NAPHTHALIMIDE-BASED MATERIALS FOR ELECTRONIC AND BIOLOGICAL APPLICATIONS

Abstract

This thesis deals with synthesis and characterization of functional materials derived from 1,8- naphthalimide as potential candidates for electronic and biological applications. The thesis is divided into seven chapters. Chapter 1 presents a review on structure-property relationship highlighting photophysical, electrochemical, electroluminescent, photovoltaic, and biological properties of mono and bis-naphthalimide-based linear and star-shaped molecules. Among the Nfunctionalized and core functionalized dyes, later resulted in elongation of conjugation since the chromophore at imidic position is orthogonal to the conjugating length. Depending upon the behavior of chromophore flanked on naphthalimide, it exhibits either typical charge transfer or π- π* extended conjugation. Bis-naphthalimides are explored for its application particularly in OSCs. The variation in the central chromophore as planar or non-planar PAH or heteroaromatic core resulted in tremendous alternation of photophysical and electrochemical properties. Star-shaped dyes provide a platform to study the effect of chromophore density and molecular symmetry on the optical properties of the dyes. However, effect of strength and variable number of electron withdrawing groups on the properties of naphthalimide is not yet explored. The modulation of aromatic core with different number of electron deficient units is limited. Also, the report on studying the effect of non-conjugated chromophores on the intrinsic properties of naphthalimide unit is narrow. In Chapter 2, the aim, scope and prospect of the work on naphthalimide-based materials is presented. Chapter 3 demonstrates the synthesis and characterization of bis-naphthalimides-based small molecules and oligomers that helped us to study the effect of different electron withdrawing acceptors and increasing number of benzotriazole unit on the properties. Linear and rigid bisnaphthalimide- based electron deficient dyes were synthesized by Pd-catalyzed Sonogashira cross coupling reaction. The dyes exhibit a red-shifted absorption band and enhanced molar extinction coefficient attributable to the increase in π-conjugation length and enhanced electronic interactions. The narrow structured emission band and small Stokes shifts is reflection of their rigid and planar structure. Solvent polarity has a negligible influence on the optical properties of the dyes. The molecules are prone for self-assembly leading to the formation of nanostructures for the as synthesized samples and those obtained from different solvents. These dyes lead to the formation of controlled morphologies of variable size and shape developed from J-aggregated solid state packing. The fluorescence images observed under optical fluorescence microscopy v exhibits multicolor emission under different light source. Electrochemical studies reveal that these dyes acquire LUMO levels in range of -3.2 to - 3.3 eV. The analysis of naphthalimide end-capped benzotriazole-based linear n-type small molecular semiconductors show that with increasing number of benzotriazole unit in the oligomer, LUMO is stabilized and HOMO is destabilized, thus leading to narrow band gap. The electronic features and coplanar arrangement of functional entities by density functional theoretical computations ensure close packing arrangement of molecules in the solid state. Furthermore, these dyes show high thermal stability. Chapter 4 deals with synthesis of pyrene-naphthalimide hybrids by Sonogashira coupling reaction and presents the effects of substitution pattern on optical, electrochemical and thermal properties. The attachment of the naphthalimide moiety is varied at the 1-, 3-, 6-, and 8-positions of the pyrene ring to alter the intrinsic properties. The subtle modifications in the architecture of dyes involving the substituent positioning, molecular symmetries and chromophore density as predicted by theoretical computations greatly influence ground and excited state properties of the functionalized pyrene compounds. The optical properties of these dyes change monotonically from mono, di, tri to tetra substituted core. Among the two isomers 1,6-di-substituted and 1,8-disubstituted, former display more red shifted absorption profile attributed to the extended linear arrangement. The molar extinction coefficient increases from mono to tetra-substituted compound attributed to the increment in chromophore density. Whereas, compared to the parent pyrene molecule, the substituted cores induce a large bathochromic shift in the absorption profile owing to the extension of conjugation. The compounds are green to orange fluorescent and display dependence on solvent polarity confessing an intra molecular charge transfer in excited state. DFT simulation reveals the probability of charge transfer from pyrene donor to naphthalimide acceptor and thus the localization of HOMO/LUMO orbitals on the donor-acceptor moieties except for tetra-substituted dye implying the significant charge transfer occurring during electronic excitation. Chapter 5 reports carbazole-naphthalimide-based linear donor-acceptor and star-shaped triazine-cored molecules. A series of ethyne-linked compounds were synthesized by a stepwise route involving a Pd/Cu catalyzed Sonogashira coupling reaction. Star-shaped dyes are synthesized in two step initiated by acid catalyzed trimerization of arylnitriles to form triazine core followed by coupling reaction. We highlight structure-property relationship by structural modification of carbazole and/ or naphthalimide. The star-shaped triazine compounds displayed superior optical, thermal and electrochemical properties when compared to linear analogs. These vi compounds act as blue to green light-emitting materials. The excited state lifetimes of linear analogs are longer than star-shaped analogs. These fluorescent chromophores are supposed to be potential candidates to acquire a space into organic electronic devices such as OSCs and OLEDs. Chapter 6 describes the effect of imidic structural variation of naphthalimide-based bipolar materials on photophysics, electrochemistry, morphology and its application in bioimaging. The photophysical and electrochemical studies reveal that these dyes are electronically similar although structurally different from each other. Whereas the structural variations helps to fine tune the morphology of the dyes at microscopic level. They reflect unique and uniform morphology depending on their molecular structure. They show considerably different morphology which is solvent and concentration dependent. Finally, potential biological applications of these dyes are evaluated by investigating their biocompatibilities and cell uptake behaviors. Chapter 7 presents the conclusion and outlook of the work embodied in the thesis. We present correlation between structural modification and optical properties of the naphthalimide-based dyes. The influence of molecular tuning for determination of the properties and behavior of the naphthalimide functional materials is addressed.