Photoinduced Strain‐Assisted Synthesis of a Stiff‐Stilbene Polymer by Ring‐Opening Metathesis Polymerization

Abstract Developing a novel strategy to synthesize photoresponsive polymers is of significance owing to their potential applications. We report a photoinduced strain‐assisted synthesis of main‐chain stiff‐stilbene polymers by using ring‐opening metathesis polymerization (ROMP), activating a macrocyclic π‐bond connected to a stiff‐stilbene photoswitch through a linker. Since the linker acts as an external constraint, the photoisomerization to the E‐form leads to the stiff‐stilbene being strained and thus reactive to ROMP. The photoisomerization of Z‐form to E‐form was investigated using time‐dependent NMR studies and UV/Vis spectroscopy. The DFT calculation showed that the E‐form was less stable due to a lack of planarity. By the internal strain developed due to the linker constraint through photoisomerization, the E‐form underwent ROMP by a second generation Grubbs catalyst. In contrast, Z‐form did not undergo polymerization under similar conditions. The MALDI‐TOF spectrum of E‐form after polymerization showed the presence of oligomers of >5.2 kDa.

Polymers that respond to al ight stimulush ave drawn tremendous attentioni nr ecent years due to their immense potential in artificial muscles, [1] photomechanical actuators, [2] drug delivery systems, [3] supramolecular systems, [4] energy storage, [5] switchable surfaces, [6] and so on. [7] Various types of molecular photoswitches, such as stilbene, [8] azobenzene, [9] spiropyran, [10] diarylethene, [2d, 11] coumarin [12] or fulgide, [12] have been incorporated into the main or side chain of the polymer to achieve reversible photoresponsive polymers. [13] Due to the light-induced molecular changes,t hese molecular switches can cause a structuralr econfigurationor change in the properties of poly-mer chains, which leads to fast, precise and remote controlo f various macroscopic properties, such as shape, [2d] wettability, [10b] opticalp roperties, [14] adhesion, [15] solubility, [16] conductivity, [14,17] and so on. Compared to the side chain polymer,t he main-chain polymer made up of photoswitchable repeating units has improved thermala nd mechanical properties that provide better photomechanical response. [18] Despite significant advancesi nt he development of photoresponsive polymers,t he synthesis of main-chain polymers necessitates complex synthesis of photoresponsive monomers, the use of toxic reagents and harsh conditions. [19] Among various photoswitching molecules, 1-(1-indanyliden)indan,c ommonly known as stiff-stilbene, has received much attention owing to 1) rigidity due to restricted rotation of phenyl ring, 2) remarkable thermal stability (a half-life of ca. 10 9 years at 300 K) due to high activation energy barrierb etweent he two isomers ( % 43 kcal mol À1 ), 3) superiorp hotostability and chemical stability,4 )high quantum yield for the photoisomerization of either isomer (50 %) and 5) easily modifiable core structure. [20] Boulatov and co-workers employed as tiff-stilbene force probe to determine the relationship between the restoring force in as tretched stiff-stilbene macrocycle and the kinetics of dissociation of different functional groupsw ithin the macrocycle. [20a,b, 21] Apart from these force probe applications,t hey have been used for dynamic supramolecular polymerization, [22] molecular machine, [23] switchable catalyst, [21] O 2 sensor [24] and stimuli-responsive proton gate. [25] Despite these encouraging applications,t he design and functiono ft he photoresponsive stiff-stilbene polymers have not been achieved until now. [26] Therefore, the development of new main-chain stiff-stilbene polymers is essential to gain more insight into this typeo fp hotomechanicals ystem and to producem ore advancedp olymers whose physical properties and functions could be controlled by external stimuli. Herein, we report, for first time, photoinduced strain-assisted ROMP synthesis of main-chain stiff-stilbene polymers by activating macrocyclic p bond connected to stiff-stilbene photoswitch through photoisomerization.
Over the past years, ROMPh as been an effective method for the synthesis of linear polymers from cyclic olefin of having a high angle or ring strains. [27] For instance, in comparison studies of ROMP of various cyclic olefins( ring sizes 5-8), 6-membered olefin (Cy6) was not subjected to ROMP due to the very low ring strain energies. [28] The ring strain of am olecule can be manipulated if the ring structure is made up of am olecular photoswitch. For example, the ring-opening reactiono fc yclobutenec onnected to C6 and C6' atomsi ns tiff-stilbene through al inker are influencedb yt he force generated duet o the increase of separation between C6 and C6' atoms during photoisomerization, since stiff-stilbenee xhibits at remendous changes in spatial extensions upon photoisomerization. [20d] We envision thatt he stiff-stilbene polymerc an be synthesized by ROMP if the p bond is attached to the stiff-stilbene through an aliphatic linker.I ns uch ad esign, the p bond can be activated by photo-triggered reversible spatial expansion [29] due to the strain generated by the linker,w hich is an external constraint attachedt oC 6a nd C6" atoms in stiff-stilbene when Z-form isomerizest oE-form ( Figure 1). [30] We synthesized the macrocycle 1 through af our-step route with good yields (Figure 2A and Scheme S1). Briefly,w eh ave treated 2-hexenoic acid (2)w ith 2ndgeneration Grubbs catalyst to yield cis-4-decenedioic acid (3). This diacid 3 further treatment with oxalyl chloride in the presence of N,N-dimethylformamide (DMF)i nd ry dichloromethane (DCM)t oy ield corresponding acyl chloride which was further converted to 6-hydroxy indinone diester (4)b yt reating 6-hydroxy-1-indanone in dry DCM in the presence of triethylamine.T he diester 4 was coupled each other using McMurry coupling with Zn dust/TiCl 4 to yield the final macrocycle 1.
The configuration of macrocycle 1 was studied by NMR spectroscopy.W ee xpected as trong nuclear Overhauser effect (NOE) interaction between H-1 and H-5( hydrogen atoms on carbonsw hich are numbered in Figure 2A)w hen the macrocycle is in the E-form, but not in the Z-form ( Figure S1). The NOE spectrum shows that there was no interaction between H-1 and H-5,i ndicating that macrocycle 1 existed in the Z-form, as shown in the previous reports. [30] As expected, the Z-form of macrocycle 1 was isomerized to the E-form when its solution in chloroform-d 3 (0.014 m)w as irradiated with UV light at a wavelength of 365 nm ( Figure 2B). Time-dependent 1 HNMR studies showedt hat the formation of an ew olefin proton signal at 4.87 ppm (peak marked as b) is due to photoisomerization ( Figure 2C). Despite photoisomerization, prolonged UV irradiation also caused the initially formed E-form to decompose, as can be seen from these NMR investigations.T he 1 HNMR showedt hat the olefin proton signal formed after photoisomerization wasd iminished with prolonged irradiation with the concomitant formation of many aromaticp roton signals. This could be attributedt ot he random polymerization of macrocycle 1 doubleb onds. This decomposition was also observedi nt he other solvents (e.g.,d ichloromethane, Figure S2). Such undesirable UV-initiated random polymerizations can easily be prevented by adding phenolic antioxidants that are knownt or eact with free radicals. We added ab utylated hydroxy-toluene (BHT), ap henolic antioxidant (0.51 equivalents to macrocycle 1)t ot he solution of macrocycle 1 in DCM-d2 (0.014 m). This solution was irradiated with 365 nm UV lamp and analyzed by 1 HNMR. An ew olefin proton signal was formed at4 .85 ppm (peak labeled asb )d ue to the E-form with ac oncomitant decrease in the signala t5 .40 ppm (peak labeled as a) due to the consumption of Z-form as can be seen from the 1 HNMR spectra ( Figure 2D). Thek inetic transformation followed ah yperbolic relationship between the isomerization and the irradiation time ( Figure 2E). After ac ertain exposure time (100 minutes), the ratio of the Z-form to the E-form (0.38) remained unchanged, indicating ap lateau stage, probably because the solution was able to reach equilibrium during this exposure time.  The UV/Vis spectroscopy gave additional evidence for the photoisomerization of macrocycle 1 in DCM solution (50 mm). Prior to irradiation, the absorption spectrum of macrocycle 1 showedt wo absorption peaks (l max = 338 nm and l max = 358 nm). Upon irradiation, these bands were blue-shifted with an increaseo fm olare xtinction coefficient (e), and the spectrum displayed as tructured absorption with vibronic replicas with am aximum of 319 nm, 333 nm and 350 nm ( Figure 3A). The hypsochromic shift in absorption is due to the fact that the E-form of the stilbene unit wasn ot ablet of orm ap lanar arrangement since the linker bound to C6 and C6" atoms serve as an external restriction. To compare the absorption spectrum of E-form with an irradiated sample of Z-form,p ure E-form was separated from the EZ mixture by column chromatography using 10 %e thyl acetate in petroleume ther as eluent. The chemicals tructure of Z-form was confirmed by NOE measurement in which as trong interaction between H-1 and H-5 protons was found ( Figure 3B). The comparison of the absorption spectrao fE-form and irradiated Z-form revealed that they were identical to each other.Itis also known that the molar extinctionc oefficient( e)o ft he E-form is highert han that of the Z-form. [30] To compare e values, both Z-a nd E-form values were prepareda tac oncentration of 50 mm in DCM solvent. As expected, the separated E-form had higher e values up to 358 nm at all wavelengths.
We also evaluated the stability of the E-form in the EZ mixture obtained after irradiation. The irradiated sample was stored atr oom temperature under dark conditions for 12 hours.I ti sw orth noting that the E-form in the irradiated sample was stable under the dark condition as the signals from 1 HNMR remained unaltered even after storage ( Figure S3).
Density functional theory (DFT) calculation revealed that the Z-form was more stable than the E-form by af actor of 13.09 kcal mol À1 (Supporting Information). In contrast to unsubstituteds tilbene, the Z-form did not have planar geometry and the planes of the phenyl rings were tilted at an angle of 37.418.I nt he E-form, the angle between these planes was too large (61.898), which caused the loss of conjugation as expected from UV/Vis spectroscopics tudies ( Figure 3C). This large angle perturbation destabilized the E-form. An umber of macrocycles have shown that stretching disulfide bonds, sulfates, or esters accelerates the ring-opening reactionw ith thiols, water,o rr educing agentsi nt heir strained E-form, making the stretched bonds easily cleavable. [20b, 29, 31] This is due to the reductioni nt he free activation energyw ith increasing strain on the macrocycles. [31] We assume that the p bond bound to a stiff-stilbene through al inker could be activated for ROMP through photoisomerization to synthesize the main-chain stiffstilbene polymers.
To investigate the reactivity of E-a nd Z-forms towardsR OMP ( Figure 4A,B), the DCM-d 2 solution of Z-form (0.014 m)w as irradiated with 365 nm UV lamps for 60 minutes in the presence of BHT (0.51 equivalents to macrocycle 1)toget the EZ mixture in which 37 % E-form and 63 % Z-form werep resent. The EZ mixturew as then treated with 5mol %2 nd generation Grubbs catalyst, and the kinetics of the reactionw as studied by timedependent 1 HNMR spectroscopy ( Figure 4C). It was observed that the reactions tarted immediately after the addition of the catalyst, which is evident from the appearance of new signals, especially those due to an ew olefin at d 5.45 (peak labeled as ci nF igure 4B). The intensitieso ft hese signals increased gradually over time, along with ac oncomitantr eduction in the intensities of signals due to the E-form (d 4.87 due to olefin (peak labeled asbin Figure 4B)). This trend continued until the reactionw as 98 %c omplete in 159 minutes. Ap lot of percentage of the reactiona gainst time showed that the reaction followed sigmoidal kinetics( Figure 4D). Despite the ROMP in the E-form, no significant change in the 1 HNMR signals of Zform was observed. This could be indirectly attributed to the inability of the Z-form of macrocycle 1 to open the ring for this polymerization by the catalystb ecause of its low strain energy.
There are two olefin bonds (stilbenea nd macrocyclic double bonds) present in the macrocycle 1,a nd it is necessaryt ou nderstandw hich bond actually opens by the catalyst. Though either of these bonds opens, the final structure of the polymer will be the same, but the newly formed double bonds may have different configurations (Supporting Information). Suppose stiff-stilbene double-bond is cleaved by the catalyst;t he polymericp roduct must have mainly Z-formo fs tiff-stilbene units in its backbone, which can be easily investigated by UV/ Vis spectroscopy.F or example, cyclic ferrocenyl olefin (both cis and trans forms) underwent ROMP by 2nd generation Grubbs catalysty ielding polymers with only cis double bond configurations in their backbone. [32] To prove this, we have separated E-form from the EZ mixture. We also carriedo ut ROMP of pure E-a nd Z-form of macrocycle 1.A so bserved in the EZ mixture, the pure E-form also underwent ROMP,a sc an be seen from NMR spectra( Figure S4). The UV/Vis spectrum of E-form after treatment with the catalyst was similart ot hat before the catalyst treatment, whichi ndicates that the E-configuration at stiffstilbene was unaltered even after polymerization ( Figure 4E). A slight red-shift might be due to the increaseinthe conjugation by the ring-opening reaction. This absorption spectrum was in good agreement with that of reported unmodified trans-stiffstilbene. [33] This reveals that only the macrocyclic double-bond underwent ROMPb yt he catalyst, andt he stiff-stilbene doublebond wasstill intact after polymerization.
The MALDI-TOF mass spectrum of reaction mass after reaction of 160 minutes showedp eaks with as pacing of 428 gmol À1 corresponding to oligomers from dimer to 12mers with an initial increase and then ag radual decrease in their intensities, indicating ROMP ( Figure 4F). The low degree of polymerization (DP) is due to the highc atalystl oading (5 mol %) and DP can be increased by reducing the amount of catalyst. It is also noted that av ery negligible amount of Zform was also formed during the reaction of E-form with Grubbs catalyst, indicating negligible E to Z isomerization by catalyst( Figure S4). No significant change was observedi nt he reactiono ft he pure Z-form with the 2nd generation Grubbs catalyst, except fort he formation of new aromatic protons ignals due to the isomerization reaction ( Figure S4). Electrospray ionization mass spectrometry (ESI-MS)a nd MALDI-TOF spectra showeda dditional evidencet os upport this claim.I nE SI-MS and MALDI-TOF spectra,m ost of the strong peaks, together with the monomeric peak, were below 1kDa, indicating that even oligomers were not formed by the reactiono ft he Z-form with the Grubbs catalyst ( Figure 4G and Figure S5).
In summary,w ehave developed an ovel strategy to synthesize main-chain stiff-stilbene polymers using ROMP by inducing as train to the macrocyclic p bond attached to as tiff-stilbene through photoisomerization.T he Z-form of macrocycle 1 was photoisomerized with am aximum conversion of 38 %t ot he E form. The DFT calculation showedt hat the E-form was more strained due to al ack of planarity,a nd therefore, the photoisomerization of the Z-form could produce as trongly strained Eform. Due to this high internal strain, E-form in the EZ mixture underwent ROMP in the presence of a2 nd generation Grubbs catalyst. In contrast, the Z-form did not undergo any polymerization. The MALDI-TOF spectrum of the E-form after treatment with the Grubbs catalyst indicated the formation of oligomers up to 12-mer.T his photoinduceds train-assisted synthesis could be of great importance in the field of stimuli-responsive polymer materials since theset ypes of advanced polymers could have excellent photoresponsive properties due to the multiple stiff-stilbene units presenti nt heir polymer backbone.