Photophysical Properties of Donor-Acceptor Stenhouse Adducts and Their Inclusion Complexes with Cyclodextrins and Cucurbit[7]Uril

molecules Article Photophysical Properties of Donor-Acceptor Stenhouse Adducts and Their Inclusion Complexes with Cyclodextrins and Cucurbit[7]uril Liam Payne 1 , Jason D. Josephson 2 , R. Scott Murphy 2 and Brian D. Wagner 1,* 1 Department of Chemistry, University of Prince Edward Island, Charlottetown, PE C1A 4P3, Canada; [email protected] 2 Department of Chemistry and Biochemistry, University of Regina, Regina, SK S4S 0A2, Canada; [email protected] (J.D.J.); [email protected] (R.S.M.) * Correspondence: [email protected]; Tel.: +1-902-628-4351 Academic Editor: Edward Lee-Ruff Received: 9 October 2020; Accepted: 22 October 2020; Published: 24 October 2020 Abstract: Donor-acceptor Stenhouse adducts (DASAs) are a novel class of solvatochromic photoswitches with increasing importance in photochemistry. Known for their reversibility between open triene and closed cyclized states, these push-pull molecules are applicable in a suite of light-controlled applications. Recent works have sought to understand the DASA photoswitching mechanism and reactive state, as DASAs are vulnerable to irreversible “dark switching” in polar protic solvents. Despite the utility of fluorescence spectroscopy for providing information regarding the electronic structure of organic compounds and gaining mechanistic insight, there have been few studies of DASA fluorescence. Herein, we characterize various photophysical properties of two common DASAs based on Meldrum’s acid and dimethylbarbituric acid by fluorescence spectroscopy. This approach is applied in tandem with complexation by cyclodextrins and cucurbiturils to reveal the zwitterionic charge separation of these photoswitches in aqueous solution and the protective nature of supramolecular complexation against degradative dark switching. DASA-M, for example, was found to form a weak host-guest 1 inclusion complex with (2-hydroxypropyl)-γ-cyclodextrin, with a binding constant K = 60 M− , but a 1 very strong inclusion complex with cucurbit[7]uril, with K = 27,000 M− . This complexation within the host cavity was found to increase the half-life of both DASAs in aqueous solution, indicating the significant and potentially useful stabilization of these DASAs by host encapsulation. Keywords: DASAs; photoswitches; supramolecular chemistry; fluorescence; cyclodextrins; cucurbiturils; host-guest inclusion; fluorescence enhancement 1. Introduction The term “photoswitch” refers to a broad palette of compounds capable of undergoing some reversible change in structure, conformation, charge, etc. in response to the absorption of light. This reversibility allows for the application of photoswitches as binary state switches, analogous to turning a light switch on or off [1]. Electronic excitation can induce bond rotation and cleavage, among other processes [2]. The ability to convert molecules between forms using light presents a wide range of applications ranging from photocontrol of enzyme activity [3], to tissue-specific drug release in vivo [4], to super-resolution fluorescence microscopy techniques [5,6]. Common examples of such photoswitchable molecules include azobenzenes [7], spiropyrans [8], hemithioindigos [9], and cyanines [10]. Donor-acceptor Stenhouse adducts (DASAs) are molecular photoswitches that can be unidirectionally controlled using visible light. This class of push-pull olefins consists of a triene skeleton with opposing Molecules 2020, 25, 4928; doi:10.3390/molecules25214928 www.mdpi.com/journal/molecules Molecules 2020, 25, x FOR PEER REVIEW 2 of 15 Molecules 2020, 25, 4928 2 of 15 Donor-acceptor Stenhouse adducts (DASAs) are molecular photoswitches that can be unidirectionally controlled using visible light. This class of push-pull olefins consists of a triene skeletonelectron-donating with opposing and electron-withdrawing electron-donating and groups electron-withdrawing (EDG and EWG, respectively) groups (EDG [1]. and Generally, EWG, respectively)the EDG is limited [1]. Generally, to a secondary the EDG amine is orlimited some toN -alkyla secondary aniline derivative,amine or some whereas N-alkyl the EWGaniline is derivative,largely customizable whereas the and EWG can beis la selectedrgely customizable to tune the absorptionand can be profile selected of theseto tune intensely the absorption colored profilecompounds of these [11 intensely]. The legacy colored of DASAs compounds spans [11]. 150 years,The legacy beginning of DASAs with thespans discovery 150 years, of Stenhousebeginning withsalts the in 1870 discovery [12]. Recentof Stenhouse renewal salts of in interest 1870 [12]. in DASA Recent compounds renewal of wasinterest sparked in DASA by a compounds 2014 paper waspublished sparked by Readby a de 2014 Alaniz paper and co-workerspublished [11by]. PreviousRead de contributions Alaniz and fromco-workers Honda [13[11].], D’Arcy Previous [14] contributionsand Šafár [15], from and Honda subsequent [13], contributionsD’Arcy [14] and from Šafár Read [15], de and Alaniz subsequent [16] and contributions Feringa [17] have from firmly Read deestablished Alaniz [16] these and molecules Feringa as [17] a novel have class firmly of visible established light-activated these molecules photoswitches as a novel that areclass synthetically of visible light-activatedaccessible in high photoswitches purity and yield. that are synthetically accessible in high purity and yield. The delocalization of the electron cloud across the extended conjugation of DASAs produces a prominent π →π* absorption band in the visible light range, giving intense color to these solids [17,18]. prominent π! π* absorption band in the visible light range, giving intense color to these solids [17,18].Additionally, Additionally, delocalization delocalization reduces reduces the double the double bond character bond character linking linking conjugated conjugated carbon carbon atoms, atoms,decreasing decreasing the energy the energy barrier barrier for rotation for rotation and thus and bestowing thus bestowing photoswitchability photoswitchability [19]. The [19]. DASA The DASAphotoswitching photoswitching mechanism mechanism is relatively is relatively complex andcomplex has been and mostly has been elucidated mostly throughelucidated a combination through a combinationof experimental of experimental and computational and computational studies [20–25 stud]: followingies [20–25]: electronic following excitation electronic with excitation visible light, with a visibleseries of light, rotations a series followed of rotations by a conrotatory followed by 4π a-electrocyclization conrotatory 4π-electrocyclization converts the linear converts triene form the linear into a trieneclosed form cyclopentenone into a closed form cyclopentenone (Figure1). Essentially, form (Figure the colored1). Essentially, “on” state the switchescolored “on” to a colorlessstate switches “o ff” tostate. a colorless Interestingly, “off” reversionstate. Interestingly, to the linearized reversio trienen to from the thelinearized cyclopentenone triene from photoisomer the cyclopentenone occurs only photoisomerthermally and occurs cannot only be photoinduced.thermally and cannot be photoinduced. Figure 1. Photoswitching mechanism of donor-acceptor StenhouseStenhouse adductsadducts (DASAs).(DASAs). Electronic excitation ofof the the linearized linearized triene triene via absorptionvia absorption of a photon of a promotesphoton promotes a series of a rotations, series of culminating rotations, culminatingin an electrocyclization. in an electrocyclizatio The cyclopentenonen. The cyclopentenone form thermally form reverts therma tolly the reverts triene form.to the triene form. By blending their uniqueunique photochemistryphotochemistry and and synthetic synthetic modularity, modularity, DASAs DASAs can can be be customized customized to tofit afit breadth a breadth of applications of applications in both in non-livingboth non-livi andng biological and biological systems. systems. Published Published applications applications include includeselective selective release release of drug of cargo drug fromcargo DASA-based from DASA-based micelles micelles [26], submicron[26], submicron photolithography photolithography [27], [27],and sensorsand sensors for amines for amines [28], metal [28], cationsmetal cations [29,30], [29, and30], changes and changes in pH [30 in]. pH Intriguingly, [30]. Intriguingly,Helmy et Helmy al. [11] etfound al. [11] that found polar that protic polar solvents protic solvents cause irreversible cause irreversible cyclization cyclization of DASAs of DASAs to a zwitterionto a zwitterion of the of thecyclopentenone cyclopentenone form, form, even even in thein the absence absence of of light. light. This This deleteriousdeleterious processprocess isis problematic for applications of DASAs in aqueous media, as the photoswitch becomes permanently locked in the Molecules 2020, 25, 4928 3 of 15 Molecules 2020, 25, x FOR PEER REVIEW 3 of 15 colorless form.form. Recently,Recently, Wang Wang et al.et [al.31 ][31] have have demonstrated demonstrated that hydrogenthat hydrogen bonding bonding between between solvent solventmolecules molecules and the carbonyland the carbonyl and ammonium and ammonium moieties of moieties the cyclopentenone of the cyclopentenone form leads to form DASA leads “dark to DASAswitching” “dark (Figure switching”2). (Figure 2). Figure 2. Water-mediated “dark switching” of a DASA, as proposed by Wang et al. [[31].31]. Favorable hydrogen bonding with the zwitterioniczwitterionic cyclopentenone form leads to irreversible stabilization.stabilization.

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