Organic Fluorescent Compounds that Display Efficient Aggregation-Induced Emission Enhancement and Intramolecular Charge Transfer

Abstract

A series of symmetric sulfone-linked organic fluorescent compounds (1a–c) was synthesized and characterized. V-shaped 1a–c were designed as aggregate of intramolecular charge transfer (ICT) and aggregation-induced emission enhancement (AIEE) processes. The 1a–c emitted intense blue violet lights in normal solvents. A large red shift of the emission wavelength and dramatic decrease of emission efficiency occurred with increasing solvent polarity. The 1a–c will function well as electron transport and blue light-emitting materials through theoretical calculations.

1. Introduction

Fluorescent organic molecules are widely used as active elements in organic light-emitting diodes (OLED), chemosensors, and bioprobes [1,2,3,4,5]. However, although most of these fluorescent organic molecules possess planar conjugated structures and display reasonable emission in dilute solutions, their emission is weak or nonexistent in the solid state because of aggregation-caused quenching. Such quenching limits the applications of fluorescent organic molecules [6,7,8]. Recently, multiple groups have independently reported an interesting phenomenon called aggregation-induced emission (AIE), wherein aggregated molecules emit more intense fluorescence than their dilute solutions [9,10,11]. Propeller-shaped molecules such as hexaphenylsilole, tetraphenylethene, and quinoline-malononitrile and their derivatives are known for their unique ability to promote AIE [12].

The integration of AIE and intramolecular charge transfer (ICT) into one molecule has attracted much attention. This type of luminophore can emit strong fluorescence not only in the aggregated form or solid state but also in dilute solution [13,14,15]. For example, Tang and coworkers developed a new donor (D)–acceptor (A) system that exhibits dual photoluminescence (PL) activity, including ICT and AIE [16]. However, the integration of AIE and ICT into one molecule is still rare and poorly understood, and thus warrants further study to clarify the relationship between molecular structure and optical properties.

Herein, we report a series of new V-shaped D-π-A-π-D fluorophores (1a–c) that exhibit strong blue emission in both solution (nonpolar solvents) and the solid state. We integrated a sulfone into 1a–c because it has been demonstrated that sulfones possess a special V-shaped structure, which is helpful to weaken π–π stacking. A 1,3,4-oxadiazole was also included in 1a–c because it is an attractive electron-accepting material that has received considerable interest for use in OLEDs.

2. Results and Discussion

2.1. Synthesis of Target Compounds 1a–c

The synthetic routes to 1a–c are described in Scheme 1. The intermediate p,p’-dihydrazinocarbonyl diphenylsulfone (4) was synthesized according to a previous report [13]. The target compounds 1a–c were obtained by condensation reactions between intermediate 4 and 4-alkoxybenzoic acid in phosphoryl chloride under reflux. Compounds 1a–c were all white crystals and their yields ranged from 37% to 43%. The structures of 1a–c were characterized by ¹H and ¹³C-NMR spectroscopy, mass spectrometry, and elemental analysis.

2.2. The Optical Properties of Compounds 1a–c

The optical properties of 1a–c in solution and the solid state were investigated by UV-vis absorption and PL spectroscopies. The data revealed that the alkyl chain did not influence the properties of these compounds. Figure S5 presents the UV-vis absorption spectra of the same concentration of 1a–c in different solvents. The absorption maxima (λ_ab) exhibit a slight red shift of a few nanometers with increasing solvent polarity. In contrast, the PL maxima of 1a–c vary dramatically with changes in solvent polarity, as shown in Figure 1 and Figure S6. The PL intensity gradually decreased with increasing solvent polarity and fluorescent quantum yields (Q_ys) decreased from 0.42 to 0.05 when the solvent was changed from toluene to DMSO. Emission maxima (λ_em) red-shifted from 387 nm in toluene to 468 nm in DMSO. The red shift and decreased QY can be ascribed to stabilization of the excited state. This behavior originates from the strong ICT character from the alkoxyl benzene to the sulfone. This result indicates that the ICT excited state has a larger dipole moment than the ground state, which is caused by substantial charge redistribution. Because of the dipole–dipole interaction of a polar solvent and excited state, the ICT state is more stable in a polar solvent than in a nonpolar one.

To determine whether 1a–c are AIE-active, fluorescence emission spectra of their diluted mixtures in THF/water and DMF/water mixtures with different water fractions (ƒ_w) were measured (Figure 2 and Figure S7). Figure 2 reveals that 1a in pure THF solution emitted intense indigo blue fluorescence at 427 nm, which was dramatically quenched when a small amount of water was added, as evidenced by its weak emission in the THF/water mixtures with ƒ_w ≤ 40 vol%. Meanwhile, λ_em red-shifted to about 449 nm upon addition of water, which was mainly attributed to the increase of solvent polarity and enhancement of the ICT effect. If a hydrophobic environment was created inside the aggregates, the ICT effect should be alleviated, and the fluorescence should recover because of the AIE effect. The pure DMF solution of 1a displayed weak blue fluorescence at λ_em = 458 nm. When ƒ_w of the DMF/water mixture was low, λ_em of 1a was dramatically quenched and slightly blue-shifted. When ƒ_w of the DMF/water mixture was high, λ_em of 1a was red-shifted and the fluorescence intensity did not recover. In this case, the solvent polarity influences the AIE effect. These data illustrate our speculation that 1a is a luminogen with both ICT and AIE characteristics. Both 1b and 1c exhibited similar AIE effects, as shown in Figure S7.

Such AIE properties were also confirmed by the vivid image of the solid powders under 365 nm UV illumination, as depicted in Figure 3. The solid powder and 1a dissolved in toluene emitted strong blue fluorescence. Conversely, the emission spectra of the solution and film samples were different, considering the broad emission peaks and red shift in Figure 3. Compound 1a exhibited dual-state emission behaviors and that demonstrated its fluorescence was strong in both solution and the aggregated state.

2.3. The Electrochemical Properties of 1a–c

The electrochemical properties of 1a–c in CHCl₃ were investigated by CV. The electrochemical data are summarized in

Table 1. Photophysical, physical, and electrochemical data for 1a–c.

Comp.	a λmax,Abs (nm)	a λmax,em (nm)	b λmax,em (nm)	cΦF	Ered/peak (V)	E1ox/peak (V)	dEgopt (eV)	e Tg/T5d (°C)
1a	323	412	420	0.42	−0.73	0.62	4.18	119/396
1b	326	411	423	0.36	−0.74	0.75	4.21	156/383
1c	328	411	423	0.32	−0.76	0.78	4.26	145/386

^a Measured in CHCl₃. ^b Measured for film. ^c Measured in CHCl₃ with quinine sulfate as a standard (QY = 0.54) [17]. ^d Calculated using Eg = 1240/λ^absonset. ^e Analyzed by DSC/TGA at a heating rate of 10 °C·min⁻¹ under N₂.

. Figure 4 displays representative CV traces for the oxidation and reduction of 1a, demonstrating that 1a undergoes a quasi-reversible reduction at the cathodic potential associated with the reduction of the electron-deficient sulfone moiety to form an anion radical. Compound 1a also exhibits an irreversible oxidation process corresponding to the removal of electrons from the terminal alkoxy phenyl group to form a radical cation.

2.4. Theoretical Calculations

All calculations were carried out using the Gaussian 09 package [18]. Calculations of the electronic ground state of compound 1a–c were carried out using density functional theory (DFT) [19] with the B3LYP exchange-correlation functional [20,21] together with the 6–31 G basis set [22]. GaussView 5.0.8 was used to calculate the interface of the structures and manipulate orbital.

DFT was used to calculate molecular orbitals to understand the photophysical behavior of the synthesized compounds. To estimate the charge-carrier injection barriers, we investigated the changes that occurred upon frontier orbital substitution. Figure 5 shows the highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) and energy gap (E_g) of 1a in toluene, THF and DMSO. The electron density distributions of representative 1a change considerably between the HOMO and LUMO. The electron density distributions of the HOMO are mostly located over the alkoxy benzene groups, whereas the LUMOs are mainly concentrated on the sulfone core. E_g between the HOMO and LUMO decreases from 4.03 to 3.97 eV from toluene to DMSO, which is consistent with the slight red shift of 1 nm of the UV-vis absorption peak in polar solvents. Generally, this type of dye molecule with such an electron distribution indicates ICT character, which is consistent with the experimental results.

The calculated vertical ionization potential IP (IP_v), adiabatic IP (IP_a), vertical electron affinity (EA_v), and adiabatic electron affinity (EA_a) for 1a are listed in

Table 2. Ionization potentials, electron affinities, extraction potentials, internal reorganization energies, and Δ = ⎹λ_h − λ_e⎸ (in eV) for the studied complexes.

Comp.	IP (v)	IP (a)	HEP	EA (v)	EA (a)	EEP	λhole	λelectron	Δ
1a	6.21	6.07	5.90	2.35	2.49	2.63	0.31	0.28	0.03
1b	6.18	6.07	5.90	2.29	2.49	2.63	0.28	0.34	0.06
1c	6.17	6.07	5.90	2.29	2.49	2.63	0.27	0.34	0.07
Alq3	IP (5.8)			EA (3.0)			0.242	0.276	0.034

. The reorganization energy (λ) can be used to estimate the charge transport rate and balance between holes and electrons; these results are also provided in Table 2. The reorganization energy for electron transport can be expressed in terms of the electron extraction potential (EEP): λ_e = EEP − EA_v. EEP represents the electron extraction potential, which is the energy difference between M and MM- (anionic) when considering M-. It was found that representative 1a had a low λ_e value of 0.28, which is similar to that of tris(8-hydroxyquinolinato)aluminum (λ_e = 0.276) [23] and good electron transfer ability. These results indicate that 1a may be a promising electron transport material.

The thermal properties of 1a–c were evaluated by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) under nitrogen atmosphere. (Table 1 and Figure S4). The TGA revealed that the compounds possessed relatively high thermal stability with an initial weight loss (5%) temperature of 383–396 °C. The glass transition temperatures of 1a–c, as shown in Table 1, were 119 °C, 156 °C and 145 °C, respectively. The excellent thermal stability of 1a–c should aid the formation of amorphous films.

3. Experimental

General Information

Ultraviolet-visible (UV-vis) spectra were recorded on a Lambda 25 spectrophotometer. Fluorescence spectra were obtained using a Shimadzu RF-5301PC fluorescence spectrophotometer. Cyclic voltammetric studies were carried out using a CHI 852C instrument with CHCl₃ as the solvent (10⁻³ M) and 0.1 M Bu₄NClO₄ as the supporting electrolyte. Counter and working electrodes consisted of a Pt wire and glass carbon, respectively, and the reference electrode was Ag/AgCl. The thermal stability of the target compounds was characterized using a Shimadzu DTG-60H thermogravimetric analyzer.

Starting compound 2 and p,p’-Dihydrazinocarbonyl diphenylsulfone (3) were synthesized according to literature method [13].

Representative procedure for the synthesis of 1a–c: A mixture of compound 2 (2.2 mmol), p,p’-Dihydrazinocarbonyl diphenylsulfone (1 mmol), and phosphorous oxychloride (8 mL) was heated at reflux for 8 h, and then poured onto crushed ice after cooling to room temperature. The white solid was obtained. The crude product was separated on a flash silica gel column (CHCl₂/Ethyl acetate) to afford 1a–c.

p,p’-Bis[2-(4-Ethoxyphenyl)-1,3,4-oxadiazol-5-yl]diphenylsulfone (1a): White powder, yield: 40%. Mp 329 °C (DSC). ¹H-NMR (400 MHz, CDCl₃): δ (ppm) 1.44 (t, J = 7.2 Hz, 6H), 4.11 (q, J =7.2 Hz, 4H), 7.01 (d, J = 8.8 Hz, 4H), 8.04 (d, J = 9.0 Hz, 4H), 8.12 (d, J = 9.0 Hz, 4H), 8.27 (d, J = 8.8 Hz, 4H); ¹³C-NMR (100 MHz, CDCl₃): δ (ppm) 14.65, 63.85, 115.12, 115.61, 127.67, 128.60, 128.79, 128.94, 143.37, 162.21, 162.51, 165.45; FTIR (KBr, cm⁻¹): 3079, 2920, 1615, 1489, 1066, 842; TOF-Ms (m/z) [M + H]⁺ = 596.1648; Anal. Calcd for C₃₂H₂₆N₄O₆S: C, 64.64. H, 4.41. Found: C, 64.71. H, 4.48.

p,p’-Bis[2-(4-Butoxyphenyl)-1,3,4-oxadiazol-5-yl]diphenylsulfone (1b): White powder, yield: 43%. Mp 325 °C (DSC). ¹H-NMR (400 MHz, CDCl₃): δ (ppm) 0.98 (t, J = 7.2 Hz, 6H). 1.52 (m, 4H), 1.79 (m, 4H), 4.03 (t, J = 6.4 Hz 4H), 7.01 (d, J = 8.8 Hz, 4H), 8.04 (d, J = 8.8 Hz, 4H), 8.13 (d, J = 8.8 Hz, 4H), 8.28 (d, J = 8.8 Hz, 4H); ¹³C-NMR (100 MHz, CDCl₃): δ (ppm) 13.79, 19.19, 31.14, 68.06, 115.14, 115.54, 127.67, 128.60, 128.80, 128.92, 143.37, 162.43, 162.50, 165.47; FTIR (KBr, cm⁻¹): 3080, 2920, 1620, 1486, 1069, 840; TOF-Ms (m/z) [M + H]⁺ = 651.1227; Anal. Calcd for C₃₆H₃₄N₄O₆S: C, 66.45. H, 5.27. Found: C, 66.46. H, 5.34.

p,p’-Bis[2-(4-Hexoxyphenyl)-1,3,4-oxadiazol-5-yl]diphenylsulfone (1c): White powder, yield: 22%. Mp 282 °C (DSC). ¹H-NMR (400 MHz, CDCl₃): δ (ppm) 0.91 (t, J = 7.2 Hz, 6H), 1.38 (m, 24H), 1.83 (m, 4H), 4.05 (t, J = 6.4 Hz, 4H), 7.03 (d, J = 8.8 Hz, 4H), 8.06 (d, J = 8.8 Hz, 4H), 8.14 (d, J = 8.8 Hz, 4H), 8.29 (d, J = 8.8 Hz, 4H); ¹³C-NMR (100 MHz, CDCl₃): δ (ppm) 14.04, 22.63, 25.72, 29.12, 31.59, 68.44, 115.20, 115.59, 127.73, 128.67, 128.87, 128.98, 143.42, 162.49, 165.54; FTIR (KBr, cm⁻¹): 3080, 2924, 1622, 1486, 1069, 844; TOF-Ms (m/z) [M + H]⁺ = 707.1256; Anal. Calcd for C₄₀H₄₂N₄O₆S: C, 67.97, H, 5.99. Found: C, 67.99. H, 6.03.

4. Conclusions

A series of D-π-A-π-D type sulfone derivatives with alkoxy D groups and sulfone A units was produced. The experimental results demonstrate that these V-shaped compounds display the features of both ICT and AIE. These blue emitters are solution processible and exhibit good film-forming ability and high thermal stability, marking them promising as electron transport materials.