Synthesis and properties of pteridine-2,4-dione-functionalised oligothiophenes

Glasgow Centre for Physical Organic Che Chemistry, University of Glasgow, Glasgow, glasgow.ac.uk Organic Semiconductor Centre, SUPA, Scho St Andrews, North Haugh, St Andrews, Fife Department of Chemistry, University of Ma † Electronic supplementary information voltammetry, uorescence spectroscopy data: http://dx.doi.org/10.5525/GLA.RESE 1053867. For ESI and crystallographic dat DOI: 10.1039/c5ra22402k ‡ These authors contributed equally. Cite this: RSC Adv., 2016, 6, 7999


Introduction
Acceptor-functionalised short-chain linear oligothiophenes 1 have received considerable attention over recent years, and have been utilised as components in organic photovoltaic (OPV) cells, organic eld effect transistors (OFETs), and organic light emitting diodes (OLEDs). 2 The juxtaposition of the thiophene donor (D) units with the acceptor (A) component affords conjugated D-A systems which typically promotes better charge carrier mobility and decreases the band gap. 3 Therefore, the synthesis of new acceptor-appended short oligothiophenes are attractive targets for the development of promising new materials with a range of optoelectronic applications.
The pteridine-2,4-dione moiety occurs widely in biological systems, and in particular, is found in avin-based redox cofactors. 4The fascinating redox properties of the latter have ensured that synthetic avin derivatives have become important building blocks for systems with molecular device and optoelectronics applications. 5Here, we report a class of functionalised oligothiphene featuring pteridine-2,4-dione acceptor units which are constructed from building block t3 (Scheme 1).An important feature of t3 is that it is possible to selectively brominate the a-position of one of the thiophene units thereby allowing the synthesis of oligothiophenes t6-t8 with symmetrically appended acceptor units.

Synthesis
The synthesis of the acceptor-functionalised oligothiophene derivatives are shown in Scheme 1.The synthesis of the key building block t3 was achieved from compound 1. 6 N-Alkylation of 3 with iodoheptane was then undertaken to confer good solubility on t3 and its resulting oligomers.The X-ray crystal structure of the t3 clearly shows that one of the thiophene residues is almost coplanar to the pteridine core, whereas the other thiophene is non-planar and therefore non-conjugated to the core acceptor moiety (Fig. 1a).Bromination of t3 furnished compound 4 in 71% yield following purication by column chromatography. 1 H NMR spectroscopy performed on compound 4 indicated that the bromination was selective, as evidence of other brominated species was not observed (see ESI †).X-ray crystallography conrmed that bromination occurs at the a-position of the more conjugated thiophene moiety (Fig. 1b).Compound 4 was then readily self-coupled (via the in situ preparation of its boronate ester) using a one-pot Suzuki-Miyaura reaction. 7Compound 4 could also be readily converted to oligothiophenes t7 and t8 using Stille methodology and the corresponding bis(tributylstannyl)thiophene.

Photophysical and electrochemical properties
UV-vis spectroscopy performed on compound t3 indicated that the longest wavelength absorption band shows solvatochromism, which is consistent with an intramolecular charge-transfer (ICT) band (see ESI †).Increasing the oligothiophene chain length from t3 to t8 shis the ICT absorption towards longer a Glasgow Centre for Physical Organic Chemistry (GCPOC), WestCHEM, School of Chemistry, University of Glasgow, Glasgow, G12 8QQ, UK.E-mail: Graeme.Cooke@ glasgow.ac.uk wavelength (547 nm for t3 to 673 nm for t8 in DCM) (Fig. 2).This chain length dependency in absorption properties suggests that the thiophene units are conjugated in the higher oligomers.The electrochemical properties of t3-t8 were determined using cyclic and square wave voltammetry (ESI † and Fig. 3).The electrochemical and optical properties are summarised in Table 1.The electrochemical data indicate that the oxidation potentials vary signicantly whilst the reduction potentials remain fairly constant for the series.Accordingly, the calculated E fund (ref.8) on going from t3 to t8 signicantly decreases, which mainly results from a lowering of the ionization potentials (IPs) due to a concomitant increase in conjugation within the thiophene backbone.However, the electron affinities (EAs) remain relatively unchanged, indicating that increasing the Scheme 1 Synthesis of t3, t6-t8.length of thiophene backbone has a limited inuence on the acceptor properties of the pteridine units.

DFT calculations
DFT calculations were undertaken to probe the structure and electronic properties of t3-t8.The calculations for the oligomers are in accordance with the X-ray data for t3, and show that the outer thiophene units are out of conjugation with the acceptor core, whereas the conjugated thiophene units are essentially coplanar.The HOMOs of t3-t8 are mainly located on the conjugated thiophene backbone whereas the LUMOs are largely localised over the pteridine acceptor cores and their conjugated thiophene units (Fig. 4

Spectroelectrochemistry
Spectroelectrochemical measurements were carried out on compounds t3 (Fig. 5) and t8 (Fig. 6).Application of increasing negative potential to the working electrode produced a profound change in their UV-vis spectra, and the applied potentials of which generally coincide with the onset of the reduction waves shown in Fig. 3.The UV-vis spectra of compound t3 undergoes a signicant change around À0.8 V with the peak at 439 nm collapsing and a new absorption at 471 nm forming.In accordance with spectroelectrochemical data recorded for related avin derivatives, we attribute this spectral change due to the formation of pteridine radical anion species. 9s further negative potential is applied to the cell up to À1.2 V, the spectra pass through an isosbestic point at around 420 nm, whilst the absorption at 471 nm collapses and a shoulder at around 387 nm forms.This second feature is presumably due to the onset of the second reduced state (as indicated in the square wave voltammetry data).Interestingly, although the spectroelectrochemistry of t8 is qualitatively similar to that of t3 when a voltage of around À0.8 V is applied, in that a longer wavelength absorption at 445 nm and a shoulder around 480 nm form, however, a signicantly longer wavelength absorption at 667 nm also forms for t8, which collapses with increasing negative potential.This new feature is likely a consequence of the addition of the second pteridine unit and the increased conjugation that occurs upon going from t3 to t8, which signicantly reduces the onset voltage required to generate the   second reduced state.For both compounds the original spectra recover upon returning the applied potential to 0 V.

Fluorescence
The solution uorescence of t3 to t8 were investigated, and experiments revealed that these materials were only weakly uorescent, presumably a consequence of their donor-acceptor architecture.We next investigated the ability of t3 and t8 to quench the uorescence of the electron rich polymer poly(3hexylthiophene-2,5-diyl) (P3HT).Fig. 7 shows the photoluminescent quantum yield (PLQY) of thin lm blends of these materials prepared from chlorobenzene solutions.Increasing the concentration of t3 and t8 to as little as 1% causes a signicant (50%) quenching of the uorescence of P3HT, thereby indicating efficient electron transfer from the donor polymer to acceptors t3 and t8. 10 At higher concentration of t3 and t8 (3%), no further quenching was observed indicating that signicant aggregation of the t3 and t8 acceptor units may occur.

Spectroscopic and voltammetry measurements
UV-vis spectra were recorded on a Shimadzu UV-3600 spectrometer using a 10 mm path length quartz cuvette.Cyclic voltammetry and square wave voltammetry was recorded with a CH Instruments Inc. 440A potentiostat.The voltammograms were recorded in solutions of tetrabutylammonium hexa-uorophosphate (TBAPF 6 ) in dry-DCM (0.1 M).Measurements were performed with degassed solutions under inert atmosphere, using a platinum working electrode, a silver wire pseudo-reference electrode and a platinum wire counter electrode.The redox potentials were determined relative to the ferrocene/ferrocenium redox couple (4.8 eV), and IPs and EAs were estimated from these values.

UV-vis spectroelectrochemistry
Spectroelectrochemical measurements were carried out using the ALS Co. Ltd.SEC-C 0.5 mm spectroelectrochemical cell featuring a platinum gauze working electrode and silver wire pseudo-reference electrode and a platinum wire counter electrode.All experiments were carried out with 1 Â 10 À4 M solutions of the analyte in DCM with TBAPF 6 (0.1 M) as the supporting electrolyte.UV-vis spectra were recorded on a Shimadzu UV-3600 spectrometer, and potential was applied using a CH Instruments Inc. 440A potentiostat.A base line spectrum of the cell and electrolyte was subtracted from the recorded spectra and the spectra were re-zeroed.

Fluoresence quenching experiments
A solution of P3HT (from Rieke metals) was prepared by dissolving 20 mg mL À1 in chlorobenzene.Solutions of t3 and t8 were prepared by dissolving 1 mg mL À1 in chlorobenzene.Both solutions were mixed by appropriate volumes to yield the desired ratio of P3HT and t3 and t8 with the concentration given as the fraction of total mass.The solutions were then spin coated in a nitrogen lled glovebox at 1000 rpm.Quenching studies were performed by measuring the photoluminescence quantum yield with a Hamamatsu U6039-05 integrating sphere.At the excitation wavelength of 500 nm the total absorption was more than 60% and the emission was collected in a range from 650 to 900 nm.PLQY was determined by the instrument and is given by the number of photons emitted divided by the number of photons absorbed.

DFT calculations
Calculations were performed using a Spartan '14 (64-bit) soware suite. 11Molecular geometries were rst optimised semiempirically (AM1) and then re-optimised using DFT (B3LYP/6-31G*).Alkyl chains are substituted with methyl groups in order to diminish calculation time.The resulting structures were local minima, as none of the vibrational frequencies generated imaginary frequencies.
The crystallographic data for compound t3 and 4 have been deposited with the Cambridge Crystallographic Data Centre with deposition number CCDC 1053866-1053867.

Conclusions
In conclusion, we report the synthesis of the symmetrically functionalised oligothiophene derivatives t6, t7 and t8 from building block t3.The increased conjugation that occurs on going from t3 to t8 results in a signicant bathochromic shi in the maximum wavelength absorption and an E fund as low as 1.5 eV.PLQY experiments reveal that these units have the propensity to act as efficient electron acceptors for the electron donor polymer P3HT. 12It is anticipated that these, and related derivatives, will have interesting optoelectronics applications (e.g.photovoltaic properties). 13Our investigations will be reported in due course.

Fig. 1 X
Fig. 1 X-ray crystal structures of derivatives (a) t3 and (b) 4. Colour scheme: C, black; H, grey; Br, pink; N, blue; O, red; S, yellow.Torsion angles with respect to central thiophene ring are provided.

Fig. 7
Fig.7The PLQY of P3HT as a function of increasing concentrations of t3 and t8.