Article pubs.acs.org/cm
Spirothiopyran-Based Reversibly Saturable Photoresist Harikrishnan Vijayamohanan, Edmund F. Palermo, and Chaitanya K. Ullal* Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180, United States S Supporting Information *
ABSTRACT: Super-resolution lithography holds the promise of achieving three-dimensional (3D) nanopatterning at deep subwavelength resolutions with high throughput. 3D super-resolution lithography schemes demonstrated thus far have all been serial in nature, primarily due to the lack of a photoresist chemistry that not only couples a saturable reversibly switchable reaction with a writing step but also has a low saturation threshold. Here, we demonstrate that combining the reversible photoisomerization of spirothiopyran with the thiol-Michael conjugate addition reaction achieves the necessary photochemical characteristics. Green light was found to saturate inhibition of the thiol-Michael addition writing step at very low intensity thresholds. By formulating a spirothiopyran-functionalized polyethylene glycol copolymer, we demonstrate spatial control over cross-linking using inhibition by green light. Kinetics measurements combined with photokinetic simulations show that interference lithography on a spirothiopyran maleimide-based writing system using conventional light sources (e.g., a 2 W green laser) should deliver super-resolution features (∼45 nm wide lines) in thick films (tens of microns) over large areas (hundreds of microns on a side). The unique combination of reversible photochromic switching of spirothiopyran with the thiol-Michael addition reaction marks an important step toward realizing a highly parallelized 3D super-resolution writing system.
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B participates in the writing process and C is the final state post locking/writing. The switching between the two states must be reversible and much faster than the kinetics of the “writing” reaction. Additionally, the system must be free of “cross-talk,” i.e., the equilibrium A ⇆ B is controlled by stimuli that do not affect the rate of the B → C writing reaction. Finally, highthroughput writing strictly requires that the system achieve low threshold saturation, i.e., very low depletion intensity effectively drives the equilibrium almost completely back to the nonwritable starting material A. A major breakthrough occurred in 2009, when the Fourkas,7 Bowman and McLeod,8 and Menon9 groups simultaneously reported the first experimental demonstrations of STEDinspired optical lithography, or super-resolution lithography, with features in the nanoscale regime. Several other groups1,10−15 have improved the capabilities of super-resolution lithography immensely. The current benchmarks for lateral and axial feature sizes patterned using super-resolution optical lithography are 101 and 53 nm,16 and the minimum separation achieved between two lines is 52 nm.14 While the superresolution writing techniques discussed above meet the requirements outlined by Hell, all 3D nanostructures fabricated using super-resolution lithography have been restricted to serial point-by-point patterning, which inherently precludes highthroughput writing over large volumes (Figure 1a). The primary reason for this one remaining limitation is that high laser intensities are required to saturate the depletion process. Thus, there is an urgent need for a photoresist platform that is
INTRODUCTION The ability to sculpt three-dimensional (3D) structures at the nanoscale is one of the long coveted goals of nanotechnology. Current state-of-the-art fabrication systems such as scanning electron beam lithography indeed produce features with nanometer resolution. However, they are serial writing techniques with high setup and usage costs and are also effectively confined to planar patterning.1 Optical interference lithography is an attractive technique to cheaply and rapidly pattern three-dimensional features in polymer photoresists.2 Unfortunately, both resolution and feature size achievable are limited by the diffraction limit of light.1 Two decades ago, Hell and Wichmann proposed stimulated emission depletion (STED) microscopy, wherein a reversible and switchable fluorophore can be used to circumvent the diffraction limit in optical microscopy.3,4 Today STED microscopy has become a well-established subdiffraction optical imaging technique capable of achieving resolutions below 10 nm.5 Hell subsequently outlined the basic requirements for a dif f raction unlimited writing system as follows: an optically switchable system having two states in equilibrium, out of which only one participates in a final locking or writing step such as polymerization or cross-linking in a photoresist6 (Scheme 1). The scheme is denoted as A ⇆ B → C, where A and B constitute the optically switchable states, of which only Scheme 1. Diffraction Unlimited Writing Using a Reversibly Switchable System
Received: February 7, 2017 Revised: May 14, 2017 Published: May 22, 2017 © 2017 American Chemical Society
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DOI: 10.1021/acs.chemmater.7b00506 Chem. Mater. 2017, 29, 4754−4760
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Figure 1. (a) STED-inspired writing techniques usually have a high saturation threshold, which limits writing to very slow point-by-point approaches. (b) Photochromic molecules, due to their inherently low saturation threshold, can allow for parallel writing with subdiffraction resolution. (c) In this work, the super-resolution lithography writing scheme using spirothiopyran was inspired by STED microscopy.
transformations. Hirai and co-workers26 demonstrated reversible light-triggered aggregation of gold nanoparticles via Au−S bonding facilitated exclusively by the MC isomer. The thiolate anion can also participate in a conjugate addition reaction with α,β-unsaturated carbonyl compounds (thiol-Michael addition).27 Experimental demonstration of cross-linking of spirothiopyran-functionalized polymers using the thiol-Michael reaction was first demonstrated utilizing photochromism by Zhu and co-workers28 and mechanochromism by Boulatov and co-workers.29 The prospect of utilizing spirothiopyran’s reversible photoswitching to create the high-efficiency inhibition pathway necessary for super-resolution writing remains unexplored to date. As an initial proof-of-concept, we performed a small-molecule model experiment: we show that when the MC isomer selectively reacts with ethyl maleimide to form the thiol-Michael addition product under UV (365 nm) irradiation, green light (532 nm) inhibits the writing step by shifting the equilibrium far toward the nonreactive SP isomer. Because the writing step is not independently triggered, the relative kinetics of the writing step and the photoisomerization are critically important. To achieve low-intensity thresholds for depletion, the photochromic switching (SP ⇆ MC) must proceed on a faster time scale than the writing step (B → C). Photoswitching and thiol-Michael addition chemistry with favorable kinetics are leveraged to demonstrate spatial control of cross-linking using a spirothiopyran and polyethylene glycol functionalized copolymer. Finally, photokinetic simulations demonstrate the potential for parallelized writing with nanoscale resolution.
reversibly saturable with a low saturation threshold. (Figure 1b) In this Article, we demonstrate a photoresist chemistry that meets these criteria and discuss its applicability to superresolution, large-area, and maskless STED lithography. As a source of inspiration, we turned to the use of photochromic molecules to achieve super-resolution in the context of interference lithography, as pioneered by the Menon and Andrew groups. Of particular note is their utilization of diarylethenes.17−20 Typically, a photochromic layer of diarylethenes is coated on the surface of a resist, and the difference in transmittance between the open and closed photoisomers of diarylethenes is used to pattern the underlying photoresist. However, having a photochromic layer separate from the photoresist limits patterning to 2D. The use of electrochemically induced oxidation states17 or exploitation of slight differences in solubility between the two isomer forms21,22 allows for the diarylethenes to be used as the resist itself. While the high extinction coefficient of the diarylethene molecule has effectively restricted even these strategies to 2D, such work strongly suggests that photochromic molecules are, in principle, prime candidates for developing a parallelized subdiffraction 3D writing system due to their fast reversible switching and low saturation thresholds. To meet the criteria for 3D superresolution photoresists herein, we present the covalent incorporation of a photochromic switching molecule into a polymeric photoresist. In our system, the writing step is based on a highly selective, high-yielding reaction of one of the two photoisomers of our photochromic system, which cross-links the linear polymer chains and produces an insoluble gel network. A possible alternative strategy is the use of photochromic molecules as photoinitiatiors.23,24 From the five classes of well-known photochromic molecules25 (diarylethenes, fulgides, spiropyrans, chromenes, and azobenzenes), we targeted spirothiopyran, a sulfo derivative of spiropyran, because one of its two photochromic isomers contains a thiolate anion that can be exploited for thiol-Michael addition chemistry. This molecule photochromically switches between its “closed” spirothiopyran (SP) and “open” merocyanine (MC) isomers. UV irradiation converts SP to MC while visible (green) light reverts the MC isomer back to SP.26 The proposed spirothiopyran-based writing scheme, shown in Figure 1c, utilizes the thiolate anion of the MC isomer, which is useful as a functional group handle for various
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EXPERIMENTAL SECTION
Materials. Reagents were purchased from Sigma-Aldrich and used without further purification. Four-arm PEG maleimide was purchased from NANOCS. Deuterated solvents were purchased from Cambridge Isotope Laboratories. Instrumentation. Proton (1H) and carbon (13C) NMR spectra were recorded on a Bruker WB 600 MHz or a Bruker SB 800 MHz spectrometer. Electrospray ionization (ESI)/mass spectroscopy (MS) analysis was carried out using a Thermo Scientific LTQ Orbitrap XL Hybrid Ion Trap-Orbitrap mass spectrometer. Gel permeation chromatography (GPC) measurements of the synthesized polymer were performed using an Agilent 1260 Infinity GPC/SEC system. A Hitachi U-2910 double-beam spectrophotometer was used for UV− visible spectroscopy measurements. Optical power reported for saturation curve measurements and two-color photopatterning was 4755
DOI: 10.1021/acs.chemmater.7b00506 Chem. Mater. 2017, 29, 4754−4760
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Chemistry of Materials measured using a Coherent LabMax-TO power meter with a LM-2 VIS silicon optical sensor. Synthesis. Spirothiopyran (Figure 1c) was synthesized according to literature procedures.26 Details of the synthetic route are given in the Supporting Information. Spirothiopyran-functionalized polymer 6 was synthesized by a previously reported route30 with modifications (Scheme 2). See Supporting Information for complete details.
Scheme 3. Schematic of Reactions Considered for Photokinetic Modelling of the Spirothiopyran Writing System; SP Represents the Spirothiopyran Isomer, MC Is the Merocyanine Isomer, and MAP Is the Resultant ThiolMichael Addition Product
Scheme 2. Synthetic Route to the Spirothiopyran and PEGFunctionalized Methacrylate Copolymers
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RESULTS AND DISCUSSION To demonstrate the feasibility of subdiffraction patterning, we first examine the yield of the conjugate addition thiol-Michael reaction as a function of depletion beam intensity (532 nm). The saturation behavior in STED microscopy is highly nonlinear and has been shown to have an exponential dependence.33 To test the depletion response of our system, spirothiopyran and N-ethyl maleimide were dissolved in deuterated acetone and exposed to a fixed UV (365 nm) intensity of 10 mW/cm2 and variable intensities of green (532 nm) light from 0 to 2.5 W/cm2. The thiol-Michael addition product (MAP) between spirothiopyran (SP) and N-ethyl maleimide has been characterized before.28 On the basis of the 1 H NMR spectra, integrated areas corresponding to the wellresolved resonances for SP at δ 6.18 ppm and MAP at δ 5.52 ppm were a convenient handle for monitoring the conversion of SP to MAP as a function of depletion intensity. The UV intensity (10 mW/cm2) and time of exposure (180 min) were held fixed in all experiments. We selected the exposure time to give >99% yield for the case of zero green light intensity. Figure 2a shows the disappearance of the spirothiopyran resonance with the concomitant appearance of the thiolMichael addition product in the 1H NMR spectra, for various depletion intensities. Figure 2b shows the MAP yield as a function of depletion (532 nm) intensity. Two important observations are to be noted. First, the MAP yield exhibits an exponential decay with increasing depletion intensity. Thus, our writing system is indeed capable of being driven to saturation, one of the key requirements for a STED-inspired lithography setup. Second, the depletion intensity required to achieve saturation is only 2.5 W/cm2, compared to that of current state-of-the-art STED-inspired techniques, which range from 104 to 107 W/cm2 depletion intensity.1,7,10−16 The prior writing systems are considered “high threshold” (Figure 2c), whereas the saturation behavior of the photochromic switch presented herein is clearly categorized as an extremely “low threshold” writing system. Low saturation threshold is a quintessential characteristic of parallelized writing systems. For example, a standard 2 W laser with a 1.1 mm beam diameter can easily produce 2.5 W/cm2 over an area as large as 5 × 5 mm2. Additionally, the kinetics are readily tailored by varying the UV and green intensities. Higher UV intensity will result in faster kinetics and, hence, lower exposure time but will require modestly higher depletion intensity to drive the system to saturation (although still orders of magnitude lower than in prior systems). Consequently, saturation can also be achieved with even lower depletion intensity but will require somewhat longer exposure times. With our small-molecule proof-of-concept in hand, we next proceeded to pattern macroscopic features in spirothiopyranfunctionalized polymer gels, in order to establish the extent to
Saturation Curve Measurements. Spirothiopyran and N-ethyl maleimide were dissolved in deuterated acetone ((CD3)2CO) and D2O. After degassing with N2, the glass vial was sealed. Collimated UV and green light (532 nm) were combined using a dichroic mirror and made incident on a continuously stirring reaction mixture. Fixed UV intensity (10 mW/cm2) and varying green intensities were used. After 3 h exposure, the UV LED was powered down, and the sample was exposed with high-intensity green light (2.5 W/cm2) for 10 min. The final reaction mixture was then analyzed by 1H NMR. Two-Color Photopatterning. Copolymer 6 and 4-arm PEG maleimide were dissolved in a water/DMF mixture. An aliquot of this solution was sandwiched between two glass slides separated by a 0.25 mm wide spacer. This assembly was exposed to green light passed through a photomask (T-shaped metal stencil) and UV light. (Details of optical setup are provided in the Supporting Information.) After exposure for 10 min, the patterned film was developed by washing the glass plates thoroughly with water/DMF mixture and acetone. Photokinetic Modeling. Kinetic analysis of photochromic systems in solution under constant irradiation has been extensively studied.9,31,32 For a photochromic system in an inert atmosphere, assuming no photodegradation pathways, the photoisomerization from ground state A to excited state B will occur under three competing rate constants, the light-induced reversible photoswitching between A and B and the thermal back-isomerization from B to A. Under constant illumination, A and B are in equilibrium. We now add a B to C writing step, which is irreversible with time. For thiol-Michael addition using spirothiopyran, this step can be modeled using an overall second-order reaction, first order with respect to both merocyanine and ethyl maleimide.27 Hence, the photokinetics of our writing system can be represented by Scheme 3 (for complete mathematical treatment and details of how photokinetic parameters are determined, see Supporting Information). We simulated a 1D interference lithography setup for the spirothiopyran/ethyl maleimide system. Absorbance in the resist is modeled using the differential form of Beer−Lambert’s law. Ethyl maleimide is transparent to both green and UV light (Table S2 in Supporting Information), and the thiol-Michael addition product is transparent to green light. Hence, their respective contributions to the net absorbance in the resist are ignored. The resist is simulated as a 2D array across x (lateral direction) and z (depth) axes. Coupled differential equations are numerically solved using the Euler finite difference method for a spirothiopyran concentration of 0.05 M. Complete details are given in the Supporting Information. 4756
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Figure 2. (a) Characteristic proton NMR chemical shift of spirothiopyran (STP) and the thiol-Michael addition product (MAP) as a function of increasing depletion intensity. (b) Plot of thiol-Michael addition reaction yield vs depletion intensity. The obtained fit for an exponential decay function (y = A0 e−kI) is also shown. Fitting parameters obtained are A0 = 87.269 and k = 1.486 cm2 W−1. (c) Schematic plot of feature size vs intensity ratio for high-threshold and low-threshold writing systems.
which the SP ⇆ MC → MAP reaction yields insoluble gel networks (as the contrast for photoresist development). Knowledge of the complex interplay between irradiation conditions (time and intensity) and the polymer characteristics (MW, concentration, cross-link density, and critical gelation point) will be required in order to translate this system into an operational STED-inspired lithography technique. We synthesized a copolymer of a methacrylate bearing a spirothiopyran side chain with PEGylated methacrylate, in order to control the solubility of the linear polymer. The spirothiopyran monomer and the PEG monomer (Mn = 360 g/mol) were statistically copolymerized in a 1:4 feed ratio by free radical copolymerization. The resulting copolymer was characterized by 1H NMR (copolymer composition f STP = 0.2) and GPC (Mw = 48 kg/ mol, Đ = 2.73). Solutions of this polymer (10% w/v) in chloroform undergo rapid gelation when irradiated with UV light in the presence of an equivalent amount of the multifunctional 4-arm star PEG maleimide polymer (Figure 3). To demonstrate the depletion of cross-linking in the polymeric spirothiopyran writing system, we patterned macroscopic features in the photoresist by simultaneous exposure to unpatterned UV (1 mW/cm2) and patterned green light (50 mW/cm2), as shown in Figure 4a and b. The regions exposed to only UV light form an insoluble gel whereas the regions exposed to UV and green are inhibited from cross-linking and are washed away during development (Figure 4c), as evidenced by the absence of material when probed using a scalpel blade. This experiment shows that the spirothiopyran−maleimide writing system meets the primary condition required for a parallelized super-resolution writing system, i.e., a reversibly switchable system that has a low saturation threshold for the wavelength that inhibits the writing process. Hence, we believe that this represents an important milestone toward a revolution in 3D super-resolution lithography. To predict the behavior of our system under super-resolution writing conditions, we simulated the response of a spirothiopyran (SP)−N-ethyl maleimide system in response to a 1D interference writing scheme as shown in Figure 5a. A plane wave of UV (365 nm) and a standing wave of green (532 nm) are incident on the photoresist for an exposure time of 45 s. The intensity of the plane wave was kept fixed (20 mW/cm2) while the intensity of the standing wave was varied (0−50 W/ cm2). As described in the Experimental Section, we combined
Figure 3. Scheme depicting photochromism and cross-linking for the spirothiopyran−PEG copolymer. The spirothiopyran monomer and PEG-methacrylate (Mn = 360, n = 6) were taken in a 1:4 molar ratio. The polymer is shown dissolved in chloroform with a 4-arm PEG maleimide polymer (Mw = 10 000) before and after cross-linking.
the chemical kinetics of photoswitching,9,31,32 an overall second-order reaction of the thiol-Michael addition27 writing step, with the differential form of Beer−Lambert’s law, in order to predict the evolution of the MAP concentration in the resist both spatially and temporally. Numerical calculations were performed using finite difference methods. Parts b and c of Figure 5 show the distributions for the MAP concentration predicted in the case of green intensities of 49 and 4.9 W/cm2, respectively, at a constant UV exposure of 4757
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intensity 20 mW/cm2. Writing occurs preferentially at the nodes of the standing wave, and feature sizes decrease with increasing depletion intensity. This behavior corroborates the experimental saturation curve in Figure 2b. The concentration of spirothiopyran in the photoresist is 0.05 M, which is the same as the effective spirothiopyran concentration for the polymer solution used in the pattern shown in Figure 4. Line widths of 100 and 45 nm are predicted for standing wave depletion intensities of 4.9 and 49 W/cm2, respectively. An important consideration in interference lithography, which applies to the use of photochromic molecules for writing in general, is that high absorbance of the photoresist will result in narrowing line widths with increasing depth for thick films.34 Decreasing the effective spirothiopyran concentration in the resist alleviates this problem. Our simulations corroborate this; we see a decrease of 20 nm in line width (44%) over 10 μm (Figure 5d). The decrease in line width is attributed to the strong UV (365 nm) extinction coefficients for the SP and MC isomers. (Table S2, Supporting Information). Simulations clearly show that it is feasible to mitigate the absorbance depth problem in our system. Figure S5 shows the effect of concentration on the line width reduction for a 1D resist. No appreciable change in line width is seen over 10 μm for SPT concentrations below 0.01 M. We can readily access such low STP concentration in our polymer solutions either by altering
Figure 4. (a) Schematic illustrating two-color patterning using the spirothiopyran photoresist. A plane wave of UV (365 nm) radiation is superimposed with a collimated green beam (532 nm) from a laser passed through a photomask. (b) Superimposed UV and green beams. (c) Resulting pattern obtained postdevelopment.
Figure 5. (a) Setup used for simulating 1D interference lithography in a spirothiopyran−N-ethyl maleimide system. Thiol-Michael addition product concentration in the resist (XZ view) for depletion intensity (b) 49 W/cm2 and (c) 4.9 W/cm2. (d) Lateral line profile of MAP concentration for (b) at top (z = 0 μm) and bottom (z = 10 μm) of resist. 4758
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Chemistry of Materials the copolymer composition ( f STP < 1:4) or by decreasing the overall polymer concentration. To retain the cross-linking ability in polymer with low spirothiopyran content, it may be necessary to synthesize higher MW polymers by a more controlled/“living” free radical technique. PEG is transparent to both UV and green intensities. For the simulation shown in Figure 5, if the overall concentration of spirothiopyran and, hence, the absorbance of the resist is reduced by a factor of 5, no appreciable decrease in full feature size should occur over a depth of several tens of microns. Thus, our model suggests that, for this simple system, a standard 2 W green laser can pattern 45 nm wide lines over a 1 mm2 area with thickness exceeding 10 μm. In fact, for effective spirothiopyran concentrations below 1 mM, three-dimensional nanopatterning with subdiffraction feature sizes over a volume exceeding 100 μm3 is achievable using this writing system, in principle. Our current efforts are focused on experimental realization of the above stated predictions. This would entail overcoming technical challenges such as ensuring high-quality nodes, eliminating standing wave and polarization artifacts due to reflections and refraction,34 and optimizing composition to balance gel point and mechanical strength. Finally, the model considered above is a good approximation for understanding the response of the spirothiopyran-based writing system but carries with it some important simplifying assumptions. We are merely mapping out the concentration of the MAP in the resist spatially and temporally; the relationship between the extent of polymer cross-linking and insolubility (the critical “gel point”) is not expressly considered. For nanopatterning, the MAP concentration must lie below the corresponding gel point at the green wave maxima and above the gel point at the green wave minima. The gel point of the photoresist can be controlled by varying the relative concentration of the multiarm PEG maleimide polymer (cross-linker) with respect to spirothiopyran. The polymer characteristics that affect gel point will require precise optimization in order to achieve such photoresist contrast. Hence, the effective resolution of the photoresist will be highly sensitive to the copolymer composition, cross-linker concentration, and molecular weight. Using tools such as RAFT, ATRP, etc., which would also allow for much tighter control over polydispersity, we plan to tune these parameters to determine their effects on the overall resolution and mechanical properties of the spirothiopyran−maleimide photoresist. Another consideration is solvent choice. The extinction coefficients for spiropyran derivatives are highly sensitive to the solvents used (Figure S5).35 The kinetics of photoswitching and cross-linking using a spirothiopyran-functionalized polymer may also deviate from the rate constants determined for a simple spirothiopyran ethyl maleimide small-molecule model system (details in Supporting Information), which were used in the simulation. Additionally, the effects of photofatigue and/or photobleaching are not incorporated (although such technical hurdles have been solved by other groups in various contexts). To precisely quantify the aforementioned parameters, detailed rheological and kinetic studies are currently underway in our laboratories. Ultimately, our goal is to fabricate large-area periodic structures with subdiffraction feature sizes.
molecule studies using ethyl maleimide and spirothiopyran demonstrated the key characteristics required for superresolution lithography: the depletion regime exhibits an extremely low saturation threshold, and the writing step irreversibly and readily participates in cross-linking. A functioning photoresist was synthesized by copolymerization of a spirothiopyran-functionalized methacrylate monomer with PEGylated methacrylate monomer. This copolymer was crosslinked with a multifunctional maleimide under UV irradiation, and green light was shown to inhibit cross-linking, with excellent spatial and temporal control. Photokinetic simulations for 1D interference lithography using a 2 W 532 nm laser show that the spirothiopyran−maleimide system is capable of delivering 45 nm lines over an area extending several hundreds of microns. These experiments lay the groundwork and mark the first steps toward realizing a highly parallelized fabrication technique with nanoscale resolution, over large volumes in three dimensions.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b00506. Detailed synthesis and characterization details of compounds used in the spirothiopyran-PEG methacrylate polymer synthesis; experimental setup for saturation curve measurement and two-color photopatterning; explicit mathematical treatment of photokinetic simulation and details of experimental determination of photokinetic simulation parameters (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Chaitanya K. Ullal: 0000-0002-7178-897X Funding
This work was supported in part by the Engineering Research Centers Program of the National Science Foundation under NSF Cooperative Agreement no. EEC-0812056 and in part by New York State under NYSTAR contract C130145. This work is based in part upon work supported by the National Science Foundation under Grant no. 1610783. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Dr. Chang Ryu for graciously allowing us to use his laboratory facilities, Dr. Jananee Narayan for helpful discussions on the spirothiopyran polymer synthesis, Cansu Ergene and Dr. Sangwoo Lee for assistance in carrying out GPC measurements, and Dr. Liping Huang for providing access to her Coherent Verdi V2 laser.
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ABBREVIATIONS SPT, spirothiopyran; SP, spiropyran isomer; MC, merocyanine isomer; MAP, thiol-Michael addition product; STED, stimulated emission depletion microscopy; PEG, polyethylene glycol; NMR, nuclear magnetic resonance; MW, molecular weight;
CONCLUSIONS In conclusion, we have laid the foundation for a superresolution lithography system that combines photochromic switching with thiol-Michael addition chemistry. Small4759
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DMF, dimethylformamide; GPC, gel permeation chromatography
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DOI: 10.1021/acs.chemmater.7b00506 Chem. Mater. 2017, 29, 4754−4760
