs (also known as IL-inoil RMs, i.e., IL/O RMs) based on a modied version of a reported approach. 16,17For this purpose, we utilized common, readily available imidazolium-based IL, such as [C 4 -mim]PF 6 , [C 4 -mim]NTf 2 and [C 4 -mim]NO 3 .The corresponding IL-RMs were prepared by using a blend of two surfactants, namely polyoxyethylene sorbitan monooleate (Tween-80) and sorbitan laurate (Span-20), in isopropyl myristate (IPM).Using a xed weight ratio of the surfactants (3 : 2 -Span-20 : Tween-80) and a xed weight fraction of the surfactant blend (20 wt%) in IPM, various sizes of each IL-RMs were prepared by the addition of the appropriate amount of ILs to the surfactant blend in IPM.Subsequently, the formation and the size of the IL-RMs was evaluated using dynamic light scattering, DLS (Fig. S1 †).According to the swelling law of RMs, 12 the incorporation of the ILs into the micelles can be conrmed using DLS if the size of the RMs increases linearly as the amount of ILs (or ratio of IL over surfactant blend, i.e., R in Fig. S1 †) increases.Interestingly, although the [C 4 -mim]X ILs (where X ¼ NO 3 , PF 6 and NTf 2 ) had differing solubility in the surfactant blend-IPM solution, no drastic variation in the IL-RMs sizes were observed (Fig. S1 †).Furthermore, we prepared IL-RMs with PD incorporated within the IL domain of the IL-RMs.It appeared that PD did not have a signicant effect on the size of IL-RMs formed, as was indicated by similar DLS intensities for the PD-containing and PD-free IL-RMs (Fig. S1 †).
With the suitable procedure for IL-RMs preparation at hand, we initially investigated the conformation of PD in [C 4 -mim]PF 6containing IL-RMs and compared it to neat [C 4 -mim]PF 6 , where PD was shown to exist largely in the planar conformation. 7In PF 6 -containing IL-RMs, although PD predominately adopted the planar conformation as indicated by the emission around 780 nm, the amount of the twisted conformation of PD was notably increased as compared to that observed in the neat [C 4 -mim]PF 6 (Fig. 2A; for absorption spectra see Fig. S2 †).In addition, a slight blue shi of the emission wavelength was also detected.Furthermore, we found that the amount of the IL and consequently concentration of PD (from 0.02 to 0.10 mM), as well as the amount of DMSO (up to 0.01% v/v) did not have an effect, as the planar conformation was observed as the major species no matter the size of the micelle, or concentration of PD.
It was previously shown that excitation of porphyrin rotors at different wavelengths (l ex ), around the Soret absorption maximum, could elucidate the effect of the media on the distribution of their conformers. 18Specically, if the planar/ twisted ratio was altered upon different l ex , then both conformers were present in a given media.On the contrary, if the ratio remained unchanged with varying l ex , then the existence of a single conformation was assumed. 18Therefore, it could also be argued that if the ratio in IL-RMs is distinct from that found in neat ILs, the environment provided by the IL-RMs and neat ILs is also distinct.Accordingly, here we investigated the conformation of PD as a function of l ex in both neat [C 4 -mim]PF 6 and [C 4 -mim]PF 6 -RM (Fig. 2B and S3 †).In both neat ILs and IL-RMs, the ratio of PD's conformations changed as a function of l ex , indicating that multiple conformations were present in both environments.However, the numerical values of the ratios in the two media were different, indicating that the natures of the IL-based media were distinct.Irrespective of the l ex , the amount of the twisted PD in IL-RM was always higher than that in the neat IL (Fig. 2B).
Next, in order to evaluate the effect of the anion (and, as such, the nature of the IL) on the conformation of PD in IL-RMs, we examined the uorescence spectra of PD in IL-RMs, which contained [C 4 -mim]NTf 2 and [C 4 -mim]NO 3 ILs and compared them to those observed in neat ILs.In [C 4 -mim] NTf 2 -RM and neat [C 4 -mim]NTf 2 (Fig. S4 †), the difference between the amount of the different conformations of PD was somewhat similar to that observed for [C 4 -mim]PF 6 -containing system (Fig. 2), as an appreciable difference between the neat IL and IL-RM was evident (Fig. S4 †).This similarity might be attributed to similar physical and/or structural properties of these two ILs.
On the contrary, in the case of the [C 4 -mim]NO 3 IL, which is known to be distinct from both [C 4 -mim]PF 6 and [C 4 -mim]NTf 2 ILs (in regard to hydrophobicity, hygroscopicity, hydrogen bonding ability, etc.), the amount of the twisted PD in the neat IL was found to be somewhat similar to the amount observed when PD was placed in the IL-RM (Fig. S5 †).However, the increase of the twisted conformation upon connement within the RM was still observed over the range of the l ex (Fig. S5 †).
Recently, it was shown that decreasing the length of the alkyl chain on the imidazolium cation, led to a decreased amount of the twisted conformation of PD. 7 If the connement of an IL within RMs increases the twisted conformation, this change would be more prominent in those ILs in which PD exhibits the smallest amount of the twisted conformation.To probe the aforementioned hypothesis, we investigated the conformation of PD in [C 2 -mim]BF 4 and [C 2 -mim]NTf 2 ILs, as well as in their respective RMs.Similar to the [C 4 -mim]-based ILs (Fig. S1 †), a linear correlation between the diameter of the IL-RMs and the amount of ILs was observed for the [C 2 -mim]-containing IL-RMs (Fig. S6 †).In both of these systems (Fig. S7 and S8 †), we found that the amount of the twisted conformation of PD increased from below 20% in neat ILs to slightly above 40% in the IL-RMs, thus conrming the aforementioned assumption that the amount of twisted conformation increases within IL-RMs.
Because the conformation of PD followed the same trend in all IL-RMs, regardless of the specic structure of the ILs, we hypothesized that the connement itself could be responsible for the observed effect.As such, this phenomenon should exist regardless of the nature of the media i.e., molecular organic solvent or IL.Therefore, we searched for a molecular organic solvent that promoted the planar conformation of PD in the bulk solvent, and was able to form RMs within the IPM-surfactant solution.DMSO was found to be a suitable solvent.It should be pointed out that unlike in neat ILs, PD in neat IPM was found to be non-uorescent.Furthermore, when the DMSO stock solution of PD was diluted with other molecular solvents including EtOH, MeOH, EtOAc, DMF, CHCl 3 , MeCN and acetone, and subsequently added to IPM as the media, no emission of PD was observed.All of the aforementioned solvents, except DMSO were found to be miscible with IPM.Therefore, the conformation of PD within DMSO-RMs was investigated.Similar to IL-RMs, the formation of DMSO-RMs was conrmed by the linear increase of the size of the RMs as a function of the amount of DMSO, i.e., the R value (Fig. S9 †).
We found that in all sizes of the DMSO-RMs, PD existed in nearly 45% of the twisted conformation (reminiscent of the behavior seen in IL-RMs), as opposed to neat DMSO where the twisted conformation of PD was around 15% (Fig. 3A and S10 †).In addition, the amount of twisted conformation of PD experienced little variation as a function of l ex in neat DMSO, while in DMSO-RMs a trend similar to those observed in IL-RMs was observed (Fig. 3B).Thus, these results supported the hypothesis that the connement might be a major factor in the control of PD's conformation.

Conclusions
We have demonstrated that conning PD within RMs could play a pronounced role on the conformational preference of PD.Specically, in RMs the amount of twisted conformation of PD was found to increase to ca. 40-45% as compared to that found in the neat solvents, i.e., 15-25%, regardless of the nature (molecular or ionic) of the media.Thus, encapsulation of PD appeared to represent an alternative method to alter the conformational preference of this molecule.These results also suggested that the effect of connement could override some specic solute-solvent interactions, which were shown to be important in control the conformation of PD in neat ILs. 7

Fig. 2
Fig. 2 Fluorescence spectra, where l ex ¼ 475 nm (A) and the amount of the twisted conformation (B, the lines were added to guide the eye) of PD in neat [C 4 -mim]PF 6 (red line and symbol) and in [C 4 -mim]PF 6 -RM (blue line and symbol).Conditions: [PD] ¼ 0.10 mM, all mixtures contained 0.010% of DMSO (v/v).