English

• 1.   Introduction

  Carotenoids mainly play two very important roles in photosynthesis, i.e., light harvesting as well as photo-protection [1, 2]. Light harvesting is fulfilled via a singlet-singlet energy transfer from excited carotenoids to bacteriochlorophyll (BChl) or chlorophyll (Chl) with variable efficiency [1-3]. Photo-protection, however, is done by direct electron transfer from carotenoid to P680⁺ in the photosystem Ⅱ reaction center [4, 5] by quenching the triplet state of BChl (or Chl) and singlet oxygen under excess solar light [6, 7], or by the ultra-fast formation of the zeaxanthin-chlorophyll chargetransfer complex in thylakoid membrane [8] and in various lightharvesting complexes Ⅱ [9-11].

  The formation mechanism and dynamics of the resulted Car triplets and carotenoid radicals have drawn extensive attention [12-22]. Carotenoids radicals can be formed on an ultrafast time scale [8] from electron transfer to the excited chlorophylls; on a subpicosecond time scale [17-19] from carotenoids S₂ state or some other dark singlet states; on a micro second scale from carotenoid triplets [18, 20]; or from direct excitation in methanol at 355 nm [21]. These studies were performed either in natural photosynthetic systems [8, 22] or in solvent [17-21].

  Solvent is one of the key factors determining the photo-induced dynamic mechanisms of carotenoids in vitro. Many studies have focused on the solvent effects on singlet excited states of carotenoids, e.g., the solvent polarizability strongly affects their S₂←S₀ absorption [23], and the solvent polarity has effects on the carotenoids' S₂-S₁ internal conversion rate [24, 25], and on the lifetime of carotenoids' S₁ state with ICT characteristics [26-37], etc. The ultrafast dynamics of carotenoid excited states in various environments was reviewed by Tomáš Polívka et al. [38].

  The study of the effects of solvents on the formation mechanism of triplets and radicals of carotenoids is relatively limited: In chloroform, no matter whether by directly excitation or triplet sensitizing, both lycopene and β-carotene (β-Car) generate radical cations [18, 20], while in n-hexane, under anthracene-sensitizing [20], only the triplet carotenoid is produced; in chloroform, the anthracene-sensitizing formation of fucoxanthin radical cation is mainly from its S₂ state, but conversely from the T₁ state in methanol [39]. A similar study on peridinin and its structural analogs upon chlorophyll anthracene-sensitized triplet excitation in acetonitrile did not generate any carotenoid cation species [40, 41]; in THF, anthracene-sensitized triplet excitation of a polar carotenoid derivative, RetInd, the production of a dication was observed [42]. Most recently, we proposed that the polar solvent, methanol, could assist two natural carbonyl carotenoids producing the cation dehydrodimers ^#[Car]₂⁺ upon anthracene-sensitized triplet excitation [43]. To investigate whether this mechanism is adaptable to a non-polar carotene, the anthracene-sensitized triplet dynamics of β-Car in n-hexane, methanol, and acetonitrile were studied with assistance of steady-state absorption studies under chemical oxidation and electrochemical oxidation as well.The generation of the cations of β-Car in methanol and acetonitrile was revealed by ns flash photolysis spectroscopy. The dynamic mechanism is discussed.

  2.   Experimental

  2.1   Sample preparation

  All trans-β-carotene (hereafter, β-Car) was purchased from Sigma (Steinheim, Germany) and twice recrystallized in n-hexane/ acetone mixture. The purity was checked by HPLC up to 95%. The pigments were kept refrigerated at -20 ℃ before use. All solvents (HPLC or spectroscopic grade) for spectroscopic measurements are from Meridian Medical Technologies, Inc. (Columbia, MD) and used directly after purchasing without further purification. Purified carotenoid was dissolved in n-hexane, in methanol, or in acetonitrile at a concentration of 1.5 × 10^-5 mol/L together with anthracene (1.5 × 10^-4 mol/L), and the solution circulated from, and to a reservoir (90 mL) through a quartz flow cell with an optical path-length of 5 mm for ns time-resolved absorption measurement.

  2.2   Spectroscopic measurement

  The ground-state absorption spectrum was measured on a Cary 50 spectrophotometer (Varian Inc., Palo Alto, CA). Spectroelectrochemistry (SEC) [44], chemical oxidation measurements and ns flash photolysis spectroscopy [44] setups were home-built and described as follows.

  For chemical oxidation measurements, β-Car (1 × 10^-5 mol/L) was oxidized by FeCl₃ (1:1 molar ratio) in methanol and in acetonitrile, the absorption spectra (300-1100 nm) was measured at r.t. in a quartz cell with an optical path-length of 1 cm.

  SEC analysis of β-Car in methanol or in acetonitrile (1 × 10^-5 mol/L) was carried out with a tri-electrode configuration consisting of a working electrode (platinum net), a counter electrode (platinum wire), and a reference electrode (Ag⁺/Ag: AgNO₃ (0.01 mol/L) and tetra-n-butylammonium hexafluorophosphate (0.1 mol/L) in methanol or in acetonitrile), which were assembled into a quartz cuvette of an optical path length of 2 mm. The electrolyte solution was tetra-n-butylammonium hexafluorophosphate (0.1 mol/L) in methanol or in acetonitrile. The electrochemical cell was mounted on a UV-3600 absorption spectrometer (190-3300 nm. Shimadzu, Japan). The applied electrical potential across the reference and the working electrodes was 1.5 V. The SEC spectra were obtained by taking the difference between the absorption spectra with and without the electrical bias.

  For the ns time-resolved absorption measurement, the excitation laser pulses of 7 ns duration at 355 nm were generated from Nd³⁺:YAG laser operated at a repetition of 10 Hz (Quanta-Ray ProSeries, Spectra Physics Lasers Inc., Mountain View, CA). The pulse energy for exciting the sample was 0.4 mJ/pulse. A laser-driven light source (Model EQ-1500, Energetiq Technology Inc.) was employed as a probe light source, which was detected with a Si PIN photodiode (Model S3071, Hamamatsu Photonics, Hamamatsu, Japan) attached to a Trivista spectrometer system (SP2500i, Princeton Instruments/Acton) after passing the excited region of the sample. During experiments, the multi-wavelength detection scheme for recording a transient spectrum at a selected delay time was employed, and the kinetic traces were stored and averaged with a digital storage oscilloscope (bandwidth 600 MHz; LeCroy WaveSurfer 64Xs, Chestnut Ridge, NY) connected to a computer. The temporal resolution of the setup was approximately 100 ns. Before experiments, oxygen was removed by repeated evacuation of the air from the cuvette and purging with solvent-saturated nitrogen (99.99%) of the cuvette at r.t. for 60 min. Kinetic traces were averaged upon photoexcitation of 1000 laser shots, yielding an △OD detection sensitivity better than 10^-4. All measurements were performed at r.t.

  3.   Results and discussion

  3.1   Solvent effects on anthracene-sensitized triplet excitation dynamics of β-Car

  Fig. 1 shows the steady-state absorption spectra of β-Car in nhexane, in methanol, and in acetonitrile, respectively. The broad and intense absorption in 19, 000-28, 000 cm^-1 (357-526 nm) originates from the S₂←S₀ absorption, which apparently has a different solvent effect from that of the carbonyl carotenoids [43]. Here, polarity of the solvents was not an obvious effect on the spectral pattern of β-Car, even when the polarity P(ε) variation is quite large (Table S1 in Supporting information). Since the polarizability R(n) of the three solvents used in this study are similar (Table S1), the S₂←S₀ absorption has no obvious shift which is reasonable [23].

  图 1

  

  图 1  Normalized absorption spectra of β-carotene, respectively, in n-hexane (solid line), methanol (dash line), and acetonitrile (dash dot line) at r.t.

  Figure 1.  Normalized absorption spectra of β-carotene, respectively, in n-hexane (solid line), methanol (dash line), and acetonitrile (dash dot line) at r.t.

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  Fig. 2 shows the transient absorption spectra at indicated delay times of β-Car in n-hexane (A), in methanol (B), and in acetonitrile (C), respectively, in the 400-1000 nm region.

  图 2

  

  图 2  Representative transient absorption spectra at indicated delay times: β-carotene in n-hexane (A), in methanol (B), and in acetonitrile (C), mixed with anthracene, excitation wavelength is 355 nm.

  Figure 2.  Representative transient absorption spectra at indicated delay times: β-carotene in n-hexane (A), in methanol (B), and in acetonitrile (C), mixed with anthracene, excitation wavelength is 355 nm.

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  In n-hexane (Fig. 2A), upon the excitation of anthracene (Ant) at 355 nm, a pair of negative and positive signals, peaking at 450 nm (for negative signal) and 510 nm (for positive signal), respectively, rise up to their maxima in several μs. The negative signal is well assigned as the bleaching of the ground state absorption (S₂←S₀) (GSB) of β-Car, with the positive signal as the triplet excited state absorption, i.e., T_n←T₁ absorption of β-Car (³Car^*) as reference [20]. These spectral changes decay in 45 μs. No transient absorption is seen in the near-infrared region. In methanol and acetonitrile upon the excitation of anthracene, both the GSB centered at 450 nm and the ³Car^* absorption at 510 nm (or 520 nm) formed in the similar time scale to that in n-hexane. Differently, in both these two polar solvents, upon excitation, a new transient absorption peaked at 720 nm rises immediately to its maxima. The decay of ³Car^* absorption in two high polar solvents has the same time scale with that in n-hexane, i.e., ~40 μs, while the decay of 720 nm absorption seems accompanied with the formation of another transient absorption peaked at 920 nm (see the identity assignments of these cationic species in the following discussion), and then the 920 nm species decays in a very long time scale.

  Fig. 3 depicts the kinetics and their fitted curves obtained from the selected wavelengths corresponding to the data sets in Fig. 2. In n-hexane, the rise and the decay of the ³Car^* absorption (510 nm) constitute a mirror image (when scaled) with respect to that of the GSB (450 nm), a result which shows transformation between the S₀ and T₁ states. The carotenoid cationic species absorption is not seen in this case based on the kinetics detected at 720 and 920 nm, respectively (Fig. 3A). From in methanol to in acetonitrile (Fig. 3B and C), the formation of ³Car^* absorption and GBS becomes gradually faster than in n-hexane (comparing the broken vertical line positions in three solvents), while the decay of ³Car^* absorption seems independent of the solvents, i.e. having almost the same decay time profile in the three solvents. In the two polar solvents, the 720 nm species generate immediately upon excitation with the decay of this species comprised of more than one component. The faster decay component of the 720 nm species in these solvents roughly constitutes a mirror image (when scaled) to the formation of 920 nm species, indicating the generation of 920 nm species from the 720 nm species. Noticeably, in methanol, both the 720 nm and the 920 nm maxima are much less than in acetonitrile. On a longer time scale (Fig. S1 in Supporting information), the 710 nm and 920 nm species have synchronous decay kinetics both in these two polar solvents. Furthermore, in acetonitrile the cationic species has a much longer lifetime than in methanol.

  图 3

  

  图 3  Kinetic traces obtained from the corresponding data sets in Fig. 2: β-Car in n-hexane (A), in methanol (B), and in acetonitrile (C), respectively. The gray scatters are the raw data, and the black lines are fitted curves. The broken vertical lines indicate the delay time when triplet absorption arrives at the maxima.

  Figure 3.  Kinetic traces obtained from the corresponding data sets in Fig. 2: β-Car in n-hexane (A), in methanol (B), and in acetonitrile (C), respectively. The gray scatters are the raw data, and the black lines are fitted curves. The broken vertical lines indicate the delay time when triplet absorption arrives at the maxima.

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  Table 1 summarizes the time constant for each component obtained by kinetic fitting based on the data sets in Fig. 3.

  表 1

  

  表 1  Rise (τ_r) and decay (τ_d) time constants at selected probing wavelengths (λ_pr) derived via fitting the kinetics traces in Fig. 3 for β-Car in various solvents.

  Table 1.  Rise (τ_r) and decay (τ_d) time constants at selected probing wavelengths (λ_pr) derived via fitting the kinetics traces in Fig. 3 for β-Car in various solvents.

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  In n-hexane, upon excitation of anthracene, the triplet β-Car excited state absorption (ESA), i.e., the T_n←T₁ absorption, is populated in~3 μs, which corresponds to the lifetime of triplet anthracene [18] and implies that the triplet-triplet excitation energy transfer (EET) is a diffusion-controlled process, and then follows the exponential decay of triplet carotene with the lifetime~10 μs. Along with the kinetics of triplet carotene ESA, the mirror kinetic behavior of GSB detected at 450 nm is also shown in Table 1.

  In methanol and in acetonitrile, upon the excitation of anthracene, the population of triplet carotene was observed in gradually faster kinetics than in n-hexane, i.e.~2.5 μs and~1.4 μs, respectively. This might be due to the solvent effect on the diffusion rate or the lifetime of triplet anthracene. Since the formation of 720 nm species is faster than the time-resolution of this study, i.e., 100 ns, the lifetime of 720 nm ESA formation (τ_r in Table 1) was then shown as ‘fast'. The decay of triplet β-Car seems not affected by the solvents, and their lifetimes are all about~10 μs. The 720 nm species decays in a multi-phase way both in methanol and in acetonitrile: Firstly, a fast decay with the lifetime~3-5 μs (τ_d1), which is similar, although a little bit longer, with the rise time constant of 920 nm. Naturally, they are kinetically correlated with each other. After the population of the maxima of 920 nm, both 720 nm and 920 nm species perform the synchronous multi-steps decay kinetics in both these two polar solvents. At this time scale, these two detect wavelengths are all belonging to the same species as shown in Fig. S2 in Supporting information, i.e., Car^·+ [18, 20]. The lifetime for Car^·+ in methanol is~400 μs, while the counterpart in acetonitrile is infinite in our measurement as shown as ‘long' (τ_d2 for GSB and 920 nm species, and τ_d3 for 720 nm species in acetonitrile) in Table 1. The two-phase characteristics of Car^·+ kinetics in both methanol and acetonitrile implies that the intrinsic mechanism is more complicated than that seen in this study. This could also explain that the recovery of GSB in these two polar solvents is biphasic kinetics without the fully matched time constants with the cationic species.

  3.2   Spectroelectrochemistry (SEC) and chemical oxidation study for the assignment of carotenoid cationic species in transient absorption

  Fig. 4 gives the absorption comparison of β-Car cationic species obtained by transient absorption (black solid line), by chemical oxidation (black broken line), and by SEC method, respectively, in methanol (A and C) and in acetonitrile (B and D). In each solvent, spectral characteristics are discussed in two regions, i.e., 550-800 nm (Fig. 4A and B) and 750-1000 nm (Fig. 4C and D).

  图 4

  

  图 4  The absorption comparison of carotenoid cationic species obtained by transient absorption (black solid line), by chemical oxidation (black dash line) and by spectroelectrochemical (SEC) method (gray line) for β-carotene measured in methanol ((A) and (C)) and acetonitrile ((B) and (D)), respectively.

  Figure 4.  The absorption comparison of carotenoid cationic species obtained by transient absorption (black solid line), by chemical oxidation (black dash line) and by spectroelectrochemical (SEC) method (gray line) for β-carotene measured in methanol ((A) and (C)) and acetonitrile ((B) and (D)), respectively.

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  In methanol, only transient absorption (by anthracene-sensitized triplet excitation) shows the cationic species peaked at~720 nm (Fig. 4A) and~920 nm (Fig. 4C), while by chemical oxidation and SEC methods, nothing could be observed. Considering that these cationic species only have less than hundreds of μs lifetimes as shown in ns flash photolysis measurement, and they are undetectable in steady-state absorption measurements under chemical oxidation or electrode oxidation (SEC), the cationic species of β-Car should be very unstable in methanol.

  In acetonitrile, both the three methods could produce the~720 nm and~920 nm centered peak (Fig. 4B and D). For 720 nm species, both the SEC and chemical oxidation give the broader, but partly overlapped absorption in comparison to the transient absorption (Fig. 4B). But they may not originate from the same species, since the 720 nm species only have a several microsecond lifetime in transient absorption measurement, while in chemical oxidation (Fig. S3B in Supporting information) and in SEC it has the lifetime of over minutes. For the 920 nm species, chemical oxidation produces well matched spectra with that obtained by transient absorption, and the SEC produces a broader spectral pattern with seemingly multi-components (Fig. 4D). Based on the very ‘long' lifetime of 920 nm species in acetonitrile detected by transient absorption, observing it at a visible time scale is reasonable. Actually the 920 nm species observed by chemical oxidation could survive in several decade minutes in acetonitrile (Fig. S3B).

  The 920 nm species observed in transient absorption is assigned as the radical cation of β-Car, i.e., Car^·+ as in previous studies [18, 20], while the 720 nm species is preferentially assigned as the cation dehydrodimers of β-Car, ^#[Car]₂⁺, due to the following reasons: the production of the cationic species of carotenoids is dependent on the oxidation condition. Gao et al. [46] reported that canthaxanthin, one kind of keto-carotenoid, preferentially forms cation dehydrodimers (^#[Car]₂⁺) in dichloromethane with irradiation and forms the di-cation (Car²⁺) without irradiation under oxidation by ferric chloride. Noticeably in this study, the transient absorption measurement has the similar oxidation condition, i.e., oxidation with irradiation, to that producing the cation dehdydrodimers in the reference [46]. Furthermore the spectral position of the dication is 70 nm blue-shifted in comparison to that of the cation dehydrodimers [46]. In this study, in acetonitrile, the~720 nm species produced by electrode oxidation (in SEC measurements) and by ferric chloride oxidation has a broader and multi-component spectral pattern, which obviously contains both the dication (~650 nm) and the cation dehydrodime (~720 nm) species (Fig. 4B). While the~720 nm species produced in ns flash photolysis has rather narrow band width, which is very similar with the~720 or 750 nm transient species obtained in our previous study and also assigned as ^#[Car]₂⁺ [43].

  3.3   Solvent effect on the triplet excitation of β-Car in comparison to the keto-carotenoids

  In our previous study, upon the anthracene-sensitization triplet, the solvent effect on the triplet excitation dynamics of two keto-carotenoids has been investigated [43]. In n-hexane, only the GSB and the excitation triplet (³Car^*) were observed, and no cationic species were detected. In methanol, the cation dehydrodimer of carotenoids (^#[Car]₂⁺) was proposed to generate from the ³Car^* which resulted in the lifetime shrinking of ³Car^*. No radical cation of carotenoid (Car^·+) is generated, and the cation dehydrodimer of carotenoids (^#[Car]₂⁺) was the only observed cationic species.

  In this study, under the same experimental condition, β-Car, a hydrophobic carotenoid, performs the same triplet excitation dynamics as the keto-carotenoids in n-hexane, i.e., only the GSB and the excitation triplet (³Car^*) were observed, and no cationic species were detected. In the two high polar solvents, methanol and acetonitrile, the excitation dynamics has the following different aspects for β-Car in comparison to the keto-carotenoids. Firstly, the dynamics of ³Car^* for β-Car is independent of solvents, i.e., β-Car owns the same decay lifetime as in the non-polar solvent, n-hexane. Secondly, the ^#[Car]₂⁺ of β-Car forms possibly through a polar solvent-assisted triplet anthracene oxidation of ground state β-Car, but not ³Car^*. Thirdly, the cation dehydrodimer of β-Car quickly transforms into the radical cation (Car^·+). Finally, the Car^·+ of β-Car has much longer lifetime in acetonitrile than in methanol.

  Therefore, the carotenoid structure must be the key to cause the aforementioned variations of solvent effect on triplet excitation dynamics. For the keto-carotenoids, due to the strong interaction between conjugated carbonyl group and high polar solvents, the polarity of the solvent has dramatic effects on the singlet excited state properties and even on the ground state absorption of the keto-carotenoids as reviewed in the Introduction [26-37]. For the hydrophobic β-Car, which actually has low solubility in high polar solvents such as methanol and acetonitrile, but has more chance to form aggregates in such solutions which may cause the formation of ^#[Car]₂⁺ directly from oxidation of the aggregated ground state β-Car. Furthermore, in comparison to the ^#[Car]₂⁺, the Car^·+ of β-Car may be more stable in the high polar solvent, which drove the former transforming into the latter quickly. Although the generation mechanism of cationic species of β-Car is the same in methanol as in acetonitrile, the different lifetimes of Car^·+ therein might be due to the property variations between methanol and acetonitrile, since the former is protic solvent and the latter is aprotic solvent.

  4.   Conclusion

  Transient absorption in the microsecond time domain is usually used to study the excitation triplets and long-lived radicals of carotenoids [18, 20, 39, 45]. And the photophysical and photochemical mechanism depends not only on sensitizer [40-42], but also the solvents [18, 20, 42, 43].

  In this study in the hydrophobic solvent n-hexane, upon anthracene-sensitized triplet excitation, β-Car triplet only performs pure decay to its ground state, while in high polar solvents, i.e., methanol and acetonitrile, the β-Car triplet has the same lifetime with that in n-hexane. And unlike the carbonyl carotenoids [43], the cation dehydrodimer (peaked at 720 nm) of β-Car generates, in these two high polar solvents, by directly excitation at 355 nm or by triplet anthracene oxidation, and subsequently transfers into its free radical cation Car^·+. The Car^·+ of β-Car has a very different lifetime in these two polar solvents of similar nature, i.e., in protic methanol with several hundred microseconds lifetime and in aprotic acetonitrile with a much longer lifetime. In comparison to the carbonyl carotenoids [43], the results of β-Car in this study prove that both solvent and the carotenoid structure determine the triplet excitation mechanism.

  Acknowledgments

  This work has been supported by the Natural Science Foundation of China (Nos. 21273282 and 21173265), the International Cooperation Project between China and Russia (NSFC-RFBR No. 21411130185), the Fundamental Research Funds for the Central Universities, and the Research Funds of Renmin University of China (No. 16XNH060).

  Appendix A. Supplementary data

  Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cclet.2016.05.032.

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  Figure 1  Normalized absorption spectra of β-carotene, respectively, in n-hexane (solid line), methanol (dash line), and acetonitrile (dash dot line) at r.t.

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  Figure 2  Representative transient absorption spectra at indicated delay times: β-carotene in n-hexane (A), in methanol (B), and in acetonitrile (C), mixed with anthracene, excitation wavelength is 355 nm.

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  Figure 3  Kinetic traces obtained from the corresponding data sets in Fig. 2: β-Car in n-hexane (A), in methanol (B), and in acetonitrile (C), respectively. The gray scatters are the raw data, and the black lines are fitted curves. The broken vertical lines indicate the delay time when triplet absorption arrives at the maxima.

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  Figure 4  The absorption comparison of carotenoid cationic species obtained by transient absorption (black solid line), by chemical oxidation (black dash line) and by spectroelectrochemical (SEC) method (gray line) for β-carotene measured in methanol ((A) and (C)) and acetonitrile ((B) and (D)), respectively.

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  Table 1.  Rise (τ_r) and decay (τ_d) time constants at selected probing wavelengths (λ_pr) derived via fitting the kinetics traces in Fig. 3 for β-Car in various solvents.

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