Photoinduced Oxidation of Water in the PyridineâWater Complex

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Photoinduced Oxidation of Water in the Pyridine-Water Complex: Comparison of the Singlet and Triplet Photochemistries Xiaojun Liu, Andrzej L. Sobolewski, and Wolfgang Domcke J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp505188y • Publication Date (Web): 18 Aug 2014 Downloaded from http://pubs.acs.org on August 20, 2014

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The Journal of Physical Chemistry

Photoinduced Oxidation of Water in the Pyridine-Water Complex: Comparison of the Singlet and Triplet Photochemistries

Xiaojun Liua, b*, Andrzej L. Sobolewskic, and Wolfgang Domckea*

a

Department of Chemistry, Technische Universität München, D-85747 Garching, Germany

b

Key Laboratory of Luminescence and Optical Information, Institute of Optoelectronic Technology,

Beijing Jiaotong University, Beijing 100044, PR China c

Institute of Physics, Polish Academy of Sciences, PL-02668 Warsaw, Poland

1 ACS Paragon Plus Environment


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Abstract It has recently been shown that low-lying dark charge-separated singlet excited states of nπ* and ππ* character exist in the hydrogen-bonded pyridine-water complex in addition to the familiar nπ* and ππ* excited states of the pyridine chromophore. The former have been shown to promote the transfer of a proton from water to pyridine, resulting in the pyridinyl-hydroxyl radical pair. In the present work, the potential-energy surfaces of the triplet excited states of the pyridine-water complex have been explored with the same ab initio electronic-structure methods (ADC(2), CASPT2). Minimumenergy reaction paths for excited-state H-atom transfer, energy surfaces in the vicinity of the barrier for H-atom transfer, as well as multi-state surface crossings have been characterized. The photochemical reaction mechanisms on the singlet and triplet potential-energy surfaces are compared and their relevance for photoinduced water oxidation with the pyridine chromophore are discussed.

Keywords water

oxidation,

proton-coupled

electron

transfer,

photochemical reaction mechanisms, conical intersections.


pyridine-water

complex,

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1. Introduction Solar water splitting, that is, the simultaneous oxidation and reduction of water with sunlight to yield to molecular oxygen and molecular hydrogen, respectively, is the dream reaction for the future generation of clean and renewable chemical energy carriers 1-5

. Since water does not absorb in the spectral range covered by sunlight on the surface

of earth, a suitable chromophore is required which absorbs visible or infrared (IR) light and reacts with water in the electronically excited state. The dissociation energy of a water molecule (in the gas phase) is 5.1 eV 6, which implies that at least two visible photons are required for the homolytic dissociation of a single OH bond of water. Therefore, the energy of at least one photon has to be stored as chemical energy until a second photon becomes available (typically within microseconds or milliseconds). The simplest conceivable reaction scheme invokes a chromophore which self-assembles with water as hydrogen acceptor and which can abstract a hydrogen atom from the water molecule upon photoexcitation 7,8:

A-H2O + hv → A*-H2O

(1)

A*-H2O → AH• + OH•.

(2)

The energy of the first photon is thus stored in the radical pair AH• + OH•. The chromophore A is regenerated (and thus becomes a catalyzer) by the photodetachment of the surplus H-atom from the intermediate AH• radical



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AH• + hv → A + H•.

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(3)

A water molecule has thus been split into OH• and H• radicals by the absorption of two photons. Alternatively to the light-induced reaction (3), two AH• radicals may recombine in a dark reaction (with a suitable catalyzer) to yield molecular hydrogen:

2 AH•→ 2 A + H2.

(4)

The realization of the photoreaction of Eq. (2) requires a strong photobase, that is, a chromophore with an excited-state hydrogen-atom affinity which is higher than the proton affinity in the electronic ground state 9. One of the simplest aromatic photobases is pyridine. Surprisingly, very few experimental data seem to be available on the photochemistry of this fundamental heterocycle in aqueous solution or in water clusters in supersonic jets. It has been known that pyridine, in contrast to the diazines, is completely non-fluorescent in aqueous solution10. Moreover, pyridine-water clusters as well as diazine-water clusters could not be detected by resonant two-photon ionization spectroscopy in supersonic jets 11. Both findings indicate an unusually short excited-state lifetime of pyridine-water complexes. More recent time-resolved measurements of the femtosecond relaxation dynamics of the excited states of pyridine in aqueous solution revealed a lifetime of 2.2 ps for the S2(ππ*) state and 9.9 ps for the S1(nπ*) state 12. These lifetimes were explained by intramolecular decay mechanisms involving conical intersections between the S2, S1 and S0 states 12. In addition, photohydration products of pyridine were observed which are believed to be formed after ultrafast internal


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conversion of pyridine to the electronic ground state via conical intersection related to valence isomers and the reaction of the hot ground state with the solvent 13,14. It has been argued that the hydrogen bond between pyridine and water may be broken in the 1nπ* excited state 10,15 although in many other chromophores hydrogen bonds with the solvent seem to be strengthened by electronic excitation

16

. Calculations by Cai and Reimers

suggested that the nearly linear N…H-O hydrogen bond in the ground state of the pyridine-water cluster is replaced by a hydrogen bond of the water molecule with the aromatic π-system in the 1nπ* excited state

17

. The large change of the hydrogen-bond

geometry would result in extraordinarily extended Franck-Condon progressions, which could explain why the pyridine-water cluster could not be detected by resonant twophoton ionization spectroscopy 17. The possibility of a direct chemical reaction of photoexcited pyridine with solvent molecules was suggested by ab initio calculations of Sobolewski and Domcke for the pyridine-ammonia cluster

18

and by Reimers and Cai for the pyridine-water cluster

19

.

While so-called proton-coupled electron-transfer (PCET) reactions in excited states of acidic chromophores (such as phenol, naphtol or indole) with ammonia and water, leading to hypervalent NH4 or H3O radicals, were extensively investigated experimentally

20-27

as well as theoretically

18,28-30

for many years, the computational

results of Refs. 18, 19 provided the first evidence of a photoinduced PCET reaction of solvent molecules with a photobase. It seems likely that ultrafast excited-state PCET processes are the reason for the lack of fluorescence of pyridine in water

10

and the

absence of a two-photon ionization signal in pyridine-water clusters 11. We have recently explored the feasibility of the photoinduced hydrogen-abstraction



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reaction (2) in the pyridine-water complex as well as of the photoinduced hydrogendetachment reaction (3) with ab initio electronic-structure calculations 8. It was shown that the H-atom transfer reaction from water to the photoexcited pyridine (abbreviated as Py in the following) is mediated by optically dark, but photochemically reactive chargetransfer (CT) states, as previously found by Reimers and Cai 19. The reaction is driven by the transfer of an electron from the 2p-shell of the oxygen atom of water to the lowest π* orbital of Py. The ensuing transfer of the proton from water to Py neutralizes the electronic charge separation, which results in a pronounced energetic stabilization of the CT states which become neutral biradical states upon the transfer of the proton. The electronic ground state of the Py-H2O complex, on the other hand, is strongly destabilized in energy by the proton transfer, which results in conical intersections of the potentialenergy (PE) functions of the CT states with the PE function of the ground state 8. The excess energy becoming available by the H-atom transfer is sufficient to break the comparatively weak hydrogen bond of the biradical, resulting in the generation of free PyH• and OH• radicals. In Ref. 8, the basic mechanisms of the photochemistry in the singlet excited-state manifold of the Py-H2O complex were investigated. It was pointed out that the photochemistry in the triplet manifold of Py-H2O also may be relevant, since the singlet and triplet states of CT or biradical character are degenerate, which may give rise to unusually efficient intersystem crossing (ISC). The understanding of the reaction paths on the triplet PE surfaces of Py-H2O is thus of considerable relevance for the theoretical modelling of the photoinduced water-splitting process. In the present work, we have explored the triplet photochemistry of Py-H2O with


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the same computational methodology as applied in Ref. 8. The mechanism of the second step of the water-splitting reaction in the Py-H2O complex, the photodissociation of the pyridinyl radical (Eq. (3)), has been characterized in Ref. 8 and will not be discussed in the present communication.

2. Results 2.1. Vertical excitation energies While the vertical singlet and triplet excitation energies of pyridine have been calculated many times (see, e. g., Refs. 31-34), only few calculations were reported for the singlet excited states of the Py-H2O complex

8,17,19

and none for the triplet excited

states. The vertical excitation energies of the lowest nπ* and ππ* triplet states obtained in the present work are given in Table 1 together with the corresponding values for the singlet states. The excitation energies of pyridine are also included for comparison. The electronic excitation energies have been calculated with the single-reference ADC(2) method and with the multi-reference CASSCF and CASPT2 methods (see the Computational Methods section). The lowest triplet state of the Py-H2O complex is of ππ* character with a calculated excitation energy of 4.45 eV (4.04 eV) at the ADC(2) (CASPT2) level. It is the lowest excited electronic state. As expected, CASPT2 tends to underestimate the energy of the ππ* excited states, while ADC(2) tends to overestimate the ππ* excitation energy. The 3

ππ* excitation energy of Py-H2O is the same as the 3ππ* excitation energy of Py at the

CASPT2 level and is slightly blue-shifted (by 0.02 eV) at the ADC(2) level. It is 0.89 eV (0.78 eV) below the 1ππ* excitation energy of Py-H2O at the ADC(2) (CASPT2) level.



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The next triplet excited state of Py-H2O is of nπ* character with a vertical excitation energy of 4.80 eV (4.70 eV) at the ADC(2) (CASPT2) level. The 3nπ* excitation energy of the Py-H2O complex is blue-shifted from the 3nπ* excitation energy of Py by 0.36 eV (0.39 eV) at the ADC(2) (CASPT2) level. The 3nπ* state of Py-H2O is located 0.53 eV (0.42 eV) below the 1nπ* at the ADC(2) (CASPT2) level. The frontier molecular orbitals (calculated as CASSCF natural orbitals) involved in the 3nπ* and 3ππ* excited states of the pyridine chromophore in the Py-H2O complex are displayed in Fig. 1. The 3nπ* state corresponds to the excitation of an electron from the 22a' (n) orbital to the 5a'' (π*) orbital. Note that the 22a' (n) orbital has density on the pyridine moiety as well as on the O-atom of water, while the 5a'' (π*) orbital is completely localized on Py, see Fig. 1. The 3nπ* state of Py-H2O thus exhibits some water-to-pyridine CT character. In fact, its wave function is to some extent mixed with the wave function of a higher-lying water-to-pyridine CT state, as will be discussed below. The mixing of the locally excited (or spectroscopic) 3nπ* state with a 3CT state is more pronounced than the mixing of the corresponding singlet states (see Fig. 2 in Ref. 8). The frontier orbitals of the spectroscopic 3ππ* state, on the other hand, are completely localized on the Py ring. The wave function of this state is a mixture of configurations which correspond to the excitation of an electron from the two highest π orbitals (3a'', 4a'') to the two lowest π* orbitals (5a'', 6a''), see Fig.1.

2.2. Energy profiles of the minimum-energy reaction-paths for excited-state H-atom transfer The minimum-energy reaction paths for the transfer of an H-atom from water to


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pyridine in the 3nπ* and 3ππ* excited states of the Py-H2O complex were constructed as relaxed scans at the ADC(2) level, adopting the bond length ROH of the OH group involved in the hydrogen bonding of the H2O molecule with the N-atom of Py as the driving coordinate (see the Computational Methods section). The Py-H2O complex was constrained to be planar (Cs symmetry) in the reaction-path calculations. The resulting energy profiles are shown in Fig. 2a for the triplet excited states and in Fig. 2b for the singlet excited states. The left-hand part of the energy profiles, corresponding to ROH < 1.1 Å, represents the PE functions of the spectroscopic states of the Py-H2O complex. Here, the S0, nπ* and ππ* energies were calculated along the reaction path optimized in the S0 state. The right-hand part of the energy profiles, corresponding to ROH > 1.1 Å and separated by the dashed vertical line for clarity, represents the PE profiles of the photochemically reactive 1,3nπ* and 1,3ππ* states. Here, the S0, nπ* and ππ* energies were calculated along the H-atom transfer reaction path optimized in the 3ππ* state (Fig. 2a) or in the 1ππ* state (Fig. 2b). The discontinuity of the S0 energy at ROH = 1.1 Å reflects the different geometries of the two reaction paths. The PE profiles of the spectroscopic states of Py-H2O on the left-hand side of Figs. 2a, b exhibit the expected pattern. The excited-state energy functions are essentially parallel to that of the S0 state, which implies that there is no significant driving force for the transfer of an H-atom from water to pyridine in the spectroscopic excited states. At the ADC(2) level, the spectroscopic 1nπ* and 1ππ* states are nearly degenerate, while the 3

nπ* state is located several tenth of an electron volt above the 3ππ* state. The optimized

energy of the 3nπ* state is 0.45 eV below its vertical excitation energy. The PE profiles for ROH > 1.1 Å represent CT states in which an electron has been transferred from the O-



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atom of the water molecule to the lowest π* orbital of Py. The stabilization of the CT states upon the transfer of the proton from H2O to Py (by about 2.3 eV for the 3ππ* state and 1.3 eV for the 3nπ* state) is remarkable. At large OH distances (when the proton is completely transferred to Py), the electronic charge separation is neutralized, which results in a hydrogen-bonded neutral PyH•-OH• biradical. The energy of the closed-shell ground state, on the other hand, is strongly destabilized by the transfer of a proton from water to pyridine. As a result, the singlet and triplet coupled biradical states are located below the S0 state for ROH > 1.8 Å and thus represent locally stable chemical species (see Fig. 2). While the spectroscopic 3nπ* and 3ππ* states are lower in energy than the corresponding singlet states, the biradical states of singlet/triplet character are strictly degenerate as a consequence of the lack of an exchange interaction between the spatially separated electrons. The energy minimum of the singlet/triplet ππ* (nπ*) biradical is 2.9 eV (3.5 eV) above the global S0 minimum at the ADC(2) level (see Fig. 2). This implies that 54% (65%) of the energy of the ultraviolet (UV) photon are stored as chemical energy in the ππ* (nπ*) biradical. The frontier molecular orbitals of the nπ* and ππ* biradical states at ROH = 2.0 Å are displayed in Fig. 3. The biradicals contain one unpaired electron in either the in-plane (n) or the out-of-plane (π) p-orbital on the oxygen atom, while the second unpaired electron is in the lowest π* orbital of the pyridinyl radical. These nπ* and ππ* biradical states should not be confused with the spectroscopic nπ* and ππ* states of the Py-H2O complex discussed above. The crossing of the energies of the nπ* and ππ* CT states in the singlet and triplet manifolds at ROH ≈ 1.2 Å (see Figs. 2a, b) is a true energy crossing (that is, a conical


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intersection), since these energies were calculated at the same geometries. Likewise, the crossings of the energies of the 1nπ* and 1ππ* biradical states with the energy of the S0 state in the vicinity of ROH = 1.8 Å (see Fig. 2b) are conical intersections. We were able to locate a three-state conical intersection involving the 1nπ*, 1ππ* and S0 states at the ADC(2) level without symmetry constraint (see the Computational Methods section). This three-state conical intersection is marked by the green star in Fig. 2b. Since the biradical singlet and triplet states are degenerate in this region of the PE surface, the green star actually marks a five-state degeneracy: the 1nπ*, 1ππ*, 3nπ*, 3ππ* and S0 states are all degenerate at this geometry. This result has been confirmed by an independent determination of the intersection of the 3nπ*, 3ππ* and S0 energies (green star in Fig. 2a). The structural parameters of Py-H2O at the 1nπ*-1ππ*-S0 and 3nπ*-3ππ-S0 three-state intersections are identical. The geometry of the optimized five-state intersection has C1 symmetry and is shown in Fig. 4 in three different views. The OH radical is twisted out of the molecular plane of the pyridinyl radical. The OH (ON) distance is 1.82Å (2.74 Å). The minimum energy of the five-state degeneracy is 3.28 eV above the global minimum of the S0 state at the ADC(2) level. At the CASPT2 level, the mean energy of these states is 3.86 eV above the S0 minimum. The energy of the 1nπ*-1ππ*-S0 three-state conical intersection is lower than the energy of another 1nπ*-1ππ*-S0 intersection point which we located earlier at the CASSCF level at a somewhat larger ROH distance 8.

2.3. Potential-energy surfaces for H-atom transfer in the 1,3nπ* and 1,3ππ* states At ROH ≈ 1.1 Å (dashed vertical line in Figs. 2a, b), the minimum-energy paths for H-atom transfer in the spectroscopic nπ* and ππ* states are connected, via a low barrier,



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with the minimum-energy paths for H-atom transfer in the nπ* and ππ* states of CT/biradical character. In earlier work, we were able to determine (at the CASSCF level) the geometry and the energy of the saddle point which connects these reaction paths in the singlet manifold 8. The molecular structure of this saddle point is of C1 symmetry (the free OH group of the water molecule is nearly perpendicular to the molecular plane of Py) and it exhibits a short and strong hydrogen bond. The structure of the electronic wave functions at this saddle point is complex, since the n and π orbitals as well as the locallyexcited and CT configurations are strongly mixed. Clearly, the most relevant nuclear coordinates involved in the barrier-crossing and/or tunnelling dynamics of the H-atom are the proton-transfer coordinate ROH and the distance RON between the O-atom atom of water (the hydrogen donor) and the N-atom of Py (the hydrogen acceptor). We now have succeeded in the characterization of the twodimensional PE surface of the lowest excited singlet state and the lowest triplet state in the vicinity of the barrier for H-atom transfer at the ADC(2) level. We have calculated these two-dimensional energy surfaces as relaxed scans, that is, for fixed ROH and RON the energy was optimized with respect to the remaining internal coordinates with Cs symmetry constraint (see the Computational Methods section). The two-dimensional PE surface of the lowest triplet state is shown in Fig. 5a. The corresponding lowest excited singlet PE surface is displayed in Fig. 5b. Overall, the singlet PE surface is about 0.4 eV above the triplet PE surface in the barrier region. Let us consider the triplet surface (Fig. 5a) in more detail. The trough in the upper left corner of the figure represents the energy minimum of the spectroscopic 3nπ* state. The deep valley in the lower right corner leads to the (triplet-coupled) PyH•-OH• biradical.


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The red star in Fig. 5a indicates the approximate location of the saddle point for the Hatom-transfer reaction in the triplet manifold. The estimated energy of the saddle point, 4.62 eV, is about 0.27 eV above the estimated minimum of the spectroscopic 3nπ* state. The saddle point is definitely below the vertical excitation energy of the 1ππ* state of the Py-H2O complex (5.34 eV at the ADC(2) level). On the singlet PE surface (Fig. 5b), the trough in the upper left corner represents the energy minimum of the spectroscopic 1nπ* state, while the valley in the lower right corner leads to the singlet-coupled PyH•-OH• biradical. The estimated energy of the saddle point for the H-atom transfer reaction on the singlet PE surface is 5.03 eV, which is about 0.4 eV higher than the saddle point on the triplet energy surface. The barrier height from the spectroscopic minimum is estimated as 0.23 eV, which is essentially the same as found for the triplet energy surface. The estimated singlet saddle point, albeit 0.4 eV higher than the triplet saddle point, is still well below the vertical 1ππ* excitation energy of 5.34 eV. The calculation of CASPT2 energies at the ADC(2)-optimized saddle points yields energies very close to the ADC(2) energies, supporting the reliability of the latter method for this difficult excited-state electronic-structure problem. Our attempts to optimize the geometries of the saddle points on the triplet and singlet energy surfaces at the ADC(2) level without any symmetry constraint were unsuccessful. However, the geometry of the estimated saddle point on the singlet surface (red star in Fig. 5b) is, apart from the orientation of the free OH group, very similar to the geometry which was obtained by an unconstrained geometry optimization on the singlet surface with the CASSCF method 8. In particular, all structures exhibit a short and strong hydrogen bond between H2O and the N-atom of Py (RNH = 1.35 Å for the estimated



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triplet saddle point, RNH = 1.41 Å for the estimated singlet saddle point). As expected, the O-N distance RON also is strongly contracted at the saddle points (RON = 2.39 Å for the triplet saddle point, RON = 2.44 Å for the singlet saddle point) compared to the groundstate equilibrium geometry (RON = 2.88 Å).

3. Discussion and conclusions We have explored in this work the PE surfaces of the lowest triplet states of the Py-H2O complex with state-of-the-art ab initio electronic-structure methods. As previously found for the singlet excited states 8, there exist hitherto unknown excited triplet states of CT character which drive a proton-transfer reaction from water to pyridine, resulting in the triplet PyH•-OH• biradical. The photochemistry of the Py-H2O complex is an example of the electron-driven proton-transfer reaction which has been shown to be a generic phenomenon in intermolecularly hydrogen-bonded organic πsystems as well as in complexes of heteroaromatic molecules with amphoteric solvent molecules, such as water or ammonia 18. The singlet and triplet reaction-path energy profiles (Fig. 2) and the twodimensional PE surfaces in the barrier region (Fig. 5) provide strong evidence that photoexcited pyridine can abstract a hydrogen atom from the water molecule which is hydrogen-bonded as H-atom donor to the N-atom of pyridine. As Fig. 2 shows, the energy profiles along the minimum-energy reaction paths for H-atom transfer in the singlet and triplet excited states are qualitatively similar. At the S0 equilibrium geometry, the 3ππ* state is about 0.9 eV lower in energy than the optically allowed 1ππ* state. The singlet and triplet biradicals, on the other hand, are degenerate. The reaction enthalpy of


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the water-oxidation reaction in the triplet states is thus lower by 0.9 eV than the reaction enthalpy in the singlet states. The topography of the PE surfaces in the region of the barrier for H-atom transfer (Fig. 5) also is similar for singlet and triplet states. The height of the barrier relative to the minimum of the spectroscopic ππ* state is about 0.3 eV in both cases. In the singlet and triplet manifolds, the spectroscopic nπ* and ππ* states are strongly vibronically coupled (as is well established for isolated pyridine

35-37

). The

spectroscopic singlet nπ* and ππ* states are, moreover, quasi-degenerate in the Py-H2O complex (see Table 1). Therefore, immediate and extensive 1nπ*-1ππ* mixing is expected upon vertical electronic excitation of the 1ππ* state. Depending on the amount of vibrational energy redistribution in the spectroscopic 1nπ* and 1ππ* states of the complex, the H-atom transfer barrier can be overcome either by above-barrier dynamics or by Hatom tunneling. For a barrier of about 0.3 eV, the tunneling time can be estimated as 10 – 100 ps. ISC from the spectroscopic 1nπ*and 1ππ* states of pyridine occurs on a time scale of tens of picoseconds 38,39. Assuming that the ISC rate in the Py-H2O complex is similar, H-atom transfer in the singlet manifold is expected to compete with ISC in the pyridine chromophore. Beyond the H-atom transfer barrier at ROH ≈ 1.1 Å, the singlet and triplet PE surfaces quickly become degenerate, see Fig. 2. This singlet-triplet degeneracy over an extended range of the reaction coordinate may give rise to unusually efficient mixing of singlet and triplet states by spin-orbit (SO) coupling. The singlet nπ* and ππ* biradicals may thus efficiently be converted to triplet biradicals during or after the Hatom transfer reaction. The singlet-triplet interconversion of the PyH•-OH• biradicals may



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be of relevance for the water-oxidation reaction for two reasons. First, wave packets on the triplet-coupled electronic surfaces will cross the S0 surface during the H-atom-transfer dynamics essentially without perturbation, since the SO coupling is weak, while wave packets on the singlet-coupled electronic surfaces hit the

1

nπ*-1ππ*-S0 conical

intersection, where the wave packets may bifurcate into a reactive component (yielding free radicals) and a non-reactive component (restoring the Py-H2O complex in the S0 state). Secondly, the PE surfaces of the triplet nπ* and ππ* biradical states are purely repulsive as a function of ROH (see Fig. 2a), while the lowest adiabatic singlet PE surface, apart from a low barrier near the S1-S0 conical intersection (see Fig. 2b), is attractive. Geminate recombination of the free radicals may therefore take place on the PE surface of the singlet biradical states, but is excluded on the PE surface of the triplet states. Singlet-to-triplet switching near the five-state degeneracy discussed in Section 2.2 may thus greatly enhance the efficiency of the water-splitting reaction. According to this qualitative picture, the rate-determining step for the water oxidation reaction should be H-atom tunnelling through the barrier which separates the PE surfaces of the spectroscopic singlet and triplet states from the PE surfaces of the reactive CT states (Fig. 5). We note that the height of this barrier may be fine-tuned by substitutions of the aromatic ring. Electron-donating substituents (e. g. CH3) will increase the energy of the CT states and thus the barrier height, while electron-withdrawing groups (e. g. F, CN) will stabilize the CT states and thus lower the barrier. There exist competing photochemical reaction channels which were not considered in the present work and in Ref. 8. In isolated pyridine, the so-called channelthree process, which has been extensively explored for the S1 state of benzene


40-42

and

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shown to involve an out-of-plane deformation of the aromatic ring 43,44, leads to efficient internal conversion with an activation energy in the S2 state of a few thousand cm-1 12,36,37 . While the barrier heights for the channel-three decay and H-atom transfer may be comparable in the Py-H2O complex, the H-atom transfer reaction is expected to be faster than the puckering of the aromatic ring due to the lower effective mass of the H-atomtransfer coordinate compared to the ring-puckering motion. A follow-up reaction to internal conversion in pyridine in aqueous solution is photohydration

13,14

. It is believed

to take place on the S0 PE surface after ultrafast internal conversion via ring-puckeringtype conical intersections. While no experimental data seem to be available on the photo-oxidation of water with pyridine, the computational results presented here indicate a rich variety of competing and partly novel dynamical phenomena in the photochemistry of the Py-H2O complex, such as quasi-degenerate ππ*/nπ* vibronic coupling, ultrafast internal conversion via ring-puckering, multi-dimensional H-atom tunnelling, ISC along extended seams of singlet-triplet degeneracy, as well as dynamics at multi-state conical intersections. The interplay and competition of these dynamical processes may lead to photo-induced water oxidation or non-reactive quenching of the energy of the UV photon. The characterization of the SO-coupling surface and the investigation of the photoinduced reaction dynamics with time-dependent nonadiabatic quantum wave-packet calculations or with quasi-classical surface-hopping trajectory calculations represent challenges for future theoretical research. While pyridine is not a suitable catalyser for solar water splitting, since it absorbs far in the UV, the Py-H2O complex represents a relatively simple molecular system for



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Page 18 of 35

which the elementary processes of photoinduced water oxidation can be investigated with comparatively accurate first-principles computational methods. On the other hand, the electronic excitation thresholds of aromatic or heteroaromatic molecules can straightforwardly be lowered by the extension of the conjugated bond system. Acridine (dibenzopyridine) and acridine orange (tetramethylacridinediamine), for example, absorb at 360 nm and 500 nm, respectively. Computational investigations of the photochemistry of hydrogen-bonded complexes of acridine and acridine-derived dyes with water are in progress.

4. Computational methods The ground-state equilibrium geometry of the Py-H2O complex was determined with the second-order Møller-Plesset (MP2) method. Excitation energies, excited-state reaction paths, excited-state potential-energy surfaces, geometries of conical intersections and saddle points were calculated with the second-order algebraic-diagrammaticconstruction (ADC(2)) method, which is a single-reference Green’s function method

45

.

The vertical excitation energies, the energies of saddle points and conical intersections were calculated, in addition, with the complete-active-space self-consistent-field (CASSCF) method

46

and the CASPT2 (second-order perturbation theory with respect to

the CASSCF reference) method 47. The active space for the Py-H2O complex was chosen as 10 electrons distributed in 9 orbitals: the three highest π orbitals and three lowest π* orbitals of Py, the n orbital of the N-atom of Py, as well as the lowest σ* orbital of H2O and one p orbital of the O-atom of H2O. This active space is the same as in our previous calculation

8

and was carefully chosen to balance the locally excited states and the CT


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states. For the calculation of the singlet states, the energies of the S0, 1ππ* and 1nπ* states were averaged with equal weights. For the calculation of triplet states, the energies of the S0, 3ππ* and 3nπ* states were averaged with equal weights. The CASPT2 calculations were carried out as single-state calculations. In all CASPT2 calculations, a level shift of 0.3 au was employed. The reaction paths for the electron-driven proton-transfer process from water to pyridine in the singlet and triplet excited states were constructed as so-called relaxed scans. For a fixed value of the driving coordinate ROH (the bond length of the OH group involved on the hydrogen bonding with Py), all other internal coordinates of the Py-H2O complex were relaxed in the respective electronic state with Cs symmetry constraint (the water molecule is constrained to be in the molecular plane of Py). The PE surfaces in the vicinity of the saddle points in the singlet and triplet excited states were constructed as two-dimensional relaxed scans, that is, for fixed values of the driving coordinates RON (the distance between the O-atom of H2O and the N-atom of pyridine) and ROH, all other internal coordinates were relaxed in the respective excited state with Cs symmetry constraint. The saddle points are not fully optimized, but are estimated from the twodimensional relaxed PE surfaces. The minimum geometries of conical intersections were optimized (without Cs symmetry constraint) using the CIOpt program developed by Martinez and coworkers 48, which has been linked to the MP2 and ADC(2) methods. Dunning’s correlation-consistent split-valence double-ϛ basis set with polarization functions on all atoms (cc-pVDZ)

49

was employed in all calculations. The MP2 and

ADC(2) calculations were carried out with the TURBOMOLE program package making use of the resolution-of-the-identity (RI) approximation


51

50

,

for the evaluation of


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the electron-repulsion integrals. The CASSCF and CASPT2 calculations were performed with the MOLPRO program package 52.

AUTHOR INFORMATION Corresponding Author * Email: [email protected] (W. Domcke); [email protected] (X. Liu) Notes The authors declare no competing financial interest.

Acknowledgements This work was supported by the National Natural Science Foundation of China (X. L., Grant No.61177017), by the National Science Center of Poland (A. L. S., Grant No 2012/04/A/ST2/00100) and by a grant of the Deutsche Forschungsgemeinschaft (W. D.). A. L. S. acknowledges support by the DFG cluster of excellence “Munich Centre for Advanced Photonics”.

References 1. New Science for a Secure and Sustainable Energy Future. Report of a Subcommittee to the Basic Energy Sciences Advisory Committee, US Department of Energy, December 2008. 2. Pagliaro, M.; Konstandopoulos, A. G.; Ciriminna, R.; Palmisano, G. Solar Hydrogen: Fuel of the Near Future. Energy Environ. Sci. 2010, 3, 279-287. 20 ACS Paragon Plus Environment

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3. Moore, G. F.; Brudvig, G. W. Energy Conversion in Photosynthesis: A Paradigm for Solar Fuel Production. Annu. Rev. Condens. Matter Phys. 2011, 2, 303-327. 4. Gust, D.; Moore, T. A.; Moore, A. L. Realizing Artificial Photosynthesis. Faraday Discuss. 2012, 155, 9-26. 5. Kärkäs, M. D.; Johnston, E. V.; Verho, O.; Åkermark, B. Artificial Photosynthesis: From Nanosecond Electron Transfer to Catalytic Water Oxidation. Acc. Chem. Res. 2014, 47, 100-111. 6. Maksyutenko, P.; Rizzo, T. R.; Boyarkin, O. V. A Direct Measurement of the Dissociation Energy of Water. J. Chem. Phys. 2006, 125, 181101. 7. Sobolewski, A. L.; Domcke, W. Photoinduced Water Splitting with Oxotitanium Porphyrin: A Computational Study. Phys. Chem. Chem. Phys. 2012, 14, 12807-12817. 8. Liu, X.; Sobolewski, A. L.; Borelli, R.; Domcke, W. Computational Investigation of the Photoinduced Homolytic Dissociation of Water in the Pyridine-Water Complex. Phys. Chem. Chem. Phys. 2013, 15, 5957-5966. 9. Rehm, D.; Weller, A. Kinetics of Fluorescence Quenching by Electron and H-Atom Transfer. Israel J. Chem. 1970, 8, 259-271. 10. Baba, H.; Goodman, L.; Valenti, P. C. Solvent Effects on the Fluorescence Spectra of Diazines. Dipole Moments in the (n, π*) Excited States. J. Am. Chem. Soc. 1966, 88, 5410-5415. 11. Wanna, J.; Menapace, J. A.; Bernstein, E. R. Hydrogen Bonded and Non-Hydrogen Bonded van der Waals Clusters: Comparison Between Clusters of Pyrazine, Pyrimidine and Benzene with Various Solvents. J. Chem. Phys. 1986, 85, 1795- 1805. 12. Chachisvilis, M.; Zewail, A. H. Femtosecond Dynamics of Pyridine in the Condensed



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Phase: Valence Isomerization by Conical Intersections. J. Phys. Chem. A 1999, 103, 7408-7418. 13. Joussot-Dubien, J.; Houdard, J. Reversible Photolysis of Pyridine in Aqueous Solution. Tetrahedron Lett.1967, 8, 4389-4391. 14. Wilzbach, K. E.; Rausch, D. J. Photochemistry of Nitrogen Heterocycles. Dewar Pyridine and its Intermediacy in Photoreduction and Photohydration of Pyridine. J. Am. Chem. Soc. 1970, 92, 2178-2179. 15. Del Bene, J. E. Molecular Orbital Theory of the Hydrogen Bond. n → π* Transitions in the Monosubstituted Pyridines and their Complexes with H2O. Chem. Phys. 1980, 50, 1-10. 16. Zhao, G.-J.; Han, K.-L. Hydrogen Bonding in the Electronic Excited State. Acc. Chem. Res. 2012, 45, 404-413. 17. Cai, Z.-L.; Reimers, J. R. The First Singlet (nπ*) and (ππ*) Excited States of the Hydrogen-Bonded Complex Between Water and Pyridine. J. Phys. Chem. A 2002, 106, 8769-8778. 18. Sobolewski, A. L.; Domcke, W. Computational Studies of the Photophysics of Hydrogen-Bonded Molecular Systems. J. Phys. Chem. A 2007, 111, 11725-11745. 19. Reimers, J. R.; Cai, Z.-L. Hydrogen Bonding and Reactivity of Water to Azines in Their S1(nπ*) Electronic Excited States in the Gas Phase and Solution. Phys. Chem. Chem. Phys. 2012, 14, 8791-8802. 20. Grabner, G.; Köhler, G.; Zechner, J.; Getoff, N. Temperature Dependence of Photoprocesses in Aqueous Phenol. J. Phys. Chem. 1980, 84, 3000-3004. 21. Mialocq, J. C.; Amouyal, E.; Bernas, A.; Grand, D. Picosecond Laser Photolysis of


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Aqueous Indole and Tryptophan. J. Phys. Chem. 1982, 86, 3137-3177. 22. Zwier, T. S. The Spectroscopy of Solvation in Hydrogen-Bonded Aromatic Clusters. Annu. Rev. Phys. Chem. 1996, 47, 205-241. 23. Kleinermanns, K.; Gerhards, M.; Schmitt, M. Electronic Spectroscopy of Aromatic Molecules in Jet-Cooled Hydrogen Bonded Clusters – Structure and Functionality. Ber. Bunsenges. Phys. Chem. 1997, 101, 1785-1798. 24. Peon, J.; Hess, G. C.; Percourt, J.-M. L.; Yuzawa, T.; Kohler, B. Ultrafast Photoionization Dynamics of Indole in Water. J. Phys. Chem. A 1999, 103, 2460-2466. 25. Pino, G.; Gregoire, G.; Dedonder-Lardeux, C.; Jouvet, C.; Martrenchard, S.; Solgadi, D. A Forgotten Channel in the Excited State Dynamics of Phenol-(Ammonia)n Clusters: Hydrogen Transfer. Phys. Chem. Chem. Phys. 2000, 2, 893-900. 26. Cohen, B.; Leiderman, P.; Huppert, D. Unusual Temperature Dependence of Proton Transfer. 2. Excited-State Proton Transfer from Photoacids to Water. J. Phys. Chem. A 2002, 106, 11115-11122. 27. Lippert, H.; Stert, V.; Schulz, C. P.; Hertel, I. V.; Radloff, W. Photoinduced Hydrogen Transfer Reaction Dynamics in Indole-Ammonia Clusters at Different Excitation Energies. Phys. Chem. Chem. Phys. 2004, 6, 2718-2724. 28. Sobolewski, A. L.; Domcke, W.; Dedonder-Lardeux, C.; Jouvet, C. Excited-State Hydrogen Detachment and Hydrogen Transfer Driven by Repulsive 1πσ* States: A New Paradigm for Nonradiative Decay in Aromatic Biomolecules. Phys. Chem. Chem. Phys. 2002, 4, 1093-1100. 29. Wohlgemuth, M.; Bonacic-Koutecky, V.; Mitric, R. Time-Dependent Density Functional Theory Excited State Nonadiabatic Dynamics Combined with Quantum



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Mechanical/Molecular Mechanical Approach: Photodynamics of Indole in Water. J. Chem. Phys. 2011, 135, 054105. 30. Nagashima, K.; Takatsuka, K. Early-Stage Dynamics in Coupled Proton-Electron Transfer From the π-π* State of Phenol to Solvent Ammonia Clusters: A Nonadiabatic Electron Dynamics Study. J. Phys. Chem. A 2012, 116, 11167-11179. 31. Kitao, O.; Nakatsui, H. Cluster Expansion of the Wave Function. Valence Excitations and Ionizations of Pyridine. J. Chem. Phys. 1988, 88, 4913-4925. 32. Walker, I. C.; Palmer, M. H.; Hopkirk, A. The Electronic States of the Azines. II. Pyridine. Chem. Phys. 1989, 141, 365-378. 33. Lorentzon, J.; Fülscher, M. P.; Roos, B. O. A Theoretical Study of the Electronic Spectra of Pyridine and Phosphabenzene. Theor. Chim. Acta 1995, 92, 67-81. 34. Cai, Z.-L.; Reimers, J. R. The Low-Lying Excited States of Pyridine. J. Phys. Chem. A 2000, 104, 8389-8408. 35. Innes, K. K.; Ross, I. G.; Moomaw, W. R. Electronic States of Azabenzenes and Aznaphtalenes: a Revised and Extended Critical Review. J. Mol. Spectrosc. 1988, 132, 492-544. 36. Villa, E.; Amirav, A.; Lim, E. C. Single-Vibronic-Level and Excitation-Energy Dependence of Radiative and Nonradiative Transitions in Jet-Cooled Pyridine. J. Phys. Chem. 1988, 92, 5393-5397. 37. Buma, W. J.; Donckers, M. C. J. M.; Groenen, E. J. J. Ab Initio Calculations on Vibronic Coupling in the Lower Triplet States of Pyridine. J. Am. Chem. Soc. 1992, 114, 9544-9551 38. Yamazaki, I.; Murao, T.; Yoshihara, K.; Fujita, M.; Sushida, K.; Baba, H. Picosecond


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Fluorescence Decays from Vibrational Levels in the S1(nπ*) State of Pyridine Vapor. Chem. Phys. Lett. 1982, 92, 421-424. 39. Sushida, K.; Fujita, M.; Yamazaki, I.; Baba, H. Electronic Relaxation Processes from Single Vibronic Levels in the S1(nπ*) State of Pyridine Vapor. Bull. Chem. Soc. Jpn. 1983, 56, 2228-2233. 40. Callomon, J. H.; Parkin, J. E.; Lopez-Delgado, R. Non-Radiative Relaxation of the Excited 1B2u State of Benzene. Chem. Phys. Lett. 1972, 13, 125-131. 41. Suzuki, T.; Ito, M. Dispersed Fluorescence Spectra of Jet-Cooled Benzene from Levels near the Channel Three Threshold. J. Chem. Phys. 1989, 91, 4564-4570. 42. Riedle, E.; Weber, T.; Schubert, U.; Neusser, H. J.; Schlag, E. W. Back to the Roots of „Channel Three“: Rotationally Resolved Spectra of the 601103 Band of C6H6. J. Chem. Phys. 1990, 93, 967-978. 43. Palmer, I. J.; Ragazos, I. N.; Bernardi, F.; Olivucci, M.; Robb, M. A. An MCSCF Study of the S1 and S2 Photochemical Reactions of Benzene. J. Am. Chem. Soc. 1993, 115, 673-682. 44. Sobolewski, A. L.; Woywod, C.; Domcke, W. Ab Initio Investigation of PotentialEnergy Surfaces Involved in the Photophysics of Benzene and Pyrazine. J. Chem. Phys.1993, 98, 5627-5641. 45. Schirmer, J. Beyond the Random-Phase Approximation: a New Approximation Scheme for the Polarization Propagator. Phys. Rev. A 1982, 26, 2395-2416. 46. Shepard, R. The Multiconfiguration Self-Consistent Field Method. Adv. Chem. Phys. 1987, 69, 63-200. 47. Andersson, K.; Malmqvist, P.-A.; Roos, B. O. Second-Order Perturbation Theory



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with a Complete Active Space Self-Consistent Field Reference Function. J. Chem. Phys. 1992, 96, 1218-1226. 48. Levine, B. G.; Coe, J. D.; Martinez, T. J. Optimizing Conical Intersections Without Derivative Coupling Vectors: Application to Multistate Multireference Second-Order Perturbation Theory (MS-CASPT2). J. Phys. Chem. B 2008, 112, 405-413. 49. Dunning, Jr., T. H. Gaussian Basis Sets for Use in Correlated Molecular Calculations. I. The Atoms Boron through Neon and Hydrogen. J. Chem. Phys. 1989, 90, 1007-1023. 50. Ahlrichs, R.; Bär, M.; Häser, M.; Horn, H.; Kölmel, C. Electronic Structure Calculations on Workstation Computers: The Program System TURBOMOLE. Chem. Phys. Lett. 1989, 162, 165-169. 51. Hättig, C.; Weigend, F. CC2 Excitation Energy Calculations on Large Molecules Using the Resolution of the Identity Approximation. J. Chem. Phys. 2000, 113, 51545161. 52. Werner, H.-J.; Knowles, P. et al. MOLPRO, Version 2006.1, a Package of Ab Initio Programs.


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Figure Captions Figure 1. Frontier molecular orbitals involved in the spectroscopic 3nπ* and 3ππ* states of the PyH2O complex. The orbital 22a' is the n-orbital, 3a'', 4a'' are the two highest occupied π orbitals and 5a'', 6a'' are the two lowest π* orbitals.

Figure 2. Energy profiles of the S0, nπ* and ππ* electronic states of the Py-H2O complex along minimum-energy paths for hydrogen transfer from water to pyridine. The triplet energies are shown in (a), the singlet energies in (b). Full symbols indicate that the reaction path has been optimized in this electronic state. Open symbols indicate the energies of electronic states which have been calculated for geometries optimized in a different electronic state. The dashed vertical line separates the reaction path optimized in the S0 state (left) from the reaction path optimized in the ππ* CT state (right). The green stars in (a) and (b) represent the S0/T1/T2 and S0/S1/S2 three-state intersections, respectively.

Figure 3. Frontier molecular orbitals involved in the 3nπ* and 3ππ* biradical states of the Py-H2O complex at ROH = 2.0 Å. The electron-donating n and π orbitals are completely localized on the water molecule, while the electron-accepting π* orbital is completely localized on pyridine.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4. Front, top and side views of the geometry of the S0/S1/S2/T1/T2 five-state degeneracy in the PyH•-OH• biradical, calculated at the ADC(2) level without symmetry constraint.

Figure 5. PE surface of the lowest excited state in the vicinity of the barrier for hydrogen transfer from water to pyridine in the Py-H2O complex, calculated with the ADC(2) method. The triplet surface is shown in (a), the singlet surface in (b). The nuclear coordinates are the OH bond length ROH of water and the distance RON of the oxygen atom of water from the nitrogen atom of pyridine. The PEs are optimized with respect to all other internal nuclear coordinates. The red star indicates the saddle point.


Page 28 of 35

Page 29 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Vertical excitation energies (in eV) and oscillator strengths (in parentheses) of the lowest triplet and singlet states of the pyridine-water complex and of pyridine.

Pyridine-H2O 3

3

ππ* (A')

3

nπ* (A'')

3

ππ* (A')

ADC(2)

4.80

4.45

4.54

4.43

CASPT2

4.70

4.04

4.31

4.04

CASSCF

5.41

3.93

4.67

3.85

1

a

nπ* (A'')

Pyridine

nπ* (A'')a

1

ππ* (A')a

1

nπ* (A'')a

1

ππ* (A')a

ADC(2)

5.33 (0.004)

5.34 (0.030)

5. 13 (0.004)

5.35 (0.025)

CASPT2

5.12

4.82

4.95

4.74

CASSCF

5.70(0.006)

5.00(0.019)

5.23(0.008)

4.95(0.011)

from ref 8



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig.1

22a ' (n)

5a'' (π*)

3a'' (π)

4a'' (π)

6a'' (π*)


Page 30 of 35

Page 31 of 35

Fig.2

6

6

(a)

3

nπ*

S0

Energy [eV]

5

5

3

ππ*

4

1

6

nπ∗

(b)

1

S0

ππ∗

1

4

3

nπ∗

3

5

nπ*

3

4

3 1

ππ*

3

ππ∗

2

2

1

2

1

S0

1

S0

0

0 0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

0

0.8

ÅÅÅÅ

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.0

1.2

1.4

1.6

ROH [ ]


1.8

2.0

2.2

2.4

2.6


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 3

n

π

π*


Page 32 of 35

Page 33 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 4



Fig.5

2.9

2.9

4.35

4.80 5.15

(a)

2.8

5.65

4.85 5.25

5.55

(b)

4.45 2.7

5.45 5.35

4.95

ÅÅÅÅ

RON [ ]

5.05

2.7

4.95

2.6

5.15

4.85 5.00

4.65 2.5

5.05 2.5

4.55 4.60

2.4

5.03

4.62 5.05

4.75

4.65 4.60

2.3

4.35 4.45

4.25

0.95

1.00

1.05

1.10

1.15

1.20

4.85 4.75

5.25 5.15 5.35

0.90

0.95

4.65

5.00

4.55

4.85 2.2 0.90

2.8

5.25

4.75

2.6

ÅÅÅÅ

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 35

2.4

4.55 4.45

2.3

4.95 1.00

1.05

ROH [ ]


1.10

1.15

1.20

2.2 1.25

Page 35 of 35

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