Photochromism of 2-(2-Hydroxyphenyl) Benzothiazole (HBT) and Its

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A: Spectroscopy, Photochemistry, and Excited States

Photochromism of 2-(2-Hydroxyphenyl) Benzothiazole (HBT) and Its Derivatives; A Theoretical Study Maryam Iravani, and Reza Omidyan J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b00266 • Publication Date (Web): 09 Mar 2018 Downloaded from http://pubs.acs.org on March 10, 2018

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The Journal of Physical Chemistry

Photochromism of 2-(2-Hydroxyphenyl) Benzothiazole (HBT) and its derivatives; A theoretical study Maryam Iravani and Reza Omidyan* Department of Chemistry, University of Isfahan, 81746-73441 Isfahan, Iran

Abstract: Hydroxyphenyl-benzothiazole (HBT), is a well-known organic system based on its special character of the excited state hydrogen transfer (ESHT) following photoexcitation. However, the capability of this system regarding photochromism and photoswitching has not been addressed yet. In this study, we have investigated this issue by the aim of the MP2, CC2, ADC(2) and CASSCF theoretical methods. Also, we have considered several electron withdrawing groups and investigated their effects on photophysical characters and spectroscopic properties of enol and keto tautomeres of titled system. It has been predicted that the main HBT and its considered substitutions fulfill the essential characters required for photochromism. Also, substitution is an effective idea for tuning the photophysical nature of HBT and its similar systems. Our theoretical results verify that different substitutions alter the UV absorption of HBT systems from 330-351 nm, and also the corresponding absorption wavelength of the γ-forms of 526-545 nm.

Introduction Molecular Photochromism is an attractive1 and magic phenomenon showing rapid and reversible change of color. It is now well established that this molecular property guarantees access to optical switches and memories operating at a single-molecule level and, therefore, meets requirements for real nanophotonic devices for future electronics2. This process has very real and many potential applications. Photochromic glasses that darken in the sunlight (protecting eyes from excessive light intensity) and bleach in dim light are today a part of everyday life. Organic Photochromic Compounds in plastic ophthalmic lenses, more comfortable to wear, are now competing with silver salts glasses, despite the longer lifetime of the inorganic system. This successful commercial application has given a new impetus to

*

E-mail adresse: [email protected], reza.omidyan@upsud. fr. Fax: (+98) 31 36689732.

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research in the general field of photochromism that had its most recent revival in the early eighties1. In recent decades, systems that exhibit the phenomenon of excited-state intramolecular hydrogen transfer (ESIPT) have been of considerable applied interest as photostabilizers and sunscreens for the protection of organic polymers and biological tissues against damage that can be caused by the UV component of sunlight3. In these systems, excitation to the first electronic excited state (S1) in Franck-Condon region triggers hydrogen transfer. This process is important in photochemistry and photophysics, happening on time scale of femtoseconds or subpicosecond4. Moreover, the ESIPT compounds have received much attention as a new group of photochromic systems. Also, these new chromophores have been applied in photostablizers and sunscreens.5-7 In these compounds, after transferring a proton and also cistrans isomerization, a new keto form produces. The keto tautomeric form with bathochromically spectral shift is responsible for photochromic effects8-9. Because of photochromic application in molecular memories and switching devices, this photochromic compounds attracted much attention during last two decades4, 9-13. In this study, we have considered the 2-(2-hyrdoxyphenyl)-benzothiazole (abbreviated to HBT), for investigation the substitution effect on photophysical nature. HBT, is a thermally stable dye, being used in laser dye activates14. In addition, HBT and its derivatives have been attractive with applications in organic light emitting diodes (OLEDS)15, molecular sensors16, receptors, optical data storage17 and also optically controlled molecular switching18-19. Also, they have been used as ligand in organic chelate metal complexes 20-22 and it has been verified that HBT derivatives form outstanding electroluminescent materials in Zn-complexes 22. Over the last decades, HBT was the subject of many experimental13, 21-27 and theoretical23, 31, 36-40

studies, containing the dynamics of hydrogen transfer in excited state and other

photophysical features. Lochbrunner studied ESIHT process of HBT with time resolution of 30 fs, and found the appearance of keto emission signal after 60 fs23-24. Some other experiments also determine that a cis keto tautomer is produced within 170 fs 25-28. The transient infrared absorption signals which record NH and CO stretching vibration determine the existence of the keto form in excited state29. Ding et al.

25

also studied the ESIPT process of HBT and

determined that following the fast ESIPT the vibrational relaxation happens. Other studies indicate that the photoexcitation of this compound may cause the torsional motion around C-C bond after hydrogen transfer in excited states30. In addition to HBT, some calculations verified that this kind of torsion occurs in 2-(2-hydroxyphenyl)-benzotriazole (TIN-H)

31

and 7-(2-

pyridyl)indole232. This torsional motion is important in photochemistry, because it provides 2 ACS Paragon Plus Environment

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important route, directing the excited system to the ground state via ultrafast internal conversion following a conical intersection. However, the fluorescence spectra and lifetime of the S1 state of HBT verifies that this torsional motion may not occur in solution 30. Concerning the previous studies on HBT, Barbatti and coworkers30 reported the life time of isolated HBT molecules to 2.6 and 100 ps in gas and cyclohexane solution respectively. The different life time is more related to the torsional motion around the benzthiazole and hydroxyphenyl moieties in gas phase. Very recently, Pijeau et al33, applied the nonadiabatic dynamic simulations to determine the structural changes that lead to internal conversion, which aid in the understanding of similar photochemical processes. They have simulated the excitedstate dynamics of HBT using a wave function-in-DFT embedding approach to the electronic structure and Ab Initio Multiple Spawning (AIMS) to treat the nuclear dynamics. It has been clarified that HBT undergoes ESIPT within 48-54 fs of photoexcitation. Following ESIPT a twisting will begin, this reaction coordinate proceeds HBT rapidly toward the intersection seam between the ground and first excited electronic state. Although the photostability of HBT has been clarified from previous studies23,

31, 36-40

its

capability for photoswitch or photochromic idea, and also the effect of different chemical substitution on their photophysics have not been investigated yet. In this work, we will address these important points.

2-Computational Details The “ab initio” calculations have been performed with the TURBOMOLE program suit (V 6.2)34, making use of the resolution-of-identity, (RI) 35 approximation for the evaluation of electron repulsion integrals. The equilibrium geometry of all systems at the ground state has been determined at the RI-MP2 (Møller−Plesset second order perturbation theory)36 level. Excitation energies and equilibrium geometry of the lowest excited singlet states have been determined at the RI-CC2 (the second-order approximate coupled-cluster)

37-39

method. The

Dunning’s correlation consistent split-valence double-ζ basis function (cc-pVDZ)40-41 and the augmented cc-pVDZ by diffuse functions on all atoms (aug-cc-pVDZ) have been used for determination of transition energies and oscillator strengths. The Minimum Energy Path (MEP) have been determined based on the RI-MP2/RI-CC2 and ADC(2)42 methods using cc-pVDZ basis set.



Although, the CC2 method is not the best choice for determination of photophysical characters and PE profiles, mainly around the conical intersection region, it has been established so far that its results in these regions are quite reliable 9, 42-50. Moreover, in a recent comprehensive theoretical study, Tuna et al.50, presented substantial evidences confirming the validity of CC2 method, and even the surface crossing between two electronic states determined by this method can be anticipated as a true conical intersection. Nevertheless, in order to determine the geometry of the lowest energy S1/S0 conical intersection, we have employed CASSCF(6,6)/cc-pVDZ level of theory implemented in the Molpro51 2015.1 program package. In all of the considered systems, the active space has been included six electrons distributed over six molecular orbitals, (three HOMOS and three LUMOS), computed at the guessed geometry of the intersection. The orbitals included in the active space are presented in the supplementary material. Moreover, the charge distribution calculations were performed based on the Natural Population Analysis (NPA) algorithm52 implemented in the TURBOMOLE program. In addition, the HBT structure, its substitutions and also the relevant photochemical reactions have been presented in scheme 1. a) HBT

b)

R=H, -NH2, -F, -NO2, -CN

Scheme 1: (a)-Photophysical relevant structures and numbering pattern of HBT and HBT derivatives under study. b) Represents the position where the substitutions have been located.

Results and discussion 4 ACS Paragon Plus Environment

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1) Electronic Structure and Transition Energies: From MP2 optimized geometry, it is predicted that the enol form of HBT is the most stable form in the ground state, having a planar structure including a N---H hydrogen bond, in agreement with previous studies based on the IR and NMR experimental reports53-55. Because the structural analysis is far from the idea of this work we ignore more discussion in this regard, instead we will attend to photophysical characters. More information about the xyz coordinates of optimized systems are presented in Table S1, ESI file.

(Cis-enol form)

(Trans keto form) HBT

State S1 (ππ*)

cc-pVDZ 3.93

aug-cc-pVDZ 3.84 (0.3845)

State S1 (ππ*)

cc-pVDZ 3.01

aug-cc-pVDZ 2.89 (0.4281)

S2 (ππ*)

4.45

4.32 (0.0448)

S2 (nπ*, σπ*)

3.04

2.97 (0.0001)

S1

3.85

NH2-HBT 3.76 (0.3424) S1

2.97

2.85 (0.4208)

S2

4.43

4.30 (0.0748)

S2

3.06

2.99 (0.0001)

S1

4.01

NO2-HBT S1 3.92 (0.3991)

2.93

2.81 (0.4915)

S2

4.41

3.96 (0.0000)

S2

3.01

2.95 (0.0001)

S1

3.95

CN-HBT 3.86 (0.3837) S1

2.94

2.83 (0.4769)

S2

4.41

4.28 (0.0182)

S2

3.02

2.95 (0.0001)

F-HBT S1

3.77

3.69 (0.3486)

S1

2.93

2.81 (0.4236)

S2

4.41

4.29( 0.0536)

S2

3.04

2.98 (0.0001)

Table 1: The two lowest singlet transitions of cis (α-form) and trans keto(γ-form) of studied derivatives of HBT, determined at the RI-CC2 level of theory. The S1 and S2 assignments for all of the considered systems have been predicted to be the same, thus we have only reported the assignments of HBT system. The values in parentheses represents the oscillator strength. We have determined the vertical excitation energies and oscillator strengths of the two lowest electronically excited states of all derivatives for corresponding α and γ forms. The results have been tabulated in Table 1. The first maximum of the S1-S0 absorption band of HBT in 5 ACS Paragon Plus Environment


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cyclohexane has been recorded by Lochbrunner24 and coworkers based on the UV-vis pumpprobe absorption spectroscopy, locating on 347 nm (3.57 eV). As shown in Table 1, according to our CC2/aug-cc-pVDZ results, a sharp absorption band has been predicted for HBT, centered at 323 nm (3.84 eV), being comparable with its relevant experimental value. It is seen that all other substituted systems have sharp peak in narrow range of 3.92-3.69 eV (317-369 nm) corresponding to the first singlet electronic transition (S1-S0). From CC2 results, it has been predicted that S1-S0 electronic transitions of all α-forms correspond mostly to HOMOLUMO single electron transition, nevertheless the HOMO-1 LUMO or HOMO-2LUMO electronic transitions have very smaller contributions. Also, it has been predicted that S2-S0 gives rise to HOMO-1LUMO and HOMO-2LUMO electronic transitions. In Figure 1, the orbitals involving in excitation energies have been presented. It is seen that the S1 and S2 excited states of all derivatives can be assigned as ππ* state. More information about electronic configuration of all considered systems has been presented in the supplementary material file. In addition, it has been determined that the first excited state of HBT γ-form is 2.89 eV (429 nm), while other substituted systems exhibit a maximum band in the 2.73-2.85eV (454-435) nm, quite in the visible range.

HOMO

HOMO-1

HOMO-2

LUMO

LUMO+1

LUMO+2


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Figure 1: Selected valence molecular orbitals of the of HBT systems (cis-enol form). Only those MOs having most important contributions on the S1 to S4 excited states have been presented. 2) Potential Energy Curves:

2-I) The Main HBT System In order to investigate the behavior of HBT especially beyond the conical intersection, we have recalculated the ground and S1 potential energy curves of HBT along the PT and twisting coordinate with a different ab initio methods (RI-CC2 and ADC(2)). The CC2 minimum energy paths (MEPs) of HBT are displayed in Figure 2, and corresponding ADC(2) results have been presented in ESI file (see Fig. S2). In Fig. 2-a, the MEP of HBT along the hydrogen transfer (HT) coordinate are depicted. As shown, the HT process from OH to the N4 is endothermic in the ground state, requiring 0.75 eV while it is exothermic in the S1 excited state (by releasing ~0.33 eV). The S1 energy profile of the HT process, determined from both of the CC2 and ADC(2) methods is along with a small barrier (0.11 eV) at the middle of reaction coordinate, which is in good agreement with the results of Pijeau et al33. They have shown that this small barrier is arisen from theoretical errors and by improving the computational level to CAS-CI, the barrier has been disappeared33. Moreover, it has been established that the barrier is effectively dependent on the method not basis set33. 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 the φ twisting coordinate (φ=C1C2-C3-N4 dihedral angle). The CC2 results have been depicted in Figurer 2-b, and the corresponding ADC(2) results are presented in Fig. S1, ESI file. Starting from the last point of the HT path, we have determined the PE curves of the S1 and S0 states to φ~180°. Although the CC2 results of the S1 MEP exhibit a small minimum in the middle of reaction coordinate, the ADC(2) predicts roughly a flat trend of the S1 MEP along the twisting coordinate (see Fig. S1, ESI). However from prediction of CC2 and ADC(2), the S0 energy curve increases to approach the S1 energy from below, resulting to the S1-S0 intersection at φ~90°.



(a) 4

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

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(b)

S1

T1 λ=540 nm

2

λ=337.5 nm

1

0 1.0

1.2

1.4

1.6

1.8

45

O-H/Å

90

/deg

135

180

Figure 2: Minimum energy paths of HBT in the S0 (circles), in the first excited singlet and triplet [S1 state (triangle) and T1 (star)] determined at the CC2/cc-pVDZ level (MP2/cc-pVDZ for the ground state) along the minimum-energy path (filled symbols) for (a) the hydrogen transfer and (b) the twisting coordinates. 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), S1(S0)), stands for the energy profile of ground state determined based on the optimized complementary electronic S1(ππ*) and S0 states. The brown color represents the Minimum Energy Path determined on the first Triplet excited state.

In a multidimensional picture, the S1-S0 curve crossing in Figure 2-b develops to a conical intersection (CI). This conical intersection can be responsible for ultrafast nonradiative relaxation of HBT, after photoexcitation to the S1 (1ππ*) excited state in the gas phase. Although, from the CI region, one possibility is proceeding the system to the end of S0 PE surface of twisting coordinate, obtaining γ-form (i.e. the trans keto form) as another photoisomer, which is responsible for photochromism of HBT, since it has a strongly redshifted absorption spectrum (λ=540 nm, f=0.420, see Table 1). There is also another possibility for returning the excited system to the global minimum of the enol form and closing the photophysical cycle by ultrafast internal conversion to the S0 ground state.


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Moreover, in photophysics of HBT molecule, containing a Sulfur as a heavy atom, the role of triplet states can be important. In order to investigate this point, we have determined the minimum energy path at the first triplet excited state along the hydrogen transfer and twisting coordinates. The results have been presented in Figure 2 (brown color). The T1 PE curve has been determined as the same trend as the S1 state in relaxed scan. As shown, the T1 state in the Hydrogen trasfer exhibites a local minimum at the end of HT coordinate. In the second pnel of Figure 2, it is seen that in the middle of twisting coordinate, where the S1/S0 conical intersection locates, the S1/T1 potential energy cruves crosses; (i.e resulting an ISC ponit). Although from Figure 2, this ISC is apparent (snice of the different optimized geometries at the S1 and T1 excited states), it is qualitatively a good insight for locating an ISC in this region. From this ISC, there is a possibility for returning the wavepacket to the local minimum of T1, which can be only slightly in competition with proceding the excited system along the twisting coordinate for obtatining the trans-keto photoisomer. The later remark, can be justified by comparing the potential energy gradient, before and after the CI (or ISC), as driving force for populating the local minimum of the T1 state, or for obtaining the trans keto-isomer respectively.

I)

AHP Derivatives:



(b)

4

4

3

3

Energy/eV

Energy/eV

(a)

2

1

2

1

0 1.0

1.2

1.4 1.6

1.8 0

45

O-H/Å

90

/deg

135

0

180

1.0 1.2 1.4 1.6 1.8

0

45

90

135

/deg

O-H/Å

180

(d)

(c)

4

4

Energy/eV

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

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3

2

2

1

1

0

0 1.0

1.2

1.4

1.6

O-H/Å

1.8 0

45

90

/deg

135

1.2

180

1.5

1.8

O-H/Å

0

45

90

135

/deg

Figure 3. Minimum energy paths of HBT derivative at the S0 (circles) and in the excited S1(1ππ*) state (squares), determined at the CC2/cc-pVDZ level (MP2/cc-pVDZ for the ground state) along the minimum-energy path (filled symbols) for the hydrogen transfer from the enol form (left panels) and the twisting of benzothiazole moiety (right panels) for: a) NH2-HBT, b) F-HBT, c) NO2-HBT, d) CN-HBT


180

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2-II) Substituted HBT Systems: Amino group (-NH2), is a well-known electron donor9, 56, and its remarkable effects on excited state energy have been studied before4,

57-58

. As the first case, we have considered this

substitution on the C8 position of HBT (See scheme 1 for numbering). Although a main concern could be positioning of substitutions on the HBT frame, our goal in this study is only to investigate the effects of different groups in the same position. Thus, the amino group is proposed to be substituted at C8 of phenolic ring (para position with respect to -OH group), and as a π-donating group58, it increases the electron density. We have considered three electron withdrawing groups; Fluoride, Nitrogen dioxide and Ciano (-F, -NO2 and –CN respectively). We have determined the electronic transition energies (see Table 1) and also potential energy curves relevant to their HT and twisting coordinate of the hydrogen acceptor moiety. The results have been presented in Figure 3. Comparison the photophysical nature of substituted HBT with their parent system three remarkable points can be presented: (1) Similar to their parent HBT, the ground state hydrogen transfer (GSPT) process is significantly endothermic in all of the substituted systems, thus the GSPT is unfavored in ground state, while the corresponding process in the S1(ππ*) state is exothermic (between 0.3045 eV, see Table 2). Also, the S1 potential energy curve for all of substituted HBT exhibits a barrier roughly amount to 0.10 eV which is quite the same as parent system. As seen in Figure 3, the substitutions do not significantly affect this barrier, thus it is expected that ultrafast ESIPT takes place with the same dynamics in all of the considered derivatives. (2) In the –NH2 substituted system, the excited state hydrogen transfer is along with strong deformation by pyramidization from C3 (out-of-plane deformation), and by twisting along C2C3 coordinate (see scheme1). Thus the S1 PE profile along the HT coordinate of this system has been determined by keeping the planarity along HT coordinate (see Figure 3 and also Figure 4, a, b). Nevertheless, in the second panel of Figure 3 (a), when the planarity constraint was lifted, strong stabilization of S1 curve has been appeared. For this reason, excited state PT process in the NH2-HBT is more exothermic than other systems (ΔE~-0.80 eV). (3) The substitution slightly shifts the S1-S0 transition energy (see Table 2), either for enol or keto form of HBT. This small shift can be important in photochromism aspects, since of the different wavelengths of absorptions in the enol or keto form of considered systems. The S1 (ππ*) state of the global enol form of substituted systems, lies in the UV range of 3.53-3.73 eV 11 ACS Paragon Plus Environment


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(352-332 nm) as well as their parent HBT, while their corresponding photoproduct of transketo (γ-Form), absorbs in the visible region of 2.26-2.36 eV (545-526 nm). The large red-shift of the absorption wavelengths from enol to keto form of substituted HBT systems is one of the most important criterion which confirms that they are appropriate for being proposed as good candidates as photoswitchable and photochromic systems as well as their HBT homologue. In all of considerate systems, the S1/S0 potential energy curves in the region where φ is around 90°, the energetic gap between the S1/S0 electronic states decreases to the lowest value.

a)

b)

c)

Figure 4, The optimized structures of a) the ground (S0), b) The S1 excited state and c) The CASSCF (6,6) optimized structure of the conical intersection of CN-HBT system. Panels a and b were determined based on the MP2 and CC2 theoretical methods respectively.

Although, the CC2 as a single-reference method, is stretched to the limit by these calculations, particularly near the perpendicular configuration of the two rings, the results of Figure 3-d (right panel) provide strong evidence for the existence of a conical intersection between the S1 and S0 surface at a dihedral angle near 90°. Starting from the assumption CI structure of the CC2 calculations, the optimization of the conical intersection at the CASSCF, confirms the CC2 results in this region. The resulting geometry is shown in Figure 4-C. The characteristic features of this geometry are the twist angle of 72° and the pyramidization of the C3 atom. More information about the geometry parameters of the CI can be found in the ESI file. In addition, we have determined the S0, S1 MEP of the CN-HBT system based on the ADC(2) method. We have presented the results in ESI file (see Fig. S1, c-d). As shown, the S1 MEP along the PT coordinate exhibits quite the same trend as the CC2 results, while for twisting coordinate, the ADC(2) predicts a flatter MEP for the S1 state than that of CC2. Nevertheless the CI region (φ~90°) predicted by ADC(2) confirms the CC2 results as well.


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Based on the seminal work of Soboloweski et al. (see Ref.9), it has been established that a molecular system should fulfill few important characters to be considered as photoswitch system: -At least two different photoisomer forms (for instance α- and γ-forms), with different UV-vis absorption wavelengths (i.e. separated absorptions) are required. -Also, a big barrier between two photoisomers is essential for prohibiting the thermal convertion of α-/γ-forms. Moreover, a conical intersection between excited state and ground state is crucial to connects two different states. Considering the potential energy curves presented in Figure 2 and 4, it is concluded that all of the considered HBT systems fulfill the required characters to be suggested as photoswitchable systems as well as their parent HBT. Hence, in order to find insights on their photophysical differences, we have tabulated the most important spectroscopic and energetic parameters arisen from Figures 3 and 4 in Table 2. As seen in Table 2, the excited state hydrogen transfer is exothermic amount to –0.26-0.80 eV, and a large barrier (ΔEγα~ 1.0 eV) hinders the thermal γ/α conversion. From the last two columns of Table 2, it is seen that the α-forms of selected systems absorb in the UV range (333351 nm) of electromagnetic radiation while the γ-forms absorb in the visible (526-545 nm). The α-form of the main HBT absorbs in 337.5 nm (as the S1-S0 transition) and substitutions result to the red- and blue- shifts in this transition. This is the same in the absorption band of the photoisomer systems (denominated as γ-form). Although the shifts are not so large, the substitution is an effective idea for tuning the photophysical nature of HBT and its similar systems.

Molecular

ΔΕαβ/eV

system

ΔΕ

ΔΕS0(γα)/

λ/nm

λ/nm

γ→α

(eV)

(α-form)

(γ-form)

HBT

-0.35

1.07

0.98

337.5

540

NH2-HBT

-0.80

1.15

0.85

342.5

537



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F-HBT

-0.26

1.12

0.96

351

544

NO2-HBT

-0.46

0.92

0.88

333

526

CN-HBT

-0.43

0.90

0.90

336.5

545

Table 2, Comparison of the crucial photo-physical properties of HBT and its studied derivatives, determined at the RI-CC2/cc-pVDZ. ΔΕ(α→β) : The energetic difference between the first- and last points of the S1 PE profile along PT coordinates (i.e it reflects the energetic value releases follow excited state hydrogen transfer process). ΔΕ(γ →α): The ground state energetic barrier between γ and α forms (prohibiting the γ →α transformation due to thermal energies. ΔΕS0(γα): Represents the energetic difference between the optimized ground state of γ-form and its corresponding α-form. The last two columns represent the absorption wavelength of the α and γ forms respectively.

On the other hand, it is known that the ground state energetic differences between α and γ forms is a critical parameter for qualification of photoswitchable systems. As shown in Table 2 (ΔΕS0(γα)), all of substitutions improve the photoswitching of HBT by decreasing the (ΔΕS0(γα)), nevertheless the most important decreasing trend has been predicted for –NH2 substitution by 0.13 eV (from 0.98 eV in HBT to 0.85 eV in NH2-HBT). Moreover, because the excited state hydrogen transfer in the NH2-HBT system is more exothermic than other substitutions and parent HBT (-0.85 eV, see Table 2), it is concluded that the most important effects on improving of photoswitching nature of HBT can be arisen from NH2 substitutions.

Conclusion We have studied the capability of a well-known ESHT system (HBT), being suggested as new class of photochromic and photoswitchable system. Also the substitution effect on electronic transition energies and photophysical characters of HBT have been investigated. The substitution effect has been investigated on the ground and excited state potential energy curves. The summary of concluding remarks can be presented as follows: Based on the RI-CC2/ADC(2) results of this study, it has been predicted that the main HBT as well as its substituted systems can be subject of photochromism and photoswitching, owing the most important required characters. The S1 potential energy profile of HBT (and other


Page 15 of 21 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


substituted systems) exhibits only a small barrier along the PT coordinate (ΔΕ=0.10 eV), which is too small to affect the dynamics rate of ESHT. Thus, it is predicted that the ultrafast excited state hydrogen transfer is common for all of the considered systems. The torsional PE functions for the hydrogen-transferred tautomer along the twisting coordinate, directs the excited HBT systems to a conical intersection, providing two possibilities for the excited wave packet; the first is proceeding following the twisting coordinate and approaching another local minimum called trans-keto form. This tautomeric form can be responsible for photochromism and photoswitching. The second possibility is returning the excited system back to the ground state minimum by ultrafast internal conversion from back-twisting and consequently back hydrogen transfer. Concerning the substitution effects on photophysics of HBT, it has been predicted that substitutions alter the S1-S0 transition of the global minimum for the HBT systems as well as its photochromic form. Although the changes are not so large, it can be significant to conclude that substitution of HBT is an effective method for tuning the absorption wavelength of the enol and trans keto form, and consequently its photochromic nature.

Supplementary material

See the Supplementary materials for the xyz coordinates, the lowest lying transition energies, relevant molecular orbitals, the ADC(2) potential energy curves and simulated UV spectra of different photochemical structures of HBT based systems available free of charge on the ACS Publications website.

Acknowledgments We appreciate the Iranian National Science Foundation (INSF) for financial support (project no. 95842111). Also the research council of University of Isfahan is acknowledged. The use of computing facility cluster GMPCS of the LUMAT federation (FR LUMAT2764) for partially performance of our calculations is kindly appreciated.



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Table of Content:

Energy

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


λ=337.5 nm

PT

Conical Intersection λ=540

nm

Twisting

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Photochromism of 2-(2-Hydroxyphenyl) Benzothiazole (HBT) and Its

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