Larger π-extended anti-/syn-aroylenediimidazole polyaromatic compounds: synthesis, physical properties, self-assembly, and quasi-linear conjugation effect

Abstract

Four π-extended anti-/syn-aroylenediimidazole (ADI) polyaromatic compounds (1, 2, 3, and 4) with 11- or 13-fused rings have been successfully synthesized via a tandem cyclocondensation reaction between tetraamines and naphthalene dicarboxylic monoanhydride monoimide. The observed optical bandgaps for 1–4 are 2.70 (458 nm), 2.34 (529 nm), 2.31 (537 nm), and 2.21 eV (561 nm), respectively, which are in accordance with the calculated bandgaps from DFT calculations for 1–4, which are 2.77, 2.49, 2.29, and 2.21 eV, respectively. Our results indicate that there are obvious anti-/syn- and π-extended effects in these molecules. The cyclic voltammetry (CV) measurements show that all the compounds exhibit quasi-reversible reduction waves. The experimental LUMO levels from CV show an interesting zigzag-curved change (zigzag-shaped curve) in sequence, which matches well with those of the theoretical calculations. Furthermore, the fitted decay lifetimes of 1–4 in CHCl₃ are 1.86, 1.32, 1.55, and 1.42 ns, which have the same trend as the above-mentioned zigzag-shaped curve. These trends are believed to be related to the intrinsically effective quasi-linear conjugation (QLC) with a theoretically calculated quasi-linear length of 1.10 nm, 1.94 nm, 1.56 nm, and 2.40 nm, respectively. The successful synthesis and characterization of four soluble π-extended ADI polyaromatic compounds could provide us with more diverse candidates for air-stable organic electronic devices.

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Introduction

For decades, larger π-extended acenes^1–4 have been strongly progressing as the most prospective candidates for application in organic-semiconductor devices, such as organic photovoltaic cells,⁴ organic light-emitting diodes (OLEDs),⁵ and organic field effect transistors (OFETs).^2b,6,7 In particular, conjugated acenes have been well-developed and some of them have been demonstrated to exhibit p-type characteristics, with the hole mobility of thin films/crystals as high as 4.28/31.3 cm² V⁻¹ s⁻¹.^3c,8 In contrast, the performance of n-type heteroacenes is far less investigated,^2f,9 although some N-substituted acenes (oligoazaacenes) have already shown some decent mobilities in FETs. Very recently, a breakthrough in solution-processed air-stable n-type organic thin-film transistors with a mobility of up to 3.50 cm² V⁻¹ s⁻¹ have been reported.¹⁰ However, searching for new organic conjugated systems to improve the electron mobility is still the main goal for most research groups.

Although the functionalization of parent frameworks with strong electron-withdrawing group such as –CN, –F, or –C=O,^1b,4b,11 has been explored to approach n-type materials, the tedious synthesis process has proved a limitation for their further versatile application. Alternatively, an effective strategy to realize n-type π-extended organic semiconductors is the replacement of CH groups in the backbone of oligoacenes with sp² N atoms. With the appropriate arrangement (numbers or positions) of sp² N atoms in the backbone,^2f,12 the lowest unoccupied molecular orbitals (LUMO) could be optimized to fall between −4.0 and −4.5 eV for air stability^9f,h,13 without oxidative degradation.^9d,14

As one of the most facile and efficient synthetic routes to approach larger electron-deficient aromatic π-systems, versatile tandem cyclocondensation with ortho-positioned carboxyls and amines has been widely employed, including diketone/hydroxyl–diamines,^2f,9,15 aldehyde–diamines,¹⁶ methylene/ketone–amines,¹⁷ carboxylic group–amines,¹⁸ anhydride–amines^11d,19 and so on.²⁰ Here, we are interested in exploring the anhydride-amine method to construct as large as 11- or 13-ring fused π-extended aroylenediimidazole (ADI) polyaromatic compounds through cyclocondensation reactions between aromatic tetraamines and commercially available or easily-prepared n-type monomer aromatic monoanhydrides. In this paper, four novel ADI polyaromatic compounds (Scheme 1), 1–4, have been synthesized and their physical properties have been fully characterized.

Scheme 1 The synthetic route of four ADI polyaromatic compounds, 1–4. Reaction condition: i. Pyridine, reflux, 6 h. Inset: pictures of the newly separated samples on TLC–SiO₂ plates (CHCl₃–MeOH).

Results and discussion

Synthesis

Four novel ADI polyaromatic compounds 1–4 were successfully synthesized via the tandem cyclocondensation reaction among benzene-1,2,4,5-tetraamine, phenazine-2,3,7,8-tetraamine hydrochloride salts,²¹ and napthanlenetetracarboxylic monoimide monoanhydride (NIA)²² according to a similar reported procedure with slight modification (Scheme 1).²³ All the as-prepared compounds 1–4 have been chromatographically purified and fully characterized by ¹H NMR, MALDI-TOF mass and high-resolution mass spectrometry. Note that it is not possible to obtain their ¹³C NMR spectra due to their poor solubility. In addition, their physical properties, such as ultraviolet-visible spectra (UV-Vis), photoluminescence spectra (PL), cyclic voltammetry (CV), fluorescence lifetime measurements, theoretical calculations, field emission scanning electron microscopy (FE-SEM), and electroluminescence devices (EL) of nanofibers of 2 are also presented in this report.

The ¹H NMR spectra (in CDCl₃, 400 MHz) of the four ADI polyaromatic compounds 1–4 are shown in Fig. 1. The two aromatic proton signals with single peaks at 9.80 (s, 1H) and 8.48 ppm (s, 1H) (Fig. 1a) belong to outer arc and inner bay protons of diimidazolo-fused phenylene in syn-type 1, respectively. The aromatic proton signal with a single peak at 9.14 ppm (s, 2H) (Fig. 1b) belongs to the two central protons of diimidazolophenylene in anti-type 2 with central symmetry. The aromatic proton signals with triple and double peaks from 7.51 to 7.38 ppm belong to end-capped phenylenes for both 1 and 2. Other signals with double or multi peaks ranging from 9.12 to 8.92 ppm correspond to their naphthalene groups. For 3 and 4, the two signals with single peaks at 9.55 ppm (s, 1H) and 8.87 ppm (s, 1H) in Fig. 1c and 9.56 ppm (s, 1H) and 8.90 ppm (s, 1H) in Fig. 1d belong to the protons of the central phenazine moieties of 3/4. Since the difference (0.67 ppm) between 9.55 ppm (s, 1H) and 8.87 (8) ppm (s, 1H) in Fig. 1c is larger than that (0.65 ppm) between 9.56 ppm (s, 1H) and 8.90 ppm (s, 1H) in Fig. 1d, we believe that Fig. 1c belongs to 3 while Fig. 1d is assigned to 4. Such assignment can be explained by the following reasons: (1) 4 has a more homogeneous π-electron delocalized system than 3, and (2) protons close to sp³-N atoms should appear at the high field while protons close to sp²-N atoms should appear at the low field. For 3, the two arc protons adjacent to two sp²-N atoms and two bay protons adjacent to two sp³-N atoms lead to the broader fieldshift distance between them compared to 4. The other signals come from naphthalenes and phenylenes and match well with 1 and 2. The distribution of the four molecular structures was also easily confirmed by the symmetry-determined polarity, as shown in the inset in Scheme 1. Thus, it is not surprising to find that anti-type 2 and 4 exhibit a smaller polarity and run faster on TLC plates than syn-type 1 and 3, respectively.

Fig. 1 ¹H NMR spectra of the separated ADI polyaromatic compounds 1 (a), 2 (b), 3 (c), and 4 (d) in CDCl₃ solutions.

Optical properties

As shown in the inset in Scheme 1, after separation by thin layer chromatography with CHCl₃–MeOH as the mobile phase and SiO₂ as the stationary phase, the molecular layer of 1–4 absorbed onto SiO₂ exhibits different colors (yellow, bright red, orange, and red-brown, respectively). This phenomenon is ascribed to the structural anti-/syn-effect. Anti-isomers 2 and 4 have well-dislocating π-electrons and lower energy levels, exhibiting deeper colors than their corresponding isomers. Meanwhile, longer π-extended 3 and 4 show deeper colors due to the enhanced intramolecular charge transfer (ICT) of larger π-aggregation compared to those of 1 and 2. This indicates that a zigzag-shaped color changing trend exists from yellow, red, orange to red-brown for 1–4, respectively.

Fig. 2a and b present the normalized UV-Vis absorption and photoluminescence (PL) spectra of 1–4 in THF solutions (1 × 10⁻⁵ mol L⁻¹). In the absorption, the shoulder peak at 529 nm of anti-type 2 can be ascribed to ICT with an obviously redshifted maximum absorption peak at 495 nm compared to that at 458 nm of syn-type 1 without ICT absorption due to the shorter linear π-conjugation. Similarly, anti-type 4 also exhibits more obvious ICT absorption peaks at 522 and 561 nm, probably resulting from the electron transfer between the sp³-N atoms and electron-deficient imidazolo-fused phenazine and carboxyl naphthalenes compared to the germinating peaks at ∼501 and ∼537 nm of syn-type 3. The calculated optical bandgaps for 1–4 are 2.70 eV (458 nm), 2.34 eV (529 nm), 2.31 eV (∼537 nm), and 2.21 eV (561 nm) with a gradually decreasing trend, respectively.

Fig. 2 (a) Normalized absorption spectra of the four compounds 1–4 in THF solutions (1 × 10⁻⁵ mol L⁻¹); (b) normalized photoluminescence spectra (excited at 458, 495, 459, and 460 nm, respectively) of the four compounds in THF solutions (1 × 10⁻⁵ mol L⁻¹). Green line-1, red line-2, pink line-3, blue line-4.

As shown in Fig. 2b, the normalized photoluminescence spectra (excited at 458, 495, 459, and 460 nm, respectively) of 1–4 in THF solutions (1 × 10⁻⁵ mol L⁻¹) show gradually redshifted emission peaks from 582, 597, 608, and 611 nm with different fluorescence colors (yellow, orange, red, and red). The maximum emission peaks at 582 and 597 nm of 1 and 2 show apparently blue shifted wavelengths compared to those at 613 and 616 nm for 3 and 4. These results indicate that the π-extended backbones and anti-type configuration could adjust the absorption, photoluminescence and optical bandgap effectively, as is typical for the π-extension effect and anti-/syn-effect.

Fluorescence decay measurement

The normalized emission decay curves for 1–4 in THF (1 × 10⁻⁵ mol L⁻¹) and their corresponding decay fittings are shown in Fig. 3. The intensity decays with time via a single-exponential function, with well-defined values of χ² at 0.9685, 0.9692, 0.9467, and 0.9822, and fitted decay lifetimes for 1–4 at 1.86, 1.32, 1.55, and 1.42 ns, respectively. These fitted lifetimes show a trend that the exciton photoluminescence lifetimes gradually change as zigzag-shaped curves, as shown in Fig. 3. Among them, syn-type 1/3 exhibit longer lifetimes compared to those of anti-type 2/4, which is ascribed to the configuration-determined longer effective quasi-linear conjugation (QLC) of the former pair than the latter, namely, the typical anti-/syn-effect. Moreover, π-extended 3/4 show shorter lifetimes than those of 1/2, which is clearly caused by the longer QLC of the former than the latter π-extension effect.^22b This zigzag-curved change might be explained by the length of the effective QLC. In particular, although the molecular size of 3 is larger than that of 2, the effective QLC of 2 is longer than 3 (1.55 ns), leading to the shorter lifetime of 2 (1.32 ns).

Fig. 3 Single-exponential fittings (lines) for 1–4 in CHCl₃ (4.0 × 10⁻⁵ mol L⁻¹); green-1, red-2, pink-3, blue-4 (excited at 400 nm). The values of χ² are 0.9685, 0.9692, 0.9467, and 0.9822, respectively.

Cyclic voltammetry

The electrochemical properties of the ADI azaacenes 1–4 were investigated in CHCl₃ in a standard three-electrode electrochemical cell using tetrabutyl-ammonium hexafluorophosphate (Bu₄NPF₆, TBAPF) (0.1 mol L⁻¹) as the electrolyte, Ag/AgCl as the reference electrode, and Pt as the working electrode and counter electrode (Pt wire). As shown in Fig. 4 and Table 1, all the ADI azaacenes 1–4 exhibit one quasi-reversible reduction wave. It is also indicated that the four ADI azaacenes 1–4 are intrinsic n-type organic semiconductors due to the absence of oxidation peaks. The half-wave reduction potentials for 1–4 are −0.84, −0.74, −0.78, and −0.66 V, respectively. Furthermore, according to the empirical equation E_LUMO = −[4.4 + E^red_1/2] eV,^15,24 the lowest unoccupied molecular orbital (LUMO) energy levels were calculated to be −3.56, −3.66, −3.62, and −3.74 eV. The anti-type structures of 2 and 4 have a big effect on the value of the HOMO and LUMO levels compared to the syn-type structures of 1 and 3. It is worth noting that the LUMOs decrease with increasing backbone length for syn-type 1 vs. 3 and anti-type 2 vs. 4, namely, with the π-extension effect and anti/syn effect. Moreover, anti-type 2 shows a lower LUMO level than syn-type 4. These results indicate that this zigzag-curved change might be ascribed to the intrinsically effective QLC length.

Fig. 4 Cyclic voltammetry curves of 1–4 in a CHCl₃ solution containing a 0.1 mol L⁻¹ TBAPF electrolyte. Scanning rate: 100 mV s⁻¹ green line-1, red line-2, pink line-3, brown line-4.

Table 1 Physical properties of the four ADI polyaromatic compounds 1–4

Entries	E^red1/2^a (V)	LUMO^b (eV)	HOMO^c (eV)	Egap/λmax^d (eV) nm^−1	LUMO^e (eV)	HOMO^e (eV)	Egap^e (eV)
a Obtained from cyclic voltammograms. Reference electrode: Ag/AgCl.b Calculated from cyclic voltammograms.c Calculated according to the formula ELUMO = −[4.4 + E^red1/2] eV, EHOMO = ELUMO − Egap.d Optical band gap, Egap = 1240/λmax of the peaks at 458 nm, 529 nm, 537 nm, and 561 nm, respectively.e Obtained from theoretical calculations.
1	−0.84	−3.56	−6.26	2.70	−3.48	−6.24	2.77
2	−0.74	−3.66	−6.00	2.34	−3.59	−6.08	2.49
3	−0.78	−3.62	−5.93	2.31	−3.58	−5.85	2.29
4	−0.66	−3.74	−5.95	2.21	−3.63	−5.84	2.21

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Calculated from the optical bandgap and LUMO levels (see Table 1), the HOMO levels of 2, 3, and 4 range from −5.93 to −6.00 eV and are higher than the values of azapetancenes (−6.03 to −6.14 eV),¹⁵ but lower than the value of pentacene (−5.14 eV)²⁵ and hexacene (−4.96 eV).^3c In contrast, the four ADI azaacenes 1–4 have lower LUMO levels (−3.56 to −3.74 eV) than those of pentacene (−3.37 eV) and hexacene (−3.56 eV).^3a,18 In particular, the LUMO level of 4 (−3.74 eV) becomes close to the air-stable limitation range (−4.0–4.5 eV).^{9d,f,h,13a–c,14} All these data could support that 1–4 might possess reasonable stability for electron injection and transportation in a device.²⁶ We believe that the π-extension effect and anti/syn effect could play an important role in the molecular design and tuning of the physical properties. To this point, the anti-type configuration has an obvious advantage over the anti-type.

Theoretical calculation

All the electronic structures of 1–4 were theoretically calculated and optimized using density functional theory (DFT) at the B3LYP/6-31G* level.²⁷ At the same level, the ground state frontier molecular orbitals of the optimized molecules were also calculated (see Fig. 5). The HOMO and LUMO orbitals of 1 and 2 and the LUMO of 3 and 4 are all delocalized over the whole ADI backbone. However, the HOMO of 3 and 4 is mainly delocalized on the diimidazolophenazine framework without naphthalene groups. In particular, 3 exhibits more homogeneous delocalization with low energy due to the anti-type structure as a quasi-linear channel for ICT.

Fig. 5 Models of the molecular orbital for the HOMO (left column) and LUMO (right column) of the ADI azaacenes 1 (row 1), 2 (row 2), 3 (row 3), and 4 (row 4).

The calculated HOMO and LUMO levels and the bandgaps of 1–4 match very well with the experimental results, which are summarized in Table 1. In particular, the change in the theoretical LUMO levels with values of −3.48, −3.59, −3.58, and −3.63 eV from 1 to 4 also appears as a zigzag-shaped curve. This special change matches very well with the change in the theoretically calculated length of the effective conjugated backbone moieties, so-called as the quasi-linear backbone length, at 1.10, 1.94, 1.56, and 2.40 nm for 1–4, respectively, not with the gradually increased molecular conjugated backbone length (1.91, 1.93, 2.39, and 2.40 nm, Fig. 6) or the whole molecular length (2.97, 3.00, 3.45, and 3.47 nm, Fig. S10, ESI†).

Fig. 6 The theoretical calculated lengths: 1.10 nm, 1.94 nm, 1.56 nm, and 2.40 nm of quasi-linear conjugated backbone moieties of 1 (a), 2 (b), 3 (c), and 4 (d), respectively.

As shown in Fig. 7, the experimental results and theoretical calculations of 1–4 not only suggest the existence of a typical π-extension effect and anti-/syn- effect, but also support the effective QLC induced zigzag-curved change of various intrinsic physical properties (e.g. different colors, fluorescence decay lifetimes, theoretically calculated and experimentally measured LUMO values).

Fig. 7 The zigzag-shaped curve of the relationship between the structures and physical properties of the ADI azaacences 1–4.

Self-assembly and electroluminescence device

The planar π-extended conjugation of 1–4 could allow them to self-assemble into a kind of nanostructure by the slow evaporation of the corresponding solutions (e.g. THF for 1, MeOH–CHCl₃ (5 : 1, v/v)) for 2 and 4, hexane-o-dichlorbenzene (5 : 1, v/v) for 3. Fig. 8a, c, e and g present field emission scanning electron microscopy (FE-SEM) images of as-prepared 1–4 nanobelts, nanofibers, nanowires, and nanobundles. The as-prepared nanostructures have widths or diameters in the range of ∼50–400 nm and lengths ranging from tens to hundreds of micrometers. Fig. 9e–h show the magnified images of the typical nanostructures (∼50 nm). The optical and polarized optical microscopy (POM) images indicate that these four nanostructures are quasi-crystalline and stack anisotropically (Fig. 8i–p).

Fig. 8 FE-SEM, magnified FE-SEM, POM and normal optical images of the abundant self-assembled nanostructures of 1–4: nanobelts of 1 (a, e, i, and m); nanofibers of 2 (b, f, j, and n); nanowires of 3 (c, g, k, and o); nanobundles of 4 (d, h, l, and p); the optical and POM images of the nanostructures (magnification: 16 × 20).

Fig. 9 The absorption (a) and photoluminescence (a.u.) spectra of the dispersed solution in MeOH (b); insert pictures: color of the aggregates of the 1–4 nanostructures and photoluminescence of their dropcast films (UV lamp, 365 nm); green line-1, red line-2, pink line-3, blue line-4; (c) single-exponential fittings (lines) for the films of the 1–4 nanostructures; green-1, red-2, pink-3, blue-4 (excited at 400 nm). The values of χ² are 0.9972 (nano-1), 0.9954 (nano-2), 0.9781 (nano-3), and 0.9820 (nano-4).

As shown in Fig. 9, the absorption spectra of the nanostructures of 1–4 exhibit an obvious self-assembly induced ∼40–50 nm red-shift of the maximum absorbed peaks from 458, 529, 537, and 561 nm (in solution) to 500, 578, 587, and 611 nm (in the nanostructure forms), respectively (Fig. 2). Meanwhile, the photoluminescence spectra of the films of the nanostructures on glasses show the gradually redshifted emission peaks from 582, 597, 608, and 611 nm (in solution) to 629, 644, 700, and 706 nm (in the nanostructure forms) with fluorescence colors of orange, orange–red, red, and deep red, respectively, which indicates that intermolecular π–π stacking and the planar π-extended conjugation synergically predominate the final color of the aggregates. Besides this, the red-shifted absorption and photoluminescence of the nanostructures of 1–4 suggest the formation of J-type aggregation through π–π stacking.^28,29 As shown in Fig. 9c, the single-exponential fittings for the fluorescence decay lifetimes of the drop cast films of the 1–4 nanostructures are also given as 1.48, 0.96, 1.15, and 0.72 ns, respectively, with an obvious QLC-induced zigzag-curved change. The values of χ² are 0.9972 (nano-1), 0.9954 (nano-2), 0.9781 (nano-3), and 0.9820 (nano-4).

Furthermore, a protocol heterojunction sandwich-like organic light-emitting device (OLED) using nanofibers of 2 was fabricated and used as an sample active layer. Fig. 10a illustrates a schematic diagram of the as-fabricated LED device containing layers of ITO/2 nanofibers/p-SiC/Al (10 nm)/Ti (80 nm)/Al (380 nm)/ITO. The Al/Ti/Al layers were first formed by e-beam evaporation on the p-SiC and then furnace annealed at 1000 °C for 5 min, protected by argon, for an alloying heat treatment. After that, the water-dispersed nanofibers were drop-cast onto the substrate and air-dried. Afterward, the substrate was annealed at 70 °C under a N₂ atmosphere for 2 h. In the final step, an ITO glass was directly pressed against the layer of nanofibers to form the top contact.

Fig. 10 (a) Schematic diagram of the sandwich-like OLED structure: quartz/ITO/nano-2/p-SiC/Al (10 nm)/Ti (80 nm)/Al (380 nm)/ITO/quartz; (b) EL spectrum of the electron deficient 2 nanofibers/p-SiC heterojunction LED biased at various forward voltages (6–12 V) with an inserted turn-on image of the OLED taken at 11 V; (c and d) current–voltage characteristics of the same OLED device under the forward and the reverse bias, respectively.

Fig. 10b presents the electroluminescence (EL) spectra of the 2 nanofibers/p-SiC heterojunction light emitting device biased at different forward voltages. An image of the EL device at a forward bias of 11 V was taken and inserted in Fig. 10b. When a constant forward bias of 11 V was biased, the detected current increased from 9.97 mA to 10.10 mA. A constant voltage was applied on the layer of nanofibers for all the EL measurements, and the EL responding spectra were measured by a photomultiplier detector equipped with a monochromator and collected from the optical fiber. The EL spectra showed broad emission spectra with a broad peak at ∼561 nm with a peak width of ∼47 nm at an intensity of 318 a.u., probably resulting from the aroylenediimidazole π-conjugated backbone with hole–electron recombination. Only the substrate zone with a Al/Ti/Al contact area in the device gives the dotted light emitting phenomena. Meanwhile, there was no light emission observed for this heterojunction OLED when a reverse bias was applied.³⁰

The current–voltage (I–V) characteristic of the device under forward and reverse bias is shown in Fig. 10c and d, respectively. From Fig. 10c, the heterojunction LEDs have a turn-on voltage of ∼3.1 V and it supports that the I–V curve exhibits a rectifying diode property and the use of n-type organic nanofibers as the electron injection layer could give an effective and low cost candidate to replace inorganic thin-film layered devices.

Conclusions

In conclusion, four novel ADI polyaromatic compounds 1–4 have been successfully synthesized from easily available compounds or intermediates via an aromatic anhydride-diamine tandem cyclocondensation. It is worth noting that 13-ring 3 and 4 are some of the longest linear-shaped n-type aroylenediimidazole/imide derivatives.^{4b,7a,b,11a,19i,31} They have been fully studied by photophysical measurements, electrochemical methods, theoretical calculations, and fluorescence lifetimes with a zigzag-curved change due to the effective QLC. In addition, the typical anti-type effect, π-extension effect and predominately π–π stacked driving force synergically lead to the formation of abundant nanostructures, one of which has been successfully employed as the active layer in an OLED device. Our results could offer an effective way to design and approach promising larger π-extended n-type materials for air-stable organic electronic devices.

Experimental section

Materials

7-(2,6-Diisopropylphenyl)-1H-isochromeno[6,5,4-def]isoquino-line-1,3,6,8(7H)-tetraone (NIA)²² was prepared according to the reported literature^19i,23 from the reaction between isochromeno[6,5,4-def]isochromene-1,3,6,8-tetraone (a) and 2,6-diisopropylaniline (b). The phenazine-2,3,7,8-tetraamine hydrochloride salt was prepared from the benzene-1,2,4,5-tetraamine hydrochloride salt (c).^15,21 Compounds a–c were purchased from Sigma-Aldrich. All the solvents were used without further purification.

General synthesis

NIA (∼0.6 mmol) (Fig. S9, ESI†) and the corresponding tetraamine hydrochloride salt (0.25 mmol) was mixed into pyridine (10 ml) and refluxed for 6 h. After the reaction finished, the pyridine was removed by rotary evaporation and the residue was purified first using silica-gel column chromatography and then using preparative thin layer chromatography with chloroform–methanol (35 : 1 for 1/2, 25 : 1 for 3/4) to afford 1 (35 mg, 15%) and 2 (39 mg, 17%) as red solid powders, 3 (41 mg, 16%) and 4 (54 mg, 21%) as reddish-brown and dark brownish-green powders, respectively.

1: ¹H NMR (CDCl₃, 400 MHz): δ 9.80 (s, 1H), 9.12 (d, J = 7.78 Hz, 2H), 9.07 (d, J = 7.70 Hz, 2H), 8.96 (d, J = 7.63 Hz, 2H), 8.94 (d, J = 7.79 Hz, 2H), 8.48 (s, 1H), 7.53 (t, J = 7.49 Hz, 2H), 7.38 (d, J = 8.39 Hz, 4H), 2.75 (t, J = 6.85 Hz, 4H), 1.19 (s, 12H), 1.18 (s, 12H). HRMS (ESI) m/z: M + H⁺ C₅₈H₄₄N₆O₆ calcd. 920.3322; found 921.3387.

2: ¹H NMR (CDCl₃, 400 MHz): δ 9.14 (s, 2H), 9.13 (d, J = 7.67 Hz, 2H), 9.07 (d, J = 7.60 Hz, 2H), 8.95–8.93 (d, J = 7.54 Hz, 2H), 8.94–8.92 (d, J = 7.62 Hz, 2H), 7.53 (t, J = 7.80 Hz, 2H), 7.38 (d, J = 8.07 Hz, 4H), 2.75 (t, J = 7.68 Hz, 4H), 1.20 (d, J = 1.31 Hz, 12H), 1.18 (d, J = 1.26 Hz, 12H). HRMS (ESI) m/z: M + H⁺ C₅₈H₄₄N₆O₆ calcd. 920.3322; found 921.2242.

3: ¹H NMR (CDCl₃, 400 MHz): δ 9.55 (s, 2H), 9.25 (d, J = 7.68 Hz, 2H), 9.10 (d, J = 7.53 Hz, 2H), 8.98 (d, J = 7.68 Hz, 4H), 8.87 (s, 2H), 7.54 (t, J = 8.10 Hz, 2H), 7.39 (d, J = 7.66 Hz, 4H), 2.75 (t, J = 6.96 Hz, 4H), 1.24 (d, J = 3.95 Hz, 12H), 1.20 (d, J = 6.77 Hz, 12H). HRMS (ESI) m/z: M + H⁺ C₆₄H₄₆N₈O₆ calcd. 1022.3540; found 1023.3851.

4: ¹H NMR (CDCl₃, 400 MHz): δ 9.56 (s, 2H), 9.26 (d, J = 7.38 Hz, 2H), 9.10 (d, J = 7.66 Hz, 2H), 8.98 (d, J = 7.53 Hz, 4H), 8.90 (s, 2H), 7.54 (t, J = 7.37 Hz, 2H), 7.39 (d, J = 7.40 Hz, 4H), 2.75 (t, J = 7.06 Hz, 4H), 1.25 (d, J = 3.82 Hz, 12H), 1.20 (d, J = 6.85 Hz, 12H). HRMS (ESI) m/z: M + 2H⁺ C₆₄H₄₆N₈O₆ calcd. 1022.3540; found 1024.3485.

Methods

Solution ¹H NMR spectra were measured on a Bruker ARX 400 spectrometer. The UV-Vis absorption and fluorescence spectra of 1–4 were recorded on a Shimadzu UV-2501 and RF-5301 spectrophotometer, respectively. MS were collected on a MALDI TOF2 AXIMA mass spectrometer and HiRes-MALDI TOF MS spectra were recorded on a Waters Q-Tof premier TM mass spectrometer. Fluorescence decay lifetimes were measured by exciting the samples in CHCl₃ (10⁻⁵ mol L⁻¹) using a laser flash photolysis spectrometer (LKS.60, Applied Photophysics), equipped with a Q-switched Nd:YAG laser (Brilliant B, Quantel), a 150 W pulsed Xe lamp, and a R928 photomultiplier was used to record nanosecond-difference absorption spectra. The samples were excited at 400 nm, and each time-resolved trace was acquired by averaging 10 laser shots at a repetition rate of 1 Hz. The room temperature fluorescence decay was conducted by exciting the samples in CHCl₃ (10⁻⁵ mol L⁻¹) with 400 nm, 150 fs laser pulses. These laser pulses were generated from a Coherent TOPAS-C optical parametric amplifier that was pumped using a 1 kHz Coherent Legend™ regenerative amplifier, which was seeded by a 80 MHz Coherent Vitesse™ oscillator. The laser pulses were focused by a lens (f = 25 cm) on the solution sample in a 2 mm-thick quartz cell. The emission from the samples was collected at a backscattering angle of 150° by a pair of lenses and directed into an Optronis Optoscope™ streak camera system, which has an ultimate temporal resolution of 10 ps.

The nanostructures of 1–4 (ADIs) were prepared as follows: 1 mg of the ADIs was dissolved in 20 ml THF, MeOH–CHCl₃ (5 : 1, v/v), hexane–dichlorbenzene (5 : 1, v/v), or MeOH–CHCl₃ (5 : 1, v/v), respectively. The mixture was stirred for 2 h and filtered due to the weak solubility of the ADIs. The transparent solutions were kept in Al-foil sealed bottles/tubes and remained at room temperature for several weeks. The nanostructures were formed at the bottom of the bottles/tubes. The as-prepared samples were coated with platinum in an ion coater for 30 s before the SEM investigation. The sizes and shapes of the nanostructures were observed on a FE-SEM (JSM-7600F, JEOL) at an accelerating voltage of 5 kV. The optical image was recorded by a Polarizing Microscope Olympus BX53. Electrochemistry was carried out with a CHI 600C potentiostat, employing a platinum button (diameter: 1.6 mm; area 0.02 cm²), a platinum wire and 0.01 M Ag/AgCl (Ag/Ag⁺) as the working, counter and reference electrode, respectively. 0.1 M of tetrabutylammonium hexafluorophosphate (TBAPF₆) in CHCl₃ was used as the electrolyte.

Device fabrication

Electroluminescence devices were fabricated. The typical fabrication method of the heterojunction LED is as follows: The HF-cleaned p-SiC substrate was coated with a metal contact (size of about 2 × 2 mm²), which consisted of a layer of a 10 nm Al film, then a ∼80 nm thick Ti film and a layer of a ∼380 nm thick Al film using e beam evaporation at room temperature. Water-dispersed nanofibers of 2 were drop-cast on the opposite surface of the metal-electrode-deposited p-SiC substrate. The top of the nanofibers of 2 was covered by the ITO coated quartz substrate, which was used as an n-type 2 nanofiber contact. The EL spectra of the heterojunction LED were measured by connecting the anode and cathode of a rectangle pulse voltage source (with a repetition rate and pulse width of 7.5 Hz and 80 ms, respectively) to the ITO coated quartz substrate and Al (10 nm)/Ti (80 nm)/Al (380 nm)/metal contact on the p-SiC, respectively. The light was collected from the uncoated side of the quartz substrate by an objective lens.

Acknowledgements

Q. Zhang acknowledges financial support from AcRF Tier 1 (RG 16/12) and Tier 2 (ARC 20/12 and ARC 2/13) from the MOE, the CREATE program (Nanomaterials for Energy and Water Management) from NRF, and the New Initiative Fund from NTU, Singapore. J. Loo acknowledges an NMRC-EDG grant (EDG09may011) and the SCELSE project (M020070110). W. Huang acknowledges the National Basic Research Program of China (2009CB930601), the National Natural Science Foundation of China (21003706, 21274064, 21144004, 60876010, 61177029, 20774043, 20704023, and 20974046). J. Zhao acknowledges Jiangsu Province Science Foundation for Youths (BK20130912). G. C. Xing and T. C. Sum acknowledge financial support by the Singapore National Research Foundation through the Competitive Research Programme under Project no. NRF-CRP5-2009-04 and the Singapore-Berkeley Research Initiative for Sustainable Energy (SinBerRISE) CREATE Programme.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra01049c

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