Polymeric Photoacids Based on Naphthols—Design Criteria, Photostability, and Light‐Mediated Release

Abstract The implementation of photoswitches within polymers offers an exciting toolbox in the design of light‐responsive materials as irradiation can be controlled both spatially and temporally. Herein, we introduce a range of water‐soluble copolymers featuring naphthol‐based chromophores as photoacids in the side chain. With that, the resulting materials experience a drastic increase in acidity upon stimulation with UV light and we systematically studied how structure and distance of the photoacid from the copolymer backbone determines polymerizability, photo‐response, and photostability. Briefly, we used RAFT (reversible addition–fragmentation chain transfer) polymerization to prepare copolymers consisting of nona(ethylene glycol) methyl ether methacrylate (MEO9MA) as water‐soluble comonomer in combination with six different 1‐naphthol‐based (“N”) monomers. Thereby, we distinguish between methacrylates (NMA, NOeMA), methacrylamides (NMAm, NOeMAm), vinyl naphthol (VN), and post‐polymerization modification based on [(1‐hydroxynaphthalen‐2‐amido)ethyl]amine (NOeMAm, NAmeMAm). These P(MEO9MAx‐co‐“N”y) copolymers typically feature a 4:1 MEO9MA to “N” ratio and molar masses in the range of 10 kg mol−1. After synthesis and characterization by using NMR spectroscopy and size exclusion chromatography (SEC), we investigated how potential photo‐cleavage or photo‐degradation during irradiation depends on the type and distance of the linker to the copolymeric backbone and whether reversible excited state proton transfer (ESPT) occurs under these conditions. In our opinion, such materials will be strong assets as light‐mediated proton sources in nanostructured environments, for example, for the site‐specific creation of proton gradients. We therefore exemplarily incorporated NMA into an amphiphilic block copolymer and could demonstrate the light‐mediated release of Nile red from micelles formed in water as selective solvent.


Introduction
Various research fields benefit from advances in the design of molecular photoswitches, [1] as light offers considerable advantages overo ther triggers (e.g.,t emperature or pH) including the possibility to control chemistry both spatially and temporally.S uch photoswitches predominantly comprise organic chromophores such as azobenzenes, [2] spiropyrans, [3] and diarylethenes, [4] and have so far been used in, for example, energy storage, chemical sensing, or in controlling both the conformation and the activity of biomolecules. [5] Alongt he same lines, material andp olymer science has been strongly affectedb yp hotoswitches, [6] offering further possibilities to controlm acromolecular conformation, charge, or polarity with externalt riggers besides pH and temperature. [7] Stimuli-responsive materials are attractive for application in diagnostics, drug delivery,o rt issue engineering and typical examples for light-responsive polymers showed as hift in solubility,t hat is, in the hydrophobic-hydrophilic balance upon irradiation. [8] Early examples of photo-responsive polymers featured azobenzenes in the side chain, [9] or block copolymer micelles capable of undergoing reversible self-assembly (disruption and reorganization) controlled via alternating UV and visible light exposure. [10] Often, these materials are classified according to whether the underlying photochemical process is reversible or irreversible,i na ddition to synthetic access to different materials. [8,11] Especially the latter experienced a boost with the advent of controlled/"living" radical polymerization techniques such as atom transfer radicalp olymerization (ATRP), [12] nitroxide-mediatedp olymerization (NMP), [13] andr eversible addition-fragmentation chain transfer (RAFT) polymerization [14] -together with post-polymerizationm odification of, for example, activatede ster moieties if direct access to ac ertain photoswitch is hampered. [15] Prominent examples of an irreversible photo-response include photo-cleavage of nitrobenzyl or pyrenyl esters and the formation of hydrophilic carboxylic acid groups along the polymer backbone, [16] whereas ar eversible photo-response is often realized using diarylethenes, azobenzenes, or spiropyran moieties. [10,17] Besides changing local polarity or charge,i tw ould nevertheless be beneficial if ap hotoswitch could additionally create a chemicalg radient upon irradiation, as experienced in the case of protong radients presenti np hotosynthesis. [18] In that respect, Meier and co-workers recently demonstrated the successfuli nsertion of the transmembrane protein proteorhodopsin as light-activated proton pump into asymmetric polymersomes from an ABC triblock copolymer. [19] Here, light absorp- tion by this "proteopolymersome" induced pumping of protons into the vesicles, and resulted in an increase in the extravesicular pH, turning this into the first successful example of oriented insertion of ap roton pump into an artificial asymmetric membrane. An alternative for the creation of al ocal proton gradient are photoacids, which have been known since the seminalw ork of Fçrster [20] and Weller [21] in the 1960s. These photoswitches feature ap rotolytic group (hydroxyl group) that experiences a strong shift in acidity upon irradiation, [22] and excited-state proton transfer (ESPT)t ot he solvent can occur. [23] In contrast to the main classes of photoswitches as discussed above,p hotoacids switch between ground and excited electronic states via irradiationw ith light of as pecific wavelength, and each state establishes an equilibrium between the acida nd its conjugate base. The rather generic term "photoacid"s trictly applies to three different types of photoacids:e xcited-state photoacids as described above,m etastable-state photoacids, and photoacid generators. [24] Metastable-statep hotoacids are usually based on spiropyran photochromes and are characterized by significantly higherl ifetimes of the conjugate base if compared to excited-state photoacids, rendering them suitablef or applicationsi np hoto-controlled proton-sensitive processes. [25] Photoacid generators typically exhibit irreversible photolysis upon irradiation releasing strong acids, andh aveb een used mainly in lithographic processes. [26] Although featuringr ather short lifetimes of the corresponding conjugate bases, excitedstate photoacids have been investigated in the context of light-triggered interactions with proteins, [27] cyclodextrins, [28] membranes, [29,30] and micelles. [30,31] Thef irst known example of ap olymeric excited-state photoacid has been reported by Itoh et al.,a lthough the irradiation has not been studied. [32] In 2017 we reported in ap roof-of-concept study on the incorporation of 1-naphthol-based photoacids into statisticalt erpolymers and could show that irradiation with light led to ad rastic increase in hydrophilicity. [33] Herein, we introduce as et of well-definedw ater-solublea nd light-responsive polymericp hotoacids, where the type and distance of the photoacid is systematically varied. Briefly,w esynthesized copolymers consisting of nona(ethylene glycol) methyl ether methacrylate (MEO 9 MA) as aw ater-soluble comonomer in combination with six different 1-naphthol ("N") derived monomersa ttached as either am ethacrylate( NMA), a methacrylamide (NMAm), am ethacrylate (NOeMA) or methacrylamide( NOeMAm) with an additional C 2 -spacer, av inylic (VN) group directly connected to the polymer backbone, and a monomer possessing amongst others, ad ivergents ubstitution pattern (NAmeMAm). These copolymers, namely P(MEO 9 MA xco-"N" y )w ith a4 :1 MEO 9 MA to "N" ratio and molar masses in the range of 10 kg mol À1 ,w ere prepared using RAFT polymerization in combinationw ith post-polymerization modification of the activated ester copolymers. After synthesis and characterization by NMR spectroscopy and size exclusion chromatography,w ecarried out thorough investigations of the photochemicalp roperties, with particular emphasis on the following scenarios: 1) Photo-cleavage or photo-degradation during irradiation in accordance with our previous assumptionsa nd observations and depending on the type and distance of the linker to the polymeric backbone. [33] 2) Occurrence of the anticipated excited state proton transfer (ESPT) mechanism to provide reversibly light-switchable and water-soluble copolymers.
Besides providingg uidelines on how to design polymerizable excited-state photoacids and am ore fundamental understandingo fp hoto-cleavage reactions during irradiation, we introduce an ew type of stimuli-responsive buildingb lock. With that, the herein derived materials can be used to introduce protong radients within various types of nanostructured materials and open up applications to, fore xample, light-mediated actuators or controlled releaseo fencapsulatedc argo. To demonstrate this, core-corona micelles are formed in water from an amphiphilic block copolymer comprising both NMA and as ufficienta mount of ap rotonizable comonomer in the hydrophobic segment. Upon irradiation,E SPT within the micellar core occurs, leading to ad rastic increase in hydrophilicity,s welling, and releaseofNile red as encapsulatedm odel cargo.

Results and Discussion
Synthesis and characterization of photoacid-containing water-soluble copolymers Our aim wast op reparew ater-soluble and well-defined polymeric materials, which are capableo fc reating al ocal proton gradient upon irradiation with light of as uitable wavelength. We therefore designed six polymericp hotoacidsw here the light-responsive comonomer is in all cases based on 1-naphthol, [34] varying only in the type of attachment to and distance from the copolymer backbone. As the substitution pattern directly influences the electron density of the naphthol moiety, we expected this to also influence both the photo-stability and photoacidity of the prepared materials. Furthermore, particular emphasis was put on the photoacidity under aqueous conditions, and therefore we chose nona(ethylene glycol) methyl ether methacrylate (MEO 9 MA) as the hydrophilic and watersoluble comonomer. First, as et of copolymers containing 1naphthold erivatives substituted in position fivew ere prepared leadingt ot he respective methacrylate, that is, 1-naphthol-5methacrylate (NMA), andt he respectivem ethacrylamide( 1naphthol-5-methacrylamide, NMAm,F igure 1A). These two compounds have been the subjecto faprevious proof-of-concept study and we observed ac ertaina mount of photocleavage or degradation duringi rradiation in deuterated DMSO. [33] We extended this now to analogousc ompounds with an ethyl spacer in betweent he polymericbackbone and the 1-naphthol (1-naphthol-5-oxyethylmethacrylate (NOeMA) and 1-naphthol-5-oxyethylmethacrylamide (NOeMAm)). Again, we are examining the difference between the presence of an ester and amide moiety (electrophilicity of the carbonyl carbon), as well as an aryl (-I effect) versusa lkyl (+ Ie ffect) substitution. Furthermore, we included NAmeMAm (1-naphthol-2-oxyamidoe- thylmethacrylamide) possessing an ortho-substitution of the naphthalene, incorporating as pacer between the chromophore and the polymeric backbone comprising an ethyl chain linked through an amide at each binding site. If we consider the naphthalene as ap rotecting group of the spacer,t he ortho-hydroxyg roup potentially leads to as ignificant increase in photo-lability. [35] Photochemical reactions of esters and amides are often described by Norrish-typer eactions involving a-cleavage and we expect this to be more pronounced in the case of NAmeMAm due to the presence of two amide groups.
Finally,w ea lso included av inyl-based comonomer( 5-vinyl-1naphthol, VN) where we anticipated al ower tendency for photo-cleavage, as well as the suppression of undesirablep hotoreactions.
As showni nF igure 1A,P (MEO 9 MA x -co-"N" y )c opolymers in the case of NMA, NMAm, NOeMA and VN were prepared through RAFT copolymerization and subsequent deprotection of the respective functional comonomers. In order to prevent side reactions during the copolymerization process (e.g.,i nhibition, retardation), the hydroxyl groupsw ere protected by using tert-butyldimethylsilyl ether (TBS, herein abbreviated t), leadingt ot he required deprotections tep after polymerization. For NOeMAm and NAmeMAm, the respective P(MEO 9 MA x -co-"N" y )c opolymers were not prepared through the polymerization of their respective methacrylamide monomers. Here, the corresponding amines, NOeA [2-{(5-hydroxynaphthalen-1-yl)-oxy}ethyl]amine andN AmeA [{(1-hydroxynaphthalen-2-amido)ethyl}amine] were used in ap ost-polymerizationm odification involving the previously prepared activated ester copolymer P(MEO 9 MA-co-FMA). [36] The synthesis of both the protected monomers and the functional amines is further outlined in the Supporting Information ( Figure S1) including characterization by 1 HNMR spectroscopy ( Figure S2) and the synthetic procedures are described in more detail in the Experimental Section. The RAFT copolymerization of the protectedm onomers (tNMA, tNMAm, tNOeMA, and tVN) and activated ester monomer (FMA) was carried out using hydrophilic MEO 9 MA as mentioned previously.B riefly,A IBN was used as at hermal initiator at ar eaction temperature of 70 8Cw ith CPDB as an established chain transfer agentf or the polymerization of methacrylates  Figure 1B shows the overall monomer conversion over time of all five copolymerizations. For clarity,the respective individual conversionso fb oth the functional comonomera nd MEO 9 MA are omitted, but can be found in the Supporting Information( Figure S3). In general, no significant differenceb etween the conversion of the functional comonomera nd MEO 9 MA is observed ( Figure S3). Except for tNMAm, MEO 9 MA is generally consumed more slowly than the functional comonomer as indicated by lower monomer conversions at any given time. Since an initial monomer feed ratio of MEO 9 MA to functional monomer of 4:1i su sed in each case, we can presume that the copolymers formed are not strictly random. However, tNMA, tNOeMA, and FMA exhibit similarp olymerization behavior as expected because they are all methacrylatebased, and are almostc ompletely consumed after 4t o5hours (81 to 97 %c onversion), which is in closea greement with the half-life of AIBN at 70 8C. In Figure 1C,t he molecular weight of each copolymer is observed to increase with reactiont ime, except in the case of tVN.I na ddition, the semi-logarithmic plot ( Figure S4) and the M n versus conversion plot ( Figure S5) also exhibit linear correlations,w hich are good indicators for well-controlled polymerization processes. This is furtherc orroborated by narrow molecular weightd istributions( 1.3), as highlighted in the SEC traces with increasing polymerization time (representatively shown for tNMAi nF igure 1D). However, after four hours of polymerization and conversions above 80 %, ah igherm olecular weights houlder appears, which may indicate the occurrence of chain-chainc oupling. From these kinetic investigations,i tc an be stated that the copolymerizations of tNMA, tNOeMA, and FMA proceed in acontrolled fashion,leading to molecular weights up to 15 kg mol À1 under these conditions.
As expected, the copolymerization behavior of tVN and tNMAm varied significantly.F igure 1B reveals av ery moderate overall monomer conversion,s ignificantly lower than the values reachedb yt he methacrylate-based comonomers. tNMAm shows al inear increase in conversion (up to 35 %) and molecular weight( up to 9kgmol À1 )t oac ertaine xtent, as showni nF igure 1C.H owever, the achieved overall monomer conversion of the tVN copolymerization is very low after one hour (ca. 10 %), and does not increases ignificantly,e ven after 24 ho fc opolymerization, reaching approximately 15 %c onversion. This would correspond to approximately 8r epeating units based on the monomer feed ratio used, andt he respective molar mass for such an oligomer is in the range of 3.5 kg mol À1 .T he molar masses obtained through SEC measurements at different reactiont imes are in agreement, with a molecular weight of approximately 4.5 kg mol À1 reacheda fter one hour ( Figure 1C), and with no further significant increase observed by NMR after this time. Furthermore, moderate molecular weightd istributions ( 1.4) were obtained, and compared to the methacrylatec omonomers, ah igh molecular weightshoulder is not observed. This is likely due to the significantly lower monomer conversionsa chieveda nd the outcome of both the semi-logarithmic plot and M n versusc onversion plot for tNMAm and tVN are shown in Figure S4 and Figure S5. Hence, the conditions reported herein are unsuitablet oo btain higher molecular weightc opolymers featuring tNMAm and tVN, and further optimization is reasonable, for example, by changing the utilized RAFT agent. Particularlyi nt he case of tVN, it might be advantageous to switch to as tyrenic-based copolymer backbone in the future. Nevertheless, we have access to as et of well-defined random copolymers with comparable molecular weights and photoacid content, with no indications thus far for ag radientstructure.
Based on the above findings, we synthesized the final copolymers using similarr eaction conditions (Table1). The copolymerizations of tNMA, tNOeMAa nd FMA were terminated Table 1. SEC and 1 HNMR characterization of the protectedc opolymers, P[MEO 9 MA x -co-t"N" y ]a nd P(MEO 9 MA x -co-FMA y ), and for the respective deprotected or functionalized copolymers, P[MEO 9 MA x -co-"N" y ].
[kg mol À1 ] [a] MEO 9 MA [b] [%] t"N"/FMA [b] [%] after three hours in order to prevent chain coupling and, hence,t he monomer feed ratios were corrected according to the efficiency of the polymerization based on our kinetic investigationst ot arget am olecular weighto fa pproximately 10 kg mol À1 .F or tVN and tNMAm, no high molecular weight shoulders wereo bserved, therefore it was not necessary to adjust the polymerization conditions. To improve the monomer conversionso btained and achieve higherm olecular weightc opolymers, copolymerizations were carriedo ut for 24 h. For the same reason, bulk conditions were applied, and also the monomer feed ratio with respect to the RAFT agent was doubled to achieve higher degrees of polymerization. Detailed characterization data of the protected copolymers P[MEO 9 MA x -cot"N" y ]a nd P(MEO 9 MA x -co-PFMA y )i ss hown in Table 1. The five copolymers prepared had similar molarm asses (ca. 9kgmol À1 ), and the copolymerc ompositions obtained are in good agreement with the monomer feed ratios used. For P[MEO 9 MA x -co-PFMA y ], it is more challenging to determine the copolymer composition as the PFMA repetition units cannot be visualized by 1 HNMR besides the backbones ignals. Despite this, the degree of polymerization of MEO 9 MA was calculated to be 29. This appears relatively high if compared to the adjusted value of 20 (according to the used monomer feed ratio) and the achieved molar mass from SEC measurements being in the expected range (10.5 kg mol À1 ).
To obtain the final materials, the as-synthesized P[MEO 9 MA xco-t"N" y ]w ere subjected to ad eprotection step, whereas in the case of P[MEO 9 MA x -co-PFMA y ], ap ost-polymerization modification was carried out using the designed photoacid functionalized amines NOeA or NAmeA (Figure 2A). The deprotection was carried out using at wo-fold excesso fT BAF with respect to the t"N" repetition units in an equimolar combination with acetic acid according to al iterature procedure. [37] 1 HNMR spectroscopy was used to characterize both the protected and deprotected copolymers ( Figure 2B). The disappearance of the signala t0 .3 ppm correspondingt ot he pro- Figure 2. A) Synthesis of the photoacid-containing copolymerst hrough ad irect deprotectionofT BS protectedcopolymers, P[MEO 9 MA x -co-t"N" y ], or through ap ost-polymerization modificationoft he activated ester copolymer P(MEO 9 MA x -co-FMA y )and the respective photoacid functionalized amines (for clarity,t he RAFT end groups of the copolymers were omitted here).B)Representative 1 HNMR spectraofP [MEO 9 MA x -co-tNOeMA y ]b efore (solid black line)and afterd eprotection (solid magenta line) in CD 2 Cl 2 .C)Representative SEC elutiont races for P[MEO 9 MA x -co-tNOeMA y ]b efore (solid blackl ine) and after deprotection (solid magental ine).D )Representative 1 HNMR spectra of P[MEO 9 MA x -co-FMA y /NOeMAm y ]before(solid blackl ine) and after post-polymerization modification (solid orange line)inC D 2 Cl 2 .E)Representative 19 FNMR spectrao fP[MEO 9 MA x -co-FMA y /NOeMAm y ]before(solid blackline) and after post-polymerization modification (solido rangeline) in CD 2 Cl 2 using TFE as an internal standard. F) Representative SECelution traces for P[MEO 9 MA x -co-FMA y /NOeMAm y ]before (solid black line) and after post-polymerizationm odification (solid orange line).  (Figure2C) show ac lears hift towards higherm olar masses as expected based on our previousobservations;stronger hydrogen bonding and potential interactions with the SEC columnd ue to the increased number of hydroxyl groups explain this behavior. [33] In order to removee xcess TBAF,p reparative size exclusion chromatography (BioBeads SX-1) with THF as eluentw as performed. However,r esidualT BAFc ould be detected even after this purification processo ns everalo ccasions. Other purification methods including precipitation and dialysis were also investigated, but didn ot alwaysi mprove the purity. The final copolymer compositions were determined after both deprotection and purification and were in good agreement with the dataobtained for the protected copolymers.
The post-polymerization modification was carried out using af ivefold excess of the functional amines NOeA or NAmeA with respectt ot he incorporated FMA units, and an equimolar amount of trimethylamine and HO-Dhbt as coupling reagent accordingt oaliterature procedure ( Figure 2A). [38] The reaction was conducted in DMF at 70 8Cfor two days to ensure full conversion, and 1 HNMR spectroscopy wasu sed to characterize the final functionalized copolymers ( Figure 2D). The appearance of aromatic signals corresponding to the 1-naphthol protons indicated as uccessful functionalization and the actual composition of both P[MEO 9 MA x -co-NOeMAm y ]a nd P[MEO 9 MA x -co-NAmeMAm y ]c ould be calculated and were 28 and 35 %, respectively.T of urther evaluate the extento ff unctionalization, 19 FNMR spectroscopy was carried out and ar epresentative 19 Fs pectra for P[MEO 9 MA x -co-NOeMAm y ]i ss hown in Figure 2E (the respective 19 FNMR spectra of P[MEO 9 MA x -co-NAmeMAm y ]c an be found in the SupportingI nformation, Figure S6). The characteristicf luorine signals expected for P[MEO 9 MA x -co-FMA y ]a re clearly visibleb efore and absent after the reaction. Only very weak signals in the respective area of the spectrum remain, which correspond to ar ather high degree of functionalization (> 99 %). Finally,as ignificant shift is observable by SEC, which clearly indicates the expectedi ncrease in molecular weight as ar esult of this post-polymerization modification ( Figure 2F).
Photophysical properties of the 1-naphthol-based("N") photoacid comonomers and the correspondingcopolymers Due to the lack of solubility in H 2 O, the UV/Vis absorption properties of the protected photoacid comonomers (t"N") and the functional amines were investigated in DMSO ( Figure 3A). While most 1-naphthol derivativesa bsorb in the range below 350 nm, NAmeA shows as tronga bsorption at longer wavelengths ranging up to 420 nm. The main absorption band located around 300 nm (in the case of NAmeA around3 50 nm) of the 1-naphthol derived monomers exhibits molar extinction in the range of 7300 (tNMAm) to 10 400 (tNOeMA) m À1 cm À1 . The absorption of 1-naphthol derivatives in this range is assigned to superimposed transitions in the first excited singlet state, 1 L b ,and to the second excited singlet state, 1 L a . [34] In case of the respective photoacid copolymers in aqueous solution,t he protonated species, ROH, of the photoacid exhib-its aa bsorption maximai nt he range between 299 and 309 nm. P[MEO 9 MA x -co-NAmeMAm y ]r eveals ab athochromically shifted absorption peak at 342 nm and the conjugate base of the photoacid copolymers, RO À ,g enerally exhibits ar edshifteda bsorption. For the 1-naphthol derivatives, the respective maximaa re localized at approximately 339 nm (354 nm for P[MEO 9 MA x -co-NAmeMAm y ]).T he protonated form of the photoacid and the conjugate base show an emission centered at approximately 350 and 440 nm, respectively. Figure 3B exemplarily shows the absorption and fluorescence spectra of P[MEO 9 MA x -co-VN y ]i na queouse nvironments at different pH values, that is, in 0.1 m HCl (red, fully protonated), H 2 O( green, pH 5.7) and 0.1 m NaOH (blue, fully deprotonated form). The dual emissiono bserved for P[MEO 9 MA x -co-VN y ] in water exhibitsc ontributionsf rom both the neutral,p rotonated form at 360 nm, andi ts conjugated, deprotonated form at 443 nm, clearly indicating the occurrence of an ESPT with water acting as ab ase. Most remaining photoacidc opolymers qualitatively show similar behavior,w hich is summarizedi n Ta ble 2( the remaining steady-state absorption and emission spectra are compiledi nt he Supporting Information;s ee Figures S7 and S8). An exception is P[MEO 9 MA x -co-NAmeMAm y ], where no distinct change in emission upon pH changes was observed, hinting towards no photoacidity for the NAmeMAm chromophore upon optical excitation. Determination of the groundand excited state acidity constants The ground state acidity constant,pK a ,o f1-naphthol in the copolymers was determined by spectrophotometric titrationa nd the values obtained range from 8.7 for P[MEO 9 MA x -co-NMA y ] to 9.7 for P[MEO 9 MA x -co-NOeMA y ]( see Figure 4A and Figure S9), consistent with literature-reported values. [39] The change in acidityo ft he polymericp hotoacids upon photoexcitation is determined using the Fçrster cycle analysis. [40] Thereby,t he change in pK a *u nits upon opticale xcitation is derived from the steady-state absorption and fluorescence spectra by use of Equation (1): In this equation N A , h, c, R and T correspond to Avogadro's constant, Planck's constant, the speed of light, the universal gas constant,a nd the temperature. The wavenumber position of the 0-0 electronic transition of the acid and base form,ñ 0À0R OH andñ 0À0R O À ,a re estimated from the intersection of the absorption andn ormalized fluorescence spectra of the corresponding acid and conjugate base forms. [41] The pK a *o f NMA, NMAm and VN were estimated to be 0.3, 0.5 and 0.8, respectively;w hereas, NOeMAa nd NOeMAm show as lightly higher acidity constant with ap K a *o f2 .9 and 2.5, respectively. The higherp K a *o ft he derivatives NOeMAa nd NOeMAm can be rationalized by considering the electron-donatinge ffect of the ether functionality used as al inker between the photoactive unit to the copolymer backbone. The change in pK a units upon photoexcitation amounts to up to À8.6 pK a units in P[MEO 9 MA x -co-NMA y ], P[MEO 9 MA x -co-NMAm y ]a nd P[MEO 9 MA xco-VN y ]. Despite the distinct increase in the acidity constant in the excited state, pK a *, full deprotonation of the "N" moieties in the copolymers is not observed under our experimental conditions. In our opinion, this can be explained with the ESPT being strongly dependent on the accessibility of ap hotoacid to water molecules. [42,43] One tentativee xplanation is that parts of the copolymers are not completely hydrated, creatingr egions where individual photoacid monomers are not accessible to water molecules,a nd we callt hese "core" regions. In this scenario, ESPT would occur predominantly in the interfacial region between the solvated copolymera nd the surrounding medium( Figure 4C). [43] Fluorescencet itration,o nt he other hand, allows for the direct estimation of the pK a *v alue, but bears certain challenges in environments where the deprotonation of the 1-naphthol units is not fully observed. Here, we estimated the pK a *b yn ormalizing the fluorescences pectra to the emission maximao f the protonated ROH form (see Figures 4B and S9). Subsequently,t he pK a *i sderived from the increasei nt he RO À emission located at approximately 450 nm as an inflection point of the sigmoidal curve and the values are in quite good agreement with the ones obtainedb yt he Fçrster equation. Here, we found 1.1 and 1.8 for NMAm and VN in P[MEO 9 MA x -co-NMAm y ]a nd P[MEO 9 MA x -co-VN y ], whereas NOeMA and NOeMAm display as lightly lowered acidity with ap K a *o f2 .4 and 2.1, respectively.T he excited state acidity constant of NMA in P[MEO 9 MA x -co-NMA y ]c ould not be determined due to the presence of an unknown emissive photoproduct( l em max = 515 nm).
The change in protonation state of the 1-naphthol in the different copolymers under continuous illumination at l exc = Table 2. Steady-state spectral properties of the photoacid-containingc opolymerst ogether with their estimated ground and exciteds tateacidity constants under aqueous conditions, pK a and pK a *, respectively.  The insets show the estimated pK a and pK a *values.C)Possible photophysical processes upon excitationof1 -naphtholi nt he photoacid copolymers within the core and interfacial regions. In this context k nr and k r refer to the respective non-radiative and radiative decay rate constantsofe lectronically excited protonated species ROH * anddeprotonateds pecies RO À À* .
Chem.E ur.J.2020, 26,2 365 -2379 www.chemeurj.org 2019 The Authors. Published by Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim 300 nm was monitored (P = 0.21 mW, I = 7mWcm À2 ,s ee Figure 5) over several emission scans until no pronounced spectralc hanges were observeda nymore. In the case of P[MEO 9 MA x -co-NOeMAm y ]a nd P[MEO 9 MA x -co-NOeMA y ], the emission at 350 nm is assigned to the protonated 1-naphthol, while the deprotonated form, RO À ,e mits at 450 nm. As seen in Figure 5, continuousi llumination of the photoacid copolymers leads to as low decrease in the ROH emission, and ac oncerted increasei nt he band assigned to the RO À form.I nc ontrast, for P[MEO 9 MA x -co-NMAm y ]a nd P[MEO 9 MA x -co-VN y ]t he steady-state equilibrium between ROH and RO À is already reachedw ithin the first emission measurement, that is, within 4.5 min, which corresponds to the experimental times needed to collect each emissions pectrum under the given experimental conditions;a ss uch, no furthers ignificant changes in the dual ROH/RO À emission are observed in subsequent measurements.H owever,c hanges in the protonation state of NOeMAand NOeMAm-based photoacid copolymers under illumination indicatet hat the copolymers undergo morphological changes, which results in ahigher degree of hydrationofthe 1-naphthol units. This leads to ah igher fraction of 1-naphthol moieties being accessible to H 2 Oa nd facilitates subsequentE SPT. Hence, 1-naphthol might also serve as ap robe for conformational changes within the copolymer strand,t hat is, the formation of domains of higherd egrees of solvation.I nt he case of P[MEO 9 MA x -co-NMAm y ]a nd P[MEO 9 MA x -co-VN y ]f eaturing excited state acidity constantsp K a * < 2, the photo-stationary condition is reached faster than in the other systems( p K a * > 2i n P[MEO 9 MA x -co-NOeMA y ]a nd P[MEO 9 MA x -co-NOeMAm y ]).

Reversibility of the deprotonationand photostability
We further investigated the reversibility of the 1-naphthol deprotonation using P[MEO 9 MA x -co-NOeMAm y ]a sabenchmark since NOeMAm showedt he most pronouncedc hanges in its protonation state during UV illumination. To evaluate the reprotonation, two series of emission spectra were collected from as ample solution that was irradiated twice under identical conditions (deaerated, l exc = 300 nm, P 300 nm = 0.69 mW, I = 12 mW cm À2 )a nd kept in the dark between measurements (except for the emission measurements, seen in Figure S10A and B). Again,a ni ncrease in the RO À emission is observed upon illumination at 300 nm until as teady state is reached. The reversibility of the process was evaluated by normalizing the first and last spectra obtainedi ne ach measurement series to the maximum emission of pure ROH (l em max ¼ 334 nm, see Figure S10 D).C omparison of the integrated emission ratio, ROH/RO À ,o ft he 1 st cycle indicates that under these conditions the reprotonation of 1-naphthol occurs only to ac ertain extent (see Ta ble 3). UV/Visspectra collected prior to and after UV illuminationo fP [MEO 9 MA x -co-NOeMAm y ]c onfirm these results ( Figure S10C). As 1-naphthols( or in general hydroxyarenes) show av ariety of photoreactions, for example, photooxidation, [44] this irreversible behavior could indicatea nother photoreactiono ccurring in parallel to the ESPT,o ri ss ubsequent to the 1-naphthol deprotonation. Decreasing the power of the illumination source to P 300 nm = 0.21 mW (I = 7mWcm À2 ,s ee Figure S11) leads to ad istinct decrease in RO À formation.M oreover,t he stationary state condition of protonated to deprotonated 1-naphthol is reachedw ithin less measurement cycles during the 2nd series, and the deprotonated form RO À is generated to al arge extent (ca. 70 %).
Af undamentalp roperty which determines the usability of a chromophore is its photostability.B esides earlier described investigationsc oncerning photocleavage, the photostability of the 1-naphthol containing copolymers was also investigated through steady-state UV/Vis absorption in H 2 Ou nder aerated conditions. Upon illumination at 365 nm, the main absorption band located at approximately 300 nm decreases, while in the region around 260 nm as light increase in absorption is observed (see Figure 6A). The degree of photodegradation was monitored as the relative change in absorbance for the main absorption band during UV light exposure anda ll investigated   [45] was conducted to investigate the absorption properties of the electrochemically oxidized1 -naphthol species and was aimed to identify ap ossible oxidative degradation pathway duringe xposure to UV light ( Figure 6B). Under oxidative conditions, we observe an absorption maximuma t245 nm and al owered absorbance in the range of 300 nm. In our opinion, these features qualitatively explain the changes in absorbance upon UV illumination:excitation of P[MEO 9 MA x -co-"N" y ]c an lead to three main pathways ( Figure 6C). As described above, ESPT occurs mainly for photoacid moieties that are accessible for H 2 O, which is accompanied by low degrees of photocleavage. In addition, a combination of photooxidation and potentialp hotocleavage can occur,l eading to the formation of aJ uglone derivative. [46] By assessment of the differential absorption spectra obtained in UV/Vis SEC and UV/Visi rradiation studies (see Figures 6C and S12) we assign the spectralc hanges upon UV illumination to the photooxidative pathway.N evertheless,aquantitative assessment to which extentp hotocleavage and photooxidation contribute to photoacid degradation occurring is not feasible, at least based on the opticalspectroscopy used herein.

Light-triggered release of cargo from photoacid-containing block copolymer micelles
After evaluating the photophysicalp roperties of photoacidcontaining water-soluble copolymers in general, we wanted to go one step further and were interested in whether photoacids within amphiphilic block copolymers can be used to release encapsulated cargo upon irradiation. We therefore designed an amphiphilic block copolymer (BCP) as ar epresentative example carrying the photoacid comonomerN MA in the hydrophobic block and featuring ah ydrophilic block of MEO 9 MA. In aqueous media, these materials undergo self-assembly into core-coronam icelles where P[MEO 9 MA] serves as coronaa nd the NMA-containing segment as core. More specifically,t he hydrophobic block consistso faterpolymer comprising methyl methacrylate (MMA/"M"),2 -(N,N-dimethylamino)ethyl methacrylate( DMAEMA/"D), andN MA (Figure 7).
Upon irradiation with light, ESPT can occur whereby adjacent DMAEMA units act as ab ase and are protonated,s hifting the overall hydrophilic/hydrophobic balance of the core-forming block and this resultsi ns welling or disruption of the micelles to enable cargo release. The underlying amphiphilic diblock quarterpolymer P[(M 33 -D 11 -NMA 13 )-b-(MEO 9 MA) 23 ]w as synthesized under comparable conditions as discussed earlier using RAFT polymerization, followed by subsequentd eprotection. In this case, we targeted a1 :1 mass ratio between both blocks ( Figure S13, the characterization data, both NMR and SEC, can be found in Ta ble 4a sw ell as in the Supporting Information, Figure S14 and Figure S15).
Micellar solutionsf rom P[M 0.32 -D 0.17 -NMA 0.19 ]-b-P[MEO 9 MA 0.32 ] were prepared by first dissolving the diblock quaterpolymer in THF as anon-selectivesolvent, followed by adding the solution dropwise into micropure water and allowing the THF to evaporate over 24 h. Nile red (NR) as am odel hydrophobic cargo was encapsulated through co-precipitation by using ac ertain amount of aNRs tock solution in THF.
Since the fluorescence quantumy ield of NR strongly depends on the polarity of the solvent, this can be taken as a measuref or the swelling of the block copolymer micelles. Upon irradiation, ad istinct decrease in emission intensity (Figure S16) hinting towards the releaseo fN Ri so bserved. Furthermore, with increasing irradiation time more pronounced changes are visible ( Figure 7B), which we interpreta st he release of approximately 30 %o ft he Nile red within 60 minutes. The formation of well-defineds pherical micelles was confirmed by cryogenic transmission electron microscopy (cryo-TEM, Figure 7C). As can be seen, narrowly dispersed spherical micelles with core sizes of about 20 nm in diameter are formed and no significant difference in morphology or micellar size could be observed after irradiation ( Figure S16C). The ability to effectively swell these micellest or elease the encapsulatedN ile red was furthers upported by aD LS experiment where we mimicked the photo-induced deprotonated state of the photoacidic moietyb yd ialyzing the prepared micelles against ap H10 buffer solution ( Figure S17).

Conclusions
The herein reported 1-naphthol containing water soluble copolymers P[MEO 9 MA x -co-"N" y ]a re interesting materials for any application where temporally ands patially controlled release of protons is desirable. Besides design of well-defined copolymers using controlled radical copolymerization (RAFT), we also investigated the ground and excited state acidity of the photoacids by means of steady-state absorption and emission spectroscopy. Thereby,w ec ould show that the local microenvironment of the 1-naphthol, that is, presumably the accessibility of the 1-naphthol units within the copolymer,s trongly affects its tendency to undergo ESPT.A mong the six different copolymers studied herein, P[MEO 9 MA x -co-NOeMA y ]a nd P[MEO 9 MA xco-NOeMAm y ]w ere characterized by al ess pronouncedi ncrease in acidity upon irradiation (pK a * > 2), and in these cases we could monitor reversible deprotonation-also accompanied by proposed morphological changes upon irradiation (increasedd egree of hydration and, with that, improved accessibility of the photoacids for water molecules). Besides the desired ESPT,w ealsoi nvestigated the photo-degradation of the herein described materials and could identify photo-oxidation as ap otential additional pathway upon illumination.A mong the materials presented herein, VN-based copolymers so far exhibit excellent photostability.I nafinal proof-of-concept experiment we demonstrated the incorporation of NMA into the core of block copolymerm icellesa nd that this can be used to trigger light-mediated release of encapsulated cargo. Although the exact mechanism and kinetics still demand further studies, we foreseeg reat potentialo fp hotoacid-containing polymeric materials for site-and time-controlledr elease applications.T he photoacid type andc ontent incorporated may prove to be a facile way to tune the release properties beyonde stablished protocols. We believe that the concept introduced here is of general interest in material science wherever (light-mediated) polarity or charge gradients are desirable on surfaces or within multilayer materials.
Nuclear magnetic resonance (NMR)s pectroscopy: 1 HNMR, 13 CNMR, and 19 FNMR spectra were measured on a3 00 or 400 MHz Bruker AC spectrometer at 298 Ku sing the residual solvent resonance as an internal standard. The chemical shifts are given in ppm.
Size exclusion chromatography (SEC):S EC was performed on an Agilent 1200 series system equipped with aG 1310A pump, a G1315D DA detector,aG 1362A RI detector,a nd PSS GRAM 30 / 1000 (10 mmol particle size, Polymer Standards Service GmbH, Mainz, Germany) columns in series at 40 8Cusing N,N-dimethylacetamide (DMAc) with 2.1 gL À1 LiCl as eluent at af low rate of 1mLmin À1 .T he system was calibrated with PMMA standards (M p = 505 to 981 000 gmol À1 ).
Mass spectrometry:F or high resolution electron ionization mass spectrometry (HR-EI-MS) aF innigan MAT95XL sector field mass spectrometer was used. For mass matching the signals were annotated relative to perfluoro kerosene signals nearby.
Steady-state UV/Vis and emission spectroscopy:T he steady-state absorption spectra were measured in quartz cuvettes with 1cm optical pathway (Hellma) using aJ asco V780 UV/Vis/NIR spectrophotometer.F or UV/Vis photostability measurements the copoly-mers dissolved in O 2 -saturated H 2 Ow ere irradiated within the UV/ Viss pectrometer using a3 65 nm LED (Thorlabs, M365LP1, P 365 nm = 85 mW). The UV/Vis spectra under illumination were collected on a Jasco V530 UV/Vis spectrophotometer.Steady state emission measurements were carried out on aF LS980 emission spectrometer from Edinburgh Instruments using aX el amp (ozone free 450 W xenon bulb) as excitation source. For measurements under deaerated conditions the solvent (i.e.,H 2 O) was deoxygenated by applying 4-5 freeze-pump-thaw cycles. UV/Vis SEC (spectroelectrochemistry) measurements were performed using at hree-electrode thinlayer spectroelectrochemical cell with ap athlength of 1mm (Hellma, Bioanalytical Systems, USA). The three-electrode system contains aPtcounter electrode, an Ag/AgCl pseudo-reference electrode and ag lassy carbon working electrode. Potential controlled monitoring was performed using ac omputer controlled VersaSTAT 3( Princeton Applied Research) potentiostat. UV/Vis spectra were recorded immediately after applying the potential to monitor the accompanied spectral changes. UV/Vis spectra were collected in transmission mode by using ap roduct of AvantesI nc.,w hich is comprised of as ingle-channel fiber-optic spectrometer (AvaSpec-ULS2048XL) equipped with ad euterium-halogen light source (Ava-Light DH-S-BAL).
Cryogenic transmission electron microscopy (cryo-TEM):C ryogenic transmission electron microscopy (cryo-TEM) measurements were performed on aF EI Te cnai G2 20 cryo-transmission electron microscope (Jena Center for Soft Matter). Acceleration voltages were set to 200 kV.S amples were prepared on Quantifoil grids (3.5/1) after cleaning by an argon plasma treatment for 120 s. The sample solutions (10 mL) were blotted using aV itrobot Mark IV. Samples were then plunge-frozen in liquid ethane and stored under nitrogen before being transferred to the microscope using a Gatan transfer stage. TEM images were acquired with a2 00 kV FEI Tecnai G2 20 equipped with a4 kx 4k Eagle HS CCD and a1 k 1k Olympus MegaView camera.
Dynamic light scattering (DLS):D ynamic light scattering (DLS) was performed using ac ustom-built ALV/DLS-90 set-up, aA LV/ CGS-3 Goniometer system, equipped with aC obolt Samba 532 nm single frequency CW diode pumped laser,a nA LV/LSE-5004 correlator,a nd af our quadrant detector.M easurements were recorded at an angle of 90 8 in UV transparent Macro Fluorescence cuvettes with 4c lear optical windows under ambient conditions. The particle size was determined using ALV-Correlator Software V-3.0 by applying aC ONTIN fit. The custom-built set-up also allowed simultaneous in situ irradiation with a3 65 nm Fiber-Coupled LED (ThorLabs, M365FP1, 9.8 mW,1 400 mA).
General procedure for the RAFT copolymerization:Solutions containing the initiator (AIBN), CTA( CPDB), and monomer in 1,4-dioxane were first prepared with a[ M]:[CTA]:[I] ratio of 25:1:0.25 in a microwave vial. The total monomer concentration was adjusted to 2 m,o ri nt he case of tNMAm and tVN, the copolymerizations were carried out in bulk. For kinetic investigations, 1,3,5-trioxane was added as an internal standard, and samples were taken before and during the polymerization to determine the monomer conversion by 1 HNMR spectroscopy in CDCl 3 .A fter sealing the reaction vessel with as uitable septum, the reaction mixture was deoxygenated by flushing with argon for 10 min. The solution polymerizations were carried out in an oil bath at 70 8Cf or 3h.T he bulk polymerizations were carried out in an oil bath at 70 8Cf or 24 h. The polymers were isolated through preparative size exclusion chromatography (Biobeads S-X1) by using THF as eluent. The resulting copolymers were precipitated twice in n-hexane and dried in vacuo.
(-(OCH 2 CH 2 -(EO) 8 -), 3.7-3.4 (-OCH 3 and -(EO) 9 -), 3.3 (-(EO) 9 -OCH 3 ), 2.  For determination of the DP,1 ,3,5-trioxane was added as an internal standard, and samples were taken before and after the terpolymerization to determine the monomer conversion by 1 HNMR spectroscopy in CDCl 3 .A fter sealing the reaction vessel with as uitable septum, the reaction mixture was deoxygenated by flushing with argon for 10 min. The terpolymerization was then initiated by placing the flask into at hermostatted oil bath pre-heated to 70 8C. After eight hours, the terpolymerization was quenched by freezing in liquid nitrogen and exposure to air.T he reaction mixture was then diluted with dichloromethane and precipitated into a1 :1 (v/ v) mixture of n-hexane and diethyl ether 3t imes before being dried in vacuo.
Post-polymerization modification of P[MEO 9 MA x -co-PFMA y ]:T he synthetic procedure was adapted and modified from al iterature recipe. [38] The respective copolymers were mixed together with 5equiv.( with respect to the FMA units) of the amines 10 or 11,3hydroxy-1,2,3-benzotriazin-4(3H)-one (HO-Dhbt) and trimethylamine. Subsequently,t he mixture was dissolved in DMF.A fterwards, the solution was deoxygenated by purging with Ar for 10 min before being heated to 70 8Cf or 48 h. The solvent was then evaporated in vacuo, and the copolymers isolated by preparative size exclusion chromatography (Biobeads S-X1) using THF as eluent.
Micelle preparation:F or micelle preparation, 10 mg of the diblock quaterpolymer were dissolved in 1mLo fT HF.T he solution was added dropwise to 10 mL of micropure water in ag lass vial under stirring. Stirring was continued in an open vial for 24 ht oa llow THF to evaporate. Subsequently,t he concentration was adjusted to 1mgmL À1 by refilling the vial with micropure water prior to further investigation.
Nile red encapsulation and release:N ile red was encapsulated into the diblock quaterpolymer micelles through co-precipitation. Briefly,6mg of the respective diblock quaterpolymer was dissolved in THF (6 mL). Then, 71.4 mLo faNile red stock solution (c = 0.14 mg mL À1 )i nT HF was added. Under vigorous stirring, 12 mL of deionized water was added dropwise. The volatiles (THF) were allowed to evaporate under continuous stirring for 24 before the suspension was diluted with 48 mL of deionized water (c block copolymer = 0.1 mg mL À1 , c Nile red = 2 10 À4 mg mL À1 ). For the investigation of light-mediated release of Nile red, the corresponding micellar solutions were directly irradiated using aT horlabs LED M365LP1 (365 nm, 1150 mW,1 7.6 mWmm À2 ,1 400 mA, equipped with aCOP1-A-Collimation Adapter,distance % 10 cm) at different irradiation times up to 60 min. After every 10 min of UV light irradiation, af luorescence emission spectrum was recorded between 525 and 750 nm using an excitation wavelength of 510 nm. The release of Nile red was calculated according to Equation (2)