Conformational and Molecular Structures of α,β-Unsaturated Acrylonitrile Derivatives: Photophysical Properties and Their Frontier Orbitals

We report single crystal X-ray diffraction (hereafter, SCXRD) analyses of derivatives featuring the electron-donor N-ethylcarbazole or the (4-diphenylamino)phenyl moieties associated with a -CN group attached to a double bond. The compounds are (2Z)-3-(4-(diphenylamino)-phenyl)-2-(pyridin-3-yl)prop-2-enenitrile (I), (2Z)-3-(4-(diphenylamino)phenyl)-2-(pyridin-4-yl)-prop-2-enenitrile (II) and (2Z)-3-(9-ethyl-9H-carbazol-3-yl)-2-(pyridin-2-yl)enenitrile (III). SCXRD analyses reveal that I and III crystallize in the monoclinic space groups P2/c with Z’ = 2 and C2/c with Z’ = 1, respectively. Compound II crystallized in the orthorhombic space group Pbcn with Z’ = 1. The molecular packing analysis was conducted to examine the pyridine core effect, depending on the ortho, meta- and para-positions of the nitrogen atom, with respect to the optical properties and number of independent molecules (Z’). It is found that the double bond bearing a diphenylamino moiety introduced properties to exhibit a strong π-π-interaction in the solid state. The compounds were examined to evaluate the effects of solvent polarity, the role of the molecular structure, and the molecular interactions on their self-assembly behaviors. Compound I crystallized with a cell with two conformers, anti and syn, due to interaction with solvent. DFT calculations indicated the anti and syn structures of I are energetically stable (less than 1 eV). Also electrochemical and photophysical properties of the compounds were investigated, as well as the determination of optimization calculations in gas and different solvent (chloroform, cyclohexane, methanol, ethanol, tetrahydrofuran, dichloromethane and dimethyl sulfoxide) in the Gaussian09 program. The effect of solvent by PCM method was also investigated. The frontier HOMO and LUMO energies and gap energies are reported.


Introduction
The study of π-conjugated organic compounds has led to the development of a rich variety of new concepts based on the interplay between their π-electronic structures and their molecular analyses to investigate different optical properties such as absorbance and fluorescence from derivatives with D-π-A type dyes (D = donor, A = acceptor) and the main packing motifs in organic solid states. The derivatives are (2Z)-3-(4-(diphenylamino)phenyl)-2-(pyridin-3-yl)prop-2-enenitrile, (2Z)-3-(4-(diphenylamino)phenyl)-2-(pyridin-4-yl)prop-2-enenitrile and (2Z)-3-(9-ethyl-9H-carbazol-3-yl)-2-(pyridin-2-yl)enenitrile ( Figure 1). The compounds bear an α,β-unsaturated nitrile and a pyridine group in the meta (I), para (II) and ortho (III) positions, respectively. We report the influence of solvent polarity and the roles of the molecular structure and the molecular interactions on their self-assembly behaviors and their optical properties. We also performed theoretical calculations in different solvent (chloroform, cyclohexane, methanol, ethanol, tetrahydrofuran, dichloromethane and dimethyl sulfoxide) using the Gaussian09 program. DFT calculations were made to determine the frontier HOMO and LUMO energies and gap energies.  Figure 1). The compounds bear an α,β-unsaturated nitrile and a pyridine group in the meta (I), para (II) and ortho (III) positions, respectively. We report the influence of solvent polarity and the roles of the molecular structure and the molecular interactions on their self-assembly behaviors and their optical properties. We also performed theoretical calculations in different solvent (chloroform, cyclohexane, methanol, ethanol, tetrahydrofuran, dichloromethane and dimethyl sulfoxide) using the Gaussian09 program. DFT calculations were made to determine the frontier HOMO and LUMO energies and gap energies.

X-Ray Crystallography and Photolysis Properties of I-III
Crystallographic data for I-III are summarized in Table 1. For compounds I-III the data were collected at 110(2) K after the crystals had been flash-cooled from room temperature. The crystal structure of I ( Figure 1) shows that the molecule of the meta position of the pyridyl ring (structure I) belongs to a monoclinic crystal with Z′ > 2 independent molecules of I per asymmetric unit, which could be attributed to a co-crystal with the solvent, (eight per unit cell, Z = 8) and to the P2/c space group. Crystal II belongs to an orthorhombic system (Pbcn, Z = 8). Crystal III also belongs to a monoclinic system with space group C2/c with a Z = 8, indicating an ordered structure. Both II and III display Z′ = 1. Notably, for I, II, and III, the unit cell contained eight molecules.

X-ray Crystallography and Photolysis Properties of I-III
Crystallographic data for I-III are summarized in Table 1. For compounds I-III the data were collected at 110(2) K after the crystals had been flash-cooled from room temperature. The crystal structure of I ( Figure 1) shows that the molecule of the meta position of the pyridyl ring (structure I) belongs to a monoclinic crystal with Z 1 > 2 independent molecules of I per asymmetric unit, which could be attributed to a co-crystal with the solvent, (eight per unit cell, Z = 8) and to the P2/c space group. Crystal II belongs to an orthorhombic system (Pbcn, Z = 8). Crystal III also belongs to a monoclinic system with space group C2/c with a Z = 8, indicating an ordered structure. Both II and III display Z 1 = 1. Notably, for I, II, and III, the unit cell contained eight molecules. The structure of (I) is modeled as ordered even though the lattice ethanol molecules are likely to be disordered as they are located at sites of two-fold axis symmetry. The elongated ellipsoids found for the lattice solvent molecule ( Figure 2) are consistent with this observation. The occupancy factor for the lattice solvent molecule was refined freely, and its value is 0.791 (7). It is possible that the void contains a mixture of disordered solvent molecules. The twin relationship corresponded to a two-fold axis found along the c axis. The batch scale factor refined to 0.3680 (16). The final refinement was performed using the HKL5 instruction (i.e., the hkl file includes the set of reflections from domain 1 and the set of overlapped reflections from component 2). The two crystallographically independent molecules of the target compound were found to be ordered ( Figure 2).  The structure of (I) is modeled as ordered even though the lattice ethanol molecules are likely to be disordered as they are located at sites of two-fold axis symmetry. The elongated ellipsoids found for the lattice solvent molecule ( Figure 2) are consistent with this observation. The occupancy factor for the lattice solvent molecule was refined freely, and its value is 0.791 (7). It is possible that the void contains a mixture of disordered solvent molecules. The twin relationship corresponded to a two-fold axis found along the c axis. The batch scale factor refined to 0.3680 (16). The final refinement was performed using the HKL5 instruction (i.e., the hkl file includes the set of reflections from domain 1 and the set of overlapped reflections from component 2). The two crystallographically independent molecules of the target compound were found to be ordered ( Figure 2).  Table 2 provides a list of torsion angles for molecules A and B, which notable differ in sign. The anti conformation is typically found in different isomeric compounds that have already been reported in the literature [28][29][30], and also in the structures of II and III reported herein.   Lattice solvent ethanol molecules act as H-bond donor bridges between molecules of conformers A along the c axis. Table 2 provides a list of H-bond interactions in the structure of I.
The N3A···O1S and O1S···O1S intermolecular distances are ca. 2.77 and 2.75 Å, respectively, which suggests that the H atom from the -OH group of the ethanol molecules is disordered (this is not surprising as the ethanol molecules are most likely disordered) [45]. The presence of lattice solvent molecules in the crystal packing of I might indicate that crystal growth is optimized with O-H(solvent)···N(compound).  Table 2 provides a list of torsion angles for molecules A and B, which notable differ in sign. The anti conformation is typically found in different isomeric compounds that have already been reported in the literature [28][29][30], and also in the structures of II and III reported herein.
Lattice solvent ethanol molecules act as H-bond donor bridges between molecules of conformers A along the c axis. Table 2 provides a list of H-bond interactions in the structure of I.
The N3A¨¨¨O1S and O1S¨¨¨O1S intermolecular distances are ca. 2.77 and 2.75 Å, respectively, which suggests that the H atom from the -OH group of the ethanol molecules is disordered (this is not surprising as the ethanol molecules are most likely disordered) [45]. The presence of lattice solvent molecules in the crystal packing of I might indicate that crystal growth is optimized with O-H(solvent)¨¨¨N(compound).  Table 2 provides a list of torsion angles for molecules A and B, which notable differ in sign. The anti conformation is typically found in different isomeric compounds that have already been reported in the literature [28][29][30], and also in the structures of II and III reported herein.  Lattice solvent ethanol molecules act as H-bond donor bridges between molecules of conformers A along the c axis. Table 2 provides a list of H-bond interactions in the structure of I.
The N3A···O1S and O1S···O1S intermolecular distances are ca. 2.77 and 2.75 Å, respectively, which suggests that the H atom from the -OH group of the ethanol molecules is disordered (this is not surprising as the ethanol molecules are most likely disordered) [45]. The presence of lattice  Table 2 provides a list of torsion angles for molecules A and B, which notable differ in sign. The anti conformation is typically found in different isomeric compounds that have already been reported in the literature [28][29][30], and also in the structures of II and III reported herein.  Lattice solvent ethanol molecules act as H-bond donor bridges between molecules of conformers A along the c axis. Table 2 provides a list of H-bond interactions in the structure of I.
The N3A···O1S and O1S···O1S intermolecular distances are ca. 2.77 and 2.75 Å, respectively, which suggests that the H atom from the -OH group of the ethanol molecules is disordered (this is not surprising as the ethanol molecules are most likely disordered) [45]. The presence of lattice   Table 3 includes a list of selected bond lengths and torsion angles for conformers A and B. The values for the pyridine ring and the p-(diphenylamino)phenyl group indicate that the N,N-diphenyl substituents are twisted through the single bonds of the N-atom, and those conformers are oppositely twisted. This behavior does not occur for structure II and for similar structures recently reported [28][29][30][31]. Table 3 also provides a list the selected bond lengths for III, as well as, the dihedral angles between the acrylonitrile linkage and the phenyl ring and with the diphenylaminophenyl or carbazole moieties. Table 3. Bond lengths (Å) and torsion angles (˝) selected in the crystal structures of I and II.
1.449 (2) When the nitrogen atom is found at the meta position, the crystal packing significantly differs from those of the analogous structures without nitrogen and those of the structures of II (para position) and III (ortho position). The compounds II and III ( Figure 5) have the Z-geometry about the ethylene bridge that links the aromatic rings and heterocyclic groups.
When the nitrogen atom is found at the meta position, the crystal packing significantly differs from those of the analogous structures without nitrogen and those of the structures of II (para position) and III (ortho position). The compounds II and III ( Figure 5) have the Z-geometry about the ethylene bridge that links the aromatic rings and heterocyclic groups. Understanding the different crystal packing modes (e.g., from herringbone to a co-facial π-stacking motif) is of importance to confer good electronic transfer properties. The molecules in I and II showed a tilted face-to-face arrangement ( Figure 6) for the aromatic ring constituted of the pyridine ring with a centroid-centroid distance of 3.71 Å, a shift distance of 1.46 Å for I, and 3.84 Å of centroid-centroid distance and a shift distance of 1.843 Å for II. These values for the π-π interactions in the structures were automatically found by the program OLEX 2 [46].  Understanding the different crystal packing modes (e.g., from herringbone to a co-facial π-stacking motif) is of importance to confer good electronic transfer properties. The molecules in I and II showed a tilted face-to-face arrangement ( Figure 6) for the aromatic ring constituted of the pyridine ring with a centroid-centroid distance of 3.71 Å, a shift distance of 1.46 Å for I, and 3.84 Å of centroid-centroid distance and a shift distance of 1.843 Å for II. These values for the π-π interactions in the structures were automatically found by the program OLEX 2 [46].
When the nitrogen atom is found at the meta position, the crystal packing significantly differs from those of the analogous structures without nitrogen and those of the structures of II (para position) and III (ortho position). The compounds II and III ( Figure 5) have the Z-geometry about the ethylene bridge that links the aromatic rings and heterocyclic groups. Understanding the different crystal packing modes (e.g., from herringbone to a co-facial π-stacking motif) is of importance to confer good electronic transfer properties. The molecules in I and II showed a tilted face-to-face arrangement ( Figure 6) for the aromatic ring constituted of the pyridine ring with a centroid-centroid distance of 3.71 Å, a shift distance of 1.46 Å for I, and 3.84 Å of centroid-centroid distance and a shift distance of 1.843 Å for II. These values for the π-π interactions in the structures were automatically found by the program OLEX 2 [46].  These distances are typical for π-π aromatic face-to-face interactions with centroid distances > 3.65 Å and offsets in the range of 1.6-1.8 Å [47] and they reflect the distribution of electronic density that minimizes the π-electrons. However, for the structure of III, as well as several crystal structure reported with the -CH=CCN-moiety, no obvious π-π interactions are found, even though isomers were present. These packing motifs may be attributed to CH¨¨¨π, edge-to-face interactions and they tune the packing structure from a herringbone mode toward more of a face-to-face π-stacking motif. Holmes et al. reported that in order to favor face-to-face π-stacking, the CH¨¨¨π interactions should be minimized [18,22,48,49]. The packing of structures I and II showed that the pyridine unit, the position of the nitrogen atom within the pyridyl core, and the p-(diphenylamino)phenylgroup play important roles as well as the solvent used for crystallization.
Bao and coworkers reported that tetracene substituted with halogen atoms at the 5 and 11 positions adopted a face-to-face π-stacking motif with enhanced charge transport [22]. In these three packing modes, the charge transport could be three-dimensionally anisotropic and the highest mobility would be observed along the major π-stacking direction ( Figure 7). In contrast, the lowest mobility would be found along the directions in which the molecules were insulated from one another. It is worth mentioning that the packing structure of III presents short contacts between the carbazole and the pyridine group. The interactions maintaining the structure of III are CH/EtCz¨¨¨-C"N of 2.74Å, and CH/(py)¨¨¨¨¨CH/π(Cz) of 2.88 Å ( Figure 8); however, there are no strong π-π stacking interactions present from centroid-to-centroid, such as in I and II, as it can be seen the short contacts values for III (Figure 8d) calculated by Mercury software [50].
From the Figure 8d it could be supposed that typical π-π interactions are present. However, in Figure 9 the distance between a pyridine plane and neighboring molecule indicated that there is not a cofacial π-π interaction arising a π-π overlap as it is exhibited in I and II. Consequently, the packing motif seems a classical herringbone packing with a strong slipped π-stacking indicating a weak π-π overlap between neighbor molecules [51]. These distances are typical for π-π aromatic face-to-face interactions with centroid distances > 3.65 Å and offsets in the range of 1.6-1.8 Å [47] and they reflect the distribution of electronic density that minimizes the π-electrons. However, for the structure of III, as well as several crystal structure reported with the -CH=CCN-moiety, no obvious π-π interactions are found, even though isomers were present. These packing motifs may be attributed to CH···π, edge-to-face interactions and they tune the packing structure from a herringbone mode toward more of a face-to-face π-stacking motif. Holmes et al. reported that in order to favor face-to-face π-stacking, the CH···π interactions should be minimized [18,22,48,49]. The packing of structures I and II showed that the pyridine unit, the position of the nitrogen atom within the pyridyl core, and the p-(diphenylamino)phenyl-group play important roles as well as the solvent used for crystallization.
Bao and coworkers reported that tetracene substituted with halogen atoms at the 5 and 11 positions adopted a face-to-face π-stacking motif with enhanced charge transport [22]. In these three packing modes, the charge transport could be three-dimensionally anisotropic and the highest mobility would be observed along the major π-stacking direction ( Figure 7). In contrast, the lowest mobility would be found along the directions in which the molecules were insulated from one another. It is worth mentioning that the packing structure of III presents short contacts between the carbazole and the pyridine group. The interactions maintaining the structure of III are CH/EtCz···-C≡N of 2.74Å, and CH/(py)⋅⋅⋅⋅⋅CH/π(Cz) of 2.88 Å ( Figure 8); however, there are no strong π-π stacking interactions present from centroid-to-centroid, such as in I and II, as it can be seen the short contacts values for III (Figure 8d) calculated by Mercury software [50].
From the Figure 8d it could be supposed that typical π-π interactions are present. However, in Figure 9 the distance between a pyridine plane and neighboring molecule indicated that there is not a cofacial π-π interaction arising a π-π overlap as it is exhibited in I and II. Consequently, the packing motif seems a classical herringbone packing with a strong slipped π-stacking indicating a weak π-π overlap between neighbor molecules [51].     The UV/Vis absorption spectra recorded in chloroform for I, II showed similar maxima of absorption bands, one in the range 416-427 nm in solution with CHCl3 and a smaller absorption band at 297-301 nm. For the solid form, only the change for compound II is reported, which underwent a bathochromic effect of 24 nm (λmax = 451nm) [52]. Only in compound II did the absorption spectrum display an obvious bathochromic shift in the solid state, which some authors suggest is due to the molecules adopting a π-aggregated form in the solid state [18]. The extended delocalization of the nitrogen electron pair onto the two phenyl rings from -N(Ph)2 accounts for the red shift of the absorption band. The emission spectra of I and II in chloroform solution in both the crystal and powder forms showed high fluorescence intensity, both in solution and in the solid state. The photoluminescence spectra of I and II were compared to reported values [52]. Crystals of II showed a strong bathochromic effect of almost 80 nm as crystals, with λem at 624 nm. However, the fluorescence emission maxima for compound I were almost the same for the powder form (535 nm) and the crystals (544 nm) (Figure 10). For III, the UV-vis absorption wavelength maximum in solution was at 380 nm, whereas in the solid state its absorption maximum (λmax) was 398 nm (III) [30]. The compound showed a typical AIE effect, because the emission maximum in the solid form was observed at 502 nm and the emission in solution was weak, which could be attributed to the carbazole group effect [30,53] (Figure 11). The UV/Vis absorption spectra recorded in chloroform for I, II showed similar maxima of absorption bands, one in the range 416-427 nm in solution with CHCl 3 and a smaller absorption band at 297-301 nm. For the solid form, only the change for compound II is reported, which underwent a bathochromic effect of 24 nm (λ max = 451nm) [52]. Only in compound II did the absorption spectrum display an obvious bathochromic shift in the solid state, which some authors suggest is due to the molecules adopting a π-aggregated form in the solid state [18]. The extended delocalization of the nitrogen electron pair onto the two phenyl rings from -N(Ph) 2 accounts for the red shift of the absorption band. The emission spectra of I and II in chloroform solution in both the crystal and powder forms showed high fluorescence intensity, both in solution and in the solid state. The photoluminescence spectra of I and II were compared to reported values [52]. Crystals of II showed a strong bathochromic effect of almost 80 nm as crystals, with λ em at 624 nm. However, the fluorescence emission maxima for compound I were almost the same for the powder form (535 nm) and the crystals (544 nm) ( Figure 10). The UV/Vis absorption spectra recorded in chloroform for I, II showed similar maxima of absorption bands, one in the range 416-427 nm in solution with CHCl3 and a smaller absorption band at 297-301 nm. For the solid form, only the change for compound II is reported, which underwent a bathochromic effect of 24 nm (λmax = 451nm) [52]. Only in compound II did the absorption spectrum display an obvious bathochromic shift in the solid state, which some authors suggest is due to the molecules adopting a π-aggregated form in the solid state [18]. The extended delocalization of the nitrogen electron pair onto the two phenyl rings from -N(Ph)2 accounts for the red shift of the absorption band. The emission spectra of I and II in chloroform solution in both the crystal and powder forms showed high fluorescence intensity, both in solution and in the solid state. The photoluminescence spectra of I and II were compared to reported values [52]. Crystals of II showed a strong bathochromic effect of almost 80 nm as crystals, with λem at 624 nm. However, the fluorescence emission maxima for compound I were almost the same for the powder form (535 nm) and the crystals (544 nm) (Figure 10). For III, the UV-vis absorption wavelength maximum in solution was at 380 nm, whereas in the solid state its absorption maximum (λmax) was 398 nm (III) [30]. The compound showed a typical AIE effect, because the emission maximum in the solid form was observed at 502 nm and the emission in solution was weak, which could be attributed to the carbazole group effect [30,53] (Figure 11). For III, the UV-vis absorption wavelength maximum in solution was at 380 nm, whereas in the solid state its absorption maximum (λ max ) was 398 nm (III) [30]. The compound showed a typical AIE effect, because the emission maximum in the solid form was observed at 502 nm and the emission in solution was weak, which could be attributed to the carbazole group effect [30,53] (Figure 11).

Electrochemical Properties.
The electrochemical characteristics of the samples I, II, and III were investigated by cyclic voltammetry (CV) (Figure 12). The onset oxidation (E ) estimated from CV curves for I, II, and III are summarized in Table 4. We determined the HOMO and LUMO energy levels of the samples, as calculated from the CV tests applying published equations [54] and using ferrocene as the external standard. All measurements were carried out at room temperature. The value of III has lower energy HOMO and LUMO levels, suggesting a lower electron injection barrier than observed for I and II, indicating that III had a stronger tendency to donate an electron, whereas the high LUMO energy level of II indicated its strong electron-withdrawing ability. The electronic energy gap values were estimated to be 2.08 eV, 1.99 eV, and 2.01eV for I, II and III, respectively (Table 4). We also determined the energy gap values, as calculated from the absorption spectrum in CHCl3 by using ΔE opt = hc/λ (ΔE opt (eV) =1237.5/λ(nm).
The energy of the lowest gap, which results from a more extended conjugation, was attributed to the (4-diphenylamino) molecule moiety in II. However, the values indicated that the position of substitution of the pyridyl unit reduced the molecule slightly, making it more easily oxidizable. In the structure, the electron withdrawing effect from the pyridine group could also affect the conjugation structure. Figure 11. Emission of compound III in solid form (´) and in solution (´) indicating that III exhibited the AIE property. Excitation wavelength for both conditions was 405 nm.

Electrochemical Properties.
The electrochemical characteristics of the samples I, II, and III were investigated by cyclic voltammetry (CV) (Figure 12). The onset oxidation (E onset Ox ) estimated from CV curves for I, II, and III are summarized in Table 4. We determined the HOMO and LUMO energy levels of the samples, as calculated from the CV tests applying published equations [54] and using ferrocene as the external standard. All measurements were carried out at room temperature.
HOMO "´pE ox´E 1{2p f erroceneq`4 .8eVq LU MO "´´E red´E1{2p f erroceneq`4 .8eVT he value of III has lower energy HOMO and LUMO levels, suggesting a lower electron injection barrier than observed for I and II, indicating that III had a stronger tendency to donate an electron, whereas the high LUMO energy level of II indicated its strong electron-withdrawing ability. The electronic energy gap values were estimated to be 2.08 eV, 1.99 eV, and 2.01eV for I, II and III, respectively (Table 4). We also determined the energy gap values, as calculated from the absorption spectrum in CHCl 3 by using ∆E opt = hc/λ (∆E opt (eV) =1237.5/λ(nm).
The energy of the lowest gap, which results from a more extended conjugation, was attributed to the (4-diphenylamino) molecule moiety in II. However, the values indicated that the position of substitution of the pyridyl unit reduced the molecule slightly, making it more easily oxidizable. In the structure, the electron withdrawing effect from the pyridine group could also affect the conjugation structure.

DFT Calculations
We carried out the Gaussian09 program optimization calculations with the M06L method using the cc-pVDZ basis set in the gas and solvent phases.
Geometries with small root-mean-square deviations were obtained with respect to the crystallographic data (the largest 8 degrees in dihedral angles). The frontier HOMO and LUMO energies and gap energies were evaluated in the different solvents ( Table 5). The effect of solvent was evaluated by the PCM method. The most energetically stable structures were obtained in solutions of methanol and dimethyl sulfoxide. Also, the highest values of dipole moment (µ) and the lowest were correlate to the solvent polarity used to obtain suitable crystals for X-ray characterization (see Table 6) [55][56][57][58]. Figure 13 represents the energies of the molecular orbitals LUMO+1, LUMO, HOMO, HOMO-1 in the gas phase and in one of the most stable solvent phase (methanol). Isosurfaces in the diagram represent the maps of HOMO and HOMO-1(bottom) and LUMO (top). The character and energy levels of HOMOs and LUMOs (and the corresponding energy gaps) were determined on fully optimized geometries and compared with electrochemical measurements. The theoretical HOMO energy levels of the three compounds I, II and III in the gas phase were close together,´4.89,´4.99 and´5.08 eV, respectively. I and II p-diphenylamino substituents caused a greater delocalization on the phenyl core than the pyridyl ring. The theoretical calculation showed a delocalization on the pyridine-carbazole backbone for III. Thus, the character of the HOMO of I and II were very different from that of III, for which the HOMO presented a clear pyridyl-phenyl-carbazole delocalization leading to the highest HOMO level (´5.08 eV).  Figure 13. Molecular orbitals of compounds obtained in gas and methanol and phases at M06L/cc-pVDZ theory level. Figure 13. Molecular orbitals of compounds obtained in gas and methanol and phases at M06L/cc-pVDZ theory level. The present calculations were almost in accordance with electrochemical data (Table 4), which led us to conclude a better delocalization in I, II and III, therefore diminishing the HOMO level from I, II to III´5.29,´5.33 to´5.50 eV. With respect to the LUMO levels of the pyridine-substituted I, II and III, they were calculated to be between´2.77, 2.91 and´2.57 eV, respectively, presenting a pyridyl character without contribution of the p-diphenylamino and carbazole moieties. The LUMO energy levels of II are 0.34 eV higher than that of III (´2.57eV) showing the less intense withdrawing effect of the pyridyl unit compared to the carbazole unit. The LUMO level of III was calculated at´2.57 eV, lower than that of I and II and the decrease of the LUMO in II emphasized the withdrawing effect of the pyridyl unit and its efficient conjugation with the carbazole unit, rendering this molecule the most easily reducible. The main tendency of the LUMO levels obtained through theoretical calculations did not agree with our electrochemical conclusions, with the lowest LUMO recorded for I. The LUMO calculated for I, II and III are close together (0.44eV), and their LUMO values determined through electrochemical measurements were also very similar (´3.21,´3.34, and´3.39 eV, respectively).
The theoretical energy gaps of I, II and III were close to 2.12, 2.08, and 2.50 eV, respectively, values closer to the energy gap values obtained from the electrochemical measurements (2.08 eV for I, 1.98 for II, and 2.01 for III) showing a good correlation between theoretical calculations. Furthermore, the values showed better fit with the theoretical calculations when the polarity of the solvent was increased.

Experimental Section
The compounds I-III were synthesized according to the reported methods [30,59]. We utilized several synthetic procedures to obtain adequate crystals of I-III to permit single crystal characterization (Table 6). In some cases, the crystals were too small to allow diffraction and in other procedures, the compounds were almost insoluble in the solvents. The majority of the isomers were crystallized after reaction or were purified by crystallization.
The optimum crystallization procedures for each compound are indicated below. For I, the solid (0.028 g) was dissolved in ethanol:cyclohexane (12 mL) 58˝C and the vessel was set aside at 4˝C and after 3 days, yellow crystals were obtained. Compound II (0.035 g) was dissolved in dimethylsulfoxide (DMSO) at 90˝C; the vessel was set aside at room temperature and after 9 days, yellow crystals were formed. Compound III (0.020 g) was dissolved in cyclohexane (15 mL) at room temperature, the vessel was set aside at 4˝C and after 4 days, tiny yellow crystals were obtained.

Single Crystal X-ray Diffraction (SCXRD)
All reflection intensities for I and II were measured at 110(2) K (after the crystals were flash cooled from RT) using a KM4/Xcalibur (detector: Sapphire3) with enhance graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) apparatus under the program CrysAlisPro (Versions 1.171.35.11 or 1.171.36.24, Agilent Technologies 2012, Santa Clara, CA, USA). Data for III were collected at 110(2) K using a SuperNova diffractometer (equipped with Atlas detector) with Cu Kα radiation (λ = 1.54178 Å) under the program CrysAlisPro (Version 1.171.36.28, Agilent Technologies 2013). The same program was used to refine the cell dimensions and for data reduction. The structures were solved with the programs SHELXS-97/SHELXS-2013, and were refined on F 2 with SHELXL-97/SHELXL-2013 [60]. Analytical numeric absorption corrections based on a multifaceted crystal model were applied using CrysAlisPro. The temperature of the data collection was controlled using the system Cryojet (Oxford Instruments, Abingdon, Oxford, UK). The H atoms were placed at calculated positions using the instructions AFIX 23, AFIX 43, AFIX 123, AFIX 137 or AFIX 147 with isotropic displacement parameters having values 1.2 or 1.5 times Ueq of the attached C or O atoms.

Absorbance and Emission (UV-VIS and PL)
The absorbance spectra were acquired on a SD2000 spectrometer (Ocean Optics, Dunedin, FL, USA) equipped with a pulse Xenon light source P-2 (Ocean Optics for the UV region (220-270 nm) and a Cary 300 Spectrometer (Agilent) equipped with a deuterium and halogen lamp. The wavelength detection range was 190-900 nm. For measurements in solution, the solvents were of spectroscopic grade and were preliminarily checked for the absence of absorbing or fluorescent impurities within the scanned spectral ranges. For powder samples, the absorption was measured using pellets prepared with KBr. A UV/Vis DT 1000 CE light source (Analytical Instrument Systems, Inc., Flemington, NJ, USA) was used for measuring absorption. Emission spectra (PL) were acquired from a QE-Pro-FL (Ocean Optics) equipped with a laser diode excitation source at a wavelength of 405 nm.

Cyclic Voltammetry (CV)
Measurements were carried out with a PGSTAT128N-serial no. AUT85577 potentiostat (Keysight, Santa Rosa, CA, USA) using a three electrode cell assembly comprised of Ag/Agì n solution of 3 M KCl as the reference electrode, a platinum wire (φ = 0.2 mm) as working electrode and counter-electrode, using a solution in CH 2 Cl 2 containing 0.1 M supporting electrolyte of tetrabutylammonium hexafluorophosphate (TBAFP 6 ), and ferrocene was used as external standard. The scanning rate was 50 mV/s.

Conclusions
In this work, we have reported three molecular structures of α,β-unsaturated acrylonitriles which crystallized into single crystals with different Z values and according to crystallography data gave information about the arrangement packing that is possible to correlate with the emission with a bathochromic shift. Such emission enhancement in the aggregate state is due to strong π-π molecular stacking in the crystal lattice, which is does not depend on of intermolecular H-bonding interactions between molecule-solvent because it is present in both I and II. The results from crystal structures with Z 1 > 1 represent a possible solution to the problem of packing molecules in three dimensions, because the three molecules I, II and III have eight molecules per unit cell, which could be due to the crystallization process, as is particularly well illustrated with I as an example where the compound adopts high Z 1 or high symmetry. Also the systems reported for I and II showed stacking via π-π interactions with values closer to those typically found for geometrical parameters of aromatic π-π interactions centroid distances. The data for I represented a rare case with complete hydrogen-bonding of the chains as I, whose structure was all the more remarkable because of the presence of both the syn and anti conformers at the double bond. One of the ultimate goals of solid-state chemistry is the ability to predict or computationally calculate the experimental crystal structure(s) of a compound solely from knowledge of its molecular structure. This factor is a remarkable problem in the computational field because the possible occurrence of more than one symmetry unique molecule greatly complicates the crystal.