Facile Synthesis of an Organic Solid State Near-Infrared-Emitter with Large Stokes Shift via Excited-State Intramolecular Proton Transfer

Abstract

An organic solid-state near-infrared (NIR)-emitter (λ_em = 738 nm) exhibiting large Stokes shift (Δλ = 293 nm) through the excited-state intramolecular proton transfer phenomenon has been synthesized and characterized. The present discovery points to the possibility of achieving a new family of solid-state NIR emitters starting from simple aldehyde and amine precursors.

1. Introduction

The interest in near-infrared (NIR)-emitting molecules has increased in recent years because of their potential applications in diverse areas including bio-imaging,^1,2 telecommunication,³ night-vision equipment,⁴ photodynamic therapy,^5,6 sensing,⁷ and optoelectronics.⁸⁻¹¹ However, the development of NIR-emitting organic molecules, especially, solid-state NIR emitters, is a formidable challenge. A pre-requisite for NIR emission is the low band gap, which is often achieved by incorporating extended π-conjugation in the molecular design.^2,8−10 However, such systems often tend to self-assemble through weak-bonding interactions leading to aggregation-caused quenching (ACQ) of fluorescence.^9,11−13 Protection of the NIR chromophore by bulky groups is a well-accepted strategy for preventing the ACQ effect.^9,11 In the literature, there are only a limited group of compounds reported as NIR emitters, which largely include certain organic dyes and their derivatives such as cyanines,² phthalocyanines,¹⁴ squarines,¹⁵ BODIPYs,¹⁶ rhodamines,¹⁷ porphyrinoids,¹⁸ as well as a number of donor–acceptor chromophores¹ and some rare-earth and transition metal complexes.^19,20 However, many of such compounds require difficult and expensive synthetic procedures. Therefore, the development of new classes of organic solid-state NIR emitters that can be crafted easily from simple building blocks is of utmost relevance. Of particular interest is the development of metal-free organic NIR emitters in view of their special advantages in comparison to the metal-based NIR emitters.

In recent years, solid-state emitters based on excited-state intramolecular proton transfer (ESIPT) phenomenon, which involves a proton shift within the intramolecularly hydrogen-bonded sites, have emerged as attractive materials for optoelectronics applications.^{8−10,21−24} Such proton transfer often causes dramatic structural and electronic changes in the molecule leading to unique fluorescence emission with a large Stokes shift.⁹ The most widely explored ESIPT fluorophores include 2-(2′-hydroxyphenyl)benzothiazole, 2-(2′-hydroxyphenyl)benzimidazole and 2-(2′-hydroxyphenyl)benzoxazole, some of which show NIR emission in solution phase.^2,22 Metal ion (Zn²⁺) binding-enabled ESIPT emission in the NIR region has been reported in the bis(benzoxazole)-based system for bio-imaging applications.²⁵

In this regard, we envisaged that a new class of compartmental Schiff bases with sufficiently extended π-conjugation could be explored as NIR emitters, because such systems are known to exhibit interesting luminescence properties as well as ESIPT.²⁶ Moreover, Schiff bases allow facile synthesis from a variety of readily available aldehyde and amine precursors, permitting easy structural and electronic fine-tuning. Herein, we propose a generalized Schiff base moiety 1, Figure 1 (inset), capable of exhibiting the ESIPT phenomenon as a potential NIR emitter, whose π-conjugation could be fine-tuned by varying substitutions at R₁, R₂, and R₃ positions. To test our hypothesis, we have developed the first example of 1, 1AA, by coupling 2-hydroxy-5-methoxybenzene-1,3-dialdehyde (dfp) with two units of 4-(methylthio)aniline (4-mta) as shown in Figure 1. Using 1AA, we could achieve solid-state NIR emission as hypothesized, the details of which are being reported in this paper.

Figure 1.

(a) Chemical structure of 1AA; and (b) ORTEP diagram (50% probability level) of 1AA. Color code: N—blue, O—red, C—gray, S—yellow, and H—black. Inset: Diagram of the generalized Schiff base moiety 1.

2. Results and Discussion

2.1. Design and Synthesis

The Schiff base 1AA was synthesized by the condensation of dfp and 4-mta in 1:2 molar ratio in ethanol; see Experimental Section and Supporting Information for more details. The molecular structure of 1AA was confirmed by single-crystal X-ray crystallography, see Figure 1b and Table S1, which unambiguously showed that 1AA consists of two units of 4-mta linked through two imine (−C=N−) bonds to the dfp unit resulting in an extended π-conjugated system as expected. The asymmetric unit consists of two molecules of 1AA as shown in Figure 1b, and these molecules show small variations in their structural features. In 1AA, the imine N of one of the two 4-mta arms (say D₁-arm) is involved in intramolecular hydrogen bonding (IHB) interaction with the phenolic −OH group. The IHB interaction in 1AA was confirmed by ¹H NMR. The ¹H NMR signal of the phenolic proton of 1AA was observed at 13.53 ppm in CDCl₃, which gets shifted to 13.84 ppm in DMSO-d₆ (see Figure S4, Supporting Information). This indicates that 1AA exists predominantly in enol form in solution.²¹ The hydrogen bond acidity (A) for 1AA is quantified by using eq 1

1

where Δδ = δ(DMSO) – δ(CDCl₃). The value of A obtained, that is, 0.047 (<0.1), establishes a strong IHB interaction in 1AA in solution.²⁷ Furthermore, the completely symmetric ¹H NMR spectra confirmed the existence of fast proton transfer in 1AA, which is an essential requirement of ESIPT.^21,28

2.2. Photophysical Properties

1AA exhibited two characteristic absorption peaks at 345 and 407 nm (molar absorptivity at λ_max, ε_1AA = 1.7 × 10⁴ M^–1 cm^–1, Figure S12) in dichloromethane (DCM) solution due to π → π* and n → π* transitions, respectively, and showed fluorescence emission at λ_em = 645 nm (λ_ex = 407 nm) with a photoluminescence quantum yield (PLQY; ϕ) of 2.0%, Figure 2 and Table 1. Furthermore, the photophysical studies performed in the polymethylmethacrylate (PMMA) matrix showed that 1AA exhibits almost similar absorption and emission properties as that observed in DCM solutions, but with a higher PLQY (ϕ = 4.0%), Figure 2 and Table 1.

Figure 2.

(a) UV–vis absorption (blue line) and fluorescence emission (green line) spectra of 1AA in DCM and PMMA matrix; and (b) DCM containing 1AA (10.0 μM) under UV light (365 nm). The rest mass of 1AA in PMMA matrix was chosen so as to give an absorbance less than 0.1 at the excitation wavelength.

Table 1. Photophysical Properties of 1AA in DCM, PMMA, and Solid Form.

1AA	emission λem (nm)	absorbance λabs (nm)	PLQY ϕ (%)	Stoke shift Δλ (nm)
DCM	645	407	2.0	238
PMMA	646	404	4.0	242
solid	738	445	9.0	293

Often, Schiff bases exhibit photo-induced electron transfer (PET) in solutions, which adversely affects their fluorescence properties.^29,301AA exhibits strong IHB interaction between the phenolic −OH and imine N of one of the 4-mta arms (D₁-arm) enabling ESIPT phenomenon, which prevents PET in this arm. However, in the non-hydrogen-bonded arm (D₂-arm), PET can still be active and that helps to transfer the electron density from D₂ to D₁-arm in the excited state,³¹ see Scheme 1. Recently, Thiel et al. reported that certain aromatic Schiff bases can undergo fast ESIPT and the keto tautomer thus generated is responsible for the fluorescence emission.³² In 1AA, fluorescence emission at 645 nm with a large Stoke shift (Δλ = 238 nm) indicates the occurrence of ESIPT leading to keto tautomer, Scheme 1.

Scheme 1. Tautomerization of 1AA Showing the Canonical Structures Contributing to the Stabilization of the NH Form.

Fluorescence studies on 1AA were also conducted in different solvents such as hexane, DCM, acetonitrile (ACN), dimethylformamide (DMF), and dimethylsulfoxide (DMSO) having different hydrogen bond acceptor ability (β). This is because the hydrogen bonding abilities of solvents are shown to affect the photophysical properties of molecules exhibiting excited-state proton transfer.³³1AA exhibited red-shifted emission at 670 nm in hexane (β = 0.0)³³ with a very low quantum yield, ϕ = ∼0.9%. However, in DMSO (β = 0.76), 1AA exhibited emission at ∼600 nm (blue-shifted) with a quantum yield ϕ = ∼5%, Figure S13 and Table 2. It is also noticed that 1AA exhibits two emission peaks, a lower wavelength emission at ∼460 nm assigned to the normal charge transfer species and a highly Stokes shifted band at ∼645 nm assigned to the proton transferred tautomer species of 1AA (Figure S13).

Table 2. Photophysical Data of 1AA in Different Solvents.

1AA	solvent β-value	absorbance λabs (nm)	emission λem (nm)	PLQY ϕ (%)	Stoke shift Δλ (nm)
hexane	0.0	405	670, 460	0.9	265
DCM	0.10	407	645	2.0	238
ACN	0.40	404	640, 463	1.9	236
DMF	0.69	402	608	4.2	206
DMSO	0.76	409	602	5.3	193

Furthermore, the fluorescence emissions were measured after sequential additions of acetic acid (AcOH) and triethylamine (TEA) to DCM solutions of 1AA in order to confirm the existence of ESIPT. 1AA was nearly nonfluorescent upon addition of TEA, probably because of the deprotonation of the phenolic proton preventing ESIPT, see Figure 3 and Scheme S1. Interestingly, 1AA became highly fluorescent (ϕ = ∼12%) upon addition of AcOH, probably because of the generation of N-protonated imine in DCM solution (Scheme S1). This observation further illustrated that the emission of 1AA was controlled by ESIPT and is in agreement with the previous reports of increased fluorescence in acidic medium in similar types of ESIPT fluorophores.² The reversible modulation of emission intensity between the N-protonated imine and deprotonated phenolic moieties (Scheme S1) by alternate additions of AcOH and TEA is shown in Figure S14. No molecular degradation was observed for at least five such cycles.

Figure 3.

Change in fluorescence emission of 1AA (10.0 μM) in DCM (a) upon addition of acetic acid (9.0 equiv); (b) upon addition of TEA (5.0 equiv) (excitation and emission slit widths = 10 nm); and (c) photo showing the fluorescence color change upon addition of acetic acid and TEA.

2.3. Photophysical Properties of 1AA in the Solid State

Considering the importance of solid-state NIR emitters, we were interested in investigating the photophysical properties of 1AA in the solid state as well. The absorption and emission spectra of 1AA in the solid state (powder form) revealed a broad absorption band in 300–450 nm range and an NIR emission band at 738 nm (λ_ex = 445 nm) with a very large Stokes shift (Δλ ≈ 293 nm) and a PLQY of ∼9% (Figure 4 and Table 1). The relatively high quantum yield of 1AA in the solid state in comparison to that in solution is probably due to the inhibition of the ultrafast excited-state deactivation channels in the rigid medium.³² The difference in emission wavelength of 1AA in solution or PMMA matrix in comparison to the solid state may be attributed to the luminescence characteristics of isolated molecules.^9,11−13 However, the absorption tails of 1AA in powder form and the PMMA matrix may indicate the existence of IHB interactions in rigid medium responsible for the photo-induced keto–enol tautomerism through ESIPT.⁹

Figure 4.

(a) UV–vis diffuse reflectance spectroscopy (DRS; black line) and fluorescence emission spectra (blue line) of 1AA in powder form; (b) solid-state image of 1AA under visible light; and (c) solid-state image of 1AA under UV light.

2.4. Crystal Structure Analysis

To understand the reasons behind the fluorescence properties exhibited by 1AA in the solid state, we analyzed its crystal structure in detail. The dihedral angles between the planes of the salicylaldimine ring of dfp and the aromatic ring of D₁-arm (H-bonded arm) are 20.5° and 25.3°, respectively, for the two molecules present in the asymmetric unit of 1AA, while the similar angles for D₂-arms were 39.7° and 46.4°, respectively, Figure 5a. In the crystal of 1AA, the molecules are stacked along the a axis, see Figure 5c. In this stacking arrangement, the salicylaldimine phenolic units face each other in a slipped configuration with a centroid to centroid distances of 4.877 and 5.168 Å, Figure 5b. It can be seen from the stacking diagram that the aromatic ring of the D₁-arms (H-bonded arm) on the adjacent molecules are packed in a parallel fashion with the −SCH₃ units on alternate molecules pointing to the opposite directions, Figure 5b. However, the aromatic rings of the D₂-arms (non-H-bonded arm) on the adjacent molecules are arranged in an almost perpendicular fashion. Such an arrangement leads to large interplanar separation in 1AA. The large interplanar separations together with the slipped configuration of the stacking of the adjacent molecules prevent any π–π stacking interaction among the molecules in the crystal lattice. The two independent molecules present in the asymmetric unit of 1AA are involved in extensive intermolecular hydrogen bonding interaction with the neighboring molecules leading to the formation of chain-like structures, see Figures 5d, S15, and Table 3. The molecular packing diagrams of 1AA along a, b, and c axes are shown in Figure S16. Earlier studies on organic solids have shown that the fluorescence emission properties can be optimized by regulating their molecular packing and intermolecular noncovalent interactions. It has been shown that the prevention of π–π stacking interactions among the molecules in the crystal helps to reduce the ACQ effect in organic fluorophores.^9,11−13 Similarly, the suppression of the molecular vibrations that can lead to nonradiative transitions through H-bonding interactions is another useful strategy employed in organic solid-state emitters. In 1AA, the π–π stacking interactions are absent because of the slipped arrangement of adjacent molecules as well as due to the large interplanar distance (Figure 5d).

Figure 5.

(a) Structures of the two molecules of 1AA present in the asymmetric unit showing the dihedral angles; (b) stacking of 1AA molecules in the crystal showing a slipped configuration with large interplanar distance; (c) crystal packing diagram of 1AA showing stacking of molecules along a axis; and (d) interconnection of adjacent molecular chain through hydrogen bonding interaction.

Table 3. Hydrogen Bonding Parameters for 1AA.

D	H	A	d(D–H)/Å	d(H–A)/Å	d(D–A)/Å	angle D–H–A/deg
O2	H2	N2	0.82	1.88	2.610(3)	147.3
O4	H4A	N4	0.82	1.91	2.641(3)	147.9
C23	H23A	O3a	0.96	2.56	3.483(4)	161.0
C1	H1B	O1a	0.96	2.36	3.319(4)	172.7
C20	H20	O4b	0.93	2.63	3.457(4)	148.6

^a+x, −1 + y, +z.

^b2 – x, 2 – y, −z.

Moreover, multiple hydrogen bonding interactions exhibited by 1AA molecules help to minimize molecular vibrations and accompanied nonradiative transitions. We believe that the unique intermolecular interactions and the crystal packing features exhibited by 1AA permit its NIR emission through the ESIPT process in the solid state. Similar cases have already been reported in the literature.⁹ For example, different alkoxy substituted (−OMe, −OEt, −OPr, and −OBt) derivatives of the ESIPT fluorophore, 2,6-bis(benzothiazol-2-yl)phenol, are reported to exhibit red- and blue-shifted fluorescence emission in the solid state compared to their emissions in the solution phase because of the differences in the intermolecular interactions in the crystalline forms.⁹

2.5. Theoretical and Electrochemical Studies

To understand the electronic structures and the enol (E)–keto (K) tautomerization in 1AA, we performed density functional theory (DFT) and time-dependent DFT calculations with the B3LYP/6-31G(d) basis set using a suite of Gaussian 09 program.³⁴ The optimized structures with the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of 1AA in enol and keto forms are shown in Figure 6. In the enol form, the D₁-arm of 1AA is not coplanar with the central phenol ring in S₀ state, whereas it is coplanar with the phenol ring in S₁ state, see Supporting Information Figure S17. Meanwhile, in the keto form, the D₁-arm of 1AA is almost coplanar with the phenol ring in both the S₀ and S₁ states, see Supporting Information Figure S17 and Table S2. This twisting of the D₁-arm from the noncoplanar to the coplanar state attributes to the proton transfer in the ESIPT process. Moreover, in the enol form, the O–H bond length increases from 0.99670 Å in the S₀ state to 1.68887 Å in the S₁ state, along with a decrease in the N···H distance from 1.70389 Å (S₀ state) to 1.04640 Å (S₁ state). However in the keto form, the changes in these bond lengths on excitation are not pronounced, see Table S3. This shows a strengthening of the hydrogen bonding interactions in the excited state, contributing to the ESIPT phenomenon.³⁵ We calculated the absorption maximum for the enol form of 1AA as λ_abs = 400 nm, see Supporting Information Figure S18, which is closely matched with the experimental value (λ_abs = 407 nm); however, the absorption of the keto form of 1AA (λ_abs = 595 nm) was red-shifted drastically by ∼195 nm to the longer wavelength, see Supporting Information Figure S18.

Figure 6.

Schematic representation of ESIPT photocycle showing the comparison of HOMO–LUMO levels between enol and keto forms of 1AA.

Upon excitation of 1AA existing in the enol form, the π-electron density located on the phenol ring and the D₂-arm gets transferred to the phenol ring, the imine linker, and the D₁-arm. As a result of the ESIPT process, there is a decrease in the energy of the LUMO levels from −2.65 eV (enol form of 1AA) to −2.74 eV (keto form of 1AA). The difference in the LUMO energy levels of the enol and keto forms of 1AA is 0.09 eV. Similarly, the theoretically calculated HOMO and LUMO energy levels of the keto form of 1AA (−5.18 eV and −2.74 eV) are in agreement with the experimentally calculated HOMO and LUMO energy levels of 1AA (−5.21 eV and −2.59 eV) by cyclic voltammetry, see Figure S19 and Table S4.

The thermal stability of 1AA was analyzed by thermogravimetric analysis (TGA), which revealed a decomposition temperature (T_d) of 306 °C. Such a high T_d value suggests the suitability of 1AA for potential optoelectronics device applications, see Figure S20.

2.6. Studies Using Control Compounds

In order to confirm the roles of ESIPT and the extended π-conjugation in 1AA in determining its deep red and NIR emitting behaviors in different environments, we synthesized and studied two control compounds 1A and 1B as shown in Figure 7. 1A has only one 4-mta unit and therefore less π-conjugation compared to 1AA. In 1B, the phenolic −OH is replaced by the −OEt group, preventing the possibility of ESIPT. The details of the synthesis, characterization, and photophysical studies of 1A and 1B are given in Supporting Information. The molecular structures of 1A and 1B were confirmed by single-crystal X-ray crystallography, see Figure 7.

Figure 7.

ORTEP diagram (50% probability level) of 1A (left) and 1B (right). Color code: N—blue, O—red, C—gray, S—yellow, and H—black.

1A showed an electronic absorption peak at 380 nm (ε = 1.8 × 10⁴ M^–1 cm^–1) and a weak fluorescence emission in DCM solution at 426 nm (less intense) and 593 nm (more intense) (λ_ex = 380 nm, ϕ = 0.03%, Δλ = 213 nm) (Figure S21) due to keto tautomer induced by ESIPT. Similar photophysical properties of 1A were also observed in the PMMA matrix (λ_em = 594 nm, λ_ex = 380 nm, ϕ = 0.89%, Δλ = 214 nm). In the solid (powdered) state, 1A exhibited fluorescence emission at 604 nm (λ_ex = 420 nm, ϕ = 15% and Δλ = 184 nm) with high quantum yield, red-shifted by ∼10 nm in comparison to the solution state, Figure 8. Crystal data demonstrated no π–π stacking in 1A, which helps to reduce ACQ. The angle between the planes of the phenol ring and the 4-mta arm is 2.29°, making almost a planar structure for 1A promoting the ESIPT phenomenon, see Supporting Information Figure S22, Table S5, and Scheme S2. The results clearly indicate that the inclusion of the extra arm of 4-mta has a significant impact on the fluorescence of 1AA by improving its Stokes shift.

Figure 8.

(a) UV–vis DRS (black line) and fluorescence emission spectra (blue line) of 1A in powder form; (b) solid-state image of 1A under visible light; and (c) solid-state image of 1A under UV light (365 nm).

On the other hand, compound 1B showed absorption peak at 375 nm, which is blue-shifted compared to that of 1AA (λ_abs = 407 nm) in DCM. However, compound 1B showed very weak or negligible fluorescence emission at 434 nm with only a small Stokes shift (59 nm) in DCM and no emission in the solid state, see Supporting Information Figures S23 and S24. This behavior is probably due to the absence of the phenolic hydrogen in 1B required for ESIPT. Also, the simple change of the replacement of phenolic −OH with −OEt group led to a totally different crystal packing arrangement in 1B compared to that of 1AA with different dihedral angles of rotations of D₁ and D₂-arms as shown in Figure 9a. Moreover, in the packing mode, the central aromatic rings face each other in a centro-symmetric manner with a centroid-to centroid distance of 3.63 Å, Figure 9b. This perhaps leads to π–π stacking interactions in 1B in solid state affecting its fluorescence properties adversely. Studies on these control compounds therefore confirm the roles of extended conjugation and ESIPT phenomenon in 1AA in determining its fluorescence properties in solution, PMMA and solid states.

Figure 9.

(a) Structure of molecule 1B showing the dihedral angles; and (b) stacking of the two molecules of 1B in the crystal showing short interplanar distance.

3. Conclusions

In conclusion, we have reported the first example, 1AA, of a potentially new class of organic solid-state NIR emitters crafted through simple Schiff base condensation. In DCM solution, 1AA exhibited deep red emission and the occurrence of ESIPT in solution phase was confirmed by large Stokes shift values and NMR studies. In powder form, 1AA emitted in the NIR region, again with a large Stokes shift due to ESIPT phenomenon. Crystallographic analyses of 1AA revealed large interplanar spacing and no π–π stacking interactions among the molecules in the crystal structure, which help to prevent ACQ of fluorescence. Similarly, the multiple H-bonding interactions exhibited by 1AA help to suppress the molecular vibrations that can induce nonradiative transitions. Theoretically calculated HOMO–LUMO energy levels and energy gap values for 1AA assuming ESIPT phenomenon were matched with the experimental values calculated through photophysical and electrochemical analyses. 1AA exhibited high thermal stability and the cyclic voltammetric studies revealed its potential for optoelectronics device applications. Studies using control compounds confirmed the roles of ESIPT and extended π-conjugation in the observed NIR emission by 1AA. Currently, we are engaged in the development of more examples of NIR emitters belonging to this series of compounds by systematically varying substitutions R₁, R₂, and R₃ on the moiety 1.

4. Experimental Section

4.1. Materials and Methods

All chemicals were purchased from Sigma-Aldrich. ¹H and ¹³C NMR spectra were recorded on a JEOL JNM ECX-500 FT-NMR spectrometer using CDCl₃ and DMSO-d₆ as solvents. Fourier-transform infrared spectroscopy spectra were recorded on an Agilent Cary-660 FT-IR spectrometer with a Diamond ATR accessory. HR-MS spectra were recorded on the Bruker Maxis Impact instrument. TGA measurements were performed on a PerkinElmer thermogravimetric analyzer (model: Pyris 1 TGA) at a heating rate of 10 °C min^–1 under an N₂ atmosphere.

4.2. Photoluminescence Measurements

Photoluminescence (PL) spectra in the solid state were measured by using a Fluorolog-3 spectrofluorometer (HORIBA-Jobin-Yvon) according to a reported procedure³⁶ and PL spectra in the liquid state were measured on an Agilent Technologies Cary Eclipse fluorescence spectrophotometer. A Shimadzu UV-2450 spectrophotometer was used for UV–vis spectra measurements. The solid-state UV–vis DRS spectra were recorded on a PerkinElmer UV/vis/NIR Lambda 750 spectrophotometer.

4.3. Photoluminescence Quantum Yield

Total PLQYs were calculated according to a reported procedure³⁶ by using a quanta-φ F-3029 sample chamber of the Fluorolog spectrometer having an integrating sphere. A spectrometer was calibrated using quinine sulfate according to a reported procedure.³⁷ The accuracy of the determination of PLQY was estimated to be ±10%.

4.4. Electrochemical Measurements

All electrochemical measurements were carried out on a Metrohm Autolab instrument using glassy carbon working electrode, Ag/Ag⁺ reference electrode, and platinum wire counter electrode. 10^–3 M solutions of the samples were prepared in DCM containing 10^–1 M tetrabutylammonium hexafluorophosphate (Bu₄NPF₆). Before each experiment, solutions were purged with argon, and the scan rate was set as 100 mV s^–1 at room temperature. The values of the potentials mentioned in this work are with respect to the F_c/F_c⁺ redox couple and the energy level of F_c/F_c⁺ was assumed to be −4.8 eV.^38,39 The HOMO energy levels of the samples were calculated using the equation E_HOMO = −(4.8 – E_1/2,Fc/Fc⁺ + E_ox,onset), where E_ox,onset is the onset oxidation potential. The LUMO energy levels of the analytes were calculated by adding the optical band gap value to the respective HOMO energy levels (E_LUMO = E_g,opt + E_HOMO).⁴⁰ The optical band gaps were taken from the onset of the absorption spectra in DCM.⁴⁰

4.5. X-ray Crystallography

Single-crystal X-ray diffraction data were collected on an Agilent SuperNova diffractometer, equipped with a dual source (Cu and Mo) and Eos CCD detector, using Cu Kα radiation (1.54184 Å) at 293 K. Absorption correction, data acquisition, and reduction were performed by using CrysAlisPRO program.⁴¹ The structures were solved by direct methods with ShelXS⁴² and refined on F² by full matrix least-squares techniques with ShelXL⁴² using the Olex² (v.1.2) program package.⁴³ Anisotropic displacement parameters were applied for all atoms, except H atoms. H atoms were calculated into their respective positions or were located from the electron density map. CCDC: 1822955, 1822956, and 1860219 contain the supplementary crystallographic data for molecules 1A, 1AA, and 1B respectively.

4.6. Procedure To Develop PMMA Film of 1A and 1AA

PMMA (200 mg) was dissolved in 2 mL tetrahydrofuran (THF) solvent and heated at 75 °C with constant stirring. After that, the required amount of sample in 20 μL of tetrahydrofuran (THF) was added into the resulting viscous solution of PMMA and stirred again at the same temperature for 2 h. The mixture was transferred into glass molds and left for slow drying at room temperature.

4.7. Theoretical Calculations

All theoretical studies on 1AA were conducted with Gaussian Software 09 using basic set DFT/B3LYP/6-31G(d).³⁴

4.7.1. Synthesis of 2-((E)-(4-(Methylthio)phenylimino)methyl)-4-methoxyphenol, C₁₅H₁₅NO₂S (1A)

An ethanolic solution of 2-hydroxy-5-methoxy benzaldehyde (mfp) (4.0 mmol, 0.61 g, 1.0 equiv) was added slowly into ethanolic solution of 4-(methylthio)aniline (4-mta) (4.0 mmol, 0.56 g, 1.0 equiv), and the resulting solution was stirred at room temperature for 3–6 h. On completion of the reaction, a precipitate was formed which was filtered and washed with cold ethanol and dried under vacuum to obtain the pure orange colored Schiff base (1A). After recrystallization from the methanol–chloroform (2:1, v/v) solvent mixture, orange crystals of 1A were obtained in 80% yield (0.87 g). IR (ATR, cm^–1): 1614 (C=N). ¹H NMR (500 MHz, CDCl₃ 300 K): δ (ppm) 12.79 (s, 1H, ArOH), 8.58 (s, 1H, HC=N), 7.30 (d, 2H, J = 8.9 Hz, ArH), 7.24 (d, 2H, J = 8.9 Hz, ArH), 6.98 (m, 2H, ArH), 6.89 (d, 1H, J = 2.7 Hz, ArH), 3.80 (s, 3H, −OCH₃), 2.5 (s, 3H, −SCH₃). ¹³C NMR (125 MHz, CDCl₃ 300 K): δ 161.4, 155.3, 152.2, 145.5, 137.3, 127.4, 121.6, 120.3, 118.7, 118.0, 115.1, 55.9 and 16.0 ppm. ESI–MS: m/z 274.04 [1A + H]⁺ (calcd 274.05).

4.7.2. Synthesis of 2,6-Bis((E)-(4-(methylthio)phenylimino)methyl)-4-methoxyphenol, C₂₃H₂₂N₂O₂S₂ (1AA)

Ethanolic solution (5.0 mL) of dfp(44) (3.0 mmol, 0.54 g, 1.0 equiv) (heated at 60 °C, until a clear yellow solution of dfp was obtained) was added slowly into 10.0 mL ethanolic solution of 4-(methylthio)aniline (4-mta) (6.0 mmol, 0.83 g, 2.0 equiv) under a nitrogen atmosphere. Subsequently, a catalytic amount of acetic acid was added, and the reaction mixture was heated to reflux with stirring overnight. The product precipitated from the solution upon cooling to room temperature and was filtered off and washed with cold ethanol and dried under vacuum to yield pure compartmental Schiff base 1AA. After recrystallization from the methanol–chloroform (2:1, v/v) solvent mixture, golden yellow crystals of 1AA were obtained in 79% (1.0 g) yield. IR (ATR, cm^–1): 1615 (C=N). ¹H NMR (CDCl₃, 500 MHz, 300 K): δ (ppm) 13.53 (s, 1H, ArOH), 8.82 (br, 2H, HC=N), 7.30 (d, 5H, J = 8.9 Hz, ArH), 7.25 (d, 5H, J = 8.9 Hz, ArH), 3.87 (s, 3H, −OCH₃), 2.50 (s, 6H, −SCH₃). ¹³C NMR (125 MHz, DMSO-d₆ 300 K): δ 157.9, 155.3, 151.6, 146.2, 136.8, 126.8, 122.0, 121.9, 117.9, 55.7, 14.8. ESI–MS: m/z 423.12 [1AA + H]⁺ (calcd 423.11).

4.7.3. Synthesis of 2-Ethoxy-5-methoxybenzene-1,3-dialdehyde, C₁₁H₁₂O₄(dfp-OEt)

To a suspension of dfp (0.5 mmol, 0.090 g, 1.0 equiv) and potassium carbonate (0.75 mmol, 0.104 g, 1.5 equiv) in 8 mL dry DMF, bromoethane (0.75 mmol, 0.056 g, 1.5 equiv) was added and heated at 60-70 °C for 6 h. After cooling to room temperature, the reaction mixture was added to ice cold water, and the precipitate was filtered off and washed with cold water to give a white colored solid in 71% (0.074 g) yield. ¹H NMR (CDCl₃, 500 MHz, 300 K): δ (ppm) 10.37 (s, 2H, HC=O), 7.61 (s, 2H, ArH), 4.14 (q, 2H, J = 6.9 Hz, −OCH₂), 3.80 (s, 3H, −OCH₃), 1.40 (t, 3H, J = 6.9 Hz, −CH₃). ¹³C NMR (125 MHz, CDCl₃ 300 K): δ 188.4, 158.2, 156.1, 131.0, 119.1, 56.0, 15.1. ESI–MS: m/z 209.06 [(dfp-OEt) + H]⁺ (calcd 209.07).

4.7.4. Synthesis of (11E)-N-(3-((E)-(4-(Methylthio)phenylimino)methyl)-2-ethoxy-5-methoxybenzylidene)-4-(methylthio)benzenamine, C₂₅H₂₆N₂O₂S₂(1B)

An ethanolic solution of dfp-OEt (0.480 mmol, 0.100 g, 1.0 equiv) was added slowly into ethanolic solution of 4-(methylthio)aniline (4-mta) (0.96 mmol, 0.133 g, 2.0 equiv), and the resulting solution was stirred at room temperature for 6 h. On completion of the reaction, a precipitate was formed, which was filtered and washed with cold ethanol and dried under vacuum to obtain the pure yellowish colored Schiff base 1B. After recrystallization from the methanol–chloroform (2:1, v/v) solvent mixture, golden yellow crystals of 1B were obtained in 83% (0.180 g) yield. IR (ATR, cm^–1): 1610 (C=N) ¹H NMR (CDCl₃, 500 MHz, 300 K): δ (ppm) 8.72 (s, 2H, HC=N), 7.75 (s, 2H, ArH), 7.25 (d, 4H, J = 8.25 Hz, ArH), 7.14 (d, 4H, J = 8.25 Hz, ArH), 3.92 (q, 2H, J = 7.1 Hz, −OCH₂), 3.80 (s, 3H, −OCH₃), 2.45 (s, 6H, −SCH₃), 1.35 (t, 3H, J = 6.9 Hz, −CH₃). ¹³C NMR (125 MHz, CDCl₃ 300 K): δ 156.6, 154.7, 154.5, 149.2, 136.3, 130.7, 127.5, 121.6, 115.4, 55.9, 15.8, 15.3. ESI–MS: m/z 451.15 [(1B) + H]⁺ (calcd 451.14).

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b02116.

• Synthesis and characterization details of the compounds reported in this study (PDF)

• Crystallographic information file (CIF)

• Crystallographic information file (CIF)

• Crystallographic information file (CIF)

The authors declare no competing financial interest.