Design of Novel Photoinitiators for Radical and Cationic

Article pubs.acs.org/Macromolecules

Design of Novel Photoinitiators for Radical and Cationic Photopolymerizations under Near UV and Visible LEDs (385, 395, and 405 nm). Jing Zhang,† Michel Frigoli,‡ Frédéric Dumur,§ Pu Xiao,*,† Laura Ronchi,‡ Bernadette Graff,† Fabrice Morlet-Savary,† Jean Pierre Fouassier,† Didier Gigmes,‡ and Jacques Lalevée*,† †

UMR CNRS 7361, ENSCMu-UHA, Institut de Science des Matériaux de Mulhouse IS2M, 15, rue Jean Starcky, 68057 Mulhouse Cedex, France ‡ UMR CNRS 8180, UVSQ, Institut Lavoisier de Versailles, 45 avenue des Etats-Unis, 78035 Versailles Cedex, France § Aix-Marseille Université, CNRS, ICR, UMR 7273, F-13397 Marseille, France S Supporting Information *

ABSTRACT: Three novel photoinitiators, namely (2,2′-bithiophen-5-yl)(4-(N,N′dimethylaminophenyl)ketone, 5,10-dimethoxybenzo[j]fluoranthene and 6,6′-(((1E,1′E)(2,5-bis(octyloxy)-1,4-phenylene)bis(ethene-2,1-diyl))bis(4,1-phenylene))bis(1,3,5-triazine2,4-diamine) applicable to different near UV or visible LEDs (385 nm, 395 nm, 405 nm or cold white LED) have been developed. When incorporated into multicomponent photoinitiating systems PISs (in the presence of iodonium salt (and optionally N-vinyl carbazole) or amine/alkyl halide couples), they exhibit quite excellent photoinitiating abilities for the cationic polymerization CP of epoxides or the free radical polymerization FRP of methacrylates under air. Compared to the corresponding camphorquinone-based systems, the newly developed photoinitiating systems display noticeably higher polymerization efficiencies under air (epoxide conversions = 31−55% vs ∼0%, halogen lamp exposure; methacrylate conversion = 56−66% vs 0−8%, LED irradiation). These systems are very interesting to overcome the oxygen inhibition. The photochemical mechanisms have been studied by steady state photolysis, electron spin resonance spin trapping, fluorescence, cyclic voltammetry, and laser flash photolysis techniques.

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INTRODUCTION Light-emitting diodes (LEDs) are attracting increasing attentions for photopolymerization applications and demonstrating an enormous potential as substitutes of traditional existing lamps due to their advantages including better light output, higher operating efficiency and lower cost.1 However, the commercial photoinitiators (e.g., benzophenone, 2, 2-dimethoxy-2-phenylacetophenone, etc.) usually exhibit good absorption in the classical UV range (300−370 nm) but often suffer from poor light absorption properties for λ > 380 nm. To successfully use LEDs in photopolymerization reactions, the design and development of photoinitiators (PIs) with adapted absorption wavelengths and excellent photochemical properties is one of the most important points.2−6 Recently, several PIs and photoinitiating systems (PISs) have been developed and found applications in near UV or visible LED-induced polymerization reactions.7−16 However, the progress must continue and it is still a challenge to develop novel PISs for both free radical polymerization (FRP) and/or cationic polymerization (CP) reactions under the available UV or visible LEDs. In the present paper, we design and develop three novel photoinitiators THBP, NANA, and T1 (Scheme 1 - (2, 2′-bithiophen-5-yl)(4-(N,N′-dimethylaminophenyl)ketone (THBP); 5,10-dimethoxybenzo[j]fluoranthene (NANA) and © 2014 American Chemical Society

6,6′-(((1E,1′E)-(2,5-bis(octyloxy)-1,4-phenylene)bis(ethene2,1-diyl))bis(4,1-phenylene)) bis(1,3,5-triazine-2,4-diamine)) (T1)) with light absorption maxima at around 400 nm. These photoinitiators are particularly adapted for exposure to near UV or blue LEDs (385, 395, 405, 455 nm) as well as to a cold white LED. They are used in the presence of iodonium salt (and optionally N-vinyl carbazole) or amine/alkyl halide couples. The mechanism and ability of the photoinitiator/ additive(s) combinations to photochemically produce reactive species (i.e., radicals and cations) are studied using steady state photolysis, electron spin resonance spin trapping, fluorescence, cyclic voltammetry, and laser flash photolysis techniques. The photoinitiating ability of the newly developed PISs for the FRP of methacrylates and the CP of epoxides upon exposure to various UV or visible LEDs or very soft polychromatic visible lights (from a halogen lamp) is investigated using real-time Fourier transform infrared spectroscopy (RT-FTIR). A comparison with reference PISs is also provided to assess the high reactivity/efficiency of the newly proposed PISs. Received: March 25, 2014 Revised: April 23, 2014 Published: May 2, 2014 2811

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Scheme 1. Chemical Structures of the Photoinitiators (THBP, NANA, and T1) Proposed in This Study

Scheme 2. Chemical Structures of Additives and Monomers

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the studied PIs in the absence and the presence of the Iod quencher, respectively; τ0 stands for the lifetime of the excited PIs in the absence of Iod). Laser Flash Photolysis. Nanosecond laser flash photolysis (LFP) experiments were carried out using a Q-switched nanosecond Nd/ YAG laser (λexc = 355 nm, 9 ns pulses; energy reduced down to 10 mJ) from Continuum (Minilite) and an analyzing system consisted of a ceramic xenon lamp, a monochromator, a fast photomultiplier and a transient digitizer (Luzchem LFP 212).19 Redox Potentials. The oxidation potentials (Eox vs SCE) of the studied PIs were measured in solutions by cyclic voltammetry with tetrabutylammonium hexafluorophosphate (0.1 M) as a supporting electrolyte (Voltalab 6 Radiometer). The working electrode was a platinum disk and the reference electrode was a saturated calomel electrode (SCE). Ferrocene was used as a standard, and the potentials determined from the half peak potential were referred to the reversible formal potential of this compound (+0.44 V/SCE). The free energy change ΔG for an electron transfer between the PIs and Iod can be calculated from the classical Rehm−Weller equation: ΔG = Eox − Ered − ES (or ET) + C, where Eox, Ered, ES (or ET), and C are the oxidation potential of the PIs, the reduction potential of Iod, the excited singlet (or triplet) state energy of the PIs, and the electrostatic interaction energy for the initially formed ion pair, generally considered as negligible in polar solvents.20 ESR Spin Trapping (ESR-ST) Experiment. ESR-ST experiment was carried out using an X-Band spectrometer (MS 400 Magnettech). The radicals were generated at room temperature upon the Xe−Hg lamp (λ > 330 nm) exposure under N2 and trapped by phenyl-N-tertbutylnitrone (PBN) according to a procedure21 described elsewhere in detail. The ESR spectra simulations were carried out using the WINSIM software. Photopolymerization Experiments. For photopolymerization experiments, the conditions are given in the figure captions. The photocurable formulations were deposited on a BaF2 pellet under air or in laminate (25 μm thick) for irradiation with different lights. The evolution of the double bond content of Bis-GMA/TEGDMA blend and the epoxy group content of EPOX were continuously followed by

EXPERIMENTAL SECTION

Materials. The investigated photoinitiators (i.e., THBP, NANA, and T1) and other chemical compounds are shown in Schemes 1 and 2. THBP, NANA, and T1 were synthesized according to the procedures presented in detail in the Supporting Information. Diphenyliodonium hexafluorophosphate (Iod), N-vinylcarbazole (NVK), methyl diethanolamine (MDEA), 2,4,6-tris(trichloromethyl)1,3,5-triazine (R′−Cl), and the other reagents and solvents were purchased from Sigma-Aldrich or Alfa Aesar with the highest purity available and used as received without further purification. (3,4epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate (EPOX) was obtained from Cytec and used as benchmark monomer for cationic photopolymerization, while bisphenol A-glycidyl methacrylate (Bis-GMA) and triethylene glycol dimethacrylate (TEGDMA) were obtained from Aldrich (the highest purity available) and used for radical photopolymerization. Computational Procedure. Molecular orbital calculations were carried out with the Gaussian 03 package. The electronic absorption spectra for the different compounds were calculated with the timedependent density functional theory at B3LYP/6-31G* level on the relaxed geometries calculated at UB3LYP/6-31G* level; the molecular orbitals involved in these transition can be extracted.17,18 The geometries were frequency checked. Irradiation Sources. Different visible lights were used for the irradiation of photocurable samples: LEDs at 385 nm (∼80 mW cm−2), 395 nm (∼80 mW cm−2), 405 nm (M405L2, ThorLabs; ∼110 mW cm−2), blue LED at 455 nm (M455L3, ThorLabs; ∼80 mW cm−2), cold white LED (400−650 nm; MCWHL5, ThorLabs; ∼80 mW cm−2) and polychromatic light from a halogen lamp (Fiber-Lite, DC-950; incident light intensity: ∼12 mW cm−2 in the 370−800 nm range). The emission spectra of the irradiation sources are given in the Supporting Information (Figures S1−S4). Fluorescence Experiments. The fluorescence properties of the investigated PIs in solution were studied using a JASCO FP-750 Spectrofluorometer. The interaction rate constants kq between the PIs and Iod were extracted from classical Stern−Volmer treatments4 (I0/ I = 1 + kqτ0[Iod], where I0 and I stand for the fluorescent intensity of 2812

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real time FTIR spectroscopy (JASCO FTIR 4100)10,22 at about 1630 and 790 cm−1, respectively.

the generation of new photochemical products during the process. As described in previous reports,8,23,24 various reactive species (radicals (Ph•; Ph−NVK•) and cations (PI•+ and Ph-NVK+)) are generated according to the reactions 1−5 between the excited photoinitiators and Iod (and optionally N-vinylcarbazole - NVK) and can initiate a cationic or a radical polymerization. In PI/MDEA/R′−Cl, different processes can also be expected (reactions 6-11; as in8,23), the generated free radicals (MDEA(−H)• and R′•) being the initiating species of the FRP process. As to THBP/MDEA system, no bleaching was observed during the irradiation (Figure S5 in the Supporting Information) indicating a poor reactivity or occurrence of back electron transfer reaction 8b. Interestingly, upon addition of another additive (R′−Cl), the steady state photolysis of THBP/MDEA/R′−Cl led to a fast increase of the absorption (Figure 3d). It could probably be ascribed to the fact that the excited state of THBP would predominantly interacts with R′−Cl (reaction 6) but not with MDEA (reaction 8) as observed in other systems.16,22,25−28 The MDEA (‑H)• and R′• are excellent initiating species of FRP (see below).

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RESULTS AND DISCUSSION 1. Light Absorption of the Studied Photoinitiators. The light absorption spectra of THBP or NANA in acetonitrile, and T1 in N,N-dimethylformamide are given in Figure 1. For

Figure 1. UV−vis absorption spectra of THBP or NANA in acetonitrile, and T1 in N, N-dimethylformamide.

PI →1 PI (hν) 1,3

THBP and NANA, the maxima are located in the UV light range (i.e., λmax = 381 nm, ε381 nm ∼ 29400 M−1cm−1 and λmax = 387 nm, ε387 nm ∼ 6700 M−1cm−1 for THBP and NANA, respectively). As to the spectrum of T1, it presents a much better light absorption (λmax = 416 nm, ε416 nm ∼ 70400 M−1cm−1). The absorption spectra of the three PIs exhibit an interesting matching with the emission spectra of the near UV or visible LEDs and the halogen lamp as well (Table 1). For THBP, the excellent absorption properties are associated with a HOMO → LUMO charge transfer transition; HOMO and LUMO stand for the highest occupied molecular orbital and the lowest unoccupied molecular orbital, respectively. In Figure 2, it can be noted that the HOMO and LUMO are mainly located on the dimethylphenylamino (donor group) and the bithiopheneketone (acceptor group) moieties, respectively. For NANA, a HOMO → LUMO charge transfer transition is observed too in which the naphthalene functionalized at the peri-positions is acting as an acceptor group and the other one as donor group. For T1, the HOMO and LUMO are delocalized; the better absorption of T1 is probably due to the highly enhanced π electron delocalization in the structure. 2. Photochemical Reactivity of the Studied Photoinitiators with Additives. The steady state photolysis of THBP/Iod, NANA/Iod or T1/Iod in acetonitrile is given in Figure 3 (a)-(c) (halogen lamp exposure under air). The fast decrease of the ground state absorption band of the THBP/Iod or T1/Iod couples demonstrates a high interaction between the two partners. For NANA/Iod, the absorption at 387 nm increased fast with the irradiation time, which was ascribed to

1

PI →3 PI

and

(1)

PI + Ph 2I+ → PI•+ + Ph 2I•

(2)

Ph 2I• → Ph• + Ph−I

(3)

Ph• + NVK → Ph−NVK•

(4)

Ph−NVK• + Ph 2I+ → Ph−NVK+ + Ph• + Ph−I

(5)

1,3

PI + R′−Cl → PI•+ + (R′−Cl)•−

(6)

PI•+ + MDEA → PI + MDEA•+ → PI + MDEA (−H)• + H+ 1,3

(7)

PI + MDEA → PI•− + MDEA•+ → PI−H• + MDEA (−H)•

(8a)

PI•− + MDEA•+ → PI + MDEA •−

PI

(8b)

•−

+ R′−Cl → PI + (R′−Cl)

(9)

PI−H• + R′−Cl → PI + H+ + (R′−Cl)•− •−

(R′−Cl)

•

(10)

−

→ R′ + Cl

(11)

The phenyl radical-PBN adducts observed in ESR spin trapping experiment (see e.g. in the irradiated THBP/Iod couple in Figure 4) support the formation of phenyl radicals in the PI/Iod electron transfer reaction (reactions 1-3). The intensity of the THBP fluorescence increased upon decreasing the solvent polarity (i.e., no fluorescence observed in acetonitrile but the fluorescence quantum yield Φfluo is very

Table 1. Light Absorption Properties of the Studied Dyes and Their Extinction Coefficients at Maximum Emission Wavelengths of the Different Irradiation Devices

a

dyes

λmax (nm)

εmax (M−1 cm−1)

ε385 nm (M−1 cm−1)a

ε395 nm (M−1 cm−1)a

ε405 nm (M−1 cm−1)a

ε455 nm (M−1 cm−1)a

THBP NANA T1

381 387b 416

29 400 6700 70 400

29 200 6300 46 600

25 700 3300 58 600

18 400 1700 66 000

600 580 24 100

for different UV or visible LEDs. bmaximum absorption wavelength in the near visible range. 2813

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Figure 2. Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of THBP, NANA, and T1 at UB3LYP/ 6-31G* level (isovalue = 0.04).

Figure 3. Steady state photolysis of (a) THBP/Iod, (b) NANA/Iod in acetonitrile, and (c) T1/Iod in acetonitrile/toluene (50%/50%, v/v) ([Iod] = 24 mM); (d) THBP/MEDA/R′−Cl in acetonitrile ([MDEA] = [R′−Cl ] = 24 mM) upon the halogen lamp exposure; UV−vis spectra recorded at different irradiation time.

weak with a value estimated of 1.1 × 10−3 in acetonitrile/ toluene (50%/50%, v/v)). Interestingly, a new fluorescence emission at 453 nm appeared and increased upon addition of Iod (Figure 5): this might be attributed to the formation of a complex between THBP and Iod (rather than to photolysis products generated after absorption of the excitation light of the fluorescence spectrometer: see in Figure S6 in the Supporting Information where no change of the UV−vis absorption of THBP/Iod was observed after the fluorescence measurement). The fluorescent properties of NANA and T1 ensure efficient interactions between their singlet states and Iod (Table 2). The results summarized in Table 2 demonstrate that (i) the 1NANA (or 1T1)/Iod interaction rate constants (kq ∼ 1010 M−1 s−1) are very high indicating that the processes are almost diffusion-controlled

(see e.g. Figure 6); (ii) the free energy changes ΔG for the PI/Iod electron transfer reactions are highly negative making the processes favorable and (iii) the 1NANA/Iod electron transfer quantum yield ΦeT is higher than that of 1T1/Iod, which illustrates the higher polymerization efficiency of 1NANA/Iod system (see below). In laser flash photolysis experiments, the transient absorption spectra of THBP, NANA and T1 following the laser excitation at 355 nm (Figure 7) are ascribed to their triplet states having lifetime values of 49.0 μs, 27.0 and 69.4 μs (Figure S7 in the Supporting Information), respectively. These transients are quenched by oxygen. Their low intensities probably result from limited triplet quantum yields due to a low intersystem crossing from the singlet to the triplet state; the 3PI/Iod interaction rate constants cannot be safely determined. Herein, the singlet route 1

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ΔG ∼ −0.57 eV; using a calculated triplet state energy level (ET) ∼ 1.82 eV at UB3LYP/6-31G* level). 3. Photoinitiating Ability of the Investigated PISs. a. Cationic Photopolymerization of Epoxides. Under the different UV or visible LEDs as well as the halogen lamp irradiation, the studied PISs initiate the ring-opening cationic polymerization of EPOX (Figure 8 and Table 3) under air. The THBP/Iod or NANA/Iod systems were very efficient with the LED at 405 nm (final conversions around 60%) and tack-free coatings are obtained, while T1/Iod was not so excellent (final conversion =33%). With the addition of another known32 additive (NVK), the efficiency was significantly improved (see T1/Iod/NVK vs T1/Iod; Figure 8c, curve 2 vs curve 1). But no effect was observed when adding NVK to the THBP/Iod or NANA/Iod systems, which may be due to the fact that the twocomponent PISs were already very efficient (i.e., the reactivities of THBP•+ or NANA•+ toward the epoxide are therefore excellent for the initiation of the CP process) and the final conversions would be limited by the high cross-linking degree of the polymer network (as in ref 33). The THBP (or NANA)/ Iod/NVK PIS was also very efficient upon the LED at 455 nm or the cold white LED exposure: tack-free coatings are also obtained. Interestingly, contrary to NANA/Iod/NVK, the THBP/Iod/NVK system can also work very well under the very soft halogen lamp (∼12 mW cm−2) irradiation (conversion = 55% vs 31%). Remarkably, all these PISs behave much more efficiently than the well-known2,4 camphorquinone (CQ) based system (CQ/Iod or CQ/Iod/NVK) which was totally inefficient under the conditions employed here (i.e., halogen lamp; Table 3). The THBP/Iod/NVK PIS was pretty stable in the EPOX formulation after 1 week of storage at room temperature as only 3% of change for the final EPOX conversion was noted (Figure 8a, curve 2 vs curve 2′). b. Free Radical Photopolymerization of Methacrylates. The investigated PISs can also initiate the FRP of methacrylates

Figure 4. ESR spectra of the radicals generated in THBP/Iod and trapped by PBN in tert-butylbenzene: (a) experimental and (b) simulated spectra. PBN/phenyl radical adducts obtained in THBP/ Iod: aN = 14.2 G, aH = 2.2 G in agreement with refs 29 and 30.

Figure 5. Fluorescence emission of THBP in the presence of different concentrations of Iod in acetonitrile/toluene (50%/50%, v/v).

predominates in the PI/Iod reaction; a triplet route would also be energetically favorable (Table 2; e.g. for 3NANA/Iod,

Table 2. Parameters Characterizing the Reactivity of the Photoinitiators: Fluorescence Quantum Yields (Φfluo) and Fluorescence Lifetime (τ) PIs f

THBP NANAg T1f

Es (eV)a

ET (eV)a

Eox (V vs SCE)b

ΔGs (eV)c

ΔGT (eV)c

kq (×1010 M−1 s−1)d

ΦeTe

Φfluo

τ (ns)

2.91 2.65 2.70

1.96 1.82 1.54

0.86 1.05 0.70

−1.85 −1.40 −1.80

−0.9 −0.57 −0.64

− 1.8 2.4

− 0.71 0.57

1.1 × 10−3 − 0.704

2.4 2.9 1.2

Singlet state energies ES extracted from the UV−vis absorption and fluorescence emission spectra as usually done;31 triplet state energies ET calculated at UB3LYP/6-31G* level. bEox values measured by cyclic voltammetry (this work). cΔGs = free energy change for the PI/Iod singlet state interaction; ΔGT = free energy change for the PI/Iod triplet state interaction; reduction potential Ered = −0.2 V2 for Iod. dkq = 1PI/Iod interaction rate constant measured by fluorescence quenching experiments. eΦeT = PI/Iod electron transfer quantum yields in the singlet state calculated according to ΦeT = kqτ0 [Iod]/(1+ kqτ0 [Iod])2, ([Iod] = 4.7 × 10−2 M). f In acetonitrile/toluene (50%/50%, v/v). gIn acetonitrile. a

Figure 6. (a) Fluorescence spectra of NANA as a function of [Iod] and (b) the corresponding Stern−Volmer plot. In acetonitrile. 2815

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Figure 7. Transient absorption spectra recorded 4 μs after the laser excitation (at 355 nm) of (a) THBP in nitrogen-saturated acetonitrile/toluene (50%/50%, v/v), (b) NANA in nitrogen-saturated acetonitrile, and (c) T1 in nitrogen-saturated acetonitrile/toluene (50%/50%, v/v).

Figure 8. Photopolymerization profiles of EPOX under air in the presence of (a) THBP-based PISs, (b) NANA-based PISs, and (c) T1-based PISs. PIs/Iod (0.5%/2%, w/w) upon the LED at 405 nm exposure (curve 1); PIs/Iod/NVK (0.5%/2%/3%, w/w/w) upon the LED at 405 nm (curve 2; curve 2′-measured after 1 week of storage at room temperature), LED at 455 nm (curve 3), cold white LED (curve 4), and halogen lamp (curve 5) exposure.

(bis-GMA/TEGDMA blend 70%/30%) under air or in laminate upon exposure to various UV or visible LEDs (Figure 9 and Table 4). THBP alone had a low efficiency under the LED

405 nm (conversion 60% were obtained upon the LED 405 or 455 nm, and 54% and 47% with the cold white LED or the halogen lamp. The NANA based PISs are a little less efficient than the THBP based systems (e.g., conversion =58% with NANA/MDEA/R′−Cl under the LED 405 nm irradiation; Figure 9b and Table 4). The efficiency of T1/Iod/NVK was very low (conversion 52%. bMeasured after 1 week of storage at room temperature. cNo polymerization.

the surrounding atmosphere to the samples was decreased). The addition of Iod significantly improved the polymerization

Figure 9. Photopolymerization profiles of Bis-GMA/TEGDMA blend (70%/30%, w/w) in the presence of the following. (a) THBP-based PISs: (curve 1) THBP alone under air; (curve 2) THBP alone in laminate; (curve 3) THBP/Iod unde air; (curve 4) THBP/Iod in laminate; (curve 5) THBP/Iod/ NVK under air; (curve 6) THBP/Iod/NVK in laminate upon the LED at 405 nm exposure; (curve 7) THBP alone in laminate; (curve 8) THBP/Iod/ NVK under air upon the LED at 455 nm exposure; (curve 9) THBP/MDEA/R′−Cl under air upon the LED at 405 nm, LED at 455 nm(curve 10); cold white LED (curve 11); halogen lamp (curve 12) exposure. (b) NANA- or T1-based PISs under air: NANA/Iod/NVK upon the LED at 405 nm (curve 1) and LED at 455 nm (curve 2) exposure; NANA/MDEA/R′−Cl upon the LED at 405 nm (curve 3) and LED at 455 nm (curve 4) exposure; T1/Iod/NVK upon the LED at 405 nm (curve 5) and LED at 455 nm (curve 6) exposure; T1/MDEA/R′−Cl upon the LED at 405 nm (curve 7) and LED at 455 nm (curve 8) exposure (THBP, NANA, or T1, 0.5 wt %; Iod or MDEA, 2 wt %; NVK or R′−Cl, 3 wt % in the formulations).

Table 4. Photopolymerization Conversions of Bis-GMA/TEGDMA Blend (70%/30%, w/w) Obtained under Air or in Laminate upon Exposure to LED at 385, 395, or 405 nm, LED at 455 nm, Cold White LED or Halogen Lamp for 300 s in the presence of THBP-, NANA-, or T1-Based PISs (THBP, NANA, or T1, 0.5 wt %; Iod or MDEA, 2 wt %; NVK or R′−Cl, 3 wt % in the Formulations); or CQ/Iod/NVK (0.5%/2%/3%, w/w/w) and CQ/MDEA (0.5%/2%, w/w) PISs as References conversion (%) PISs THBP THBP/Iod THBP/Iod/NVK THBP/MDEA/R′−Cl NANA/Iod/NVK NANA/MDEA/R′−Cl T1/Iod/NVK T1/MDEA/R′−Cl CQ/Iod/NVK CQ/MDEA a

LED 385 nm

61a

LED 395 nm

62a

LED 405 nm

LED 455 nm