Green Perylene Bisimide Dyes: Synthesis, Photophysical and Electrochemical Properties

Three asymmetric amino-substituted perylene bisimide dyes with different n-alkyl chain lengths (n = 6, 12, or 18), 1-(N,N-dialkylamino)perylene bisimides (1a–1c), were synthesized under mild condition in high yields and were characterized by 1H NMR, 13C NMR (nuclear magnetic resonance), HRMS (High Resolution Mass Spectrometer), UV-Vis and fluorescence spectra, as well as cyclic voltammetry (CV). These molecules show intense green color in both solution and solid state and are highly soluble in dichloromethane and even in nonpolar solvents, such as hexane. The shapes of the absorption spectra of 1a–1c in solid state and in solution were found to be virtually the same, indicating that the long alkyl chains could efficiently prevent aggregation. They exhibit a unique charge transfer emission in the near-infrared region, of which the peak wavelengths show strong solvatochromism. The dipole moments of the compounds have been estimated using the Lippert-Mataga equation, and upon excitation, they show larger dipole moment changes than that of 1-aminoperylene bisimide (2). Furthermore, all of the compounds exhibit two quasi-reversible one-electron oxidations and two quasi-reversible one-electron reductions in dichloromethane at modest potentials. Complementary density functional theory (DFT) calculations performed on these dyes are reported in order to rationalize their molecular structures and electronic properties.

To date, a useful strategy for introducing substituents onto the PBIs core is bromination of perylene dianhydride. Subsequently, nucleophilic substitutions and metal-catalyzed cross-coupling reactions can then be executed. However, these reactions are usually accompanied by extensive debromination [78] and stringent reaction conditions, such as high temperatures and absence of water and oxygen. In an effort to expand the scope of PBI-based chromophores available for designing systems for colorful dyes and charge transport, we synthesized a series of purple dyes based on 1-aminoperylene bisimides [76]. We now report on the introduction of different long alkyl chains of 1-aminoperylene bisimide (2) affording chromophores (1a-1c) that are intense green in color and that readily undergo two quasi-reversible one-electron oxidations and two quasi-reversible one-electron reductions.

Synthesis of 1-Nitroperylene Bisimide (3)
A mixture of bisimide 4 (900 mg, 1.6 mmol), cerium (IV) ammonium nitrate (CAN) (1.2 g, 2.2 mmol), nitric acid (0.1 M, 3.0 mL) and dichloromethane (150 mL) was stirred at 25 °C under N 2 for 2 h. The mixture was neutralized with 10% KOH and extracted with CH 2 Cl 2 . After the solvent was removed, the crude product was purified by silica gel column chromatography with eluent CH 2 Cl 2 to afford 3 (920 mg, 95%). Characterization data for 3: 1 (2) Tin chloride dihydrate (5.0 g, 22 mmol) and 3 (0.9 g, 1.5 mmol) were suspended in 50 mL of THF and stirred for 20 min. The solvent was refluxed with stirring for 2 h at 80 °C. THF was removed from the rotary evaporator, and the residue was dissolved in ethyl acetate and washed with 10% NaOH solution and brine. The organic layer was dried over anhydrous MgSO 4 , and the filtrate was concentrated under reduced pressure. The crude product was purified by silica gel column chromatography with eluent ethyl acetate/n-hexane (2/3) to afford 2 (680 mg, 80%). Characterization data for 2: 1

General Procedure for Alkylation (1a-1c)
A mixture of a solution of 2 (400 mg, 0.70 mmol), sodium hydride (97%, 100 mg, 4.00 mmol) and dry THF (50 mL) was stirred at 0 °C under N 2 for 30 min. Alkyl iodide (1.60 mmol) was then added, and the resulting mixture was stirred for 2 h. The resulting mixture was diluted with 15 mL of water and extracted with CH 2 Cl 2 . The crude product was purified by silica gel column chromatography with eluent ethyl acetate/n-hexane (1/2) to afford 1a (1b or 1c) in an 85% yield. Characterization data for 1a: 1

Synthesis
Scheme 1 depicts the chemical structures and synthetic routes of asymmetric amino-substituted PBIs (1a-1c). Synthesis starts from an imidization of perylene dianhydride (5) by reaction with cyclohexylamine. The mono-nitration can then be achieved by a reaction of perylene bisimide (4) with cerium (IV) ammonium nitrate (CAN) and HNO 3 under ambient temperature for 2 h [58], giving 3 in high yields of ca. 90%. The reduction of 1-nitroperylene bisimide (3) by tin (II) chloride dihydrate (SnCl 2 ·2H 2 O) in refluxing THF obtained 1-aminoperylene bisimide (2). Finally, three highly soluble perylene bisimide derivatives (1a-1c) with different n-alkyl chain lengths (n = 6, 12, or 18, Figure 1 Figure 3 shows the steady-state absorption spectra of the green dye 1a in solvents of varying polarity, and pertinent photophysical data for 1a-1c are summarized in Table 1. The spectra of all of the amino-substituted PBIs (1a-1c and 2) are dominated by broad absorption bands that cover a large part of the visible spectrum (350-750 nm). These broad bands are representative for perylene bisimide derivatives N-substituted at the bay-core positions, due to charge transfer absorption [78]. The longest wavelength absorption bands of 1a-1c in various solvents are found to be almost the same, which indicates that different N-alkyl chain lengths do not significantly affect the band gap energies. Moreover, the longest wavelength absorption band of 1a-1c exhibits a red shift when the solvent polarity increases (Table 1), which is consistent with the previous studies [76].

Optical Properties
The steady-state emission spectra of 1a in different solvents of varying polarity are shown in Figure 4. Unlike the small shift in absorption spectra, the fluorescence spectra of 1a-1c are largely red-shifted if there is any increase of the solvent polarity, which indicates strong intramolecular charge transfer (ICT) characteristics for the excited states of the compounds, 1a-1c (Table 1). We further used the well-established fluorescence solvatochromic shift method [87] to measure the stabilization of the excited states of 1a-1c.
The plot of the Stokes shift

Quantum Chemistry Computation
To gain more insight into the molecular structures and electronic properties of 1-4, quantum chemical calculations were performed using density functional theory (DFT) at the B3LYP/6-31G** level [88,89]. The highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) of 1a and 2 are shown in Figure 6. The HOMO of all amino-substituted PBIs (1a-1c and 2) is delocalized mainly on the amino group and the perylene core, while the LUMO is extended from the central perylene core to the bisimide groups. The calculated and experimental parameters for perylene bisimide derivatives 1-4 are summarized in Table 2. It is apparent that the HOMO/LUMO energy levels of 1a-1c and 2 are higher than those of 3 and 4; this can be explained by the fact that the amino (nitro) substituent is a strong electron-donating (electron-withdrawing) group and hence increases (decreases) both the HOMO and LUMO energy levels. Additionally, the relative band gap energies estimated from the longest absorption maxima of 1-4 are in good agreement with the theoretical calculations ( Table 2). DFT calculations also show that the ground-state geometries of the perylene core have different core twist angles (Figure 7), i.e., approximate dihedral angles between the two naphthalene subunits attached to the central benzene ring; these are ~9.40° and ~13.43° for 1a, ~9.42° and ~13.45° for 1b, ~9.45° and ~13.49° for 1c, ~9.23° and ~17.49° for 2 and ~7.89° and ~15.87° for 3 (Table 2); and all are larger than those of 4 (~0.00°). As a whole, the core twist angles of the mono-substituted PBIs (1)(2)(3) are larger than that of the non-substituted compound (4).

Electrochemical Properties
The cyclic voltammograms of 1a-1c are illustrated in Figure 8. These dyes undergo two quasi-reversible one-electron oxidations and two quasi-reversible one-electron reductions in dichloromethane, which clearly indicates that all of these processes can be attributed to the successive addition or removal of electrons to the orbitals. Table 3 summarizes the redox potentials and the HOMO and LUMO energy levels estimated from cyclic voltammetry (CV) for 1-4. It appears that both the first oxidation and the first reduction potentials can be shifted toward more negative (positive) values by introducing strongly electron-donating (electron-withdrawing) groups onto the perylene core, while both the HOMO and LUMO energy levels increase (decrease) with the trend.   Figure 9 shows the absorption spectra recorded for thin drop-cast films of 1a-1c. The shapes of the absorption spectra of 1a-1c in solid state and in solution are found to be virtually the same, in view of the wavelength range and peak positions. The results clearly show that it is difficult for compounds 1a-1c to form π-aggregates; this can be explained by the fact that the long alkyl chains can efficiently prevent aggregation. In contrast, the absorption spectrum of the drop-cast film chromophore 2 is red-shifted, as well as broadened compared to its respective spectrum in nonpolar cyclohexane, which can be attributed mainly to intermolecular π-π interactions in the solid state [76].

Influence of the Acidic Condition on the Optical Properties of Dyes
The effects of strongly acidic conditions on the absorption and emission spectra of 1a were also examined. Figure 10 shows the absorption and emission spectra of 1a in concentrated HCl. The absorption spectrum of 1a in such an acidic condition loses its typical absorption band over 590 nm and, instead, shows non-substituted PBIs centered at 520 nm, as also observed by the red color of the analyzed solution. The most likely explanation is that the nitrogen atom on the bay-core is protonated in this medium, so that charge transfer is no longer possible (Scheme 2). Moreover, the protonation-dependent change of color is completely reversible.

Conclusions
We have successfully synthesized three green dyes based on alkylamino-substituted PBIs (1a-1c). All of the new PBI dyes are soluble in most organic polar and nonpolar solvents. These molecules show a unique charge transfer emission in the near-infrared region, and the associated peaks exhibit solvatochromism. Upon excitation, they show larger dipole moment changes than that of 2; the dipole moments of these compounds have been estimated using the Lippert-Mataga equation. Furthermore, they display reversible redox properties, as well as good optical stability. Research on their applications to organic photovoltaic cells is currently in progress.

Acknowledgments
The project was supported by the Ministry of Science and Technology (MOST 103-2113-M-035-001) in Taiwan. The authors appreciate the Precision Instrument Support Center of Feng Chia University for providing the fabrication and measurement facilities.

Author Contributions
Che-Wei Chang designed and performed the experiments. Hsing-Yang Tsai measured and analysed the data. Kew-Yu Chen supervised the project.

Conflicts of Interest
The authors declare no conflict of interests.