Naphthalene diimide-based non-fullerene acceptors for simple, efficient, and solution-processable bulk-heterojunction devices

Abstract

Two solution processable, non-fullerene electron acceptors, 2,2′-(((2,7-dioctyl-1,3,6,8-tetraoxo-1,2,3,6,7,8-exahydrobenzo[lmn][3,8]phenanthroline-4,9-diyl)bis(thiophene-5,2-diyl))bis(methanylylidene))dimalononitrile (R1) and (2Z,2′Z)-3,3′-((2,7-dioctyl-1,3,6,8-tetraoxo-1,2,3,6,7,8-hexahydrobenzo[lmn][3,8]phenanthroline-4,9-diyl)bis(thiophene-5,2-diyl))bis(2-(4-nitrophenyl) acrylonitrile) (R2), comprised of central naphthalene diimide and two different terminal accepting functionalities, malononitrile and 2-(4-nitrophenyl)acetonitrile, respectively, were designed and synthesised. The central and terminal accepting functionalities were connected via a mild conjugated thiophene linker. Both of the new materials (R1 and R2) displayed high thermal stability and were found to have energy levels matching those of the archetypal electron donor poly(3-hexylthiophene). A simple, solution-processable bulk-heterojunction device afforded a promising power conversion efficiency of 2.24% when R2 was used as a non-fullerene electron acceptor along with the conventional donor polymer poly(3-hexylthiophene). To the best of our knowledge, the materials reported herein are the first examples in the literature where synchronous use of such accepting blocks is demonstrated for the design and development of efficient non-fullerene electron acceptors.

---

Introduction

Becquerel first noted the photovoltaic effect in 1839,¹ however, Tang's 1989 work indicated that the same effect could be observed in organic systems, meaning that organic solar cells can be a reality in the near future.² Since then the research field of organic solar cells has attracted interest from all over the world and two major technologies namely dye-sensitized and bulk heterojunction solar cells have been investigated with great effort.^3,4 In regards to bulk-heterojunction (BHJ) devices, the design and development of new functional materials and improvements in device fabrication strategies have been carried out to achieve properties such as light weight, low cost and flexibility.^5–7

A typical BHJ device is an interpenetrating network of an electron donor and acceptor entities where a solution-processed blend film is coated atop an active conducting surface. Conventional donors, such as poly(3-hexylthiophene) (P3HT), and acceptors, such as [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM), have been studied to understand device architecture, surface morphology and active surface thicknesses to gain insight of new material designs, either conjugating polymers or small molecular solids, and optimum fabrication conditions. Research based on new conjugating donor structures and conventional acceptors, such as PC₆₁BM or its C₇₁ analogue (PC₇₁BM), has been able to achieve power conversion efficiencies in the range of 10%,^8,9 which encourages continued research on the design and development of new chromophores. Multiple advantages with the development of small molecular solids exist as they can be synthesized with reproducibility, less batch to batch contamination and with simple purification strategies. Though the fullerene acceptors are dominant in the research field of organic photovoltaics,¹⁰ they are afflicted with a number of significant disadvantages such as electronic tuning via structural modification and weak absorption in the visible spectrum. Moreover, a large electron affinity can result in low open-circuit voltage (V_oc).¹¹ Such disadvantages and problems with fullerenes provide strong incentives to researchers to design and develop new non-fullerene acceptors that can have crucial photo-physical properties such as efficient absorption over the visible spectrum, excellent solubility, high charge carrier mobility and matching energy levels with those of potential donors.

Recent research on the design and development of non-fullerene acceptors for use in BHJ devices has seen a dramatic surge in efficiency, indicative of research advancing materials' design and device fabrication strategies. Recent reviews and reports ratify this logic and we, along with others, find it convincing to carry out research on a molecular level.^12–20 We understand that in order to have any potential in competing and outperforming C₆₀ derivative in efficiency, a chromophore must address the limitations of C₆₀, namely structural diversity, strong absorption in the visible range and electronic tunability. However, it must also have the properties that make C₆₀ attractive such as strong accepting strength, solubility and thermal stability. One such functionality is naphthalene diimide (NDI) which has all of the aforementioned properties and is a host of simple and scalable synthetic strategies. Chromophores based on NDI framework have been used as active components for field-effect transistors, two-photon absorption and as electron-accepting semiconductors.²¹ We and others have previously shown the utility of NDI in developing non-fullerene acceptors,^22–25 from which we demonstrated that NDI can be a substantial ally with a myriad of favourable properties.

Herein we report the design, synthesis and characterisation of two novel non-fullerene acceptors, 2,2′-(((2,7-dioctyl-1,3,6,8-tetraoxo-1,2,3,6,7,8-hexahydrobenzo[lmn][3,8]phenanthroline-4,9-diyl)bis(thiophene-5,2-diyl))bis(methanylylidene))dimalononitrile (R1) and (2Z,2′Z)-3,3′-((2,7-dioctyl-1,3,6,8-tetraoxo-1,2,3,6,7,8-hexahydrobenzo[lmn][3,8]phenanthroline-4,9-diyl)bis(thiophene-5,2-diyl))bis(2-(4-nitrophenyl)acrylonitrile) (R2), where we have used two different terminal accepting functionalities, malononitrile and 2-(4-nitrophenyl)acetonitrile, respectively, along with the central NDI block (Fig. 1). New materials R1 and R2 are the first reported examples in the literature where synchronous use of such blocks is demonstrated to develop potential non-fullerene acceptors. We were able to demonstrate that R1 and R2, when blended with P3HT in solution-processable BHJ solar cells, can indeed work as non-fullerene acceptors and the BHJ device performance confirms our design motif. It is important to mention that we are highly interested to develop novel materials for organic electronics, organic solar cells in particular, and the current work is a continuation of our efforts on the design and development of small molecular chromophores for such applications.^26–29

Fig. 1 Molecular structures of the NDI-based, novel, non-fullerene electron acceptors, R1 and R2, investigated in this study.

Experimental

Materials and methods

All the reagents and chemicals used, unless otherwise specified, were purchased from Sigma-Aldrich Co. India. Solvents were received from commercial sources and purified by standard methods where required. ¹H NMR and ¹³C NMR spectra were recorded on ADVANCE 300 MHz spectrometer with TMS as an internal reference, and CDCl₃, DMSO-d₆ and deuterated TFA were used as processing solvents. A low resolution mass spectrometry analysis was done using electron spray ionization (ESI-MS) technique with an Agilent Technologies 1100 series (Agilent ChemStation) software. High Resolution mass spectra (HRMS) were recorded by using ESIQTOF mass spectrometry. FT-IR Spectra were taken on a Perkin Elmer FT-IR 400 spectrometer. Synthesis of 5,5′-(2,7-dioctyl-1,3,6,8-tetraoxo-1,2,3,6,7,8-hexahydrobenzo[lmn][3,8]phenanthroline-4,9-diyl)bis(thiophene-2-carbaldehyde) was reported previously.³⁰

Details of device fabrication and characterization of photovoltaic devices have been reported previously.²⁰

Synthetic strategy

R1 and R2 were synthesized as per the synthetic protocol depicted in Scheme 1.

Scheme 1 Synthetic strategy for the synthesis of R1 and R2.

2,2′-(((2,7-dioctyl-1,3,6,8-tetraoxo-1,2,3,6,7,8-hexahydrobenzo[lmn][3,8]phenanthroline-4,9-diyl)bis(thiophene-5,2-diyl))bis(methanylylidene))dimalononitrile (R1). To a mixture of 1 (200 mg, 0.28 mmol) and malononitrile (73.0 mg, 1.12 mmol) in 20.0 mL of DCM : CH₃CN (1 : 1) was added piperidine (41.0 μL, 0.40 mmol), and the resultant reaction mixture was refluxed for 48 h. The completion of reaction was confirmed by TLC. The reaction mixture was cooled and poured into precooled water containing hydrochloric acid and was extracted with dichloromethane. The organic layer was dried over Na₂SO₄, filtered and evaporated under reduced pressure. The crude product was purified by silica gel column chromatography using dichloromethane : hexane (9 : 1, v/v) mixture as an eluent to afford title product as a light red solid (100 mg, 44%). Mp: 228–230 °C; ¹H NMR (CDCl₃ : DMSO-d₆ 4 : 1, 300 MHz) δ (TMS, ppm): 8.71–8.75 (s, 2H), 7.55–7.56 (s, 2H), 7.52–7.55 (d, J = 3 Hz, 2H), 7.33–7.36 (d, J = 3 Hz, 2H), 4.03–4.11 (m, 4H), 1.65–1.72 (m, 4H), 1.20–1.37 (m, 20H), 0.81–0.90 (t, 6H); ¹³C NMR (CDCl₃ : DMSO-d₆ 4 : 1, 75 MHz) δ ppm: 157.7, 154.6, 147.0, 142.4, 139.6, 136.7, 136.5, 135.2, 129.1, 126.4, 121.1, 113.3, 76.2, 45.3, 32.4, 29.8, 28.6, 27.7, 23.2, 22.6, 14.3; FT-IR (KBr, cm⁻¹): ν 3437, 2925, 2857, 2340, 2186, 1700, 1659, 1629, 1450, 1311, 1251, 1185, 957, 614; (ESI) m/z: (M)⁺ 807; HRMS m/z: calculated for C₄₆H₄₂N₆O₄S₂Na 830.0370, found (M + Na)⁺ 830.0380.

(2Z,2′Z)-3,3′-((2,7-dioctyl-1,3,6,8-tetraoxo-1,2,3,6,7,8-hexahydrobenzo[lmn][3,8]phenanthroline-4,9-diyl)bis(thiophene-5,2-diyl))bis(2-(4-nitrophenyl)acrylonitrile) (R2). To a mixture of 1 (60.0 mg, 0.08 mmol) and 4-nitrophenylacetonitrile (19.0 mg, 0.16 mmol) in 15 mL of DCM : CH₃CN (1 : 1) was added piperidine (41 μL, 0.40 mmol), and the resultant mixture was refluxed for 48 h. The reaction mixture was cooled and poured into water containing hydrochloric acid, and extracted with dichloromethane. The organic layer was dried over Na₂SO₄, filtered and evaporated under reduced pressure. The crude product was purified by silica gel column chromatography (dichloromethane : hexane (9 : 1, v/v)) to afford title compound as a brownish solid (75.0 mg, 88%). Mp: 314–315 °C; ¹H NMR (CDCl₃ : deuterated TFA 1 : 0.1, 300 MHz) δ (TMS, ppm): 8.83 (s, 2H), 8.37–8.34 (m, 4H), 7.95–7.97 (s, 2H), 7.83–7.88 (d, J = 9 Hz, 4H), 7.80–7.83 (d, J = 6 Hz, 2H), 7.38–7.41 (d, J = 6 Hz, 2H), 4.10–4.17 (m, 4H), 1.63–1.70 (m, 4H), 1.22–1.36 (m, 20H), 0.81–0.89 (t, 6H); ¹³C NMR (CDCl₃ : deuterated TFA 1 : 0.1, 75 MHz) δ ppm: 163.1, 154.5, 152.7, 150.8, 147.6, 146.8, 139.9, 138.7, 136.8, 135.9, 129.3, 127.7, 126.5, 125.6, 124.8, 117.7, 105.6, 42.1, 31.7, 29.7, 29.1, 27.8, 26.9, 22.5, 13.7; FT-IR (KBr, cm⁻¹): ν 3428, 2956, 2921, 2851, 2360, 1706, 1665, 1629, 1596, 1517, 1416, 1341, 1185, 1080, 963, 820, 718; (ESI) m/z (M + NH₄)⁺ 1018; HRMS m/z: calculated for C₅₆H₅₁N₆O₈S₂ 999.1607, found (M)⁺ 999.1622.

Results and discussion

R1 and R2 were both prepared in moderate to high yields (Scheme 1), characterized spectrally and their optoelectronic properties were investigated.

The ultraviolet-visible (UV-Vis) absorption spectra of R1 and R2 were measured in their chloroform solutions (Fig. 2). As expected, dye R2 had a significantly higher extinction coefficient and absorption maximum when compared with R1 (R2 = 77 446 L M⁻¹ cm⁻¹ at 387 nm against R1 = 65 834 L M⁻¹ cm⁻¹ at 340 nm). This bathochromic shift for the solution spectrum of R2 was observed as a result of using aromatic functionality (phenyl group) at both the terminals. The use of additional phenyl group can provide extended conjugation over the whole molecular backbone which is responsible for this bathochromic shift as well as enhancing the solubility profile. An additional peak in the solution spectrum of R2 was also observed at 501 nm (molar extinction coefficient = 16 031 L M⁻¹ cm⁻¹) which is indicative of strong conjugation within the molecular format of R2. With the insertion of terminal phenyl rings the enhanced profile allowed a larger amount of the solar spectrum to be absorbed, thus exhibiting greater intramolecular charge transfer (ICT) transition. This type of control of altering the absorption profile through the use of an acceptor with greater strength could be expected to lead to enhanced light harvesting, which in fact can be advantageous for achieving higher photocurrent density. The film absorption of R2 showed a strong response in pristine and blend forms covering most of the visible region, which, as previously mentioned, is an important property for a potential non-fullerene acceptor. Unfortunately, the film absorption of R1 wasn't feasible in low boiling solvents, such as chloroform, mainly due to its low solubility. The use of a high boiling solvent, such as o-dichlorobenzene, demonstrated very broad absorption pattern covering most of the visible region. We suspect that the low solubility of R1 may be due to the presence of excessive-cyano functionalities.

Fig. 2 UV-Vis absorption spectra of R1 and R2 in chloroform solutions (R1 S and R2 S), as pristine as-casted films (R1 F and R2 F) and as a blend with P3HT [1 : 1.2 P3HT : R2 (R2 B)].

The energy levels of both the dyes were estimated using a combination of photoelectron spectroscopy in air (PESA) and UV-Vis spectroscopy. PESA measurements were conducted in powder form rather than the film form in order to maintain consistency in experimental results. This was done in practice as R1 failed to give smooth films using low-boiling solvents. The HOMO energy levels using R1 and R2 powders were estimated at −5.73 eV and −5.92 eV respectively. The HOMO value of R2 was lower due to the presence of a stronger electron withdrawing functionality, 2-(4-nitrophenyl)acetonitrile, at the terminal positions. Furthermore, the overall band gap of R2 was lower when compared with R1, a finding supported by the theoretical density functional theory (DFT) calculations. For energy level diagram and PESA curves, see Fig. 3 and S1 and S2 in the ESI† respectively. The energy level diagram indicated that the band gaps of these materials are all in the range that are required of an electron acceptor material in terms of fabrication with a donor such as P3HT.

Fig. 3 Energy level diagram depicting the band gaps of R1 and R2 in comparison with P3HT, an archetypal donor, and PC₆₁BM, a conventional acceptor.

DFT calculations were carried out using the Gaussian09 suite of programs³¹ and the B3LYP/6-311+G(d,p)//B3LYP/6-31G(d) level of theory, and indicated that orbital densities are evenly distributed over the whole molecular backbone, a result that is quite common with small-molecule non-fullerene acceptors.^19,20 Though the design of R1 and R2 incorporated different terminal acceptors, the LUMO densities were found to be resided within the central NDI functionality, primarily due to its strong electron withdrawing capacity, however, LUMO+1 energy levels presided along the terminal moeties (please see Fig. 4 and S3† for DFT calculations). Furthermore, the computed absorption spectra (see Fig. S4†) show the first transition peaks at 591.98 nm and 653.36 nm, respectively, for R1 and R2. These first peaks are described as HOMO → LUMO transition: excited state 1: 2.0958 eV 591.58 nm f = 0.4574; excited state 1: 1.8976 eV 653.36 nm f = 0.7449 for R1 and R2, respectively. It is vital to mention that the theoretical optical absorption transitions of R1 and R2 shown in Fig. S4 (ESI†) verify our experimental findings. It was further realised that despite the presence of interesting and gainful optoelectronic properties, organic semiconducting materials must possess thermal stability in order to endure rigid device fabricating conditions. In accordance with this requirement, we conducted thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) analyses. TGA and DSC curves revealed that thermal stability of R1 and R2 is excellent, a finding that supports high temperature annealing of P3HT : R1/R2 devices (for TGA and DSC curves see Fig. S5 and S6 in the ESI†). Furthermore, X-ray powder diffraction (XRD) study indicated the amorphous nature of R1 and R2, a finding consistent with the physical appearance of newly synthesized materials (for XRD patterns please see Fig. S7 in the ESI†).

Fig. 4 Orbital density distribution for the HOMOs (bottom) and LUMOs (top) of R1 and R2.

As the energy level diagram indicated that both R1 and R2 had promising optical and electrochemical properties, solution-processable BHJ devices using R1 and R2 as non-fullerene acceptors were fabricated. The BHJ devices were fabricated using the classical, low cost commercial donor polymer P3HT.³² We realised that in order to demonstrate feasibility of new type of non-fullerene acceptors, we must consider their foremost suitability with easily synthesizable and commercially available donors, such as P3HT, in order to gain an understanding of device processing conditions and blend film morphology, to name a few. The BHJ device architecture used was ITO/poly(3,4-ethylenedioxythiophene)polystyrenesulfonate (PEDOT:PSS, 38 nm)/active layer (∼50–60 nm)/Ca (20 nm)/Al(100 nm) where the active layer was a 1 : 1.2 blend of P3HT : R1/R2, spin-cast from o-dichlorobenzene on top of the PEDOT:PSS surface. This simple device architecture was chosen due to its stability, reproducibility, solution processability and ease of fabrication.

As R1 indicated its insufficient solubility in low boiling solvents, o-dichlorobenzene was chosen as the processing solvent. It is imperative to mention that the use of a high boiling solvent is beneficial during the fabrication of photovoltaic devices comprising P3HT and a non-fullerene acceptor as is demonstrated by us and others.^20,33 Moreover, the use of a high boiling solvent is preferable from processing point of view and can result in smooth and consistent films. The blend films using an optimal ratio of 1 : 1.2 (P3HT : acceptor) worked excellently with R2, however, poor film characteristics were observed for R1. Although an inconsistency of blend surface (P3HT : R1) in terms of surface evenness was observed, the device still performed and gave a V_oc of 1.02 V, short circuit current density (J_sc) of 2.15 mA cm⁻², fill factor (FF) of 0.36 and a PCE of 0.79%, respectively, indicating that R1 indeed has a capability to work as a non-fullerene acceptor. In contrast, the V_oc, J_sc, FF and maximum PCE reached 0.87 V, 6.77 mA cm⁻², 0.38 and 2.24%, respectively, for a device based on P3HT : R2 when fabricated under similar conditions. Though we also tried other donor : acceptor combinations such as 1 : 1 and 1 : 2, the 1 : 1.2 combination was found superior than either of the combinations as it provided best blend film characteristics and device processing. Lower V_oc and higher J_sc values observed for the blend of P3HT : R2 are in good agreement with energy levels and optical absorption when compared with P3HT : R1 blend. It was evident that R2 blended excellently with P3HT, a result rationalizing the literature recommendation of using high boiling solvent. It was also observed that annealing of active films resulted in minor cracks to film surfaces and poor photovoltaic performance was recorded. The respective current–voltage curves for the best devices are represented in Fig. 5 and Table S1 (ESI†) displays current–voltage characteristics for other donor : acceptor combinations along with average values.

Fig. 5 Current–voltage curves for the best devices based on P3HT : R1/R2 blends (1 : 1.2 w/w) under simulated sunlight (AM1.5, 1000 Wm⁻²).

Incident-photon-to-current conversion efficiency (IPCE) measurement of the blend film of R2 with a donor–acceptor weight ratio of 1 : 1.2 is shown in Fig. 6. Broad spectrum IPCE over most of the visible region was observed with a maximum IPCE value of approximately 32% at around 505 nm. In view of this potential IPCE, good optical profile and appropriately positioned energy levels we surmise that R2 could also be a suitable accepting component for other electron-donating low-band gap conjugated polymers as well as small molecular chromophores. Microstructure analysis of the P3HT : R2 blended surface deposited on ITO coated glass was examined by the conventional atomic force microscopy (AFM) in tapping mode. The actual surface morphology of the blend film of P3HT : R2 (1 : 1.2 w/w) is shown in Fig. 7. The blend appears to have crystalline grains with a root-mean-square (RMS) roughness of ∼7 nm. Though the crystalline grains may be a result of P3HT self-organization, a finding that is beneficial to charge transportation and ordered structure formation, an interpenetrating network of donor–acceptor component was observed which is beneficial to exciton dissociation and can result in an enhanced efficiency of photovoltaic devices. Though it was evident that R2 exhibited an ability to interweave well with P3HT it didn't disrupt the effect of P3HT dominating the blend morphology, a finding consistent with blend film absorption. Furthermore, the PCE of the corresponding P3HT : R2 device remained nearly steady after a week's hold whereas the outcome of the P3HT : R1 device dropped below 70%. This phenomenon further indicates that it is very critical for a non-fullerene acceptor, such as R2, to plait well with a donor, such as P3HT, in order to generate favourable morphology and for the futuristic practical applications of BHJ devices.

Fig. 6 IPCE curve of the best BHJ device based on a blend of P3HT : R2.

Fig. 7 AFM image for the blend film of P3HT : R2 (1 : 1 w/w).

It is worth mentioning that even though the PCEs with the use of non-fullerene acceptors are surging, central NDI-based chromophores which comprise terminal accepting counterparts still lag in design, development, and efficiency. Having said this, the design of R1 and R2 not only advocates the use of electron-accepting building blocks to generate a highly conjugated target non-fullerene acceptor but also provides a solid platform for other accepting fragments that can be used at terminals along with central NDI functionality.

Conclusions

In summary, we have been successful to creating two small-molecular non-fullerene electron acceptors which were thoughtfully designed, synthesized and displayed promising optoelectronic properties. Design envision of these chromophores allowed the concomitant use of a variety of accepting blocks and to produce an interesting structural concept for the design and development of non-fullerene acceptors. Solution-processable BHJ devices incorporating R2 as an acceptor along with the conventional donor polymer P3HT afforded a worthwhile outcome, which is paramount for the advancement of research of non-fullerene acceptors. This work achieved one of the best efficiency outcomes based on the current design and has provided an opportunity for insightful selection of various accepting blocks for their union to seek high device performance. Efforts are ongoing to further improve the efficiency of BHJ devices and future studies will look at exploiting the commercially available conjugating polymers.

Acknowledgements

Sid. V. B. would like to thank the TAPSUN programme for financial assistance under the project NWP0054. S. V. B. (RMIT) acknowledges financial support from the Australian Research Council (ARC), Australia, under a Future Fellowship Scheme (FT110100152). D. S. acknowledges CSIR, New Delhi for SRF support. A. G. is thankful to the Alfred Deakin Fellowship Scheme at the Deakin University, Waurn Ponds Victoria. The CSIRO Division of Materials Science and Engineering Clayton Victoria is acknowledged for providing support through a visiting fellow position for A. G. We also acknowledge the fabrication and analytical facilities at RMIT University, Deakin University and Bio 21 Institute, Melbourne University, Melbourne Victoria Australia.

Notes and references

1. E. Becquerel, C. R. Acad. Sci., 1839, 9, 561.
2. C. W. Tang, Appl. Phys. Lett., 1986, 48, 183.
3. W. Xiang, A. Gupta, M. K. Kashif, N. Duffy, A. Bilic, R. A. Evans, L. Spiccia and U. Bach, ChemSusChem, 2013, 6, 256.
4. G. Yu, J. Gao, J. C. Hummelen, F. Wudl and A. J. Heeger, Science, 1995, 270, 1789.
5. Y. Huang, E. J. Kramer, A. J. Heeger and G. C. Bazan, Chem. Rev., 2014, 114, 7006.
6. A. J. Heegar, Adv. Mater., 2014, 26, 10.
7. Y. Lin, Y. Li and X. Zhan, Chem. Soc. Rev., 2012, 41, 4245.
8. B. Kan, Q. Zhang, M. Li, X. Wan, W. Ni, G. Long, Y. Wang, X. Yang, H. Feng and Y. Chen, J. Am. Chem. Soc., 2014, 136, 15529.
9. J. You, L. Dou, K. Yoshimura, T. Kato, K. Ohya, T. Moriarty, K. Emery, C.-C. Chen, J. Gao, G. Li and Y. Yang, Nat. Commun., 2013, 4, 1446.
10. Y. He and Y. Li, Phys. Chem. Chem. Phys., 2011, 13, 1970.
11. R. Y. C. Shin, P. Sonar, P. S. Siew, Z. K. Chen and A. Sellinger, J. Org. Chem., 2009, 74, 3293.
12. P. Sonar, J. P. F. Lim and K. L. Chan, Energy Environ. Sci., 2011, 4, 1558.
13. Y. Lin and X. Zhan, Mater. Horiz., 2014, 1, 470.
14. S. Holliday, R. S. Ashraf, C. B. Nielsen, M. Kirkus, J. A. Rçhr, C.-H. Tan, E. C. Fregoso, A.-C. Knall, J. R. Durrant, J. Nelson and I. McCulloch, J. Am. Chem. Soc., 2015, 137, 898.
15. J. Zhao, Y. Li, H. Lin, Y. Liu, K. Jiang, C. Mu, T. Ma, J. Y. L. Lai, H. Hu, D. Yu and H. Yan, Energy Environ. Sci., 2015, 8, 520.
16. Y. Lin, Z.-G. Zhang, H. Bai, Y. Yao, Y. Li, D. Zhu and X. Zhan, Energy Environ. Sci., 2015, 8, 610.
17. Y. Lin, Z.-G. Zhang, H. Bai, Y. Li, D. Zhu and X. Zhan, Adv. Mater., 2015, 27, 1170.
18. H. Patil, W. X. Zu, A. Gupta, V. Chellappan, A. Bilic, P. Sonar, A. Rananaware, S. V. Bhosale and S. V. Bhosale, Phys. Chem. Chem. Phys., 2014, 16, 23837.
19. A. M. Raynor, A. Gupta, H. Patil, A. Bilic and S. V. Bhosale, RSC Adv., 2014, 4, 57635.
20. (a) H. Patil, A. Gupta, B. Alford, D. Ma, S. H. Privèr, A. Bilic, P. Sonar and S. V. Bhosale, Asian J. Org. Chem., 2015, 4, 1096; (b) A. M. Raynor, A. Gupta, H. Patil, D. Ma, A. Bilic, T. J. Rook and S. V. Bhosale, RSC Adv., 2016, 6, 28103.
21. (a) S. V. Bhosale, C. Jani and S. Langford, Chem. Soc. Rev., 2008, 37, 331; (b) C.-C. Lin, M. Velusamy, H.-H. Chou, J. T. Lin and P.-T. Chou, Tetrahedron, 2010, 66, 8629.
22. A. Gupta, X. Wang, D. Srivani, B. Alford, V. Chellappan, A. Bilic, H. Patil, L. A. Jones, S. V. Bhosale, P. Sonar and S. V. Bhosale, Asian J. Org. Chem., 2015, 4, 800.
23. A. Gupta, R. V. Hangarge, X. Wang, B. Alford, V. Chellapan, L. A. Jones, A. Rananaware, A. Bilic, P. Sonar and S. V. Bhosale, Dyes Pigm., 2015, 120, 314.
24. G. Ren, E. Ahmed and S. A. Jenekhe, Adv. Energy Mater., 2011, 1, 946.
25. Y. Liu, L. Zhang, H. Lee, H.-W. Wang, A. Santala, F. Liu, Y. Diao, A. L. Briseno and T. P. Russell, Adv. Energy Mater., 2015, 5, 1500195.
26. A. Gupta, A. Ali, A. Bilic, M. Gao, K. Hegedus, B. Singh, S. E. Watkins, G. J. Wilson, U. Bach and R. A. Evans, Chem. Commun., 2012, 48, 1889.
27. A. Gupta, V. Armel, W. Xiang, G. Fanchini, S. E. Watkins, D. R. MacFarlane, U. Bach and R. A. Evans, Tetrahedron, 2013, 69, 3584.
28. R. J. Kumar, Q. I. Churches, J. Subbiah, A. Gupta, A. Ali, R. A. Evans and A. B. Holmes, Chem. Commun., 2013, 49, 6552.
29. A. Gupta, A. Ali, T. B. Singh, A. Bilic, U. Bach and R. A. Evans, Tetrahedron, 2012, 68, 9440.
30. S. R. Bobe, R. S. Bhosale, S. P. Goskulwad, A. L. Puyad, S. V. Bhosale and S. V. Bhosale, RSC Adv., 2015, 5, 100697.
31. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, et al., Gaussian 09, revision D.01, Gaussian Incorporation, Wallingford CT, 2013.
32. J. H. Bannock, S. H. Krishnadasan, A. M. Nightingale, C. P. Yau, K. Khaw, D. Burkitt, J. J. M. Halls, M. Heeney and J. C. de Mello, Adv. Funct. Mater., 2013, 23, 2123.
33. Y. Lin, P. Cheng, Y. Li and X. Zhan, Chem. Commun., 2012, 48, 4773.

---

Footnote

† Electronic supplementary information (ESI) available: PESA, theoretical optical absorption, TGA, DSC, XRD and experimental spectra. See DOI: 10.1039/c6ra05538a

---