Oxidized Hemithioindigo Photoswitches—Influence of Oxidation State on (Photo)physical and Photochemical Properties

Abstract The photophysical and photochemical properties of sulfoxide and sulfone derivatives of hemithioindigo photoswitches are scrutinized and compared to the unoxidized parent chromophores. Oxidation results in significantly blue‐shifted absorptions and mostly reduction of photochromism while thermal stabilities of individual isomers remain largely unaltered. Effective photoswitching takes place at shorter wavelengths compared to parent hemithioindigos and high isomeric yields can be obtained reversibly in the respective photostationary states. Reversible solid‐state photoswitching is observed for a twisted sulfone derivative accompanied by visible color changes. These results establish oxidized hemithioindigo photoswitches as promising and versatile tools for robust light‐control of molecular behavior for a wide range of applications.

Abstract: The photophysical and photochemical properties of sulfoxidea nd sulfone derivatives of hemithioindigo photoswitches are scrutinized and compared to the unoxidized parent chromophores. Oxidationr esults in significantly blue-shifted absorptionsa nd mostly reductiono f photochromism while thermals tabilities of individual isomers remain largely unaltered. Effective photoswitching takes place at shorterw avelengthsc omparedt op arent hemithioindigos and high isomeric yields can be obtained reversibly in the respective photostationary states. Reversible solid-statep hotoswitchingis observed for at wisted sulfoned erivativea ccompanied by visible color changes. These resultse stablisho xidized hemithioindigo photoswitches as promising and versatile tools for robustl ightcontrolo fmolecular behavior for aw ide range of applications.
The hemithioindigo( HTI) [1] structural motiveb elongs to the class of indigoid chromophores [2] and has been knowns ince 1906 when it was first synthesized by Paul Friedländer. [3] It took somet ime since its discoveryu ntil HTI hasb een recognized to undergo light-induced photoisomerization, which was first reported in 1961 by Mostoslavskii and co-workers. [4] Since then HTI has been applied as photoswitch by ac ouple of groups [5] including our own, [6] andh as been further developed into am ore mature photoswitching system primarily by the mechanistic works of Rück-Braun, Cordes,Z inth,[5f,h,7] de Vivie-Riedle, [8] Riedle, [9] and our own group. [6a, 9-10] Other research teamsa lso have joined these efforts in recenty ears. [11] In many studies the effects of variouss ubstitution patterns at different positions of the chromophores tructure on the photo/physical properties have been explored quantitatively,w hich now enables conscious design of different property profiles of this class of photoswitches. At present it is thus welle stablished that HTI chromophoresp ossess many advantages permitting highlyr obusta nd efficient photoswitching within the visible part of the electromagnetic spectrum. When further exploring the structuralm otive of HTI, our group used oxidation of the sulfura tom to the corresponding sulfoxide as as traight-forward methodt oi ntroducec hiralityi nto HTI chromophores with fourfold [12] substituted central doubleb onds. Such structures enabledu st od evelop av ariety of different molecular motort ypes that are responsive to visible light and perform unidirectionalm otions upon irradiation. [9,13] Despite this progress, the effects of sulfur oxidation on HTI photoswitches bearing three substituents at the central photoisomerizabled ouble bond have to the best of our knowledge not been explored so far.S ome photophysical data of mainly sulfone HTIs are described in the literature butn op hotoisomerization reactions are scrutinized. [14] In this work we set out to fill in this gap and provide as urvey of the effects of sulfur oxidationo nt he photoswitching properties of HTI. To this end we comparef ive parentH TI structures (1)(2)(3)(4)(5)w ith the corresponding sulfoxide (HTI-SO 1-5)a nd sulfone (HTI-SO 2 1-5)d erivatives as shown in Figure1a. The specific substitution variations on the stilbene fragment were chosenb ecause electronically neutralo rd onating substituents are generally beneficiary for HTI photoswitching. [6a, 10a] Likewise twisted structures such as in HTIs 4 and 5 have also been reported by our group to elicit interesting (photo)physical properties, especially with regard to high thermal bistabilitya nd switching performance. [10c, f] In this context it was deemed most effective to consistently increaset he push-pull character across the central double bond and evaluate the effects in as ystematic way.F or these reasons we increased acceptor strength of the thioindigo fragment by successive increasei nt he oxidation state of the sulfur atom and pairedit with donor substituents on the stilbenefragment.
Synthesis of oxidized HTI derivatives 1-5 followede ither of two different approaches, oxidation of established HTI photoswitches using H 2 O 2 /AcOH or NaBO 3 ·4H 2 O/AcOH, or condensation of oxidized benzothiophenones (sulfoxide 6 or sulfone 7) with the correspondinga ldehydes 8 or 9 (Scheme 1). Synthetic procedures are detailed in the Supporting Information together with comprehensive characterization (see also Figures S14-S25 for NMR spectra).M ixtures of the correspondingH TI-SO and HTI-SO 2 products were usually obtainedu nder the first conditions, which could easily be separatedb yc onventional columnc hromatography.T ypically the isolated yields of HTI-SOs were much lower than of the corresponding HTI-SO 2 sb ecause of prolonged reactiont imes and an excesso fH 2 O 2 added (HTI-SO :H TI-SO 2 = 5% :5 8% for 2,2 6% :3 5% for 4). However,i nt he case of 5,t he opposite behavior was observed Scheme1.Differentsynthetic approaches to obtain oxidizedH TI photoswitches 1-5. Chem. Eur.J.2020, 26,10712 -10718 www.chemeurj.org 2020 The Authors. Published by Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim (HTI-SO :H TI-SO 2 = 95 %:5%). HTI-SO 1 was obtained after oxidation of the correspondingH TI 1 with NaBO 3 ·4H 2 Oi n2 0% yield. HTI-SO 3 was synthesized in 40 %y ield by condensation of 9-formyljulolidine (8)w ith knownb enzothiophenone-sulfoxide (6). [15] HTI-SO 2 derivatives 1 and 3 were obtained by condensation of the respective aldehydes 9 and 8 with commercially available benzothiophenone-sulfone (7)i n5 0% and 67 % yield, respectively.C rystals suitable for structural analysis were obtained for the Z isomers of HTI 4,H TI-SO 1 to 4,a nd HTI-SO 2 derivatives 1-5 as shown in Figure 1b (see also Tables S4-S8 in the Supporting Information). In the crystalline state it can be observedt hat the moleculars tructures of oxidized HTIs 1-3 are still almost completely planarstructures facilitating efficient conjugation between the central photoisomerizable double bond and the stilbene fragments. This finding shows that oxidation at the sulfur atom does not induce substantial sterical hindrance at this position to lead to clashes with adjacent molecular fragments. However,t he molecular structures of derivatives 4 and 5 are substantially distorted primarily because of the increased sterical hindrance induced by their methoxy and methyls ubstituents. The stericale ffect is much more pronounced in the methyl-substituted derivatives of HTI 5.
For many applications of photoswitchest hermally stable switchings tates are required to ensure full light-addressability of the system. Since metastable states are usually generated by at least one photoreaction, the kineticso ft heir spontaneous thermald ecays are important parameters for photoswitches. For HTIs the E isomersa re typicallym etastable and slowly decay thermally back to the Z isomers. We have measured the thermald ecay of the E isomersf or oxidized HTIs in toluene solution and analyzed them with the Eyring equation to derive the corresponding free activatione nthalpies DG°(for details see Figures S1-S3 and Tables S1 and S2 in the Supporting Information). All DG°values are reported in Ta ble 1. The free activation enthalpies fort hermald ouble-bond isomerizations range from 21.4 kcal mol À1 (HTI 3)t o> 33.0 kcal mol À1 (HTIÀSO and HTIÀSO 2 5), which is quite high compared to many conventional photoswitching systems. Ap ossible reason for the significantly higherb arriers of thermali somerization of oxidized derivatives of HTI 5 may be rooted in ap ositive interaction between the oxygen atoms on the sulfur atom and the hydrogen atoms of the neighboring ortho-methyl groups. Indications for this additional stabilization are derived from the relatively short distance between the oxygen atom and the corresponding methyl-carbon atom in the crystal structures. When comparing the resultsw ith the unoxidized parent HTIs it can be seen that oxidation of the sulfur atom usually does not lead to significant effects with respect to the thermal isomerization reactions and no linear trends are observed for the changes seen. Free activation enthalpy differences DG°in the range of 1.4 to 4.5 kcal mol À1 are observed upon oxidation of HTIs. These effectsa re not very strongf or thermald oubleb ond rotations rendering oxidized HTIs very bistable photoswitchesa s well. The only notable exception to this behavior is HTI system 4 for whicho xidation leads to as ignificant reduction in thermal stabilityo fu pt o8 .2 kcal mol À1 for HTI-SO 4.T he resulting barriero f2 3.3 kcal mol À1 is nevertheless still high enough to warrant bistable photoswitching for most applications. For all chromophores reported here the molara bsorptions were recorded in CH 2 Cl 2 and/or toluene solutionf or both the Z and respective E isomers (see Figures S4-S6 in the Supporting Information). As it can clearly be seen in Figure 2o xidation of the sulfur atom leads to an oticeable hypsochromic shift of the absorption. Interestingly,t he first oxidation to the sulfoxide HTI-SO derivatives induces the strongest hypsochromic shift of the absorption.F urther oxidation to the sulfones HTI-SO 2 does not result in further hypsochromics hift. Instead the absorption remains essentially the same as compared to the corresponding sulfoxides. Apparently the electronice ffects of sulfoxideo r sulfone-oxidation states are very similar in these chromophores.H ypsochromic shifting occurs for both isomers,b ut stronger so for the E isomers in most cases.T he result is an overall reduction in the photochromism of these compounds. This observation can at least partly be explained by the more Figure 2. Comparison of molara bsorptivities e for the Z and E isomersofH TIs and the corresponding oxidized derivatives. Molar absorptivities werem easured in CH 2 Cl 2 solution for the series 2, 4,a nd 5 and in toluenesolution for 1 and 3.Spectra of HTIs are shown in grey,ofs ulfoxides HTI-SOs in red, and of sulfones HTI-SO 2 si nblue, of Z isomers as solid and of E isomers as broken lines (see legend in sub- Figure a). a) Molar absorptivities of HTI series 1.b )Molar absorptivities of HTI series 2.c )Molar absorptivitieso fH TI series 3.Note thatmolar absorptivities of the E isomer of 3-SO 2 could not be obtained due to inefficientp hotoisomerization and strongsignal overlapping of Z and E isomer in 1 HNMR spectra.d )Molar absorptivitieso fH TI series 4.e)Molar absorptivitieso f HTI series 5.f)Enlarged section of the molarabsorptivities of HTI series 5. similar electron withdrawing character of the carbonyl ando xidized sulfur functions connected to the central double bond. Because of this similarity the electronic character of the molecule does not change very much when switching from the Z to the E isomer.T he notable exception to this trend is the series of HTIs 4 where photochromism is actually improved slightly as shown in Figure 2d.U pon oxidation of the sulfur atom molara bsorption increases in comparison to the unoxidized parent HTIs in the series 1, 2,a nd 4 butdecreases slightly for the series 3.F or series 5 the effects are more complicated with notable decreases of the Z isomer absorptions while the E isomers absorptions actually increase significantly upon oxidation. In all cases the typical two-band structureo ft he absorptiono bserved for unoxidized HTIs developsi nto am ainly single-band structuref or both HTI-SO and HTI-SO 2 derivatives. With regard to solventc hanges the absorptions of oxidized HTIs are not particularly solvatochromic except for derivatives bearings trong electron-donor groups,t hat is, 3 (see Figure S7 and Table S3 in the Supporting Information). Absorption maximaa nd molara bsorptions are summarized in Ta ble 1f or all compounds.
Because of the less pronouncedp hotochromism of oxidized HTI derivatives their photoisomerization reactions could be rendered less efficient in generating excess of ap articular isomer.T ot est the photoswitching behavior,s olutionsw ith the respective oxidized HTI in either CH 2 Cl 2 or toluene were irradiated to the photostationary state (pss) at different wavelengths and the maximum attainable isomer enrichment was quantified using 1 HNMR spectroscopy (see Figures S8-S13 in the Supporting Information). The results are summarized in Ta ble 1. As it turns out oxidized HTIs in general undergo quite efficient photoisomerizationsw ith E isomer enrichmentr eaching up to 77 %a nd Z isomer enrichment reaching > 90 %f or the best performing derivatives as (showne xemplarily in Figure 3f or HTI-SO 3 and 4 and HTI-SO 2 2 and 4). For all oxidized derivatives-except in the series of 3-light with < 400 nm is most efficient for the Z to E photoisomerization direction but also blue light of 405 nm is still effective in most cases. In general it can be seen that there is some decline in the achievable isomer enrichment in the photoisomerization reactions if the sulfur atom is oxidized. HTI-SO derivatives are most of the time the least efficient photoswitchesw ithin ap articular series and thus again no linear trend with increasing oxidation state is observed. In series 5 HTI-SO is more efficient than HTI-SO 2 in the Z to E photoisomerization most likely because of the significantly higher absorption of the E isomer across the spectrum for the latter.T he best performing sulfoxide photoswitches are HTI-SO 3 and 4 reaching 73 %a nd 75 % E isomer enrichment as well as 100 %a nd 76 % Z isomer enrichment in the pss, respectively.T he best performings ulfone photoswitches are HTIÀSO 2 2 and 4 reaching 70 %a nd 77 % E isomer enrichmenta sw ell as 92 %a nd 83 % Z isomer enrichment in the pss, respectively.O xidized HTI derivatives in the series 3 possessing stronge lectron donating groups lose photoswitching capability in more polar solvents, whichi si ndicative of ap ossible competing deexcitation pathway that involves significant charget ransfer in the excited state. [10b, c] For this reason photoisomerization reactions of derivatives 3 were measured in the less polar toluene solvent exclusively.
It should be noted at this time that strong isomer enrichment is desirable for most photoswitching applicationsb ut is not necessary for applications in molecular machineb uilding, especially molecular motors,w here efficient photoswitching of both isomersisd esirable at actually the same wavelength of irradiation.Therefore, it can be stated that oxidized HTIs possess high potential tos erve as valuable molecular motives for both (visible)-light induced photoswitching and molecular machine building.
As it was mentioned already above photochromism is reduced wheno xidizingt he sulfur atom and as ar esult changes in isomer composition are oftentimes not readily visible by the naked eye.T he latter is also true for many unoxidized HTI photoswitches. This situation is different for the oxidized HTI series 1 where only the absorptions of the E isomers are crossing the 400 nm threshold. As ar esult colorless solutions of pure Zi somers turn yellow upon photoswitchingf or HTI-SO 1 and HTI-SO 2 1 (see Figure 4a lso foracomparison of the photochromism within the full series 1). As imilar behavior is also observed for HTI-SO 2 5.H owever,f or HTI-SO 2 5 this easily visible color change is not confined to solution but also occurs in the solid and crystalline state evidencing effective photoswitching of this derivativei nm uch more restricted environments. To illustrate this, we have irradiated amorphic films of the pure compound with alternating 365 nm and 420 nm light to reversibly affect color changes in the solid state ( Figure 4d). Such behavior is of great interestf or opticalm aterials applications for example,for information storage.
In summary we have compared the performance of as eries of novel oxidized HTI photoswitches, both sulfoxide and sulfone derivatives, with their parent unoxidized HTI chromophores.W eh ave shown that oxidized HTIs represent valuable novel photoswitches providing efficient photoisomerization reactions and high thermal stabilities of their metastable states. Particularly interesting are twisted derivatives that allow the observation of the photoisomerization process with the naked eye both in solutiona nd solid state. Scrutiny of the chiroptical properties of chiral sulfoxide HTI-SOsi sc urrently underway in our laboratories and will be reported in due course. With these findings we hope to bring novel photoswitching motives to the attention of molecular scientists seekingr eliable light responsivet ools that elicit predictable structuralc hanges upon irradiation.