Theoretical study on the thermal cis - Trans isomerization of novel acylhydrazone photoswitches

Organic Electronics 48 (2017) 154e164

Contents lists available at ScienceDirect

Organic Electronics journal homepage: www.elsevier.com/locate/orgel

Theoretical study on the thermal cis - Trans isomerization of novel acylhydrazone photoswitches Ting-Ting Yin, Zeng-Xia Zhao*, Li-Ying Yu, Hong-Xing Zhang** International Joint Research Laboratory of Nano-Micro Architecture Chemistry, Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin University, 130023 Changchun, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 21 April 2017 Received in revised form 15 May 2017 Accepted 31 May 2017 Available online 2 June 2017

Quantum chemical calculations of a potent class of photoswitches acylhydrazones have been carried out with the goal to describe their thermal cis / trans isomerization. The effects of substituents, in particular their number, position and nature of the substituents (electron donating/withdrawing groups (EDGs/ EWGs)), on activation energies incorporating solvent effects have been systematically researched with the ultimate goal to illuminate the isomerization process. Our results show that most parameters are highly dependent on the substitution pattern on the crucial positions (R1 and R2 position) of the backbone fragment, which in turn has a significant impact on the absorption spectra, the energy levels of molecular orbitals, the transition properties for the trans/cis isomers, the half-lives t1/2 and the rate constants for the thermal cis / trans isomerization of the compounds. In summary, both relative strength of the donor/accept groups and solvent polarity have significant impact on the electric properties. Finally, the nature of the transition state(s) and its dependence on substituents and the environment are discussed. An ingenious approach to the construction of reaction path was realized, and energy barriers were determined from two-dimensional potential energy surfaces of the ground states. © 2017 Elsevier B.V. All rights reserved.

Keywords: DFT Acylhydrazone Substituent Isomerization Transition state

1. Introduction Molecular photoswitches are ubiquitous in various fields ranging from the interface with biology and materials contexts [1e5], supramolecular and organic chemistry [6e8], photopharmacology and optochemical genetics [3,6,7] to materials science and data storage [6e9], all the way to the interface with physics [10e12]. In the past few decades, albeit it is clear to us that there exists several classes of these highly interesting functional molecules (azobenzenes, stilbenes, diarylethenes and spiropyrans, etc.) that have been heavily investigated and optimized, inherent limitations and distinct obstacles remain. Acylhydrazones as an innovative, yet under-exploited class of photochromic molecules based on the imine structural motif owns characteristic structural properties. As a result, the important and pervasive acylhydrazones have attracted much attention and were widely used in the past few decades from medicine, agriculture and chemistry as

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H.-X. Zhang).

(Z.-X.

http://dx.doi.org/10.1016/j.orgel.2017.05.053 1566-1199/© 2017 Elsevier B.V. All rights reserved.

Zhao),

[email protected]

complexones [13], photo-thermochromic compounds and precursors for organic synthesis [14,15], to dynamic combinatorial chemistry [16] and covalent organic frameworks [17,18], etc. In many cases, properties and reaction ability of acylhydrazones are determined by their conformations and/or configurations. The existence of the carbonyl oxygen atoms and the imine nitrogen atoms in acylhydrazones encourages the formation of a chelate binding core. The facile synthesis, high stability enhanced stability toward hydrolysis in aqueous media and functional diversity of this azomethine group, which is characterized by the triatomic structure NeN]C, can be cited as reasons for their popularity. The acylhydrazones functional group is perfectly suited to address the challenge as its trans/cis isomerization can be activated by both light and chemical inputs. This dual control over the rotary motion of a molecular system showcases the uniqueness of the acylhydrazone functional group. A quick survey of the structures of acylhydrazones (Fig. 1) reveal that configurational isomerism stemming from the intrinsic nature of the imine-like N]C double bond, altering the configuration and hence the steric and electronic relationship between the two parts on either side of the acylhydrazone center part (-N]C-) [19]. These structural motifs give acylhydrazone its chemical and physical properties, in addition to

T.-T. Yin et al. / Organic Electronics 48 (2017) 154e164

155

acylhydrazones, bearing different substituents in all four crucial positions of the backbone fragment, exhibited some basic properties which they observed experimentally. But the data displayed here could not permit them to make comments on the property of the N]C isomerization (i.e., inversion vs. rotation). Inspired by the work of Hecht [30] and having this information in mind, we focused our attention on the elusive isomerization mechanism of acylhydrazones. We present herein a systematic study describing the effect of different substitutions on the overall performance of this new class of highly promising photoswitchable acylhydrazones (Fig. 1) by describing the effect of different substitution on the R1 and R2 positions, as well as assess the strong solvent impact on geometric and properties of acylhydrazones derivatives. The photochemical and photophysical properties of all compounds were investigated, together with the thermal stability of the cis forms using hybrid density functional theory (DFT). In particular, the vertical absorption spectra were computed using timedependent DFT (TD-DFT) and thermal back-isomerization rates were predicted using Eyring transition-state theory, and last but not least, the preferred isomerization mechanism was investigated. This work may act as a systemic supplement that provides valuable insights for future experimental studies of searching highly promising photoswitches and guide the synthesis of new photoswitches with specific characteristics designed beforehand. 2. Computational details 2.1. Ground-state calculations Fig. 1. Mechanism for cis / trans interconversion of acylhydrazone complexes investigated for structure-property relationship.

playing a vital part in determining the scope of applications it can be included in. Acylhydrazones produce an especial interesting scaffold, allowing more feasible photochemical isomerization than hydrazones [20] and acylhydrazone photoswitches have been testified invaluable in some intriguing applications [21]. Isomerization of acylhydrazones induces configurational change around the N]C double bond which similar to stilbene-, azobenzene-, and indigo derived photo-switches. Unlike azobenzenes [22,23], acylhydrazone photoswitches have lots of superior properties: (i) the geometry of both trans- and cis-acylhydrazone are tunable, which is a main advantage and provides a higher degree of control over their properties; (ii) acylhydrazones are more robust and resistant to oxidation; (iii) relative ease of synthesis from esters or acyl chlorides via their hydrazide precursors; (iv) importantly, their thermal stability can be controlled with no need to specifically stabilize the cis-isomers; (v) moreover, electron-donating and -withdrawing, heteroaromatic and large aromatic groups such functional groups could be introduced with no inhibiting the photochromic function, which opens the door to a good deal of applications to these photoswitches. However, acylhydrazones and their photoisomerization are far less researched, the systems reported in the literature thus far provide only scattered indications of photochromic properties of this class of compounds [24], including: Lehn [19,25,26] and Aprahamian's research [27e29]. Considerable synthetic efforts and complex structure-property relationship render the exploitation and development of this new photoswitches family difficult. Interestingly, in spite of a few reports have been investigated experimentally, however, concerning the thermal cis / trans isomerization there is not much known from the theoretical side. For a comprehensive review, we referred to the work of Hecht et al. [30], and relevant references are cited herein. They mostly designed and synthesized more than a dozen different photochromic molecules based on the imine structural motif

In this paper, we report quantum chemical calculations on acylhydrazone derivatives by conducting computational modeling of the trans and cis isomers of them. In order to further understand the effects of different substituents on the photophysical and photochemical properties of this class of complexes, six acylhydrazone derivatives were studied theoretically in this work. The donor strength is modified by substituting the R1 and R2 positions of such isomers with the groups depicted in Fig. 1. Among them is the parent acylhydrazone molecule 2, starting from 2, we focused on (i) change the nature of para-substituents on R2 position (-phenyl or -2-thiophene); (ii) keep R2 unchanged, we alter the nature of para-substituents on R1 position (electron-donating groups or electron withdrawing groups). All calculations were performed using Gaussian 09 suite of programs [31]. All the stable structures and the transition states were fully optimized unrestrainedly by density-functional theory (DFT) [32,33] with the B3LYP [34] functional. More-over, all elements were assigned the 6311þþG(d,p) basis set based on the test of basis sets and computational costs consideration (Table S2). Solvent effect was simulated using the self-consistent reaction field (SCRF) method based on the polarizable continuum model (PCM) [39], with parameters taken from the acetonitrile (ACN). In order to assess the influence of solvent on the isomerization properties of the acylhydrazone complexes, solutions of 2, 11, 12, 13, 14 and 16 in solvents with varying polarities (gas phase, chloroform, THF, acetone, ethanol, acetonitrile, and water (increasing solvent polarity)) were prepared. Normal mode analysis of the transition structures delivered a single imaginary frequency, and the zero-point energy (ZPE) correction for each structure was obtained. Furthermore, the intrinsic reaction coordinate (IRC) theory was applied to identify the transition states connecting reactants and products. The natural bonding orbitals (NBO) analysis [40] was performed using NBO 3.1 program as implement in the Gaussian 09 package. The kinetic parameters such as reaction rate constants of the thermal cis / trans isomerization at the temperature of 298.15 K (room temperature) were studied using conventional transition

156

T.-T. Yin et al. / Organic Electronics 48 (2017) 154e164

state theory (TST) [41,42] as implemented in the KiSThelP program [43]. Thermal transition rates are given in Eyring theory as Equation (1):

kðTÞ ¼ kðTÞs

    kB T Q z ðTÞ DS0 ðTÞ DH0 ðTÞ exp  exp  h Q R ðTÞ R RT

(1)

is calculated with the reaction path degeneracy s given under ideal gas conditions. Here, k is the transmission coefficient, T is the temperature in kelvin, kB is the Boltzmann's constant, h is Planck's constant. DH0, DS0, Q z and QR are the relative enthalpy, the relative entropy, the total partition function of the transition state and the reactant, respectively. One can also calculate the s, this number is correctly given by Ref. 43, Equation (2):

s¼

nz  sR nR  sz

(2)

Here, nz , nR and sz , sR are the number of chiral isomers at the transition state and the reactant, and the rotational symmetry numbers, respectively. 2.2. Excited-state calculations Time dependent density-functional theory (TD-DFT) [38,44] with CAM-B3LYP/6-311þþG(d,p) were used for the excited-state calculations. In order to ensure the reliability of the selected functionals used in the present calculation again, these abovementioned B3LYP, PBE0 [35], CAM-B3LYP [36], M06 [37], M06-2X [37] are considered as the five most applicable functionals. Evidently, CAM-B3LYP exhibits very good performance reproducing the examined absorption spectra (Fig. S1 and Table S1). Potential energy curves (PECs) were constructed starting from the cis isomer optimized geometry by constraining either the NeN]CeC dihedral angle or one of the NeN]C bond angles while optimizing all the other structural parameters. The constrained angle was incremented in 10 steps between the cis and trans minima, and the energy of the optimized structure obtained at each point. 3. Results and discussion In this work, we were motivated to elucidate the mechanism of thermal cis - trans isomerization of azoheteroarene photoswitches. As a first step, we carefully studied acylhydrazone molecules 13, 16, 11, 2, 14 and 12 previously synthesized by Hecht et al. [30] and depicted in Fig. 1. 3.1. Optimized ground-state geometry The B3LYP(PCM)/6-311þþG(d,p) optimized ground-state structures of the simplest acylhydrazone molecule 2 (R1 ¼ phenyl, R2 ¼ -phenyl) are presented in Fig. 2 along with the numbering of some key atoms, important bond distances, angles and dihedrals are summarized in Table 2 together with different R1para-substitution isomers 13 (R1 ¼ -N-methyl), 16 (R1 ¼ -2thiophene), 11 (R1 ¼ -phenyl), 14 (R1 ¼ -4-Pyridine) and 12 (R1 ¼ -Nitryl). They all belong to the (R2 ¼ substituents) 2thiophene derivatives, because through Hecht and co-workers's exploration, they illustrated these molecules are the most superior ones after several important aspects research. Going from molecule 2 to 13, 16, 11, 14 and 12, the main lightspot is that the latter systems undergo a 2-thiophene group substituting (R2 position) rather than phenyl ring, then R2 remains fixed, the R1 position is replaced by a donor or accept group instead of phenyl group, where thiophene-

derivatives group is formed. Structurally, for each acylhydrazone molecule studied (Table 2), the N2]C3 bond of the cis isomer is longer than that of the trans one. Acylhydrazone 2 has the shortest N2]C3 distance for both conformations, while compounds 13, 16, 11, 14 and 12 have the same N2]C3 bond distance. The N1eN2 distances were quite similar between the acylhydrazones ranging from 1.361 to 1.368 Å for the cis isomers and 1.359 to 1.365 Å for the trans isomers. The different substituents appear to contribute only slightly to the N2]C3 bond as evidenced by the very small increase in bond length upon para-substitution with different electrondonating/withdrawing groups on the R1 position. Likewise, all bond distances were not overly affected by changing the donor/ accept groups attached to the p-bridge. Analogously, like the N2]C3 distance, the N1eN2]C3 angles are very similar. For the trans conformation, the angles range from 116.3 to 116.9 , while the range for the cis isomers were from 119.3 to 120.0 . N2]C3eC4 dihedral also belongs to the same situation. In our calculation, the N1eN2]C3eC4 dihedral angles are almost 180.0 , while the N1eC5eC6eC7 dihedral angle is 159.5 , 163.6 , 180 , 150.4 , 180.0 and 147.8 , respectively, indicating that right heterocyclic ring with the whole ligand is nearly a planar configuration except the left heterocycle part bowed to some extent from the right moiety (see Fig. 2 trans-vertical view and trans-side view). The non-planarity of cis-acylhydrazones are instead substantial, especially, the two methyl groups in acylhydrazone cis-2 are predicted to force these species into a twisted conformation with N1eN2]C3eC4 ¼ 2.0 and N1] C5eC6eC7 ¼ 151.6 , respectively. Follow a myriad of information above, we see that almost all bond distances and angles were not significantly modified by isomeric effects. Energetically, Table 2 displays the relative energy (DEZPE) with respect to the cis-form for all species. The trans isomers are more stable for all molecular groups, the differences between the cis and trans ground-state energies were calculated and found to be very similar ranging from 4.7 kcal mol1 for 13, 4.9 kcal mol1 for 16, 14, and 12, and 4.1 kcal mol1 for 11, which are all lower than that of 2 counterpart, the energy barriers vary very small (<0.8 kcal mol1) between different substituents. It indicates that different aromatic ring instead of benzene ring (molecule 2) increases the stability of the trans isomer, that is to say lowers the energy gap of trans and cis isomers. To avoid proliferation of tables and figures, we present only the gas-phase and acetonitrile (ACN) results for some isomers herein and the rest cases are presented in Supporting Information, see Table S3 and Table S4. We elected these geometric parameters to explore the variation in bond distances and angular distortion of the p-conjugated framework between donor and acceptor groups when circumscribed by polar or non-polar solvents. From Table 1 and Table 2, we see that almost all bond distances were not significantly modified by solvents. Concerning the geometrical aspects, inspecting the bond distances and dihedral angles, we see that the distances and dihedral angles were weakly affected with the solvent's existence. Now let's look at the energy as the change trend of solvents. Fig. 3 compares the barrier height calculated with the PCM model in different solvents. The barrier heights of TS passing from 27.0 kcal mol1 in gas phase to 29.1 kcal mol1 in water solution according to the PCM calculations, and 6.0 to 2.4 kcal mol1 in gas phase to water solution of intermediate structure M, we take molecule 2 for example, we can see from Fig. 3 that the energy of TS and trans isomers become bigger as the greater the polarity of solvents, while it becomes smaller and smaller for M. The situation of the other molecules is the same, for data details, see Table S5 in Support Information.

T.-T. Yin et al. / Organic Electronics 48 (2017) 154e164

157

Fig. 2. Optimized equilibrium geometries and structure of the transition state (TS) of acylhydrazone molecule 2 in the S0 state computed at B3LYP(PCM)/6-311þþG(d,p) level. Table 1 Selected geometrical parameters and relative energies of the stationary points on the S0 surface in gas phase computed at B3LYP/6-311þþG(d,p).

13

16

11

2

14

12

cis trans TS-inv M cis trans TS-inv M cis trans TS-inv M cis trans TS-inv M cis trans TS-inv M cis trans TS-inv M

R(N2C3)

R(N1N2)

A(N1N2C3)

A(N2C3C4)

D(N1N2C3C4)

D(N1C5C6C7)

DEZPE

1.290 1.285 1.255 1.288 1.286 1.282 1.256 1.288 1.286 1.282 1.255 1.289 1.283 1.280 1.255 1.286 1.286 1.282 1.256 1.289 1.286 1.282 1.256 1.289

1.362 1.360 1.287 1.366 1.355 1.355 1.285 1.367 1.354 1.356 1.288 1.368 1.360 1.356 1.287 1.370 1.357 1.358 1.288 1.370 1.358 1.359 1.288 1.371

120.0 116.8 177.4 121.6 120.4 117.2 177.5 121.5 120.4 117.1 177.5 121.3 119.4 117.3 177.5 121.4 120.2 117.0 177.6 121.1 120.1 117.0 177.6 121.1

133.2 122.0 125.3 119.5 133.0 122.1 125.4 119.6 133.1 122.2 125.4 119.6 129.8 122.2 125.5 119.9 133.1 122.2 125.4 119.7 133.1 122.2 125.4 119.7

2.6 179.6 1.0 179.3 2.2 179.6 2.8 179.5 1.9 180.0 1.9 179.3 1.6 179.3 1.9 179.3 2.0 179.5 0.8 179.4 2.1 179.4 2.7 179.3

157.5 159.4 157.4 159.2 164.4 165.2 170.0 165.7 152.2 180.0 150.6 152.7 155.6 151.9 149.7 152.6 151.8 151.6 149.3 152.5 150.3 150.6 148.3 151.8

0.0 3.9 25.8 6.9 0.0 4.0 26.6 6.0 0.0 3.4 26.8 6.2 0.0 4.7 27.0 6.0 0.0 3.8 27.2 5.9 0.0 3.7 27.4 5.8e3.64897065

Notes: Bond distances (in Å), angles (in degrees), relative total energies between cis and trans isomers (in kcal mol1), (Ets-Ecis) in kcal mol1 and zero-point energies (ZPE) correction included.

3.2. Absorption properties and electronic excitation energies The absorption results of the TD-DFT calculations and simulated absorption spectra for the trans-acylhydrazones are shown in Fig. 4 and Table 3. Moreover, the corresponding vertical transition

wavelength lmax, the oscillator strength f are listed in Table 3 for direct comparison to experimental data. Fig. 4 shows the main absorption bands for all trans isomers in ACN. Our calculation predicts the positions of the maximum absorption bands of all trans isomers (the order as follows: 2 > 12 > 13 > 16 > 11 > 14), the

158

T.-T. Yin et al. / Organic Electronics 48 (2017) 154e164

Table 2 Selected geometrical parameters and relative energies of the stationary points on the S0 surface in acetonitrile computed at B3LYP(PCM)/6-311þþG(d,p).

13

16

11

2

14

12

cis trans TS-inv M cis trans TS-inv M cis trans TS-inv M cis trans TS-inv M cis trans TS-inv M cis trans TS-inv M

R(N2C3)

R(N1N2)

A(N1N2C3)

A(N2C3C4)

D(N1N2C3C4)

D(N1C5C6C7)

DEZPE

1.289 1.285 1.257 1.289 1.289 1.285 1.257 1.289 1.289 1.285 1.257 1.289 1.285 1.282 1.256 1.286 1.289 1.285 1.257 1.290 1.289 1.285 1.257 1.290

1.361 1.359 1.289 1.366 1.366 1.362 1.288 1.368 1.365 1.363 1.290 1.369 1.366 1.363 1.290 1.370 1.368 1.364 1.291 1.371 1.368 1.365 1.291 1.371

120.0 116.9 178.5 122.0 119.5 116.6 178.6 121.8 119.6 116.5 178.6 121.6 119.3 116.6 178.7 121.7 119.3 116.4 178.7 121.4 119.4 116.3 178.6 121.3

133.1 122.0 125.3 119.8 133.2 122.0 125.4 119.8 133.3 122.0 125.4 119.8 130.3 122.3 125.4 120.2 133.3 122.0 125.4 119.9 133.3 122.1 125.4 119.9

2.6 179.6 5.0 179.3 3.1 179.6 11.4 179.4 2.7 180.0 6.2 179.3 2.0 179.4 0.4 179.3 3.1 179.5 1.9 179.5 3.1 179.4 8.5 179.3

157.6 159.5 158.9 159.2 162.3 163.6 170.0 162.7 149.7 180.0 149.6 149.9 151.6 150.4 149.3 149.6 147.0 180.0 146.9 148.5 146.4 147.8 146.2 147.4

0.0 4.7 29.0 2.6 0.0 4.9 29.1 2.6 0.0 4.1 29.3 2.8 0.0 5.0 29.0 2.5 0.0 4.9 29.6 2.8 0.0 4.9 29.7 2.7

Notes: Bond distances (in Å), angles (in degrees), relative total energies between cis and trans isomers (in kcal mol1), (Ets-Ecis) in kcal mol1 and zero-point energies (ZPE) correction included.

Fig. 3. Schematic plot of the energy barrier (kcal mol1) of compound 2 to interconversion in the gas phase and different solutions and zero-point energies (ZPE) correction included.

changes in the absorption band of each trans-acylhydrazone with EDGs and EWGs corresponding to that of the 2 mainly depend on the electron-donating ability of the para-substituent (the order of the electron-donating ability: 13 > 16 > 11 > 14 > 12), which is in agreement with the experimental observation [30]. It is worth noticing the case of 12, which, although the weakest electrondonating ability, results in the largest hypochromatic shift compared to 2. In this work, we demonstrated that attaching electron accepting/donating groups (EWGs/EDGs) at the paraposition instead of the phenyl group leads to a larger blue shift in the absorption spectra, especially the EWGs. Obviously, these spectra show little solvent dependence, Fig. 4 (right one) (acylhydrazone 2) and Table S6 show the absorption spectra in different solvents, the peak wavelengths differ only by 8 nm at most among the spectra in gas and different solvents for the compounds investigated. In general, an appreciable red shift is recognized on

going from gas phase to these prescribed solvents, while there seems no significant difference among these given solvents. It is interesting to compare differences between the trans excitations. TD-DFT predicts low-intensity n-p* absorbance than p-p* for all isomers as is seen in the experimental spectra [30]. The n-p* transitions all have very weak oscillator strength, while the first transition, p-p*, shows some intensity. Table 3 shows the vertical excitation of the acylhydrazones which involves two main electronic transitions. The structures and energy levels of the MOs are displayed in Fig. 5. For all trans complexes, it is apparent that the contribution to the highest occupied molecular orbital HOMO (H) and the lowest unoccupied molecular orbital LUMO (L) come from the p N(2) ¼ C(3) bond/ring and p* antibonding N(2) ¼ C(3) bond/ ring, respectively. The S0 / S1 transitions are mainly attributed to the transitions from HOMO to LUMO (pNC / p*NC and pring / p*ring). Another group of the S0 / S5, 6, 7, 8 consists of the HOMO-X / LUMO (X ¼ 6, 7) transition that originated from the lone electron pairs in the n orbital arising from the N(2) atom and the p* antibonding N(2) ¼ C(3) bond, respectively. The difference is S0 / S9 transition comes from molecular 12, it belongs to the lone-pair n orbital to p* antibonding orbital (LUMOþ1) of N(2) ¼ C(3) and phenyl group, this system shows some additional charge transfer to their NO2 substituents. Molecules with small HOMO-LUMO energy gaps are more polarizable than ones having large energy gaps [45]. Structural modifications and solvent inclusion are two methods to reduce the HOMO-LUMO energy gap. Our calculations indicate that the EDGs in the para-position increase the energy level of p and p* molecular orbitals and decrease the n orbital energy. The HOMO orbitals on the energy scale are stable than the corresponding LUMO orbitals, leading to lower the HOMO / LUMO gap. The isomeric effect caused the HOMO-LUMO energy gap to decrease, the narrowing of the energy gap up to 1.33 eV relative to compound 2. However, the solvent effects on HOMO-LUMO energy gap values are almost negligible. This is observed for all derivatives, irrespective of their conformation, when moving from nonpolar (chloroform) to polar (water) solvents (Table S7), indicating that this property is not appreciably affected by the solvent.

T.-T. Yin et al. / Organic Electronics 48 (2017) 154e164

159

Fig. 4. The absorption spectra of trans isomers of acylhydrazone compounds 2-16 and absorption spectra of trans isomer of 2 in various solvents of different polarity.

Table 3 Spectroscopic properties of the trans isomers in ACN at 298.15 K, including vertical transition energies (in nm/eV) and oscillator strength (f), and molecular orbitals involved in the excitation features of the trans isomers. n - p*

p - p*

Structures

state

Transition

13 16 11 2 14 12

S8 S7 S6 S5 S7 S9

H H H H H H

-

7 6 6 6 7 6

/ / / / / /

L L L L L Lþ1

coeff./gap (eV)

lmax/E

f

state

Transition

0.61/9.26 0.59/9.17 0.62/9.17 0.52/9.27 0.53/9.18 0.56/9.41

224.0 223.7 224.9 227.0 224.7 225.4

0.0020 0.0071 0.0004 0.0249 0.0026 0.0015

S1 S1 S1 S1 S1 S1

H H H H H H

(5.535) (5.544) (5.514) (5.463) (5.517) (5.502)

/ / / / / /

L L L L L L

coeff./gap (eV)

lmax/E

f

Expt.

0.51/6.05 0.67/6.50 0.68/6.55 0.68/6.94 0.66/6.51 0.50/5.61

318.6 311.6 309.1 285.3 308.1 319.6

1.3270 0.9414 0.9150 1.0561 0.8414 0.8224

338 323 319 295 320 325

(3.891) (3.979) (4.011) (4.346) (4.024) (3.879)

Fig. 5. Frontier molecular orbital surfaces and molecular orbital energy level diagrams of trans acylhydrazone compounds.

3.3. Natural bond orbital analysis and dipole moment We have analyzed the charge separation by computing the NBO

atomic charges, splitting the molecular systems into three important moieties: the region of R1 substituent groups, the region of R2 substituent groups and the remaining part R3. The results obtained

160

T.-T. Yin et al. / Organic Electronics 48 (2017) 154e164

for NBO charge distribution are clearly presented in Table S8, we mainly analyze the partial charge of R1. From Table S8, we can see that a significant charge separation occurs in both trans/cis and TSinv isomers. Evidently, the region bearing the donating group exhibits a positive net charge (compounds 13, 16, 11 and 2), whereas the region bearing the acceptor group carries an opposing negative net charge (compounds 14 and 12), as depicted in Table S8 for the net charges on the partitioned moieties. It can now be stated with confidence that 13, 16, 11 and 2 have electron donating groups, 14 and 12 have electron withdrawing groups. In addition, we can see that the charge distribution of R1 substituent part for the TS-inv (the square brackets) compared to cis isomers (parentheses) are increased, meaning that the aromatic ring of R1 portion acts as an electron donor. Accordingly, the electron donating groups (EDGs) are advantageous to the inversion mechanism of acylhydrazones. The B3LYP(PCM)/6-311þþG(d,p) static results for dipole moment m is presented in Fig. 6 and Table S9/S10. From Table S9/10 we can see that for any class of these molecules those from the cis conformation have greater values of m with respect to its trans counterpart. In Fig. 6, the results show that type-13 isomers have the highest m values because of the presence of stronger donor group at the R1 position, which strengthens the electron-donating capacity. When it comes to the solvent effects on m, the solvent polarity has a significant impact. From Fig. 6 we clearly see that in general, for these given solvents, the molecules displayed an appreciable increment in the m values as the solvents polarity increased, the different molecules predict qualitatively the same trend for both cis and trans conformers, and the dipole moment m nearly saturates at ethanol and therefore is not significantly influenced by more polar solvents. Another interesting finding originates from the dipole moments (m) change along the reaction path. For almost all acylhydrazones, calculated m values decrease when approaching the TS from the cis side (Table S9). A smaller dipole at TS is less effectively stabilized by the interacting, polarizable continuum than a larger cis dipole. In accordance with an antecedent theory based on the free energy of dipoles embedded in a spherical cavity in a polarizable continuum [46,47], lnkc/t increases proportionally to the difference (mTS)2 e (mcis)2, where mTS and mcis are the dipole moment of the TS and cis isomer, respectively [48]. The dipole changes are particularly large for 14 (28.2 D2) and 12 (28.0 D2) but less pronounced for 13 (19.7 D2). This may help to expound the reason for 14 and 12 are thermally more stable than other

molecules. 3.4. Thermal stability of the cis isomers Perhaps the most remarkable feature of these acylhydrazones is the extremely long thermal half-life of the cis isomers in virtue of their stabilization by the electron-donating and electronwithdrawing substituents. To insight into the effect of substitution on the thermal stability, the half-live t1/2 of this kind of derivatives were investigated in detail in ACN at 298.15 K to obtain data over a reasonable theoretical time frame (Table 4). Moreover, the Eyring activation parameters and the discrepancies of the thermal cis / trans isomerization rates kc/t for the acylhydrazone series have been taken into account in the ACN environment, together with the thermal half-lives calculated from Eyring rates as t1/2 ¼ ln2/kc/t. We can see that acylhydrazone 2 exhibits a half-life of t1/ 4 2 ¼ 1.23  10 h (Table 4), whereas cis-16 was found to be more stable, with a thermal half-life of t1/2 ¼ 1.87  104 h at 298.15 K. All compounds described in this work are thermally more stable than the experimental value, in the order 14 > 12 > 11 > 13 > 16 > 2. Interestingly, although structurally quite similar, the nature of the substituent groups exerts a significant effect on the rate of cis / trans thermal isomerization, with cis-14 and cis- 12 (EWGs) showing longer half-life than cis-13/16/11/2 (EDGs), that is to say, replacing the electron-donating group substituent at the R1 position by an electron-withdrawing group substituent raises the halflife obviously. Therefore, it seems that introducing an electrondonating group at the R1 position such as acylhydrazone 2 should be avoided if one is seeking cis isomers with very long thermal halflives, by contrast, 14 and 12 are preferred which possess electronwithdrawing groups. We have ensured that its property should be inferred by comparison of the thermal relaxation rates t1/2 experimentally observed [30]. In previous studies [49,50], the thermal cis / trans isomerization kc/t can be accelerated in parasubstituent with electron-donating groups or electronwithdrawing groups. The fortified stability of acylhydrazones may come as a surprise in view of previous findings [48], whereby pacceptors with a -M effect such as eNO2 in positions para or ortho results in smaller activation energies for cis / trans isomerization, together with shorter lifetimes. In order to assess the influence of solvent on the isomerization

Fig. 6. B3LYP(PCM)/6-311þþG(d,p) results for dipole moment m (in Debye) of trans and cis acylhydrazone isomers in gas phase (ε ¼ 1.00), chloroform (ε ¼ 4.71), tetrahydrofuran (ε ¼ 7.43), acetone (ε ¼ 20.49), ethanol (ε ¼ 24.85), acetonitrile (ε ¼ 35.69) and water (ε ¼ 78.36).

T.-T. Yin et al. / Organic Electronics 48 (2017) 154e164

161

Table 4 Activation parametersa, rate constants for the cis / trans isomerization and the half-lives for the cis isomersb. Compounds

DGz [kJ mol1]

DHz [kJ mol1]

DSz [J mol1K1]

kc/t [s1]

t1/2 [h]

13 16 11 2 14 12

120.98 118.97 122.63 117.90 123.59 123.34

118.98 119.31 120.69 116.77 121.94 122.15

6.69 1.12 6.52 3.80 5.55 3.99

4.5765  109 1.0282  108 2.3552  109 1.57  108 1.6006  109 1.7724  109

4.21 1.87 8.18 1.23 1.20 1.09

     

Expt. t1/2 [min] 104 104 104 104 105 105

stable 541 stable 145 340 355

Notes: a The obtained activation free energies, enthalpies and entropies are presented, respectively. b The half-lives of the experiment adopts different units [30].

properties of the synthesized complexes, solutions of 13, 16, 11, 2, 14 and 12 in solvents (chloroform, tetrahydrofuran, acetone, ethanol, acetonitrile and water) with varying polarities as evaluated by the dielectric constant ε, ranging from 1.00 to 78.36, were prepared. The results are reported in Table 5. We can see that for most complexes studied, t1/2 (cis) increases continually with increasing solvent polarity (Fig. 7). This general trend observed can be understood in term of a better solvation of cis complexes (exhibiting a higher permanent dipole m than trans isomers) in polar solvents, thus increasing their thermal stability and consequently the t1/ 2(cis) values.

3.5. Thermal cis / trans isomerization pathway It is well established that acylhydrazones and derivatives undergo a thermal cis to trans isomerization (Fig. 1) in the ground state, hence we will analyze this isomerization mechanism in deeper detail below. In passing we note that, 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 acylhydrazones in gas and condensed phase, and also showed clearly the solvent can affect the isomerization mechanism both in the ground and excited states. Therefore, a further step is the research of the ground state two-dimensional potential energy surfaces for each compound (Fig. 8). Cis to trans barrier heights were determined as described in Tables 1 and 2. For every acylhydrazone molecule studied here, linear or almost linear transition states with N1eN2]C3 z 180 were found. Suitable identification of these constructions as true transition states were done, checking for the being of only one imaginary frequency in normal mode analysis. Based upon analysis, each of the acylhydrazone molecule has an inversion transition state TS-inv with the angle N1eN2]C3 of 180 , and it is assigned to the N(2) stretching. Geometry parameters other than N1eN2]C3 and C5eN1eN2]C3 are generally not so much affected in the TS. The N2]C3 distance changes along each pathway in the acylhydrazones following nearly the same trend. Along this reaction path, the N2]C3 distance decreases from the cis isomer to the TS-inv and then increases in length as it approaches trans-S0 for all molecules studied in ACN. TS-inv presents the strongest N2]C3 double bond character along the trajectory. The inversion transition states of acylhydrazones nearly have the same length 1.257 Å. These distances indicate that the central nitrogen and carbon of the inversion transition state have a double bond between them. We find that inversion of the N1eN2]C3 angle does not cause conspicuous change of the N1eN2]C3eC4 angle, only leads to a slight compression of N2]C3 bond. As the cis / trans isomerization reaction proceeds through the inversion pathway, the N1eN2]C3 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 N1eN2]C3eC4 angle is an important parameter in defining the critical structures for the inversion mechanism, as it can be seen in Table 2. The torsional N1eN2]C3eC4 angle is from 2.0 of cis-S0 to the value not far from 0.4 in the transition state (take molecule 2 as an example). Therefore, this transition state is somewhat a mixture between the inversion and rotational paths. And also hybridization of the N atom converts from near sp2 hybridization to near sp in transition state. There exists greater steric hindrance between the lone pairs on the central nitrogen and p orbitals of the phenyl ring close to the 180 N1eN2]C3 angle. From this perspective, the thermal cis - trans isomerization is not a pure inversion along N1eN2]C3, but it follows a rotation-assisted inversion mechanism where the N1eN2]C3 angle must reach the value close to 180 but where the N1eN2]C3eC4 angle can take any value. The smallest energy barrier appears along the inversion pathway is acylhydrazone 13, it lies about 29.0 kcal mol1, making it difficult to rule out this pathway as a possible isomerization mechanism for these acylhydrazones on the basis of barrier height alone. This suggests that the preferred mechanism of isomerization in the ground state of 13 system is the inversion of the N1eN2]C3 angle which is on the same side as the R2 substituent portion. This system has the lowest inversion energy barrier of all the acylhydrazones studied. The transition state is stabilized by the vacant orbitals of the acylhydrazone substituted phenyl rings accepting electron density from the lone pairs on the central nitrogen (-N]C). The lone pairs are parallel to the vacant p orbitals on this phenyl ring and they are also perpendicular to the occupied orbitals of the substituted phenyl ring which has a stabilizing effect as it minimizes the electron-electron repulsion. The combination of these effects results in the 13 having the lowest inversion energy barrier. Having established the reaction scope, the mechanism was investigated by means of DFT calculations to have a better understanding of the potential energy profile. An ingenious approach to the construction of reaction path was realized, we calculated the reaction path from TS-inv by scanning the N1eN2]C3 angle as shown in Fig. 8 (we take molecule 2 as an example, all other molecules yield qualitatively similar profiles), which is started from the TS-inv along the forward and reverse pathway, respectively. Interestingly, it is the transition state for cis / M belongs to the N(2) stretching and the energy difference between the cis and M is 26.5 kcal mol1. It is visualized from the PES that there is an energy barrier during the rotation around the C5eN1eN2]C3 axis from M to trans form, requiring energy of about 3.7 kcal mol1 for M to trans. The optimized geometries of M show little variation in the N2]C3 bond length and the most bond parameters, but show overt variations in the C5eN1eN2]C3 and N1eN2]C3eC4 dihedral angles. Isomer M is not predicted to be stable because it should be easily depopulated by transformations to cis and trans forms. We have also tried to optimize the rotational transition state, looking for a hypothetical transition state geometry, such a

T.-T. Yin et al. / Organic Electronics 48 (2017) 154e164

4.31 6.21 9.75 1.79 1.91 2.11 2.39 10 109 109 109 109 1010 1010 3.07 3.29 7.72 1.42 1.54 1.71 1.98 10 109 109 109 109 109 1010

Fig. 7. B3LYP(PCM)/6-311þþG(d,p) results for the half-lives (t1/2, h) of cis acylhydrazone isomers in gas phase (ε ¼ 1.00), chloroform (ε ¼ 4.71), tetrahydrofuran (ε ¼ 7.43), acetone (ε ¼ 20.49), ethanol (ε ¼ 24.85), acetonitrile (ε ¼ 35.69), and water (ε ¼ 78.36).

10 104 104 104 104 105 105 10 109 109 109 109 109 109 10 104 104 104 104 104 104 10 108 109 109 109 109 109 10 104 104 104 104 104 104 2.82 1.64 2.79 5.65 6.00 6.62 7.43 10 108 109 109 109 109 109 1 4.7113 7.4257 20.493 24.852 35.688 78.355 gas phase chloroform tetrahydrofuran acetone ethanol acetonitrile water

6.82 1.18 6.90 3.41 3.21 2.91 2.59

      

7

t1/2

      

2

1.75 1.55 6.93 3.52 3.28 2.96 2.60

      

7

1.10 1.24 2.78 5.48 5.87 6.51 7.41

      

3

1.28 6.87 4.45 2.22 2.04 1.79 1.51

      

7

1.50 2.80 4.32 8.66 9.44 1.08 1.27

      

3

8.41 9.05 6.07 3.49 2.92 3.26 2.50

      

10 109 109 109 109 109 109

2.29 2.13 3.17 5.52 6.59 5.91 7.69

      

10 104 104 104 104 104 104

6.28 5.85 2.49 1.35 1.25 1.13 9.71

      

8 3

8

t1/2 kc/t [s1]

2

t1/2 kc/t [s1]

11

kc/t [s1] kc/t [s1]

t1/2 16 13 ε Solvents

Table 5 Rate constants for the cis / trans isomerization and the half-lives (t1/2, h) of cis isomers of all complexes in various solvents (ranked by order of increasing polarity parameter).

t1/2

      

10 104 104 105 105 105 105

4.47 3.10 1.97 1.08 1.01 9.13 8.06

      

8

kc/t [s1] kc/t [s1]

3

12 14

t1/2

      

103 104 104 105 105 105 105

162

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 ACN, we could not get a stationary point along that path, neither in water, in ethanol, in acetone, in tetrahydrofuran, in chloroform and in gas phase. Maybe our finding verified the previous research [51,52], the isomerization takes place by inversion in the ground state and by rotation in the excited state. Thus we decided to examine again this problem limiting ourselves to the ground state, that is to say the inversion mechanism. Based on the above discussion, we can present an intuitive point of view about the global thermal isomerization pathway of acylhydrazone system, the two steps i and ii differ mainly in two geometric parameters: N1eN2]C3 inversion and the torsion (or twist) of the C5eN1eN2]C3 group. The obtained results show the conversion of form cis into trans as a stepwise process with intermediacy of form M, including a concerted process involving simultaneous change of N1eN2]C3 inversion and the torsion of the C5eN1eN2]C3 group. From these observations it follows that the whole process of cis / trans isomerization for acylhydrazone system can be depicted as follows: (i) the transition state of the overall reaction is connected to two equilibrium structures cis and intermediate M, respectively, by the inversion of the N1eN2]C3 angle; (ii) intermediate M reaches trans isomer through an energy barrier no higher than 3.7 kcal mol1 by the rotation of the C5eN1eN2]C3 dihedral angle. In the isomerization process, the TS-inv structure can only be changed as the N1eN2]C3 inversion increase or decrease, because the acylhydrazone molecule needs to remain its symmetry. At the same time, its change is also restricted by the carbonyl and imine moieties in the symmetrical position. Therefore, the rotation of the heterocyclic ring is involved in the thermal inversion isomerization process. 4. Concluding remarks In this work, a novel class of acylhydrazone photoswitches 13, 16, 11, 2, 14 and 12 with a broad range of unique properties, depending on their substituents has been analyzed and showcased, and which also takes into account the solvent (ACN) used in the synthesis of these compounds. For all of them, cis and trans isomers have been calculated and transition states have been searched for. To allow for a systematic theoretical study, as a

T.-T. Yin et al. / Organic Electronics 48 (2017) 154e164

163

Fig. 8. Potential energy profiles along the inversion of N1N2C3 angle (a) and rotation around C5N1N2C3 dihedral angle (b).

compromise between accuracy and computational cost, the B3LYP/ 6-311þþG(d,p) and CAM-B3LYP/6-311þþG(d,p) model based on DFT and TDDFT methods have been chosen, in conjunction with Eyring transition state theory and the PCM to treat solvents. We herein detailed a comprehensive investigation of the acylhydrazone family, and demonstrated that they hold significant potential and uniqueness. The impact of conformational modifications and solvent effects are all very significant for the properties of acylhydrazone family. The conformation of the acylhydrazones are highly dependent on the substitution pattern on the crucial positions of the backbone fragment, which in turn has a significant impact on the electronic properties of the compounds, such as the more stabilization of the frontier molecular orbitals, decreased energy barriers of cis / trans thermal reversions and faster isomerization rates. Especially noteworthy feature is the very long thermal half-life (1.20  105 h and 1.09  105 h), the optimal structures modeled in this work were acylhydrazone 14 and 12 with EWGs substituents. These outstanding features for acylhydrazones are readily tuned, using steric interactions alone, by choice of the substituents on the R1 and R2 position. Actually, all calculated properties appear corresponding regularity with the increase of the polarity of the solvents. The inclusion of solvents has shown similar trends and we observed that the greater the solvent polarity, the stronger the property, except the absorption spectra, which is not appreciably affected by solvent. Last but not least, our results reveal that the isomerization reaction proceeds through a linear transition state indicative of an inversion mechanism. The transition state, however, is not reached by pure inversion along the N1eN2]C3 angle but rather by simultaneous rotation around the C5eN1eN2]C3 dihedral angle, that is a rotation-assisted inversion mechanism. In view of our theoretical calculation results, it may provide useful guidelines to modify the photophysics of the compounds of interest. These complicating factors must be addressed in the construction of more efficient molecular photoswitches and we

hope that our theoretical study of acylhydrazone photoswitches will afford unique application to the field of designing molecular machinery and different advanced scientific fields, etc., through access to photochemical and photophysical properties not achievable in the more commonly used hydrazones. From a more general perspective, the results of our theoretical calculation may pave the way towards the rational design of novel photoswitches that fulfill a set of desired characteristics. Acknowledgments 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). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.orgel.2017.05.053. References [1] A. Goulet-Hanssens, C.J. Barrett, Photo-control of biological systems with azobenzene polymerspolym, Polym. Chem. 51 (2013) 3058e3070. [2] J. Broichhagen, J.A. Frank, D. Trauner, A roadmap to success in photopharmacology,, Acc. Chem. Res. 48 (2015) 1947e1960. [3] W.A. Velema, W. Szymanski, B.L. Feringa, Photopharmacology: beyond proof of principle, J. Am. Chem. Soc. 136 (2014) 2178e2191. €nberger, M. Althaus, M. Fronius, W. Clauss, D. Trauner, Controlling [4] M. Scho epithelial sodium channels with light using photoswitchable amilorides, Nat. Chem. 6 (2014) 712e719. [5] A. Kocer, M. Walko, B.L. Feringa, Synthesis and utilization of reversible and irreversible light-activated nanovalves derived from the channel protein MscL, Nat. Protoc. 2 (2007) 1426e1437. [6] A.A. Beharry, G.A. Woolley, Azobenzene photoswitches for biomolecules, Chem. Soc. Rev. 40 (2011) 4422e4437.  ski, J.M. Beierle, H.A.V. Kistemaker, W.A. Velema, B.L. Feringa, [7] W. Szyman Reversible photocontrol of biological systems by the incorporation of

164

T.-T. Yin et al. / Organic Electronics 48 (2017) 154e164

molecular photoswitches, Chem. Rev. 113 (2013) 6114e6178. [8] J. Broichhagen, D. Trauner, The in vivo chemistry of photoswitched tethered ligands, Curr, Opin. Chem. Biol. 21 (2014) 121e127. [9] A. Fihey, A. Perrier, W.R. Browne, D. Jacquemin, Multiphotochromic molecular systems, Chem. Soc. Rev. 44 (2015) 3719e3759. [10] M. Alemani, M.V. Peters, S. Hecht, K.H. Rieder, F. Moresco, L. Grill, Electric field-induced isomerization of azobenzene by STM, J. Am. Chem. Soc. 128 (2006) 14446e14447. €rjesson, M. Herder, D.T. Duong, J.A. Hutchison, C. Ruzie, [11] M.E. Gemayel, K. Bo G. Schweicher, A. Salleo, Y. Geerts, S. Hecht, E. Orgiu, P. Samorì, Optically switchable transistors by simple incorporation of photochromic systems into small-molecule semiconducting matrices, Nat. Commun. 6 (2015) 6330. [12] C. Dri, M.V. Peters, J. Schwarz, S. Hecht, L. Grill, Spatial periodicity in molecular switching, Nat. Nanotechnol. 3 (2008) 649e653. [13] P. Vicini, F. Zani, P. Cozzini, I. Doytchinova, Hydrazones of 1,2-benzisothiazole hydrazides: synthesis, antimicrobial activity and QSAR investigations, Eur. J. Med. Chem. 37 (2002) 553e564. [14] M. Hidai, Y. Mizobe, Recent advances in the chemistry of dinitrogen complexes, Chem. Rev. 95 (1995) 1115e1133. [15] R. Lazny, A. Nodzewska, N, N-Dialkylhydrazones in organic synthesis. from simple N, N-dimethylhydrazones to supported chiral auxiliaries, Chem. Rev. 110 (2010) 1386e1434. [16] Y.H. Jin, C. Yu, R.J. Denman, W. Zhang, Recent advances in dynamic covalent chemistry, Chem. Soc. Rev. 42 (2013) 6634e6654. [17] D.N. Bunck, W.R. Dichtel, Bulk synthesis of exfoliated two-dimensional polymers using hydrazone-linked covalent organic frameworks, J. Am. Chem. Soc. 135 (2013) 14952e14955. [18] X.P. Zhou, Y. Wu, D. Li, Polyhedral metal-imidazolate cages: control of selfassembly and cage to cage transformation, J. Am. Chem. Soc. 135 (2013) 16062e16065. [19] M.N. Chaur, D. Collado, J.M. Lehn, Configurational and constitutional information storage: multiple dynamicsin systems based on pyridyl and acyl hydrazones, Chem. Eur. J. 17 (2011) 248e258. [20] D.G. Belov, B.G. Rogachev, L.I. Tkachenko, V.A. Smirnov, S.M. Aldoshin, Photochemistry of arylhydrazides in solution, Russ. Chem. Bull. 49 (2000) 666e668. [21] G. Vantomme, S. Jiang, J.M. Lehn, Adaptation in constitutional dynamic libraries and networks, switching between orthogonal metalloselection and photoselection processes, J. Am. Chem. Soc. 136 (2014) 9509e9518. [22] H.M.D. Bandara, S.C. Burdette, Photoisomerization in different classes of azobenzene, Chem. Soc. Rev. 41 (2012) 1809e1825. [23] H. Asanuma, X. Liang, H. Nishioka, D. Matsunaga, M. Liu, M. Komiyama, Synthesis of azobenzene-tethered DNA for reversible photo-regulation of DNA functions: hybridization and transcription, Nat. Protoc. 2 (2007) 203e212. [24] L.A. Tatum, X. Su, I. Aprahamian, Simple hydrazone building blocks for complicated functional materials, Acc. Chem. Res. 47 (2014) 2141e2149. [25] G. Vantomme, N. Hafezi, J.M. Lehn, A light-induced reversible phase separation and its coupling to a dynamic library of imines, Chem. Sci. 5 (2014) 1475e1483. [26] L. Greb, J.M. Lehn, Light-driven molecular motors: imines as four-step or twostep unidirectional rotors, J. Am. Chem. Soc. 136 (2014) 13114e13117. [27] H. Qian, I. Aprahamian, An emissive and pH switchable hydrazone-based Hydrogel, Chem, Commun 51 (2015) 11158e11161. [28] X. Su, M. Lokov, A. Kütt, I. Leito, I. Aprahamian, Unusual para-substituent effects on the intramolecular hydrogen-bond in hydrazone-based switches, Chem. Commun. 48 (2012) 10490e10492. [29] D. Ray, J.T. Foy, R.P. Hughes, I. Aprahamian, A switching cascade of hydrazonebased rotary switches through coordination-coupled proton relays,, Nat. Chem. 4 (2012) 757e762. [30] D.J.V. Dijken, P. Kovarícek, S.P. Ihrig, S. Hecht, Acylhydrazones as widely tunable photoswitches, J. Am. Chem. Soc. 137 (2015) 14982e14991. [31] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J. Nakatsuji, R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H.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, Jr. Montgomery,

[32]

[33]

[34] [35]

[36]

[37]

[38]

[39]

[40] [41] [42] [43]

[44] [45]

[46]

[47] [48]

[49]

[50]

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.A. Yazyev, 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, O. Farkas, J.B. Foresman, J.V. Ortiz, J. Cioslowski, D.J. Fox, Gaussian 09 Revision D.01, Gaussian, Inc., Wallingford, CT, 2009. S. Knippenberg, M.V. Bohnwagner, P.H. Harbach, A. Dreuw, Strong electronic coupling dominates the absorption and fluorescence spectra of covalently bound BisBODIPYs, J. Phys. Chem. A 119 (2015) 1323e1331. M. Frenette, M. Hatamimoslehabadi, S. Bellinger-Buckley, S. Laoui, J. La, S. Bag, Shining light on the dark side of imaging: excited state absorption enhancement of a Bis-styryl BODIPY photoacoustic contrast agent, J. Am. Chem. Soc. 136 (2014) 15853e15856. A.D. Becke, Density-functional thermochemistry. III. The role of exact exchange, J. Chem. Phys. 98 (1993) 5648e5652. C. Adamo, V. Barone, Toward reliable density functional methods without adjustable parameters: the PBE0 model, J. Chem. Phys. 110 (1999) 6158e6170. T. Yanai, D.P. Tew, N.C. Handy, A new hybrid exchange-correlation functional using the Coulomb-attenuating method (CAM-B3LYP),, Chem. Phys. Lett. 393 (2004) 51e57. Y. Zhao, D.G. Truhlar, The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: two new functionals and systematic testing of four M06-class functionals and 12 other functionals, Theor. Chem. Acc. 120 (2007) 215e241. Y. Kubota, Y. Ozaki, K. Funabiki, M. Matsui, Synthesis and fluorescence properties of pyrimidine mono- and bisboron complexes, J. Org. Chem. 78 (2013) 7058e7067. V. Barone, M. Cossi, J. Tomasi, A new definition of cavities for the computation of solvation free energies by the polarizable continuum model, J. Chem. Phys. 107 (1997) 3210e3221. E.D. Glendening, A.E. Reed, J.E. Carpenter, F. Weinhold, NBO, Version 3.1, University of Wisconsin, Madison: TCI, 1998. H. Eyring, The activated complex and the absolute rate of chemical reactions, Chem. Rev. 17 (1935) 65e77. D.G. Truhlar, B.C. Garrett, S.J. Klippenstein, Current status of transition-state theory, J. Phys. Chem. 100 (1996) 12771e12800. S. Canneaux, F. Bohr, E. Henon, KiSThelP: a Program to predict thermodynamic propertiesand rate constants from quantum chemistry results, J. Comput. Chem. 35 (2014) 82e93. E. Runge, E.K.U. Gross, Density-functional theory for time-dependent systems, Phys. Rev. Lett. 52 (1984) 997e1000. C. Albayrak, R.J. Frank, Spectroscopic, molecular structure characterizations and quantum chemical computational studies of (E)-5-(diethylamino)-2-[(2fluorophenylimino)methyl] phenol, J. Mol. Struct. 984 (2010) 214e220. S. Miertus, E. Scrocco, J. Tomasi, Electrostatic interaction of a solute with a continuum. A direct utilizaion of AB initio molecular potentials for the prevision of solvent effects, Chem. Phys. 55 (1981) 117e129. K.J. Laidler, P.A. Landskroener, The influence of the solvent on reaction rates, Trans. Faraday. Soc. 52 (1956) 200e210. J. Doki c, M. Gothe, J. Wirth, M.V. Peters, J. Schwarz, S. Hecht, P. Saalfrank, Quantum chemical investigation of thermal Cis-to-trans isomerization of azobenzene derivatives: substituent effects, solvent effects, and comparison to experimental data, J. Phys. Chem. A 113 (2009) 6763e6773. M.M.M. Raposo, A.M.C. Fonseca, M.C.R. Castro, M. Belsley, M.F.S. Cardoso, L.M. Carvalho, P.J. Coelho, Synthesis and characterization of novel diazenes bearing pyrrole, thiophene and thiazole heterocycles as efficient photochromic and nonlinear optical (NLO) materials, Dyes. Pigm 91 (2011) 62e73. M.M.M. Raposo, M.C.R. Castro, M. Belsley, A.M.C. Fonseca, Pushepull bithiophene azo-chromophores bearing thiazole and benzothiazole acceptor moieties: synthesis and evaluation of their redox and nonlinear optical properties, Dyes. Pigm 91 (2011) 454e465.