Theoretical study of the cis–trans isomerization mechanism of a pendant metal-bound azobenzene

Abstract

In this paper, we performed density-functional theory (DFT) at the B3LYP/6-311++G(d,p) & LANL2DZ quantum level to probe the thermal cis → trans isomerization of Re(CO)₃–AB, a diimine ligand with an azo group substituent. We have searched for the transition states (TS) on the S₀ and T₁ potential energy surfaces and detected two inversion transition states TS-inv-1 and TS-inv-2 following a rotation-assisted inversion mechanism. In the T₁ state, only the rotation pathway exists with the energy barrier 1.88 eV. We also investigated the photoisomerization pathway, the potential energy profiles of the vertical excitation for the excited states T₁, S₁, T₂ and S₂ were calculated, and we analyzed two possible isomerization routes through the inversion and rotation pathways. In addition, we probed the properties of the isomerization of the azo portion caused by excitation of the MLCT bands.

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1. Introduction

Over the last few decades, the development of photo-responsive materials has become an intensive area of research, whose properties and eventual functionality are controlled by changes of the environment-light irradiation. These systems have been already implemented in an extensive range of modern materials and devices for daily applications like sunglass lenses, memory devices, photochromic inks and so forth.^1–3 Transition metal compounds that show metal to ligand charge transfer transitions (MLCT), as seen in many d⁶ metal complexes such as rhenium, are perfect for transferring energy to an external group.^4–9 Because of the importance and versatility of transition-metal complexes, the photochemical characters of rhenium tricarbonyl complexes with this type of ligands were extensively studied by Stufkens as early as in the 90's.^10–12 And the focus of people's research is from how metal coordination could affect the photochromism of the organic switch located at the organic ligand shifted toward the modification of the properties and eventually the functionality of the metal complex upon isomerization of the photochromic fragment present in the ligands.⁶ Interest in this type of photoluminescent compounds remains strong as evidenced by a number of recent reviews.^10,13,14 In principle, photo-responsive metal complexes can be acquired by incorporation of organic photochromic units in the structure of their ligands.^1,15–17 Azobenzene (AB) is the photochromic fragment most frequently used for this purpose. AB does experience a reversible cis to trans photoisomerization which induces not only structural changes but also important electronic modifications in the ligands.^18–26 The photoinduced isomerization of AB is a well-studied example of a light driven process.^18,22,23,27 AB has been appended to a number of chromophores, including transition metal complexes, in an attempt to couple the cis → trans isomerization process to an external electronic transition.^9,17,28–32

Amazingly, in spite of the wide number of transition metal complexes of AB-containing Re(CO)₃ have been investigated experimentally, however, concerning the thermal cis → trans isomerization there is not much known from the theoretical side. To the best of our knowledge, only the group of Hasheminasab and co-workers showed example of transition metal complex of AB-containing Re(CO)₃ (Scheme 1).⁴ They designed and synthesized Re(CO)₃–AB, shown UV-visible absorption bands resulted from both the AB unit and the Re(CO)₃ diimine unit. Excitation of each compound led to isomerization of the azo-benzene bond, which they observed experimentally. But the data displayed here could not permit them to make comments on the nature of the AB isomerization (i.e., inversion vs. rotation). The fusion of the properties of transition metal complexes together with the photochromism has created a new generation of multifunctional metallic compounds able to react to an external stimuli, the light.³³

Scheme 1 Photoisomerization of Re(CO)₃–AB.

Much theoretical work has been done to investigate the photochemistry of photo-triggered system, and has been proven (TD)DFT^34,35 techniques are instrumental in interpreting photophysical behavior and spectra. DFT techniques with hybrid functionals are successfully used to visualize singlet and triplet CT states and reveal their characters by displaying differences of electron density distribution upon excitation and excited-state spin-density distributions.^35–37 To demonstrate this photo-triggered process theoretically, density functional theory (DFT) and time-dependent (TD-DFT) calculations are performed to investigate the activity of a Re(CO)₃ diimine complex with a pendant azo-benzene group at a molecular level. Here we wish to find out the most efficient isomerization routes in the S₀ and T₁ states according to a detailed study of their potential energy profiles of Re(CO)₃–AB as well as discover how the organometallic group might influence the cis → trans isomerization of the pendant AB group. It may also act as a systemic supplement that provides valuable insights for future experimental studies of searching effective photo-responsive materials.

2. Computational details

2.1. Ground-state calculations

To understand the properties of the potential energy surfaces of Re(CO)₃–AB, we conducted computational modeling of the trans and cis isomers of it. All the stable structures and the transition states for the thermal isomerization of Re(CO)₃–AB in the S₀ state were fully optimized unrestrainedly by density-functional theory (DFT) with the B3LYP^38,39 (i.e., the long-range corrected version of the Becke's three-parameter hybrid exchange functional with the Lee–Yang–Parr correlation functional, using the Coulomb-attenuating method) functional. In another comparison test, several other functionals (CAM-B3LYP,⁴⁰ M06,⁴¹ M06-2X⁴¹ and PBE0 (ref. 42)) were employed as supplements in order to avoid the possible shortcomings of DFT methods (Table S1 and Fig. S1†). All elements except rhenium were assigned the 6-311++G(d,p) basis set and the “double-ζ” quality basis set LANL2DZ with relativistic effective core potentials was employed for the rhenium atom based on the test of basis sets and computational costs consideration. These functional and basis sets have been justified in literature.^43–45 To investigate the isomerization mechanism, the potential energy surfaces (PESs) were generated by scanning the N₁N₂C₃ and the C₅N₁N₂ angles from 90° to 280° at a 10.0° interval. The frequencies of all the constructions were computed at the same level to identify the nature of the stationary points and the transition states, and the zero-point energy (ZPE) corrections for each structure were obtained. Furthermore, the intrinsic reaction coordinate (IRC) theory was applied to identify the transition states connecting reactants and products.

2.2. Excited-state calculations

Time dependent density-functional theory (TDDFT) with B3LYP/6-311++G(d,p) & LANL2DZ were used for the excited-state calculations as they were found to give authentic results.^46,47 The excited-state potential energy surfaces were produced by counting single point vertical excitation energies for each of the points in the ground-state potential energy surfaces. All calculations were performed using Gaussian 09 suite of programs.⁴⁸ Solvent interactions were simulated using a polarizable continuum model (PCM),⁴⁹ with parameters taken from the water, acetonitrile, tetrahydrofuran, hexane and gas phase, for the sake of completeness. In order to compare with the experimental results, we chose THF at last.

3. Results and discussion

3.1. Optimized ground-state geometry

The optimized structures of trans and cis isomers for Re(CO)₃–AB in the S₀ state have been illustrated in Fig. 1 and Table 1, and compared with the value of AB obtained in previous theoretical^23,50–54 and experimental work (Table 1).^55,56

Fig. 1 Optimized equilibrium geometries and structures of the two transition states (TS) of Re(CO)₃–AB in the S₀ state computed at B3LYP(PCM)/6-311++G(d,p) & LANL2DZ level.

Table 1 Some main geometrical parameters of equilibrium geometries and transition states (bond lengths in Å and bond angles in °) for Re(CO)₃–AB in the S₀ and T₁ states

Parameter	trans-S0	cis-S0	M	TS-inv-1	TS-inv-2	AB-trans/cis^a	Expt^b^,^c	trans-T1	cis-T1	TS-T1
a Ref. 52.b Ref. 56.c Ref. 55.
R(C5N1)	1.417	1.435	1.434	1.450	1.427	1.424/1.430	1.427/1.449	1.365	1.365	1.369
R(N1N2)	1.254	1.245	1.245	1.219	1.216	1.229/1.231	1.260/1.253	1.292	1.291	1.234
R(N2C3)	1.417	1.435	1.435	1.332	1.442	1.417/1.430	1.427/1.449	1.371	1.371	1.379
R(Re7Cl8)	2.545	2.546	2.547	2.545	2.552	—	—	2.546	2.546	2.551
R(Re7N9)	2.219	2.220	2.219	2.219	2.212	—	—	2.216	2.216	2.212
R(Re7N10)	2.225	2.225	2.224	2.222	2.244	—	—	2.235	2.234	2.246
R(Re7C11)	1.928	1.928	1.928	1.929	1.925	—	—	1.927	1.927	1.924
R(Re7C12)	1.915	1.914	1.915	1.915	1.912	—	—	1.914	1.914	1.913
R(Re7C13)	1.932	1.932	1.931	1.932	1.931	—	—	1.932	1.933	1.932
R(C11O14)	1.154	1.154	1.154	1.153	1.155	—	—	1.154	1.154	1.155
R(C12O15)	1.159	1.159	1.158	1.158	1.159	—	—	1.159	1.159	1.159
R(C13O16)	1.153	1.153	1.153	1.153	1.154	—	—	1.153	1.153	1.154
A(C5N1N2)	115.3	124.2	124.2	117.6	179.5	118.2/126.3	113.6/121.9	122.6	122.6	130.4
A(N1N2C3)	115.7	123.9	124.0	179.9	118.5	117.9/126.3	113.6/121.9	123.1	123.2	148.4
D(C6C5N1N2)	−179.2	136.4	54.1	−180.0	119.5	0.0/46.5	0.0/53.3	−176.1	175.2	179.4
D(C5N1N2C3)	−179.8	−9.9	9.6	33.4	−30.3	180.0/9.5	180.8/8.0	−108.9	109.5	1.1
D(N1N2C3C4)	0.3	−49.3	48.5	−123.4	0.5	0.0/46.5	0.0/53.3	5.3	175.2	−91.5

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Analysis of the crystallographic data exhibits that Re(CO)₃–AB crystallizes in the trans form with a N₁N₂ bond length of 1.254 Å that is almost identical to that of AB (average 1.25 Å), the C₅N₁N₂C₃ dihedral angle is −179.8°, while the N₁N₂C₃C₄ dihedral angle is 0.3°, so both the optimized metal diimine and AB groups are essentially planar, but these two units are non-coplanar. The non-planarity of cis-Re(CO)₃–AB is instead substantial, with N₁N₂C₃C₄ = −49.3° and C₅N₁N₂C₃ = −9.9°. While the geometrical parameters of metal diimine group Re(CO)₃ have almost no change in the two isomers. Table 2 displays the energy (E), the zero-point energy (ZPE) and the relative energy with respect to the trans-Re(CO)₃–AB for all the species. The trans isomer is 0.59 eV more stable than the cis isomer, due to twisting of the benzene rings around the C–N bond to avoid steric hindrance, which is lower than that of AB reported in the literature.^{26,47,52,54,57,58} It indicates that binding metal diimine group to benzene rings for AB greatly increases the stability of the trans isomer, that is to say lowers the energy gap of trans and cis isomers. It is interesting to note that although there are small differences between gas-phase and solution-phase, especially in the values of the NNC and CNN angles (Table S2†), the geometries of cis and trans on the ground-state (S₀) surface show an almost negligible dependency on the solvent, and the effect of the solvent on the cis-to-trans barrier height is rather small (Table S2†).

Table 2 The energies of all the stationary points for the potential energy profile of the S₀ and T₁ states computed at B3LYP(PCM)/6-311++G(d,p) & LANL2DZ level^a

	E/a.u.	ZPE/a.u.	ΔEZPE/eV
a ΔE is the relative energy with respect to the trans-S0.b Ref. 23.c Ref. 58.d Ref. 63.e Ref. 52.
trans-S0	−1793.1738426	0.31010	0.00
cis-S0	−1793.1519706	0.30993	0.59
M	−1793.1520029	0.31003	0.59
TS-inv-1	−1793.11056	0.30782	1.72
TS-inv-2	−1793.1176145	0.30811	1.48
trans-T1	−1793.130336	0.30813	1.13
cis-T1	−1793.1298957	0.30813	1.14
TS-T1	−1793.1020197	0.30722	1.88
AB-trans/cis^b	—	—	0.71
AB-trans/cis^c	—	—	0.70
AB-trans/cis^d	—	—	0.80
AB-trans/cis^e	—	—	0.83
AB-TS^e	—	—	2.28

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3.2. Potential energy surface

Another point worthy of discussion is the potential energy surface (PES), numerous theoretical studies have been carried out to characterize the PES connecting the two possible isomers of AB (trans- and cis-AB). They expounded that the photoisomerization proceeds through torsion of the central CNNC fraction and planar inversion of the NNC angle either independently or simultaneously, or through hybridizations of torsion and inversion movement. For the rotation type mechanism, two dihedral angles CNNC and NNCC are the most significant variables. For the inversion type isomerization route, two bond angles NNC and CNN are the most important variables. For these points the varying angles were fixed, all the remaining parameters optimized, then the potential energy surfaces were carried out. We will discuss the two isomerization pathways separately in the following part by two-dimensional potential energy surfaces.

3.2.1. Isomerization pathway in the S₀ state. It is well established that AB and derivatives undergo a thermal cis to trans isomerization (Scheme 1) in the ground state, hence we will analyze this isomerization mechanism in deeper detail below.

The cis → trans isomerization can, in principle, proceed along different reaction pathways and through diverse transition states, enormous theoretical efforts have devoted to the study of the isomerization of ABs in gas and condensed phase, and also pointed out the solvent can affect the isomerization mechanism both in the ground and excited states.^58–62

In accordance with previous studies, a transition state structure (TS-inv-1) (Fig. 1) was found to be responsible for inversion mechanism, being characterized by an almost linear N₁N₂C₃ moiety (Table 1). Suitable identification of this construction as a true transition state was done, checking for the being of only one imaginary frequency in normal mode analysis. Based upon analysis, TS-inv-1 is assigned to the N₂ stretching. Along this reaction path, the N₁N₂ distance decreases from the cis isomer to the TS-inv-1 and then increases in length as it approaches trans-S₀. TS-inv-1 presents the strongest N₁N₂ double bond character along the trajectory. An in-plane inversion of the N₁N₂C₃ angle is from 123.9° for cis-S₀ to approximate 180° for TS-inv-1, while this angle is 115.7° for trans-S₀. Inversion of the N₁N₂C₃ angle does not cause obvious change of the C₅N₁N₂ angle, only leads to a slight compression of N₁N₂ and N₂C₃ bonds and the elongation of C₅N₁ bond. As the cis-to-trans isomerization reaction proceeds through the inversion pathway, the N₁N₂C₃ angle increases up to approximately 180° at the transition state. This is not to say that the rotation coordinate is frozen and it does not play any role. In fact, the C₅N₁N₂C₃ angle is an important parameter in defining the critical structures for the inversion mechanism, as it can be seen in Table 1. The torsional C₅N₁N₂C₃ angle is from −9.9° of cis-S₀ to the value not far from 33.4° in the transition state, and −179.8° in the trans form. Therefore, this transition state is somewhat a mixture between inversion and rotational paths. And the hybridization of one nitrogen atom changes from sp² to sp because the coupling between the two nitrogen lone-pairs weakens. There exists greater steric hindrance between the lone pairs on the central nitrogens and p orbitals of the phenyl ring close to the 180° N₁N₂C₃ angle. From this perspective, the thermal cis–trans isomerization is not a pure inversion along NNC, but it follows a rotation-assisted inversion mechanism where the NNC angle must reach the value close to 180° but where the CNNC angle can take any value.

To have a better understanding of the potential energy profile, we calculated the reaction path from TS-inv-1 by scanning the N₁N₂C₃ angle as shown in Fig. 2(a), which is started from the TS-inv-1 along the forward and reverse pathway, respectively. This transition state suggests that along the lowest energy path connecting the cis and trans isomers, these two structures are characterized, with respect to TS-inv-1, by a sort of motion of the perpendicular phenyl ring increases (cis-S₀) or decreases (trans-S₀) of the N₁N₂C₃ angle, starting to define which will be the final isomers, upon surmounting a barrier of 1.13 eV on the potential energy surface, in AB this value is 2.05 eV.⁵² Manifesting that adding metal diimine group to the benzene rings of AB would lessen the cis–trans barrier height, making the thermal cis to trans isomerization easier than in AB. That is due to the MLCT effect minishes the energy demands for the isomerization reaction.

Fig. 2 Potential energy profiles along the inversion of N₁N₂C₃ angle (a), C₅N₁N₂ angle (b) and rotation around C₆C₅N₁N₂ dihedral angle (c).

Because the left and right side of this molecule is quite asymmetric, unlike AB and its derivatives have the same substitution of both sides, so we scan the C₅N₁N₂ angle from cis isomer with the aim to determine the second inversion pathway see Fig. 2(b) and (c). An invertomer TS-inv-2 can be envisaged, with C₅N₁N₂ angle constrained to be 179.1°, it is the transition state for trans → M belongs to the N₁ stretching and the energy difference between the trans and M is 0.59 eV. Actually, this transition state is found at slightly lower energy, 1.48 eV, compared with TS-inv-1. It is visualized from the PES (Fig. 2(c)) that there is an energy barrier during the rotation around the C₆C₅N₁N₂ axis from M to cis-S₀, requiring energy of about 0.11 eV (Table 2) for M to cis. The optimized geometries of M show little variation in the N₁N₂ bond length and the most bond parameters, but show overt variations in the C₆C₅N₁N₂ and C₅N₁N₂C₃ dihedral angles.

We have also tried to optimize the rotational transition state, looking for a hypothetical transition state geometry, such a transition state might influence the thermal isomerization mechanism. The conclusion is negative, although we tried a number of different initial structures for the TS-rot to obtain the saddle point for the rotational pathway in THF, we could not get a stationary point along that path, neither in water, in acetonitrile, in hexane and in gas phase. But we can find the inversion transition states (only tested TS-inv-1) in all the above condition except acetonitrile, geometrical parameters and energies are depicted in Table S3.†

Based on the above discussion, we can present an intuitive point of view about the global thermal isomerization pathways of Re(CO)₃–AB. The whole process of cis → trans isomerization for Re(CO)₃–AB can be depicted as follows: the transition state of the overall reaction is connected to two equilibrium structures cis-S₀ and trans-S₀, respectively, by the inversion of the N₁N₂C₃ angle. However, another transition state can link to intermediate M and trans-S₀ by the inversion of the C₅N₁N₂ angle. Through an energy barrier around 0.11 eV reaching cis isomer from M by the rotation of the C₆C₅N₁N₂ dihedral angle. This isomerization mechanism can thus be accounted in terms of successive rotation, inversion, and rotation processes. In the isomerization process, the TS-inv-1(S₀) and TS-inv-2(S₀) structures can only be changed as the N₁N₂C₃ and C₅N₁N₂ inversion increase or decrease, because the Re(CO)₃–AB molecule needs to remain its symmetry. At the same time, its change is also restricted by the metal complex (Re(CO)₃ moiety) in the symmetrical position. Therefore, the thermal inversion isomerization process involves the rotation of the phenyl ring.

3.2.2. Isomerization pathway in the T₁ state. The molecular structures of trans-T₁ and cis-T₁ are shown in Fig. 3. Remarkably, their stable geometries are twisted. There has an obvious multi-configuration character in the T₁ state of Re(CO)₃–AB molecular because the mixed of nπ* and ππ* nature, and the weight of the ππ* constituent enlarges following the N₁N₂ rotation, that is why the molecule is non-planar. There is no remarkable difference except the C₅N₁N₂C₃ dihedral angle between the geometries of the two isomers. The structural parameters of the two isomers are summarized in Table 1. The C₅N₁N₂C₃ is −108.9° for trans-T₁ and 109.5° for cis-T₁. As can be seen from the data in Table 2, the energy of trans-T₁ is approximately equal to that of cis-T₁.

Fig. 3 Optimized structures of all the stationary points of the T₁ state.

Next we studied the potential energy profile of T₁ state (Fig. 4). The geometries and energies of the two minimum points on the potential energy profile are consistent with trans-T₁ and cis-T₁. And we show the construction of the transition state TS-T₁ in Fig. 3, which is characterized by C₅N₁N₂C₃ = 1.1° (Table 1). We find the two phenyl rings of TS-T₁ are almost perpendicular, and the N₁N₂ double bond character still remains throughout the isomerization process. The barrier height for this isomerization is 1.88 eV (Table 2), which is determined by subtracting from the energy of trans-T₁. It exhibits an obvious preference for the rotation route. Accordingly, the Re(CO)₃–AB molecule can isomerize via the rotation manner at trans or cis point when it is excited to the T₁ state.

Fig. 4 Potential energy profile of the T₁ state along the C₅N₁N₂C₃ rotation coordinate.

3.3. Isomerization pathways of excited states

Electronic energies (E) and oscillator strengths (f) of the compound was computed by means of time dependent DFT calculations at the B3LYP(PCM)/6-311++G(d,p) & LANL2DZ level of theory. The data associated with the main transitions computed for this compound are presented in Table 3, and the shape of the involved frontier molecular orbitals are depicted in Fig. 5.

Table 3 Some selected vertical electronic transitions of Re(CO)₃–AB computed at B3LYP(PCM)/6-311++G(d,p) & LANL2DZ level

Electron state		Key excitation	Character	E (eV)/λ (nm)	f
a Ref. 58.b Ref. 52.c Ref. 26.
trans	S0 → S1	H → L (96%)	dπ(Re) → L(π*)	2.29 (542)	0.0004
	S0 → S2	H−3 → L (54%)	Ln → L(π*)	2.48 (500)	0.0009
		H−3 → L+1 (42%)	Ln → L(π*)
	S0 → T1	H−3 → L (33%)	Ln → L(π*)	1.79 (692)	0.0000
		H−3 → L+1 (59%)	Ln → L(π*)
	S0 → T2	H−4 → L (22%)	dπ(Re) → L(π*)	2.05 (606)	0.0000
		H−2 → L (44%)	Lπ → L(π*)
		H−2 → L+1 (20%)	Lπ → L(π*)
		H → L (19%)	dπ(Re) → L(π*)
cis	S0 → S1	H−1 → L (90%)	dπ(Re) → L(π*)	2.33 (531)	0.0004
	S0 → S2	H → L (59%)	dπ(Re) → L(π*)	2.43 (510)	0.1407
		H → L+1 (26%)	dπ(Re) → L(π*)
	S0 → T1	H → L (18%)	dπ(Re) → L(π*)	1.62 (765)	0.0000
		H → L+1 (70%)	dπ(Re) → L(π*)
	S0 → T2	H−1 → L (58%)	dπ(Re) → L(π*)	2.18 (569)	0.0000
		H → L (14%)	dπ(Re) → L(π*)
AB-trans/cis^a	S0 → S1	—	n → π*	3.24 (383)/3.36 (369)	—
AB-trans/cis^b	S0 → S1	—	n → π*	3.08 (403)/3.77 (329)	—
AB-trans/cis^c	S0 → S1	—	n → π*	2.53 (490)/2.72 (456)	—

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Fig. 5 Most relevant MOs associated with the vertical excitations and its energy in eV shown in Table 4. Occupied and unoccupied orbitals are represented by purple & blue or yellow & green surfaces respectively (top: trans form, bottom: cis form).

3.3.1. Electronic structure. The energies and compositions of some selected molecular orbitals are given in Table 4. The electron density of HOMO (H) and H−1 are mainly the mixed contribution of the dπ(Re) (53%/52%) and π*(CO) (19%/17%) orbitals of ligand while the low-lying virtual orbitals, LUMO (L) and L+1 are mainly composed of diazo antibonding orbital of ligand localized on both the diimine and functionalized AB moieties, 83% and 60% respectively. Though the crystal structures present that the two planes of the ligand are non-coplanar in the solid state, the single CN bond between the imine and the pendant phenyl of the AB unit is free to rotate. This would bring about at lowest partial overlap between the pyridine imine orbitals and the AB antibonding orbitals. The H−3 is residing at –N=N– moiety (34%) and π*(L) (66%).

Table 4 Frontier molecular orbital composition (%) in the ground state (S₀) for trans-Re(CO)₃–AB

Orbital	Energy (eV)	% of composition				Main bond type
		Re	CO	Ligand
				N=N	Aromatic system
L+1	−2.584	3	2	35	60	π*(L)
L	−3.259	5	4	8	83	π*(L)
H	−6.339	53	19	0	27	dπ(Re) + π*(CO) + π(L)
H−1	−6.445	52	17	0	31	dπ(Re) + π*(CO) + π(L)
H−2	−6.584	16	6	12	66	dπ(Re) + π*(L)
H−3	−6.677	0	0	34	66	n(L) + π*(L)
H−4	−6.890	64	20	2	13	dπ(Re) + π(CO) + π*(L)

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3.3.2. Electronic spectra. For the trans isomer of Re(CO)₃–AB, the calculated vertical electronic transitions show one weak transition at 536 nm (2.31 eV) corresponding to H → L transition having dπ(Re) → π*(L) character, internal metal to ligand (MLCT) transitions (Table 3, Fig. 5 and 6). The transition energy is much lower than the energy gap of AB.^26,52,58 This phenomenon indicates that introduction of metal complex can reduce the excitation energies of AB. The S₁ ← S₀ transition occurs at about the similar energy for both trans and cis. Unlike the trans excitations, the S₁ ← S₀ transition for the cis isomer shows slight intensity owing to the loss of symmetry making the transition possible. And transition appears at 503 nm (2.47 eV) due to intra-ligand charge transfer (ILCT) character which is connected with the Ln → Lπ* (63%) character delocalized throughout the entire molecule originating from the lone pair on the central nitrogens. The S₂ ← S₀ transition for the cis isomer is a lot more intense and slightly lower in energy than that of the trans isomer. It makes to be transformed the forbidden n → π* for the trans to a slight allowed transition as much the result of the loss of symmetry (from C_i to no symmetry) when it converts the trans to cis. The S₂ ← S₀ transition is a greater difference than the S₁ ← S₀ transition between the cis and trans. The cis isomer shows the lesser energy gap than trans. For the triplet vertical excitations of trans and cis, the excitation energy of trans for the T₂ ← S₀ transition is 2.05 eV. This energy of cis is slightly higher than the trans isomer. The T₁ ← S₀ transition for the cis isomer is slightly lower in energy than that of the trans isomer. Thus in summary the three absorption bands, assigned to the S₀ → S₂, S₀ → T₁ and S₀ → T₂ transitions, all have intra-ligand charge transfer (ILCT) character, involving rearrangement of electron density.

Fig. 6 Energy level diagram of MOs involved in singlet–singlet and singlet–triplet vertical excitations of trans form.

The orbital energies nearest the initial H–L gap are displayed in Fig. 5 and 6 (trans form), and one can see that the H and L energies are strongly affected by the Re(CO)₃ group. The substitution pattern has also an important effect on the computed H–L gaps. Our results show that the electron withdrawing group Re(CO)₃ substituted compound present a gap of about 3.05 eV lower than the pure AB counterpart 3.86 eV.⁶⁴

Encouraged by the results from above, we shifted our focus to the isomerization pathways of excited states, we calculated the ground state single point vertical excitation energy to obtain the T₁, S₁, T₂ and S₂ states potential energy profiles along the inversion and rotating ways.

3.3.3. Inversion way. On studying the PESs along the inversion way of excited states of Re(CO)₃–AB, Fig. 7(a) and Table 3, it is found that the S₂ ← S₀ transition is feasible. In the inversion way, at both sides of the cis- and trans-Re(CO)₃–AB, the absolute values of the slope of the S₂ profile are big, this demonstrates that isomerization reaction can't happen along the S₂ state potential energy surface when excited to S₂ state from the cis (trans) isomer. On the potential energy profile of T₂, the absolute values of the slope on both sides of the curve are also very large. And the S₂ and T₂ states are close in energy, it needs to overcome larger energy barrier as well, so the probability of this isomerization is not very great at this stage. Regarding the potential energy profile of T₂ state, corresponding minimum point of cis is below the potential energy profile of S₁ state, that is near the cis side, the potential energy profiles of T₂ and S₁ states exist cross at NNC = 130°, showing that N₁N₂C₃ potential energy surface of inversion coordinates of T₂ state provides a quick channel for its decay. Transferring our attention to the T₁ and S₁ states, both path-ways run downhill from cis isomer, whatever the mechanism, the larger slope of the curves on the cis side and their general asymmetry explain why the decay of cis in S₁ is faster than that of trans and we can give this hypothesis that the cis → trans quantum yields are larger than the trans → cis ones.

Fig. 7 Potential energy profiles in the excited states for Re(CO)₃–AB along the N₁N₂C₃ inversion coordinate (a) and the C₅N₁N₂C₃ rotation coordinate (b).

Along the inversion route, the possible isomerization pathway can be depicted as follows: cis-isomer would absorb light (hν) excited to the S₂ state from S₀ state and reached to the minimum point of S₂ state. Because there is a high energy barrier of S₂, so it can only be relaxed to a lower energy state for isomerization reaction to occur. Although the lifetime of the T₂ state is long, the transition to the T₂ state is forbidden, the possibility of occurrence of such intersystem crossing is less likely, thus relaxes to the S₁ minimum fast. The potential energy surfaces of S₁ and T₁ states are very close, it may achieve this transition through spin–orbit coupling effect. Along the T₁ state potential energy surface, near the maximum value close to the S₀ state, through intersystem crossing to the potential energy profile of S₀ state and gets to the trans isomer. The isomerization pathways are indicated by the arrow in Fig. 7(a). Along the isomerization pathway of inversion coordinates, rapid distribution of energy would occur due to repeated vibrational relaxation, when this process is completed, little energy is left to the coordinates associated with isomerization, leading to a low isomerization efficiency, and therefore the chance of this reaction is vanishingly small.

The CNN potential energy surface is depicted in Fig. S2,† by compared the potential energy curves with NNC, we find there is no significant difference, so we don't discuss here.

3.3.4. Rotation way. Along the excited state potential profile of the rotating way is shown in Fig. 7(b). Corresponding to the cis and trans minimum value area, the absolute values of the slope are bigger on the potential energy profiles of the S₂ and T₂ states, both path-ways run uphill from cis or trans, the larger energy barrier makes it difficult for cis and trans isomerization carried out on the two excited states. There is a conical intersection (CI) between S₀ and S₁ states potential energy surfaces, whereby the decay to S₀ occurs. If we consider the ππ* photochemistry, we can find that it is important to take into account the first excited singlet (S₁) with CNNC in the range 0° to 180°, with a deep minimum around 90°. The energy of S₁ state minimum value is very close to the maximum value of S₀ state. For the conical intersection (CI) of S₁/S₀, C₅N₁N₂C₃ dihedral angle is about 90°. The N atoms on the N₁N₂ and N₂C₃ bonds are the on the σ orbitals between sp/sp² hybridization in the CI, the lone pair electrons need to increase the nature of p orbitals, and the lone pair electrons on the N atoms need to keep its sp² orbitals in plane configurations as well. Along the rotating coordinates, n → π* occupies leading position, with the increase of the dihedral C₅N₁N₂C₃ rotation angle, the coupling of the lone pair electrons on the two N atoms fades gradually, making nπ* character taking absolute predominance in one of the benzene rings, leading to energy redistribution for excitation. Therefore, making two benzene rings relative torsional.

Intriguingly, we found that the maximum of T₁ state potential energy profile is not on top of the energy barrier of S₀ state, but below the maximum of S₀ state energy profile, this is because in the vicinity of the maximum, it forms a biradical system with the breakage of N₁N₂ bond, therefore, T₁ state is more stable at this time. Such a feature might influence the thermal isomerization mechanism. Due to the fact that S₀ state's potential energy surface would visit T₁ state at the dihedral angle C₅N₁N₂C₃ spinning area, isomerization process can be realized through the S₀–T₁–S₀, making the thermodynamic isomerization go through a recession of T₁ ← S₀ intersystem crossing. This isomerization process has a low energy barrier of 1.88 eV, but modification of the spin multiplicity is needed.

The calculations suggest that along the isomerization way of coordinate-rotation begins with absorbing light energy of cis isomer excited to S₂ state from S₀ state, because there is a high energy barrier of S₂ state, therefore it reaches the nadir of S₁ state potential energy profile after releasing its energy through fast vibration relaxation. With this, there are two pathways: (1) along the S₁ state potential energy profile, it can transit to the maximum of S₀ state potential energy surface without radiation by way of the CI of the S₁ and S₀ states potential energy surfaces, subsequently gains isomerization product cis isomer along the S₀ state potential energy profile; (2) from S₁ state to T₁ state by intersystem crossing, then reaches isomerization product along the potential energy profile of T₁ state by the intersection of S₀ and T₁ states (Fig. 7(b)).

4. Concluding remarks

In this work, we have theoretically studied the influence brought by the additional metal complex on AB isomerization and analyzed the thermal cis → trans isomerization mechanism of Re(CO)₃–AB.

Calculations reveal that the energy transfer process of metal complex occurs due to partial diazo antibonding character of the LUMO orbital, allowing the MLCT band to induce isomerization. We have found that binding metal diimine group to benzene rings for AB drastically lowers the energy barrier of the cis → trans isomerization, making it easier to isomerize. It is concluded that the thermal isomerization of Re(CO)₃–AB mainly follows the NNC rotation-assisted inversion mechanism in the S₀ state. We also searched the potential energy profiles of the vertical excitation for the excited states (T₁, S₁, T₂ and S₂) along the inversion and rotation pathways. By detailed analysis, we presented a hypothesis that can be accomplished in the following steps: (1) the isomerization can easily occur through the S₀/S₁ conical intersection (CI) and descend to the S₀ state; (2) a relaxation to the T₁ state from the S₁ state may occur and the reaction could take place via the S₀–T₁–S₀ path.

These complicating factors must be addressed in designing more efficient switching in organo rhenium complexes. We are continuing our work on searching effective photo chemical materials.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 21173096) and the State Key Development Program for Basic Research of China (Grant No. 2013CB834801).

Notes and references

1. J. Pérez-Miqueo, A. Telleria, M. Muñoz-Olasagasti, A. Altube, E. García-Lecina, A. de Cózar and Z. Freixa, Dalton Trans., 2015, 44, 2075–2091.
2. H. Bouas-Laurent and H. Dürr, Pure Appl. Chem., 2001, 73, 639–665.
3. J. Żmija and M. Małachowski, Issues, 2010, 1, 2.
4. A. Hasheminasab, L. Wang, M. Dawadi, J. Bass, R. Herrick, J. Rack and C. Ziegler, Dalton Trans., 2015, 44, 15400–15403.
5. M. Busby, P. Matousek, M. Towrie and A. Vlček, Inorg. Chim. Acta, 2007, 360, 885–896.
6. C. C. Ko and V. W. W. Yam, J. Mater. Chem., 2010, 20, 2063–2070.
7. T. Yutaka, I. Mori, M. Kurihara, J. Mizutani, K. Kubo, S. Furusho, K. Matsumura, N. Tamai and H. Nishihara, Inorg. Chem., 2001, 40, 4986–4995.
8. A. Sakamoto, A. Hirooka, K. Namiki, M. Kurihara, M. Murata, M. Sugimoto and H. Nishihara, Inorg. Chem., 2005, 44, 7547–7558.
9. M. E. Moustafa, M. S. McCready and R. J. Puddephatt, Organometallics, 2012, 31, 6262–6269.
10. M. S. Jana, A. K. Pramanik, S. Kundu, D. Sarkar and T. K. Mondal, Inorg. Chim. Acta, 2013, 399, 138–145.
11. B. D. Rossenaar, D. J. Stufkens and A. Vlček, Inorg. Chem., 1996, 35, 2902–2909.
12. B. D. Rossenaar, E. Lindsay, D. J. Stufkens and A. Vlček, Inorg. Chim. Acta, 1996, 250, 5–14.
13. I. R. Farrell and A. N. Vlček, Coord. Chem. Rev., 2000, 208, 87–101.
14. K. K. W. Lo, W. K. Hui, C. K. Chung, K. H. K. Tsang, T. K. M. Lee, C. K. Li, J. S. Y. Lau and D. C. M. Ng, Coord. Chem. Rev., 2006, 250, 1724–1736.
15. O. S. Wenger, Chem. Soc. Rev., 2012, 41, 3772–3779.
16. H. Nishihara, Coord. Chem. Rev., 2005, 249, 1468–1475.
17. M. Kurihara and H. Nishihara, Coord. Chem. Rev., 2002, 226, 125–135.
18. H. D. Bandara and S. C. Burdette, Chem. Soc. Rev., 2012, 41, 1809–1825.
19. P. Cattaneo and M. Persico, Phys. Chem. Chem. Phys., 1999, 1, 4739–4743.
20. L. Duarte, R. Fausto and I. Reva, Phys. Chem. Chem. Phys., 2014, 16, 16919–16930.
21. F. Zapata, M. Á. Fernández-González, D. Rivero, Á. Álvarez, M. Marazzi and L. M. Frutos, J. Chem. Theory Comput., 2014, 10, 312–323.
22. T. Schultz, J. Quenneville, B. Levine, A. Toniolo, T. J. Martínez, S. Lochbrunner, M. Schmitt, J. P. Shaffer, M. Z. Zgierski and A. Stolow, J. Am. Chem. Soc., 2003, 125, 8098–8099.
23. A. Cembran, F. Bernardi, M. Garavelli, L. Gagliardi and G. Orlandi, J. Am. Chem. Soc., 2004, 126, 3234–3243.
24. A. W. Adamson, A. Vogler, H. Kunkely and R. Wachter, J. Am. Chem. Soc., 1978, 100, 1298–1300.
25. C. W. Chang, Y. C. Lu, T. T. Wang and E. W. G. Diau, J. Am. Chem. Soc., 2004, 126, 10109–10118.
26. I. Conti, M. Garavelli and G. Orlandi, J. Am. Chem. Soc., 2008, 130, 5216–5230.
27. G. Tiberio, L. Muccioli, R. Berardi and C. Zannoni, ChemPhysChem, 2010, 11, 1018–1028.
28. T. Yutaka, I. Mori, M. Kurihara, J. Mizutani, N. Tamai, T. Kawai, M. Irie and H. Nishihara, Inorg. Chem., 2002, 41, 7143–7150.
29. K. Yamaguchi, S. Kume, K. Namiki, M. Murata, N. Tamai and H. Nishihara, Inorg. Chem., 2005, 44, 9056–9067.
30. T. Yutaka, I. Mori, M. Kurihara, N. Tamai and H. Nishihara, Inorg. Chem., 2003, 42, 6306–6313.
31. C. J. Lin, Y. H. Liu, S. M. Peng and J. S. Yang, J. Phys. Chem. B, 2012, 116, 8222–8232.
32. R. Sakamoto, S. Kume, M. Sugimoto and H. Nishihara, Chem.–Eur. J., 2009, 15, 1429–1439.
33. A. Telleria, J. Pérez-Miqueo, A. Altube, E. García-Lecina, A. de Cózar and Z. Freixa, Organometallics, 2015, 34, 5513–5529.
34. M. Turki, C. Daniel, S. Záliš, A. Vlček Jr, J. van Slageren and D. J. Stufkens, J. Am. Chem. Soc., 2001, 123, 11431–11440.
35. A. Cannizzo, A. M. Blanco-Rodríguez, A. Nahhas, J. Sebera, S. Záliš, A. Vlček Jr and M. Chergui, J. Am. Chem. Soc., 2008, 130, 8967–8974.
36. A. M. Blanco-Rodríguez, A. Gabrielsson, M. Motevalli, P. Matousek, M. Towrie, J. Sebera, S. Záliš and A. Vlček Jr, J. Phys. Chem. A, 2005, 109, 5016–5025.
37. A. Gabrielsson, M. Busby, P. Matousek, M. Towrie, E. Hevia, L. Cuesta, J. Perez, S. Záliš and A. Vlček Jr, Inorg. Chem., 2006, 45, 9789–9797.
38. A. D. Becke, J. Chem. Phys., 1993, 98, 5648–5652.
39. P. Stephens, F. Devlin, C. Chabalowski and M. J. Frisch, J. Phys. Chem., 1994, 98, 11623–11627.
40. T. Yanai, D. P. Tew and N. C. Handy, Chem. Phys. Lett., 2004, 393, 51–57.
41. Y. Zhao and D. G. Truhlar, Theor. Chem. Acc., 2008, 120, 215–241.
42. C. Adamo and V. Barone, J. Chem. Phys., 1999, 110, 6158–6170.
43. K. Potgieter, T. Gerber, R. Betz, L. Rhyman and P. Ramasami, Polyhedron, 2013, 59, 91–100.
44. V. Baranovskii and D. Maltsev, Comput. Theor. Chem., 2014, 1043, 71–78.
45. R. Sarkar and K. K. Rajak, J. Org. Chem., 2015, 779, 1–13.
46. K. Ichimura, Photochromic materials and photoresists, Photochromism: Molecules and Systems, ed. H. Dürr and H. Bouas-Laurent, Elsevier, Amsterdam, 1990, pp. 903–918.
47. C. R. Crecca and A. E. Roitberg, J. Phys. Chem. A, 2006, 110, 8188–8203.
48. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery Jr, J. E. Peralta, F. Ogliaro, M. J. Bearpark, J. Heyd, E. N. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. P. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, N. J. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox, Gaussian 09 Revision D.01, Gaussian, Inc, Wallingford, CT, 2009.
49. V. Barone, M. Cossi and J. Tomasi, J. Chem. Phys., 1997, 107, 3210–3221.
50. H. Fliegl, A. Köhn, C. Hättig and R. Ahlrichs, J. Am. Chem. Soc., 2003, 125, 9821–9827.
51. M. Quick, A. L. Dobryakov, M. Gerecke, C. Richter, F. Berndt, I. N. Ioffe, A. A. Granovsky, R. Mahrwald, N. P. Ernsting and S. A. Kovalenko, J. Phys. Chem. B, 2014, 118, 8756–8771.
52. L. Yu, C. Xu and C. Zhu, Phys. Chem. Chem. Phys., 2015, 17, 17646–17660.
53. M. Pederzoli, J. Pittner, M. Barbatti and H. Lischka, J. Phys. Chem. A, 2011, 115, 11136–11143.
54. R. L. Klug and R. Burcl, J. Phys. Chem. A, 2010, 114, 6401–6407.
55. T. Tsuji, H. Takashima, H. Takeuchi, T. Egawa and S. Konaka, J. Phys. Chem. A, 2001, 105, 9347–9353.
56. D. L. Beveridge and H. Jaffe, J. Am. Chem. Soc., 1966, 88, 1948–1953.
57. N. Kurita, S. Tanaka and S. Itoh, J. Phys. Chem. A, 2000, 104, 8114–8120.
58. M. Pederzoli, J. Pittner, M. Barbatti and H. Lischka, J. Phys. Chem. A, 2011, 115, 11136–11143.
59. J. C. Corchado, M. L. Sanchez, I. F. Galvan, M. E. Martín, A. M. Losa, R. B. Morgado and M. A. Aguilar, J. Phys. Chem. B, 2014, 118, 12518–12530.
60. T. Cusati, G. Granucci, E. M. Nunez, F. Martini, M. Persico and S. Vazquez, J. Phys. Chem. A, 2012, 116, 98–110.
61. J. Dokic, M. Gothe, J. Wirth, M. V. Peters, J. Schwarz, S. Hecht and P. Saalfrank, J. Phys. Chem. A, 2009, 113, 6763–6773.
62. L. M. Tiago, S. I. Beigi and S. G. Louie, J. Chem. Phys., 2005, 122, 094311.
63. N. Kurita, S. Tanaka and S. Itoh, J. Phys. Chem. A, 2000, 104, 8114–8120.
64. Y. J. Lee, S. I. Yang, D. S. Kang and S. W. Joo, Chem. Phys., 2009, 361, 176–179.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra10880f

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