Solution processable luminescent fluorinated perylene derivatives showing high n-type mobility and columnar self-assembly

Abstract

Unsymmetrically substituted fluorinated perylenes and their corresponding all-alkyl derivatives were designed and synthesized using a microwave reactor. Perfluoro diester perylene (PEIF) was found to stabilize the columnar phase at high temperature, whereas the other fluoro derivative, perfluoro imide perylene (PBIF), exhibits crystalline properties. Both the alkyl derivatives, perylene ester imide (PEIH) and perylene bisimide (PBIH), are crystalline. All four molecules exhibit good solution processability and excellent thermal stability at high temperatures. All four derivatives are highly fluorescent with their relative quantum yield values reaching as high as ∼93% with respect to Rhodamine-6G. Compounds PEIF and PEIH in particular showed intense red emission in the solid state as well. The hydrophobic nature of these materials was also characterised by water contact angle measurements. A systematic investigation of SCLC devices revealed that PBIF exhibits exceptional out-of-plane bulk electron mobility of 1.3 × 10⁻² cm² V⁻¹ s⁻¹. This performance is attributed to its superior molecular packing, low surface roughness, optimal energy alignment, and high crystallinity. These findings underscore the critical role of structural order and interface engineering in the development of high-performance organic electron transport materials.

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I. Introduction

Over the last decade, there have been significant strides made by researchers in the field of organic electronics. The use of organic semiconducting materials as functional components in various optoelectronic devices has gained significant attention.^1–3 Yet the expansion of the field of n-type semiconductors is still very challenging and not as fully explored as compared to the already established segment of p-type semiconductors. Perylene derivatives have emerged as one of the leading contenders for n-type materials due to their high electron affinity, low energy LUMOs and good charge transport behavior.^4–8 Perylene based materials are promising candidates for various optoelectronic devices because of their excellent thermal and chemical stability, and the ability to fine-tune the molecular design, coupled with their exceptional electronic and optical properties. They are being extensively used in various optoelectronic applications, such as photovoltaic solar cells, organic light emitting diodes (OLEDs), sensors, energy storage devices and organic field effect transistors (OFETs).^9–15

Amongst π-conjugated polyaromatic materials, molecular ordering and arrangement is crucial for the realization of high performance electronic and photonic properties. Self-organizing liquid crystalline (LC) perylene derivatives offer significant advantages due to the additional degree of ordering present in the mesophase, as well as their potential for self-healing and solution processability.^16–19 Perylene, being a polyaromatic core, suffers from limited solubility in common organic solvents and hence in its film forming ability, which is very crucial for various applications. Imbuing liquid crystallinity in perylene derivatives with the introduction of flexible chains eliminates these drawbacks by increasing their solubility in common solvents and hence their processability.^20–25 The columnar phase displayed by liquid crystalline perylene derivatives is characterized by a co-facial π-stacked arrangement of the perylene cores, which facilitates the uniaxial charge transport along the stacking axis. Several perylene bisimides (PBIs) and perylene tetraesters (PTEs) have been studied extensively, whereas the perylene diester imides (PEIs), and unsymmetrical PBIs are less reported due to the inherent synthetic limitations.^26–28 Various structural modifications can be done at the imide position with the incorporation of straight/branched alkyl chains, oligoethoxy chains, an oligosiloxane moiety and a fluorinated chain to tune the self-assembly and functional behavior.^29–33 A well proven strategy to increase the n-type semiconductor property of organic semiconductors is to attach a strongly electron-withdrawing fluorine atom or perfluoalkyl chain to the central aromatic core. The addition of a perfluoro chain into the thiophene core has already demonstrated high mobility n-type semiconductor properties.^34–39 Chen et al. have reported several PBI derivatives bearing electron withdrawing –F and –CF₃ moieties on the two phenyl rings at the imide position, which exhibited electron charge–carrier mobilities as high as 0.066 cm² V⁻¹ s⁻¹.⁴⁰ Recently, Yao et al. have shown the use of core-fluorinated PDI as a cathode interlayer material (CIL) in organic solar cells (OSC) achieving an impressive power conversion efficiency (PCE) of 18%.⁴¹ The addition of a perfluoroalkyl chain in PDI makes it an even stronger electron acceptor. Additionally, most n-type semiconductors suffer from oxidative degradation over time; however, this issue can be addressed effectively by the introduction of highly electronegative moieties. These groups lower the energy of the lowest unoccupied molecular orbital (LUMO), making the charge carrier less susceptible to oxidation in air.^42–45

Herein, we describe the design and synthesis of perfluoroalkyl chain appended unsymmetrical perylene derivatives and their corresponding alkyl derivatives. Perfluoro chains featuring perylene ester imide (PEIF) and unsymmetrical perylene bisimide (PBIF) were systematically investigated, and their properties were comprehensively compared with those of their alkyl-substituted counterparts, PEIH and PBIH. Interestingly, one of the derivatives, PEIF, has shown liquid crystalline properties at elevated temperature revealing an ordered molecular arrangement that could be beneficial for advanced material applications. All molecules have shown remarkably high relative quantum yields (∼90%) and could be utilized for organic light emitting diodes (OLEDs). Charge carrier mobility measurements of all derivatives with the help of the space charge limited current (SCLC) method showed that they are good n-type semiconductors.

II. Results and discussion

II.1 Synthesis and molecular structural characterization

Detailed procedures for the synthesis of all four final molecules and the intermediates are given in Schemes 1 and 2. Commercially available perfluoroalkyl iodide was treated with phthalimide salt to give the corresponding perfluoroalkyl phthalimide (1), which was then subjected to reductive hydrolysis to produce the important intermediate perfluoroalkyl amine (2). In a similar fashion, the other non-fluorinated alkyl amine (4) was prepared starting from 1-bromodecane.⁴⁶ Perylene tetracarboxylic dianhydride was first hydrolyzed and then refluxed with 1-bromodecane after pH adjustment to yield perylene tetraester (8).^47,48 The tetraester was then subjected to partial hydrolysis by treating with p-toluenesulphonic acid and a toluene–dodecane mixture to give the intermediate perylene diester monoanhydride (9).^49,50 Perylene bisimide (6) was prepared by reacting perylene tetracarboxylic dianhydride with swallow-tail amine (5) and the resultant product then undergoes partial hydrolysis in the presence of KOH to afford the vital intermediate perylene imide monoanhyride (7).^50,51 The diester monoanhydride (9) is finally treated with the perfluoalkyl amine and decyl amine under microwave conditions to give the final products PEIF and PEIH. The other two derivatives PBIF and PBIH were also prepared in a similar manner when perylene imide monoanhydride (7) was treated with fluorinated and non-fluorinated amine, 2 and 4, respectively.^52,53 All molecules were analysed and characterized thoroughly by several common techniques like ¹H NMR, ¹³C NMR, IR spectroscopy and MALDI-TOF mass spectrometry.

Scheme 1 Reagents and conditions: (i) potassium phthalimide, DMF, 80 °C, 12 h (61%); (ii) hydrazine hydrate, methanol, reflux, 12 h (90–95%); (iii) potassium phthalimide, DMF, 80 °C, 12 h (93%); (iv) hydrazine hydrate, methanol, reflux, 12 h (90–95%); (v) NH₄OAc, NaBH₃CN, MeOH, rt, 72 h (95%); (vi) 12-aminotricosane, imidazole, Zn(OAc)₂, 140 °C, 6 h (91%); (vii) KOH, t-butanol, 80 °C, 55 min (39%); (viii) KOH, H₂O, 70 °C, 30 min, 1 M HCl, aliquat 336, KI, n-bromodecane, reflux, 24 h (77%); (ix) p-TsOH·H₂O, n-dodecane/toluene (5/1), 95 °C, 5 h (70%).

Scheme 2 Reagents and conditions: (i) 2, imidazole, Zn(OAc)₂, 145 °C, 45 min, MW (65%); (ii) 4, imidazole, Zn(OAc)₂, 145 °C, 45 min, MW (68%); (iii) 2, imidazole, Zn(OAc)₂, 140 °C, 40 min, MW (72%); (iv) 4, imidazole, Zn(OAc)₂, 140 °C, 40 min, MW (74%).

II.2 Thermal behavior

The thermal behavior of all four final molecules was investigated using polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and variable temperature X-ray diffraction (XRD) measurements. The thermal stability of all four derivatives was probed by the thermogravimetric analysis (TGA) technique. All molecules showed robust thermal stability, with 5% decomposition noted at temperatures ≥360 °C (Fig. 1a). Mesomorphic behavior of all molecules was investigated using POM attached with a programmable hot stage to check if any characteristic mesophase was present. The findings from POM were later corroborated with DSC studies, which gave ample information about the enthalpy change during the phase transition. These complementary studies revealed that only PEIF exhibited a mesophase of short range out of the four molecules discussed here (Fig. 1b and Table 1). The symmetry and molecular arrangement in the liquid crystalline phase were eventually confirmed by powder X-ray measurements conducted at different temperatures.

Fig. 1 TGA plots (heating rate of 10 °C min⁻¹, nitrogen atmosphere) showing the temperatures at which 5 wt% decomposition occurs (a) and a bar graph showing the crystalline and mesophase range for compounds PEIF, PBIF, PEIH and PBIH (plotted from the first cooling cycle of DSC measurements) (b).

Table 1 Phase transition temperatures (°C), corresponding enthalpies (kJ mol⁻¹)^a and decomposition temperatures obtained by TGA

Compounds	Phase sequence (kJ mol^−1)		T 5 (°C)
	Second heating	First cooling
a Peak temperatures in the DSC thermograms obtained during the second heating and first cooling cycles at 5 °C min^−1. Cr = crystal; Colh = columnar hexagonal phase; I = isotropic liquid phase. b Temperature at which 5 wt% decomposition was observed in TGA.
PEIF	Cr 167.4 (35.9) Colh 183.3 (1.9) I	I 182 (2.1) Colh 146.2 (35.3) Cr	361
PBIF	Cr 191.3 (19.8) I	I 183.2 (19.3) Cr	413
PEIH	Cr1 72.6 (18) Cr2 155 (20.1) I	I 135.2 (20.4) Cr2 67.2 (15) Cr1	362
PBIH	Cr 158.9 (16) I	I 148.2 (15.5) Cr	364

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Compound PEIF was taken in between a clean glass slide and a coverslip and placed in the hot stage under POM. The sample was then subjected to steady heating at a rate of 5 °C min⁻¹. At a temperature of ∼160 °C, we observed an increase in birefringence along with fluidity suggesting that the compound transitioned from crystalline to liquid crystalline phase. On further heating, the LC phase transforms into an isotropic liquid state (Fig. S33). In the heating cycle of the DSC thermogram, two peaks were observed (Fig. 2a). The first peak at ∼167 °C corresponds to crystal to mesophase transition that involves a large enthalpy change (ΔH = 35.89 kJ mol⁻¹) whereas the second transition indicating mesophase to isotropic transition corresponds to a relatively small change (ΔH = 1.88 kJ mol⁻¹, at 183 °C) in enthalpy (Table 1). This pattern of enthalpy change is in accordance with earlier literature reports. On cooling the compound from the isotropic liquid state, batton-like optical texture starts growing from homeotropic domains at ∼182 °C (ΔH = 2.1 kJ mol⁻¹) indicating the presence of liquid crystalline phase (Fig. 2b). A decrease in birefringence as well as fluidity, was noted at ∼146 °C, with a huge enthalpy change (ΔH = 35.31 kJ mol⁻¹) suggesting crystallisation. This crystalline texture remains unaltered at room temperature. Though the optical texture showed similarity to the previous reports on columnar phases, the nature and symmetry of the mesophase were confirmed by powder X-ray diffraction measurements carried out at different temperatures. At 180 °C, the X-ray profile of PEIF displayed a strong reflection in the small angle region (2θ ≈ 3°) corresponding to a Bragg spacing d ≈ 29.8 Å. Two other peaks were observed in the mid angle region (2θ ≈ 5–10°) with corresponding d-spacing values of 9.93 Å and 8.53 Å, respectively (Fig. 2c). These three reflections can be indexed to a columnar hexagonal lattice with Miller indices of 10, 30 and 22 in the ratio 1 : 1/√9 : 1/√12. Other than these three peaks, two diffuse peaks were also observed in the wide-angle (2θ ≈ 16–26°) region. The first diffuse peak having a d-spacing value of 5.24 Å corresponds to the packing of the flexible perfluoro and alkyl tails. The second diffuse peak with a d-spacing of 3.53 Å indicates core–core interaction within a column, which corresponds to the core–core distance (h_c). The intense reflection at low angle gives the distance between adjacent (10) lattice planes d₁₀, which was used later to determine the lattice parameter ‘a’. The value of the hexagonal lattice parameter was found to be 34.4 Å, which is considerably less compared to the theoretical diameter of PEIF. This indicates possible interdigitation of the alkyl chains in neighbouring columns as well as close proximity of the perfluorinated chains between two molecules present in a single column. The value of lattice parameter was used to calculate the lattice area and molecular volume and they are found to be 1024.2 Ų and 4108.1 ų, respectively (Table 2). The number of molecules present in a columnar slice was also calculated from these values and it was found to be two (Z ≈ 2). A single disc is formed by the parallel arrangement of two molecules to form a disc, where the fluorous domains of both molecules interact with each other (Fig. 2d).⁵⁴ In the columnar phase, aromatic units stack one above the other and aliphatic units segregate separately leading to 1D self-assembly. Meanwhile, on addition of one more perfluoroalkyl chain, which is more rigid and incompatible with the alkyl chains, it gets phase segregated into a different zones (fluorophobic effect).⁵⁵ In such arrangements, the alkyl part remains outside thereby surrounding the column, by being more flexible and liquid like. As a result, π-stacking interactions increase and stabilize the columnar phase. These dimeric arrangements, when stacked upon one another, form cylindrical, column-like structures. (Fig. S34c, SI). Similar columnar hexagonal arrangements with two perylene-based molecules involved in the formation of a disc-like unit have been reported earlier.^52,53,56 X-ray measurement at lower temperature reveals multiple reflections in the wide-angle indicating transformation of the mesophase into a crystalline phase. Surprisingly, the peaks in the small angle region retain a hexagonal columnar pattern but the presence of multiple peaks in the mid and wide-angle region confirms the crystallinity at low temperature (Fig. S34, SI). The other perfluoro derivative, PBIF, was also investigated using POM and was found to be crystalline as no characteristic texture for the mesophase was observed (Fig. S33, SI). DSC thermograms suggest the same as there was only one peak in the heating and cooling segment with a large enthalpy change indicating crystal–isotropic liquid transition taking place (Fig. S32, SI). The alkyl chain derivative of the perylene ester imide displayed two distinct crystalline phases as evidenced from their DSC graph and POM images. In the DSC curve, two transitions were observed both in the heating and cooling cycle (Fig. S32c, SI). In the heating cycle, the first peak at 73 °C corresponds to crystal to crystal transition involving an enthalpy value of 18 kJ mol⁻¹, and the other peak with a similar large enthalpy value at 155 °C corresponds to crystal to isotropic liquid transition. These two crystalline phases are denoted by Cr₁ and Cr₂ in the DSC thermogram. When observed under POM, on cooling from the isotropic state, PEIH displayed two distinct textures corresponding to two different crystalline phases present in the molecule (Fig. S33, SI). The other derivative containing an alkyl chain, PBIH, also did not show liquid crystallinity as evidenced from the POM images and DSC thermogram (Fig. S32 and S33, SI).

Fig. 2 DSC thermogram for the first cooling (blue trace) and second heating (red trace) taken at 5 °C min⁻¹ for compound PEIF (a); POM image of the Col_h phase at 175 °C (scale bar 100 µm) (b); powder XRD pattern obtained for the Col_h phase at 180 °C (c); schematic diagram showing the self-assembly of PEIF to form the Col_h phase (by considering the XRD pattern obtained at 180 °C) (d).

Table 2 Results of (hkl) indexation of the XRD profiles of the compound PEIF at a given temperature (T) of the mesophase

Compounds (D/Å)	Phase (T/°C)	d obs (Å)	d cal (Å)	Miller indices hk	Lattice parameters a (Å), lattice area S (Å^2), molecular volume V (Å^3)
Colh = columnar hexagonal: a = lattice parameter = d10/cos 30°; lattice area S = a^2 sin 60°; lattice volume V = a^2 sin 60° × hc; no of molecules per slice of column (Z) = (√3 × Na × ρ × a^2 × h)/2M; Na = Avogadro number; ρ = density (considered as 1); a = lattice parameter; hc = core–core distance; ha = distance between the alkyl chains between the two discs in a column; M = molecular weight.
PEIF	Colh	29.79	29.79	10	a = 34.39
(37.53 Å)	(180 °C)	9.93	9.93	30	S = 1024.22
		8.53	8.53	22	V = 4108.11
MW: 1136		5.24 (ha)			Z ≈ 2
		3.53 (hc)

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II.3 Photophysical properties

Photophysical properties of all four derivatives were studied extensively by recording their absorption as well as emission spectra in micromolar chloroform solution and also in thin films deposited on a quartz substrate. Table 3 depicts the summarised data obtained from all photophysical studies. As in the case of the perylene derivative, four well-defined vibronic bands appeared in the absorption spectra comprising a maximum and three shoulder peaks in the shorter wavelength region (Fig. 3). The maximum absorption range for all four molecules spreads around 505–527 nm. The presence of different substituents on the imide nitrogen does not affect the absorption spectra. The maximum absorption wavelength for PEIF and PEIH are very close to each other while those for PBIF and PBIH are identical (λ_max = 526 nm). Both ester imide derivatives (PEIF and PEIH) have shown a hypsochromic shift (λ_max = 507 nm and 505 nm) in their maximum absorption wavelength compared to the bisimide derivatives. As is typically observed in the case of conjugated systems like perylene, all four derivatives have shown large molar extinction coefficient values (ε: 44 700–50 200 L mol⁻¹ cm⁻¹). The concentration dependent studies of all four derivatives reveal absorbance being a function of concentration; increasing concentration results in increased absorbance (Fig. S35, SI). The emission spectra of all four molecules appear as mirror images of their absorption spectra, each showing two prominent peaks when excited at their respective absorption maxima. The emission maxima of all four derivatives are centered within the range of 530 nm–539 nm (Fig. 4). PEIF has its emission maximum at 532 nm with a Stokes shift of 927 cm⁻¹ whereas PEIH shows an emission maximum at 530 nm having a large Stokes shift value of 934 cm⁻¹. PBIF and PBIH have shown maximum emission at 539 nm and 534 nm, respectively. The Stokes shift values thus calculated are found to be 459 cm⁻¹ for PBIF and 285 cm⁻¹ for PBIH (Table 3). The ester imide derivatives have shown higher Stokes shift values than their bisimide counterparts. Different concentrations of micromolar solution were studied to determine the effect of concentration on intensity. It was found that for all derivatives, intensity decreases on decreasing concentration (Fig. S36, SI). When kept under long wavelength UV light (λ = 365 nm), micromolar solutions of all four derivatives showed very bright yellowish fluorescence. As all the molecules were showing very bright fluorescence, relative quantum yield was measured with respect to Rhodamine-6G in very dilute solution. All four derivatives have shown excellent quantum yields with PEIH and PBIF recording the highest value of 0.93 (Fig. S37, SI). Photophysical studies were done in the thin film state as well. The films were prepared on a clean quartz substrate by drop casting a millimolar solution of the compound in toluene. On comparison with the well-defined spectra from the solution state, the thin film spectra are broad in nature. In all cases, the thin film absorption maxima show a hypsochromic shift when compared with solution spectra (Fig. 3). Only a single peak was observed in the emission spectra of the thin films (Fig. 4). The emission maxima in the thin films undergo a bathochromic shift compared to the maxima in the solution state. In thin films due to aggregation, the energy levels come closer, accounting for the apparent red shift in the emission spectra. For the absorption spectra in the thin film state, the hypsochromic shift indicates the formation of H-aggregates where molecules stack one above another. This results in allowed transition to the upper level, which leads to a blue-shifted absorption band compared to that in the solution state. The thin films of PEIF and PEIH show very bright red emission under UV illumination (λ = 365 nm) while those of PBIF and PBIH are weakly emissive. The absolute quantum yields of all four derivatives were measured using the powder sample. PEIH displayed the highest absolute quantum yield value whereas PBIF exhibits the minimum fluorescence in the solid state (Table 3). Micromolar chloroform solution (20 µM) of all four derivatives was then used to investigate the excited states via time resolved photoluminescence spectroscopy, utilising a 505 nm laser source. The average life-time (τ₁) of the excited species of all four derivatives is comparable with PIBF recording the highest value of 4.14 ns (Fig. S38 and Table S5, SI).

Table 3 Photophysical properties of perylene derivatives in solution^a and the thin film^b state

Entry	Absorption [nm]	Emission^c [nm]	Stokes shift (cm^−1)	Molar extinction coefficient (ε) (L mol^−1 cm^−1)	Quantum yield^d (QS)	ΔEg, opt^e [eV]	Absorption [nm]	Emission^f [nm]	Stokes shift (cm^−1)	Solid state quantum yield
a Micromolar solution in CHCl3. b Prepared by drop casting of millimolar solution in toluene. c The excitation wavelength λex = 507, 526, 505, and 526 nm for PEIF, PBIF, PEIH, and PBIH. d Relative quantum yield is calculated with respect to Rhodamine-6G (λex = 530 nm) in ethanol solution as the standard and compound in CHCl3. e Calculated from the red edge of the absorption band. f Excited at the absorption maximum.
PEIF	420, 445, 477, 507	532, 565, 617	927	44 700	0.89	2.33	456, 496, 533	608	5482	23.6
PBIF	431, 458, 489, 526	539, 577, 623	459	48 850	0.93	2.29	469, 540, 574	619	5166	1.8
PEIH	421, 444, 475, 505	530, 562, 612	934	50 200	0.93	2.35	434, 493, 528	627	7092	8.1
PBIH	431, 457, 488, 526	534, 473, 620	285	47 500	0.88	2.29	468, 497, 575	637	5398	10.5

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Fig. 3 Overlay of the absorption spectra (a)–(d) in micromolar chloroform solution (solid line) and drop casted thin films (dotted line) of PEIF, PBIF, PEIH and PBIH.

Fig. 4 Overlay of the emission spectra (a)–(d) in micromolar chloroform solution (solid line) and drop casted thin films (dotted line) of PEIF, PBIF, PEIH and PBIH. Inset images of solution and thin films illuminated with UV light (λ = 365 nm).

II.4 Electrochemical properties

Electrochemical properties of the final molecules were explored by cyclic voltammetry studies. This helps to determine the energy levels of the frontier orbital (HOMO and LUMO) and gives us valuable insight about the reversibility of the oxidation–reduction process. All experiments were conducted in 0.5 mM concentration of the compound in anhydrous dichloromethane solution. Tetrabutylammonium perchlorate (TBAP) solution (0.1 M) was employed as the supporting electrolyte, and for all cyclic voltammograms the scan rate was kept at 0.1 mV s⁻¹. All experiments were conducted using Ag/AgNO₃ (0.1 M) as the reference electrode, platinum wire as the counter electrode and glassy carbon as the working electrode. All final molecules demonstrated distinct, reversible reduction peaks, in line with the electron-deficient properties of perylene imide derivatives (Fig. 5a). The first reduction potential value was used to determine the energy levels of the lowest unoccupied molecular orbital (LUMO). As the oxidation peak was absent, the energy of the highest occupied molecular orbital (HOMO) was calculated utilising the optical energy band gap, which in turn was derived from the red edge of the absorption spectra. The results obtained from cyclic voltammetry study are tabulated in Table 4. The effect of the electronegative perfluoro chain is visible in the LUMO energy level (Fig. 5b).^36,57–60 The perfluoro containing ester imide derivative PEIF has a LUMO energy level at −3.19 eV, slightly lower in comparison to its alkyl chain counterpart PEIH having a LUMO level at −3.13 eV. The other perfluoro derivative PBIF also has a lower LUMO energy level (−3.41 eV) than the alkyl counterpart PBIH which has a LUMO energy level of −3.35 eV. The highly electronegative perfluoro chain decreases the electron density from the perylene core making the LUMO more stabilized, i.e. lower in energy. The energy level values obtained experimentally from CV studies were very similar to the theoretical values obtained from DFT.

Fig. 5 Cyclic voltammogram of PEIF, PBIF, PEIH and PBIH (a); compared energy band level diagram showing the HOMO and LUMO energy levels of PEIF, PBIF, PEIH and PBIH (b).

Table 4 Electrochemical^ab data and data obtained from DFT^h calculations for PEIF, PBIF, PEIH and PBIH

Electrochemical data					Data from DFT calculations
Entry	E 1st red	E LUMO	E HOMO	ΔEg, opt^d^g	E LUMO	E HOMO	ΔEg^d^h
a 0.5 mM dichloromethane solutions. b Experimental conditions: Ag/AgNO3 as reference electrode, glassy carbon working electrode, platinum wire counter electrode, TBAP (0.1 M) as a supporting electrolyte, room temperature. c In volts (V). d In eV. e Estimated from the formula by using ELUMO = −(4.8 − E1/2,Fc/Fc+ + Ered,onset) eV. f Estimated from the formula EHOMO = (ELUMO − Eg,opt) eV. g Calculated from the red edge of the absorption band of each compound. E1/2,Fc/Fc+ = 0.50 V. h Obtained from DFT calculations by employing the combination of Becke3-Lee–Yang–Parr (B3LYP) hybrid functional and 6-31G(d,p) basis set using the Gaussian 09 package.
PEIF	−1.11	−3.19	−5.52	2.33	−3.21	−5.85	2.64
PBIF	−0.89	−3.41	−5.69	2.28	−3.58	−6.11	2.53
PEIH	−1.17	−3.13	−5.48	2.35	−3.07	−5.72	2.65
PBIH	−0.95	−3.35	−5.63	2.28	−3.44	−5.97	2.53

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II.5 Surface morphology and contact angle measurements

Surface wettability is one of the key factors that determines the hydrophobic nature of materials. Water contact angle (WCA) values were measured for thin films of all the molecules. The presence of the perfluoroalkyl group in PEIF and PBIF is supposed to make these molecules more hydrophobic in nature compared to the other two all alkyl derivatives. The contact angle values obtained for all these molecules are provided in Fig. 6. Millimolar solution of the respective compounds in toluene was drop-casted in a previously clean glass slide and air-dried. The contact angles of the perfluoro perylene derivatives are found to be higher than those of their alkyl counterpart. The fluorinated chains tend to have low surface energy, low polarizability and chemically inert nature.^61,62 This makes the perfluorinated compounds more hydrophobic than the all hydrocarbon derivatives. This is highly critical in the case of organic electronics where fluorinated layers repel moisture or water, which tend to degrade the semiconductor adversely affecting its performance.

Fig. 6 Image showing water contact angle values of all four compounds PEIF (a), PBIF (b), PEIH (c) and PBIH (d).

The morphology of the thin films was further investigated by atomic force microscopy and scanning electron microscopy (Fig. S41, SI). Millimolar solution of the respective compounds in toluene was prepared and drop-casted on a clean glass substrate. All the derivatives exhibit good film forming ability (Fig. 7). The smoothness of the films was evidenced by the very small RMS roughness value (Fig. 7) obtained from AFM studies for all molecules. The smooth film forming ability along with the hydrophobic nature of the film makes these derivatives highly attractive towards device application.

Fig. 7 AFM topography images with RMS roughness values of the spin coated films of all four compounds PEIF (a), PBIF (b), PEIH (c) and PBIH (d).

II.6 Density functional theory studies

Density functional theory was employed to conduct a comprehensive theoretical analysis of all four final molecules. A hybrid model combining Becke's three-parameter exchange functional with the Lee–Yang–Parr correlation functional and 6-31G(d,p) basis set utilizing the Gaussian 09 package was used for all clculations. This study helps to visualize the optimized structure, MOs corresponding to HOMO and LUMO and the HOMO–LUMO gap, i.e., theoretical band gap. The contours of HOMO–LUMO and their energy level and corresponding diffrences are shown in Fig. 8. It can be seen that both the HOMO and LUMO spread over the central aromatic core and not on the imide substituents because of the presence of nodes on the imide nitrogen atoms. The theoretical band gaps obtained for PEIF, PBIF, PEIH and PBIH are 2.64 eV, 2.52 eV, 2.65 eV and 2.53 eV, respectively. The values and trends among different derivatives match well with the experimental results. Molecular electrostatic potential surfaces were also generated and visualised. The central aromatic core marked by blue color indicates its elecron deficient nature and the more electron-rich heteroatom in the imide and ester group is marked by red color (Fig. 9).

Fig. 8 Frontier molecular orbitals of PEIF (a), PBIF (b), PEIH (c) and PBIH (d) obtained from DFT calculations at the B3LYP/6-31G(d,p) level; E_H and E_L denote energies of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), respectively; ΔE denotes band gap energy.

Fig. 9 3D molecular electrostatic potential contour map of the optimized structure of PEIF (a), PBIF (b), PEIH (c) and PBIH (d). In the mapped electro-static potential surface, the red and blue colors refer to the electron-rich and electron-poor regions, respectively, whereas the green color signifies the zero electrostatic potential; chain length is limited to isopropyl for clarity purpose.

II.7 Charge carrier mobility studies

We further investigated that bulk charge transport properties of these semiconductors, by fabricating electron only diodes. There have been several reports of charge carrier properties investigation of fluorinated perylene derivatives by various research groups. Piliego et al. reported on bottom-contact bottom-gate OFETs based on spin-coated films of N,N′-1H,1H-perfluorobutyl dicyanoperylenecarboxydiimide and compared them with the N-alkyl functionalized perylene.⁶³ The former PBI with a perfluorinated chain showed mobility of 0.15 cm² V⁻¹ s⁻¹ when employed in a simple and inexpensive solution–deposition manner. Perfluoro functionalized perylene diimides with varying numbers of fluorine substitutions at the bay positions were reported by Schimdt et al., where the perfluoro perylene bisimide with two bay fluorine atoms exhibited higher mobilities than the PBI with four bay fluorine substitutions.⁵⁸ The higher mobilities were attributed to the higher grain size and more regular grain boundaries observed in the thin films of the PBI with two fluorine atoms. Another study by Yao et al. investigated the use of bay-fluorine substituted PBIs as cathode interlayer materials in organic solar cells, highlighting the impact of surface morphology on the performance of the material.⁴¹ The single fluoro-substituted derivative exhibited a smoother film with better charge carrier mobility in comparison to disubstituted PBI. In another comparative study of the fluoro substituted perylene derivative and its non-fluoro analogue materials used as an acceptor in organic solar cells, superior performance was demonstrated by the fluoro-substituted derivative, which has been attributed to the efficient exciton dissociation and charge separation, as well as suppressed charge recombination of the fluorinated derivative, contributing to improved power conversion efficiency.³⁶ The high mobility observed for a bis(heptafluorobutyl)-substituted PBI based n-type semiconductor has been facilitated by very dense and parallel arrangement of undistorted PBI π-planes as reported by Jak Oh et al.⁵⁷

Here, electron only devices were preferred considering the general n-type semiconducting behaviour associated with perylene-based semiconductors. For device fabrication, an ITO-coated glass substrate was used as the bottom electrode, on top of which the semiconducting layer were drop casted resulting in a thickness of ∼ (1–2) µm. This is followed by annealing the device at 110 °C for 30 minutes. An Ag electrode was then evaporated on the top to complete the device (Schematic: Fig. 10a and Fig. S45). We also fabricated spin coated devices with all four perylene derivatives. However, these devices exhibited a linear response (Fig. S46). In the I–V characteristics of the drop casted devices, we consistently observe two regimes of transport: the low voltage Ohmic regime where J ∝ V and then at higher voltage we have the space-charge limited regime (SCLC) with J ∝ V². From the Ohmic regime of the I–V characteristics, we observed that PBIF exhibited the highest conductivity of 30 µS m⁻¹, whereas PEIF exhibited the lowest conductivity of ∼2 µS m⁻¹ (Fig. 10). Charge carrier mobility was subsequently obtained using the Mott–Gurney equation, where, J is the current density measured at an applied voltage V, ε₀ is the permittivity of free space, ε_r is the dielectric constant of the semiconducting material, µ is the charge carrier mobility of the material and L is the thickness of the semiconductor film. The µ value measured on PBIF devices reaches a maximum value of ∼1.3 × 10⁻² cm² V⁻¹ s⁻¹ at 300 K (µ_average ∼ 1.2 × 10⁻² cm² V⁻¹ s⁻¹). Interestingly, upon alkyl group substitution both PEIH or PBIH exhibited a decrease in mobility with a mobility value in the range of (1–1.5) ×10⁻³ cm² V⁻¹ s⁻¹ and decreases to a value of ∼5.5 × 10⁻⁵ cm² V⁻¹ s⁻¹ for PEIF (Fig. 10 and Fig. S47–S50). To elucidate the unusually high electron mobility observed in PBIF, we correlate it first with the microstructural properties obtained from AFM characterization, which indicate exceptionally low r.m.s roughness of ∼1.37 nm, which is among the lowest in all the molecules. This reduced roughness of the semiconductor contributes to an improved interface to facilitate charge transport. In addition to the surface morphology, the LUMO energy level of PBIF is the lowest among all the molecules thereby minimizing the injection barrier between the metal and semiconductor and promoting efficient charge injection for efficient mobility. Furthermore, the bulk carrier mobility is significantly influenced by the alignment of the material.⁶⁴ Polarized optical microscopy (POM) images (Fig. S33, SI) illustrate that PBIF films exhibit more pronounced crystalline domains in comparison to the other materials indicating higher crystallinity. Such molecular ordering enhances π–π interaction, facilitating continuous pathways for charge transport while reducing energetic disorder. This behaviour can possibly be attributed to the rigidity and planarization associated with bis-imide groups. All studied molecules contain a central electron-deficient core, which, in principle, should facilitate electron acceptance and promote high mobility. However, despite the similarity in electron-deficient moieties and crystalline features, PEIF exhibits substantially lower mobility in comparison to the others. This decline in performance may be attributed to its higher contact angle, on a hydrophilic ITO surface, which suggests poorer wetting characteristics. This is expected to hinder film formation thereby potentially increasing surface roughness and inducing more defects. Moreover, ester-imide groups introduce more flexible and less planar linkages, often resulting in less ordered morphologies and weaker intermolecular interactions which can disrupt π–π stacking, leading to lower charge carrier mobilities. The trap density (n_t) was computed using the equation where V_tfl is the trap-filled-limit voltage, q is the elementary charge, L is the film thickness, and ε_r is the relative permittivity of the sample. The resulting trap density values were consistent with the transport trends.

Fig. 10 Schematic diagram illustrating the electron-only device (a). I–V curves illustrating different transport regimes (Ohmic and SCLC regions) represented by varying shades of color for electron-only devices of PEIF (b), PEIH (c), PBIF (d), and PBIH (e). Bar plots of mobility for all compounds. Error bar is the standard deviation over 5–8 devices (f).

III. Conclusions

A series of unsymmetrical perylene derivatives incorporating different electronegative substituent groups has been synthesized and thoroughly characterized. Out of these four molecules, perylene diester imide with a perfluoro chain, PEIF, was found to stabilize the columnar hexagonal phase (Col_h) at elevated temperatures. The other three unsymmetrical derivatives, PBIF, PEIH and PBIH, were crystalline in nature. These compounds show well-structured strong absorption with a high extinction coefficient, which indicates their promising role in solar cell applications. The low lying LUMO level observed for PBIF highlights its potential as a strong candidate for use as a non-fullerene acceptor moiety in solar cell applications. Additionally, their hydrophobic nature ensures increased stability against degradation through air oxidation. Their excellent fluorescence nature revealed by high relative quantum yield values (>90%) points to their great potential in light emitting devices. The charge transport measurements carried out by the space charge limited current (SCLC) method for all derivatives show very promising results. Among these molecules, PBIF shows remarkably high electron carrier mobility values of 1.3 × 10⁻² cm² V⁻¹ s⁻¹. In conclusion, this work investigates the effect of an electronegative perfluorinated substituent on the charge transport behavior of different perylene derivatives. These molecules represent a family of promising solution-processable n-type organic semiconductors, with a bright potential for application in organic electronic devices.

Conflicts of interest

There are no conflicts of interest to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: The data supporting this article have been included as part of the SI. Materials and methods; experimental details; molecular structural characterization data (¹H and ¹³C NMR spectra, MALDI-TOF mass data); TGA curves; POM images; DSC scans; and data from X-ray diffraction, cyclic voltammetry, computational, photophysical studies and charge carrier mobility studies (PDF). See DOI: https://doi.org/10.1039/d5tc03259h.

Acknowledgements

ASA sincerely thanks Science and Engineering Board (SERB) DST, Govt. of India and BRNS-DAE for funding this work through project CRG/2018/000362 and No. 2012/34/31/BRNS/1039, respectively. We thank the Ministry of Human Resource and Development for the Centre of Excellence in FAST (F. No. 5-7/2014-TS-VII). SPS acknowledges funding support from Intensification of Research in High Priority Areas (IRHPA) from the Science and Engineering Research Board (IPA/2021/000096), and funding from the Department of Atomic Energy (DAE), Government of India through RIN-4001 & RNI-4011. IM acknowledge support from DAE, Government of India for the fellowship. IM acknowledges Abhinandha Asokh for assisting with measurements.

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