Luminescent boron containing materials find numerous applications in modern technologies such as display/lighting, bio-imaging and sensing. Thus, investigations of structure-property relationships in organic luminescent compounds to understand their molecular and bulk properties are of fundamental importance. The main thrust of this thesis is the development of facile synthetic routes for boron containing novel polyads and study their structure-property correlations and to utilize this information to design functional materials with desired properties such as multiple emission, bio imaging, anion sensing and organic photo voltaic characteristics. This thesis contains seven chapters and the contents of each chapter are described below. Chapter 1 This chapter is a concise overview of the recent developments in the chemistry of boron based molecular systems such as triarylborane and BODIPYs. This chapter also highlights the basic nature of broad emissive materials. In addition, an advance in the frontier areas such as bio imaging is discussed in brief. Chapter 2 This chapter describes the structure and optical properties of a new triad (Borane-Bithiophene-BODIPY) 1. Triad 1 exhibits unprecedented tricolour emission when excited at borane centred high energy absorption band and also acts as a selective fluorescent and colorimetric sensor for fluoride ion with ratiometric response. The experimental results are supported by computational studies. Chapter 3 Two fluorescent compounds with similar absorption profiles and complementarily emissive properties can be regarded as the ideal couple for the generation of white-light. Two structurally close and complementarily fluorescent boron based molecular siblings 2 and 3 were prepared. The luminescence properties of individual triads were modulated to an extent to complement each other by controlling the intramolecular energy transfer in triads by fine-tuning the dihedral angle between fluorophores in 2 and 3. A binary mixture of 2 and 3 emitted white-light. Chapter 4 This chapter deals with a straight forward strategy for the generation of white-light emission in aqueous media. Using a blue-emissive AIE-active (aggregation-induced emission) 1, 8-naphthalimide- based sensitizer and a boron-dipyrromethene based red emitter as a dopant, water dispersible nanostructures with tunable emission features are produced. The white-light emissive (WL) nano-aggregates are stable at neutral pH and have been elegantly utilized for four-colour cell imaging (including near- infrared imaging). Chapter 5 This chapter describes the design and development of a NIR emitting triarylborane decorated styryl-BODIPY (4) via a facile synthetic route. Incorporation of TAB entities results in a significantly red shifted broad emission in 4 (compared to compound M3 which is devoid of TAB unit). The near coplanar orientation of Ar3B planes and BODIPY core results in a highly efficient (TAB to BODIPY) EET process in 4. Conjugate 4 acts as a highly selective and sensitive fluoride sensor with naked eye visual response as well as ratiometric fluorescent response. The dual emission in fluoride bound 4 possibly results from the restricted partial TAB to BODIPY energy transfer. Chapter 6 This chapter describes how the energy of transitions of the broad emissive molecular triads can be fine-tuned by judiciously changing the spacer oligothiophene length. A series of triarylborane and BODIPY conjugates (TAB-π-BODIPY) has been designed, and synthesized by a combined strategy of changing the connection mode between the two units, extending the conjugation size by introducing terthiophene, quaterthiophene, and pentathiophene units. The electrochemical and photophysical behavior of these conjugates were investigated. The experimental findings were rationalized by density functional theory calculations. Chapter 7 This chapter describes design and development of boron based novel electron acceptor BDY for the bulk-heterojunction solar cell. The electron mobility values of BDY was found to be of the order of standard PCBM. Bulk-heterojunction was fabricated using BDY as the electron acceptor layer. The power conversion efficiency of the newly developed solar cells with BDY as electron acceptor is much higher than the value obtained for standard cells with PCBM as the electron acceptor.

Luminescent materials find numerous applications in recent times and have enriched human lives in several different ways. From display and lighting technologies to security, sensing and biological investigations, luminescent organic compounds have become indispensible and often preferred over their inorganic counterparts. The versatility of organic materials arises from their comparative low costs, ease of fine-tuning, low toxicity and the possibility to develop flexible devices. Even until very recent times, the investigations and usage of organic luminescent materials were mostly limited to solution-state properties. However, with progress of available characterisation techniques and parallel development of their usage in solid-state devices and other applications (e.g. security, forensics, sensing etc.), significantly greater attention has been paid to the development and investigations of solid-state emissive organic materials. In solid-state applications, apart from the molecular properties of any given material, their cumulative i.e. bulk physical properties are of even greater importance. Thus, investigations of structure-property relationships in organic luminescent compounds to understand their molecular and bulk properties are of fundamental interest. In this thesis, NPI (1,8-naphthalimide) and BODIPY (boron-dipyrromethene) dyes were investigated to provide a broad overview of their structure-property correlations. Among commonly encountered organic luminescent materials, NPIs and BODIPYs have emerged as two broad classes of luminescent organic compounds, finding applications as functional luminescent materials in various fields. However, lack of understanding for controlling the cumulative emissive properties of these compounds has limited their usage as active solid-state emitters in various applications. This thesis presents several new insights into the molecular and bulk emissive properties of these two classes of luminescent dyes (NPIs and BODIPYs). The contents of the six chapters contained in this thesis are summarised below. Chapter 1 summarises the available understanding of the basic concepts of photoluminescence and the design strategies to develop solid-state luminescent and AIE (aggregation-induced emission) active materials. This chapter also emphasises in the basic nature of the NPI and BODIPY compounds, their substitution patterns and their inherent characteristics and touches upon the relatively unexplored properties of NPI and BODIPY based materials. The importance and scope of the work reported in the thesis is outlined at the end of the chapter. Chapter 2 describes a detailed investigation of a series of seven (4-oxoaryl substituted) NPI compounds (1-7) providing an insight into the molecular and cumulative photophysical behaviour of these compounds. The low ICT characteristics of the NPIs, coupled with the twisted geometry, facilitated solid-state luminescence in these materials. The solution and solid-state luminescent properties of these compounds can be directly correlated to their structural rigidity, nature of substituents and solid-state intermolecular interactions (e.g. π-π stacking, C-H•••O interactions etc.). The solid-state crystal structures of the NPI siblings are profoundly affected by the pendant substituents. All of the NPIs (1-7) show antiparallel dimeric π-π stacking interactions in the solid-state which can further extend in parallel, alternate, orthogonal or lateral fashion depending on the steric and electronic nature of the C-4′ substituents. Structural investigations including Hirsfeld surface analysis methods reveal that while strongly interacting systems show weak to moderate emission in their condensed states, weakly interacting systems show strong emission yields under the same conditions. The nature of packing and extended structures also affects the emission colors of the NPIs in the solid-state. DFT computational studies were utilized to understand the molecular and cumulative electronic behavior of the NPIs. Apart from the investigation of solid-state luminescence, other functional potentials of these NPIs were also explored. One of the compounds (i.e. 4) shows chemodosimetric response towards aqueous Hg(II) species with a ‘turn-on’ response. Also, depending on the molecular flexibility of the compounds, promising AIEE (aggregation-induced emission enhancement) features were observed in these NPIs. Later (in Chapter 3), we developed a systematic investigation in a series of purely organic NPIs, restricting various parameters, to attain a thorough understanding of such AIEE properties. Chapter 3 describes a detailed experimental and computational study in order gain an insight into the AIE (aggregation-induced emission) and AIEE mechanisms in NPI compounds. Systematic structural perturbation was used to fine tune the luminescence properties of three new 1,8-naphthalimides (8-10) in solution and as aggregates. The NPIs (8-10) show blue emission in solution state and the fluorescence quantum yields depend on their molecular rigidity. In concentrated solutions of the NPIs, intermolecular interactions were found to result in quenching of fluorescence. In contrast, upon aggregation (in THF:H2O mixtures), two of the NPIs show aggregation-induced-emission-enhancement (AIEE). The NPIs also show moderately high solid-state emission quantum yields (~10-12.7 %). The AIEE behaviors of the NPIs depend on their molecular rigidity and nature of intermolecular interactions. The NPIs (8-10) show different extents of intermolecular (π-π and C-H•••O) interactions in their solid-state structures depending on their substituents. Detailed photophysical, computational and structural investigations suggest that only an optimal balance of structural flexibility and intermolecular communication is the effective recipe for achieving AIEE characteristics in these NPIs. Chapter 4 presents the design, synthesis and detailed investigations and potential applications of a series of NPI-BODIPY dyads (11-13). The NPI and BODIPY moieties in these dyads are electronically separated by oxoaryl bridges and the compounds only differ structurally with respect to methyl substitutions on the BODIPY fluorophore. The NPI and BODIPY moieties retain their optical features in these molecular dyads (11- 13). Dyads 11-13 show dual emission in solution state originating from the two separate fluorescent units. The variations of the dual emission in these compounds are controlled by the structural flexibility of the systems. The dyads also show significant AIES (Aggregation-Induced-Emission Switching) features upon formation of nano-aggregates in THF-H2O mixtures with visual changes in emission from green to red color. Whereas the flexible and aggregation prone system (i.e. compound 11) shows aggregation-induced enhancement of emission, rigid systems with less favorable intermolecular interactions (i.e. compound 12-13) show aggregation-induced quenching of emission. The emission-intensity vs. the structural-flexibility correlations were found to be reverse in solution and aggregated states. Photophysical and structural investigations suggest that the intermolecular interactions (e.g. π-π stacking etc.) play major role in controlling emission of these compounds in aggregated states. Similar trends were also observed in the solid-state luminescence of these compounds. The applications of the luminescent dyads 11-13 as live-cell imaging dyes was also investigated. Chapter 5 describes investigations of photophysical properties of a series of six BODIPY dyes (14-19) in which there is a systematic alteration of a common -C6H4Si(CH3)3 substituent. Inrelated constitutional isomers, the systematic increment of steric congestion and lowering of molecular symmetry around the BODIPY core result in a steady increment of solution and solid- state fluorescence quantum yields. The increasing fluorescence quantum yields (solution, solid state) with increasing steric congestions show that the molecular free rotation and aggregation-induced fluorescence quenching of BODIPYs can be successfully suppressed by lowering the flexibility of the molecules. Photophysical and DFT investigations reveal that the electronic band gap in any set of these constitutional isomers remain almost similar. However, the crystal structures of the compounds reveal that the solid-state colour and quantum yields of the compounds in solid-state are also related to the nature of intermolecular interactions. Chapter 6 demonstrates the use of DFT computational methods to understand the effect of alkyl groups in governing the basic structural and electronic aspects of BODIPY dyes. As demonstrated in Chapter 4 and Chapter 5, apparently electronically inactive alkyl groups can be of immense importance to control the overall photophysics of BODIPYs. In this context, a systematic strategy su was utilized considering all possible outcomes of constitutionally-isomeric molecules to understand the effects of alkyl groups on the BODIPY molecules. Four different computational methods were employed to ascertain the unanimity of the observed trends associated with the molecular properties. In line with experimental observations, it was found that alkyl substituents in BODIPY dyes situated at 3/5-positions effectively participate in stabilization as well as planarization of such molecules. Screening of all the possible isomeric molecular systems was used to understand the individual properties and overall effects of the typical alkyl substituents in controlling several basic properties of such BODIPY molecules.