On the Charge-Transfer Excitations in Azobenzene Maleimide

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

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 J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b02082 • Publication Date (Web): 07 Jun 2019 Downloaded from http://pubs.acs.org on June 7, 2019

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The Journal of Physical Chemistry

On the Charge-Transfer Excitations in Azobenzene Maleimide Compounds. A Theoretical Study Dragoș Lucian Isac1, Anton Airinei1, Dan Maftei2, Ionel Humelnicu2, Francesca Mocci,1,3, Aatto Laaksonen1,4*, Mariana Pinteală1* 1

“Petru Poni” Institute of Macromolecular Chemistry Iasi, Grigore Ghica Voda Al. No. 41A,

700487 Iasi, Romania 2

Department of Chemistry, “Alexandru Ioan Cuza” University of Iasi, Carol I Blvd. No 11,

700506 Iasi, Romania 3

Department of Chemical and Geological Sciences, University of Cagliari,

I-09042 Monserrato, Italy, 4Department

of Materials and Environmental Chemistry, Division of Physical Chemistry,

Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract

Photo-switchable systems with charge transfer transitions have gained much attention during the recent years, due to their many emerging applications. Charge transfer (CT) transitions themselves are of fundamental importance from physical, chemical, engineering and molecular modeling point of view because they depend on the modified intramolecular electronic structure. Charge transfer transitions in azobenzene (AB) were observed when substituted with 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 charge transfer 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 CT character. Calculations using both density functionals theory (DFT) and highend molecular orbital (MO) 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 design of responsive materials.

1. INTRODUCTION


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A large number of studies, especially in the fields of photochemistry,1-3 photophysics1 and photobiology4 have focused on molecular systems based on azobenzene. Moreover, AB and its derivatives have been used in a wide variety of applications including photoactive materials,5 photo-switchable units,1 optical data storage,6 activated optical control of photoinduced birefringence materials7 and photorefractive polymers.8 Azobenzene 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 near-UV region corresponds to a π  π* symmetry-allowed transition (So  S2), whereas the absorption band located in 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 involved in the transition. The introduction of the push-pull functional groups on the azobenzene 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 effect. Photoswitchable systems such as the azobenzene derivatives in which the presence of charge transfer transitions have been confirmed, represent a 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 azobenzene photo-switches compounds,12 in



azobenzene dye semiconductor complexes,13 into self-adaptive switches enabling a complete charge separation in optoelectronic molecular systems,14 or in light-driven compounds.15 The presence of CT transitions in azobenzene 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 molecule is an organic compound having 2 π-electrons, containing a bridgehead N atom and 8 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 Diels-Alder reaction19) and as active electrophilic reagents in the synthesis of pyridazine derivatives20 and polymethine dyes.21 The MI structure can induce a charge transfer (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 due to 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 azobenzene-functionalized poly-(Nsubstituted maleimide-alt-styrene) has been used for photo-stimulated phase separation encapsulation.28 Other applications of AB–MI systems that have received increasing interest during the last years include photo-switches for visible light control of an ionotropic glutamate receptor,29 red light photocontrol of conformation of some azobenzene di-maleimide compounds30 and photo-switches designed for glutamate receptor optogenetics.31 Excited states of several azobenzene derivatives have been recently studied theoretically32,

33, 34

using molecular orbital representation, obtained either from TD–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 azobenzene core), n → π* (involving the


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lone nitrogen pairs and antibonding azo orbital), as well as charge transfer transitions, occurring when additional substituent groups were added to the azobenzene moiety.34,35 In contrast to substituted azobenzene derivatives, the transitions in an unsubstituted AB skeleton do not show any charge transfer effect because it is a centrosymmetric molecule. The substitution of AB with electron releasing or 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 charge transfer 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) multiconfiguration 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 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.

2. TARGET COMPOUNDS AND COMPUTATIONAL DETAILS



The eight azobenzene maleimide derivatives in trans form (E) and their cis isomers (Z) shown in Figure 1 were investigated and they 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 (1Hpyrrole-2, 5-dione); (Z2): (Z)-1, 1’-(4-(p-tolyldiazenyl)-1, 3-phenylene) bis (1H-pyrrole-2, 5dione); (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, 5-dione); (E4): (E)-4-(2, 5dioxo-2H-pyrrol-1(5H)-yl)-N-(4-(phenyldiazenyl) phenyl) benzamide; (Z4): (Z)-4-(2, 5– dioxo-2H-pyrrol-1 (5H)-yl)-(4-(phenyldiazenyl) phenyl) benzamide. The preparation and characterization of these azobenzene maleimide 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 [32]. This study does not, however, contain any calculations or discussion of possible CT effects in the presence of the added MI moieties, which is the focus in this work.


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Figure 1. Target compounds and custom numbering scheme adopted for selected atoms. 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 6–31+G(d) basis set to characterize the AB–MI systems. TD–DFT 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 azobenzene and substituted azobenzenes.42-44 However, it has been reported that also the combination of the parameter-free PBE0 functional39 and the CAM– B3LYP method with the 6–31+G(d) basis set can 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 gas phase two additional basis sets were employed, namely those of Pople48 with 6–311+G(d,p) and 6–311++G(2df,2pd), respectively, to further increase the accuracy. The excitation energies and oscillator strengths (f) were analyzed using the classical linear response theory (LR–PCM) with non-equilibrium 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 6– 31+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, CIS(D)53 methods. All 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 low-lying CT is difficult to represent due to the high sensitivity of this


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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 complete active space multiconfigurational self-consistent field method (SA–CASSCF)57,58 , in particular when supplemented by the n-electron valence states multireference perturbation theory SA–CASSCF–NEVPT259-61 (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 complete active space (CAS) chosen based on single point DFT calculations (CAM–B3LYP/6– 31+G(d)) using both unrestricted natural orbitals (UNOs) and quasi-restricted orbitals (QROs).

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. Ground state equilibrium geometries The CAM–B3LYP/6–31+G(d) optimized molecular geometries of the AB–MI derivatives, are shown in Figure 2. The most relevant geometrical parameters of the geometry both at the CAM–B3LYP and PBE0 level are summarized in Table S1, showing that the central AB unit is planar. Experimental values quoted in Table S1, used for comparison were taken from refs. 63, 64 and 65.



Figure 2. Ground state equilibrium geometries of the AB–MI derivatives.


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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 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 in solution a twisted conformation.68 Our results show that the E1 isomers are planar in contrary to a previous X-ray report63 where the MI substituent to azobenzene 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 due to 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 with other experimental results on the unsubstituted azobenzene,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 ground state 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 scans (PES) and by analyzing both the spatial orientation and through-space interactions of the MI group (ESI, Figures S2-S5). The scan involved changing the orientation of the MI group by small steps of 10° of the angle C4ꞌ–C5ꞌ–N8ꞌ–C9ꞌ, 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 as well as unpaired electron repulsion from the proximity of the N1ꞌ via N8ꞌꞌ (from ortho position of MI) were found to lead to non-planar geometries for the compounds E2 and E3 and their cis isomers (Z2, Z3). The specific orientation of the MI moiety (around 140° for C4ꞌ–C5ꞌ–N8ꞌ–C9ꞌ 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 C6ꞌ and C4ꞌ when the AB and MI rings are in-plane. The deviation of AB and MI rings from coplanarity is caused by the steric repulsion due to the electron lone-pairs of the N atoms of the –N=N– moiety and of oxygen atoms of MI (in ortho position in compounds II and III) which repel each other, counteracting the intramolecular hydrogen bond (favored when the C2ꞌ–C3ꞌ–N8ꞌꞌ–C9ꞌꞌ dihedral angle is planar). The balancing of these effects leads to a global minimum having a dihedral angle C2ꞌ– C3ꞌ–N8ꞌꞌ–C9ꞌꞌ 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, 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–


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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 while for the cis isomers a slight decrease is observed (Table S1). On the other hand, the lengths of the adjacent bonds (N1–C2, N1ꞌ–C2ꞌ, respectively) are shorter in the trans isomer derivatives and become longer in the cis isomer derivatives (Table S1). The 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 solvent (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 gas phase, especially in the case of the –N=Nꞌ– 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 molecular orbitals using the Kohn-Sham frontier orbitals theory. Hybrid exchange-correlation functional using the Coulomb-attenuating method (CAM–B3LYP) with 6–31+G(d) basis set was chosen for the MOs representation in ground and the first singlet excited states in Figures 3-6 and Figures S6-S9, together with the relevant data concerning the transitions. 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 gas phase. Beside the main π → π* and n → π* electronic transitions occurring in all azo compounds, the quantum calculations predict also charge transfer (CT) (ππ∗, nπ∗) transitions in AB-MI derivatives. Several studies have confirmed the presence of intramolecular charge transfer transitions in azo dyes.34,36,71 Thus, it is not surprising that ππ∗, nπ∗ pure transitions, mixed nπ∗ + ππ∗ or CT (ππ∗, 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 azobenzene 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 Figures 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 are represented in green (dark) and white (intense) were drawn at 0.004 a.u. iso-density level. Ten vertical excitations were considered to describe the involved 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, Figures S6-S9 and Table 1 only the electronic transitions with higher contributions (C%) of molecular orbitals 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 maleimide fragments (especially in compounds II and III where two MI fragments are in ortho and in para positions (Figures 4 and 5, Figures 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).


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The transition intensities, given by the oscillator strength (f), obey the following trend: ππ* > ππ*CT > nπ* for trans isomers (except for 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, Figures S6-S9, and Table 1).

Figure 3. Representation of Kohn-Sham frontier molecular orbitals using CAM–B3LYP/6– 31+G(d) level of theory for electronic transitions of Compound I (E1 derivative).



Figure 4. Representation of Kohn-Sham frontier molecular orbitals using CAM–B3LYP/6– 31+G(d) level of theory for electronic transitions of Compound II (E2 derivative). In most cases the electron charge transfer occurs between HOMO → LUMO frontier molecular orbitals. 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 MI group is not directly linked to the main AB structure in compound IV) can be also considered (Figures 3-6, Figures S6-S9). The values of the vertical transition energies (ΔEv) and the corresponding configuration interaction coefficients (C%) of the CT states were found in most cases close to those of the main transitions (Figures 3-6,


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Figures S6-S9, Table 1). The oscillator strength values of the CT transitions, calculated with CAM–B3LYP functional (Figures 3-6, Figures S6-S9), compared to those determined by PBE0 method (Table 1), are very similar.

Figure 5. Representation of Kohn-Sham frontier molecular orbitals using CAM–B3LYP/6– 31+G(d) level of theory for electronic transitions of Compound III (E3 derivative).



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 para position of the trans isomers and when the MI group is in ortho position of the cis isomers (Table 1).

Figure 6. Representation of Kohn-Sham frontier molecular orbitals using CAM-B3LYP/631+G(d) level of theory for electronic transitions of Compound IV (E4 derivative). 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


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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 due to the presence of MI moiety on the AB structure (Table S7), especially in compounds II and III (Figures 3-6, Figures 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 CT state, provide a small shift of ππ* transition wavelength to higher energies while the spectral shift of nπ* transitions was made to lower energies. Also, the oscillator strength data, based on the TD– PBE0/6-31+G(d) results in Table S8, indicate a slight decrease of ππ* transition intensities in E3 isomer in DMF due to the CT affinity to polar solvent. Experimental absorption values quoted in Table S8, were taken from refs. 73 and 32.

Table 1. Vertical transition energies ΔEv with the oscillator strengths (f) and the percentage of the composition of molecular orbitals calculated in (C, %) of the AB-MI derivatives at TD–PBE0/6– 31+G(d) level of theory. Compound

state/assignment

C (%)

f

ΔEv (eV)

E1

1nπ*

98

0.0000

2.61

2ππ*CT

93

0.0028

3.07

5ππ*

98

0.9840

3.76

8ππ*

92

0.0222

4.27

10ππ*

89

0.0163

4.37

1nπ*

89

0.0484

2.60

2nπ*CT

96

0.0012

2.64

4ππ*CT

73

0.0026

3.55

6ππ*

75

0.1597

4.12

9ππ*

71

0.0271

4.27

1nπ*

91

0.0040

2.60

2ππ*CT

94

0.0022

2.87

3ππ*CT

95

0.0002

3.10

Z1

E2



6ππ*

92

10ππ*CT Z2

E3

Z3

E4

Z4

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0.9850

3.53

0.0047

3.88

1nπ*

64

0.0395

2.61

2nπ*CT

96

0.0010

2.67

3nπ*CT

73

0.0110

2.70

8ππ*CT

55

0.0036

3.79

9nπ*CT

85

0.0067

3.85

10ππ*CT

40

0.0052

3.95

1nπ*

60

0.0193

2.54

2ππ*CT

92

0.0021

2.87

3ππ*CT

96

0.0003

3.07

4nπ*

89

0.0010

3.34

6ππ*

60

0.8155

3.54

9ππ*

94

0.0003

3.68

10ππ*

91

0.0003

3.87

1nπ*

73

0.0370

2.61

2nπ*CT

96

0.0010

2.70

3nπ*CT

81

0.0064

2.72

4ππ*CT

56

0.0020

3.52

7ππ*CT

70

0.0014

3.73

8ππ*CT

61

0.0001

3.74

9nπ*CT

38

0.0008

3.83

10ππ*CT

42

0.0022

3.84

1nπ*

94

0.0000

2.62

2ππ*CT

98

0.0002

2.75

3nπ*CT

98

0.0000

3.20

4ππ*CT

74

0.0036

3.33

5ππ*

99

1.3772

3.40

1nπ*CT

99

0.0001

2.54

2nπ*

77

0.0819

2.56

3ππ*CT

41

0.0031

3.27

4ππ*CT

55

0.0006

3.52

6ππ*

38

0.4884

3.81


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8ππ*CT

82

0.0000

3.91

9ππ*

47

0.0516

3.94

10ππ*

85

0.0000

4.07

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 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 calculations data in Tables 1 and S9 and Figures 3-6 show that the ππ∗CT states corresponds 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, Figures S6S9, Table 1). Now, three situations can be encountered for the AB–MI derivatives: (i) the nπ∗ transition corresponds to the S1 energy level for cis isomer, (ii) the nπ∗ transition can be replaced by the nπ∗CT transition and (iii) all AB–MI derivatives show CT transitions (from azobenzene to the vicinal molecular orbitals of the maleimide unit). It is worth noting that the PBE0 functional predicts the same order of ππ∗CT and nπ∗CT transition 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 TD/CAM– 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 6–31+G(d) basis set provide a good estimation of 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 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 due to 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, CIS(D) with 6–31+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 TD–DFT calculations, compared to using the 6–31+G(d) basis set (Figures 3-6, Figures S6-S9 and Tables 1, 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 configuration interaction calculations, a different transition order was found when


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compared to the results using the TD–DFT method (Table 1, Figures 3-6, Figures 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 configuration interaction 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 CIS method is higher and comparable with the corresponding values of the classical ππ∗ transition (Table S10). Although the CIS method overestimates the excitation energies 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 used of CAMB3LYP computational model predict a mixed character of the molecular orbitals in AB–MI derivatives, especially in compounds II and III, as the effect of the nearly 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 case of bismaleimide (compounds II and III) the presence of maleimide 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 molecular orbitals 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 isomers E1 and Z1; the corresponding representations for compounds E2, Z2, E3, Z3, E4, Z4 are shown in the Supporting information (Figures S18S23). The color representation of energy levels is the same as in Figure 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, while PBE0, CAM–B3LYP, 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 TDPBE0. The CT transition energy obtained with the CIS(D) method is close to that obtained with CASSCF–NEVPT2 (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), while the rest of methods appear to overestimate the CT transitions 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 indicate that the first transition (S1) has an nπ*CT character, while with the other methods it has 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 doesn't mean that these computations are necessarily more accurate than those obtained using CAM–B3LYP in case of our, or 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


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wave functions. Because the molecular orbitals 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.

Figure 7. Graphical representation of relative energy levels with respect to the ground state minimum (for E1 isomer) as calculated with PBE0//CAM–B3LYP//CIS//CIS(D)//CASSCF– NEVPT2/6–31+G(d) levels of theory in the gas phase.



Figure 8. Graphical representation of relative energy levels with respect to the ground state minimum (for Z1 isomer) as calculated with PBE0//CAM–B3LYP//CIS//CIS(D)//CASSCF– NEVPT2/6–31+G(d) levels of theory in the gas phase. 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, E4) show an increasing number of low-lying excited states (nπ* and nπ*CT), whereas for cis isomers (Z2, Z3, Z4) the number of nπ*CT and ππ*CT excited states increased. 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


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closer agreement between the values of the transition energy in gas phase and the values obtained with multi-determinant methods (Figure7, Table S11, Figures S18-S21). On the other hand, the TD–PBE0 functional gives a reasonable transition energy as compared to 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 the both density functionals give results comparable with experimental determinations, but 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 configuration interaction CIS(D) functional in gas phase. In predicting the ICT transition mechanisms and studying how they occur, both TD– DFT functionals (PBE0 and CAM–B3LYP) 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 and it suggests two intramolecular charge transfer mechanisms, planar (PICT) and twisted (TICT). They were found in each structure studied in this work (Figure 24). This charge transfer 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, while both isomers Z2, Z3 prefer a twisted conformation.. These results were supported by the involved frontier orbitals (see composition of molecular orbitals calculated in



the configuration interaction coefficients C(%) and oscillator strengths from Figures 3-6 and 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 C4ꞌ–C5ꞌ–N8ꞌ–C9ꞌ dihedral angle (for notation see Figure 1) from 0 to 180˚. Each point also contains calculations of ten upper first low-lying excited states (see the Computational Details section) with respect to the ground state profile of E1 and Z1 isomers. Figures 9 and 10 can be used to follow the ICT mechanisms, based on the excited state patterns. The ordinary ππ* and nπ* transitions are in blue, while 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 maleimide 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 130150°, respectively), estimated from CAM-B3LYP calculations. Also, a conical intersection between the excited state of trans-AB and state of MI occurs at 90° based on PBE0 calculations. At this stage, the maleimide structure can introduce in the azobenzene low lying transitions (see Table S7). Therefore, the states can become interchanged between the ππ* or nπ* levels of the AB core with the ππ*/nπ* states of the MI group and the CT transition appears in the ABMI system as a low-lying transition. In 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


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introducing low lying transitions in the AB systems (Figures 10 (b), 25, 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 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 molecular orbitals 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 ortho position through the negative region of the nitrogen atoms from the azo double bond and of the nitrogen heteroatom from MI moiety. However, neither the oscillator strength nor the CI coefficients do indicate a transfer from azo unit to maleimide for isomers E2 and E3.



Figure 9. Evolution of ground and first ten excited states energy during the PES rotation of the C4ꞌ–C5ꞌ–N8ꞌ–C9ꞌ dihedral angle (for E1 isomer) across 0 o – 180 o with respect to the AB structure based on the (a) TD-CAM-B3LYP and (b)TD-PBE0 calculations in gas phase.


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Figure 10. Evolution of ground and first ten excited states energy during the PES rotation of the C4ꞌ–C5ꞌ–N8ꞌ–C9ꞌ dihedral angle (for Z1 isomer) across 0o – 180o with respect to the AB structure based on the (a) TD-CAM-B3LYP and (b)TD-PBE0 calculations in gas phase. As a general remark, even if the CT states do not strongly influence the spectral properties of azobenzene-maleimide derivatives, these compounds belong to the azobenzene 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 azobenzene. 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 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 molecular orbitals, energy excitations and transfer mechanisms. The main observation is that the CT occurs from the azo moiety acting as a donor to the maleimide 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 number of low-lying (nπ*, nπ*CT, 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 case of trans isomers and from nπ* to nπ*CT in 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 Supporting Information The additional material with computational results include 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. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] Note There are no conflicts of interest to declare ACKNOWLEDGEMENTS 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 22014 SupraChem Lab. FM and AL thank the COST Action CM1405 MOLecules In Motion (MOLIM). This work was supported also 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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On the Charge-Transfer Excitations in Azobenzene Maleimide

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