Near-Infrared Light-Induced Deep Curing of Thiol–Epoxy Networks Based on Upconversion Photochemistry

Abstract

Thiol–epoxy photopolymerization offers exceptional advantages for high-performance protective coatings, yet efficiently curing thick formulations remains a significant challenge due to the limited penetration depth of conventional UV light. Herein, we report a novel near-infrared (NIR) light-activated photopolymerization system for deep-curing applications, strategically integrating upconversion nanoparticles (UCNPs) as NIR-to-UV converters, isopropylthioxanthone (ITX) as a photosensitizer, and a liquid N-phenylglycine-based photobase generator (NPG-TBD) with enhanced resin solubility. Upon 980 nm NIR irradiation, photogenerated TBD efficiently catalyzes thiol–epoxy polymerization through an anionic mechanism, enabling uniform network formation with epoxy and thiol functional group conversions greater than 90% throughout samples exceeding 2.5 cm in thickness. The resulting coatings exhibit excellent mechanical properties including 3H pencil hardness, strong adhesion (0 grade), and good flexibility (2 mm), significantly outperforming conventional UV systems limited to approximately 1.5 mm. Additionally, the cured materials demonstrate multifunctional characteristics including distinctive upconversion luminescence and dual-responsive shape memory behavior. This approach addresses critical limitations in deep-photocuring technology while offering significant potential for applications in protective coatings for marine infrastructure, chemical storage facilities, and smart materials requiring both substantial barrier properties and programmable responsiveness.

1. Introduction

Efficient photopolymerization systems for deep curing applications represent a fundamental challenge in coating technology, particularly for thick or pigmented protective surfaces [1]. Traditional ultraviolet (UV) light-induced polymerization approaches face inherent limitations due to their restricted penetration depth (typically < 500 μm), which severely compromises the through-cure capabilities in thick coatings while introducing additional challenges related to material degradation and potential safety concerns [2,3,4,5]. With the emergence of advanced applications in protective coatings, adhesives, and smart materials demanding uniform network formation throughout substantial thicknesses, overcoming this penetration limitation has become increasingly critical for industrial implementation [6,7,8,9,10].

Near-infrared (NIR) light-induced polymerization utilizing lanthanide-doped upconversion nanoparticles (UCNPs) has emerged as a promising strategy to address these constraints [11,12,13,14]. The strategic selection of UCNPs, particularly NaYF₄:Yb/Tm nanocrystals, offers several distinctive advantages [15,16]: (i) efficient conversion of deeply penetrating 980 nm NIR radiation into UV/visible emissions through sequential energy transfer processes, (ii) exceptional photostability enabling sustained photon upconversion during prolonged irradiation periods, and (iii) tunable emission profiles that can be precisely matched to specific photo-initiator absorption bands. These characteristics, combined with the significantly reduced tissue absorption and scattering of NIR light, enable deep photopolymerization while maintaining precise spatial and temporal control over the curing process [17,18,19].

Among various photopolymerization mechanisms, anionic systems offer distinct advantages for coating applications, including oxygen insensitivity, minimal shrinkage, and non-corrosive behavior toward metal substrates [17,20]. The thiol–epoxy click reaction proceeding via an anionic mechanism is particularly valuable for protective coatings due to its excellent chemical resistance and the formation of beneficial hydroxyl functionalities [18,21,22]. These hydroxyl groups significantly enhance coating adhesion to metal substrates while enabling subsequent functionalization for smart coating development [18,23,24,25].

Our group reported the first example of UCNP-assisted thiol–epoxy photopolymerization through a two-component system integrating UCNPs with a thioxanthone-based unimolecular photobase generator (TX-TBD) [26]. Thioxanthone-based unimolecular photobase generators, featuring covalent linkages between thioxanthone and carboxylate moieties, effectively eliminate diffusion-controlled processes and enhance electron transfer efficiency. The irreversible photodecarboxylation mechanism further suppresses back electron transfer, leading to improved base generation kinetics. However, these generators are severely constrained by limited solubility in coating formulations due to their high molecular weight and rigid molecular architecture, which has significantly restricted their practical application in industrial coating processes.

In this work, we present a novel three-component photosensitizing system that strategically integrates (i) UCNPs as efficient NIR-to-UV converters, (ii) isopropylthioxanthone (ITX) as a photosensitizer specifically designed to match the characteristic 345 and 361 nm UCNP emissions, and (iii) a novel N-phenylglycine-based photobase generator (NPG-TBD) featuring significantly enhanced resin solubility to overcome previous limitations. This liquid photobase generator NPG-TBD represents a significant advancement over crystalline alternatives, offering not only superior formulation compatibility but also efficient photodecarboxylation kinetics. Through a scanning-based NIR laser system, this design achieves uniform network formation throughout samples exceeding 2.5 cm in thickness, while the resulting networks exhibit unique dual-responsive shape memory behavior. This approach demonstrates new possibilities for advanced coating materials with integrated functionalities, particularly for applications requiring thick protective barriers and programmable shape transformation capabilities.

2. Materials and Methods

2.1. Materials

N-Phenylglycine (NPG), 1,5,7-triazabicyclo [4.4.0]dec-5-ene (TBD), and 4-isopropylthioxanthone (ITX) were obtained from Adamas. Bisphenol A diglycidyl ether (BADGE) was purchased from Macklin. Lanthanide-doped upconversion nanoparticles (UCNPs, NaYF₄:Yb/Tm) were supplied by Shanghai Scientific Optoelectronics Technology Co., Ltd., Shanghai, China. Pentaerythritol tetrakis (3-mercaptobutyrate) (PE-1) was acquired from Showa Denko K.K. in Japan.

2.2. Methods

2.2.1. Synthesis of NPG-TBD

NPG (0.15 g, 1.0 mmol), dichloromethane (20 mL), and TBD (0.14 g, 1.0 mmol) were sequentially added to a 50 mL round-bottom flask and stirred at room temperature for 1 h. The solution was concentrated by rotary evaporation, followed by freeze-drying to yield a pale yellow viscous product (0.27 g, 93.1% yield). ¹H NMR (400 MHz, Chloroform-d) δ 10.49 (s, 2H), 7.19–7.10 (m, 2H), 6.67–6.59 (m, 3H), 3.73 (s, 2H), 3.31 (t, J = 5.8 Hz, 4H), 3.26 (t, J = 6.0 Hz, 4H), and 1.98 (p, J = 5.9 Hz, 4H), 1.25 (s, 1H).

2.2.2. UV-Vis Absorption and Steady-State Photolysis

UV-vis absorption spectra of NPG-TBD and ITX in acetonitrile (concentration range: 10⁻³–10⁻⁵ mol/L) were recorded on a Shimadzu UV-3600 Plus spectrophotometer over the 200–800 nm range. Molar extinction coefficients (ε) were determined according to the Lambert–Beer law.

For the steady-state photolysis experiments, NPG-TBD/ITX acetonitrile solutions (3 mL) containing 0.3 wt% UCNPs were placed in sealed quartz cuvettes and subjected to intermittent 980 nm NIR irradiation (8.9 W/cm²). The sample temperature was kept below 60 °C. UV-vis spectra were collected at predetermined intervals to monitor photochemical changes.

2.2.3. ¹H NMR Characterization of Photolysis

Structural changes during photolysis were investigated using ¹H NMR spectroscopy. A solution of NPG-TBD/ITX (20 mg/mL) with 0.3 wt% UCNPs in DMSO-d₆ was prepared and transferred to an NMR tube. Spectra were collected before and after 980 nm NIR irradiation (17.85 W/cm² for 1 h) on a Bruker AVANCE III 400 MHz spectrometer, with the sample temperature maintained below 60 °C.

2.2.4. Evaluation of Photo-Induced Base Generation

A methanol solution containing NPG-TBD (10⁻³ mol/L), ITX (10⁻⁴ mol/L), and UCNPs (1 wt%) was subjected to intermittent 980 nm NIR irradiation (8.9 W/cm², 40 °C). The pH was continuously recorded in real time using a PHS-3CU pH meter.

For colorimetric observation, an acetonitrile solution of NPG-TBD (1 × 10⁻³ mol/L, 3 mL) containing 0.01 g UCNPs was irradiated under 980 nm NIR (10 W/cm²). Immediately after irradiation, three drops of saturated phenolphthalein indicator were added to visualize base generation through the characteristic color change.

2.2.5. Formulation of the NIR-Curable Thiol–Epoxy System

The thiol–epoxy resin system was prepared by mixing BADGE and PE-1 at a 1:1 molar ratio of epoxy to thiol functional groups. The photo-initiating system consisting of ITX (1 wt%), NPG-TBD (3 wt%), and UCNPs (1 wt%) was then incorporated into the resin mixture. The formulation was ultrasonically dispersed for 30 min under dark conditions to ensure homogeneous distribution of all components.

2.2.6. Monitoring of Photopolymerization by FTIR

Samples were irradiated with a 980 nm NIR laser (13 W/cm² for 10 min) and then analyzed by Fourier transform infrared (FTIR) spectroscopy in the 4000–500 cm⁻¹ range (32 scans, resolution 4 cm⁻¹). Subsequently, the specimens were heated at 100 °C, and time-resolved spectra were obtained at 5 min intervals. The carbonyl peak at 1720 cm⁻¹, which remains unchanged during polymerization, served as the internal reference. The peak areas of the sulfhydryl (2570 cm⁻¹) and epoxy (860 cm⁻¹) groups were normalized separately; the difference before and after irradiation was calculated accordingly.

2.2.7. DSC Analysis of Polymerization Kinetics

Polymerization kinetics were evaluated using differential scanning calorimetry (NETZSCH photo-DSC 204 F1Phoenit). Resin samples (20 mg) were pre-irradiated with 980 nm NIR (13 W/cm²) for various durations and analyzed under nitrogen atmosphere (20 mL/min flow rate) using two modes:

Non-isothermal mode: Temperature ramp from 30 °C to 160 °C at 5 °C/min.

Isothermal mode: Constant temperature maintained at 70 °C for 1 h.

2.2.8. Deep Photocuring Protocol and Conversion Profiling

A cubic PTFE mold (2.5 cm × 2.5 cm × 2.5 cm) was filled with the resin and irradiated using a near-infrared light source under the following scanning parameters: light intensity 27.2 W/cm², scanning speed 1 mm/s, 20 scans, line length 2.5 cm, and line width 0.25 cm. Afterward, the sample was post-cured in an oven at 100 °C for 30 min.

To examine curing efficiency at various depths, the cured samples were analyzed by attenuated total reflection FTIR (ATR-FTIR; resolution 8 cm⁻¹, 16 scans, range 4000–500 cm⁻¹). The normalized peak area ratios of the sulfhydryl (2570 cm⁻¹) and epoxy (860 cm⁻¹) groups were calculated separately using the stable carbonyl peak (1720 cm⁻¹) as the internal standard.

2.2.9. Preparation and Activation of Shape Memory Samples

Rectangular specimens (100 mm × 8 mm × 1 mm) were prepared in PTFE molds. Dynamic shape programming was carried out either by photothermal activation under near-infrared light (30 W/cm²) or by conventional thermal stimulation (120 °C).

2.2.10. Coating Application and Performance Evaluation

Tinplate substrates (150 mm × 70 mm × 0.28 mm, E4 tin coating, T52 hardness) were mechanically abraded with sandpaper, then thoroughly cleaned and dried. The coating formulation was applied using a BGD 206/3 film applicator and cured by 980 nm NIR scanning irradiation (27.2 W/cm², scan rate 1 mm/s, 20 cycles).

Coating properties were evaluated in accordance with national standards:

Gloss: Measured at a 60° angle using a BGD 516/3 gloss meter (GB/T 9754-2007) [27]. Five measurements were taken and averaged.

Adhesion: Assessed with a BGD 502/2A cross-cut tester (GB/T 9286-2021) [28]. Grade 0 indicates optimal adhesion.

Pencil hardness: Determined with a BGD 506/2 tester (GB/T 6739-2022) [29].

Flexibility: Evaluated via a BGD 564 cylindrical mandrel bender with diameters from 2 to 10 mm (GB/T 6742-2007) [30].

3. Results and Discussion

3.1. Design and Characterization of Base-Releasing System

N-phenylglycine (NPG) derivatives have been widely recognized as efficient hydrogen-donating co-initiators in various photopolymerization systems due to their exceptional electron-donating capabilities and favorable photochemical properties [31,32,33]. The α-amino acid structure of NPG, featuring an electron-rich nitrogen atom and a readily abstractable α-hydrogen, enables efficient electron transfer interactions with excited photosensitizers, particularly aromatic ketones [34,35]. Upon photo-initiation, NPG undergoes rapid decarboxylation through a well-established photochemical pathway: initial electron transfer to the excited sensitizer generates an NPG radical cation, followed by deprotonation at the α-carbon position and subsequent decarboxylation, yielding highly reactive carbon-centered radicals [36,37,38]. This mechanistic pathway offers several distinct advantages including (i) high quantum efficiency for radical generation, (ii) the rapid decarboxylation process that effectively suppresses back electron transfer, and (iv) the ability to function under relatively mild irradiation conditions with various wavelengths when paired with appropriate photosensitizers.

Based on these advantageous characteristics of NPG, we designed and synthesized a novel N-phenylglycine-based photobase generator (NPG-TBD) by incorporating the strong guanidine base 1,5,7-triazabicyclo [4.4.0]dec-5-ene (TBD) through a straightforward acid-base neutralization reaction (Figure 1). The synthetic approach involved simply mixing NPG and TBD at room temperature, resulting in salt formation through proton transfer from the carboxylic acid group of NPG to the basic nitrogen of TBD (Figures S1 and S2). Unlike conventional crystalline photobase generators with limited solubility (<3 wt%) [26], NPG-TBD exists as a viscous liquid at ambient temperature, providing exceptional solubility in epoxy and thiol-containing formulations. This enhanced solubility greatly facilitates formulation design and processability, thereby improving the versatility and scalability of thiol–epoxy systems in industrial applications. The molecular design strategically positions the TBD moiety to be released upon NIR-triggered photodecarboxylation, enabling precise spatial and temporal control over base generation.

As shown in Figure 2, UV-vis absorption spectroscopy revealed that ITX exhibits a strong absorption band centered at 380 nm (n–π* transition of the carbonyl chromophore) with additional peaks at 240–270 nm, while NPG-TBD shows absorption primarily in the deep UV region (248 nm and 299 nm). The β-NaYF₄:18%Yb, 0.5%Tm UCNPs display characteristic emission bands in the UV-A region at 345 and 361 nm (¹I₆ → ³F₄ and ¹D₂ → ³H₆ transitions of Tm³⁺, respectively) [39].

The absorption profile of ITX demonstrates good spectral overlap with the UCNP emission peaks, ensuring efficient absorption of upconverting fluorescence under 980 nm NIR excitation. The high molar extinction coefficient of ITX (ε = 3.2 × 10⁴ L·mol⁻¹·cm⁻¹ at 380 nm) facilitates effective capture of the upconverted photons. While NPG-TBD shows minimal direct absorption in the UCNP emission region, its photochemical activation occurs primarily through electron transfer from excited ITX molecules.

3.2. Investigation of Photo-Induced Base Generation

To elucidate the photolysis mechanism, steady-state photolysis of NPG-TBD/ITX in acetonitrile was conducted under both 980 nm NIR laser (with UCNPs) and 405 nm LED irradiation (Figure 3). To minimize potential thermal effects during NIR exposure, an intermittent irradiation protocol was implemented with sample temperature maintained below 80 °C throughout the process.

Under 405 nm irradiation (Figure 3a), a blue shift in the absorption peak at 285 nm was observed, along with enhanced absorption around 400 nm, corresponding to the consumption of ITX during the photosensitization process. The NIR irradiation (Figure 3b) produced similar spectral changes, though requiring longer exposure time (30 min) due to the limited upconversion efficiency. This limitation stems primarily from the multiphoton nature of the energy-transfer process of UCNPs, coupled with surface quenching phenomena and potential lattice distortions [40,41,42,43]. The comparable spectral evolution under both irradiation conditions confirms that the NIR-triggered photolysis follows the same mechanistic pathway as direct LED activation. The slight inconsistency in absorption peaks at around 300 nm between the two light sources can be attributed to light scattering effects from residual UCNPs in solution after centrifugation prior to measurement.

To further confirm the photolytic mechanism, ¹H NMR spectroscopy was employed to analyze the structural changes in the NPG-TBD/ITX system before and after irradiation (Figure 4). The spectrum of the NIR-irradiated sample revealed the selective disappearance of the characteristic methylene proton signal of NPG at 3.37 ppm, while the proton signals associated with TBD (1.87, 3.15, 3.25 ppm) remained intact. Notably, the NMR spectra of both UV-405 nm and 980 nm-NIR irradiated samples exhibit consistent changes, with both showing the selective disappearance of the 3.37 ppm peak, further confirming that both irradiation methods follow the same photolytic mechanism. This selective cleavage pattern provides direct evidence for the proposed photodecarboxylation mechanism. Importantly, the control sample subjected to thermal treatment (80 °C for 1 h) exhibited no significant spectral changes compared to the original sample, confirming that the observed decomposition under NIR irradiation was indeed photochemically driven rather than thermally induced.

To quantitatively evaluate the generation of the active base species under NIR irradiation, real-time pH monitoring and phenolphthalein colorimetry experiments were conducted (Figure 5). Upon exposure to 980 nm NIR irradiation, the methanol solution containing NPG-TBD/ITX and UCNPs exhibited a significant pH increase from 7.71 to 9.66 within 20 min (Figure 5a), indicating the progressive release of the strong base TBD. This pH change corresponded directly with a visible color transformation in the phenolphthalein-containing solution from pale yellow to orange, as evidenced by the marked increase in absorption at approximately 570 nm (Figure 5b). Control experiments, in which any one of the three essential components (NPG-TBD, ITX, or UCNPs) was omitted from the system, showed no appreciable pH change or color transformation under identical NIR irradiation conditions. These results conclusively demonstrate that all three components are necessary for effective NIR-triggered base generation, confirming the proposed synergistic mechanism involving upconversion, photosensitization, and subsequent photodecarboxylation processes.

To further elucidate the thermodynamic feasibility of the electron transfer process between NPG-TBD and ITX, the redox properties of both components were characterized and summarized in

Table 1. Data of photophysical and electrochemical properties of the NPG-TBD and ITX.

Photo-Initiator	λmax nm	εmax L/(mol·cm)	λs nm	${\mathit{E}}_{\mathit{o}\mathit{x}}$ eV	${\mathit{E}}_{\mathit{r}\mathit{e}\mathit{d}}$ eV	${\mathit{E}}_{\mathit{s}}$ eV	$\mathit{∆}\mathit{G}$ eV
NPG-TBD	299	2005	-	0.6	−1.212	-	-
ITX	383	6860	398	1.441	−0.837	3.116	−0.463

, Figures S3 and S4. The negative Gibbs free energy change (ΔG = −0.463 eV) confirms that the electron transfer from NPG-TBD (oxidation potential: 0.6 eV) to the excited state ITX is energetically favorable, providing the thermodynamic driving force for the subsequent photodecarboxylation process.

Based on the spectroscopic investigations and established literature on analogous PBG systems [44,45,46], we propose a mechanism for NIR-induced base generation, as illustrated in Figure 6. Initially, UCNPs absorb 980 nm NIR radiation and emit UV-vis light. These upconverted photons excite ITX to the singlet state (¹ITX*), which rapidly undergoes intersystem crossing to form the longer-lived triplet state (³ITX*). The triplet state ITX then abstracts an electron from NPG-TBD (supported by the favorable ΔG = −0.463 eV), generating an NPG radical cation that undergoes sequential deprotonation and decarboxylation. This process releases free TBD (the active base catalyst), carbon dioxide, and a phenylamine radical species which is able to induce free radical polymerization. The spectroscopic changes, pH measurements, and colorimetric tests collectively confirm this electron-transfer-initiated photodecarboxylation mechanism, enabling NIR-triggered base generation for deep-curing applications.

3.3. Polymerization Study

To evaluate the catalytic activity of photogenerated TBD in thiol–epoxy polymerization, differential scanning calorimetry (DSC) analysis was conducted on formulations subjected to varying NIR irradiation times (13 W cm⁻²). Figure 7a reveals a progressive reduction in polymerization onset temperature with increasing irradiation duration, with the exothermic peak shifting significantly toward lower temperatures after 30 min of exposure. This temperature shift directly correlates with the concentration of photo-released TBD catalyst. Complementary isothermal DSC measurements at 70 °C (Figure 7b) further demonstrate the dramatic acceleration of curing kinetics in irradiated samples. While the unirradiated reference formulation exhibited minimal reactivity at this temperature, samples pre-irradiated for 30 min showed pronounced exothermic activity with sharply reduced induction periods. The progressive enhancement of polymerization rate with increasing irradiation time provides evidence for the effective photogeneration of catalytically active TBD and confirms its central role in initiating the anionic thiol–epoxy click reaction.

Beside DSC, FTIR was employed to further study the photothermal dual curing of the base-catalyzed thiol–epoxy polymerization. As shown in Figure 8a, during the initial NIR irradiation phase (13 W cm⁻²), both functional groups exhibited relatively modest conversion rates (below 20%). Upon subsequent thermal treatment at 120 °C, polymerization dramatically accelerated, ultimately achieving final conversions of 99% and 90% for epoxy and thiol groups, respectively. This photothermal synergistic mechanism provides both spatial control through photoactivation and optimal network formation via thermal acceleration.

Building upon the established advantages of NIR light’s superior penetration depth due to reduced Rayleigh scattering compared to UV-visible radiation, we further explored the scalability of UCNPs-assisted thiol–epoxy photopolymerization. While our previous work demonstrated remarkable curing depths (11.8 cm) using concentrated NIR laser point sources [26], practical applications in coating processes demand capabilities for large-area processing. To address this challenge, we developed a scanning-based approach utilizing an NIR line laser system.

The photosensitive formulations were cast into a cubic PTFE mold (2.5 cm × 2.5 cm × 2.5 cm) and subjected to exposure using a 980 nm NIR line laser mounted on a programmable conveyor system (Figure S5). Through optimized scanning parameters (scanning speed: 1 mm s⁻¹, number of scans: 20, line length: 2.5 cm, line width: 0.25 cm, light intensity: 27.2 W cm⁻²), followed by thermal post-curing at 100 °C for 30 min, uniform network formation was achieved throughout the bulk sample (Figure 8b). In contrast, conventional ultraviolet (UV) curing suffers from limited penetration depth (<500 μm), resulting in inadequate photocuring depth and often leading to uneven material properties and compromised mechanical performance [1,47,48]. By comparison, our approach successfully achieved a 2.5 cm photocuring depth with excellent material uniformity.

Depth-profiled FTIR analysis (Figure 8c) revealed consistent reduction patterns of characteristic epoxy and thiol absorption bands throughout the sample volume, indicating uniform network formation regardless of depth from the irradiated surface. Quantitative conversion analysis (Figure 8d) confirmed this uniformity, with both epoxy and thiol functional groups exhibiting consistent conversion percentages across the entire 2.5 cm depth profile. This approach enables the fabrication of thick, uniformly crosslinked coatings showing great potential in applications where coating thickness directly correlates with barrier performance and service longevity. Control experiments using 405 nm LED failed to achieve bulk curing, highlighting the unique deep-penetration capabilities of the NIR-induced system.

3.4. Multifunctional Properties of the Polymeric Materials

UCNPs can play not only the role of internal light sources during photopolymerization but also function as luminescent fillers embedded within the formed polymeric networks. Under 980 nm NIR irradiation, the cured sample becomes a “glowing strip” emitting bright bluish violet light (Figure 9a). Notably, this strong fluorescence is solely derived from UCNPs, as ITX undergoes intersystem crossing (ISC) with high quantum yield without fluorescence [49]. This assertion is supported by the observation that no comparable fluorescence can be observed when the sample is irradiated under UV-405 nm light (Figure 9b).

The crosslinked thiol–epoxy networks, featuring a unique molecular architecture comprising flexible thioether linkages and rigid aromatic segments, exhibit excellent shape memory properties [50]. To demonstrate this functionality, photocured strips (100 mm × 8 mm × 1 mm) were fabricated (Figure 9c) and subsequently deformed into a prescribed temporary configuration when heated above the glass transition temperature (Figure 9d).

The shape memory behavior can be triggered through two distinct pathways: conventional thermal activation and NIR-induced recovery. Under direct thermal stimulus, the temporary shape (Figure 9c) undergoes complete recovery to its original configuration (Figure 9g), driven by the entropic elasticity of the crosslinked network. The thermal-induced shape memory performance demonstrates excellent cycling stability, maintaining complete shape recovery even after multiple consecutive deformation-recovery cycles.

However, conventional thermal activation suffers from several inherent limitations, including slow heating rates, poor spatial control, and inability to achieve selective actuation in specific regions. These limitations can be effectively addressed through NIR-triggered shape recovery (Figure 9e,f). Upon 980 nm NIR irradiation, the sample temperature rapidly increases to the transition temperature through a combination of direct NIR absorption and UCNP-mediated thermal conversion, enabling spatiotemporal and programmable control over the shape recovery process. Multiple temporary shapes can be sequentially recovered through controlled NIR exposure patterns, offering unprecedented flexibility in shape transformation design.

Although shape memory materials activated by near-infrared (NIR) light have been extensively investigated [51,52], the invisibility of NIR radiation to the naked eye creates challenges for achieving precise shape memory effects. By leveraging UCNPs with their distinctive blue upconversion luminescence, this approach enables real-time visual guidance for the otherwise invisible NIR activation process (Figure 9e). This unique visualization capability ensures accurate manipulation of the shape transformation, particularly valuable for applications requiring precise spatial control.

This dual-responsive shape memory behavior, combined with the deep-curing capabilities and visual monitoring function, demonstrate significant potential for applications such as smart robotics, biomedical devices, and adaptive structures where programmable shape transformation and real-time visualization are essential for precise actuation and control.

3.5. Coating Performance Analysis

The coating performance metrics outlined in

Table 2. Mechanical properties of thiol–epoxy coatings with varying thicknesses prepared by NIR and LED curing methods.

Parameter	Thickness/μm
	50	100	150	1500 a	1500 b	2000
Adhesion (grade)	0	0	0	-	-	-
Pencil hardness	3H	3H	3H	3H	H	3H
Flexibility (mm)	2	2	2	2	6	4

^a under 980 nm NIR irradiation; ^b under 405 nm LED irradiation.

demonstrate exceptional consistency across varying thicknesses for conventional thin coatings (50–150 μm). The adhesion performance (0 grade, indicating excellent adhesion with no detachment), pencil hardness (3H), and flexibility (2 mm mandrel bending without cracking) remain remarkably stable across these thickness ranges, highlighting the uniform network formation throughout the coating volume.

Most significantly, the NIR-based photopolymerization system demonstrates good capability for ultra-thick coating preparation, successfully producing uniform coatings up to 2000 μm thick with maintained hardness (3H) and only moderate reduction in flexibility. In contrast, conventional approaches using 405 nm LED irradiation cannot effectively produce coatings beyond approximately 1500 μm. Furthermore, the LED-cured thick coatings exhibit notably inferior performance characteristics compared to the NIR-cured counterparts of equivalent thickness, with compromised hardness and substantially decreased flexibility.

The capability to significantly increase coating thickness has profound implications for anti-corrosion coatings, abrasion-resistant coatings, and thermal insulation coatings, where barrier properties directly correlate with thickness. This advancement is particularly crucial for demanding application scenarios such as offshore marine infrastructure, chemical storage tanks, and aerospace components that operate in extreme environments where conventional thin coatings provide insufficient protection against aggressive chemical agents and mechanical wear.

4. Conclusions

In this work, we have successfully developed a novel three-component photosensitization system for NIR-induced deep curing of thiol–epoxy networks. The strategic integration of lanthanide-doped UCNPs, photosensitizer ITX, and a liquid N-phenylglycine-based photobase generator (NPG-TBD) enables efficient photopolymerization with superior solubility and formulation compatibility. Through a photosensitization mechanism with favorable thermodynamics (ΔG = −0.463 eV), the system effectively generates the strong organic base TBD under NIR irradiation, catalyzing uniform thiol–epoxy polymerization. The developed approach enables consistent network formation throughout 2.5 cm cubic samples with exceptional functional group conversion, demonstrating the system’s potential for practical applications in thick coating fabrication. The successful preparation of coatings up to 2 mm in thickness with maintained mechanical properties (3H hardness) significantly outperforms conventional LED-cured systems limited to thinner applications. Moreover, the observed dual-responsive shape memory behavior, triggered by both thermal and NIR stimuli with real-time visual monitoring through upconversion luminescence, shows promising potential for smart protective coatings.