Design and synthesis of near-infrared absorbing diketopyrrolopyrroles

Abstract

Diketopyrrolopyrrole (DPP) is a 8π electron bicyclic system containing two lactam units. DPP is an electron acceptor building block widely used in the field of organic photovoltaics. DPP derivatives often exhibit broad optical absorption, high thermal stability and have been used for various applications including aggregation induced emission, solar cells, photodynamic therapy, bioimaging, laser dyes, conductive polymers, photoconductive materials and fluorescent sensors. The substitution of the donors and acceptors at 3- and 6-position of the DPP perturbs the photonic properties of DPP based molecular systems significantly. The donoracceptor (D–A) architectures with strong absorption in visible to near-infrared (NIR) region and low HOMO–LUMO gap are used as potential candidates for organic photovoltaics. In order to improve the absorption towards NIR region and lower the HOMO–LUMO gap, DPP unit has been functionalized with various donors, acceptors and linkers in symmetrical and unsymmetrical way. The effect of substitution of various donor and acceptor units on the photonic, thermal and electrochemical properties was investigated. The photophysical, electrochemical properties and HOMO–LUMO gap of DPP based D–A systems can be tuned either by altering the strength of donor or acceptor units.We have explored variety of donor (carbazole, triphenylamine, ferrocene etc.) and acceptor (TCBD) functionalized DPP based D–A systems and investigated their photophysical, thermal, electrochemical and computational properties which indicate their applicability for photovoltaic applications. The DPPs in this work have been classified as symmetrical (DPPs 2–4) and unsymmetrical (DPPs 5–7) on the basis of functionalization at the 3‐ and 6‐ positions of DPP. The substitution of same donor or acceptor units on both the positions on the DPP and by making dimers willresult in symmetrical DPP, whereas the substitution at one side or substitution of different donor/acceptor units on 3‐ and 6‐ positions resulted in unsymmetrical DPPs (Figure 1). The main objectives of present study are:  To synthesize donor functionalized symmetrical and unsymmetrical DPPs of the type D––A––D and D––A.  To design and synthesize donor/acceptor functionalized symmetrical and unsymmetrical DPPs and to compare their photophysical, thermal and electrochemical properties.  To study the effect of systematic variation of various donors on the photophysical, thermal and electrochemical properties of DPP.  To develop a strategy for tuning the HOMO–LUMO gap in donor functionalized DPPs.  To study the effect of metal functionalization on photophysical, thermal and electrochemical properties of DPP.Chapter 1: Introduction This chapter describes the synthesis and functionalization approaches of DPP derivatives, and their applications in various fields. Chapter 2: Materials and experimental techniques This chapter summarizes the general experimental methods, characterization techniques and details of instruments used for characterization.Chapter 3: N-phenyl carbazole functionalized diketopyrrolopyrroles In chapter 3, we have designed and synthesized ethyne bridged N-phenyl carbazole substituted DPPs (DPPs 8 and 9) and their TCBD derivatives (10 and 11) by the Pd-catalyzed Sonogashira cross-coupling and [2+2] cycloaddition-retroelectrocyclization reactions respectively. The effect of incorporation of tetracyanobutadiene (TCBD) unit on photophysical, thermal and electrochemical properties of N-phenyl carbazole functionalized DPPs was investigated, which show substantial donor–acceptor interaction between carbazole and DPP. The N-phenyl carbazole substituted DPPs exhibit excellent thermal stability. The TCBD derivatives of DPP 10 and 11 show red shifted broad charge-transfer (CT) bands in the visible region with low HOMO – LUMO gap compared to ethyne bridged N-phenyl carbazole substituted DPPs 8 and 9 (Figure 3). The electrochemical study reveals, additional reduction waves in 10 and 11 at low voltage corresponding to the TCBD moiety.Chapter 4: Triphenylamine functionalized diketopyrrolopyrroles In Chapter 4, a set of triphenylamine (TPA) based unsymmetrical and symmetrical diketopyrrolopyrroles (DPPs) 12 and 13 were designed and synthesized by the Pd-catalyzed Sonogashira cross-coupling reaction. The [2+2] cycloaddition-retroelectrocyclization reaction of DPPs 12 and 13 with tetracyanoethylene (TCNE) resulted in tetracyanobutadiene (TCBD) derivatives DPPs 14 – 16 respectively. The effect of TPA as donor and TCBD as acceptor on the photophysical properties of DPP and their donor-acceptor interaction was evaluated. The photophysical, thermal and electrochemical properties of TPA based DPPs 12 and 13 were compared with their TCBD derivatives 14 – 16. The TCBD derivatives DPPs 14 – 16 show red shifted absorption (Figure 4), decrease in thermal stability and increase in strength of D – A interaction compared to DPPs 12 and 13.Chapter 5: Ferrocenyl functionalized diketopyrrolopyrroles Chapter 5 describes the synthesis of unsymmetrical and symmetrical ferrocenyl diketopyrrolopyrroles 17 and 18 by Sonogashira cross-coupling and their TCBD derivatives DPPs 19–21 by [2+2] cycloaddition-retroelectrocyclization reaction respectively. The incorporation of TCBD in ferrocenyl DPPs facilitate the reduction and show low HOMO – LUMO gap. The TCBD substituted DPPs show systematic red shift in absorption (Figure 5) with enhanced thermal stability. The photophysical, electrochemical, and computational studies show substantial donor–acceptor interaction. High thermal stability and low HOMO – LUMO gap ofChapter 6: Metal functionalized diketopyrrolopyrroles In chapter 6, a symmetrical cobalt-dithiolene functionalized diketopyrrolopyrrole (DPP) Co-DPP-Co was designed and synthesized. Its photophysical, thermal, electrochemical and computational studies were compared with symmetrical ferrocenyl Fc-DPP-Fc. The electronic absorption spectrum of Co-DPP-Co exhibited absorption in Vis-NIR region with shoulder band in longer wavelength region (Figure 6). The thermogravimetric analysis shows good thermal stability of Co-DPP-Co. The density functional theory (DFT) investigation exhibited the cis geometry of cobalt-dithiolene ring with respect to DPP in Co-DPP-Co whereas trans geometry was observed for ferrocenyl unit in Fc-DPP-Fc.Chapter 7: Diketopyrrolopyrrole based monomer, dimers and trimer In chapter 7, we have designed and synthesized diketopyrrolopyrrole (DPP) based monomer (22), dimers (23 and 24) and trimer (25) in order to see the effect of number of DPP units on the photophysical and electrochemical properties (Figure 7). The absorption spectra show that the red shift in the absorption after dimerization and trimerization of DPP, with broadening of absorption bands in dimer analogues (23 and 24). The TGA analysis shows the dimerization and trimerization of DPP improves the thermal stability. The electrochemical study exhibited additional oxidation peak in trimer 25 related to the oxidation of triphenylamine unit. The effect of connecting bridge between DPP units in dimers and trimer of DPP was investigated which reveals the ethene bridged dimer exhibit good electronic communication between two DPP units.Chapter 8: Conclusions and future scope This chapter summarizes the salient features of the work and future prospective to develop new materials for optoelectronic applications.