Photoinduced WaterâHeptazine Electron-Driven Proton Transfer

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Photoinduced Water-Heptazine Electron-Driven ProtonTransfer: Perspective for Water-Splitting with g-CN 3

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Naeem Ullah, Shunwei Chen, Yan-Ling Zhao, and RuiQin Zhang J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b01248 • Publication Date (Web): 16 Jul 2019 Downloaded from pubs.acs.org on July 18, 2019

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The Journal of Physical Chemistry Letters

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Photoinduced Water-Heptazine Electron-Driven Proton-Transfer: Perspective

2

for Water-Splitting with g-C3N4

3

Naeem Ullaha, Shunwei Chena, Yanling Zhaoa,b, Ruiqin Zhanga,c

4 5

aDepartment bShenzhen

Research Institute, City University of Hong Kong, Shenzhen 518057, China

6 7

of Physics, City University of Hong Kong, Hong Kong, China

cBeijing

Computational Science Research Center, Beijing 100193, China

8

Heptazine assembled polymeric carbon nitride (CN) materials have fascinated the

9

research community as a photocatalyst for hydrogen evolution while less attention

10

has been devoted to the mechanistic features of the host materials. Using excited-

11

state non-adiabatic dynamics simulations, the molecular-level picture of the

12

decomposition of heptazine hydrogen bonded to water molecule(s) (heptazine-water

13

complex) into heptazinyl and hydroxyl biradical products is revealed. Dynamics

14

simulations show that hydrogen detachment from the water molecule to the

15

heptazine occurs within tens of femtoseconds and suggest that excited-state

16

deactivation via N−H∙∙∙∙∙∙O−H electron-driven proton-transfer (EDPT) is the dominant

17

and most relevant excited-state deactivation process in heptazine-water complexes

18

leading to conical intersection. The observation of photorelaxation-induced water

19

splitting by heptazine is proof of the water-splitting reaction principle, which presents

20

further challenges for computational and experimental investigations of the

21

deactivation of heptazinyl and OH biradical products for efficient hydrogen evolution.

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*Corresponding Author: [email protected]

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Keywords: Heptazine, EDPT reaction, water-splitting, g-C3N4, TDDFT

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Table of Content

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ACS Paragon Plus Environment

The Journal of Physical Chemistry Letters 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

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The mechanism of the photocatalytic water-splitting reaction is currently of great

2

importance to accompany future technologies for clean and sustainable energy

3

production. Graphitic carbon nitride (CN) represents an alternative to the usual

4

photocatalysts based on semiconductor materials and metal-organic complexes.1

5

Heptazine-based polymeric CN possesses enhanced photostability; it is low cost and

6

made of earth-abundant elements, and its photocatalytic activity in the photoinduced

7

conversion of water into hydrogen and oxygen molecules has been experimentally

8

established.1-6 Even with significant experimental and theoretical investigations, the

9

atomic-level dynamical process involving the water splitting is still not well-

10

couched.7,8 Recently, a theoretical interpolation9 of the potential energy surface for a

11

model heptazine-water system and an experimental10 detection of the OH radical in

12

the heptazine-water system revealed the electron-driven proton transfer (EDPT) low-

13

barrier reaction for water splitting. To reveal the dynamics of the light-driven reaction

14

in the heptazine-water system, we ran an excited-state dynamics investigation

15

leading to the molecular-level delineation of the deactivation of the heptazine-water

16

system and eventually to water splitting.

17

During our investigations, we noticed that a single unit of hydrogen-terminated

18

heptazine can detach a hydrogen atom from a water molecule, yielding heptazinyl

19

and OH radicals and storing the hydrogen at the outer nitrogen site. This behaviour

20

is crucial for photoinduced water splitting by g-C3N4 materials as, compared with the

21

multielectron charge separation process at the mesoscopic level, a small heptazine

22

system may better describe the photochemical reaction with water. Several studies

23

have reported that small heptazine-based oligomers exhibit higher hydrogen

24

evolution rates than the standard mesoscopic scale CN materials.6,11-13 Our findings

25

report a major excited-state deactivation pathway for heptazine within 300 fs,

26

supplementing the previously proposed heptazine-water complex excited-state

27

pathway, water oxidation and proton-transfer reaction9 and the experimentally

28

reported proton-coupled electron transfer reaction with OH radical detection.10

29

The primary inspiration for investigating the non-adiabatic dynamics of UV-Vis light

30

excited heptazine-water molecular clusters is that understanding how a hydrogen-

31

bonded heptazine (building block of CN materials) reacts with water upon

32

photoexcitation is a key step for the molecular-level exploration of water splitting with

33

CN materials. Usually, the photoinduced water-splitting reaction is described in terms


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of the initial exciton generation by light absorption, followed by the exciton

2

dissociation, the transport of the electrons and holes to the solid/liquid interfaces

3

neutralized proton and water oxidation by suitable catalysts.6,10,14-21 On the other

4

hand, theoretical investigations of the heptazine-water complex and bulk water have

5

been conducted to provide a molecular-level understanding of the photochemical

6

water-splitting reaction.9,22 These studies have proposed that heptazine may abstract

7

a hydrogen atom from a water molecule by an electron/proton-transfer reaction

8

producing the heptazinyl and hydroxyl radicals. Recently, using the non-adiabatic

9

excited-state surface hopping dynamics, Pang et al.23 revealed the hydrogen

10

abstraction form water molecule to pyridine via electron-driven proton-transfer

11

(EDPT) reaction. Inspired by the fundamental framework for molecular-level water

12

splitting, we investigated the excited-state dynamics of the heptazine-water cluster to

13

reveal the predicted reaction. Although this dynamical molecular-level framework is

14

in itself important, the findings of the present dynamics simulations are more general,

15

with potential implications for the photoinduced water-splitting mechanism in diverse

16

fields of g-C3N4 based photocatalysis research.

17

Determining the relaxation mechanism of solvated CN materials using dynamic

18

simulations has been challenging due to the size of the system and the lack of

19

adequate methods for shaping the excited state potential energy surfaces. Recently,

20

the development of excited-state non-adiabatic dynamics with the time-dependent

21

density functional theory (TDDFT),24 as implemented in the Gaussian09 package25

22

interfaced with the NewtonX code,26 has made it possible to simulate the reaction of

23

the heptazine-water system to ultraviolet (UV) excitation (see supporting information

24

(SI) for computational details). Herein, our working principle is that the main impact

25

of the solvation on the excited-state evolution of the hydrogen-bonded heptazine

26

molecule in a water cluster environment. Therefore, we ran excited-state non-

27

adiabatic

28

(heptazine+w1) or a five water molecule (heptazine+w5) cluster using the surface

29

hopping method within TDDFT based femtosecond (fs) evolution of the potential

30

energy.

31

The ground state optimized heptazine-water complexes are shown in Figure 1a,b.

32

These ground-state structures are optimized using the ωB97XD functional27,28 along

33

with choosing the 6-31G(d,p) basis set29 (see supporting information (SI) for details

dynamics

simulations

of

heptazine


with

one

water

molecule


1

on the geometries). The reliability and accuracy of the ωB97XD functional over the

2

common long-range corrected functionals has been tested in practice it well

3

reproduced the excited-state charge-transfer energies compared to the expensive

4

ADC(2)30 and high-level ab initio CASPT2 calculations.31 In the heptazine-w1

5

complex, heptazine and water molecules are coplanar, while in the heptazine-5w

6

system, a 5w water cluster forms a parallel hydrogen-bonded planar cluster above

7

the heptazine molecule (Figure S1). The absorption spectra of the heptazine (gas

8

phase), heptazine-w1, and heptazine-w5 are shown in Figure 1e while the vertical

9

excitation energies are given in the SI.

10 11

Figure 1. Ground state (a) heptazine-w1 and (b) heptazine-w5 optimized geometries.

12

Excited-state relaxation induced S0/S1 intersection geometries for selected

13

geometries of (c) heptazine-w1 and (d) heptazine-w5 complexes. (e) Simulated

14

absorption cross section of heptazine, heptazine+w1 and heptazine-w5. Important

15

atoms and corresponding bond lengths are labelled as numerical values in

16

Angstrom.

17

It is well known that the excited-state charge-transfer states cannot be correctly

18

determined with the usual DFT functionals,31,32 whereas this discrepancy has largely

19

been removed using the rang-separated ωB97XD functional.30,31 The vertical

20

excitation energies (Table S1) are blue shifted compared to the ADC(2) predicted

21

values9, however, the overall profile of the transitions are well matched except the

22

CT state appearance at 5.17 eV. This transition features a S8 state, which is a nπ*

23

transition in the ADC(2) calculations with no explicitly mentioned energy of CT state.

24

This CT state is well above the S1 state and has no major role in the ground-state

25

local photoexcitation of heptazine. To ensure the accuracy of the present method,


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we traced out the minimum energy reaction path of the H-atom abstraction from

2

water molecule and attachment to the heptazine via H-N covalent bonding (see

3

Figure S3). The relaxed scan revealed a barrier of 0.68 eV for water-heptazine

4

hydrogen transfer in agreement with the ADC(2) calculations (0.75 eV),9 ensuring

5

the accuracy of the present method.

6

Starting from the ground state optimized geometries, spectral points in the 3.0 eV ±

7

0.3 domain are used to start the dynamics on the first singlet excited-state (S1)

8

potential energy surface. The simulation results show that the relaxation of excited-

9

state heptazine-water complex(es) contributes to the longer time constant, where

10

only ~23% of the trajectories deactivate via conical intersection (CI) (Figure 2). Most

11

of the trajectories (≈77%) require more than 300 fs (the maximum simulation time in

12

this study) to relax; the remaining trajectories follow a pathway ending at CI point

13

due to the occurrence of water-heptazine electron-driven proton-transfer (EDPT)

14

reaction (Figures 1c,d). For the heptazine in the gas phase, no trajectory deactivate

15

to the ground state within the maximum simulation time. As the dominant intersection

16

geometry causes by water-heptazine EDPT reaction within tens of femtoseconds,

17

here, our main concern is water splitting via the EDPT reaction than the statistical

18

enumeration of the relaxation dynamics.

19 20

Figure 2. Splitting ratio (in percentage) of the total trajectories in the deactivated

21

(electron-driven proton-transfer (EDPT)) and non-deactivated (still in S1 state until

22

the maximum simulation time, i.e. 300 fs) trajectories of heptazine-water complexes.

23

At the proton-transfer intersection, the excited state S1 involves transition from the

24

oxygen of a water molecule to the heptazine π* state. The rotation of the water

25

molecule facing the heptazine N-atoms is the main distinctive aspect inducing the

26

intersection, where N1∙∙∙∙H1OH2∙∙∙∙N2 conformation in the ground-state geometry

27

changes to the N2H1∙∙∙∙OH2 conformation at the point of intersection. The intersection



1

geometry consists of the heptazinyl and OH radicals in the potential metastable

2

ground state of the original heptazine-w1 complex (Figure 1a,c). To provide a

3

representative trajectory with a proton-transfer intersection, the relative NH and OH

4

distances with time coordinates are shown in Figure 3, which features the main

5

components shared by all the trajectories following the EDPT pathway, where the

6

trajectory starts in the first excited state followed by the S1/S0 intersection within 40

7

fs.

8 9

Figure 3. Bond distances vs simulation time of a representative heptazine-w1

10

complex trajectory. Structural features at the initial and CI point of the trajectory are

11

also given for a clearer assessment of the geometry during the dynamics.

12

For the EDPT conical intersection, the relative character of the S1 excitation in the

13

relaxation of the excited-state is of vital importance in interpretation of the driving

14

force for such reactions to happen. Regarding the charge character of the orbitals

15

involved in the excitation, Figure 4a shows the results of a state analysis through

16

charge density difference distributions in the S1 state. Initially, the ππ* state is

17

populated at the start of the dynamics. A fast excited-state relaxation of the

18

heptazine-w1 complex leads to a charge transfer (CT) populated state. This CT

19

populated state is due to the CT from the O-atom of the water molecule to the π*

20

state of the heptazine molecule. This electronic CT is essentially an instantaneous

21

effect due to fluctuations of the charge donor (O) and acceptor (N) atoms involved in

22

hydrogen bonding. The charge density difference distribution effectively changes,

23

where the hole transfer from the heptazine to the water molecule, a strong force due

24

to the electronic charge separation, drives the proton from the water molecule to the

25

heptazine. The S1/S0 intersection geometry shown in Figure 1c represents the CI


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1

occurring during the non-adiabatic dynamics. The S1-S0 energy gap along the

2

trajectory is shown in Figure 4a; the energy gap at the intersection geometry is 0.09

3

eV, producing the hydrogen-bonded heptazinyl and the OH biradical.

4 5

Figure 4. Time evolution of the energy gap for a single trajectory in the S1 state for

6

(a) the heptazine-w1 complex and (b) the heptazine-w5 complex. Isosurafaces with a

7

green-blue (negative-positive) color represent the charge density difference

8

distribution in the given state.

9

The simulation results for the heptazine-w5 complex show that, in fact, the relaxation

10

to the intersection geometry can be thought of as the CT from the water molecules

11

followed by the proton transfer. The main mechanistic feature is again the

12

rearrangement of the water molecules against the light irradiation following the

13

production of heptazinyl and OH radicals upon electron-driven proton-transfer EDPT

14

(Figures 1d and 4b). In the initial state, the water molecule (w5) connects to the

15

heptazine though hydrogen bonding, and fluctuation of the hydrogens triggers the

16

geometry resulting in the CT from the oxygen atom to the π* state of heptazine.

17

Water molecules W2 and W5 contribute to the CT, while the proton transfer is

18

triggered from W5 to the nearest nitrogen atom (N) of heptazine. The heptazinyl

19

radical essentially represents a planar geometry, and the slightly out-of-plane atomic

20

bulging is due to the dynamic fluctuational launch at each step and to rearrangement

21

after the excess accumulated charge and proton capture. The charge density

22

difference distributions from the indicated state character and the S1-S0 energy gap



1

along the trajectory are shown in Figure 4b, where the energy gap at the CI

2

geometry is 0.12 eV.

3

Although the dynamic study of the S1 relaxation shows the pronounced reaction

4

channel to the heptazinyl—OH biradical products, the S1 (dark state with zero

5

oscillator strength) may not represent the direct excitation but rather a vibrational

6

relaxation from the higher energy bright states (Table S1). Starting from the

7

generation of initial conditions from the absorption spectrum (shaded area in Figure

8

5a), we simulated over 200 points for the first ten excited states of heptazine+w1 (for

9

dynamics of heptazine+w5 in S6 state, please refer to SI file), dynamics start in the

10

bright S6 state, which is ~1.68 eV higher in energy than the S1 state. Through

11

ultrafast dynamics relaxation, the hydrogen detachment from water to heptazine for a

12

representative trajectory took place within 41 fs (Figure 5b), reaching the CI

13

geometry. From the average population of six excited states over 30 trajectories,

14

according to the Kasha’s rule, fast radiationless relaxation from S6 state populate the

15

S1 state within 300 fs (Figure 5c). The findings here represent the nature of the

16

ultrafast non-adiabatic reaction channel, as the bright ππ* excited state is replaced

17

by the CT state through vibrational relaxation within tens of femtoseconds and

18

eventually S1-S0 intersection.

19

An inspection of the relaxation process from the bright ππ* excited state S6 to S1

20

reveals the main feature shared by all the trajectories following the EDPT pathway.

21

Figure 5d,e illustrates the time evolution of the energy gap between the state of the

22

system and the ground state. Initially, the bright ππ* state is populated, and a fast

23

relaxation reveals the mixed state of ππ* and nNπ* for most of the time. With the CT

24

contribution from the oxygen atom of the water molecule to π* of heptazine (CT

25

state), the hole transfer from the heptazine to the water molecule and the

26

electrostatic force derives the proton transfer from the water to the heptazine (Figure

27

5e). This EDPT reaction brings the excited state system to the CI point with the

28

ground state, and water-heptazine complex splits into hydrogen bonded heptazinyl

29

and hydroxyl biradical. Since the electron along with proton transfers from the water

30

to the heptazine, the excited-state at CI point is the lowest and represents the neutral

31

heptazinyl-hydroxyl biradical product with an average hydrogen bond length of 1.98

32

Å. Also, in pursuance of a realistic intersection picture, the complete active space

33

self-consistent field (CASSCF)33 method was used to cross-check the TDDFT


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1

dynamics results. Single-point CASSCF calculations (please refer to the SI for

2

detailed discussion) were done using the starting and S1/S0 intersection geometries

3

for the representative trajectories discussed above (heptazine-w1 complex shown in

4

Figure 1c). The findings of the present study confirm the water-splitting capability of

5

heptazine, thus joining the experimentally reported10 detection of free OH radicals in

6

an ultrafast spectroscopic study of the heptazine based photocatalyst and a

7

theoretical linear interpolation study9.

8 9

Figure 5. Representative trajectory with conical intersection in the heptazine-w1

10

complex. (a) Simulated absorption spectrum of the heptazine-w1 complex for the first

11

ten excited states, with 200 points for each state. Initial conditions for dynamics were

12

sampled from the shaded area, with an energy interval of 4.9 ± 0.2 eV. (b) Potential

13

energy surfaces (smoothed over a 2 fs interval) against the simulation time for a



1

representative trajectory. Dynamics were started in the bright sixth excited state

2

(S6). Black dots indicate the state of the system at a given time. (c) The average

3

population (taken over 30 trajectories) of the heptazine-w1 complex run for 300 fs.

4

The population was smoothed over a 39 fs interval. (d) The instantaneous state-

5

ground-state (Sn-S0) energy gap evolution of the trajectory represented by (b). (e)

6

Charge density difference distributions for a selected trajectory illustrated with

7

isosurface green-blue (positive-negative) charges at a given simulation time.

8

We provide a mechanistic picture of the photoreaction for the heptazine-water

9

complex system through non-adiabatic dynamics simulations. Populating a higher

10

Franck-Condon state via photo-absorption, through fast radiationless transitions, the

11

lowest excited state is populated within 300 fs. Twenty-five percent of the state-

12

relaxation trajectories show the crossing of the S1 state by the CT state, where the

13

CT from the oxygen of the water molecule to the π* heptazine state leads to the

14

EDPT from the water molecule to the heptazine. The EDPT reaction results in the

15

generation of the heptazinyl-hydroxyl biradical in the vicinity of < 50 fs and is a

16

suitable scenario for water splitting and subsequent hydrogen evolution using a

17

catalyst via the dark recombination of heptazinyl radicals. This CT state carries the

18

reaction energy through dissociating the initially formed ππ* state with a CT from the

19

water to the heptazine followed by the proton transfer. The active role of the proton34

20

in neutralizing the system on a fast timescale (tens of femtoseconds) has been

21

suggested as a barrierless34,35 reaction due to the excessive vibrational energies of

22

the system. In the present heptazine-water complex, the excessive energy available

23

through the radiationless relaxation from the higher bright state to S1 and the solvent

24

rearrangement will suppress any barriers (Ref. 9; linear interpolation via relative

25

distances of the proton with donor oxygen and accepted nitrogen atoms in the

26

heptazine-water system). The predicted photochemical picture of the heptazine-

27

water complex for water splitting (with heptazine, the backbone of g-C3N4 materials)

28

reveals the possibility of biradical detection on an experimental scale, as does the

29

earlier reported generation of OH radicals with titanium oxide materials.36-40

30

Recently, it has been reported that CN-based materials can harvest and store

31

sunlight as long-lived trapped electrons for redox chemistry in the dark, resulting in

32

ultra-long-lived radicals that can reductively produce hydrogen in the presence of a

33

hydrogen evolution catalyst in the dark on demand.41 In the dark, the addition of


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1

colloidal Pt produces H2 evolution from the long-lived radicals and regenerates the

2

original material. Our non-adiabatic dynamics predicted that the generation of

3

radicals upon photoexcitation provides a scenario for H2 generation in the dark with

4

catalyst (Pt, Pd) dispersion from the heptazinyl radicals. Interestingly, the water-

5

splitting mechanism presented here is in agreement with a recent static calculation

6

deduced from the excited state energy profile against the N-H and O-H distances.9

7

Although, present TDDFT calculated EDPT reaction barrier (Figure S3) is agreement

8

to a previous ADC(2) prediction,9 the EDPT reaction via dynamics should take long

9

enough (>10 ps) for such large barrier to cross. Additionally, a recent experiment10

10

on 2,5,8-tris(4-methoxyphenyl)-1,3,4,6,7,9,9b-heptaazaphenalene (TAHz) reveal

11

fluorescence quenching around 16 ns which is typical of a fast emission process. For

12

the first case, the relaxed scan is a constrained coordinate scheme where an

13

electron and proton (hydrogen atom) both transfer from water to heptazine at the

14

same time. In excited-state dynamics,42,43 electron-transfer occur initially thereby

15

forming a strong coulomb field and exert a strong electrostatic force to couple the

16

proton-transfer. Also, at each dynamics step (0.5 fs), there is a rearrangement of

17

atomic positions and velocities for each atom adopted from the initial frequency

18

modes.44 These velocities provide relative motion of atoms to perturb the atomic

19

distances, not a constrained motion which is a usual case in relaxed scan. Similar

20

situation has been found in a relaxed scan ADC(2) calculation45 of adenine, where

21

deactivation via C2, C6 and EDPT revealed a barrier (~0.40 eV), however, the

22

ADC(2) level dynamics46 shown deactivation within ~200 fs (with a minimum of 19 fs

23

for an individual trajectory), in agreement to the experiment. Importantly, crossing a

24

barrier around ~0.40 eV will require tens of picoseconds which are far greater than

25

the experimental deactivation time (~1ps). Similarly, a combined experimental and

26

computational study revealed that TDDFT dynamics in agreement to experiment

27

population decay time.47

28

In case of the experimental10 study on TAHz, the time difference for fluorescence

29

quenching and present EDPT reaction is large and the fluorescence quenching

30

around 16 ns represent the a typical fast emission process without a clear

31

femtosecond/sub-picosecond level understanding detection of hydroxyl radical

32

detection. Similarly, hydroxyl radical detection revealed, however, a clear

33

understanding of the reaction time and detection is still lacking due to experimental



Page 12 of 18

1

constraints. Additionally, the assembly of TAHz and water revealed additional bright

2

CT state (from the substituent to the center of molecule) possessing an excitonic

3

character. Hence the femtosecond level comparison of the experimental10 results

4

and the present investigation while the fundamental electronic structures are

5

different and lack of ultrafast experimental resolution data, the two will not agree in

6

details. Furthermore, more sophisticated experimental investigations are required to

7

reveal

8

photochemistry data of the heptazine based oligomers.

9

In fact, this study aimed at the occurrence of the excited-state relaxation induced by

10

the water-heptazine EDPT reaction as the intersection may be affected by increasing

11

the cluster or bulk affect. The EDPT conical intersection, however, provides the

12

prospect for water splitting and the internal conversion of heptazine in aquatic media

13

through the ultra-fast rearrangement of the solvent.

14

Here, we have shown that photorelaxation by water-heptazine electron-driven

15

proton-transfer (EDPT) is a dominant pathway connecting to the conical intersection

16

for heptazine-water clusters. Dynamics simulations show that hydrogen detachment

17

from the water molecule to the heptazine occurs within tens of femtoseconds

18

producing the heptazinyl and hydroxyl biradical product, a generic water-splitting

19

event and a suitable scenario for H2 evolution. Hence, the use of the catalytic

20

materials for the precise H2 evolution and OH scavengers is required to avoid the

21

possible recombination of the product at the conical intersection. Furthermore, there

22

are major challenges for the computational simulations, including the investigation of

23

this reaction pathway in the assembly of the heptazine oligomers, how EDPT

24

intersection will evolve in the bulk water, and the ultimate OH scavengers required to

25

avoid the recombination. On the physical scale, this study presents experimental

26

challenge of providing a more robust characterization, with the possible time-

27

resolved photoelectron spectroscopy of the entitled heptazine oligomers with small

28

clusters as well as with bulk water.

29

ASSOCIATED CONTENT

30

SUPPORTING INFORMATION

31

Computational Details, Ground State Geometries and Vertical Excitations,

32

Absorption Spectra, Relaxed Scan Result, Complete Active Space Self-Consistent

the

sub-picosecond

events

in

order

to


acquire

the

femtosecond

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1

Field (CASSCF) Calculations, Dynamics started in S6 (heptazine+w5), Additional

2

References. (PDF)

3

CONFLICTS OF INTEREST

4

There are no conflicts to declare.

5

ACKNOWLEDGEMENTS

6

The work described in this paper was supported by grants from the Research Grants

7

Council of the Hong Kong SAR (CityU 11334716, 11304415), the National Natural

8

Science Foundation of China (11874081), and the Science Technology and

9

Innovation Committee of Shenzhen Municipality (JCYJ20170818104105891).

10

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T.

Cyclohexadiene

Revisited:

A

Time-Resolved


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