Self-Assembly of Octanuclear PtII/PdII Coordination Barrels and Uncommon Structural Isomerization of a Photochromic Guest in Molecular Space.

Two tetragonal molecular barrels TB1 and TB2 were successfully synthesized by coordination-driven self-assembly of a tetrapyridyl donor (L) of the thiazolo[5,4-d]thiazole backbone with cis-blocked 90° Pd(II) and Pt(II) acceptors, respectively. The single-crystal structure analysis of TB1 revealed the formation of a two-face opened tetragonal Pd8 molecular barrel architecture. In contrast, the isostructural Pt(II) barrel (TB2) is water-soluble. The large confined hydrophobic molecular cavity including wide open windows and good water solubility of the barrel TB2 made it a potential molecular container for the encapsulation of guests with different sizes and properties. This has been exploited to encapsulate and stabilize the open form of a photochromic molecule (G2) in water, while the same photochromic molecule exists exclusively in a cyclic zwitterionic form in aqueous medium in the absence of the barrel TB2. This cyclic form is very stable in water and does not go back to its parent open form under common external stimuli. Surprisingly, reverse switching of the cyclic form to a colored hydrophobic open form was also possible instantly in water upon addition of the solid barrel TB2 into an aqueous solution of G2. Such a fast reverse isomerization of an irreversible process in aqueous medium by utilizing host-guest interaction of the barrel TB2 and the guest G2 is interesting. The barrel TB2 was also capable of encapsulating the water-insoluble radical initiator G1 in aqueous medium.

Introduction

Nature is the unique dictator of self-assembly by elegantly utilizing a wide range of weak noncovalent interactions to carry out limitless biological phenomena.[1−4] Many attempts to mimic natural self-assembly phenomena in the laboratory have failed to get discrete finite assemblies of well-defined shapes and sizes due to the lack of strong directionality in many noncovalent interactions. In this regard, the evolution of coordination-driven self-assembly in the last three decades has enabled researchers to design and construct a wide variety of discrete 2D and 3D architectures with predefined shapes and sizes.[5−15] These self-assembled architectures possess unique nanocavities that have been widely used in host–guest chemistry, drug delivery, catalysis, and other applications.[16−26] Despite having a large internal confined space, some of the supramolecular 3D architectures suffer from narrow windows, which restrict the easy access of large guest molecules.[27−37] However, molecular barrels with two open faces have become very promising candidates in utilizing their full internal cavity size for encapsulation and de-encapsulation processes.[38−43] The kinetic inertness of the Pt–N bond limits the formation of discrete supramolecular architectures. As a consequence, a very limited number of Pt(II) barrels have been reported to date, but they are more robust and stable in comparison to the Pd(II) analogues.[44−48] Stang et al. reported several phosphine-blocked Pt(II) macrocycles/cages having diverse structural and impressive functional properties, but they suffer from poor water solubility.[49−54] Thus, the synthesis of Pt(II) water-soluble barrels to explore their ability to act as molecular containers has been a challenging task.

Species with a π-electron-deficient thiazolo[5,4-d]thiazole (TzTz) core have been widely employed for the construction of organic polymers, optical devices, and semiconducting materials.[55,56] The TzTz unit shows interesting electrochemical and photophysical properties due to its electron-deficient nature and planar rigid structure when it is incorporated into small molecules or extended polymers.[57−59] However, TzTz containing tetradentate donor has not yet been explored in designing a discrete supramolecular coordination architecture, and this prompted us to explore TzTz as an electron-deficient, robust backbone for designing metal-coordinated molecular containers.

The donor–acceptor Stenhouse adduct DASA (G2), an important class of photochromic molecule, easily photoisomerizes from a colored open form to a colorless cyclic form by irradiation of visible light in nonpolar organic solvents.[60,61] In protic solvents such as water and methanol, the colorless cyclic zwitterionic form of DASA is stable and does not revert to the colored open form by common external stimuli. The large window of the molecular container may promote the reversible conversion of DASA from a closed to an open form in water very quickly, a process that is impossible in general in the absence of a molecular vessel.

Herein, we report the design and synthesis of two new giant tetrafacial molecular barrels (TB1 and TB2) by treating the TzTz-based tetrapyridyl linker L separately with cis-[(tmen)Pd(NO3)2] (M) and cis-[(en)Pt(NO3)2] (M) as 90° acceptors (tmen = N,N,N′,N′-tetramethylethane-1,2-diamine, en = ethane-1,2-diamine) (Scheme ). The structure of TB1 was established in the solid state by single-crystal X-ray diffraction studies. While the barrel TB1 is insoluble in water, the Pt analogue TB2 becomes readily soluble in water. The water solubility of the Pt analogue is presumably due to the possibility of H-bonding between the blocking ethylenediamine and water. The large confined hydrophobic pocket and good water solubility make the molecular barrel TB2 an efficient host for the encapsulation of a polyaromatic radical initiator in aqueous medium. Furthermore, TB2 has been exploited for the encapsulation of a photochromic Stenhouse adduct (G2) as a guest. While the cyclic zwitterionic form of G2 exclusively exists in water, the presence of the molecular barrel TB2 stabilizes G2 in its open form. Moreover, the cyclic zwitterionic form of G2 in water converts instantly to its colored open form upon addition of a solid or aqueous solution of the barrel. Thus, in the presence of the molecular barrel (TB2) G2 acts in an way opposite to its normal behavior in aqueous medium in the absence of the barrel. To the best of our knowledge, such an instantaneous reverse switching of the cyclic form of the photochromic guest G2 to its open form in aqueous medium in the presence of a molecular container has not been explored previously.

Scheme 1

Schematic Representation of the Synthesis of Tetrafacial Barrels TB1 and TB2 from a Tetrapyridyl Donor (L) and Unusual Stabilization of the Open Form of Photochromic Guest G2 in Aqueous Medium

Results and Discussion

The tetrapyridyl donor 2,5-bis(3,5-bis(pyridin-4-yl)phenyl)thiazolo[5,4-d]thiazole (L) was synthesized by a condensation reaction of 3,5-di(pyridin-4-yl)benzaldehyde (P) with dithiooxamide under reflux conditions in dry DMF (Scheme S2). 3,5-Di(pyridin-4-yl)benzaldehyde (P) was prepared through a Suzuki coupling reaction of 3,5-dibromobenzaldehyde with 4-pyridylboronic acid under nitrogen atmosphere at 100 °C for 2 days (Scheme S1). All of the intermediates and the building block (L) were characterized by several spectroscopic techniques (Figures S1–S6). The tetrapyridyl building block (L) was insoluble in all common solvents. However, the protonation of four pyridine groups by adding dilute trifluoroacetic acid in D2O made it soluble and enabled us to characterize it by multinuclear 1D and 2D NMR studies. The 1H NMR spectrum of the protonated ligand showed two singlets and two doublets in the aromatic region due to the presence of the phenyl and pyridyl protons, respectively (Figure S3). A 1:2 treatment of the insoluble ligand L with the metal acceptor M in DMSO at 60 °C gave an almost clear brown solution after overnight stirring, which upon addition of excess ethyl acetate yielded TB1 as a brownish precipitate in good yield (Scheme ). The 1H NMR spectrum of TB1 in DMSO-d displayed four peaks between 8.39 and 9.44 ppm with an integration ratio of 4:1:4:2 (Figure S7). The 1H DOSY NMR invariably confirmed the formation of a single self-assembled species with a single diffusion band (D = 5.75 × 10–11 m2/s) for all of the peaks (Figure S8). The PF6– analogue of TB1 was similarly obtained by a self-assembly of ligand L, M, and KPF6 in 1:2:4 molar ratio at 65 °C in CD3CN (Scheme S3), and it was also characterized by 1D and 2D NMR studies in CD3CN (Figures S9–S11). Moreover, an ESI-MS analysis of the PF6– analogue confirmed a ML composition with the appearance of several prominent peaks at m/z 1482.8052, 1157.2352, 940.2, and 785.2024 corresponding to [TB1 – 4PF6–]4+, [TB1 – 5PF6–]5+, [TB1 – 6PF6–]6+ and [TB1 – 7PF6–]7+ charge fragments, respectively (Figures S12 and S13). Finally, suitable single crystals were grown by slow diffusion of dioxane vapor into a DMSO solution and used for the X-ray structural characterization by synchrotron radiation. The crystal structure analysis unambiguously established the formation of a tetragonal molecular barrel.

The octanuclear barrel (TB1) crystallized in monoclinic space group C2/m, where the asymmetric unit is one-fourth of the whole complex (of symmetry C2). The asymmetric unit comprises three Pd acceptors (two of which are located on a mirror plane), one tetrapyridyl organic ligand, counterbalanced by PF6– anions, and several dioxane/DMSO solvent molecules. The eight (tmen)PdII units are connected by four tetradentate ligands (L) with the similar binding mode μ4-η1:η1:η1:η1 to form a tetragonal barrel (TB1) architecture that consists of two terminal and four narrow side windows. The terminal open square windows in the tubular arrangement consist of four Pd(II) ions with Pd–Pd–Pd angles in the range of 89.48–90.93°. Diagonal and adjacent Pd–Pd distances are in the ranges of 18.93–19.12 and 13.41–13.49 Å, respectively. The distance between two opposite windows is roughly 17.52 Å. The two dihedral angles between the TzTz core and the adjacent benzene rings in the ligand backbone are 1.22 and 2.86°, which create almost planar walls limiting the molecular barrel. The inner cavity dimension of the barrel is approximately 13.4 × 13.4 × 17.5 Å3 (Figure ).

Figure 1

(a) Top and (b) side views of the single crystal structure of TB1. (c) Capped-stick model of the energy-optimized structure of TB2. (d) Experimental ESI-MS isotropic distribution pattern of the fragment [TB2 – 7PF6–]7+. Color codes: golden, C; blue, N; green, S; red, Pd; purple, Pt.

Although the barrel TB1 has open windows with a large nanocavity, it is marred by poor water solubility, which restricts its application as an efficient host for binding hydrophobic guest molecules in water. However, treatment of L with a cis-[(en)Pt(NO3)2] acceptor (M) in a 1:2 molar ratio in DMSO at 90 °C yielded a clear brown solution after overnight stirring (Scheme ). The product was obtained as a light brown precipitate by adding excess ethyl acetate to the DMSO solution followed by washing with diethyl ether. Interestingly, the obtained solid is readily soluble in water. 1H NMR of the newly synthesized complex in D2O showed the appearance of four distinct proton peaks in the range 7.8–9.0 ppm (Figure S14). Furthermore, diffusion ordered NMR spectroscopy (DOSY, D2O) confirmed the formation of the single self-assembled entity TB2 that displayed a single diffusion coefficient (D = 1.12 × 10–10 m2/s) for all of the protons associated with the complex (Figure S15). The molecular composition of the barrel TB2 was determined by ESI-MS spectroscopy. For better fragmentation in mass spectrometric analysis, the PF6– analogue of TB2 was prepared by mixing an aqueous solution of TB2 with the salt KPF6 salt followed by isolation of the precipitate. The appearance of several prominent peaks with appropriate isotropic patterns at m/z 1209.2751, 983.5664, 822.3523, and 701.4386 for the [TB2 – 5PF6–]5+, [TB2 – 6PF6–]6+, [TB2 – 7PF6–]7+, and [TB2 – 8PF6–]8+ charge fragments, respectively, confirmed the formation of a M8L4 species (Figures S19 and S20). Several attempts to grow suitable single crystals of TB2 for XRD analysis were unsuccessful. However, an energy-optimized structure was obtained by the DFT method, which revealed structural features analogous to those of TB1 characterized by an SC-XRD analysis (Figure ). Thus, spectroscopic and DFT studies indicated the formation of a two-face opened tetragonal molecular barrel structure analogous to TB1. The presence of terminal open windows associated with a large hydrophobic interior cavity (13.9 × 13.9 × 17.5 Å3) combined with good water solubility make the molecular barrel TB2 an ideal candidate for the encapsulation of different kinds of hydrophobic guest molecules of various shapes and sizes.

Polyaromatic hydrocarbon based derivatives as radical initiators have been used to synthesize useful polymers.[62,63] However, radical initiators are insoluble/sparingly soluble in water, restricting their chemistry mainly in organic solvents. The newly engineered molecular barrel TB2 is found to act as an effective host to solubilize the radical initiator naphthalen-1-ylmethyl-2-bromo-2-methylpropanoate (G1) in water. Guest encapsulation was performed by adding solid G1 to a D2O solution of TB2 at room temperature with continuous stirring for 2 days. The resulting solution was centrifuged, and the examined 1H NMR spectrum of the host–guest complex (G1⊂TB2) showed a downfield shift of the barrel α-pyridyl protons at 9.16 ppm with the appearance of aromatic peaks due to G1 in the range 5.8–6.32 ppm along with a distinct methyl peak at 0.73 ppm of the propionate chain (Figures S22 and S23). Additionally, 2D DOSY NMR confirmed the formation of a host–guest complex by the appearance of a single diffusion band (D = 1.02 × 10–10 m2/s) for all of the peaks associated with the complex (Figure S24). The host–guest binding ratio was calculated to be 1:2 from 1H NMR integration of the guest methyl protons with host α-pyridyl protons (Figure S22). Such an observation of aromatic guest encapsulation into an aqueous molecular barrel prompted us to check the efficiency of TB2 toward encapsulation of less-explored photochromic compounds such as the donor–acceptor Stenhouse adduct (G2). Such Stenhouse species (DASA) are new-generation photochromic compounds that show reversible photochromism under visible light at room temperature, differently from other common photochromic compounds, which mainly show a color change upon irradiation with UV light or warm-up. The Meldrum acid based DASA molecule (G2) is soluble in common organic solvents.[64] It converts from a red open form to a pale yellow cyclic zwitterionic isomeric form very slowly in organic solvents such as dioxane, toluene, etc. upon irradiation with visible green light. Again, the closed form reverts to its parent open species by keeping it in the dark or by heating. However, such reversible photoswitching is not observed in protic solvents such as water and methanol. In water, G2 exclusively remains in its zwitterionic cyclic form and does not turn back to its open form under common external stimuli.[65] Due to restricted conformational freedom in the confined space of molecular cages, photochromic molecules can behave differently in cages from their bulk behavior.[66−70] Depending on the window size of such cages, the rate of photoswitching can also be controlled. Larger windows are expected to make the guest encapsulation and de-encapsulation processes faster, leading to facile photoswitching. The barrel TB2 has opposite large windows with an appropriate nanocavity suitable for the encapsulation of hydrophobic aromatic compounds. Thus, the hydrophobic confined pocket of the barrel TB2 is expected to have the potential to encapsulate the hydrophobic open form of the photochromic guest (G2) and can make its reverse photoswitching phenomenon in aqueous medium faster. Consequently, the host–guest chemistry with the freshly prepared TB2 barrel was investigated with G2 as a guest. To an aqueous solution of TB2 were added 2 equiv of solid G2, and the mixture was stirred at room temperature in visible light. The barrel solution turned deep red, and it remained stable in the presence of visible light for several days. This indicates that the barrel encapsulates G2, stabilizing its open form in the hydrophobic confined pocket in aqueous medium even though the open species is known to readily isomerize to the cyclic form in aqueous medium in the absence of the barrel. Such a stabilization of the hydrophobic open form of G2 in aqueous medium was not observed in the presence of either the metal acceptor or the donor L alone (Figure S29). This reflects the significant contribution of the molecular barrel in driving the reverse photoswitching of the photochromic molecule G2 in aqueous medium. Thus, in the presence of TB2 the photochromic guest G2 behaves exactly opposite to its expected behavior in aqueous medium.

Furthermore, to confirm this unusual observation further, UV–vis spectroscopic studies were performed. Solid G2 was added to an aqueous solution of TB2, and the mixture was stirred. The mixture was centrifuged, and the UV–vis spectrum of the resulting red solution shows an absorption band at 488 nm, which clearly reveals the existence of the open form (Figure a) of G2. G2 alone in aqueous solution shows an absorption band at 264 nm instead of 488 nm, which implies the existence of the cyclic form of G2 in aqueous medium in the absence of the barrel (Figure b). Moreover, such a cyclic form is very stable and does not revert to its parent open form in water under external stimuli. To check the feasibility of the reverse reaction (cyclic to open form) in aqueous medium, the solid barrel was added to the pale yellow aqueous solution of G2. Surprisingly, the resulting solution immediately turned red (Scheme ), indicating the facile conversion of the cyclic form of G2 to a hydrophobic open form.

Figure 2

UV–vis spectra of (a) the barrel TB2 by itself and after addition of G2 to TB2 solution and (b) closed form of G2 solution in water and after addition of solid TB2 into an aqueous solution of G2 recorded at 298 K in micromolar concentration.

Scheme 2

Forward and Reverse Isomerism of G2 in Water in the Presence of the Barrel TB2 at 298 K

The UV–vis spectrum of the resulting solution was recorded immediately to check the appearance of the open form of the G2 molecule (Figure b). The absorption maximum of the cyclic form of G2 at 264 nm shifts to 276 nm which resembles that of just the barrel’s (TB2) absorption profile in water. On the other hand, a strong band appears at 488 nm situated exactly at the same position when forward encapsulation took place. Therefore, a complete diminishing of cyclic form’s absorption maximum with the reappearance of an absorption maximum for the open form confirmed the reverse isomerization of DASA to the open form in water. This fast reverse isomerism of an established irreversible process is otherwise hard to achieve, the zwitterionic cyclic form of G2 being very stable in water.

Furthermore, the host–guest complex formation was investigated by 1H NMR study (Figure ). A 2 equiv amount of the guest G2 was added to a D2O solution of the barrel TB2 and checked by 1H NMR. The 1H NMR spectrum of the host–guest complex (G2⊂TB2) showed broadening and splitting of the host’s aromatic peaks with the appearance of the guest’s C-methyl and N-methyl protons at 1.52 and 1.21 ppm, respectively. The host:guest binding ratio was found to be 1:2 by integrating the α-pyridyl proton of the barrel with the guest’s methyl proton (Figure S27). The encapsulation of G2 was supported by control experiments using different amounts of G2 with respect to the barrel TB2. At first, an excess amount of guest G2 was used with a low barrel (TB2) concentration in D2O and checked by 1H NMR. The 1H NMR spectrum showed the guest’s two aromatic peaks at 7.84 and 6.70 ppm along with aliphatic doublet (−CH−) and broad (−CH2−) peaks at 3.64 and 3.34 ppm, respectively, similarly to the cyclic form of guest G2 in D2O (Figure d). However, after the barrel concentration was increased to a host:guest ratio of 1:3, G2 aromatic peaks at 7.84 and 6.70 ppm gradually diminished with the appearance of new sets of aromatic peaks at 7.67 and 6.52 ppm. Similarly, in the aliphatic region doublet and broad peaks at 3.64 and 3.34 ppm diminished with the appearance of a new multiplet at 3.46 ppm (Figure c). Finally, the 1H NMR spectrum for a 1:2 host:guest mixture showed the complete disappearance of aromatic and aliphatic peaks due to the closed form of the guest G2 with the appearance of a new set of aromatic peaks along with a characteristic multiplet for the open form of G2 at 3.46 ppm (Figure b). Thus, the control NMR experiment confirmed again that the cyclic form of G2 converts into the open form upon increasing the TB2 concentration and full conversion occurs when a 1:2 host:guest ratio is maintained. These experimental observations account for the hydrophobic cavity of TB2 being responsible for driving G2 into its hydrophobic open form from its polar zwitterionic cyclic form in water.

Figure 3

Stacked 1H NMR plots (at 298 K) of (a) TB2, (b) a mixture of TB2:G2 in a 1:2 molar ratio, (c) a mixture of TB2:G2 in a 1:3 molar ratio, (d) a mixture of TB2 with excess G2, (e) the cyclic form of free G2 in D2O and (f) the open form of free G2 in CDCl3.

Finally, a 2D NOESY experiment was carried out to get structural insights into the host–guest complex. In the 1H–1H NOESY spectrum of the host–guest complex (G2⊂TB2), a clear correlation was observed between the G2 methyl protons with the host’s α- and β- pyridyl protons (Figure S28). A strong correlation between the methyl protons of G2 with the β-pyridyl protons of barrel TB2 indicates a closer proximity of the guest toward the host’s inner wall. Cross peaks between N-methyl protons of G2 with its own olefinic proton further indicate a head to tail orientation of two G2 molecules inside the barrel. The geometry of the host–guest complex was also examined computationally by the DFT method, where the energy-optimized structure supported these correlations (Figure S30). Thus, efficient host–guest complexation between TB2 and G2 helped the unusual reverse switching of DASA to occur in water. To the best of our knowledge, the present work represents the first report on the fast reverse isomerism of DASA molecule G2 in water employing a molecular container.

Conclusions

In conclusion, the self-assembly of a 90° cis-Pd(II) acceptor (M) with a tetrapyridyl ligand (L) containing a TzTz backbone yielded an octanuclear PdII8 tetragonal molecular barrel (TB1) whose structure was confirmed by a single-crystal X-ray diffraction study. TB1 suffers from poor water solubility, which limits its use as a molecular host for guest encapsulation in water. A similar reaction replacing the acceptor unit with an ethylenediamine-blocked cis-Pt(II) acceptor (M) yielded an analogous water-soluble PtII8 barrel (TB2), which was exploited as an efficient molecular container for encapsulation of different kinds of guest molecules. The photochromic Stenhouse adduct G2 is known to exist in a strongly colored open form in nonpolar organic solvents or in the solid state. In nonpolar solvents, upon visible light irradiation, it readily isomerizes to the weakly colored zwitterionic cyclic form that easily reverts to its parent open form in the dark. However, in aqueous/polar medium the open form converts irreversibly to a closed form, but the inverse reaction does not occur under common external stimuli. Surprisingly, in the presence of the barrel TB2 the photochromic guest (G2) was found to exist in the open form in aqueous medium without its usual conversion to the cyclic form. Moreover, while the zwitterionic cyclic form of G2 is known to be stable in aqueous medium, addition of TB2 to an aqueous solution of G2 readily drove the reverse reaction to yield the open form of G2. Such a reverse and fast conversion of the cyclic form of photochromic G2 to its open form in aqueous medium in the presence of the molecular container TB2 is remarkable, and this unusual conversation is mainly driven by the favorable host–guest interaction between the open form of G2 and TB2. Furthermore, the barrel showed efficient encapsulation of a hydrophobic radical initiator (G1) containing a naphthalene backbone in aqueous medium. The present work demonstrates that the behavior of photochromic G2 in the presence of TB2 is exactly opposite to its usual behavior in bulk aqueous medium.