3.1. Spectroscopy Analysis
The DADB derivatives were synthesized by C–C coupling Heck reaction following the procedure described by Oliveira et al. [
13,
14]. In terms of this mechanism, it is important to mention that the preference of structural form obtained is the trans-trans DADB. In the second step of synthesis, DEADP is obtained by reduction of double bounds, using KCOOH in the presence of catalyzed Pd. These compounds present different spectroscopy properties. The modifications in the substituent of the ring are of fundamental importance in obtaining derivatives with better properties to be used in biological systems.
As related by Oliveira et al. [
13,
14], the absorption and emission spectra of DEADP, gives three well-defined absorption bands at 356, 375 and 396 nm, and emission bands at 400, 424 and 449 nm, whereas DADB has a broad band of absorption and fluorescence spectra, due to the presence of the double bonds at the 9,10 position of the anthracene ring producing an electron delocalization in the structure, with a maximum absorption at 260 and 405 nm and a maximum fluorescence emission at 525 nm with excitation at 405 nm in acetonitrile. The quantum yield of fluorescence at room temperature to DADB and DEADP compared to Rhodamine B (at a concentration of 1 × 10
−6 mol·L
−1 in MeCN;
= 400 nm, Φ = 0.65) was Φ = 0.230 ± 0.002 and 0.352 ± 0.01 in MeCN, respectively [
13,
14]
The infrared spectrum of the DADB derivative obtained is presented along with the spectrum of the DEADP derivative in
Figure 2. The spectrum shows peaks in the region of 2900–3000 cm
−1 referring to the C–H and C=C groups and it is observed the influence of double bond in the intensity of signals in this region, the presence of the C=O at 1600 cm
−1 and peaks in the 1200 cm
−1 region characteristic of the C–O group. It is observed the higher contribution of signals characterized by C–H in the DEADP structure in the range of 1300–1400 cm
−1.
3.4. Solid State Studies
DADB is an anthracene analogue in which two acrylate groups are
para-bonded to aromatic ring B. This compound crystallized in the monoclinic crystal system and space group P2
1/c. Its unit cell measures a = 13.0556(3) Å, b = 4.03270(10) Å, c = 18.0384(4) Å and
= 90.7920(10)°. Besides crystallographic symmetry, DADB has molecular symmetry as an inversion center
i above the gravity center of aromatic ring B. An Ortep diagram showing the atom displacement with 50% probability level and numbering scheme of DADB is presented in
Figure 5a. DEADP is the result of the addition to the acrylate group, presenting two
sp3 carbons bound to ring B. DEADP crystallized in the centrosymmetric space group C2/c, with eight asymmetric units (AU) in the unit cell and half molecular unit per UA. The ORTEP representation of DEADP is shown in
Figure 5b, as well as the numbering scheme used in the structural description. The refinement indicated that DEADP has a dynamic disorder in O
1, O
2, C
11 and C
12 atoms in which the dominant conformation corresponds to 85%. The main crystallographic data are tabulated in
Table 1.
Geometric parameters of DADB showed that it has a non-planar conformation by analyzing the angle formed between the planes of acrylate and anthracene groups (ω1 = 48.72°). On the other hand, the acrylate portions can be considered planar since the unique planarity deviation is a slight twist around the σ-bond C9–C10 (ω1 = 12.09°). A conformational analysis was performed on two flexible dihedral angles C8–C9–C10–O1 (carbonyl and olefin groups) and C2–C1–C8–C9 (olefin and anthracene groups), indicating that carbonyl groups adopt an anti-periplanar conformation regarding olefin moiety, while the last one has a syn-clinal conformation regarding the anthracene group. Like DADB, the ethyl portion of DEADP is nonplanar to the anthracene group (the angle formed between the planes, ω1, is 25.38°).
For DADB, the carbonyl group often assists in aggregation via two weak C–H
O interactions. These interactions are responsible for assembling the acrylate groups as follows: the first one [C
11–H
11 ⋯ O
2i; d
(D-H) = 0.970 Å; d
(D-A) = 3.137 Å; d
(HA) = 2.777 Å;
= 102.69°] gives rise to a 1D
zigzag chain with
C(13) motif, represented by the color green in
Figure 6a. Then, the second one [C
11i–H
11i O
2; same geometric parameters of C
11–H
11 O
2i] forms a complementary 1D chain, also with
C(13) motif that connects two green chains as a layer almost parallel to (
0 3), represented by the color blue in
Figure 6b,c. The crystal packing of DEADP is like that of DADB, since it is stabilized by two C–H
O interactions. The first interaction [C
5–H
5 O
1i; d (D-H) = 0.931 Å; d (D-A) = 3.541 Å; d (H⋯A) = 2.641 Å; ∡ = 162.74°] forms a chain along the
c axis, while the second interaction [C
5i–H
5i O
1; d (D-H) = 0.931 Å; d (D-A) = 3.541 Å; d (H⋯A) = 2.641 Å; ∡ = 162.74°] links these chains to form a layer parallel to (100).
The supramolecular arrangements of DADB and DEADP were also evaluated from HS analysis. By combining
(distance from the inner molecule to the surface) and d
e (distance from the outer molecule to the surface) in the function of their Van der Waals radii, it is possible to generate a surface named
. This surface is based on a scale color ranging from blue (less intense) to red (more intense). In this sense, the weak intermolecular interactions of DADB are shown as red dots on the surface, as seen in
Figure 7a. The 2D fingerprint plot of O···H contacts is also shown, indicating their percentage. Although interactions (1) and (2) present high intensity on
surface, the fingerprint indicates
and
around 1.5 Å as weak interactions. On the other hand, interactions (3) and (4), represented in both the HS and fingerprint plots of
Figure 7b, presented
and
around 1.4 Å for DEADP.
The π delocalization presented on the anthracene portion provides a planar conformation to it. Hence, the crystal packing of DADB is also stabilized by π
π, as represented in
Figure 8a, and observed in all aromatic rings (Cg
Cg = 4.033 Å). The confirmation of this interaction is given from the shape index surface (
Figure 8b) as red and blue triangular shapes above aromatic rings, representing the places where two molecules meet each other.
In addition, π
π interactions have two specific features in the 2D fingerprint plots of C
C contacts: (a) a triangular shape around 2.2Å >
and
> 1.7 Å; and (b) high incidence of contacts with
1.8, concerning the stacking of aromatic rings. Even having as anthracene portion, DEADP adopts a conformation which stabilizes the DEADP crystal packing with C–H
π interactions, shown in
Figure 8c [C
8-H
8A Cg
A; d (D–H) = 0.969 Å; d (D–A) = 3.628 Å; d (H
A) = 2.987 Å; ∡ = 124.67°; C
8–H
8B Cg
B; d (D–H) = 0.969 Å; d (D–A) = 3.803 Å; d (H
A) = 3.151 Å; ∡ = 126.06°; C
8–H
8A Cg
A; d (D–H) = 0.931 Å; d (D–A) = 3.937 Å; d (H
A) = 3.256 Å; ∡ = 131.75°]. These contacts are 21.2% of the total and are characterized both as red regions over aromatic rings A, B and C (acceptor regions) and blue regions over ethyl hydrogens (donor regions) (
Figure 8d).
3.5. Molecular Modeling Analysis
Molecular geometry plays an important role in understanding the chemical structure of the compounds. The lengths and angles bond molecules of the DADB and DEADP compounds are shown in
Table 2 and
Table 3. The results obtained by DFT/M06-2X/6-311G++(d, p) level of theory agree with the experimental data, according to the correlation factor R
DADB = 0.9825 and R
DEADP = 0.9816 to bond lengths, and R
DADB = 0.9339 and R
DEADP = 0.9786 to angle and show by graphs present by
Figure 9 and
Figure 10.
The bond lengths obtained by theoretical calculations in the aromatic region of the compounds also agree with the values found for the anthracene molecule by measuring X-ray diffraction [
32]. The average lengths of the –C–O– and –C=O bonds in the carboxyl group in both compounds correspond to 1.35 Å and 1.22 Å, respectively. Unsaturation in the vinyl group of DADB (–C
8=C
9–) has a length equal to 1.34 Å, whereas the saturation of the same carbon atoms in DEADP (–C
8–C
9–) has an average length of 1.53 Å. The aliphatic group covalently connected to the anthracene B ring through the –C
1–C
8– bonds in the DADB with an average length of 1.48 Å and the –C
7–C
8– bonds in the DEADP with an average length of 1.51 Å. The average length of –C
1–C
8– bonds in DADB is 2.3% shorter than the equivalent –C
7–C
8– bond in DEADP. In addition, the average length of –C
9–C
10– bonds in DADB is 2.6% shorter than in DEADP. On the other hand, the average length of the –C=O bonds of the carbonyls in DADB is 2.4% higher than in DEADP. The aforementioned differences are related to the presence of π electrons in the –C
8=C
9– bonds of the DADB that resonate with the π electrons of the aromatic ring of anthracene. On the other hand, the π electrons that resonate in this region result in the repulsion of the electrons from the –C=O bond, causing its elongation.
In fact, the largest deviations observed for the angles between the two compounds are found at the atoms of C8 and C9. The angles –C1–C8–C9– and –C8–C9–C10– are respectively 5.29° and 3.04° larger, compared to the 120° value of the plane trigonal geometry in the DADB. In the DEADP composite, the angles of the tetrahedral geometry are altered in relation to the value of 109.5°, which increase 2.53° and 2.44° in –C7–C8–C9– and –C8–C9–C10–, respectively. Both the anthracene aromatic ring and the aliphatic groups are coplanar in both compounds. However, these groups of molecules are in different planes when their dihedral angles are evaluated. In the DADB molecule, the aliphatic and aromatic groups are located at 56.60°, when we look at the dihedral angle –C2–C1–C8–C9–; on the other hand, in the DEADP molecule, these groups are at an average distance of 88.01°, when we look at the dihedral angle –C2–C7–C8–C9–.
The energies of the frontier molecular orbitals (HOMO and LUMO), obtained by the DFT calculations, for the compounds studied in this paper, are shown in
Figure 11. These data are important parameters in the electronic description of the chemical compound, as they help in the understanding of chemical reactivity and kinetic stability of molecules. HOMO corresponds to the ability that the molecule must donate electrons, while LUMO, to receive electrons. and, thus, for large values of the difference between their ΔE
HOMO-LUMO energies, the molecule is highly stable, which means that the compound has low reactivity in chemical reactions, and vice versa. ΔE
HOMO-LUMO calculations showed that the DADB molecule is more reactive than the DEADP molecule. The differences between the energies of the molecular orbitals, ΔE
HOMO-LUMO, show that the DADB molecule is chemically less stable than the DEADP molecule. This is justified by the presence of π electrons located in the unsaturation between atoms C
8 and C
9, in DADB. Furthermore, the energies of the frontier orbitals play an important role in describing the electronic structure, as well as the chemical reactivity of molecules. Important descriptors can be drawn from these values, such as electronegativity
, chemical potential
, hardness
and softness
, defined by the expressions
where
is the energy of the system,
is the number of electrons,
E
HOMO is the ionization potential,
E
LUMO is the electron affinity [
33].
Table 2 brings the reactivity indexes for the DADB molecules and the DEADP molecule.
The electronic isodensities [
25,
29] for the DADB and DEADP molecules are shown in the electrostatic potential maps in
Figure 12. The MEP maps show that the regions with the highest electron density are predominantly concentrated in the red regions of the molecules; that is, the oxygen atoms of the carbonyl groups are prone to electrophilic attacks.
The regions in blue, located on the hydrogen atoms, are the Van der Waals surfaces of lower electron density. The lowest potential values calculated on the electrophilic regions of the DADB and DEADP molecules are −31.39 and −29.96 kcal.mol−1, respectively. Therefore, the carbonyl groups of both molecular structures are sites with high electronic density charge, which can exhibit Lewis base behavior in chemical processes.
3.6. Supramolecular Arrangement
DADB crystals are formed through two specific arrangements, observed experimentally: the first arrangement is a zig-zag structure, where the molecules are joined frontally through –C=O ⋯ C
11 interactions; the second arrangement forms a second structure that interacts axially with the first (
Figure 13a). The calculated values for the complexation energies in the frontal and axial interactions between molecules in the DADB crystals are −3.12 and −17.01 kcal.mol
−1, respectively, corrected by the counterpoise theory [
30].
The first arrangement appears essentially between the region of higher electron density of a molecular unit, around the carbonyl group –(CO)–, with the region of low electron density of the second molecular unit, the ethyl group (
Figure 13a). The second arrangement takes place with the axial contact between the molecules of the compound; its complexation energy is about 5.5 times more intense due to the large surface area of the structures in which contact occurs.
Similarly, the DEADP crystals are formed in two arrangements: the first in zig-zag and the second in layers (
Figure 13b). In this case, the formation of the first arrangement occurs through the contact between the O atom of the carbonyl group with the H atom of the anthracene ring while the second also axially (
Figure 13b). The theoretical calculation showed that the complexation energy of the frontal contact has an energy equal to −3.69 kcal·mol
−1 while the axial contact resulted in an energy of −12.69 kcal·mol
−1. The axial contact between DEADP molecules is justified by the large surface area, which is in contact with the crystals, resulting in an interaction 3.4 times greater compared to the frontal contact.
Through NBO calculations, it was possible to determine the stabilizing interactions between the donor bonding orbitals (Lewis type) and acceptor antibonding (non-Lewis type) in the molecules of the crystals of DADB (
Table 3) and DEADP (
Table 4). The interactions present between the bonding orbitals (donors) and the antibonding orbitals (acceptors) have low hyperconjugation energies. The frontal contact between two DADB molecules showed that there are preferential hyperconjugations between the bonding σ orbitals the C
11–H and C
11i–H bonds—where the donor orbitals have an occupancy slightly higher than 1.98e, while the acceptor orbitals have an occupancy of 0.02e. The isolated pairs of electrons from the O atom of the carbonyl of a molecular unit also participate in this intermolecular interaction through hyperconjugation with antibonding σ* orbitals of the O
1–C
11 and C
12–H bonds of the second molecular unit. In the axial contact between the DADB molecules, it is possible to observe a large amount of hyperconjugation between the binding π orbitals of one molecular unit with the antibonding π* orbitals of the second molecular unit. These hyperconjugations have low energies, but in general, higher than 0.1 kcal·mol
−1, especially in the interactions that occur in the region of the aromatic ring of anthracene. In DEADP crystals, frontal contact between molecular units occurs weakly through σ and π hyperconjugations, and through isolated pairs of electrons on the O atom of the carbonyls of one molecular unit with the antibonding orbitals of the other unit. The axial contact results in numerous interactions that are weakly stabilized thanks to the large contact surface in the planar region of the aromatic ring of the anthracene core. Hyperconjugations of the bonding π orbitals with the antibonding π* orbitals have a greater stabilizing energy, as well as occur in the layered structure of DEADP crystals.
The topological parameters at the critical bond point between the nuclear attractors of the intermolecular interactions are shown in
Table 5 and
Table 6 for compounds DADB and DEADP. The critical bond points, as well as the bond pathways, are represented by the molecular graphs in
Figure 14 and
Figure 15. The values of these parameters showed that the interactions between the molecules in the crystals of both compounds are of the
closed-shell type. This means that the interactions that occurred present an electrostatic character due to the low electronic density between the nuclear regions of the molecular pairs.
The frontal interaction between two DADB units is explained through two BCPs: the first, occurs exactly in the intranuclear region –C=O ⋯ C
11; the second, in the C
11–H ⋯ H–C
11 region. These two regions have a low electron density (<0.2 au). The axial interaction between two molecular units is observed through five BCP, all with low electron density. The larger contact surface between the molecules in the second case increases the number of interactions. In addition, the large number of delocalized π electrons in the region of the aromatic rings contributes to the weak interaction between the molecules of the compound in the crystal. Similarly, the interactions between molecules observed in DEADP interact, as shown in
Figure 13b, being
closed-shell interactions. Frontal contact between two molecules results in an interaction involving three BCP. The results showed that the electron density in the intranuclear regions C
8–H ⋯ H–C
AR, C
AR–H ⋯H–C
AR and C
AR–H ⋯ O
1=C
10 are low, according to the values presented in
Table 6.
The axial interaction between two molecules in the crystals of this compound is observed by several BCP, whose electron density is similarly low among nuclear attractors (
Table 6). In this last contact, the interaction takes place at various points in the molecules due to the large contact surface. Therefore, the molecules of DADB and DEADP, interact in their respective crystals through low-intensity intermolecular forces.