Cyclopentadithiophene-Based Organic Semiconductors: Effect of Fluorinated Substituents on Electrochemical and Charge Transport Properties

Abstract

b S Supporting Information S olution processable organic semiconductors have generated tremendous interest because of their applicability in organic microelectronics, light-emitting diodes, and photovoltaics. 1-6 Both p-type and n-type materials, with commensurate mobilities, are required for effective functioning of many of these devices. Significant progress has been made in the development of materials possessing high charge mobilities with materials based on heteroaromatic molecules, especially thiophenes. Whereas thiophene-based materials are often p-type materials, 7,8 it has been shown that the incorporation of fluorinated substituents as terminal functionalities causes these materials to exhibit n-type characteristics. 9-15 This behavior has often been attributed to the lowering of LUMO energy level, making electron injection easier and hence imparting n-type character to the molecule. 11,13 However, it is not clear whether the factors underlying stabilization of frontier orbital energy levels have correlations to the transport of the injected charges. We were interested in testing this issue with a molecular scaffold that satisfies the following design criteria: (i) changing the hydrocarbon substituent to a fluorocarbon substituent should have minimal effect on the frontier orbital energy level and (ii) the scaffold allows for tuning the frontier orbital energy level independent of the fluorocarbon substituent. For this purpose, we report here the design, syntheses, and charge transport characteristics of solution-processable systems based on cyclopenta[2,1-b:3,4-b 0 ]dithiophene.The cyclopenta[2,1-b:3,4-b 0 ]dithiophene, referred here as simply cyclopentadithiophene (CPD), unit can be said to be a combination of structural motifs found in fluorenes and oligothiophenes. The rigid fused ring structure in this molecule lowers the reorganization energy, a parameter that strongly affects the rate of intermolecular charge hopping and therefore the charge carrier mobility in organic semiconductors. 16 The CPD unit has two locations where substituents can be incorporated: (a) the R-positions in the thiophenes, where hydrocarbon or fluorocarbon substituents can be incorporated, and (b) the bridgehead position, where substituents can be incorporated to alter significantly the frontier orbital energy levels. In fact, it has been shown previously that variations at these positions in such fused thiophene structures can be used to tune the frontier orbital energy levels. [17][18][19] The molecules that satisfy our design criteria and potentially address the issue in hand are shown in Chart 1. The CPD core is functionalized at the bridgehead position with the electronwithdrawing carbonyl or dicyanomethylene functionality, whereas the R-positions are substituted with either phenyl or pentafluorophenyl moieties. Installation of electron-withdrawing functionalities, such as carbonyl group, at the bridgehead position of the CPD leads to lowering of bandgap, which has been attributed to the stabilization of the quinoid fo...

Structure‐Property Optimizations in Donor Polymers via Electronics, Substituents, and Side Chains Toward High Efficiency Solar Cells

“…However, even though polymers and small molecules synthesized with CPD-based systems possess very low electrochemical band gaps, the optical absorption in the low energy portion of the spectrum is typically very poor. [ 62 ] Similar poor absorption coeffi cients in the infrared portion of the absorption spectrum are seen in the case of benzo[1,2-c ;4,5-c ′] bis[1,2,5]thiadiazole-based copolymer systems as well, due to the same issues. [ 63,64 ] Therefore, when designing new acceptors for ICT polymers, emphasis .…”

Organic and Organometallic Fluorinated Materials for Electronics and Optoelectronics: A Survey on Recent Research

“…Cyclopentadithiophene (CPD) derivatives with the corresponding fluorinated derivatives have also been reported. Beside the introduction of fluorine atoms, electron withdrawing groups (carbonyl, dicyanomethylene) are also present on the bridgehead position of the CPD.…”

Core and double bond functionalisation of cyclopentadithiophene-phosphaalkenes

“…ketone (II) and dicyanoalkenes (III) have led to interesting opto-electronic materials and building blocks for donor-acceptor polymers, in which the exocyclic fulvenoid repeating unit acts as an outstanding acceptor unit. 4,5 Recently, examples of imine functionalised CPDT units have been incorporated into polymers allowing controlled modifications of the HOMO and LUMO levels of the molecular components. 6 The diagonal relationship of phosphorus and carbon has led to a variety of conjugated compounds in which carbene fragments of double bonds are replaced by isolobal phosphinidenes giving rise to phosphaalkenes.…”