Electrochemical, Photophysical, and Magnetic Properties of Green Emitting bis(2,5-Hexyloxy)-Phenylene-alt-Thiophene Fluorescent Conducting Oligomer Addended Fullerene-Diol Dyad

Towards the development of potential new organic photovoltaic and optoelectronic materials, a simple route to synthesize flexibly ether linked fullerene-bis[oligo-(phenylene-alt-thiophene)] and evaluation of electrochemical, photophysical and magnetic properties is presented. Flexible ether linking of oligo-phenylene-thiophene chain to 1, 2 C60(OH)2 is achieved employing Williamson’s ether synthesis. 7-chain phenylene-thiophene chain fluorescent conducting oligomer is synthesized using Grignard coupling reaction with preservation of bromo end groups. Oligomer is highly ordered and soluble in all organic solvents while on linking to fullerene-diol, solubility of adduct restricts only to dimethyl sulfoxide (DMSO). All the synthesized materials are characterized through spectroscopic techniques and molecular weight is determined by mass spectrometry and GPC. Properties of the material indicate the substantial effect of fullerene. High quenching in fluorescence intensity and strong paramagnetic property are observed in this material.


Introduction
Several fullerene-based donor-acceptor dyads [1][2][3][4] synthesized so far through the attachment of polymeric or oligomeric chain to fullerene have resulted in improved photovoltaic efficiency [5,6]. The search for flexibly attached donor moiety to fullerene core is, however, almost negligible. Ether linkages provide better flexibility and help in film formation. Away from conventional methods resulting in rigid cyclo-additions [7,8], exohedral addition of hydroxyl groups on fullerene provides several sites for flexible attachment for other molecules [9][10][11][12][13][14][15][16][17][18]. Fullerenol behaves as excellent nucleophile because of the electrophilic nature of fullerene which makes hydroxyl hydrogen highly acidic. Interestingly, fullerenol is appeared to be very notorious owing to non-specified attachment of hydroxyl groups onto its surface but sup-posed to be the most applicable molecule due to its high reactivity, stability and solubility. Controlled synthesis of fullerenol with fewer number of hydroxyl groups has opened up the way to use this magical derivative of fullerene and in the present study 1, 2 C 60 (OH) 2 used as the fullerene source [19]. Due to attachment of only two exohedral OH groups, fullerene's symmetry is not perturbed to a great extent and it preserves its high electrophilic character making the efficient nucleophile.

  2 60
C O  One of the important issues to improve performance of photovoltaic material is to design donor moiety having effective donating capability. Extensive research is on to prepare different classes of donor molecules like polymers, macrocycles, etc., but polymers find most extensive application. Among the polymers, highly regioregular poly 3-hexylthiophene (rrP3HT) is being extensively used with the best results [20][21][22]. Substitution on the main chain and control over regio-regularity can tune the band gap of polythiophene from 1.0 -2.5 eV [22]. The lowest band gap allows the creation of polymer light-emitting diodes in the near infra red range and matches well with ultimate requirement for solar spectrum. The main drawback for commercial use of polymeric materials is, however, associated with their poor stability to air/oxygen resulting in larger off-currents & lower on/off ratio and also positive shift in the threshold voltage in case of organic thin film transistor (OTFTs). The stability towards oxidative doping is related to ionization potential, i.e., the highest occupied molecular orbital (HOMO) levels in vacuum and lowering of HOMO-level improves its stability. This can be achieved by a number of methods. Introduction of electron donating groups, such as alkoxy or thio-alkyl groups, at β-position of thiophene unit increases the band gap by lowering HOMO level [23,24]. Insertion of suitably substituted phenylene rings into the thiophene backbone is another efficient structural modification approach for improving photoluminescence (PL) efficiency of thiophene-based conjugated polymers [25]. Substitutions on both phenylene and thiophene rings remarkably affect the PL quantum efficiency of the resulting polymers and also provide better air stability.
Present article describes a simple and convenient method for stepwise synthesis of regioregular bromo end capped 7-chain phenylene-alt-thiophene oligomer (5) and its further attachment to 1,2-fullerene-diol to achieve novel photovoltaic materials (6). One of the strong advantages of this synthesis route is that the use of both cryogenic temperatures and highly reactive metals is not required and consequently this method offers quick and easy preparation of oligomer in large scale. These reactions are carried out either at room temperature or in reflux condition. Covalent attachment of 5 onto fullerene core by nucleophilic substitution of the terminal bromo group with the hydroxyl groups of fullerenol produces fullerenol-adduct 6. Evaluation of thermal, photophysical and magnetic properties of the material indicates substantial effect of fullerene on the thermal behavior as well as high quenching in fluorescence intensity and strong paramagnetic property.

Materials
[60] fullerene was obtained from MER Co. (purity > 99.5%). The sample quality was checked by UV/vis absorption, 13 C NMR, and was used without further purification. Thiophene and hydroquinone were purchased from E Merck and used without further purification. NBS was recrystallized and dried freshly. All solvents were purified and dried before use. All glass wares were flame dried under argon before reaction.

Synthesis of Diiodothiophene (1
In a 200 mL three necked flask, 4.2 g (0.05 M of thiophene is added into 30 mL DCM. 64.5 g of iodine (0.25 M is added in portions with high speed stirring (me-chanical. A solution of 68 mL nitric acid: water (1:1 is transferred to a dropping funnel fitted to the flask and ~10 mL of this nitric acid: water mixture is added slowly with vigorous stirring. Once initiated, the reaction proceeds vigorously with the evolution of brown oxides of nitrogen. After the evolution of gases is subsided, the remaining nitric acid is added drop wise and reaction proceeds smoothly at room temperature. After all nitric acid is added, the solution is heated under refluxing for 30 min. The reaction mixture is allowed to stand and the red organic layer is collected and washed several times with water and dried over anhy. Na 2 SO 4 and concentrated to yellow oil. On cooling the yellow oil freezes and collected as yellow crystals of diiodothiophene. The product is characterized by 1

Synthesis of 1,4 Dihexyloxy Phenylene (2
To the degassed alkaline (12.5 g NaOH  methanolic solution of hydroquinone (11 g, 100 mM in 150 mL methanol, 1-bromo hexane (35.09 mL, 250 mM is added slowly at room temperature. The reaction mixture is then refluxed for 24 h and finally extracted with diethyl ether (4 times, 50 mL each. Combined organic fractions were washed with 10% NaOH solution (2 -3 times; 50 mL each and further with water to remove the impurity of hydroquinone and alkali respectively. Organic layer is dried over anhy. Na 2 SO 4 & concentrated to obtain white crystals. Washed several times with chilled methanol and further re-crystallized with dicholomethane.

Synthesis of 7-Chain Oligomer (5
Grignard of 1 (0.3359 g, 1 mM, prepared as described above, is added to the ice-cooled dry THF solution of 4 (2 g, 2.5 mM and Ni (ddpe Cl 2 (30 mg) under inert atmosphere in stirring condition. Temperature is raised to r. t (25˚C) and stirring continued for 24 h. Similar work-up steps to that of product 4ii are employed and is recrystallized with dichloromethane. FTIR (KBr, cm −

Synthesis of Fullerene-Oligomer Adduct (6)
To the benzene solution of fullerenol (0.020 g, 0.027 mM), K 2 CO 3 (anhy, 0.0398 g, 2.88 mM) and 18-crown-6-ether (catalytic amount) are added at room temperature (25˚C). The benzene solution of oligomer 5 (0.2177 g, 1.44 mM in 10 mL) is added drop wise with stirring at room temperature. The temperature of the reaction mixture is now raised to reflux and after 24 h of refluxing; the solid mass is collected by centrifugation and washed several times with methanol: water (80:20) mixture to remove the impurity of K 2 CO 3
Oligomer (5) is a pale yellow crystalline solid soluble in most of the organic solvents, while the dyad (6) is black powder with solubility only in DMSO.
Product structures are established by spectroscopic characterization. The critical 13 C peaks assignment of the oligomer (5) suggest three non equivalent carbons (C 1 , C 2 , C 3 ) for inner phenylene ring and six for terminal  (3) 2,5-diiodothiophene (1) 1,4-bis(hexyloxy)benzene (2) i, ii 1,2 C 60 (OH) 2 phenyl ring, of which C 1 and C 2 is common for both. Thiophene has two non-equivalent carbons (C 2 , C 3 ). Overall one should expect eight 13 C peaks for (5). On the other hand, 1 H NMR assigns four types of non-equivalent protons; inner phenyl ring proton at C 3 , two different protons for the terminal ring at C 3 and C 6 and single proton for thiophene at C 3 . The FTIR spectrum of (5) also nicely accords with the proposed structure. The inter-ring C-C & C-Br streching peak is appearing in product (5).
The molecular weight of the oligomer estimated by GPC using polystyrene as standard confirms the 7-ringed oligomeric phenylene-thiophene structure with poly-dispersity ~1.1. The molecular ion peak in ESI-MS (methanol, M + -1 at m/z 1521) also adheres with the GPC result. Attachment of oligomer to fullerene core is evident from the disappearance of all the typical peaks of fullerenol and appearance of aromatic & alkyl stretching and bending peaks in FTIR spectrum of (6). The spectrum also shows peak for C-Br ( at 1104 cm −1 suggesting single terminal bromo group involvement in the reaction. Fol-O-C ether linkage is ascertained from the ap- pearance of broad (-band at 1020 cm −1 . All eight non-equivalent carbon peaks, similar to that of (5), still exist in 13 C NMR of the dyad (6). The broad peaks in 1 H NMR are shifted down-field due to the attachment of electronegative fullerene into oligomer. Sp 3 hybridized fullerene (at the point of attachment) as well as hexyloxy carbon peaks are present. High symmetry of the dyad is as well be determined by NMR spectroscopy. Broad triplet for methyl group of hexyloxy chain, followed by methylene proton peaks between 2.2 -2.6 ppm are observed in 1 H NMR of (6). Appearance of single triplet for methylenoxy protons envisaged the equivalent nature of the addend oligomeric chain. Aromatic proton peaks appear between 7.1 -7.3 ppm. Due to paramagnetic nature of the dyad material, NMR signals are not very sharp.

Molecular Modeling
Effect of exohedral addition of OH-groups in fullerenol and oligomers in dyad is compared with that of fullerene by performing molecular modeling at semiemperical AM1 level using DS visualizer (Figure 1) and with SPARTAN for the lowest energy molecule (1597.3744 Kcal/mol) (Figure 2). C 1 -C 2 bond length of fullerene moiety in derivatives has increased due to addition. Molecular modeling calculation measures C 1 -C 2 bond length of 1.55 -1.609 Å in diol to compared to 1.38 Å in pristine fullerene suggesting conversion of sp 2 carbon into sp 3 . A further nominal increment by ca. 0.03 Å. of C 1 -C 2 bond length has been observed on addition of oligomeric chain. Slight change in bond angle is also observed on oligomer addition.

Thermal Properties
Oligomer 5 is a crystalline solid material possessing good OH HO    (Figure 3). The oligomer (heating rate of 10˚C/min in nitrogen atmosphere) shows thermal stability up to 150˚C followed by a sharp degradation. First derivative TGA trace indicate single degradation step (crest temp 318.4˚C) owing to high symmetric structure. It is attributed to the formation of more rigid planar interdigited π-stack 3D structure and conformational symmetry due to thermal ordering of hexyloxy groups attached in regular fashion at a particular distance. Flexible ether linkages of hexyloxy groups might have assisted in attending such geometry.
Symmetric structure of 5 is perturbed on chemical attachment to fullerene and is clearly evident from the TGA and first derivative TGA thermogram of 6. Adduct 6 still maintain initial thermal stability up to 150˚C and the small weight loss (6.7%) within this temperature range is due to removal of low boiling solvents physically absorbed on the surface. Stepwise removal of addended oligomers is also recorded in first derivative before the structural degradation of fullerene. First step is associated with long temperature zone 150˚C -410˚C (crest temp 330.78˚C) followed by two sharp steps in the temperature range 425˚C -520˚C (crest temperature 480.16˚C) and 520˚C -560˚C (crest temperature 550˚C) respectively (Figure 4). Weight loss associated with each step corresponds to sequential release of terminal bromo  ontaining phenylene and attached thiophene ring fol-

Electrochemical Properties
with a computer  DSC of adduct 6 recorded at a heating rate of 10˚C /min under N C lowed by middle phenylene and thiophene rings respectively. The required amount of absorbed energy associated during sequential release of addended aromatic units is calculated to be 224.5 J/g (DSC of 6 in N 2 atm., heating rate 10˚C/min).
Cyclic voltammograms are recorded controlled Autolab model 302 Potentiostat at a constant scan rate of 25 mV/s using 0.1 M tetrabutyl ammonium perchlorate (n-Bu 4 NH 4 ClO 4  in acetonitrile as supporting electrolyte. A three-electrode configuration undivided cell is used: platinum disc working electrode, platinum wire counter electrode and Ag/AgCl (3 M KCl and saturated Ag/Cl separated with a diaphragm as reference electrode. Typical cyclic voltammogram of 5 (scans window between -1.2 to 2.0 V) is displayed in Figure 5 and the electrochemical data are presented in Table 1 [27] Electron affinity (LUMO level and ionization potential (HOMO level is calculated as 3.74 eV and 5.87 eV and the difference in energy between I p and E a yields the band gap of the material. For 5, the value is 2.13 eV. The measured low band gap value in 5 compared to pristine oligo-thiophene ggest a more regular structure in solution and incorporation of alkoxy substituted phenylene ring alternate to each thiophene can be a better design for obtaining these -conjugated systems. Reports suggest that for thiophene-phenylene oligomers, the standard formal oxidation potential (E ox  increases with the introduction of p-phenylene rings into the oligomer [28]. This increase in oxidation potential is ca 0.15 V for one p-phenylene ring and subsequent introduction of p-phenylene rings shift its oxidation potential by ca. 0.10 V. It means that the thio-  (6) could not be measured due to solubility problem of the material. Operating window of DMSO falls in the range -1 to +1 V and the adduct (6) does not show any characteristic redox peak in this range.

Photo
Electrical and optical properties of copolymers mainly depend on mean length and -electron delocalization [29,30]. The nature of the substituted group generally affects the electronic and optical properties of the conjugated polymers in two ways [31]; the electronic feature of the substituted group and steric hindrance arising from the substituted group. The side chains do not take part directly in the -bonds delocalization, but their steric hindrance could induce a considerable inter-ring twisting, giving rise to a substantial reduction of conjugation length. It has been demonstrated both experimentally and theoretically that alkyl substitution in polythiophene gives rise to stronger steric hindrance and increases the torsional angles between aromatic rings (effectively reduces conjugation length) resulting spectral blue shift in absorption [32]. On the other hand, introduction of alkoxy chains at 2-and 5positions of the phenylene rings induces spectral red shift of the same polymers [33][34][35]. Strong electron-donating property and less steric hindrance of alkoxy groups compared to alkyl substituents are considered responsible for the spectral red shift and by far, hexyloxy substituents at 2,5-position of phenylene rings have the highest effect on spectral red shift [33]. Theoretical works [36] on the copolymers suggest that the resultant band gap is the weight average band gap value of the individual unit in polymer composed of alternative low (i.e., thiophene mental observation records 20 -40 nm blue shifts of the absorption maxima (higher energy) in dilute chloroform solution on inserting a single phenylene unit in 5-ring polymer of thiophene analogue [28].

Absorption Properties
Absorption spectrum of (4 in chlorof preciably red-shifted (lower e individual building blocks (1) and (3) because of easy - * transition on increasing the conjugated chain length. A blue-shift (higher energy) in absorption maxima by about 50 nm compared to 3-ring thiophene homologue (  on increasing the chain length from 3 to 7 (Figure 6). This result indicates that the absorption maxima remains uneffected on inserting 1,4-he ylene group between two thiophene units, although optical gap is slightly decreased by ca. 0.17 eV. The optical gap is however significantly higher compared to 7-ring thiophene homo-polymer (~2.8 eV and the difference is about 0.8 eV and the optical band gap is more comparable to 7-ring oligo p-phenylene homo-polymer. The inductive effect due to electron-donating character of an alkoxy group present in 2,5-positions of phenylene ring would expect to contribute red shifting of abs xyloxy substituted phen orption an ol d emission maxima. Indeed, this effect is substantial in fluorescence but in absorption it contributes for blueshifting. It is attributed that although these alkoxy groups are positioned head-to-tail in adjacent rings but still yields a slightly greater preference for non-coplanar geometry in ground state. Thus, no shifting of absorption maxima on increasing the conjugation chain length can be rationalized by combining the total effect of alkoxy substituted phenylene with thiophene and chain extension. The absorption spectrum in methanol also shows similar spectral features. This indicates that polarity of the solvent does not influence the absorption property. Insolubility of the adduct (6) in common solvent (soluble only in DMSO) prohibits comparison of absorption property. UV-vis spectrum of (6) recorded in DMSO shows high absorption in UV-region and extended tailing in the entire visible region, typically matched with the characteristic of the fullerene derivatives (Figure 6).

Emission Properties
Comparative emission spectra of oligomer 5, fulleren   Figure 7 and photophysical data are complied in Table 2. Fluorescence spectra of 5 show strong single emission peak in green region (512 nm/2.43 eV). A small red-shifting (10 nm) of emission peak in oligomer 5 compared to 4 is observed. It is attributed that this small red-shifting is net balance of two opposing factors operating simultaneously; the presence of phenyl ring in the backbone and contribution of thiophene ring and chain extension. However, both oligomer 5 and monomer 4 show a large Stokes' shift between absorption and emission peaks. The Stokes' shift is more than 200 nm and can be ascribed to the formation of more rigid planar inter-digited π-stack 3D structure and conformational symmetry occurring after photo-excitation [37][38][39] allowing a possible exciton migration to long conjugation segments. The large Stokes' shift of these materials thus promises a possible use as active medium in laser diodes.
Fullerene-diol and adduct 6 shows similar spectral patrn and the emission peaks appear at 416 and 436 nm in symmetry of fullerene in both fullerenol and adduct (6) result similar spectral patterns. Fullerene core governs the emission peaks positions and are independent of the nature of addends. Fluorescence intensity of the adduct (6) is however highly quenched compared to both fullerenol and oligomer (5) and is also blue shifted compared to oligomer 5. This indicates an efficient intra-and inter-molecular electron transfer from oligo-phenylenealt-thiophene (donor) to the fullerene (acceptor) producing a stable charge-transfer exciplex in photo-excited (Figure 8). Electron acceptor property of fullerene is well known and the functionalization with oligophenylenealt-thiophene molecules undergoes fast charge-separation (CS) and slow charge-recombination. [40] The stronger quenching in fluorescence intensity in C 60 -oligothiophene-C 60 triads compared to oligo-thiophene-C 60 dyads is observed earlier [41][42][43][44][45].
In addition, adduct (6) also show high blue-shift in photoluminescence spectra ( gation defect and conformational disorder arises due to steric repulsion and distortion of torsional angle between phenylene-thiophene rings on chemical attachment of C 60 results blue-shifting [33].

Magnetic Properti
Oligomer 5 in its pristine un-d indicating an absence of free tronically defect free symmetric structure in solid state. The oligomer behaves like a p-donor in presence of strong acceptor and creates an electronic imbalance within the adduct on chemical attachment to fullerene (acceptor) making it EPR active (Figure 9) [46]. Higher g-value (g = 2.005) compared to both free electron (g = 2.0023) and fullerenol (g = 1.99) owing to strong electronic interaction between oligomer donor chain and fullerene acceptor producing resultant dipole moment within the dyad molecule. Deviation of g-factor from the free-spin value is a useful indication of the extent of spin-orbit coupling in the paramagnetic species and provides information about the molecular environment of the unpaired electrons. Absorption at higher magnetic field (3515 G) compared to free electron (3300 G) indicate electron-electron interaction within the molecule in ground state needing higher energy for electronic transition. Peak to peak width is inversely proportional to the spin-spin relaxation time (

Conclusio
tes and the mo n d bis-(2,5-hexyloxy) substitutes ene oligomer and its attachment to Bromo-end cappe phenylene-alt-thioph fullerene-diol are explored for their thermal, electrochemical, photophysical and magnetic properties. Synthesis of oligomer and formation of its adduct with fullerenol are successfully carried out. Thermal disorder of oligomer on chemical attachment to fullerenol is evi-dent from the multi-step degradation of the adduct (6) compared to single step degradation in oligomer (5). Also, the chemical attachment induced paramagnetic character to otherwise EPR inactive material. Low bandgap of the oligomer in solution indicates symmetric planar structure and good electron donating contribution of alkoxy substituents towards easy migration of -electrons. Attachment to fullerene moiety results partial distortion of co-planarity and is easily reflected from the   blue shifting of emission peak. There is considerable charge-transfer interaction between p-type oligomeric chain and n-type fullerene core resulting in the high quenching of fluorescence intensity. Optical properties of the polymer can be tuned by suitable manipulation of the back-bone chain and also by changing the substituents on both phenylene and thiophene rings. Long chain alkoxy pendant groups substantially improve the solubility yet ontribute to retai These types of fullerene-addended materials may find potential in solar cells and photo-sensing devices.

Intermolecular CS
The article highlights the dyad characteristics of phenyl-altthiophene addended fullerene diol through inter and intra molecular CS leading to quenching and red shifting of emission spectra . Significant effect of fullerene on magnetic and electro chemical properties are also observed.