pubs.acs.org/OrgLett Letter Dual-Wavelength Gated oxo-Diels−Alder Photoligation Marc Villabona, Sandra Wiedbrauk, Florian Feist, Gonzalo Guirado, Jordi Hernando,* and Christopher Barner-Kowollik* Cite This: Org. Lett. 2021, 23, 2405−2410 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: The control of chemical functionalization with orthogonal light stimuli paves the way toward manipulating materials with unprecedented spatiotemporal resolution. To reach this goal, we herein introduce a photochemical reaction system that enables two-color control of covalent ligation via an oxo-Diels−Alder cycloaddition between two separate light- responsive molecular entities: a UV-activated photocaged diene based on ortho-quinodimethanes and a carbonyl dienophile appended to a diarylethene photoswitch, whose reactivity can be modulated upon illumination with UV and visible light. under illumination with two different wavelengths (λ1 and λ2). B y enabling chemical functionalization of molecules and materials with high efficiency and spatiotemporal control, photoinduced ligation reactions1 are finding applications in a As a result, bond formation proceeds at λ1 and is halted upon the dual impact of λ1 and λ2. variety of fields ranging from polymer network formation2 to To reach this goal, we herein focus our attention on a surface patterning,3 3D printing,4 and bioconjugation.5 In most photoligation process that has been seldomly explored for of the cases, these photoreactions are driven by irradiation with light-induced chemical functionalization: the oxo-Diels−Alder a single light source and, therefore, their time and spatial cycloaddition, a hetero-Diels−Alder reaction between dienes profile evolution is determined by illumination intensity and and carbonyl groups (Figure 1b).10 When driven under normal the reagents’ concentration and properties. A much more electron demand, catalyst-free oxo-Diels−Alder cycloadditions precise control of photoinduced reactivity can, however, be are highly favored for electron-rich dienes and electron-poor achieved by regulating the ligation processes with two dienophiles,11 a condition on which we capitalize herein to independent optical signals, i.e., two different colors of light reach two-color control of photoreactivity at ambient temper- that serve as “and” gates for the λ-orthogonal activation and ature. Specifically, we select o-quinodimethanes (o-QDMs) deactivation of the desired reaction on demand.1b,c Such a photochemically generated from o-methyl benzaldehydes (o- strategy can, among other advantages, grant access to novel MBAs) as electron-rich dienes (Figure 1c),10,12 which have lithographic processes6 and finely controlled postmodification been widely exploited for other types of photoligation reactions strategies for materials,7 which constitute two key concepts for (e.g., homo-Diels−Alder cycloadditions13 and self-dimeriza- the development of advanced functional structures.8 tion14). As for the electron-poor dienophile, we choose Current methodologies applied to the dual-wavelength trifluoromethyl ketones,15 which are tethered to a diaryl- control of photoligation chemistry involve time or spatially ethene photoswitch (DAE) to reversibly alter their reactivities resolved two-color irradiation of one of the reagents of a upon photoisomerization (Figure 1c). DAEs16 have already bimolecular reaction, which can be either molecules capable of been used to photomodulate homo-Diels−Alder cycloadditions undergoing photocycloaddition and photocycloreversion pro- by generating and removing reactive diene functionalities when cesses (e.g., anthracenes7c) or molecular photoswitches whose interconverting between their open and closed states;17 however, it is another property of these systems that we take reactivity can be reversibly toggled upon photoisomeriza- tion.6b−e,7a,b,9 To control light-induced reactivity with finely selected colors of light, we herein pioneer a reaction strategy Received: January 4, 2021 that ceases photochemical bond formation upon the Published: February 23, 2021 simultaneous irradiation of two photoreactive entities (Figure 1a): a photocaged reagent, which becomes activated upon irradiation with one color of light (λ1), and a photoswitch reversibly isomerizing between reactive and nonreactive states © 2021 American Chemical Society https://dx.doi.org/10.1021/acs.orglett.1c00015 2405 Org. Lett. 2021, 23, 2405−2410 Organic Letters pubs.acs.org/OrgLett Letter Figure 2. (a) Synthesis of DTE1open. (b) UV−vis absorption spectra of DTE1open, DTE1closed, and o-MBA1 (X = SBu, Figure 3a) in toluene. (c) Variation of the molar fraction of DTE1closed (as determined from UV−vis absorption data) upon consecutive cycles of irradiation of a toluene solution of this compound with UV (λexc = 365 nm) and visible (λexc = 650 nm) light to trigger photo- isomerization between its open and closed states. of DTE1 (DTE1open) was readily obtained in 60% yield (Figure 2a). DTE1 preserves the characteristic photochromic behavior of DAEs. First, its colorless open isomer (λabs < 400 nm, Figure 2b) rapidly transforms into the closed DTE1closed state upon irradiation with UV (or even violet) light (Figure S1). In our hands, this light-induced process proceeded with a high quantum yield (Φo‑c = 0.37 in toluene) and efficiency, as we determined by 1H NMR that the photostationary state produced at a λexc of 365 nm in toluene contained up to 96% of the closed isomer. DTE1closed was found to be thermally stable in the dark at room temperature, both in solution and in the Figure 1. (a) Strategy for dual-wavelength control of photoligation solid state. As shown in Figure 2b, it strongly absorbs in the reactions by combining photocaged and photoswitchable reagents. (b) Normal electron demand oxo-Diels−Alder cycloaddition selected visible region (λabs,max = 635 nm in toluene) and, when as a photoligation reaction (EDG = electron-donating group; EWG = irradiated with green or red light, quantitatively back- electron-withdrawing group). (c) General structures of the photo- photoisomerizes to the open isomer with a modest quantum generated o-quinodimethane dienes and the photoswitchable DAE- yield (Φc‑o = 0.031 in toluene, Figure S1). Finally, DTE1 trifluoromethyl ketone dienophile employed, which should undergo shows excellent fatigue resistance and can be reversibly oxo-Diels−Alder cycloaddition selectively for the closed state of interconverted between its open and closed states without DTE1. apparent photodegradation (Figure 2c). According to our molecular design, the trifluoromethyl advantage of: the change in electronic communication between carbonyl groups in DTE1closed should efficiently undergo oxo- the substituents of the aryl rings upon photoisomerization.7a,16 Diels−Alder cycloaddition with electron-rich dienes. To For the herein utilized symmetric dithyenylethene DTE1, the validate this hypothesis, we explored the reaction between carbonyl group electron density of the two electron-with- DTE1closed and methylbenzaldehyde o-MBA1 (Figure 3a), a drawing trifluoromethyl carbonyl side groups should be model photocaged precursor of diene o-QDM1 that was dramatically decreased in the closed state of the switch, prepared following the methodology previously reported by us where they are electronically coupled (Figure 1c). Thus, these (Scheme S1).20 o-MBA1 was selected on the basis of the well- interdependent entities become more active dienophiles for documented capacity of o-MBA derivatives to undergo the photogenerated o-QDM dienes, thereby allowing for sequential intramolecular H-transfer and isomerization upon photomodulation of the corresponding oxo-Diels−Alder photoexcitation, which produces o-QDM dienes that can be reaction. trapped in a Diels−Alder reaction with electron-deficient enes To synthesize DTE1, a classic approach for the preparation (Figure 1c).6b,20,21 In addition, by introducing an alkylthio of trifluoromethyl ketones was employed, which consists of the group in o-MBA1, its absorption spectrum bathochromically trifluoroacetylation of organometallic reagents.18 Thus, when shifts relative to regular nonsulfurated o-MBAs, reaching 1,19 a common intermediate for the preparation of a variety of maximal photoreactivity at λexc ∼ 350−390 nm (Figure DAE switches, was sequentially treated with tert-butyllithium 2b).13,20 At such spectral range, DTE1open also shows high and ethyl trifluoroacetate in anhydrous THF, the open isomer photoconversion into DTE1closed, thus ensuring efficient light 2406 https://dx.doi.org/10.1021/acs.orglett.1c00015 Org. Lett. 2021, 23, 2405−2410 Organic Letters pubs.acs.org/OrgLett Letter light absorption by DTE1closed. This, in combination with the photochromic properties of DTE1, made light-induced transformation of the closed isomer into the open form negligible at our experimental conditions, as proven by the very low intensity registered along the process for the 19F NMR signal of DTE1open (δ = −71.73 ppm) (Figure 3b). In addition, photodegradation of DTE1closed was not observed either under equivalent illumination conditions in the absence of o-MBA1 (Figure S3). Thus, the evolution of new resonances in the 19F NMR spectrum of the reaction mixture under UV irradiation selectively reported on the products formed by direct photoligation between DTE1closed and o-MBA1. In practice, a complex set of NMR resonances was obtained after illumination for 90 min (Figure 3b), where (a) the 19F NMR resonance of DTE1closed at δ = −71.26 ppm fully vanished, thus indicating a complete reaction with o-MBA1; (b) three slightly downfield shifted signals appeared at δ = −70.81, −70.82, and −70.87 ppm, which could be assigned to the trifluoromethyl carbonyl groups of the photoproducts; (c) four additional intense peaks developed at rather lower chemical shifts (δ = −77.12, −77.74, −78.40, and −78.41 ppm), which is compatible with the CF3 substituents of the isochroman moiety formed through cycloaddition.22 It must be noted that, though such a diversity of NMR resonances could indicate the occurrence of competitive photoreactions, they could simply originate from the large number of regio- and stereoisomers that can be produced by light-induced oxo-Diels−Alder cycloaddition between DTE1closed and o-MBA1 (Scheme S2). Interestingly, the complex 19F NMR spectrum obtained for the photoreaction mixture of DTE1closed and o-MBA1 was largely simplified after irradiation with visible light (LED λmax = 625 nm, Figure S2), where only two sets of different signals attributable to trifluoromethyl carbonyl (δ = −71.57 and −71.60 ppm) and trifluoromethyl isochromanyl (δ = −78.90 Figure 3. (a) oxo-Diels−Alder reaction of DTE1closed with the diene and −79.80 ppm) groups were found (Figure 3b). As color photogenerated from o-MBA1 under UV illumination (LED λmax = bleaching of the sample was concomitantly observed (Figures 365 nm, Figure S2) and ambient temperature. The adduct A1closed can 3b and S4), we hypothesize that quantitative ring-opening of be converted to its open isomer A1open upon irradiation with visible the DAE structure of the cycloadducts formed by the reaction light (LED λmax = 625 nm, Figure S2). (b) 19F NMR spectra (565 between DTE1closed and o-MBA1 (A1closed) occurred, thereby MHz, CD3CN) of (i) DTE1open; (ii) DTE1closed; (iii) a mixture of yielding the corresponding colorless open-ring products with a DTE1closed and o-MBA1 after UV illumination (LED λmax = 365 nm, lower number of stereoisomers (A1open, Figure 3a and Scheme 0.017 mW cm−2) for 45 min in deuterated acetonitrile; (iv) a mixture S2). Actually, this process could be reverted with UV of DTE1closed and o-MBA1 after UV illumination (LED λmax = 365 nm, 0.017 mW cm−2) for 90 min in deuterated acetonitrile, which irradiation and, as such, it is reminiscent of DTE1 photo- resulted in nearly quantitative transformation into A1closed; (v) chromism (Figure S4). A1closed upon irradiation with visible light (LED λmax = 625 nm, 35 Due to its reduced complexity, we subsequently focused on mW cm−2) for 2 min in deuterated acetonitrile, which led to further characterizing A1open from which the structure of quantitative photoisomerization into A1open. For clarity, the 19F NMR A1closed was derived. In this way, A1open and A1closed were resonances in each spectrum arising from DTE1open (blue ▲), proven to consist of inseparable mixtures of two and four DTE1closed (pink ▼), A1closed (red ●), and A1open (black ■) are diastereoisomeric pairs of enantiomers, respectively, whose indicated with different symbols. The changes in color of the reaction general structures are shown in Figure 3a (see Scheme S2 for mixture before illumination (ii, DTE1closed) and after UV (iv, A1closed) the exact stereoisomers). Specific experimental data underpin and subsequent visible (v, A1open) irradiation are shown as insets in (b). this assignment. First, a single molecular mass was determined for A1open by ESI-MS, which matches the value expected for the monocycloaddition products of DTE1 with one unreacted activation of both substrates of the cycloaddition reaction trifluoromethyl carbonyl group (Figure S5). This result is using a single irradiation wavelength. corroborated by the NMR spectra registered for A1open, which The photoligation reaction between the activated dienophile also demonstrate selective hemiacetal formation and 1,3,8- DTE1closed and o-MBA1 was tested at ambient temperature in trisubstitution of the isochroman moiety generated, i.e., regio- acetonitrile and toluene under UV illumination (LED λmax = specific cycloaddition between DTE1closed and o-MBA1 365 nm, Figures 3a and S2), and equivalent results were (Scheme S2 and Figure S6), a typical feature of oxo-Diels− obtained in both solvents. A large excess of o-MBA1 was Alder reactions.9 Finally, all attempts to resolve the employed (30:1 molar ratio) to favor complete conversion into diastereoisomeric mixture of A1open were found to be the cycloaddition products, while minimizing competing UV unsuccessful (e.g., by LC-MS) and, indeed, variation of the 2407 https://dx.doi.org/10.1021/acs.orglett.1c00015 Org. Lett. 2021, 23, 2405−2410 Organic Letters pubs.acs.org/OrgLett Letter stereoisomer relative ratio was observed in time and upon In the case of DTE1closed, such an activation effect could just treatment with acidic media (Figure S7). As previously be inhibited by photoisomerization into DTE1open with visible reported for other cycloadducts formed between o-QDMs light, where the two side trifluoromethyl carbonyl groups are and trifluoromethyl ketones,15 this is a clear signature of no longer electronically connected (Figure 1c). In this way, we mutual interconversion between these stereoisomers by could reach visible light-induced control of the UV-triggered hemiacetal epimerization (Figure S7). oxo-Diels−Alder reaction between DTE1 and o-MBA1. To Despite the large o-MBA1 excess used in our photoreactivity demonstrate this concept, we monitored the evolution of the experiments with DTE1closed, selective UV-induced formation cycloaddition process between DTE1open and o-MBA1 at of the monocycloaddition adduct A1closed was found, which did ambient temperature under simultaneous illumination with UV not further evolve into the dicycloaddition product B1closed. In (LED λmax = 365 nm) and visible (LED λmax = 625 nm) light fact, B1closed could only be detected when exposing the (Figure 4b). In the absence of visible light, full conversion of reaction mixture to very long irradiation times (>10 h) that DTE1open into adduct products (92:8 A1closed/A1open mixture) also led to photodegradation (Figure S8). Therefore, this result was observed after 120 min, though with a somewhat slower evidences that the rate of the oxo-Diels−Alder process critically rate than when o-MBA1 was directly reacted with DTE1closed. decreases when converting one of the trifluoromethyl carbonyl Hence, efficient UV-induced ligation occurred even when groups of DTE1closed into a hemiacetal moiety, i.e., by starting from the open isomer of DTE1, as it rapidly removing the electron-withdrawing effect imparted on the photoisomerizes into the activated closed state. In contrast, a other reactive trifluoromethyl carbonyl group. To confirm our dramatic reduction in photoreactivity was observed when the conclusion, we prepared monotrifluoromethyl ketone 2 reaction mixture was concomitantly irradiated with red light of (Scheme S3) and tested its light-induced reactivity with o- increasing intensity (LED λmax = 625 nm). Since none of the MBA1 under equivalent conditions as for DTE1closed (Figure initial reagents (DTE1open and o-MBA1) absorb red light, its 4a). Clearly, no cycloadduct formation was observed in this effect on the oxo-Diels−Alder reaction is exclusively attributed case, which further corroborates that fast oxo-Diels−Alder to the photoactivity of red-absorbing DTE1closed. In particular, reactivity of trifluoromethyl ketones with o-QDMs at ambient illumination with visible light converts the UV-generated, conditions requires activation by effective conjugation to the activated dienophile DTE1closed back to DTE1open (Figure S9) electron-withdrawing groups (EWGs). and, accordingly, it must decrease the cycloaddition process with the photoinduced o-QMD1 diene. Indeed, the fact that this reaction was virtually suppressed under sufficiently intense red illumination at which DTE1closed concentration is nearly residual proves that the oxo-Diels−Alder cycloaddition rate for the open isomer is negligible at ambient temperature. Therefore, our results unambiguously demonstrate light- induced modulation of DTE1 reactivity with o-QMDs and, as such, the viability of our dual-wavelength gated photo- ligation strategy; i.e., whereas UV irradiation triggers the oxo- Diels−Alder reaction by concurrently generating the reactive DTE1closed and o-QDM1, illumination with visible light halts the process by photoisomerizing DTE1closed back to inactive DTE1open. In summary, we herein introduce a dual-color controlled photoligation process that relies on the oxo-Diels−Alder reaction between two light-responsive compounds. Specifically, λ1 (365 nm) induces the activation of a photocaged diene, while λ2 (625 nm) regulates the reactivity of a photoswitchable carbonyl ene. As a result, covalent bond formation can be effectively induced and halted on demand by λ1 and λ2. Such dual-color reaction control offers critical advantages to photoligation chemistry that cannot be reached by merely using a single photoactivating excitation source (e.g., turning on and off λ1). First, it allows true confinement of the photoreaction in time and space using patterned two-color Figure 4. (a) Formation rate of the adduct produced by oxo-Diels− illumination, which inhibits the effects of the λ1-photoactivated Alder cycloaddition of the diene photogenerated from o-MBA1 under reagent diffusing out of the irradiated region or surviving UV illumination (LED λmax = 365 nm, 0.017 mW cm−2) and two longer than the photoexcitation time. Second, it may allow the trifluoromethyl ketones in toluene: DTE1closed and model trifluor- reduction of the spatial resolution of the photoligation process omethylthienylketone 2 (Scheme S3). (b) Two-color modulation of to the nanometer scale by applying advanced lithographic the formation rate of the oxo-Diels−Alder cycloadduct formed by the reaction between DTE1 open and o-MBA1 in toluene upon techniques6 on the basis of on−off photoswitchable reactions. simultaneous irradiation with constant UV intensity (LED λmax = Therefore, our photochemical system can find direct 365 nm, 0.017 mW cm−2) and variable visible light power density application in the preparation of multiresponsive photoresists (LED λmax = 625 nm; 0 to 1300 mW cm−2). for the development of miniaturized functional structures. 2408 https://dx.doi.org/10.1021/acs.orglett.1c00015 Org. Lett. 2021, 23, 2405−2410 Organic Letters ■ pubs.acs.org/OrgLett Letter ASSOCIATED CONTENT Control of Chemical Processes. Nat. Rev. Chem. 2019, 3, 706−722. (e) Kumar, G. S.; Lin, Q. Light-Triggered Click Chemistry. Chem. * sı Supporting Information Rev. 2020, DOI: 10.1021/acs.chemrev.0c00799. The Supporting Information is available free of charge at (2) (a) Blasco, E.; Wegener, M.; Barner-Kowollik, C. Photochemi- https://pubs.acs.org/doi/10.1021/acs.orglett.1c00015. cally Driven Polymeric Network Formation: Synthesis and Applications. Adv. Mater. 2017, 29, 1604005. (b) Chatani, S.; Further details on compound synthesis and character- Kloxin, C. J.; Bowman, C. N. The Power of Light in Polymer Science: ization and light-controlled oxo-Diels−Alder cyclo- Photochemical Processes to Manipulate Polymer Formation, addition (PDF) Structure, and Properties. Polym. Chem. 2014, 5, 2187−2201. ■ (3) (a) Arumugam, S.; Popik, V. V. Attach, Remove, or Replace: AUTHOR INFORMATION Reversible Surface Functionalization Using Thiol−Quinone Methide Photoclick Chemistry. J. Am. Chem. Soc. 2012, 134, 8408−8411. Corresponding Authors (b) Delaittre, G.; Goldmann, A. S.; Mueller, J. O.; Barner-Kowollik, C. Jordi Hernando − Department de Química, Universitat Efficient Photochemical Approaches for Spatially Resolved Surface Autònoma de Barcelona, 08193 Cerdanyola del Vallès, Functionalization. Angew. Chem., Int. Ed. 2015, 54, 11388−11403. Spain; orcid.org/0000-0002-1126-4138; (4) (a) Barner-Kowollik, C.; Bastmeyer, M.; Blasco, E.; Delaittre, G.; Email: jordi.hernando@uab.cat Mü ller, P.; Richter, B.; Wegener, M. 3D Laser Micro- and Christopher Barner-Kowollik − Centre for Materials Science, Nanoprinting: Challenges for Chemistry. Angew. Chem., Int. Ed. 2017, 56, 15828−15845. (b) del Barrio, J.; Sánchez-Somolinos, C. School of Chemistry and Physics, Queensland University of Light to Shape the Future: From Photolithography to 4D Printing. Australia (QUT), Brisbane, Queensland 4000, Australia; Adv. Opt. Mater. 2019, 7, 1900598. (c) Jung, K.; Corrigan, N.; Ciftci, orcid.org/0000-0002-6745-0570; M.; Xu, J.; Seo, S. E.; Hawker, C. J.; Boyer, C. Designing with Light: Email: christopher.barnerkowollik@qut.edu.au Advanced 2D, 3D, and 4D Materials. Adv. Mater. 2020, 32, 1903850. (5) (a) Lim, R. K. V.; Lin, Q. Photoinducible Bioorthogonal Authors Chemistry: A Spatiotemporally Controllable Tool to Visualize and Marc Villabona − Department de Química, Universitat Perturb Proteins in Live Cells. Acc. Chem. Res. 2011, 44, 828−839. Autònoma de Barcelona, 08193 Cerdanyola del Vallès, Spain (b) Herner, A.; Lin, Q. Photo-Triggered Click Chemistry for Sandra Wiedbrauk − Centre for Materials Science, School of Biological Applications. Top. Curr. Chem. 2016, 374, 1. Chemistry and Physics, Queensland University of Australia (6) (a) Liaros, N.; Fourkas, J. T. Ten Years of Two-Color (QUT), Brisbane, Queensland 4000, Australia Photolithography. Opt. Mater. Express 2019, 9, 3006. (b) Müller, P.; Zieger, M. M.; Richter, B.; Quick, A. S.; Fischer, J.; Mueller, J. B.; Florian Feist − Centre for Materials Science, School of Zhou, L.; Nienhaus, G. U.; Bastmeyer, M.; Barner-Kowollik, C.; Chemistry and Physics, Queensland University of Australia Wegener, M. Molecular Switch for Sub-Diffraction Laser Lithography (QUT), Brisbane, Queensland 4000, Australia by Photoenol Intermediate-State Cis−Trans Isomerization. ACS Nano Gonzalo Guirado − Department de Química, Universitat 2017, 11, 6396−6403. (c) Vijayamohanan, H.; Palermo, E. F.; Ullal, Autònoma de Barcelona, 08193 Cerdanyola del Vallès, Spain C. K. Spirothiopyran-Based Reversibly Saturable Photoresist. Chem. Complete contact information is available at: Mater. 2017, 29, 4754−4760. (d) Müller, P.; Müller, R.; Hammer, L.; https://pubs.acs.org/10.1021/acs.orglett.1c00015 Barner-Kowollik, C.; Wegener, M.; Blasco, E. STED-Inspired Laser Lithography Based on Photoswitchable Spirothiopyran Moieties. Chem. Mater. 2019, 31, 1966−1972. (e) Regehly, M.; Garmshausen, Notes Y.; Reuter, M.; König, N. F.; Israel, E.; Kelly, D. P.; Chou, C.-Y.; The authors declare no competing financial interest. Koch, K.; Asfari, B.; Hecht, S. Xolography for Linear Volumetric 3D ■ ACKNOWLEDGMENTS The current work was supported by Agencia Estatal de Printing. Nature 2020, 588, 620−624. (7) (a) Kathan, M.; Kovaříček, P.; Jurissek, C.; Senf, A.; Dallmann, A.; Thünemann, A. F.; Hecht, S. Control of Imine Exchange Kinetics with Photoswitches to Modulate Self-Healing in Polysiloxane Investigación (PID2019-106171RB-100/AEI/10.13039/ Networks by Light Illumination. Angew. Chem., Int. Ed. 2016, 55, 501100011033) and Generalitat de Catalunya (2017 13882−13886. (b) Fuhrmann, A.; Göstl, R.; Wendt, R.; Kötteritzsch, SGR00465). M.V. thanks the Spanish Ministry for Education, J.; Hager, M. D.; Schubert, U. S.; Brademann-Jock, K.; Thünemann, Culture and Sports for his predoctoral FPU fellowship. C.B.-K. A. F.; Nöchel, U.; Behl, M.; Hecht, S. Conditional Repair by Locally acknowledges the Australian Research Council (ARC) for Switching the Thermal Healing Capability of Dynamic Covalent funding in the form of a Laureate Fellowship underpinning his Polymers with Light. Nat. Commun. 2016, 7, 13623. (c) Claus, T. K.; photochemical research program as well as the Queensland Telitel, S.; Welle, A.; Bastmeyer, M.; Vogt, A. P.; Delaittre, G.; Barner- University of Technology (QUT) for key continued support. Kowollik, C. Light-Driven Reversible Surface Functionalization with The Central Analytical Research Facility (CARF) at QUT is Anthracenes: Visible Light Writing and Mild UV Erasing. Chem. gratefully acknowledged for access to analytical instrumenta- Commun. 2017, 53, 1599−1602. (8) Bialas, S.; Michalek, L.; Marschner, D. E.; Krappitz, T.; Wegener, tion, supported by the Faculty of Science and Engineering at M.; Blinco, J.; Blasco, E.; Frisch, H.; Barner-Kowollik, C. 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