Synthesis and photophysical properties of amino-substituted benzodithiophene-based fluorophores

Abstract

An amino-substituted benzodithiophene was synthesized by a TiCl₄ promoted reaction. It was used to construct a series of acetylene-containing fluorophores, which exhibit intense fluorescence in solutions and solid states with large Stokes shifts. The emission colour can be modulated from green to red by changing the terminal groups. These fluorophores may have potential applications in optoelectronic devices and fluorescent sensors.

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Organic fluorescent molecules have attracted continuous attention in the past decades, due to their wide applications in electroluminescence,¹ biomedical imaging,² fluorescent sensor,³ and so on. The fluorophores emitted in solution are often used in biological science and analytical chemistry, and many of them are already commercially available.⁴ In recent years, motivated by the development of organic optoelectronic devices, it is strongly desired to search for the fluorophores with solid-state luminescence.⁵ Due to the aggregation-caused-quenching (ACQ) effect of fluorescent, it is much challenging to develop solid-state fluorophores with high quantum efficiency and desirable emission colors.⁶ What's more, the organic fluorophore structure can be readily modified through incorporation of functional group and thus the emission color can be carefully modulated.⁷ This characteristic helps to realize the full-color emission applied in organic optoelectronic devices.

Benzodithiophene (BDT) is one of the most powerful structures for the construction of conjugated materials for organic solar cells (OSCs).⁸ In order to further improve the photovoltaic performance, various groups including alkoxyl,⁹ alkylthio,¹⁰ thienyl¹¹ and so on, have been introduced into BDT moiety to modulate the energy levels and carrier mobilities. On the other hand, amino is an important functional group in designing organic optoelectronic material. The lone pair electrons on nitrogen atom can provide it with strong electron-donating characteristics. Some nitrogen-containing moieties such as triphenylamine (TPA)¹² and carbazole¹³ play important roles in the field of organic optoelectronic material. However, there is no report of incorporating nitrogen atom into BDT, which may exhibit unique optoelectronic properties.

We report herein a TiCl₄-promoted reaction to synthesize an amino-substituted BDT (NBDT) structure, which can be used to construct fluorescent molecules through introduction of electron-withdrawing alkynyl groups (Scheme 1). These NBDT-based fluorophores show intense fluorescence both in solutions and solid states as well as large Stokes shifts. The chemical structure-dependent photophysical properties are discussed in detail.

Scheme 1 Chemical structures of NBDT-based fluorophores.

Benzodithiophene unit is usually synthesized from dinone 1 with reductant of Zn or SnCl₂, which is applied to construct BDT with substituents of O, C or S.^9–11 For the amino substituted BDT structure, we used a TiCl₄-mediated reaction to synthesize the compound 2 (Scheme 2).¹⁴ This is a simple approach to introduce two amino groups to the 4, 8-positions of BDT, while the reaction yield is not very satisfying. After that, 2 was brominated by BuLi/CBr₄ to give 3, which was then coupled with trimethylsilylacetylene through Sonogashira reaction to give compound 4 in good yield. Compound 5 was obtained by treating 4 with KOH/MeOH to remove the TMS group. In order to extend the conjugation, phenyl was coupled to the terminal acetylene to give compound 6.¹⁵ And as to further modulate the emission color, electron accepting ester group was introduced to give compound 7. The compounds of 4–7 are soluble sufficiently in common organic solvents such as dichloromethane (CH₂Cl₂), ethyl acetate (EtOAc), tetrahydrofuran (THF) and so on.

Scheme 2 Chemical structures and synthetic routes to compounds 4–7.

Firstly, the UV-vis absorption and photoluminescence (PL) properties of 4–7 in cyclohexane solution were investigated (Fig. 1, Table 1). All the fluorophores show moderate absorption band at 336 nm to 363 nm (log ε ∼ 4.6). In the region over 380 nm, a weaker peak is observed for each UV spectrum, which is assigned to the intramolecular charge transfer (ICT) transition from HOMO to LUMO, as evidenced by the calculation results (Table S1†).¹⁶ Fluorophores 4 and 5 exhibit similar UV and PL spectra due to their similar structures. Compared to 4, the elimination of TMS group for 5 results in slightly blue-shift of absorption and emission spectra in cyclohexane. The fluorescent maxima of 514 nm and 502 nm with moderate quantum yields of 0.39 and 0.38 were obtained for 4 and 5, respectively. For conjugation extended 6 the emission peak is red-shifted to 531 nm as a yellowish green fluorescence with a quantum yield of 0.29. Compound 7 contains two electron-withdrawing ester groups, as a result the emission peak is further red-shifted to 575 nm with a low quantum yield 0.17. All the fluorophores exhibit large Stokes shifts of 107 nm to 158 nm in cyclohexane, which might be induced by the ICT effect.¹⁷

Fig. 1 UV-vis (a) and PL ((b), λ_ex = 365 nm) spectra of compounds 4–7 in cyclohexane.

Table 1 Photophysical properties of 4–7 in cyclohexane and films

Compound	λabs^a^,^b/nm	λem^c/nm (Φ)
	(εmax/dm^3 mol^−1 cm^−1)	Solution^b^,^d	Thin film^e^,^f	PS film^e^,^g
a Absorption maximum.b 1 × 10^−5 M in cyclohexane.c Emission maximum.d The Φ values were determined using quinine sulfate (Φ = 0.54) in 0.1 M H2SO4 as the standard.e Absolute quantum yield determined with a calibrated integrating sphere system.f Prepared by spin-coating from a solution in CH2Cl2.g Dispersed in a polystyrene (PS) film.h N/A stands for not available.
4	332 (33 182), 347 (54 598), 395 (15 806)	514 (0.39)	522 (0.12)	522 (0.56)
5	321 (24 096), 336 (37 798), 389 (12 199)	502 (0.38)	531 (0.11)	518 (0.48)
6	363 (47 995), 407 (21 599)	531 (0.29)	550 (N/A)^h	538 (0.37)
7	346 (29 284), 359 (38 896), 417 (9500)	575 (0.17)	643 (0.05)	594 (0.23)

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This series of fluorophores also show intense fluorescent in solid state. Their photophysical properties are summarized in Table 1. The fluorophores of 4 and 5 in thin films emit green light. The quantum yield is obtained in low values of about 0.10. Interestingly, 4 and 5 dispersed in PS films show stronger fluorescent with good quantum yield of about 0.50, which may be ascribed to the reduction of aggregation-caused quenching (ACQ) effect.¹⁸ Fluorophore 6 shows quantum yield of 0.37 in PS film, while it's not available in pure thin film, which is probably caused by strong quenching effect. The ester derivative 7 emits red light with quantum yield of 0.05 in thin film and 0.23 in PS film. All the fluorophores show stronger quantum yields in PS films than solutions and pure thin films, indicating PS was good medium for inhibition of fluorescent quenching.

Environment sensitive dyes are of special interest as probes. Compound 7 exhibits remarkable solvatochromism of fluorescence, as shown in Table 2 and Fig. S2.† The emission maxima are significantly red-shifted from 575 nm in cyclohexane to 654 nm in DMF. The bathochromic shift is highly dependent on the dielectric constant of the solvents.¹⁹ However, as for the absorption spectra, only a little shift is observed in the solvents with different polarity (Fig. S2†). As a result, it showed a large Stokes shift of 232 nm for compound 7 in DMF, which is potentially valuable for multiplexing experiments.

Table 2 Spectroscopic properties of fluorophore 7

Entry	Solvent (ε)	λabs^a/nm	λem^b/nm	Φ^c
a Absorption maximum.b Emission maximum.c The Φ values were determined using quinine sulfate (Φ = 0.54) in 0.1 M H2SO4 as the standard.
1	Cyclohexane (2.02)	346, 359, 417	575	0.17
2	CCl4 (2.24)	347, 361, 423	588	0.08
3	Toluene (2.37)	363, 423	611	0.06
4	EtOAc (6.02)	358, 418	620	0.06
5	THF (7.58)	360, 421	620	0.05
6	CH2Cl2 (9.08)	363, 424	633	0.06
7	DMF (37.6)	361, 422	654	0.01

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The nanostructures of the fluorophores were explored using confocal laser scanning microscope (CLSM). The samples were prepared by depositing the mixture solution of THF–H₂O (1 : 1) onto glass slides. As shown in Fig. 2, compounds 4 and 5 show amorphous structures. Compound 6 displays a uniform rod-like structure with the width of about 3 μm, which resulted from the ordered arrangement of molecule, as evidenced by the X-ray crystallography analysis. As for 7, a nanofiber structure is observed with red light fluorescence, which is consistent with its emission spectra.

Fig. 2 PL microscopy images of fluorophores 4 (a), 5 (b), 6 (c) and 7 (d). The scale bar is 20 μm.

To get insight into the photophysical performance, single crystal of 6 was obtained by slow evaporation from CH₂Cl₂/EtOAc and applied for X-ray crystallography analysis. The crystals of 6 are in the triclinic space group P1̄ with an inversion centre at the centroid of the benzene ring (Fig. 3). In the crystals, there is an intermolecular hydrogen bond between C–H of benzene ring and lone pair electrons of nitrogen (C–H⋯N, 2.749 Å), which connect the molecules to form a large coplanar structure. As a result, a well ordered nanostructure is formed, as shown in Fig. 3c and 2c. The amino group forms a large dihedral angle (∠C12–C13–N1–C14) of 88.4° to the central BDT unit. It is nearly perpendicular to the conjugated plane, which results in a large distance of 6.281 Å between adjacent planar layers and is beneficial to reduce the intermolecular stacking and prevent fluorescence quenching.

Fig. 3 (a) Ellipsoid-style molecular structure of 6. (b) Planar layers in the crystals. (c) Intermolecular hydrogen bonds in the crystals. Ellipsoids are set at 50% probability; hydrogen in (a) is omitted for clarity.

Conclusions

In summary, we have developed a series of new fluorophores based on the acetylene containing amino substituted benzodithiophene (NBDT) structure. These compounds show good fluorescence in solution, thin film and polystyrene film, and the Stokes shift is ranging from 107 nm to 158 nm in cyclohexane. The emissive colour can be modulated from green to red by modifying the functional-end groups on acetylene moiety. The ester terminated compound 7 shows obvious solvatochromism of the fluorescence, and the emission wavelength can be tuned from 575 nm in cyclohexane to 654 nm in DMF and large Stokes shift of 232 nm is observed. Compound 6 can form a uniform rod-like nanostructure, which is confirmed by X-ray crystallography. The interesting characteristics make them promising candidates for fluorescent materials. Further studies about the applications in sensing and optoelectronic devices are in progress.

Acknowledgements

This work was supported by National Natural Science Foundation of China (21202181, 21402219, 21274161, 51173199), and Department of Science and Technology of Shandong Province (ZR2012BQ021, ZR2011BZ007).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Details of the methods, synthesis and characterization of all compounds, computational data and crystal structural data for 6. CCDC 1030217. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra14186e

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