Synthesis of novel star-shaped truxene cored small molecules and applications in organic solar cells

The synthesis of two new star-shaped organic small molecules with a truxene core, molecules 11 and 12, were carried out. They varied in sidechain length with molecule 11 having hexahexyl and molecule 12 having hexadodecyl side chains. The physical and photovoltaic properties were measured and characterized. Both molecules showed activity in UV-visible light and electrochemical processes. Molecule 11 had optical and electrochemical HOMO-LUMO energy gaps of 2.15 and 2.45 eV, respectively, while molecule 12 had 2.12 and 2.50 eV gaps. Photovoltaic device fabrication and testing revealed bifunctional acceptor-donor properties of the molecules in bulk-heterojunction organic solar cells. The device PCEs for molecule 11 were 0.003% and 0.002%, while the PCEs for 12 were 0.004% and 0.03%. This improvement was attributed to the ease of processing and self-assembly induced by the long alkyl side chains of molecule 12.


INTRODUCTION
The unique two to three-dimensional geometry of star-shaped molecules make them attractive in varied optoelectronic applications such as colour converters  Functionalization of the truxene core is typically achieved at the C2, C7, and C12 positions, as well as the C5, C10, and C15 atoms.However, other positions can also be substituted.Alkyl groups are usually substituted at the C5, C10, and C15 positions (Shi et al., 2015).Furthermore, the incorporation of alkyl side chains is found to increase the solubility of the molecules, as well as, improve charge transport, morphology, and device performance (Gadisa et al., 2009).Additionally, these electronically inert alkyl side chains which are orthogonal to the truxene plane control packing through steric hindrance, even though they may hinder molecular π−π stacking (Roncali, 2007).The length and position of these side chains also contribute to the optoelectronic properties of the molecules (Huang et al., 2012;Tao et al., 2014).
Thiophene π-linkers have been utilized in the synthesis of scalable novel semiconducting organic molecules for solar cell applications (Bin et al., 2016).They induce stabilized planarity in the host molecule, through attractive heteroatom interactions (S-O/S-N), which may be fortified in crystal formation (Raychev and Guskova, 2017).This further improves the solid-state packing leading to favorable current density and charge transport properties (Schueppel et al., 2008).Additionally, the termination of novel semiconducting organic small molecules with the strong electron-withdrawing 3-ethyl rhodanine unit has been extensively utilized in organic solar cells (Antwi et al., 2016;Ni et al., 2015;Wan et al., 2017).This includes the synthesis of novel electron donors Fan et al., (2017) and non-fullerene acceptors (Holliday et al., 2015).
The electron-withdrawing thioketone and ketone groups on the rhodanine periphery increase the internal electronic push-pull (Privado et al., 2017).Therefore, end-capping scalable organic photoactive molecules with 3-ethyl rhodanine have been found to enhance the photovoltaic performance of molecules through an efficient HOMO-LUMO energy gap engineering, photon absorption, and favorable device morphology (Wu et al., 2018).Similarly, the 2D geometry and extended π−conjugations of truxene have been explored in the synthesis of non-fullerene acceptors Wu et al., (2018), electron donors Isla et al., (2012), and electroncollecting interlayers for organic solar cells (Xu et al., 2015).Here in, we report a newly synthesized truxene derivative achieved by coupling 3-ethyl rhodanine to the terminals of the truxene core through thiophene linkers.Substitution of hexahexyl and hexadodecyl chains at the C5, C10, and C15 positions were separately carried out to yield, molecules 11 and 12 respectively, (Scheme 1).
The physical properties as determined showed that the molecules were thermally stable with a 5% degradation temperature beyond 300 °C.Also, they exhibited sufficient photon absorption in visible light with a molar absorptivity constant between 1.63×105 (11) and 1.47×105 (12).Electrochemical processes displayed a HOMO-LUMO gap of 2.45 eV and 2.5 eV for 11 and 12 respectively.Organic solar cell applications showed a bifunctional property of the molecules.The molecules acted as both donor and acceptor units to PC61BM and P3TH respectively, even though their recorded photovoltaic performances were not as expected.

METHODOLOGY
Truxene and reagents were purchased from Sigma-Aldrich at 99.9% purity.To a stirred suspension of truxene, (1 eq) in tetrahydrofuran (5.4 ml per gram) under nitrogen, n-BuLi (3.8 eq) was added dropwise at 0 °C over 30 min.The temperature was held below 15 °C for the duration of the addition.The truxene solid was dissolved and the colour changed to deep red, which disappeared during addition but persisted on the complete addition of n-BuLi.The solution was stirred at room temperature for 30 min then 1-bromoalkane (3.8 eq) was added over 10 min at 0 °C.The mixture was stirred at room temperature for 4 h and a second portion of n-BuLi (3.8 eq) was added over 10 min at 0 °C.After stirring at room temperature for 30 min, a second portion of 1bromoalkane (3.8 eq) was added over 10 min at 0 °C.The reaction mixture was then stirred at room temperature for 18 h (TLC monitoring).Upon reaction completion, it was quenched with saturated aqueous ammonium chloride and extracted with petroleum ether (5 times).
The combined organic fractions were washed with water, dried over anhydrous MgSO4, and the solvent evaporated.The material was purified by column chromatography on silica gel, eluting with petroleum ether to yield the product.OPV device performances were investigated using the bulk-heterojunction architecture with indium tin oxide (ITO) and calcium as the electrodes and poly (3, 4-

RESULTS AND DISCUSSION
The synthesis of the truxene core 2 was completed by the cyclotrimerization of 1-indanone 1 in an acidic medium, Scheme 1.The completed batch of truxene core was separated into two portions to allow alkylation using brominated hexyl and dodecyl alkanes.
In this process, the three sp2 carbons of the truxene core were di-substituted to yield hexahexyltruxene 3 and hexadodecyltruxene 4 units, Scheme 1.The Truxene core had positions C2, C7, and C12 brominated as shown in compounds 5 and 6, Scheme 1, to facilitate the coupling of thienyl groups to the truxene core.tested by thermogravimetric analysis.Thermograms of the molecules are shown in (Figure 2).The onsets of 5% degradation temperatures were found at 390.94 οC and 364.29 οC for 11 and 12, respectively, (Table 1).This is an indication that the molecules are suitably thermally stable for applications in organic solar cell fabrications.The optical properties of molecules 11 and 12 were determined by the UV-vis spectroscopic measurements.Plots of the normalized absorption spectra for the solution and solid state are shown in (Figure 3).486 nm, respectively, (Figure 3a).Molecule 11 showed a slightly higher absorbance with an extinction coefficient of 1.63×105 L mol-1 cm-1, (Table 1).In the solid state, Figure 3b, a broader absorption peak compared to the narrow peak in solution indicates strong aggregation and packing of molecules when deposited as a film.The π-π stacking interaction of the molecules resulted in shoulder peaks in the solid state, for 11 at 499 nm and 12 at 497 nm compared to those in solution.
The difference in shoulder peaks can be attributed to the substituted alky chains on the molecules.It is anticipated that the strong aggregation of 11 due to hexahexyl chain substituents may have influenced the shoulder peak at 499 nm, while the octadecyl chain on 12 caused a weak aggregation, leading to a slightly lower shoulder peak at 497 nm.Both molecules 11 and 12 exhibited narrow optical HOMO-LUMO energy gaps, Table 1, which were calculated from the leading edge of absorptions.The HOMO-LUMO energy gap narrowed in the solid state for both molecules.12 showed the narrower energy gap, 2.12 eV, compared to 2.15 eV for 11, in the solid state.
The oxidation and reduction potentials for 11 and 12 are shown in (Figure 4).They exhibited oxidation and reduction potentials in an electrochemical process, which indicates their ability to lose and gain electrons.11 exhibited two oxidation potentials.One irreversible at +0.57V and one reversible at half-wave potential +0.73 V.An irreversible reduction wave was observed at -1.87 V.
Similarly, 12 showed one irreversible oxidation and reduction process with potentials of +0.93 V and -1.57V and one quasireversible oxidation wave with a half-wave potential of +1.10 V.The oxidation potentials are attributable to electrons lost by the thiophene linkers and the reduction waves due to the electron gained by 3-ethyl rhodanine acceptors (Xu et al., 2015).The molecules, 11 and 12, exhibited slightly different electrochemical HOMO-LUMO energy gaps, 2.45 eV and 2.50 eV respectively, which is backed by the dissimilar HOMOs (-5.37 vs 5.73 eV) and LUMOs (-2.92 vs -3.23), (Table 1).This is attributable to the different lengths of the alkyl sidechains on 11 and 12.
[52] The LUMO (3.2 vs 3.2) and HOMO (5.7 vs 5.1) energy levels for 12 and P3HT promoted electron extraction and hole transport to the electrodes at a power conversion efficiency of 0.03%, which is an over tenfold efficiency compared to 0.002% for 11.
Additionally, the favorable device morphology induced by the pendant dodecyl chains and the complimentary absorption of 12 in the visible region contributed to its better performance over 11.

CONCLUSION
Two truxene derivatives 11 and 12 bearing hexahexyl and hexadodecy pendant chains on the backbone have been successfully synthesized.Their physical and photovoltaic properties have been measured and characterized.The optical and electrochemical HOMO-LUMO energy gaps were 2.15 and 2.45 eV for 11 and, 2.12 and 2.50 eV for 12 respectively.They showed photovoltaic performances as electron-donating and withdrawing units in a binary bulk-heterojunction solar cell and indicated their potential to act as bifunctional molecules which could be explored in ternary bulk-heterojunction organic solar cells.Molecule 12, displayed a better device performance (PCE of 0.03%), as an electron acceptor, compared to 11 (PCE of 0.002%).
Products 7 and 8 were only isolated following Stille cross-coupling reactions in moderate yields, Scheme 1.With compounds 7 and 8 in hand, subsequent formylation was attempted by lithiation, and Vilsmeier-Haak techniques to give compounds 9 and 10 moderate yields.Formation of the targeted molecules was attempted via coupling the appropriate acceptor unit to molecules 9 and 10 using the Knoevenagel condensation reaction, Scheme 1.In the event, the 3-ethyl rhodanine acceptor was successfully coupled to both 9 and 10 to give compounds 11 and 12 as orange powders in 45% and 65% yield, respectively.Details of the above experiments, the NMR, DSC, MALDI-MS, and elementals are outlined in the supporting information, SI-1 to SI-5.The thermal stability of 11 and 12 was

Figure 2
Figure 2 TGA plots of 11 and 12 measured at 10 °C min-1 under argon.

Figure 3
Figure 3 Normalized absorption spectra of 11, and 12 (a) in chloroform solution (10-5 M) and (b) drop cast film on quartz glass.

Table 1
Electrochemical, thermal, and optical characteristics of 11 and 12.

Table 2
Table of best device performances for 11 and 12