Experimental and DFT Study of the Photoluminescent Green Emission Band of Halogenated (-F, -Cl, and -Br) Imines.

The morphological, optical, and structural changes in crystalline chiral imines derived from 2-naphthaldehyde as a result of changing the -F, -Cl, and -Br halogen (-X) atoms are reported. Scanning electron microscopy (SEM), optical absorption, photoluminescence (PL), and powder X-ray diffraction (XRD) studies were performed. Theoretical results of optical and structural properties were calculated using the PBE1PBE hybrid functional and compared with the experimental results. Differences in surface morphology, absorbance, XRD, and PL of crystals were due to the change of halogen atoms in the chiral moiety of the imine. Absorption spectra exhibited the typical bands of the naphthalene chromophore located in the ~200-350 nm range. Observed absorption bands in the UV region are associated with π→π* and n→π* electronic transitions. The band gap energy was calculated using the Tauc model. It showed a shift in the ~3.5-4.5 eV range and the crystals exhibited different electronic transitions associated with the results of absorbance in the UV region. XRD showed the monoclinic→orthorhombic crystalline phase transition. PL spectra displayed broad bands in the visible region and all the samples have an emission band (identified as a green emission band) in the ~400-750 nm range. This was associated with defects produced in the morphology, molecular packing, inductive effect and polarizability, crystalline phase transition, and increase in size of the corresponding halogen atoms; i.e., changes presumably induced by -C-X…X-, -C-X…N-, -C-N…π, and -C-X…π interactions in these crystalline materials were associated with morphological, optical, and structural changes.

1. Introduction

In the last decades, much effort has been devoted to synthesizing organic crystals with interesting morphological, optical, and structural properties to elucidate the relationships that exist between these parameters and proposed theoretical models. Therefore, organic crystals have been considered for the fabrication of some electroluminescent (EL) devices [1]. Several reports have investigated the emission bands in the visible region and this optical phenomenon is related to crystalline defects such as vacancies and interstices [2,3,4,5]. In this regard, they are ideally suited for multicolor display applications [6,7]. However, scarce reports can be found in the literature regarding studies using the photoluminescence (PL) technique explaining the origin of green, blue, and red emission bands generated by crystal defects [8]. On the other hand, the composing units of organic crystals have an anisotropic structure at the molecular level, as well as diverse chemical and physical parameters that have influence and are associated with different crystalline defects. Furthermore, in order to discuss the lattice defects of organic crystals, information is needed not only on the changes of the position of each functional group in the diverse positions of the organic molecule, but also on the influence of the resulting molecular orientations, such as their rotation and inclination around a lattice point as well as the effect of chirality, which distorts the organic molecule and as a consequence, generates different emission bands [9,10]. Thus, for solid-state organic crystals, the constituent molecules may form strong intermolecular interactions and assemble into packed structures; these are some of the key parameters affecting the optical behavior of these materials. Organic crystals are of crucial interest because of their wide range of applications in various fields [11]. On the other hand, regarding green chemistry, the solvent-free approach is encouraging because is a simple synthetic method, fast and more environmentally acceptable, and can achieve high yields and shorter reaction times in the synthesis of imines (I). Furthermore, such procedures allow one to obtain crystals of small molecules (less than 60 atoms) [12]. The molecules synthesized by our group involve a stereogenic carbon atom, which is considered as an additional feature in regard to the parameters examined here [13]. The photoluminescence (PL) spectroscopic technique is applied in different organic and inorganic materials to investigate emission bands in the visible region. These bands are associated with crystalline defects such as vacancies and interstices. However, they have been also observed in rare earth oxides and these are associated with f→f transitions [14]. The introduction of functional groups in I crystals causes a PL signal increase of the characteristic emission peaks of I [15] by mesomeric and inductive effects, as the electron density of the aromatic ring is increased with the π→π* and n→ π* electronic transitions, etc., along with hyperconjugation [16]. In summary, the objective of this paper is systematically synthesize four crystals and modify them by adding the −X halogen atom and to characterize the morphological, structural, and optical relationships associated with the change of −X atoms. A structural parameter is the crystalline phase presented by some materials, showing a monoclinic→orthorhombic variation which modifies the signals observed by means of the PL and, in this report, the aforementioned signals in the visible region of the electromagnetic spectra are observed in all the organic crystals. This phenomenon of crystalline phase transition has been scarcely studied, unlike intense works of this behavior in inorganic semiconductors. PL is associated with this phenomenon by rearrangement of the ions that make up the inorganic crystal. In the last decade, the −C−X…X−C− interactions have been widely studied in this kind of system. These interactions are related to van der Waals forces of short and long range. Within these interactions, −C−N…X−, −C−N…H−, −C−H…π, −C−X…π, and −C−N…π are the most relevant [17].

2. Experimental Part

The crystalline growth technique of the synthesized imines has been previously reported by our research group [18]. The diagrams of the molecular structures, names, and the labels used of the corresponding molecule, ionic radii, as well as the polarizability of the halogen [19] atoms added are shown in Table 1. Selected crystallographic data are shown in Table 2. Morphological images were achieved by the scanning electronic microscopy (SEM) utilizing Voyager II X-ray quantitative microanalysis in an 1100/1110 SEM system from Noran Instruments. The powder X-ray diffraction (XRD) patterns were acquired with a D8 Bruker Discover Series 2 Diffractometer (Bruker AXS GmbH, Östliche Rheinbrückenstr 49, 76187 Karlsruhe Germany) with Cu Kα radiation of wavelength λ ~1.54 Å. The absorbance spectra were registered using a UV-Vis spectrophotometer (Cary-5000). PL spectra were characterized by a main peak under optical excitation provided by an Ar+ laser beam with a pump power of 10 mW and excitation at 425 nm using a Science-Tech model 9040 apparatus. The initial geometries for theoretical calculations were generated from X-ray data for all crystals by using standard Cartesian coordinates through the utilities of the Mercury program [20]. Optimized geometries, HOMO and LUMO energies, and gap energies (Eg) were obtained using density functional theory (DFT) with the PBE1PBE hybrid functional [21,22] with standard 6-311G(d,p) basis set on all atoms [23]. Based on the optimization geometry, TD-DFT (TD-PBE1PBE/6-311G(d,p)) was used to calculate the lowest-lying electronic energies of allowed transitions for 20 excitations (nstates = 20). All theoretical calculations were carried out in the Gaussian 09 package and the results were visualized with Chemcraft 1.8 and Gaussian View 6.0 programs [24].

Table 1

Structures of the chiral imines: (S)-(+)-2-(((1-Phenylethyl)imino)methyl)naphthalene (I), (R)-(–)-2-((((4-Fluorophenyl)ethyl)imino)methyl)naphthalene (I−F), (S)-(+)-2-((((4-Chlorophenyl) ethyl)imino)methyl)naphthalene (I−Cl) and (S)-(+)-2-((((4-Bromophenyl)ethyl)imino)methyl) naphthalene (I−Br).

Sample	Structure	Ionic Radii (Å)	Polarizability (Å3)
I		-------	------
I−F		F ~ 1.33	~0.6
I−Cl		Cl ~ 1.81	~2.2
I−Br		Br ~ 1.96	~3.1

Table 2

Selected crystallographic data of the chiral imines.

	I	I−F	I−Cl	I−Br
Crystal data
Chemical formula	C19H17N	C19H16FN	C19H16ClN	C19H16BrN
M r	259.33	277.33	293.78	338.24
Crystal system, space group	Monoclinic, P21	Monoclinic, P21	Orthorhombic, P212121	Orthorhombic, P212121
Temperature (K)	298	298	298	298
a, b, c (Å)	14.9328(2), 6.01143(10), 33.9985(7)	7.5950(11), 5.8997(9), 16.996(3)	6.0567(5), 7.6139(5), 33.853(3)	6.0526(2), 7.6671(4), 33.9712(19)
α, β, γ (°)	90, 102.6011 (17), 90
V (Å3)	2978.45 (9)	751.3 (2)	1561.1 (2)	1576.46 (13)
Z	8	2	4	4
Radiation type	Cu Kα	Mo Kα	Mo Kα	Mo Kα
μ (mm−1)	0.51	0.08	0.24	2.60
Crystal size (mm)	0.49 × 0.17 × 0.10	0.74 × 0.21 × 0.09	0.38 × 0.32 × 0.14	0.36 × 0.22 × 0.19
Data collection
Diffractometer	Agilent Xcalibur Atlas Gemini	Agilent Xcalibur Atlas Gemini	Agilent Xcalibur Atlas Gemini	Agilent Xcalibur Atlas Gemini
Absorption correction	Analytical (CrysAlis PRO; Agilent, 2013)	Analytical (CrysAlis PRO; Agilent, 2013)	Multi-scan (CrysAlis PRO; Agilent, 2013)	Multi-scan (CrysAlis PRO; Agilent, 2013)
Tmin, Tmax	0.862, 0.960	0.970, 0.994	0.437-0.703	0.876, 0.920
No. of measured, independent and observed [I > 2σ(I)] reflections	33,420, 10,553, 9322	7823, 2627,1640	21,167, 2752, 2014	19,221,3120,2017
R int	0.045	0.0.051	0.065	0.047
Refinement
R[F2 > 2σ(F2)], wR(F2), S	0.039, 0.092, 1.04	0.066, 0.153, 1.25	0.057, 0.139, 1.07	0.044, 0.098, 1.02
No. of reflections	10,553	2627	2752	3120
No. of parameters	722	190	191	191
No. of restraints	1	1	0	0
H-atom treatment	H-atom parameters constrained	H-atom parameters constrained	H-atom parameters constrained	H-atom parameters constrained
Δρmax, Δρmin (e Å−3)	0.13, −0.12	0.23, −0.19	0.18, −0.29	0.32, −0.44

3. Results and Discussion

3.1. Molecular Structure

An investigation associating the optical, morphological, and structural changes presented by the different crystals modified by introducing −X halogen atoms in the I organic molecule was carried out. Evidently, the different ionic radii and polarizability effect of the −X atoms differ, so this effect is observed in the corresponding packing array. This structural behavior is considered with the systematic incorporation in the imine I molecule of the corresponding −X. On the other hand, prediction is a fundamental aim of the calculations based on the quantum chemistry methods, and several physicochemical properties depend largely on the more stable conformation of the molecular system in its fundamental state. Thus, the optimized molecular structure of I and I−X crystals can be compared in order to analyze the changes in the properties, as the −X atoms were substituted in the para-position of the phenyl group [25]. Molecular structures and schematic representation of the (a) I, (b) I−F, (c) I−Cl, and (d) I−Br crystals are showed in Figure 1.

Figure 1

Molecular structures and schematic representation of (a) I, (b) I−F, (c) I−Cl, and (d) I−Br compounds.

The initial geometries were minimized using the DFT method at PBE1PBE/6-311G(d,p) theory level without any restriction of symmetry. The experimental and optimized molecular structures were compared by superposition using the UCSF Chimera package [26], the optimized structure (in green) and the experimental structure (in cyan) of which are shown in Figure 2. X-ray data and optimized structures of I and I−X crystals are provided in Table S1. An agreement in two of the four compounds between optimized and experimental values can be observed. Experimental crystal structures (CCDC 1425659, 1425662, 1425663, and 1425664) show no π−π interactions; however, C−H…π intermolecular contacts are observed [27]. Halogen bonds were not observed in molecular packing from CIF file structures. Halogen bonds C−X…H were additionally investigated by means analysis of Hirshfeld surfaces and fingerprint plots [28] using CrystalExplorer17 software [29]. Contributions of 5.9% for F…H and 5.2% for H…F, 7.5% for Cl…H and 4.1% for H…Cl, 8.0% for Br…H, and 3.8% for H…Br were found. This analysis indicated that the most predominant noncovalent interactions in compounds are H…C (14.1−17.5%), H…H (45.7−57.2%), and C…H (9.6−22.3%), see Figure S1 in SM. Values of RMSD for structures (a) I and (b) I−F are 0.10, 1.54, and 1.99 for (a) I, while for (b) I−F were 0.09, 0.95, and 3.04, for bond lengths, valence angles, and dihedral angles, respectively. For compounds (c) I−Cl and (d) I−Br, larger deviations were obtained: 0.09, 1.12, and 16.04 for (c) I−Cl, while for (d) I−Br these were 0.09, 1.17, and 15.41. Values of 16.04 and 15.41 for RMSD for dihedral angles indicate the great conformational flexibility of compounds (c) and (d) arising from torsional motions in these compounds. RMSD of atomic positions for (a) I, (b) I−F, (c) I−Cl, and (d) I−Br are 1.46, 1.72, 8.44, and 8.11, respectively. Values of RMSD between X-ray experimental and calculated structures for internal coordinates are collected in Table S2, using the numerical convention of Figure S2. In Figure 2, it is observed that optimized geometries of (a) I and (b) I−F crystals do not present significant changes compared to the X-ray coordinates. The phenyl groups in the optimized and experimental structures practically overlap, as it can be observed from their small RMSD values. The main difference is the distortion of the dihedral angle corresponding to the phenyl group in (c) I−Cl and (d) I−Br crystals. In the optimized structures, the phenyl groups are not coplanar in regard to those in the experimental data. The calculated dihedral angles of N-Cchiral-Cring-Cring are ~149.25° and ~147.12°, while the experimental values are ~97.92° and ~98.04° for (c) I−Cl and (d) I−Br crystals, respectively. DFT calculations including dispersion correction were used for testing if the difference in ring tilt for structures (c) and (d) was improved; however, similar values were obtained at PBE1PBE and PBE1PBE-D3, including dispersion correction, see Table S3. These deviations in molecular structure affect electronic transitions such as π*→π, n→π, etc. [30] of the naphthyl group. The difference between the calculated and experimental structure can be attributed to different effects; for example, molecular distortion generated by the chiral carbon, and inductive effect and ionic radii of the corresponding −X atom, as well as morphological effects arising from crystalline packing of these materials. The substitution in the para position of the −X group of the largest size on the phenyl group implies a large deviation in the torsional angle N−Cchiral−Cring−Cring and therefore an influence on the electronic interaction in the solid state crystalline form can be presented.

Figure 2

Optimized structure (in green) and the experimental structure (in cyan) of (a) I, (b) I−F, (c) I−Cl, and (d) I−Br compounds.

As can be seen, a chiral carbon atom exists in the four structures. In structures (a), (c), and (d), the methyl group linked to the chiral carbon is set in the back of the molecular plane, while in the structure (b) I−F, the methyl group is found at the front of the phenyl group. This fact was corroborated by theoretical calculations of electronic circular dichroism (ECD) [31,32] using TD-DFT methodology. These results are summarized in Figure S3 and Table S3 in SM.

An important quantity in the mechanical-quantum calculations is the molecular electrostatic potential (MEP). It is described as a first-order interaction measurement between the molecular charge (nuclei and electrons) and a unit positive charge in any point surrounding the molecule. Electrostatic potential is a real physical property, which can be determined by computational methods, as well as by experimental methods [33]. The distribution of electronic and nuclear charge in a molecule can be achieved through the MEP and can be used as a reactivity index for the interaction study between receptors or between two molecules. The isosurfaces of the electrostatic potential are mapped on the electron density using an isovalue = 0.07 a.u. for (a) I, (b) I−F, (c) I−Cl, and (d) I−Br calculated at the PBE1PBE/6-311G(d,p) theory level as shown in Figure 3. Low electron density is indicated by blue regions while high electron density is indicated by red regions [34] in a −3.0 × 10−2 to 3.0 × 10−2 range.

Figure 3

Isosurface of the total electron density mapped with molecular electrostatic potential (isovalue 0.07 a.u.) of (a) I, (b) I−F, (c) I−Cl, and (d) I−Br compounds.

The electronic charge distribution of I and I−X crystals by using MEP based on the static charge distributions of the nuclei and electrons was examined to predict the reactivity of the corresponding organic molecule [34]. The MEP at a given point ρ(x,y,z) in the vicinity of a molecule is the force acting on a positive test charge (a proton) located at ρ through the electrical charge cloud generated through the molecules electrons and nuclei. Despite the fact that the molecular charge distribution remains unperturbed through the external test charge (no polarization occurs), the electrostatic potential of a molecule is still a good guide in assessing the molecule’s reactivity towards positively or negatively charged reactants. The MEP is typically visualized through mapping its values onto the surface, reflecting the molecule’s boundaries. The latter will be used here to answer the question of which site in the systems are altered by the −X group substitution, generating a higher electronic density zone where the halogen is attached. In order to map the MEP on the molecular surface using a color-coded scheme, it is observed that the zones at the hydrogen-plane atoms of the rings of phenyl and naphthyl groups become more electrophilic regions, resulting in lower electronic density (blue regions) [35]. Halogen atoms in halo-organics are typically considered as sites of high electron density because of their high electronegativity. Consistent with this well-established understanding, it is commonly accepted that −X atoms can form attractive interactions by functioning as electron donor sites (i.e., nucleophiles) [36]. The most electrophilic region follows a reactivity trend: I−F > I−Cl > I−Br > I. In addition, it is observed that the negative potential regions (red zones) are found around the π-systems of the three aromatic rings, showing high activity of the nitrogen atom in the I−F compound. When the −X electron-releasing substituents are included, the behaviors of the MEP for the aryl group are slightly more positive (green color) and these results showed that this process modulates the charge distribution. They are caused by the inductive effect of the −X atoms and imply that the aryl rings stabilize the negative charge more efficiently when the −X atoms are changed [37]. A thorough analysis of this behavior has been reported by Clark et al. [38], considering that the three unshared pairs of electrons produce a belt of negative electrostatic potential around the central part of −X, leaving the outermost region positive. On the other hand, some atomic properties, such as an atomic charge, are isotropic, but when these derivations become anisotropic, the plotting of the MEP onto the electron density surface can exhibit some anisotropic behavior; quantifying these changes can be a challenge. It is well known that halogen atoms can form so-called halogen-bonds of the type C−X…H in crystal structures with a near perfect 180° angle. This has led to the discovery of σ-holes in halogens. σ-holes are electrophilic portions of the anisotropic electrostatic potential in an otherwise nucleophilic atom. We calculated the MEP for four compounds at the same level of theory and the mapping shows that there is no evidence of such σ-holes, as seen in Figure 3. In addition, Figure S3 corroborates the nonexistence of halogen bonds. We wanted to quantify the strength of the σ-holes to effectively attract a nucleophile and perhaps perform C−X…H interactions. The calculations showed that this effect is not found in the case of I−X compounds, for which the combination of its high electronegativity and significant sp-hybridization causes an influx of electronic charge that neutralizes the σ-hole.

3.2. Crystal Structure and Morphological Properties

The chemical composition was investigated by energy dispersive X-ray spectroscopy (EDS). Using this technique, the elements are identified by their characteristic emission lines on the emitted X-ray spectra and this is associated with the electronic transitions corresponding to the different atomic layers [39]. Percentages of C, F, Cl, and Br atoms (%) are shown in Table 3.

Table 3

Percentages of atoms (%) of the I−X crystals.

Sample	Elements (%)
I−F	C 88.1	F 11.9
I−Cl	C 99.8	Cl 0.2
I−Br	C 97.7	Br 2.3

The typical EDS spectra of I−X crystals are shown in Figure 4a–c and in the inset the corresponding peaks of C and halogen F, Cl, and Br atoms.

Figure 4

EDS spectra of (a) I−F, (b) I−Cl, and (c) I−Br. Inset shows the corresponding peaks associated with F, Cl, and Br elements.

SEM images of (a) I, (b) I−F, (c) I−Cl, and (d) I−Br are shown in Figure 5, with two different amplifications (100 μm and 500 μm) in order to appreciate the corresponding morphology. Different morphologies can be seen when compared with each other. For (a) I image, laminar stacking of small thickness with a smooth and compact surface can be observed. In general, this morphology is associated with crystals having lamellar morphology, commonly corresponding to organic molecules with aromatic rings and/or conjugated bonds [40]. With the incorporation of the –X atoms, the morphology undergoes drastic changes and typical compact surface morphology of thin films can be observed [41]. Thus, images (b−d) show a rough surface and a defined grain boundary with conglomerates in the crystals. The crystals grew in a lamellar form and the corresponding −X effect modified the crystalline molecular packing drastically.

Figure 5

SEM images of (a) I, (b) I−F, (c) I−Cl, and (d) I−Br samples.

The structural properties of these crystals have already been reported by our research group [27]. By using Bragg’s law, the structural changes of all crystals were examined [42]. As the powder X-ray diffraction (XRD) technique is scarcely used in organic crystals, there are no patterns available in the databases. However, we used the results obtained by using the powder XRD technique to obtain additional information of these materials. XRD diffractograms of I and I−X crystals are shown in Figure 6. The sweep was performed in the 2θ ~5°–60° angular range. In these diffractograms, only four reflections of different intensities can be observed. The 2θ diffraction angles corresponding to the four most intense crystal planes are displayed in these diffractograms.

Figure 6

XRD patterns of I, I−F, I−Cl, and I−Br crystals.

These phases, as reported in Table 2, are I (monoclinic), I−F (monoclinic), I−Cl (monoclinic), and I−Br (orthorhombic), respectively. Worth mentioning is the monoclinic→orthorhombic transition as an effect produced by the −X atom in the crystalline structure of I−X, a drastic change, and only the I−F and I−Cl crystals present two reflections in the same direction with similar intensity. The effect of −X atoms is significant and the intensity of the reflection of the crystal planes corresponding to an intensity decrease in the order I < I−Br < I−F < I−Cl can be observed. I−Br and I have different crystalline phases and low intensity peaks. However, the peak located at 2θ ~ 10.73° only changed in intensity in I, I−F, and I−Br crystals. The monoclinic→orthorhombic phase transition has been analyzed in detail in various materials, such as inorganic semiconductors, and its behavior is associated with photoluminescence (PL) spectroscopy, see Section 3.3.

3.3. Optical Properties

A study of the optical properties of these materials in solid state was performed to compare their behavior associated with the interaction of radiation in the UV-region of the electromagnetic spectrum. An important aspect is the study of the relationship between the chemical structure and the photophysical properties of these optical materials. The experimental model applied in this paper has been reported in previous reports [25,43]. The correlation of the morphological properties associated with the optical response has motivated careful analysis of optical materials [44]. We chose of study in this region is because these materials generally absorb in the UV-region [45]. The absorbance was examined in the UV-region (~200–400 nm or ~6.20–3.10 eV), which is associated with π→π* transitions that have been examined earlier [43]. These electronic transitions are generally assigned in this region of the electromagnetic spectra according to previous reports [15,46]. The absorbance vs. wavelength (nm) spectra of the I and I−X samples are shown in Figure 7.

Figure 7

Absorbance vs. wavelength (nm) spectra of I, I−F, I−Cl, and I−Br samples in solid state.

These absorbance spectra exhibit the typical bands of the naphthalene chromophore located at ~200–350 nm (~6.2–3.5 eV) [47]. Electronic transitions can be appreciated in the I sample as the broad band located at ~242 nm (~5.12 eV). The I−X crystals, whose absorbance bands presented better definition and clear decoupling, exhibited two bands located at ~234–248 nm (~5.29–5.00 eV), this region is shown with a blue strip and is associated with the degeneration effect and a small shift to shorter UV wavelengths, and is probably linked to the effects mentioned in the theoretical part induced by effects such as molecular distortion, ionic radii size, resonance effect, and chirality. These bands may be attributed to π*→π (~343 nm or 3.61 eV) and n→π* (~512 nm or ~2.42 eV) electronic transitions [48]. However, other reports ascribed these transitions at a different range, which implies that these electronic transitions are located at different wavelengths according to the molecular arrangement of the corresponding molecule [49]. In our opinion, this behavior is related to the structural and morphological properties associated with the crystalline growth and the related parameters. In the I−Br sample, there are four bands located in the ~208–269 nm (~5.90–4.60 eV) range. The small and weak band located at ~269 nm has been reported by Kose et al. and was associated with the protonation of the imine compounds [50]. The bands with maximum intensity located at region ~239–250 nm (~5.18–5.00 eV) can be assigned to benzenoid (π→π* and n–π*) electronic transitions [30]. On the other hand, three absorption bands located at ~208 nm (~5.96 eV), ~224 nm (~5.53 eV), and ~286 nm (~4.33 eV) are associated with the benzene ring [51]. The appearance as well as the almost complete disappearance and shift of the aforementioned bands can be probably associated with the molecular packing, geometric distortion induced by chirality electron affinity, polarizability, and the inductive effect [52]. Experimental wavelengths of I and I−X samples quantified by absorbance results are showed in Table 4.

Table 4

Experimental wavelengths of I, I−F, I−Cl, and I−Br samples quantified by absorbance results.

Sample	λ1 (nm)	λ2 (nm)	λ3 (nm)	λ4 (nm)
I	242	286	298	---
I−F	239	250	---	---
I−Cl	208	224	239	248
I−Br	234	243	253	---

The band gap energy (Eg) of organic molecules has been previously reported [53]. Eg of organic molecules can be qualitatively derived from its dependence on the bond length alternation pattern, planarity, resonance, and/or electron-withdrawing/releasing substitutions as well as other related structural and morphological properties. Although useful in describing certain trends, these properties are ultimately dependent on the electronic structure of the molecule, which must hence be incorporated in any quantitative description. The experimental numerical quantification of Eg of optoelectroactive materials is challenging because, in principle, it requires knowledge of the two-particle Green’s function propagator. Applying the absorbance experimental results, the Eg of the samples was calculated. The optical behavior associated with electronic transitions appearing in these organic molecules was also examined, applying the model used to investigate electronic transitions in semiconductors [38,54], oxides [14], and some dielectrics materials [55]. A typical model to quantify the Eg for a direct band is the classic Tauc model, which is simple, versatile, and very powerful to provide adequate experimental results in different energy intervals of the photon (hν). This model is given by the (αhν)n = A(Eg − hν) equation, where A is constant and the exponent n depends on the type of transition (½, 2, 3/2, and 3), corresponding to allowed direct and indirect transitions Eg. The near-edge region can be fitted to an equation in which the intercept gives Eg, the fitting exponent (n) identifies the electronic transition as direct or indirect, and these are often called Tauc plots [56]. Extrapolation of straight-line proportions to zero absorption coefficient (α = 0) yields experimental estimation of the Eg values. The optical absorption spectra of I and I−X crystals are shown in Figure 8, which are linear at the absorption edge, indicating a direct allowed transition.

Figure 8

(αhν)2 vs. (hν) plot of I−X crystals. Inset shows (αhν)2 vs. (hν) of I spectra. In I−F and I−Cl samples, the so-called Urbach tail generated by crystal defects located in the ~3.7–4.3 photon energy range can be seen.

In the inset of Figure 8, the spectra of the I sample is shown. In particular, the I sample showed a small transition located at ~3.5 eV and the fundamental band, located at ~3.8 eV, is well defined. In the spectra, different optical behaviors and shifts towards shorter wavelengths and therefore an increase in photon energy for I−X samples (see Table 5) can be seen. From these experimental results, in addition to identifying the corresponding fundamental transition, other weak transitions are observed that can be associated with the electronic transitions observed by absorbance in the UV region. For I−F and I−Cl samples, similar optical behavior can be observed. On the contrary, worth noting is that the I−Br compound has three electronic transitions in addition to the fundamental located at ~3.46–4.16 eV. In the I−Cl and I−F samples, the so-called Urbach tail can also be observed, which is generally associated with crystalline defects that generate energy levels within Eg [57]. On the other hand, the Eg > 3.6 eV could correspond to π→π* and n→π* electronic transitions. For I crystal, the high-energy peak at ~3.8 eV may correspond to the transition of the valence band to the conduction band [58].

Table 5

Experimental electronic transitions (Eg) of I, I−F, I−Cl, and I−Br crystals.

Sample	E1 (eV)	E2 (eV)	E3 (eV)	E4 (eV)
I	3.50	3.8	---	---
I−F	----	----	4.03	4.07
I−Cl	3.46	3.77	3.84	3.95
I−Br	3.46	3.77	4.02	4.16

From a theoretical point of view, in various systems of organic molecules with conjugated bonds, as is the case of I and I−X compounds, the analysis of the molecular orbitals (MO) and properties such as energy associated with the different electronic transitions are considered to explain their optical behavior. This fact allows predicting the aforementioned electronic transitions presented by these compounds [59]. Frontier molecular orbitals (HOMO/LUMO), the Eg, and the total electronic energies of each compound in their ground state were calculated using the same theory level. The values associated with the energies of the frontier molecular orbitals (HOMO/LUMO), and the gap energy Eg, can provide information about the electronic transitions and the physicochemical properties of the molecules. For a small value of Eg (ELUMO-EHOMO), a greater polarizability is reflected in a molecule and this is associated with the thermodynamic probability of chemical reactivity or with low chemical kinetic stability [60]. HOMO energy is related to the ionization potential of a molecule, while the LUMO energy is associated with the electron affinity; therefore, they can be considered as molecular orbitals with the ability to donate or accept electrons, respectively. The isosurfaces of frontier molecular orbitals with an isovalue of ~0.022 for (a) I, (b) I−F (c) I−Cl and (d) I−Br compounds are shown in Figure 9.

Figure 9

Isosurfaces of the frontier molecular orbitals with isovalue of 0.022 a.u. of (a) I, (b) I−F, (c) I−Cl, and (d) I−Br compounds.

One of our purposes in this work was to investigate the electronic transitions of these compounds using TD-DFT. Only the lowest excited electronic singlet states were considered. The maximum absorption wavelengths λmax (nm), oscillator strengths (f), transition energy (Eg-theo, eV), experimental transition energy (Eg-exp, eV),% of contribution, and assignment for the I and I−X compounds are displayed in Table 6. These values exhibited four fundamental transitions of HOMO→LUMO for each compound with low-strength oscillator f ~0.0732, ~ 0.0183, ~0.0806, and ~0.0755 at λmax of ~299.78, ~301.33, ~301.29, and ~300.67 nm for I and I−X, respectively. However, the second transitions corresponding to HOMO→LUMO+1 showed the major oscillator strength of 0.1431, 0.1582, 0.1500, and 0.1430 found at λmax ~286.0, ~239.0, ~234.0, and ~277.0 nm. All of these transitions correspond to an assignment of π→π*. In addition, at 208 nm an electronic transition π →δ* was found for I−Br.

Table 6

Calculated maximum absorption wavelengths λmax (nm), oscillator strengths (f), transition energy (Eg-theo, eV), experimental transition energy (Eg-exp, eV),% of contribution, and assignment of I, I−F, I−Cl, and I−Br compounds. Calculated results at TD-PBE1PBE/6-311G(d,p) theory level.

Compound	Transition Type	λ max	f	Eg-theo	Eg-exp	%Major Contribution	Assignment
I	HOMO→LUMO	242	0.0732	4.13	3.50	58%	π→π*
	HOMO→LUMO+1	286	0.1431	4.41	3.8	35%
I–F	HOMO→LUMO	243	0.0755	4.13	4.03	60%	π→π*
	HOMO→LUMO+1	239	0.1430	4.40	4.07	38%
I–Cl	HOMO→LUMO	239	0.0806	4.12	3.95	62%	π→π*
	HOMO→LUMO+1	234	0.1500	4.41	4.02	37%
I–Br	HOMO→LUMO	298	0.0813	4.11	3.46	71%	π→π*
	HOMO→LUMO+1	277	0.1582	4.40	3.77	35%
	HOMO→LUMO+2	224	0.0050	4.47	3.46	49%
	HOMO→LUMO+3	208	0.0062	4.58	3.77	19%	π→δ*

As seen in Figure 9 in all molecules, the HOMO orbital has a character of π-electron type, located on the naphthalene ring on the double bond of the I group and, in a minor contribution, on the single bonds −C−C− of the chiral carbon atoms. Therefore, the HOMO→LUMO transitions in the I−X compounds imply a transfer of electron density exclusively involving the carbon atoms of the naphthalene ring and the double bond of I. For HOMO→LUMO+1 transition, minor participation of the I group is observed. Such interactions involving HOMO→LUMO and LUMO+1 orbitals can be considered as π→π* electronic transitions. The most intense absorption bands represent the major probability transitions π→π*, while the lowest intense absorption corresponds to less probable transitions n→π*. The calculations showed a certain degree of influence of attractor character of the −X atoms on the λmax, i.e., the value only increases by ~1.0 nm in regard to nonsubstituted I, owing to the nonsubstituted −X group on the naphthalene ring which is not contributing directly to the formation of the frontier molecular orbitals. The other two transitions with lower oscillator strength were compared with the experimental values for the I−Br sample. The electronic transitions arise where the molecular orbitals HOMO, LUMO+2, and LUMO+3 are involved [60]. The HOMO→LUMO+2 transitions with oscillator strength f ~0.0050 implied an electronic transfer of the rings of the naphthalene group to the phenyl ring and to the orbitals of the −Br atom. This transition can be considered of π→π* character. Finally, the fourth transition in I−Br as a δ→π* transition with an oscillator strength slightly greater than the previous transition (f ~0.0062) can be observed. Here, the molecular orbitals involved are those located on the carbon atoms of the phenyl ring and, in the major part, on the −Br atom and the bond between the −Br and the C atom of the phenyl ring, see Figure 9.

On the other hand, several descriptors of global and local chemical reactivity can be calculated using the energies of HOMO/LUMO orbitals. As can be observed in Table 7, the increment in the stability displays the following trend: I−Br > I−Cl > I−F > I due to the less electronegative effect of the −X atom substituted in the para position on the phenyl ring, making the molecular system more stable. A trend in decreasing order is found for the values of Eg, showing a decrease of −0.0194, −0.0188, and −0.0104 eV in I−Br, I−Cl, and I−F, respectively. Therefore, the Eg decrease confirms the possibility of I−Br as a semiconductor, providing transfer of intramolecular charge. Finally, the value of μ also presents an increase when the halogen group is less electronegative. Such increment occurs due to the minor inductive effect of the substituent in the para-position on the phenyl ring, which causes an increase in μ for the system with the −Br and −Cl atoms.

Table 7

Molecular orbital energies (EHOMO and ELUMO, hartree), gap energies (ΔEg, eV), total energy (ETot, hartree), and dipole moment (Debyes) of I, I−F, I−Cl, and I−Br compounds calculated at PBE1PBE/6-311G(d,p) theory level.

Compound	EHOMO	ELUMO	ΔEg	ETot	μ
I	−0.2350	−0.0566	4.8535	−788.2530	1.4943
I−F	−0.2371	−0.0591	4.8431	−887.4288	2.9426
I−Cl	−0.2382	−0.0605	4.8347	−1247.7157	3.5991
I−Br	−0.2380	−0.0604	4.8341	−3361.4753	3.4362

Finally, photoluminescence (PL) technique allows one to investigate defects commonly present in crystalline arrays of different materials, and these defects can be associated with bands located in the visible region of the electromagnetic spectrum. PL reports have identified visible-region bands located at ~450–800 nm (~2.7–1.5 eV) due to interstitial defects, vacancies, etc., in some inorganic crystals. In some rare earth oxides, the emission bands located in the UV-Vis region are associated with electronic transitions of the f→f orbitals [14]. The PL spectra at room temperature for I and I−X crystals is shown in Figure 10, displaying emission bands for I located at ~599 nm (~2.13 eV), I−F: ~568 nm (~2.18 eV), I−Cl: ~553 nm (~2.24 eV), and I−Br: ~599 nm (~2.07 eV), respectively. Generally, in inorganic materials, this range corresponds to green, yellow, and red emission bands [15,25]. In order to obtain more information about the PL spectra, it is possible to perform the deconvolution of the Gaussians, which in this case are asymmetric, usually involving two overlapping bands. Thus, the inset of Figure 10 shows the deconvolution of the PL spectrum of I crystal. The PL spectrum of sample I−F crystal does not show significant changes with respect to the I sample and thus was not analyzed. The intensity of the PL signal is significantly increased with the size of the corresponding −X atom (in the following order: F < Cl < Br), and this change in intensity is accompanied by a shift to shorter wavelength and therefore increased energy. In our opinion, the increase in the atomic radius of the corresponding −X atoms contributes to the molecular distortion caused by chirality.

Figure 10

Photoluminescence spectra at room temperature of I, I−F, I−Cl, and I−Br crystals. In the inset, the PL deconvolution of I crystal can be seen.

The intensity of this band is also increased by the inductive effect. Generally, the phenomenon of crystalline phase transition is manifested as changes in the emission signals of the PL [61]. These experimental results of PL are in agreement in with the monoclinic→orthorhombic transition of the crystalline phase observed by XRD. According to the SEM results, these crystals have defects of crystal packing, which are associated with changes in the surface morphology, exhibiting a bright emission with a maximum at ~560 nm (~2.21 eV), and a small emission band located at ~654 nm (~1.89 eV). Generally, these PL emission bands are associated with π→π* and n→π electronic transitions, as reported in studies on cyclic aromatic molecules [62]. Such reports and our own research have demonstrated that face-to-face intermolecular π→π* interactions can have a dramatic impact on the PL properties of organic materials and lead to shifts of emission spectra due to excimer emission formation [63,64]. Thus, the emission bands in the visible region can be associated with the crystalline defects that are generally observed in visible region for other types of materials, such as semiconductors and rare earth oxides [14]. The PL spectra of the I−Cl crystal is shown in Figure 11. λ and two bands of different intensity can be seen at ~553 nm (~2.24 eV) and another at ~478 nm (~2.58 eV), respectively.

Figure 11

Photoluminescence deconvolution of the Gaussian corresponding to I−Cl spectra. The inset shows the deconvolution corresponding to the I−Br sample.

The corresponding −X atom effect is minor compared to the I−Br and I−Cl samples, respectively. However, the band located at ~676 nm (~1.83 eV) is asymmetrical and has a steep tail, which is less intense compared to the band seen at ~553 nm (~2.24 eV). The PL spectrum of the I−Br crystal is shown in the inset of Figure 11. The band located at ~553 nm can be associated with the effect of the –X size and polarizability effect (see Table 1), which caused the crystal defects generating a distorted molecular packaging caused by chirality that produces different molecular distortion in each crystal. According to the morphology recorded by SEM of these crystals, superficial differences are observed, and this fact generates crystalline defects, such as a compact, smooth morphology as well as small conglomerates, and can tentatively be associated with important optical changes. Worth noting in our crystalline solids are the −C−X.....X−C− stacking interactions as well as −C−X.....π interactions. Several reports have showed that −X….N− interactions have the property of forming strong interactions in the condensed phase according to the following order: −I > −Br > −Cl > −F [. An extensive study has shown that the electronic distribution has anisotropic character and is located around the covalent bond in X−X [.

In summary, PL spectroscopy is associated with: (i) drastic change in the surface morphology of the crystals that induces crystalline defects; (ii) monoclinic→orthorhombic crystalline phase transition that generated preferential orientation in two crystalline planes; (iii) change in the size of the atom radius; (iv) a polarizability effect of the corresponding −X atom; and (v) −X…X− interactions that produce drastic changes in the electronic transitions associated with the crystalline and optical effects. These effects are associated with the intensity of emission (green, red, and blue) bands caused by distortion in the crystal packing, i.e., the chiral carbon atoms in these crystals produce molecular dissymmetry, which leads to distortion in the packing as can be seen in PL spectra [67]. Worthy mentioning is that protonation of the molecule is a parameter affecting the intensity of emission bands in PL, as reported earlier, but this was not the case in our study [15,25]. The I−Br crystal has stronger intermolecular interactions, which could lead to its emission’s red shift [68].

4. Conclusions

The morphological, optical, and structural changes in four crystals of chiral imines as building blocks to obtain different luminescent crystals based on −X atoms (where: −F, −Cl, and −Br) were reported. The change of −X atoms afforded significant variations in the aforementioned properties. Monoclinic→orthorhombic phase transition can be seen from the XRD analysis. The structural phenomenon was associated with an intensity increase of the green emission (GE) band and the presence of the stereogenic carbon atom induced molecular distortion effects, which in turn affect the molecular packing and polarizability. These experimental results evidenced that it is possible to systematically control some properties of our crystals by just changing one end (functional group) of the chiral moieties. These crystals have properties similar to those reported in inorganic oxides and, likewise, their Eg is in the ~3.5−4.5 eV range, exhibiting several electronic transitions, which were confirmed by the absorbance results. Thus, the most relevant issue was the existence of bands observed by PL in the visible region, with the increase of the green emission band that is commonly associated in oxides with the existence of different crystalline defects.