Controlling the end-groups of Poly (3- Hexylthiophene) via Vilsmeier-haack reaction

π-Conjugated oligomeric and polymeric semiconductors have been the focus of intense research over the past few decades as alternatives to inorganic semiconductors for lowcost electronic applications such as organic thinfilm transistors (OTFTs), light-emitting diodes (OLEDs), and photovoltaics (OPVs). These materials enable vaporor solution-phase fabrication of large-area, lightweight electronic devices and are compatible with plastic substrates for mechanically flexible, conformable, and wearable electronics. In this research, we aim at modification of the H/Br end groups of poly (3-hexylthiophene) to CHO/Br end groups via Vilsmeier-haack reaction using POCl3 and DMF as the catalytic system in toluene medium. The end groups of the obtained polymer were determined via FT-IR spectroscopy and were further confirmed by Maldi-ToF. The result showed that completion of the Vilsmeierhaack reaction was obtained after 24 h at 75 °C.


INTRODUCTION
Polythiophenes have become the subject of extensive study. These materials are viewed as potentially useful components in field-effect transistors, optical and electronic sensors, lightemitting devices, non-linear optical materials, etc [1,4].

T -H T T T -H T H H -H T H H -T T
However, these polymers exhibit an intrinsic insolubility which makes purification, as well as chemical modification and processability, difficult. To overcome this major inconvenience, the introduction of long alkyl chains in the βposition of the thiophene ring has been envisaged. In fact, the poly(3-alkylthiophenes) P3AT shows a very good solubility in common organic solvents. However, the alkyl substituent in a thiophene ring can be incorporated into a polymer with different regioregularities head-to-tail (HT), tail-to-tail (TT) and head-to-head (HH) [5][6][7].
Recent research on P3AT has focused on the HT regioregularity.
Indeed, these HT regiospecific polymers have improved electroconductivity, optical nonlinearity and magnetic properties over regiorandom polymers in which more sterically hindered HH linkages can cause defects in the conjugated polymer chains and reduce the desired physical properties of the materials.
Therefore, in this study, we aim to modify the end groups of P3HT via Vilsmeier-Haack reaction using POCl3 and DMF as the catalytic system. The end-groups of P3HT were characterized via FTIR and Maldi-Tof spectroscopies.
All reactions were performed in oven-dried glassware under purified nitrogen.

Characterization
Attenuated total reflection Fourier transform infrared (ATR FT-IR) spectra were recorded using BIO-RAD Excalibur spectrometer equipped with an ATR Harrick Split PeaTM. 1 lL aliquots of those solutions were applied onto the target area already bearing the matrix crystals, and air-dried. For the recording of the single-stage MS spectra, the quadrupole (rf-only mode) was set to pass ions from 750 to 3000 Th, and all ions were transmitted into the pusher region of the time-offlight analyzer where they were mass analyzed with 1 s integration time

Preparation of P3HT with controlled endgroups
Into the flask under nitrogen atmosphere containing 2-bromo-3-hexyl-5-iodothiophene (2,36 g, 6,31 mmol) was added dry THF (30.0 mL) via a syringe, and the mixture was stirred at 0°C. i-PrMgCl (2.0 M solution in THF, 3,16 mL, 6,31 mmol) was added via a syringe, and the mixture was stirred at 0°C for 0.5h. A suspension of Ni (dppp)Cl2 (72 mg, 0.13 mmol) in THF (10.0 mL) was added to the mixture via a syringe at 0°C, and then the mixture was stirred at room temperature.
After the reaction mixture was stirred for 1 day, the HCl aqueous solution 5N was added drop by drop. The mixture was stirred for another 0.5h and then precipitated in cold MeOH. The product was washed well with MeOH to afford a purple solid, > 98.5% rr-HT-P3HT (1,2 g, 72%, Mn = 7606, PDI = 1.12).

Optimization of Vilsmeier-Haack reaction
H-P3HT-Br (Mn=4061, PDI=1.24) (300 mg, 0.074 mmol) was dissolved in anhydrous toluene (80 mL) under N2. N,N-Dimethylformamide (1 mL, 13 mmol) and POCl3 (0.7 mL, 7.6 mmol) were added. The reaction was carried out first at 75 °C for 3h. The solution was cooled to room temperature, followed by the addition of saturated aqueous solution of sodium acetate. The solution was stirred for another 2h. The polymer was precipitated in cold methanol and washed well with water, then methanol.

RESULTS AND DISCUSSION
A formyl-de-hydrogenation of the as-obtained α-bromo P3HT (Br-P3HT-H) protic end-group was successfully performed by Vilsmeier reaction (second step). The evolution of the reaction was followed by MALDI-ToF analysis until complete transformation of the α-bromo P3HT (Scheme 1) to α-bromo, ω-formyl P3HT (Scheme 1). The mechanism of the Vilsmeier-Haack reaction can be described as in Scheme 1.  The FT-IR spectroscopy in Figure 1 shows the characteristic peak at 1650 cm -1 attributed to the CHO groups of P3HT.
The MALDI-ToF result of α-bromo, ω-formyl is presented in Figure 2. The results showed that the end-groups may be controlled with the HCl addition rate. To confirm this observation, two P3HT samples, with the same Mn (Dp=30), were synthesized with different quenching rates. With a fast quenching, only H/Br end-groups were obtained (Figure 2, above), whereas with a slow quenching, a mixture of H/Br and Br/Br endgroups was obtained (Figure 2, below). MALDI-ToF analysis showed that the H-P3HT-Br was partially transformed into CHO-P3HT-Br. This product was re-used to carry out the Vislmeier reaction until finding the optimal reaction time (that ensures the completion of the end group modification).  As seen in Figure 3, the -H end group was totally transformed into -CHO end group after almost 48h. Therefore, the Vislmeier reaction was carried out in 24h and the DMF/POCl3 quantity was doubled. In fact, with this condition, another H-P3HT-Br (Mn=3481, PDI=1.24) was totally transformed into CHO-P3HT-Br (Mn=3504, PDI=1.23) with 94% yield (Figure 4).

Conclusion
The end-groups of P3HT were successfully transferred from H/Br to CHO/Br via Vislmeier-