IntroductionManufacturers use free-radical polymerization to make 45% of the world’s plastics1. Thermally cured polymers often employ thermal initiators, which necessitate high activation temperatures and extended processing times, leading to high energy consumption. Photoinitiators are an energy-saving alternative in which light initiates polymerization and are commonly employed for acrylate resins. Photoinitiation benefits include rapid polymerization ( 97%), and (Ir[dF(CF3)ppy]2(dtbpy))PF6 (IrPC; Mw = 1121.91 g·mol−1), phosphate buffered saline (PBS, pH 7.2–7.6), 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone (Irgacure 2959; Mw = 224.26 g·mol−1; > 98%), toluene (Mw = 92.14 g·mol−1, > 99.9%), 2-ethylhexyl acrylate (2-EHA; 184.27 g·mol−1, 98%), aluminum oxide (Al2O3, activated basic), cyclohexane (CHX, anhydrous, 99.5%), tetrahydrofuran (THF, anhydrous, ≥99.9%), hexafluorobenzene (HFB, NMR grade, ≥99.5%) and AlamarBlue were purchased from Sigma Aldrich, Singapore. All reagents were used as received. Methanol (MeOH, analytical reagent grade, ≥99.9%), N,N-dimethylformamide (DMF, HPLC grade), and dichloromethane (DCM, HPLC grade, ≥99.8%) were purchased from Fisher Scientific. Chloroform-d (CDCl3, D, 99.8%) is from Cambridge Isotope Laboratories.Detailed procedures performed in this work are included in the Supplementary Information.ResultsScope: initiation across wavelength, concentration, solvent, and demonstrator conditionsWe investigate aryl diazirines with various pendant groups to assess their ability to initiate acrylate polymerization under various resins, wavelengths, and solvent conditions. Acrylate-based resins serve as the model system due to their commercial relevance. TPO serves as a performance benchmark. Table S1 summarizes the aryl diazirine structures, most of which are commercially available. A graphical summary of the current applications, scope, and mechanisms evaluated is provided in Fig. 1. Primary structure–property relationships focus on wavelength, initiator concentration, joule dose, and solvents correlated to G*, normal force, cytotoxicity, and leachates. Photorheometry links the chemical and viscoelastic properties. Chemical and in vitro toxicity assays benchmark attributes missing in current photoinitiators.Fig. 1: Outline of aryl diazirine as a photoinitiator.Full size image(Top) Current applications of aryl diazirines39,40. (Middle left) Previous work showed that a carbene-based bioadhesive initiated polymerization of a liquid acrylate upon irradiation with 365 nm UVA light (increase in G*). (Middle right) Overview of the advantages of aryl diazirine photoinitiators derived in this work. (Bottom) A carbene or diazoisomer from diazirine activation is hypothesized to be responsible for the initiation of vinyl monomers through a triplet carbene. R1 denotes the different aryl substituents that were evaluated in this work.Diazirines initiate PEGDA polymerization in both UVA and visible lightWe investigate the influence of activation wavelength, initiator concentration, joule exposure, and formulation environments (e.g., neat, aqueous, organic solvent) on the polymerization processes initiated by the aryl diazirine, and subsequently compare the results obtained with those achieved using a commercial photoinitiator (TPO). If aryl diazirines initiate polymerization, photorheometry allows correlations to viscoelastic and chemical properties. Four wavelengths cover the most common light-activation scenarios: LEDs at 365, 385, 405, and 445 nm. The latter two evaluate lower-energy visible spectra sought in consumer and biomedical applications. UV-Vis spectroscopy reveals that all aryl diazirines exhibit molar absorptivities greater than 200 M−1·cm−1 at 365 nm, making them suitable for pairing with UVA LEDs (Fig. 2). Electron-rich diazirine VI absorbs both UVA and visible light, a potential replacement for TPO. UVA (defined as 315–400 nm) poses less risk than UVB or UVC, but exposure should be below the regulatory guideline of 27 J·cm−2 9.Fig. 2: Emission of LEDs and photoinitiator absorbance.Full size imageThe left axis is the molar absorptivity M−1·cm−1 against wavelength (nm) of the different photoinitiators in chloroform. The right axis shows the emission spectra of normalized intensity versus wavelength (nm) for Thorlabs LED sources with peak wavelengths of 365 nm, 385 nm, 405 nm, and 445 nm. Measured emission values are normalized to 1.Irradiating a PEGDA-based formulation containing 1 mol.% I with 365 nm UVA light results in an increase in the G* from 0.08 MPa (liquid) to above 10 MPa (solid) (Fig. 3A); also observed using TPO as the photoinitiator (Fig. S6), but not in the PEGDA control samples. The result achieved by PEGDA with 1 mol.% I aligns with observations of CaproGlu-cured resin5. Compound VI’s ethylene glycol substitution extends its absorbance to overlap with the emission profile from a 405 nm LED and achieves a similar G* of 10 MPa (Fig. 3C). The complex modulus (G*) of PEGDA initiated with aryl diazirines at 365 nm shows equivalent results (no statistical difference) to TPO, as shown in Fig. 3C. At 445 nm, the aryl diazirines have molar absorptivities of 50 MPa; NSD to BAPO) at 365 nm, as shown in Fig. 3D. Curing with IV and VI is completed within 2 J·cm−2 at 405 nm, whereas V is not due to its lack of molar absorbance at 405 nm (Fig. S9). While technically feasible, diazirines require higher doses (~5 times that of BAPO) to achieve 10 MPa in these studies (Fig. S10), thus limiting commercial and high-throughput applications. Instead, printing with diazirines would be suitable for applications where non-toxicity requirements dominate, such as millimeter-sized scaffolds for tissue engineering.Aryl diazirines initiate PEGDA polymerization from 0.1 to 10 mol.%The concentration of photoinitiators influences the mechanical properties of cured resin. This is especially true for aryl diazirines, which assume different roles depending on concentration: at higher mol.%, they act as foaming agents due to nitrogen gas evolution and as adhesive crosslinkers via carbene insertion chemistry. However, higher amounts enhance crosslinking but reduce molecular weight; thus, it is important to evaluate the mechanical properties across different concentration ranges to determine the future utilities of aryl diazirine photoinitiators. Aryl diazirines were tested at 0.1 mol.%, 1 mol.%, and 10 mol.% for modulus and force (Fig. 4A, B). All three levels produce similar G* within 1 J·cm−2. The normal force increases with aryl diazirine concentration, causing shrinkage at low concentrations and expansion at high concentrations. This suggests that shrinkage can be offset by trial and error with aryl diazirines, a unique advantage over other photoinitiators.Fig. 4: Photorheometry of PEGDA with I at 0.1, 1, and 10 mol.% concentrations irradiated at 365 nm for 10 J·cm−2.Full size imageA Zoomed-in complex modulus (G*, Pa) against time (min) for 0.1 to 10 mol.% I in PEGDA exposed up to 1 J·cm−2. B Normal force (N) against time (min) and dose (J·cm−2) for samples exposed to up to 10 J·cm−2 at 365 nm measured simultaneously. C Complex modulus (G*, Pa) against time (min) and dose (J·cm−2) for 1:4, 1:1, and 4:1 ratios of sample (PEGDA with 1 mol.% III) to water irradiated at 365 nm. It is noted that delamination occurs for the samples containing 50% water. This was not observed for the other ratios. D Complex modulus (G*, Pa) against time (min) and dose (J·cm−2) of 1:4, 1:1, and 4:1 ratios of sample (PEGDA with 1 mol.% III) to toluene irradiated at 365 nm. Normal force results are included in Fig. S11.Diazirines initiate in diluted water and toluene solventsLaboratory syntheses of free-radical polymers generally occur in dilute solvents, ideal for subsequent analysis12. Aryl diazirine solubility outperforms all commercial initiators—miscibility in organics (I, II, IV, V, VI), aqueous (III, IV), and neat resins (I, II, IV, V, VI) presents novel dispensing and formulation opportunities.Aryl diazirines were evaluated in dilute environments against TPO, BAPO, I2959, and LAP (Fig. S12, see Table S4 for structures). PEGDA of 20, 50, and 80 wt.% were screened in dH2O and toluene, chosen for their radical-scavenging abilities—see Table S1 for solubility13,14,15. Compound III displays the best performance, summarized in Fig. 4C, D. PEGDA (20 wt.%) in water yields a 100 kPa elastic film with transparent, flexible properties. It also outperforms TPO when comparing G* (100 kPa; III vs. 10 kPa; TPO, Table S5) in water and toluene (2.1 MPa; III vs. 1 MPa; TPO, Table S5).The monomer degree of conversion (DC) or molar mass could account for these observations, as summarized in Tables S6 and S7 and aligned with previously reported results16. Neat samples show no statistical difference between the 1 mol.% aryl diazirine samples and the controls. All aryl diazirines outperform LAP, likely due to its poor solubility. Under water dilution, the aryl diazirine DC% decreases considerably. Compounds I, II, and VI fail to polymerize PEGDA, as evidenced by DC values below 2%. No statistical difference was observed between the DC% of BAPO and I2959 against III and IV. Note that these studies limited irradiation to 2 J·cm−2.Benchmarking the utility of aryl diazirines to TPOTable 1 summarizes the performance of the aryl diazirines evaluated in the study compared to TPO.Table 1 Benchmark of aryl diazirines to TPO as studied in this work.Full size tableThis section benchmarks the aryl diazirines against TPO, the current market-leading photoinitiator, across a broad range of criteria. Five of the six (all except III) are liquids at 25 °C, whereas TPO is a crystalline solid, simplifying handling and eliminating the dissolution step required for solid initiators. All evaluated photoinitiators, including TPO, are soluble in neat PEGDA and toluene, whereas only III and IV are soluble in water. All photoinitiators activate under UV light (