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Excited State Proton Transfer and Deactivation Mechanism of 2#(4#-Amino-2#-hydroxyphenyl)#1H#imidazo[4,5#c]pyridine and its Analogues: A theoretical study Reza Omidyan, and Maryam Iravani J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b12122 • Publication Date (Web): 26 Jan 2016 Downloaded from http://pubs.acs.org on January 27, 2016

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The Journal of Physical Chemistry

Excited State Proton Transfer and Deactivation Mechanism of 2‑ ‑(4′-Amino-2′hydroxyphenyl)‑ ‑1H‑ ‑imidazo-[4,5‑ ‑c]pyridine and its Analogues: A Theoretical Study Reza Omidyan* and Maryam Iravani Department of Chemistry, University of Isfahan, 81746-73441, Isfahan, Iran

Abstract In the present study, the results of comprehensive theoretical exploration on the nonradiative relaxation of three hydroxyphenyl-imidazole based organic compounds; (abbreviated by AHP, HPIP and HPBI), in the gas phase are presented. Having small structural differences, the selected systems are common with excited state intramolecular proton transfer process (ESIPT). The ground and S1 excited state potential energy profiles of titled systems have been determined based on the RI-MP2 and RI-CC2 methods, and the effect of small structural distinctions on their photophysical characters will be extensively addressed. Although, in the presence of solvent, high fluorescence quantum yield is another character of AHP and HPBI, owing to accessible conical intersections between S1/S0 state potential energy profiles of both systems, non-radiative relaxation can be proposed as the most important feature of these two systems in gas phase. These conical intersections (CIs) are responsible for ultrafast deactivation of excited systems via internal conversions to the ground state. The nonradiative deactivation mechanism determined in this work, deals with the remarkable photostability of the AHP and HPIP molecules.

1-Introduction Excited state hydrogen/proton transfer (ESHT/PT) and electron transfer are two important phenomena in biological and photochemical processes causing dual emission1-8. Following photoexcitation, the ESPT/HT happens from a protic acid to basic group, occurring by an intrarmolecular hydrogen bond, resulting to produce phototautomer in the excited state1-4. The intramolecular charge transfer (ICT) is another character for some organic compounds occurring when charge transfers from charge-donor to accepter5-8. Based on the experimental studies of *

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

1 ACS Paragon Plus Environment


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Krishnamoorthy and coworkers8-21, in the presence of different solvents, it has been established that ESIPT systems may show ICT emissions. Nevertheless, the inherent photophysical characters of such important compounds are still obscure. Whether the fluorescent nature of such systems depends on the presence of solvent or not, is a basic question, without clear response so far8-21. Fortunately, the photophysics of organic systems can be well studied nowadays by the aim of advanced ab initio methods, on the basis of potential energy surfaces (PESs)22-26. These PESs, directly govern the relaxation mechanism of molecular systems.

The conical intersections

between potential energy surfaces27 have been recognized as crucial to enabling population transfer between different electronic states

27-28

. So far, it has been well-known that nuclear

dynamics driving the CIs, capable the molecular systems for many applications in sunscreens, photochromic systems and photostabilizers

29, 27, 30-31

. Photostability can either be an intrinsic

character of chromophore or related to intermolecular interactions with solvent or other parts of molecule32-33. The dynamics of excited-system relaxations of large molecules to the ground is a complicated subject, especially in the presence of solvent. Most probably, the solvents play the role of quencher or on the other hand, it can block the quenching processes and lead the excited system to act as an emitter state. In preset work, we have selected few organic molecules having strongly emission character in the solvent phase and will explore their relaxation mechanism in the vacuum, via accurate ab initio methods. We briefly introduce our selected systems: 2‑(4′-Amino-2′-hydroxyphenyl)‑1H‑imidazo-[4,5‑c]pyridine, abbreviated by AHP here after, has been introduced as an ICT emitter following ESIPT. Krishnamoorthy18 observed the dual fluorescence from AHP in several solvents, such as cyclopenthan, buthanol and other alcohols. The system shows two intense fluorescence peak in the UV-vis region, the shorter wavelength has been assigned to the normal emission of the trans- AHP and the longer wavelength has been assigned to be related to keto tautomer18. Also, Fasani et al.34, for the first time observed the ICT on [2-(4′-aminophenyl)-1H-imidazo[4,5-c]- pyridine], which is quite a close structure to AHP. However, in both of aprotic and protic solvents the ESIPT process suppresses the ICT process in AHP system18. 2-(2′-hydroxyphenyl)-1H-imidazo[4,5-c]pyridine, abbreviated by HPIP; (the second structure considered in this study), is another homologue of AHP, in which the substituted -NH2 on the 2 ACS Paragon Plus Environment

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hydroxyphenyle part, is replaced with hydrogen atom. The HPIP is also well-known as a dual emitting compound in solvent phase35. Based on Behera et al.18, the HPIP displays twisted ICT (TICT) emission in protic solvents, resulting from of hydrogen bonding of protic solvents with >NH and nitrogen of imidazole which twists the acceptor (imidazopyridine) part. 2-(2-Hydroxyphenyl)-1H-benzimidazole, abbreviated by HPBI, is the last system considered in this work. Comprehensive theoretical study on the ESIPT of the HPBI and its analogues have been reported by Chipem et al.9 at the CIS and DFT level of theory. From their experimental and theoretical investigations, it has been remarked that HPBI and its substituted analogues at least have two rotamer (cis- and trans-enol forms). Also, it has been clarified that excited state proton transfer to the >N, leads to producing a keto tautomer. Most importantly, Chipem et al. proposed that twisting of the tautomer can play the role of the non-radiative channels for these type structures. More accurately, we will explore the potential energy curves of these systems (AHP, HPIP, HPBI) along the PT and twisting reaction coordinates, by the aim of RI-MP2 and RI-CC2 methods, which are more convenient for considering the electron correlations rather than DFT or CIS methods. Thus, we will report the PE functions obtained by the RI-CC2 method for ESIPT and detail mechanisms on the nonradiative deactivations of AHP, HPIP and HPBI systems. We discuss and explain the geometry and electronic structures of titled compounds along with their transition energies and oscillator strengths. Then, the photophysical characters of these systems will be extensively interpreted. The RI-CC2 method has been selected because it gives reasonable results for medium size organic molecules for a moderate computational time36-40.

2-Computational Details: The RI-MP2/RI-CC2 calculations have been performed using TURBOMOLE program suit (V 6.3)41-42, making use of the resolution-of-identity (RI) approximation43 for the evaluation of electron repulsion integrals. The equilibrium geometry and dipole moment of all systems at the ground state has been determined at the RI-MP2 level (Moller-Plesset second order perturbation theory)44-45. Excitation energies, equilibrium geometry, and dipole moment of the lowest excited singlet states have been determined at the RI-CC2 (the second-order approximate coupled-cluster method)46-47.



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(a)

(c) 12 13 11

10

15

1

9

4 2

89

14

16

19

17

3

87

55

(d)

18

6

(e)

(b)

(f)

Figure 1, MP2 optimized structures and numbering pattern of a) AHP cis-enol , b) AHP transenol, c)HPIP cis -enol d) HPIP, trans enol, e) HPBI, cis-enol f) HPBI, trans enol

The Dunning’s correlation consistent split-valence double-ζ basis set (cc-pVDZ)47-48 and the augmented cc-pVDZ, by diffuse functions on all atoms (aug-cc-pVDZ)47, have been employed for determination of electronic transition energies and oscillator strength, while the potential energy profiles have been calculated using cc-pVDZ basis function. Additionally, the abbreviations of AHP, HPIP and HPBI will be used indicating to 2-(4′-Amino2′-hydroxyphenyl)-1H-imidazo-[4,5-c]pyridine,

2-(2’-hydroxyphenyl)-1H-imidazo[4,5-

c]pyridine and 2-(2-Hydroxyphenyl)-1H-benzimidazole respectively.

3-Results and Discussions: 3-1-Ground State Structures and Excitations Energies,


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The MP2 geometry optimization shows planar structures for AHP, HPIP and HPBI. The optimized structures of two rotamers for each system (cis and trans enol forms) have been presented in Figure 1. Further details on analyzing of geometry parameters have been presented in ESI file. As shown in Figure 1, for enol form of all three compounds, two equilibrium geometries, (cis and trans) have been predicted. The calculations predict that cis-enol structure of AHP, HPIP, and HPBI is more stable by 0.28, 0.26, and 0.29 eV respectively; (~ 29 kJmol-1). In enol structure, the dihedral angle between imidazole and phenyl ring has been determined to be 0.0° and 0.7° respectively for HPIP/HPBI and AHP, indicating to planar structures for all systems at ground state. There is a strong O-H….N hydrogen bond which is necessary for the formation of excited state intermolecular proton transfer. Also, the RI-MP2 calculation of enol structure reveals that H…N bond is 1.754 Ǻ, 1.754 Ǻ, 1.744 Ǻ respectively in the AHP, HPIP and HPBI (see ESI file). Furthermore, the O-H….N bond angle also, has been determined to be 148.4°, 148.0°, and 148.6° respectively in AHP, HPIP and HPBI. In addition, in all compounds, the C3-N4 and C3-N5 bonds in imidazole ring lie between 1.34 Ǻ and 1.38 Ǻ respectively. Also, the N5-H6 of imidazole ring has been estimated to 1.013 Ǻ for all molecules. Moreover, the bond length of C2-C3 for all compounds is similar to each with the value of ~1.460 Ǻ. We have determined the lowest transition energies of four singlet excited states of AHP, HPIP and HPBI. The results have been tabulated in Table 1. As shown, the S1-S0 electronic transition energy of AHP has been determined to be 4.19 eV and 4.06 eV at the RI-CC2 level respectively with cc-pVDZ and aug-cc-pVDZ basis functions. Although there is no gas phase electronic spectrum to be compared with our theoretical results, the only experimental spectrum is related to the work of Behera et al.18, where, they have reported the maximum wavelength of UV absorption of AHP to 296 nm (4.18 eV), recorded in dioxane solvent, which is in the excellent agreement with our RI-CC2/cc-pVDZ result.

AHP (cis)

AHP (trans)



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State

cc-pVDZ

S0

Aug-cc-pVDZ -

Dipole Moment (Debye)

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cc-pVDZ

5.81

Aug-cc-pVDZ -

Dipole Moment (Debye) 8.32

S1 (1ππ*)

4.19 (0.6327)

4.06

7.28

4.42 (0.5616)

4.24

8.94

S2 (1ππ*)

4.72 (0.2446)

4.52

8.11

4.73 (0.3219)

4.55

8.45

S3 (1σπ*)

4.97 (0.0016)

4.63

4.13

4.93 (0.0006)

4.59

3.70

S4 (1ππ*)

5.17 (0.0010)

4.85

6.80

5.13 (0.0025)

4.91

7.07

HPIP (cis)

HPIP (trans)

-

S0

4.85

-

6.91

S1(1ππ*)

4.17 (0.3541)

4.07

5.12

4.51 (0.5145)

4.37

6.64

S2 (1ππ*)

4.87 (0.2372)

4.70

5.25

4.81 (0.1239)

4.67

6.57

S3 (1σπ*)

4.90 (0.0081)

4.77

5.71

4.84 (0.0006)

4.75

3.98

S4(1ππ*)

5.24 (0.0067)

5.04

4.64

5.07 (0.0140)

4.95

5.02

S0

HPBI (cis) -

HPBI (trans) -

3.79

4.62

S1 (1ππ*)

4.16 (0.4439)

4.05

2.81

4.41 (0.6315)

4.27

4.51

S2 (1ππ*)

4.73 (0.1329)

4.56

3.95

4.75 (0.0341)

4.61

4.36

S3 (1ππ*)

5.12 (0.0006)

4.90

5.53

4.87 (0.1011)

4.71

4.31

S4 (1ππ*)

5.67 (0.2274)

4.92

4.36

5.48 (0.0011)

4.90

2.43

Table 1, The four lowest singlet transitions of two enol forms (cis and trans) of AHP, HPIP and HPBI systems, determined at the RI-CC2 level of theory. The values in parenthesis represent the oscillator strengths.


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In Table 1, we have also presented the first four singlet transitions of HPIP and HPBI as well. As shown, the first electronic transitions of HPIP, HPBI systems are not only close to each other but also they are quite near to that of AHP (4.16-4.17eV at RI-CC2/cc-pVDZ and 4.07- 4.05 eV at RI-CC2/aug-cc-pVDZ level of theory ). The UV electronic spectra of HPIP and HPBI were recorded by Krishnamoorthy’s group9, 35. The first maximum bands of HPIP and HPBI systems have been reported to 328 nm35(3.80 eV), and 332 nm9 (3.73 eV) respectively. Both experimental values are comparable with our theoretical results (Table 1) with an approximate error of +0.25 eV. The error can be related either to overestimation of RI-CC2 or it can be related to distinctions between solvent and gas phase nature of experimental and theoretical results respectively. In addition, from our RI-CC2 results, it has been predicted that S1-S0 electronic transitions of all three systems (AHP, HPIP and HPBI) corresponds mostly to HOMO-LUMO, electronic transition. Also, it has been predicted that S2-S0 transition arises from MOMO-1-LUMO and HOMO-LUMO+1 single electronic transition. In Figure 2, the frontier MOs with most important contributions in the first two singlet electronic transitions have been presented. As seen, both excited S1, S2 states of all three compounds can be assigned to 1ππ* nature. Moreover, the first two singlet transition energies of trans-enol form of all titled compounds; (AHP, HPIP and HPBI), have been determined base on the RI-CC2 method with two basis sets. The results have been tabulated in Table 2. As shown the S1-S0 transition of trans enol lies +0.20-0.30 eV higher than that of cis-enol form for all compounds, with slightly larger oscillator strengths. Nevertheless, the nature of transition for both S1 and S2 excited states have been predicted to be the 1ππ* state. Moreover, we have determined the ground and excited state dipole moment of the cis and trans forms of AHP, HPIP and HPBI (see Table 1). As shown, the ground state of cis conformers of all three compounds is moderately polar (5.81-3.79 D). Nevertheless, the ground state polarity of trans forms of all three compounds has been predicted to be greater than that of cis conformer (8.32-4.62 D). Also, the S1-S0 electronic transition for AHP and HPIP is along with slightly increasing of dipole moment while that is along with slightly decreasing in HPBI.



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HOMO

AHP HOMO-1

HOMO-2

LUMO

LUMO+1

LUMO+2

HOMO

HPIP HOMO-1

HOMO-2

LUMO

LUMO+1

LUMO+2

HOMO

HPBI HOMO-1

HOMO-2

LUMO

LUMO+1

LUMO+2

Figure 2: Selected valence molecular orbitals of the of AHP, HPIP and HPBI systems (cis-enol forms). Only those MOs having most important contributions on the S1 to S4 excited states have been presented. The significant values of the ground state dipole moments for studied compounds indicate to the significant interaction of these systems with polar solvents, resulting to the spectral shifts and perhaps photophysical alterations in the presence of polar solvents. Also, the strong emission of AHP and HPBI in solvents, results from the polar character of low-lying excited states. 3-Potential Energy Profiles and Conical Intersections:


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Although the spectroscopic properties of AHP, HPIP and HPBI have been extensively studied in different solvents, there is no accurate and comprehensive study dealing with the optical characters of these systems, in their individual form (i.e. in the vacuum or gas phase). Thus, we have determined and explored the potential energy curves of AHP, HPIP and HPBI at the RI-MP2 and RI-CC2 computational methods. Furthermore, the credibility of the RI-CC2, as a single reference method, for determination of excited state potential energy profiles, has been investigated49 by comparing the RI-CC2 results with accurate CASPT2 and MR-AQCC data. It has been established that RI-CC2 predicts qualitatively reliable energy profiles and, its results are reliable for the qualitative determination of PE curves24, 50-55.

I) AHP In Figure 3, the potential energy profiles calculated along the minimum-energy path for proton transfer (O12-H13 stretching coordinate) and ring twisting coordinate, the N5-C3-C2-C1 dihedral angle of the enol and N4-C3-C2-C1 dihedral angle of keto tautomer of AHP in the S0 and S1 electronic states are shown. The potential curves have been determined based on the RIMP2/RI-CC2 methods for the S0 and S1 states at their optimized geometries respectively. For each point of PECs, only the bond length or dihedral angle that was defined as reaction coordinate became frozen and all of other parameters left free to be optimized. Because, the PE curves of ground state and excited state have been determined respectively on their corresponding optimized geometries, their PE curves have been denoted to the S0 (S0) and S1 (S1) respectively. Also, the S0(S1) and S1 (S0) indicate the energy of the S0 and S1 states calculated along the reaction path optimized in the S1 and S0 state respectively. In Figure 3-a, the potential energy curves for rotation of phenyl part of AHP around C2-C3 bond (i.e. in respect to the plane of imidazole ring), have been presented. As shown, the S0

(S0)

curve

exhibits a barrier for ring torsion of the enol form in the S0 state (~0.52 eV, ~50 kJmol-1), and the energy profile of the S1 state indicates a larger barrier (~0.90 eV, ~87 kJmol-1) in the S1 state. In order to prevent the proton transfer in the S1 optimization of enol form, it was mandatory to freeze an additional O-H coordinate in the few points at the beginning of reaction coordinate of Figure 3-a. The last point of reaction coordinate in Fig. 3-a; (i.e. dihedral angle of N5-C3-C29 ACS Paragon Plus Environment


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C1=0°) corresponds to another local minimum of AHP, introduced as the trans-enol form, in previous sections, which is roughly 29 kJmol-1 less stable than cis conformer (see Figure 1-b). However, from PE profile of Figure 3-a, it can be remarked that cis-trans transformation of AHP is neither favored in the ground nor in the excited states due to the large barriers existing in the middle of corresponding reaction coordinates. The energy profiles along the hydrogen-transfer reaction path are displayed in Figure 3-b. As shown, the proton transfer process from O-H group to the N4 atom of imidazole ring is endothermic in the ground state (requiring 0.61 eV, 59.0 kJ.mol-1 energy), while it is exothermic in the S1 excited state. The S1 PE profile of the PT process is quite barrier free, which facilitates the PT processes in the S1 excited state. Nevertheless, the planar keto-type S1 structure lies 3.52 eV above the global minimum of the ground state (Figure 3-b, last point of the reaction coordinate). As reported by Behra et al.56, the AHP exhibits normal and tautomer emissions in a protic and aprotic solvents due to ESIPT. It means that in the presence of protic and aprotic solvents, the S1 keto-form of AHP is stable, thus, fluorescence plays the most important deactivation channel of enol and keto type AHP in solvent. The stability of S1 keto type of AHP in protic solvents can be arising from the significant interaction of polar solvents with this structure, owing to its moderate dipole moment of 5.14 D. However, in the gas phase, and for individual AHP molecule, it is possible that another reaction coordinate, beyond the ESIPT, directs the excited keto type system to the ground, via a conical intersection. As mentioned above, the S1 state of individual keto structure is unstable with respect to out-of-plane distortions. The S1 geometry optimization without planarity constraint results in twisting and leads to the conical intersection of the S1 with the S0 surface. Additionally, In Figure 3-b, the S1(S1) and S0(S1) potential energy profiles (redcolored solid and dash curves respectively), determined along the PT reaction coordinate, without planarity constraint. As shown the S1 profile at the beginning of reaction path consists with blue profile (planarity constraint), while it is ~0.30 eV stabilized at the end of reaction path, where proton entirely transferred. This stabilization arises from twisting of phenoxy ring around the C2-C3 bond near to 40°. Also a small out-of-plane deformation of imidazole ring at the C3 region is another consequence of geometry optimization of S1 state without planarity constraint. In order to examine the nature of the potential-energy curves with respect to ring twisting, we have determined the S1, S0 energy profiles along φ= N4-C3-C2-C1 dihedral angle. 10 ACS Paragon Plus Environment

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The results have been depicted in Figurer 3-c. Starting from the last point of PT reaction coordinate (φ =40°), we have determined the PE curves of the S1 and S0 states to φ~90°. It is seen (Figure 3-c) that pure twisting around the C2-C3 bond (φ≈60°), is accompanied by the stabilization of the S1 state by about 0.45 eV with respect to the planar structure. In addition to decreasing trend of S1 PE sheet along the twisting reaction coordinate, the S0 energy (circles) increases to approach the S1 energy (triangles) from below, resulting to the S1-S0 intersection at φ=N4-C3-C2-C1~ 65°. In a multidimensional picture, S1-S0 curve crossing in Figure 3-c develops to a conical intersection (CI). This conical intersection can be responsible for ultrafast nonradiative relaxation of AHP, after photoexcitation to the S1 (1ππ*) excited state in the gas phase. In addition, after the conical intersection, a back proton transfer from N5-H6 to the O12 oxygen atom in the region of φ>120° is the most important reaction, resulting to produce the global enol form of AHP at the end of reaction coordinate of panel c.

5

(a)

(c)

(b) S1(S0)

4 S1(S1)

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


3

2

1 S0(S1) S0(S0)

0 45

90

135

1.0

1.2

1.4

1.6

O-H/Å

N5C3C2C1 /deg


40

50

60

70

N4C3C2C1/deg


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Figure 3: CC2/cc-pVDZ energy profiles of the S0 state (circles) and S1 state (triangles) as the function of the torsional reaction path (a, c) and the hydrogen transfer reaction path (b). Solid lines represent the minimum energy profiles of reaction paths determined in the same electronic state (S0(S0), S1 (S1),…), while the dashed lines (S0(S1)), stands for the energy profile of ground state determined based on the optimized complementary electronic S1(ππ*) state. In panel b, the red colored curves represent the S1(S1) and S0(S1) PE profiles determined without considering the planarity constraint.

II) HPIP In Figure 4, the RI-CC2 PE profiles calculated along the minimum-energy path for proton transfer and ring twisting dihedral angle of N5-C3-C2-C1of the enol and N4-C3-C2-C1 of keto tautomers in the S0 and S1 states of HPIP system are shown. The potential curves have been determined in the same trend as we have already discussed in previous section for AHP. In contrary to the AHP, there is a small barrier of 0.13 eV in the middle of the S1 PE profile of HPIP along PT process. Although, the existing barrier is not so large that hinders the PT process, the dynamics rate of the PT process in HPIP shall be affected by this barrier, resulting to the lower enol-keto transformation rate in HPIP rather than AHP. From unconstraint S1 PE curve along PT coordinate of HPIP (i.e., red colored curves, for which, no planarity for PT reaction has been imposed) and consequently the blue curve MEP of panel c, it can be concluded that the keto-type photo product of HPIP, resulting from ESIPT, is not stable. The S1 PE profile of keto type HPIP along the twisting reaction coordinate, (N4-C3-C2C1dihedral angle), indicates that a conical intersection connects the S1/S0 potential energy curves at the region of φ around 80°. This CI can be responsible for ultrafast deactivation of HPIP to the ground state via internal conversion. Because the RI-CC2 method is not adequate to predict the real position of conical intersections, it can be only qualitatively reliable. It should be emphasized that HPIP compound is an effective fluorescent species in the protic and aprotic solvents. Nevertheless there is no report on the optical activity of this compound in absence of any solvent. Thus, from our ab initio results, it can be remarked that, the conical intersection between S1/S0 potential energy surfaces, plays the most important rule in the nonradiative deactivation of individual HPIP, as well as AHP via ultrafast internal conversion of the excited system to the ground state.


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(a)

(c)

(b)

5

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


3

2

1

0 0

45

90

135

N5C3C2C1/deg

1.0

1.2

1.4

O-H/Å

45

1.6

60

N4C3C2C1/deg

Figure 4: CC2/cc-pVDZ energy profiles of the S0 and S1 states as a function of the torsional reaction path (a, c) and the hydrogen transfer reaction path (b) of HPIP system. Moreover, both the ground and S1 state potential energy profiles of enol form of HPIP along the C2-C3 bond, show barriers with 0.51 eV and 1.15 eV high respectively. Thus there is no significant possibility for twisting of enol form of HPIP around C2-C3 bond in the gas phase since of the large barrier along the cis-trans transformation coordinate.

III) HPBI In Figure 5, the minimum potential energy curves calculated along the proton transfer and ring twisting dihedral angle of N5-C3-C2-C1 of the enol and the N4-C3-C2-C1 dihedral angle of keto tautomers in the S0 and S1 states of HPBI system are shown (full curves). Inspecting of Figure 5, there are few important points relevant to photophysics of HPBI:



1-From Figure 5-a, it is seen that similar to AHP and HPIP (in enol form), the torsion of phenolic part along the C2-C3 is neither favorable in ground nor in the excited state, since of the existing large barriers. 2-The S1-S0 transition energy in the FC region of HPBI is quite similar to AHP and HPIP. Nevertheless, the S1 PE profile of HPBI along the PT reaction coordinate (i.e. from OH to N4 nitrogen) is along with a barrier of 0.14 eV high (13.5 kJmol-1), in the same quality with that of HPIP, indicating that ESIPT process should take place with the same dynamics rate in these two systems (i.e. HPIP and HPBI). 3-Inspite of AHP and HPIP, the S1 proton transfer in HPBI is neither spontaneously accompanied with a significant torsion of proton acceptor moiety along C2-C3 bond, nor that is along with an out-of-plane deformation from C3 carbon cite. Thus the S1 PE profile for PT process of HPBI (Figure 5-b) has been calculated without any planarity constraint. As shown in Figure 5, the border region between panel b and c, is equivalent to the end of PT process and beginning of twisting reaction coordinate. However, the right side of panel b is a local minimum

(a)

(b)

(c)

5

4

Energy (ev)

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3

2

1

0 0

45

90

135

N5C10C15C1 /deg

1.0

1.2

1.4

1.6

1.8

O-H/Å


45

90

135

N4C3C2C1/ deg

180

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Figure 5: CC2/cc-pVDZ energy profiles of the S0 and S1 states of HPBI as function of the torsional reaction path (a, c) and the hydrogen transfer reaction path (b). at S1 state. Thus, there is a large possibility for fluorescent emission of HPBI keto system in gas phase. 4-The most important distinction between photophysical behavior of HPBI and two earlier compounds (AHP, HPIP), can be concluded from panel c of Figure 5. As shown, the S1 PE profile of keto type HPBI along ring twisting around C2-C3 (dihedral angle of N4-C3-C2-C1), shows a small barrier of ~0.10 eV beyond a flat trend at the beginning of reaction coordinate (i.e. at the φ~70°). Nevertheless, the S1 PE profile shows a decreasing pattern after φ~70°, which exhibits the smallest energetic gap with S0 state at φ~90°. Nevertheless, the S1-S0 gap is so large (~0.74 eV, 70 kJmol-1) to be adequate for preventing the electronic coupling between these two electronic states, indicating to low possibility of non-radiative relaxation of HPBI in its keto type. However, in contrast to previous two molecular systems, there is no conical intersection between S1/S0 PE profiles of HPBI system, resulting to quite different photophysical character of HPBI compared to AHP and HPIP systems. Finally, it can be remarked that main-frame structure of proton acceptor part of such molecules; (i.e. benzoimidazole moiety), strongly affects their photophysical trends and optical activities. Nevertheless, a more comprehensive potential energy exploring is required to answer an essential question; what is the effect of position and number of substituted nitrogen(s) on photophysical character of these type systems?. Certainly, additional nitrogen or another hetroatoms may strongly affect the PE profiles, electronic couplings and relaxation dynamics of these type molecular systems.

4-Conclusions: The accurate RI-MP2/RI-CC2 methods have been employed to determine the electronic transition energies as well as the ground and excited state potential energy profiles of few imidazole based organic compounds; AHP, HPIP and HPBI, having the common excited state proton transfer characters. The considered compounds are very close in the main farm structures,



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with only small differences. The first electronic transitions of all three compounds, having 1ππ* nature, has been predicted to lie in very close range, but with different oscillator strengths. In contrary, the photophysics of these systems has been predicted to be significantly different. The S1 PT profile of AHP, has been determined to be barrier-less indicating to a fast dynamics mechanism. Nevertheless, our RI-CC2 results predict a barrier of 0.13-0.14 eV high roughly at the middle of PT reaction coordinate in HPIP and HPBI systems. Following the PT process, it has been predicted that twisting of phenoxy group in respect to imidazole plane, may play the most important role in non-radiative deactivation of these systems. For AHP and HPIP, a conical intersection between S1 and S0 has been predicted to be responsible for ultrafast deactivation of S1 excited systems, via internal conversion to ground state. The nonradiative deactivation mechanism determined in this work, indicates to efficient photostability of AHP and HPIP. Acknowledgment The Iranian National Science Foundation (INSF) is acknowledged for financial support (project no. 93036500). Also, the use of computing facility cluster GMPCS of the LUMAT federation (FR LUMAT2764) for partially performance of our calculations is kindly appreciated.

Supporting Information The xyz coordinates and additional information about geometry parameters of AHP, HPIP and HPBI are available free of charge via the Internet at http://pubs.acs.org.

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Graphical Abstract

CC2 potential energy profiles of the ground and excited states of AHP system along the proton transfer and twisting reaction coordinates.

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