Synthesis, photophysics, ion detection and DFT investigation of novel cyano-substituted red-light chromophores with a triphenylamine donor

Abstract

A series of novel triphenylamine-based red-light chromophores with multiple electron-withdrawing cyano substituents were synthesized by the Knoevenagel condensation reaction and characterized in detail. Compounds 3–5 showed bright green-yellow emission in dichloromethane solution and red-light emission in the solid state, respectively. The interesting solvatochromic behavior in different polar solvents was observed, varying from positive solvatochromism for compound 3 to negative solvatochromism for compound 5. In addition, the notable optical response of metal ions in DMF solution for the cyano-substituted chromophores was investigated. Especially, with the addition of Hg²⁺ ions, the blue shift in the absorption spectra and the decrease in the emission spectra suggested that the new metal complexes possibly formed between the cyano substituents and metal ions or metal ion-induced optical quenching happened. Density functional theory calculations were used to further understand the effect of the cyano substituents on the photoelectron properties of the donor–acceptor molecules.

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Introduction

Since a PPV-based light-emitting diode (LED) was reported in 1990 by Burroughes and Friend, PPV derivatives, containing polymer molecules and oligo(phenylene vinylene), have attracted considerable attention due to their potential extensive applications, such as in organic light-emitting devices,^1–3 solar photovoltaics,^4,5 field-effect transistors and biochemical sensors,^6,7 and so on. In the past decades, the remarkable progress in the design and development of organic photoelectron materials was made by regulating the π-conjugated systems or the side-chain substituents on the molecular backbone.^8–11

Recently, triphenylamine (TPA) has widely been introduced in the design and preparation of organic photoelectron materials due to its strong electron-donating characteristic and modifiable molecular structure. Therefore, organic compounds with a donor–π-conjugated bridge–acceptor (D–π–A) structure have attracted considerable interest for their broad absorption from intramolecular charge transfer (ICT) in the visible region and their lower LUMO levels.^12–15 For example, two star-shaped D–π–A molecules with TPA as the donor, dicyanovinyl as the acceptor, and 4,4′-dihexyl-2,2′-bithiophene or 4,4′-dihexyl-2,2′-bithiophene vinylene as the π-bridge reported by J. Zhang et al.¹⁶ possessed a broad absorption from 296 to 585 nm and a lower LUMO level of −3.42 eV. In addition, the optical properties of the molecules with electron-accepting and donating units could be easily tuned via controlling the extent of intramolecular charge transfer (ICT). Thus, introducing stronger electron-withdrawing groups was preferable to induce efficient ICT. The cyano group as a well-known strong electron acceptor has been widely used for red-light chromophores,¹⁷ which could promote efficient ICT and enhance the exciton dipole in the excited state.^18–20 In particular, introducing multi-cyano groups into a conjugated system resulted in a stronger electron-accepting effect.^21–23 So far, the TPA-based compounds with a D–π–A structure and multi-cyano groups as the electron acceptor have been reported. For example, the new TPA-based compounds with tetracyanoethylene groups reported by M. Grigoras showed board absorption over the solar spectrum,²⁴ and were desirable candidates for bulk heterojunction and dye-sensitized solar cells. Y. Yang et al. reported the multi-branched TPA derivatives with dicyanovinyl as the electron acceptor, presenting outstanding solvatochromic behaviors.²⁵ Besides, the optical probe of the TPA-based chromophores was recently reported as being due to the coordination of metal ions by cyano groups.²⁶ Consequently, the introduction of cyano substituents on the TPA-based derivatives could finely tune the electron energy levels.

In this report, the synthesis, photophysical performances and electron structures of novel D–π–A molecules with TPA as the donor, cyano groups as the acceptor, and ethynylbenzene units as the π-conjugated linkage were investigated systematically. Cyano substituents on the conjugated system could facilitate tuning of the optical properties. These compounds are red-emitting in the solid state as expected, and possess a broad visible absorption band. Furthermore, theoretical calculation was performed to further understand the effect of the cyano substituents on the TPA-based molecules on the optical properties.

Experimental section

Materials

Triphenylamine (TPA), potassium tert-butoxide (t-BuOK), N-bromosuccinimide (NBS), CuCN, malononitrile, and metal salts (Hg(CH₃COO)₂, SnCl₂·2H₂O, and so on) are commercial products from Aldrich and were directly used without further purification. Solvents such as triethylamine, dimethylformamide (DMF), phosphorus oxychloride (POCl₃) and tetrahydrofuran (THF) were purchased from commercial sources. Compound 1,4-dibromo-2,5-bis(diphenylaminophenyl) benzene (1) was synthesized according to the reported literature.²⁷

General characterization

¹H and ¹³C NMR spectra were recorded with a Bruker Avance III 600 MHz spectrometer. ESI-TOF MS data were obtained from a Bruker esquire 2000 mass system. The infrared (IR) spectra were measured on a Shimadzu FTIR-8400 spectrometer with KBr pellets. X-ray diffraction (XRD) was carried out on an XD-3 powder diffractometer (Purkinje General Instrument Co., Beijing, China). Scanning electron microscopy (SEM) images were observed by an S4800 (Hitachi, Japan) at 20 kV. The thermal behaviors of these compounds were analyzed with a TA SDTQ600 thermal gravimetric analyzer (USA) and differential scanning calorimetry (DSC, model 200F3-Maia, Germany) under a dry nitrogen gas flow at a heating rate of 10 °C min⁻¹. Ultraviolet-visible (UV-vis) absorption and emission spectra in solution, a thin film and powder were performed on a UV-2550 spectrometer (Shimadzu, Japan) and F-7000 fluorescence spectrometer (Hitachi, Japan). The fluorescence quantum yield of these compounds in different polar solvents at room temperature was recorded using a FL-TCSPC time resolved fluorescence spectrometer (Horiba, Jobin Yvon Inc, France). The luminescence decay lifetimes were recorded using a fluorescence lifetime spectrometer (FL-TCSPC, Horiba, Jobin Yvon Inc, France). Cyclic voltammetry was recorded on a CHI-660D electrochemical workstation (Chenghua Co., China) with platinum wire as the counter electrode, Pt as the working electrode and a saturated calomel electrode as the reference electrode. Theoretical calculation was performed with time dependent density functional theory (TD-DFT, B3LYP/6-31G (d)) on the Gaussian 09 quantum chemical program package.

Synthesis of triphenylamine-malononitrile derivatives 2–5

4,4((((1E,1′E)-2,5-Dibromo-1,4-phenylene)bis(ethene-2,1-diyl)bis(4,1-phenylene))-bis(phenylazanediyl))dibenzaldehyde (compound 2). POCl₃ (2.4 mL, 25.75 mmol) was dropwise added at 0 °C to 1,4-dibromo-2,5-bis(diphenylamino-phenyl)benzene (1.6 g, 2.06 mmol) in dry DMF solution, and the mixture was heated and kept at 90 °C for 12 h. Then POCl₃ (1.2 mL, 12.88 mmol) was added again and reacted for 12 h. After the reaction, the mixture was cooled to room temperature, poured into ice water, and neutralized with aqueous 2 M NaOH solution in turn. And then the solvent was removed under a reduced pressure. The residue was washed with distilled water and separated by column chromatography (silica gel, petroleum ether/EtOAc, 6 : 1, v/v). Finally, the product was obtained as a yellow powder (1.43 g, 83% yield). ¹H NMR (600 MHz, CDCl₃): δ (ppm): 9.84 (s, 2H), 7.87 (s, 2H), 7.72 (d, J = 6.0 Hz, 4H), 7.52 (d, J = 8.4 Hz, 4H), 7.38–7.35 (m, 4H), 7.33 (d, J = 16.2 Hz, 2H), 7.2–7.19 (m, 6H), 7.17 (d, J = 9 Hz, 4H), 7.10 (d, J = 6 Hz, 4H), 7.05 (d, J = 16.2 Hz, 2H). ESI-TOF MS: calcd for C₄₈H₃₄Br₂N₂O₂ 828.1; found: 828.5. (Scheme 1)

Scheme 1 Synthetic routes for compounds 3–5.

2,5-Bis(4-((4-formylphenyl) (phenyl)amino)styryl)terephthalonitrile (compound 3). A mixture of compound 2 (0.5 g, 0.6 mmol), and CuCN (0.14 g, 1.5 mmol) was dissolved in 50 mL DMF under nitrogen atmosphere and stirred at 155 °C for 48 h. The reaction mixture was cooled to room temperature (RT), poured into water, filtered, and washed with ammonia (15%) and water in turn. And then, the residue was purified with column chromatography (silica gel, petroleum ether/EtOAc, 6 : 1, v/v). Finally, compound 3 was obtained as a yellow powder (0.32 g, 73% yield). ¹H NMR (600 MHz, CDCl₃): δ (ppm): 9.88 (s, 2H), 8.05 (s, 2H), 7.77 (d, J = 8.4 Hz, 4H), 7.57 (d, J = 9 Hz, 4H), 7.42–7.39 (m, 4H), 7.32 (d, J = 6 Hz, 4H), 7.22 (d, J = 6 Hz, 2H), 7.21–7.2 (m, 8H), 7.15 (d, J = 8.4 Hz, 4H). ¹³C NMR (150 MHz, CDCl₃): δ (ppm): 190.4, 152.7, 147.5, 145.9, 138.9, 134.2, 131.5, 131.3, 130.2, 130.0, 129.7, 128.7, 126.6, 125.6, 125.2, 121.1, 120.9, 116.6, 114.9. ESI-TOF MS: calcd for C₅₀H₃₄N₄O₂ 722.2; found: 722.7. FT-IR (KBr): ν (cm⁻¹): 3032 (=C–H), 2221 (–C≡N), 1686 (C=O), 1585, 1581 (C=C), 1285 (C–N), 1319, 1165 (C–CN), 964, 822, 698 (C–H).

2,2′-(4,4′-(4,4′-(1E,1′E)-2,2′-(2,5-Dicyano-1,4-phenylene)bis(ethene-2,1diyl)bis(4,1-phenylene))bis(phenylazanediyl)bis(4,1-phenylene))bis(methan-1-yl-1-ylidene) dimalononitrile (compound 4). To compound 3 (0.32 g, 0.44 mmol) in anhydrous DCM (30 mL) was added malononitrile (0.64 g, 0.97 mmol) at RT and 5 drops of triethylamine were added. After being stirred for 24 h at RT, the reaction mixtures were concentrated and purified with column chromatography (silica gel, petroleum ether/EtOAc, 6 : 1, v/v) to obtain compound 4 as a red powder (0.25 g, 69% yield). ¹H NMR (600 MHz, CDCl₃): δ (ppm): 7.91 (s, 2H), 7.80 (d, J = 3 Hz, 4H), 7.58 (d, J = 7.2 Hz, 4H), 7.56 (s, 2H), 7.44–7.42 (m, 4H), 7.38 (d, J = 16.2 Hz, 2H), 7.24–7.21 (m, 6H), 7.22 (d, J = 8.4 Hz, 4H), 7.06 (d, J = 9 Hz, 2H), 7.04 (d, J = 9 Hz, 4H). ¹³C NMR (150 MHz, CDCl₃): δ (ppm): 157.8, 152.8, 146.4, 145.0, 134.1, 132.9, 130.2, 129.8, 128.8, 128.7, 128.5, 126.9, 126.5, 126.2, 126.0, 123.8, 121.8, 119.8, 119.5, 115.1, 113.8. ESI-TOF MS: calcd for C₅₆H₃₄N₈ 818.2; found: 817.7. FT-IR (KBr): ν (cm⁻¹): 3032 (=C–H), 2218 (–C≡N), 1568, 1500, 1323 (C=C), 1292 (C–N), 1184 (=C–CN), 960, 822, 694 (C–H).

2,2′-(4,4′-(4,4′-(1E,1′E)-2,2′-(2,5-Dibromo-1,4-phenylene)bis(ethene-2,1-diyl)bis(4,1-phenylene))bis(phenylazanediyl)bis(4,1-phenylene))bis(methan-1-yl-1-ylidene) dimalononitrile (compound 5). To a solution of compound 2 (0.5 g, 0.6 mmol) in anhydrous DCM (30 mL) was added malononitrile (0.87 g, 1.32 mmol) at RT to which was added 8 drops triethylamine. After being stirred at 24 h at RT, the reaction mixtures were concentrated and purified with column chromatography (silica gel, petroleum ether/EtOAc, 6 : 1, v/v) to yield compound 5 as a red powder (0.38 g, 68% yield). ¹H NMR (600 MHz, CDCl₃): δ (ppm): 7.91 (s, 2H), 7.80 (d, J = 8.4 Hz, 4H), 7.58 (d, J = 6.6 Hz, 4H), 7.57 (s, 2H), 7.44–7.42 (m, 4H), 7.38 (d, J = 16.2 Hz, 2H), 7.24–7.21 (m, 6H), 7.23 (d, J = 1.2 Hz, 4H), 7.06 (d, J = 9 Hz, 4H), 7.05 (d, J = 6 Hz, 2H). ¹³C NMR (150 MHz, CDCl₃): δ (ppm): 157.8, 153.1, 145.4, 145.1, 137.4, 134.1, 133.0, 131.2, 130.4, 130.1, 128.4, 126.8, 126.4, 126.3, 126.0, 123.4, 123.1, 119.3, 115.0, 113.9. ESI-TOF MS: calcd for C₅₄H₃₄Br₂N₆ 922.1; found: 921.6. FT-IR (KBr): ν (cm⁻¹): 3032 (=C–H), 2218 (–C≡N), 1566, 1497, 1323 (C=C), 1296 (C–N), 1184 (=C–CN), 1053, 814, 694 (C–H).

Results and discussion

Thermal properties and morphology

In order to evaluate the thermal properties of compounds 3–5, thermo-gravimetric analysis (TGA) and differential scanning calorimetry (DSC) were carried out under a nitrogen atmosphere and the corresponding data are summarized in Table 1. As shown in Fig. S1,† the TGA results exhibited that the degradation temperatures (T_d) of compounds 3–5 were 350 °C, 390 °C and 390 °C (5% weight loss), respectively, and the higher degradation temperatures of compounds 4 and 5 were attributed to multi-cyano end substitutes which resulted in a strong intramolecular interaction. It demonstrated that the thermal stability of the compounds was regulated by the amount and position of the cyano substituents. In addition, DSC (Fig. S2†) analysis indicated that the compounds only showed a relatively weak endothermic step-transition in the scanning region, which could be attributed to the glass phase transition. The glass transition temperature of compounds 3–5 was 111 °C, 116 °C and 108 °C, respectively. From the above data it is obvious that the cyano-substituted compounds have a certain amorphous characteristic.

Table 1 The photophysical data and thermal properties of compounds 3–5

Compound	λabs/nm (sol)	λem/nm (sol)	λabs/nm (film)	λem/nm (film)	λem/nm (solid)	Tg/°C	Td/°C	τ^a/ns
a Luminescence decay lifetimes of compounds 3–5 were measured in dichloromethane solution.
3	439	542	458	572	576	111	350	2.68
4	469	557	497	609	619	116	390	1.58
5	463	539	522	608	623	108	390	1.86

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In order to further evaluate the morphological nature of the compounds in the solid state, the morphology investigations have been performed using powder X-ray diffraction (XRD) analysis. As shown in Fig. S3,† a series of sharp peaks for compound 3 were observed due to the restriction in the molecular ordered aggregation and crystallinity by the terminal aldehyde groups.²⁸ Meanwhile, compound 4 shows a sharper peak, which demonstrated that end multi-cyano substituted molecules showed a crystallization tendency in the solid state. However, compound 5 displays a series of more intensive peaks, compared to those of the first two compounds, demonstrating the difference in the crystal phase or intermolecular interaction in the solid state. Furthermore, the XRD profile of compound 4 shows one strong peak (d = 1.022 nm) in the small-angle region except for several weak peaks in the wide-angle region (Fig. S3†). Compound 3 exhibits similar XRD profiles, which consist of one strong peak (d = 0.883 nm) in the small-angle region and a series of relatively strong peaks in the wide-angle region, indicating the similar packing mode of compound 4. In contrast, the XRD profile of compound 5 exhibits a series of relatively strong peaks in the small- and wide-angle regions, with one strongest peak in the small-angle region (d = 1.061 nm) and wide-angle region (d = 0.408 nm), respectively.

Photophysical properties

The UV-vis absorption and photoluminescence spectra of compounds 3–5 in dichloromethane solution are displayed in Fig. 1 and the corresponding photophysical data are summarized in Table 1. All novel synthesized chromophores, containing two electron-donating TPA terminals and a bis(styryl)benzene backbone, presented two distinct absorption bands around 260 nm and 470 nm. The absorption around 260 nm was attributed to a π–π* or n–π* transition on the TPA donor, while the absorption around 470 nm was assigned to the intramolecular charge transfer (ICT) from the TPA donor to the centre bis(styryl)benzene backbone acceptor except for the contribution of the cyano substituents. In addition, it should be noted that a weak absorption at about 320 nm was observed for the compounds, probably due to the combined effect of the π–π* transition of the molecular backbones and the electron-withdrawing cyano substituents on the TPA parts.²⁹

Fig. 1 Normalized UV-vis absorption and emission spectra of compounds 3–5 in dichloromethane solution (1 × 10⁻⁵ M).

In addition, the absorption spectra of the compounds show a red shift compared with those of their TPA-based analogues without cyano substituents reported in the literature (410 nm).³⁰ The lower energy absorption band for compound 3 was around 439 nm. Compounds 4 and 5 presented a more remarkable red shift in the absorption maximum than compound 3 (30 nm and 24 nm, respectively), probably attributed to the stronger influence of the cyano substituent on the terminal TPA parts in comparison with those on the center benzene ring. All of the compounds displayed a relatively strong green-light emission in solution. An obvious red shift emission of compound 4 was attributed to the combined effect of the cyano substituents on different benzene rings compared with compounds 3 and 5. In addition, compounds 4 and 5 showed lower PL quantum efficiencies, which suggested that the effect of the terminal cyano substituents on the TPA units may result in more non-radiative transitions of the compounds (Table 1).

Compared with the UV-vis absorption and emission spectra in solution, compounds 3–5 in the thin film exhibited a larger red shift, and broad and structureless absorption bands, demonstrating strong intermolecular π–π* interaction or aggregation in the solid state (Fig. 2). Compound 3 showed bright orange fluorescence (572 nm) in the thin film, while compounds 4 and 5 showed red emission at 609 and 608 nm in the thin film. In addition, red fluorescence for compounds 4 and 5 was observed at 619 and 623 nm in the powder state, respectively, indicating a more remarkable red shift, which demonstrated a much stronger intermolecular π–π* interaction or aggregation in the powder state than in the film (Fig. S4 and S5†).

Fig. 2 UV-vis absorption and PL emission spectra of compounds 3–5 in the thin film.

Moreover, the concentration dependence of the emission of the compounds in dichloromethane solution was also investigated (Fig. 3 and S6†). As shown in Fig. 3, the emission spectra of compound 5 shows a notable red shift except for the concentration quenching on increasing the solution concentration. It demonstrated that the intermolecular π–π* interaction or aggregation was present in a high solution concentration.

Fig. 3 The concentration dependence of the emission spectra of compound 5 in dichloromethane solution (mol L⁻¹).

The solvatochromism of the novel red compounds was assessed in different polar solvents. As shown in Fig. S7,† the absorption maxima of these TPA derivatives showed a slightly negative solvatochromism with the increase of solvent polarities. In other words, compound 3 exhibited a blue shifted absorption with increasing the solvent polarity, shorter wavelength absorption maxima in DMF (435 nm) and a longer wavelength in TOL (445 nm) (Table 2). Similarly, compounds 4 and 5 showed red shifted absorption maxima in non-polar TOL solvent (464 and 458 nm) and blue shifted absorption maxima in polar DMF solvent (460 and 436 nm), respectively. The results suggested that the dipole moment of the molecules in the excited state was decreased due to the effect of the electron-withdrawing cyano substituents.

Table 2 Data of optical properties of compounds 3–5 in different polarity solvents

Solvent	3				4				5
	λabs/nm	λem/nm	Δν/cm^−1	ΦF^a/%	λabs/nm	λem/nm	Δν/cm^−1	ΦF^a/%	λabs/nm	λem/nm	Δν/cm^−1	ΦF^a/%
a Absolute quantum yields ΦF were determined in the absorption maximum (10^−6 M L^−1) corresponding to the solvents.
Toluene	445	515	3054	0.765	464	544	3169	0.171	458	560	3977	0.069
CH2Cl2	439	542	4329	0.732	469	556	3336	0.014	463	539	3045	0.01
THF	437	538	4296	0.703	462	543	3229	0.017	456	524	2846	0.012
EA	434	535	4350	0.707	459	557	3833	0.018	455	535	3286	0.012
DMF	435	556	5003	0.619	460	559	3850	0.28	436	545	4587	0.16

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Furthermore, the emission solvatochromism of the compounds was examined with the increase of solvent polarity from a non-polar solvent (toluene) to a polar solvent (DMF) (Fig. 4 and S8†). Interestingly, varying from a non-polar solvent to a polar solvent, the emission spectra of compound 3 show a progressive red shift (515 nm in TOL solutions and 556 nm in DMF solutions) and emission broadening (Fig. 4a), indicating that the intramolecular charge transfer (ICT) occurred in the excited state from the electron donor to the electron acceptor. However, compound 4 only shows a weaker positive solvatochromism with the increase of solvent polarity. This suggested that the combined effect of the cyano substituents on the different positions weakened the ICT abilities in the excited state (Fig. 4b). In contrast, compound 5 indicated an obviously negative solvatochromism, with a blue shift of 36 nm as the solvent polarity varied from a non-polar solvent to a polar solvent (Fig. 4c). The result implied that the solvent polarity-induced distortion of the molecular planarity restricted the intramolecular charge transfer transition, resulting in the blue shift emission.³¹

Fig. 4 Normalized emission spectra of compounds 3–5 in different polar solvents and solvatochromic shift.

The linear variety of the Stokes shift of compounds 3–5 to the Lippert–Mataga solvent polarity parameter, Δf,³² is plotted in Fig. 4d. Compound 3 showed an obvious increase of Stokes shift along with an increase of Δf, suggesting that it had a strong ICT characteristic in the excited state. For compound 4, the fit line relationship between the Stokes shift and Δf has a comparably gentle slope and exhibited a slight deviation in the experimental data. This suggested that compound 4 underwent a small dipolar change in the excited process from a non-polar to polar solvent. In contrast, the slope value of the line relationship for compound 5 was negative, which demonstrated that compound 5 had a negative solvatochromism. In other words, the ICT character of compounds 3–5 could be tuned by the terminal cyano groups.

The fluorescence quantum yields (Φ_F) of compounds 3–5 were studied in different solvents to get a deep understanding of the solvatochromism behavior. Φ_F of compound 3 was observed to be as high as 0.765 in the low polar solvent of toluene (Table 2), and gradually decreased with the increase in polarity, indicating that a strong solvent polarity could result in an increase in the non-radiative transition. However, compounds 4 and 5 exhibited a relatively higher Φ_F in high polar solvents, while only a lower Φ_F was observed in a non-polar solvent, demonstrating that the cyano substituents on the TPA units were not bad for Φ_F.

The luminescence decay lifetimes of compounds 3–5 were measured in dichloromethane solution (Table 1). Compound 3 (2.68 ns) exhibited a relatively higher luminescence decay lifetime than that of compounds 4 and 5 (1.58 ns and 1.86 ns), demonstrating that introducing the cyano substituents to the TPA units led to an enhanced ICT character which changes the decay of the singlet excited states. Actually, the high strength ICT led to the increase of intramolecular rotation and the distortion which increased the energy loss of the non-radiative transition, and thus the luminescence decay lifetime was decreased.

Metal ion detection

The cyano substituents on the compounds were able to form a coordination complex with the metal ions.³³ Therefore, in order to explore the potential application of these compounds, the effect of the addition of metal ions on the optical properties of compounds 3–5 was recorded in DMF solution (Fig. S9–S11†). The results indicated that the presence of the metal ions, including Ba²⁺, Co²⁺, Cr³⁺, Cu²⁺, Hg²⁺, Pb²⁺, Sn²⁺ and Zn²⁺, could cause the change in the absorption and emission spectra, deviating from the initial positions. Compounds 4 and 5 showed a hypsochromic shift in the absorption spectra to a certain extent and an increase or a decrease in the emission spectra with the addition of metal ions.

The metal ion Hg²⁺ was chosen as an example to further assess the influence of metal ions on the absorption and emission spectra of compounds 3–5. The concentration of compounds 3–5 was fixed at 1 × 10⁻⁵ M L⁻¹ and the metal ion concentration varied from 1 × 10⁻⁴ to 9 × 10⁻⁴ M L⁻¹. The UV-vis absorption and PL spectra were recorded immediately after the addition of metal ions (Fig. 5 and S12†). The absorption and emission maximum of compound 3 in DMF solution only displayed a gradual decrease without a notable red shift with the addition of Hg²⁺ up to the concentration of 9 × 10⁻⁴ mol L⁻¹, which may be attributed to the Hg²⁺ ion-induced decrease in the ICT band.

Fig. 5 Absorption spectra of compounds 3–5 in DMF (1 × 10⁻⁵ M L⁻¹) with the addition of Hg²⁺in DMF solution (1 × 10⁻⁴ M L⁻¹).

In the case of cyano-substituted TPA derivatives, the absorption spectra of compounds 4 and 5 show a remarkable hypsochromic shift and a new peak appeared at around 330 nm after Hg²⁺ ions were added. Simultaneously, with increasing the Hg²⁺ ions, the peak intensity of the absorption spectra decreased. The results may be attributed to a result of ICT between the electron-withdrawing cyano and electron-donating TPA units induced by Hg²⁺ ions, or to the new coordinative complex possibly formed between the cyano groups and Hg²⁺ ions. In addition, the presence of Hg²⁺ ions had a certain influence on the emission intensity, which presented firstly an increase, and then a decrease as the Hg²⁺ ions increased.

Electrochemical properties

The electrochemical behaviors of compounds 3–5 were studied using cyclic voltammetry (CV) in dimethylformamide (DMF) in the presence of tetrabutylammonium perchlorate (TBAP) as a supporting electrolyte (Fig. 6, Table 3 and Table S1 in the ESI†). The CV curves of the compounds exhibit an irreversible oxidation process in the positive scanning region and several successive irreversible reduction processes in the negative scanning region, which were assigned to the oxidation of the TPA units and the reduction of the combination of the conjugated distyrylbenzene core and cyano substituents, respectively. The anodic scanning shows an onset oxidation potential of 0.97 V for compound 3, 0.99 V for compound 4 and 0.96 V for compound 5. In contrast, compounds 3–5 show an onset reduction potential of −1.38, −1.30 and −1.36 V, respectively. According to equations reported in the literature,³⁴ the energy levels of the highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), and the electrochemical band gap (E_g) of compound 3 were estimated to be −5.67, −3.32 and 2.35 eV, respectively. Similarly, for compound 5, the HOMO, the deeper LUMO and the corresponding E_g were obtained at −5.66 eV, 3.34 eV and 2.32 eV, respectively. Meanwhile compound 4 possessed deeper HOMO (−5.69 eV) and LUMO (−3.40 eV) levels, resulting in a narrower band gap (2.29 eV), due to the presence of more electron-withdrawing cyano substituents on the TPA parts. The electrochemical band gaps of compounds 3–5 were in agreement with the optical measurements. Consequently, the electron structures of the synthesized compounds succeeded in being fine-tuned by the cyano substituent on the TPA-based derivatives.

Fig. 6 Cyclic voltammetry of compounds 3–5 in DMF solution at a scanning rate of 100 mV s⁻¹.

Table 3 The electrochemical data and energy levels of compounds 3–5

Compound	CV/V		Energy^a/eV − exp			E^optg^b
	Eox/onset	Ered/onset	EHOMO	ELUMO	Eg
a Electronic structures were determined from the formula: EHOMO = −(Eox + 4.7) (eV) and ELUMO = −(Ered + 4.7) (eV).b E^optg = 1240/λ (absorption band edge).
3	0.97	−1.38	−5.67	−3.32	2.35	2.37
4	0.99	−1.30	−5.69	−3.40	2.29	2.15
5	0.96	−1.36	−5.66	−3.34	2.32	2.15

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DFT investigation

In order to further clarify the experimental results, the space geometries of compounds 3–5 were optimized as shown in Table 4, and the corresponding parameters are summarized in Table 5. Compounds 3–5 showed a slight twist along the conjugated distyrylbenzene skeleton, respectively (Table 5). The small twist dihedral angles along C₅–C₆–C₉–C₁₀ and C₉–C₁₀–C₁₉–C₂₀ for compound 3 were 2.20° and 2.67°, respectively. Meanwhile compound 4 presented smaller twist dihedral angles (1.10° and 2.01°) along C₅–C₆–C₉–C₁₀ and C₉–C₁₀–C₁₉–C₂₀ due to the combined influences of the cyano substituents in the middle and terminal regions. Interestingly, larger twist dihedral angles along C₅–C₆–C₇–C₈ and C₁₆–C₁₅–C₈–C₇ for the terminal cyano-substituted compound 5 were 13.93° and 5.57°, in agreement with the optical characteristics. Moreover, the three phenyl rings on the TPA group displayed different twist dihedral angles to each other. The twist dihedral angles among the three rings on TPA decreased from −73.80° to −79.64° along C₂₃–C₂₄–C₂₆–C₂₈ (for compounds 3 and 4) or C₁₉–C₂₀–C₂₂–C₂₄ (for compound 5). Meanwhile the twist dihedral angles increased from −63.10° to −61.26° along C₂₁–C₂₄–C₃₂–C₃₅ (for compounds 3 and 4) or C₁₇–C₂₀–C₂₈–C₃₁ (for compound 5) and −71.24° to −67.80° along C₂₇–C₂₆–C₃₂–C₃₃ (for compounds 3 and 4) or C₂₃–C₂₂–C₂₈–C₂₉ (for compound 5), respectively. Similar alteration in the plane angle for compounds 3–5 is given in Table 5. Thus, such varieties further confirmed the effect of the electron-withdrawing cyano substituents on the molecular space geometries.

Table 4 The electronic structure data by the theoretical calculation of compounds 3–5

Compound	EHOMO	ELUMO	Steric structure
3
4
5

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Table 5 Optimized geometry parameters of bond lengths, bond angles and dihedral angles for compounds 3–5 (bond lengths are in Å, and bond angles and dihedral angles are in degrees)

3		4		5
Bond length
O39–C38	1.22	N92–C90	1.16	C86–N88	1.16
C38–C37	1.47	C90–C89	1.43	C86–C85	1.43
C32–N25	1.41	C89–C38	1.37	C85–C34	1.37
C26–N25	1.43	C38–C37	1.44	C34–C33	1.44
C24–N25	1.42	C32–N25	1.40	C28–N21	1.40
C10–C19	1.46	C26–N25	1.43	C22–N21	1.43
C9–C10	1.35	C24–N25	1.42	C20–N21	1.42
C6–C9	1.46	C10–C19	1.46	C15–C8	1.46
C2–C7	1.43	C9–C10	1.35	C8–C7	1.35
C7–N8	1.16	C6–C9	1.46	C7–C6	1.46
		C2–C7	1.43	C2–Br95	1.92
		C7–N8	1.16
Bond angle
O39–C38–C37	124.91	C90–C89–C38	125.18	C86–C85–C84	125.18
C24–N25–C26	119.01	C89–C38–C37	131.70	C34–C33–C32	117.33
C26–N25–C32	119.75	C24–N25–C26	118.34	C20–N21–C22	118.24
C24–N25–C32	121.24	C26–N25–C32	120.09	C22–N21–C28	120.40
C9–C10–C19	126.95	C24–N25–C32	121.57	C20–N21–C28	121.36
C6–C9–C10	126.40	C9–C10–C19	126.87	C7–C8–C15	126.72
		C6–C9–C10	126.38	C6–C7–C8	126.04
Dihedral angle
C23–C24–C26–C28	−73.80	C23–C24–C26–C28	−78.71	C19–C20–C22–C24	−79.64
C21–C24–C32–C35	−63.10	C21–C24–C32–C35	−61.26	C17–C20–C28–C31	−62.31
C27–C26–C32–C33	−71.24	C27–C26–C32–C33	−69.32	C23–C22–C28–C29	−67.80
C9–C10–C19–C20	2.67	C9–C10–C19– C20	2.01	C16–C15–C8–C7	5.57
C5–C6–C9–C10	2.20	C5–C6–C9–C10	1.10	C5–C6–C7–C8	13.93

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The HOMO and LUMO for compounds 3–5 are reported in Table 4. The HOMO was delocalized over the entire molecule, while the LUMO was mainly localized on the center conjugated moieties through the π-bridge. In addition, the LUMO for compounds 4 and 5 extended the terminal substituted cyano branches on the TPA moieties. Especially, for compound 5, the terminal substituted cyano group presented a stronger influence on the LUMO, which further verified that the charge distribution on such molecules was extremely influenced by the terminal cyano groups. The results would lead to the difference in the photophysical properties among compounds 3–5 when the intramolecular charge transfer happened during an excited process.

The HOMO energy levels (E_HOMO) and LUMO energy levels (E_LUMO) are estimated in Table 4. E_HOMO and E_LUMO for compound 3 were −5.28 and −3.01 eV, respectively. Introducing the electron-withdrawing cyano group on the benzene ring resulted in a remarkably low E_LUMO with an increasing number of cyano groups. Compared with compound 3, introducing dicyanovinyl arms on the TPA parts for compound 4 resulted in lower E_HOMO and E_LUMO energies. However, in comparison with compound 4, compound 5 exhibited a slight increase in E_HOMO and E_LUMO due to the absence of cyano substituents on the phenyl core, which was in agreement with the electrochemical experiment. In other words, the cyano substituents on the TPA-based derivatives could finely regulate the electron structures, resulting in the red-light properties.

Conclusions

In this work, three new TPA-based chromophores with electron-withdrawing cyano substituents were synthesized. Their relevant characterizations were performed by TGA, DSC, UV-vis, PL spectroscopy and cyclic voltammetry. Compounds 3–5 showed bright green-yellow emission in dichloromethane solution and red-light emission in the solid state, respectively. Interestingly, compounds 3 and 4 showed a significant positive solvatochromism and a weaker positive solvatochromism with increasing the polarity of the solvent, respectively, while a negative solvatochromism for compound 5 was observed, due to the effect of terminal cyano substituents. Furthermore, the compounds showed an optical response to metal ions. Especially, with a change of Hg²⁺ concentration, the blue shift in the absorption spectra and the decrease in the emission intensity demonstrated that the compounds were potential materials for highly selective and sensitive optical sensors. The electrochemical characterization and DFT calculation validated the fine regulation in the HOMO and LUMO from the variety of cyano substituents. In addition, DFT calculation revealed the essence that the cyano substituents regulated the photophysical properties, electrochemical characteristics and electron structures of the donor–acceptor molecules.

Acknowledgements

We gratefully acknowledge the Fundamental Research Funds for the Central Universities (XDJK2015C020) for the financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Electrochemical parameters, thermal properties, solid morphology, photophysical properties, and metal ion detection of the compounds. See DOI: 10.1039/c5ra24341f

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