Photoluminescent green emission band induced by systematic

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Photoluminescent green emission band induced by systematic change of the -CH3, -OCH3 and -Naphthyl groups in chiral imines O. Portillo Moreno, F. J. Meléndez Bustamante, M. Chavez Portillo, G. E. Moreno Morales, G. Hernández Tellez, A. Sosa Sanchez, M. E. Araiza Garcia, E. Rubio Rosas, P. Sharma, and R. Gutiérrez Pérez Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00851 • Publication Date (Web): 08 Jan 2018 Downloaded from http://pubs.acs.org on January 11, 2018

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Crystal Growth & Design

Photoluminescent green emission band induced by systematic change of -CH3, -OCH3 and -Naphthyl groups in chiral imines

O. Portillo Morenoa*, F.J. Meléndez Bustamanteb, M. Chávez Portilloa, G.E. Moreno Moralesa, G. Hernández Télleza, A. Sosa Sáncheza, M.E. Araiza Garcíaa, E. Rubio Rosasc, P. Sharmad, R. Gutiérrez Péreza* a

Lab. Síntesis de Complejos. Fac. Ciencias Químicas, Universidad Autónoma de Puebla,

Edif. FCQ-6, C.U. Av. San Claudio y 22 Sur, C.P. 72592, Puebla, Pue. México. b

Lab. de Química Teórica, Centro de Investigación, Dpto. de Fisicoquímica, Fac. Ciencias

Químicas, Universidad Autónoma de Puebla, Edif. 105-I, C.U. Av. San Claudio y 22 Sur, C.P. 72570, Puebla, Pue. México. c

Centro Universitario de Vinculación y Transferencia Tecnológica, Universidad Autónoma

de Puebla, C.U., C.P. 72001, Puebla Pue. México d

Instituto de Química-UNAM, Circuito exterior, C.U. Coyoacán, C.P. 04510, México, D.F.

E-mail: [email protected] Tel.: (+52)(222)229-55-00 Ext. 2825. E-mail: [email protected] Tel.: (+52)(222)229-55-00 Ext. 7519.

Abstract: Herein, we report the morphological, optical and structural modifications induced by the change of different functional groups in the para-position of the benzene ring in a series of chiral imines. These organic compounds were examined using Scanning Electron Microscopy (SEM), Optical Absorption (OA), X-Ray Diffraction (XRD) and Photoluminescence (PL) techniques. SEM images showed drastic morphological changes and the absorbance results showed significant changes in the bands located in the ~200-400 nm range, associated with π→π*, δ→δ* and n→π* transitions. An optical behavior similar to that of semiconductors (in UV region), with two transitions in the ~3.3-4.3 eV range was observed for the compounds. The results obtained by PL spectra exhibited changes in intensity, with gradually shifting increases in the green band emission. However it is more

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intense with the crystals of the imine bearing the -OCH3 functional group, considering that the aforementioned green band is associated to the different morphology in these crystals. Keyword Imines, Photoluminescence, green emission band, band gap energy, chiral imines.

1. Introduction The condensation of aldehydes and ketones with amines yielding imines is a well- known reaction in organic chemistry, a classical one indeed. This reaction was first discovered by Hugo Schiff, and imines are normally referred to as Schiff bases. The chemistry of imines presents interest for both theoretical and applied organic chemistry1,4. Imines find a wide range of applications in organic synthesis, as the starting compounds for the synthesis of bio-active and natural-ocurring products and, therefore, molecules with an imine group play an significant function in living organisms. In this regard, imines display a widespread scope of useful biological activities such as, inter alia, anti-inflammatory, analgesic, antibacterial, antiviral, antifungal, anticonvulsant, antimalarial, antitubercular, anticancer, antioxidant, anthelmintic, antipyretic, antiglycation, and antidepressant activities. Imines can display fluorescent variability with metals, along with photochromism and/or thermochromism, and have also been employed as catalysts, pigments, polymer stabilizers, as precursors for the preparation of nanoparticles, corrosion inhibitors, etc. On the other hand, in the field of solid organic materials, as molecules can form strong intermolecular interactions, such reciprocal influences enables the molecular assembly to fit as closely packed structures and, as a result, the properties of the crystalline materials are determined by the whole collective arrangement rather than by the constituents molecules. As such, the performance of organic crystal-based devices is mostly determined by the assembly of the molecules and, then, the comprehension and control of such molecular arrays in the solid state are crucial matters to obtain the desired physico-chemical properties in organic crystal-based materials5; in other words, the understanding of assembly features could allow the control of intermolecular interactions and molecular packing structures and, as a result, the structural, optical and electronic properties of the new high-performance organic compounds could be tuned in the solid state6. The most common research in organic crystals is related to noncovalent intermolecular interactions such as hydrogen bonding and


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π→π stacking which are two kinds of important intermolecular interactions in the design of supramolecular systems influencing strongly the final packing structure7. Different morphological arrangements may have very different physico-chemical properties and the molecular assembly of optical and electronic materials based on the collective interactions of hydrogen bonding and π→π stacking is still an appealing challenge in the area of crystalbased organic materials, owing to their potential applications in organic electroluminescent devices, organic light-emitting diodes,8,9etc. Herein, four organic molecules, namely imines1-4, prepared under solvent-free conditions are reported, which can act as building blocks to obtain luminescent crystals with well-defined morphological arrangements based on bonding and π-stacking interactions. The presence of electron-donating groups can produce the PL characteristic emission bands of imines 1-4 to the red shift by inductive and mesomeric effects, given that the aromatic electronic density in the phenyl ring is increased due to the δ→π hyperconjugation effect, and this effect is particularly noticed by the introduction of different groups in the para-position of the benzene ring linked to the chiral moiety of the imines.

2. Experimental details Optically pure primary amines were allowed to react with 2-naphthaldehyde to yield chiral imines 1-4 under solvent-free conditions, as reported earlier by us10. The imines were obtained in excellent yields and no further purification processes were needed. The crude products were recrystallized from CH2Cl2 obtaining pure crystals 1-4. An UV-vis spectrophotometer (Cary-5000) was used for the absorbance studies. The morphological images were registered by Scanning Electron Microscopy (SEM) by means of a Voyager II X-ray in an 1100/1110 SEM system from Noran Instruments. X-Ray Diffraction (XRD) patterns were obtained in a D8 Bruker Discover Series 2 Diffractometer with Cu Kα radiation of wavelength λ ~1.5408 Ǻ. The Photoluminescence (PL) spectra was characterized by a main peak, under optical excitation provided by an Ar+ laser beam with a pump power of 10 mW, 325 nm as excitation using a Science-Tech model 9040 apparatus. Theoretical calculations were supported by calculations based on the Time DependentDensity Functional Theory (TD-DFT) in order to evaluate the frontier molecular orbitals and maxima absorption wavelengths on the four imines.



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2. Results and discussion Selected crystallographic data are showed in Table 1. Table 1. Selected crystalline data 1 2 3 4 Chemical formula C19H17N C20H19N C20H19NO C23H19N Mr 259.33 273.36 289.36 309.39 Crystal system, Monoclinic, Orthorhombic, Orthorhombic, Monoclinic, space group P21 P212121 P212121 P21 Temperature (K) 298 298 298 298 a(Å) 14.93 (2), 6.09 (5), 6.10(5), 7.85 (5), b (Å) 6.01 (10), 7.57 (5), 7.72 (7), 7.87 (4), c (Å) 33.99 (7) 34.04 (3) 34.22 (4) 14.04 (9) α, β, γ (°) 90, 90, 90, 90, 102.60 90, 90, 99.85 (6), (17), 90 90 90 90 V (Å3) 2978.45 (9) 1571.4 (2) 1615.6 (3) 856.01 (9) Z 8 4 4 2 −1 µ (mm ) 0.51 0.51 0.57 0.07 Crystal size (mm) 0.49 × 0.17 0.49 × 0.13 × 0.39 × 0.25 × 0.45 × 0.35 × 0.10 0.05 0.23 × 0.11 Rint 0.04 0.04 0.06 0.04 −1 (sin θ/λ)max (Å ) 0.62 0.59 0.62 0.62 2 2 R[F > 2σ(F )], 0.039, 0.044, 0.112, 0.063, 0.155, 0.048, wR(F2), S 0.092, 1.04 1.08 1.08 0.119, 1.01 ∆ρmax, ∆ρmin (e 0.13, −0.12 0.09, −0.11 0.15, −0.20 0.12, −0.14 Å−3)

Structures and names of 1-4 are showed in Figure 1, respectively. CH3

H

CH3

H N

N

H

H

CH3

1

2


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CH3

H N CH3

H

H

N H O CH3

3

4

Figure 1. Structures of (S)-(+)-2-(((1-phenylethyl)imino)methyl)naphthalene (1), (S)(+)-2-((((4-Methylphenyl)ethyl)imino)methyl)naphthalene

(2),

(R)-(–)-2-((((4-

Methoxylphenyl)ethyl)imino)methyl)naphthalene

and

(S)-(+)-2-((((1-

(3)

Naphthyl)ethyl)imino)methyl)naphthalene (4)

Packing plots of 1-4 crystals are showed in Figure 2. As it is well-known, different packing structures may result in different morphologies of crystals11, i.e. morphologies in the solid state are dependent on the molecular packing structures of the crystals, as disclosed by Xray analysis. In the crystals, the molecules are packed into II D molecular lamellar based on π→π interactions along the c axis.

1

2

4 3

Figure 2. Packing plots of 1-4 crystals, displaying the three-dimensional packing of the chiral imines.



These molecular lamellar layers further assemble into III D lamellar layers, and the neighbouring columns within the layers are held together by C-H….O and C-H….N bonding interactions. Although molecular crystals have been studied extensively, their mechanical behaviour is poorly understood, as no systematic studies have been conducted11. The morphology of the crystals was investigated by the SEM technique, and such images are showed in Figure 3 for 1-4 crystals, respectively. Two images were selected for each crystal, using two different scales: 10 µm and 50 µm, respectively.

1)

2)

3)


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4)

Figure 3. SEM images for 1-4, showing significant morphological changes associated with the presence of different functional groups. Significant changes in surface morphology can be observed in the images when compared to each other. In Fig.1, for images 1 and 3, a lamellar morphology can be appreciated, with compact sheets between them forming stacked plates, and this morphology is commonly found in crystals with lamellar morphology corresponding to organic molecules with conjugated bonds and/or aromatic rings

12

. For imine 3, the images display an uniform

surface and compact plates with no holes, as well as uniform grains of polycrystalline nature, similar to the morphology that occurs in thin solid films13, 14. In Fig. 3, imine 4 shows crystals with a morphology completely different to those mentioned before: in these images a rough morphology can be appreciated and this morphological change is directly associated with the functional group of the molecule, i.e., the naphthyl group, which modified the crystalline molecular packing dramatically. Worth-noting is that the morphology exhibiting these crystals are rough and non-planar surfaces like the images of the previous crystals. Furthermore, the change in the corresponding functional group significantly modifies the morphological properties of these crystals. The absorbance spectra of 1-4 (in solution) in the UV-vis (~200-400 nm) region was examined and the optical absorption coefficient (α) was calculated for all the imines using the T = (1-R)2exp(αt) equation, in which T is transmittance and R is reflectance. The absorbance vs. wavelength (nm) spectra for imines 1-4 is displayed in Fig. 4.



231 240

222

4

238 Absorbance (a. u)

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3

226

267 2 242

300 1 200

250

300

350

Wavelenght (nm)

Figure 4. Absorbance vs. wavelength (nm) spectra of 1-4 solutions. Absorption spectra displayed the classic features for the naphthalene chromophore15, and different spectra can be observed when compared to each other. It is known that the band located at ~222 and ~240 nm corresponds to the benzene molecule16. Bands with a maximum intensity located at ~222-267 nm range can be assigned to the benzenoid π→π*, δ→δ* and n→π* transitions of imines. The n→π* transition of the 1 crystal shifted to the longer wavelength in the π→π* and δ→δ* transitions, displayed in the ~212-289 nm range17. The widening of two strong bands located at ~226 nm and ~267 nm can be observed, and the effect of the -CH3 group is generated by splitting, therefore the solution band exhibited a split with a shift to UV-vis wavelengths and such bands may be attributed to π*→π transitions18. From these experimental results, we can see a marked difference in the absorbance spectra, and the plausible causes may be: (i) the size of the functional group as well as the hybridization is a crucial parameter (ii) the corresponding functional group display different polarity, and such polarity is associated with the different hybridizations and the resonance effect present in each molecule (iii) The aforementioned electronic transitions produced by π*→π and δ–δ* and n→π* transitions. The optical behavior


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associated with electronic transitions appearing in these organic molecules was also examined. In this report, we apply the model used to investigate electronic transitions in semiconductors13, oxides20 and some dielectrics materials21. The near-edge region can be fitted in an equation in which the intercept gives the band-gap energy (Eg) and the fitting exponent can be associated to the electronic transition as direct or indirect, and are called Tauc plots22. Extrapolation of straight-line proportions to zero absorption coefficient (α = 0) leads to the experimental estimation of the Eg values. Considering the values obtained of the absorbance, the optical Eg of all the crystals were calculated using a standard equation (Tauc model): αhν = A(Eg - hν)n, where A is constant, the exponent n depends on the type of transition (½, 2, 3/2 and 3) corresponding to allowed direct, allowed indirect forbidden and indirect transitions, respectively. The plot of (αhν)2 vs. (hν) is shown in Figure 5, which is linear at the absorption edge, indicating a direct allowed transition23. In the inset of the same Fig. 5 the spectrum of crystal 1 is shown.

2

2 (α hν )

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


3 4 3.5

4.0 E (eV)

4.5

Figure 5. Spectra of (αhν)2 vs. hν of 2-4 crystals. The inset shows the spectrum of 1 crystal.



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In these spectra, two transitions can be seen in all samples and experimental values are showed in Table 2. This spectra exhibited a behavior similar to that of direct-band semiconductors13,23, and in this case, for crystal 1, E1 ~3.4 and E2 ~4.2 eV were obtained, respectively. Table 2. Electronic transitions (Eg) of the crystals. Sample

E1 (eV)

(eV)

1

3.4

4.2

2

3.6

4.0

3

4.5

---

4

3.3

4.0

E2

The corresponding transitions observed in these spectra showed the following increasing order: 4

Photoluminescent green emission band induced by systematic

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