Turn-on-type emission enhancement and ratiometric emission color change based on the combination effect of aggregation and TICT found in the hexaazatriphenylene-triphenylamine dye in an aqueous environment

Abstract

Turn-on-type emission enhancement and ratiometric emission color change in star-shaped donor–acceptor dyes in an aqueous medium are reported. In a THF/water medium, the hexaazatriphenylene dye, bearing six triphenylamine moieties 2, indicated that the emission was significantly quenched in low water volumes, but was recovered and enhanced in high water volumes. This turn-on-type emission enhancement is attributed to aggregate formation, which restricts emission quenching. In addition to emission enhancement, an emission color change was observed in another donor–acceptor dye 1 bearing an additional benzene spacer between the hexaazatriphenylene and triphenylamine moieties. The initial orange light emission at 0% water fraction changed to a green light emission at 40–90%, with the quenching of a red light emission at 10–30%. This unusual double emission change was attributed to the combination effect of aggregation and twisted intramolecular charge-transfer. In contrast, both turn-on-type and ratiometric emission changes were not found in dyes 3 and 4, which lacked triphenylamine donor moieties.

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Introduction

Organic fluorescence dyes have attracted much interest due to their potential applications in materials science, including as light-emitting diodes and solid lasers, as well as their biological applications, including as sensors and probes.^1–4 In biological applications, an attractive change in emission behavior is essential to achieve turn-on-type detection techniques based on emission enhancement and ratiometric detection techniques involving emission color changes. Photo-induced electron-transfer (PET)⁵ and fluorescence resonance energy-transfer (FRET)⁶ have been selected as versatile strategies to achieve efficient emission changes. In general, emission changes due to PET and FRET phenomena are achieved at the monomer level.

However, a new emission change system has been created in a biological aqueous environment as the result of an aggregation phenomenon; the so-called aggregation-induced emission.^7–9 This interesting emission system has the potential to produce turn-on-type biological imaging reagents due to changing from an emission quenching state at the monomer level to a strong emission state at the aggregate level.^10,11 Recently, interesting longer-wavelength emissions, including yellow, orange, red, and near-infrared light, were generated by the aggregation of donor–acceptor-type dyes.^12–19 These red and near-infrared emissions are of particular importance in biological applications, because they lie in the biological optical window, in which light maximally penetrates biological tissues.²⁰ These attractive donor–acceptor aggregate systems can generally be developed, because the necessary longer-wavelength emission can be created by simply combining donor and acceptor moieties and controlled by tuning donor–acceptor strengths. In these studies, various acceptors, such as cyanostilbene,¹² pyran-malononitrile,¹³ quinoline-malononitrile,¹⁴ triazine,¹⁵ carborane,¹⁶ benzothiadiazole,^17,18 quinoxaline,¹⁷ thiadiazolopyridine,¹⁷ and naphthobisthiadiazole,¹⁹ were combined with two or three triphenylamine donor moieties. In all systems, the donor–acceptor-type dyes were structurally limited in the two or three triphenylamine system composed of a small acceptor moiety. To further extend the generality of the aggregate emission, new structural features of donor–acceptor dye are desirable.

Hexaazatriphenylene is a candidate for a new acceptor moiety because it has an expanded π-electron system and three fused pyrazine rings with electron-deficient character.^21–23 The expanded star-shaped structure can be used to introduce six functional groups (Fig. 1). We previously reported the preparation of hexaazatriphenylenes bearing six triphenylamine donor moieties, and investigated the liquid crystalline and organogel supramolecular assemblies arising from their expanded π-electron system.²³ The star-shaped donor–acceptor structure is a fascinating prospect for extending the generality of aggregate emission. In this paper, we report the emission behavior of these star-shaped donor–acceptor dyes in the aggregate state. In addition to turn-on-type emission enhancement in an aqueous medium, the hexaazatriphenylene-triphenylamine dyes provide a ratiometric emission color change from an initial orange light emission to an intermediate red light emission, and then a final green light emission.

Fig. 1 Molecular structure of hexaazatriphenylene dyes 1, 2, 3, and 4.

Results and discussion

Absorption and emission behavior

The UV/vis absorption spectra of hexaazatriphenylene dyes 1–4 scarcely changed due to changing solvent polarity. Solvent polarity dependence was observed in the fluorescence spectra, indicating fluorescence solvatochromism (Table 1, Fig. 2 and S1†). In 2, bearing triphenylamine donor moieties, the fluorescence emission band shifted bathochromically (from 505 nm to 608 nm) along with a reduction in fluorescence quantum yield (Φ_FL), from 0.72 to 0.14, with an increase in solvent polarity from toluene to THF, DCM, and finally DMF (Fig. 2b). The change in fluorescence spectra is attributed to an intramolecular charge-transfer (ICT) arising from the donor–acceptor character, which changes the dye molecule to a polarized excited state.^24,25 In contrast, a slight spectral change was found in weak donor–acceptor dyes 3 and 4, which lacked triphenylamine donor moieties (Fig. S1†).

Table 1 Spectral data for 1, 2, 3, and 4

Comp.	Solvent	λabs^a (nm)	ε	λex (nm)	λem^b (nm)	ΦFL^c	Δλ (nm)
a at 2 × 10^−6 M.b at 2 × 10^−7 M.c Determined relative to fluorescein (ΦFL 0.97, ex. 455 nm) in ethanol at 2 × 10^−7 M.d Determined relative to quinine sulfate (ΦFL 0.55, ex. 350 nm) in 1 N sulfuric acid at 2 × 10^−7 M.e Absolute fluorescence quantum yield determined by an integrating sphere system.
1	Toluene	418	134 320	420	509	0.87	91
		312	155 180
	THF	414	129 350	420	596	0.59	182
		309	147 880
	DCM	416	115 830	420	625	0.25	209
		311	149 020
	DMF	411	120 120	420	661	<0.005	250
		309	144 770
	Solid			420	552	0.18^e
2	Toluene	452	103 760	440	505	0.72	53
		302	139 270
	THF	448	96 600	440	542	0.75	94
		300	130 180
	DCM	447	84 770	440	570	0.72	113
		302	130 880
	DMF	448	91 290	440	608	0.14	160
		300	135 660
	Solid			440	571	0.10^e
3	Toluene	392	100 350	390	436	0.23^d	44
		345 (sh)	78 550
	THF	388	103 180	390	439	0.22^d	51
		345 (sh)	83 550
	DCM	390	96 570	390	448	0.35^d	58
		345 (sh)	77 350
	DMF	387	100 260	390	457	0.35^d	70
		345 (sh)	83 100
	Solid			390	477	0.17^e
4	Toluene	365	58 670	350	414	0.02^d	49
		326	51 300
	THF	362	57 010	350	416	0.02^d	54
		324	49 530
	DCM	365	57 530	350	420	0.03^d	55
		324	52 620
	DMF	362	52 490	350	422	0.03^d	60
		324	46 940
	Solid			350	447	0.06^e

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Fig. 2 Fluorescence spectra of 1 (ex. 420 nm) and 2 (ex. 440 nm) in toluene, THF, DCM, and DMF (2.0 × 10⁻⁷ M).

A significant bathochromic shift and fluorescence quenching were observed in 1 bearing a benzene spacer between hexaazatriphenylene and triphenylamine moieties (Fig. 2a). In polar DMF solution, the emission band shifted to a longer wavelength, at 661 nm, together with a lower Φ_FL value of <0.005 (Table 1). The observed solvatochromic behavior was analyzed using the Mataga–Lippert plot,²⁶ in which the Stokes shift was plotted against the solvent polarity parameter (Δf). In the plot, the slope of 1 (18 200) was larger than that of 2 (11 900), indicating that the excited state in 1 was more polarized than that of 2 (Fig. 3). The highly polarized excited state in 1 is explained by a twisted intramolecular charge-transfer (TICT),^27,28 which is generated by introducing a benzene spacer, as found in donor–acceptor dyes bearing benzene spacers.²⁹ Thus, the excited state in 1 is significantly stabilized by the TICT phenomenon, leading to a large bathochromic shift of the fluorescence emission band and significant fluorescence quenching. In contrast, dye 2, without benzene spacers, indicated moderate stabilization of the excited state arising from ICT. This TICT was supported by molecular orbital DFT calculations (Fig. S3†). The HOMO orbital was located mainly on the triphenylamine donor moiety, while the LOMO orbital was on the hexaazatriphenylene acceptor moiety.

Fig. 3 Plots of the Stokes shift (Δν = ν_abs − ν_FL) against the solvent polarity parameter (Δf) for 1–4 in toluene, THF, DCM, and DMF. Slopes: 18 200 in 1, 11 900 in 2, 4450 in 3, and 2280 in 4. Goodness of fit parameter, r²: 0.98 in 1, 0.87 in 2, 0.76 in 3, and 0.88 in 4. The relationship between the Stokes shift and the solvent polarity parameter was evaluated by Mataga–Lippert equations (see Experimental section).

Emission behavior in aggregate state

The fluorescence emission in aqueous environments was studied using a THF/water medium, in which the dye concentration was maintained at 2 × 10⁻⁶ M, and only the water fraction (vol%) was varied from 0% to 90%. In all dyes, the UV/vis absorption and fluorescence spectra were dependent on the water fraction (Fig. 4 and S4–S6†).

Fig. 4 (a) UV/vis absorption and (b) fluorescence spectra of 1 in THF/water (0, 10, 20, 30, 40, 50, 60, 70, 80, and 90% water fractions) at 2.0 × 10⁻⁶ M excited at 410 nm.

In 1, the emission was quenched significantly in low water volumes, but was recovered and enhanced in high water volumes (Fig. 6). A large Φ_FL value (0.50) at 0% water changed suddenly to a low value (0.02) at 10–20% water (Fig. 5c). In these low water volumes, dye molecule 1 existed as its monomeric form and fluorescence quenching was enhanced by increasing solvent polarity (water fraction), as observed in DMF. A bathochromic shift of the emission band from 596 nm (0% water) to 634 nm (20% water) supported the fluorescence quenching (Fig. 5b). However, once the water fraction exceeded 40%, the Φ_FL value was enhanced to, and maintained around, 0.4 (Fig. 5c). In this high water volume, aggregation started mainly via hydrophobic interactions and some π-stacking interactions. The aggregate provides a hydrophobic space inside its structure, which restricts fluorescence quenching, resulting in turn-on-type emission enhancement.¹⁷ The emission found in this aggregate state is attributed to inherent emission behavior in the aggregated solid state; i.e., a greenish yellow emission at 552 nm with Φ_FL of 0.18 (Table 1). Thus, fluorescence emission can be achieved also in the star-shaped structure of the present donor–acceptor dye, leading to a generalized aggregate emission.

Fig. 5 Plots of (a) the absorption band shift, (b) the fluorescence band shift, and (c) the fluorescence quantum yield versus the water fraction (vol%) for 1, 2, 3, and 4 in THF/water (0, 10, 20, 30, 40, 50, 60, 70, 80, and 90% water fractions) at 2.0 × 10⁻⁶ M. The absorption bands were detected at 414 nm for 1, 448 nm for 2, 388 nm for 3, and 362 nm for 4 (at 0% water), and at 432 nm for 1, 471 nm for 2, 394 nm for 3, and 382 nm for 4 (at 90% water). The fluorescence bands were detected at 596 nm for 1, 542 nm for 2, 455 nm for 3, and 416 nm for 4 (at 0% water), and at 542 nm for 1, 546 nm for 2, 469 nm for 3, and 422 nm for 4 (at 90% water).

Fig. 6 Fluorescence images of 1, 2, 3, and 4 in THF/water (0, 10, 20, 30, 40, 50, 60, 70, 80, and 90% water fractions, from left to right) at 2.0 × 10⁻⁶ M under UV light irradiation.

In 1, aggregate formation was supported by the shifts of both absorption and emission bands. A bathochromic shift (Δλ_abs) of the absorption band was observed, moving from 414 nm (0% water) to 432 nm (90% water) (Fig. 5a). The Δλ_abs value of 18 nm is likely due to the formation of π-stacked aggregates, as found in liquid crystalline and organogel supramolecular assemblies reported previously.²³ In addition, the emission band shifted to a shorter wavelength, from 634 nm (20%) to 542 nm (90%), despite an increase in solvent polarity (Fig. 5b). The observed hypsochromic shift of 92 nm is likely due to the local hydrophobic environment inside the aggregate structure, as described above. Dynamic light scattering (DLS) provided direct evidence for aggregate formation (Fig. S7 and S8†). In 1, the formation of particles with several hundred nm in size was detected above a 40% water fraction, whereas no particles were detected below 30%. Atomic force microscopy (AFM) showed an anisotropic spherical aggregate morphology, with diameters of 250–350 nm (50% water fraction) and 200–450 nm (90%) (Fig. 7). The spherical aggregate was morphologically similar to the less ordered aggregate found in triphenylamine-based dyes reported previously.¹⁷ The three-dimensional non-planar structure of the triphenylamine moieties tends to prevent ordered packing, producing less ordered aggregates.³⁰ However, as described above, molecules of 1 formed π-stacked aggregates, as found in the columnar liquid crystal in the bulk state.²³ The spherical aggregate structure found by AFM was different to the one-dimensional aggregate structure in the bulk state. To clarify this, X-ray diffraction (XRD) analysis was performed on the solid aggregate samples, which were obtained by reprecipitation from concentrated THF/water solutions containing different water fractions (30, 40, 70, and 90% water) (Fig. 8). In general, a broad XRD pattern was observed, except in the small angle region, indicating low-order aggregation. In low water volume samples, a relatively strong reflection was observed at around 3.0–3.5 degree, in accordance with the (100) reflection (3.1 degree) found in the liquid crystal with hexagonal columnar packing.²³ The reflection intensity in the small angle weakened with an increase in water fraction, although the reflection remained as a shoulder even at 90% water fraction. Thus, the spherical aggregate found by AFM observation would be composed of microscopic one-dimensional aggregates in a low-order packing fashion. The low ordering of spherical aggregation in these aqueous media would be attributed to hydrophobic interactions, which are favored in aqueous environments and accelerated by increased water fractions. The inherent π-stacking nature of hexaazatriphenylene leads to ordered one-dimensional aggregation, whereas hydrophobic interactions result in less ordered aggregation.

Fig. 7 AFM images of 1, 2, 3, and 4. The samples were prepared by drop casting from the 1.0 × 10⁻⁵ M THF/water (50% and 90% water fraction) solutions on freshly cleaved HOPG, with subsequent vacuum evaporation.

Fig. 8 XRD patterns of 1. Samples were obtained by reprecipitation from THF/water (30, 40, 70, and 90% water fractions) solutions at 1.0 × 10⁻⁴ M.

A different emission behavior in the aqueous environment was found for 3 and 4, which lacked triphenylamine moieties. In low water volumes, the Φ_FL value remained constant, but reduced in high water volumes (50–90% in 3 and 80–90% in 4). In 3, the maximum Φ_FL value of 0.37 with 40% water fraction dropped to 0.15 at 90% water. Similarly, the Φ_FL value of 0.03 at 60% water fraction changed to 0.005 at 90% in 4, although the inherent emission ability of 4 was low (Fig. 5c). Aggregate formation in high water volumes was suggested by the observed bathochromic shift of the absorption band in high water fractions (Fig. 5a). Aggregate formation in high water volumes was suggested by DLS; the formation of particles with several hundred nm in size was detected above 50% water fraction in 3 and above 80% in 4 (Fig. S7 and S8†). The emission reduction observed in 3 and 4 can be rationalized by different aggregate morphologies. Isotropic rod-shaped aggregates were indicated by AFM observation, although the spherical aggregate was still afforded in 3 at 90% water fraction (Fig. 7). Also, differences were found in the XRD reflection patterns. In 3 and 4, the reflection intensity in the small angle region was stronger than that of 1, but relatively weak and broad reflection patterns were found in the whole region (Fig. S9†). The relatively ordered aggregation in 3 and 4 was attributed to their lacking triphenylamine moieties.

Double emission change

As found in 1, the transition from the initial emission state to the final emission state via the quenching state was observed in 2 (Fig. 5 and 6). In low water volumes, the degree of emission quenching in 2 was weaker than in 1. This trend is consistent with the findings of solvatochromic quenching in DMF, with strong quenching in 1 and weak quenching in 2. Aggregate formation in the emission state was supported by DLS, AFM, and XRD (Fig. 7 and S7–S9†). A spherical aggregate was only detected in the emission region of a 50–90% water fraction.

In 2, emission quenching in the monomeric form and subsequent emission enhancement in the aggregate form can be explained by shifts in absorption and emission bands, as seen for 1 (Fig. 5a and b). The only difference between 1 and 2 was the degree of hypsochromic shift in the emission band observed for the aggregate state. In 1, a significant hypsochromic shift of 92 nm was observed from 20% to 90% water fraction, whereas only a 37 nm shift from 40% to 90% was observed in 2. Due to the significant shift of the emission band, dye 1 indicated unusual emission color changes in two steps from the initial orange light emission (596 nm) to the intermediate red light emission (634 nm), and to the final green light emission (542 nm). In contrast, in 2, the initial and final emission colors (542 and 546 nm) remained unchanged; green via intermediate orange light emission (583 nm) (Fig. 6). The emission color change in 1 is explained as a combination effect of aggregation and TICT phenomena (Fig. 9). In low water volumes, the emission band appeared at a longer wavelength due to the TICT effect,²⁸ providing orange (0% water fraction) and red (20%) light emissions. In high water volumes, the as-formed aggregate provides a hydrophobic effect to enforce the shorter wavelength shift,¹⁷ leading to the green light emission (90%). Thus, the emission efficiency and the emission color can be controlled simultaneously by the combination of aggregation and TICT.

Fig. 9 Emission enhancement and color change based on the combination effect of aggregation and TICT phenomena.

Conclusions

In conclusion, we have demonstrated that fluorescence emission can be generated, even in a star-shaped donor–acceptor structures composed of hexaazatriphenylene acceptor and triphenylamine donor moieties due to aggregate formation in an aqueous environment. This is rare example for the multi-triphenylamine-based donor–acceptor dyes using expanded π-electron system of hexaazatriphenylene. The aggregate formation efficiently restricts emission-quenching arising from the enhanced donor–acceptor polarization in the polar environment by providing a less polar hydrophobic space inside the aggregate structure, leading to emission enhancement. By introducing an additional benzene spacer between the hexaazatriphenylene and triphenylamine moieties, a TICT phenomenon was generated, which produced significant emission quenching together with a large bathochromic shift in the emission band. The bathochromically shifted emission band then moved hypsochromically upon aggregate formation, leading to an emission color change from orange to green light emission. The combination effect of aggregation and TICT resulted in an unusual double emission change of turn-on-type and ratiometric emission changes. The present double emission change will be developed in the biological science field as a new fluorescence imaging and detection technique.

Experimental

Compounds

Hexaazatriphenylene dyes 1–4 were prepared according to a method reported previously.²³

Instrumentation

UV/Vis spectra were measured on a JASCO V-570 spectrophotometer in a 1.0 cm width quartz cell. Fluorescence spectra were measured on a HITACHI F-4500 fluorescence spectrophotometer in a 1.0 cm width quartz cell. Fluorescence quantum yields were estimated using fluorescein (0.97 in ethanol) as the reference for 1 and 2, and quinine bisulfate (0.55 in 1 N sulfuric acid) as the reference for 3 and 4. Absolute fluorescence quantum yield for solid samples was determined using a Hamamatsu Photonics C9920–02 Absolute PL Quantum Yield Measurement System. Dynamic light scattering (DLS) was measured on a Photal OTSUKA ELECTRONICS ELSZ-1000 equipped with a 785 nm red laser source, using a fixed angle (90°). DLS experiments were performed at 25 °C in THF/water solution at 2 × 10⁻⁶ M, using a 1.0 cm width quartz cell. Atomic force microscopy images were obtained on a SII SPA400 DFM (tapping mode) using SI-DF20-type tips. X-ray diffraction measurements were performed on RIGAKU RINT-TTR III and carried out with Cu(Kα) radiation from an X-ray tube with a 0.5 × 10 mm² filament operated at 50 kV × 300 mA (15 kW).

Aggregate preparation

THF stock solutions containing the dyes were prepared at 2.0 × 10⁻⁵ M. A portion (1 mL) of the stock solution was mixed with appropriate volume of THF and distilled water, to a total volume of 10 mL. In all samples, the concentration was kept at 2.0 × 10⁻⁶ M, with only the water fraction (vol%) changed, from 0% to 90%.

Mataga–Lippert plot

The relationship between the Stokes shift and the solvent polarity parameter was evaluated by Mataga–Lippert equations:

Δν = ν_abs − ν_FL = 2Δf(μ_e − μ_g)²/(hca³)

Δf = (ε − 1)/(2ε + 1) − (n² − 1)/(2n² + 1),

where h is the Plank constant, c is the speed of light, a is the Onsager cavity radius, μ_e and μ_g are the dipole moments in the excited and ground states, respectively, and ε and n are the dielectric constant and refractive index of the solvents, respectively.

Acknowledgements

We thank Professor Dr Masumi Miyazaki (AIST) for absolute fluorescence quantum yield measurements. This work was partially supported by Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Culture, Sports, and Technology of Japan (No. 26410105) and by the Cooperative Research Program of Network Joint Research Center for Materials and Devices (Institute for Materials Chemistry and Engineering, Kyushu University).

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Footnote

† Electronic supplementary information (ESI) available: The fluorescence images in various solvents, the fluorescence spectra in various solvents, the HOMO and LUMO orbitals, the UV/Vis and fluorescence spectra in THF/water, the DLS charts in THF/water, and the XRD patterns. See DOI: 10.1039/c6ra18320d

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