Photophysics of Protonated and Microhydrated 2-Aminobenzaldehyde

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A: Kinetics, Dynamics, Photochemistry, and Excited States

Photophysics of Protonated and Microhydrated 2-Aminobenzaldehyde: Theoretical Insights into Photoswitchability of Protonated Systems Mohammad Salehi, Zahra Heidari, and Reza Omidyan J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b09930 • Publication Date (Web): 26 Oct 2018 Downloaded from http://pubs.acs.org on October 27, 2018

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Photophysics of Protonated and Microhydrated 2-Aminobenzaldehyde: Theoretical Insights into Photoswitchability of Protonated Systems Mohammad Salehi, Zahra Heidari, and Reza Omidyan* Department of Chemistry, University of Isfahan, 81746-73441, Isfahan, Iran Abstract The photoswitchability of a protonated aromatic compound (2-Aminobenzaldehyde, 2ABZH+) in its individual and microhydrated states have been addressed based on the RI-MP2/RICC2 theoretical methods. Our calculated results give insight into ultrafast nonradiative deactivation mechanism of the 2ABZH+, driving by a conical intersection between the S1/S0 potential energy surfaces. Also, it has been predicted that protonation accompanies with a significant blue shift effect on the first 1ππ* excited state of 2ABZ by 0.87 eV (at least 50 nm).

1-Introduction The interpretation of spectroscopy and dynamics of protonated aromatic systems (AH+) is of essential interest in physical organic chemistry1-10. These short-lived species are important to insight the mechanism and selectivity of chemical processes, since of their crucial roles in electrophilic aromatic substitution reactions (EAS)

11

. For instance, under physiological

conditions, rare probability for an Imino tautomer of adenine is expected. However, once it is formed, it can bind to cytosine instead of guanine and lead to mutation12. Also, tryptophan is well known as the most ﬂuorescent amino acid, nevertheless its fluorescence13 signiﬁcantly weakens in the acidic environment. There was not so clear interpretation on these phenomena at least until two decades ago. Fortunately, recent developments in the laser spectroscopy and quantum chemical methods enabled characterization of these species in the ground and excited electronic states9, 14-18. Consequently, several experimental7, 9, 15, 19 and theoretical10, 20-22 studies have been carried out so far on AH+ systems. As a result, it has been shown that protonation either effectively moves the S1-S0 transition energy23-26 or affects the S1-excited state lifetime1, 27-33. For instance

*

Corresponding author, E-mail: [email protected], [email protected], Fax: (+98) 311 6689732

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Naphthalene,25 Anthracene and other PAHs (Polycyclic Aromatic Hydrocarbons)26 belong to the former group. In such cases, the possibility of recording their well-resolved electronic spectra confirms their stable S1 electronic state. In contrast, for protonated benzene (BH+)35 and benzenedimer (BBH+), no resolved electronic spectra has been reported so far. Instead, for the BBH+ a broad and structures-less spectrum has been recorded by Jouvet’s group36. Hence, for the BH+ and its homologues based on theoretical studies35,37, a ring-puckering was suggested to play the main role for deactivation of the excited system to the ground, via a conical intersection. This deactivation pathway justifies their short-lived S1 state at least theoretically35. Also, it has been predicted that the photophysics of functionalized aromatic systems (e.g., Phenol38-39, Naphthol40 and Anisole41) is even more complicated than BH+. In fact, a competition between other deactivation mechanisms with the ring puckering have been established in such functionalized systems42. In the present work, we have considered a specific bifunctional aromatic compound; 2Aminobenzaldehyde, abbreviated by 2ABZ. It is an interesting and simple aromatic compound having two proximate functional groups (-CHO and –NH2). 2ABZ plays a key role as an intermediate in the heterocyclic synthesis43-44, especially for quinolines45. However, only rare reports have been devoted either to its neutral structure or its similar systems46-47. Thus, the lack of information on spectroscopy and physical characters of the considered system is noticeable in the literature. Accordingly, the electronic transitions, oscillator strength, protonated-isomers’ relative stability and finally the deactivation pathways of 2ABZH+ will be addressed in our present work. In addition, the hydration of AH+ ions has an essential influence on their structure and dynamics as well as their chemical reactivity18-19, 48-61. However, performing any theoretical study on relaxation mechanism of solvated chromophores has been challenging, since of the large size of the questioned systems. It has been clarified62 that the main effect of solvent arises from the small number of water molecules directly connected to the solute via Hydrogen-bond. Therefore, it is a suitable model to investigate the solute-solvent interactions and also the solvent effects on deactivation mechanism following photoexcitation19,

48-49, 63-64

. There are several different

positions on 2ABZ subject to Hydrogen bond with water molecules. Thus, in addition to its



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individual form, the hydration effect on the nonradiative deactivation pathways of 2ABZH+ will be addressed as well.

a)

b)

c)

d)

Figure 1. Optimized structures of: (a) neutral 2ABZ and numbering outline, (b) 2ABZH+ (N9 isomer, α-form), (c) the CASSCF optimized geometry of the conical intersection between the S0 and S1 states of 2ABZH+, (d) the β-form photoisomer of 2ABZH+. 2- Computational Details All of the calculations in this work were performed by the TURBOMOLE program suit (V 6.3)65-66. The optimized geometry of all systems at the ground and excited states have been determined respectively by the RI-MP267-68 and RI-CC269-70 methods. Also, the cc-pVDZ and the aug-cc-pVDZ71 basis sets have been employed for determination of the optimized geometries and transition energies, respectively. Also, the potential energy curves have been calculated using the aug-cc-pVDZ basis set for the –CHO/-NH3+ groups and the cc-pVDZ for the ring atoms. The CC2 as a method of choice is not the most appropriate case for determination of the PE profiles. Nevertheless, it has been recently established that CC2 results even in the CI regions could be reliable72-79. More recently, Tuna et al.79 have presented substantial results confirming the validity of the CC2 method in describing the CI region at least for the small size molecules. Moreover, in order to perform the geometry optimization of the S1/S0 conical intersection, the CASSCF(6,6) implemented in the Molpro80 2015.1 program has been used. The orbitals included in the active space are depicted in the ESI file (Table S1). Also, the charge distribution were calculated on the basis of natural population analysis (NPA) algorithm72 using TURBOMOLE program. 3 ACS Paragon Plus Environment

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The abbreviations of 2ABZ, 2ABZH+ and 2ABZ-Wn will be employed instead of neutral, protonated and water clusters of 2-aminobenzaldehyde respectively. The numbering model of atoms has been presented in Figure 1.

3. Results and Discussions 3-A. Geometry and Electronic Structures: 2ABZ and 2ABZH+ As discussed before, rare experimental or theoretical study has been devoted to 2aminobenzaldehyde so far. Thus, we have performed the relevant calculations to obtain significant insights on the geometry, electronic transition energies and electronic structures of neutral 2ABZ and then we attend to its protonated analogue. The optimized structures of the neutral and the most stable protonated isomer of 2ABZ have been presented in Figure 1. Also, in Table 1, we have tabulated the selected bond-lengths and bond-angels of the neutral 2ABZ at the ground and the first 1ππ* electronic states. As shown in Figure 1 and Table 1, the optimized structure of 2ABZ is roughly planar and stabilized by a strong intramolecular NH⋯O=C Hydrogen bond (RH⋯O=1.977 Å). According to our calculated results, the aromatic bond lengths have been determined to be 1.397-1.426 Å. In addition, the C7=O8 bond length has been determined to 1.231 Å. Moreover, the C-C bond lengths show the lowest value of alteration in neutral 2ABZ following photoexcitation to the first 1ππ* by 0.001-0.021 Å while other parameters exhibit more alteration. Nevertheless, the excited state hydrogen/proton transfer has not been predicted to occur following the 1ππ* geometry optimization of 2ABZ.

Excited state (1ππ*)

Ground state

cc-pVDZ

aug-cc-pVDZ distances/Å

cc-pVDZ

aug-cc-pVDZ

C1-C2

1.426

1.428

1.450

1.449

C7-O8

1.231

1.239

1.329

1.397

O8-H12

1.977

1.975

1.159

1.652

C2-N9

1.382

1.378

1.411

1.412

N9-H11

1.015

1.012

1.032

1.021

N9-H12

1.019

1.017

1.333

1.044



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angles/deg C1-C2-C3

117.8

117.9

120.3

120.3

C1-C7-H10

113.9

114.5

120.9

123.7

C1-C7-O8

126.1

125.8

120.5

122.2

N9-H12-O8

131.8

130.1

154.6

131.0

dihedral angles/deg O8-C7-C1-C2

0.8

0.5

19.2

5.7

C7-C1-C2-N9

6.8

4.2

8.3

12.4

H12-N9-C2-C3

164.9

169.4

165.9

173.1

O8- H12-N9-C2

24.2

16.8

14.4

38.9

Table 1. Selected equilibrium geometry-parameters of neutral 2ABZ at the ground and the first 1

ππ* excited states. In order to find the most attractive site for protonation of 2ABZ, the MP2 geometry

optimization for the possible protonated isomers has been employed. Obviously, one may expect several protonation sites on the neutral 2ABZ resulting from the various carbon, oxygen, and nitrogen sites. On this basis, we have considered seven protonated isomers for 2ABZ. Evidently in 2ABZ, the amino group as an electron-reach position can be rather attractive for protonation in respect to other sites. In Figure 2, we have presented the ground state relative stabilities of isomers resulted from protonation of 2ABZ. For determination the ground state energetic level of each isomer and the corresponding internal energy arisen from the relevant optimized geometry, we have compared each isomer with the N9 (which has been assigned as the most stable isomer, see Table S2, ESI file). As shown, other isomers have been predicted to be 0.43-1.80 eV less stable than N9 isomer. On the other hand, based on the large internal energy of C1, C4, and C6 isomers, less probability for populating of these type isomers in any experiment could be expected. Thus, we have ignored obtaining more details on these type isomers.


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2.0

1.89 C4

1.82 C6

1.5

Energy/eV

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


1.0

1.09 C1

0.80 C3

0.63 C5

0.5

0.43 O8 0.00 N9

0.0

Protonated Isomers of 2ABZ Figure 2. MP2 calculated results indicating relative stability of different isomers associated with protonation of 2ABZ. From inspection of Figure 2, it is seen that protonation of side groups (-NH2 and -C=O) is more favored than aromatic ring. It can be arisen from larger negative-charge located over nitrogen atom of 2ABZ which makes it more attractive for H+ (see Table S3, ESI file). The natural population analysis (NPA) predicts the charge of -0.88 and -0.69 e (e is atomic unit of charge) respectively on N and O atoms while the local charge predicted for C atoms of the ring has been estimated to be significantly lower than those of O and N (+0.3 to -0.3 e). In addition, the proton affinity of 2ABZ has been determined amount to 9.46 eV (913.04 kJmol-1) which is significantly more than those of Benzaldehyde and Aniline (8.64 and 9.15 eV respectively81). The detailed information on the optimized geometry parameters and xyz coordinates for protonated 2ABZ have been presented in the ESI file (Table S2 and S4). Thus, we have briefly attended to the most stable protonated structure here. As shown in Figure 1, the equilibrium structure of 2ABZH+ (N9 isomer) in its ground electronic state is planar from its benzene ring, stabilized by strong intramolecular NH+⋯O=C Hydrogen bond. We have also determined the vertical and adiabatic transition energies for the S1 and S2 electronic states of neutral and protonated species. Only the lowest lying protonated isomers (i.e., N9, C3, C5, O8) have been considered and we have ignored other isomers owing to the high values of internal energy of ground state. The vertical transitions were calculated based on the ground 6 ACS Paragon Plus Environment


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state optimized geometry while the adiabatic S1−S0 transitions have been determined at the corresponding S1 excited state optimized geometries. To perform an assignment for the lowest lying electronic transitions, the valence orbitals of the neutral and protonated N9 isomer of 2ABZ have been analyzed. The first S1-S0 transition of 2ABZ corresponds to Lumo+5 ← Homo (~83% contribution) and its S2-S0 arises from Lumo+5 ← Homo-2 (~74%). Also, in the most stable protonated isomer of N9, the S1-S0 and S2-S0 have been predicted to originate respectively from Lumo ← Homo-2 (~84%) and Lumo ← Homo-1 (~62%). The frontier molecular orbitals (MOs) of neutral and protonated 2ABZ (the most stable isomer) are depicted in Table 3. As shown, the first and second electronic transitions in neutral 2ABZ can be assigned as 1ππ*, 1nπ* states respectively. Protonated isomers and relative energies/eV C3 0.80 C5 0.63 O8 0.43

Vertical Transition/eV

Vertical Transition/eV

state

state cc-pVDZ

aug-cc-pVDZ

S1(1nπ*)

3.57 (0.0000)

S1(1nπ*)

3.47 (0.0000)

S2(1ππ*)

3.85 (0.2190)

S2(1ππ*)

3.71 (0.2103)

S1(1nπ*)

3.93 (0.0002)

S1(1nπ*)

3.87 (0.0002)

S2( nπ )

4.52 (0.0000)

S2( nπ )

4.40 (0.0001)

S1(1ππ*)

3.10 (0.1060)

S1(1ππ*)

3.00 (0.0947)

S2(1ππ*)

4.85 (0.3159)

S2(1ππ*)

4.75 (0.3101)

S1(1nπ*)

4.10 (0.0004) 4.73 (0.0267)

1

*

1

*

4.84 (0.0229) N9 0.00

S1(1ππ*) 3.62 S2(1ππ*)

5.53 (0.2446)

S2(1ππ*)

S1(1ππ*)

3.97 (0.0764)

S1(1ππ*)

S2(1nπ*)

4.15 (0.0226)

S2(1nπ*)

3.80 (0.0944) Neutral 2aminobenzaldehyde

3.31 3.99 (0.0067)

Table 2. The lowest lying electronic transition energies of neutral and protonated 2ABZ. aThe values presented in the blue and italic form reveal the S1-S0 adiabatic transition energies corrected with ΔZPE (the difference between the zero-pint vibrational energy of the ground and excited states). We have determined the adiabatic S1−S0 transition energy for 2ABZ resulted to 3.31 eV and its corresponding value for 2ABZH+ (N9) to 3.62 eV. To best of our knowledge, there is no spectroscopic study devoted to 2ABZ either in neutral or protonated state to be compared with our 7 ACS Paragon Plus Environment

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theoretical results. Nevertheless, it has been established from the joint theoretical and experimental studies of Jouvet’s group that the CC2 method gives reasonable results for protonated aromatic systems compared to experiment5, 20, 26. The basis set effect has also been examined on adiabatic S1-S0 electronic transition energy, and the average error of CC2 method for the 0-0 band of the S1S0 transition is +0.15 eV. Therefore, according to our CC2 results for electronic transitions of the neutral 2ABZ the four lowest-lying electronic transitions have been predicted to be in the UV range (3.80–5.07eV, 327-245 nm). In protonated 2ABZ system, four electronic transitions are located in the same range (4.10–6.40 eV, 392-194) which are slightly blue-shifted as compared to their neutral homologue.

HOMO-2 (n)

HOMO (π)

LUMO+5 (π*)

HOMO-2 (n)

HOMO-1 (π)

LUMO (π*)

2ABZ

2ABZH+

Table3. Selected HF molecular orbitals of neutral and protonated 2ABZ involved in the first 1ππ* and 1nπ* electronic states.

3-B. Neutral and Protonated 2ABZ-Wn Clusters:

The MP2 method has been employed to investigate the most stable structures for microhydration of selected monomers. Based on the Hydrogen bond interaction resulted from the CO, NH and also the CH bonds of benzene ring, we have considered several positions for locating the water molecule(s). The relevant geometry optimizations were performed to investigate their



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relative stabilities. In addition, the O-H⋯π interaction has been taken into account even though it resulted to the local minima relevant to other interactions.

2ABZ-Wn

W1

W2

W3

W4

W5

Neutral

Protonated

Figure 3. The optimized structures obtained for the most stable clusters of the neutral and protonated 2ABZ.

As shown in Figure 3, the most stable water clusters for 2ABZ-Wn (n=1-5) are resulted from gathering the water molecules around -NH2 moiety. This is the same for almost of the lowest lying water clusters of mono- to penta-hydrated 2ABZ. For the 2ABZ-W1, the only isomer obtained involving the HCO⋯OH2 Hydrogen bond is 5.79 kJmol-1 less stable than its a-2ABZ-W1 analogue (see Table S5, ESI file). Similar to the neutral 2ABZ, the most stable structures for [2ABZ-Wn]H+ complexes have been resulted from locating of water molecule(s) near to the -NH3+ group being stabilized by linear or cyclic H-bonds. In contrast to the neutral clusters, the positive charge of 2ABZH+ leads to forming the Hydrogen bonds between the C-H of benzene ring and water, resulting to the second-order stabilized isomers of [2ABZ-Wn]H+ (see Table S6, ESI file).

3.C. Potential Energy Profiles and Internal Conversions In this section, we present our results on proton transfer at the ground and excited states of 2ABZH+ and then the photophysical character of its clusters with 1-4 water molecules will be discussed. We have determined the minimum energy paths (MEPs) for all selected systems based on the RI-MP2 and RI-CC2 methods respectively for optimization the ground and excited state structures. As mentioned before, the aug-cc-pVDZ basis set has been employed for the atoms 9 ACS Paragon Plus Environment

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involving in the -CHO/-NH3+ groups while the cc-pVDZ basis set has been considered for the rest atoms. I) The Individual 2ABZH+ In Figure 4, we have presented the potential energy profiles along the proton transfer and torsion coordinate of proton acceptor group (the O8-C7-C1-C2 dihedral angle denoted by Φ hereafter) of the 2ABZH+ in the ground and the two lowest excited 1nπ*, 1ππ*states. To distinguish the 1nπ*, 1ππ* excited states we have imposed the Cs symmetry constraint only for the calculations responsible for determination of the MEPs along the PT coordinate (Figure 4-a). Within the Cs symmetry, the nπ* and ππ* states transform with different irreducible representations of a' and a″, respectively. For obtaining the PE curves of the S0 and 1nπ*/1ππ* states, we have performed the relaxed scan. The responsible geometries have been optimized by freezing the reaction coordinates (i.e., a bond length or a dihedral angle) and all of other parameters left free to be optimized. From inspection of Fig. 4-a, it is shown that the PE profiles of the ground and the lowest valence 1nπ* excited states increase with increasing the PT coordinate while the PE profile of the 1

ππ* state is essentially repulsive. The decreasing pattern of the 1ππ* PE profile results to the curve

crossing with the 1nπ* PE sheet at the beginning of reaction coordinate. The ground state proton transfer process from -NH3+ moiety to the O atom of -CHO group is significantly endothermic, needing 0.29 eV excess energy (28.0 kJ.mol-1). It, however, is significantly exothermic in the S1 excited state (releasing more than 1.0 eV following the excited state PT process). The 1ππ* PE profile of the PT process is barrier-free, resulting to the ultrafast process in the 1ππ* excited state. Nevertheless, the obtaining proton transferred species (the last point of Fig. 4a, shown by a star shape), have been predicted to be at least 3.90 eV less stable than the global minimum of the 2ABZH+ (α-form, see Fig. 4-a and insets). This latter point indicates to extreme instability of proton transferred structure on the potential energy surface (star shape). However, it has been previously shown that the twisting coordinate of the proton acceptor group, may play an important role for nonradiative deactivation via a conical intersection60. To examine this suggestion, we have obtained the 1ππ* and S0 energy profiles along the Φ twisting coordinate. The results have been depicted in Figure 4-b. Starting from the last point of the PT reaction coordinate (star shape, Φ ~0°), we have calculated the MEPs of the 1ππ* and S0 states up to the Φ ~165°. It is noteworthy that for preventing the proton transfer in the 1ππ* and S0 optimization, it is needed to freeze an additional O-H coordinate in the few points of the beginning of Figure 4-b. As shown, the S0(S0) 10 ACS Paragon Plus Environment


curve exhibits a large barrier along the torsion coordinate of the S0 state (~1.58 eV, ~152 kJmol-1) while the 1ππ* PE profile of the enol form with respect to the torsion process is totally barrier-less. Also, it is seen that proceeding the system along the Φ-twisting coordinate to 105° results to a large stabilization of the 1ππ* state by 1.33 eV compared to the first point of 1ππ* curve (star shape). On the other hand, the ground state PE sheet (S0S1 shown by circles in Fig. 4-b) substantially increases to approach the 1ππ* energy (triangles) from below resulting in the 1ππ*-S0 intersection roughly at Φ =105° (see Figure 1-c, the CASSCF optimized structure of CI). On this region, the S0/1ππ* potential energy profiles cross one another and obtain a conical intersection in multidimensional feature. This conical intersection can be responsible for ultrafast non-radiative deactivation of 2ABZH+ following photoexcitation to the first 1ππ* excited state in the gas phase. Moreover, beyond the conical intersection by following this trend until the region of Φ =165°, another protonated isomer of 2ABZH+ will be obtained at the final positions of reaction coordinate in panel b (i.e., denoted by the β-form of 2ABZH+, see Figure 1). The β-form of 2ABZH+ photoisomer, produced following deactivation of 2ABZH+, has been determined to be involving 0.58 eV of internal energy rather than the α-form global minimum of 2ABZH+. According to Sobolewski et al. (see Ref.82), it has been established that a molecular system which fulfills following crucial circumstances can be suggested as a photoswitchable system: 1) A light induced equilibrium between two different isomers having various UV-vis absorption wavelengths (i.e., separated absorptions). 2) A large barrier between the ground state PE profiles of two photoisomers for hindering the thermal conversion process between two different forms. 3) A conical intersection between the excited/ ground states to link two different photoisomer forms. Therefore, inspection of Figure 4, indicates that protonated 2ABZ accomplishes the mentioned criteria thus, it can be suggested as a good choice for ionic photoswitchable system82. In the other word, the 2ABZ system in the solvent and the presence of low pH following protonation acts as a photoswitch system. Particularly, the α- and β-forms of 2ABZH+ have been predicted to have essentially separated electronic absorption spectra. Although the α-form absorbs the UV light (i.e., 276 nm, 4.5 eV), it has been predicted that its corresponding photoisomer 11 ACS Paragon Plus Environment

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absorbs in the visible region at 416 nm (2.98 eV). Finally, the large barrier of the ground state MEP hinders the thermal conversion of these two isomers while a conical intersection facilitates the photoisomer equilibrium following photoexcitation.

5

1 4

Energy/eV

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


ππ*

nπ*

1

3

λ=276 nm λ=517 nm

2

1

S0(ππ*) S0(S0)

0 -0.4

-0.2

0.0 0.2 NH-OH/Å

0.4

30

60

90

120

(O8-C7-C1-C2)/deg

150

Figure 4: The potential energy profiles of the S0 (circles) and the S1 (triangles) states as the function of proton transfer coordinate (a) and twisting coordinate (b). The star stands for the last point of the 1ππ* state along PT coordinate. II) Micro-hydration Effect on Photophysics of 2ABZH+ The Hydrogen-bonded networks of water molecules biomolecular systems due to hydration play an important role on their physical properties. Upon to hydration, water molecules bound to the surface of the peptides and the size of water networks get larger with increasing hydration level62. Because of the large size of chromophore and solvent, it is not practical to perform ab initio studies in the presence of the whole solvent for modeling the solvent effect. More importantly, the photoswitchability of 2ABZ originates from protonation, consequently, the solvent effect on its photophysical character is quite essential. Thus, we have studied here only the microhydration effect on photophysical nature of 2ABZH+.



The CC2 energy profiles of [2ABZ-Wn]H+, n=1-4, have been determined along the minimumenergy paths (MEPs) for proton transfer and torsion coordinate of [2ABZ-Wn]H+ in the S0 and the 1

nπ*, 1ππ* excited states. The MEPs have been obtained based on the same attitude that we have

already discussed in the previous section for 2ABZH+. The results have been presented in Figure 5. From investigation of results and also comparing with Figure 4, the following remarkable points can be presented:

i.

The 1ππ* MEP of the PT process is barrier-free in 2ABZH+ which facilitates the PT processes in the 1ππ* excited state. Nevertheless, in [2ABZ-W1-4]H+, there is a small barrier from 0.04-0.25 eV in the middle of the 1ππ* MEP along the PT process. The barrier is not so large to hinder the ESIPT. Thus, it may only affect the dynamics rate of the PT process following the excitation of solvated systems.

ii.

In the twist coordinate of [2ABZ-W1-4]H+ similar to 2ABZH+, there is a conical intersection between 1ππ* and S0 PE profile being responsible for nonradiative deactivation of [2ABZ-Wn]H+ systems via ultrafast internal conversion. Inspection of Figure 4 and 5 shows that the conical intersection in the bare 2ABZH+ appears around Φ =105° while it locates roughly at Φ=60° and 45°, respectively, in [2ABZ-W3]H+ and [2ABZ-W4]H+. Therefore, microhydration facilitates the nonradiative deactivation by shortening the pathway which connects the last point of proton transferred coordinate to the CI region.


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2) 5

5

4

4

Energy (eV)

Energy (eV)

1)

3

3

2

2

1

1

0 -0.4

-0.2

0.0

0.2

0.40

30

/Å

60

90

/deg

120

0 -0.4

150

0.0

0.2

0.4 0

30

60

90

120

150

/deg

4) 5

4

4

Energy (eV)

5

3

2

3

2

1

1

0 -0.4

-0.2

/Å

3)

Energy (eV)

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


-0.2

0.0

0.2

NH-OH/Å

0.4 0

10

20

30

deg

40

0 -0.4

50

-0.2

0.0

0.2

/Å

0.4 0

15

30

45

60

75

/deg

Figure 5: The potential energy profiles of the S0 (circles) and S1 states (triangles) as the function of proton transfer and twisting coordinates for [2ABZ-W1-4]H+, respectively.

iii.

The ground state intramolecular proton-transfer (GIPT) from -NH3+ to the carbonyl group (-CHO) has been predicted to be endothermic needing 0.29 eV in the bare 2ABZH+. Thus, it is not favored even in the bare system. On the other hand, the main effect of hydration on MEP is considerably increasing the required energy for such process following the hydration in the ground state. The energetic values required for GIPT in the mono- to tetrahydrated system (2ABZW1-4]H+) have been predicted to be 0.57-0.97 eV. Therefore, 14 ACS Paragon Plus Environment


microhydration stabilizes the positive charge on the -NH3+ moiety at the ground state. As a result, it can be concluded that in solvated 2ABZH+, the excess proton prefers staying on the amino group rather than transferring to solvent.

Conclusion The electronic transition energies and potential energy profiles of protonated 2aminobenzaldehyde (2ABZ) in their individual and microhydrated forms with 1-4 water molecules have been determined. The additional proton prefers the amino group producing the N9-2ABZH+ isomer being 0.43 eV (41.5 kJmol-1) more stable than other isomers. The MEPs for the first 1ππ* state along the PT coordinate has been determined to be barrier-less indicating to an ultrafast ESIPT dynamics. Also, a conical intersection along the twisting coordinate of aldehyde group following the PT process directs either the excited system to the ground state by back proton transfer resulting in the global minimum of 2ABZH+ or it leads the excited system to continue the twisting coordinate resulting in the β-form of 2ABZH+. These results bring insights into possibility of photoswitching in protonated system. In addition, the hydration effect on deactivation mechanism of 2ABZH+ for 1-4 water molecules has been investigated. It has been shown that even in presence of water, the nonradiative deactivation mechanism of 2ABZH+ is essentially preserved. From spectroscopy point of view, it has been predicted that the strong blue shift effect on the first 1

ππ* transition energies is the most important result of protonation. In overall, 2ABZ is a suitable

choice for studding protonation and micro hydration effects simultaneously, owing to strongly selective protonation sites and also the large energetic gap between protonated isomers in contrast to its benzaldehyde analogue20. Thus, it would be interesting if one studies that by an appropriate experimental setup.

Acknowledgment:

The research council of University of Isfahan is kindly appreciated for financial supports. The use of computing facility cluster GMPCS of the LUMAT federation (FR LUMAT2764) for partial performance of our calculations is kindly appreciated.


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