Visible‐Light‐Driven Rapid 3D Printing of Photoresponsive Resins for Optically Clear Multifunctional 3D Objects

Light‐driven 3D printing is gaining significant attention for its unparalleled build speed and high‐resolution in additive manufacturing. However, extending vat photopolymerization to multifunctional, photoresponsive materials poses challenges, such as light attenuation and interference between the photocatalysts (PCs) and photoactive moieties. This study introduces novel visible‐light‐driven acrylic resins that enable rapid, high‐resolution photoactive 3D printing. The synergistic combination of a cyanine‐based PC, borate, and iodonium coinitiators (HNu 254) achieves an excellent printing rate and feature resolution under low‐intensity, red light exposure. The incorporation of novel hexaarylbiimidazole (HABI) crosslinkers allows for spatially‐resolved photoactivation upon exposure to violet/blue light. Furthermore, a photobleaching mechanism inhibited by HNu 254 during the photopolymerization process results in the production of optically‐clear 3D printed objects. Real‐time Fourier transform infrared spectroscopy validates the rapid photopolymerization of the HABI‐containing acrylic resin, whereas mechanistic evaluations reveal the underlying dynamics that are responsible for the rapid photopolymerization rate, wavelength‐orthogonal photoactivation, and observed photobleaching phenomenon. Ultimately, this visible‐light‐based printing method demonstrates: (i) rapid printing rate of 22.5 mm h−1, (ii) excellent feature resolution (≈20 µm), and (iii) production of optically clear object with self‐healing capability and spatially controlled cleavage. This study serves as a roadmap for developing next‐generation “smart” 3D printing technologies.


Introduction
Recently, light-based 3D printing, commonly referred to as a vat photopolymerization, has garnered significant attention as a exchange reactions or reversible covalent bond breakage and recombination in response to external triggers like heat, light, pH, and chemicals. [4]The incorporation of dynamic covalent bonds into additive manufacturing (AM) provides a promising solution to existing challenges, thereby paving the way for the fabrication of next-generation designer materials. [5]Recent research has delved deeply into this field, leading to successful examples that integrate desirable properties, such as reshaping ability, [6] selfhealing capability, [7] and recyclability, [8] into 3D printed objects.Among these various stimuli, light has gained considerable attention due to its capacity for localized activation in both time and space, as well as its potential to trigger reaction remotely.Most photochemical reactions can be conducted under ambient conditions, making them viable in heat-sensitive environments such as displays and semiconductor materials. [9]Moreover, when combined with vat polymerization, this approach offers the advantage of accurately and rapidly producing 3D objects that respond to light.Despite these advantages, 3D printing of photoresponsive materials within the framework of vat photopolymerization remains relatively unexplored.While the creation of photoresponsive 3D objects using DLP 3D printing has been reported by Zhang et al. and Honda et al., [10] the approaches described in these studies did not fully harness the potential of vat photopolymerization, including fast build speeds and high printing resolutions.
Employing light as the trigger mechanism poses inherent challenges when striving for effective 3D printing of photoresponsive materials.The inclusion of multiple chromophores comprising photoactive moieties and PISs within a single resin can lead to unintended interference.This interference can involve electron/energy transfer between the photoactive moieties and PISs, and/or potential light attenuation due to the spatial overlap between the absorption spectra of the photoresponsive functional groups and the emission spectra of the light source. [11]These issues can result in reduced printing rate, compromised feature resolution, and sluggish activation rate for photoactive units.For instance, Zhao et al. employed blue light (490 nm) for network formation, which, to the best of our knowledge, is the longest incident light wavelength reported to date.Subsequently, violet light (405 nm) was used to activate the photoresponsive units within the printed 3D objects.10e] One potential solution is to employ light sources with longer wavelengths for 3D printing to ensure absorption orthogonality with the photoresponsive component.In addition, it is crucial to gain a deep understanding of the operating mechanisms, and the photochemical and photophysical interactions of chromophores, in their roles as both PISs and photoresponsive materials.This understanding is vital to achieve wavelength-orthogonal activation, effectively minimize cross-reactivity, and attain a rapid photopolymerization rate.
Moreover, a significant challenge arises from the coloration of the printed object, which is a result of the need to incorporate substantial amounts of photocatalysts (PCs) for visible-light absorption.While several strategies, such as the development of highly efficient PISs that require minimal amounts of PCs, [12] or the design of PCs capable of photobleaching, [13] have been successfully implemented in the broader field of photopolymerization, their adoption in vat photopolymerization has rarely been reported.
In this study, we present a visible-light-driven photochemistry that enables the 3D printing of acrylic resins containing photoresponsive components with remarkable printing rate (22.5 mm h −1 , 8 s/layer) and excellent feature resolution (≈20 μm).Notably, this method produces multifunctional 3D printed objects that are optically clear, capable of changing color, possessing self-healing properties, and even undergoing selective degradation in response to light.To embed these light-responsive multifunctional properties, we integrated a novel hexaarylbiimidazole (HABI)-based crosslinker, designed to respond to visible light (≤455 nm), into a red-light-curable acrylic resin.To mitigate interference from the HABI moiety and ensure optical transparency, we selected a red-light-absorbing cyanine analog as a PC, given its nonoverlapping spectrum with HABI and a well-established photobleaching pathway.Additionally, we carefully chose the coinitiators, taking into account the redox potential of the PC, to achieve a high printing rate and outstanding feature resolution.This process was implanted using a DLP system equipped with a red LED (620 nm).The efficacy of our approach was validated through real-time Fourier transform infrared (RT-FTIR) spectroscopy, confirming the rapid photopolymerization of the HABIcontaining acrylic resin under red light irradiation.A comprehensive mechanistic evaluation was conducted to elucidate the underlying dynamics responsible for the rapid photopolymerization rate and the resulting photobleaching phenomenon.

Results and Discussion
To achieve high-performance photoactive 3D printing characterized by fast build speed, exceptional feature resolution, and rapid photoactivation, we developed novel panchromatic photocurable resins.These resins are a combination of red-light-absorbing PISs, photoresponsive components (namely, HABI analogs) activated by violet/blue light, and acrylic resins.Earlier approaches predominantly used violet/blue light to incorporate UV/violet light-active components, resulting in a lower curing rate, diminished feature resolution, and slow photoactivation.These limitations likely stem from the spatial overlap of absorption peaks for both the photoactive moiety and PISs, and/or unintended photochemical and photophysical interactions between them.Previous studies in this area are summarized in Table S1 in the Supporting Information.In this study, we examined a three component PIS (i.e., PC + two coinitiators) that absorbs red light to achieve rapid visible-light curing and spatiallyresolved absorption with HABI analogs.The PIS comprised red-light-absorbing cyanine derivatives (HNu 640MP) as the PC, 2-(butyryloxy)-N,N,N-trimethylethan-1-aminium butyltriphenylborate (Borate V) as the electron donor (D) coinitiator, and [4-(octyloxy)phenyl](phenyl)iodonium hexafluoroantimonate (HNu 254) as the electron acceptor (A) coinitiator.HNu 640MP is composed of 30-40 wt% 1-butyl-2- [5-(1-butyl-1,3-dihydro-3,3-dimethyl-2H-indol-2-ylidene)-penta-1,3-dienyl]-3,3-dimethyl-3Hindolium hexafluorophosphate (Infrared absorber 643, namely, IA 643), and 60-70 wt% of Borate V. To eliminate the influence of Borate V present in HNu 640MP, we chose to employ IA 643 rather than HNu 640MP for a more in-depth mechanistic study.The optimal composition for PIS in this study, unless otherwise specified, consists of 0.1 wt% HNu 640MP, 0.2 wt% Borate V, and 2 wt% HNu 254 (see Table 1 for their chemical structures).Notably, the optimal resin composition was determined with the photobleaching process of the cyanine dye in mind, which is to be performed post-photocuring.For this study, two HABI derivatives were synthesized and utilized both as crosslinkers and photoactive moieties: bis(acryloyl)-functionalized HABI (BA-HABI) and tetrakis(acryloyl)-substituted HABI (TKA-HABI).Further details can be found in the Supporting Information (see "Synthesis of HABI derivatives" section and Figures S1-S7, Supporting Information).Additionally, 2,2′-bis(2-chlorophenyl)-4,4′,5,5′tetraphenyl-1,2′-biimidazole (o-Cl HABI) was employed as an additive for a control study (Table 1).Isobornyl acrylate (IBOA) and 2-ethylhexyl acrylate (EHA) served as the resin monomers.We effectively controlled network stiffness by adjusting the ratio of stiff IBOA (with a glass transition temperature (T g ) of ≈94 °C in its homopolymer form) and soft EHA (with a T g of approximately −50 °C in its homopolymer form) in combination with the HABI crosslinker.HABI derivatives were selected as the photoactive units due to their unique attributes and the absence of overlap in their absorption bands with those of PISs.These HABI derivatives displayed a faint yellow color, indicative of their absorption in the blue region of the spectrum (Figure 2a).HABIs are photochromic compounds that undergo reversible, homolytic cleavage of the carbon−nitrogen (C−N) bond between the imidazole rings when exposed to UV light.This process efficiently yields two indigo-colored 2,4,5-triarylimidazoyl (lophyl) radicals.Remarkably, these lophyl radicals can recombine to regenerate the original imidazole dimer without requiring an external trigger (refer to Figure 1c and Figures S8-S10 in the Supporting Information which display photographs of the HABI solutions).Unlike most organic radicals, the lophyl radicals produced through HABI homolysis are insensitive to atmospheric oxygen and have extremely slow recombination rates.These characteristics are attributed to their distinctive chemical structures, which provide stabilization through steric hindrance and electron delocalization.Given their exceptional stability, slow recombination rate, and reversibility, HABI derivatives are well-suited for rapid photoactivation at low irradiation intensities compared to other well-known photoactive compounds, such as cinnamate and coumarin derivatives. [14]14b,15] Furthermore, lophyl radicals can abstract hydrogen when hydrogen-donating molecules (H-donors) are present, transitioning into irreversible imidazole compounds.In the absence of H-donors, HABI itself showcases no initiation activity in (meth)acrylate formulations.Upon photoexcitation, lophyl radicals recombine with propagating radicals, effectively suppressing free-radical polymerization in (meth)acrylate formulations.Consequently, HABIs are also referred to as photoinhibitors and have been used in various applications, including rapid and continuous AM, [16] sub-diffraction photolithography, [17] and others.Despite their remarkable attributes, conventional UV/violet light sources, in which HABIs mainly function as photoinhibitors, are unsuitable for printing HABI-based dynamic 3D objects.
We hypothesized that photoactivation of the HABI crosslinker upon violet/blue light irradiation and red-light-induced photopolymerization, driven by HNu 640MP and the two coinitiators, would operate independently due to their distinct absorption spectra.During red light irradiation, HABI remains inactive, contributing neither to free-radical polymerization nor functioning as a photoinhibitor.Figure 2a illustrates the UV-vis absorption spectra of the HABI-based crosslinker (TKA-HABI; violet, solid line) and HNu 640MP (red, solid line), along with the emission spectra of the violet LED (405 nm, violet, dashed line) and the red LED (620 nm, red, dashed line).Although the absorption spectrum of the HNu 640MP aligned well with the emission spectrum of the red LED, it did not match the absorption spectrum of TKA-HABI.The absorption spectrum of the HABI derivative partially overlapped with that of the violet LED, facilitating rapid photoactivation following red-light-induced curing (or network formation).To validate this hypothesis, RT-FTIR was employed to monitor the conversion of the vinyl peak in the acrylic resin over time.To preclude dark polymerization, the LED light was turned on 10 s after the experiment began.In the absence of HABI, a rapid photopolymerization (10.9% s −1 ) was observed under 405 nm irradiation (3 mW cm −2 ).However, with 10 wt% of TKA-HABI present, the acrylate conversion was negligible (<5%) (see Figure 2b; Table S2, Supporting Information).It was assumed that the lophyl radicals produced upon blue light exposure recombined with the propagating radicals, thereby preventing the progress of free radical polymerization, which is consistent with our predictions.Subsequently, we evaluated the red-light-reactive PIS (Figure 2c; Table S2, Supporting Information).Regardless of the presence of HABI, no generation of lophyl radicals was observed during red light irradiation, yielding a rapid photocuring rate (4.6 and 4.2% s −1 in the presence and absence of HABI, respectively).We tested a range of incident light wavelengths (365-620 nm) to determine the impact of incident light wavelength on the photopolymerization rate (see Figure S11 and Table S3, Supporting Information).Upon exposure to light at 365, 405, and 455 nm, only negligible polymerization rates were observed.However, the photocuring rate increased with the application of longer wavelengths, with the rates as follows: 0.2% s −1 (455 nm), 0.08% s −1 (405 nm), and 0.0% s −1 (365 nm).The red-light-absorbing PIS exhibited the highest polymerization rate (4.6% s −1 ).We also assessed the various compositions of the three-component system for resin optimization, with the detailed results presented in Figures S12 and S13 and Table S4 in the Supporting Information.
To impart adaptability to the cross-linked network, the HABI component must be incorporated into the backbone of the polymer strands.To achieve this, we synthesized new bis(acryloyl)-and tetrakis(acryloyl)-functionalized HABI crosslinkers equipped with two and four polymerizable acrylic groups linked to the HABI core, as depicted in Table 1.Before deploying the HABI crosslinkers for 3D printing applications, we conducted spectroscopic analyses.This was done to understand the influence of substituents and HABI structure on factors such as the absorption wavelength, lophyl radical generation capability, and kinetics of HABI photolysis, as well as the subsequent dark recombination of lophyl radicals.The UV-vis spectrum of o-Cl HABI reveals a prominent absorption band around 270 nm, attributed to the -* transition of biimidazole.Notably, the solutions of both HABI crosslinkers (that is, BA-HABI and TKA-HABI) displayed a significant bathochromic shift compared to o-Cl HABI.This shift stems from the introduction of electron-donating groups (EDGs), as evidenced by the increased absorption intensities in the visible region, particularly at 405 nm.The order of absorption enhancement was as follows: TKA-HABI (0.18) > BA-HABI (0.09) > o-Cl HABI (0.08).The absorption tail of TKA-HABI extended beyond 450 nm in the blue spectrum, as shown in Figure 3a.Under irradiation at 405 nm (10 mW cm −2 for 3 min), new absorption bands were detected in the 500-600 nm range, indicating the generation of lophyl radicals (Figure 3b).As more EDGs were incorporated into the HABI core, peak intensities at  max (550, 592, 585 nm for o-Cl HABI, BA-HABI, TKA-HABI, respectively) increased.S6, Supporting Information).
To further investigate the influence of the substituents and structure of HABIs on their properties, we conducted kinetic experiments using UV-vis spectroscopy to study photodissociation and lophyl radical recombination.Since the lophyl radical absorption peak does not overlap with the absorbance spectra of the original HABI solutions, we simply monitored  max (Figure 3d).Under 405 nm irradiation, the radical concentrations in all samples rapidly increased to plateau values, then decreased to undetectable levels within several minutes to tens of minutes after the irradiation ceased.Among these, TKA-HABI displayed the most potent lophyl radical generation capability, consistent with the findings from our UV-vis absorption and ESR spectroscopy studies.The equilibrium peak intensities at  max of HABI solutions were 0.38, 0.26, and 0.15 for TKA-HABI, BA-HABI, and o-Cl HABI, respectively.This is likely due to the enhanced solubility of the functionalized HABI crosslinkers and their corresponding lophyl radicals in the medium, coupled with superior photon absorption, as is evident from the increased overlap with the 405 nm LED emission (Figure 3f).Notably, of the three HABIs examined in this study, TKA-HABI exhibited the lowest recombination rate.These patterns persisted under 365 and 455 nm irradiation, implying that, regardless of the irradiation wavelength, the recombination dynamics were primarily governed by the structural characteristics of HABI (Figures S19 and S20, Supporting Information).Owing to its superior lophyl radical generation capability and slow recombination rate, TKA-HABI emerged as the primary HABI cross-linker candidate for subsequent experiments.Furthermore, TKA-HABI displayed remarkable fatigue resistance, as validated by a repetitive cycle test (as shown in Figure 3e and additional Figures S21 and S22, Supporting Information).
Three HABI crosslinkers were introduced as photoactive components into a red-light-curable acrylic resin, with pentaerythritol tetraacrylate (PETTA) serving as a nonphotoreactive crosslinker for the control study (Table 2).The monomers comprised a mixture of EHA and IBOA.In Entry 2, the nonfunctionalized HABI additive (o-Cl HABI) was incorporated to analyze its impact on the photopolymerization behavior while maintaining the same concentration (3.1 × 10 −4 mol, 2 wt%) of PETTA to assess the influence of crosslinking density on the photocuring rates.Entry 3 and 4 contained the same concentration (3.1 × 10 −4 mol, 5.5 and 10 wt%, respectively) of BA-HABI and TKA-HABI, respectively, ensuring the same HABI concentration in the resin formulation.Figure 4a illustrates the conversion of the vinyl groups over time.Entry 1 and 2 showcased nearly identical photocuring trajectories.The introduction of the HABI additive (o-Cl HABI), which may function as a plasticizer within the resin, slightly enhanced the photocuring rate and maximum curing rate (M p ) compared to those in Entry 1. Incorporating HABI crosslinkers (BA-HABI in Entry 3 and TKA-HABI in Entry 4) slightly reduced the induction period, suggesting that HABIs might play a minor role in the photoinduced electron/energy transfer reactions.However, no significant changes were observed in the photocuring or maximum curing rates (M p ).To verify the stability of HABI during the polymerization process and its wavelength orthogonality within the acrylic resin, an excessive amount of HABI was introduced into the resin formulation, which was subsequently subjected to RT-FTIR spectroscopy (Figure 4b).Remarkably, even at elevated concentrations of TKA-HABI (10, 30, and 50 wt%), the photocuring behavior remained consistent, as summarized in Table S5 in the Supporting Information.
The key parameters in AM, especially for vat photopolymerization, revolve around attaining rapid build speeds, while maintaining high feature resolution.For our system to be on par with conventional UV/violet-based DLP printing, [1a] and visible-light 3D printing, [11d,18] it should deliver features with a lateral resolution of ≤100 μm and a build speed of ≥10 mm h −1 .11d] In this evaluation, each resolution print included a set of squares, all printed simultaneously, but with varying exposure times/layers (Figure 4c).The lower half of each square featured a series of smaller patterns, ranging from 1 to 16 pixels width (with each pixel measuring 20 μm).This setup enabled us to measure the lateral (x, y) resolution (Figure 4d).The thickness of all samples was 50 μm.c) Digital projection layer for the "resolution print" method at 3 s when all squares were illuminated simultaneously (white regions correspond to exposure).d) An expanded view of a square with 8 s exposure/layer, displaying the line and pixel array for the projection.Resolution print obtained from e) Entry 1 and f) Entry 4, with magnified image observed using optical microscopy.The thickness of each layer was 50 μm.
The resolution prints for Entry 1, 2, 3, and 4 are displayed in Figures S25-S28 in the Supporting Information.At 8 s of exposure time/layer (corresponding to a build speed of 22.5 mm h −1 without repositioning and recoating processes), all resolution prints exhibited excellent feature resolution, with distinctly visible 20 μm wide pixels and line arrays (Figure 4e,f).Magnified optical microscopic images showcased line pattern spanning one, two, and four pixels (each pixel being 20 μm wide).Their measured widths (20.0 ± 1.2, 39.6 ± 2.0, 80.3 ± 1.4 μm, respectively) aligned closely with theoretical values.Additionally, we created an array of squares to determine the minimum distance between lines.As depicted in Figure S29 in the Supporting Information, the minimum achievable distance was approximately 21 μm, consistent with our previous findings.It is important to note that this limitation arises from the printer's pixel size, not our resin.Consequently, the rapid build speed and superior feature resolution achieved through our novel, photoactive, visible-light-driven 3D printing method are comparable to those of conventional UVbased DLP 3D printers.
We then conducted a mechanistic study to identify the factors contributing to rapid polymerization kinetics; to remove the influence of Borate V present in HNu 640MP, we instead used IA 643 as the PC for the characterization and mechanistic study.Initially, the redox potentials of cyanine-based PC and the coinitiators were measured through cyclic voltammetry (CV) using acetonitrile (CH 3 CN) as the solvent (Figure S30, Supporting Information).Additionally, the photophysical properties of PC were determined by UV-vis absorption and photoluminescence (PL) spectroscopy in ethyl acetate (EtOAc), as illustrated in Figure S31 in the Supporting Information.Subsequent measurement of the electron transfer rate constants (k PET , M −1 s −1 ) between photoexcited PC (E red * = 1.05 V vs SCE) and the coinitiators were facilitated by Stern-Volmer plots, which monitored the changes in the maxima of steady-state PL emission intensity in EtOAc at room temperature (Figure 5b; Figure S33a, Supporting Information); EtOAc was selected as the solvent due to its similar polarity with the acrylic monomers employed.Among the additives used in the polymerization, only Borate V quenched the PL emission of PC, indicating that our PIS worked under the reductive quenching cycle (Figure 5a).Remarkably, an extraordinarily fast electron transfer rate constant (k PET = 3.7 × 10 13 M −1 s −1 ) with Borate V (E ox 0 = 0.58 V) was measured, exceeding the diffusion-limit (≈10 10 M −1 s −1 in EtOAc).This suggests that the electron-transfer process in our PIS is likely based on a pseudo-unimolecular reaction without the need for bimolecular collisions. [19]We hypothesized that an encounter complex is formed between cyanine dye (Cy + ) and the borate anion (BR 4 − ) through Coulombic interactions in nonpolar solvents, thereby facilitating intramolecular electron transfer. [20]o provide direct evidence for our hypothesis, we monitored the changes in the UV-vis absorption spectra of IA 643 upon the addition of Borate V in both a nonpolar solvent (EtOAc) and a polar solvent (DMSO) (Figure 5c; Figure S32a, Supporting Information).It was evident that  max of the samples red-shifted upon the addition of Borate V in EtOAc, indicating the generation of new PC species with altered electronic structures.In contrast, the addition of the same amount of Borate V did not shift the maximum absorption peak of DMSO.These direct observations suggest that IA 643 and Borate V exist as solventseparated ion pairs (SSIP) in DMSO.20a] These suggestions are supported by the previously reported dissociation constant (K D ) of cyanine dyes, which fluctuates by approximately 10 5 times between polar solvent (CH 3 CN) and less polar solvent (EtOAc), correlating with different ion-pairing stabilization energies (E D ) in CH 3 CN ( = 36.2,E D ≈ 0.06 eV) and in EtOAc ( = 6.0,20a] To further support this argument, the electron transfer rate constants in DMSO were measured (Figure S33b, Supporting Information).The measurements in DMSO revealed that, in contrast to EtOAc, the PL emission of IA 643 remained unquenched, indicating a very slow electron transfer rate constant that was beyond our measurement range, even with the use of 10 4 -10 5 times higher concentrations of Borate V than those in EtOAc.Thus, it is confirmed that the formation of ion-pairing complex of Cy + BR 4 − in a nonpolar environment can promote the electron transfer rate and rapidly generate the radical species for polymerization (Figure 5a).
With a solid understanding of the wavelength orthogonality of this novel resin, we investigated the photomediated activation of 3D printed acrylic objects containing HABIs. Figure 6a depicts a schematic of the localized activation of HABIs into lophyl radicals within the polymer network upon exposure to light.This transformation is fully reversible, making it an ideal candidate for photomediated healable materials.As an exemplary case, we 3D printed "the Statue of Liberty" using Entry 4, achieving an impressive build speed (8 s of exposure time/layer with a 50 μm slice thickness).Initially, the object exhibited a blue hue due to the residual cyanine dye; however, its color changed to yellow during postcuring (Figure 6b).Upon selective exposure of the statue's torch to 405 nm light, it transformed into a teal or greenish-blue color, confirming the generation of lophyl radicals.The torch subsequently reverted to its original yellow color after several minutes without light exposure.For a more detailed comparison, a series of films was 3D printed using Entry 1-4 (Figure S34, Supporting Information).The sample from Entry 1, which contained PETTA, showed no color change under 405 nm irradiation.Conversely, all samples infused with HABI compounds (o-Cl HABI, BA-HABI, and TKA-HABI, for Entry 2, 3, and 4, respectively) underwent a color change due to lophyl radical formation in the right half of the film, which was irradiated under 405 nm light.We further explored the effects of various light wavelengths (365, 405, 455, and 530 nm) on these films (Figure S35, Supporting Information).Predictably, the clarity of the irradiated area decreased with increasing incident wavelength in the following order: 365 nm > 405 nm > 455 nm.Given that HABI compounds lack an absorption peak beyond 500 nm, the 530 nm LED lamp produced no discernible color variation in this study.
Photoactive 3D printing was used to demonstrate the healing capability.A 3D printed film, prepared using the same method as before, had its surface intentionally scratched using a razor blade.It was then irradiated with 405 nm light at 10 mW cm −2 for 10 min.Figure 6c presents optical microscopic images of the samples before and after light irradiation.No healing was observed for the sample prepared from Entry 1.However, the samples from Entry 3 and 4 demonstrated pronounced crack-healing capabilities, with the scratches fully vanishing after only 10 min of light exposure (Figure 6c).Importantly, the healing ability was spatially controlled; only the irradiated regions exhibited photomediated healing (as evidenced in Figure S35, Supporting Information).UV and blue light (365 and 455 nm) LED lamps were also employed, with each lamp showcasing the potential for photomediated healing (Figures S36 and S37, Supporting Information).
One of the interesting attributes of HABIs is their potential for chemical deactivation, which allows for the selective dis-abling of their dynamic behavior.In our experiments, we used 2-ethylhexanehtiol as a model thiol compound to explore the thiol-mediated irreversible transformation into imidazole compounds.This was examined in the context of developing photomediated degradable materials.Our initial setup involved creating a 3D object with the letters "KRICT" engraved on its surface using the resin from Entry 3, employing a DLP 3D printer (Figure 6d(i)).The sample was then immersed in EtOAc containing an excess quantity of thiols (200 times the amount relative to the HABI compounds).It underwent selective irradiation with 405 nm light focused on the letters "KR" via a photomask, which resulted in the erasure of the letter sequence "KR" (Figure 6d(ii)).Subsequently, the remaining letters "ICT" were also targeted with 405 nm light, leading to the complete erasure of all engraved letters on the surface of 3D objects (Figure 6d(iii)).For photomediated erasing of 3D printed object, we exclusively utilized difunctional HABI (BA-HABI), resulting in a crosslinked network consisting of linear acrylic strands connected only by the HABI crosslinker.Upon violet/blue light irradiation, the HABI transformed into the lophyl radical, causing a loss of connectivity between each polymeric strand.The HABI-containing crosslinked network underwent a transformation into a linear polymer, which readily dissolves in an organic solvent (see Figure S39 in the Supporting Information for schematic illustration of photo-mediated degradation).As evidence, films produced from Entry 1 and 4 remained unaltered after 405 nm irradiation for a day (Figure S40, Supporting Information).We evaluated the fragmentation of the films during photomediated degradation using gel permeation chromatography (GPC) (Figure S41, Supporting Information).The sample prepared from Entry 3 yielded linear polymers post-light exposure, with an M n of 16 500 g mol −1 and a broad polydispersity index (PDI) of 3.6 (Table S7, Supporting Information).NMR analyses further highlighted the emergence of an imidazole peak at 12.5 ppm, in with its intensity increased with the duration of light exposure (Figure S42, Supporting Information).
In addition, we conducted an in-depth analysis of the photobleaching mechanism of the PC.Initially, photobleaching experiments were performed in EtOAc under the illumination of a 3 W 630 nm LED (30 mW cm −2 ), screening combinations of IA 643 (1.0 × 10 −5 M) and all additives (2.0 × 10 −4 M) in the polymerization conditions (Figure S43, Supporting Information).The results revealed that in the control experiments, there was no observable photobleaching behavior in the PC itself, and furthermore, the addition of o-Cl HABI did not result in photobleaching.In the presence of HNu 254, approximately 20% of PC contents were degraded within 5 min.In contrast, with Borate V, PC were fully degraded within 1 min, even at lower concentrations (2.0 × 10 −5 M) under lower LED intensities (5 mW cm −2 ), indicating that Borate V plays a key role in the photobleaching behaviors.These photobleaching trends correlated with those of the electron transfer rate constants, and PC was not bleached in DMSO under the same conditions (Figure S44, Supporting Information).
20a,21] However, given the rapid rate of photobleaching in our observations, the concentrations of the cyanine and alkyl radicals generated after the photoinduced electron transfer process are too low to undergo Initially, the object had a blue hue owing to the remaining cyanine dye.However, the color then changed to yellow during the postcuring process.When selectively exposed to 405 nm light on the torch, the irradiated area changes to a greenish-blue color, confirming the generation of lophyl radicals.The torch returns to its original yellow color after a few minutes without light exposure.c) Optical microscopic images demonstrating the crack-healing capability of HABI-containing films using Entry 1 and 4.After 10 min of 405 nm irradiation at 10 mW cm −2 , the crack on the surface of the HABI-containing film (Entry 4) was completely healed, while the wound on the film made from Entry 1 remained intact.d) Schematic representation of the selective erasing process upon light exposure (top) and optical microscopic images of 3D printed objects featuring the engraved letters "KRICT" on the surface (bottom).In Entry 3, excessive amount of 2ethylhexanethiol (×200 times relative to HABI compound) was added as H-donor.Upon selective irradiation with 405 nm light, the engraved letters on the product underwent sequential removal.bimolecular coupling.20a,21b] To identify the cyanine species (i.e., Cy + or Cy•) predominantly responsible for photobleaching, we employed a thermal initiator, azobisisobutyronitrile (AIBN), as the radical source instead of Borate V, excluding the generation of cyanine radicals (Figure 7a; Figure S45, Supporting Information).We monitored the changes in UV-vis absorption spectra of the solution of IA 643 (1.0 × 10 −5 M) and AIBN (2.0 × 10 −3 M) in degassed EtOAc at 60 °C under dark conditions over time.The use of AIBN fully bleached the IA 643 solution, indicating that Cy + is likely the major contributor to the PC degradation; the use of AIBN took approximately 1 h to observe PC bleaching, likely due to the slow generation of radicals from AIBN at 60 °C (T 1/2, 60 °C ≈ 12.5 h). [22]A radical coupling reaction between cyanine and alkyl radicals cannot be entirely excluded.
The 3D printed acrylic object retained its color for a substantial duration.Therefore, we speculated that the factors inhibiting PC photobleaching were present within the 3D printing conditions.After screening combinations of IA 643, Borate V, and other components, it was found that an excess of HNu 254 hindered the formation of the Cy + BR 4 − ion pair complex.To further support our observations, we monitored the photobleaching kinetics of the IA 643 and Borate V solutions under the same conditions while varying the HNu 254 content (Figure 7b).Kinetics analysis indicated that excess iodonium significantly inhibited photobleaching.This inhibited photobleaching behavior can be attributed to the formation of an ion-pair complex between iodonium and BR 4 − , leading to the reduced formation of Cy + BR 4 − .To address these counterion exchanges, we measured the UV-vis absorption spectra of solutions of IA 643, Borate V, and HNu 254, and monitored the changes in their absorption maxima (Figure S32b, Supporting Information).As the concentration of HNu 254 increased, the absorption maxima were blue-shifted close to the original maxima of IA 643; these changes in the UV-vis spectra shape, induced by counterion exchange, aligned with a previous report. [23]Thus, HNu 254 is likely to serve contradictory roles; (i) as an electron-accepting coinitiator to close efficiently catalytic cycle by regenerating PC and (ii) as an inhibitor to form Cy + BR 4 − , leading to a slow PET rate.These observations were further supported by the dependence of the curing rates on the HNu 254 content (Figure S47 and Table S8, Supporting Information).
After establishing the photobleaching mechanism of the redlight-curable acrylic resins, we explored their ability to produce optically-clear 3D objects.Initially, we compared the UV-vis absorption spectra of the photopolymerized and post-cured films.
The resin formulation (Entry 4) was exposed to 620 nm light under the same conditions used in the 3D printing process (3 mW cm −2 , 8 s) and then post-cured (620 nm, 10 mW cm −2 , and 5 min).In Figure 7c, the UV-vis absorption spectra of these films are displayed.Prior to post-curing, a significant drop in transmittance in the red region (550-700 nm) and a small drop in the violet region (400-450 nm) were observed.These changes were attributed to the presence of cyanine dye and HABI derivatives (see photographic images in Figure S48 in the Supporting Information).After post-curing, the transmittance reached almost 100%, indicating the complete consumption of cyanine dye.However, a small drop in transmittance in the violet region persisted, which was attributed to the HABIs not participating in photobleaching.Similar trends were observed when Entry 1 and 3 were tested at different thickness (50 and 100 μm) under same irradiation conditions (Figure S49, Supporting Information).As a final demonstration, we printed glass using Entry 1 to showcase the capability of our red-light-curable resin to produce optically-clear 3D objects with visible light.Figure 7d presents the digital rendering and a photograph of the printed object.While the 3D printed object initially had a blue hue due to the remaining cyanine dye, it became transparent over the course of post-curing.We also conducted tests on the printing-dependent properties of our novel HABI-containing resin and gathered comprehensive material properties of the resulting 3D printed object (Figures S50 and S51, and Tables S9 and  S10, Supporting Information).These results collectively affirm that our novel resins are comparable to conventional thermoset 3D printing resins in terms of their performance.

Conclusion
This study describes and systematically examines novel visiblelight-driven acrylic resins designed for creating optically transparent multifunctional materials, with a focus on their application in DLP 3D printing.A tricomponent system, comprising a cyanine derivative, borate (electron donor), and iodonium salt (electron acceptor) was developed to achieve an outstanding build speed (22.5 mm h −1 ) and feature resolution (≈20 μm) under lowintensity red light exposure (3 mW cm −2 ).In addition, novel HABI-based crosslinkers were incorporated to enable spatiallyresolved photoactivation under violet/blue light irradiation.The utilization of red-light-reactive PC ensured spatially-resolved absorption with a photoactive unit, allowing swift photoactivation and rapid photocuring, even at excessive loading of HABIs (up to 50 wt%).Mechanistic studies utilizing CV and PL spectroscopy revealed the exceptionally fast electron transfer rate constant with Borate V, attributed to the formation of an encounter complex between PC (Cy + ) and borate anion (BR 4 − ), leading to subsequent intramolecular electron transfer.Novel bis(acryloyl)-and tetrakis(acryloyl)-functionalized HABIs demonstrated enhanced visible-light absorption and remarkable lophyl radical generation capabilities.The wavelength orthogonality of the novel resin facilitated (i) rapid wound healing under violet/blue light irradiation and (ii) selective photomediated erasing in the presence of an H-donor.The photobleaching pathway, leveraging HNu 254, played dual roles as an electron acceptor for photoinduced electron transfer and as an inhibitor, influencing the Cy + BR 4 − formation and slowing down the photocuring rate.The consumption of all the remaining PC during post-curing resulted in the pro-duction of optically-clear 3D printed materials.This fundamental study provides new possibilities for advancing photoactive 3D printing, enabling the seamless integration of multiple photoactive units while mitigating loss or deactivation during the photopolymerization process.In the field of 3D printing, this breakthrough holds immense promise for diverse applications ranging from tissue engineering to the development of adaptable structures for soft robotics.

Figure 1 .
Figure 1.Schematic illustration of photoactive DLP 3D printing.a) Redlight-reactive PIS enables rapid photopolymerization (and corresponding swift build speed) and excellent feature resolution.b) The incorporation of violet/blue light-reactive photoactive units, HABI crosslikers imparts multiple functionalities (i.e., crack-healing and degradation) to the 3D printed objects.c) The reversible/irreversible chemical transformation of HABIs.

Figure 3 .
Figure 3. Spectroscopic analysis of the three HABIs utilized in this study.Concentration of all samples was 0.5 × 10 −3 M in EtOAc.UV-vis absorption spectra of three HABI solutions: a) prior to irradiation and b) under 405 nm irradiation at 10 mW cm −2 .c) ESR spectra of HABI solutions at equilibrium under irradiation with 405 nm light at 10 mW cm −2 .d) Absorbance at  max versus time, as determined by UV-vis spectroscopy.All solutions were irradiated from 10 s to 5 min, and were otherwise kept in the dark.e) Evolution of absorbance at  max during repetitive switching cycles comprising alternating 405 nm irradiation (10 mW cm −2 , 3 min) and darkness (20 min).f) UV-vis absorption spectra of the three HABI solutions and the normalized emission spectrum of 405 nm LED lamp.

Figure 4 .
Figure 4. Rapid high-resolution 3D printing of HABI-containing photoactive polymer: Plot of double bond conversion versus time for red-light-curable resin containing a) various crosslinkers (PETTA and three HABI derivatives) and b) different ratios of TKA-HABI.The thickness of all samples was 50 μm.c) Digital projection layer for the "resolution print" method at 3 s when all squares were illuminated simultaneously (white regions correspond to exposure).d) An expanded view of a square with 8 s exposure/layer, displaying the line and pixel array for the projection.Resolution print obtained from e) Entry 1 and f) Entry 4, with magnified image observed using optical microscopy.The thickness of each layer was 50 μm.

Figure 5 .
Figure 5. Mechanistic study to understand rapid polymerization in 3D printing.a) Proposed mechanism of photocatalyzed, visible-light-driven free radical polymerization.Formation of encounter complex between cyanine-based PC and borate anion (i.e., BR 4 − ) enable a rapid photoinduced electron transfer beyond diffusion-limit.b) Measurement of photoinduced electron transfer rate constant (k PET ) between IA 643 and Borate V. Stern-Volmer plots were obtained by the photoluminescence (PL) emission quenching of the maxima of PL intensity at  ex = 420 nm using undegassed solution of IA 643 (1.0 × 10 −5 M) in EtOAc, along with the addition of Borate V (0-1 × 10 −4 M) at RT. c) Direct observation of encounter complex formation between IA 643 and Borate V in EtOAc (left) and DMSO (right).Changes in the maxima of absorbance ( max ) were monitored with UV-vis absorption spectra using undegassed solution of IA 643 (1.0 × 10 −5 M) in EtOAc, along with the addition of Borate V (0-5 × 10 −4 M) at RT.

Figure 6 .
Figure 6.Photoresponsive, "smart" 3D printing.a) Illustration of a 3D printed polymeric network containing HABI, showcasing the reversible generation of lophyl radical upon light exposure.b) A 3D printed "Statue of Liberty" created using Entry 4.Initially, the object had a blue hue owing to the remaining cyanine dye.However, the color then changed to yellow during the postcuring process.When selectively exposed to 405 nm light on the torch, the irradiated area changes to a greenish-blue color, confirming the generation of lophyl radicals.The torch returns to its original yellow color after a few minutes without light exposure.c) Optical microscopic images demonstrating the crack-healing capability of HABI-containing films using Entry 1 and 4.After 10 min of 405 nm irradiation at 10 mW cm −2 , the crack on the surface of the HABI-containing film (Entry 4) was completely healed, while the wound on the film made from Entry 1 remained intact.d) Schematic representation of the selective erasing process upon light exposure (top) and optical microscopic images of 3D printed objects featuring the engraved letters "KRICT" on the surface (bottom).In Entry 3, excessive amount of 2ethylhexanethiol (×200 times relative to HABI compound) was added as H-donor.Upon selective irradiation with 405 nm light, the engraved letters on the product underwent sequential removal.

Figure 7 .
Figure 7. Production of optically-clear 3D object via photobleaching.a) Observation of PC bleaching with azobisisobutyronitrile (AIBN).Changes in the UV-vis absorption spectra of the samples were monitored as a function of time using degassed solution of IA 643 (1.0 × 10 −5 M) and AIBN (2.0 × 10 −3 M) in EtOAc at 60 °C.Images of the bleached sample were captured after 60 min at 60 °C (inset).b) Inhibited photobleaching of PC via addition of HNu 254.Changes in the UV-vis absorption spectra of samples were monitored under the illumination of a 3 W 630 nm LED (5 mW cm −2 ) for 60 s using undegassed solution of IA 643 (1.0 × 10 −5 M) and Borate V (2.0 × 10 −5 M) in EtOAc, along with addition of HNu 254 (0-2 × 10 −3 M), at RT.The kinetics of photobleaching of each sample were recorded under dark conditions (inset).c) UV-vis transmittance spectra of photopolymerized film using Entry 4 (100 μm thickness) before and after post-curing.d) Digital rendering of glass, and photographs of 3D printed object (from Entry 1) before and after post-curing.

Table 2 .
Resin formulations and results from RT-FTIR measurements shown in Figure4a.
porting Information for ESR spectroscopy studies).Enhanced radical generation was observed at shorter wavelengths, following the order: 365 nm > 405 nm > 455 nm.This is likely due to the greater overlap of HABIs' absorption peaks of the HABIs with the emission spectra of the individual LED lamps(Figures  S17 and S18 and Table