Base-Free Synthesis and Photophysical Properties of New Schiff Bases Containing Indole Moiety.

Schiff bases represent an essential class in organic chemistry with antitumor, antiviral, antifungal, and antibacterial activities. The synthesis of Schiff bases requires the presence of an organic base as a catalyst such as piperidine. Base-free synthesis of organic compounds using a heterogeneous catalyst has recently attracted more interest due to the facile procedure, high yield, and reusability of the used catalyst. Herein, we present a comparative study to synthesize new Schiff bases containing indole moieties using piperidine as an organic base catalyst and Au@TiO2 as a heterogeneous catalyst. In both methods, the products were isolated in high yields and fully characterized using different spectral analysis techniques. The catalyst was reusable four times, and the activity was slightly decreased. The presence of Au increases the number of acidic sites of TiO2, resulting in C=O polarization. Yields of the prepared Schiff bases in the presence of Au@TiO2 and piperidine were comparable. However, Au@TiO2 is an easily separable and recyclable catalyst, which would facilitate the synthesis of organic compounds without applying any hazardous materials. Furthermore, the luminescence behavior of the synthesized Schiff bases exhibited spectral shape dependence on the substituent group. Interestingly, the compounds also displayed deep-blue fluorescence with Commission Internationale de l'Éclairage (CIE) coordinates of y < 0.1. Thus, these materials may contribute to decreasing the energy consumption of the emitting devices.

Introduction

Schiff bases, synthesized through the condensation reactions between primary amines with ketones or aldehydes under optimized conditions, are widely used as antitumor, antiviral, antifungal, and antibacterial active substances.[1−5] For instance, Schiff bases derived from indole-3-carboxaldehyde showed antimicrobial and antitumor activities.[5−7] They are commonly used for stabilizing metal cations, resulting in enhancing their catalytic, industrial, and biological applications.[7−10] The synthesis of these bases can be performed in the presence of homogeneous or heterogeneous catalysts. Homogeneous catalysts, such as organic bases, inorganic bases, and Lewis acids, are not recoverable and require careful disposal to avoid environmental hazards. Therefore, the use of recoverable catalysts acquired much interest.

Recently, the use of heterogeneous catalysts received attention in the organic synthesis field due to their high efficiency and ease of recovery.[11−15] The use of these recoverable heterogeneous catalysts reduces the environmental risks that can arise from the use of nonrecoverable homogeneous catalysts without a significant reduction in the yield of synthesized compounds.[16] In the presence of UV light, FeCu@N-doped carbon is an efficient catalyst that can be applied for converting amines with alcohols into Schiff bases.[17] Also, bifunctionalized cobalt/zinc-incorporated mesoporous silica nanoparticles were used for synthesis of Schiff bases from aryl amine and benzyl alcohol, and the reaction was performed at 120 °C for 3 h in the presence of airflow.[18] The mixture of P2O5 and Al2O3 was used to catalyze the synthesis of Schiff bases from the carbonyl compounds and primary amines in the absence of a solvent.[19] Co nanoparticles embedded in mesoporous nitrogen-doped carbon were found effective in converting nitro groups into amino groups followed by coupling with carbonyl compounds in the presence of formic acid at 190 °C.[20]

On the other hand, Schiff bases are perfect candidates for photovoltaic solar cell, sensor, and organic light-emitting diode (OLED) applications; they can emit in a specific range.[21,22] Recently, we have reported some simple Schiff bases as luminescent coatings with white luminescence from a single chromophore.[23] Moreover, studying the excited state and its correlation with the molecular structure or molecular aggregation can help us to specify the optical properties of the organic materials.[24−29] Some indole derivatives and other heterocyclic compounds have been designed and showed aggregation-induced emission properties.[22,30−32] Thus, Schiff bases with indole moieties may have some interesting optical properties. However, limited studies have exclusively examined such materials.

In this work, Au@TiO2 was utilized as a base-free catalyst in the synthesis of six new Schiff bases derived from indole, and the product yields were compared with the yields obtained from the use of piperidine. The developed catalyst was characterized by X-ray diffraction analysis (XRD), infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDX). The synthesized Schiff bases were characterized by different spectral analyses such as IR, 1H NMR, 13C NMR, mass spectrometry, UV–Vis absorption, PL, and TCSPC. The recovery and reusability of the catalyst were investigated for four cycles. Also, the photophysical properties of the synthesized Schiff bases were studied.

Results and Discussion

Catalyst Characterization

For the hydrothermal development of Au@TiO2, TiO2 and the Au precursor (HAuCl4·3H2O) were heated in a Teflon autoclave at 180 °C in the presence of ethanol. Figure a shows the XRD patterns of TiO2 (P25) before and after Au loading. Characteristic diffraction peaks located at 27.3, 35.9, 41.1, 54.9, 56.5, 68.9, and 70.0° are indexed to (110), (101), (111), (211), (220), (301), and (112) crystallographic planes for rutile TiO2, respectively.[33−36] Peaks at 25.1, 37.6, 47.8, 53.7, 62.7, 75.0, and 82.5° were equivalent to the planes (101), (004), (200), (105), (204), (215), and (303), respectively, indicating the tetragonal structure of anatase TiO2.[33,34,37,38] Additional XRD peaks at 44.2, 64.4, and 77.3° were observed, which are indexed to the (200), (220), and (311) crystallographic planes of Au, respectively.[39] When the XRD spectra of TiO2 were subtracted from the spectra of Au@TiO2 as illustrated in Figure b, the overlap between these spectra was deconvoluted, resulting in new peaks at 38.0 and 81.5°, which are indexed to the crystallographic planes (111) and (222) of Au, respectively.[39,40] From the XRD data, the loading of Au on TiO2 was successfully achieved without observing changes in the phases of TiO2. At the same time, the deposited Au nanoparticles had a face-centered cubic (fcc) structure.[39,40]

Figure 1

XRD patterns of (a) TiO2 and Au@TiO2 and (b) Au@TiO2 after subtracting TiO2 patterns.

Figure S1a,b shows the TEM images of TiO2(p25), where the size of TiO2 particles was <30 nm. Figure S1c illustrates the selected area electron diffraction (SAED) patterns, which indicates the polycrystallinity of TiO2 (P25) due to the presence of both anatase and rutile TiO2 nanoparticles. Figure S1d shows the high-resolution (HR)-TEM data that the interlayer distance between the lattice fringes was 0.34 nm, which is close to the d-spacing of the (101) plane in anatase TiO2. After deposition of Au (1%) on TiO2, Au nanoparticles with a size of <10 nm were observed, as shown in Figure a,b. The polycrystallinity of the developed Au@TiO2 is concluded from the SEAD image (Figure c), where the estimated d-spacing values indicate the presence of anatase TiO2, rutile TiO2, and Au nanoparticles. The HR-TEM image (Figure d) illustrates the lattice fringes with a d-spacing of 0.22 nm, corresponding to the (111) plane of the Au fcc structure.[41,42]Figure S2a shows the EDX spectrum of Au(1%)@TiO2, which indicates the presence of Au with a percentage of 1.2%. The TEM image of Au(5%)@TiO2 is illustrated in Figure S2b. These results indicated that the grafting of Au@TiO2 through the hydrothermal method was successfully performed, and the size of Au was <10 nm. EDX mapping of Au(1%)@TiO2 (Figure S3) illustrates the dispersion of Au nanoparticles on TiO2. From these results, TiO2 worked as a support, where the Au ions were dispersed. The FTIR spectrum of Au@TiO2 (Figure S4) shows absorption bands at 1345–1625 and 3342 cm–1, which are ascribed to Ti-OH and the absorbed water, respectively.[43−46] Also, the absorption bands at 400–700 cm–1 were ascribed to TiO2.[43−46]Figure S5 shows the chemical compositions of the Au(1%)@TiO2 composite investigated by XPS. In the Ti2p spectrum (Figure S5a), two peaks were observed at 464.5 and 459.0 eV, which are attributed to Ti2p1/2 and Ti2p3/2, respectively.[47] The XPS O1s spectrum (Figure S5b) shows a peak at 529.9, which is attributed to titanium oxide.[47]Figure S5c shows the Au4f spectrum, where two peaks are shown at 83.5 and 87.3 eV, which are attributed to Au(0)4f5/4 and Au(0)4f7/2 nanoparticles, respectively. From the abovementioned results, the hydrothermal treatment induced the reduction of the dispersed Au ions into Au(0) nanoparticles without changing the morphology and crystallinity of TiO2.[49,50]

Figure 2

(a, b) TEM, (c) SAED pattern, and (d) HR-TEM images of Au(1%)@TiO2.

Synthesis of Schiff Bases

As shown in Scheme , the starting material 3-chloro-1H-indole-2-carbaldehyde (1) was prepared according to the previously published procedure,[51] which then was allowed to react with different aromatic amines represented in 4-anisdine, 4-aminoacetophenone, 1-naphthyl amine, and 6-aminonaphthalene-2-sulfonic acid in the presence of piperidine as a homogeneous catalyst. The reaction mixture was refluxed for ≥ 4 h, and the used solvent was ethanol. All products (3a–3d) were isolated in high yields (85–90%). Besides, heterocyclic amines including 6-methoxy-2-aminobenzothiazole and N-ethyl-9-aminocarbazole have been tolerated in this transformation, which efficiently delivered the products (3e, 3f) in high yields (90–92%). The six new Schiff bases were successfully characterized using different spectral analysis techniques. The IR spectra (Figure ) exhibited new bands around 1620 cm–1, characteristic for the CH=N group. Also, the absorption band at 1661 cm–1, which was attributed to the stretching absorption bands of the C=O group in the starting aldehyde 1 as previously reported, disappeared, indicating the occurrence of condensation between the C=O group of aldehyde and NH2 groups of amines.[52] Moreover, the 1H-NMR analysis revealed new signals in the region 8–9 ppm corresponding to the CH=N protons for all products.

Scheme 1

Substrate Scope for Amines

Unless noted otherwise, the reaction of 1 (6 mmol) and 2a (1 equiv) was carried out with piperidine (5 drops) in EtOH at 90 °C for the mentioned time.

Figure 3

IR spectra of the synthesized Schiff bases.

Based on the abovementioned advantages of heterogeneous catalysis in organic synthesis, the base-free alternative way was used to synthesize the six Schiff bases in Scheme . To the best of our knowledge, the use of Au@TiO2 as a catalyst for the synthesis of these Schiff bases has not been reported before. To begin this study, aniline 2 was used as the model amine substrate for the reaction with 3-chloro-1H-indole-2-carbaldehyde (1) and EtOH as a solvent. No products were detected in the absence of catalysts even if the reaction temperature was increased to 65 °C (Table ). Adding TiO2 only delivered product (3) in traces with 10% yield when the reaction continued for 1 h in ethanol (entry 2). To our delight, the Schiff base product was obtained in 60% yield using the Ag@TiO2 catalyst in ethanol when the reaction was conducted for 1 h (entry 3), which suggests that the Ag particles have a considerable effect on the polarization of the carbonyl group, which in turn favors the condensation reaction.[53,54] The Au(5%)@TiO2 and Au(1%)@TiO2 catalysts interestingly delivered the Schiff base product in 72% and 70% yield (entries 4 and 5), respectively. Carrying out the reaction for a longer time boosted the reaction yield, where the product was isolated in 75% and 85% yield after 2 and 3 h, respectively (entries 6 and 7). Moreover, the examination of other solvents illustrated that EtOH allowed the reaction to give the highest yield, while the reaction gave only a 30% yield in H2O (entry 8). Trace amounts of Schiff bases were synthesized in the presence of CH2Cl2 and CH3CN as solvents (entries 9 and 10, respectively). Furthermore, the catalyst load was also investigated, where the amount of Au catalyst has a considerable effect on the product yield. We found that lowering the loading of the catalyst to 0.5 and 5% led to a diminished yield (entries 11 and 12). However, increasing the catalyst load to 20% did not significantly increase the reaction yield (entry 13). Keeping all reaction conditions unchanged and using piperidine as a homogeneous catalyst instead of Au@TiO2 led to the formation of 3 in 50% yield (entry 14). Using piperidine as an organic catalyst under reflux in ethanol for 5 h delivered the product in 85% yield (entry 15). These findings indicated that the optimized procedures for the synthesis of Schiff bases performed the reaction in ethanol at 65 °C in the presence of Au(1%)@TiO2 with a load of 10%.

Table 1

Reaction Optimizationa

entry	catalyst	load (%)	solvent	T (°C)	t (h)	yield (%)
1			EtOH	65	1
2	TiO2	10	EtOH	65	1	10
3	Ag(5%)@TiO2	10	EtOH	65	1	60
4	Au(5%)@TiO2	10	EtOH	65	1	72
5	Au(1%)@TiO2	10	EtOH	65	1	70
6	Au(1%)@TiO2	10	EtOH	65	2	75
7	Au(1%)@TiO2	10	EtOH	65	3	85
8	Au(1%)@TiO2	10	H2O	65	3	30
9	Au(1%)@TiO2	10	CH2Cl2	65	3	traces
10	Au(1%)@TiO2	10	CH3CN	65	3	25
11	Au(1%)@TiO2	0.5	EtOH	65	3	traces
12	Au(1%)@TiO2	5	EtOH	65	3	30
13	Au(1%)@TiO2	20	EtOH	65	3	85
14	piperidine	5 drops	EtOH	65	3	50
15	piperidine	5 drops	EtOH	reflux	5	85

Unless noted otherwise, the reactions of 1 (3 mmol) and aniline (1 equiv) were carried out with Au@TiO2 in 10 mL of solvent and at 65 °C for the mentioned time (t) in hours.

The substrate scope for the Schiff base formation was explored (Table ) using the optimized procedures. Aryl substituents, including electronically rich or poor ones, have been efficiently tolerated in this transformation to afford Schiff base products in high yields. Aromatic amines with the electron-donating group p-anisidine gave the corresponding Schiff base product 3a in a very good yield (85%). Aromatic amines containing an electron-withdrawing group (4-aminoacetophenone) provided the Schiff base product 3b only without detecting the chalcone byproduct, and the reaction needed a longer time to complete. Furthermore, the scope of substrates containing more substituted arenes with a naphthalene ring has been successfully examined, and the desired products 3c and 3d were synthesized in high yields (88 and 85%, respectively). Finally, the substrate of heterocyclic amine was also tolerated in this transformation to give the Schiff bases in high yields. Both 3e and 3f Schiff bases obtained from benzothiazole and carbazole substrates were isolated in 90% yield.

Table 2

Substrate Scope for Aminesa

entry	product	t (h)	yield (%)
1	3a	3	85
2	3b	4	80
3	3c	3	88
4	3d	3	85
5	3e	2	90
6	3f	2	90

Unless noted otherwise, the reactions of 1 (3 mmol) and 2 (1 equiv) were carried out with a 10% load of Au(1%)@TiO2 in EtOH at 65 °C for 3 h.

Comparing the results obtained by the two methods, it is obvious that the base-free method is a promising alternative route and gave a comparable yield with the conventional method using an organic base (such as piperidine). This strategy is beneficial for the synthesis of different kinds of Schiff bases and may be extended to include the design of new reactions that need organic bases such as Knoevenagel condensation or aldol condensation.

The catalyst was easily separated by dissolving the synthesized Schiff bases, followed by filtration, washing, and drying at 80 °C for 3 h. The catalyst was effective for synthesizing compound 3 for four cycles without a significant decrease in the separated yield as shown in Figure a. Comparing the applied conditions during the use of Au(1%)@TiO2 to those during the use of base illustrates the mild conditions during the use of Au@TiO2 with effective catalytic activity and recyclable usage. Also, the Schiff base can be synthesized in water (Table ), which is considered a green approach. Figure b illustrates the plausible mechanistic routes of Schiff’s bases on Au@TiO2, where the presence of Au(0) in the catalyst would presumably facilitate the coordination with the oxygen of the carbonyl groups of indole-aldehyde or amino groups of amines.[53−57] TiO2 is considered a weak acid support, and the deposition of Au(0) would enhance the reactivity toward Schiff base synthesis by increasing the number of Lewis acid and Bronsted acid sites. The CHO groups would be polarized due to the presence of acidic sites. Subsequently, amino groups of amines nucleophilically attacked the polarized CHO groups, resulting in the formation of N–C bonds. Then, rearrangements occurred to form OH, which would be elaborated from the compounds with the H atom as H2O to give Schiff bases.[53−57] Also, the amino groups have an affinity to bond with Au atoms through the donation–backdonation mechanism, which might facilitate their reaction with formyl groups.[58,59] No changes were observed in the FTIR spectrum of recycled Au@TiO2 (Figure S6), which could refer to the stability and reusability of Au@TiO2 after the synthesis of Schiff bases. The characteristic diffraction peaks of Au NPs and TiO2 (anatase and rutile) were observed in the XRD spectrum of the recycled Au@TiO2 (Figure S7). No changes in the XPS Au 4f, Ti 2p, and O 1s spectra of the recycled Au(1%)@TiO2 catalyst were observed as illustrated in Figure S8. Also, the XPS N 1s spectra of the as-prepared and the recycled Au(1%)@TiO2 are shown in Figure S9, where nitrogen did not exist on the catalyst. Namely, the recycled catalyst after regeneration was clean without contaminating it with the Schiff base. The content of Au(0) in the catalyst decreased to 70% after recycling, which could describe a slight decrease in the reactivity of the catalyst.

Figure 4

(a) Reuse of Au(1%)@TiO2 in the synthesis of 3 compared to the use of piperidine as a base. (b) Plausible synthetic routes of Schiff bases on Au(1%)@TiO2.

Photophysical Behavior

The photophysical behavior of the selected Schiff bases has been studied in the solution state using DMSO as a solvent. The UV–Vis absorption spectra of the solutions (1 × 10–7 mol L–1 for 3b, 1 × 10–9 mol L–1 for 3a, 3c, and 3f) are displayed in Figure a. Due to the weak absorption of 3b, its spectrum is presented separately as an inset of Figure a. The studied compounds showed absorption bands in the region of 290–440 nm with peaks at λmax around 365 nm for 3a and 3c, 315 nm for 3b, and 395 nm for 3f. These absorption peaks can be ascribed to the π → π* transitions. The peaks at 280–320 nm have resulted from the π → π* transitions of the aromatic core. The bands at the range of 300–360 nm involve π → π* transitions of the C=N groups. The longer wavelength absorptions reflect the extended conjugation in the whole molecule. On the other hand, these materials are transparent in the region of 450–700 nm. The different absorption spectra of these Schiff bases suggest different ground states of their solutions due to having different substituents.

Figure 5

Photophysical behavior of DMSO solutions of 3a, 3b, 3c, and 3f at room temperature: (a) UV–vis absorption spectra (1 × 10–7 mol L–1 for 3b and 1 × 10–9 mol L–1 for 3a, 3c, and 3f). (b) Emission spectra with excitation at 320 nm (1 × 10–7 mol L–1). (c) CIE chromaticity diagram of the emission colors. (d) Photoluminescence decay profiles with excitation at 320 nm.

Furthermore, the emission spectra of the studied compounds have been measured as illustrated in Figure b. It has been reported that inserting electron-donating or electron-withdrawing groups into Schiff bases can decrease or increase the energy gap (Egap) between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) to cause a red or blue shift, respectively.[60] However, there is no significant change in λmax of the emission spectra for 3a and 3b (about 415 nm for both) with the electron-donating (methoxy) and -withdrawing (acetyl) groups, respectively. Egap of the gaseous state could be different from that of the solution or solid states because some interactions disturb the electronic distribution around the molecule and subsequently change the Egap. Therefore, we speculate that attaching these groups to the phenyl ring does not affect Egap of these Schiff bases due to the interaction of these polar molecules with the solvent molecules (DMSO). On the other hand, replacing the phenyl ring of 3a and 3b with naphthyl and carbazolyl rings in 3c and 3f red-shifted the emission λmax to 430 and 450 nm, respectively. This result implies the ability of the substituent groups to increase EHOMO in the order carbazolyl > naphthyl > phenyl. The emission colors were determined quantitatively using a Commission Internationale de l’Éclairage (CIE) chromaticity diagram, Figure c. CIE of 3a showed light-blue emission with coordinates of (0.17, 0.15), while 3b, 3c, and 3f gave deep-blue emission with coordinates of (0.16, 0.09), (0.16, 0.07), and (0.15, 0.09), respectively. Blue light-emitting dyes are basic for realizing the full-color display in OLEDs.[61]

Additionally, the smaller the CIE coordinates of the blue light, the less the power consumption of OLEDs, where it has been reported that deeper blue emitters are predicted to boost white OLED performance due to the fact that power consumption diminishes as blue saturation increases.[62−65] The photoluminescence lifetime of the examined compounds was also estimated in the solution state. Figure d displays the logarithmic graph of the emission decay curve. Despite having different spectral shapes owing to the different molecular structures, the studied Schiff bases showed similar biexponential decay profiles with emission lifetimes on the order of nanoseconds. Thus, the emission of the materials can be assigned as fluorescence emitted from singlet excited states. This fluorescence was also confirmed from the close λmax of both the absorption and emission spectra given in Figure a,b. The compounds displayed similar decay profiles because their luminescent centers are comparable. The detailed parameters with the corresponding relative amplitudes (Ai) are summarized in Table S1.

Conclusions

In this report, new Schiff bases have been synthesized for the first time on the Au@TiO2 heterogeneous catalyst under mild conditions. The optimal procedures for accomplishing the synthesis of Schiff bases were mixing the aldehydes and amines in ethanol as a solvent at 65 °C for 3 h in the presence of Au(1%)@TiO2. In water, the isolated yield of Schiff base was 30%, which is considered a green approach for synthesizing Schiff bases. The catalyst was easily recycled from the reaction mixture and showed reusability for four cycles without a significant decrease in the isolated yield. Additionally, the synthesized Schiff bases demonstrated some interesting luminescence behaviors, such as the dependence of the spectral shape on the substituent group and the emission of deep-blue fluorescence with CIE coordinates of y <0.1. Therefore, these dyes may help design new OLEDs with reduced energy consumption.

Experimental Section

Synthesis of Au@TiO2

In total, 1 g of TiO2 (P25, Degussa, 20 nm) was transferred into a Teflon container. Then, 20 mL of water and 20 mL of ethanol were mixed and transferred into the Teflon container. The Teflon container was sonicated for 20 min, and subsequently, the appropriate amount of HAuCl4·3H2O (BDH, 1% w/v) was added into the Teflon container and sonicated for an additional 20 min. The Teflon container closed well in a stainless-steel autoclave and was heated at 180 °C for 12 h. Then, Au@TiO2 was separated through centrifuging at 4000 rpm, washed using deionized water three times, and dried at 80 °C for 8 h. The amount of HAuCl4·3H2O was calculated to prepare Au(1%)@TiO2 and Au(5%)@TiO2.

Synthesis of Schiff Bases

Base-Catalyzed Synthesis of the Schiff Bases

All the Schiff base derivatives were synthesized by refluxing an ethanolic solution of 3-chloro-1H-indole-2-carbaldehyde (1) (6 mmol, 0.5 gm) and the corresponding aromatic or heterocyclic amine (6 mmol) in 1:1 stoichiometric ratio for the mentioned time in the presence of piperidine catalyst (five drops).[22,51,52] The solid precipitate formed after cooling the reaction mixture was filtered off, dried, and recrystallized from ethanol to afford the desired pure compounds.

Base-Free Synthesis of the Schiff Bases

In a typical reaction procedure, 3-chloro-1H-indole-2-carbaldehyde 1 (0.5 gm, 3 mmol), aniline (0.28 gm, 3 mmol), ethanol (15 mL) as the solvent, and catalyst (50 mg) were added into a 50 mL reaction vessel. The resulting mixture was stirred at 65 °C in an oil bath for 3 h. After confirming the reaction completion using TLC, ethyl acetate was added to the reaction mixture, and the catalyst was filtered. The solvent was evaporated at reduced pressure using a rotary evaporator to give the desired product.

Characterization Instruments and Photophysical Measurements

Structural analysis was studied using a Philips 1700 version diffractometer X-ray powder diffractometer (XRD) equipped with Cu Kα radiation. A transmission electron microscope (TEM, JEOL, JEM-2100F, Japan) was used for investigating the morphologies and crystallinity of the developed catalyst, where an accelerating voltage of 200 kV was applied. XPS spectra of samples were collected using a Kratos AXIS Supra, Japan, with the monochromatic AL anode (1486.6 eV). CasaXPS software was utilized for data analysis. All melting points were measured on a Fisher-John apparatus. IR spectra were registered on a Pye-Unicam Sp-100 spectrophotometer utilizing the KBr wafer method. NMR analyses were performed on Bruker BioSpin GmbH (1H: 500 MHz, 13C: 125 MHz) spectrometers using tetramethylsilane (TMS) as an internal standard and DMSO-d6 as a solvent. Analytical TLC was completed on silica gel plates (Fluka 70643-50EA, Sigma-Aldrich, Germany) utilizing UV light. Absorption spectra were recorded with a SPECORD S600 UV/Vis spectrophotometer (Analytik Jena, Jena, Germany). PL spectra were collected using an FP-8500 spectrofluorometer (Jasco, Groß-Umstadt, Germany). The excitation wavelength was 320 nm with a bandwidth of 5 nm. The PMT voltage was set to 580 V. All measurements were performed in ambient air and at room temperature. The sample solution was excited using a pulsed LED PLS320 at 320 nm (Fluo Time 100, PicoQuant, FWHM ∼800 ps). For all samples, a long-pass filter at 390 nm was inserted routinely to block stray light of the excitation source.