Development of a D–π–A pyrazinium photosensitizer possessing singlet oxygen generation

Abstract

(D–π–)₂A pyrazinium dyes (OEJ-1 and OEJ-2) bearing a counter anion (X⁻ = Br⁻ or I⁻) have been newly developed as a photosensitizer possessing singlet oxygen (¹O₂) generation. The two dyes show specific solvatochromism, leading to a large bathochromic shift of the photoabsorption band in halogenated solvents, compared to polar and non-polar solvents. The effects of the counter anion and solvents on the ¹O₂ generation efficiency such as Φ_Δ and the rate constant (K_obs) have been investigated. It was revealed that OEJ-2 (X⁻ = I⁻) exhibits a higher ¹O₂ quantum yield (Φ_Δ) than OEJ-1 (X⁻ = Br⁻). This result indicates that the (D–π–)₂A pyrazinium dyes possess the ability to generate ¹O₂ under visible light irradiation, due to the effective intersystem crossing (ISC) from the singlet excited state of the photosensitizer (¹S*) to the triplet excited state (³S*) by the superior heavy-atom effect of I⁻ ion as the counter anion. Moreover, it was found that THF and dichloromethane are favorable solvents for the (D–π–)₂A pyrazinium dyes to efficiently generate ¹O₂ compared with the polar solvents such as acetonitrile and DMSO. On the basis of the ¹O₂ quantum yield, the rate constant for ¹O₂ generation, the HOMO and LUMO energy levels of OEJ-1 and OEJ-2, and density functional theory (DFT) calculation, the photoabsorption and ¹O₂ generation properties of the D–π–A pyrazinium dyes are discussed.

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Introduction

Photosensitizers possessing the ability to generate singlet oxygen (¹O₂) have received considerable attention in recent years from the viewpoint of not only fundamental study in photochemistry and photophysics, but also their potential applications in photodynamic therapy (PDT).^1–41O₂ generally occurs through the following processes: initially the photosensitizer absorbs light (hν) to generate the singlet excited state of the photosensitizer (¹S*), then the photoexcited dye (¹S*) undergoes intersystem crossing (ISC) to generate the triplet excited state (³S*). Subsequent energy transfer from the photoexcited dye (³S*) to triplet oxygen (³O₂) produces ¹O₂. Thus, to enhance ISC efficiency is one of the most effective strategies to generate high ¹O₂ quantum yield. For this purpose, many kinds of photosensitizers exhibiting high ¹O₂ generation efficiency for PDT have been developed, including organic dyes such as methylene blue⁵ and rose bengal,⁶ porphyrin dyes,^7,8 phthalocyanines,⁹ boron dipyrromethene (BODIPY) dyes,^10–12 fullerene derivatives^13,14 and ruthenium (Ru)¹⁵ and iridium (Ir) complexes,¹⁶ and the mechanisms of ¹O₂ generation by the photosensitizers were investigated.^1–4,17 However, there have been few efforts to develop new organic photosensitizers possessing the ability to generate ¹O₂.¹⁸

Thus, in this work, to gain insight into a direction in molecular design toward creating new photosensitizer family possessing ¹O₂ generation, we have developed (D–π–)₂A pyrazinium dyes (OEJ-1 and OEJ-2) bearing bromide ion (Br⁻) or iodide ion (I⁻) as a counter anion (Scheme 1). The heavy atoms such as bromine and iodine would be expected to facilitate ISC by the heavy-atom effect. Moreover, D–π–A pyrazinium dyes have an advantage over the conventional photosensitizers in carrying out the fundamental study on ¹O₂ generation, that is, we can obtain a great deal of useful knowledge for the relationship between the molecular structures and ¹O₂ generation efficiency, by easily exchanging the counter ion. Interestingly, it was found that the (D–π–)₂A pyrazinium dyes show specific solvatochromism, leading to a large bathochromic shift of absorption band in halogenated solvents, compared to polar and non-polar solvents. Therefore, the effects of the counter anion and solvents on the ¹O₂ generation efficiency have been investigated. On the basis of ¹O₂ quantum yield (Φ_Δ), rate constant (K_obs) for ¹O₂ generation, the HOMO and LUMO energy levels of OEJ-1 and OEJ-2, and density functional theory (DFT) calculation, the photoabsorption and ¹O₂ generation properties of the (D–π–)₂A pyrazinium dyes are discussed.

Scheme 1 Chemical structures of (D–π–)₂A pyrazinium dyes OEJ-1 and OEJ-2.

Results and discussion

Synthesis

The (D–π–)₂A pyrazinium dyes (OEJ-1 and OEJ-2) were synthesized from the (D–π–)₂A fluorescent dye OUK-2 (ref. 19) and the corresponding n-butyl halide (Scheme 2).

Scheme 2 Synthesis of (D–π–)₂A pyrazinium dyes OEJ-1 and OEJ-2.

Photoabsorption properties

The photoabsorption spectra of OEJ-1 and OEJ-2 in various solvents (THF, acetonitrile, DMSO and dichloromethane) are shown in Fig. 1 and their optical data are summarized in Table 1. The two dyes show a broad absorption band (λ^abs) at around 500–700 nm, which is assigned to the intramolecular charge-transfer (ICT) excitation from electron donor moiety (diphenylamino group) to electron acceptor moiety (pyrazinium group). In all the four solvents, the λ^abs for ICT band of OEJ-2 occurs at a longer wavelength than of OEJ-1. Interestingly, the two dyes showed the specific solvatochromism as with the previously reported D–π–A pyridinium dyes,²⁰ leading to a large bathochromic shift of absorption band in halogenated solvent such as dichloromethane, compared with that in polar and non-polar solvents; the λ^abs for ICT bands of OEJ-1 and OEJ-2 in dichloromethane occurs at a longer wavelength by ca. 30 nm and ca. 70 nm, respectively, than those in acetonitrile. It is worthy to note here that the specific solvatochromism depends on the counter anion of the (D–π–)₂A pyrazinium dyes, that is, the bathochromic shifts of ICT band for OEJ-2 bearing I⁻ ion is larger than that of OEJ-1 bearing Br⁻ ion.

Fig. 1 Photoabsorption spectra of (a) OEJ-1 and (b) OEJ-2 in THF, acetonitrile, DMSO and dichloromethane (CH₂Cl₂).

Table 1 Optical data of OEJ-1 and OEJ-2, and ¹O₂ quantum yield (Φ_Δ) and first-order rate constant (K_obs) for the photooxidation of DPBF using OEJ-1 and OEJ-2 as photosensitizer

Dye	Solvent	λ ^abs/nm for ICT band	ε/M^−1 cm^−1@λ^abs = 509 nm	Φ Δ ^a	K obs ^b/min^−1
a ^1O2 quantum yield (relative decomposition rate of DPBF), with Rose Bengal (RB) as standard (ΦΔ = 0.80 in methanol,^15 see Fig. S4) and 1,3-diphenylisobenzofuran (DPBF) as ^1O2 scavenger. These values were estimated under an assumption that the reactivity of singlet oxygen is independent of the kind of solvents. b First-order rate constant for the reaction of DPBF with ^1O2 generated upon photoexcitation of OEJ-1 or OEJ-2. The Kobs for RB is 0.250 min^−1 (see Fig. S5). c Too low. d Estimated from the slope for the range of 5–10 min in Fig. 9b.
OEJ-1	THF	490	28 200	0.19	0.034
	Acetonitrile	490	21 200	—^c	—^c
	DMSO	490	26 000	—^c	0.004
	Dichloromethane	520	24 900	—^c	0.006
OEJ-2	THF	500	3600	0.22	0.016
	Acetonitrile	500	7600	0.05	0.032
	DMSO	500	8500	0.03	0.006
	Dichloromethane	570	9700	0.07	0.160^d

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Electrochemical properties

The electrochemical properties of OEJ-1 and OEJ-2 were determined by cyclic voltammetry (CV) in acetonitrile containing 0.1 M tetrabutylammonium perchlorate (Bu₄NClO₄). The potentials were referred to ferrocene/ferrocenium (Fc/Fc⁺) as the internal reference (Fig. 2). For OEJ-2, the oxidation wave for the iodide counter ion was observed at around 0.05 V. The oxidation waves were observed at 0.43 V for OEJ-1 and 0.35 V for OEJ-2, respectively, vs. Fc/Fc⁺ (Table 1). The corresponding reduction waves appeared at 0.36 V for OEJ-1 and 0.18 V for OEJ-2, respectively. However, the oxidation and corresponding reduction waves are reversible for OEJ-1, but irreversible for OEJ-2. In fact, at second cycle OEJ-1 also showed the reversible oxidation wave, but OEJ-2 showed cathodic shift by ca. 0.1 V for the oxidation wave as well as disappearance of the oxidation wave for the iodide counter ion (Fig. S3†). The HOMO energy level vs. vacuum level is −5.20 eV for OEJ-1 and −5.07 eV for OEJ-2, respectively, which was evaluated through equation −[E^ox_1/2 + 4.8]eV from the half-wave potential for oxidation (E^ox_1/2 = 0.40 V vs. Fc/Fc⁺ for OEJ-1 and 0.27 V vs. Fc/Fc⁺ for OEJ-2). On the other hand, the LUMO energy level is −3.20 eV for OEJ-1 and −3.07 eV for OEJ-2, respectively, which was estimated from the HOMO and the onset of photoabsorption spectra (620 nm; 2.0 eV for both OEJ-1 and OEJ-2) in acetonitrile.

Fig. 2 Cyclic voltammograms of (a) OEJ-1 and (b) OEJ-2 in acetonitrile containing 0.1 M Bu₄NClO₄ at the first cycle. The arrow denotes the direction of the potential scan.

Theoretical calculations

In order to examine the HOMO and LUMO of OEJ-1 and OEJ-2, the molecular orbitals of the (D–π–)₂A pyrazinium dye cation (OEJ) was calculated using density functional theory (DFT) at the B3LYP/6-31G(d,p) level (Fig. 3).²¹ The DFT calculation for the dye cation indicates that the HOMO is mostly localized on the diphenylamine–carbazole moiety containing a thiophene ring. On the other hand, the LUMO is mainly concentrated on pyrazinium moiety. Accordingly, the DFT calculations reveal that excitation of the dye upon light irradiation induces a strong ICT from the diphenylamine–carbazole moiety to the pyrazinium moiety.

Fig. 3 (a) HOMO and (b) LUMO of OEJ cation by the density functional theory (DFT) calculations at B3LYP/6-31G(d,p) level.

¹O₂ generation by (D–π–)₂A pyrazinium dye

¹O₂ generation by (D–π–)₂A pyrazinium dyes OEJ-1 and OEJ-2 in various solvents (THF, acetonitrile, DMSO and dichloromethane) was evaluated by monitoring the photoabsorption spectral change of the known ¹O₂ scavenger 1,3-diphenylisobenzofuran (DPBF) accompanied by the reaction of DPBF with the generated ¹O₂, that is, DPBF can trap ¹O₂ through its photooxidation.²² All the solvents were bubbled with air for 15 min. The air-saturated solution containing the dye (OEJ-1 or OEJ-2) and DPBF was irradiated with 509 nm (160 μW cm⁻², see Table 1 for ε/M⁻¹ cm⁻¹@λ^abs = 509 nm) obtained by passage of xenon light through monochromator. The absorption band of DPBF at around 410 nm decreased with the increase in the photoirradiation time (Fig. 4 and 5), which indicate the reaction of DPBF with ¹O₂ generated upon the excitation of (D–π–)₂A pyrazinium dyes. To gain insight into the effect of the solvent and the counter anion on the efficiency of DPBF photooxidation, the changes in optical density (ΔOD) of DPBF are plotted against the photoirradiation time (Fig. 6), and the slope (m_sl) is used to estimate the ¹O₂ quantum yield (Φ_Δ) for OEJ-1 and OEJ-2. It was revealed that the m_sl value for OEJ-2 becomes steeper in the following order: DMSO (−0.4 × 10⁻³) < acetonitrile (−0.5 × 10⁻³) < dichloromethane (−0.8 × 10⁻³) < THF (−2.6 × 10⁻³), that is, the m_sl value in THF is larger than those in the other solvents. It was also found that the m_sl value of OEJ-2 is larger than that of OEJ-1 (−2.0 × 10⁻³ in THF). Consequently, this result indicates that THF is a favorable solvent for the (D–π–)₂A pyrazinium dyes to present a high DPBF-oxidation efficiency compared with the other solvents. Moreover, the plots demonstrate that OUJ-2 bearing I⁻ ion exhibits higher DPBF-oxidation efficiency than OUJ-1 bearing Br⁻ ion. Thus, the Φ_Δ values of OEJ-1 and OEJ-2 were estimated by the relative method using Rose Bengal (RB) (Φ_Δ = 0.80) in methanol as the standard (Table 1). The Φ_Δ value of OEJ-2 is 0.03, 0.05, 0.07 and 0.22 in DMSO, acetonitrile, dichloromethane and THF, respectively, which is in good agreement with the m_sl value. A higher Φ_Δ value in THF is ascribable to that as for the (D–π–)₂A pyrazinium dyes the ISC from ¹S* to the ³S* may be facilitated by THF, although further study for the solvent effects on ¹O₂ generation is necessary to ensure the hypothesis. It is worth noting that the Φ_Δ values of OEJ-2 in all the four solvents are higher than those of OEJ-1. Therefore, the high Φ_Δ value of OEJ-2 relative to OEJ-1 is attributed to the fact that I⁻ ion possesses superior heavy-atom effect rather than Br⁻ ion, resulting in the facilitation of the ISC.

Fig. 4 Photoabsorption spectral changes for the photooxidation of DPBF (4.6 to 5.4 × 10⁻⁵ M) using OEJ-1 as photosensitizer under photoirradiation with 509 nm (160 μW cm⁻²) in (a) THF (7.8 × 10⁻⁶ M), (b) acetonitrile (1.1 × 10⁻⁵ M), (c) DMSO (9.3 × 10⁻⁶ M) and (d) dichloromethane (9.7 × 10⁻⁶ M).

Fig. 5 Photoabsorption spectral changes for the photooxidation of DPBF (2.8 to 6.1 × 10⁻⁵ M) using OEJ-2 as photosensitizer under photoirradiation with 509 nm (160 μW cm⁻²) in (a) THF (7.2 × 10⁻⁵ M), (b) acetonitrile (3.3 × 10⁻⁵ M), (c) DMSO (3.1 × 10⁻⁵ M) and (d) dichloromethane (2.6 × 10⁻⁵ M).

Fig. 6 Plots of ΔOD for DPBF against the photoirradiation time for the photooxidation of DPBF using (a) OEJ-1 and (b) OEJ-2 as photosensitizers under photoirradiation with 509 nm (160 μW cm⁻²) in THF, acetonitrile, DMSO and dichloromethane.

In order to evaluate the photosensitizing ability of the (D–π–)₂A pyrazinium dyes, the ln(C_t/C₀) is plotted against the photoirradiation time, where C_t is a concentration of DPBF at the reaction time (t) and C₀ is the initial concentration of DPBF before photoirradiation (Fig. 9). All the four solvents (THF, acetonitrile, DMSO and dichloromethane) were bubbled with air for 15 min. The air-saturated solution containing the dye (OEJ-1 or OEJ-2) and DPBF was irradiated with visible light (>510 nm, 6 mW cm⁻²) obtained by passage of xenon light through a 510 nm long path filter. The photoabsorption spectral changes for the photooxidation of DPBF using OEJ-1 and OEJ-2 under photoirradiation with the visible light in the four solvents are shown in Fig. 7 and 8, respectively. The ln(C_t/C₀) decreased almost linearly with the increase in the photoirradiation time, although the linear relationship for the ln(C_t/C₀) for OEJ-2 in dichloromethane become steeper after the photoirradiation for 5 min under this photoirradiation. Thus, this result indicates the ln(C_t/C₀) bears a linear relationship with the photoirradiation time to provide the first-order rate constants (K_obs) for the photooxidation of DPBF using OEJ-1 and OEJ-2 as the photosensitizer (Table 1). The K_obs values for OEJ-2 are greater than those of OEJ-1, although the K_obs value for OEJ-2 in THF is lower than that of OEJ-1 due to the low photoabsorption property of OEJ-2 in THF. Interestingly, the plot of OEJ-2 in dichloromethane show a non-linear relationship, but the slope for OEJ-2 become steeper after 5 min of photoirradiation and the K_obs value for OEJ-2 in dichloromethane is greater than those in the other solvents. This interesting observation may be attributed to not only the bathochromic shift and broadening of absorption but also the enhancement of heavy-atom effect with the increase in the photoirradiation time in dichloromethane, that is, the significant specific solvatochromic behavior of the (D–π–)₂A pyrazinium dye bearing I⁻ ion. Therefore, this result demonstrates that OEJ-2 exhibits more efficient photosensitizing ability compared to OEJ-1, due to a superior heavy-atom effect of I⁻ ion.

Fig. 7 Photoabsorption spectral changes for the photooxidation of DPBF (5.0 × 10⁻⁵ M) using OEJ-1 (1.0 × 10⁻⁵ M) as photosensitizer under photoirradiation with visible light (>510 nm, 6 mW cm⁻²) in (a) THF, (b) acetonitrile, (c) DMSO and (d) dichloromethane.

Fig. 8 Photoabsorption spectral changes for the photooxidation of DPBF (5.0 × 10⁻⁵ M) using OEJ-2 (1.0 × 10⁻⁵ M) as photosensitizer under photoirradiation with visible light (>510 nm, 6 mW cm⁻²) in (a) THF, (b) acetonitrile, (c) DMSO and (d) dichloromethane.

Fig. 9 Plots of ln(C_t/C₀) for DPBF against the photoirradiation time for the photooxidation of DPBF using (a) OEJ-1 and (b) OEJ-2 as photosensitizers under photoirradiation with visible light (>510 nm, 6 mW cm⁻²) in THF, acetonitrile, DMSO and dichloromethane.

In addition, we performed an electron paramagnetic resonance (EPR) method with 2,2,6,6-tetramethyl-4-piperidone (4-oxo-TEMP) as the spin-trapping agent, which can react with ¹O₂ to produce 4-oxo-TEMPO as a stable nitroxide radical.^14,23 When the air-saturated solution containing OEJ-2 and 4-oxo-TEMP was irradiated with visible light (>510 nm, 14 mW cm⁻²) obtained by passage of xenon light through a 510 nm long path filter, the ESR spectrum of 4-oxo-TEMPO was clearly observed as a characteristic 1:1:1 triplet (Fig. S6†). Moreover, to obtain the direct evidence of ¹O₂ generation by (D–π–)₂A pyrazinium dyes, a phosphorescence spectrum of ¹O₂ was measured in air-saturated THF solution of OEJ-2. The phosphorescence maximum of ¹O₂ produced upon the excitation of OEJ-2 at 467 nm was clearly observed at around 1270 nm (Fig. S7†).^6b,14,24 Consequently, this work demonstrated that (D–π–)₂A pyrazinium dyes possess the ability to generate ¹O₂ under visible light irradiation.

Conclusions

(D–π–)₂A pyrazinium dyes bearing a counter anion (X⁻ = Br⁻ or I⁻) which show specific solvatochromic behavior leading to the bathochromic shift of photoabsorption band in halogenated solvents, have been designed and developed as a photosensitizer possessing singlet oxygen (¹O₂) generation. This work demonstrated that the (D–π–)₂A pyrazinium dyes possess the ability to generate ¹O₂ under visible light irradiation, due to the effective intersystem crossing (ISC) from the singlet excited state of the photosensitizer (¹S*) to the triplet excited state (³S*) by the heavy-atom effect of the counter anion. It was found that the ¹O₂ quantum yield (Φ_Δ) of OEJ-2 bearing I⁻ ion is higher than that of OEJ-1 bearing Br⁻ ion. Consequently, this result indicates that the high Φ_Δ value of OEJ-2 relative to OEJ-1 is attributed to the fact that I⁻ ion possesses superior heavy-atom effect rather than Br⁻ ion, resulting in the facilitation of the ISC. Moreover, it was found that THF is a favorable solvent for the (D–π–)₂A pyrazinium dyes to provide higher Φ_Δ value compared with the polar solvents such as acetonitrile and DMSO. Thus, this result suggests that as for (D–π–)₂A pyrazinium the ISC from ¹S* to the ³S* may be facilitated by THF, although much effort for the solvent effects on ¹O₂ generation is necessary to ensure the hypothesis. Interestingly, the first-order rate constants (K_obs) for the photooxidation of DPBF using OEJ-2 in dichloromethane is greater than those in the other solvents, which is attributed to the bathochromic shift and broadening of absorption in dichloromethane, that is, the significant specific solvatochromic behavior of the (D–π–)₂A pyrazinium dye bearing I⁻ ion. Further study to gain greater insight into the effects of the molecular structure of pyrazinium dyes on the ¹O₂ generation efficiency is now in progress by developing the (D–π–)₂A pyrazinium photosensitizers possessing strong photoabsorption property in body therapeutic window (650–900 nm) and water solubility.

Experimental

General

Melting points were measured with a Yanaco micro melting point apparatus MP model. IR spectra were recorded on a SHIMADZU IRAffinity-1 spectrometer by ATR method. High-resolution mass spectral data were acquired on a Thermo Fisher Scientific LTQ Orbitrap XL. ¹H NMR spectra were recorded on a Varian-400 (400 MHz) FT NMR spectrometer. Photoabsorption spectra were observed with a HITACHI U-2910 spectrophotometer. Cyclic voltammetry (CV) curves were recorded in and acetonitrile/Bu₄NClO₄ (0.1 M) solution with a three-electrode system consisting of Ag/Ag⁺ as reference electrode, Pt plate as working electrode, and Pt wire as counter electrode by using a Electrochemical Measurement System HZ-7000 (HOKUTO DENKO).

Synthesis

1-Butyl-3,5-bis(5-(9-butyl-7-(diphenylamino)-9H-carbazol-2-yl)thiophen-2-yl)pyrazin-1-ium bromide (OEJ-1). A solution of OUK-2 (ref. 19) (0.15 g, 0.15 mmol) and 1-bromobutane (1.28 g, 9.4 mmol) in DMF (8 ml) was stirred at 80 °C for 5 days under an argon atmosphere. After concentrating under reduced pressure, the resulting residue was subjected to reprecipitation from dichloromethane–hexane. The reprecipitation solid was chromatographed on reverse-phase silica gel (chloroform–methanol = 1 : 3 as eluent) to give OEJ-1 (0.078 g, yield 46%) as dark red solids; mp 152–153 °C; IR (ATR): [small nu, Greek, tilde] = 1624, 1591, 1526, 1487, 1445, 1427 cm⁻¹; ¹H NMR (400 MHz, DMSO-d₆) δ = 0.83 (t, J = 7.4 Hz, 6H), 1.01 (t, J = 7.5 Hz, 3H), 1.19–1.26 (m, 4H), 1.46–1.52 (m, 2H), 1.65–1.73 (m, 4H), 2.07–2.14 (m, 2H), 4.35 (t, J = 7.0 Hz, 4H), 4.58 (t, J = 7.3 Hz, 2H), 6.89 (dd, J = 1.8 and 8.4 Hz, 2H), 7.04–7.09 (m, 12H), 7.18 (d, J = 1.7 Hz, 2H), 7.31–7.36 (m, 8H), 7.66 (dd, J = 1.5 and 8.3 Hz, 2H), 7.98 (d, J = 4.0 Hz, 2H), 8.06 (s, 2H), 8.10 (d, J = 8.4 Hz, 2H), 8.16 (d, J = 8.1 Hz, 2H), 8.25 (d, J = 4.0 Hz, 2H), 9.51 (s, 2H) ppm; ¹³C NMR (100 MHz, DMSO-d₆) δ = 13.49, 13.71, 19.06, 19.75, 30.68, 32.23, 34.40, 41.80, 104.80, 106.26, 116.73, 117.48, 117.68, 120.58, 121.62, 122.90, 123.03, 123.59, 123.71, 129.38, 129.50, 136.45, 140.89, 140.95, 142.12, 146.34, 147.57, 147.63, 151.89, 152.04 ppm; HRMS (ESI): m/z (%): calcd for C₇₂H₆₅N₆S₂⁺ 1077.47066; found 1077.47144.

1-Butyl-3,5-bis(5-(9-butyl-7-(diphenylamino)-9H-carbazol-2-yl)thiophen-2-yl)pyrazin-1-ium iodide (OEJ-2). A solution of OUK-2 (ref. 19) (0.045 g, 0.04 mmol) and 1-iodobutane (0.81 g, 4.4 mmol) in DMF (3 ml) was stirred at 80 °C for 3 days under an argon atmosphere. After concentrating under reduced pressure, the resulting residue was subjected to reprecipitation from dichloromethane–hexane to give OEJ-2 (0.042 g, yield 77%) as dark red solids; mp 227–228 °C; IR (ATR): [small nu, Greek, tilde] = 1624, 1591, 1528, 1487, 1445, 1425 cm⁻¹; ¹H NMR (400 MHz, DMSO-d₆) δ = 0.83 (t, J = 7.5 Hz, 6H), 1.01 (t, J = 7.4 Hz, 3H), 1.18–1.27 (m, 4H), 1.44–1.52 (m, 2H), 1.65–1.73 (m, 4H), 2.06–2.14 (m, 2H), 4.35 (t, J = 7.0 Hz, 4H), 4.58 (t, J = 7.7 Hz, 2H), 6.89 (dd, J = 1.8 and 8.4 Hz, 2H), 7.04–7.09 (m, 12H), 7.18 (d, J = 1.7 Hz, 2H), 7.31–7.36 (m, 8H), 7.66 (dd, J = 1.4 and 8.1 Hz, 2H), 7.98 (d, J = 4.0 Hz, 2H), 8.06 (s, 2H), 8.10 (d, J = 8.4 Hz, 2H), 8.16 (d, J = 8.2 Hz, 2H), 8.25 (d, J = 4.0 Hz, 2H), 9.50 (s, 2H) ppm; ¹³C NMR (100 MHz, DMSO-d₆) δ = 13.49, 13.71, 19.06, 19.75, 30.69, 32.22, 41.79, 104.81, 106.28, 116.75, 117.50, 117.68, 120.59, 121.61, 122.89, 123.04, 123.69, 129.31, 129.37, 129.50, 136.43, 140.89, 142.12, 146.33, 147.56, 151.90, 152.06 ppm (one aliphatic and two aromatic carbon signals were not observed owing to overlapping resonances); HRMS (ESI): m/z (%): calcd for C₇₂H₆₅N₆S₂⁺ 1077.47066; found 1077.47131.

Evaluation of ¹O₂ quantum yield

Quantum yields (Φ_Δ) for singlet oxygen (¹O₂) generation by (D–π–)₂A pyrazinium dyes (OEJ-1 and OEJ-2) in various solvents (THF, acetonitrile, DMSO and dichloromethane) were evaluated by monitoring the photoabsorption spectral change of the known ¹O₂ scavenger 1,3-diphenylisobenzofuran (DPBF) accompanied by the reaction of DPBF with the generated ¹O₂, that is, DPBF can trap ¹O₂ through its photooxidation. All the solvents were bubbled with air for 15 min. The absorbance of DPBF was adjusted to around 1.0 in air-saturated solvent. Concentration of OEJ-1 or OEJ-2 was adjusted with an absorbance of 0.2–0.3 at the irradiation wavelength (509 nm). The air-saturated solution containing the photosensitizer (OEJ-1 or OEJ-2) and DPBF was irradiated with 509 nm (160 μW cm⁻²) obtained by passage of xenon light through monochromator. The photoabsorption spectral change of DPBF with the photoirradiation was monitored with an interval of 5 s up to 40 s. The absorption band of DPBF at around 410 nm decreased with the increase in the photoirradiation time. The changes in optical density (ΔOD) of DPBF are plotted against the photoirradiation time, and the slope is used to estimate the Φ_Δ of OEJ-1 and OEJ-2. The Φ_Δ of OEJ-1 and OEJ-2 was estimated by the relative method using Rose Bengal (RB) (Φ_Δ = 0.80) in methanol as the standard. Therefore, the ¹Φ_Δ values were calculated according to the following eqn (1):

Φ_Δsam = Φ_Δref × [(m_sam/m_ref) × (L_ref/L_sam)] (1)

where Φ_Δsam and Φ_Δref are the ¹O₂ quantum yield of photosensitizer (OEJ-1 or OEJ-2) and RB, respectively, m_sam and m_ref are the slope of the difference (ΔOD) in the change in the absorption maximum wavelength of DPBF (around 410 nm) which are plotted against the photoirradiation time, L_sam and L_ref are the light harvesting efficiency, which is given by L = 1 − 10^−A (“A” is the absorbance at the photoirradiation wavelength).

Photosensitizing ability

Photosensitizing ability of the (D–π–)₂A pyrazinium dyes (OEJ-1 and OEJ-2) in various solvents (THF, acetonitrile, DMSO and dichloromethane) was evaluated by plotting the ln(C_t/C₀) against the photoirradiation time, where C_t is a concentration of DPBF at the reaction time (t) and C₀ is the initial concentration of DPBF before photoirradiation. All the solvents were bubbled with air for 15 min. The air-saturated solution containing the photosensitizer (1 × 10⁻⁵ M for OEJ-1 and OEJ-2, 1 × 10⁻⁶ M for RB) and DPBF (5 × 10⁻⁵ M) was irradiated with visible light (>510 nm, 6 mW cm⁻²) obtained by passage of xenon light through a 510 nm long path filter. The absorbance of DPBF was adjusted to around 1.0 in air-saturated solvent. The photooxidation of DPBF with the photoirradiation was monitored by following the decrease in the photoabsorption at around 410 nm with an interval of 20 s up to 10 min. The concentration (C_t) of DPBF at the reaction time (t) was calculated based on Lambert–Beer law (A_DPBF = εcl). The ln(C_t/C₀) decreased almost linearly with the increase in the photoirradiation time due to the photooxidation of DPBF, that is, the slope was used to estimate the rate constants (K_obs).

¹O₂ detection by EPR spin-trapping method with 4-oxo-TEMP

The EPR spectra were recorded on a JEOL JES-RE1X spectrometer under the following experimental conditions: temperature 298 K, microwave power 1 mW, microwave frequency 9.439 GHz, field modulation 0.2 mT at 100 kHz, and scan time 4 min. The air-saturated THF solution containing OEJ-2 (0.01 mM) as the photosensitizer and 4-oxo-TEMP (50 mM) as the spin-trapping agent was irradiated with visible light (>510 nm, 14 mW cm⁻² for 30 min) obtained by passage of xenon light through a 510 nm long path filter. The ESR spectrum of 4-oxo-TEMPO which is formed by the reaction of 4-oxo-TEMP with ¹O₂, was clearly observed as a characteristic 1:1:1 triplet (Fig. S6†).

Phosphorescence measurement of ¹O₂

Phosphorescence spectrum of ¹O₂ was recorded on a HORIBA NanoLog spectrometer equipped with a 450 W xenon lamp and a photomultiplier tube (NIR-PMT R5509-43 liquid nitrogen configurations, Hamamatsu photonics). The phosphorescence maximum of ¹O₂ produced upon the excitation of OEJ-2 (0.07 mM THF solution) at 467 nm was clearly observed at around 1270 nm (Fig. S7†).

Acknowledgements

This work was supported by Grants-in-Aid for Scientific Research (B) from the Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Number 15H03859 and by Electric Technology Research Foundation of Chugoku. We would also like to thank Dr Yasushi Nakata of HORIBA, Ltd. for support with phosphorescence measurement of singlet oxygen.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra26647e

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