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

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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 S Supporting Information *

ABSTRACT: Hydroxyphenyl benzothiazole (HBT), is a well-known organic system based on its special characteristic 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 the photophysical characteristics and spectroscopic properties of the enol and keto tautomers of the titled system. It has been predicted that the main HBT and its considered substitutions fulfill the essential characteristics 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 to 351 nm and also the corresponding absorption wavelength of the γ-forms of 526−545 nm. responsible for photochromic effects.8,9 Because of the photochromic application in molecular memory and switching devices, photochromic compounds have attracted much attention during the last two decades.4,9−13 In this study, we have considered 2-(2-hyrdoxyphenyl)benzothiazole (abbreviated to HBT) for investigation of the substitution effect on photophysics. HBT is a thermally stable dye being used in laser dye activities.14 In addition, HBT and its derivatives have been attractive with applications in organic light emitting diodes (OLEDs), 15 molecular sensors, 16 receptors, optical data storage,17 and also optically controlled molecular switching.18,19 Also, they have been used as ligands 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 few decades, HBT was the subject of many experimental13,21−27 and theoretical23,31,36−40 studies pertaining to the dynamics of hydrogen transfer in the excited state and other photophysical features. Lochbrunner studied the ESIHT process of HBT with a time resolution of 30 fs and found the appearance of a keto emission signal after 60 fs.23,24 Some other experiments also determine that a cis-keto tautomer is produced within 170 fs.25−28 The transient infrared absorption signals that record NH and CO stretching vibration determined the existence of the keto form in the excited state.29 Ding et al.25 also studied the ESIPT process of HBT and determined that following the fast ESIPT, vibrational relaxation happens. Other studies indicate that the photoexcitation of this compound may

1. INTRODUCTION Molecular photochromism is an attractive1 phenomenon showing a 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 electronics.2 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 research in the general field of photochromism that had its most recent revival in the early eighties.1 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 sunlight.3 In these systems, excitation to the first electronic excited state (S1) in the Franck−Condon region triggers hydrogen transfer. This process is important in photochemistry and photophysics, happening on the time scale of femtoseconds or subpicoseconds.4 Moreover, the ESIPT compounds have received much attention as a new group of photochromic systems. Also, these new chromophores have been applied in photostabilizers and sunscreens.5−7 In these compounds, after proton transfer and also cis−trans isomerization, a new keto form is produced. The keto tautomeric form with a bathochromic spectral shift is © 2018 American Chemical Society

Received: January 10, 2018 Revised: March 9, 2018 Published: March 9, 2018 3182

DOI: 10.1021/acs.jpca.8b00266 J. Phys. Chem. A 2018, 122, 3182−3189

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The Journal of Physical Chemistry A

Molpro51 2015.1 program package. In all of the considered systems, the active space has included six electrons distributed over six molecular orbitals, (three HOMOS and three LUMOS), computed at the estimated geometry of the intersection. The orbitals included in the active space are presented in the Supporting Information (SI). 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.

cause torsional motion around the C−C bond after hydrogen transfer in the excited state.30 In addition to HBT, some calculations verified that this kind of torsion occurs in 2-(2hydroxyphenyl)-benzotriazole (TIN-H)31 and 7-(2-pyridyl)indole.32 This torsional motion is important in photochemistry, because it provides an 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 lifetime of isolated HBT molecules to be 2.6 and 100 ps in gas and cyclohexane solution, respectively. The different lifetimes are more related to the torsional motion around the benzthiazole and hydroxyphenyl moieties in the gas phase. Very recently, Pijeau et al.33 applied 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 excited state 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; these reaction coordinates accelerate HBT rapidly toward the intersection seam between the ground and first excited electronic state. Although the photostability of HBT has been clarified from previous studies,23,31,36−40 its capability for photoswitching or photochromism and also the effect of different kinds of chemical substitution on their photophysics has not been investigated yet. In this work, we will address these important points.

Scheme 1. (a) Photophysical Relevant Structures and Numbering Pattern of HBT and HBT Derivatives and (b) Position Where the Substitutions Have Been Located

3. RESULTS AND DISCUSSION 3.1. Electronic Structure and Transition Energies. From the 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 an N···H hydrogen bond, in agreement with previous studies based on the IR and NMR experimental reports.53−55 Because the structural analysis is far from the idea of this work, we ignore more discussion in this regard; instead, we will attend to the photophysical characteristics. More information about the xyz coordinates of optimized systems are presented in Table S1 of the SI file. We have determined the vertical excitation energies and oscillator strengths of the two lowest electronically excited states of all derivatives for the 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 cyclohexane has been recorded by Lochbrunner24 and coworkers based on UV−vis pump−probe absorption spectroscopy, located at 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 a sharp peak in a narrow range of 3.92−3.69 eV (317−369 nm) corresponding to the first singlet electronic transition (S1−S0). From the CC2 results, it has been predicted that the S1−S0 electronic transitions of all α-forms correspond mostly to a HOMO → LUMO single-electron transition; nevertheless, the HOMO-1→ LUMO or HOMO-2 → LUMO electronic transitions have very small contributions. Also, it has been predicted that S2−S0 gives rise to the HOMO-1 → LUMO and HOMO-2 → LUMO electronic transitions. In Figure 1, the orbitals involving excitation energies have been presented. It is seen that the S1 and S2 excited states of all derivatives can be assigned as the ππ* state. More information about the

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 the equilibrium geometry of the lowest excited singlet states have been determined at the RI-CC2 (the second-order approximate coupled-cluster)37−39 method. 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) has been determined based on the RI-MP2/RI-CC2 and ADC(2)42 methods using the ccpVDZ basis set. Although the CC2 method is not the best choice for the determination of photophysical characteristics 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 evidence confirming the validity of the 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 the CASSCF(6,6)/cc-pVDZ level of theory implemented in the 3183

DOI: 10.1021/acs.jpca.8b00266 J. Phys. Chem. A 2018, 122, 3182−3189

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The Journal of Physical Chemistry A Table 1. Two Lowest Singlet Transitions of cis- (α-Form) and trans-Keto (γ-Form) of Studied Derivatives of HBTa (cis-enol form)

(trans-keto form) HBT

state S1 (ππ*) S2 (ππ*)

ccpVDZ 3.93 4.45

S1 S2

3.85 4.43

S1 S2

4.01 4.41

S1 S2

3.95 4.41

S1 S2

3.77 4.41

aug-cc-pVDZ

state

3.84 (0.3845)

S1 (ππ*)

S2 (nπ*, σπ*) NH2−HBT 3.76 (0.3424) S1 4.30 (0.0748) S2 NO2−HBT 3.92 (0.3991) S1 3.96 (0.0000) S2 CN−HBT 3.86 (0.3837) S1 4.28 (0.0182) S2 F−HBT 3.69 (0.3486) S1 4.29(0.0536) S2 4.32 (0.0448)

ccpVDZ 3.01

aug-cc-pVDZ 2.89 (0.4281)

3.04

2.97 (0.0001)

2.97 3.06

2.85 (0.4208) 2.99 (0.0001)

2.93 3.01

2.81 (0.4915) 2.95 (0.0001)

2.94 3.02

2.83 (0.4769) 2.95 (0.0001)

2.93 3.04

2.81 (0.4236) 2.98 (0.0001)

Figure 2. Minimum-energy paths of HBT in the S0 (circles), in the first excited singlet and triplet [S1 (triangle) and T1 (star)] states 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)), stand for the energy profile of the ground state determined based on the optimized complementary electronic S1(ππ*) and S0 states. The brown color represents the minimumenergy path determined on the first triplet excited state.

a

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 the HBT system. The values in parentheses represents the oscillator strength.

the middle of reaction coordinates, which is in good agreement with the results of Pijeau et al.33 They have shown that this small barrier has arisen from theoretical errors, and by improving the computational level to CAS-CI, the barrier has disappeared.33 Moreover, it has been established that the barrier is effectively dependent on the method and not the basis set.33 In order to examine the nature of the potential-energy curves with respect to ring twisting, we have determined the S1- and S0-energy profiles along the φ twisting coordinates (φ = C1− C2−C3−N4 dihedral angle). The CC2 results have been depicted in Figure 2b, and the corresponding ADC(2) results are presented in Figure S1 in the SI file. Starting from the last point of the HT path, we have determined the PE curves of the S1 and S0 states to be φ ≈ 180°. Although the CC2 results of the S1 MEP exhibit a small minimum in the middle of reaction coordinates, the ADC(2) predicts roughly a flat trend of the S1 MEP along the twisting coordinates (see Figure S1 of the SI). However, from the predictions of CC2 and ADC(2), the S0-energy curve increases to approach the S1 energy from below, resulting in the S1−S0 intersection at φ ≈ 90°. In a multidimensional picture, the S1−S0 curve crossing in Figure 2b develops to a conical intersection (CI). This conical intersection can be responsible for the 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 the S0 PE surface of the twisting coordinates and obtaining the γ-form (i.e., the trans-keto form) as another photoisomer, which is responsible for photochromism of HBT, since it has a strongly red-shifted 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

Figure 1. Selected valence molecular orbitals of the HBT systems (cisenol form). Only those MOs having most important contributions on the S1 to S4 excited states have been presented.

electronic configuration of all considered systems has been presented in the Supporting Information file. In addition, it has been determined that the first excited state of the HBT γ-form is 2.89 eV (429 nm), while other substituted systems exhibit a maximum band in 2.73−2.85 eV (454−435) nm, quite in the visible range. 3.2. Potential-Energy Curves. 3.2.1. 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 coordinates with 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 the SI file (see Figure S2). In Figure 2a, the MEP of HBT along the hydrogen transfer (HT) coordinates are depicted. As shown, the HT process from O−H 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 has a small barrier (0.11 eV) at 3184

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Figure 3. Minimum-energy paths of the HBT derivative at the S0 (circles) and in the excited S1(1ππ*) state (squares), determined at the CC2/ccpVDZ 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.

for obtaining the trans-keto photoisomer. The later remark can be justified by comparing the potential-energy gradient before and after the CI (or ISC) as the driving force for populating the local minimum of the T1 state, or for obtaining the trans-keto isomer, respectively. 3.3. AHP Derivatives. 3.3.1. Substituted HBT Systems. The amino group (−NH2) is a well-known electron donor,9,56 and its remarkable effects on excited state energy have been studied before.4,57,58 For 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 the positioning of the 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 the phenolic ring (para position with respect to the −OH group), and as a π-donating group,58 it increases the electron density. We have considered three electron withdrawing groups: fluoride, nitrogen dioxide, and a cyano group (−F, −NO2, and −CN, respectively). We have determined the electronic transition energies (see Table 1) and also the potential-energy curves relevant to the HT and twisting

closing the photophysical cycle by ultrafast internal conversion to the S0 ground state. Moreover, in the photophysics of the HBT molecule, which contains 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 to have the same trend as the S1 state in the relaxed scan. As shown, the T1 state in the hydrogen transfer exhibits a local minimum at the end of the HT coordinates. In the second panel of Figure 2, it is seen that in the middle of the twisting coordinates, where the S1/S0 conical intersection is located, the S1/T1 potential-energy curves cross; (i.e., resulting in an ISC point). Although from Figure 2, this ISC is apparent (because of the different optimized geometries at the S1 and T1 excited states), and 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 proceeding the excited system along the twisting coordinates 3185

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The Journal of Physical Chemistry A Table 2. Comparison of the Crucial Photophysical Properties of HBT and Its Studied Derivativesa molecular system

ΔEαβ/eV

ΔE/γ→α

ΔES0(γα)/eV

λ/nm (α-form)

λ/nm (γ-form)

HBT NH2−HBT F−HBT NO2−HBT CN−HBT

−0.35 −0.80 −0.26 −0.46 −0.43

1.07 1.15 1.12 0.92 0.90

0.98 0.85 0.96 0.88 0.90

337.5 342.5 351 333 336.5

540 537 544 526 545

Determined at the RI-CC2/cc-pVDZ. ΔE(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). ΔE(γ→α): The ground state energetic barrier between γ and α forms (prohibiting the γ→α transformation due to thermal energies. ΔES0(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. a

coordinates of the hydrogen acceptor moiety. The results have been presented in Figure 3. In comparison of 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 the ground state, while the corresponding process in the S1(ππ*) state is exothermic (between 0.30 and 45 eV, see Table 2). Also, the S1 potential-energy curve for all of the substituted HBT exhibits a barrier roughly equal 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 occurs along with strong deformation by pyramidalization from C3 (out-of-plane deformation) and by twisting along the C2−C3 coordinates (see Scheme 1). Thus, the S1 PE profile along the HT coordinates of this system has been determined by keeping the planarity along the HT coordinates (see Figure 3 and also Figure 4a,b). Nevertheless, in the

(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 the enol to keto form of the substituted HBT systems as well as their HBT homologue is one of the most important criteria confirming whether they are appropriate for being proposed as good candidates as photoswitchable and photochromic systems. In all of the considered systems, the energetic gap between the S1/S0 electronic states decreases to the lowest value for the S1/S0 potential-energy curves in the region where φ is around 90°. 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 3d (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 assumed 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 4C. The characteristic features of this geometry are the twist angle of 72° and the pyramidalization of the C3 atom. More information about the geometry parameters of the CI can be found in the SI file. In addition, we have determined the S0 and S1 MEP of the CN−HBT system based on the ADC(2) method. We have presented the results in the SI file (see Figure S1c,d). As shown, the S1 MEP along the PT coordinates exhibits quite the same trend as the CC2 results, while for the twisting coordinates, 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. Based on the seminal work of Soboloweski et al. (see ref 9), it has been established that a molecular system should fulfill a few important characteristics to be considered as a photoswitching system • At least two different photoisomer forms (for instance, αand 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 conversion of the α-/γ-forms. Moreover, a conical intersection between the excited state and ground state is crucial to connect two different states. Considering the potential-energy curves presented in Figures 2 and 4, it is concluded that all of the considered HBT systems fulfill the required characteristics to be suggested as photoswitchable systems as well as their parent HBT. Hence, in order

Figure 4. 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 the CN−HBT system. Panels a and b were determined based on the MP2 and CC2 theoretical methods, respectively.

second panel of Figure 3a, when the planarity constraint was lifted, a strong stabilization of the S1 curve had appeared. For this reason, the excited state PT process in NH2−HBT is more exothermic than that of other systems (ΔE ≈ −0.80 eV). (3) The substitution slightly shifts the S1−S0 transition energy (see Table 2), either for the enol or keto form of HBT. This small shift can be important in photochromism aspects because 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 (352−332 nm) along with that of their parent HBT, while their corresponding photoproduct of the trans-keto 3186

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The Journal of Physical Chemistry A 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 and equals −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 region (333−351 nm), while the γ-forms absorb in the visible region (526−545 nm). The αform of the main HBT absorbs at 337.5 nm (as the S1−S0 transition) and substitutions result in the red- and blue-shifts of this transition. This is the same in the absorption band of the photoisomer systems (denominated by the 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. On the other hand, it is known that the ground state energetic differences between the α and γ forms are critical parameters for the qualification of photoswitchable systems. As shown in Table 2 (ΔES0(γα)), all of the substitutions improve the photoswitching of HBT by decreasing (ΔES0(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 that of other substitutions and the parent HBT (−0.85 eV, see Table 2), it is concluded that the most important effects on the improvement of the photoswitching nature of HBT arise from NH2 substitutions.

HBT is an effective method for tuning the absorption wavelength of the enol and trans-keto forms and consequently its photochromic nature.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.8b00266. See the Supporting Information 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 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] and [email protected]; Fax: (+98) 31 36689732 ORCID

Reza Omidyan: 0000-0003-4538-2500 Notes

The authors declare no competing financial interest.

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

4. 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 characteristics of HBT has been investigated. The substitution effect has been investigated on the ground and excited state potential-energy curves. A summary of the 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 the subject of photochromism and photoswitching, owing to the most important required characteristics. The S1 potential-energy profile of HBT (and other substituted systems) exhibits only a small barrier along the PT coordinates (ΔE = 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 coordinates and directs the excited HBT systems to a conical intersection, providing two possibilities for the excited wave packet; the first is proceeding following the twisting coordinates and approaching another local minimum called the 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 the 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 the substitution of

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REFERENCES

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