Influence of Solubilizing Group Removal Rate on the Morphology and Crystallinity of a Diketopyrrolopyrrole-Based Compound Shabi Thankaraj Salammal,* ,† Jean-Yves Balandier, † Saroj Kumar, ‡ Erik Goormaghtigh, ‡ and Yves Henri Geerts † † Laboratory of Polymer Chemistry and ‡ Laboratory for Structure and Function of Biological Membrane, Faculte ́ des Sciences, Universite ́ Libre de Bruxelles (ULB), Boulevard du Triomphe, 1050 Brussels, Belgium * S Supporting Information ABSTRACT: Thermally cleavable solubilizing groups have been introduced on a π- conjugated core, which can be cleaved and volatilized via heat treatment directly after the thin film fabrication. The X-ray and atomic force microscopic investigations of “3,6-di(2,2′-bithiophen-5-yl)pyrrolo[3,4-c]pyrrole-1,4-(2H,5H)-dione” (DPP-4T) synthesized from its precursor (di-tert-butyl-3,6-di(2,2′-bithiophen-5-yl)-1,4-dioxo- pyrrolo[3,4-c]pyrrole-2,5-(1H,4H)-dicarboxylate) through thermal conversion re- veals that the solubilizing group removal rate plays a crucial role on the ultimate grain size and crystallinity of the final compound (DPP-4T). This means that, when the heating rate is decreased from 50 to 0.1 °C/min, the crystallinity of DPP-4T powder decreases from 49% to 34%, and the grain size of DPP-4T film reduces from 976 to 344 nm. The crystallite size of the films can be further reduced to 185 nm by decarboxylating the films isothermally at 120 °C. The Fourier transform infrared spectra reveals that the reduction of crystallinity with the heating rate could be attributed to the trapping of gaseous byproducts inside the lattice, during the decarboxylation of the solubilizing group. The increment in intermolecular N-H···OC hydrogen bond length together with the trapped gaseous byproducts results in a blue shift in the UV-vis absorption spectra while decreasing the heating rate as well as the isothermal decarboxylation temperature. ■ INTRODUCTION The excellent electrical and optical properties of dioxo-3,6- diarylpyrrolo[3,4-c]pyrroles, commonly abbreviated as DPP, along with their environmentally benign features have found their profound application in optoelectronic sectors. 1-3 The DPP-based polymers and small molecules have been generally used to fabricate organic solar cells 1,2,4 due to their ease at tuning the band gap by flanking the electron-deficient pyrrolopyrrole with the electron-rich aromatic moieties, such as thiophene or thienothiophene, phenyl, etc. 5,6 Such DPP- based compounds are not soluble in common organic solvents because of strong intermolecular N-H···OC hydrogen bonds and π-π interactions, which complicate the fabrication of organic electronic devices further. 1,7,8 In most cases, the solubility of such conjugated molecules is obtained through grafting the flexible side chains. Though the significance of alkyl side chains enables us to engineer the crystal structure and resulting thin film morphology, their insulating properties tend to hamper the realization of efficient electronic devices. 9-11 Moreover, these solubilizing groups also promote the soft characteristics of functional organic materials, inducing conformational defects and polymorphism. 10 Therefore, the devices are in need of different morphologies to meet their respective applications. For example, edge- and face-on oriented crystallites are mandatory for the fabrication of efficient organic field effect transistors (OFETs) and solar cells, respectively. 9,12 It has been documented that the aforementioned problems can be solved by grafting the solubilizing groups such as t-butoxycarbonyl (t-BOC) and ester or tetrahydropyran, which can be cleaved and volatilized via thermal or acid treatment directly after the film preparation. 7,13-17 Moreover, the insolubility of thermally or acid-treated films assists to fabricate heterostructures without dissolving the prepatterned layers through solution processing techniques. 15,18 Various solubilizing group free organic small molecules and polymers, such as dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP-phenyl), dinaphtho[2,3-b:2′,3′-f ]thieno[3,2-b]thiophene (DNTT), polythiophenes, poly(p-phenylene vinylene), penta- cene, hexane, and tetrabenzoporphyrin (BP), have been synthesized from their precursors via heat treatment. 7,13,14,19-21 For example, insoluble pentacene could be synthesized from its precursor (13,6-N-sulfinylacetamidopentacene) through a thermochemical process, which releases N-sulfinylamide during the decomposition of its solubilizing group. 22 Similarly, Soeda et al. have successfully grown single crystalline pentacene and DNTT platelets during the removal of the solubilizing group from their precursor solution via heat treatment. 13 On the Received: October 30, 2013 Revised: November 25, 2013 Published: December 2, 2013 Article pubs.acs.org/crystal © 2013 American Chemical Society 339 dx.doi.org/10.1021/cg401625p | Cryst. Growth Des. 2014, 14, 339-349