Dynamic Supramolecular Ruthenium‐Based Gels Responsive to Visible/NIR Light and Heat

Abstract A simple supramolecular crosslinked gel is reported with a photosensitive ruthenium bipyridine complex functioning as a crosslinker and poly(4‐vinylpyridine) (P4VP) as a macromolecular ligand. Irradiation of the organogels in H2O/MeOH with visible and NIR light (in a multiphoton process) leads to cleavage of pyridine moieties from the ruthenium complex breaking the cross‐links and causing degelation and hence solubilization of the P4VP chains. Real‐time (RT) photorheology experiments of thin films showed a rapid degelation in several seconds, whereas larger bulk samples could also be photocleaved. Furthermore, the gels could be reformed or healed by simple heating of the system and restoration of the metal–ligand crosslinks. The relatively simple dynamic system with a high sensitivity towards light in the visible and NIR region make them interesting positive photoresists for nano/micropatterning applications, as was demonstrated by writing, erasing, and rewriting of the gels by single‐ and multiphoton lithography.

Abstract: As imple supramolecular crosslinked gel is reported with ap hotosensitive ruthenium bipyridine complex functioning as ac rosslinker and poly(4-vinylpyridine) (P4VP) as am acromolecular ligand. Irradiation of the organogels in H 2 O/MeOH with visible and NIR light (in am ultiphotonp rocess) leads to cleavage of pyridine moieties from the ruthenium complex breaking the cross-links and causingd egelation and hences olubilizationo ft he P4VP chains.R eal-time (RT) photorheology experiments of thin films showed ar apid degelationi ns everal seconds, whereas largerb ulk samples could also be photocleaved. Furthermore, the gels could be reformed or healedb y simple heatingof the systema nd restoration of the metal-ligand crosslinks. Ther elatively simple dynamic system with ah igh sensitivity towardsl ight in the visible and NIR regionm ake them interestingp ositive photoresists for nano/micropatterning applications, as was demonstrated by writing, erasing, and rewriting of the gels by single-and multiphoton lithography.
The field of supramolecular polymers aims to use directional intermolecular forces [1] to prepare dynamic materials. These so called "dynamers" are designed to undergo reorganization by reversible chemical reactions under the influence of external factors. [2] The dynamicity can be incorporated either by reversible covalent or noncovalent bonding, such as defined hydrogen-bonding moieties, or alternatively by ionic bonding or coordinatingm etal bonds. [2] Metallopolymers can offer unique properties and an umber of supramolecular systemsh ave been developed based on dynamic ligand-metali nteractions. [3] Such materials have been investigated in aw ide field of appli-cations,f or example the transductiono fm echanicalf orce into chemicalr eactions, [4] as well as in dynamic self-healing [5] and optically healable materials. [6] Also an umber of ruthenium-containingm etallopolymers have been reported,w ith ap articular focus on their use as stimuli-responsivepolymers. [7] Meanwhile, photocleavable polymers,t hat is polymers whichc leave or degrade in response to irradiation with certain wavelengths of light have been recently developed. [8] Incorporation of known photocleavable protecting groups,a lso sometimes referred to as photocages, [9] into macromolecular systems is one route to achieve this goal. Examples include the incorporation of coumarin [10] or o-nitrobenzyl [11] groupsi nto polymericm aterials [12,10] to achieve selective photodegradation.
While materials responsive to UV-light have long been established,t here has been ar ecent shift in the field towards the use of photochemical processes, which respond in the visible and NIR region [13, 11a] due to the poor penetration of high-energyU V-lighta sw ell as its incompatibility with biological environments. [14] Also light-sensitive ruthenium-based complexesh ave been shown to be responsive to light with long wavelengths, [15] for example, for photodynamic therapy with deep-tissue penetration. Various complexes based on a cis-[Ru II (bpy) 2 (L 1 )(L 2 )] 2 + structure are established as photocages [16] and showarapid cleavage of ligands upon excitationw ith visible light, as well as at wo-photon process in the NIR region. Furthermore, such complexesh ave recently been incorporated into hydrogels to achieve degelation and enable microphotopatterning [17] and the light-triggeredr elease of enzymes. [18] Herein, we report the use of rutheniumb ipyridine complexes as ac rosslinker for the polymerp oly(4-vinylpyridine) (P4VP), which acts as am acromolecular ligand, binding to the ruthenium centers and hence cross-linking the polymers. The procedure of the gel preparation was identical for all gels, and just the concentrationo ft he ruthenium complex wasv aried. Briefly,p oly(4-vinylpyridine)( P4VP; M W % 60 000 gmol À1 )w as crosslinked with [Ru(bpy) 2 Cl 2 ]t hroughc omplexation with the polymer-bound pyridine ligandsi nawater/methanol (v/v = 2:3) mixturea t8 08Cf or 16 h( Scheme 1) upon which gelation occurred (see the Supporting Information forf ull experimental details). The ligand exchange at the ruthenium atom with P4VP can be observed by UV/Viss pectroscopy (Figure 1), shown as an example for the gel with 5mol %o ft he ruthenium complex. At hin layer of the gel (1 mm) was appliedb etween two glass slides and measuredi nt he UV/Vis spectrometer.F or comparison, the spectrum of [Ru(bpy) 2 Cl 2 ]i si ncluded in Figure 1w ith its characteristic MLCT transition observed at 491 nm. The complexation with the pyridine moieties to the ruthenium atom shifts the peak maximum to 461 nm with a shouldera t% 435 nm. The observed shift is typical for the MLCT peak of comparable ruthenium-pyridine complexes. [17,19] Above % 600 nm no significant absorption is observed (see To compare different crosslinker ratios, bulk gels with 5, 10, 15, and 20 mol %o f[ Ru(bpy) 2 Cl 2 ]p er pyridine unit were prepared (see the Supporting Information). Accordingly,w ith higher ruthenium ratios, the color of the obtained gel becomes darkera nd the gel more brittle.A dditionally,t he crosslinking ratio also impacts the gelation time. P4VP gels with 20 mol %o f[ Ru(bpy) 2 Cl 2 ]a re formed in about 15 minutes while standinga tr oom temperature andg els with 5mol %r equire up to 60 minutes for complete gelation.
All obtained gels remain solid in the dark for at least > 12 months. The sensitivity to light was investigated by photorheology for polymers with 5% ruthenium cross-linker content ( Figure 2). Real-time-(RT)-photorheology experimentsi nw hich thin films are exposed to al ight source were directly measured on the rheometer plates. [20] The storage modulusG ' and the loss modulus G'' of the gels were measured in oscillation mode (1 %d eformation,1 0rad s À1 ,f or furtherd etails and explanations see the Supporting Information). Upon irradiation (365 nm, 25 000 mW cm À2 )a ni mmediate and rapid decrease in moduli was observed, indicative of ad e-crosslinking of the gel [21] through cleavage of the pyridine moiety of the P4VP from the complex.
Degelation of the bulk samples was also investigated. Upon irradiation of the P4VP gel in a1 0mLp yrex culture tube with visible light > 395 nm the gel becomes liquid.T he kinetics differ from the RT-rheology experimentsd ue to the different light source used and different thickness andhence penetrability of the samples. As expected,t he degree of crosslinking ratio also impacts the cleavage time due to the increased number of crosslinks but predominantly due to the increased absorbance of the incident light by the ruthenium chromophore. While the P4VP with 5mol %R uc rosslinker becomes dissolved within 10 minutes, the sample with 15 mol %r equires up to 60 minutes until the reaction mixture becomes liquid (see Figures S1-S3, Supporting Information). Meanwhile for the P4VP gel with 20 mol %o f[ Ru(bpy) 2 Cl 2 ]n od egelation was observed, presumably due to highera bsorption of light and hence the lower penetration of light into the system. The cleavage of the ligand can also be followed by UV/Vis spectroscopy( Figure S4, Supporting Information). Since only one of the pyridinel igandsi sc leaved in water [17,22] the change in the absorption spectra is indicated through as light shift of the MLCT band.   It wasp ostulatedt hat since the gelation reactioni si nitiated by an exchange of H 2 Ol igands in the ruthenium complex with pyridine ligands, [17] and since [Ru(bpy) 2 (H 2 O)(py-P)] 2 + (py-P = one pyridine group of P4VP) is known to be the product of the photoreaction, then the gelation reaction would be thermally reversible (Scheme 2). Simple heating of the sample in the (RT)-photorheology experiments indeed showed an increase in moduli ( Figure S5, Supporting Information), although heatingo ft hin films caused some solvent evaporation, which may interferew ith the read-out. The process was thus investigated on bulk samples with ar otational viscometer in ap yrex culture tube. The viscosity of the % 15 %P 4VP [Ru(bpy) 2 Cl 2 ] 2 + mixture increased to am aximum of 12 800 mPasu pon heating for 16 hours at 80 8C ( Figure 3). Upon irradiation with visible light > 395 nm, the value decreases to near its initial value of around3 20 mPas, suggesting an ear complete degelation and hence solubilization of the P4VP chains.
Interestingly the reversible gelation/degelation process could be repeated with an umber of cycles,a ss hown for the gel with % 15 %R ui nF igure 3a nd Figure S5, Supporting Information. The maximum viscosityo ft he gel appearst od ecrease with increasing number of cycles,s uggesting ar educed extent of crosslinking as the system is reheated.I ts hould, however, be stressed, that the viscosity experiments had to be carried out under ambient conditions. After 8cycles gelation could not be achieved for this cross-linker concentration. This is presumably due to some oxidation of the rutheniumc entrest o Ru III over time. It is wellk nown that Ru II -bpy complexes are efficient photoreductants. [23] The formed Ru III complexes are in general less pronet ol igand exchange reactions and to our knowledge no analogous photosubstitution has been reported for Ru III -bpy complexes.Hence, the decreasing number of reactive ruthenium(II) centres availablef or crosslinking leads to a loss of gelation ability.
Since it was anticipated that the photocleavage is locally restricted, agel containing test tube was covered with aluminum foil leaving as trip of 1cmu ncovered. The tube was then exposed to visible light leadingt ot he degelation and solubilization of the P4VP chains ( Figure S7, SupportingI nformation). Upon subsequent immersion of the test tube in aw ater bath at 80 8C, ac omplete gel was obtained again, suggesting ap otential use of the polymers as healable materials.
Furthermore, to demonstrate the potentialf or micropatterning, logos were written into gel portions ( Figure 4) containing 5a nd 15 mol %o fruthenium crosslinker.T hese polymer samples were enclosed between two glass slides and properly sealed. The micropatterning could be achieved both by focused NIR-laserl ight at 1028 nm (2 mW 15 %g el, 20 mW 5% gel) in am ultiphoton process, [24,25] as well as by single-photon laser at single-photonl ithography (1PL) at 514 nm (2 mW1 5% gel). To verify the micropatterning, images were acquired directly after writing with the same setup using an infinity corrected NIR air microscopy objective (50 /0.42). For imaging we used either an industrial camera for white light images or a single photon avalanche diode for backscattered images (see Figure S8, Supporting Information). Upon smoothly heating the gel samples on ah ot plate at 60 8Cf or one hour,t he logos Scheme2.Dynamic reversible gels formed by binding of ruthenium to the P4VP macromolecular ligand. were observed to disappear.S ubsequently,t he same area of the gel was rewritten using the same MPL and 1PL conditions. All acquired images were contrastenhanced.
In summary,asimple crosslinked gel was prepared based on P4VP as am acromolecular ligand cross-linked with [Ru(bpy) 2 Cl 2 ]c omplexes. Photocleavage of the metal-ligand bond upon irradiation broke the linkage between the macromolecules leading to degelation and hence solubilisation of the PV4P chains. RT-rheology experiments of thin films showed ar apid degelation in several seconds while larger bulk samples could also be photocleaved in severalm inutes,d epending on the concentrationo fr uthenium and penetrabilityo fl ight into the gels. Furthermore the system wass hown to be reversible with the gel being reformed upon heating,aprocess which could be cycled an umber of times. The gels were prepared by simple mixing and heatinga nd the properties could be tuned by the ratio of crosslinker to polymer.T he relative simplicity of the system combined with the high sensitivity of the systems to light in the visible and NIR region make them highly interesting novel materials for lithographic applications.I nt his context micropatterning of the gels was demonstrated by multiphoton and single photon lithography at 1028 and 514 nm, respectively and it was shown that the gels could be positive photoresist erased, healed, and rewritten on the same region of the gel.