Design and synthesis of donor-acceptor truxenes

Abstract

Truxene (10,15-dihydro-5H-diindeno[1,2-a;10,20-c]fluorene) is a planar polyaromatic hydrocarbon, which represents a fusion of three fluorene moieties in such a way, that it leads to a C3 symmetric structure. Truxene scaffold appears to be a potential building block for the construction of larger molecular architectures, owing to its easy functionalization, rigid structure, high thermal and chemical stability. Literature reveals that the substitution at para position (2, 7 and 12 positions) of truxene core results in a variety of π-delocalized molecular systems. Truxene based donor-acceptor (D–A) systems are potential candidates for applications in two-photon absorption, organic light-emitting diodes (OLED), and organic fluorescent probes.The electronic and photonic properties of the π‐conjugated D–A molecular systems can be tuned by altering the HOMO‐LUMO gap. The HOMO‐LUMO gap in D–π–A molecular systems can be tuned either by altering the strength of D/A units or by varying the π-bridge. A variety of donors (triphenylamine, phenothiazene, ferrocene) and acceptors, tetracyanoethylene (TCNE) and 7,7,8,8-tetracyanoquinodimethane (TCNQ), naphthalimide have been explored in the design and synthesis of truxene based donor–acceptor systems.The substitution of the donors and acceptors at 2, 7 and 12 position perturbs the photonic properties of truxene derivatives significantly. The effect of substitution SYNOPSIS of various D/A systems on the photophysical and electrochemical properties were studied. The main objectives of the present study are:  To study the effect of substitution of different donor and acceptor units on truxene core and exploring the donor-acceptor interaction by tuning the HOMO-LUMO gap.  To study the influence of various D/A units on photophysical, thermal and electrochemical properties.  To improve the photonic properties of truxene derivatives by incorporating TCNE and TCNQ acceptors.  To design and synthesize new donor-acceptor molecules for optoelectronic applications.Chapter 1: Introduction This chapter describes the synthesis and functionalization approaches of truxene derivatives, and their applications in different fields. Chapter 2: Materials and Experimental Techniques Chapter 2 summarizes the general experimental methods, characterization techniques and details of instruments used for characterization. Chapter 3: Star Shaped Ferrocenyl Truxenes: Synthesis, Structure and PropertiesChapter 3 describes a series of donor–acceptor ferrocenyl substituted truxenes, synthesized by the Pd-catalyzed Sonogashira cross-coupling and Cycloaddition reactions. The electronic absorption and electrochemical studies of these truxenes show effective electronic interaction, which can be tuned by the introduction of different spacers.Chapter 4: Strategy Towards Tuning the Emission Behaviour of Star Shaped Tetraphenylethene Substituted Truxenes Chapter 4 summarizes the synthesis of star shaped, C3-symmetric, tetraphenylethene (TPE) and 2,3,3-triphenyl acrylonitrile (TPAN) substituted truxenes by the Pd-catalyzed Suzuki and Sonogashira cross-coupling reactions. The TPE substituted truxenes show aggregation-induced emission (AIE) behavior, whereas TPAN substituted truxene shows aggregation-caused quenching (ACQ) effect in THF/water medium due to the - stacking. The computational calculation on truxenes was performed, which reveals that, electron density transfers from truxene to TPAN core.Chapter 5: C3-Symmetric Star Shaped Donor-Acceptor Truxenes: Synthesis, Photophysical, Electrochemical and Computational Studies Chapter 5 reports the design and synthesis of donor and acceptor substituted truxenes using Pd-catalyzed Sonogashira cross-coupling and [2+2] Cycloaddition-retroelectrocyclization reactions. Their photophysical, electrochemical and computational studies were explored, which exhibits strong donor-acceptor interaction and effective tuning of the HOMO–LUMO gap. The computational studies reveal that the TCNE and TCNQ substituted truxenes exhibit lower HOMO–LUMO gap. The reaction pathway of [2+2] Cycloaddition-retroelectrocyclization was studied by computational calculations, which reveals that, the donor substituted truxene is favourable, whereas acceptor substituted truxene is not favourable for Cycloaddition-retroelectrocyclization reactions.Chapter 6: Phenothiazene Based 1,1,4,4–Tetracyanobuta–1,3–Diene (TCBD) Substituted Donor-Acceptor Truxenes: Synthesis, Photophysical and Electrochemical Properties Chapter 6 describes the synthesis of phenothiazene substituted truxenes, 1,1,4,4–tetracyanobuta–1,3–diene (TCBD) and cyclohexa–2,5–diene–1,4–ylidene–expanded TCBD functionalized donor-acceptor truxenes by using the Ullmann coupling, Pd-catalyzed Sonogashira cross-coupling and [2+2] Cycloadditionretroelectrocyclization reactions. Their photophysical, electrochemical and thermal properties were studied. The effect of substitution through different positions of phenothiazine unit on truxene core was explored. The substitution through N-position (10-position) enhances the thermal stability of truxene compared to 3-position of phenothiazine. The incorporation of TCNE and TCNQ leads to red shifted absorption resulting in low HOMO–LUMO gap which was supported by DFT calculations.Chapter 7: β-Substituted Truxene Porphyrins: Synthesis and Photophysical Properties The Chapter 7 describes design and synthesis of the -conjugated, β-Substituted truxene porphyrin and its metalated derivative by using the Pd-catalyzed Sonogashira cross-coupling and metalation reaction. Their photophysical properties were explored, which reveals that the substitution of porphyrin results in bathochromic shift of absorbance and fluorescence maxima. The resultsindicate that there is considerable electronic communication between truxene core and porphyrin ring.Chapter 8: Conclusions and Future Scope. Chapter 8 summarizes the salient features of the work and its future prospects to develop the new materials for optoelectronic applications.