Theoretical Study on Photoisomerization Effect with a Reversible

Article pubs.acs.org/JPCA

Theoretical Study on Photoisomerization Effect with a Reversible Nonlinear Optical Switch for Dithiazolylarylene Ping Song,† Ai-Hua Gao,† Pan-Wang Zhou,† and Tian-Shu Chu*,†,‡ †

State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China ‡ Institute for Computational Sciences and Engineering, Laboratory of New Fiber Materials and Modern Textile, The Growing Base for State Key Laboratory, Qingdao University, Qingdao, 266071, China S Supporting Information *

ABSTRACT: DFT and TDDFT methods have been performed to investigate the photoisomerization effect for dithiazolylarylene on solution. The weak S···N interaction and CH···N hydrogen bond restrain the rotation of the side-chain thiazolyl ring in open-isomer 1a, the higher stability of which prefers to show a high quantum yield of photoisomerization. The calculated UV−Vis spectrum at around 320 nm for open-isomer 1a is bathochromically shifted to 647 nm for closed-isomer 1b, in excellent agreement with the experimental photochromic phenomenon. The electron transition in ECD (electron circular dichroism) spectra for closed-isomer 1b with two chiral carbon atoms is dominated by ICT (intramolecular charge transition) and LE (local excitation) corresponding to one positive (440 nm) and one negative Cotton effect (650 nm), respectively, where the two chiral carbon atoms play a slight role in these transitions. The PES in the S1 and S0 states, respectively, indicates that the cyclization reaction from open-isomer 1a to closed-isomer 1b is allowed in the photoexcited state with high-conversion quantum efficiency, while it is forbidden in the thermodynamic process. In addition, the second-order nonlinear optical response for closed-isomer 1b is nearly six times larger than that for open-isomer 1a. It is also confirmed that the photoirradiation evokes the photoisomerization character to show dramatic difference in the second-order NLO response, which can be applied to designing photochromic materials and reversible NLO switches.

1. INTRODUCTION Photochromism is the reversible chemical phenomenon between two isomers, which exhibits distinct absorption spectra by photoirradiation.1 In lots of organic photochromic compounds, such as anilines, disulfoxides, hydrazone derivatives, azobenzen, diarylethenes, and fulgides,2−7 the photochromism effect can take place through photocyclic reactions, cis−trans isomerizations, intramolecular hydrogen transfer, and electron transfers (oxidation−reduction) when the photoirradiation has been executed. The typical photochromic compounds terarylenes and diarylethenes contain a hexatriene structure composed of three heteroaromatic rings, and they can undergo a photocyclization reaction from the ring-open structure to convert into a ring-closed formation of cyclohexadiene by the photoirradiation. Consequently, a coplanar structure has been built owing to the reversible photoinduced cyclization reaction. The corresponding modulated reversible chemical and physical properties lead them to be excellent photochromic materials in display, optical memory media, and photoptical switches.8−14 Many groups have investigated cyclization and cycloreversion reactions for dithienylethene derivatives in experiment.15−17 Recently, a new photochromic dithiazolyarylene molecule has been reported,18 and it is very © 2012 American Chemical Society

interesting that the weak interaction between the adjacent heteroaromatic rings suppresses the rotation of the thiazolyl rings. Similar to the diarylethenes and terarylenes, the hexatriene backbone built by three heteroaromatic rings turns to cyclohexadiene upon UV irradiation. Nonlinear optics (NLOs) are very useful in information processing, electron−optical switching,19 redox-switchable optical materials,20−23 and the design of photonic devices such as optical memory media.24−28 The photoisomerization phenomenon29 has been applied to not only linear optics but the NLO field to induce different NLO responses. The relationship between photoisomerization and NLO properties can provide a specific way to design materials for light-induced second-order NLO switching.30,31 That is to say, the novel reversible switching effect for photoisomerization materials can reversibly modulate the NLO response to produce NLO switching materials. Some excellent theoretical work in photoisomerization or the photochromic effect has been performed in recent years,32−34 while the study of the Received: March 16, 2012 Revised: May 6, 2012 Published: May 16, 2012 5392

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Scheme 1. Optimized Geometries of the Ground States (a) and Excited States (b) for Compounds 1a and 1b with Part of the Geometry Parametersa

a

The bond distances in red are the calculated values for the current study at the M06/TZVP level, and the black and blue ones in parentheses are those calculated at the B3LYP/6-31G* level and the experimental values, respectively, in ref 18.

ref 18 with N1 and S3 atoms on the same side. The corresponding ring-closed isomer 1b is constructed based on the orientation of 1a. The optimized geometries have been illustrated in Scheme 1, and all hydrogen atoms expect for the H1 atom have been omitted for clarity. It is visible that the C1 and C2 atoms with the sp2 hybrid in open-isomer 1a have turned into an sp3 hybrid in closed-isomer 1b, and the methyl group connected to the C1 atom is oriented outwardly with the one linked to C2 atom inwardly. Therefore, the two chiral carbon atoms in 1b can build (R,R)-stereochemical geometry.

relationship between photoisomerization and NLO properties in theory is relatively rare. In order to investigate the photoisomerization character of the dithiazolyarylene molecule, density functional theory (DFT) and time-dependent density functional theory (TDDFT) have been employed to study the properties and the photochemistry process.

2. COMPUTATIONAL DETAILS It is well-known that DFT can conveniently treat the groundstate electronic structures for organic molecules, and it can take into account electron correlation and relativistic effects. In addition, the moderate efficiency, accuracy, and time-savings lead DFT to be the appropriate choice to study the ground state of organic molecules compared with other theoretical computational methods.35−39 Meanwhile, TDDFT has been confirmed to be an effective candidate to investigate the excited state and electronic spectra. Therefore, geometry optimizations for the ground state and the first singlet electronic excited state were performed using DFT and TDDFT methods, respectively, in the Gaussian 09 program40 in the current paper. The hybrid meta functional of Truhlar and Zhao (M06) has been confirmed to have good accuracy for main group thermochemistry41 and has also been tested to be the reasonable one for the current systems (see the Supporting Information), and the triple-ζ valence quality with one set of polarization functions (TZVP)42 basis set was assigned. In order to reproduce the experimental results, the solvent effect in n-hexane solution was employed in the SCRF calculations by using the conductor-like screening model (COSMO) method.43 All electronic structure calculations were completed with no constraints for symmetry, and all of the local minima were in the absence of an imaginary mode in vibrational frequency analysis calculations (see the Supporting Information). The group orientation of the initial geometry for 2,3-dithiazolylbenzothiophene 1a in the current paper has been optimized based on the crystal structure in the

3. RESULTS AND DISCUSSIONS 3.1. Optimized Geometries for the Compounds 1a and 1b. It can be seen for compound 1a from Scheme 1 that the length between N1 and S3 (3.16 Å) is shorter than the sum of the van der Waals radii of N (1.55 Å) and S (1.85 Å), and N2···H1 has the distance of 2.63 Å, also shorter than the sum of the van der Waals radii of N (1.55 Å) and H (1.20 Å). The two distances are not only in good agreement with the former calculations by 3.20 and 2.60 Å, respectively, but they reproduce the experimental values well (2.95 and 2.69 Å, respectively). The weak interaction of N1···S3 and N2···H1 prohibits the rotation of the thiazolyl ring around the C3−C5 axis, and consequently, the three heteroaromatic rings prefer to be nearly coplanar. However, the sp3 hybrid of C1 and C2 with the steric methyl groups causes the thiazolyl ring to twist slightly, leaving the dihedral angle of N1C5C3S3 by 47°. The distance between the two reactive C1 and C2 atoms is up to 3.47 Å in the open-ring isomer 1a. The photoirradiation induces the three heteroaromatic rings of 1b to be coplanar, with the distance between C1 and C2 of 1.52 Å. The distance between N2 and H1 has been shortened to 2.47 Å, with the remaining distance between N1 and S3. Another initial structure 1a-2 with the dihedral angle of N1C5C3S3 to locate N1 and S3 atoms at the opposite side is tested at the same level, and the optimized geometry 1a-2 in 5393

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Figure 1. PES as a function of the dihedral angle for N1C5C3S3 in ring-open isomer 1a.

(nearly zero) but is much smaller than that in 1a. Thus, this analysis indicates that the S···N weak interaction contributes significantly to the stability. 3.2. UV−Vis and Electronic Circular Dichroism (ECD) Spectra. For the purpose of examining the optical properties, the UV−Vis and ECD spectra for compounds 1a and 1b have been simulated by calculation and compared further with experimental values. The excitation energies, oscillator strengths, and rotational strengths of the 20 lowest-energy electronic excitations have been calculated at TDDFT/M06/ TZVP level based on the optimized geometries. All of the main transitions have been listed in Table S2 in the Supporting Information, and the related orbitals have been illustrated in Figure 2.

Figure S1 (Supporting Information) is 8.17 kJ/mol higher in energy than 1a, with a dihedral angle of N1C5C3S3 of −136°. In other words, the structure tethered by the weak interaction of S···N and N···H exhibits better thermal stability. In order to further investigate the stability effect by the weak interaction of N1···S3 and N2···H1, another ring-open structure replacing the N1 and N2 atoms with the C atoms has been chosen to be in comparison (see Figure S2 in the Supporting Information). The substituted ring-open compound 2a with C1′ and S3 atoms on the opposite side with a dihedral angle of −134° for C1′C5C3S3 shows the same stability as the isomer 2a2 with C1′ and S3 atoms on the same side, indicating the absence of the stability effect between C1′ and S3 with C2′ and H1. In other words, the weak noncovalent S···N interaction and the CH···N hydrogen bonding44−46 in compound 1a play important roles in the thermal stability. Confirmation of the influence of the weak interaction of N1···S3 and N2···H1 on stability properties for the ring-open isomer 1a can be further provided by the potential energy surface (PES) as a function of the dihedral angle of N1C5C3S3 in 1a, as depicted in Figure 1. It is constructed by the frozen geometry with only the variable parameter of the dihedral angle of N1C5C3S3 from 0 to −90° in steps of 10°. It is visualized from the PES that there are two barriers during the rotation around the C3−C5 axis from 1a to 1a-2, requiring energies of 19.96 and 6.51 kJ/mol for 1a to reach 1a-2, while the lower energies of 12.02 and 6.30 kJ/mol are needed, leading 1a-2 to easily turn to 1a. This indicates that the thermal stability of 1a is due to the weak interaction of S···N and N···H. All of these confirmed the stability effect from the weak interaction of N···S and N···H to keep coplanarity in 1a. To further estimate the respective contribution of the S···N and N···HC weak interactions to the stability, the secondperturbation energy E(2)47 has been obtained by NBO analysis (see Table S1 in the Supporting Information), which can be used to describe the weak interaction. Three geometries (1a, 1a-120, and 1a-2) on the PES in Figure 1 have been chosen for discussion. Here, 1a-120 denotes the geometry with the N1C5C3S3 dihedral angle of 120°. From Table S1 (Supporting Information), we can see that the weak interaction for S···N is much larger than that for N···HC in 1a, while there is no weak interaction for N···HC in 1a-120 and 1a-2. Moreover, the weak interaction for S···N in 1a-2 is larger than that in 1a-120

Figure 2. The main molecular orbitals for the dominant electron transition in UV−Vis spectra for compounds 1a and 1b.

As seen, the experimental absorption spectra in Figure 3 for open-isomer 1a and closed-isomer 1b have been well reproduced at the TDDFT/M06/TZVP level. In detail, the maximal electronic absorption (320 nm) for open-isomer 1a with the largest oscillator strength of 0.6519 is in good agreement with the experimental result (307 nm), and the S0 → S3 excitation mainly composed of H → L+2 is assigned from the π-electron delocalized over the whole molecule to localize 5394

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Figure 3. Simulated UV−Vis spectra for 1a and 1b and ECD spectra for 1b. (The black line is the experimental UV−Vis spectrum.)

Figure 4. PESs as a function of R(C1−C2) for the S0 and S1 states, respectively obtained at the M06/TZVP and TDDFT/M06/TZVP levels. The left panel is the PES for the S0 state, and the right one is that for the S1 state.

on the benzothiophene part. Herein, the covered S0 → S6 excitation at 299 nm with the larger oscillator strength of 0.2832 is assigned to H−2 → L, in which the electron is transferred from the benzothiophene and phenylthiazole segment with an S3 atom to another phenylthiazole segment with an S2 atom. Upon irradiation with the UV light, a new absorption band for closed-isomer 1b appears at 647 nm, which exhibits one main electron transition of S0 → S1 assigned to H → L with the electron from two localized thiazole rings and the benzothiophene segment to the two phenylthiazole segments. Moreover, the sharp peak in compound 1b at around 312 nm has a slight blue shift by 8 nm compared with that in compound 1a, corresponding to the situation where the electron transfers from a localized π-orbital over two phenyl rings and the loneelectron pair of S1 and S2 atoms to delocalize over the whole molecule. This character of electron transition for compound 1b at 312 nm is quite distinct from that for compound 1a, where the ring-closed effect induces the electron for the LUMO orbital to delocalize over the two phenylthiazole segments. Moreover, the contribution of the two chiral carbon atoms to the LUMO in compound 1b is much larger than that in compound 1a, indicating that the ground-state geometry plays an important role in the photoisomerization reaction. Thus, the reaction prefers to form a closed-ring isomer by light excitation. The reactive carbon atoms C1 and C2 in closed-isomer 1b present chiral character, which can be analyzed with ECD spectra. The TDDFT method is confirmed to be reliable to simulate the ECD spectra to be in accordance with the experiments.48−53 The rotational strengths are calculated using both length and velocity representations. To study the chiroptical property of 1b and to which extent the two chiral C atoms may contribute to it, the positive and negative peaks for the ECD spectra have been analyzed, and electron density difference maps (EDDMs) (see Table S2 in the Supporting

Information) have been utilized to clarify the origin of the ECD spectra by the Gauss-Sum2.2.5 software package.54 It can be seen from the ECD spectra in Figure 3 that there are two Cotton effects between 350 and 800 nm. Herein, the positive Cotton effect at 440 nm is ascribed to excited state S2 around 424 nm, which is assigned to the intramolecular electron transition (ICT) from the two terminal phenyl rings to three heterocycles including S atoms. In addition, the negative Cotton effect at 650 nm is composed of the negative rotational strength at around 647 nm, mainly due to the local excitation (LE) for H → L over the two thiazole rings and one benzothiophene segment. It is obvious from the transition for ECD spectra that there is little contribution from the two chiral carbon atoms to the chiroptical properties in ECD spectra. 3.3. PES and Electronic Properties in S1 States for Compounds 1a and 1b. In order to further investigate the photoisomerization process, the differences in the electronic properties of the first excited state (S1 state) for 1a and 1b have been studied at the TDDFT/M06/TZVP level. The optimized geometry structures of the S1 states have also been displayed in Scheme 1. Compared with the ground state, the distance between S3 and N1 atoms in the S1 state of open-isomer 1a has been compressed to 3.03 Å, with also the decreased distance of 2.46 Å between N2 and H1 atoms. Moreover, C1 and C2 atoms are close to each other by the distance of 3.33 Å, and the dihedral angles of N1C5C3S3 and N2C6C4C7 decrease to 36 and 44°, respectively. By contrast, the geometry structure for the S1 state of closed-isomer 1b has almost no change compared with that for the S0 state, except for the slightly increased distance by 2.51 Å between N2 and H1. What should be considered next is the stability of the photogenerated closed-ring isomers; therefore, the PESs of the translation between open- and closed-isomers have been obtained and are presented in Figure 4. In order to better 5395

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where βx, βy, and βz are the components of the second-order polarizability tensor along the x-, y-, and z-axes, respectively. The vector component of β (βi) is defined by eq 2

describe the energy path, we have calculated the PES as a function of the distance between reactive C1 and C2 atoms (denoted as R(C1−C2) for simplicity) on the basis of partially optimized geometries, referring to the work by Yabushita’s group.32 It can be seen from the PES of the S0 state that a large energy barrier by more than 170 kJ/mol makes the cyclization difficult in thermodynamics. In addition, the activation energy from 1b to 1a with ∼130 kJ/mol provides some probability for thermal cycloreversion, and this calculated energy value is in good agreement with the experimental value. In contrast, a very small activation energy (i.e., less than 10 kJ/mol) exists in the cyclization reaction in the S1 state from open-isomer 1a to close-isomer 1b, indicating the permission for the cyclization reaction in the S1 state with the nearly zero barrier process. However, much larger energy barriers (about 142.5 kJ/mol) for the cycloreversion process from 1b to 1a can prevent the cycloreversion reaction in the S1 state. Nevertheless, the photon energy of 383 kJ/mol corresponding to the shortest excited wavelength at 312 nm (with the largest oscillator strength) from Table S2 (Supporting Information, which is larger than the sum (327.5 kJ/mol) over this larger energy barrier (142.5 kJ/mol) and the vertical excitation energy between S0 and S1 (185 kJ/mol, corresponding to 647 nm in Table S2, Supporting Information), is sufficient for overcoming the S1−PES barrier and thus can evoke a small probability for cycloreversion to take place. In other words, the cyclization process from openisomer 1a to closed-isomer 1b is allowed in the photoexcited state but forbidden in thermodynamics. The above results are in good agreement with the high (0.98) cyclization conversion quantum yield and nonzero (0.008) cycloreversion quantum yield in experiment.18 To further study the properties of the S1 state, the fluorescence spectra and electron transition have been investigated by TDDFT at the M06/TZVP level, all of which have been listed in Table S3 in the Supporting Information. It shows that two dominant electron transitions for the emission of 1a focus on the S3 → S0, S5 → S0, and S1 → S0 transitions around the wavelengths of 355, 320, and 399 nm, respectively. From the EDDMs of the transition in Table S3 (Supporting Information), the localized state for the S3 → S0 transition is delocalized over the three heteroaromatic rings including S atoms. By contrast, the intramolecular charge transition (ICT) character from the benzothiophene segment to the phenylthiazole segment belongs to the S1 → S0 transition, the same as that for the transition for S5 → S0. In addition, the electron transition for 1b also presents two main transitions of S1 → S0 and S8 → S0, which exhibit localized state and ICT characters, respectively. The localized state is mainly over the three heteroaromatic rings, and the ICT is assigned from one side of the phenylthiazole ring to another side including the cyclohexa1,3-diene ring. 3.4. Second-Order NLO Switch Based on the Photoisomerization Effect. The photoisomerization reaction results in the reversible transfer between open- and closedisomers 1a and 1b, and better conjugated formation has been presented in closed-isomer 1b, which indicates better nonlinear optical response. In connection to this, the finite field (FF) approach at the MP2 level was executed to calculate the static second-order polarizability in hexane solution. The total second-order polarizabilities β0 for the studied compounds are defined with the following equation β0 = (βx2 + βy2 + βz2)1/2

βi = βiii +

1 3

∑ (βijj + βjij + βjji) j≠i

(2)

Herein, βiii is the diagonal tensor. It is revealed that the second-order NLO response for openisomer 1a is 3.42 × 10−30 esu, while that for closed-isomer 1b is increased to 20.33 × 10−30 esu due to the better conjugated ring-closed structure resulting from the photoisomerization reaction. This NLO value for 1b is nearly five times larger than the prototype of a conventional D-π-A model (p-nitroaniline, PNA) with large NLO response,55 which indicates that compound 1b is potentially an excellent NLO material. It is therefore confirmed that the photoirradiation can play a dramatic role in the second-order NLO response, and the reversible photochromic character can be applied to designing NLO switches.

4. CONCLUSION DFT and TDDFT methods indicate that the dithiazolyarylene represents a photoisomerization phenomenon in solution by light excitation. In the open-isomer 1a, the weak S−N heteroatom contact interaction and CH−N hydrogen bond tether the side-chain thiazolyl ring to favor the coplanarity of the three heteroatomatic rings. The distinct UV−Vis spectra and electron transition indicate the photochromic phenomenon between the open- and closed-isomers. The chiral carbon atoms presented in the closed-isomer 1b show one positive and one negative Cotton effect at around 440 and 650 nm, respectively, for ECD spectra, and the electron transition is assigned to ICT and LE, respectively. The potential energy surfaces for the S1 and S0 states respectively speculate that the cyclization reaction from 1a to 1b is allowed in the photoexcited state but forbidden in the thermodynamic process. Moreover, closedisomer 1b presents larger second-order nonlinear optical response than open-isomer 1a. In other words, the photoirradiation makes the compound dithiazolylarylene not only a photochromic material but also a reversible NLO switch.

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ASSOCIATED CONTENT

S Supporting Information *

The test for different functionals in the current paper. The relative energies for compounds 1a and 1a-2 with the optimized geometry. The optimized geometries for compounds 2a and 2a-2 with atom replacement and part of the geometry parameters. The UV−Vis spectra with the main electron transitions for compounds 1a and 1b. The fluorescence emission with electron transitions for compounds 1a and 1b. The XYZ coordinates (Å) and SCF energies (au) for compounds 1a, 1a-2, 1b, 2a, and 2a-2. Vibrational frequencies for compounds 1a, 1a-2, 1b, 2a, and 2a-2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86-411-84379029. Fax: +86-411-84675584. E-mail: [email protected] or [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from the General Financial Grant from the China Postdoctoral Science Foundation (No. 2011M500582).

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