UVâVis, Fluorescence, and Resonance Raman Spectroscopic and

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

UV-Vis, Fluorescence, and Resonance Raman Spectroscopic and Density Functional Theoretical Studies on 3-Amino-1,2,4-Triazole: Microsolvation and Solvent-Dependent Nonadiabatic Excited State Decay in Solution Shuang Meng, Aimin Duan, Jiadan Xue, Xuming Zheng, and Yanying Zhao J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b07384 • Publication Date (Web): 08 Oct 2018 Downloaded from http://pubs.acs.org on October 11, 2018

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The Journal of Physical Chemistry

UV-Vis, Fluorescence, and Resonance Raman Spectroscopic and Density Functional Theoretical Studies on 3-Amino-1,2,4-triazole: Microsolvation and Solvent-Dependent Nonadiabatic Excited State Decay in Solution Shuang Meng, Aimin Duan, Jiadan Xue, Xuming Zheng, Yanying Zhao* Department of Chemistry and Engineering Research Center for Eco-dyeing and Finishing of Textiles, Key Laboratory of Advanced Textiles Materials and Manufacture Technology, Ministry of Education, Zhejiang Sci-Tech University, Hangzhou, 310018, China

ABSTRACT The microsolvation and photophysics of 3-amino-1,2,4-triazole (3AT) after excitation to the light-absorbing S2(nπ*) state were studied by using resonance Raman spectroscopy and single component artificial force-induced reaction (SC-AFIR) in a global reaction route mapping (GRRM) strategy. The vibrational spectra were assigned on the basis of experimental data and density functional theory (DFT) calculations. The resonance Raman spectra of 3AT were measured to probe the excited state structural dynamics in the Franck– Condon region. The conformations of 3AT(CH3CN)1, 3AT(CH3OH)2, and 3AT(H2O)2 clusters

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were determined by combining vibrational spectrum experiments and B3LYP/6-311++G(d,p) computations. DFT calculations were carried out to obtain the minimal excitation energies of the lower-lying singlet excited states, and the curve-crossing points. It was revealed that the shorttime structural dynamics of 3AT were dominated by the N–N stretching coordinates. An excited state decay mechanism is proposed: 3AT is initially excited to the S2(nπ*) state, then the conical intersection (CI) of the S2(nπ*)/S1(ππ*) potential energy surfaces is crossed, and 3AT then decays to the lower solvent-dependent excited state S1(ππ*). It subsequently returns to the S0 state, accompanied by a large Stokes fluorescence shift, which was interpreted as the stabilized S1(ππ*) excited state bonding to several water molecules via intermolecular hydrogen bonding.


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INTRODUCTION Solute–solvent interactions, which are typically significantly sensitive toward changes in the solvation environment, play an important role in chemical reactions.1,2 Therefore, investigating the interactions between solvents and the substances they solvate is of utmost importance. Indeed, comprehending the photophysical and photochemical process of solvent-depandent interactions is also the basis of understanding the influence of other interesting surfaces, DNA, proteins, and polymeric matrices environments.3-6 However, much less is known about the consequences of changes to the specific microsolvation due to the solute-solvent hydrogen bonding interactions.7-9 The formation of weakly bonded molecular aggregates also involves noncovalent interactions due to induced or permanent dipoles, which may be dominated by electrostatic, induction, or dispersion interactions. Such weakly bonded molecular assemblies can be further classified ionic, metallic and weakly bonded molecular cluster according to the nature and the type of the forces. 10−13

Due to a significant contribution from the dispersion interactions, not only the individual

water molecules aggregate by hydrogen bonds (HB), but also acetylene and benzene can hold together bound by π…π and CH…π interactions.14 The nature of these intermolecular interactions contribute significantly to the structural stability, and physical and chemical properties of molecular clusters. Hence, understanding the phenomenon of magic numbers15,16 in clusters has been an important subject of many physical, chemical and biological sciences. Several reviews over the last three decades have addressed these aspects by summarizing experimental and theoretical studies on clusters. 13-15,17 The spectroscopy of molecular clusters in solvents, especially a combination of the absorption, vibration and laser spectroscopic techniques, has proven to be a popular and forceful experimental technique for providing direct and accurate structure and solute–solvent interaction sites. In particular, a combination of vibration


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spectroscopic techniques with mass spectra has successfully been applied to investigate the microsolvation phenomenon (solute–solvent clusters) combining with quantum chemical calculations.18−26 These studies will undoubtedly bring progress to many scientific fields and industrial applications that might benefit from an in-depth understanding of microsolvation, such as corrosion science,27 drug delivery,28 and polymer synthesis.29 Triazoles,30 a family of compounds with five-membered heterocyclic rings, play an important role in numerous biological activities due to their three electronegative nitrogen atoms. These species have great value of research as human therapeutic agents, as they exhibit a broad spectrum of pharmacological

activities,

including

antifungal,31

antitumor,32

anti-inflammatory,33

antibacterial,34 antidepressant,35 and antiviral36 properties. In Ar and Xe, two isolated tautomers of 3-amino-1,2,4-triazole (1H-3AT, 2H-3AT) were observed and characterized by PagaczKostrzewa et al.37 They found that UV could trigger interconversion of the two tautomers into each other via proton transfer, and concluded that this experimentally observed photo-induced tautomerization process followed the 1H-3AT(S1) → CI → 1H-3AT(S0)/2-3AT(S0) pathway; the S1/S0 conical intersection (CI) plays a crucial role in the photo-physical and chemical reaction mechanism. This type of proton transfer is generally produced between a reagent and product with one proton attached at two different positions of the heteroring and no exocyclic groups involved.38, 39 Furthermore, 3AT is very sensitive to the environment.40 The vibrational spectra analysis identified that the most stable form of 3AT is the solid 3-amino-1,2,4-2Htriazole (2H-3AT) dimer due to the intermolecular hydrogen bonding interaction, while in a polar solvent, 2H-3AT monomer is isolated by solvent molecules due to the stronger hydrogen bonding interaction between the solvent and 3AT.


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In this study, we will clarify the microsolvation and solvent-dependent excited state decay mechanism of 2H-3AT in polar solvents. Further, we combine the experimental examination with time-dependent density functional theory (TD-DFT) calculations. The microsolvation phenomenon in solvents was illustrated by 3AT(CH3CN)1, 3AT(CH3OH)2, and 3AT(H2O)2 clusters on the basis of the vibration and emission spectra. DFT calculations were undertaken to acquire the minimal excitation energies of the lower-lying singlet excited states, and the curvecrossing points. A large Stokes fluorescence shift can be interpreted as arising from a stabilized S1 excited state bonding to several water molecules to form a cluster via intermolecular hydrogen bonding.

EXPERIMENTAL AND COMPUTATIONAL DETAILS UV absorption spectra in water, methanol, and acetonitrile were measured using an ultraviolet/visible spectrometer at a concentration of ∼10−4 mol·L−1. The fluorescence spectra were recorded on an F-4600 FL spectrophotometer at a concentration of ∼10−5 mol·L−1. For the resonance Raman experiments, The harmonics of a nanosecond Nd:YAG laser and their hydrogen Raman shifted laser lines were utilized to generate the 200, 204, 208, 217, and 223 nm excitation wavelengths , which have been described previously.41 The excitation laser beam used a ~100 J pulse energy loosely focused to a 0.5-1.0 mm diameter spot size onto a flowing and circulating liquid stream of sample. Solution-phase samples were used at a concentration of ∼6.0 × 103 mol·L−1 3AT (99% purity) in water and spectroscopic-grade acetonitrile and methanol (99.5% purity). The Raman signal was recorded for about 100 s before being read out to an interfaced PC computer; 80–100 of these readouts were combined to obtain a resonance Raman spectrum with a 2 cm−1 resolution. The Raman shifts of the resonance Raman spectra were


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calibrated with the known vibrational frequencies of the cyclohexane solvent Raman bands, and the solvent Raman bands were then subtracted from the resonance Raman spectra using an appropriately scaled solvent spectrum. Sections of the resonance Raman spectra were fitted to a baseline plus a sum of Lorentzian bands to determine the integrated areas of the Raman bands of interest. The visible excitation laser line (488 nm) was produced with a CVI MellesGriot argon ion laser (543-AP-A01). A backscattering geometry was employed for collection of the Raman scattered light by reflective optics that imaged the light onto a liquid nitrogen cooled CCD mounted on the exit of the spectrograph. Raman backscattering spectra were recorded using a 20× objective lens with a ~2 cm−1 resolution TriVista Spectrometer System equipped with a research-grade PAHL-20 microscope. The complete geometry optimization and vibration analysis were carried out at the B3LYP/6-311++G(d,p) level of theory. The vertical transition energy for S 0 → Sn and the Sn minimum were estimated at B3LYP-TD/6-311G** by employing a self-consistent reaction field (SCRF) and the polarized continuum overlapping spheres model (PCM). The ground state and vertical transition computations were obtained using the Gaussian 09 program.42 The minimum energies and the optimized geometric structures in S 0, S1, and S2 were calculated using the single component artificial force-induced reaction (SC-AFIR) in the global reaction route mapping (GRRM) program43–46 which runs in combination with Gaussian 09. What’s more, to avoid the erroneous description of weak interaction, DFT including long-range dispersion effects (DFT-D) by Grimme 47-49 are involved in all optimized geometries.

RESULTS AND DISCUSSION


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Vibrational Spectra. We have previously reported that 3AT displays significant band shifts and Raman intensity patterns in the solid state and in different solvents. The obvious large wavenumber differences are induced by hydrogen bonding perturbation along the intermolecular hydrogen bonds on the five-membered N-heterocyclic ring.40 In this study, DFT calculations were performed to further support the above experimental assignments and determine the bonding sites between 3AT and the solvent H2O, CH3OH, and CH3CN molecules. The E/G energies of the optimized geometries are shown in Figure S1. First, one to three solvent molecules were added near the polar sites of the 3AT molecule (C–NH2 and NH groups) to calculate possible structures of 3AT(solvent)n clusters using B3LYP/6-311++G(d, p) theory. The number of solvent molecules were resolved for 3AT(H2O)2, 3AT(CH3OH)2, and 3AT(CH3CN) H-bonding-related clusters (Figure S2(a) 2d, (b) 2a-d, and (c) 1a). The results indicate that 3AT(solvent)n (n = 2–6) clusters can be greatly stabilized via NH⋯N H-bonding in CH3CN, or via cyclic NH⋯O and N⋯OH H-bonding in CH3OH and H2O. In addition, the B3LYP/6-311++G(d,p) theory predicted that the hydrogen bonds between 3AT and the solvent molecules, 3AT(solvent)n, are moderately strong, in accordance with the classification of Grabowski, which states that the stabilization energy from weak to moderate H-bonding is 0.9– 15 kcal·mol−1.50

-5

-7

-4.5

-8

-4.0

-9

-3.5

-10 -3.0

-11 -12

-2.5 0

1

2 Number of CH3CN

3

4

-5

-3

(b)

-5.2 -10

-5.4

-10 Energy (kcal/mol)

-5.0

-6

Per CH3CN (kcal/mol)

-5

-5.0

-15

-5.6

-20

-5.8

-25

-6.0

-30

-6.2

-35

-4

-15 -5 -20 -6 -25 -7

-30

Per Methanol (kcal/mol)

0

-5.5

Energy (kcal/mol)

-6.0 (a)

-4

Per Water (kcal/mol)

-3

Energy (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


-6.4

-40 0

1

2

3

4

5

6

Number of Water

-35

-8 0

1

2

3

4

5

Number of Methanol

Figure 1. Cluster binding energy (in kcal/mol) as a function of the number of (a) acetonitrile, (b) methanol and (c) water solvent molecules (blue line/symbols) at B3LYP-D3(BJ)/6-311++(d, p)


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level with PCM solvent model. Note the larger average stabilization gain from n = 1 to 2 than any other step, shown dramatically in the binding energy per solvent plot (right axis, red line/symbols). To confirm the stability of the solute–solvent clusters, Figure 1 shows the average and total binding energy as a function of n; another cluster combination energy E/G is shown in Figure S1. There are large gaps of 4.44 kcal·mol−1 and 5.42 kcal·mol−1 between the n = 1 and n = 2 binding energies for water and methanol clusters; subsequent water molecule or methanol molecular additions consistently have a binding energy of ∼3.0 kcal·mol−1. The result is a slope change or “kink” in the curve at n = 2. Enhanced binding per water or methanol for n = 2 is highlighted by the per molecule binding energy, which is also plotted in Figure 1. Stabilization at n = 2 implies that the “preferred” number of H2O or CH3OH molecules per acid molecule is two. Therefore, the dihydrate is expected to be over-represented in a system containing populations of various 2H-3AT(H2O)n units, while in acetonitrile, 2H-3AT exists as a monomer. Shultz and coworkers reported that pTSA−water clusters behave in a similar manner. 51 Figure 2 indicates that the 488 nm Raman spectrum is in good agreement with the calculated Raman intensity spectrum of the 2H-3AT(H2O)2 cluster, whose geometry is shown in Figure S1(a) 2d. Similar to our recent systems53,54, the three O…H hydrogen bond distances are 1.906, 1.777 and 1.773 Å, which are shorter than that in the triazine–water system.52 Table S2 summarizes the band positions of the vibrational frequencies of the 2H-3AT(H2O)2 cluster (conformation shown in Figure S2(a) 2d), including the experimental and calculated vibration frequencies and modes as well as potential energy distribution (PED) assignments. The conformation of the 2H-3AT(H2O)2 cluster is shown in Figure 3, while the calculated and


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experimental spectra of the clusters 2H-3AT(H2O)n, n = 1–6, are shown together for comparison



3AT(H2O)2



 























in Figure S2.

Raman Intensity

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


Experiment

400

600

800

1000

1200

1400

1600

1800

2000

-1

Wavenumber(cm )

Figure 2. The 488 nm and calculated Raman intensity spectra of 3AT(H2O)2 in H2O. UV-Vis Absorption Spectra. Figure 4 shows the UV absorption spectra of 3AT in water, methanol, and acetonitrile with the excitation wavelengths for the resonance Raman experiments indicated as numbers (in nm) above the spectra. The recorded spectrum displayed is only a halfband centered at about 200 nm. TD-DFT calculations are listed in Table 1 for comparison with the experiments conducted in water. Tables S2 also shows the calculated electronic transitions in acetonitrile and methanol obtained with the PCM model. The calculated electronic transitionallowed band at ~190 nm with an oscillator strength 0.1152 is in agreement with the experimentally observed band at  = 199.8 nm with f = 0.1294. The values of max for absorption in the three solvents acetonitrile, methanol, and water are 4.44 × 10 3, 5.43 × 103, and 5.21 × 103 Lmol−1cm−1, respectively. Almost no band shifts are observed due to the solvation and/or hydrogen bonding interactions for the major electronic transitions. To interpret the experimental


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results, hydrogen bonding interactions are considered by adding a defined number of solvent molecules close to the –NH and/ or –C=N moieties of 2H-3AT. There are obvious changes in either the transition energies or the corresponding oscillator strengths due to the different number of water coordination , as shown in Table S5.

Figure 3. Bond lengths (in Å) of 2H-3AT(H2O)2 cluster at B3LYP-D3(BJ)/6-311++G(d, p) level with PCM=H2O solvent model. Figure 5 presents the molecular orbitals associated with the UV-absorption electronic transitions of 2H-3AT. The highest occupied molecular orbital 22 (HOMO) and the lowest unoccupied molecular orbital 23 (LUMO) are -type orbitals, respectively designated as H and L*. The second highest occupied molecular orbital 21 (HOMO−1) is an n-type orbital of the N atom on the heterocyclic ring. Orbital 24 (LUMO+1) is a diffuse orbital with the majority of its density localized on the N1C5 and –NH2 moieties, and is designated as Rdy1. The third lowest unoccupied molecular orbital 25 (LUMO+2) and the third highest occupied molecular orbital 20 (HOMO−2) are diffuse and  orbital, respectively, whose major densities are distributed over the entire molecular frame, including the H atoms of the heterocyclic ring.


10

208.8nm

H2O CH3OH 217.8nm

CH3CN

228.7nm 223.1nm

-1

-1

ε (L· mol · cm )

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


199.8nm

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25000

30000

35000

40000

45000

50000

-1

Wavenumber(cm )

Figure 4. UV absorption spectra of 2H-3AT in water (black line), methanol (green line) and acetonitrile (red line) with the excitation wavelengths labeled for the resonance Raman spectra.

Table 1. Observed maximum absorption bands in water and singlet electron transition energy and oscillator strengths predicted at B3LYP-TD (nstates=15)/6-311G** level of theory with PCM model for 2H-3AT.

States

Orbitals

Character

Transition Energy (nm)

Oscillator strength f

Cal.

Cal.

Exp.

Exp.

At B3LYP-TD/6-311G** level of theory for 2H-3AT in water solvent employing PCM solvent model (for molecular orbitals associated the electronic transitions listed below see Figure 5) S1

S2

22→23(0.35199)

πH→πL*

22→24(0.60308)

πH→Ryd1

195.6

22→23(0.58056)

πH→πL*

189.7

20→23(0.10335)

πH-2→πL*


0.0374 199.8

0.1152

0.1294

11


22→24(0.35898)

πH→Ryd1

S3

21→23(0.69897)

n→πL*

S4

20→23(0.18104)

πH-2→πL*

22→25(0.60616)

πH→πL+2*

22→26(0.28243)

πH→Ryd2

20→23(0.20193)

πH-2→πL*

22→25(0.21640)

πH→πL+2*

22→26(0.62553)

πH→Ryd2

S5

20πH-2

21n

22πH

23πL*

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183.44

0.0039

172.44

0.0702

167.5

0.0429

24Ryd1

25πL+2*

26Ryd2

Figure 5. Molecular orbitals associated with the electronic transitions of the UV absorptions of 3AT. Resonance Raman Spectra and Structural Dynamics. Figures 6–8 display the resonance Raman spectra and the vibrational assignments in water (the figures show only the largest Raman band contributions to each Raman feature in the 1000–1100 cm−1 range). Compared to the 488 nm Raman spectrum in the corresponding solvents, the intensities of some Raman bands are greatly altered. In addition, some bands become very broad and cannot be resolved due to the effects of the solvents. Figure 6 compares the resonance Raman spectrum of 3AT in different solvents obtained with 208 nm and 217 nm excitation wavelengths. One can see, comparing Figure 6 and Figure S5, that the different solvents have almost no influence on the Raman intensities and fundamental modes. Thus, most of the resonance Raman features can be assigned


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to fundamental, overtone, and combination bands of about 10 Franck–Condon active vibrational modes on the basis of the information given in Table S4. As shown in Figure 7, the excitation wavelength is important for the 8 and9 modes, whose intensities are greatly influenced by this experimental parameter. Using 208 and 200 nm, the intensities of these two vibrational modes almost vanish in water and weaken in methanol (Figure S7); 8 and9 are assigned as N4C3, N1C5, and N2C3 stretching, combined with H7N2N1 in-plane bending and N1C5, H7N2N1 in-plane bending, respectively. In acetonitrile, the high-energy excitation light does not reduce the intensities of these two peaks. Therefore, the highly excited state would relax to the S2 state by internal crossing, followed by further decay. 7 is assigned to N2C3 and N1C5 stretching motions and H8C5N4 in-plane bending; 10 is assigned to N4C5/N1C5 stretching and H8C5N1/N1C5N4 inplane bending; 11 is assigned to N4C5 stretching and H8C5N4 in-plane bending; 12 arises from N4C3 stretching and N2C3N4, H9N6C3, H8C5N1 in-plane bending; 13 is assigned to N2C3N4/H9N6C3 and C3N4C5 in-plane bending; 18 is assigned as N2C3N4/C3N4C5 in-plane bending and N6C3 stretching. Figure 8 displays the vibrational assignments of fundamental, overtone, and combination bands of active vibrational modes with large contributions in the Franck–Condon region. In order to interpret the experimental resonance Raman spectra, Raman spectrum intensity calculations were carried out. Figure S12 indicates the calculated spectra of the first and second singlet states, and the 217 nm resonance Raman spectra. As shown in Figure S12, the Raman spectrum intensities of the S1 state are in better agreement with the experimental 217 nm resonance Raman spectrum than those of the and S2 state. Thus, the experimental resonance Raman spectrum is likely to be responsible for the S1 state of 3AT.


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Raman Intensity





 

**

 

 



CH3CN





*

 

 

CH3CN

Raman Intensity

CH3OH

*

H2 O

CH3OH

H 2O

*

500

1000

500

1500

1000

-1

1500

Wavenumber(cm )

-1

Wavenumber(cm )

Figure 6. 208 (Left) and 217 nm (Right) resonance Raman spectra in acetonitrile, methanol and



228nm

 

 

*



water.

223nm

Raman Intensity

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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217nm

208nm

*

200nm

* 500

1000

1500

2000

-1

Wavenumber(cm )

Figure 7. 228, 223, 217, 208 and 200 nm resonance Raman spectra of 2H-3AT in water.


14

1000

1500

2000

2500

3000

9+10+18

18+13

8 18

500

9+18 8+18 11+13 7+18 10+13 211 8+13/10+11  + 9+10 9 11 7+11 8+9 28 7+8 27

10 9

11

13

Raman Intensity

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


7

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3500

-1

Wavenumber(cm )

Figure 8. Expanded view of 217.8 nm resonance Raman spectra of 2H-3AT with the tentative vibrational assignments indicated above the spectrum.

Excited State Decay Dynamics Mechanism. Based on the above-mentioned results, we clarified the microsolvation phenomenon due to 3AT(CH3CN)1, 3AT(CH3OH)2, and 3AT(H2O)2 clusters in polar solvents combining the resonance Raman spectra examination with timedependent density functional theory (TD-DFT) calculations. DFT calculations were undertaken to obtain the minimal excitation energies of the lowest-lying singlet S1 excited state in good agreement with the experimental spectra in the Franck-Condon region.

What’s more, the

excitation wavelength is located in the band region of the second singlet excited state. Thus it is concluded that 2H-3AT is vertically excited to higher singlet S2 state and then fastly decay to the lower S1 singlet state via conical intersection, further radiates back to the ground state, as illustrated schematically in Figure 9. As the starting point, an energy-minimum structure denoted


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S1min in the C1 point group is determined for the light-absorbing S1(ππ*) state, which is likely to be a twisted charge-transfer counterpart of the planar S2min structure in the Franck–Condon region, initially formed from the πH−1 → πL* electronic transition. Thepredicted vertical transition energies of S1(ππ*) and S2(nπ*) are 150.7 and 160.4 kcal·mol−1, respectively, which are greater than the 132.3 kcal·mol−1 for S2/S1 in the Franck–Condon region. The structure at the S1/S2 CI is not a flat surface, because the dihedral angle D (H7-N2-C3-N6) is −18.0°, the D (N2N1-C5-N4), D (N1-C5-N4-C3), and D (C3-N2-N1-C5) angles are 8.9°, 3.8°, and −9.1°, respectively; thus the triazole ring is also non-planar. The bond angles for N4C5N1 and H7N2C3 at S1/S2 are 114.3°and 123.3°, respectively. Some related bond lengths, for example N1N2, C5N2, and N2H7, are respectively 1.342 Å, 1.403 Å and 1.305 Å. The triazole ring of S1min is greatly deformed relative to the pseudo-planar ground state; the bond angles N4C5N1 and H7N2C3 move from 115.8° to 102.7° and from 129.4° to 116.4°, respectively. The bond lengths of N1N2 and C5N1 are respectively elongated from 1.447 Å to 1.374 Å and 1.404 Å to 1.316 Å, while the N2H7 bond is shortened from 1.015 Å to 1.008 Å. A large deformation occurs for dihedral angles D (H7-N2-C3-N6), D (N2-N1-C5-N4), D (N1-C5-N4C3), and D (C3-N2-N1-C5): from 39.8°to 2.5°, from 37.9°to −0.1°, from −26.2°to 0.1°, and from −34.7°to 0.1°, respectively. Other bond angles, bond lengths, and dihedral angles of the lower excited states are shown in Table S6. The vertically excited S2(nπ*) state of 2H-3AT relaxed to the (nπ*)/(ππ*) CI, where the nπ* and ππ* proportions are each 0.48. Subsequently, crossing the CI, the molecule relaxes back to 2H-3AT on the S1,min(ππ*) surface. After this, S1 returns to S0 by fluorescence emission. Figure 10 depicts the emission spectra for 3AT in CH3CN and H2O at room temperature, corresponding to the emission peaks observed at 292 and 300 nm. As shown in Table 2, the emission


16

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wavelength of the S1 excited state is red-shifted as the number of solvent molecules in the clusters increases, which indicates that intermolecular hydrogen bonding interactions play an important role in the excited state decay process. Thus, the fluorescence band observed at 300 nm in H2O should be assigned to the 3AT(H2O)n cluster, as calculated at 301 nm for the S1 state. Similarly, the calculated bands in acetonitrile and methanol also correspond to the solventdependent fluorescence. The large Stokes shift is considered to be caused by the intermolecular hydrogen bonding interaction between 3AT and the solvent molecules, which stabilizes the (ππ*) excited state. This finding demonstrates that ground and excited state conformations of 3AT are controlled by the polarity and hydrogen bonding capability of the solvent.16,17 The energy of S1min is 124.8 kcal·mol–1 relative to S0,min, but the ring is twisted significantly along with the N– N bonding elongation to 1.447 Å. The S2,min state has been found to have two stable geometries; the first S2min state energy is a dissociation along N-N bond with 106.2 kcal·mol–1 lower than the S1min state; the second one is an N–H dissociation state with energy 136.8 kcal·mol–1, N–H bond length 1.865 Å, and N–N bond length 1.335 Å (see S2min−1 in Table S6).


17


kcal/mol

FC

S2(160.4)

(n π*)

S1(150.7)

(π π*)

140

132.3 S1xS2 (nπ*/ ππ*)

130

124.8 S1min(π π*)

110

S

Fluorescence

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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S0 1.375

1.452

N-N/Å

1.472

Figure 9. Potential energy surfaces diagram for 2H-3AT relaxation mechanism with PCM model (H2O) from hot structures to the local minima along N-N bond. The energies (kcal/mol) of ground state (S0) , vertical transition energies, S2min, S1min and (CI)S2/S1 are estimated at B3LYPTD/6-31G** level..


18

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UV Fluorescence

1

UV Fluorescence

1

(a)

(b)

0 200

250

300 -1

Wavelength(cm )

350

0 200

250

300

350

Wavelength(nm)

Figure 10. Normalized absorption and emission (λex=200 nm) spectra of 3AT in (a) acetonitrile, (b) water at 298 K.


19


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Table 2. Observed emission and calculated S1 excited state of 3AT(solvent)n (solvent=CH3CN, H2O; n=1,2,3,4,6) clusters at B3LYP/6-311++G(d,p) level. Cluster

3AT

3AT(H2O)1

3AT(H2O)2

3AT(H2O)4

3AT(H2O)6

Experiment

(nm)

260

262

270

301

312

300

CONCLUSION In this study, we clarified the microsolvation and solvent-dependent excited state decay mechanism of 2H-3AT in polar solvents by using resonance Raman spectroscopy and artificial force induced reaction (SC-AFIR) methods in a GRRM strategy. The most stable conformations of the 3AT(CH3CN)3, 3AT(CH3OH)4, and 3AT(H2O)4 clusters were determined by combining resonance Raman spectroscopy experiments and DFT calculations. The vibrational spectra were assigned on the basis of the experimental measurements and the B3LYP/6-311++G(d,p) computations, as well as normal mode analysis. The A-band resonance Raman spectra of 3AT were measured to probe the structural dynamics in the Franck–Condon region; these showed that different solvents have very little influence on the Raman intensities and fundamental modes. 8 (N4C3/N1C5/N2C3 stretching and H7N2N1 in-plane bending) and 9 (N1C5 stretching and H7N2N1 in-plane bending) almost vanished for 208 and 200 nm wavelength excitations in water, and were weaker in methanol and acetonitrile. The intermolecular >NH⋯N and >NH⋯O H-bonding interactions are considered to be the origin of the stability of the 3AT molecular structure. The solvent-dependent photophysics of 3-amino-1,2,4-triazole (3AT) after excitation to the light absorbing S2(nπ*) state were studied in solution by UV-vis, fluorescence, and resonance Raman spectra combined with DFT calculations. DFT calculations were undertaken to obtain the minimal excitation energies of the lower-lying singlet excited states, S1 and S2, and the curve-


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crossing point, S2/S1. It was revealed that the short-time structural dynamics of 3AT is dominated by the N–N stretching coordinates. DFT calculation combined with GRRM obtained the minimal excitation energies and geometric structures of the lower-lying singlet excited states, and the curve-crossing points. A detailed decay mechanism was proposed: 3AT is excited to S2(nπ*), then crosses the S2(nπ*)/S1(ππ*) CI to decay to the lower solvent-dependent excited state S1(ππ*); in a subsequent step, it returns to S0 by fluorescence relaxation. The observed large Stokes fluorescence shift was interpreted as the stabilized S1 excited state bonding to several water molecules to form a cluster via intermolecular hydrogen bonding interactions.

AUTHOR INFORMATION Corresponding Author  Yanying Zhao E-mail: [email protected] Phone number: +86-571-868-436-27 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We gratefully acknowledge financial support from the National Natural Science Foundation of China (Grant Nos. 21473162 and 21273202). Y. Zhao is grateful for the Project Grants 521 Talents Cultivation of Zhejiang Sci-Tech University. We are also grateful for support from the


21


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Zhejiang Provincial Top Key Academic Discipline of Chemical Engineering and Technology of Zhejiang Sci-Tech University.

Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Optimized bond lengths (in Å) and zero-point correction E/G energies (in kcal/mol) of 2H3AT of 3AT and 2H- 3AT···(solvent)n (solvent = H2O, CH3OH and CH3CN; n = 1–6) (Figure S1~S3); 488 nm and calculated Raman spectra of

2H-3AT(solvent)n (n=1~6) in solvents

(solvent = H2O, CH3OH and CH3CN; n = 1–6) (Figure S4~S6); 488 and 217 nm Raman spectra in water (Figure S7); 208 nm Resonance Raman spectra in acetonitrile, methanol and water (Figure S8); 217 nm resonance Raman spectra in acetonitrile, methanol, water (Figure S9); 200, 208, 218, 223 and 228 nm resonance Raman spectra in CH3OH and CH3CN (Figure S10~11). 217 nm resonance Raman spectra in water and calculated Raman intensity spectra of S1 and S2 excited states (Figure S12); Optimized bond lengths (in Å) and zero-point correction E/G energies (in kcal/mol) of 2H-3AT and 2H-3AT(H2O)n (n=1~6) clusters, at B3LYP-D3(BJ)/6311++G(d,p)level with PCM solvent model(Figure S13~15); Selected cluster binding energy (kcal·mol−1) at B3LYP/6-311++G(d, p) level (Table S1); 488 nm experimental and calculated Raman frequencies of 2H-3AT(H2O)2 clusters at B3LYP/6-311++G(d, p) level (Table S2); Experimental, calculated singlet electronic transition energies and the corresponding orbitals and oscillator strengths with the electronic transition character (Table S3); 217 nm resonance Raman spectra and calculated vibrational frequencies at B3LYP/6-311++G(d, p) level and assignments of 2H-3AT in H2O (Table S4); Experimental and calculated S2 electronic transition energies at B3LYP-TD/6-311G** on the optimized ground state geometry of 2H-3AT(H2O)n clusters, and the corresponding orbitals and oscillator strengths with the electronic transition character (Table S5); Calculated geometries of the ground, excited S1, S2 states and conical intersection S2/S1, orbital transitions at B3LYP/6-311G** level with H2O PCM model (Table S6)(PDF).


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Table of Contents (TOC) Image

Large Stokes Shift observed experimentally!!


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Table of Contents (TOC) Image

Large Stokes Shift observed experimentally!!


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