Selenium‐Atom‐Enhanced Triplet Exciton Kinetics in MR‐TADF Photocatalysts for Ultrafast, High‐Resolution, and Open‐Air 3D Printing

ABSTRACT

The photoinitiating system is a decisive factor governing the speed, resolution, and operational robustness of digital light processing (DLP) 3D printing. However, most existing systems rely on inert atmospheres, high irradiation intensities, or prolonged exposure times, severely limiting printing efficiency and practical applicability. Here we report a molecular design paradigm that fundamentally redefines the functional role of multiple‐resonance thermally activated delayed fluorescence (MR‐TADF) materials, transforming them from efficient light emitters into highly active triplet photocatalysts through selenium‐atom engineering. By integrating carbonyl‐assisted n–π*/π–π* state coupling with selenium‐induced spin‐orbit enhancement, the resulting photocatalyst QPSO achieves a near‐unity intersystem crossing quantum yield together with an exceptionally large forward‐to‐reverse intersystem crossing rate constant ratio, thus efficiently channeling exciton flux into long‐lived, redox‐active triplet states. When combined with a hypervalent iodonium co‐initiator, this system enables rapid photopolymerization in ambient air using low‐intensity blue light. As a result, single‐layer curing is completed within only 1.5–2 s across a broad thickness range, affording a printing resolution down to 10 µm and a record‐high build speed of up to 72 cm h⁻¹ at 400 µm layer thickness. The platform supports the fabrication of complex hierarchical architectures while exhibiting excellent biocompatibility.

Selenium‐atom engineering of a carbonyl‐embedded multiple‐resonance TADF (MR‐TADF) framework dramatically enhances spin‐orbit coupling, boosting the intersystem crossing quantum yield to near‐unity (Φ _ISC = 0.97) and achieving an exceptionally high k _ISC/k _RISC ratio of 9.76 × 10⁴. This maximizes triplet exciton flux for efficient photoredox initiation, enabling ultrafast, high‐resolution DLP 3D printing under low‐intensity light in ambient air.

1. Introduction

Three‐dimensional (3D) printing has emerged as a powerful rapid prototyping and manufacturing technology, enabling the efficient fabrication of complex architectures with high geometric freedom and the direct integration of functional components [1, 2, 3, 4]. These capabilities have accelerated its adoption across a wide range of applications, including personalized manufacturing [5], soft robotics [6, 7], and biomedical engineering [8, 9]. Among the various 3D printing modalities, digital light processing (DLP) has become one of the most widely implemented techniques for producing high‐resolution structures, owing to its area‐projection exposure mode, excellent spatial fidelity, and favorable system integrability (Figure 1a) [10, 11, 12, 13, 14]. At the heart of DLP printing lies a photopolymerization process whose efficiency, speed, and reliability are critically determined by the performance of the photoinitiating system. Conventional photoinitiators are broadly classified into Type I systems, which rely on unimolecular bond cleavage upon high‐energy ultraviolet excitation, and Type II systems, which generate initiating radicals through bimolecular or multimolecular reactions [15]. While Type I initiators typically demand UV irradiation that can induce material degradation and limit biocompatibility, Type II systems, although compatible with visible light, often suffer from intrinsically slower reaction kinetics and pronounced sensitivity to oxygen inhibition [16, 17, 18]. As a consequence, most existing DLP photoinitiating systems require inert atmospheres, high light intensities, or prolonged exposure times, thereby substantially compromising printing speed, resolution, and operational practicality. Developing DLP printing strategies that can operate efficiently under ambient conditions using low‐intensity visible light, while simultaneously maintaining high printing speed and precision, therefore remains a central challenge.

FIGURE 1.

(a) Schematic diagram of the digital light processing (DLP) 3D printing. (b) Mechanistic framework of visible‐light‐driven photoredox polymerization via triplet excited states. (c) Conceptual shift from emitter to photocatalyst: reprogramming thermally activated delayed fluorescence (TADF) excited‐state kinetics by simultaneously maximizing intersystem crossing quantum yield (Φ _ISC) and forward‐to‐reverse intersystem crossing rate constant ratio (k _ISC/k _RISC) to preferentially funnel exciton flux into productive triplet‐mediated chemical transformations rather than radiative decay. (d) Selenium‐engineered multiple‐resonance TADF (MR‐TADF) photocatalyst enabling ultrafast, high‐resolution, and open‐air DLP 3D printing.

With the rapid maturation of light‐emitting diode (LED) technologies, visible‐light‐driven photoredox catalytic polymerization has emerged as a compelling alternative to conventional UV‐based photoinitiation, offering improved safety, energy efficiency, and operational flexibility (Figure 1b) [19, 20, 21]. In this approach, organic photocatalysts (PCs) are photoexcited under visible light to mediate polymerization via single‐electron transfer processes, with the efficiency of radical generation being dictated by the lifetime, redox potential, and reactivity of their excited states [22, 23]. In comparison to singlet excited states, which typically possess nanosecond lifetimes, often necessitating high catalyst loadings that aggravate inner‐filter effects, triplet excited states exhibit much longer lifetimes (microseconds to milliseconds) [24, 25]. These extended lifetimes increase the probability of productive electron‐transfer events, thereby enabling a higher steady‐state concentration of initiating radicals even under oxygen‐rich conditions. Consequently, efficient population and utilization of triplet excitons is widely regarded as a key requirement for achieving rapid and oxygen‐tolerant photopolymerization. However, for most purely organic chromophores, intersystem crossing (ISC) from the singlet to the triplet manifold is inherently inefficient due to weak spin‐orbit coupling (SOC) [26]. To overcome this limitation, strategies such as heavy‐atom incorporation (e.g., Ru, Br, or I) [27, 28] or the construction of twisted charge‐transfer excited states [29, 30, 31] have been widely adopted, leading to photocatalysts based on BODIPY, naphthalimide, and related chromophores with enhanced intersystem crossing quantum yields (Φ _ISC). Despite these advances, the relatively large singlet‐triplet energy gaps (ΔE _ST ≈ 0.5–1.0 eV) commonly associated with such systems often lower the effective redox driving force of the triplet excited state, thereby requiring the addition of sacrificial electron donors, most commonly aliphatic or aromatic amines, to achieve efficient initiation [28, 31, 32, 33]. This added complexity not only compromises formulation simplicity and biocompatibility, but also fails to fully alleviate kinetic limitations and oxygen inhibition, ultimately restricting their applicability in high‐speed DLP 3D printing under ambient conditions.

Recently, thermally activated delayed fluorescence (TADF) molecules have attracted increasing attention as organic photocatalysts for photoredox polymerization, owing to their efficient population of triplet excited states and broadly tunable excited‐state redox potentials [34, 35, 36, 37]. A defining feature of TADF systems is their small singlet‐triplet energy splitting, which enables thermally assisted bidirectional interconversion between singlet and triplet manifolds. From a photocatalytic perspective, such a small ΔE _ST offers a dual advantage: it facilitates rapid ISC, ensuring efficient triplet generation, while simultaneously maintaining a sufficiently high triplet energy that supports strong excited‐state redox activity [38]. Despite their success in solution‐phase polymerization, the application of TADF molecules in rapid photocuring processes relevant to 3D printing, where polymerization must proceed on the timescale of seconds, remains largely unexplored. To date, only a limited number of studies have reported the use of cyano‐carbazole‐based [39] or boron–nitrogen‐embedded [40] TADF emitters in direct laser writing (DLW) or DLP printing, with the achieved curing rates and oxygen tolerance falling well below the theoretical potential of this material class. This performance gap is likely associated with the fact that most reported TADF structures were originally designed and optimized for electroluminescent applications, rather than for catalytic reactivity. In organic light‐emitting diodes (OLEDs), TADF emitters are engineered to promote fast radiative decay and efficient reverse intersystem crossing (RISC) for exciton recycling [41], whereas in photoredox catalysis, the primary objective is to maximize the utilization of triplet excitons in chemical transformations before deactivation occurs (Figure 1c). Consequently, transforming TADF molecules from efficient light emitters into effective photocatalysts requires not only a high Φ _ISC, but also the deliberate suppression of the RISC process. Achieving a large forward‐to‐reverse intersystem crossing rate constant ratio (k _ISC/k _RISC) is therefore essential to steer exciton flux toward the reactive triplet channel and enable efficient initiation under the demanding conditions of rapid photocuring.

In contrast to donor‐acceptor (D‐A) TADF molecules that operate through long‐range charge transfer (LRCT) excited states, multiple‐resonance TADF (MR‐TADF) systems based on short‐range charge transfer (SRCT) typically exhibit more rigid and planar π‐conjugated frameworks, moderately larger ΔE _ST, and intrinsically slower RISC kinetics [42, 43, 44]. These structural and electronic features effectively suppress non‐radiative energy dissipation and, more importantly, favor directional exciton flux toward the triplet manifold, thereby enabling the simultaneous realization of high Φ _ISC and a large k _ISC/k _RISC ratio [45]. We therefore envisioned that MR‐TADF molecules represent a promising platform for developing high‐performance organic photocatalysts capable of meeting the stringent initiation efficiency and oxygen tolerance required for ultrafast photocuring. Guided by this hypothesis, we designed and synthesized a new class of MR‐TADF photocatalysts based on a rigid N/C═O‐containing polycyclic framework (Figure 1d). In this architecture, the incorporation of carbonyl functionality introduces n‐orbital contributions that enhance vibronic coupling between n‐π* and π‐π* excited states, thereby promoting efficient ISC [46]. To further amplify SOC and accelerate ISC, heavy atoms such as chalcogen atoms (S and Se) were strategically embedded into the MR core [47]. Among the resulting derivatives, the selenium‐doped analogue QPSO exhibits the strongest SOC, affording a near‐unity Φ _ISC (0.97) together with an exceptionally high k _ISC/k _RISC ratio on the order of 10⁴, thus enabling highly efficient and directional triplet exciton utilization. Owing to its sufficiently high triplet excited‐state oxidation potential, QPSO can be effectively coupled with a hypervalent iodine co‐initiator [bis(4‐tert‐butylphenyl)iodonium hexafluorophosphate, Iod], forming a robust photoinitiating system for visible‐light‐driven polymerization. Under realistic printing conditions using a commercial layer‐by‐layer DLP printer operating at 405 nm in ambient air with a low light intensity of 4.0 mW cm⁻², this system delivers outstanding performance even at an ultralow photocatalyst loading of 0.02 mol%. Single‐layer exposure times of only 1.5–2 s are sufficient across a broad thickness range of 10–400 µm, enabling high‐resolution printing at 10 µm (2.4 cm h⁻¹) and record‐high build speeds of up to 72 cm h⁻¹ at 400 µm layer thickness (excluding platform translation time). Moreover, the resulting platform supports the rapid fabrication of complex architectures spanning multiple length scales, while maintaining excellent biocompatibility. Collectively, these results demonstrate that selenium‐atom engineering of MR‐TADF cores provides a powerful and general strategy for developing practical, high‐performance photoinitiators tailored for ultrafast, high‐precision additive manufacturing under ambient conditions.

2. Results and Discussion

2.1. TD‐DFT Calculations and Photophysical Properties

To validate the proposed molecular design strategy, namely, the construction of heavy‐atom‐engineered MR‐TADF photocatalysts based on an N/C═O framework, we synthesized four target compounds, denoted as MQAO, QPXO, QPO, and QPSO (Figure 2a; see Section S2 for detailed synthetic procedures). Time‐dependent density functional theory (TD‐DFT) calculations reveal that all four molecules feature rigid MR cores appended with peripheral substituents, giving rise to well‐defined SRCT excited states (Figure 2a). Their frontier molecular orbitals exhibit the characteristic alternating electron‐density distribution within the MR skeleton, confirming a pronounced multiple‐resonance effect. SOC calculations further highlight the critical role of heavy‐atom engineering. As the heteroatom is varied from oxygen to sulfur and subsequently to selenium, the SOC constants between the singlet (S₁) and triplet (T₁/T₂) excited states increase monotonically. In particular, the selenium‐substituted derivative QPSO displays SOC constants that are 6–10 times larger than those of its oxygen analogue QPXO, indicative of substantially enhanced spin‐orbit interaction. Such a pronounced SOC enhancement is expected to markedly accelerate ISC, thereby promoting efficient population of reactive triplet excitons, a prerequisite for high‐performance photoredox catalysis.

FIGURE 2.

(a) Molecular structures, calculated frontier molecular orbital distributions, energy level alignment, and SOC matrix elements of four target compounds, denoted as MQAO, QPXO, QPO, and QPSO. (b) UV–vis absorption spectra in toluene (1 × 10⁻⁵ M). (c) Delayed PL decays measured in doped films. (1 wt% in poly(methyl methacrylate), PMMA).

Photophysical studies show that all compounds exhibit strong absorption in the visible region, spanning 350–450 nm, with maxima located between 405 and 412 nm (Figure 2b and Table 1). The molar extinction coefficients at 405 nm range from (0.61–1.15) × 10⁴ M⁻¹ cm⁻¹, ensuring efficient light harvesting under low‐intensity blue irradiation. Spectral deconvolution suggests that the high‐energy bands below 400 nm mainly originate from π‐π* and n‐π* transitions, whereas the lower‐energy absorption features between 400 and 450 nm are dominated by SRCT character. In dilute toluene solution, the compounds display fluorescence maxima in the range of 441–496 nm (see Figure S2 for details). By combining low‐temperature (77 K) fluorescence and phosphorescence spectra (see Figure S4 for details), the ΔE _ST values were estimated to be 0.27–0.31 eV. These moderately large ΔE _ST values, characteristic of MR‐TADF systems, imply intrinsically suppressed RISC, which is advantageous for sustaining high triplet exciton populations under photocatalytic conditions. Time‐resolved photoluminescence measurements corroborate this interpretation, revealing long‐lived delayed fluorescence components with lifetimes extending from 243.1 to 1102.6 µs (Figure 2c), indicative of efficient ISC and slow RISC dynamics. Photoluminescence quantum yield (PLQY) measurements in doped films further reflect the impact of heavy‐atom substitution. While MQAO, QPXO, and QPO exhibit moderate to high PLQYs of 0.72, 0.83, and 0.49, respectively, the selenium‐doped QPSO displays a markedly reduced PLQY of only 0.07. This pronounced quenching of fluorescence is consistent with its substantially enhanced SOC, which strongly favors non‐radiative ISC over radiative decay, thereby effectively channeling exciton flux into the triplet manifold.

TABLE 1.

Photophysical properties of four MR‐TADF PCs.

Entries	λ max a (nm)	ΔE ST b (eV)	τ d (µs) c	Ф PL d	k r e (107 s−1)	k ISC f (108 s−1)	k RISC g (103 s−1)	Ф ISC h	k ISC/ k RISC i (104)
MQAO	405	0.31	592.4	0.72	3.45	0.68	2.88	0.66	2.36
QPXO	412	0.29	1102.6	0.83	3.19	0.41	1.44	0.56	2.85
QPO	407	0.27	552.6	0.49	2.18	0.71	2.56	0.76	2.77
QPSO	405	0.29	243.1	0.07	1.73	5.71	5.85	0.97	9.76

^{a )}Maximum absorption peak measured in toluene.

^{b )}Singlet‐triplet energy gap measured in toluene.

^{c )}Delayed PL decays measured in doped films (1 wt% in PMMA).

^{d )}Photoluminescence quantum yield measured in doped films (1 wt% in PMMA).

^{e )}Radiative decay rate constant.

^{f )}Intersystem crossing rate constant.

^{g )}Reverse intersystem crossing rate constant.

^{h )}Intersystem crossing quantum yield.

^{i )}Forward‐to‐reverse intersystem crossing rate constant ratio.

Furthermore, the key exciton dynamic rate constants were quantitatively evaluated to elucidate the impact of selenium‐atom engineering on excited‐state kinetics (Table 1). All four MR‐TADF photocatalysts exhibit rapid ISC, with k _ISC values on the order of 10⁷–10⁸ s⁻¹. Among them, the selenium‐substituted QPSO displays the highest k _ISC of 5.71 × 10⁸ s⁻¹, which can be directly attributed to the markedly enhanced SOC imparted by selenium incorporation. Consistently, the corresponding Φ _ISC values are uniformly high across the series (0.56–0.97), with QPSO approaching a near‐unity value of 0.97. This exceptional efficiency originates from the synergistic combination of accelerated ISC and a substantially suppressed radiative decay rate (k _r), underscoring the pivotal role of selenium in promoting efficient singlet‐to‐triplet conversion. Meanwhile, the RISC rate depends not only on SOC but also on the Boltzmann factor exp(−ΔE _ST/k_BT) for the endothermic process. The rigid MR backbones afford a ΔE _ST of approximately 0.27–0.31 eV. Although this moderate gap helps suppress non‐radiative losses, it exponentially slows down the thermally activated RISC. Consequently, all compounds maintain relatively low k _RISC (≈ 10³ s⁻¹), resulting in exceptionally large k _ISC/k _RISC ratios of up to 10⁴, reaching 9.76 × 10⁴ for QPSO. This pronounced kinetic asymmetry enforces a strongly directional exciton flux from the singlet to the triplet manifold, thereby maximizing the steady‐state population of long‐lived, redox‐active triplet excitons. Collectively, these kinetic characteristics highlight selenium‐atom substitution as a powerful strategy for amplifying triplet utilization efficiency at the molecular level, laying a robust photophysical foundation for subsequent photochemical applications.

2.2. Photochemical Properties

To assess the photoinitiating capability of this MR‐TADF series, each photocatalyst was combined with the commercially available hypervalent iodine co‐initiator bis(4‐tert‐butylphenyl)iodonium hexafluorophosphate (Iod), and their photochemical behaviors were systematically investigated through complementary spectroscopic and electrochemical analyses. Steady‐state photolysis experiments under 405 nm irradiation reveal that, in the absence of Iod, all photocatalysts (MQAO, QPXO, QPO, and QPSO) exhibit negligible spectral changes, confirming their excellent intrinsic photostability (see Figure S7 for details). In stark contrast, upon addition of Iod, all systems display characteristic and reproducible spectral evolution (Figure 3a–d): the absorption band near 400 nm initially intensifies and subsequently diminishes with a slight blue shift, accompanied by the new emergence of a weak band around 500 nm. The temporal evolution of these features is indicative of the rapid formation of charge‐transfer (CT) complexes between the MR‐TADF photocatalysts and Iod, followed by photoinduced electron transfer that triggers Iod decomposition and generates reactive aryl radicals. The generation of these aryl radicals was confirmed by TEMPO (2,2,6,6‐tetramethylpiperidine‐1‐oxyl) radical trapping experiments and EPR (electron paramagnetic resonance) spin‐trapping experiments (see Figures S8 and S9 for details).

FIGURE 3.

Steady state photolysis experiments of four MR‐TADF PC/Iod systems (1.0 × 10⁻⁵ M/2.0 × 10⁻⁵ M in dichloromethane) under 405 nm blue LED (5 W) irradiation. Steady state photolysis curves of (a) MQAO/Iod, (b) QPXO/Iod, (c) QPO/Iod, and (d) QPSO/Iod systems. Fluorescence quenching curves of (e) MQAO/Iod, (f) QPXO/Iod, (g) QPO/Iod, and (h) QPSO/Iod systems. (i) Stern–Volmer treatment for fluorescence quenching of four MR‐TADF PC/Iod systems. (j) Schematic of ATR‐IR analytical techniques. (k) Photopolymerization kinetic curves (monomer conversions vs. irradiation time) of four MR‐TADF PC/Iod systems (MR‐TADF PC, 1.0 × 10⁻⁵ mol g⁻¹; Iod, 2.0 × 10⁻⁵ mol g⁻¹) upon irradiation by 405 nm LED within 600 s. Insert figure: photopolymerization kinetic curves upon irradiation by 405 nm LED within 15 s.

To further probe the interaction in the singlet excited state, fluorescence quenching experiments were conducted under ambient conditions (Figure 3e–h). Progressive addition of Iod leads to efficient quenching of the fluorescence emission of all photocatalysts, consistent with electron‐transfer‐mediated deactivation of the S₁ state. Analysis of the corresponding Stern‐Volmer plots affords quenching constants (K _SV) and enables estimation of the singlet excited‐state electron transfer quantum yields (Φ _S1 ^et), which are determined to be 64% (MQAO), 71% (QPXO), 64% (QPO), and 54% (QPSO) (Figure 3i and Table 2). Notably, QPXO exhibits the highest Φ _S1 ^et, whereas QPSO displays a comparatively lower value, suggesting a more efficient singlet‐level interaction between QPXO and Iod.

TABLE 2.

Experimental electronic properties of four MR‐TADF PCs.

Entries	K SV a (M)−1	Ф S1 et b	E S1 ox c (V vs. SCE)	E S1 red d (V vs. SCE)	E T1 ox e (V vs. SCE)	E T1 red f (V vs. SCE)	ΔG S1 et g (eV)	ΔG T1 et h (eV)
MQAO	87	0.64	−1.51	1.11	−1.20	0.80	−0.81	−0.50
QPXO	123	0.71	−1.54	1.00	−1.23	0.69	−0.84	−0.53
QPO	87	0.64	−1.47	1.04	−1.21	0.78	−0.77	−0.51
QPSO	58	0.54	−1.52	1.10	−1.24	0.82	−0.82	−0.54

^{a )}Fluorescence quenching constants determined by the slope of fluorescence quenching Stern–Volmer treatment.

^{b )}Singlet excited state electron transfer quantum yield calculated from: Ф _S1 ^et = K _SV [Iod]/(1 + K _SV [Iod]), Iod concentration = 0.02 M.

^{c )}Singlet excited state oxidation potential.

^{d )}Singlet excited state reduction potential.

^{e )}Triplet excited state oxidation potential.

^{f )}Triplet excited state reduction potential.

^{g )}Singlet excited state electron transfer Gibbs free energy change: ΔG _S1 ^et = E _S1 ^ox (PCs)—E _red (Iod), E _red (Iod) = –0.7 V vs. SCE.

^{h )}Triplet excited state electron transfer Gibbs free energy change: ΔG _T1 ^et = E _T1 ^ox (PCs)^— E _red (Iod).

To elucidate the thermodynamic driving forces underlying these processes, the ground‐state redox potentials of the photocatalysts were determined by cyclic voltammetry (see Figure S11 for details), from which their excited‐state redox potentials were derived (Table 2). All compounds possess strong excited‐state reducing ability in both the singlet (E _S1 ^ox = –1.54 to –1.47 V vs. SCE) and triplet manifolds (E _T1 ^ox = –1.24 to –1.20 V vs. SCE), enabled by their relatively small ΔE _ST. Importantly, these values are substantially more negative than the reduction potential of Iod (–0.70 V vs. SCE) [48], providing a clear thermodynamic impetus for electron transfer from either excited state. Correspondingly, Rehm‐Weller analysis yields negative free energy changes (ΔG _S1 ^et and ΔG _T1 ^et) for all systems [49], confirming that electron transfer from both S₁ and T₁ states to Iod is thermodynamically favorable. Taken together, these results demonstrate that the present MR‐TADF photocatalysts can directly engage in efficient photoinduced electron transfer with Iod, generating initiating radicals without the need for external electron donors. This dual‐channel reactivity, combined with the exceptionally high triplet utilization efficiency imparted by selenium engineering, underpins their outstanding performance as photoinitiators for rapid and oxygen‐tolerant photopolymerization.

2.3. Photopolymerization

Subsequently, photocuring experiments were conducted using attenuated total reflection infrared (ATR‐IR) spectroscopy to monitor the curing kinetics of ethoxylated trimethylolpropane triacrylate (ETPTA) (Figure 3j) [50]. The decrease in the characteristic C═C stretching band at 1636 cm⁻¹ was employed to quantify monomer conversion. Remarkably, at low catalyst loadings (1.0 × 10⁻⁵ mol g⁻¹ PC and 2.0 × 10⁻⁵ mol g⁻¹ Iod) and under 405 nm irradiation in air, all systems exhibited rapid polymerization. As shown in Figure 3k, QPSO/Iod delivered outstanding performance, achieving 40% conversion within 5 s, 72% within 10 s, and quantitative conversion (>99%) within 300 s. In comparison, MQAO/Iod, QPO/Iod, and QPXO/Iod displayed progressively slower initial rates, reaching 40%, 20%, and 15% conversion after 10 s, respectively, although all attained approximately 80% conversion after 300 s (see Figures S13 and S14 for details). We attribute this difference to the substantially lower triplet utilization efficiency of the other three systems relative to QPSO/Iod (see Table 1), which leads to insufficient radical generation in the later stages of polymerization, thereby failing to overcome limitations such as diffusion constraints, oxygen inhibition, and increased viscosity. Notably, the efficient curing of all formulations under ambient atmosphere highlights the exceptional oxygen tolerance imparted by the MR‐TADF PC/Iod platform. These ultrafast kinetics, combined with robust air operability, provide a strong foundation for high‐speed, high‐precision 3D printing applications.

To further assess spatiotemporal controllability, intermittent light on/off experiments were conducted in air using the optimal QPSO/Iod system. Polymerization initiated immediately upon illumination, ceased when the light was turned off, and promptly resumed upon re‐exposure (see Figures S15–S17 for details). This reversible switching behavior demonstrates precise photocontrol with high temporal fidelity, highlighting the excellent responsiveness of the MR‐TADF PC/Iod system to photon input and its suitability for finely controlled photopolymerization processes.

Integrating the photophysical, photochemical, and kinetic data affords insight into the origin of the observed performance disparities. Although QPXO exhibits the highest singlet state electron‐transfer efficiency (Ф _S1 ^et = 0.71), its comparatively low triplet population efficiency (Φ _ISC = 0.56) leads to the slowest polymerization kinetics. In contrast, QPSO, despite possessing the lowest Ф _S1 ^et (0.54), delivers the fastest polymerization owing to its near‐unity Φ _ISC (0.97) and exceptionally high k _ISC/k _RISC ratio. This inverse correlation unequivocally identifies efficient triplet exciton generation and accumulation rather than singlet‐state reactivity as the primary determinant of overall photopolymerization performance. Accordingly, the MR‐TADF PC/Iod systems operate predominantly through a triplet‐mediated electron‐transfer mechanism: upon photoexcitation, electron transfer can proceed from both singlet and triplet states, but the triplet channel dominates and drives the highly efficient radical generation and subsequent polymerization (see Figure S18 for details).

2.4. 3D Printing

Leveraging the ultrafast photoinitiation kinetics, precise spatiotemporal controllability, and outstanding oxygen tolerance of the MR‐TADF PC/Iod systems, we systematically evaluated their performance in DLP 3D printing under practically relevant conditions. All printing experiments were conducted on a commercial bottom‐up DLP printer (λ _max = 405 nm, irradiance = 4.0 mW cm⁻²) in ambient air. The printing formulation consisted of MR‐TADF PC, Iod, N,N‐dimethylacrylamide (DMA), and trimethylolpropane triacrylate (TMPTA) in a fixed molar ratio of 0.02:0.30:92.28:7.72 (Figure 4a). Under these conditions, the optimized systems exhibited exceptional printing efficiency, requiring a single‐layer exposure time of only 1.5–2 s. This translated into a build rate of 2.4 cm h⁻¹ at a 10 µm layer thickness (high‐resolution mode) and up to 72 cm h⁻¹ at a 400 µm layer thickness (high‐speed mode), excluding platform translation time. Mechanical characterization of the printed specimens (see Figure S22 and Table S7 for details) revealed the expected build‐direction anisotropy. The flat orientation delivered the optimal balance of stiffness (Young's modulus = 1190.8 ± 37.9 MPa), strength (yield strength = 63.9 ± 1.3 MPa), and ductility (fracture strain = 14.6 ± 0.3%), demonstrating that the MR‐TADF/Iod formulation yields robust mechanical performance suitable for functional applications.

FIGURE 4.

(a) Schematic of DLP 3D printer and resin components. (b) Comparisons of printing speed and resolution with prior literature. (c) 3D printed “Maya pyramid” models using MR‐TADF PC/Iod and commercial photoinitiators (DETX/Iod, TPO, and BAPO) systems without any post‐curing treatment (see Section S7.4 for detailed optimization of printing parameters).

To rigorously probe both the attainable speed limit and the fidelity of fine‐feature reproduction, a “Maya pyramid” model (26 × 26 × 11 mm), featuring complex textures and gradient cross sections, was employed as a standardized benchmark (see Section S7.4 for details). At a fixed layer thickness, the exposure time was progressively reduced until structural failure—manifested as feature blurring, interlayer delamination, or over‐curing—occurred. Notably, all printed constructs were evaluated without post‐curing. At a layer thickness of 200 µm and an exposure time of 5 s, only the QPSO/Iod and QPO/Iod systems achieved defect‐free printing, whereas QPXO/Iod, MQAO/Iod, and commercial photoinitiators (DETX/Iod, TPO, and BAPO) systems failed to deliver acceptable fidelity. Upon further reducing the exposure time to 3, 2, and 1.5 s, only the QPSO/Iod system maintained high‐resolution, artifact‐free printing. At an extreme exposure of 1 s, the overall geometry remained intact, but fine top‐layer features became partially compromised. These results unequivocally demonstrate the superior initiation efficiency and ultrafast curing kinetics of the QPSO/Iod system under stringent temporal constraints.

The ultimate speed limit of the QPSO/Iod platform was further explored by fixing the exposure time at 2 s while progressively increasing the layer thickness. High‐quality prints were obtained at 300 and 400 µm, whereas hollow defects emerged at 500 µm owing to insufficient through‐layer curing. Under optimized conditions (400 µm per layer, 2 s exposure), a build speed of 72 cm h⁻¹ was achieved, which, to the best of our knowledge, represents the highest reported rate for layer‐by‐layer DLP printing to date (Figure 4b). Resolution tests further confirmed the system's capacity for precision fabrication: at a layer thickness of 10 µm and an exposure time of 1.5 s, sharp edges and fine surface textures were faithfully reproduced while maintaining a build rate of 2.4 cm h⁻¹ (Figure 4c). This combination of ultrafast speed and micrometer‐scale resolution highlights the exceptional versatility and performance of the selenium‐engineered MR‐TADF photoinitiating system across disparate length and time scales. Comprehensive benchmarking against previously reported DLP systems in terms of printing speed, resolution, and exposure time is summarized in Table S9 [20, 28, 31, 32, 33, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63], underscoring the distinct performance advantages of the present platform for rapid, high‐precision additive manufacturing.

The exceptional printing performance achieved with the QPSO/Iod system can be directly traced to the synergistic interplay between its favorable photophysical properties and optimized exciton dynamics. The high molar extinction coefficient at 405 nm, excellent photostability, and suitably positioned excited‐state redox potentials collectively provide a robust thermodynamic basis for efficient radical generation under low catalyst loading and low photon flux. More critically, the exciton kinetic landscape plays a decisive role. Selenium‐induced enhancement of SOC elevates the Φ _ISC value to near unity (0.97), thereby ensuring highly efficient triplet exciton population. Concurrently, the exceptionally large k _ISC/k _RISC ratio (≈ 10⁴) effectively suppresses exciton back‐transfer to the singlet manifold, maximizing the availability and lifetime of reactive triplet states. This unique combination enables sustained generation of a high flux of initiating radicals within ultrashort exposure windows, while the long‐lived triplet excitons concurrently promote efficient quenching of dissolved oxygen, thereby mitigating inhibition effects. Ultimately, these features establish a molecular‐level foundation for ultrafast, high‐fidelity DLP printing under ambient atmosphere and low light intensity.

To further demonstrate the versatility and manufacturing capability of the MR‐TADF PC/Iod platform for complex, multiscale architectures, a series of intricate 3D models were fabricated. Representative examples include an “octet truss” with hierarchical structural motifs (Figure 5a), the intricate “benchy” benchmark (Figure 5b), a biomimetic “dental mandible” model (Figure 5c), and “golfball spire” structures featuring hollow geometries and internal lattice networks (Figure 5d). All constructs were printed with high dimensional accuracy at layer thicknesses of 10–50 µm, while the optimized QPSO/Iod formulation required remarkably short exposure times of only 1.5–5 s per layer (see Table S11 for details), thereby unifying high throughput with high resolution. Notably, the printed objects exhibited uniform blue or green fluorescence under 365 nm UV irradiation, reflecting excellent structural integrity and imparting additional optical functionality. An expanded library of complex printed architectures, including “octet unit cells”, “vases”, “diagrid bracelets”, “dice towers”, “Eiffel towers”, and “anatomical heart keychains”, is provided in Figures S51–S62, collectively underscoring the robustness, reproducibility, and geometric adaptability of the printing platform. From a practical standpoint, the printed parts retained their mechanical integrity without observable delamination or cracking after six months of ambient storage, while the corresponding resin formulations displayed excellent dark stability, highlighting the system's technological maturity and potential for scalable, customized additive manufacturing.

FIGURE 5.

3D printed high complexity models using the MR‐TADF PC/Iod system: (a) “Octet truss”, (b) “Benchy”, (c) “Dental mandible”, and (d) “Golfball spire”, along with its corresponding fluorescence image captured under 365 nm LED irradiation.

2.5. Biocompatibility

To assess the biocompatibility of the MR‐TADF photocatalysts, in vitro cell viability assays were conducted using human umbilical vein endothelial cells (HUVECs) and human hepatocellular carcinoma cells (HepG2) (see Section S8 for details). In HUVECs, all four PCs exhibited high cell viability at concentrations ranging from 6.25 to 25 µM, with only a moderate decrease observed at 50 µM; notably, three PCs maintained viability above 70%, while QPXO showed slightly lower tolerance. In HepG2 cells, all MR‐TADF PCs displayed favorable biocompatibility across the entire concentration range, retaining cell viabilities of approximately 80% even at 50 µM. Furthermore, leachates from cured printed samples after immersion in water and saline for 5–7 days were analyzed by UV–vis absorption and ICP‐MS (inductively coupled plasma mass spectrometry). The results indicated that only a negligible amount of photocatalyst was released from the polymer matrix (see Figure S67 and Table S20 for details). Together, these findings demonstrate the generally low cytotoxicity and good biocompatibility of this class of materials, supporting their potential for applications in biomedical fabrication and environmentally friendly manufacturing.

3. Conclusion

In conclusion, we have established a rational molecular design strategy that enables the functional transformation of MR‐TADF emitters into highly efficient triplet photocatalysts for DLP 3D printing. Strategic incorporation of a heavy selenium atom into an N/C = O‐based MR‐TADF framework markedly enhances SOC, thereby profoundly reshaping the excited‐state kinetics. As a result, the developed QPSO photocatalyst exhibits a near‐unity Φ _ISC (0.97) and an exceptionally large k _ISC/k _RISC ratio of 9.76 × 10⁴, which effectively channels photoexcited singlet excitons into long‐lived triplet states and maximizes their participation in photoredox processes. Benefiting from this optimized triplet exciton management, together with suitably matched redox potentials, the QPSO/iodonium initiating system operates efficiently under practically demanding conditions, including ambient atmosphere, low irradiation intensity, and ultralow catalyst loading. This unique combination enables DLP 3D printing with an unprecedented integration of ultrafast fabrication speed (up to 72 cm h⁻¹), high spatial resolution (10 µm), faithful replication of complex multiscale architectures, and excellent material biocompatibility. Beyond demonstrating a high‐performance photocatalyst for additive manufacturing, this work highlights MR‐TADF materials as a versatile platform for engineering excited‐state spin dynamics and photochemical reactivity. More broadly, it provides a feasible and generalizable strategy for advancing high performance, high‐throughput, and user‐friendly photopolymerization‐based 3D printing technologies.

Author Contributions

Yuyang Tang: conceptualization, methodology, formal analysis, investigation, writing – original draft, writing – review and editing. Tingting Yang: methodology, validation, writing – review and editing, formal analysis. Xingliang Wang: methodology, validation, writing – review and editing, formal analysis. Guangying Tan: conceptualization, writing – review and editing, funding acquisition, project administration, formal analysis, supervision, resources. Dongle Li: writing – review and editing, validation, formal analysis. Yuanzhi Xu: methodology, investigation, formal analysis, writing – review and editing. Yanbing Wang: validation, writing – review and editing, formal analysis. Jingsong You: project administration, supervision, writing – review and editing, funding acquisition, resources. Haozheng Sun: validation, formal analysis, writing – review and editing.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File 1: anie72815‐sup‐0001‐SuppMat.pdf.

The authors have cited additional references within the Supporting Information [64, 65, 66, 67, 68, 69, 70, 71, 72].

Data Availability Statement

The data that supports the findings of this study are available in the Supporting Information of this article.