On the Charge-Transfer Excitations in Azobenzene Maleimide

Article pubs.acs.org/JPCA

Cite This: J. Phys. Chem. A 2019, 123, 5525−5536

On the Charge-Transfer Excitations in Azobenzene Maleimide Compounds: A Theoretical Study Dragos Lucian Isac,† Anton Airinei,† Dan Maftei,‡ Ionel Humelnicu,‡ Francesca Mocci,†,§ Aatto Laaksonen,*,†,∥ and Mariana Pinteala*̆ ,† †

“Petru Poni” Institute of Macromolecular Chemistry Iasi, Grigore Ghica Voda Al. No. 41A, 700487 Iasi, Romania Department of Chemistry, “Alexandru Ioan Cuza” University of Iasi, Carol I Blvd. No 11, 700506 Iasi, Romania § Department of Chemical and Geological Sciences, University of Cagliari, I-09042 Monserrato, Italy ∥ Department of Materials and Environmental Chemistry, Division of Physical Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden

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‡

S Supporting Information *

ABSTRACT: Photoswitchable systems with charge-transfer (CT) transitions have gained much attention during the recent years because of their many emerging applications. CT transitions themselves are of fundamental importance from physical, chemical, engineering, and molecular modeling points of view because they depend on the modified intramolecular electronic structure. CT transitions in azobenzene (AB) were observed when substituted with the maleimide (MI) functional group. This work represents a systematic theoretical study of excited states of the AB−MI structures of eight azo derivatives. In addition to the two main azo transitions (π → π* and n → π*), our calculations show a CT occurring between the azo moiety as a donor and the MI group as an acceptor. The CT mechanism can be characterized based on both the number and the position of the MI fragments. MI groups in the azo structure result in low-energy transitions, changing the order of the main transitions by introducing a CT character. Calculations using both density functional theory (DFT) and high-end molecular orbital theories confirm the CT character of these derivatives, although the order of excited states was found to differ depending on the chosen level of theory. We present here the first theoretical investigation of the electronic excited states (nπ*CT and ππ*CT) and corresponding transitions for this class of compounds. The computational results showed that the CT mechanism in AB−MI derivatives can occur via two pathways: planar and twisted. Our findings are expected to be of substantial interest, especially in the area of molecular optoelectronics and in the design of responsive materials.

1. INTRODUCTION A large number of studies, especially in the fields of photochemistry,1−3 photophysics,1 and photobiology,4 have focused on molecular systems based on azobenzene (AB). Moreover, AB and its derivatives have been used in a wide variety of applications including photoactive materials,5 photoswitchable units,1 optical data storage,6 activated optical control of photoinduced birefringence materials,7 and photorefractive polymers.8 An AB molecule can be activated by photo-irradiation to become a photochromic molecule. When exposed to UV light, AB compounds can undergo an isomerization process between the trans (E) and cis (Z) configurations that are interconvertible both photochemically and thermally.1,3 Previous studies of AB derivatives have identified two well-separated absorption bands in the UV−vis range. The strong absorption band in the near-UV region corresponds to a π → π* symmetry-allowed transition (So → S2), whereas the absorption band located in © 2019 American Chemical Society

the visible region, much weaker in intensity, arises from an n → π* forbidden transition (So → S1).2,9,10 In the case of the cis isomer (metastable), the absorption band assigned to the S0 → S1 transition is more intense than that in the trans isomer (thermodynamically more stable) as a result of the suppression effect of the orthogonality of the molecular orbitals (MOs) involved in the transition. The introduction of the push−pull functional groups on the AB rings can extend the π-electronic conjugation and reorganize the electronic structure. The π-electronic conjugation from push (acts as a donor) to pull functional groups (acts as an acceptor) involves a charge-transfer (CT) effect. Photoswitchable systems such as the AB derivatives, in which the presence of CT transitions has been confirmed, represent a Received: March 5, 2019 Revised: June 3, 2019 Published: June 7, 2019 5525

DOI: 10.1021/acs.jpca.9b02082 J. Phys. Chem. A 2019, 123, 5525−5536

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The Journal of Physical Chemistry A

tion method by including the n-electron valence state perturbation correction (NEVPT2) in order to reliably predict the nature, order, and character of the electronic transitions. All of the employed methods predict, for the first time to the best of our knowledge, the presence of excited CT states (nπ*CT and ππ*CT) in AB−MI derivatives.

hot topic which have attracted much attention because this class of compounds can be used in many applications in both industrial and fundamental scope: in coordination compounds with photochromic ligands,11 in recognition-gated AB photoswitch compounds,12 in AB dye semiconductor complexes,13 into self-adaptive switches enabling a complete charge separation in optoelectronic molecular systems,14 or in lightdriven compounds.15 The presence of CT transitions in AB derivatives introduces a polarizability effect through the π → π* excitation energy,16 enhancing the linear and nonlinear optical properties17 or can induce a spectral shift as an effect of the π-electronic extension. The maleimide (MI) molecule is an organic compound having 2 π-electrons, containing a bridgehead N atom and eight lone-pair electrons on the oxygen atoms. The MI systems have been used as photoinitiators,18 as reagents in 1,3 dipolar reactions (active dienophiles in the Diels−Alder reaction19) and as active electrophilic reagents in the synthesis of pyridazine derivatives20 and polymethine dyes.21 The MI structure can induce a CT interaction which is a key condition for initiation and transfer reactions during polymerization.22−24 AB structures, substituted with MI groups, have been investigated because of their ability to exhibit enantiotropic crystalline behavior,25 to improve the optical properties and the thermal stability of some polymer matrices.26,27 Furthermore, the AB-functionalized poly-(N-substituted MI-altstyrene) has been used for photostimulated phase separation encapsulation.28 Other applications of AB−MI systems that have received increasing interest during the last years include photoswitches for visible light control of an ionotropic glutamate receptor,29 red light photocontrol of conformation of some AB di-MI compounds,30 and photoswitches designed for glutamate receptor optogenetics.31 Excited states of several AB derivatives have been recently studied theoretically32−34 using MO representation, obtained either from TD-density functional theory (DFT) calculations with PBE0, CAM-B3LYP, and B3LYP functionals or ab initio methods of type RASSCF-RASPT2. These studies confirmed the two main electronic transitions: π → π* (involving the bonding and antibonding orbitals of the AB core) and n → π* (involving the lone nitrogen pairs and antibonding azo orbital), as well as CT transitions, occurring when additional substituent groups were added to the AB moiety.34,35 In contrast to the substituted AB derivatives, the transitions in an unsubstituted AB skeleton do not show any CT effect because it is a centrosymmetric molecule. The substitution of AB with electron-releasing or electron-withdrawing groups can induce electron depletion from a donor to acceptor region. Moreover, the presence of push−pull moieties on the AB structure induces an intramolecular CT transition (ICT). The MI moiety has a withdrawing character, and both the methyl and azo groups provide a donor effect, as confirmed in previous studies of the atomic charge distributions,32,36 all making the AB−MI derivatives to an excellent prototypic system to be studied more closely. The main goal of this study is to investigate the electronic structure of some AB−MI derivatives in order to point out the influence of the CT transitions on the excitation energy and the order of the main azo π → π* and n → π* transitions. Theoretical calculations were performed using the time-dependent density functional theory (TD-DFT), configuration interaction (CI) with single and double excitations [CIS and CIS(D)], and the complete active space self-consistent field (CAS-SCF) multiconfigura-

2. TARGET COMPOUNDS AND COMPUTATIONAL DETAILS The eight AB−MI derivatives in the trans form (E) and their cis isomers (Z) shown in Figure 1 were investigated, and they

Figure 1. Target compounds and custom numbering scheme adopted for selected atoms.

will be considered for discussion as follows: (E1): (E)-1-(4(phenyldiazenyl) phenyl)-1H-pyrrole-2,5-dione; (Z1): (Z)-1(4-(phenyldiazenyl)phenyl)-1H-pyrrole-2,5-dione; (E2): (E)1,1′-(4-(p-tolyldiazenyl)-1,3-phenylene)-bis-(1H-pyrrole-2,5dione); (Z2): (Z)-1,1′-(4-(p-tolyldiazenyl)-1,3-phenylene)bis-(1H-pyrrole-2,5-dione); (E3): (E)-1,1′-(4-(o-tolyldiazenyl)-1,3-phenylene)-bis-(1H-pyrrole-2,5-dione); (Z3): (Z)-1,1′(4-(o-tolyldiazenyl)-1,3-phenylene)-bis-(1H-pyrrole-2,5dione); (E4): (E)-4-(2,5-dioxo-2H-pyrrol-1(5H)-yl)-N-(4(phenyldiazenyl)phenyl)benzamide; and (Z4): (Z)-4-(2,5dioxo-2H-pyrrol-1(5H)-yl)-(4-(phenyldiazenyl)phenyl)benzamide. The preparation and characterization of these ABMI compounds have been described previously.25,27,37 Spectral properties of the E4 isomer, including the solvatochromic behavior applying different solvation model parameters, partial atomic charges in the ground and excited states, the molecular electrostatic potential diagrams, and the solute−solvent interactions, have been previously discussed in ref 32. This study does not, however, contain any calculations or discussion 5526


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3. RESULTS AND DISCUSSION This section is divided into two parts: (i) analysis of the ground state (GS) geometries and (ii) assignment of the electronic spectra based on computations. 3.1. GS Equilibrium Geometries. The CAM-B3LYP/631+G(d)-optimized molecular geometries of the AB−MI derivatives are shown in Figure 2. The most relevant

of possible CT effects in the presence of the added MI moieties, which is the focus in this work. Geometry optimization calculations were performed using the Gaussian 09 suite of programs.38 All compounds were first minimized at their ground states (S0) and two different DFT functionals were selected: hybrid density functional PBE039 and the long-range-corrected CAM-B3LYP40 with the 631+G(d) basis set to characterize the AB−MI systems. TDDFT calculations41 using the CAM-B3LYP functional have been reported to provide reliable results that balance the description of the optimized geometries with a good estimation of the first electronic transitions in AB and substituted ABs.42−44 However, it has been reported that the combination of the parameter-free PBE0 functional39 and the CAM-B3LYP method with the 6-31+G(d) basis set can also provide adequate results with a reasonable computational cost.45,46 Ultrafine integration grid and tight convergence thresholds were used in the geometry optimization. Calculations of vibrational frequencies gave no negative eigenvalues of the Hessian matrix. The AB−MI geometries were further optimized taking into account the effect of the solvents [tetrahydrofuran (THF ε = 7.43), dichloromethane (DCM, ε = 8.93), and N,N-dimethylformamide (DMF, ε = 37.22)] at the same level of the theory as described above using the polarized continuum model (PCM).47 In the optimization in the gas phase, two additional basis sets were employed, namely, those of Pople48 with 6-311+G(d,p) and 6-311++G(2df,2pd) to further increase the accuracy. The excitation energies and oscillator strengths (f) were analyzed using the classical linear response theory (LR−PCM) with nonequilibrium solute−solvent coupling49,50 using the standard implementation of the integral equation formalism (IEFPCM)51 in Gaussian 09, involving TD density levels of theory with PBE0 and CAM-B3LYP functionals and the 631+G(d) basis set. LR−PCM was also applied with TD-PBE0 and TD-CAM-B3LYP using 6-311+G(d,p) and 6-311++G(2df,2pd) basis sets, respectively, for a good estimation of the first electronic transitions in AB−MI derivatives, similarly as described elsewhere.52 To describe the transition states and the CT excited states, all potential ICTs were analyzed in detail first by using both the PBE0 method and the long-range-corrected CAM-B3LYP functionals in TD-DFT and thereafter applying the CIS and CIS(D)53 methods. All of these calculations were performed to find possible underestimation/overestimation of the CT state energy when these common functionals are used in studying spurious long-range CT* excited states.45,54,55 A spurious lowlying CT is difficult to represent because of the high sensitivity of this characteristic transition, especially in cases of weak donor−acceptor interactions. Even, the CIS and CIS(D) methods can overestimate the CT* state energies resulting in an incorrect order.56 The state-averaged CAS multiconfigurational self-consistent field method (SA-CASSCF),57,58 in particular when supplemented by the n-electron valence state multireference perturbation theory SA-CASSCFNEVPT259−61 (and strongly contracted SC-NEVPT2), provides a more appropriate scheme to obtain reliable results. High-level SA-CASSCF-NEVPT2 computations were performed with the Orca program package62 with the CAS chosen based on single-point DFT calculations (CAM-B3LYP/ 6-31+G(d)) using both unrestricted natural orbitals and quasirestricted orbitals.

Figure 2. GS equilibrium geometries of the AB−MI derivatives.

geometrical parameters of the geometry both at the CAMB3LYP and PBE0 levels are summarized in Table S1, showing that the central AB unit is planar. Experimental values quoted in Table S1 used for comparison are taken from refs.63−65 The geometry of the AB compounds has been long debated concerning their planar versus twisted orientations. A closer survey of previous theoretical and experimental studies indicates that the unsubstituted AB structure is planar both in the solution (MP2 calculations66) and in the crystal.64 On the other hand, electron diffraction data in the gas phase67 reveals that the unsubstituted trans form is twisted around the NN bond, having a value of ca. 30° for the C−NN−C dihedral angle. The same dihedral has been found slightly twisted (ca. 15°) when the AB core is substituted.34 Other diazo compounds display a twisted conformation in the solution.68 Our results show that the E1 isomers are planar in contrary to a previous X-ray report63 where the MI substituent to AB did cause a slight twisting of ca. 24° of the C3−C2−N1 N1′ dihedral angle of the AB main core. This discrepancy can be attributed because of the packing forces present in the crystal, while absent in the calculations. To further validate the DFT results concerning the optimized geometries, we performed an additional geometry optimization at the MP2/ 6-311++G(2d,2p) level of theory (see Table S1) and, in line 5527


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The Journal of Physical Chemistry A with other experimental results on the unsubstituted AB,69,70 a planar structure of the AB central core was obtained. The dihedral angles C3−C2−N1N1′ in AB−MI derivatives (see Table S1) indicate that the azo core is slightly twisted in the E2 isomer and the values of this angle increase considerably in the E3 isomer as an effect of a steric hindrance imposed by the CH3 group in the ortho position. On the contrary, the E4 isomer has a planar structure of the azo core, comparable with that in the E1 geometry. Cartesian atomic coordinates (in Å) for the GS geometry of AB−MI isomers are included in the electronic Supporting Information (ESI, List S1). The conformational preferences of the studied compounds were verified by potential energy surface (PES) scans and by analyzing both the spatial orientation and through-space interactions of the MI group (Supporting Information, Figures S2−S5). The scan involved changing the orientation of the MI group by small steps of 10° of the angle C4′−C5′−N8′−C9′, across the range 0−180°, with respect to the AB structure (for each trans and cis isomer, respectively). During the PES scan, the methyl group from compounds II and III was kept frozen because the rotation of this functional group did only insignificantly affect the potential energy. The large volume of phenyl and MI rings and unpaired electron repulsion from the proximity of the N1′ via N8″ (from the ortho position of MI) were found to lead to nonplanar geometries for the compounds E2 and E3 and their cis isomers (Z2 and Z3). The specific orientation of the MI moiety (around 140° for C4′−C5′−N8′−C9′ dihedral angle, see numbering scheme in Figure 1) with respect to the AB plane depends most likely on two competing interactions: intramolecular hydrogen bond and steric effect. Indeed, a weak intramolecular C−H···O interaction could be formed between the electron-rich oxygen (MI) and the hydrogen atoms from the adjacent aromatic sp2 C6′ and C4′ when the AB and MI rings are in-plane. The deviation of AB and MI rings from coplanarity is caused by the steric repulsion because of the electron lone-pairs of the N atoms of the −NN− moiety and of oxygen atoms of MI (in the ortho position in compounds II and III) which repel each other, counteracting the intramolecular hydrogen bond (favored when the C2′−C3′−N8″−C9″ dihedral angle is planar). The balancing of these effects leads to a global minimum having a dihedral angle C2′−C3′−N8″−C9″ around 120° with all levels of theory used in our calculations. Indeed, the MI group in the para position, and thus distant from the azo group, has a smaller deviation from coplanarity (torsion of ca. 140°). Additional computations using PBE0 and CAM-B3LYP functionals with the 6-311+G(d,p) and 6-311++G(2df,2pd) basis sets were performed for all AB−MI derivatives. All computational methods employed were found to provide structural results in close accordance with experimental data (Tables S2 and S3). By a comparative analysis of the results, obtained with different computational methods, it can be seen that the presence of the MI unit on the structural parameters of the AB−MI derivatives induces a slight change in the −NN− bond length compared to the unsubstituted AB. The same bond length increases slightly in trans isomers of the AB−MI derivatives, whereas for the cis isomers, a slight decrease is observed (Table S1). On the other hand, the lengths of the adjacent bonds (N1−C2, N1′−C2′, respectively) are shorter in the trans isomer derivatives and become longer in the cis isomer derivatives (Table S1). Both DFT functionals PBE0/6-

31+G(d) and CAM-B3LYP/6-31+G(d) provide good results compared with experimental data. Concerning the effect of the solvents (here THF, DCM, and DMF) on the structural parameters, we see only a minor influence on the structures of the azo-derivatives. Bond lengths become slightly longer in all solvents compared to those obtained in the gas phase, especially in the case of the −N N′− bond increasing slightly as the solvent polarity increases (Tables S4−S6). Overall, the computations did not reveal any significant differences between the values of the valence and dihedral angles obtained for AB−MI derivatives, compared to unsubstituted AB (Tables S1−S6). 3.2. Excited Low-Lying Electronic States. A description of the electron transitions, present in the UV−vis spectra, can be based on the representation of the MOs using the Kohn− Sham frontier orbital theory. A hybrid exchange−correlation functional using the Coulomb-attenuating method (CAMB3LYP) with the 6-31+G(d) basis set was chosen for the MO representation in ground and the first singlet excited states in Figures 3−6 and Figures S6−S9, together with the relevant

Figure 3. Representation of Kohn−Sham frontier MOs using CAMB3LYP/6-31+G(d) level of theory for electronic transitions of compound I (E1 derivative).

data concerning the transitions. The CAM-B3LYP functional has been recommended to compute and to predict the vertical transition (even for CT character) into the electronic spectra of organic compounds.54 The frontier MOs, involved in the electronic transitions of AB−MI derivatives, are determined at the CAM-B3LYP/6-31+G(d) level of theory. Transition energies were calculated at the TD-CAM-B3LYP/6-31+G(d) level of theory in the gas phase. Beside the main π → π* and n → π* electronic transitions occurring in all azo compounds, the quantum calculations also predict CT (ππ*, nπ*) transitions in AB−MI derivatives. Several studies have confirmed the presence of intramolecular CT transitions in azo dyes.34,36,71 Thus, it is not surprising that ππ*, nπ* pure transitions, mixed nπ* + ππ*, or CT (ππ* and nπ*) states were found in the AB−MI derivatives (Figures 3−6, Figures S6−S9). The mixture of the transition states is due to their degenerate nature. The major contribution to the specific MOs from AB regions is labeled in blue, and the electron density depletion region from AB to MI units (CT) is labeled in red in Figures 3−6 and S6−S9. The MOs labeled with black color represent the other transitions which are closer to the character of the ππ*, nπ*, or to the CT states, but with a minor contribution. The excited states are represented by corresponding numbers. Electron density surfaces in the ground and excited states represented in green (dark) and white (intense) 5528


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transition states including even the lowest energy transitions. For a comparison, the electronic transitions calculated at the TD-PBE0/6-31+G(d) level of theory are presented in Table 1. In Figures 3−6, S6−S9 and Table 1, only the electronic transitions with higher contributions (C %) of MOs are reported. The CT transition is present in both the coplanar and twisted structures of the azo derivatives. It is also important to point out that the CT excited-state energy depends both on the number and position of the MI fragments [especially in compounds II and III where two MI fragments are in ortho and para positions, respectively (Figures 4, 5, S7 and S8)]. Even when the MI unit is not directly bound to the AB structure (compound IV), the CT transition is observed (Figures 6 and S9). The transition intensities, given by the oscillator strength (f), obey the following trend: ππ* > ππ*CT > nπ* for trans isomers (except for the E3 isomer, where nπ* > ππ*CT) and ππ* > nπ* > nπ*CT in cis isomers, except for the Z4 isomer, where ππ* transitions were replaced by ππ*CT transitions (see Figures 3−6, S6−S9, and Table 1). In most cases, the electron CT occurs between HOMO → LUMO frontier MOs. However, other transfer pathways such as HOMO → LUMO + 1, HOMO − 3 → LUMO + 1, HOMO − 4 → LUMO + 1 (when the number of MI functional groups is two), and HOMO − 3 → LUMO + 1 (when the MI group is not directly linked to the main AB structure in compound IV) can also be considered (Figures 3−6 and S6−S9). The values of the vertical transition energies (ΔEv) and the corresponding CI coefficients (C %) of the CT states were found in most cases close to those of the main transitions (Figures 3−6, S6−S9, and Table 1). The oscillator strength values of the CT transitions, calculated with CAMB3LYP functional (Figures 3−6, S6−S9), compared to those determined by the PBE0 method (Table 1), are very similar. Generally, the global hybrid PBE0 functional gives a good estimation of the absorption maximum wavelengths of singlet excited states in organic molecules;46,54 however, sometimes, the anticipated CT transition energies can be overestimated.45,54 In the present study, the PBE0 model predicts higher CT oscillator strength values when the MI fragment is fixed in the para position of the trans isomers and when the MI group is in the ortho position of the cis isomers (Table 1). The ππ*CT transitions are present in all of the trans isomers and nπ*CT transitions in all of the cis isomers. The calculations also showed the presence of the nπ*CT transitions in one of the trans isomers (E4) and ππ*CT transitions in all of the cis isomers (Z1−Z4) of AB−MI derivatives (Table 1) but having small values of the oscillation strength. An important conclusion of these results is that the presence of CT states leads to an increase of the probability and number of the low-lying transitions of nπ* states type because of the presence of the MI moiety on the AB structure (Table S7), especially in compounds II and III (Figures 3−6, S6−S9, Tables 1 and S7). Computational data in Table S8 indicate that the inclusion of the solvent effect in the calculations of the vertical transition energies, as well as the presence of the CT state, provides a small shift of ππ* transition wavelength to higher energies, whereas the spectral shift of nπ* transitions was made to lower energies. Also, the oscillator strength data, based on the TDPBE0/6-31+G(d) results in Table S8, indicate a slight decrease of ππ* transition intensities in the E3 isomer in DMF because

Figure 4. Representation of Kohn−Sham frontier MOs using CAMB3LYP/6-31+G(d) level of theory for electronic transitions of compound II (E2 derivative).

Figure 5. Representation of Kohn−Sham frontier MOs using CAMB3LYP/6-31+G(d) level of theory for electronic transitions of compound III (E3 derivative).

were drawn at 0.004 au iso-density level, respectively. Ten vertical excitations were considered to describe the involved 5529


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Figure 6. Representation of Kohn−Sham frontier MOs using CAM-B3LYP/6-31+G(d) level of theory for electronic transitions of compound IV (E4 derivative).

excitation energies as compared to the same functionals having larger basis sets: 6-311+G(d,p) and 6-311++G(2df,2pd). Consequently, the differences in the excitation energies are smaller than 0.10 eV. On the contrary, significant differences appear (>0.50 eV) when compared to computational results obtained with different functionals (PBE0 with CAM-B3LYP). The order and the nature of the transition states are in good agreement with PBE0 and CAM-B3LYP methods except for isomer E2, where the use of CAM-B3LYP/6-311++G(2df,2pd) found that the ππ* transition corresponds to the S2 energy level and ππ*CT to the S3 level. In the case of E4, DFT methods, used in the computational analysis, found that the ππ* transition followed a S0 → S2 path as in the unsubstituted AB. Therefore, the main excited electronic transitions ππ* (S0 → S2) and nπ* (S0 → S1) became modified in some AB−MI derivatives relating to the unsubstituted AB because of the MI moiety, inducing low-energy transition states changing both the nature and order of the transition states. To further analyze the nature and order of the electronic transitions in AB−MI derivatives, calculations using higher level CASSCF-NEVPT2 method and CIS and CIS(D) with 631+G(d) basis set were performed. The more time-consuming 6-311+G(d,p) and 6-311++G(2df,2pd) basis sets were not employed because no significant differences in the oscillator strengths, excitation energy values, and state order in the TDDFT calculations, compared to using the 6-31+G(d) basis set (Figures 3−6, S6−S9, Tables 1, and S9), were observed. The excitation state character and oscillator strengths of the AB− MI derivatives calculated with CIS and CIS(D) methods are collected in Table S10. After analyzing the results of the CI calculations, a different transition order was found when compared to the results using the TD-DFT method (Table 1, Figures 3−6, and S6−S9). Both CIS and CIS(D) overestimated the excited-state energies for all AB−MI compounds, the CT states appearing at higher values of energies (>3.50 eV). The description of the CT states by using CI calculations appears different for the E2 isomer (Table S8) where the first transition is of nπ* type. The oscillator strengths for 2ππ*CT transition from isomer E2 determined by the CIS method are higher and comparable with the corresponding values of the

of the CT affinity to the polar solvent. Experimental absorption values quoted in Table S8 are taken from refs 73 and 32. The solvent effects were considered in order to see if the CT excited states can influence the excitation energy and intensity of the azo nπ* and ππ* transitions. By comparing the available experimental and computed data, both reported in Table S8, it can be seen that CAM-B3LYP/6-31+G(d) results are generally in the best agreement for E1, Z1, E2, Z2, and E4 compounds. The differences between computed and experimental values are smaller than 0.20 eV. For compound Z1, in particular, a very good agreement was found between the computed and experimental data using the CAM-B3LYP functional in the case of n → π* transition. Only for compound E3, the PBE0 functional predicted better values of excitation energies than CAM-B3LYP. Our TD calculation data in Tables 1 and S9 and Figures 3−6 show that the ππ*CT states correspond to S0 → S2 transition (except E4 isomer) which usually characterizes a π → π* transition for trans isomers. Regarding the cis isomers, depending on the nature of the compound, the nπ*CT transitions can have either the same energy order of the ππ*CT transitions, observed in the trans isomers, or they can have higher values of energy and be forbidden, as indicated by the low values of their oscillator strength (Figures 3−6, S6−S9, Table 1). Now, three situations can be encountered for the AB−MI derivatives: (i) the nπ* transition corresponds to the S1 energy level for the cis isomer, (ii) the nπ* transition can be replaced by the nπ*CT transition, and (iii) all AB−MI derivatives show CT transitions (from AB to the vicinal MOs of the MI unit). It is worth noting that the PBE0 functional predicts the same order of ππ*CT and nπ*CT transitions as the CAM-B3LYP method (Table 1). However, the computations with the PBE0 functional assign a CT character to the S0 → S1 transition in compound Z4 and not to the nπ* transition (Table 1). An analysis is performed by computing the transitions with TDCAM-B3LYP/PBE0//6-311+G(d,p)/6-311++G(2df,2pd), using values listed in Table S9. The results from Table S9 indicate that the TD-CAM-B3LYP and TD-PBE0 functionals with a 6-31+G(d) basis set provide a good estimation of 5530


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The Journal of Physical Chemistry A Table 1. Vertical Transition Energies ΔEv with the Oscillator Strengths (f) and the Percentage of the Composition of MOs Calculated in (C %) of the AB−MI Derivatives at the TD-PBE0/6-31+G(d) level of Theory compound

state/assignment

C (%)

f

ΔEv (eV)

E1

1nπ* 2ππ*CT 5ππ* 8ππ* 10ππ* 1nπ* 2nπ*CT 4ππ*CT 6ππ* 9ππ* 1nπ* 2ππ*CT 3ππ*CT 6ππ* 10ππ*CT 1nπ* 2nπ*CT 3nπ*CT 8ππ*CT 9nπ*CT 10ππ*CT 1nπ* 2ππ*CT 3ππ*CT 4nπ* 6ππ* 9ππ* 10ππ* 1nπ* 2nπ*CT 3nπ*CT 4ππ*CT 7ππ*CT 8ππ*CT 9nπ*CT 10ππ*CT 1nπ* 2ππ*CT 3nπ*CT 4ππ*CT 5ππ* 1nπ*CT 2nπ* 3ππ*CT 4ππ*CT 6ππ* 8ππ*CT 9ππ* 10ππ*

98 93 98 92 89 89 96 73 75 71 91 94 95 92

0.0000 0.0028 0.9840 0.0222 0.0163 0.0484 0.0012 0.0026 0.1597 0.0271 0.0040 0.0022 0.0002 0.9850 0.0047 0.0395 0.0010 0.0110 0.0036 0.0067 0.0052 0.0193 0.0021 0.0003 0.0010 0.8155 0.0003 0.0003 0.0370 0.0010 0.0064 0.0020 0.0014 0.0001 0.0008 0.0022 0.0000 0.0002 0.0000 0.0036 1.3772 0.0001 0.0819 0.0031 0.0006 0.4884 0.0000 0.0516 0.0000

2.61 3.07 3.76 4.27 4.37 2.60 2.64 3.55 4.12 4.27 2.60 2.87 3.10 3.53 3.88 2.61 2.67 2.70 3.79 3.85 3.95 2.54 2.87 3.07 3.34 3.54 3.68 3.87 2.61 2.70 2.72 3.52 3.73 3.74 3.83 3.84 2.62 2.75 3.20 3.33 3.40 2.54 2.56 3.27 3.52 3.81 3.91 3.94 4.07

Z1

E2

Z2

E3

Z3

E4

Z4

64 96 73 55 85 40 60 92 96 89 60 94 91 73 96 81 56 70 61 38 42 94 98 98 74 99 99 77 41 55 38 82 47 85

compounds II and III, as the effect of the near degeneration between states evidenced also in using the TD-PBE0 method. To obtain a clearer picture of the transition order and energy, CASSCF-NEVPT2 calculations were performed for AB−MI derivatives. On the basis of the eigenvectors obtained in TD-DFT calculations, CAS (6,5) was chosen for compound I and CAS (6,6) for the remaining compounds. In the case of bismaleimide (compounds II and III), the presence of MI unit can introduce a supplementary orbital, which can be filled with electrons in the excited state. Also, in the compound IV, the presence of amide−phenyl sequences can introduce virtual MOs in the active space. CAS (6,5) and CAS (6,6) include the lowest (nπ*, ππ*CT and nπ*CT) and highest-lying (ππ*) orbitals (Figures S10−S17), which correspond to 6 electrons in the occupied orbitals that are allowed to promote in 5, 6 unoccupied orbitals. As results from TD-DFT, CIS, CIS(D), and single-point CAS determinations, all computations indicate that the first six singlet states of low-lying energy have a major contribution to electronic transitions of AB−MI derivatives. Therefore, these six average singlet low-lying electronic states (6SA-CASSCF-NEVPT2) were used to fit the electronic spectra of the molecules in the AB−MI derivatives. A graphical representation of the energy levels and order of the excited states is shown in Figures 7 and 8 for the two

Figure 7. Graphical representation of relative energy levels with respect to the GS minimum (for the E1 isomer) as calculated with PBE0//CAM-B3LYP//CIS//CIS(D)//CASSCF-NEVPT2/631+G(d) levels of theory in the gas phase.

classical ππ* transition (Table S10). Although the CIS method overestimates the excitation energy values in comparison with other azo compounds,1,3,26,32,34,72 better results were obtained for the E3 isomer (Table S10) with the DFT method. The use of the CAM-B3LYP computational model predicts a mixed character of the MOs in AB−MI derivatives, especially in

Figure 8. Graphical representation of relative energy levels with respect to the GS minimum (for the Z1 isomer) as calculated with PBE0//CAM-B3LYP//CIS//CIS(D)//CASSCF-NEVPT2/631+G(d) levels of theory in the gas phase. 5531


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The Journal of Physical Chemistry A isomers E1 and Z1; the corresponding representations for compounds E2, Z2, E3, Z3, E4, and Z4 are shown in the Supporting Information (Figures S18−S23). The color representation of energy levels is the same as in Figures 3−6 and S6−S9. In the case of E1, there is some discrepancy in the character and values of the excited-state energies obtained at different theory levels. Indeed, although PBE0, CAM-B3LYP, and CIS(D) predict the ππ*CT transition at lower energies with respect to the ππ* transition, CASSCF-NEVPT2 and CIS methods show the opposite trend, with the ππ* transition located at lower energies than the ππ*CT. The energy difference between the ππ*CT and ππ* transition is ca. 0.85 eV at the TD-PBE0. The CT transition energy obtained with the CIS(D) method is close to that obtained with CASSCFNEVPT2 (Figure 7, Tables S10 and S11). In the case of the cis isomer Z1 (Figure 8), the PBE0 and the CASSCF methods predicted enough close values of the first CT transition energy (difference around 0.04 eV), whereas the rest of methods appear to overestimate the CT transition energies. Some differences in the state character prediction between the results from using the most high-level method compared to other calculations were observed as the NEVPT2 procedure indicates that the first transition (S1) has an nπ*CT character, whereas with the other methods, it has an nπ* character (Figures S18−S23). Even if CASSCF-NEVPT2 is qualified as a high-end computation method and used as a reference for CT transitions, it does not mean that these computations are necessarily more accurate than those obtained using CAMB3LYP in the case of our, or other similar compounds.73,74 On the other hand, CAM-B3LYP calculations converge faster with larger basis sets describing accurately the n → π* and π → π* states of AB (Table S8). The TD-DFT method is also expected to give a good estimation for the CT transitions. The differences in results may depend on the formal differences in describing the wave functions. Because the MOs in our calculations become mixed, there is a possibility for those energy levels to be nearly degenerate. This is the case for S3 and S2 in the isomer E1 (Figure 7), after CAS calculation, and also for S2 and S1 in the isomer Z1 (Figure 8) after TD-PBE0 computation, or simply the character of states may change (Z1, Figure 8). In summary, although we find some deviations between the results obtained at different levels of theories from TD-DFT to CASSCF-NEVPT2 computations, all confirm a clear CT character of the transitions occurring in AB−MI derivatives. The high-end CASSCF-NEVPT2 calculations indicate that during photoexcitation of the AB−MI derivatives, multiple π → π*CT and n → π*CT transitions appear. The number of CT transitions is much higher than that of the π → π* and n → π* transitions (Table S11). In summary, the computational studies indicate the existence of CT excited states that can substitute the lowest excited state, namely, nπ*, especially in the case of cis AB−MI. For cis derivatives, the high-level computations predict that the nπ*CT transition occurs during S0 → S1 excitation. The CI coefficients determined from CAS calculations have values exceeding 0.40, leading to increased probabilities for the CT states. The MI units in the coplanar forms (especially E2, E3, and E4) show an increasing number of low-lying excited states (nπ* and nπ*CT), whereas for cis isomers (Z2, Z3, and Z4), the number of nπ*CT and ππ*CT excited states increased. The TD-CAM-B3LYP functional gives a good prediction for the bright π → π* transition energies in comparison with the

experimental values. Also, this level of theory provides a closer agreement between the values of the transition energy in the gas phase and the values obtained with multideterminant methods (Figure 7, Table S11, Figures S18−S21). On the other hand, the TD-PBE0 functional gives a reasonable transition energy as compared to the multiconfigurational wave function (Figures 8, S19, S21,S23 and Table S11) especially for nπ*CT transition and experimental data. Concerning the dark n → π* transition, both density functionals give results comparable with experimental determinations, but the CAM-B3LYP method predicts values somewhat closer to experimental data. The calculations based on the CIS method indicate higher values of the transition energies as compared to the other used methods. The results become somewhat overestimated, even when using the perturbed CI CIS(D) functional in the gas phase. In predicting the ICT transition mechanisms and studying how they occur, both TD-DFT functionals (PBE0 and CAMB3LYP) were used because they gave the ππ* and nπ* excitation energies, especially at the CAM-B3LYP level of theory, in a reasonably good agreement with experimental estimations. The PBE0 functional also predicted a reasonable energy, as well as the transition order for the nπ*CT states, in comparison with the multideterminant methods used in this study. The CT excited-state structures (Figure S24) were first optimized, suggesting two intramolecular CT mechanisms, planar (PICT) and twisted (TICT). They were found in each structure studied in this work (Figure S24). This CT in the AB−MI systems depends on the internal structural degrees of freedom of the MI fragments. Theoretical results indicate that the isomers E1, Z1, E2, E3, E4, and Z4 have planar orientation in the CT excited state, whereas both isomers Z2 and Z3 prefer a twisted conformation. These results were supported by the involved frontier orbitals (see composition of MOs calculated in the CI coefficients C(%) and oscillator strengths from Figures 3−6, S6−S9, Tables 1, S8 and S9) between the donor (AB) and acceptor (MI). To better understand the transfer of energy from AB to MI, evolution of the energy, both in the ground and excited states levels, was followed for compounds E1 and Z1 both having a planar CT excited state. The CT mechanism was studied by single-point PES scans starting from a planar conformation and rotating the C4′−C5′−N8′−C9′ dihedral angle (for notation see Figure 1) from 0 to 180°. Each point also contains calculations of 10 upper first low-lying excited states (see the Computational Details section) with respect to the GS profile of E1 and Z1 isomers. Figures 9 and 10 are used to follow the ICT mechanisms, based on the excited-state patterns. The ordinary ππ* and nπ* transitions are in blue, whereas the CT transitions are in red. The black lines represent other transitions with ππ*/nπ* character or weak CT states. The pink color represents the internal transition of the MI moiety where the azo core has no contribution. The results show a conical intersection occurring between the excited state of trans-AB and state of MI close to the Franck−Condon (FC) region (around 30−50° and 130− 150°, respectively), estimated from CAM-B3LYP calculations. Also, a conical intersection between the excited state of transAB and state of MI occurs at 90° based on PBE0 calculations. At this stage, the MI structure can introduce in the AB low lying transitions (see Table S7). Therefore, the states can become interchanged between the ππ* and nπ* levels of the 5532


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Figure 9. Evolution of ground and first 10 excited states energy during the PES rotation of the C4′−C5′−N8′−C9′ dihedral angle (for E1 isomer) across 0°−180° with respect to the AB structure based on the (a) TD-CAM-B3LYP and (b) TD-PBE0 calculations in the gas phase.

Figure 10. Evolution of ground and first 10 excited states energy during the PES rotation of the C4′−C5′−N8′−C9′ dihedral angle (for Z1 isomer) across 0°−180° with respect to the AB structure based on the (a) TD-CAM-B3LYP and (b) TD-PBE0 calculations in the gas phase.

AB core with the ππ*/nπ* states of the MI group, and the CT transition appears in the AB−MI system as a low-lying transition. In the case of cis conformations, the TD-DFT calculations do not show a conical intersection between the excited states from AB with MI group. The ICT appears because the energy states become nearly degenerate during the excitation as an effect of the MI group introducing low-lying transitions in the AB systems (Figures 10 (b), S25, and Table S7) and the nπ* state can become interchanged with nπ*CT. The energy profile of the CT excited state in isomers E1 and Z1 in the scan exhibits a low-energy value around 180° of the C4′-C5′-N8′-C9′ dihedral angle, meaning that these isomers are planar. The PICT transfer is favored because an intramolecular hydrogen bond provides a channel for the electron transfer. In the case of the AB−MI derivatives, substituted into the ortho position, this protocol was not employed because of the strong repulsion between the lone pair electrons of the oxygen atoms (from the MI moiety) and the lone pair electrons of the nitrogen atoms (from −NN−), hindering the formation of a planar structure between MI functional group and AB moiety. From the analyses of the frontier MOs according to Figures S7, S8, Tables 1, S8, S9 and the graphical representation of optimized structure in ICT excited state, the TICT mechanism was confirmed for Z2 and Z3 isomers. In the case of TICT mechanism, the electron transfer can occur to the unoccupied MI orbitals in the ortho position through the negative region of the nitrogen atoms from the azo double bond and of the

nitrogen heteroatom from the MI moiety. However, neither the oscillator strength nor the CI coefficients do indicate a transfer from azo unit to MI for isomers E2 and E3. As a general remark, even if the CT states do not strongly influence the spectral properties of AB−MI derivatives, these compounds belong to the AB class according to the Rau classification.75

4. CONCLUSIONS In this paper, the results from both TD-DFT and multiconfiguration MO methods show that the order and character of the main transitions in AB−MI derivatives can be altered from those of the unsubstituted AB. The main electron transitions change from Sn to CT or from Sπ to CT type in the case of the lowest n → π* transitions. The existence of such excited nπ*CT and ππ*CT states is shown here for the first time for this group of molecular systems. The binding of the MI groups to the AB structure induces low-energy transitions that can change the order of the main transitions or replace them. The analysis of the CT states was performed with methods based on both TD-DFT (PBE0 and CAM-B3LYP) functionals and post Hartree−Fock methods (CIS, CIS(D) and CASSCF-NEVPT2, respectively). Both categories of methods indicate the presence of CT states but do not predict the same order for the transitions. In overall, the vertical energies (ΔEv) calculated using the both methodologies are in 5533


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(5) Dinçalp, H.; Yavuz, S.; Haklı, Ö .; Zafer, C.; Ö zsoy, C.; Durucasu, I.;̇ Iç̇ li, S. Optical and photovoltaic properties of salicylaldimine-based azo ligands. J. Photochem. Photobiol., A 2010, 210, 8−16. (6) Ho, A.; Natansohn, A.; Rochon, P. Azo polymers for reversible optical storage. 7. The effect of the size of the photochromic groups. Macromolecules 1995, 28, 6124−6127. (7) Fukuda, T. Rewritable high-density optical recording on azobenzene polymer thin film. Opt. Rev. 2005, 12, 126−129. (8) Iftime, G.; Labarthet, F. L.; Natansohn, A.; Rochon, P.; Murti, K. Main chain-containing azo-tetraphenyldiaminobiphenyl photorefractive polymers. Chem. Mater. 2002, 14, 168−174. (9) García-Iriepa, C.; Marazzi, M.; Frutos, L. M.; Sampedro, D. E/Z photochemical switches: syntheses, properties and applications. RSC Adv. 2013, 3, 6241−6266. (10) Merino, E.; Ribagorda, M. Control over molecular motion using the cis−trans photoisomerization of the azo group. Beilstein J. Org. Chem. 2012, 8, 1071−1090. (11) Ko, C.-C.; Yam, V. W.-W. Coordination compounds with photochromic ligands: ready tunability and visible light-sensitized photochromism. Acc. Chem. Res. 2018, 51, 149−159. (12) Ouyang, Y.; Yuan, Z.; Wang, J. A recognition-gated azobenzene photoswitch. New J. Chem. 2018, 42, 5660−5663. (13) Torres, A.; Prado, L. R.; Bortolini, G.; Rego, L. G. C. Charge transfer driven structural relaxation in a push-pull azobenzene dyesemiconductor complex. J. Phys. Chem. Lett. 2018, 9, 5926−5933. (14) Wu, Z.; Zhang, P.; Luo, Y.; Jiang, J.; Jiang, J. Self-adaptive switch enabling complete charge separation in molecular-based optoelectronic conversion. J. Phys. Chem. Lett. 2018, 9, 837−843. (15) Rombouts, J. A.; Ehlers, A. W.; Lammertsma, K. A quantitative analysis of light-driven charge transfer processes using voronoi partitioning of time dependent DFT-derived electron densities. J. Comput. Chem. 2017, 38, 1811−1818. (16) Haghdani, S.; Davari, N.; Sandnes, R.; Åstrand, P.-O. Complex frequency-dependent polarizability through the π → π* excitation energy of azobenzene molecules by a combined charge-transfer and point-dipole interaction model. J. Phys. Chem. A 2014, 118, 11282− 11292. (17) Pal, A. K.; Duignan, T. J.; Autschbach, J. Calculation of linear and nonlinear optical properties of azobenzene derivatives with Kohn−Sham and coupled-cluster methods. Phys. Chem. Chem. Phys. 2018, 20, 7303−7316. (18) Miller, C. W.; Jönsson, E. S.; Hoyle, C. E.; Viswanathan, K.; Valente, E. J. Evaluation of N-aromatic maleimides as free radical photoinitiators: A photophysical and photopolymerization characterization. J. Phys. Chem. B 2001, 105, 2707−2717. (19) Das, K. T.; Banerjee, M. DFT study of the 1,3-dipolar cycloaddition of azomethine ylides with maleimide, maleic anhydride, methylacrylate and some simple substituted alkenes. J. Phys. Org. Chem. 2009, 23, 148−155. (20) Tominaga, Y.; Yoshioka, N.; Kataoka, S. Synthesis of aminopyrimidopyidazines as chemiluminescent compounds by reaction of functionalized maleimide with various amine derivatives. Heterocycles 1996, 43, 1597−1600. (21) Shigemitsu, Y.; Komiya, K.; Mizuyama, N.; Tominaga, Y. Reaction of functionalized maleimides with versatile nucleophiles. Synthesis, electronic spectra and molecular orbital study. Dyes Pigm. 2007, 72, 271−284. (22) Zhang, X.; Li, Z.-C.; Li, K.-B.; Lin, S.; Du, F.-S.; Li, F.-M. Donor/acceptor vinyl monomers and their polymers: Synthesis, photochemical and photophysical behavior. Prog. Polym. Sci. 2006, 31, 893−948. (23) Hall, H. K.; Padias, A. B. Organic and polymer chemistry of electrophilic tri- and tetrasubstituted ethylenes. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 2845−2858. (24) Ullrich, G.; Herzog, D.; Liska, R.; Burtscher, P.; Moszner, N. Photoinitiators with functional groups. VII. Covalently bonded camphorquinone-amines. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 4948−4963.

reasonably good mutual agreement. Our results indicate that the presence of the CT states can be explained in a satisfactory manner in the picture of MOs, energy excitations, and transfer mechanisms. The main observation is that the CT occurs from the azo moiety acting as a donor to the MI group through either a planar or twisted structure in an intramolecular process. The existence of CT depends on both the number and positions of MI moieties. In addition to these two mechanisms, the excited states were stabilized by an internal motion of the MI group. Also, the presence of the MI moiety in the azo structure induces an increase in the number of low-lying (nπ*, nπ*CT, and ππ*CT) excited states both in the trans and cis isomers. The evolution of the excited states shows that a stepwise mechanism occurs in a transfer from ππ* to ππ*CT in the case of trans isomers and from nπ* to nπ*CT in the case of the cis compounds. The presence of the nearly degenerate energy levels in AB−MI derivatives and a polarization introduced by the MI groups on the AB structure in the excited states favors the appearance of the mixed and novel type of CT transitions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.9b02082.

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Additional material with computational results including structure optimizations with DFT-CAM-B3LYP/ PBE0//6-311+G(d,p)/6-311++G(2df,2pd) methods, excitation energies, MO plots, and the optimized atomic coordinates (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (A.L.). *E-mail: [email protected] (M.P.). ORCID

Aatto Laaksonen: 0000-0001-9783-4535 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS D. Isac and M. Pinteala acknowledge the project that has received funding from the European Union’s Horizon 2020 Research and Innovation agreement no. 667387 WIDESPREAD 2-2014 SupraChem Lab. F.M. and A.L. thank the COST Action CM1405 MOLecules In Motion (MOLIM). This work was also supported by a grant of Ministry of Research and Innovation, CNCS-UEFISCDI, project number PN-III-P4-ID-PCCF-2016-0050, within PNCDI III.

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REFERENCES

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On the Charge-Transfer Excitations in Azobenzene Maleimide

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