Substituent Effect on AIE mechanism of two coumarin derivatives: uncommon TICT fluorescence in aggregation state

Two coumarin derivatives, 7-diethylamino-3-(4-nitrophenyl)coumarin (DNC) and 7-hydroxy-3-(4-nitrophenyl)coumarin (HNC), were synthesized via Knoevenagel condensation of salicylaldehyde derivatives with 4-nitrophenylacetonitrile and then cyclization reaction. Both of them were characterized by single-crystal X-ray diffraction. The molecules of DNC are stacked via π-π interaction, while the hydrogen bond interactions instead of π-π interaction were observed in the crystal packing of HNC. Both of DNC and HNC showed solvatochromic properties and aggregation-induced emission (AIE) activities, but the AIE characteristics of them were entirely different. HNC exhibited an AIE phenomenon as the result of the restriction of twisted intramolecular charge transfer (TICT), while DNC emitted peculiar dual fluorescence which was assigned to the emission based on the inhibition of TICT state formation and the emission from the TICT state respectively.


Introduction
Conventional luminophores often show intense fluorescent emission in dilute solutions but meet with emission quenching in aggregation/solid state, which is widely known as aggregation-caused quenching (ACQ). In recent decades, a class of new functional materials with aggregation-induced emission (AIE) property has emerged as a rising hot topic for their wide applications in organic light-emitting diodes [1,2], dye-sensitized solar cells [3], bioimaging [4], and cancer theranostics [5]. Exploring the relationships between molecular structures and fluorescence properties are essential for the understanding of the AIE mechanism and designing of the high performance AIE-active materials. The generally accepted explanations for AIE mechanisms are the restriction of intramolecular rotation/vibrations/motions (RIR/RIV/RIM) [6]. In addition to the mechanisms based 162.7, 160. 1, 155.9, 147.1, 143.7, 142.4, 131.1, 129.8, 123.8, 120.3, 114.2, 112.2, 102.3. ESI-MS m/z calculated for [M-H] − 282.04, found 282.0 ( Fig. S4-6).

Crystallography
Single crystals of DNC and HNC suitable for X-ray analysis were obtained by slow evaporation of their THF solutions at room temperature. The diffraction data collection was performed at 293 K on a Gemini A Ultra diffractometer (MoKα, λ = 0.71073 Å). The intensity data were corrected for Lorentz and polarization effects. The structures were solved by direct methods and all non-hydrogen atoms were refined anisotropically by the full-matrix least-squares technique using the SHELXL-2014/7 package [14]. All the hydrogen atoms were set in geometrically calculated positions and refined isotropically. The elementary crystal data were provided in supplementary materials (Table S1). For more information, see the crystallographic information files that have been deposited in the Cambridge Crystallographic Data Centre (CCDC 2,130,543 for DNC and 2,130,544 for HNC).

Results and discussion
Crystal structure DNC crystallizes in the monoclinic space group P2 1 /c. As shown in Fig. 1 A, the 4-nitrophenyl moiety and the coumarin moiety are nonplanar with a dihedral angle of 36.6°. The N atom of the diethylamino moiety exhibits sp 2 hybridization and form a p-π conjugated system with the coumarin moiety. The dihedral angle between the plane through N(1), C(16), C(18) and the coumarin plane is 15.0°. The molecules are alternately stacked in an antiparallel manner to form one-dimensional molecular chains along the b-axis (Fig. 1B). The adjacent molecules in the molecular chain interact via π-π stacking interaction with a distance of 3.6170 Å between the centroid of benzene and the coumarin plane.
HNC crystallizes in triclinic system, space group P-1. There are two independent molecules in crystal lattice and they exhibit distinct conformation differences. In one molecule the dihedral angle between the plane through phenyl moiety and the plane through coumarin moiety is 20.1°, while it is 25.1° in the other molecule ( Fig. 2 A). The two types of unique molecules stack separately along the c-axis to form two molecular chains which are linked by two hydrogen bonds (O(42) − H(42)···O (21) and O (20) − H(20)···O(41) ) state is extremely rare because the molecular aggregation is usually unfavorable for the formation of the photoinduced TICT state.

Reagents and apparatus
All chemicals were obtained from commercial suppliers. 1 H NMR and 13 C NMR spectra were recorded on a Bruker Av400 NMR spectrometer. ESI-MS spectra were performed on a Bruker Esquire HCT mass spectrometer. Fluorescence spectra were taken on a Hitachi F-7000 fluorescence spectrometer. The X-ray diffraction data was collected on an Agilent Gemini A Ultra diffractometer.

General synthetic procedure for DNC and HNC
Salicylaldehyde derivative (4-diethylaminosalicylaldehyde or 4-hydroxysalicylaldehyde, 4 mmol) and 4-nitrophenylacetonitrile(0.648 g, 4 mmol)were dissolved in absolute ethanol (5 mL). After a tiny amount of piperidine was added for catalysis, the mixture was stirred for 2 h at 60 ℃. A red solid precipitated out. The precipitate was collected by filtration and then washed several times with ethanol to give the intermediate KCP. KCP (2 mmol) in acetic acid (5 mL) was stirred for 0.5 h at 110 ℃ during which time a solid precipitated out. The precipitate was collected by filtration and then washed several times with a small amount of ethanol respectively to afford DNC and HNC.
DNC in toluene were observed. The emissions of DNC in THF, MeCOOEt and CHCl 3 were so faint that it is hard to be distinguished. In MeCN, EtOH and MeOH, no luminescence was perceived. HNC displayed similar solvatochromic property as DNC. With the increase of the solvent polarity (from petroleum ether to acetonitrile), the emission peak gradually shifted to long-wavelength region and the fluorescence intensity is enhanced except for MeCN in which the emission intensity decreased. In EtOH and MeOH the fluorescence emission was neatly quenched (Fig. 3E). The fluorescence color change from blue to orange was observed when the solvent changed from petroleum ether to acetonitrile. In the more polar solvents such as ethanol and methanol, the fluorescence of HNC was quenched due to the formation of dark TICT state (Fig. 3 F).

AIE Properties
The AIE characteristics of DNC and HNC in the binary mixtures of acetonitrile and water with varied volume ratios were investigated. The concentration-dependent fluorescence spectra of HNC and DNC in CH 3 CN/H 2 O (1:9, V/V) indicated that HNC emitted obvious aggregation-induced fluorescence until its concentration was up to 50 µmol/L, while DNC showed a visible AIE phenomenon at a lower (Table S2). No π-π stacking interactions were found in the crystal packing (Fig. 2B).

Solvatochromic Properties
Both of DNC and HNC are the typical donor-acceptor type luminophores which commonly show solvent-dependent photophysical property [15,16]. Therefore, the solvent polarity effect on the absorption and emission of them was firstly investigated and the absorption and emission spectra in different solvents are shown in Fig. 3. Both DNC and HNC exhibited abnormal low, short-wavelength absorption in petroleum. In other solvents, the absorbances were similar, and the maximum absorption showed slight changes with the variation of solvent polarity which can be ascribed to the intramolecular charge transfer (ICT) (Fig. 3 A and D). The emission peak of DNC appears at 475 nm in petroleum ether, which shifted to 505 nm in toluene due to the solventenhanced intramolecular charge transfer. In more polar solvents such as THF, MeCOOEt and CHCl 3 the luminescence intensities of the DNC were dramatically weakened, and even were quenched in MeCN, EtOH and MeOH owning to the conversion from the locally excited state to the dark twisted intramolecular charge transfer state (Fig. 3B). The emission colors of DNC in different solvents under 365 nm UV lamp were illustrated in Fig. 3 C. The blue luminescence in petroleum ether and the strong green luminescence  Crystal packing showing one-dimensional molecular chain along b-axis via π-π stacking interaction emission enhancement at 495 nm. The appearance of the dual fluorescence should be ascribed to the formation of molecular aggregates, which was further supported by the absorption changes in MeCN/H 2 O mixtures when the water volume fractions were higher than 60% (Fig. S9B). Unfortunately, the AIE effect of DNC was inconspicuous on account of the π-π stacking interaction in the aggregates of DNC (Fig. 1B). The shorter-wavelength emission (495 nm) of the dual fluorescence was assigned to the relaxation of the locally excited state, while the longer-wavelength emission (625 nm) should be ascribed to the radiative relaxation of TICT state. It suggested that the aggregation cannot entirely inhibit the TICT state formation of DNC, and the TICT state in the aggregation state tend to relax back to the ground state via radiative channel. The TICT fluorescence in solution is rarely observed because the TICT state often relaxes back to the ground state via nonradiative pathways [15]. The aggregation-state TICT fluorescence is rarer because the molecular aggregation generally cause the restriction of the photoinduced TICT state formation [24]. On the contrary, the aggregation-induced TICT restriction can commonly lead to local fluorescence [16]. Therefore, the aggregation-state TICT fluorescence of DNC is a very interesting finding.
The photos of DNC and HNC in solid states under UV/ natural light irradiation were shown in Fig. 5. DNC was orange under natural light (Fig. 5 C). When excited by UV irradiation, it emitted strong red fluorescence both in the crystal state and in the powder state (Fig. 5 A and  5B), whereas the aggregation-induced fluorescence in MeCN/H 2 O mixtures is pink or rose (different mixtures of green and red fluorescence, Fig. 4 F). It suggested that DNC in solid states is more propitious to emit TICT fluorescence than its aggregates in MeCN/H 2 O mixtures. In general, organic compounds in the crystal state were unable to concentration of 10 µmol/L (Fig. S7), so 50 µmol/L HNC and 10 µmol/L DNC were used for the AIE research. Their aggregation behaviors in MeCN/H 2 O (1:9, V/V) were validated using dynamic light scattering (DLS). The aggregates of HNC with average particle size 160 nm were observed, while the average size of DNC aggregations was up to 459 nm (Fig. S8). As illustrated in Fig. 4 A − C, HNC emitted strong fluorescence in pure acetonitrile. When increasing the water fraction from 0 to 10%, its emission was dramatically quenched due to the formation of TICT nonemissive state. In MeCN/H 2 O solutions with water fractions between 10% and 70%, little changes in emission spectra were observed. When the water fraction was increased to 80%, the fluorescence intensity was distinctly enhanced and accompanied by a little blue shift, which was ascribed to the fact that the formation of the molecular aggregates inhibited the formation of TICT state and thereby local fluorescence appeared. The absorption curves of HNC tended to be flatter and broader when the water fractions were up to 80%, which was a further proof of the aggregate formation of HNC (Fig.  S9A). Unlike HNC exhibiting AIE characteristics based on the TICT mechanism that has been widely reported [17][18][19][20][21][22][23], DNC exhibited peculiar dual fluorescence in the aggregate state. As shown in Fig. 4D − F, DNC emitted weak green fluorescence at 485 nm in pure acetonitrile. Upon adding water, the emission intensities were reduced slightly and about 10 nm red-shift was observed. When increasing the water fraction from 60 to 99%, a new emission peak at about 625 nm emerged, which was companied by a slight  TICT fluorescence because the tight, regular molecular arrangement in crystal lattices restricts the molecular geometry changes in the formation of the TICT state. Hence the TICT fluorescence in solid state is extremely rare except for the particular-shaped molecules [25]. HNC in solid states was yellow under natural light (Fig. 5 F)

Conclusions
In summary, we have obtained two coumarin-based donoracceptor type luminophores, which were briefly named as DNC and HNC, through a facial synthetic strategy. HNC exhibited typical AIE characteristics based on the mechanism of the restriction of TICT state formation in the aggregate state. Significantly, DNC in aggregate state emitted dual fluorescence which was ascribed to the emission based on the restriction of TICT state formation and the TICT emission respectively. The TICT emission in the aggregate state is very rare.
Author contributions EW contributed to the study conception and design, and was the major contributor in writing the manuscript. The synthesis, spectrum test, data analysis and so on were performed by QL with the assistance of YZ. The X-ray diffraction data collection and structure determination was performed by ZN.
Funding This work is financially supported by the National Natural Science Foundation of China (22061016) and Program for Innovative Research Team in University (IRT-16R19).
Availability of data and material/Data availability The crystallographic information files that have been deposited in the Cambridge Crystallographic Data Centre (CCDC 2,130,543 for DNC and 2,130,544 for HNC) which are freely available for all. The link for CCDC: https:// www.ccdc.cam.ac.uk/.
Code Availability not applicable.

Conflicts of Interest/Competing interests
The authors have no conflicts of interest to declare that are relevant to the content of this article.
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