Flor
Rodríguez-Prieto
*ab,
Carlos Costa
Corbelle
b,
Berta
Fernández
b,
Jorge A.
Pedro
a,
M. Carmen
Ríos Rodríguez
*ab and
Manuel
Mosquera
*ab
aCentro Singular de Investigación en Química Biolóxica e Materiais Moleculares (CIQUS), Universidade de Santiago de Compostela, E-15782 Santiago de Compostela, Spain. E-mail: flor.rodriguez.prieto@usc.es; carmen.rios@usc.es; manuel.mosquera@usc.es
bDepartamento de Química Física, Facultade de Química, Universidade de Santiago de Compostela, E-15782 Santiago de Compostela, Spain
First published on 24th November 2017
N-Methylquinolinium cation (MQ+) in its first-excited singlet state is a strong oxidant commonly used as a photosensitizer, whose fluorescence is therefore quenched by electron donors. Interestingly, the fluorescence of MQ+ is also quenched by hydroxy compounds such as water and alcohols, more difficult to oxidize. We investigated the quenching mechanism of MQ+ fluorescence by small amounts of water and alcohols in acetonitrile solution. The fluorescence intensities and lifetimes exhibited a nonlinear dependence on the quencher concentration. We found evidence that emissive exciplexes MQ+*-ROH are formed between the excited quinolinium and the hydroxy compounds. An accurate quantitative description of the results was obtained with a model in which the exciplex reacts with a second molecule of the hydroxy compound, which quenches the fluorescence. The rate constant of this process increased as the quencher ionization energy decreased. We showed also that a low basicity of the hydroxy compound inhibits the quenching process. These results are consistent with the existence of a concerted photoinduced proton-coupled electron transfer (PCET) involving an intermediate complex of the excited quinolinium with a H-bonded molecular pair of the hydroxy compounds. In these pairs, a water or an alcohol molecule is able to donate an electron to the photoexcited quinolinium cation and a proton to the second H-bonded hydroxy molecule, showing an enhanced reducing power in comparison with the isolated molecule. The structure of the intermediate complex was investigated using high-level quantum mechanical calculations. At high water concentrations in acetonitrile/water mixtures, the quenching process is slowed down, indicating that higher water aggregates are less effective for a PCET process. The results obtained may be relevant to the study of water oxidation and electron transfer in biological systems.
Electron transfer is frequently made possible by the rate increase brought about by photoexcitation of the reactants, through coupling proton and electron transfer, or other enhancement mechanisms.14–18 Electronic excitation brings about a strong increase of molecular electron donor and acceptor abilities. Photoexcitation is therefore a method of choice to promote electron transfer processes, as it is also the coupling of electron and proton transfer, which catalyses redox processes by avoiding high-energy intermediates. PCET processes are at the heart of the most important chemical and biochemical energy conversion and storage processes, such as photosynthesis, respiration, and solar fuels.16–23 The coordinated movement of electrons and protons is also implicated in other fundamental biological processes, like DNA mutation and repair, biological photoprotection mechanisms, and the function of proteins.15,17,21–25
Photoinduced PCET processes combine the two above-mentioned enhancement mechanisms and lead therefore to a large increase in the rate of electron transfer processes. This pattern is used by green plants in the photosynthetic process, and is also at the heart of many artificial photosynthesis schemes that pursue water splitting to form hydrogen and oxygen, or water reduction of CO2 to carbon-based fuels.16–23,26,27 Photoinduced PCET is therefore a cornerstone for the design of renewable energy-conversion systems, which, together with the need to understand the involvement of PCET in water and in biological systems, triggered increasing interest in the study of its detailed mechanism from both the experimental and the theoretical points of view.14–31
In this paper, we report an in-depth quantitative investigation of the fluorescence quenching mechanism of the N-methylquinolinium cation by water and aliphatic alcohols in acetonitrile. The results provide evidence for a novel photoinduced PCET quenching mechanism, which involves an electron transfer from a pair of molecules of the hydroxy compounds to the photoexcited quinolinium. This knowledge will contribute to the understanding of biological processes and energy-converting systems where PCET involving hydroxy compounds plays a central role.
Fluorescence decays were measured using the time-correlated single-photon counting technique in an Edinburgh Instruments LifeSpec-ps time-resolved spectrometer equipped with a sub-nanosecond pulsed LED from PicoQuant as the excitation source (308 nm). The reconvolution analysis software supplied by the manufacturer was employed.
MQ+ has an iodide ion as a counterion and this anion quenches its fluorescence at a diffusion-controlled rate.4 The effect of the constant iodide concentration on MQ+ fluorescence is entirely negligible for the steady-state fluorescence measurements due to the low concentration used for these experiments (∼10−5 mol dm−3), but has a small effect that must be considered in the time-resolved experiments because of the 10-times higher concentration of the N-methylquinolinium iodide employed for these measurements. This small effect must be taken into account for the global analysis of steady-state and time-resolved measurements. For this reason, we have introduced in the analysis the linear dependence of the deactivation rate constant of excited MQ+ on the iodide concentration in acetonitrile (the quenching rate constant is 5.22 × 1010 mol−1 dm3 s−1, measured for concentrations up to 2 × 10−4 mol dm−3).
The ab initio calculations were performed with the Gaussian 09 program package.37
Solvent | Φ |
---|---|
Acetonitrile | 0.422 |
Water | 0.232 |
Methanol | 0.00398 |
Ethanol | 0.00221 |
We investigated the decrease of the fluorescence intensity of MQ+ in acetonitrile upon addition of water, methanol, ethanol, 1-propanol, 1,2-ethanediol, and 1,3-propanediol. Plots of the fluorescence intensity ratio in the absence and presence of the quencher (F0/F) versus quencher concentration showed nonlinear Stern–Volmer relationships with an upward curvature in all cases (Fig. 2 and Fig. 5S–9S, ESI†). As can be seen from these plots, F0/F gradually changed with the emission wavenumber, which revealed that the shape of the emission spectra changed with the addition of quenchers. These results suggest that a second fluorescent species is formed. This species must exist only in the excited state, as the excitation spectra remained unchanged in all cases upon addition of quenchers (cf.Fig. 2 and Fig. 5S–9S, ESI†).
To examine the possible effect of hydrogen bonding on the quenching process, we studied the influence of 2,2,2-trifluoroethanol on the fluorescence of MQ+ solutions in acetonitrile. We chose this alcohol due to its extremely low hydrogen-bond basicity, originated by the strong electron-withdrawing effect of the fluorine atoms.38,39 We found that no fluorescence quenching at all was observed in the alcohol concentration range of 0 to 0.39 mol dm−3 (Fig. 10S, ESI†).
The fluorescence decay of MQ+* in acetonitrile in the absence of quenchers was monoexponential, with a lifetime of τ0 = 23.37(4) ns. It remained monoexponential upon addition of hydroxy compounds (some examples are shown in Fig. 11S, ESI†), but the lifetime τ decreased as their concentration increased. Nonlinear Stern–Volmer plots of τ0/τ versus quencher concentration were obtained in all cases, but their curvatures were significantly different from those obtained for the intensity ratio F0/F (cf.Fig. 2 and Fig. 5S–9S, ESI†). This behaviour, together with the change of the spectral shape observed upon addition of hydroxy compounds, revealed that a quenching mechanism more complex than static and/or dynamic quenching must be operating.
Principal-component analysis applied to the series of fluorescence emission spectra of MQ+ at different quencher concentrations showed that two independent spectral components are needed to reproduce the spectral series.40,41 This indicates that a new emissive species appears upon addition of quenchers, which we identify with an exciplex E* formed by the excited quinolinium cation and the quencher (see Scheme 1).
Scheme 1 Excitation and deactivation patterns proposed for MQ+ in acetonitrile in the presence of water or alcohols. |
To explain the complex fluorescence intensity and lifetime dependence on the quencher concentration (Fig. 2 and Fig. 5S–9S, ESI†), we propose that after excitation of MQ+ and formation of the exciplex E*, a second molecule of the hydroxy compound approaching E* induces its nonradiative deactivation. Furthermore, as the fluorescence decay of MQ+* was monoexponential, we assume that a fast equilibrium is established between MQ+* and E*. To test the proposed mechanism, a thorough data analysis was carried out, as explained below.
Any emission spectrum F of the series must be a linear combination of the spectra of the emissive species MQ+* and E*. In eqn (1), F0MQ and F0E represent the fluorescence spectra that would be obtained for MQ+* and E* if each absorbed photon formed an excited molecule of the respective species, and only the unimolecular deactivation of these excited species would be operative (see the kinetic model, ESI†). The coefficients CMQ and CE are the contributions of the MQ+* and E* spectra to the experimental spectrum, and their values depend on the quencher concentration.
F = CMQF0MQ + CEF0E | (1) |
From the mechanism shown in Scheme 1, one can easily derive eqn (2)–(4), which show the predicted dependence on the quencher concentration of the emission coefficients CMQ and CE, and of the lifetime ratio τ0/τ (see the kinetic model in the ESI†).
(2) |
(3) |
(4) |
In these equations (see Scheme 1), kMQ and kE are the unimolecular deactivation rate constants of the excited quinolinium cation and the exciplex, respectively. They correspond to the sum of the radiative kr and the nonradiative knr deactivation constants of the respective species. kq represents the quenching rate constant of the exciplex by ROH, K is the exciplex formation equilibrium constant, and τ0 is the lifetime of MQ+* in the absence of quenchers.
For each quencher, we analysed using Principal-Component Global Analysis (PCGA),40,41 with the set of eqn (1)–(4), the series of fluorescence spectra and lifetimes of MQ+ obtained at different quencher concentrations. From these analyses, we obtained the fluorescence spectra F0MQ and F0E, together with the optimized values of K, kMQ, kE, and kq, collected in Table 2. Fig. 3 shows graphically the fit results obtained for methanol: part (A) displays the optimized spectra F0MQ and F0E, and parts (B), (C) and (D) show the experimental fluorescence lifetimes τ and the coefficients CMQ and CE as a function of quencher concentration, together with plots of the fitted curves calculated with the single set of parameters shown in Table 2. It is seen that the model quantitatively reproduces the fluorescence intensity and lifetime data. The same goodness of fit was obtained for water and the rest of the alcohols investigated (Fig. 12S–16S, ESI†), which reflects the model consistency.
Quencher | E i/eV | η 0E | |||||||
---|---|---|---|---|---|---|---|---|---|
a T = 293 K. b From ref. 42 c From ref. 43 | |||||||||
Water | 12.62b | 0.83 | 0.51 | 0.217(5) | 4.41(3) | 2.81(5) | 0.250(3) | 1.4 | 1.4 |
2,2,2-Trifluoroethanol | 11.49b | — | — | — | — | — | — | — | — |
Methanol | 10.84b | 0.56 | 0.75 | 0.112(3) | 4.35(1) | 2.6(1) | 1.90(4) | 2.0 | 0.6 |
Ethanol | 10.48b | 1.9 | 0.22 | 0.123(5) | 4.33(2) | 8.1(3) | 6.5(2) | 1.8 | 6.3 |
1-Propanol | 10.22b | 2.4 | 0.18 | 0.115(6) | 4.35(1) | 10.4(6) | 9.0(4) | 1.8 | 8.6 |
1,2-Ethanediol | 10.16b | 5.8 | 0.073 | 0.28(2) | 4.34(1) | 24(1) | 6.2(3) | 1.7 | 22 |
1,3-Propanediol | 9.7c | 3.2 | 0.13 | 0.169(5) | 4.34(1) | 16.9(7) | 12.1(5) | 2.2 | 14.7 |
We show in Fig. 3(A) that the fluorescence spectrum recorded for MQ+ in neat methanol almost completely overlaps with the spectrum F0E obtained by PCGA for the exciplex MQ+*-methanol in acetonitrile (the same is also true for ethanol, see Fig. 13S, ESI†). We interpret this fact as an indication that the exciplex is the main emissive species in pure methanol, favoured by the high concentration of the solvent.
The integrated areas of the fluorescence spectra F0MQ and F0E obtained from PCGA contain information on the quantum yields of these species in the solvent used, acetonitrile. The ratio of the areas equals the quotient Φ0MQ/η0E, where Φ0MQ denotes the fluorescence quantum yield of MQ+ in acetonitrile in the absence of quenchers, and η0E represents the fluorescence quantum efficiency of E* if only the unimolecular photophysical deactivation of this excited species would be operative (η0E = kEr/kE, see details in the ESI†). The values of Φ0MQ/η0E for various quenchers are collected in Table 2. As Φ0MQ is the fluorescence quantum yield of MQ+ in neat acetonitrile (0.422), the values of η0E for the different exciplexes in this solvent can be calculated, together with their radiative and nonradiative deactivation constants (Table 2). For water and methanol, the η0E value is slightly greater than Φ0MQ, which means that for these species, the fluorescence quenching is entirely due to the bimolecular reaction process with a second molecule of the hydroxy compound. For the rest of the alcohols, η0E is somewhat lower than Φ0MQ, but the main quenching comes also from the bimolecular deactivation process with rate constant kq (compare for example the results obtained on going from water to 1,3-propanediol: a 4-fold decrease of η0E but almost a 50-fold increase in the bimolecular quenching rate constant kq). Therefore, we conclude that the main process responsible for the strong fluorescence quenching observed is not the formation of the exciplex, but its bimolecular quenching by a second molecule of the hydroxy compound. In the following, we discuss the nature of this quenching process.
Fig. 4 Dependence of the quenching constant kq on the ionization energy of the quenchers listed in Table 2. |
As we pointed out above, the main contribution to the quenching comes from the reaction of the exciplex with a second quencher molecule. The linear decrease of log kq with increasing Ei of the quencher (Fig. 4) strongly supports the hypothesis that the process involves an electron transfer from the quencher to the exciplex. However, an explanation is required for the fact that 2,2,2-trifluoroethanol does not quench the MQ+ fluorescence, despite having an intermediate Ei value between those of water and methanol (Table 2). Even the solvent acetonitrile has an Ei value (12.20 eV) lower than water,42 but nevertheless, the fluorescence quantum yield of MQ+ is higher in acetonitrile than in water (Table 1). What distinguishes 2,2,2-trifluoroethanol and acetonitrile from water and other alcohols is their different hydrogen-bond ability and acid–base character. This led us to think that, in addition to the electron transfer, a proton transfer may also have a role in the quenching process.
The optimized excited-state geometry of the MQ+*–2H2O complex (Fig. 5) shows a water dimer coordinated through an O atom to two H atoms of the quinolinium molecule. The main differences between the ground- and the excited-state geometries are due to the intermolecular interactions between the water dimer and the MQ+ molecule. In the excitation, the water molecule that is closest to the MQ+ unit suffers a displacement towards the closest hydrogen atom in the methyl group, and the distance between the two water molecules increases (the O–O distance varies from 2.801 Å to 2.822 Å). The distance between the water O and the methyl H (2.304 Å) is slightly larger than the distance between the O and the H atom of the quinolinium ring at position 2 (2.229 Å), and both are indicative of the existence of C–H⋯O hydrogen bonds.53 The values of the equilibrium constant K for exciplex formation are similar for water and alcohols (Table 2), indicating that the strength of the interaction of H2O or ROH with MQ+* is about the same in all cases studied. This result is in accordance with the similar hydrogen-bond basicity of water and these alcohols,54 so we presume the complexes of MQ+* with H2O or ROH to have an analogous structure.
Fig. 5 B3LYP/aug-cc-pVTZ optimized geometry of the MQ+*−2H2O complex in the first-excited singlet state. |
The very low hydrogen-bond basicity of 2,2,2-trifluoroethanol38,39 would hinder the formation of the exciplex and the trimolecular reactive complex. Moreover, its proton basicity is also extremely low, as deduced from its very low gas basicity58 and its inability to accept the proton of relatively strong photoacids in liquid solution.59 These characteristics impede the PCET process for 2,2,2-trifluoroethanol. Acetonitrile has also a very low hydrogen-bond basicity and proton basicity in liquid solutions,38,39 and is unable to participate in a PCET reaction due to the lack of ionisable protons. These facts explain why acetonitrile and 2,2,2-trifluoroethanol do not quench the fluorescence of the N-methylquinolinium cation, in spite of being more easily oxidized than water.
With the quenching constants obtained for methanol and ethanol solutions in acetonitrile (Table 2), we can calculate an extrapolated value of the fluorescence quantum yield of MQ+ in pure alcohol solutions (24.70 mol dm−3 methanol and 17.13 mol dm−3 ethanol). The predicted values are Φ = 5.5 × 10−3 for methanol and Φ = 6.2 × 10−3 for ethanol. These results are in reasonable agreement with the experimental values (Table 1), taking into account the large extrapolation from dilute solutions and the change in the medium from acetonitrile to alcohol. Nevertheless, the fluorescence quantum yield of MQ+ predicted in the same way as that for pure water solution (Φ = 1.1 × 10−2) is much lower than the experimental value (Table 1). This led us to think that a change in the quenching mechanism must occur upon increasing the water concentration. To test this hypothesis, we measured the fluorescence intensities of MQ+ in the whole concentration range from pure acetonitrile to pure water. The fluorescence spectral shape hardly changes for these mixtures (cf.Fig. 1 and Fig. 5S(C), ESI†), but the fluorescence intensity decreases with increasing water content until a molar fraction of xH2O ≈ 0.5, increasing afterwards. Fig. 6 shows the influence of the water content on the fluorescence intensity quotient F0/F, with a maximum at equimolar amounts of water and acetonitrile.
Fig. 6 Dependence of the relative fluorescence intensity of MQ+ on the molar fraction of water in mixtures acetonitrile/water (exc = 31650 cm−1, em = 24400 cm−1). |
The behaviour observed for MQ+ in acetonitrile/water mixtures is probably due to the microheterogeneity of these solutions, which has been demonstrated by a range of different techniques.60,61 At low water concentrations, the water molecules form small aggregates of a few molecules, also associated with the acetonitrile molecules. As the water content is increased, microscopic domains of self-associated water exist, as well as acetonitrile domains, until the water content is so high that an extensive hydrogen-bonded network is established.60 Our results (Fig. 6) point to the fact that the ability of the water molecules to cooperate via PCET to donate an electron is higher for small aggregates than for an extensive hydrogen-bonded network of water. This can be related to the higher ability of a water molecule that does not participate in the ordinary water structure to donate an electron pair toward a hydrogen bond than one that does.61
A close parallelism exists between the behaviour of MQ+ here described and that of the stronger oxidant methyl viologen. The fluorescence of MV2+ is also quenched by water, methanol and ethanol, but not by acetonitrile or 2,2,2-trifluoroethanol.45 Femtosecond transient absorption experiments in bulk methanol have shown that the radical cation of methyl viologen is produced at an ultrafast rate (<180 fs) as a result of the oxidation of a solvent molecule by the photoexcited MV2+.45 To explain the rapid quenching of the first-excited singlet state of MV2+ in aqueous solution, Kohler and co-workers proposed a concerted proton–electron transfer reaction.62 By using ultrafast spectroscopic techniques, they presented convincing evidence that the strongly oxidizing excited state of MV2+ triggers the proton-coupled oxidation of a water molecule, which transfers a proton to the bulk solvent and an electron to MV2+* to form the hydroxyl radical. Although the MV˙+/OH˙ radical pair was not detected probably due to fast back electron transfer, a photoproduct was identified as the charge-transfer complex formed between ground-state MV2+ and a hydroxide ion. We propose here that a similar mechanism can take place for the weaker oxidant MQ+* in acetonitrile solution, with a water or alcohol molecule in a H-bonded pair acting as an efficient electron-donating entity towards MQ+* through the concerted proton transfer to the second hydroxy molecule of the pair. This result is in accord with previous quantum-mechanical calculations, which show a strong decrease in Ei of water upon dimer formation.63 The concerted action of a pair of water or alcohol molecules in proton transfer reactions has also been demonstrated.64–67
Electron transfer from water or alcohols to other excited chromophores has also been established. Femtosecond experiments in bulk solvents showed that excited oxazine 750 is reduced by different aliphatic alcohols.68 Moreover, several quantum-mechanical calculations on excited chromophores clustered with water or alcohols predict that an electron transfer from the hydroxy compound to the chromophore can take place. Examples comprise oxazine 750 clustered with two ethanol molecules68 or 7H-adenine clustered with several water molecules.69 In some cases, the initial electron transfer is followed by a proton transfer from water to the excited chromophore (for example, in the complexes pyridine-H2O,70 benzoquinone-H2O,71 1-methylcytosine-(H2O)2,72 9H-adenine-(H2O)573 and acridine-H2O).74 The capacity of the triplet excited state of acridine orange to split the O–H bond of phenol derivatives through PCET has also been experimentally demonstrated.75
According to our proposal, the quenching process of MQ+* by small amounts of water and alcohols in acetonitrile solution can be described as a concerted multiple-site electron-proton transfer (MS-EPT),22 also called bidirectional PCET,18 in which the electron and proton transfers occur from a single donor (water or alcohol) to different acceptors. This type of PCET plays a major role in a variety of biological processes and in a wide range of chemical systems involving hydroxy compounds as electron and proton donors.18,22,62,76–81
Recognition of the relatively high efficiency of water pairs as electron donors can contribute to understanding the puzzling photorelaxation and electron-transfer mechanisms of biomolecules, where water molecules have been shown to play a fundamental role.82,83 Our results are also relevant to the issues of relaxation mechanisms of excited molecules in hydroxylic solvents, solar water splitting and solar fuel production.
Our findings support that water and alcohol dimers show a much stronger reducing power than the isolated molecules in acetonitrile. The results obtained may be relevant to the study of water oxidation and electron transfer in biological systems.
Footnote |
† Electronic supplementary information (ESI) available: Kinetic model, the supplementary experimental results and data analyses with supporting figures, and the results of the ab initio calculations. See DOI: 10.1039/c7cp07057h |
This journal is © the Owner Societies 2018 |