Materials Letters 157 (2015) 252–255

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Materials Letters

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The influence of molecular geometry on photophysical properties andself-assembly of phthalimide end-capped thiophene-based organicmolecules

Ala’a F. Eftaiha a,n, Arthur D. Hendsbee b, Jon-Paul Sun c, Ian G. Hill c

a Department of Chemistry, The Hashemite University, P.O. Box 150459, Zarqa 13115, Jordanb Department of Chemistry, Dalhousie University, 6274 Coburg Road, Halifax, Nova Scotia, Canada B3H 4R2c Department of Physics, Dalhousie University, 1459 Oxford Street, Halifax, Nova Scotia, Canada B3H 4R2

a r t i c l e i n f o

Article history:Received 21 March 2015Received in revised form20 May 2015Accepted 27 May 2015Available online 28 May 2015

Keywords:Small organic moleculeOrganic photovoltaicsPhthalimide end-capped oligothiopheneBulk heterojunctionSelf-assembly

x.doi.org/10.1016/j.matlet.2015.05.1367X/& 2015 Elsevier B.V. All rights reserved.

esponding author. Tel.: +962 5 3903333 (ext.ail address: alaa.eftaiha@hu.edu.jo (A.F. Eftaih

a b s t r a c t

The synthesis of π-conjugated organic small molecules with n-octyl substituted thiophene phthalimideend-capping units have been reported. A combination of absorption and emission spectroscopy, cyclicvoltammetry, ultraviolet photoelectron spectroscopy, density functional theory calculations and atomicforce spectroscopy were used to elucidate the impact of molecular dimensionality on photophysicalproperties and self-assembly.

& 2015 Elsevier B.V. All rights reserved.

1. Introduction

Organic π-conjugated materials offer a platform for a broadrange of optical and electronic applications such as light-emittingdiodes, organic photovoltaics, and organic field-effect transistors[1–6]. This is mainly attributed to their optoelectronic character-istics, low fabrication costs and ease of processing [7–9].

Key geometries of organic semiconductors realized by re-searchers are propeller or pin-wheel architectures (often referredas star-shaped) [10–14] and non-planar, dimeric scaffolds [15–17].These architectures showed superior solar cell performance overplanar structures due to their tendency to suppress molecularaggregation which result in formation of small domains on theorder of exciton diffusion length [14,16]. Zhan and co-workersreported a star-shaped perylene diimide (PDI) electron acceptorwith triphenylamine core for solution processed organic solarcells. The achieved high power conversion efficiency (PCE,�3.3%)in bulk hetero-junction (BHJ) solar cells was explained by thequasi-three-dimensional structure that facilitate miscibility withthe donor [14]. Non-planar PDI dimer showed promising perfor-mance in BHJ solar cell as electron acceptor with �4% PCE [16].Very recently, a three dimensional molecular structure based on

5113); fax: +962 5 3903349.a).

tetraphenylethylene core with four PDI units was exploited as anelectron acceptor to construct a non-fullerene organic solar cellwith a high PCE of 5.53% [18].

Recently, our group has reported the optimized synthesis andopto-electronic characteristics of phthalimide end-capped oli-gothiophene small molecules that bear terminal alkyl chains[19,20]. When combined with poly(3-hexylthiophene) (P3HT) inBHJ solar cell, bithiophene core molecule (PthTh2Pth (1), Fig. 1A)exhibited a high open circuit voltage (VOC�0.94 V) and low PCE(�0.2%). The low efficiency was attributed to the tendency ofPthTh2Pth to undergo crystallization from donor–acceptor blends,which result in formation of micron-size domains compared toexciton diffusion length [21]. So, improvement the molecular de-sign of PthTh2Pth still needed toward respectable PCE values [22].In this study, we have investigated the influence of moleculargeometry of small molecules based on phthalimide thiophenebuilding block on the photophysical properties and molecular self-assembly which are limiting factor in achieving high efficiencyorganic solar cells.

2. Results and discussion

Similar to PthTh2Pth [19,20], (PthTh)3BZ (2) and (PthTh)3TPA(3) (Fig. 1B and C, respectively) were synthesized using a

Fig. 1. Chemical structures and absorption–emission spectra in solution and thin films of A: 5,5′-([2,2′-bithiophene]-5,5′-diyl)bis9(2-octylisoindoline-1,3-dione) (PthTh2Pth),B: 5,5′,5″-(5,5′,5″-(benzene-1,3,5-triyl)tris(thiophene-5,2-diyl))tris(2-octylisoindoline-1,3-dione) ((PthTh)3BZ) and C: 5,5′,5″-(5,5′,5″-(nitrilotris(benzene-4,1-diyl))tris(thio-phene-5,2-diyl))tris(2-octylisoindoline-1,3-dione) ((PthTh)3TPA).

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combination of Stille and direct hetero-arylation C–C bond form-ing reactions. Full synthetic and characterization data of com-pounds 2 and 3 are provided in the electronic supplementaryinformation.

The photophysical properties of compounds 1, 2 and 3 weremeasured by UV–vis and fluorescence spectroscopy. Fig. 1 showsthe normalized absorption and emission spectra of the threecompounds in chloroform solution and their optical absorption inthin films. In solution, PthTh2Pth exhibits absorption and emissionmaxima at 420 and 510 nm, respectively. The absorption max-imum is blue-shifted to 385 nm after transitioning from solutioninto thin-film, due to the tendency of PthTh2Pth to self-assemblein an ordered face to face orientation (H-aggregates) [19,20]. Theabsorption maxima of (PthTh)3BZ and (PthTh)3TPA in thin filmsare red-shifted compared with those obtained in solution (390versus 380 nm and 445 versus 433 nm, respectively). This can beexplained by the improved molecular aggregation in solid state.

In comparison to PthTh2Pth, the absorption maximum of

Table 1Energy of frontier molecular orbitals of PthTh2Pth, (PthTh)3BZ and (PthTh)3TPA obtaine

CV UPS/Optical

HOMO LUMO Eg HOMO LUM

PthTh2Pth, (1) �5.5519 �3.1019 2.4519 �6.020,21 �3.(PthTh)3BZ, (2) �5.60 �3.03 2.57 �6.3 �3.(PthTh)3TPA, (3) �5.11 �3.05 2.06 �5.7 �3.

(PthTh)3BZ in solution is shifted hypsochromically (380 versus420 nm). This is attributed to themeta-substituents at the benzenering which result in reducing the π-conjugation length [23,24]. Onthe other hand, the increased conjugation length along the TPAcore molecule compared to PthTh2Pth and (PthTh)3BZ led to abathochromic shift in its absorption maximum in solution. Inter-estingly, the progressive increase in Stokes shift of compounds 1, 2and 3 (90, 94, and 160 nm, respectively) suggests a less energeticstructural reorganization upon photoexcitation in solution.

The electrochemical properties of PthTh2Pth, (PthTh)3BZ and(PthTh)3TPA were investigated using voltammetry (CV), ultravioletphotoelectron spectroscopy (UPS) and density functional theory(DFT). The highest occupied molecular orbital (HOMO) and lowestunoccupied molecular orbital (LUMO) energy levels of the threecompounds are presented in Table 1. CV measurements indicatedthat the three compounds exhibit similar LUMO levels, whichmeans equal electron injection ability. The higher HOMO level of(PthTh)3TPA compared to the other two compounds is explained

d using CV, UPS and DFT.

DFT

O Eg HOMO LUMO Eg

420,21 2.620,21 �5.7120 �2.6820 3.0320

5 2.8 �6.01 �2.53 3.483 2.4 �5.15 �2.41 2.74

Fig. 2. Depictions of the optimized molecular geometries, HOMO and LUMO orbitals obtained from B3LYP/6-31G(d,p) calculations for PthTh2Pth, (PthTh)3BZ and(PthTh)3TPA.

A.F. Eftaiha et al. / Materials Letters 157 (2015) 252–255254

by the significant contribution of the electron rich triphenyl amine(TPA) moiety to the HOMO orbital (vide infra), which results in arelatively low optical band gap compared to PthTh2Pth and(PthTh)3BZ.

The UPS measurements, which are more relevant toward theusage of organic semiconductors in optoelectronic applications,indicated that compound 2 exhibits a slightly deeper HOMOcompared to compounds 1 and 3. In comparison with the dataobtained by the CV measurements, the energy of the frontiermolecular orbitals estimated from the UPS measurements arefound to be lower in energy and following a similar trend [25].This difference in energy is justified by the different proceduresfollowed to evaluate the orbitals energy, which influence sig-nificantly the obtained results [26,27]. DFT calculations showed asimilar trend to the experimental data. The differences betweenthe two sets of data is mainly attributed to the lower accuracy ofDFT in predicting the HOMO–LUMO energies [28].

Depictions of the HOMO and LUMO frontier molecular orbitalsobtained from DFT calculations (Fig. 2) show fully delocalized or-bitals over the entire framework of PthTh2Pth. The orbital locali-zation throughout the backbone of (PthTh)3BZ and (PthTh)3TPAare highly dependent on the identity of the central core unit.

Fig. 3. AFM height mode images (90 mm�90 mm) of neat and blend films spun from CHCE: D: 1:1 P3HT-(PthTh)3BZ and F: 1:1 P3HT-(PthTh)3TPA.

The LUMO and HOMO orbitals of (PthTh)3BZ are centered at thephenyl core unit, but they are not fully delocalized over the threesubstituted phthalimide terminals. On the other hand,(PthTh)3TPA possess a highly localized LUMO and HOMO orbitalscentered on the substituted phthalimide terminals and the TPAcore, respectively. In this regard, the nitrogen lone pair act in a“donating fashion” and result in a less low lying HOMO.

Atomic force microscopy (AFM) is a useful technique for in-vestigating the morphology of solid-supported films at the nan-ometer scale. Fig. 3 shows AFM images of neat films comprised ofPthTh2Pth, (PthTh)3BZ and (PthTh)3TPA and their blends withP3HT. As shown in Fig. 3A, PthTh2Pth tend to form stick-like shapedomains that had lengths ranging from �3 to 5 mm. Film of(PthTh)3BZ (Fig. 3B) consists of semi-circular domains of �0.5–1 mm in diameter. Images of neat films comprised of (PthTh)3TPA(Fig. 3C) indicate the formation of micron size, flower-shapeddomains. The correlation of the obtained domain shapes with themolecular dimensionality of the investigated compounds is in-tuitive, where the stick-like shape, the semi-circular and theflower-shaped domains represent the one, two and three dimen-sional analogous of compounds 1, 2 and 3 respectively. So, thehuge morphological differences displayed by the neat films of the

l3 solutions of A: PthTh2Pth, B: (PthTh)3BZ, C: (PthTh)3TPA, D: 1:1 P3HT-PthTh2Pth,

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three compounds emphasize how structural differences in mole-cular shape impact the self-assembly of small organic molecules insolution processed thin films. Blending P3HT with PthTh2Pth or(PthTh)3BZ (Fig. 3D and E, respectively) diminish the aggregationtendency of the two molecules. The interfacial organization ofP3HT-(PthTh)3TPA composite (Fig. 3F) indicates the formation ofmicron-size interconnected stands of either component or theirmixture. Further studies still needed to examine the impact offilms processing strategies on the microstructure of donor–ac-ceptor composites with an ultimate goal of obtaining an inter-penetrating phase separated donor–acceptor domains toward highdevice performance.

3. Conclusions

We have reported on the design and synthesis of three π-conjugated small molecules based upon n-octyl substituted thio-phene phthalimide end-capping terminals. The topology of thebithiophene core molecule had an influence on the optical prop-erties through the localization/delocalization of frontier molecularorbitals over the molecular backbone. Atomic force microscopymeasurements indicated the profound effect of molecular di-mensionality on aggregation properties in thin films, which can beexploited to optimize films morphology for optical or electronicapplications.

Acknowledgments

Funding for this work was provided by the Natural Sciencesand Engineering Research Council of Canada (NSERC, Grant #435715), NSERC Discovery Grants (RGPIN-04809), the CanadaFoundation for Innovation (CFI, Grant # 228364) and CFI LeadingEdge Fund (CFI LEF 19994). Dalhousie University is acknowledgedfor providing access to instrumentations. A.D.H is grateful for fi-nancial support from NSERC. A.F.E acknowledges NSERC CREATEDREAMS (Dalhousie Research in Energy, Advanced Materials andSustainability, http://dreams.irm.dal.ca/) and the Hashemite Uni-versity for financial support.

Appendix A. Supplementary material

Supplementary data associated with this article can be found inthe online version at http://dx.doi.org/10.1016/j.matlet.2015.05.136.

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