Highly Soluble Monoamino-Substituted Perylene Tetracarboxylic Dianhydrides: Synthesis, Optical and Electrochemical Properties

Three dialkylamino-substituted perylene tetracarboxylic dianhydrides with different n-alkyl chain lengths (n = 6, 12 or 18), 1a–1c, were synthesized under mild conditions in high yields and were characterized by 1H NMR, 13C NMR and high resolution mass spectroscopy. Their optical and electrochemical properties were measured using UV-Vis and emission spectroscopic techniques, as well as cyclic voltammetry (CV). This is the first time that the structures and the properties of monoamino-substituted perylene tetracarboxylic dianhydrides have been reported. These molecules show a deep green color in both solution and the solid state and are soluble in most organic solvents. They all show a unique charge transfer emission in the near-infrared region, and the associated peaks exhibit solvatochromism. The dipole moments of the compounds have been estimated using the Lippert-Mataga equation, and upon excitation, they show slightly larger dipole moment changes than those of corresponding perylene diimides, 2a–2c. Additionally, Compounds 1a–1c undergo two quasi-reversible one-electron oxidations and two quasi-reversible one-electron reductions in dichloromethane at modest potentials. Complementary density functional theory calculations performed on these chromophores are reported in order to gain more insight into their molecular structures and optical properties.

PDIs and PTCDs suffer from serious problems, such as poor solubility and aggregation. A number of synthetic methods to prepare PDI and PTCD derivatives with improved solubility have been reported [53,55,59]. The synthesis of highly soluble PDI and PTCD derivatives is very important for process ability and for the preparation of their thin films to be used in optoelectronics applications, such as OFETs, OLEDs and OSCs. Soluble PDI derivatives can be obtained by introducing long and bulky substituents at the perylene core and/or at the imide nitrogen atoms, while soluble PTCDs can only be prepared by introducing substituents at the perylene core. Several diamino-substituted PTCDs based on this concept have been synthesized and studied so far [64,65]. However, to the best of our knowledge, the molecular structures, as well as the optical and electrochemical properties of monoamino-substituted PTCDs have not been reported yet. To expand the scope of highly soluble PTCD-based chromophores available for designing systems for colorful dyes and charge transport, we report here the detailed synthesis and characterization of monoamino-substituted PTCDs (1a-1c), shown in Scheme 1. Furthermore, the optical, electrochemical and complementary density functional theory calculations of the newly synthesized PTCDs are investigated.

Synthesis
Scheme 1 depicts the chemical structures and synthetic routes of monoamino-substituted asymmetrical PTCDs (1a-1c). In brief, the synthesis of 1a-1c started from an imidization of perylene dianhydride 6 and cyclohexylamine in the presence of acetic acid, followed by the mononitration of perylene diimide 5, giving a nitro compound 4. The reduction of 1-nitroperylene diimide (4) by tin (II) chloride dihydrate (SnCl2·2H2O) in refluxing THF afforded 1-aminoperylene diimide (3). Next, three highly soluble PDI derivatives (2a-2c) with different N-alkyl chain lengths (n = 6, 12 or 18) were prepared by the alkylation of 3 with the corresponding alkyl halides. Finally, alkylamino-substituted PDIs 2a-2c were converted to the respective PTCDs via saponification to afford 1a-1c. The asymmetric structure of 1a-1c can be verified by the presence of seven signals (one singlet and six doublet signals) at δ 8.3-9.4 ppm in the 1 H NMR spectrum (Figure 1), which indicates that there are seven different kinds of protons in the perylene core. Detailed synthetic procedures and product characterization are provided in the Experimental Section and Supplementary Materials (Figures S1-S10). Scheme 1. The synthetic routes for 1a-1c.

X-ray Structure
The structure of 2a was further confirmed by single-crystal X-ray diffraction analysis ( Figure 2). To the best of our knowledge, this is the first time that the X-ray structure of monosubstituted PDI has been resolved. Compound 2a crystallizes in the triclinic space group P-1, with a = 9.8924 (6), b = 14.6338 (10), c = 15.6221 (10) Å, α = 105.098 (3)°, β = 103.386 (2)°, γ = 107.772 (3)° and Z = 2. The central six-membered ring of 2a is twisted with dihedral angles of 11.6 (3)° and 14.9 (3)° associated with bay area carbon atoms C7-C8-C15-C16 and C11-C10-C13-C22, respectively, these values being smaller than those of disubstituted and tetrasubstituted PDIs [39,60]. All C-C bond lengths of the perylene moiety range between 1.358 and 1.486 Å (Table 1), which indicates the presence of π-conjugation for all C-C bonds. The longest bonds in the perylene backbone are the C8-C15 and C10-C13 bonds that connect the two naphthalene rings at a distance of 1.47 Å. The bond length is almost identical to the lengths observed for other PDIs [39,60]. These results demonstrate that the degree of conjugation of the perylene unit is similar in both unsubstituted and core-substituted PDIs.
119.3 (2) 119.2 C(11)-C(10)-C (13) 123.4 (2) 123.3 C(13)-C(14)-C (19) 120.6 (2) 120.  Figure 3 shows the absorption spectra of the green dyes 1a and 2a, the purple dye 3 and the red dye 4 in dichloromethane. The absorption spectrum of 1-nitroperylene diimide (4) is almost identical with the spectrum of the non-substituted perylene diimide (5), but it does not show fluorescence [61]. On the other hand, the reduction of 4 to 3 switches the substituent from an electron-withdrawing nitro group to an electron-donating amino group and causes a significant red shift. The spectra of 1a, 2a and 3 are dominated by very broad absorption bands that cover a large part of the visible spectrum (350-750 nm). These broad bands are representative of perylene diimide (dianhydride) derivatives N-substituted at the bay-core positions, due to charge transfer absorption [53]. The longest wavelength absorption band of 1-(N,N-dihexylamino)perylene diimide (2a: 610 nm) is red-shifted relative to that of 1-aminoperylene diimide (3: 578 nm), but it is blue-shifted relative to that of 1-(N,N-dihexylamino)perylene tetracarboxylic dianhydride (1a: 650 nm). It appears that the inductive effect of the alkyl groups in 1a and 2a causes an additional red shift. Interestingly, the longest wavelength absorption band of 1a is 40-nm red-shifted relative to that of 2a; the decrease in the energy band gap is attributed to a great decrease in the LUMO energy level (vide infra).   Figures S11 and S12). 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 characteristics for the excited states of 1a-1c (Table 2). Using the fluorescence solvatochromic shift method [66], we measured the stabilization of the excited-states of 1a-1c and compared these results to those of 2a-2c. The change of magnitudes for dipole moments between ground and excited states, i.e., μ μ μ e g Δ = −   , can be estimated by the Lippert-Mataga Equation (1) and expressed as:

Optical Properties
where h is the Planck constant, c is the speed of light, 0 a denotes the cavity radius in which the solute resides, is the Stokes shift of the absorption and emission peak maximum, and f Δ is the orientation polarizability as shown in Equation (2).
The plot of the Stokes shift    Figure 6 shows the cyclic voltammograms of 1a and 2a. Both undergo two quasi-reversible one-electron oxidations and two quasi-reversible one-electron reductions in dichloromethane at modest potentials. One can clearly see that both the first oxidation and the first reduction potentials of 1a are larger than those of 2a; this can be explained by the fact that the anhydride group is a stronger electron-withdrawing group than the imide group. Table 3

Quantum Chemistry Computation
To gain deeper insight into the molecular structures and electronic properties of 1a-1c and 2a-2c, quantum chemical calculations were performed using the density functional theory (DFT) at the B3LYP/6-31G** level. Figure 7 depicts the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) of 1a and 2a. The HOMO of 1a (2a) is delocalized mainly on the amino group and the perylene core, while the LUMO is extended from the central perylene core to the dianhydride (diimide) groups. The calculated and experimental parameters for 1a-1c and 2a-2c are summarized in Table 4. It is apparent that both the HOMO and LUMO energy levels of 1a-1c are lower than those of 2a-2c and are in good agreement with the experimental data.

Stacking Behaviors of Dyes in Solution and Solid State
The stacking behaviors of 1a-1c and 3 in ethyl acetate were investigated by concentration-dependent UV-Vis measurements (from 10 −4 to 10 −6 M). The absorption spectra of 1a at different concentrations in ethyl acetate are shown in Figure 9; the absorption spectra of 1b, 1c and 3 at different concentrations in ethyl acetate are shown in the Supplementary Materials (Figures S13-S15). At high concentrations, a clear red shift was observed for 1-aminoperylene bisimide 3, indicating the formation of J-type aggregates [51].
However, compounds 1a-1c show no significant red shift at high concentrations of ethyl acetate, which can be explained by the fact that the long alkyl side chains of 1a-1c decrease the π-π interactions. Figure 10 shows the absorption spectra recorded for the thin drop-cast films of 1a-1c. The shapes of the absorption spectra of 1a-1c in the solid state and in solution show significant differences, in view of the wavelength range and peak positions. The absorption spectra of all of the drop-cast film chromophores were broadened, as well as red-shifted compared to their respective spectra in cyclohexane solution, which indicates aggregation. This spectral change can be mainly attributed to intermolecular π-π interactions in the solid state [67].

General
The starting materials, such as perylene-3,4,9,10-tetracarboxylic dianhydride, acetic acid, cyclohexylamine, cerium(IV) ammonium nitrate (CAN), 1-methyl-2-pyrrolidinone (NMP), tetrahydrofuran (THF), tin (II) chloride dihydrate (SnCl2·2H2O), KOH and 2-propanol, were purchased from Merck (Whitehouse Station, NJ, USA), ACROS (Pittsburgh, PA, USA) and Sigma-Aldrich (St. Louis, MO, USA). Solvents were distilled freshly according to the standard procedure. Column chromatography was performed using silica gel Merck Kieselgel si 60 (40-63 mesh). 1 H or 13 C NMR spectra were recorded in CDCl3 on a Bruker 400 or 500 MHz instrument (Palo Alto, CA, USA). Mass spectra were recorded on a VG70-250S mass spectrometer (Tokyo, Japan). The absorption and emission spectra were measured using a Jasco V-570 UV-Vis spectrophotometer (Tokyo, Japan) and a Hitachi F-7000 fluorescence spectrophotometer (Tokyo, Japan), respectively. The Gaussian 03 program (Pittsburgh, PA, USA) was used to perform the ab initio calculation on the molecular structure. Geometry optimizations for Compounds 1a-1c and 2a-2c were carried out with the 6-31G** basis set to the B3LYP functional. Vibrational frequencies were also performed to check whether the optimized geometrical structures for all compounds were at energy minima, transition states or higher order saddle points.

Synthesis of 1-Aminoperylene Diimide (3)
Tin chloride dihydrate (2.5 g, 10.2 mmol) and 4 (1.0 g, 1.7 mmol) were suspended in THF (50 mL) and stirred 20 min. The solvent was refluxed at 80 °C with stirring for 2 h. THF was removed at the rotary evaporator, and the residue was dissolved in ethyl acetate and washed with 10% sodium hydrate solution and brine. The organic layer was dried over anhydrous MgSO4, 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 3 in an 80% yield. Characterization data: 3: 1

General Procedure for Alkylation (2a-2c)
A mixture of a solution of 3 (400 mg, 0.70 mmol), sodium hydride (97%, 100 mg, 4.00 mmol) and dry THF (50 mL) was stirred at 0 °C under N2 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 CH2Cl2. The crude product was purified by silica gel column chromatography with eluent ethyl acetate/n-hexane (1/2) to afford 2a (2b or 2c) in an 85% yield. Characterization data: 2a: 1   2a (2b or 2c, 0.27 mmol) was taken in 2-propanol (30 mL), and subsequently, KOH (1.9 g, 33.8 mmol) was added. The reaction mixture was stirred under N2 at reflux for 4 h. After being cooled to room temperature, the reaction mixture was poured into acetic acid (50 mL) and stirred overnight. The resulting green precipitate was collected by filtration, washed with water and methanol and dried. The crude product was purified by silica gel column chromatography with eluent CH2Cl2 to afford 1a (1b or 1c) in a 75% yield. Characterization data: 1a: 1

Crystal Structural Determination
A single crystal of 2a with dimensions of 0.48 mm × 0.12 mm × 0.03 mm was selected. The lattice constants and diffraction intensities were measured with a Bruker Smart 1000 CCD area detector radiation (λ = 0.71073 Å) at 150 (2) K. An ω-2θ scan mode was used for data collection in the range of 2.22 ≤ θ ≤ 26.507. A total of 32,126 reflections were collected, and 8084 were independent (Rint = 0.0626), of which 4733 were considered to be observed with I > 2σ (I) and used in the succeeding refinement. The structure was solved by direct methods with SHELXS-97 [68] and refined on F 2 by a full-matrix least-squares procedure with Bruker SHELXL-97 packing [69]. All non-hydrogen atoms were refined with anisotropic thermal parameters. The hydrogen atoms refined with riding model position parameters isotropically were located from the difference Fourier map and added theoretically. At the final cycle of refinement, R = 0.0644 and wR = 0.1601 (w = 1/[σ 2 (Fo 2 ) + (0.0871P) 2

Conclusions
We have successfully synthesized three monoamino-substituted asymmetrical perylene tetracarboxylic dianhydrides with different n-alkyl chain lengths (1a-1c). These molecules show an intense green color in both solution and the solid state and are soluble in most organic solvents and even in nonpolar solvents, such as hexane. They show a unique charge transfer emission in the near-infrared region, of which the peak wavelengths exhibit strong solvatochromism. Upon excitation, they show slightly larger dipole moment changes than those of corresponding perylene diimides 2a-2c; the dipole moments of these compounds have been estimated using density functional theory calculations and the Lippert-Mataga equation. Furthermore, they undergo two quasi-reversible one-electron oxidations and two quasi-reversible one-electron reductions in dichloromethane at modest potentials. Research on their applications to dye-sensitized solar cells (DSSCs) 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
Kew-Yu Chen supervised the project. Che-Wei Chang measured the data. All authors read and approved the final manuscript.