Nanosecond Laser Flash Photolysis Study of the Photochemistry of 2-Alkoxy Thioxanthones

Abstract

Laser flash photolysis studies (λ = 355 nm) on the characterization and reactivity for the triplet excited state of 2-methoxythioxanthone, 2-benzyloxythioxanthone, and 2-(n-propoxy) thioxanthone revealed that the transients generated in their excitation, in acetonitrile, have absorption bands with maxima at 310 and 620 nm. These transients decayed by a mixed 1^st and 2^nd order kinetics, with a lifetime of 5.3 μs for 2-methoxythioxanthone and benzyloxythioxanthone and 3.9 μs for 2-(n-propoxy) thioxanthone. In methanol, ethanol, and 2-propanol, a new absorption at 430-460 nm was observed, which was attributed to their corresponding ketyl radical (the second-order quenching rate constant (k_q) is ca. 10⁵ L mol^-1 s^-1). The energy transfer rate constants for trans-stilbene, 1-methylnaphthalene, and 1,3-cyclohexadiene were diffusion-controlled, indicating a triplet energy for these alkoxy thioxanthones higher than 61 kcal mol^-1. The quenching rate constant observed for 1,4-cyclohexadiene (ca.10⁹ L mol^-1 s^-1) suggests that the lowest energy triplet excited state for these thioxanthones must have nπ* character. For phenols and indole (k_q ca. 10⁹ L mol^-1 s^-1), the reaction mechanism must involve the initial formation of a triplet exciplex, followed by a coupled electron/proton transfer. For triethylamine and 1,4-diazabicyclo[2.2.2]octane (k_q ca. 10⁹ L mol^-1 s^-1) it was observed the formation of the radical anion of the alkoxy thioxanthones formed from electron transfer (λ = 410 nm).

Keywords:
laser flash photolysis; thioxanthones; energy transfer; hydrogen transfer; electron transfer

Introduction

The photophysical properties of many aromatic ketones are strongly influenced by the configuration of their electronically excited states. The initial study of the solvent effect in these cases aims to understand the reaction mechanisms in the ground and excited states of molecules.¹,²,³,⁴,⁵

The nature of the excited state of thioxanthone (9H-thioxanton-9-one, TX) and its derivatives have been extensively studied. The energy for the triplet excited state of TX is approximately 65.5 kcal mol^-1. The lowest energy singlet state has a pπ* configuration and, in hydroxylic solvents, shows intense fluorescence.⁶ Thioxanthone can form free radicals in the triplet excited state and is often used as a photoinitiator for polymerization, a photosensitizer, and a photocatalyst.⁷,⁸,⁹,¹⁰,¹¹,¹²,¹³,¹⁴,¹⁵,¹⁶,¹⁷,¹⁸,¹⁹,²⁰,²¹

The efficient formation of the triplet excited state of thioxanthones is attributed to the high value for the intersystem crossing rate constant between systems involving the S₁ (ππ*) to T₂ (nπ*) states, followed by an internal conversion T₂ (nπ*) to T₁ (ππ*).²² Intersystem crossing processes between systems that involve configuration changes are about three orders of magnitude more efficient than similar processes that involve S₁ and T_n states with the same configuration.²³,²⁴

Early studies on the spectroscopy and kinetics of several alkoxyl and halogen substituted thioxanthones employing conventional flash photolysis showed that irradiation in the highly efficient hydrogen donor solvent 2-propanol leads to the formation of the corresponding ketyl radicals. Under this condition, the recorded transient absorption spectra showed absorption maxima between 340 and 420 nm for the 1-methoxy, 3-methoxy, and 2-methoxythioxanthone.²⁵ Similar results were reported for the photolysis of 2-benzyloxythioxanthone in tetrahydrofuran. In this case, a broad absorption band was observed with a maximum at 440 nm, associated with the ketyl radical derived from this thioxanthone.²⁶ In acetonitrile, the triplet excited state of these thioxanthones presents an intense absorption in the 580-640 nm region.²⁷ Laser flash photolysis studies on 2-chlorothioxanthone in acetonitrile revealed that the T-T absorption generated upon 355 nm laser excitation has absorptions at 310 and 630 nm and depletion in the 350-400 nm region.²⁸

In the presence of a tertiary amine, which acts as an electron donor, the triplet excited state of thioxanthone generates a pair of radical ions through the initial formation of an exciplex. These radical ions then decay to the alkylamino/ketyl radical pair, which has been used as the effective initiator in polymerization reactions.²⁶

A study of the solvent polarity variation on the triplet-triplet absorption spectrum of thioxanthone, as well as of thioxanthones containing substituents on the aromatic ring, shows that the triplet-triplet absorption spectrum for these compounds presents three different bands in the regions between 305-330, 400-450 and 600-640 nm.⁷,²⁵,²⁹ The absorption at 400-450 nm is usually attributed to the semi-reduced form of the ketone, resulting from hydrogen abstraction from the solvent by the triplet excited state of thioxanthone.²⁵

Results from our group³⁰ on the photophysical and photochemical properties of thioxanthone using the laser flash photolysis technique show that its triplet excited state is easily obtained upon 355 nm irradiation. The triplet lifetime and the λ_max of the triplet-triplet absorption band of thioxanthone increase with the change from a non-polar to a polar solvent.³⁰ When hydrogen donor solvents such as methanol, ethanol, and 2-propanol were employed, it was possible to observe the appearance of a band at 410 nm in the transient absorption spectrum, which was attributed to the thioxanthone-derived ketyl radical.³⁰,³¹ Quenching rate constants for hydrogen-donor compounds ranged from 1.7 × 10⁶ L mol^-1 s^-1 for toluene to ca. 10⁹ L mol^-1 s^-1 for phenol and its derivatives containing polar substituents, and indole. The quenching rate constants employing electron donors such as 1,4-diazabicyclo[2.2.2]octane (DABCO) and triethylamine³⁰ were also on the order of 10⁹ L mol^-1 s^-1.

Several publications¹¹,³²,³³,³⁴,³⁵,³⁶,³⁷ from other groups deal with the study of the solvent effect on the photochemical and photophysical properties of thioxanthone, and its interaction with different compounds. Thioxanthone and its derivatives have been widely used as polymerization photoinitiators and photosensitizers for several decades.⁹,¹⁴,²¹,²⁶,³¹,³⁸,³⁹,⁴⁰,⁴¹,⁴²,⁴³,⁴⁴,⁴⁵,⁴⁶,⁴⁷ However, nowadays, the study of the photophysics and photochemistry of thioxanthones remains a topic of great relevance, as demonstrated by recent review articles on the subject, which highlight both their ability to act as a potent photocatalyst for organic reactions as well as to promote photopolymerization processes and the 3D printing of photocurable resins under visible light.²¹,⁴⁸ In addition, they have also been used in the study of protein folding,⁴⁹ in the development of a new efficient photocaging system,⁵⁰ and as new drugs for the treatment of solid tumors.⁵¹,⁵²,⁵³

The present work aims to study the reactivity of the triplet excited state of 2-methoxythioxanthone (MeOTX), 2-benzyloxythioxanthone (BzOTX), and 2-(n-propoxy) thioxanthone (PrOTX) (Scheme 1) in the presence of different quenchers acting by energy, hydrogen, or electron transfer, employing the nanosecond laser flash photolysis technique.

Scheme 1
Structure for the 2-alkoxy thioxanthones investigated in the present work.

Experimental

Materials

Solvents (acetonitrile, methanol, ethanol, and 2-propanol, spectroscopic grade) were purchased from Sigma-Aldrich (São Paulo, Brazil). trans-Stilbene, 1-methylnaphthalene, 1,4-diazabicyclo[2.2.2] octane (DABCO), triethylamine, indole, and the phenols (H, 4-cyano, 3-cyano, 4-methyl, 3-methyl, 4-methoxy, 3-methoxy, 3-fluor, 3-chloro) were purchased from Sigma-Aldrich (São Paulo, Brazil) and used as received. 1,3-Cyclohexadiene (Sigma-Aldrich, São Paulo, Brazil) and 1,4-cyclohexadiene (Sigma-Aldrich, São Paulo, Brazil) were distilled bulb-to-bulb immediately before use.

Synthesis of substituted thioxanthones

The 2-methoxy-, 2-benzyloxy-, and 2-(n-propoxy) thioxanthones used in this work were kindly provided by Prof Dr Miguel Neumann (University of São Paulo, São Carlos). The synthesis of substituted thioxanthones consisted of a simple alkylation reaction between 2-hydroxythioxanthone and the corresponding alkyl halide (methyl iodide, benzyl chloride, or 1-propyl chloride). Briefly, 1.2 g of 2-hydroxythioxanthone (ca. 5 mM) was dissolved in N,N-dimethylformamide (10.0 mL), and then the corresponding alkyl halide (5 mM) was added to the reaction medium, maintaining a constant temperature of 60 ºC throughout the procedure. Then, 2 mL of an aqueous potassium hydroxide solution (5% m/v) were slowly added to the medium to form a red-colored precipitate (2-hydroxythioxanthone phenoxide). After the reaction, filtration was carried out, and the resulting solid was recrystallized from dioxane/water, yielding a yellow-colored product.²⁵,²⁶,²⁷,³¹ The spectroscopic and spectrometric data for the 2-alkoxythioxanthones obtained agree with the proposed structures.

Laser flash photolysis

The nanosecond laser flash photolysis experiments were performed using a Luzchem model mlfp 112 system (Gloucester, Canada) employing a 175 W ceramic xenon lamp, a CUI Laser Corporation Digikröm CM 110 monochromator, a Hamamatsu photomultiplier, a Tektronix oscilloscope TDS 2012 model, with the entire system being controlled by a personal computer containing a National Instruments Labview software.⁵⁴ The samples were irradiated with a continuum Nd/YAG laser, model Surelite II, using the third harmonic (λ = 355 nm, 10 ns pulse, ca. 40 mJ per pulse).

Kinetic and spectroscopic data about the excited state of the thioxanthones used, as well as the reaction rate constants between these transients and a quencher, were obtained using the nanosecond laser flash photolysis technique implemented in the Photochemistry Laboratory of the Instituto de Química at the Universidade Federal do Rio de Janeiro-Brazil, in the Lablaser of the Instituto de Química at the Universidade Federal da Bahia-Brazil, and at the Photochemistry Laboratory at the Universidade de São Paulo-São Carlos-Brazil and also using a Luzchem model mlpf 112 system.

For the quenching experiments, ca. 10^-4 mol L^-1 solutions of MeOTX, BzOTX, and PrOTX in acetonitrile (absorbance ca. 0.3 at 355 nm, measured in a Varian DMS-80 UV-Vis spectrophotometer, Palo Alto, California, USA) were prepared. Solutions of quenchers were also prepared in varying concentrations using acetonitrile as a solvent so that there was no change in the reaction medium while quencher solutions were added. All experiments used a 10 × 10 mm² static quartz cell containing 3.0 mL of the thioxanthone solution sealed with a rubber septum. The samples used in the laser flash photolysis experiments were degassed immediately before irradiation by passing a stream of oxygen-free nitrogen for 30 min. In the quenching experiments, aliquots of the different quencher solutions were gradually added with a microliter syringe.

Second-order quenching rate constants for the triplet excited state of MeOTX, BzOTX, and PrOTX were obtained from Stern-Volmer plots (equation 1),⁵⁵ and, in all cases, linear plots were obtained, showing very good correlation coefficients.

(1) ${\text{k}}_{\text{obs}}={\text{k}}_0+{\text{k}}_{\text{q}}\left[\text{Q}\right]$

where k_obs is the triplet decay rate constant in the presence of the quencher; k₀ is the triplet decay rate constant in the absence of the quencher Q; k_q is the second-order quenching rate constant, expressed in L mol^-1 s^-1; and [Q] is the quencher concentration in mol L^-1.

Results and Discussion

Laser flash photolysis (λ = 355 nm) of nitrogen degassed acetonitrile solutions of 2-methoxythioxanthone (MeOTx), 2-benzyloxythioxanthone (BzOTX), or 2-(n-propoxy) thioxanthone (PrOTX) led to the formation of readily detectable transients with absorptions in the 309 and 620 nm regions and a depletion in the 350-400 nm region (Figure 1). In a previously published work, it was reported that in ethyl acetoacetate the transient generated in the laser excitation of these alkoxyxanthones showed absorptions at 310 and 640 nm.²⁹

Figure 1
Triplet-triplet absorption spectra (λ_exc = 355 nm) in acetonitrile (ca. 1 × 10^-4 mol L^-1), recorded at different times after the laser pulse, for: (a) 2-methoxythioxanthone; (b) 2-benzyloxythioxanthone; (c) 2-(n-propoxy) thioxanthone.

The decay associated with these absorptions showed mixed first and second-order kinetic, a usual behavior for long-lived triplets (Figure 1). The contribution of 2^nd order decay in triplet aromatic ketones is usually associated with a deactivation process by triplet-triplet annihilation.⁵⁶ As a consequence, the decay for these transients is concentration dependent, and for acetonitrile as a solvent, the following self-quenching rate constants were measured: (3.9 ± 0.2) × 10⁷ L mol^-1 s^-1 for MeOTX; (2.0 ± 0.2) × 10⁷ L mol^-1 s^-1 for BzOTX, and (3.1 ± 0.1) × 10 ⁷ L mol^-1 s^-1 for PrOTX. From the decays in acetonitrile, a triplet lifetime of 5.3 ± 0.2 μs was obtained for MeOTX and BzOTX, and 3.9 ± 0.2 μs for PrOTX.

The transients obtained on laser irradiation of MeOTX, BzOTX, and PrOTX were assigned to their corresponding triplet excited state because they were quenched at a diffusion-controlled rate constant (k_diff for acetonitrile was calculated as 1.68 × 10¹⁰ L mol^-1 s^-1 by employing the Smoluchowski-Stokes-Einstein theory)⁵⁷ with trans-stilbene (triplet energy (E_T) = 49 kcal mol^-1), 1,3-cyclohexadiene (E_T = 53 kcal mol^-1). For 1-methylnaphthalene (E_T = 61 kcal mol^-1) (Table 1),⁵⁷ the energy transfer rate constant was one order of magnitude slower than for trans-stilbene and 1,3-cyclohexadiene, i.e., ca. 10⁹ L mol^-1 s^-1. Such difference in behavior suggests that the triplet energy for the alkoxy thioxanthones studied in this work must be at least isoenergetic to that of 1-methylnaphthalene. This result is fully in accord with literature data on the triplet energy for thioxanthones.⁶

Table 1
Second-order rate constants (k_q) for the quenching of 2-methoxythioxanthone, 2-benzyloxythioxanthone, and 2-(n-propoxy) thioxanthone in acetonitrile by energy, hydrogen, and electron transfer

Quencher	kq / (L mol-1 s-1)
	MeOTX	BzOTX	PrOTX
trans-Stilbene	(1.1 ± 0.1) × 1010	(1.1 ± 0.1) × 1010	(1.2 ± 0.1) × 1010
1,3-Cyclohexadiene	(1.1 ± 0.1) × 1010	(1.1 ± 0.1) × 1010	(1.0 ± 0.1) × 1010
1-Methylnaphthalene	(2.6 ± 0.1) × 109	(3.3 ± 0.1) × 109	(3.4 ± 0.1) × 109
Methanol	(1.1 ± 0.1) × 105	(1.3 ± 0.1) × 105	(6.4 ± 0.2) × 104
Ethanol	(1.8 ± 0.1) × 105	(2.1 ± 0.1) × 105	(1.2 ± 0.1) × 105
2-Propanol	(2.2 ± 0.1) × 105	(2.6 ± 0.1) × 105	(1.8 ± 0.1) × 105
1,4-Cyclohexadiene	(4.1 ± 0.1) × 109	(2.4 ± 0.1) × 109	(3.0 ± 0.1) × 109
DABCO	(6.5 ± 0.1) × 109	(6.1 ± 0.2) × 109	(6.6 ± 0.1) × 109
Triethylamine	(2.9 ± 0.1) × 109	(3.5 ± 0.2) × 109	(3.4 ± 0.1) × 109

• DABCO: 1,4-diazabicyclo[2.2.2]octane; MeOTX: 2-methoxythioxanthone; BzOTX: 2-benzyloxythioxanthone; PrOTX: 2-(n-propoxy) thioxanthone.

In addition to the absorptions observed at 310 and 620 nm for triplet MeOTX, BzOTX, and PrOTX, transient absorption spectra for these thioxanthones in hydroxylic solvents such as methanol, ethanol, or 2-propanol revealed the presence of a new absorption in the 420-440 nm region. Figure 2 shows a representative transient spectrum for PrOTX in 2-propanol. As these solvents are known as hydrogen donating species,²⁹,⁵⁸,⁵⁹ this new absorption can be assigned to the ketyl radical resulting from a hydrogen abstraction reaction from the solvent by the triplet excited state of MeOTX, BzOTX, and PrOTX.³⁰

Figure 2
Transient absorption spectra (λ_exc = 355 nm) for 2-(n-propoxy) thioxanthone in 2-propanol as solvent.

This absorption can be associated with the ketyl radical derived from BzOTX, MeOTX, or ProTX, not only because the formation of this transient had previously been reported for triplet thioxanthone,²⁹,³⁰ but mainly because it presents a much slower decay than their corresponding triplet excited state. Figure 3 shows a comparison between the decay of PrOTX triplet in acetonitrile, monitored at 610 nm, and that of its respective ketyl radical in 2-propanol, monitored at 440 nm. It is important to note that besides the difference in the decay rate constants, the trace corresponding to the ketyl radical clearly indicates a growth kinetics for this transient. This suggests the ketyl radical formation from the triplet excited state of alkoxythioxanthone through a hydrogen abstraction mechanism, with the solvent 2-propanol being the hydrogen donor.

Figure 3
Decay trace for the triplet excited state of 2-(n-propoxy) thioxanthone (PrOTX) in acetonitrile, monitored at 610 nm (red); in black, growth and decay for the transient generate upon laser photolysis of PrOTX in 2-propanol (λ_mon = 440 nm).

Alcohols (methanol, ethanol, 2-propanol), 1,4-cyclohexadiene, phenols, and indole were used as quenchers in the study of the triplet reactivity of MeOTX, BzOTX, and PrOTX in processes involving the hydrogen abstraction reaction, whereas tertiary amines (DABCO and triethylamine) were employed in the electron transfer quenching process. All reactions were performed in acetonitrile as solvent.

The rate constants obtained for the quenching of MeOTX, BzOTX, and PrOTX by methanol, ethanol, and 2-propanol are shown in Table 1. From these results, dependence of the quenching rate constant of the thioxanthones with the structure of the alcohol used as a quencher is indicated. Thus, the highest hydrogen abstraction rate constant value corresponds to the alcohol having the weakest H-COH bond. In other words, the most stable ketyl radical derived from the alcohol is formed, with hydrogen abstraction rate constants being in the order k_2-PrOH > k_EtOH > k_MeOH for all alkoxy thioxanthones under study. It is worth noting that ketyl radicals derived from these alcohols cannot be detected spectroscopically with our experimental setup.

The second-order quenching rate constants obtained for reactions of MeOTX, BzOTX, and PrOTX with methanol, ethanol, and 2-propanol suggest that their reactive triplet excited state has nπ* character (Table 1). Time-dependent density functional theory calculations indicated the T₁ states for thioxanthones have ππ* character, regardless of solvent polarity.⁶⁰ Even though there are examples where the ππ* triplet state shows some reactivity,⁵⁸ based on the quenching rate constants observed in the current study, we may neglect its contribution to the thioxanthones reactivity, at least for hydrogen abstraction reactions from alcohols and 1,4-cyclohexadiene. Since these rate constants have the same order of magnitude as those obtained for a typical aromatic carbonyl compound with a triplet excited state which is known to behave as a pure nπ* triplet state, such as benzophenone, it is reasonable to assume that the reactive triplet states for MeOTX, BzOTX, and PrOTX have the same configuration.⁶¹ For comparison only, benzophenone reacts with methanol, ethanol, and 2-propanol with rate constants of 3 × 10⁵, 6 × 10⁵, and 1 × 10⁶ L mol^-1 s^-1, respectively.⁶²

To further confirm the nπ* character of the reactive triplet states for MeOTX, BzOTX, and PrOTX, hydrogen abstraction rate constants were measured employing the excellent hydrogen donor 1,4-cyclohexadiene as a quencher.⁶³ The high k_q values obtained, close to the diffusion-controlled rate constant for acetonitrile, confirm that the nπ* triplet is the state responsible for these thioxanthones reactivity. For comparison, the triplet state of typical nπ* aromatic ketones, such as 10,10-dimethylanthrone and benzophenone, reacts with 1,4-cyclohexadiene with rate constant of 6.6 × 10⁸ and 2.9 × 10⁸ L mol^-1 s^-1, respectively.⁶⁴,⁶⁵ This is almost two orders of magnitude faster than the H-atom abstraction (k_q = 1.4 × 10⁷ L mol^-1 s^-1) by aromatic ketones having a triplet state with ππ* character, such as 2,2’-dinaphthyl ketone.⁶⁷

A mechanistic proposal is presented in Scheme 2 for the reaction between the triplet excited state of MeOTX, BzOTX, PrOTX and 1,4-cyclohexadiene (1,4-CHD). Similar mechanism can be applied to the hydrogen abstraction reaction from alcohols. The thioxanthone ketyl radical formation derived from these 2-alkoxythioxanthones is also indicated in this scheme.

Scheme 2
Mechanistic proposal for the hydrogen abstraction reaction between alkoxyxanthones and 1,4-cyclohexadiene in acetonitrile.

The electron transfer reaction for MeOTX, BzOTX, and PrOTX was conducted using the tertiary amines (DABCO) and triethylamine as electron donors. As these amines do not have a hydrogen atom attached to nitrogen, the reaction occurs only by full electron transfer, confirmed by the high quenching rate constants observed (Table 1). It is well known that reactions involving electron transfer are significantly faster than those for hydrogen abstraction.⁶² The reaction mechanism for electron transfer between excited carbonyls and tertiary amines described in the literature suggests that these reactions lead to the formation of an ion-radical pair.⁵⁹ For example, in the absorption spectrum of transients generated after the excitation of an acetonitrile solution containing MeOTX and excess triethylamine, recorded 13.6 μs after the laser pulse (Figure 4), it was possible to observe a new absorption at 410 nm, which can be attributed to the radical anion derived from this thioxanthone.³⁰ Under this condition the absorption relative to the alkoxythioxanthone triplet completely disappeared. Scheme 3 depicts a mechanistic proposal for the reaction between the triplet state of MeOTX, BzOTX, or PrOTX with DABCO.⁶⁷

Figure 4
Transient absorption spectrum generated after the excitation of an acetonitrile solution containing MeOTX and excess triethylamine, recorded 13.6 ms after the laser pulse.

Scheme 3
Mechanistic proposal for the electron transfer reaction between alkoxyxanthones and 1,4-diazabicyclo[2.2.2] octane (DABCO) in acetonitrile.

The triplet excited state for MeOTX, BzOTX, and PrOTX is efficiently quenched by phenols via a hydrogen transfer process (k_q ca. 10⁹ L mol^-1 s^-1) in acetonitrile (Table 2). Figure 5 shows a considerable decrease in the intensity of triplet absorption bands of MeOTX at 610 nm when this thioxanthone was quenched by 4-methoxyphenol. In addition, one can also observe the appearance of new weak absorptions at 410 and 430 nm, which can be attributed to the 4-methoxyphenoxyl⁶⁸ and the ketyl radical from MeOTX, respectively. Similar results were found in the quenching process of triplet MeOTX, BzOTX, and PrOTX by the several phenols employed in this work.

Table 2
Second-order rate constants (k_q) for the quenching of 2-methoxythioxanthone (MeOTX), 2-benzyloxythioxanthone (BzOTX), and 2-(n-propoxy) thioxanthones (PrOTX) by phenols containing polar substituents, and indole

Quencher	kq / (L mol-1 s-1)
	MeOTX	BzOTX	PrOTX
Phenol	(5.0 ± 0.2) × 109	(5.0 ± 0.2) × 109	(4.1 ± 0.1) × 109
4-Methoxyphenol	(7.0 ± 0.1) × 109	(5.5 ± 0.3) × 109	(3.7 ± 0.1) × 109
3-Methoxyphenol	(3.3 ± 0.1) × 109	(4.0 ± 0.1) × 109	(4.5 ± 0.1) × 109
3-Chlorophenol	(3.6 ± 0.1) × 109	(3.7 ± 0.2) × 109	(4.8 ± 0.2) × 109
3-Fluorophenol	(3.4 ± 0.1) × 109	–	(4.9 ± 0.2) × 109
4-Methylphenol	(3.3 ± 0.1) × 109	(3.4 ± 0.1) × 109	(6.6 ± 0.2) × 109
3-Methylphenol	(4.0 ± 0.1) × 109	(2.8 ± 0.1) × 109	(2.9 ± 0.1) × 109
4-Cyanophenol	(2.4 ± 0.1) × 109	(1.8 ± 0.2) × 109	(6.6 ± 0.1) × 109
3-Cyanophenol	(2.0 ± 0.1) × 109	–	(4.0 ± 0.1) × 109
Indole	(2.3 ± 0.1) × 108	(4.8 ± 0.1) × 108	(2.2 ± 0.1) × 109

Figure 5
Transient absorption spectrum for 2-methoxythioxanthone in the presence of large excess of 4-methoxyphenol solution, in acetonitrile and recorded 12.2 ms after the laser pulse.

Phenols and indole quench the triplet excited state of MeOTX, BzOTX, and PrOTX by hydrogen abstraction with the quenching rate constants measured for these reactions being close to the diffusion-control in all cases (Table 2). As can be seen by inspecting Tables 1, 2, the quenching rate constants for phenols are higher than those observed for alcohols because the hydrogen abstraction reaction occurs by different mechanisms in these two cases. In reactions involving the triplet excited state of carbonyls and alcohols, saturated aliphatic hydrocarbons (such as cyclohexane), and dienes (such as 1,4-cyclohexadiene) a mechanism called a “pure” alkoxyl hydrogen abstraction, due to the similarity between the reaction of carbonyl triplets and alkoxyl radicals, such as the tert-butoxyl radical, is operating.⁶⁹,⁷⁰ On the other hand, hydrogen abstraction reaction from phenols is usually described by a mechanism in which, after the initial formation of a triplet exciplex, the electron transfer process is followed by an ultra-rapid proton transfer, ultimately resulting in the aryloxyl-ketyl radical pair (Scheme 4).⁷¹,⁷²,⁷³,⁷⁴,⁷⁵,⁷⁶,⁷⁷,⁷⁸,⁷⁹,⁸⁰

Scheme 4
Mechanistic proposal for the phenolic hydrogen abstraction by the triplet excited state of 2-methoxythioxanthone, 2-benzyloxythioxanthone, and 2-(n-propoxy) thioxanthone in acetonitrile.

Indole reacts with carbonyl compounds through the very same mechanism proposed for phenols (Scheme 4) involving formation of a triplet exciplex intermediate, followed by an electron transfer coupled with an ultra-fast proton transfer (Table 2). The proposed mechanism for the reactions of MeOTX, BzOTX, and PrOTX with indole results in the formation of the indolyl radical, which has absorption around 500 nm,⁸¹,⁸²,⁸³,⁸⁴ and the ketyl radical derived from the thioxanthones. A representative spectrum is shown in Figure 6 for the reaction of triplet MeOTX with indole, in which it was possible to observe the presence of the indolyl radical at 510 nm and the ketyl radical from the alkoxyxanthone with maximum absorption at 430 nm.

Figure 6
Transient absorption spectrum generated on the excitation (λ = 355 nm) of 2-methoxythioxanthone in presence of large excess of indole solution, in acetonitrile, and recorded 6.0 μs after the laser pulse.

In conclusion, it was demonstrated that the spectroscopic and kinetic properties of MeOTX, BzOTX, and PrOTX, as well as their photochemical reactivity towards hydrogen and electron transfer reactions, are independent of the structure of the substituent. These results agree with those reported in the literature for the photochemistry of thioxanthones.²⁰,²⁵,²⁷,²⁹,³⁰,³¹

Conclusions

The absorption spectra of transients generated in the excitation (λ = 355 nm) of MeOTX, BzOTX, and PrOTX show absorption at 309 and 620 nm and a depletion in the 350-400 nm region. In 2-propanol a new absorption in the 419-469 nm region could be observed, which corresponds to the respective ketyl radicals derived from the alkoxy thioxanthones and generated by a hydrogen transfer process from the solvent. Confirmation of the transient formed in the irradiation of these thioxanthones as their triplet excited state was made by energy transfer quenching studies, from which one can conclude that the triplet energy for these alkoxy thioxanthones is close to 61 kcal mol^-1. For electron transfer reactions with DABCO and triethylamine, it was possible to observe absorption bands in the region of 460 and 500 nm (MeOTX) assigned to the formation of the corresponding anion radical. The hydrogen abstraction rate constants by these triplets varied from 10⁵ L mol^-1 s^-1, for alcohols, to 10⁹ L mol^-1 s^-1, for 1,4-cyclohexadiene (1,4-CHD). The high value of k_q for 1,4-CHD indicates that the reactive triplet excited state for MeOTX, BzOTX, and PrOTX must have a nπ* character. The high value for the quenching rate constants for phenols (10⁹ L mol^-1 s^-1) suggest a mechanism in which there is the initial formation of a triplet exciplex, followed by a coupled electron/ proton transfer. For indole, the same mechanism that was previously proposed for phenol reaction must be operating. In this case, the measured quenching rate constants were (2.3 ± 0.1) × 10⁸ (MeOTX), (4.8 ± 0.1) × 10⁸ (BzOTX), and (2.2 ± 0.1) × 10⁹ L mol^-1 s^-1 (PrOTX).

Acknowledgments

This work was funded by the Brazilian agencies Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), and Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ).

A generous gift by Prof Dr Miguel Neumann (University of São Paulo-São Carlos) of 2-methoxy-, 2-benzyloxy-, and 2-(n-propoxy) thioxanthones used in this work is greatly acknowledged. The authors acknowledge Prof Dr Cristina Quintella (Institute of Chemistry at Universidade Federal da Bahia, Brazil) and Prof Dr Carla Schmitt (Institute of Chemistry at Universidade de São Paulo-São Carlos, Brazil) for making available the laser flash photolysis facilities.

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Edited by

Editor handled this article: Izaura C. N. Diógenes (Associate)

Publication Dates

• Publication in this collection
  05 Apr 2024
• Date of issue
  2024

History

• Received
  29 Oct 2023
• Accepted
  07 Mar 2024