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The high molar absorptivity of thioxanthone in the near-UV spectral region makes this chromophore attractive as a photoinitiator for free-radical polymerization. Recent advances in blue LEDs have made these low-cost, highly energy-efficient light sources available for use in photo-curing tools, which generates demand for blue-light-absorbing photoinitiators. In this chapter we review recent progress to shift the absorbance of thioxanthone into the visible spectral region using heterocyclic extension with thiophene, benzothiophene, indole, triazole and others. First, the photophysical properties of these blue-light-absorbing chromophores are discussed followed by examining their photoreactivity. Finally, the efficacies of these heterocyclic extended thioxanthones as photoinitiators for free-radical polymerization of acrylates are evaluated. The polymerization efficacies are compared to photoinitiation with the popular camphorquinone/amine system. The improved performance of these new thioxanthone derivatives compared to camphorquinone is mostly caused by the orders of magnitude higher molar absorptivity of the heterocyclic extended thioxanthones.

For many decades, photopolymerization has been the basis for commercial applications in coatings, adhesives, dental composites, inks, printing plates, and microelectronics.1,2  The key ingredient in these photopolymerization formulations is the photoinitiator.3  A good match of the absorption spectrum of the photoinitiator with the light source is of major importance. In the past, photo-curing tools with light sources emitting in the near-UV spectral region (350–400 nm) have dominated. For that reason, most commercially used photoinitiators are tailored to the near-UV spectral region. For example, thioxanthone-based photoinitiators show some of the best light absorption properties in the near-UV spectral region and are therefore widely used.4  However, UV light penetration of pigmented coatings and composite materials is poor and can lead to incomplete curing, especially for thicker layers. Shifting the initiator absorbance and light source into the visible, blue spectral region has been shown to improve the light penetration problem. In addition, recent progress in the development and mass production of blue LEDs has made these low-cost, highly energy efficient light sources attractive in photo-curing tools, which has increased the demand for blue-light absorbing photoinitiators.

Bathochromatically shifted absorption of thioxanthone can be achieved with appropriate substituents. Table 1.1 lists such examples, including thioxanthone derivatives, that have been used as photoinitiators for polymerization. Only small bathochromic shifts were observed for most substituents. Dibromination (6) shifts the absorption by ∼15 nm while the triplet energy and photoreactivity remains high.5  A much larger bathochromic shift was observed with amino substituents (7). However, amino substitution alters the thioxanthone photoreactivity.

Table 1.1

Thioxanthone derivatives with electron-donating substituents: Absorption maximum at the longest wavelength peak (λmax) and molar absorptivity (ε).

Thioxanthone derivativeλmax (nm)ε at λmax (M−1 cm−1)
 380 (benzene)6  6600 (benzene)6  
381 (DMF)5  6328 (DMF)5  
 386 (benzene)6  6900 (benzene)6  
 383 (THF)7  3857 (THF)7  
 388 (benzene)6  6700 (benzene)6  
 388 (DMF)5  4941 (DMF)5  
 396 (DMF)5  5234 (DMF)5  
 438 (THF)8  4470 (THF)8  
Thioxanthone derivativeλmax (nm)ε at λmax (M−1 cm−1)
 380 (benzene)6  6600 (benzene)6  
381 (DMF)5  6328 (DMF)5  
 386 (benzene)6  6900 (benzene)6  
 383 (THF)7  3857 (THF)7  
 388 (benzene)6  6700 (benzene)6  
 388 (DMF)5  4941 (DMF)5  
 396 (DMF)5  5234 (DMF)5  
 438 (THF)8  4470 (THF)8  

In this chapter, we explore heterocyclic extended thioxanthone derivatives, which shift the absorption bathochromically more significantly than simple substitutions. The structures are shown in Scheme 1.1. We discuss their photophysical properties, photoreactivity and the mechanisms to initiate polymerization.

Scheme 1.1

Heterocyclic extended thioxanthones.

Scheme 1.1

Heterocyclic extended thioxanthones.

Close modal

Heterocyclic extended thioxanthones are synthetically accessible by the condensation reaction of thiosalicylic acid with heterocyclic compounds such as benzothiophene, dibenzothiophene, carbazole and benzotriazole in the presence of concentrated H2SO4. Examples are shown in Scheme 1.2.9–12 

Scheme 1.2

Synthesis of heterocyclic extended thioxanthones.

Scheme 1.2

Synthesis of heterocyclic extended thioxanthones.

Close modal

The absorption spectra of thioxanthone and heterocyclic extended thioxanthones are shown in Figure 1.1. The spectra reveal that the cyclic extension causes major bathochromic shifts of up to 100 nm in addition to large variations in molar absorptivities. Extension of thioxanthone with thiophene (8) has little impact on the absorption peak position, however, it increases the molar absorptivity by ∼40%. Benzothiophene extension (9) causes a major bathochromic shift, but reduces the molar absorptivity. Absorbance in the blue spectral region with high molar absorptivity was achieved by bridging two thioxanthone molecules with thiophene (10). This chromophore showed the highest molar absorptivity in the visible spectral region whereas the largest bathochromic shift was observed for 11.

Figure 1.1

Absorption spectra of thioxanthone derivatives in acetonitrile (8, 10) and DMF (9, 11, 12, 13).

Figure 1.1

Absorption spectra of thioxanthone derivatives in acetonitrile (8, 10) and DMF (9, 11, 12, 13).

Close modal

The photophysical parameters of thioxanthone derivatives 813 are summarized in Table 1.2. All compounds show low fluorescence quantum yields (Φf), which is consistent with efficient intersystem crossing into the triplet state, the excited state which generates the radicals to initiate polymerization. Unsubstituted thioxanthone has a triplet quantum yield (ΦT) of 0.76.5  Heterocyclic extension with thiophene (8) increased the triplet quantum yield to 0.93.9  However, the thiophene bridged thioxanthone derivative (10) shows a slightly reduced triplet quantum yield of ΦT=0.5.9  Another important triplet state property is the triplet lifetime (τT). The triplet lifetimes were measured by laser flash photolysis (Table 1.2). A long enough triplet lifetime allows for efficient reactions with co-initiators to generate radicals (see Section 1.3). The unsubstituted thioxanthone shows a triplet lifetime of 45 µs in deoxygenated acetonitrile solutions.5  Although the triplet lifetimes were reduced by heterocyclic extension (see Table 1.2), the triplet lifetimes are long enough for efficient reaction with most co-initiators. The triplet energy (ET), which was determined from the low-temperature phosphorescence spectra, is another property of the triplet state that is relevant to its reactivity. As expected, together with the bathochromic shifted absorption, the triplet energy also decreases by heterocyclic extension. The phosphorescence lifetimes (τp) at 77 K give information on the nature of the triplet state (* vs ππ*).13  The long phosphorescence lifetimes of the investigated heterocyclic extended thioxanthones (Table 1.2) are indicative of a ππ* configuration of the lowest triplet states.

Table 1.2

Heterocyclic extended thioxanthone derivatives: Photophysical properties in different solvents.a

TX-BT 8bTX-DBT 9bTX-DBT-TX 10bTX-FN 11cTX-Cz 12TX-N3 13d
λmax (nm)e 380 (ACN) 403 (DMF) 425 (ACN) 455 (DMF) 434 (DMF)n 393 (DMF) 
ε (M−1 cm−1)f 8760 (ACN) 2610 (DMF) 3830 (ACN) 762 (DMF) 2014 (DMF)n 1208 (DMF) 
ES (kJ mol−1)g 307 (ACN) 257 (Tol) ∼262 (ACN) 237 (DMF) 260 (EtOH)n  
ET (kJ mol−1)h 236 (EtOH) 223 (Tol) 221 (Tol) 218 (MeTHF) 221 (EtOH)n 238 (EtOH) 
Φfi 0.028 (ACN) 0.009 (Tol) 0.003 (ACN) 0.127 (DMF) 0.11 (EtOH)n 0.15 (EtOH) 
0.002 (Tol) 
ΦTj 0.93 (ACN)  0.5 (ACN)    
τP (ms)k 192 (Tol) 85 (Tol) 42 (Tol) 49.9 (MeTHF) 53 (EtOH)n 73 (EtOH) 
222 (EtOH) 64 (EtOH) 
τT (µs)l 18 (ACN)  5.6 (ACN) 7.63 (DMF) 17.5 (ACN)n  
19 (ACN)m 
TX-BT 8bTX-DBT 9bTX-DBT-TX 10bTX-FN 11cTX-Cz 12TX-N3 13d
λmax (nm)e 380 (ACN) 403 (DMF) 425 (ACN) 455 (DMF) 434 (DMF)n 393 (DMF) 
ε (M−1 cm−1)f 8760 (ACN) 2610 (DMF) 3830 (ACN) 762 (DMF) 2014 (DMF)n 1208 (DMF) 
ES (kJ mol−1)g 307 (ACN) 257 (Tol) ∼262 (ACN) 237 (DMF) 260 (EtOH)n  
ET (kJ mol−1)h 236 (EtOH) 223 (Tol) 221 (Tol) 218 (MeTHF) 221 (EtOH)n 238 (EtOH) 
Φfi 0.028 (ACN) 0.009 (Tol) 0.003 (ACN) 0.127 (DMF) 0.11 (EtOH)n 0.15 (EtOH) 
0.002 (Tol) 
ΦTj 0.93 (ACN)  0.5 (ACN)    
τP (ms)k 192 (Tol) 85 (Tol) 42 (Tol) 49.9 (MeTHF) 53 (EtOH)n 73 (EtOH) 
222 (EtOH) 64 (EtOH) 
τT (µs)l 18 (ACN)  5.6 (ACN) 7.63 (DMF) 17.5 (ACN)n  
19 (ACN)m 
a

Acetonitrile (ACN), dimethylformamide (DMF), toluene (Tol), ethanol (EtOH), 2-methyltetrahydrofuran (MeTHF).

b

Data from ref. 9.

c

Data from ref. 14.

d

Data from ref. 12.

e

Absorption maximum at the longest wavelength peak.

f

Molar absorptivity.

g

Energy of the first excited singlet state determined from the interception of the fluorescence excitation and emission spectra.

h

Triplet state energy determined from the first peak of the phosphorescence spectra at 77 K.

i

Fluorescence quantum yield.

j

Triplet quantum yield.

k

Phosphorescence lifetime at 77 K.

l

Triplet lifetime at room temperature.

m

This work.

n

Data from ref. 10.

Thioxanthone, a type II photoinitiator, requires a co-initiator, such as amines, to generate initiating radicals from its triplet excited state (Scheme 1.3). An initial electron transfer reaction generates a radical ion pair, which rapidly undergoes a proton transfer reaction resulting in carbon-centered radicals.15,16 

Scheme 1.3

Photoreaction of thioxanthone with tertiary amines to generate initiator radicals.

Scheme 1.3

Photoreaction of thioxanthone with tertiary amines to generate initiator radicals.

Close modal

Similar to unsubstituted thioxanthone, the discussed thioxanthone derivatives in this chapter function most efficiently as photoinitiators in the presence of amines as the co-initiator. A high bimolecular rate constant for the reaction of the triplet states of the thioxanthone derivatives with amines (kamine) is essential for commercial applications. This rate constant can be determined by laser flash photolysis.17  We will show the methodology to measure the rate constant on the example of the triplet state reaction of 12 with methyldiethanolamine (MDEA), a popular co-initiator used in polymer formulations. Excitation of 12 in an inert solvent, such as acetonitrile, with laser pulses (355 nm, 5 ns pulse width) generates triplet excited states of 12, which can be detected by transient absorption spectroscopy. Figure 1.2 (left) shows the transient spectrum at the end of the laser pulse (integrated for a period of 3 µs). The negative absorption values at ∼300 nm and 430 nm correspond to ground state bleaching of 12 and the positive absorptions were assigned to the triplet state. The inset shows the triplet absorption decay trace monitored at 800 nm, which was fitted to an exponential decay function corresponding to the triplet lifetime of 12 (τT=19 µs). To determine kamine, triplet decay traces were recorded in the presence of different concentrations of MDEA. The inverse lifetimes of these decays were plotted against the MDEA concentration (Figure 1.2, right). The bimolecular rate constant of triplet state quenching by MDEA (kamine) is determined by the slope of this plot.

Figure 1.2

Left: Transient absorption spectrum of 12 in deoxygenated acetonitrile solution recorded 0–3 µs after pulsed laser excitation (355 nm, 5 ns pulse length). Inset: kinetic trace of the triplet absorption decay monitored at 800 nm. Right: Determination of the bimolecular rate constant (kamine) of quenching of triplet states of 12 by methyldiethanolamine (MDEA) from the plot of the inverse triplet lifetimes of 12 determined by laser flash photolysis and monitored at 800 nm vs. the MDEA concentration. These experiments were performed analogous to work previously published for 8 and 10.9 

Figure 1.2

Left: Transient absorption spectrum of 12 in deoxygenated acetonitrile solution recorded 0–3 µs after pulsed laser excitation (355 nm, 5 ns pulse length). Inset: kinetic trace of the triplet absorption decay monitored at 800 nm. Right: Determination of the bimolecular rate constant (kamine) of quenching of triplet states of 12 by methyldiethanolamine (MDEA) from the plot of the inverse triplet lifetimes of 12 determined by laser flash photolysis and monitored at 800 nm vs. the MDEA concentration. These experiments were performed analogous to work previously published for 8 and 10.9 

Close modal

A high rate constant (kamine) of the triplet state reaction of the photoinitiator with amines is essential for efficient initiator radical generation (Scheme 1.3). The rate constants for several thioxanthone derivatives are listed in Table 1.3. High rate constants of 109 M−1 s−1 were observed for all listed thioxanthone derivatives, which ensures efficient initiator radical formation.

Table 1.3

Rate constants of the reaction of triplet states of thioxanthone derivatives with methyldiethanolamine (MDEA).

1281012
kamine (M−1 s−19×109 (Bz)a 6×109 (Bz)a (2.0±0.1)×109 (ACN)b (6.4±0.2)×109 (ACN)b (1.4±0.1)×109 (ACN)c 
1281012
kamine (M−1 s−19×109 (Bz)a 6×109 (Bz)a (2.0±0.1)×109 (ACN)b (6.4±0.2)×109 (ACN)b (1.4±0.1)×109 (ACN)c 
a

Data from ref. 6.

b

Data from ref. 9.

c

Data from this work.

Most thioxanthone derivatives require a co-initiator, such as an amine, to generate initiator radicals from their triplet excited states. However, the carbazole derivative 12 contains a secondary amine, which can react with triplet excited states, as shown in Scheme 1.4, to generate an amino radical (16) and a ketyl radical (17).

Scheme 1.4

Reaction of triplet excited states of 12 with another molecule of 12 in the ground state.

Scheme 1.4

Reaction of triplet excited states of 12 with another molecule of 12 in the ground state.

Close modal

The rate constant for this intermolecular self-reaction (kself) was determined by laser flash photolysis by measuring the triplet state lifetime of 12 over a wide range of concentrations of 12. The slope of the plot, shown in Figure 1.3, gives a rate constant of 4×108 M−1 s−1, which is high enough so that 12 can be used as one-component photoinitiator in the absence of amines as co-initiator.

Figure 1.3

Determination of the bimolecular rate constant (kself) of triplet state quenching of 12 by ground state molecules of 12 from the plot of the inverse triplet lifetimes of 12 determined by laser flash photolysis (355 nm, 5 ns pulse length) monitored at 800 nm vs. the concentration of 12.

Figure 1.3

Determination of the bimolecular rate constant (kself) of triplet state quenching of 12 by ground state molecules of 12 from the plot of the inverse triplet lifetimes of 12 determined by laser flash photolysis (355 nm, 5 ns pulse length) monitored at 800 nm vs. the concentration of 12.

Close modal

As shown in Schemes 1.3 and 1.4, photoreaction of thioxanthone derivatives with amines generates ketyl radicals (e.g.14 and 16) and α-aminoalkyl radicals (15) or amino radicals (17). Radicals 15 and 17 are able to add efficiently to olefinic monomers, such as acrylates, to initiate polymerization. However, the thioxanthone ketyl radicals (e.g.14 and 16) are known to have insufficient reactivity with monomers. Scheme 1.5 shows possible reaction pathways for the ketyl radicals.18  Upon encounter of two ketyl radicals, coupling would generate a pinacol derivative (18). In addition, disproportionation regenerates thioxanthone and thioxanthole (19). Regeneration of thioxanthone can also occur by oxidation with molecular oxygen. Termination reactions of the polymer chain with thioxanthone ketyl radicals leads to incorporation of thioxanthole moieties (20) into the polymer. The reactions shown in Scheme 1.5 for unsubstituted thioxanthone ketyl radicals should also apply to the ketyl radicals generated from the heterocyclic extended thioxanthone derivatives discussed in this chapter.

Scheme 1.5

Possible reactions of thioxanthone ketyl radicals.18 

Scheme 1.5

Possible reactions of thioxanthone ketyl radicals.18 

Close modal

Extensive photopolymerization studies of methyl methacrylate (MMA) were performed to assess the efficiency of the heterocyclic extended thioxanthones (813) as photoinitiators and were reported previously.9–12,14  All thioxanthone derivatives listed in Scheme 1.1 showed efficient initiation of polymerization in the presence of a co-initiator, such as MDEA. This indicates that the reaction mechanism shown in Scheme 1.3 is dominant to generate α-aminoalkyl radicals (15) as the initiating radical. In case of the carbazole derivative 12, which contains a secondary amine that can act as co-initiator, addition of an amine is not essential for efficient photoinitiation of polymerization. Photopolymerization experiments of MMA were performed using 12 as photoinitiator in the presence and absence MDEA (co-initiator).10  The generated polymers were quantified gravimetrically. Experimental conditions were selected such that only low conversion was attained so that differences between the systems can be observed more clearly. Using 12 as the initiator, in the absence of MDEA as co-initiator similar conversion was achieved (4.1% conv.) compared to in the presence of MDEA (3.5% conv.; Table 1.4), demonstrating the 12 can act as a one-component photoinitiator. Furthermore, under our experimental conditions, slightly more polymer formation was observed in air-saturated solutions, suggesting that small amounts of molecular oxygen can be beneficial in some cases, especially for regeneration of the thioxanthone chromophore from the ketyl radical as shown in Scheme 1.5 (bottom left). Because of the bathochromically shifted absorption spectrum into the visible light range, 12 can be used to initiate polymerization as demonstrated in ref. 11. An N-ethyl derivative of 12 has been reported that shows similar properties, but higher solubility in organic solvents, especially non-polar solvents.19  Similar to 12, this N-ethyl derivative also can act as one-component photoinitiator to initiate free radical polymerization due to its amine functionality.

Table 1.4

Photopolymerization of MMA (4.68 M) in DMF using 12 or 21 as photoinitiators.a

[MDEA] (mM)DeoxygenatedConv.b %
1221
10 − 4.7 
10 3.5 2.1 
− 4.1 
[MDEA] (mM)DeoxygenatedConv.b %
1221
10 − 4.7 
10 3.5 2.1 
− 4.1 
a

Irradiation with 400 W medium pressure mercury lamp for 15 min.

b

Data from ref. 10.

Camphorquinone (21) in conjunction with amines is a popular visible light photoinitiator used extensively in dental restorative materials.20,21  Its absorption spectrum is shown in Figure 1.4, which exhibits absorbance in the blue spectral region similar to the heterocyclic extended thioxanthones (Figure 1.1). However, the molar absorptivity of the lowest energy transition of camphorquinone is only 38 M−1 cm−1 owing to the forbidden nature of n,π* transitions. The heterocyclic extended thioxanthones have orders of magnitudes higher molar absorptivities owing to π,π* transitions (Table 1.2), which allows the use of much lower initiator concentrations.

Figure 1.4

Absorption spectrum of camphorquinone (21) in DMF.

Figure 1.4

Absorption spectrum of camphorquinone (21) in DMF.

Close modal

The polymerization efficacy was evaluated under identical experimental conditions using camphorquinone (21) or the thioxanthone derivative 12 as initiators. As shown in Table 1.4, when 21 was used as the photoinitiator, no polymer was observed in the absence of amine or when the polymerization was conducted in the presence of air. Furthermore, significantly lower amounts of polymer were observed under deoxygenated conditions and in the presence of MDEA for 21 (2.1% conv.) compared to 12 (3.5% conv.).

Another example demonstrating the superior performance of thioxanthone derivatives compared to camphorquinone is shown in Figure 1.5. Lauryl acrylate formulations containing MDEA as the co-initiator and 1, 12, or 21 as the photoinitiator were polymerized inside a photo-DSC setup under identical conditions. Significantly faster conversion was achieved with thioxanthones 1 and 12 compared to camphorquinone, 21.14 

Figure 1.5

Conversion of monomer to polymer vs. irradiation time as measured by photo-DSC. Polymerization of lauryl acrylate using 0.1% (w/w) thioxanthone (1), a heterocyclic extended thioxanthone derivative (12) or camphorquinone (21) as the initiator in the presence of 1% (w/w) MDEA.14 

Figure 1.5

Conversion of monomer to polymer vs. irradiation time as measured by photo-DSC. Polymerization of lauryl acrylate using 0.1% (w/w) thioxanthone (1), a heterocyclic extended thioxanthone derivative (12) or camphorquinone (21) as the initiator in the presence of 1% (w/w) MDEA.14 

Close modal

In this chapter we have reviewed recent advances made to shift the absorbance of thioxanthone into the visible spectral region using heterocyclic extension. These blue-light absorbing chromophores exhibit similar photoreactivity compared to the parent thioxanthone. High rate constants of the reaction of triplet excited states with co-initiators, such as MDEA, ensure efficient initiator radical generation for free radical polymerization. The orders of magnitude higher molar absorptivity in the blue spectral region compared to camphorquinone should make some of these thioxanthone derivatives attractive as a replacement for camphorquinone in photopolymer formulations. Although this chapter focused on the use of these heterocyclic extended thioxanthones to initiate free radical polymerization, some of these chromophores can also be used for free radical-promoted cationic polymerization.19,22 

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Figures & Tables

Scheme 1.1

Heterocyclic extended thioxanthones.

Scheme 1.1

Heterocyclic extended thioxanthones.

Close modal
Scheme 1.2

Synthesis of heterocyclic extended thioxanthones.

Scheme 1.2

Synthesis of heterocyclic extended thioxanthones.

Close modal
Figure 1.1

Absorption spectra of thioxanthone derivatives in acetonitrile (8, 10) and DMF (9, 11, 12, 13).

Figure 1.1

Absorption spectra of thioxanthone derivatives in acetonitrile (8, 10) and DMF (9, 11, 12, 13).

Close modal
Scheme 1.3

Photoreaction of thioxanthone with tertiary amines to generate initiator radicals.

Scheme 1.3

Photoreaction of thioxanthone with tertiary amines to generate initiator radicals.

Close modal
Figure 1.2

Left: Transient absorption spectrum of 12 in deoxygenated acetonitrile solution recorded 0–3 µs after pulsed laser excitation (355 nm, 5 ns pulse length). Inset: kinetic trace of the triplet absorption decay monitored at 800 nm. Right: Determination of the bimolecular rate constant (kamine) of quenching of triplet states of 12 by methyldiethanolamine (MDEA) from the plot of the inverse triplet lifetimes of 12 determined by laser flash photolysis and monitored at 800 nm vs. the MDEA concentration. These experiments were performed analogous to work previously published for 8 and 10.9 

Figure 1.2

Left: Transient absorption spectrum of 12 in deoxygenated acetonitrile solution recorded 0–3 µs after pulsed laser excitation (355 nm, 5 ns pulse length). Inset: kinetic trace of the triplet absorption decay monitored at 800 nm. Right: Determination of the bimolecular rate constant (kamine) of quenching of triplet states of 12 by methyldiethanolamine (MDEA) from the plot of the inverse triplet lifetimes of 12 determined by laser flash photolysis and monitored at 800 nm vs. the MDEA concentration. These experiments were performed analogous to work previously published for 8 and 10.9 

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Scheme 1.4

Reaction of triplet excited states of 12 with another molecule of 12 in the ground state.

Scheme 1.4

Reaction of triplet excited states of 12 with another molecule of 12 in the ground state.

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Figure 1.3

Determination of the bimolecular rate constant (kself) of triplet state quenching of 12 by ground state molecules of 12 from the plot of the inverse triplet lifetimes of 12 determined by laser flash photolysis (355 nm, 5 ns pulse length) monitored at 800 nm vs. the concentration of 12.

Figure 1.3

Determination of the bimolecular rate constant (kself) of triplet state quenching of 12 by ground state molecules of 12 from the plot of the inverse triplet lifetimes of 12 determined by laser flash photolysis (355 nm, 5 ns pulse length) monitored at 800 nm vs. the concentration of 12.

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Scheme 1.5

Possible reactions of thioxanthone ketyl radicals.18 

Scheme 1.5

Possible reactions of thioxanthone ketyl radicals.18 

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Figure 1.4

Absorption spectrum of camphorquinone (21) in DMF.

Figure 1.4

Absorption spectrum of camphorquinone (21) in DMF.

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Figure 1.5

Conversion of monomer to polymer vs. irradiation time as measured by photo-DSC. Polymerization of lauryl acrylate using 0.1% (w/w) thioxanthone (1), a heterocyclic extended thioxanthone derivative (12) or camphorquinone (21) as the initiator in the presence of 1% (w/w) MDEA.14 

Figure 1.5

Conversion of monomer to polymer vs. irradiation time as measured by photo-DSC. Polymerization of lauryl acrylate using 0.1% (w/w) thioxanthone (1), a heterocyclic extended thioxanthone derivative (12) or camphorquinone (21) as the initiator in the presence of 1% (w/w) MDEA.14 

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Table 1.1

Thioxanthone derivatives with electron-donating substituents: Absorption maximum at the longest wavelength peak (λmax) and molar absorptivity (ε).

Thioxanthone derivativeλmax (nm)ε at λmax (M−1 cm−1)
 380 (benzene)6  6600 (benzene)6  
381 (DMF)5  6328 (DMF)5  
 386 (benzene)6  6900 (benzene)6  
 383 (THF)7  3857 (THF)7  
 388 (benzene)6  6700 (benzene)6  
 388 (DMF)5  4941 (DMF)5  
 396 (DMF)5  5234 (DMF)5  
 438 (THF)8  4470 (THF)8  
Thioxanthone derivativeλmax (nm)ε at λmax (M−1 cm−1)
 380 (benzene)6  6600 (benzene)6  
381 (DMF)5  6328 (DMF)5  
 386 (benzene)6  6900 (benzene)6  
 383 (THF)7  3857 (THF)7  
 388 (benzene)6  6700 (benzene)6  
 388 (DMF)5  4941 (DMF)5  
 396 (DMF)5  5234 (DMF)5  
 438 (THF)8  4470 (THF)8  
Table 1.2

Heterocyclic extended thioxanthone derivatives: Photophysical properties in different solvents.a

TX-BT 8bTX-DBT 9bTX-DBT-TX 10bTX-FN 11cTX-Cz 12TX-N3 13d
λmax (nm)e 380 (ACN) 403 (DMF) 425 (ACN) 455 (DMF) 434 (DMF)n 393 (DMF) 
ε (M−1 cm−1)f 8760 (ACN) 2610 (DMF) 3830 (ACN) 762 (DMF) 2014 (DMF)n 1208 (DMF) 
ES (kJ mol−1)g 307 (ACN) 257 (Tol) ∼262 (ACN) 237 (DMF) 260 (EtOH)n  
ET (kJ mol−1)h 236 (EtOH) 223 (Tol) 221 (Tol) 218 (MeTHF) 221 (EtOH)n 238 (EtOH) 
Φfi 0.028 (ACN) 0.009 (Tol) 0.003 (ACN) 0.127 (DMF) 0.11 (EtOH)n 0.15 (EtOH) 
0.002 (Tol) 
ΦTj 0.93 (ACN)  0.5 (ACN)    
τP (ms)k 192 (Tol) 85 (Tol) 42 (Tol) 49.9 (MeTHF) 53 (EtOH)n 73 (EtOH) 
222 (EtOH) 64 (EtOH) 
τT (µs)l 18 (ACN)  5.6 (ACN) 7.63 (DMF) 17.5 (ACN)n  
19 (ACN)m 
TX-BT 8bTX-DBT 9bTX-DBT-TX 10bTX-FN 11cTX-Cz 12TX-N3 13d
λmax (nm)e 380 (ACN) 403 (DMF) 425 (ACN) 455 (DMF) 434 (DMF)n 393 (DMF) 
ε (M−1 cm−1)f 8760 (ACN) 2610 (DMF) 3830 (ACN) 762 (DMF) 2014 (DMF)n 1208 (DMF) 
ES (kJ mol−1)g 307 (ACN) 257 (Tol) ∼262 (ACN) 237 (DMF) 260 (EtOH)n  
ET (kJ mol−1)h 236 (EtOH) 223 (Tol) 221 (Tol) 218 (MeTHF) 221 (EtOH)n 238 (EtOH) 
Φfi 0.028 (ACN) 0.009 (Tol) 0.003 (ACN) 0.127 (DMF) 0.11 (EtOH)n 0.15 (EtOH) 
0.002 (Tol) 
ΦTj 0.93 (ACN)  0.5 (ACN)    
τP (ms)k 192 (Tol) 85 (Tol) 42 (Tol) 49.9 (MeTHF) 53 (EtOH)n 73 (EtOH) 
222 (EtOH) 64 (EtOH) 
τT (µs)l 18 (ACN)  5.6 (ACN) 7.63 (DMF) 17.5 (ACN)n  
19 (ACN)m 
a

Acetonitrile (ACN), dimethylformamide (DMF), toluene (Tol), ethanol (EtOH), 2-methyltetrahydrofuran (MeTHF).

b

Data from ref. 9.

c

Data from ref. 14.

d

Data from ref. 12.

e

Absorption maximum at the longest wavelength peak.

f

Molar absorptivity.

g

Energy of the first excited singlet state determined from the interception of the fluorescence excitation and emission spectra.

h

Triplet state energy determined from the first peak of the phosphorescence spectra at 77 K.

i

Fluorescence quantum yield.

j

Triplet quantum yield.

k

Phosphorescence lifetime at 77 K.

l

Triplet lifetime at room temperature.

m

This work.

n

Data from ref. 10.

Table 1.3

Rate constants of the reaction of triplet states of thioxanthone derivatives with methyldiethanolamine (MDEA).

1281012
kamine (M−1 s−19×109 (Bz)a 6×109 (Bz)a (2.0±0.1)×109 (ACN)b (6.4±0.2)×109 (ACN)b (1.4±0.1)×109 (ACN)c 
1281012
kamine (M−1 s−19×109 (Bz)a 6×109 (Bz)a (2.0±0.1)×109 (ACN)b (6.4±0.2)×109 (ACN)b (1.4±0.1)×109 (ACN)c 
a

Data from ref. 6.

b

Data from ref. 9.

c

Data from this work.

Table 1.4

Photopolymerization of MMA (4.68 M) in DMF using 12 or 21 as photoinitiators.a

[MDEA] (mM)DeoxygenatedConv.b %
1221
10 − 4.7 
10 3.5 2.1 
− 4.1 
[MDEA] (mM)DeoxygenatedConv.b %
1221
10 − 4.7 
10 3.5 2.1 
− 4.1 
a

Irradiation with 400 W medium pressure mercury lamp for 15 min.

b

Data from ref. 10.

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