A detailed study on the thermal, photo-physical and electrochemical properties and OFET applications of D–π–A–π–D structured unsymmetrical diketopyrrolopyrrole materials

Abstract

A series of seven unsymmetrical diketopyrrolopyrrole (DPP) derivatives with donor–pi–acceptor–pi–donor (D–π–A–π–D) architecture have been designed, synthesized and well characterised. The effect of the electron donating capacity and extent of electronic conjugation of the end-capping units on the thermal, photo-physical and electrochemical properties of the synthesized materials was thoroughly investigated using various experimental techniques and theoretical calculations. Organic field-effect transistors (OFETs) were fabricated using these materials to obtain their hole/electron transporting characteristics. All these materials showed moderate to good hole transporting ability, and the OFET fabricated using the DPP-derivative with benzofuran and pyrene end groups exhibited a hole mobility of 6.7 × 10⁻⁴ cm² V⁻¹ s⁻¹ with V_T of −9 V. The observed photo-physical, electrochemical, thermal, and charge carrier properties of the synthesized DPP-derivatives indicated their applicability in various areas like organic photovoltaics, disposable electronics and biomedical devices.

---

Introduction

During the last couple of decades, organic materials with donor–acceptor (D–A) architecture have attracted a great deal of attention from researchers owing to their promising applications in various fields, like organic field-effect transistors (OFETs), organic photovoltaics (OPV) and organic light-emitting devices (OLEDs).¹ Organic materials with the combination of donor (D) and acceptor (A) moieties can show broad visible region absorption and narrow optical band gaps due to the intramolecular charge transfer (ICT) character between the donor and acceptor moieties. Broad visible region absorption and narrow optical band gap are prerequisite for the materials to show efficient light harvesting properties over the entire solar spectrum and high charge carrier mobility.² Hence numerous reports are available in the literature based on the materials with D–A architecture.³ In the last few years extensive efforts have been made by the researchers to diversify D–A materials and materials with various structural motifs, like symmetrical D–A–D or A–D–A, or even more complex combinations such as linear A–D–A–D–A and D–A–D–A–D have been developed for optoelectronic applications.⁴

Among the various building blocks, diketopyrrolopyrrole (DPP) has been extensively studied to develop materials for various applications owing to its facile synthetic procedures and excellent properties, like good thermal and photo stabilities, high planarity and ambipolar charge carrier transport.⁵ Many high performance DPP-based polymers are available in the literature for OPV and OFET applications.^5d The excellent intermolecular solid state aggregation of the DPP derivatives in their polymers improves their charge carrier mobilities and leads to highly efficient organic semiconductors.^2b,6 Small molecular semiconducting materials are advantageous over their polymer counterparts in terms of their simpler synthesis and purification procedures, lower batch-to-batch variation, and greater reproducibility.⁷ After the pioneering work of Nguyen et al. on DPP functionalized oligothiophene for solution-processed BHJ solar cells, several small molecular DPP derivatives have been reported with a variety of applications.⁸

Modulation of the molecular backbone with variations in their donor aromatic groups as end-capping units has a great impact not only for improving the molecular optoelectronic properties but also to control aggregation of the materials.^4b,9 Even though several DPP based small molecular semiconducting materials with symmetrical D–A–D architecture are reported, studies on unsymmetrical DPP materials are very rare. Keeping this in mind, in the present work we have synthesized a series of unsymmetrical DPP derivatives with different end capping units by anticipating that selection of appropriate end capping units can impart better properties compared to their symmetrical analogues. The synthesized unsymmetrical DPP materials have benzofuran at one end and chemical entities with different electron donating capacity and various extents of electronic conjugation are at the second side. The optoelectronic properties of the synthesized DPP-derivatives were studied using UV-visible, fluorescence and time-resolved fluorescence spectroscopic techniques and cyclic voltammetric analysis. OFETs were fabricated using the synthesized DPP derivatives to understand the effect of the end-capping units on their charge carrier mobilities. The observed properties of the synthesized unsymmetrical DPP derivatives are compared with their symmetrical analogue dibenzofuran-DPP DPP(TBFu)₂.

Results and discussion

Molecular structures and thermal properties

The molecular structures of the synthesized DPP-derivatives are shown in Fig. 1. All the DPP-derivatives reported here contain benzofuran as one end-capping unit and they differ structurally only in use of the other end-capping group. The DPP-derivatives, BFTPADPP, BFHTDPP and BFTFDPP contains electron donating triphenylamine (TPA), 2-hexylthiophene (HT) and electron withdrawing p-trifluoromethylphenyl (TF) moieties, respectively, as end-capping units, and the derivatives BFPhDPP, BFNaDPP, BFAnDPP and BFPyDPP possess phenyl (Ph), naphthyl (Na), anthracenyl (An) and pyrenyl (Py) end groups that have various extents of electronic conjugation, respectively. A triple bond is introduced for the derivatives BFPhDPP, BFNaDPP, BFAnDPP and BFPyDPP by assuming that insertion of a triple bond can provide sufficient space to minimize the steric hindrance between the neighboring moieties and be helpful to improve the planarity of the materials. These structural manipulations among the seven compounds are certainly helpful for an effective investigation on the effects of end-capping substitutions on their optoelectronic properties. The synthetic scheme and detailed synthetic procedure of the reported DPP-derivatives are provided in ESI† and the synthesized materials were thoroughly characterized using NMR (Fig. S1–S14, ESI†) and ESI-HRMS (Fig. S15–S21, ESI†) analyses. The synthesized DPP-derivatives had good solubility in common organic solvents such as CHCl₃, THF, O-dichlorobenzene and toluene, which is essential for solution processability in device fabrication.

Fig. 1 Chemical structures of the synthesized DPP-derivatives.

High thermal stability is an important requirement for the materials for practical applications. Therefore, thermal properties of the synthesized DPP-derivatives were investigated using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The TGA curves and the corresponding data of these materials are provided in Fig. 2 and Table 1, respectively. All the synthesized materials showed decomposition temperatures (T_d, defined as the temperature at which 5 wt% loss is observed) of above ∼348 °C, indicating their high thermal stability and suitability for vacuum deposition techniques. DSC thermograms of these DPP-derivatives are provided in Fig. S22, ESI.† All the derivatives except BFAnDPP showed sharp endotherms corresponding to their melting temperatures. Among the derivatives BFPhDPP, BFNaDPP and BFPyDPP, the melting temperatures gradually increased from BFPhDPP to BFPyDPP and BFPyDPP showed the highest melting temperature (193.7 °C) while BFPhDPP displayed the lowest (121.7 °C). This observed gradual increment in the melting temperatures might be due to the increase in their molecular weights as well as improvement in their molecular planarity that results in better intermolecular stacking.¹⁰ Among the derivatives BFTPADPP, BFHTDPP and BFTFDPP, BFTFDPP exhibited the highest melting temperature (207.0 °C) followed by BFTPADPP (190.3 °C) and BFHTDPP (140.1 °C). Apart from their melting endotherms, the derivatives BFTPADPP, BFPyDPP and BFHTDPP displayed exothermic peaks at 124.3, 130.9 and 103.1 °C, respectively, corresponding to their crystallization temperatures upon first heating and first cooling cycles. The melting (T_m), glass transition (T_g) and crystallization temperatures (T_c) of the synthesized DPP-derivatives are provided in Table 1.

Fig. 2 TGA thermograms of the synthesized DPP-derivatives.

Table 1 Thermal data of the synthesized DPP-derivatives

Material code	Decomposition temperature (Td) °C	Melting temperature (Tm) °C	Glass transition temperature (Tg) °C	Crystallization temperature (Tc) °C
BFTPADPP	373.5	190.3	74.7	124.3
BFHTDPP	348.7	140.1	—	103.1
BFTFDPP	352.4	207.0	—	—
BFPhDPP	364.8	121.7	—	—
BFNaDPP	387.2	185.3	—	—
BFAnDPP	390.9	—	—	—
BFPyDPP	379.7	193.7	79.2	130.9

---

Photo-physical characteristics

The UV-visible absorption characteristics of the reported DPP-derivatives were measured in both the solution and thin film states. Fig. 3a shows the UV-visible absorption spectra of these materials in chloroform solution, and the corresponding data is provided in Table 2. All the DPP-derivatives showed two primary absorption bands in the visible region ranging from 370–480 nm and 490–700 nm that are quite different from their mono-bromo synthetic precursor (Fig. S23, ESI†) and indicated efficient electronic coupling between the DPP core and the end-capping moieties. The absorption bands in the shorter wavelength region are attributed to the localized π–π* transitions and the absorption bands in the longer wavelength region are ascribed to the charge transfer (CT) between the end-capping units and DPP central core.⁸ Among the DPP-derivatives reported here, BFTPADPP with the electron donating TPA end group exhibited the highest absorption maxima (λ_max, ∼630.6 nm) and BFTFDPP with the electron withdrawing TF end group showed the lowest λ_max of 613.5 nm. The DPP-derivatives BFPhDPP, BFNaDPP, BFAnDPP and BFPyDPP with Ph, Na, An and Py end-capping groups had similar absorption maxima of ∼619.0, 622.0, 625.2, and 629.4 nm, respectively. These observations clearly suggested that compared to the extent of electronic conjugation, the electron donating tendency of the end-capping units had a significant effect on their absorption maxima. The effect of solvent polarity on the ground state absorption of the synthesized DPP-derivatives was estimated by observing their UV-visible absorption patterns in different solvents, and the corresponding absorption spectra are provided in Fig. S24, ESI.† All the derivatives except BFPyDPP showed only modest variations in their absorption patterns in different solvents, revealing that solvent polarity has an insignificant role in their ground state absorption. The observed significant deformation in the absorption band of BFPyDPP might be ascribed to the improved intermolecular interactions in MeOH solution. Absorption spectra of the DPP-derivatives in their thin film state are shown in Fig. 3b. Compared to the solution state (Fig. 3a), the absorption bands of the DPP-derivatives were significantly broadened and developed new CT bands in the longer wavelength region.^10,11 Interestingly, among BFTPADPP, BFHTDPP and BFTFDPP, the CT bands are more pronounced for BFTPADPP, followed by BFHTDPP and BFTFDPP. This marked difference in the thin film state absorption profiles of BFTPADPP, BFHTDPP and BFTFDPP could be attributed to the variations in their solid-state arrangement as a result of the intermolecular donor–acceptor interactions between the DPP central core and the end-capping units.¹² Since TPA is electron-rich, it can couple strongly to the electron deficient DPP core of adjacent molecules, resulting in head-to-tail type J-aggregation. As a consequence of the low electron donating capacity of HT and TF compared to TPA, they could not maintain strong intermolecular donor–acceptor interactions with the DPP central core of the neighbouring molecules, which hence resulted in low intensity CT bands in their thin film absorption spectra.¹² Similarly in the case of the derivatives BFPhDPP, BFNaDPP, BFAnDPP and BFPyDPP, the intensity of the longer wavelength CT bands was increased with the extent of electronic conjugation of the end groups, which might be due to the improved donor–acceptor (D–A) interactions in the materials with end-capping units having high electronic conjugation.¹⁰ It is imperative to note that in the thin film state, all the synthesized DPP-derivatives have enhanced visible region light harvesting ability compared to their symmetrical DPP-analogues,^10,13 which could be attributed to the improved planarity and efficient ICT between the end-capping units and DPP central core of these materials. The optical band gaps of the synthesized DPP-derivatives were estimated from the onsets of their thin film absorption spectra and the values are provided in Table 2.

Fig. 3 UV-visible absorption spectra of the synthesized DPP-derivatives (10 μM) in chloroform solution (a) and their normalized absorption spectra in thin film state (b).

Table 2 Photo-physical data of the synthesized DPP-derivatives

Material code	λmaxAbs (nm) (ε, M^−1 cm^−1)	λmaxEmi (nm)	Emission lifetime (τ, ns)	λonset film (nm)	Band gap (Eopt)
BFTPADPP	630.6 (64 454)	671.4	2.82	789.7	1.57
BFHTDPP	627.3 (41 885)	653.6	1.95	760.1	1.63
BFTFDPP	613.5 (35 230)	635.2	2.68	729.3	1.70
BFPhDPP	619.0 (46 667)	632.7	2.46	734.3	1.69
BFNaDPP	622.0 (37 066)	641.4	2.40	742.2	1.67
BFAnDPP	625.2 (38 443)	648.1	1.97	754.2	1.64
BFPyDPP	629.4 (54 432)	655.5	1.66	771.9	1.61

---

Fig. 4a depicts the emission spectra of the synthesized DPP derivatives in chloroform solution. All the synthesized derivatives showed two well-resolved bands in their emission spectra that are characteristic to DPP.¹⁴ Among all the derivatives synthesized, BFTPADPP showed the highest (671.4 nm) and BFPhDPP and BFTFDPP (632.7 and 635.2 nm, respectively) showed the lowest emission maxima in accordance with their absorption pattern, indicating that the TPA end group imparted improved charge delocalization compared to the other end groups used in this study. Emission spectra of these derivatives in different solvents were recorded, and the spectra are provided in Fig. S25, ESI.† The observed solvent dependent variations in the emission maxima of the synthesized DPP-derivatives were pronounced compared to the solvent dependent changes in their absorption maxima (Fig. S24, ESI†). These results indicated that solvent–solute interactions are stronger in their excited state compared to the ground state.

Fig. 4 Emission spectra (a) and decay lifetimes (b) of the synthesized DPP-derivatives in chloroform solution.

In sharp contrast, the synthesized materials in their thin film state displayed a broad and structureless emission in the region of 700–810 nm (Fig. S26, ESI†). Moreover, the fluorescence emission intensity of these materials in their thin film state was drastically reduced relative to their fluorescence emission in solution state (Fig. 4a). These observed differences in the fluorescence dynamics of the synthesized DPP-derivatives in thin film and solution states clearly indicated the possibility for intermolecular electron transfer among the DPP-derivatives in their thin film state that facilitates non-radiative deactivation of the excited state materials.¹⁵ The fluorescence lifetimes of these DPP-derivatives were estimated in the solution state using the time correlated single photon counting (TCSPC) technique (Fig. 4b) to understand the lifetime of the excitons generated upon absorption of light. All the derivatives showed nearly equal excited singlet state lifetime profiles, and the observed fluorescence lifetimes are provided in Table 2. The emission decay profiles with their corresponding fit curves are shown in Fig. S27, ESI.†

To understand the origin of the spectroscopic properties of the DPP-derivatives observed in solution, density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations were performed on these molecules using Gaussian 09 software with the functionals B3LYP (Tables S1 and S2, ESI†), M06-2X (Table S3, ESI†) and cam-B3LYP (Table S4, ESI†) combined with the 6-31G(d,p) basis set. The alkyl groups of the DPP-derivatives were replaced with methyl groups to simplify the calculations. The optimized structures of the DPP-derivatives and their corresponding highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) wavefunctions are illustrated in Fig. 5. Both the HOMO and LUMO wavefunctions are delocalized over the DPP, thiophene and end-capping fragments. All the DPP-derivatives except BFTPADPP (bulky TPA end-capping unit) have planar molecular geometry. No considerable intramolecular twist was observed by changing the end-capping units (Fig. 5). All the DPP-derivatives show low ground state dipole moments (Table S1, ESI†) and these low ground state dipole moments could be the reason for the observed mild sensitivity of their absorption profiles towards solvents with various polarities. TD-DFT calculations were also conducted, and the theoretically derived absorption spectra obtained in chloroform solvent phase using the B3LYP/6-31G(d,p) method (Fig. S28, ESI†) are in good agreement with the experimental results (Fig. 3a),¹⁶ whereas the λ_max of these derivatives obtained using M06-2X/6-31G(d,p) and cam-B3LYP/6-31G(d,p) in chloroform solvent are slightly underestimated, although the trend is reproduced. According to results of the TD-DFT calculations, the observed long wavelength absorption band (630–700 nm) of the DPP-derivatives corresponds to the S₀ → S₁ transitions and is dominated by the HOMO → LUMO transitions (Table S2, ESI†). The results of the TD-DFT calculations also indicated that these molecules have high ground to excited state transition dipole moments. The HOMO and LUMO energy levels, adiabatic ionization potential (IP_A) and electron affinity (EA_A) values of the synthesized DPP-derivatives estimated from the theoretical calculations and the values experimentally obtained are in good agreement.

Fig. 5 HOMO and LUMO distributions of the synthesized DPP-derivatives.

Electrochemical characteristics

The electrochemical properties of the DPP based materials reported here were investigated using cyclic voltammetry (CV). All the CV experiments were performed at room temperature with a conventional three electrode setup consisting of a glassy carbon working electrode and a standard calomel electrode (SCE) and platinum wire as reference and counter electrodes, respectively. The potential of the reference electrode was calibrated using a ferrocene internal standard. All the measurements were conducted in anhydrous dichloromethane media, under a nitrogen atmosphere using Bu₄NClO₄ (0.1 M) as the supporting electrolyte at a scan rate of 100 mV s⁻¹. All the derivatives displayed reversible (and quasi reversible) oxidation and reduction behaviour (Fig. 6 and S29, ESI†), and the respective onset oxidation and reduction potentials are provided in Table 3. The oxidation potentials of the DPP-derivatives were sensitive to the nature of the end groups. Among all the DPP-derivatives, BFTPADPP showed lower oxidation potential (+0.67 V) owing to the easily oxidisable nature of the electron donating TPA end-capping unit, and the oxidation potentials of the DPP-derivatives increased with decreasing electron donating tendency of the end groups (0.72 and 0.81 V respectively for BFHTDPP and BFTFDPP). On the other hand, the extent of electronic conjugation of the end groups had negligible effect on their oxidation potentials.¹⁰ The DPP-derivatives BFPhDPP, BFNaDPP, BFAnDPP and BFPyDPP had similar oxidation potentials of 0.85, 0.84, 0.83 and 0.82, respectively. The corresponding HOMO energy levels of these derivatives were estimated from their oxidation potentials using the ferrocene reference value of −4.4 eV below the vacuum level,¹⁷ and the values are provided in Table 3. The LUMO values of the synthesized DPP-derivatives were calculated from their onset reduction potentials obtained from cyclic voltammetry, and the values are included in Table 3. The calculated HOMO and LUMO energy levels of these DPP-derivatives (5.07–5.25 and 3.41–3.49 eV, respectively) were well matched with the work functions of commonly used gold and silver cathodes (5.1 and 4.7 eV, respectively) indicating the possibility for efficient charge injection and improved charge carrier mobility.¹⁸ It is interesting that even though all the synthesized DPP-derivatives have extended visible region light harvesting ability, their HOMO energy levels were considerably low-lying (except for BFTPADPP and BFHTDPP) compared to the well studied symmetrical dibenzofuran-DPP, DPP(TBFu)₂ (−5.14 eV).^13a These low-lying HOMO energy levels can certainly help to improve the open circuit voltage (V_oc) of the BHJ devices fabricated using these materials. Hence the results of the UV-visible and CV experiments suggested that by selecting appropriate end-capping units, unsymmetrical DPP-derivatives can show better properties over their symmetrical analogues.

Fig. 6 Cyclic voltammograms representing the reversible oxidation behaviour of the synthesized DPP-derivatives.

Table 3 Electrochemical data of the synthesized DPP-derivatives

Material Code	E^+/0+1/2 (V)	HOMO (eV)	E^0/−1/2 (V)	LUMO (eV)	Band gap (Eechem)
BFTPADPP	0.67	−5.07	−0.99	−3.41	1.66
BFHTDPP	0.72	−5.12	−0.96	−3.44	1.68
BFTFDPP	0.81	−5.21	−0.94	−3.46	1.75
BFPhDPP	0.85	−5.25	−0.98	−3.42	1.83
BFNaDPP	0.84	−5.24	−0.94	−3.46	1.78
BFAnDPP	0.83	−5.23	−0.93	−3.47	1.76
BFPyDPP	0.82	−5.22	−0.91	−3.49	1.73

---

Charge carrier mobility measurements

The charge transport properties of the synthesized DPP-derivatives were measured in organic field-effect transistors (OFETs). The device configuration was bottom gate bottom contact with gold electrodes as source and drain contacts. The gate dielectric was 230 nm thick SiO₂. The gate electrode was heavily doped silicon. All the materials were spun on the substrate from chloroform solution. The output characteristics were measured by sweeping the drain voltage between 0 and −100 V and holding the gate voltage at various constant potentials (Fig. 7a). All the synthesized DPP-derivatives exhibited hole transporting ability in this configuration and displayed clear linear and saturation regimes (Fig. S30–S33, ESI†). It should be noted that the linear regime didn’t start at 0 V, indicating the possibility of contact resistance and charge traps.¹⁹ The charge carrier mobility was calculated using linear and saturation regimes.²⁰ The OFET parameters for these DPP-derivatives are summarized in Table 4. All the synthesized DPP-derivatives showed moderate to good hole transporting ability and among all these derivatives, the highest hole mobility was measured for BFPyDPP (6.7 × 10⁻⁴ cm² V⁻¹ s⁻¹) with a V_T of −9 V (Fig. 7b). The highest mobility in BFPyDPP is likely due to the better packing between the molecules due to the presence of flat pyrene as one terminal moiety. In line with the results of the DSC analysis (Fig. S22, ESI†), the DPP-derivatives with crystalline nature (BFTPADPP, BFHTDPP and BFPyDPP) displayed better hole mobilities compared to the other DPP-derivatives reported here. However, it is not appropriate to comment on the absolute solid-state packing of these materials without single-crystal structures. Among the synthesized DPP-derivatives, BFHTDPP (1.1 × 10⁻⁴ cm² V⁻¹ s⁻¹) and BFPhDPP (2.2 × 10⁻⁴ cm² V⁻¹ s⁻¹) displayed comparable hole transporting characteristics with DPP(TBFu)₂ (1.6 × 10⁻⁴ cm² V⁻¹ s⁻¹), while BFPyDPP exhibited four times better hole transporting ability.²¹ The BFTFDPP output and transfer characteristic curves showed ambipolar charge transport (Fig. S31, ESI†) owing to the presence of the electron withdrawing TF end-capping unit. The hole mobility was found to be 2.2 × 10⁻⁵ cm² V⁻¹ s⁻¹. However, it was not possible to calculate the electron mobility due to the absence of clear linear and saturation regimes.

Fig. 7 Typical output curve (a) and transfer curve (b) of BFPyDPP.

Table 4 Field-effect transistor data of the synthesized DPP-derivatives

Material code	Mobility (cm^2 V^−1 s^−1) saturation regime	Mobility (cm^2 V^−1 s^−1) linear regime	VT (V)	Ion/off
BFTPADPP	8.85 × 10^−5	4.95 × 10^−5	−5	2.02 × 10^2
BFHTDPP	1.12 × 10^−4	1.02 × 10^−4	−20	9.17 × 10^2
BFTFDPP	2.25 × 10^−5	1.35 × 10^−5	−38	3.01 × 10^2
BFPhDPP	2.16 × 10^−4	2.02 × 10^−4	−18	1.12 × 10^2
BFNaDPP	2.70 × 10^−5	4.22 × 10^−5	−11	7.92 × 10^1
BFAnDPP	4.72 × 10^−6	3.37 × 10^−6	−10	8.30 × 10^2
BFPyDPP	6.74 × 10^−4	4.11 × 10^−4	−9	7.02 × 10^1

---

Conclusions

Seven unsymmetrical DPP-derivatives, namely BFTPADPP, BFHTDPP, BFTFDPP, BFPhDPP, BFNaDPP, BFAnDPP and BFPyDPP, with D–π–A–π–D architecture have been synthesized and were well characterised using NMR and ESI-HRMS analytical techniques. Optoelectronic properties of the synthesized DPP derivatives were measured and the effect of the end capping units on their optoelectronic properties was explored. The obtained values were compared with their symmetrical analogue dibenzofuran-DPP DPP(TBFu)₂. All the synthesized derivatives show improved UV and visible region light harvesting ability compared to DPP(TBFu)₂. Among all the DPP-derivatives studied, BFTPADPP with the electron donating TPA end-capping unit displayed the highest light absorption capacity, indicating that the electron donating capacity of the end capping units has a significant role in their absorption profiles. The estimated HOMO and LUMO energy levels of the synthesized materials are in agreement with the work functions of the most commonly used cathode materials (Au and Ag) and as well as with the fullerene acceptors (PC₆₀BM and PC₇₀BM). The HOMO energy levels of the synthesized DPP derivatives are considerably low-lying (except for BFTPADPP and BFHTDPP) compared to DPP(TBFu)₂. OFETs were fabricated using these materials, and they showed moderate to good charge carrier mobilities. Among the OFETs fabricated, the OFETs with BFPyDPP exhibited the best performance characteristics with the hole mobility of 6.7 × 10⁻⁴ cm² V⁻¹ s⁻¹ with a V_T of −9 V, and the observed mobility is four times better compared to OFETs with DPP(TBFu)₂. Excellent thermal stability, good light harvesting capacity with appropriate energy levels and good charge carrier mobilities signified the utility of these materials in various applications like organic BHJ solar cells, disposable electronics and biomedical devices. Moreover, this study also reveals that the development of unsymmetrical DPP-based materials by selecting suitable end-capping units gives us a wide scope for obtaining materials with appropriate photo-physical, electrochemical and charge transport properties.

Acknowledgements

N. R. Ch. thanks the CSIR, India, for CSIR-Nehru Science Post-doctoral Fellowship. D. B., M. S. and B. S. thank the CSIR, India, for research fellowship. Financial support from CSIR project NWP0054 is greatly acknowledged. Both the authors D. B. and S. C. have equally contributed to this work.

Notes and references

1. (a) Organic Electronics: Materials, Manufacturing and Applications, ed. H. Klauk, Wiley-VCH, Weinheim, Germany, 2006; (b) C. B. Nielsen, M. Turbiez and I. McCulloch, Adv. Mater., 2013, 25, 1859; (c) J. D. Yuen and F. Wudl, Energy Environ. Sci., 2013, 6, 392; (d) S. Radhakrishnan and N. Somanathan, J. Mater. Chem., 2006, 16, 2990; (e) J.-H. Jou, S. Kumar, P.-H. Fang, A. Venkateswararao, K. R. J. Thomas, J.-J. Shyue, Y.-C. Wang, T.-H. Li and H.-H. Yu, J. Mater. Chem. C, 2015, 3, 2182.
2. (a) S. Roquet, A. Cravino, P. Leriche, O. Aleveque, P. Frere and J. Roncali, J. Am. Chem. Soc., 2006, 128, 3459; (b) C. Kanimozhi, N. Yaacobi-Gross, K. W. Chou, A. Amassian, T. D. Anthopoulos and S. Patil, J. Am. Chem. Soc., 2012, 134, 16532; (c) Y. J. Cheng, S. H. Yang and C. S. Hsu, Chem. Rev., 2009, 109, 5868.
3. (a) Y. Shirota and H. Kageyama, Chem. Rev., 2007, 107, 953; (b) K. Müllen and G. Wegner, Electronic Materials: The Oligomer Approach, Wiley-VCH, Weinheim, Germany, 1998; (c) B. Tieke, A. R. Rabindranath, K. Zhang and Y. Zhu, Beilstein J. Org. Chem., 2010, 6, 830.
4. (a) B. Walker, C. Kim and T. Q. Nguyen, Chem. Mater., 2011, 23, 470; (b) Y. Z. Lin, Y. F. Li and X. W. Zhan, Chem. Soc. Rev., 2012, 41, 4245; (c) A. Mishra and P. Bauerle, Angew. Chem., Int. Ed., 2012, 51, 2020; (d) J. Li, K. H. Ong, S. L. Lim, G. M. Ng, H. S. Tan and Z. K. Chen, Chem. Commun., 2011, 47, 9480; (e) S. Loser, C. J. Bruns, H. Miyauchi, R. P. Ortiz, A. Facchetti, S. I. Stupp and T. J. Marks, J. Am. Chem. Soc., 2011, 133, 8142.
5. (a) H. Bronstein, Z. Chen, R. S. Ashraf, W. Zhang, J. Du, J. R. Durrant, P. S. Tuladhar, K. Song, S. E. Watkins, Y. Geerts, M. M. Wienk, R. A. J. Janssen, T. Aothopoulos, H. Sirringhaus, M. Heeney and I. McCulloch, J. Am. Chem. Soc., 2011, 133, 3272; (b) S. Y. Qu and H. Tian, Chem. Commun., 2012, 48, 3039; (c) L. T. Dou, J. B. You, J. Yang, C. C. Chen, Y. J. He, S. Murase, T. Moriarty, K. Emery, G. Li and Y. Yang, Nat. Photonics, 2012, 6, 180; (d) Y. N. Li, P. Sonar, L. Murphy and W. Hong, Energy Environ. Sci., 2013, 6, 1684.
6. (a) J. Lee, A. R. Han, H. Yu, T. J. Shin, C. Yang and J. H. Oh, J. Am. Chem. Soc., 2013, 135, 9540; (b) I. Kang, H. J. Yun, D. S. Chung, S. K. Kwon and Y. H. Kim, J. Am. Chem. Soc., 2013, 135, 14896; (c) I. Kang, T. K. An, J. A. Hong, H. J. Yun, R. Kim, D. S. Chung, C. E. Park, Y. H. Kim and S. K. Kwon, Adv. Mater., 2013, 25, 524.
7. (a) Y. Sun, G. C. Welch, W. L. Leong, C. J. Takacs, G. C. Bazan and A. J. Heeger, Nat. Mater., 2012, 11, 44; (b) A. K. K. Kyaw, D. H. Wang, V. Gupta, J. Zhang, S. Chand, G. C. Bazan and A. J. Heeger, Adv. Mater., 2013, 25, 2397; (c) D. Ye, X. Li, L. Yan, W. Zhang, X. Hu, Y. Liang, J. Fang, W. Y. Wong and X. Wang, J. Mater. Chem. A, 2013, 1, 7622; (d) B. Walker, A. B. Tamayo, X. D. Dang, P. Zalar, J. H. Seo, A. Garcia, M. Tantiwiwat and T. Q. Nguyen, Adv. Funct. Mater., 2009, 19, 3063; (e) Y. Liu, X. Wan, F. Wang, J. Zhou, G. Long, J. Tian, J. You, Y. Yang and Y. Chen, Adv. Energy Mater., 2011, 1, 771.
8. A. B. Tamayo, B. Walker and T. Q. Nguyen, J. Phys. Chem. C, 2008, 112, 11545.
9. (a) J. E. Coughlin, Z. B. Henson, G. C. Welch and G. C. Bazan, Acc. Chem. Res., 2014, 47, 257; (b) L. Dou, J. You, Z. Hong, Z. Xu, G. Li, R. A. Street and Y. Yang, Adv. Mater., 2013, 25, 6642; (c) Y. Li, Acc. Chem. Res., 2012, 45, 723; (d) S. D. Dimitrov and J. R. Durrant, Chem. Mater., 2014, 26, 616.
10. Y. J. Kim, J. Y. Back, S.-O. Kim, C.-W. Jeon, C. E. Park and Y.-H. Kim, Chem.–Asian J., 2014, 9, 2505.
11. Q.-C. Yu, W.-F. Fu, J.-H. Wan, X.-F. Wu, M.-M. Shi and H.-Z. Chen, ACS Appl. Mater. Interfaces, 2014, 6, 5798.
12. Y. Ren, A. M. Hiszpanski, L. Whittaker-Brooks and Y.-L. Loo, ACS Appl. Mater. Interfaces, 2014, 6, 14533.
13. (a) J. Liu, B. Walker, A. Tamayo, Y. Zhang and T.-Q. Nguyen, Adv. Funct. Mater., 2013, 23, 47; (b) S.-Y. Liu, M.-M. Shi, J.-C. Huang, Z.-N. Jin, X.-L. Hu, J.-Y. Pan, H.-Y. Li, A. K.-Y. Jen and H.-Z. Chen, J. Mater. Chem. A, 2013, 1, 2795; (c) V. S. Gevaerts, E. M. Herzig, M. Kirkus, K. H. Hendriks, M. M. Wienk, J. Perlich, P. Muller-Buschbaum and R. A. J. Janssen, Chem. Mater., 2014, 26, 916.
14. G. Zhang, H. Li, S. Bi, L. Song, Y. Lu, L. Zhang, J. Yu and L. Wang, Analyst, 2013, 138, 6163.
15. H. Liu, H. Jia, L. Wang, Y. Wu, C. Zhan, H. Fu and J. Yao, Phys. Chem. Chem. Phys., 2012, 14, 14262.
16. (a) M. R. Silva-Junior, M. Schreiber, S. P. A. Sauer and W. Thiel, J. Chem. Phys., 2008, 129, 104103; (b) M. J. G. Peach, P. Benfield, T. Helgaker and D. J. Tozer, J. Chem. Phys., 2008, 128, 044118.
17. M. C. Hwang, J.-W. Jang, T. K. An, C. E. Park, Y.-H. Kim and S.-K. Kwon, Macromolecules, 2012, 45, 4520.
18. A. Kahn, N. Koch and W. Gao, J. Polym. Sci., Part B: Polym. Phys., 2003, 41, 2529.
19. M. Abbas, A. Pivrikas, E. Arici, N. Tekin, M. Ullah, H. Sitter and N. S. Sariciftci, J. Phys. D: Appl. Phys., 2013, 46, 495105.
20. (a) S. S. Dharmapurikar, A. Arulkashmir, C. Das, P. Muddellu and K. Krishnamoorthy, ACS Appl. Mater. Interfaces, 2013, 5, 7086; (b) A. Arulkashmir, R. Y. Mahale, S. S. Dharmapurikar, M. K. Jangid and K. Krishnamoorthy, Polym. Chem., 2012, 3, 1641.
21. Y. Zhang, J. Liu and T.-Q. Nguyen, ACS Appl. Mater. Interfaces, 2013, 5, 2347.

---

Footnote

† Electronic supplementary information (ESI) available: NMR, ESI-HRMS, DSC, solvent dependent changes, theoretical calculations and charge transport properties are provided. See DOI: 10.1039/c5ra16653e

---