Two Conical Intersections Control the Luminol Chemiluminescence

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Reaction Mechanisms

Two Conical Intersections Control the Luminol Chemiluminescence Ling Yue, and Ya-Jun Liu J. Chem. Theory Comput., Just Accepted Manuscript • DOI: 10.1021/acs.jctc.8b01114 • Publication Date (Web): 04 Feb 2019 Downloaded from http://pubs.acs.org on February 5, 2019

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Journal of Chemical Theory and Computation

Two Conical Intersections Control the Luminol Chemiluminescence Ling Yue*†, and Ya-Jun Liu*‡ †Key Laboratory for Non-Equilibrium Synthesis and Modulation of Condensed Matter, Ministry of Education, School of Sciences, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China. ‡Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education, College of Chemistry, Beijing Normal University, Beijing 100875, China. KEYWORDS: luminol, chemiluminescence, cyclic peroxide, cyclic hydrazide, nonadiabatic molecular dynamics

ABSTRACT: Luminol, the first discovered and man-made effective chemiluminescence (CL) system, is the best known and one of the most widely used CL materials. The chemiluminescent process of the luminol CL has not yet been fully elucidated, although the decomposition of 1,2dioxane-3,6-dione dianion (CP2-) is verified to be the key step to produce light emitter. However, the mechanisms of the CP2- decomposition and the effective singlet chemiexcitation are totally unknown, which is the outstanding obstacle to comprehending luminol CL. In present work, by means of the state-of-the-art multireference computation and the nonadiabatic molecular dynamics (NAMD) simulation, the decomposition mechanism of CP2- is clearly revealed. A stepwise single electron transfer from the aminophthaloyl to the O–O bond initiates the decomposition of CP2-,

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and the light emitter is produced via the two crossings of the potential energy surfaces of the ground state (S0) and the first singlet excited state (S1). The NAMD simulated quantum yield of the light emitter demonstrates that the located two conical intersections control an effective nonadiabatic pathway in luminol CL. The proposed mechanism of “two conical intersections” is not only suitable to luminol but also the other CL materials with cyclic peroxide as the chemical energy provider.

1. INTRODUCTION Luminol (5-amino-2,3-dihydro-1,4-phthalazinedione, Scheme 1), a cyclic hydrazide whose chemiluminescence (CL) phenomenon was firstly described by Albrecht in 1928,1 is the best known and one of the most widely used chemiluminescent substrates.2 When mixed with appropriate oxidizing agents, such as H2O2 or O2, and catalysts like the iron ions in hemoglobin, luminol exhibits chemiluminescence, with a blue glow.3 Luminol CL is thus famous for the detection of trace amounts of blood at crime scenes and also has important applications in analytic, environmental, forensic, biomedical, and clinical sciences.2,4-17 Since 1960s, White et al. have investigated the chemical and light emission step in CL of luminol and relative organic hydrazides.18-22 The singlet excited-state (S1) 3-aminophthalate dianion (AP2-*) is usually considered as the light emitter. However, the mechanism of the luminol CL has not yet been fully understood, especially the key step directly leading to the effective S1 chemiexcitation. Several mechanisms were proposed

in which the cyclic peroxide (CP2-), 1,2-dioxane-3,6-dione is

considered as the key intermediate for the chemiexcitation (Scheme 1).23-27


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Scheme 1. The structures of luminol and the cyclic peroxide intermediate. The atomic labels are marked for CP2-. Recently, Griesbeck et al.28 synthesized a series of dialkyl luminol derivatives and observed a 20-fold increasing of CL efficiency in comparison with the intact luminol. According to the theoretical computations, they suggested a chemiluminescent mechanism of luminol, based on CP2- which decomposes to S1-state AP2- through a S0/S1 conical intersection (CI) as shown in Scheme 2. The enhancement of CL is attributed to be a direct consequence of steric gearing which facilitates the transition from the intermediate endoperoxide to the electronically excited phthalate dianion (AP2-*). Please see ref 28 for detailed discussion about the steric gearing effect. Actually, in Scheme 2, the chemiexcitation details of CP2- and the quantum yields (Φ) of AP2-* (in singlet and triplet states) are unknown. Due to the large computational costs on nonadiabatic coupling vectors and the difficulty to consider the singlet/triplet intersystem crossings, theoretically evaluating the quantum yields of chemiluminescence via nonadiabatic molecular dynamics (NAMD) simulation is a big challenge, especially for the current chemiluminescent reaction in which several degenerated states and open-shell biradicals are involved. Recently, the improved Zhu–Nakamura algorithm doesn’t need to calculate the nonadiabatic couplings and can handle singlet and triplet states simultaneously, which enhances the accuracy and efficiency of the surface hopping technique,29-41 and makes the NAMD simulation on the current system possible. The purpose of this article is to investigate and elucidate the detailed decomposition and chem-


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excitation mechanism of the cyclic peroxide intermediate (CP2-) by density functional and multiconfigurational methods. To confirm the chemiexcitation process, the quantum yield of luminol CL is evaluated for the first time by means of the newly developed surface hopping algorithm.

Scheme 2. A plausible chemiluminescent mechanism of CP2- proposed by Griesbeck et al.28 2. COMPUTATIONAL METHODS Geometric and Electronic Structure. The unrestricted open-shell density functional theory (DFT) with the CAM-B3LYP functional42 and the complete active space self-consistent field (CASSCF)43-44 with state-averaging over the four lowest-energy roots are used to perform the geometry optimizations of the critical stationary points on S0 potential energy surface (PES) including the local Min, TS and CIs. The single point energy corrections for both ground and excited states are also performed by means of the second-order multiconfigurational perturbation (CASPT2)45, based on the geometries optimized by CAM-B3LYP or SA-CASSCF. The active space used in the CASSCF and CASPT2 is 18-in-14: the σ and σ* orbitals of O–O bond, the two oxygen lone-pair orbitals perpendicular to the ring, the two π and two π* orbitals on the two carbonyl groups, plus the three π and three π* on the benzene. The 6-31G** basis set are used in both DFT and SA-CASSCF optimization and CASPT2 energy correction. The water and DMSO solvent are also considered in the decomposition of CP2- at both CAM-B3LYP and CASPT2 computational levels by the conductor-like polarized continuum model (C-PCM),50 and the


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calculated results show a qualitative consistence with the ones in gas phase, respectively. All the DFT computations are finished using Gaussian 09 program suit.47 The SA-CASSCF optimization of CP2- and TSO–O and all CASPT2 computations are performed by the MOLCAS 8.0.48 The optimizations of CIs at SA-CASSCF level are completed in Molpro 2012.49-50 Please see the computational details in Supporting Information. On-the-Fly Trajectory Surface Hopping Dynamics. The decomposition of CP2- is investigated by on-the-fly TSH dynamics51-52 with Zhu–Nakamura global switching probability, which was tested to have good performance on the NAMD simulations involving internal conversion and intersystem crossing.37-39 In the initial samplings, random velocities at a constant temperature of 300 K and geometry of the CP2- Min are sampled as an initial condition for 600 trajectories, according to the algorithm proposed by Sellner et al.49, where the statistical behavior of the trajectories can be described by the canonical (NVT) ensemble. The zero-point vibrational energy of the CP2- at SA-CASSCF(14,10)/6-31G** level, 3.41 eV is chosen as the initial kinetic energy. The on-the-fly switching or transition probability between adiabatic electronic states are computed by the Zhu–Nakamura algorithm, by using the spin-diabatic representation where the lowest four singlet and lowest four triplet roots in SA-CASSCF consist of the basis vectors. The potential energies, nuclear gradients and spin-orbit couplings required by the NAMD are calculated by SA-CASSCF(14,10)/6-31G at each time step. The intersystem crossing dynamics with Zhu–Nakamura algorithm is performed in the newly modified version of Newton-X software package.53 The SA-CASSCF computations in NAMD simulation are computed by the MOLCAS 8.0,48 with being interfaced to the Newton-X packaged. Please see more details about the NAMD simulation in Supporting Information. 3. RESULTS


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The energies and key geometric parameters of the local minimum (Min) and transition state (TS) on S0 PES located by CAM-B3LYP/6-31G** and the S0/S1 CIs located by SACASSCF(14,10)/6-31G** are summarized in Table S1. Based on the reaction path optimized by CAM-B3LYP/6-31G**, the potential energy curves (PECs) of the ground (S0) and excited states (S1~S3 and T1~T4) are computed at the CASPT2(18,14)/6-31G** level in Figure S1. According to the pseudo natural orbitals and the occupations (Figures S2-S4), the diabatic CASPT2 PECs (Figure 1) are plotted diabatically from the adiabatic CASPT2 PECs by means of the diabatization proposed by Nakamura and Truhlar54. The diabatic PECs hence follow particular electronic configurations through conical intersections between two states of the same spin multiplicity. For the Min structure of CP2-, the S0 state mainly consists of a typical closed-shell configuration, in which the lowest π* antibonding orbital of the phenylamine ring (π1*) are doubly occupied, in comparison with the neutral 1,2-dioxane-3,6-dione (CP) where π1* is unoccupied. The primary configuration of CP2- is denoted as [CP](π1*)2, shortly (π1*)2, where [CP] represents the closed-shell electronic configuration of CP. With the stretching of the O−O bond, the σ and σ* orbitals are moving closer in energy, and finally they become the two nonbonding p orbitals of oxygen. For the Min structure of AP2-, the characteristic of the S0 state is [CP](π1*)0(σ*)2. Thus, we express the S0 state as (π1*)2/(σ*)2 (as described in Figure 1). Meanwhile, the transition characteristic of the S1 state changes from 1[CP](π1*)1(π2*)1 of CP2- to 1[CP](π1*)1(σ*)1 of AP2-, where π2* is the second π* orbital on phenylamine. The 1[CP](π1*)1(σ*)1 is a typical biradical with the singly occupied π1* of phenylamine and σ* of O−O. The S1 state is hence shortly denoted as 1(π *,π *)/1(π *,σ*). 1 2 1

Similarly, the S2 state is denoted as 1(π1*,σ*)/1(n,π1*), and S3 as (π2*)2/1(n, π1*),

where the n is the out-of-plane nonbonding p orbital on peroxide, and the π2* is doubly occupied.


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The π1, π1*, π2*, σ* and n orbitals are described in the insets of Figure 1. T1, T2, and T3 have the same transition characteristics as S1, S2 and S3, respectively. The expectation value of S2 operator () by CAM-B3LYP along the reaction path is plotted in Figure 2A. The CP2- is divided into four parts including the benzene, amino (NH2), two carbonyl (C=O×2) and O–O moieties. The aminophthaloyl (APA) moiety is the summation of the benzene, NH2, and C=O×2 moiety. The Mulliken charge populations along the reaction path by CAM-B3LYP and CASPT2/(18,14) are plotted in Figure 2B-D. Two-dimensional distributions of the O−O bond length and C10−C9−C1−O2 dihedral angle of the nonadiabatic hopping spots in NAMD simulation are plotted in Figure 2. The time evolution of the ensemble-averaged fractional occupation of the S0-S3 and T1-T4 states, the ensemble-averaged Mulliken charge population for all successful, individual ground and excited trajectories are plotted in Figure 3. The experimental and the NAMD simulated efficiency of luminol CL and Φ of the ground- and excited-state AP2are summarized in Tables 1 and S2. 4. DISCUSSION Resonance Structure of CP2-. The commonly adopted resonance structure of CP2- is the enol form (see Scheme 1), in which the two negative charges are uncritically labeled on the carbonyl oxygen without any experiment or theoretical supporting.28 The doubly occupied π1* shows that the two electrons mainly populate on benzene but not on carbonyl groups. Besides, the optimized C1–O11 and C4–O12 bond lengths by CAM-B3LYP/6-31G** (CASSCF(14,10)/6-31G**), 1.252 (1.239) and 1.270 (1.253) Å, are not the typical C–O single bond length (about 1.43 Å) but very close to the common double bond length of C=O (about 1.20 Å). Therefore, the keto-form resonance structure of CP2- is suggested in Scheme 1.


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100 80

TSO-O

60

CI1

40

E (kcal/mol)

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

20 0

1

(

1

(,)

1

(,)/

1

()2/

3

CI2

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)2 1 1 1

(,)/

(,)

(n,)

(,) 3

(,)

AP2-*

CP2-

-20 -40



-60

double ET

-80 -100 -120 -5.0



0.0

2.5

1

AP2-

 1*

 2* -2.5

n

single ET

*

5.0

7.5

10.0

12.5

15.0

1/2

Reaction Path (amu bohr)

Figure 1. The diabatic PECs of the decomposition of ground- and excited-state CP2- calculated by CASPT2//CAM-B3LYP. Two SETs induce the decomposition of CP2-. According to the DFT and SA-CASSCF optimizations, the unique transition state (TSO–O) is located for the decomposition of CP2-. As shown in Table S1, the optimized O–O bond length of CP2- Min and TSO−O by CAM-B3LYP/631G** are 1.452 and 1.572 Å respectively. Vibrational analyses show that the imaginary vibrational mode of the TSO−O corresponding to the O–O stretch. The low active energies of the TSO–O, 0.6 and 4.3 kcal/mol at the CASPT2(14,10)//CAM-B3LYP and CAM-B3LYP//CAM-B3LYP levels, respectively, indicate the dissociation of peroxide is very fast. After the O−O bond cleavage, the C10–C9–C1–O2 and C9–C10–C4–O3 dihedral angles (Table S1 and Figure S5) are becoming larger. The

electronic energy of the S0 state decreases rapidly after TSO−O as shown in Figures 1 and S1. Along


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the intrinsic reaction coordinate (IRC) from 0.000 to 2.223 amu1/2·bohr, the S0 state is of biradical characteristic. This can be evidenced by the CAM-B3LYP calculated expectation value of S2 operator () shown in Figure 2A and the SA-CASSCF predicated natural orbitals shown in

1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 -0.1 -5.0 -2.5

0.0

gas water DMSO

A

-0.4 -0.6 -0.8 -1.0

-1.4 -5.0 -2.5

benzene NH2

APA O-O C=O2

-0.4

 (|e|)

biradical

-0.8 -1.0

-1.4 -5.0

benzene NH2

-0.2

-0.4 -0.6

-1.2

0.0 2.5 5.0 7.5 10.0 12.5 15.0 Reaction Path (amu1/2bohr)

0.0

APA O-O C=O2

-0.2

B

-1.2

0.0 2.5 5.0 7.5 10.0 12.5 15.0 Reaction Path (amu1/2bohr)

0.0

benzene NH2 C=O2 O-O APA

-0.2

E (kcal/mol)

Figure S3.

 (|e|)

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


biradical

-0.6 -0.8 -1.0

C

S0 state -2.5

0.0

2.5

5.0

7.5

Reaction Path (amu1/2bohr)

10.0

12.5

-1.2

15.0

-1.4 -5.0

D

S1 state -2.5

0.0

2.5

5.0

7.5

10.0

12.5

15.0

Reaction Path (amu1/2bohr)

Figure 2. The (A) in gas, water and DMSO; (B) Mulliken charge populations along reaction path by CAM-B3LYP/6-31G**, and the Mulliken charge populations computed at the (C) S0 and (D) S1 states along the reaction path, by CASPT2(18,14)/6-31G**//CAM-B3LYP/6-31G**.


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Figure 3. Two-dimensional distribution of the O−O bond length and C10−C9−C1−O2 dihedral angle at (A) S0 ↔ S1 hopping (B) S0 ↔ T1 hopping (C) S0 → S1 excitation (D) S0 → T1 excitation (E) S0 ← S1 de-excitation (F) S0 ← T1 de-excitation spots. The numbers of hopping spots are shown in the parentheses. The number of hopping spots in (A) is the sum of the ones in (C) and (E). The number of hopping spots in (B) is the sum of the ones in (D) and (F).


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Two CIs control the chemiexcitation of CP2-. As shown in Figure 1, with the elongation of O−O distance, the 1(π1*,σ*) PEC is downhill rapidly and close to the diabatic 1(π1*)2 PEC. The smallest energy gap (0.7 kcal/mol) between 1(π1*)2 and 1(π1*,σ*) states at IRC=0.376 amu1/2·bohr indicates there could be a CI between the S0 and S1 PESs (see adiabatic PECs in Figure S1). This CI (denoted as CI1) is located by SA-CASSCF(14,10)/6-31G** (Figure 1 and Table S1). The NAMD simulation clearly shows that the nonadiabatic transitions between S0 and S1 mainly occur at the biradical region with O−O bond length of about 1.6 to 2.8 Å (Figure 3A). For the S0 → S1 excitation, the distribution center is CI1 with 1.9 Å O−O bond length (Figure 3C). During the decomposition of CP2-, the time evolution of the ensemble-averaged fractional occupation in Figure 4A shows that the population of the S0 state quickly decreases from 1.00 to 0.85 within the beginning 50 fs, meanwhile the population of the S1 state increases from zero to about 0.049. This variation occurs at the CI1 region. The transition 1(π1*)2→1(π1*,σ*) through CI1 is corresponding to a single electron transfer (SET) from π1* on aminophthaloyl (APA) to σ* on O–O moiety. Figure 4B shows that about 0.08 e transfer from APA to O–O within the initial 50 fs for all successful trajectories (see details in Figure S6). For individual S0 and excited (S1~S3 and T1~T4) trajectories, the electrons on O–O moiety increase by nearly the same amount, as shown in Figure 4C. This SET initiates the decomposition of CP2-, which is very similar with the catalyzed luminescence of 1,2-dioxetane and 1,2-dioxetanone where the electrons transfer from the electron donor to the O−O antibonding orbital producing a biradical.55-62


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Figure 4. The time evolution (0~800 fs) of (A) the ensemble-averaged fractional occupation of S0-S3 and T1-T4 states; (B) the ensemble-averaged Mulliken charge population on APA and O−O moieties for all successful trajectories.; (C) the ensemble-averaged Mulliken charge population on


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APA and O−O moieties for those ground (terminate on S0) and excited (terminate on S1~S3 and T1~T4) trajectories. The APA is the summation of the benzene, NH2, and two C=O moieties. After CI1, O−O continues to elongate till 1.915 Å at 2.223 amu1/2·bohr IRC (Figure 1), where S0 changes from the biradical 1(π1*,σ*) state to the closed-shell (σ*)2 one, whereas S1 changes from (σ*)2 to 1(π1*,σ*). The S0/S1 energy gap at 2.223 amu1/2·bohr computed by CASPT2(18,14)/6-31G** is about 4.0 kcal/mol (Figure S1), indicates another CI (denoted as CI2) between the 1(π1*,σ*) and (σ*)2 states. CI2 is confirmed by the geometry optimization at the SA-CASSCF(14,10)/6-31G** level (Figure 1 and Table S1), which is probably the CI located by Griesbeck et al.28 After CI2, the S0 or S1 AP2- are produced. The S1-state AP2- decays to its S0 state and produces chemiluminescence. According to the hopping spots distribution for S0 ← S1 de-excitation (Figure 3E), the two centers are exactly at CI1 and CI2 with about 1.9 Å and 2.4 Å O−O bond, respectively. The time evolution of fractional occupation shows the population of S0 state continues to slowly decrease to 0.606 and that of the excited S1 state increase to 0.161 in the CI2 region after the initial 50 fs (Figure 4A). The de-excitation via CI2 is corresponding to the other SET process from π1* of phenylamine to σ* of O–O. Figure 4B shows that the electrons on O–O and APA continues to increase and decrease after 50fs, respectively. The increasing of electrons on O–O is quicker on the S0 trajectories than on the excited-state trajectories (Figure 4C). At the end of the simulation, about 0.21 and 0.46 e transfer from APA to O–O in the excited-state and ground trajectories, respectively. The second SET controlled by CI2 de-exciting S1 to S0 for the excited-state CP2- or exciting the S0 CP2- to S1 state. Both the SA-CASSCF optimizations and the hopping spots distributions clearly demonstrate the existence of the two CIs between the S0 and S1 PESs. According to the gradient difference (GDV) and derivative coupling vectors (DCV) shown in Figure S7, the GDVs of two CIs are close and follow the reaction path of TSO–O, hence both CIs


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provide high probability of the nonadiabatic transition. As shown in Figure 3C, the distribution of the S0 → S1 excitation centers at the region of CI1, while that of the S1 → S0 de-excitations centers at both CI1 and CI2 region. Therefore, CI1 provides more S0 → S1 excitation than CI2, and might have more responsibilities for the nonadiabatic transition in luminol CL. In addition, the distributions of CI1 and CI2 region are connected with each other and have no sharp divisions as shown in Figure 3. Maybe the two CIs belong to a same crossing seam, i.e. they are two local minima in the same CI subspace. As described in Figure 1 and Scheme 3, along the reaction path, the PESs of the S1 state with (π1*,σ*) transition characteristic and the S0 state with (π1*)2/(σ*)2 characteristic two intersects. Through CI1 crossing, one channel leads to a transient closed-shell dianion CP2- at the S1-(σ*)2 state via two SETs from APA to O–O moiety, while the other channel goes to a transient biradical anion CP2- at the S0-(π1*,σ*) state via once SET. Thus, at CI2 crossing, the S0-(π1*,σ*) CP2- could stay on the 1(π1*,σ*) PES to produce the S1-(π1*,σ*) AP2-, which is the light emitter. Alternatively, the S0-(π1*,σ*) CP2- could decay and produce the S0-(σ*)2 AP2- through a second SET from APA to O–O moiety. Similarly, the S1-(σ*)2 dianion CP2- could produces S1-AP2- through a back SET or stay on the (σ*)2 PES producing the S0-AP2-. In brief, two final branches end in the S0-AP2- and two final branches end in S1-AP2-, which are called dark paths and light paths, respectively. The two SETs and two CIs during CP2- decomposition are important for luminol CL.


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light path

S1/S0 two crossings CI1 (S1) O

dark path

O O O



O O

O-O stretch

CI1 (S0) O ET O

O NH2

S0

O

-()2

CP2-

T SE

S0-( 

O

S0/S1 CI1

O

O

NH2

O

)

CI2 (S0) O O

O NH2

O

O

O

two SETs

back SET

O NH2

S1

*

O

O

NH2

O O

ET

NH2

CI2 (S1) O SET

O

dS 2n

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


O

-()2

NH2

O

S0/S1 CI2

ba

ck

T SE

NH2

O



S1-(  ) AP2-* O

O

O

O

O NH2

O

S0-()2 AP2-

Scheme 3. CP2- decomposition to producing light emitter via two SETs and two CIs. Similarly, the triplet 3(π1*,σ*) state also intersects twice with the 1(π1*)2/1(σ*)2 state as shown by the diabatic PECs in Figure 1 and hopping spots distributions in Figure 2B, D and F. The triplet chemiexcitation might be possible in luminol CL. As shown in Figure 3, the time evolution of T1 can be divided into two stages. Before 50 fs, the T1 population quickly increases from zero to about 0.098, along with the first SET. After 50 fs, it slowly increases to about 0.180, following the second SET. The T2 population slowly increases from 0 to 0.038 during the dynamic evolution starting from 70 fs. The adiabatic PECs of higher excited states (S2, S3, T3 and T4) are far from the S0 PEC during the dissociation of O–O as shown in Figure S1, so that they have no contributions to the effective CL. The populations of these states indeed almost keep zero all the time in the NAMD simulation (Figure 3A). The chemiexcitation of the triplet AP2- hence is similar with that of the singlet one, so no more tautologies here. In addition, the similar charge-transfer induced decomposition and “two conical intersections” chemiexcited mechanism are also observed in the chemi- and bioluminescence of four-membered cyclic peroxide, 1,2-dioxetane derivatives.57-61,63 Thus, the charge-transfer induced “two conical intersections” mechanism is not only suitable to


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luminol and other cyclic hydrazides but also the other CL materials with cyclic peroxide as the chemical energy provider. Quantum yield of the excited-state AP2-. According to the Zhu–Nakamura trajectory surface hopping (TSH) simulation, the final Φ of the ground and excited-state product are computed using the Monte Carlo method with equal weight to all trajectories, i.e. quantum yield Φ𝑖 = 𝑁𝑖 𝑁, with the standard error using 𝜎𝑖 = (𝑁 ― 𝑁𝑖) (𝑁 × 𝑁𝑖),

where N is total number of successful

trajectories and Ni is number of successful trajectories terminated on state i. In the successful 533 trajectories, 96 and 20 trajectories are terminated on T1 and T2 PESs at the end of the simulation, the final Φ values of the T1 and T2 products are computed to be 0.180±0.092 and 0.038±0.219, respectively. However, due to the quenching of phosphorescence by triplet oxygen, the CL from the triplet products are usually not observed in solution. Meanwhile, 86 trajectories are terminated on S1 PESs at the end of the simulation, thus the final Φ of S1 AP2- (ΦS) is simulated to be 0.161 ±0.099, which represents the probability of populating the S1-state AP2- via the decomposition of CP2-. ΦS is in line with the experimentally estimated excitation efficiency ΦE (in Tables 1 and S2) and justifies that the “two conical intersections” indeed provides the high probability of chemiexcitation. In fact, the chemiluminescence efficiency ΦCL is the total quantum yield of luminol chemiluminescence, which is determined by ΦE and the fluorescence efficiency of the S1-state AP2-, i.e. ΦCL = ΦE × ΦFL = ΦP × ΦS × ΦFL. ΦP is the product yield of CP2- from reactant luminol, ΦS represents the probability of populating the S1-state AP2- via the decomposition of CP2-, and ΦFL is the fluorescence quantum yield of the S1-state AP2-. ΦP is likely to be less than unity due to side reactions and the efficiency of the light path of luminol → CP2-, which is strongly influenced by the oxidant, catalyst and solvent. Due to the complicated processes and influence factors of


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luminol → CP2- and instability of CP2-, the ΦP is difficult to be obtained directly experimentally and theoretically. Combined the values of experimental ΦE and theoretical ΦS, ΦP could be evaluated in different solvents by ΦP = ΦE/ΦS (Table 1). The discrepancy of ΦP in protic or aprotic solvent might be due to the different oxidation mechanism of luminol in protic or aprotic solvent. Considering the competitive dark reaction without light emission, the NAMD simulated ΦS should be the upper limit of ΦE. This is the reason that the simulated ΦS (0.161±0.099) is larger than the experimentally estimated ΦE values (0.03~0.09). Suppose ΦP =1, the theoretical maximum value of ΦCL (Φmax CL = ΦS × ΦFL) is hence estimated to be 0.006~0.062, which is roughly in accord with the experimental ΦCL of 0.003~0.012 (Table 1). Furthermore, many experiments show that the triplet products dominate the excited states in the uncatalyzed thermolysis of cyclic peroxide like the neutral 1,2-dioxetanes.64-66 The ratios of T1/S1 of the uncatalyzed decomposition of 1,2-dioxetanes are more than 140.64 In present work, the T1/S1 ratio is 1.1. Despite of the unavoidable deviation between experimental measurements and theoretical simulation, the striking contrast between the uncatalyzed and catalyzed CL indicates that the ET indeed significantly enhancing the efficiency of the singlet chemiexcitation in the decomposition of cyclic peroxide.

Table 1. The simulated ΦCL of luminol CL in present work and the experimental values in ref 67. Solvent

ΦE (Exp.) ΦFL (Exp.) ΦCL (Exp.) Φmax CL (Sim.)d ΦP (Sim.)

water

0.04

0.30

0.0124

0.049

0.248

PrG (50%)a

0.03

0.38

0.012

0.062

0.186

PrG (90%)

0.03

0.38

0.012

0.062

0.186

DMSOb

0.09

0.14

0.0124

0.023

0.559


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DMFc

0.08

0.05

0.004

0.008

0.497

THFd

0.08

0.04

0.003

0.006

0.497

aPrG:

propylene glycol, bDMSO: dimethylsulfoxide, cDMF: dimethylformamide, tetrahydrofuran, dsuppose ΦP=1, Φmax CL (Sim.)=ΦS (Sim.)×ΦFL (Exp.).

dTHF:

5. CONCLUSION In summary, the keto-form resonance structure is suggested to express the structure formula of CP2- instead of the enol-form; the decomposition of CP2- is initiated by a stepwise single electron transfer from the aminophthaloyl to the O–O bond and the two S0/S1 CIs control an effective nonadiabatic pathway, which could be expanded to the other CL materials with cyclic peroxide as the chemical energy provider; the simulated ΦCL of CP2- agrees with the experimentally estimated one.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: The following files are available free of charge. The computational details, the supplementary figures and tables and the optimized Cartesian coordinates (PDF) AUTHOR INFORMATION Corresponding Author *[email protected]


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*[email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by grants from the National Natural Science Foundation of China (Grant Nos. 21503156, 216730202 and 21421003). REFERENCES

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Two Conical Intersections Control the Luminol Chemiluminescence

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